Journal of Metals, Materials and Minerals, Vol.20 No.3 pp.127-131, 2010
Densification and Grain Growth in BaO.Bi2O3.ZnO Varistor Ceramics
Niti YONGVANICH1,2*, Pranuda JIVAGANONT3, Fariya SAKASUPHALERK1,
Porntip HUAYHONGTHONG1 and Weerawat SUWANTEERANGKUL1
1
Materials Science and Engineering, Faculty of Engineering and Industrial Technology,
Silpakorn University, Nakornpathom 73000
2
Center of Excellence for Petroleum, Petrochemicals, and Advanced Materials,
Chulalongkorn University, Bangkok 10330
3
National Metal and Materials Technology Center, Pathumthani 12120
Abstract
The effect of barium addition on the densification and grain growth of ZnO-based ceramics was
investigated. ZnO-based varistors have been employed as surge arrestors to prevent damages from voltage
surges in electrical or energy-related equipments such as transformer and consumer products. These devices
have been commercially incorporated into energy-supplying grids. The selected composition was ZnO +
0.0025Bi2O3 + (x) BaCO3 where x = 0 and 0.005. Both solid-state processing and precipitation method were
investigated. It was shown that the amount of Bi addition was too low to be effective for improving grain
growth; however, it was sufficient to enhance densification. When barium was further added, the averaged
grain size was more than doubled, possibly due to liquid-phase sintering. This result suggests the possibility
of BaO-containing liquid-phase formation along grain boundaries. However, the main drawbacks of barium
addition were a slight decrease in density, which was presumed to be caused by closed pores trapped in the
grain interior during the sintering process, and a reduction in the threshold voltage.
Key words: Grain growth, Varistor, Zinc oxide, Doping
Introduction
ZnO-based ceramics have been widely
used in varistor application due to their suitable
electrical properties. The highly nonlinear currentvoltage characteristics and high current handling
capability enables them to be efficient overvoltage
protectors in electronic circuits, electric power
system and equipment for power transmission.(1-5)
Different oxide compounds are usually added
during the processing to tune various electrical
characteristics such as non-linearity and breakdown
voltage. Bi2O3 is always included in the composition
as its derivatives have been reported to be responsible
for the non-ohmic properties. Other well-known
dopants and additives include CoO, Nb2O5, Cr2O3,
MnO and Sb2O3.(1-5) Among various microstructural
aspects, the grain size appears to frequently dictate
the breakdown voltage. Several reports have found
that high thresholds tend to be associated with
small-grained varistor samples.(6-7, 9-10) Control of
grain growth is one major subject of research, and
wet chemical route has appeared to be one potential
solution to tailor the final grain size in the varistor
piece.(11-14) Since grain growth is a complex process and
can be affected by different dopants and additives
in the system, it would be interesting to investigate
these compounds in detail. This study examined
the influence of barium addition (at 0.5 mol%) on
densification and grain growth in ZnO-based
varistor ceramics. The starting precursors were
prepared from both solid-state processing and
chemical precipitation method in order to
investigate the effect of the processing condition
on the final microstructure.
Materials and Experimental Procedures
The chemicals for the solid-state processing
were reagent-grade ZnO, Bi2O3 and BaCO3. They
were weighed according to the composition: ZnO +
0.0025Bi2O3 + (x) BaCO3 where x = 0 and 0.005.
The mechanical mixture was ball-milled with
alumina balls in ethanol for 24 hours. For the
precipitation method, reagent-grade ZnCl2,
Bi(NO3)3.5H2O and C4H6BaO4 were dissolved in
distilled water (1M). The pH of the system, using a
1M NaOH solution, was adjusted to be 14. After
one-hour of continuous stirring, the solutions were
dried at 120°C and washed thoroughly to remove
chloride ions. Thermogravimetric analysis (TGA,
7 Perkin-Elemer) was employed to determine the
*Corresponding author E-mail: niti.yongvanich@gmail.com
128
YONGVANICH, N. et al.
Results and Discussion
The XRD patterns of the calcined nanopowders
are shown in Figure 1. TGA showed complete
decomposition of organic residues at 400°C (data
not shown) which was chosen to be the calcination
temperature. No impurity peaks were present within the
detection limit of XRD. The averaged crystallite
sizes calculated by the Scherrer’s formula are
approximately in between 45 and 50 nm for both
x = 0.00 and x = 0.005 samples. No preferential
orientation was observed, and all major peaks can
be indexed as a zincite phase according to the
JCPDS database (75-0576). The morphology of the
crystals (Figure 2) displays both equiaxed and slightly
elongated character. The latter might be yielded from
the hexagonal structure of ZnO. Agglomeration of
the particles is clearly seen as evidenced by high
specific surface area and grain coalescence at the
calcination temperature of 400°C.
