POLITECNICO DI MILANO
Scuola di Ingegneria Industriale e dell'Informazione
Corso di Laurea Magistrale in Ingegneria Elettrica
Preparation and Optimization of Multicomponent Oxide
Ceramics with Micro Capillary Structure
Relatore: Prof. Giovanni Dotelli
Correlatore: Dr. Renato Pelosato
Tesi di Laurea Magistrale di:
Mengshan Hu
Matr. 796136
Anno Accademico 2014-2015
TABLE OF CONTENTS
TABLE OF CONTENTS
TABLE OF CONTENTS....................................................................................................... 1
LIST OF FIGURES ............................................................................................................... 4
LIST OF TABLES ................................................................................................................. 6
ABSTRACT .......................................................................................................................... 7
ABSTRACT .......................................................................................................................... 8
1
Introduction ............................................................................................................. 9
1.1
Research Status of Ceramic Resistors ................................................................... 11
1.2
Vacuum Surface Flashover Theory........................................................................ 15
1.3
Research Status of Heat Dissipation Techniques................................................... 18
1.4
Molding Process of Micro Capillary Ceramic Composites ................................... 21
1.5
Research Ideas, Contents and Significance ........................................................... 22
1.6
2
1.5.1
Research Ideas ......................................................................................... 22
1.5.2
Research Contents and Significance........................................................ 24
Technical Route of This Research ......................................................................... 25
Fabrication and Research of Sintering Method ..................................................... 26
2.1
Fabrication of Materials......................................................................................... 26
2.1.1
Instruments .............................................................................................. 26
2.1.2
Preparation of Samples ............................................................................ 28
2.1.3
Analysis of Microstructure ...................................................................... 30
2.1.4
Performance Testing ................................................................................ 30
1
TABLE OF CONTENTS
2.2
3
Summary................................................................................................................ 40
Preparation and Properties of Mo-doped Alumina Ceramics ................................ 41
3.1
The influence of Mo Doping for Alumina Ceramics in Phase Composition and
Microstructure .................................................................................................................. 41
3.2
3.1.1
The Phase Composition ........................................................................... 42
3.1.2
Microstructure ......................................................................................... 42
The influence of Mo Doping for Alumina Ceramic in Sintering and Mechanical
Properties ......................................................................................................................... 47
3.3
3.2.1
Sintering Properties ................................................................................. 47
3.2.2
Mechanical Properties ............................................................................. 48
The influence of Mo Doping for Alumina Ceramic in Dielectrical Properties ..... 49
3.3.1
The Influence of Mo Doping for Alumina Ceramic in Dielectrical
Properties ............................................................................................................. 49
3.3.2
The Influence of Mo Doping for Alumina Ceramic in Flashover
Performance and Analysis of TSDC .................................................................... 50
3.4
4
Summary................................................................................................................ 53
The Discussion and Optimization of Ceramics with micro capillary structure ..... 54
4.1
Preparation of Ceramics with micro capillary structure ........................................ 54
4.2
Test Platform Establish and Analysis of Properties ............................................... 57
4.3
5
4.2.1
Test Platform............................................................................................ 57
4.2.2
The Properties of Ceramics with micro capillary .................................... 59
Summary................................................................................................................ 61
Stability of Resistance in Mo Doping Alumina Ceramic Doping with SiC, TiC .. 62
5.1
The Influence of SiC, TiC Doping for Alumina Ceramics in Microstructure ....... 62
5.2
The Influence of SiC, TiC Doping for Alumina Ceramics in Phase Composition 66
2
TABLE OF CONTENTS
5.3
The Influence of SiC, TiC Doping for Alumina Ceramics in Dielectrical Properties
68
5.4
Summary................................................................................................................ 72
6
CONCLUSIONS ................................................................................................... 74
7
BIBLIOGRAPHY ................................................................................................. 75
3
LIST OF FIGURES
LIST OF FIGURES
Fig. 1-1 SVC equipment in 35/10kV substation.................................................................... 9
Fig. 1-2 Machine in the magnetic levitation train ............................................................... 10
Fig. 1-3 Machine in the metallurgical industry ................................................................... 10
Fig. 1-4 Voltage-current characteristic of ZnO ceramic resistor ......................................... 12
Fig. 1-5 The influence of graphite doping content on ceramic composites resistivity ........ 13
Fig. 1-6 Mo/Al2O3 ceramic with A-B-A laminated structure .............................................. 15
Fig. 1-7 Micro-capillary group heat sink ............................................................................. 20
Fig. 1-8 Main technical route ............................................................................................. 25
Fig. 2-1 Sintering process of Mo-doped alumina ceramic .................................................. 29
Fig. 2-2 Sintering step of Mo-doped alumina ceramic ........................................................ 29
Fig. 2-3 Vacuum equipment and Pulse voltage generator ................................................... 34
Fig. 2-4 Sample and electrode system ................................................................................. 35
Fig. 2-5 Cleaning process .................................................................................................... 36
Fig. 2-6 Vacuum surface flashover experimental process ................................................... 37
Fig. 2-7 TSDC Test scheme ................................................................................................ 38
Fig. 2-8 CaO -Al2O3-SiO2 phase diagram (high content of Al2O3) ..................................... 39
Fig. 2-9 MgO-Al2O3-SiO2 phase diagram (high content of Al2O3) ..................................... 40
Fig. 3-1 XRD patterns for the alumina/molybdenum composite ........................................ 42
Fig. 3-2 SEM micrographs showing ceramic microstructure (a) 0 wt.% of molybdenum
doping, (b) 1 wt.% of molybdenum doping.(c) 3 wt.% of molybdenum doping.(d) 5 wt.%
of molybdenum doping........................................................................................................ 44
Fig. 3-3 SEM micrographs showing ceramic microstructure (a) 10 wt.% of molybdenum
doping, (b) 20 wt.% of molybdenum doping.(c) 30 wt.% of molybdenum doping.(d) 40
wt.% of molybdenum doping .............................................................................................. 46
Fig. 3-4 EDS of doping-Mo 30wt.% ceramic and BSE line scan analysis.......................... 47
Fig. 3-5 Relationship between Mo doping and relative density of Mo doped alumina
ceramic composites.............................................................................................................. 48
Fig. 3-6 Relationship between bending strength and Mo content ....................................... 49
Fig. 3-7 Resistivity and permittivity of Mo-doped ceramic composites ............................. 50
4
LIST OF FIGURES
Fig. 3-8 The flashover performance of Mo-doped alumina ceramic surface ...................... 51
Fig. 3-9 TSDC test results of undoped alumina ceramic ..................................................... 51
Fig. 3-10 TSDC test results of Mo doped alumina ceramic ................................................ 52
Fig. 3-11 The same area of 5wt.% SEM and BSE of 5wt.% Mo doped ceramics .............. 53
Fig. 4-1 Equipment setting for resistor preparation in factory ............................................ 56
Fig. 4-2 Ceramic component sample with micro capillary ................................................. 57
Fig. 4-3 Mo-Al2O3 ceramic component with micro capillary ............................................. 57
Fig. 4-4 Large-power thermal performance test platform structure .................................... 58
Fig. 4-5 Ceramic resistor test system .................................................................................. 59
Fig. 4-6 The variation in resistivity for increase in temperature for samples with different
resistivity ............................................................................................................................. 60
Fig. 4-7 Power-temperature relationship ............................................................................. 61
Fig. 5-1 SEM micrographs showing ceramic microstructure (a) 0 wt.% of SiC doping, (b) 1
wt.% of SiC doping.(c) 2 wt.% of SiC doping.(d) 3 wt.% of SiC doping, (e) 5 wt.% of SiC
doping .................................................................................................................................. 64
Fig. 5-2 SEM micrographs showing ceramic microstructure (a) 0wt.% of TiC doping, (b)
1wt.% of TiC doping.(c) 2wt.% of TiC doping.(d) 3wt.% of TiC doping, (e) 5wt.% of TiC
doping .................................................................................................................................. 66
Fig. 5-3 XRD of ceramic with different doping content of SiC .......................................... 67
Fig. 5-4 XRD of ceramic with different doping content of TiC .......................................... 68
Fig. 5-5 the influence on volume resistivity of ceramic doping with SiC, TiC, respectively.
............................................................................................................................................. 69
Fig. 5-6 The influence on the resistivity standard deviation of ceramics by the doping
amount of SiC, TiC, respectively. ........................................................................................ 70
Fig. 5-7 the dielectric constant (25℃, 1MHz) and dielectric loss of different SiC content of
Mo-doped alumina ceramics ............................................................................................... 71
Fig. 5-8 the dielectric constant (25℃, 1MHz) and dielectric loss of different TiC content of
Mo-doped alumina ceramics ............................................................................................... 72
5
LIST OF TABLES
LIST OF TABLES
Table 1-1 The application of the two theories and limitations ............................................ 17
Table 2-1 Raw material for alumina ceramic preparation ................................................... 26
Table 2-2 Equipments for preparation of samples ............................................................... 27
Table 2-3 Equipment for Analysis and test .......................................................................... 27
Table 2-4 Composition of ceramic with CaO-MgO-Al2O3-SiO2 quaternary system .......... 28
Table 3-1 Value of the formulations in the ceramic ............................................................. 41
Table 3-2 Energy trap level of alumina ceramic.................................................................. 52
Table 5-1 Shannon periodic of ionic radius ......................................................................... 68
6
ABSTRACT
ABSTRACT
Con il crescente sviluppo dell'industria dell'informazione e la richiesta di componenti
elettronici sempre più piccoli, lo studio della resistenze per applicazioni di grande potenza
è divenuto un argomento di fondamentale importanza. In questa tesi sono stati preparati
campioni di materiale resistivo ceramico a base di allumina drogata con molibdeno; ne è
stata indagata la microstruttura, le proprietà dielettriche e meccaniche. L’obiettivo è quello
di migliorare la stabilità e controllabilità della resistività dei materiali e ottimizzare la
dissipazione del calore dei materiali utilizzando la tecnica micro-capillare.
In primo luogo, è stat preparata una serie di campioni ceramici basati sul sistema CaOMgO-Al2O3-SiO2 contenenti percentuali crescenti di fase conduttiva (Mo) a 1600 °C. I
risultati mostrano l’effetto del contenuto di Mo sulla resistività del composito,
evidenziando la soglia di percolazione: quando il Mo è presente in quantità inferiore al
20% in peso, la resistività del composito in ceramica cambia solo leggermente; quando la
quantità di Mo supera 20%, la resistività del materiale composito diminuisce drasticamente
(< 1010 ∙cm), in quanto il Mo passa da fase dispersa a fase continua. Quando la quantità
di anti-drogaggio di Mo supera il 40wt.%, la resistività del composito è estremamente
bassa (<10∙cm) e non subisce più cmbiamenti all’aumentare del Mo, grazie alla
percolazione della fase conduttiva. nell’intervallo tra il 20 ed il 40%, il materiale mostra un
comportamento conduttivo con andamento ohmico (lineare voltaggio -corrente).
