Effect of B2O3 Nano-coating on the Sintering Behaviors and Electrical
Microwave Properties of Ba(Nd2-xSmx)Ti4O12 Ceramics
Li-Chun Chang*1, Bi-Shiou Chiou2
1
1001 Ta Hsueh Rd., Hsinchu, 300, Taiwan, R.O.C. / Department of Electronics Engineering and Institute
of Electronics, National Chiao Tung University / lcchang.ee87g@nctu.edu.tw
2
1001 Ta Hsueh Rd., Hsinchu, 300, Taiwan, R.O.C. / Department of Electronics Engineering and Institute
of Electronics, National Chiao Tung University / bschiou@mail.nctu.edu.tw /
Tel : 886-3-5131927 / Fax : 886-3-5131590
Abstract
Ba(Nd2-xSmx)Ti4O12 ceramics prepared by conventional solid-state
sintering have a dielectric constant about 80 and nearly zero temperature
coefficient of resonant frequency; however, the sintering temperature is
above 1350℃. Doping with B2O3 (up to 5wt%) promotes the densification
and dielectric properties of Ba(Nd2-xSmx)Ti4O12 ceramics. It is found that
coating Ba(Nd2-xSmx)Ti4O12 powder with thin B2O3 layer of about 180nm
reduces the sintering temperature to below 1020℃. The effect of B2O3
nano
coating
on
the
dielectric
microwave
properties
and
microstructures of Ba(Nd2-xSmx)Ti4O12 ceramics are investigated.
Keywords
oxide, microwave dielectric properties, ceramics, liquid phase sintering
1
the
1. Introduction
As the rapid development in telecommunications and microelectronic
technology, dielectric materials are continuing to play a very important role.
These materials are key in realization of low-loss temperature-stable
resonators and filters for broadcasting equipment, and in many other
microwave devices.
High dielectric-constant materials are critical to
miniaturization of wireless system, both for the terminals and base-stations,
as well as for handset [1, 2]. The BaO-PbO-Nd2O3-TiO2 material has a
high dielectric constant of 90 [3]. This material is widely used at lower
frequencies of around 1GHz. Its Q value of 5000 is high enough for use
at these frequencies, but with lead-pollution concern.
Processes of
BaO-Nd2O3-TiO2 series materials were, however, extremely difficult due to
complicated interactions between the constituents [4].
The dielectric
properties were not sensitive toward minor deviations in Nd2O3 content;
moreover, the temperature stability and phase composition were sensitive
to the ratio of TiO2/BaO. Kolar et al. [4], used Bi2O3 to improve the
temperature coefficient of resonant frequency by an appropriate mixture
with second phase bismuth titanate.
A sintering temperature of
1350~1370℃ is needed to obtain a ceramic disc with good dielectric
2
properties, k = 105 and Q = 1000 at 1 MHz, but τf (temperature coefficient
of resonant frequency) = -104 ppm/℃. Laffez [5] sintered BaO-(Nd2O3,
Sm2O3)-TiO2 system at 1400℃and obtained a k = 74 ~ 81, Q×f up to
9000GHz at 3 GHz, and τf
around +9 ppm/℃.
They found that
addition of 1 or 2 wt% WO3, MnO2, and CaO to BaO-(Nd2O3, Sm2O3)-TiO2
system to densified ceramics and improve the microwave properties. The
WO3 and MnO2 doped ceramics has a higher dielectric constant,
accompanied second phase and lower Q×f value, however, the sintering
temperature is above 1350℃.
Chemical processing and small particle sizes of the starting materials
are generally advantageous to reduce the sintering temperature of dielectric
materials [6, 7].
However, they required a flexible procedure that
increased the cost and time to fabricate a dielectric device. The liquid
phase sintering by adding glass or other low melting point materials was
found to effectively lower the firing temperature of ceramic [6].
The
dielectric microwave properties of dielectric resonators were deeply
affected by the liquid sintering temperature and the reaction between host
material and addition. In this paper, B2O3 was chosen as a sintering aid to
lower the sintering temperature of BaO-(Nd2O3, Sm2O3)-TiO2 (hereafter
3
referred to as BNST) ceramics. The crystalline phase, the microstructures,
and the dielectric microwave properties of B2O3 – doped BNST ceramics
will be investigated.
4
2. Experiments
Host material (BNST) powders were mixed with B2O3( ＞ 99%)
powders and distilled water. The mixture was stirred at 90℃ for 3 hr.