Figure 2. TEM image of the ZnO nanoparticles calcined
at 400°C for 6 hours.
Figure 3 shows the relationship between
bulk density and sintering temperature of all
samples. Similar densities were obtained from both
solid-state processing and precipitation method.
The undoped pellets display a continuous increase
in density from 66% (900°C) to the maximum of
94% (1300°C) where as the density values of the
(Ba,Bi)-doped pellets never exceeded 90%. In fact,
they fluctuate in the range between 84% and 90%.
The greatest improvement in sintering was
observed in the samples of Bi-doped compositions
among which a relative density of 94% was readily
acquired at 1100°C.
100
Relative
RelativeDensity
Density (%)
(%
calcination temperature. The precipitates were
calcined at 400°C for 6 hours to remove inorganics.
The morphology of the nanosized powders was
examined through Transmission Electron Microscope
(TEM, Philips EM201C). The powders were pressed
into disks with a diameter of 8 mm and a thickness
of ~ 2 mm. These disks were sintered in air for
6 hours at 900, 1000, 1100, 1200, 1300 and 1400°C,
with heating and cooling rates of 5°C/min. Sinterability
was characterized by the geometric (bulk) density
measurement. Scanning electron microscopy (SEM,
Hitachi S-3000N) was used to examine the microstructure.
The averaged grain size was calculated by the lineintercept method. Phase formation was confirmed
by X-rays diffraction (XRD, D/MAX 2000 Rigaku).
The non-linear response was measured by a simple
variac and multimeters.
95
90
85
80
75
70
65
60
900
Bi-doped
(Ba,Bi)-doped
Undoped
1000
1100
1200
1300
1400
Sintering Temperature (C)
Figure 3. Relationship between density and sintering
temperature of the undoped, Bi-doped and
(Ba,Bi)-doped samples. Similar trends were
observed for both solid-state processed and
precipitated samples.
Figure 1. XRD patterns of the ZnO nanopowders
(undoped and doped) calcined at 400°C for
6 hours.
The sintered samples were crushed and
x-rayed to examine phase purity. Figure 4 shows
the XRD pattern of the (Ba,Bi)-doped sample (x =
0.005) sintered at 1300°C for 6 hours. The major
peaks belong to a zincite phase with very small
traces of secondary peaks. The intensities of these
129
Denssification andd Grain Groowth in BaO.B
Bi2O3.ZnO Varistor
V
Ceraamics
mine
minor peakks are too loow to accurately determ
their correspponding phaases.
Fig
gure 6. SEM
M image of tthe (Ba,Bi)-d
doped sample
sinteered at 1300°C for 6 hourrs. (a) Solidstate processing. (bb) Precipitatio
on method.
Figure 4. XRD
X
pattern of the (Ba,Bi)--doped sample (x
= 0.005) sinteered at 1300°°C. The impuurity
peeaks are denotted as asteriskks.
Thhe microstrucctures of the sintered
s
sampples
(1300°C, 6 hours) weree shown in Figures
F
5 andd 6.
Most grainns appeared to be equiaaxed with soome
being a litttle elongatedd. There seemed to be no
significant intergranularr pores. Thee averaged grrain
sizes of thee undoped annd Bi-doped samples are 4.0
μm for sollid state proocessing andd 15.9 μm for
precipitation. A dramaatic change in grain size,
s
however, was
w
observved in the (Ba,Bi)-doped
sample (10.5 μm for soolid-state proocessing and 6.6
μm for preccipitation). Note
N
that diffferent trends in
grain size upon
u
addition of barium were obtainned:
an increasee in the soolid-state proocessing buut a
decrease in the precipittation methodd. Figure 6 also
a
displays a number of small pores trapped witthin
these large (Ba,Bi)-dopeed grain; theese intragranuular
pores couldd be responssible for a loower densityy in
this (Ba,Bii)-doped sam
mple. Also,, the Bi-doped
precipitatedd sample (Figure 5(b)) displays sm
mall,
discrete graains of approoximately 1 to 2 μm aloong
grain bounddaries.
Figure 5. SE
EM image off the Bi-dopedd sample sinteered
att 1300°C for 6 hours. (a) Soliid-state processsing.
(bb) Precipitationn method.