In secondo luogo, secondo la tecnica micro capillare, permette di unire alla resistenza ad
alta potenza anche una più efficiente dissipazione di calore ai composti preparati. In questo
processo, il materiale ceramico è stato formato con un opportuno processo nella struttura
microcapillare desiderata. Le prove effettuate mostrano che i manufatti ottenuti soddisfano
i requisiti di elevata dissipazione del calore.
Infine, i composti con il 30% di Mo sono stati drogati con diverse quantità (1 – 5%) di
semi-conduttore (SiC, TiC); il drogaggio con TiC ha un effetto più significativo, e può
aiutare a stabilizzare la resistività del materiale.
Parole chiave: Resistenza ad alta potenza, Cerme, Micro capillare.
7
ABSTRACT
ABSTRACT
With the development of information industry, the trend of electronic components become
smaller and with larger power, the topic of high power resistance is hot among domestic
and foreign countries. In this paper, we prepared alumina ceramics doped with
molybdenum, to investigate their microstructure, mechanical and dielectric properties. In
the premise of maintaining excellent mechanical and dielectric properties, we focused on
the improvement of stability and controllability of resistivity, and using micro capillary
technic to optimize the heat dissipation of ceramics.
First, we prepared alumina ceramics doped with high conductive phase (Mo) at 1600°C,
which are formed by CaO-MgO-Al2O3-SiO2 system. The results indicates that the effect of
Mo content in resistivity showed significant percolation characteristics: When doping
amount below 20wt.%, the resistivity of composite ceramic changed slightly; When the
doping amount of Mo exceeds 20wt.%, because the Mo shifted from the dispersed phase to
continuous phase, the resistivity of the composite material was drastically decreased
(<1010Ω∙cm). When the doping amount of Mo reached 40wt.%, since the Mo has formed
continuous phase of the material into conductive material (<10 Ω∙cm), no significant
change in resistivity. When the doped content is between 20wt.% and 40wt.%, the ceramic
become conductive with straight volt ampere characteristic curve.
Second, according to the micro capillary technic, we combine the most efficient heat
dissipation technic with high power resistance, to product multicomponent oxide ceramics
with micro capillary. In this process, the ceramic is formed by specific craft, molded with
customized tool, and meet the requirement for heat dissipation.
Finally, with different amount of semi-conductive phase (SiC, TiC) adopted into Al2O3/Mo
ceramics, but TiC doping has a more significant effect, which may help to obtain specific
resistivity of ceramic materials. When doping amount of TiC is 3wt.%, the standard
deviation of resistivity reach to the lowest.
KEY WORDS: High power resistor, Cermet, Micro capillary.
8
CHAPTER 1
1 Introduction
The continuous development of science and technology gives a rapid enhancement of
highly integrated, high-power miniature electronic equipment. With these advancements,
the production of high power density but small sized resistors has become the focus of
research at home and abroad, thus in recent years, investment in power-related industries
increased significantly1. In High Voltage Direct Current process, several challenges
actually require a solution such as maximizing the transmission line capacity, improving
system stability and solving the tough problem of high-power impact load and
unbalanced2. Applying Static VAR Compensator (Fig.1-1) for reactive power
compensation is one of the cost-effective solution for the problems mentioned above. The
core part of SVC is the rectifier inverter (also known as converter valve), which is
composed by high-power resistors and other components. Power resistors are widely used
in not only the electric power industry, but also high-speed rail, magnetic levitation train,
emergency braking systems as well as the metallurgical industry, (see Fig.1-2). Reaching
an acceptable reliability, high stability and efficiency, has been an issue of great concern to
the users.
Fig. 1-1 SVC equipment in 35/10kV substation
9
CHAPTER 1
Fig. 1-2 Machine in the magnetic levitation train
Fig. 1-3 Machine in the metallurgical industry
The so-called high-power resistor means that the power can reach hundreds of watts, in
which the power, the size and the raw material have a significant difference compared with
carbon film resistors, metal film resistors etc.. When the power of compact resistor element
exceeds 1kW (up to 3kW) it can be considered as a large power resistor element.
The improvement of electrical equipment technique requires the resistors to be high-power
miniatures, so the requirement of heat flux density becomes more stringent. However, the
common methods actually used such as heat pipe cooling, thermal capillary pump loop,
10
CHAPTER 1
forced liquid single phase convection cooling, etc., hardly meet the requirements for the
heat removal and space occupancy.
Studies have shown that the restrictive key factors to the reliability of high-power
resistance equipment is the heat flow capacity and the operating current efficiency. Thus, in
order to achieve an efficient and reliable cooling system, under the premise of the current
flow capacity, work equipment that meet the more stringent requirements has become one
of the key factors in promoting HVDC, SVC and other development.
This thesis is about the production and synthesis of resistive materials that approach to be
the ceramic resistors having excellent global properties with appropriate additives to
control the resistivity and to increase the stability in order to meet the actual requirements
for the electrical power industry. Existing micro-capillary technology is used to optimize
the resistance because it improves the heat dissipation performance.
1.1 Research Status of Ceramic Resistors
Current research on the resistive materials, mainly focus on some filed such as ceramic
resistors, zinc oxide resistors, graphite resistors, silicon carbide resistors, carbon film
resistors etc.; a brief introduction of each kind of resistor will follow in the next sections.
(1) Wire-wound resistors and cement resistors
Wire-wound resistors3 and cement resistors are currently used in electrical equipment.
Power type wire wound resistors4 are typically associated with high-power semiconductor
devices. Heat dissipation of resistor increases surrounding ambient temperature, the
thermal effects is too prone to lead to current explosion5, at the same time, the volume of
the cement resistance is too large, which is not beneficial to the miniaturization of
electrical equipment.
(2) Zinc oxide (ZnO) ceramic resistors
ZnO ceramic linear resistor has a great advantage in various circuit applications, especially
under the situation of low inductance, shock energy, peak power or high-pressure etc. Zinc
oxide ceramic resistor presents the advantages of a linear voltage-current characteristic
curve, resistivity controllability, and small positive temperature coefficient that plays an
important role as neutral grounding resistor in 10kV-110kV transmission line6. It has been
11
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demostrated7 that Zinc Oxides samples (30mm x 5 mm sized) can crack during the
electrifying process if the temperature exceeds 150 °C, because made by a kind of
semiconductor. Under certain conditions, by changing thickness and resistance value,
although the linear characteristic, resistor values can meet the dual requirements of
controllability and linearity, because of the thinner grain boundaries and non ideal thermal
conductivity that is prone to the impact of high current burst. Moreover, only under a
constant current value, linearity of volt-ampere characteristics in zinc oxide ceramic
resistor we can see from Fig.1-4.
1000
Voltage/V
100
10
1
0.1
1
10
100
Current/A
Fig. 1-4 Voltage-current characteristic of ZnO ceramic resistor7
(3) Graphite ceramic composite resistors
Graphite ceramic composite resistor is widely used in the aerospace, military, heating
construction and other specific fields because of its good conductivity, high temperature
withstand, good insulation, etc. Li Yan8 tried to add black carbon and graphite to kaolin
soil to prepare composite ceramic resistance materials. He found out that when graphite
doping amount is greater than 25%, the temperature of material caused by heat dissipation
could reach up to 400 °C, maintaining good electrical conductivity. Chen Wei et al.9 made
an experiment of impulse voltage on graphite ceramic resistor. It shows that the resistance
changes insignificantly at 10s intervals and voltage coefficient is relatively low with the
increasing shock times and the resistance tends to a constant coefficient. Zheng Xin10
realized that the increase of graphite content leads to a sharp decrease of the resistivity of
12
CHAPTER 1
graphite/ceramic composite conductive material, when the doping is greater than 15%,
while the resistivity tendency is to be smooth. Although resistance of graphite ceramic is
small and meet voltage-current characteristics, its disadvantages, such as oxidation,
volatilization over 300 °C and difficulties in metallization, make it not able to meet the
requirements of high-power resistance at high temperature (Fig.1-5). Similarly, other
ceramic matrix/graphite composites also exist with this problem even at low temperature.
11
10
⊥
9
∥
Fraction
Fraction
Resistivity/Ω·m
8
7
6
5
4
3
2
1
0
5
10
15
20
25
30
35
40
Graphite Doping/wt.%
Fig. 1-5 The influence of graphite doping content on ceramic composites resistivity
(4) Silicon carbide (SiC), titanium carbide (TiC) and metal oxide ceramic composite
resistors
Due to the chemical stability property, high thermal conductivity, small thermal expansion
coefficient, good durability, SiC is prospected in a wide range of applications such as home
appliances, building heating, healthcare, etc.11. Zheng Xin12 found out that when the
content of SiC rises from 1% to 3%, both the porosity and the resistivity increases, but the
bending strength is reduced. When the silicon carbide content is up to 9%, the porosity
decreases, the bending strength increases and the resistivity significantly drops. However,
the main drawback of SiC is represented by the non-linearity of the volt-ampere
characteristic resistivity curve that means SiC cannot meet the high power requirement.
13
CHAPTER 1
Titanium carbide ceramics are rapidly developing among the transition metal materials.
Doping TiC particles improves thermal shock and thermal conductivity of material and
enhance the strength and toughness of Al2O3 ceramic13. This result is due to the dispersion
of the TiC particles that impedes Al2O3 grain growth. However, the resistance of TiC is
unable to meet the linear ampere-voltage characteristic and results difficult to weld with
metal like SiC14.
Mo/Al2O3 ceramic composite materials are better at high temperature, presenting good
thermal conductivity and thermal shocks. Zhang Huijun et al.15 fabricated composite mixed
Mo and Al2O3 powder with different molar ratio. He found out that with the increasing
content of Mo porosity is reduced with enhanced strength, while Mo mainly transfers from
the dispersed phase into the continuous phase. Liu Kaiqi et al.16 studied the oxidation
behavior of Mo/Al2O3 and realized that sintering at temperature higher than 500 °C in air,
leads to an increase of MoO2 weight, while when the temperature is higher than 700 °C
samples lose weight due to the evaporation of MoO3. Nawa et al.17 produced Mo/Al2O3
composites at nanoscale and they found hardness and fracture toughness of material
increased than before dispersing Mo.
Shengtao Li et al. 18 did research on Al2O3 /Mo composite as dielectric material to suppress
vacuum surface flashover, achieving an Al2O3 /Mo cermet - Al2O3 ceramic - Mo/Al2O3
cermet (A-B-A) with gradient bonded insulation structure. Experimental study of the
structure and its DC and pulsed vacuum surface flashover performance (Fig.1-6) revealed
that DC and pulsed vacuum surface flashover voltage increased by 52% and 86%
respectively. S. Hussain19 found that when the Mo content exceeds 20 vol% by volume the
conductivity increases rapidly. But Mo only as an additive metal incorporation (<10 wt.%)
and Mo as a resistive material has not been reported.