After cooling, the mixture was milled for 12 hr with zirconia balls. After
milling, the slurry was hydrated and granulated by mixing with 3wt%
polyvinyl alcohol solution.
Pellets with 10mm in diameter and either
5.2mm or 1.2mm in thickness were pressed using uni-axial pressing. A
pressure of 750kg/cm2 was used for all samples. After debinding, these
pellets were sintered in the temperature range of 900 ~ 1100℃ for 1 hour
in air.
The X-ray diffraction (XRD, MAC science MXP18) spectra were
collected by using CuKα (λ= 1.5406 Å) radiation with 30kV and 20mA in
the 2θ range of 20 to 50°, and precise integral intensity measurements of
the main peak. The microstructure observations of sintered samples were
performed by scanning electron microscopy (SEM, Hitachi S4700) and
transmission electron microscopy (TEM, Hitachi, H-7500).
observations were performed on the as-sintered surfaces.
The SEM
An electron
probe microanalyzer (EPMA, Joel JXA-8800M) was used to analyze the
composition of the sintered samples. The bulk densities of the sintered
5
specimens were measured by the Archimedes’ method. According to the
rule of mixing, the density of BNST-B2O3 ceramics is:
Dcal = (W1+W2)/(W1/D1+W2/D2)
(1)
Where W1 and W2 are the weight per cent of the BNST dielectric and
B2O3 in the mixtures, respectively; D1 and D2 are the densities of the BNST
and B2O3, respectively. The dielectric characteristics were measured by
Impedance/Gain -Phase Analyzer (Hewlett Packard, HP4194) in the
frequency range of 100Hz ~ 15MHz.
electrodes.
Silver paste was used for the
The dielectric constant (k) and the quality factor (Q×f) at
microwave frequencies were measured using the Hakki-Coleman dielectric
resonator method which was modified and improved by Courtney [11].
The dielectric resonator was positioned between two brass plates.
A
system combined with a HP8510 network analyzer and a HP8350 sweep
oscillator was employed in the measurement. The Q×f factor was used for
evaluating the loss quality, where f is the resonant frequency.
6
3. Results and Discussions
X-ray diffraction patterns for BNST specimens with various contents
of B2O3 and sintered at 900 ~ 1100℃ for 0.5 ~ 2.0 hr are shown in Fig. 1.
No interaction between additive and host materials is observed.
The
phase detected is Ba(Nd,Sm)2Ti4O12 for the entire sintering temperature.
While for the boric oxide doped BNST, the intensity of (002) peak
increases with the increase of B2O3 content, as exhibited in Fig. 1(a), (c),
and (g). With increasing sintering temperature, the main peak positions
distinctly transfer from (511) to (002), as exhibited in Fig. 1(d) ~ (f). The
peak intensity ratio of I(002)/I(511) increases from 0.16 to 5.23 as the sintering
temperature is increased from 960℃ to 1100℃, as shown in Fig. 2, as
compared to 0.36 of undoped BNST.
Fig. 3 shows that the BNST ceramics sintered at 1350℃ for 2 hr just
could reveal a pore-free structure and indicate homogeneous columnar
crystals. As the BNST bulks were sintered below 1350℃, the structures
of these samples are granular and porous. The one sintered at 1450℃
exhibits preferred direction of grain growth which formed a columnar
feature accompanying voids. The abnormal grain is over 5μm in length
and 0.35μm in diameter. In the work of Chen et al. [9], they argued that
7
the BNST system acquired little liquid phase when sintering temperature
exceeded 1350℃ because BNST ceramics had a eutectic temperature
between 1350 and 1375℃. The liquid phase would improve the sintering
density and the columnar grain growth. The measured densities of the
samples with 1wt% B2O3 addition sintered at the temperature ranged from
900 to 1100℃ for 1 hr were 71.29 ~ 96.23% of the estimated densities as
shown in Fig. 4. Nearly equi-axis fine crystals were observed in the
BNST ceramics sintered below 1150℃ for 2 hr in air, while specimens
with 1wt% B2O3 sintered at 1020℃ for 1.0 hr in air reveals homogeneous
columnar structure as shown in Fig. 4(c). For BNST ones with 2wt% or
5wt% oxide content sintered at various temperatures are shown in Fig. 5.
The formation of columnar structure is apparent for samples doped
with 5wt% (equal to 11.0 vol%) B2O3 at temperature as low as 960℃.
