Effect of the crystallinity of BaTiO3 powders prepared using the Merker method on the properties of PTCR ceramics

Article abstract of DOI:10.1134/S0020168512060179

Barium titanate powders differing in particle size (110-740 nm) were prepared by calcining barium titanyl oxalate precipitated by the Merker method. The powders were sintered to produce PTCR ceramics with the composition 100(Ba_0.89Ca_0.08Pb_0.03)TiO₃ + 0.8TiO₂ + 0.7Y + 0.1Mn + 2.5SiO₂ and electrical properties of the ceramics were studied. The results demonstrate that improving the crystallinity of the barium titanate powder suppresses recrystallization of the ceramics and has a significant effect on their resistance ratio and electric strength. We found the optimal range of calcination temperatures (950-1000°C) for barium titanyl oxalate which ensures the highest electric strength of thermistors with a resistance of 31 Ω. The average crystallite size of the parent barium titanate powder is -250-320 nm. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S0020168512060179

ISSN 0020ꢀ1685, Inorganic Materials, 2012, Vol. 48, No. 6, pp. 648–654. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © V.N. Shut, S.V. Kostomarov, V.L. Trublovsky, 2012, published in Neorganicheskie Materialy, 2012, Vol. 48, No. 6, pp. 746–752.
Effect of the Crystallinity of BaTiO3 Powders Prepared
Using the Merker Method on the Properties of PTCR Ceramics
V. N. Shut, S. V. Kostomarov, and V. L. Trublovsky
Institute of Technical Acoustics, Belarussian Academy of Sciences, pr. Lyudnikova 13, Vitebsk, 210023 Belarus
eꢀmail: shut@vitebsk.by
Received November 7, 2011
Abstract—Barium titanate powders differing in particle size (110–740 nm) were prepared by calcining barꢀ
ium titanyl oxalate precipitated by the Merker method. The powders were sintered to produce PTCR ceramꢀ
ics with the composition 100(Ba0.89Ca0.08Pb0.03)TiO3 + 0.8TiO2 + 0.7Y + 0.1Mn + 2.5SiO2 and electrical
properties of the ceramics were studied. The results demonstrate that improving the crystallinity of the barium
titanate powder suppresses recrystallization of the ceramics and has a significant effect on their resistance
ratio and electric strength. We found the optimal range of calcination temperatures (950–1000
titanyl oxalate which ensures the highest electric strength of thermistors with a resistance of 31
crystallite size of the parent barium titanate powder is ~250–320 nm.
°
Ω
C
) for barium
. The average
DOI: 10.1134/S0020168512060179
INTRODUCTION
to a heated titanyl oxalic acid solution, and the resultꢀ
ant BTO precipitate is separated. The precipitation
procedure and BTO calcination conditions have a sigꢀ
nificant effect on the characteristics (yield and particle
size) of barium titanate powders [9].
Earlier, we studied the electrical properties of
PTCR ceramics fabricated from oxalateꢀderived barꢀ
ium titanate powders synthesized by the Clabaugh
method [10–12]. The use of reactive, fineꢀparticle
(100–200 nm) powders allowed us to lower the sinterꢀ
Positive temperature coefficient of resistance
(PTCR) semiconducting barium titanate ceramics are
used to produce PTC thermistors, which find applicaꢀ
tion in industrial and consumer electronics [1–3]. The
wide range of applications for such materials leads to a
variety of requirements on their electrical characterisꢀ
tics, which stimulates extensive research into their
property–composition relationships. No less imporꢀ
tant is the effect of the characteristics of the starting
materials and fabrication procedure on their electrical
properties [4, 5]. This issue has recently attracted
increased attention.
The most commercially successful approach to the
preparation of barium titanate powder is solidꢀstate
synthesis. “Classic” precursors are titanium dioxide
and barium carbonate. The key features and capabiliꢀ
ties of this process have been studied in sufficient
detail. The best known alternative approach is oxalate
route [6]. In the first step, ТiCl4, ВаСl2, and Н2С2О4
solutions are reacted to form barium titanyl oxalate
ing temperature by 50–60°C relative to materials proꢀ
duced from submicronꢀ and micronꢀsized particles
and to improve the performance parameters of therꢀ
mistors (electric strength and resistance jump).
The purpose of this work was to study the microꢀ
structure and properties of PTCR ceramics produced
from barium titanate powders prepared through the
Merker process.