14
CHAPTER 1
(a) Sample
(b) Sketch
Fig. 1-6 Mo/Al2O3 ceramic with A-B-A laminated structure18
1.2 Vacuum Surface Flashover Theory
Over years, the mechanism theory of vacuum surface flashover during the first and the
third period has reached a consensus, which is initiated by electron emission at a threepronged point of contact the insulating material-the cathode-vacuum surface, and
15
CHAPTER 1
ultimately form channel in the adsorbed gas layer of insulating material surface20.
However, different models are used to explain the development process of vacuum surface
flashover theory; two models are widely accepted to explain the theory: Secondary
Electron Emission Avalanche (SEEA) 21 and Electron Triggered Polarization Relaxation
(ETPR) 22.
(1) Vacuum Surface Flashover Theory
The initial electronics, which formed at three-pronged point of the insulating material
surface-the cathode–vacuum, will hit the insulating material surface under external electric
field, to produce new electrons called secondary electrons. of which there are part of the
secondary electrons impinge insulation material surface to produce more electrons,
wherein, the ratio of the number of emitted electrons with the number of incident electrons
is called secondary electron emission coefficient (δ).
When δ> 1, the amount of emitting electrons is more than the incident electrons, so that the
secondary electron emission will leave positive charge on surface of the insulating
material, resulting in surface charging. electrons collide with the launch occurred in
succession on insulating material surface, resulting in secondary electron avalanche, and
push secondary electrons move toward the anode.
Secondary Electron Emission Avalanche model shows that the initial electron emission and
the formation of secondary electron avalanche are two key factors of vacuum surface
flashover, while positive charge on insulating material surface also plays an important
effect on electron avalanche development. Secondary Electron Emission Avalanche model
can be used to explain the nanosecond delay and the insulating material surface
electrification of the vacuum surface flashover effect under micro/nanosecond pulse
voltage.
(2) Electron Triggered Polarization Relaxation (ETPR).
Secondary electron avalanche theory explains many vacuum surface flashover
phenomenon, but cannot explain the flashover without applied electric field, in response to
this issue, Blaise and Gressus22 proposed Electron Triggered Polarization Relaxation
theory.
16
CHAPTER 1
By this theory, applying an external electric field or electron beam, original defect of
insulating material itself and trap center by polarization, caused by non-uniform dielectric
constant, will constantly trap charge. Positive or negative charge center is formed with the
charged trap of insulating material surface in different surface area. These charge centers
will produce an electrostatic field to form a very unstable equilibrium. Any slight change in
external factors, are likely to destroy the balance.
The trapped charges will escape from the trap after getting enough energy by disturbance
of external radiation, particles, heat, light, electric field and destroy the original balance,
which cause polarized energy release. Many high-energy electrons and emitted photons
accompany the whole process, leading to an enhancement of the electron multiplier stage,
hence to vacuum surface flashover phenomenon. ETPR model is used to explain flashover
second delay under DC pressure and vacuum surface flashover without applying external
voltage.
(3) Comparison and Analysis of Two Models (SEEA and ETPR)
SEEA and ETPR, these two models explain physical phenomena in the flashover process,
but not for every flashover mechanism. Tab.1-1 summarizes the applications and the
limitations of the two theories23.
Table 1-1 The application of the two theories and limitations23
Theory
Explanation
Non-Explanation
Nano-second flashover
Analysis gas Adsorption-Desorption
SEEA
In response to the magnetic field
DC flashover
Secondary electron emission coefficient of correlation
DC flashover
Analysis gas Adsorption-Desorption
ETPR
Insulator characteristics associated
Nano-second flashover
Flashover with accelerator
Flashover initiated from Anode
17
CHAPTER 1
1.3 Research Status of Heat Dissipation Techniques
Increasing HVDC and SVC device power level leads to a stricter demand for ceramic
material with heat dissipation properties. Nowadays, the common systems for heat
dissipation are heat pipe cooling, thermal capillary pump loop, liquid single-phase forced
convection cooling, micro-channel liquid single-phase forced cooling, forced air-cooling,
thermoelectric cooling and thermal cooling micro-capillary group. In the next sections a
brief introduction for each configuration will follow.
(1) Heat pipe cooling
Heat pipe cooling mainly consists of a tube shell, wick and cover, which transfers heat by a
thin film with Meniscus shape. At the ends of the tube there are the evaporator and the
condenser separated by the adiabatic section. After the liquid is vaporized in the
evaporator, heat is released to form liquid again in the condenser under small pressure
difference of micro-capillary action. Because of the very effective heat transfer capacity,
strong heat flux conversion capabilities, as well as the characteristics of the special design
of the thermostat, heat pipe is always used to help computer chips in dissipating heat24.
(2) Capillary pumped loop (CPL) loops and loop heat pipe (LHP)
Capillary pumped loop (CPL) loops and loop heat pipe (LHP) transfers heat by thin film
with Meniscus shape. The main source of heat separate to two parts, evaporation end and
heat sink side for cooling and solidification 25.
CPL and HLP structure avoids many of the shortcomings of traditional heat pipe. However,
Maidanik26, found that in the part of the triggering mechanism boiling has limits in CPL
cooling device. Still the problem of heat leakage occurs because of the structure design of
CPL and LHP, while the micro-capillary inner wall will have a greater pressure drop to the
inner core that makes it is easy to reach and even exceed the kernel boiling limit.
(3) Liquid single-phase forced convection cooling applications
Liquid single-phase forced convection cooling applications with regular size is mainly
applied to the cooling electronics devices, optoelectronics devices, and other precise small
devices for a long time. In addition, it makes use of special cooling fluid, heat pump part
and connective circulate pipe by forced one-directional fluid cooling of high-efficient heat
18
CHAPTER 1
conducting liquid to transfer heat from inner to outside. This system temperature can be
reduced and lower energy will be consumed in pump with conventional size, which is
suitable for long distance heat transport. However, due to the high complexity of its
installations, during the operation it needs to overcome the pressure drop of the water cycle
while the entire cooling system in power electronics intensive area, so the requirements for
airtightness in such cooling system is very high27.
(4) Microchannel liquid single-phase forced convection cooling
In the heat dissipation aspect of microelectronics and optoelectronics, micro-channel liquid
cooling method has a great advantage in unilateral convection, which is based on principles
of microscale heat. Although this forced convection cooling method has strong cooling
capacity for the entire device with small resistance, the pressure in differential system is
relatively large because of its very small size, resulting in a large temperature gradient. It is
likely to result in incoherent pipeline, accumulates dirt, so the demand of micro-pump
performance is very high28.
(5) Forced air cooling
Combination of heat sink and fan for heat dissipation in electronic or optoelectronic
devices is typically used, but integration level and power of precision devices increase
performance in heat generation, so that this way makes load close to the limit29.
(6)Thermoelectric cooling
Thermoelectric cooling exploits the temperature difference (Peltier effect). Nowadays,
electrical materials are generally made of alloy metal with doped semiconductor. Thus, the
thermoelectric cooling is also called semiconductor cooling. Due to the absence of sliding
parts in semi-conductor refrigerator, this kind of cooling system is usually used in high
reliability requirements, where the space is limited and with small thermal inertia,
controllable precise temperature. However, currently there is no breakthrough on material
aspect, so thermoelectric cooling conversion efficiency is not high, which also need largescale integration with high-cost30.
19
CHAPTER 1
(7) Micro-capillary group heat sink
Micro-capillary group heat sink has advantages in heat-taking capability, temperature
controllability, and long-distance heat transportation, low-cost. Tang Dawei, Dr. Hu
Xuegong et.al. [31-32] conducted in-depth research on micro-capillary group heat sink. They
found that two mechanisms are in micro capillary with rectangular shape, which all have
high pressure drop and strong intensity, first is heat transfer only by evaporation under low
heat load, the other is joint heat exchange with the liquid in boiling nuclear states under
high load. Both of the two heat transfer mechanisms are with high strength. They also
found that the geometry of the micro-capillary groups has a significant effect on wetting
characteristics of the liquid micro-tank. For a capillary with 0.2mm width and 0.7mm
depth, the density of the tank was 2500l/m, the structure allows a better wetting, so that
heat transfer and cooling effect are both improved.
(a) Open micro capillary structure
(b) shape in the micro capillary
Fig. 1-7 Micro-capillary group heat sink
Liquid flow under pressure from micro capillary structure, in the same time, a liquid thin
film is formed near the three-phase contact line (solid, liquid, gas), and with the high
strength evaporation capacity, high density of heat exchange can be achieved.
Derjagui33, first pointed out that liquid evaporation rate is related to the thickness of the
film. Evaporation rate is low in thin film area, while very high in thick part. Ayyaswamy34,
define the friction factor coefficient K under liquid flowing, which provides a theoretical
basis for the calculation for micro capillary heat transmission.
Ji and Yan35 made several researches on the thermal conductivity in the case of a microchannel heat sink gas-liquid-solid phase contact surface nearby using the molecular
20
CHAPTER 1
dynamic theory. The simulation results show the presence of nano-scale liquid film on
solid heat transfer surface. Since the film is not easy to evaporate, and according to the
simulation result, they also proposed the thickness of three-phase contact line under
different liquid temperature of micro-capillary bottom. They found out that the thickness
changed just a little in case of various number of molecules, but reduce when underlying
micro fluid heating temperature increase, which indicate that the strong interaction among
solid-liquid molecules.
Ayyaswamy, Catton and Edwards36 established two-dimensional steady laminar flow
motion equations with Galerkin boundary method, wherein, the contact angle of the
triangular micro capillary and meniscus surface are important parameters. Theoretical
results show that, average speed of laminar flow movement, hydraulic diameter and area,
as well as the friction coefficient relevant diagrams based on the hydraulic diameter under
the contact angle between 0-60 °C, and highly matched with experimental data.
Renk and Wayner37, 38 measured and studied the surface of ethanol meniscus component, to
obtain the function related to the meniscus shape, wherein, the liquid evaporation rate is an
independent variable. The results show that when the meniscus in a stable condition, with
the heat flux between 0 and l.36W/cm2, meniscus shape is related to heat flux. This proves
that meniscus shape changes can make liquid flow replenish in the stable evaporating
meniscus region, which lead to a very high local flux density, also taking into account the
liquid surface tension.
Ma and Peterson 39, 40 reported that the tension produced in the meniscus curve with more
liquid is weaker than friction hinder, due to liquid flow in evaporated film and absorbent
layer. Therefore axial flow depends on liquid layer region, and evaporation occurs
primarily on both sides of the wall of the thin film area in micro capillary.
1.4 Molding Process of Micro Capillary Ceramic
Composites
To meet the requirements of small power products, we produced Mo / Al2O3 composite
ceramic material to cool resistor by micro capillary structure. For alumina ceramic,
molding the alumina ceramic is the main process that lumps the dispersed material system
into a geometric bulk with certain strength. Several forming method can be used for
21
CHAPTER 1
alumina ceramics, such as hot casting, grouting, extrusion, molding method multiple static
pressure casting, centrifugal grouting etc. In this work we used isostatic press technique.