Exaggeratedly grown columnar structure with a preferred orientation of
(002) is observed, especially for 5wt% (11.0 vol%) B2O3 additive sintered
at 1020℃ as shown in Fig. 5(f) and (g). This implies that B 2O3 both
reduces the sintering temperature of the dielectric and contributes to (002)
preferred orientation. Boric oxide was first dissolved in hot water (~ 90℃)
to become boric acid (H3BO3), boric acid would be partially reduced to
8
form
H+
and
H2BO3-.
In
the
ABO3
structure
of
BNST
( Ba(Nd,Sm)2Ti4O12 ), larger ions Ba2+ ( CN=6, ionic crystal radius = 1.36
Å ) [10] are partially substituted by smaller ones Nd3+ and/or Sm3+ ( CN=6,
r = 1.00 and 0.96 Å ) , which simultaneously generates vacancies at A-site
to cause the surface of BNST particle negative potential. The H + ions are
absorbed by the surface. Then adequately well mixed with host materials
by H2BO3- through H+. After dehydrating, the BNST power was coated
uniformly boric oxide particle about 180nm in diameter as shown in Fig. 6.
In the classic liquid-phase sintering, rearrangement, solution-reprecipitation,
and final-stage sintering are the three stages of densification [11].
It
seems that the additive content up to 2wt% and sintering temperature above
or equal to 960℃ were the criterion in this study on liquid formation
which implied a burst of rearrangement densification, followed by
solution-reprecipitation with concomitant grain growth and grain shape
accommodation. The typical columnar structure could be observed for the
samples even doped with 1wt% boric oxide only and sintered at 960℃.
And the phenomenon was more evident for the 5wt% additive ones. On
the contrary, the undoped ceramics sintered under 1100 ℃ revealed
spherical and porous structure but no columnar grains. Fig. 7 shows that
9
the ratios of length to diameter of the columnar grain, l/d, increase with the
sintering temperature in the range of 900 to 1100℃ for B2O3-doped
specimens. The value of l/d increases as the amount of B2O3 increases.
The slopes of l/d versus sintering temperature curves are 2.38×10-2, 3.13×
10-2, and 6.53×10-2 per ℃ for 1wt%, 2wt%, and 5wt% B2O3 doping,
respectively.
In sintered specimens, no specific boron compound was
observed at grain boundaries, and the EPMA analysis implied that the
boron content of the sintered matrix is same as that of the sample before
sintered.
No segregation of boron was observed.
Maljuk et al. [12]
estimated that it is possible to neglect boron volatilization from the liquid
during 12hr, 1240℃ sintering in air. Thus, it is argued that the boron
atoms diffuse into BNST matrix. The diameter of B3+ (0.23 Å) is very
small compared with those of Ba2+, Nd3+, Sm3+, and/or Ti4+ (0.61 Å), one
can expect that boron would occupy the interstitial site of the matrix grains.
The dielectric properties of BNST sintered at various temperatures are
illustrated in Table 1.
Both the dielectric constant (k) and Q×f value
increase initially and then decrease with increasing sintering temperature.
For B2O3–doped BNST ceramics, the dielectric properties versus sintering
temperatures are shown in Fig.8. The dielectric constants increase with
10
the relative sintering density. With 2wt% B2O3 addition, a k value of 50 is
obtained for BNST ceramics sintered at 960℃. The microwave dielectric
loss was simultaneously affected by several factors, mainly caused not only
by the lattice vibration modes, but also by densification, pores, grain
sizes/boundaries, and secondary phase [13]. Relative density and pores
are important factors in controlling the dielectric loss, and have been shown
for other microwave dielectric materials. In this study, the Q×f value
increases with increasing sintering temperature first and then decreases,
since the l/d ratio of columnar grain increases with sintering temperature.
The larger l/d ratio accompanies a lot of amount of voids in the ceramics
and reduces quality factor.
Specimen with 1wt% B2O3 exhibits the
highest Q×f value (~ 5500GHz) and a moderate k (~43), among all
composition studied.
The boric oxide procures to be effective in the
densification of specimens, the reduction of the sintering temperature when
added appropriately, and columnar structure with high ratio of length to
diameter.
However, the role of the columnar microstructure plays in
improving the dielectric microwave properties is very complicated and
subjected to further investigation.