EXPERIMENTAL
(BTO), ВаТiO(С2О4)2
⋅
4Н2О, as a reaction intermeꢀ
The starting material used was BTO, BaTiO(C2O4)2
⋅
diate. In the second step, the BTO is calcined at temꢀ 4Н2О, precipitated by the Merker method. To obtain
peratures above 700
ВаТiO3) powder.
BTO can be precipitated by several techniques,
which differ in mixing sequence, process temperature,
°
C
to give barium titanate barium titanate powders differing in crystallinity, BTO
was calcined in air for 1 h at temperatures from 800 to
(
1200
°С.
Figure 1 plots the average crystallite size of the barꢀ
and Ba/Ti and Ba/C2O4 molar ratios in the starting ium titanate powders and the percentage of residual
mixture. The Clabaugh and Merker processes are used phases against BTO calcination temperature. The
most widely. In the Clabaugh process [7], a mixture of characteristics of the barium titanate powders were
BaCl2 and TiCl4 solutions is added to a hot oxalic acid described in greater detail elsewhere [9]. Calcium
solution. In the Merker process [8], an aqueous TiCl4 titanate was prepared by solidꢀstate reaction between
solution is added to oxalic acid to give titanyl oxalic calcium carbonate and titanium dioxide in air at
acid, H2TiО(C2O4)2. Next, barium chloride is added 1000°C. Ground barium titanate and calcium titanate
648

EFFECT OF THE CRYSTALLINITY OF BaTiO3 POWDERS
649
BT, nm
powders were mixed with TiO2, SiO2, and PbO by
d
grinding with polyamide media in deionized water to
give the overall composition
12
8
600
400
200
2
100(Ba0.89Ca0.08Pb0.03)TiO3
1
+ 0.8TiO2 + 0.7Y + 0.1Mn + 2.5SiO2.
Yttrium (donor dopant) and manganese (acceptor)
were introduced in the form of aqueous YCl3 and
MnSO4 solutions. To precipitate yttrium and mangaꢀ
nese carbonates, (NH4)2CO3 was added to the starting
mixture. The mixture was pressed into compacts with
a green density of 3.0 g/cm3. The compacts were fired
4
0
800
0
900
1000
Temperature, °C
1100
in air at temperatures of up to 1320
°С. The heating
Fig. 1. (1) Percentage of residual phases and (2) average
crystallite size of barium titanate as functions of BTO calꢀ
cination temperature.
rate was 350°C/h. Cooling to 800 was performed at
°С
a rate of 110 to 300°C/h, depending on the required
resistivity of the ceramics.
The complex impedance was measured at room
temperature with a HewlettꢀPackard 4285A LCR
bridge at frequencies from 75 kHz to 10 MHz and with
a BM538 impedance meter at frequencies from 5 to
110 MHz. In impedance measurements, we used
ceramic samples 5.6
× 5.6 × 2.6 mm in dimensions.
RESULTS AND DISCUSSION
Figure 2 illustrates the microstructure of the PTCR
ceramics prepared using the Merker process followed
by calcination at different temperatures. Figure 3 plots
30
μ
m
(а)
(b)
(c)
the average grain size of barium titanate (
synthesis temperature ( synth). The average grain size of
the ceramics is seen to decrease monotonically with
increasing synth. The average grain size varies from
6.8 m at tsynth = 800°C to 4.7 m at tsynth = 1200°C.
dBT) against
t
t
μ
μ
The microstructure ranges widely in grain size, with an
increased content of a glass phase, which is encounꢀ
tered on the surface of the samples. These features are
related to the morphology of the parent barium titanꢀ
ate powder and the diffusion of ionic additives during
sintering [13].
There is a strong correlation between the donor
concentration, the surface composition of the grains,
and the microstructure and electrical properties of the
ceramics. Rareꢀearth enrichment at grain boundaries
inhibits the recrystallization process. This effect has
been demonstrated in a number of studies concerned
with both semiconducting [14–16] and dielectric
(capacitor) barium titanate ceramics [17–20]. As the
overall rareꢀearth content increases, the grainꢀboundꢀ
ary rareꢀearth concentration increases more rapidly
than the bulk concentration because of segregation
processes [21]. As a result, the rareꢀearth elements
form a zonal shell on the outer layers of the grains. This
considerably reduces the mobility of the grain boundꢀ
aries, because of the slow diffusion of the donors.
Below the sintering temperature, the surface and
grainꢀboundary diffusion coefficients exceed the volꢀ
ume diffusion coefficient by more than two orders of
magnitude [22]. During sintering, the Y dopant is first
relatively uniformly distributed over the surface of priꢀ
30 μm
30 μm
Fig. 2. Microstructure of PTCR ceramics produced from
barium titanate synthesized at (a) 800, (b) 1000, and
(c) 1200°C (firing at 1320°C for 40 min).