Isostatic press41 is mainly divided into three types, such as cold isostatic pressing, heat
isostatic pressing and hot isostatic pressing in accordance with the temperature of molding
and solidification. The mold usually is made of rubber or plastic for an outer sleeve and the
pressure medium is mostly water.
Hot Isostatic Pressing (HIP) technique has the following characteristics: (1) The sample
produced by using isostatic pressing, whose compact density is 5%~10% higher than the
average pressure of bi-die direction. (2) The powder by the isostatic pressing force in each
direction is uniform, thus the density difference of each sample is less than 1%. (3) In
molding process, it is possible to prepare complex samples with large aspect size ratio
because of uniform pressure. (4) Isostatic pressing process, different from conventional
molding process, does not require additives, which not only reduces material pollution, but
also reduces the processing steps. (5) Sintering temperature of isostatic press is low, when
compared with other molding process, achieving excellent physical properties.
In summary, cold isostatic pressing stress is more uniform, by far, which is the most widely
used and cost-effective molding technic.
1.5 Research Ideas, Contents and Significance
1.5.1 Research Ideas
At present, the qualified rate of domestic production of ceramic linear resistance is low,
because the impact factor and regularity of ceramic linear resistance properties are still not
completely clear, so that materialize on the formulation and process optimization can not
be achieved. Based on these current problems, the focus is on the formula and the process
of ceramic material to study the factors affecting the performance of the resistance and to
do a preliminary exploration on the firing mechanism and the relationship between the
morphology and ceramic dielectric properties.
For the problem of power control and heat dissipation of ceramic materials, this study
focuses on the preparation of the material, not only for obtaining good mechanical
properties of the material, but also for the improvement of the dielectric properties; while
22
CHAPTER 1
taking advantage of already available technology that combine micro capillary with high
power resistors to develop heat dissipation effects and then to explore the relationship
between microstructure and their electrical properties of the material and optimize the
components of the formulation and preparation of process.
(1) Research on Ceramic linear resistance formula and Key Preparation Technique
Compared with alumina, Mo has a vapor pressure at lower temperature with more
significant volatility, so that in the process of composite material preparation, the sintering
temperature should be reduced in order to avoid high temperature volatilization of Mo. In
the requirement of actual production, different types and amounts of additives are added to
reduce the sintering temperature and the production needs of the actual product. According
to different mechanism, addictives can be divided into two categories: First, additives
which can form a solid solution with aluminum, which mainly belongs to variable valence
oxide such as TiO2, Cr2O3, MnO2 and Fe2O3, etc., second is liquid phase additives formed
with alumina, such as kaolin, SiO2, CaO, MgO, etc.
CaO-MgO-Al2O3-SiO2 system quaternion components used in this paper belongs to the
second category of additive. This kind of Ceramic material has advantages of low firing
temperature, small grains, organizational dense structure, antacid ability, strong corrosion
resistance. Meanwhile, Mo can not only reduce the secondary electron emission
coefficient, but also as a first type of additives, which can form a solid solution with Al2O3,
activate Al2O3 lattice.
Here, we do experiment to obtain a sample with a suitable porosity and resistivity in
required size, by the linear mechanical resistance and dielectric properties affected by
material formula, sintering temperature, pressure molding . The above study finally was to
set a better process recipe, and lay the foundation for future research.
(2) Research on microstructure, conductivity mechanism and SiC, TiC doped with
Mo/Al2O3 ceramic composites
Conductivity mechanism analysis here to study the relationship between morphology and
resistivity of ceramic material.
23
CHAPTER 1
Based on the study of Mo-doped alumina ceramics, by introducing highly conductive
phase (SiC, TiC) and co-doped with Mo in the formulations, the effect of microstructure
and stability of resistor by adding high conductivity material to ceramic / metal composite.
(3) The combination of micro-capillary and ceramic resistance
Using formulation process resulting from (1), the micro-capillary composite phase-change
cooling technology combined with high-power ceramic resistors, finally, ceramic resistor
can be produced with controllable resistivity, ampere-voltage linear characteristic and a
high efficiency of heat dissipation, which is adapt to large-scale industrial production, to
meet the actual requirements of electrical equipment.
1.5.2 Research Contents and Significance
For high power density resistance materials, the use of Mo/Al2O3 ceramic composite has
some benefits. First, Molybdenum has a high melting point, and its resistance presents liner
characteristic over a wide temperature range; Second, Mo/Al2O3 cermet with merit of high
temperature, thermal shock and current resistance, etc., so that to meet the technical
requirements of high-power density resistance components
In this study, we applied micro-capillary structure to the Mo/Al2O3 ceramic as cooling
technology, which will improve thermal performance and reliability of resistors. In the
working process, heat-taken device (resistor) and heat-spread part (micro-capillary) has
been integrated in the cermet, therefore the working temperature of high-power resistor can
be reduced significantly.
On the basis of the preparation of CaO-MgO-Al2O3-SiO2 ceramics, we can observe the
phase composition, microstructure, mechanical properties, and dielectric properties of
cermet by certain series of test.
The importance of this study consist on the preparation technique and electrical properties
tests of alumina composite ceramic materials with different dopants. We explored the
relationships among dopants, ceramic resistance and material stability, while based on
good mechanical properties of ceramic materials and dielectric properties. We tried to
improve the stability of the ceramic material, to achieve large-scale production of industrial
commodities. At the same time, the use of micro-capillary technique for heat dissipation,
take more possibility to future production.
24
CHAPTER 1
1.6 Technical Route of This Research
The main technical route in this paper is shown in Fig.1-8.
Fig. 1-8 Main technical route
25
CHAPTER 2
2 Fabrication and Research of Sintering Method
2.1 Fabrication of Materials
This chapter introduces the experimental methods to prepare the materials, focusing on
equipment used, components of raw materials, structure analysis, test properties and
research performed related to the ceramic sintering process. The main raw materials for
preparing alumina ceramic doped Mo are shown in Tab.2-1, where the particle size of
alumina powder is controlled in the sub-micron scale.
Table 2-1 Raw material for alumina ceramic preparation
Name
Molecular Formula
Purity
Manufacturer
Alumina
α-Al2O3
CP(A.R.)
Silica
SiO2
CP(A.R.)
SCR
Magnesia
MgO
CP(A.R.)
SCR
Calcium oxide
CaO
CP(A.R.)
SCR
Polyvinyl alcohol
[C2H4O]n
CP(A.R.)
SCR
CH3CH2OH
CP(A.R.)
SCR
Mo
99.9%
Jinduicheng Co.,Ltd
Ethanol
Molybdenum powder
Sinopharm Chemical Reagent Co.,Ltd
2.1.1 Instruments
(1) Instruments for ceramic preparation
The equipments used for the preparation of ceramics are shown in Tab.2-2. The pressure
forming machine for producing alumina ceramic provide a pressure of 200 MPa and the
cold isostatic press machine applies a pressure of 100 MPa. Electronic balance here for
weigh, whose scale has an accuracy of 0.001g.
26
CHAPTER 2
Table 2-2 Equipments for preparation of samples
Name
Model
Electronic balance
XY300JB
Roller mill
QM-1F
Planetary ball mill
QM-3SPO4
Oven
DGG-9053
Pressure forming machine
HPC-63
Cold isostatic press
KIC1.3/5
Hand presser
YS 210
Muffle furnace
N11-H
Nabertherm furnace
LHT02/17
Electric resistance furnace
YFG200×250-16-YC
(2) Test instruments
The equipment used to analyze and test the prepared ceramic samples are shows in Tab.23. For testing, the electronic balance for weighing raw material quality has accuracy of
0.0001g, which is different from the electronic balance used for ceramic sample
preparation equipment.
Table 2-3 Equipment for Analysis and test
Name
Model
electronic balance
FA2004N
X - ray diffraction
D/MAX-2400
SEM
Quanta 250FEG
Laser particle size analyzer
Rise 2008
Universal tensile testing machine
Instron1195
high resistance meter
Agilent 4339B
The dielectric and dielectric loss tester
HIOKI 3532
27
CHAPTER 2
2.1.2 Preparation of Samples
The alumina ceramic material prepared is based on CaO-MgO-Al2O3-SiO2 quaternary
system, with mass proportions shown in Table 2-4. In particular the SiO2 / CaO (mass
ratio) is 4.8: 1, in order to ensure sufficient liquid phase that can be formed during the
sintering process and for promoting the sintering of alumina ceramic.
The components reported in Tab.2-4 are doped with 0, 1, 3, 5, 7, 10, 20, 30, 50wt.% of Mo,
respectively. The sample preparation process is shown in Fig.2-1. In order to mix
uniformly Mo with alumina, the raw materials powders have been put into a milling tank,
together with ethanol and balls, and rolled for 12 hours.
Table 2-4 Composition of ceramic with CaO-MgO-Al2O3-SiO2 quaternary system
Component
Mass fraction /wt.%
SiO2
2.7
Al2O3
95-97
MgO
1-1.5
CaO
0.6
After mixing and drying the mixture, the mass powder is stirred in a mortar with PVA
added. Wherein, the PVA mass fraction is of 10wt.%, so that PVA and powders can be
mixed enough and then the green bodies were produced at 100 MPa pressure force. The
precise granular powder is obtained after crushed in a 40 mesh sieve. The last samples with
different sizes are molded under bidirectional 200MPa pressure force.
28
CHAPTER 2
Fig. 2-1 Sintering process of Mo-doped alumina ceramic
10
C
·min -1
Temperature/C
1600C
RT
RT
0
2
4
6
8
10
Time/h
Fig. 2-2 Sintering step of Mo-doped alumina ceramic
The green body is placed in a muffle furnace for PVA evacuation, and then it is transferred
to a high-temperature sintering furnace, where the sintering process is realized as shown in
Fig. 2-2. The temperature of sintering in a furnace up to 1600 °C and hold for 2 hours
29
CHAPTER 2
where the heating rate of 10 °C·min-1 is controlled during the sintering process. In order to
keep the sample heated evenly, buried powder sintering is used, and then the sample can be
cooled automatically after sintering furnace shut down. At the end of the process,
compounds doped with Mo, SiC, TiC are ready to be tested.
2.1.3 Analysis of Microstructure
(1）Phase analysis
In this experiment, X'pert pro X-ray diffractometer (Philips Company) was used for phase
analysis of Cu Kα line with a target wavelength of 1.5406 Å. It is scanned with a step size
of 0.02 °2.
(2）Morphology analysis
Field emission SEM (Quanta 250FEG) was used to observe the surface fracture of the
samples and surface morphology with spectrometer (EDS, Model Oxford INCA, UK) for
the elemental composition of specimen fracture and distribution.
2.1.4 Performance Testing
(1) Testing for Linear shrinkage, relative density and apparent porosity
Significant dimensional changes occur after sintering. Linear shrinkage is to measure the
density of ceramic. Vernier is used to measure the line length of two parallel front of the
compounds before and after sintering; each material is tested three times to obtain the
average value results.

Lo  Ls
100%
Lo
(2-1)
where：
α ——shrinkage in%,
Lo —— line length before sintering in mm,
Ls ——line length after sintering in mm.