11
Conclusions
In this study, the effects of B2O3 on the properties of BNST dielectric
are investigated. Ninety-six percent of theoretical densities is obtained for
specimens coated with 2wt% B2O3 sintered at 960℃. Addition of boric
oxide also influences the microstructure evolution of the dielectric. For
samples without B2O3, a columnar structure is obtained when the sintering
temperature exceeds 1350℃.
However, exaggerated columnar grains
grow at 1100℃ for specimens doped with 1wt% B2O3. For samples with
5wt% boric oxide, a columnar structure with grain length 8 times longer
than grain diameter is obtained.
X-ray diffraction patterns show
significant (002) preferred orientation. For BNST dielectrics with 1wt%
boric oxide sintered at 1020℃, Q×f is more than 5500 GHz at 6 GHz,
which is equal to that of BNST sintered at 1350℃ (Q×f = 6000GHz at 8
GHz), but the dielectric constant is 43 as compared to 80 of BNST.
12
Acknowledgement
This work is supported by the Phycomp Taiwan Ltd. under contract
No.C90195.
13
References
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L.J. Golonka, K.J. Wolter, A. Dziedzic, J. Kita, and L. Rebenklau,
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Romania, 2001), p. 73.
2.
S. Jerry Fiedziuszko, Ian C. Hunter, T. Itoh, Y. Kobayashi, T.
Nishikawa, S. N. Stitzer, and K. Wakino, IEEE Trans. Microwave
Theory Tech., 50, 706 (2002).
3.
K. Wakino, K. Minai, and H. Tamura, J. Am. Ceram. Soc., 67, 278
(1984).
4.
D. Kolar, S. Gaberscek, Z. Stadler, and D. Suvorov, Ferroelectrics, 27,
269 (1980).
5.
P. Laffez, G. Desgardin, and B.Raveau, J. Mater. Sci., 27, 5229 (1992).
6.
T. Takada, S.F. Wang, S. Yoshikawa, S.J. Jang, and R. E. Newnham, J.
Am. Ceram. Soc., 77, 1909 (1994).
7.
V. Tolmer, and G. Desquardin, J. Am. Ceram. Soc., 80, 1981 (1997).
8.
B.W. Hakki and P.D. Coleman, IRE Transactions on Microwave
Theory and Techniques, 8, 402 (1960).
9. X.M. Chen, Y. Suzuki, and N. Sato, J. Mat. Sci.: Materials in Electrics,
6, 10 (1995).
10. R.D. Shannon and C.T. Prewitt, Acta Cryst., B25, 925 (1969).
11. Randall M. German, Sintering theory and practice (John Wiley &
14
Sons, Inc., 1996), p. 228.
12. A. Maljuk, S. Watauchi, I. Tanaha, and H.K. Kojima, Journal of
Crystal Growth, 212, 138 (2000).
13.
W.S. Kim, H.T. Hong, E.S. Kim, and K.H. Yoon, Jpn. J. Appl. Phys.,
37, 5367 (1998).
15
List of Tbales
Table 1 The dielectric properties of BNST ceramics.
16
Figures Caption
Fig. 1
XRD patterns for Ba(Nd, Sm)2Ti4O12 ceramics doped with x wt%
B2O3 addition and sintered at various conditions. (a) x = 1, 960℃
for 1 hr, (b) x = 2, 960℃ for 0.5 hr, (c) x = 2, 960℃ for 1 hr, (d) x
= 2, 960℃ for 2 hr, (e) x = 2, 1020℃ for 1 hr, (f) x = 2, 1100℃
for 1 hr, and (g) x = 5, 960℃ for 1 hr.
Fig. 2
Intensity ratios of I(002)/I(511) as a function of sintering
temperature for the specimens with various B2O3 content sintered
for 1 hr in air.
Fig. 3
Surface micrographs and relative density of BNST ceramics
sintered for 2hr at (a) 1250℃, (b) 1350℃, and (c) 1450℃ in air.
Fig. 4
Surface micrographs and relative density ( Dmeasured / Dcalculated with
mixing rule
×100% ) of BNST with 1wt% B2O3 ceramics sintered at
(a) 900℃, (b) 960℃, (c) 1020℃, and (d) 1100℃ for 1 hr in air.
Fig. 5
SEM micrographs of BNST with 2wt% B2O3 ceramics sintered at
(a) 900℃, (b) 960℃, (c) 1020℃, and with 5wt% B2O3 ceramics
sintered at (d) 900℃ (e) 960℃, (f) 1020℃ for 1.0 hr in air.
Fig. 6
TEM of BNST particle coated with 2wt% B2O3 .