INORGANIC MATERIALS Vol. 48
No. 6 2012

650
SHUT et al.
used, it is reasonable to assume the presence of excess
yttrium on the grain boundaries of the ceramic
because rareꢀearth ions have a small volume diffusion
coefficient [23]. As a result, the recrystallization proꢀ
cess slows down.
7
6
5
4
Note that, in ceramics prepared from powders synꢀ
thesized using the Clabaugh process [10, 11], dBT
(8.5–4.5 m) is a stronger function of tsynth (800–
μ
1200°C). This correlates well with the observed morꢀ
phology of the asꢀprepared barium titanate powders:
when the Clabaugh process was used, the increase in
800
900
1000
1100 tsynth, °С
the size of primary crystallites with increasing
more pronounced.
tsynth was
Fig. 3. Average grain size of PTCR ceramics as a function
of barium titanate synthesis temperature.
Figure 4 shows the 25°C resistivity of the PTCR
ceramics produced from barium titanate synthesized
at various temperatures. All of the samples were sinꢀ
ρ25
,
Ω
сm
1200
800
tered at a maximum temperature of 1320
and then cooled at a rate of 215°C/h. The ρ25
curve has a minimum at tsynth 950 , which is due to
°C
for 40 min
(
tsynth
)
=
°C
a competitive effect of two factors. On the one hand,
raising the BTO calcination temperature leads to a
reduction in the residual barium carbonate content of
the synthesized barium titanate. This would be
expected to reduce the resistivity of the PTCR ceramic
through a reduction in the volume of intergranular
glass phases in the sintered samples because this rules
out reaction between BaCO3 and SiO2. On the other
hand, as mentioned above, the use of macrocrystalline
400
0
800
900
1000
1100 tsynth, °С
Fig. 4. 25 C resistivity of PTCR ceramics as a function of
barium titanate synthesis temperature.
°
starting powders (at increased tsynth) leads to yttrium
enrichment at grain boundaries. It is well known that,
at low donor concentrations, the charge of the donor
ions is compensated by free electrons, which results in
semiconducting properties. With increasing donor conꢀ
mary barium titanate grains. Subsequent recrystallizaꢀ
tion and diffusion processes bring the donor to the
bulk of the grains.
In our experiments, the percentage of yttrium in centration, the charge of the donors is compensated by
the starting mixture was the same for all of the samples. negatively charged cation vacancies, which leads to a
rise in the resistance of grainꢀboundary regions (to the
point of formation of a dielectric layer) [24].
As a result, when microcrystalline barium titanate
powder, with a large grainꢀboundary area, was used
(
tsynth = 800–900°C), the initial donor concentration
PTCR behavior is a grainꢀboundary effect and is
best described in terms of the Heywang–Jonker
model. According to this model, the PTCR effect is
due to a grainꢀboundary double Schottky barrier
formed by electron traps. Actually, the grainꢀboundary
barrier has a more complex structure. It comprises a
Schottky barrier and an adjacent diffusion layer with a
on the surface of primary crystallites was lower. The
volume diffusion path was thus shorter, and the recrysꢀ
tallization process was more active. This led to the forꢀ
mation of PTCR ceramics with a uniform yttrium disꢀ
tribution over the grains and a relatively coarseꢀ
grained microstructure. Because macrocrystalline
barium titanate powders have a smaller specific surface reduced conductance, due to a nonuniform distribuꢀ
area, the yttrium content on the surface of primary tion of defects and impurities (donors and acceptors)
crystallites will be higher. When such powders are [25–27]. To analyze such structure, it is necessary to
examine an equivalent circuit (Fig. 5) whose impedꢀ
ance is the sum of three components, related to the
Rb
R2
R1
*
*
grain boundaries ( ), diffusion layers ( ), and grain
Z1 Z2
*
bulk ( ). The three components are given by
Zb
*
Zi = Ri (1 + jωRiCi)(i = 1,2,b),
where Ri and Ci are the equivalent resistances and
capacitances, which are adjustable parameters of the
model.
Cb
C2
C1
In this approach, the contribution of each layer can
be estimated by analyzing Cole–Cole diagrams. In parꢀ
Fig. 5. Equivalent circuit of PTCR ceramics.