30
CHAPTER 2
The relative density is the ratio between the bulk density and the theoretical density; the
bulk density is calculated by the formula (2-2).
ρ=
m0
ρ
m2 -m1 water
(2-2)
Where,
ρ ——bulk density / g·cm-3,
m0 ——Mass of dry sample/g,
m1 ——Sample mass in water after sufficient absorbent /g,
m2 ——Sample mass in air after sufficient absorbent /g,
ρwater ——Density of distilled water / g·cm-3.
Apparent porosity is the ratio between open pores and the total volume. Apparent porosity
is calculated by equation (2-3)
P
m2 -m0
 100%
m2 -m1
(2-3)
Where,
P ——Apparent porosity /%,
(2) Flexural strength test
Load value per unit area is measured in flexural strength test (Universal Tensile Tester)
where the test pieces are placed between the two points until destruction. Method of
bending three-point was used here to test with a span of 16 mm, loading rate of 0.5
mm·min-1, the samples with surface polished 3mm x 4mm x 35mm,which is in a standard
size. Five set of each samples is tested and the average value of the failure load was taken
in each set of the five samples. The flexural strength is calculation using the formula below
(2-4).
31
CHAPTER 2

3F  L
2b  h 2
(2-4)
Where,
σ ——flexural strength /MPa,
F —— destruction load/N,
L ——span between two supporting points /m,
b —— width/m,
h —— thickness/m.
(3) Dielectric Performance Testing
High impedance meter (HP 4339B) is used to measure the volume resistivity of the
material. The DC resistance measurement ranges from1.0×103 Ω to 1.6×1016 Ω which is a
very wide range of resistance measurement. It takes 60s to obtain average value of volume
resistance.
Rv 
v  d
S
(2-5)
Where,
Rv —— volume resistance/Ω·m,
ρv —— volume resistivity/Ω·m,
S ——superficial area /m2,
d —Thickness/m.
So, the volume resistivity can be calculated as (2-6),
v 
Rv  S
d
(2-6)
Permittivity and dielectric loss tester (HIOKI 3532 ) is used for measuring the relative
dielectric constant of the material by the bridge method and each sample measurement is
32
CHAPTER 2
repeated three times to obtain an average value. In addition, the device can be used directly
to measure dielectric loss.
r 
Cd
0S
(2-7)
Where,
C ——Capacility/F；
d ——Thickness/m；
S —— Area/m2；
 0 ——permittivity of free space, equal to 8.85×10-12 F·m-1.
(4) Vacuum Surface Flashover Test System
The test system consists of pulse voltage generator, vacuum chamber, the electrode system
and oscilloscope measurement system. In the test, can be observed in real-time if the
flashover occurs on the side surface of the sample.
The pulse voltage waveform generated by pulse voltage generator, as shown in Figure 2-3
(a), with a standard lightning wave (1.2/50μs), whose voltage is in a range of 0~200kV
with boost gradient 2kV, the sub-pressure used here is 4000:1. Artificial simulating
lightning wave is to test the lightning impulse, withstand voltage ability of electrical
equipment insulator here.
In fact, the lightning wave is negative. In addition, negative standard lightning is used in
the test. Two-polar capacitor is applied in charging process; therefore, voltage displayed on
the operation panel is half of the actual gap voltage in actual discharged process.
Experimental study of creeping flashover is carried out in a high vacuum (10-5~10-1Pa)
environment, so vacuum chamber is used here to obtain a vacuum environment. With JTK200 vacuum system, as shown in Fig.2-3(b), the ultimate vacuum is 5.0×10-4Pa. Creeping,
the degree of vacuum chamber maintained at about 3×10-3Pa during flashover test.
33
CHAPTER 2
(a) Vacuum equipment
(b) Pulse voltage generator
Fig. 2-3 Vacuum equipment and Pulse voltage generator
Sample and electrode system are shown in Fig.2-4, the electrode system uses flat electrode,
which made by stainless steel. The sample size is the diameter of 30mm, with height 1015mm.
34
CHAPTER 2
Fig. 2-4 Sample and electrode system
The oscilloscope is used to display waveforms during the test, to understand intuitively if
the flashover occurs at the sample surface, the oscilloscope is set for horizontal 5μs per
division, ordinate to 5V per division, in order to show the entire lightning pulse display
changes in the process completely. The relationship between actual voltage and the voltage
showed on oscilloscope is 4000:1.
Test methods used are mainly divided into the following steps.
1) In order to reduce the influence of pollutants and moisture on vacuum surface flashover
performance, the counter electrode and the sample should be thoroughly washed and
dried by ultrasonic cleaning equipment. Cleaning process is summarized in Fig.2-5.
35
CHAPTER 2
Fig. 2-5 Cleaning process
2) After cleaning and drying, the sample is assembled with electrode into vacuum chamber
as shown in Fig.2-4. The upper and lower electrodes are respectively connected to the high
voltage terminal and to the ground, the vacuum chamber is closed and vacuum device is
opened. During the test, although surface flashover will cause deflation, a degree of
vacuum still can be maintained 10-3Pa.
3) When the vacuum degree reach 10-3Pa, gap voltage reach 32kV flashover and the
experiment starts: voltage is exerted 5 times at each level without interval, increasing by
2kV each time until first surface flashover occurs. This can be considered as first
breakdown voltage (Ufb). After first flashover, voltage is increased until flashover occurs 3
times continuously; the retrieved value of the voltage is the conditioned voltage (Uco).
Then voltage is decreased by 2kV each time until no more flashover occurs. This voltage
value is called hold-off voltage. The complete experimental process can be seen from
Fig.2-6.
36
CHAPTER 2
Fig. 2-6 Vacuum surface flashover experimental process
(4) TSDC test system
Thermal stimulation theory42 is developed based on the Dielectric physics. A variety of
different H or  charged particles in materials are easily separated by thermal stimulation to
obtain respective parameter, so it is a very effective means to research on
dielectric material, insulating material and semiconductor material.
The Japanese company SEISAKU-SHD manufactures TSDC test system used. Polarization
voltage range of 0-4kV, temperature range of -160 °C-230 °C, heating rate of 0.5-5°C·min1
. Depolarization current is measured through 6517 meter, the measuring range of 1×10-12-
1×10-3A. LabVIEW software acquires temperature and current data into a computer for
storage.
Sample size diameter 30mm, thickness 1mm. Before the test, sample pre-treatment
includes polishing, ultrasonic cleaning, drying, gold-ions-sputtering on both end. Test
process, reported in Fig.2-7, works in vacuum environment. Polarization voltage is
different for each sample, but the polarization current fixed by10μA.
37
CHAPTER 2
Fig. 2-7 TSDC Test scheme
Current caused by the trapped electrons and holes, which TSDC formula is42:
J (T )  A exp[
H B T
H
  exp( )dT ]
kT  T 0
kT
(2-8)
in which:
A, B—constant,
H——Trap levels /eV,
k ——Boltzmann's constant,
-23
k=1.38×10 J· K-1,
T ——Temperature/K;
Formula (2-8) can be simplified as
38
CHAPTER 2
J (T )  A exp( 
H
)
kT
(2-9)
taking logarithm from both sides (2-9), we can obtain:
ln J (T )  ln A 
H
kT
(2-10)
As can be seen from the above formula, ln J (T) and 1/T is in a linear relationship, the
slope of the line can figure out the trap levels.
We choose CaO-MgO-Al2O3-SiO2 quaternary system, which is according to CaO-Al2O3SiO2, MgO-Al2O3-SiO2 phase diagram in sintering process of this part of work. Fig.2-8
represents the content of Al2O3 in CaO-Al2O3-SiO2 phase diagram. Mineral material used to
balance composite are α-Al2O3, 3Al2O3·2SiO2, CaO·Al2O3·2SiO2, and CaO·6Al2O3.
Fig. 2-8 CaO -Al2O3-SiO2 phase diagram (high content of Al2O3)43
Fig.2-9 is MgO-Al2O3-SiO2 phase diagram with high content of Al2O3, mineral material
used to balance composite are MgO·Al2O3·3Al2O3·2SiO2.
39
CHAPTER 2
Fig. 2-9 MgO-Al2O3-SiO2 phase diagram (high content of Al2O3)43
When the mass ratio of SiO2/CaO is larger than 2.16:1，the composite of balanced mineral
material are MA, CAS2, A3S2 and α-Al2O3。In this paper, we choose the mass ratio of
SiO2/CaO 4.8:1, so the composites are mainly large amount of α-Al2O3 phase and MA
phase at 1600 °C.
2.2 Summary
According to the CaO-Al2O3-SiO2, MgO-Al2O3-SiO2 Phase diagram, we prepared the
alumina ceramic composite with SiO2/CaO mass ratio equal to 4.8:1 that is composed by
large amount of α-Al2O3 phase and small amount of MA phase at 1600 °C.
40
CHAPTER 3
3 Preparation and Properties of Mo-doped Alumina
Ceramics
The sintering behavior under different doping content of Mo, the distribution of Mo in
alumina ceramics and its impact on the Mo phase will be studied based on the preparation
of CaO-MgO-Al2O3-SiO2 quaternary system in this chapter. By controlling the content of
Mo, materials with better conductivity and insulating properties can be obtained.
Mechanical properties test and dielectric properties test have been carried out in order to
evaluate the performance of the produced alumina ceramic.
3.1 The influence of Mo Doping for Alumina Ceramics in
Phase Composition and Microstructure
The amount (0, 1, 3, 5, 10, 20, 30 and 40% in weight) of molybdenum (with average
particle size=100 mesh) is added and mixed with CaO-MgO-Al2O3-SiO2 system. The
impact of phase and microstructure properties, caused by the distribution of Mo in alumina
lattice, are analyzed. Formulations of the mixture are reported in Tab.3-1.
Table 3-1 Value of the formulations in the ceramic
No./Materials
Al2O3/wt.%
CaO/wt.%
SiO2/wt.%
Mo/wt.%
1
96.0
0.6
3.40
0
2
93.1
0.58
3.30
3
3
91.2
0.57
3.23
5
4
89.3
0.56
3.16
7
5
86.4
0.54
3.06
10
6
76.8
0.48
2.72
20
7
67.2
0.42
2.38
30
8
57.6
0.36
2.04
40
41
CHAPTER 3
3.1.1 The Phase Composition
XRD patterns for the alumina/molybdenum composite are shown in Fig.3-1. The material
is mainly composed of Al2O3 phase and Mo phase. It is difficult to observe the XRD
characteristic peaks with Mo powder doped under 5wt.%, while increasing Mo doping
contents, diffraction peaks of Mo gradually become more visible. When the doping amount
of Mo reaches to 40wt.%, there is still no new element created.