Figure 7 shows thee microstrucctures of the
(Baa,Bi)-doped precipitated
p
saample sintereed at 1300°C
C
forr 6 hours att different m
magnification
ns. A greatt
deg
gree of poroosity can be seen with siizes varying
fro
om approxim
mately 5 to 220 μm. Accu
umulation off
porres to form macroscopic
m
vvoids (larger than 50 μm)
occcurred extensively, confirm
ming sluggish
h sinterabilityy
as evidenced byy the density data in Figu
ure 3. Hence,
thee (Ba,Bi)-dopping tended too yield both intergranularr
and
d intragranullar pores. Fiigure 7(b) also shows a
miccrostructure at a higher m
magnificatio
on. It is veryy
inteeresting to see
s smaller grains trapp
ped within a
larg
ge grain (in the middle oof the figuree). This type
of grain entrapm
ment was observed locatin
ng randomlyy
thrroughout thee sample. Itt is believed
d that suchh
occcurrence might be caussed by abno
ormal grainn
gro
owth typicallly associated with multticomponentt
sysstems withinn which addditives posssessing low
w
meelting temperatures or aare capable of inducing
forrmation of a liquid phase with other oxides.
o
Fig
gure 7. SEM image
i
of the (Ba,Bi)-doped
d precipitatedd
samplle sintered att 1300°C forr 6 hours att
differeent magnificattions.
Increasses in both ddensity and grain
g
growthh
at the temperaatures higherr than 1200°C for bothh
Bi--and (Ba,Bii)-doped sam
mples could
d likely be
exp
plained by liquid phasse formation
n; differentt
sysstems would yield different mechanisms of suchh
forrmation. Adddition of Bi2O3 has been
n reported to
ressult in drastic improvem
ment in densiification andd
graain growth kinetics.
k
Thee sintering temperatures
t
bettween 1100°°C and 1300°C were much
m
higherr
thaan the eutecctic temperaature (~750°°C) for the
ZnO-Bi2O3 sysstem.(1, 3) Thhe formed liquid
l
phase
130
YONG
GVANICH, N.
N et al.
enhances diffusion
d
durring sinteringg. Nevertheless,
this study shows thaat there waas virtually no
difference in
i grain sizee between thhe undoped and
Bi-doped samples.
s
Succh finding could
c
likely be
explained by
b the amounnt of Bi addiition which was
w
very low in
i this studyy. Previous studies usually
employed Bi
B addition of
o more than 0.5 mol%.(3, 8-9)
In addition, a very small amount of Bii2O3 (0.04 mool%)
has been staated to yield microstructura
m
al inhomogenneity
with noticeable porosityy.(2) Hence, there
t
seemedd to
exist a criticcal amount of
o Bi additionn beyond whhich
significant grain
g
growthh could occurr.
milarly, the inncrease in graain size of more
m
Sim
than two-ffold in the (Ba,Bi)-dooped solid-state
sample shoould not be correlated to Bi addittion
alone. Nevvertheless, thhe theory of
o liquid phhase
sintering coould still be valid
v
in spitee of the abseence
of observabble intergranuular liquid films in Figurre 6.
The larger ionic radiuss of Ba2+ (0.134 nm) woould
likely segreegate along grain bounddaries due too its
insolubilityy with the sm
maller Zn2+ ioon (0.074 nm
m).(4)
The BaO liqquid phase, along
a
with poossible Ba-Znn-O
compoundss, was reporrted to help enhance grrain
growth ratee.(4)
A typical
t
I-V characteristic is shownn in
Figure 8 forr (Ba,Bi)-doped solid-statee sample sinteered
at 1300°C for 6 hourss. Other sam
mples displaayed
similar I-V format. No significant difference
d
in the
threshold voltage
v
was obtained
o
bettween the soolidstate and precipitatedd samples. The threshhold
voltages at 0.1
0 mA/cm2 are approxim
mately 120 V/cm
V
for the Bi-dooped sampless and 90 V/cm
m for the (Ba,B
Bi)doped sampples. These relatively
r
low
w values of the
threshold vooltage mightt be explaineed by ineffecttive
barrier alonng the grain boundary duue to inefficiient
amount of bismuth-cont
b
taining liquidd phase.
Co
onclusions
The unndoped pelleets display a continuous
inccrease in density to thhe maximu
um of 94%
(13
300°C). On the contraryy, the densitty values off
thee (Ba,Bi)-dooped pellets only fluctu
uated in the
ran
nge between 84% and 90%. The best sinterabilityy
waas observed in the Bi--doped samp
ples with a
relaative densityy of 94% readdily obtained
d at 1100°C.
The averaged grain
g
sizes w
were found to
t be similarr
bettween the undoped and Bi-doped samples.
s
Ann
inccrease of moore than twoo-fold in graain size was
obsserved in thee (Ba,Bi)-dopped sample. Differences
in both densiffication and grain growtth could be
exp
plained by the mechaanism of liiquid phase
forrmation as well as poossible inho
omogeneous
disstribution off additives. The amou
unt of Biadd
dition of onnly 0.5 mool% in this study was
suffficient to grreatly improvve sintering but did nott
seeem to be hiigh enough to producee a requiredd
am
mount of liqquid phase. Ba addition
n, however,
yieelded possiblly a differentt liquid phasee compoundd
wh
hich was efffective for eenhancing grain growthh
butt not forr eradicatioon of po
orosity norr
imp
provement inn the non-linnear responsee.
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