17
Fig. 7
The ratios of l/d as a function of sintering temperature for the
specimens with various B2O3 contents sintered for 1.0 hr in air (l
and d are the length and diameter of the columnar grain; f is the
diameter of the granular grain).
Fig. 8
Dielectric properties as a function of sintering temperature for
BNST ceramics with B2O3 additive sintered in air for 1.0 hr. Data
in the parentheses are Dmeasured/Dcalculated with mixing rule ×100% .
18
Table 1 The dielectric properties of BNST ceramics.
Tsintering (℃)
Dm/Dcal (%)
k
Q×f (GHz)
1250
95.3
46.7
1500
1350
98.6
78.8
4839
1450
97.1
61.0
2380
Dm: measured density
Dcal: calculated density with mixed rule
19
(002)
(341)
(10 2 0)
(231)
(611)
(511)
(230)
(620)
(411)
Relative Intensity
(g)
(f)
(e)
(d)
(c)
(b)
(a)
20
25
30
35
40
45
50
2(degree)
Fig. 1
XRD patterns for Ba(Nd, Sm)2Ti4O12 ceramics doped with x wt%
B2O3 addition and sintered at various conditions. (a) x = 1, 960℃
for 1 hr, (b) x = 2, 960℃ for 0.5 hr, (c) x = 2, 960℃ for 1 hr, (d) x
= 2, 960℃ for 2 hr, (e) x = 2, 1020℃ for 1 hr, (f) x = 2, 1100℃
for 1 hr, and (g) x = 5, 960℃ for 1 hr.
20
0wt%
1wt%
2wt%
5wt%
6.0
5.0
I(002)/I(511)
4.0
3.0
2.0
1.0
0.0
900
950
1000
1050
1100
1150
1200
o
Sintering Temperature( C)
Fig. 2 Intensity ratios of I(002)/I(511) as a function of sintering temperature
for the specimens with various B2O3 contents sintered for 1 hr in air.
21
(a) Relative density = 95.3%
(b) Relative density = 98.6%
(c) Relative density = 97.1%
Fig. 3
Surface micrographs and relative density of BNST ceramics
sintered for 2hr at (a) 1250℃, (b) 1350℃, and (c) 1450℃ in air.
22
(a) Relative density = 71.29 %
(b) Relative density = 91.59 %
(c) Relative density = 95.48 %
(d) Relative density =96.23 %
Fig. 4 Surface micrographs and relative density ( Dmeasured / Dcalculated with
mixing rule
×100% ) of BNST with 1wt% B2O3 ceramics sintered at
(a) 900℃, (b) 960℃, (c) 1020℃, and (d) 1100℃ for 1.0 hr in air.
23
(a)
(d)
(b)
(e)
(c)
(f)
Fig. 5 SEM micrographs of BNST with 2wt% B2O3 ceramics sintered at
(a) 900℃, (b) 960℃, (c) 1020℃, and with 5wt% B2O3 ceramics
sintered at (d) 900℃ (e) 960℃, (f) 1020℃ for 1.0 hr in air.
24
150nm
Boric oxide
BNST
Fig. 6 TEM of BNST particle coated with 2wt% B2O3 .
25
5 wt%
2 wt%
1 wt%
0 wt%
8
l/d (m/m)
6
4
2
5
2
1
0
= 1.05
= 0.77
= 0.61
= 0.62
0
 = 0.63
 = 0.63
900
950
1000
 = 0.72
1050
1100
o
Sintering Temperature ( C)
Fig. 7 The ratios of l/d as a function of sintering temperature for the
specimens with various B2O3 contents sintered for 1.0 hr in air (l
and d are the length and diameter of the columnar grain;  is the
diameter of the granular grain).
26
6000
1wt%
2wt%
5wt%
5000
Qxf (GHz)
4000
3000
2000
1000
9001wt%
2wt%
5wt%
Dielectric Constant
50
950
1000
1050
1100
o
Sintering Temperature( C)
(83.1)
(96.3)
40
(95.5)
30
(73.6)
(91.5)
(96.2)
20
(72.4)
(79.4)
(91.6)
(71.3)
(71.2)
10
(63.8)
0
900
950
1000
1050
1100
o
Sintering Temperature( C)
Fig. 8
Dielectric properties as a function of sintering temperature for
BNST ceramics with B2O3 additive sintered in air for 1.0 hr.
Data in the parentheses are Dmeasured/Dcalculated
100%.
27
with mixing rule
×