 Al2O3
 Mo
Relative Intensity

40wt.%
30wt.%
20wt.%
10wt.%
7wt.%
5wt.%
3wt.%
0wt.%
20


30


40

50

60


70

80
2 Theta/°
Fig. 3-1 XRD patterns for the alumina/molybdenum composite
3.1.2 Microstructure
The microstructure of the alumina ceramic composites doped with molybdenum powder
(0, 1, 3, 5, 10, 20, 30 and 40% in weight) is shown in Fig.3-2 and Fig.3-3 by BSE. Since
atomic number of each element in CaO-MgO-Al2O3-SiO2 quaternary system and Mo are
quite different, it is possible to observe the distribution of Mo on alumina ceramics better
by BSE.
As shown in Fig.3-2, Mo is mainly dispersed in the grain boundaries of ceramics for Mo
doping lower than 10% in weight. As the doping amount increases, the grain size of Al2O3
reduces due to the dispersion of microsize particle within the composite grains, which
prevents the growth of the ceramic grains. Whereas the majority of the particles were
located at the grain boundaries of alumina
42
CHAPTER 3
Mo particles dispersed inside the composite prevents the growth of the ceramic grains.
Increasing the Mo content, it is easy to form a stable grain boundary in the composite
sintering process; the grains can be fined by easier diffusion of grain boundary.
5μm
(a)
5μm
(b)
43
CHAPTER 3
5μm
(c)
5μm
(d)
Fig. 3-2 SEM micrographs showing ceramic microstructure (a) 0 wt.% of molybdenum doping, (b) 1
wt.% of molybdenum doping.(c) 3 wt.% of molybdenum doping.(d) 5 wt.% of molybdenum doping
44
CHAPTER 3
5μm
5μm
（a）
5μm
（b）
45
CHAPTER 3
5μm
5μm
（c）
5μm
（d）
Fig. 3-3 SEM micrographs showing ceramic microstructure (a) 10 wt.% of molybdenum doping, (b) 20
wt.% of molybdenum doping.(c) 30 wt.% of molybdenum doping.(d) 40 wt.% of molybdenum doping
As shown in Fig.3-3, when Mo content is beyond 10% in weight, the majority of Mo
particles appears to agglomerate, which results in a large amount of pores inside the
composites, thus decreasing the density of composites by preventing the densification of
Al2O3 substrate (Fig.3-3 (c), (d)). Fig.3-3 shows also the phase transition from diffusion
phase to continuous phase increasing the amount of Mo doping, as reported by Yansheng in
his paper44.
46
CHAPTER 3
In order to define the composition of white part in BSE line, scan analysis have been done
on the Mo/Al2O3 composites, resulting in Mo particles, as reported in Fig.3-4. The results
obtained from XRD, BSE and EDS showed that Al2O3 ceramics doped with Mo has the
Counts/a.u.
structure of Mo / Al2O3 composite.
Al
Mo
5μm
0
2
4
6
8
10
12
14
16
Distance/m
Fig. 3-4 EDS of doping-Mo 30wt.% ceramic and BSE line scan analysis
3.2 The influence of Mo Doping for Alumina Ceramic in
Sintering and Mechanical Properties
3.2.1 Sintering Properties
The relationship between Mo content and relative density of Mo doped alumina ceramic
composites can be seen in Fig.3-5. When the doping amount is less than 10%, the relative
density of the composite is still more than 90%. While the doping amount increases above
10%, the relative density significantly decreases, which is the same as SEM analysis
showed in Fig.3-3.
47
CHAPTER 3
Relative Density
1.0
0.9
0.8
0.7
0.6
0
20
40
Mo/wt.%
Fig. 3-5 Relationship between Mo doping and relative density of Mo doped alumina ceramic composites
3.2.2 Mechanical Properties
Mo Doping also has an impact on flexural strength of alumina ceramic. Fig.3-6 shows the
flexural strength changing curve for different Mo doping. There is an increase from the
initial flexural strength when the doping is less than 5%, while the flexural strength
decreases with doping above 5%. Thus, while alumina ceramic is doped with a small
amount of Mo, mechanical properties can be improved due to its positive role in grain
refinement. However, with the increase of Mo content, the flexural strength of the ceramics
starts to decrease significantly, due to the connective intensity of biphasic interface that is
lower than the average connective intensity of same phase interface45. When Mo doping
content is 5wt.%, the flexural strength of the material reaches its maximum (270MPa).
48
Bending Strength/MPa
CHAPTER 3
260
240
220
200
180
0
10
20
30
40
Mo/wt.%
Fig. 3-6 Relationship between bending strength and Mo content
3.3 The influence of Mo Doping for Alumina Ceramic in
Dielectrical Properties
3.3.1 The Influence of Mo Doping for Alumina Ceramic in
Dielectrical Properties
The resistivity and relative permittivity of Mo doped composites is reported in Fig.3-7. For
Mo doping amount lower than 10wt.% there is no significant change in the resistivity and
relative permittivity. These behaviors is still similar to the undoped alumina ceramics ones,
which means Mo did not form a conductivity channel. However, a small amount of Mo
dispersed in the grain boundaries (as shown in Fig.3-2 (b), (c)) leads to the distortion of the
internal electric field and changes the state of polarization which results in a sharp
permittivity increase. When the Mo-doped is greater than 20wt.%, the through channel has
been formed, the resistivity decrease sharply, resulting in a steep rise of permittivity.
However, Mo doping more than 40wt.% the resistivity of the ceramic is close to metals
and dielectric constant is reduced at this level of doping.
49
CHAPTER 3
Relative Permitivity
Permitivity
Volumn
Resistivity
12
10
40
8
30
6
4
20
lg(Vol. Resistivity/cm)
50
2
10
0
10
20
30
40
0
50
Mo/wt.%
Fig. 3-7 Resistivity and permittivity of Mo-doped ceramic composites
3.3.2 The Influence of Mo Doping for Alumina Ceramic in
Flashover Performance and Analysis of TSDC
The flashover test results of Mo-doped alumina ceramic surface is reported in Fig.3-8.
When the doping amount of Mo was 5wt.%, the ceramic has the highest flashover voltage,
in which, withstand voltage is 55.5·kVcm-1, 26% higher than the undoped sample.
50
CHAPTER 3
Fig. 3-8 The flashover performance of Mo-doped alumina ceramic surface
-11
Current/10 A
15
10
5
0
-150
-100
-50
0
50
100
Temperature/C
150
200
250
Fig. 3-9 TSDC test results of undoped alumina ceramic
51
CHAPTER 3
9
-11
Current/10 A
12
6
3
-150
-100
-50
0
50
100
150
200
250
Temperature/C
Fig. 3-10 TSDC test results of Mo doped alumina ceramic
The TSDC test results of undoped alumina ceramic and 5wt.% Mo doping is shown in
Fig.3-9 and Fig.3-10 respectively. Comparing these two figures, the TSDC results of Modoped samples and undoped samples of alumina are similar, there are only two
depolarization peak.
The surface energy trap levels of Mo-doped composite after the peak separation for both
the doped and undoped samples are listed in Tab.3-2. It also shows a significant decrease
of energy trap level on the surface of ceramics after Mo doping where the shallow trap
levels decrease by 38% and the deep trap levels decrease by 62%。Meanwhile, the surface
flashover performance is increased by 26% (shown in Fig.3-8).
Table 3-2 Energy trap level of alumina ceramic
Sample
Peak A/eV
Peak B/eV
undoping
0.60
1.80
5wt.% Mo
0.37
0.67
52
CHAPTER 3
5μm
(a) SEM
5μm
(b) BSE
Fig. 3-11 The same area of 5wt.% SEM and BSE of 5wt.% Mo doped ceramics
Considering that Mo belongs to low resistance additives, it’s probably that the mechanism
influence of flashover property of Mo doped alumina ceramic is due to the additive nature,
which changes electric field distribution on the surface of ceramic, inhibiting the flashover.
From Fig.3-11 we can see that compounds doped with Mo show a bulk density of the
ceramic decrease with an increase of the porosity, and its gas desorption leads to the drop
of flashover voltage.
3.4 Summary
1) The sintering method is used to fabricate ceramic composites with CaO-MgO-Al2O3SiO2 quaternary system under different doping content of Mo, XRD shows that the
samples phase consist of Al2O3 and Mo without other new phase. BSE shows that Mo
mainly disperse on the grain boundaries of Al2O3.
2) The surface flashover performance increases by 26%
3) The analysis on microstructure and resistivity of composite ceramic materials shows that
when the Mo-doped greater than 20wt.%, the through channel is formed, the resistivity
decrease sharply, resulting in a fast rise of permittivity. However, Mo doping more than
40wt.% leads to the resistivity close to metals and the dielectric constant is reduced at this
percentage of doping.
53
CHAPTER 4
4 The Discussion and Optimization of Ceramics with
micro capillary structure
This chapter is for preparing reliable and small-size high power resistor, based on the open
micro capillary heat transfer technique. Then the characteristics of the prepared new
ceramic resistor with large power, small size under micro-capillary will be discussed.
4.1 Preparation of Ceramics with micro capillary
structure
The sample production which is consist of Ti, Mo, CaO-MgO-Al2O3-SiO2 system, as well
as the platform for testing the samples are done at ShenFei Ceramics Co. Ltd.
Ceramic materials with micro capillary are prepared using cold isostatic molding method
(undoped sample with micro-capillary is shown in Fig.4-2). Based on the result seen in
Fig. 4-8, a fine adjustment on samples has been done according to the factory actual
situation. The sintering temperature is higher than 1450℃ with a holding time of 2h where
Hydrogen is used as protective atmosphere.
Preparation equipment can be seen in Fig.4-1.
(a) High temperature tunnel kiln
54
CHAPTER 4
(b) Horizontal steam boiler
(c) Electroplating equipment
(d) X-ray layer thickness meter
55
CHAPTER 4
(e) Cold isostatic press machine
(f) Spray drying tower
Fig. 4-1 Equipment setting for resistor preparation in factory
56
CHAPTER 4
Fig. 4-2 Ceramic component sample with micro capillary
After performing the sintering, cleaning, drying, metallic drying and finally metallizing,
the ceramic resistor is brazed with copper electrode using Ag-Cu-Ti as filler metal. The
soldering temperature is maintained higher than 850 ℃, holding time more than 40min, in
order to obtain an efficient heat dissipation for a ceramic resistor with large power (Fig.43).
Fig. 4-3 Mo-Al2O3 ceramic component with micro capillary
4.2 Test Platform Establish and Analysis of Properties
4.2.1 Test Platform
After packaging the Electrode, ceramic resistors is connected with capillary microgroove
cooling system, which is a large-power thermal performance test platform (as shown in
57
CHAPTER 4
Fig.4-4). The test platform mainly consists of three basic parts: ceramic resistor as heater,
copper tube as a channel for heat transfer and fluid transportation and the micro-condenser
for cooling. Ceramic resistor with Micro-capillary structure is connected with a small
condenser, in order to form a pressure differential that allows liquid flows by micro
capillary structure, transporting heat from the heater to condenser.
Fig. 4-4 Large-power thermal performance test platform structure
Ceramic resistor test system is shown in Fig.4-5. In this system, the micro capillary group
is fabricated directly in large power resistor, which is able to leads to the heat flux with
high density by transferring heat in the micro capillary group structure. Then the formed
steam will remove heat and release it to environment by natural convection or forced air at
the condenser.