INORGANIC MATERIALS Vol. 48
No. 6
2012

EFFECT OF THE CRYSTALLINITY OF BaTiO3 POWDERS
651
ticular, the diagrams of samples prepared from barium
titanate synthesized using the Clabaugh process were
similar in shape to a semicircle, with highꢀfrequency
distortions [10]. This attests to a nearly single relaxation
and homogeneity of potential barriers, and the distorꢀ
tions are well represented by the contribution of the difꢀ
fusion layer to the total resistance of the samples.
Figure 6 presents impedance spectroscopy data for
the ceramics produced from barium titanate powders
prepared using the Merker process. The three diaꢀ
Z
'',
Ω
(а)
240
160
80
0
'',
160
320
480
640
(b)
Z
Ω
''
'
grams are seen to be severely distorted (
<
Z max Z max
0.38). This means that the potential barrier height varꢀ
ies markedly from grain to grain. It is then necessary to
introduce a soꢀcalled arc depression factor, αi [25].
The complex impedance is then given by
75
50
25
αi
*
Zi = Ri [1 + ( jωτi) ] (i = 1, 2, b) .
0
50
100
150
200
The αi value characterizes the relaxation time distriꢀ
bution of grains and ranges from 0 to 1. At higher αi
values, the grain boundaries are more uniform and the
shape of the Cole–Cole diagram approaches a semiꢀ
circle.
According to analysis in this model (Fig. 6), the
rightꢀhand semicircle corresponds to the complex
Z
'',
Ω
(c)
400
300
200
100
*
Z1
impedance
(boundary layer), and the leftꢀhand
*
Z2
semicircle, to
(adjacent diffusion layer). It can be
0
200
400
600
800
Z
',
Ω
seen that the depression of the Cole–Cole diagram
increases with an increase in the calcination temperaꢀ
ture of barium titanate. The diffusion layer makes an
appreciable contribution to the total resistance of the
samples. This contribution increases with increasing
barium titanate crystallinity.
The Schottky barrier height can be effectively conꢀ
trolled by varying the cooling rate, which influences
the density of grainꢀboundary acceptor states [12, 28].
Reducing the cooling rate increases the density of
grainꢀboundary electron traps and vice versa. This
allows one to achieve a constant resistance R25 of
samples prepared from barium titanate powders difꢀ
Fig. 6. Cole–Cole diagrams for samples prepared from
barium titanate synthesized at (a) 800, (b) 1000, and (c)
1200
°
C (firing at 1320°C for 40 min, cooling at 215 C/h).
°
The large semicircle corresponds to the calculated comꢀ
*
Z1
plex impedance
(Schottky barrier), and the small
*
semicircle, to
(diffusion layer). The dots represent the
Z2
measured total impedance.
×
105
290
1
14
10
270
250
fering in dBT. This approach is of importance for the
ability to produce PTC thermistors with a predeterꢀ
mined resistance.
2
6
2
230
210
Adjusting the cooling rate in the range from 1320 to
800
°
C
, we achieved the same roomꢀtemperature resisꢀ
360
320
tance (R25 = 31
Ω
) in ceramics produced from barium
titanate powders of different particle sizes. Figure 7
plots the electric strength and resistance ratio against
BTO calcination temperature for thermistors with
3
280
240
R25 = 31 . Also given in Fig. 7 are the cooling rates of
Ω
200
160
samples prepared from various barium titanate powꢀ
ders with R25 = const. The electric strength of PTCR
ceramics is known to increase with increasing resisꢀ
800 850 900 950 1000 1050 1100 1150
tsynth, °С
tance ratio and decreasing voltage sensitivity of
ρ
(varistor effect) [29, 30]. The varistor effect is, in turn,
weaker at a smaller grain size and a more uniform
microstructure of ceramics because the grainꢀboundꢀ
ary voltage drop is then smaller [31].
Fig. 7.
tions of barium titanate synthesis temperature for samples
with the same ; ( ) cooling rate that ensures = const
(1) Electric strength and (2) resistance ratio as funcꢀ
ρ
3
ρ
25
25
(sintering at 1320 C for 40 min).
°
INORGANIC MATERIALS Vol. 48
No. 6 2012

652
SHUT et al.
states cannot be compensated by spontaneous polarꢀ
ization in the ferroelectric phase. It is worth pointing
out that, when barium titanate is codoped with donors
(Y) and acceptors (Mn), the latter may not only preꢀ
vent rareꢀearth accumulation at grain boundaries but
also segregate [33–35]. This process may also play an
important part not only during cooling of ceramics but
also during highꢀtemperature synthesis [12, 32, 36].
For this reason, in the case of prolonged highꢀtemperꢀ
ature synthesis, manganeseꢀcontaining ceramics
degrade rather rapidly.