This cooling system is based on a micro-capillary, by driving the cooling media flowing,
which is relied on its own capillary pressure gradient, lead to heat flux with high density.
The heat transfer coefficient up to 106·W·m-2·K-1 which is much higher than the forced
water-cooling heat exchanger technique.
58
CHAPTER 4
Fig. 4-5 Ceramic resistor test system
4.2.2 The Properties of Ceramics with micro capillary
(1) Stability under various temperature tests
Five sets of samples with different resistivity, from small to large (numbered 1# to 5#),
with measured resistivity of 12, 23, 32, 49 and 70Ω·cm respectively are tested for
resistivity change at different temperature by resistance-temperature characteristics tester.
Fig.4-6 shows the variation in resistivity increasing the temperature for samples with
different resistivity. The temperature is increased from room temperature (25℃) to 190℃.
The maximum temperature dependency of the resistivity of all samples is less than 7%.
The alumina ceramic itself has the advantages of good thermal stability and thermal shock
resistance, thus it ensures long-term stability of the ceramic resistor under thermal class
insulation H level and above operating temperature.
59
CHAPTER 4
Fig. 4-6 The variation in resistivity for increase in temperature for samples with different resistivity
(2) The thermal efficiency test
The five sets of ceramic samples mentioned above were connected into ceramic resistor
heat dissipation test system, by regulating current in order to control the power and the
stability. Here, the power we set in the experiment are 80, 160, 240, 320, 400, 480, 560 and
640W, separately, the temperature of micro-condenser condenser is at room temperature 25
℃. The result of the temperature measured for applied is shown in Fig.4-7.
Among five sets of samples with micro capillary structure, when the power rises from 80W
to 640W, the temperature of the entire cooling system increases from 30℃ to 70℃,
showing a very good heat dissipation performance.
60
CHAPTER 4
70
T/C
60
50
1#
2#
3#
4#
5#
40
30
0
100
200
300
400
500
600
700
P/W
Fig. 4-7 Power-temperature relationship
4.3 Summary
Ceramic resistor with controllable resistivity with doping TiC is obtained in the factory.
After electronic package, samples are tested by heat-dissipation system for temperature
stability and heat dissipation performance. The results are reported from room temperature
(25℃) to 190℃. The maximum temperature dependency of the resistivity among all
samples is less than 7%. When the power rises from 80W to 640W, the temperature of the
entire cooling system increases from 30 ℃ to 70 ℃ showing a very good heat dissipation
performance.
61
CHAPTER 5
5 Stability of Resistance in Mo Doping Alumina
Ceramic Doping with SiC, TiC
As can be seen from the results of Chapter 3, ceramic compounds have good mechanical
properties and high surface flashover voltage with Mo doping less than 10wt.%, thus can
be applied in the field of vacuum insulated equipment. However, when the doping amount
of Mo more than 20wt.%, the resistivity of the materials begin to decrease; moreover, if the
amount of Mo exceeds 30wt.%, this makes the resistivity decrease sharply. Therefore,
based on the advantages of Al2O3 ceramic, such as thermal shock resistance, corrosion
resistance, excellent mechanical properties, it is possible to obtain certain resistance of
composite ceramic between 10 and 1012 Ω·cm, which can be used as large power ceramic
resistance46,47, optimizing the doping amount of Mo.
When Mo doping amount is around 30wt.%, the resistivity changes very rapidly; even if
the amount of Mo changes by 1wt.%, the resistivity can vary up to several orders of
magnitude, therefore materials formula need to be optimized further to improve the
stability of the resistivity.
TiC, SiC, as semiconductor materials, have been reported that can improve the mechanical
properties of the ceramic, which is also easy to sinter with alumina ceramic. In order to
improve the stability of resistance, we tentatively adjustment Mo resistivity by doping low
amount of TiC/SiC. 0, 1, 2, 3, 5, wt.% amount of SiC and TiC were added into the basic
ceramic formula, respectively, which is comprised of 30wt.% Mo and CaO-MgO-Al2O3
system. Here, each team of formula, 10 pieces of samples (φ30mm×10mm) were prepared
a total amount of 100 slices.
5.1 The Influence of SiC, TiC Doping for Alumina
Ceramics in Microstructure
As shown in Fig.5-1, with the increase in amount of SiC, density of the ceramic is not
improved, porosity is still high, and we also can see that dispersibility of Mo is not good.
62
CHAPTER 5
5μm
(a)
5μm
(b)
5μm
(c)
63
CHAPTER 5
5μm
(d)
5μm
(e)
Fig. 5-1 SEM micrographs showing ceramic microstructure (a) 0 wt.% of SiC doping, (b) 1 wt.% of SiC
doping.(c) 2 wt.% of SiC doping.(d) 3 wt.% of SiC doping, (e) 5 wt.% of SiC doping
SEM pictures reported in Fig. 5-2 shows the ceramic microstructure with different TiC
doping amount, we can obtain that increasing the amount of TiC, the porosity of ceramic
decreases, while the density improves obviously.
64
CHAPTER 5
5μm
(a)
5μm
(b)
5μm
(c)
65
CHAPTER 5
5μm
(d)
5μm
(e)
Fig. 5-2 SEM micrographs showing ceramic microstructure (a) 0wt.% of TiC doping, (b) 1wt.% of TiC
doping.(c) 2wt.% of TiC doping.(d) 3wt.% of TiC doping, (e) 5wt.% of TiC doping
5.2 The Influence of SiC, TiC Doping for Alumina
Ceramics in Phase Composition
66
CHAPTER 5
Fig. 5-3 XRD of ceramic with different doping content of SiC
In Fig.5-3, it is possible to see that after SiC doping, the Mo2C phase can be observed, but
with very low intensity. When doping amount of SiC is 3wt.% and 5wt.%, molybdenum
silicide (Mo5Si3) is formed. Thanks to the XRD analysis, we can conclude that, during the
sintering process, SiC reacted with Mo and created molybdenum carbide (Mo2C) and
molybdenum silicide (Mo5Si3), which decrease the content of Mo and SiC.
SiC and Mo react to molybdenum silicide above 1200℃, in which, Mo5Si3 is stablest,
which the temperature is above 1600 °C, Mo2C is more stable48.
67
CHAPTER 5
Fig. 5-4 XRD of ceramic with different doping content of TiC
After TiC doping, only Al2O3 and Mo phase can be observed in Fig.5-4, without any
characteristic peaks of TiC. We suppose that the ion radius of Ti4+ (60.5 pm) and Al3+ (53.5
pm) reported in Tab.5-1 are close to each other, and therefore can form a continuous solid
solution.
Table 5-1 Shannon periodic of ionic radius
Atomic Number
Atomic name
Valence of ion
Ionic radius (nm)
13
Al
+3
0.0535
14
Si
+4
0.04
22
Ti
+2,+3,+4
0.086,0.067,0.605
5.3 The Influence of SiC, TiC Doping for Alumina
Ceramics in Dielectrical Properties
In order to investigate the influence on the stability of ceramic resistance led by SiC and
TiC doping, all samples were analyzed with standard volume resistance deviation. Due to
poor stability of undoped samples (range from 1 Ω·cm to 1×103Ω·cm), a large standard
deviation resulted. Standard deviation of all samples are reported in logarithm in Fig.5-6.
68
CHAPTER 5
Fig. 5-5 the influence on volume resistivity of ceramic doping with SiC, TiC, respectively.
The resistivity, reported in Fig.5-1, does not change significantly with SiC and TiC doping
and fluctuates around 107Ω • cm. This means that, when Mo doping content of Mo is 30%,
the ceramic composite is still insulator but may form particular conductive path, leading to
very high values of the resistivity. The doping content of SiC and TiC is very small, thus
the resistivity of composites will not have obvious change, which will not have a big
influence on the final resistivity of ceramic composite.
69
CHAPTER 5
Fig. 5-6 The influence on the resistivity standard deviation of ceramics by the doping amount of SiC,
TiC, respectively.
As can be seen from Fig.5-6, with different SiC and TiC doping, the stability of ceramics
has different trend: SiC does not influence it, while for the doping amount of TiC of
3wt.%, the lowest standard deviation of resistivity is achieved. As explained, SiC and TiC
have different impacts on the resistivity stability of the materials.
70
CHAPTER 5
Fig. 5-7 the dielectric constant (25℃, 1MHz) and dielectric loss of different SiC content of Mo-doped
alumina ceramics
The free electron was divided by SiC or Al2O3 phase, which lead to electron relaxation
polarization under alternating electric filed. Thus the possible mechanisms for the
polarization in SiC are: electronic polarization, relaxation polarization including electron
relaxation polarization and micro inter phase relaxation polarization49.
As we can see from Fig.5-7, dielectric constant decreases with the increasing of SiC
doping amount, when compared to materials doped only with Mo, SiC reacts with Mo to
semi-conductor Mo2C and Mo3Si5. Dielectric loss increases with SiC doping, mainly
because of a decreasing ceramic density, which can be seen from the Fig.5-1.
71
CHAPTER 5
Fig. 5-8 the dielectric constant (25℃, 1MHz) and dielectric loss of different TiC content of Mo-doped
alumina ceramics
Here, we can see from Fig.5-8. With the addition of TiC, dielectric constant has an upward
trend and later decline, so as the dielectric loss. Due to TiC content of up to 5% Mo
conductive phase changes, demonstrating that the trend is not obvious. And we can see
from the XRD analysis of TiC doped materials, that we cannot find TiC phase in the
composite.
Due to the difference in conductivities of the Mo and Ti4+ solid solutied Al2O3 grains, the
moveable electrons and ions respond quickly and gather at the Mo/Al2O3 interface, which
leads to the formation of interfacial polarization50. With the increase of TiC content, the
interfacial polarization is enhanced for the increasing interface area between the Mo and
Al2O3. Therefore, permitivity increased with increasing TiC content.
5.4 Summary
Sic doping leads to a reaction of the dopants with Mo to create Mo2C and Mo3Si5; in
addition it also forms more pores with increasing dopant content, which lead to higher
dielectric loss and lower dielectric constant. Whereas increasing TiC doping amount,
dielectric loss increases first, then decreases and dielectric constant rises slightly.
72
CHAPTER 5
Considering SiC or TiC doping, the stability of materials has different trend: SiC doesn’t
influence it so much, while the doping amount of TiC to achieve the lowest standard
deviation of resistivity is 3wt.%.
73
CONCLUSIONS
6 CONCLUSIONS
The sintering method is used to fabricate ceramic composites with CaO-MgO-Al2O3-SiO2
quaternary system under different doping content of Mo, here, we have research on the
relationships among doping material, properties, and fabricating technics. We produced
high power resistor with micro capillary to meet the requirement of heat disspation, based
on the open micro capillary heat transfer technic developed by the Chinese Academy of
Sciences. Following main conclusions are obtained.
The influence of doping content of Mo on resistivity shows obvious seepage
characteristics. For Mo doping amount lower than 10wt.% there is no significant changes
in the resistivity (>1012Ω·cm) and relative permittivity of the Mo doped alumina ceramic,
when the Mo-doped greater than 20wt.%, the through channel has been formed, the
resistivity decrease sharply (<1010Ω·cm), resulting in a fast rise of permittivity.
However, Mo doping more than 40wt.% the resistivity of the ceramic is close to metals and
dielectric constant is meaningless at this level of doping. When the doping amount of Mo
was 5wt.%, the ceramic has the highest flashover voltage, in which, withstand voltage is
55.5 kV·cm-1, 26% higher than the undoped sample. Research also shows that, TiC doping
can significantly improve the stability of the electrical resistivity of material, which is
helpful to obtain the specific resistivity of ceramic resistance material.
Micro capillary was built on ceramic resistor based on the open micro capillary heat
transfer technique, which can dissipate heat very fast on a large scale, so that reliable and
small-size high power resistor can be obtained. The results shows that temperature from
room temperature (25℃) to 190 °C the maximum resistivity rate of temperature change of
all samples is less than 7% when the power rises from 80W to 640W. The temperature of
the entire cooling system increases from 30 °C to 70 °C, showing a very good heat
dissipation performance.
74
BIBLIOGRAPHY
7 BIBLIOGRAPHY
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
Tan Qin, et al. National Grid security plan and analysis [J]. China power,2015,48(1)．
Yue Bin. SVC application and impact to power system stability in the power system [D]. Nanjing
University of Technology and Engineering,2012．
Zhang Xiaonian. Analysis wire wound resistor surface temperature and power relations [D]. China
Electronic Business,2012(03)．
Chen Jun,et al. Failure characteristic impulse current cement resistors and capacitors [J]. Journal
of Hubei University of Technology,2010(01)．
Cao Qin, Xie Hui Cai, Wei Bingyu. Carbon fiber reinforced cement resistance test [J]. Journal of
Shantou University (Natural Science),2002(01)．
Yu Qunai. Preparation of aluminum-doped zinc oxide ceramics [J]. Journal of Zhejiang University
of Technology,2011，28(6)：28～29．
Yuanfang Li, Cheng Jie, Ji Endicott. High energy linear resistance of zinc oxide ceramics [J].
Journal of Northwest Institute of Light Industry,1992，03(10)．
Li Yan, Xia Jintong, Shao Haoming, et al. Study on electric heating property of carbon/ceramic
composites [J]. Journal of Aeronautical Material, 2006, 26(2):57~61.
Wang Junbo, Chen Sixiang, Miao silver silver, Chen. Experimental study on the impact of the
ceramic resistor voltage [J]-graphite. Electromagnetic arrester,2013(06)．
Zheng Xin, Liu Juncheng, Bai Jiahai. Preparation and properties of graphite/ceramic conductive
component [J]. Material composite Journal, 2009, 26(4), 107~109.
Liu Jincheng. Infrared radiation conductive substrate sauna room. Chinese patent, 200620160962.
4. 2007-1121．
Zheng Xin. Content on silicon carbide carbon/ceramic composite conductive material
microstructure and properties of [D]. Modern technical ceramics, 2013, 03(137):30-31.
Zhao Ming, LI Qin, Gu Jinbao. Spectral Properties Preparation and Resistivity DLC films [J].
Vacuum,2007(06)．
Li.X.S., L.M.Low. An Introduction to Key Engineering Materials [J]. Ceramic Cutting Tools,
1994, 9(6):245-251.
Zhang Huijun, Liu Kaiqi. Development of high temperature Mo-Al2O3 cermet [J]. Metal Materials
and Engineering, 2007, 36(1):282-284．
Liu Kaiqi, Pan Wei. Oxidation and corrosion behavior of slag Mo-Al2O3 cermet [J]．Metal
Materials and Engineering,2007,36(1):238-240．
M.Nawa, T.Sekino, K.Niihara. Fabrication and mechanical behavior of nano-composites
Mo/Al2O3 [J]. Journal of Material Science, 1994, 29(12):3185-3192.
Li ST, Zhang T, Huang QF, et al. Improvement of Surface Flashover Performance in Vacuum of
A-B-A Insulator by Adopting ZnO Varistor Ceramics as Layer A [J]. IEEE Transactions on Plasma
Science, 2010, 38(7):1656-1661.
S.Hussain, I.Barbariol, S.Roitti, O.Sbaizero. Electrical conductivity of an insulator matrix
(alumina) and conductor partical (molybdenum) composites [J]. Journal of the European Ceramic
Society, 2003,23:315-321.
Bergeron KD. Theory of the secondary electron avalanche at electrically stressed insulatorvacuum interfaces [J]. J. Appl. Phys., 1977, 48(7):3073-3080.
Anderson RA, Brainard JP. Mechanism of pulsed surface flashover involving electron stimulated
desorption [J]. J. Appl. Phys., 1980, 51(3):1414-1421.
Tourreil CH, Srivastava KD. Mechanism of surface charging of high-voltage insulators in vacuum
[J]. IEEE T. Dielect. El. In., 1973, 8(1):17-21.
Miller HC. Flashover of insulators in vacuum: Review of the phenomena and techniques to
improve holdoff voltage [J]. IEEE T. Dielect. El. In., 1993, 28(1):512-527.
Ma Tongze, Hou Zengqi. Heat pipe [M]. Beijing,Science Press,1983．
Cotter,T. Principles and prospects of micro heat pipe. Proc. 5th International Heat Pipe
75
BIBLIOGRAPHY
Conference[C]. Tsukuba, Japan, 1984, 328~335.
[26] Maidanik.Y, Solodovnik.N and Fershtater.Y. Experimental and Theoretical Investigation of Startup
Regimes of Two-Phase Capillary Pumped Loop [D]. Society of Automotive Engineers. Paper 932305, July, 1993.
[27] Shoji.S., Esashi, M. Micro-flow Devices and Systems[J]. Micromechanics and Microengineering,
1994,4:157~171.
[28] Qu, W., Mudawar, I. Experimental and Numerical Study of the Pressure and Heat Transfer in a
Single-Phase Micro-Channel Heat Sink [J]. Int.J. Heat Mass Transfer, 2002, 45:2549-2565.
[29] El-Genk, M.S, Saber, H.H. High Efficiency Segmented Thermoelectric Unicouple for Operation
between 973 and 300K[J]. Energy Conversion and Management, 2003, 44:1069-1088.
[30] Caillat, T., Fluerial, J.P., Snyder, G.J. Thermoelectric Properties of the Incommensurate Layered
Semiconductor GexNBTe2 [J]. Materials Research, 2000, 15:2789-2793.
[31] Hu, X.G., Yan, X.H., Zhao, Y.H. An Experimental Investigation on Application of Micro Capillary
Groove Evaporator to Electronic Chip Cooling[C]. Proceedings of 6th International Symposium on
Heat Transfer, Beijing, China, June, 15-19, 2004, 165-169.
[32] HU Xue Gong, Yan Xiaohong, Zhao Yaohua. Micro-groove evaporator used in electronic chip
cooling aspect of [D]. Journal of Chemical Technology,2005,56(3):412-416．
[33] Darjaguin, B.V. and Zorin, Z.M. Optical Study of the Absoprtion and Surface Condensation of
Vapors in the Vicinity of Saturation on a Smooth Surface[C]. Proceedings 2nd International
Congress on Surface Activity, London, 1957, 2:145-152.
[34] Ayyaswam, yRS., Catton,1., Edwards, D.K. Capillary Flow in Triangular Grooves[J]. Applied
Mechanics, 1974, 41:332~336.
[35] C.Y.Ji, Y.Y.Yan. A molecular dynamics simulation of liquid-vapor-solid system near triple-phase
contact line of flow boiling in a micro-channel [J]. Applied Thermal Engineering, 2008, 28:
195~202.
[36] Bazzo, E., Riehl, R.R. Operation Characteristics of a Small Scale Capillary Pumped Loop [J].
Applied Thermal Engineering, 2003, 23(6):687~705.
[37] Maidanik,Y., Solodovnik, N. and Fershtater, Y. Experimental and Theoretical Investigation of
Startup Regimes of Two-Phase Capillary Pumped Loop[D]. Society of Automotive Engineers,
Paper 93~23,05,July 1993.
[38] Hamdan, M., Gerner, F.M, Henderson,H.T. Steady State Model of a Loop Heat Pipe(LHP) with
Coherent Porous Silicon(CPS) Wick in the Evaporator Proceedings of Semiconductor Thermal
Measurement and Management Symposium[J]. Ninteenth Annual IEEE, San Jose, California,
2003,88~96.
[39] G.P.Peterson, H.B. Ma. Theoretical analysis of the maximum heat transport in triangular grooves:
a study of idealized micro heat pipes [J]. Journal of Heat Transfer,1996,118(3):731~739.
[40] H.B.Ma, G.P.Peterson. Experimental investigation of the maximum heat Transport in triangular
grooves [J]. Journal of Heat Transfer, Transactions of the ASME,1996,118:740~746.
[41] Li Ying Ying.. Hot die casting process mold design Methods [C]. Chinese Institute of Electronics
vacuum electron credits Nineteenth Annual Conference Proceedings (the book), 2013.
[42] Wang Liheng. Medium heat stimulation theory and its application [M]. Beijing: Science Press,
1988.
[43] Chen Jidan, Liu Ziyu. Dielectric Physics [M]. Beijing, Mechanical Industry Press,1982.
[44] Yin Yansheng, Conservancy and Hydroelectric Power. Alumina ceramics and composite materials
[M]. Beijing, Chemical Industry Press, 2001.
[45] S Hussain, I Barbariol, S Roitti. Electrical conductivity of an insulator matrix (alumina) and
conductor particle (molybdenum) composites [J]. Journal of the European Ceramic Society,
2003,23(2):312-321.
[46] Yuan Fanga, Yongsheng Zhang, Junjie Songa. Influence of structural parameters on the
tribological properties of Al2O3/Mo laminated nanocomposites [J]. Wear, 2014, 320(1-2):152-160.
[47] Jin Haiyun. A composite alumina ceramic resistance and its preparation method as power resistor
elements used. China,CN103319161A[P]．
[48] A.E. Martinelk, R.A.L. Drew, Microstructural development during diffusion bonding of a-silicon
carbide to molybdenum, Materials Science and Engineering A, 191 (1995):239-247.
[49] Huihui Song∗ , Wancheng Zhou, Fa Luo, Zhibin Huang, Yuchang Qing, Malin Chen, Yang Mu,
Temperature dependence of dielectric properties of SiCf/PyC/SiC composites, Materials Science
76
BIBLIOGRAPHY
and Engineering B, 195 (2015):12–19.
[50] Yuan Wang, Fa Luo, Wancheng Zhou, Dongmei Zhu,Dielectric and electromagnetic wave
absorbing properties of TiC/epoxy composites in the GHz range, Ceramics International,
40(2014):10749–10754.
77