×
105
370
330
20
16
12
1
2
290
250
8
4
The microstructure of the ceramics produced by
sintering at the highest temperature for 15 min was
essentially identical to that of the samples prepared by
heat treatment for 40 min. At the same time, their
roomꢀtemperature resistance was lower in comparison
with the materials prepared by prolonged sintering
(provided the cooling rate was 215°C/h in all cases).
200
150
100
50
3
Therefore, to obtain thermistors with R25 = 31 , this
Ω
800 850 900 950 1000 1050 1100 1150
regime allows one to reduce the cooling rate and,
accordingly, to raise the density of grainꢀboundary
acceptor states.
The electrical properties of thermistors held at the
highest temperature for 15 min are presented in Fig. 8.
It can be seen that the above trends persist and that the
breakdown voltage and resistance ratio are higher
when microcrystalline active powders are used.
tsynth
, °С
Fig. 8.
tions of barium titanate synthesis temperature for samples
with the same ; ( ) cooling rate that ensures = const
(1) Electric strength and (2) resistance ratio as funcꢀ
ρ
3
ρ
25
25
(sintering at 1320 C for 15 min).
°
The Rmax
from BTO powders calcined in the range 800–1000
increases with increasing synth, which is due to the
/Rmin ratio of the thermistors produced
°C
t
CONCLUSIONS
increase in the density of acceptor states and the drop
in grain size. This is accompanied by an increase in
breakdown voltage. The use of coarser powders
reduces the breakdown voltage, even though the
ceramic has a small grain size and rather large PTCR.
This points to an increase in the magnitude of the
varistor effect in such materials. The increase in the
magnitude of the varistor effect may be due to the
severe inhomogeneity of grainꢀboundary barriers, as
evidenced by the above impedance spectroscopy data.
Increasing the calcination temperature of BTO
precipitated by the Merker method increases the averꢀ
age crystallite size of the barium titanate powder from
110 ( synth = 800°C) to 740 nm (tsynth = 1200°C) and
t
reduces the percentage of residual phases. The syntheꢀ
sized barium titanate powders were used to produce
PTCR ceramics with the composition
100(Ba0.89Ca0.08Pb0.03)TiO3 + 0.8TiO2
+ 0.7Y + 0.1Mn + 2.5SiO2.
The optimal tsynth is 950°C. The size of primary barium
titanate crystallites is then 250–300 nm.
To improve the performance of thermistors, we
studied samples sintered at 1320°C for a short time
The average grain size of the ceramics decreases
with increasing barium titanate synthesis temperature
owing to the donor impurity enrichment at grain
boundaries.
(15 min). Reducing the sintering time (or sintering
temperature) reduces the roomꢀtemperature resisꢀ
tance of manganeseꢀcontaining ceramics [12]. During
synthesis, Мn2+ ions occupy Ti4+ sites, and the excess
The PTCR ceramics produced from the powders
synthesized at 900–1000
(250–300 cm), and the grainꢀboundary barriers are
°C has the lowest resistivity
charge is compensated by oxygen vacancies ( i i ),
Ω
VO
less uniform at higher BTO calcination temperatures.
Adjusting the cooling rate, we obtained PTC therꢀ
which form various (Mn)n–VO complexes, for examꢀ
i i
ple,
''
Mn Ti
–
[32]. During cooling, the grain boundꢀ
VO
mistors with a roomꢀtemperature resistance of 31
The samples prepared from the barium titanate powꢀ
ders synthesized at 950–1000 offer the highest elecꢀ
Ω
.
i i
aries oxidize, and the
concentration drops. As a
VO
result, the oxidation state of the manganese ions
changes from 2+ to 3+ or 4+ and they act as electron
°C
tric strength: 350 V/mm. The average crystallite size of
the parent barium titanate powder is 250–320 nm.
Our results demonstrate that the use of active
traps, increasing the electron trap density NS
.
The increase in NS is accompanied by an increase
in resistance ratio. However, at high NS values, the microcrystalline materials with a small percentage of
resistivity ρmin also rises because the grainꢀboundary residual phases helps to optimize the dopant distribuꢀ
INORGANIC MATERIALS Vol. 48
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EFFECT OF THE CRYSTALLINITY OF BaTiO3 POWDERS
653
15. Urek, S. and Drofenik, M., PTCR Behavior of Highly
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states, and to improve the electrical performance of
the thermistors.
16. Buscaglia, M.T., Viviani, M., Buscaglia, V., et al.,
Incorporation of Er3+ into BaTiO3, J. Am. Ceram. Soc.
,
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