ELSEVIER

August 1994

Materials Letters 20 (1994) 275-278

Electrical and structural properties of CuO-SrPb03-Ag ceramic composites

Masahiro Tanaka, Sachiko Fukushima, Masayuki Suzuki, Masaaki Isobe, Takaaki Ami Sony Corporation Research Center, I74 Fujitsuka-cho Hodogaya-ku, Yokohama 240. Japan

Received 9 May 1994; accepted 19 May 1994

Abstract

CuO-SrPbOs-Ag ceramic composites were synthesized at 1148 K in an oxygen ambient. The electrical conductivity of the composites strongly depends on the oxygen pressure during the sintering process. Samples synthesized under oxygen pressures higher than 10 MPa show metal-like conductivity due to the percolation channels formed by SrPbOs and Ag.

In order to obtain the most suitable ceramic mate- rials for electronic devices, many researchers have

been trying to create new ceramic composite systems. For example, a dielectric material being used in a fre- quency region higher than sub-GHz consists of more

than two complex compounds so that the tempera- ture dependence of the dielectric constants can be tuned to the temperature tendency of circuit con- stants, which are mainly realized by controlling the

volume fraction of these constituents. In this work, we examined how the electrical con-

ductivity and the ceramic structure of CuO-SrPb03- Ag multi-composite conductive ceramic systems were influenced by the oxygen pressure during the sinter-

ing process. If these ceramic composites are to be ap-

plied to resistive components in a hybrid ceramic de- vice, this information is very important for designing the structure of the device and planning of produc- tion processes.

CuO, SrO, SrCOJ, PbO and Ag,O powders with purities greater than 0.999 were used as starting ma- terials. SrO powder was calcined at 1234 K for 20 h at an oxygen pressure of 0.1 MPa before weighing. The raw materials were weighed and mixed using a

ball mill to achieve the composition 12 CuO-Sr- Pb03-Ag. The raw materials were ground by a wet

type ball mill using ethyl alcohol. This mixture was calcined at 1148 K for 30 h at an oxygen pressure of

0.1 MPa. The calcined sample was reground and pressed into a pellet under an uniaxial pressure of 56.8

MPa and then sintered at 1148 K for 5 h at an oxygen pressure of 0.02-12.8 MPa.

X-ray diffraction data (Cu Ka radiation) were col-

lected at room temperature using both 28-w and 28 scan modes (w was fixed at 0.052 rad). The conduc- tivity measurements were performed in the temper-

ature range 10 to 300 K by a normal four-point probe method at a He pressure of 0.1 MPa. The size of the

sample was typically 3 x 1.5 x 14 mm3. Gold thin film electrodes and Au wire leads were connected by a sil-

ver paint. The resistivities of the samples sintered at 1148 K

strongly depended upon the oxygen pressure during the sintering process. Fig. 1 shows the resistivities of the samples. The densities of the samples and the ox- ygen pressures during the sintering process are tabu- lated in Table 1. The densities of the samples are not seriously affected by the oxygen pressure. To check

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276 M. Tanaka et al. /Materia~s~tiers 20 (1994/ 275-278

2 20 10 60

2% (‘) -4 I

0 50 100 150 200 250 300

TEMPERATURE f K 1

Fig. 1. Temperature dependence of sample resistivities synthe- sized at 1148 K. Curves (B)-(E) correspond to samples as- signed in Table 1.

Table 1 Sample names, densities and the ambient oxygen pressure during the sintering process for samples synthesised at 1148 K

Sample Oz pressure

@@a)

Density Density ratio (kg/m’) (exp./calc.)

(A) #3013(3014) 0.0785 4.90 0.71 (B) #3023(3024) 0.686 5.45 0.79 (C) #301 l(3012) 5.93 4.95 0.72 (D) #3021(3022) 10.0 5.42 0.79 (E) #3015(3016) 12.8 5.33 0.77

the structure and homogeneity of each sample, X-ray diffraction patterns were obtained under various dif- fraction conditions. The X-ray diffraction pattern of the surface layer from the as-sintered sample is the same as that from the central part of the sample cut in half. Both patterns are also the same as that ob- tained by a normal powder diffraction using 28-w scan mode. Fig. 2 shows typical X-ray diffraction patterns of the samples sintered at 1148 IS. In each case, CuO [ 11, SrPb03 [ 2 ] and Ag are assigned as the origin of the pattern. Namely, it is shown that each sample sintered at 1148 K has good homogeneity and consists of CuO, SrPb03 and Ag. The lattice con- stants, estimated from the X-ray diffraction data of CuO and SrPb03 which are included in the sample, are shown in Table 2. Any influence of the oxygen pressure during the sintering process on the lattice parameters of CuO and SrPb03 was not observed in our experimental preciseness.

The X-ray diffraction patterns for the samples cal-

s

Fig. 2. X-ray diffmction patterns of (a) sample (B) #3024 and (b) sample (E) 83016 (see Table 1). Circles, triarqles and x

show the Brag8 peaks of SrPbOh CuO and Au which are used for standard materials in diffraction data.

Table 2 The lattice constants (8, and deg units) of CuO and SrPbOs which are included in the composites sintered at 1148 K. Sample names correspond to those in Table 1

#3024 #3016 ASTM data

CUO a=4.683(5) a=4.685(2) a=4.684

(C 2/c) b=3.428(4) b=3.425(1) b= 3.425 c-5.125(5) c=5.127(2) c=5.129 j&=99.54(8) 8=99.55(4) p= 99.47

SrPbOs a=5.857(2) a=5.850(4) a= 5.8595 (P bmm) b=5.951(2) b=5.947(4) b= 5.9568

c=8.324(3) c=8.327(9) c=8.3253

cined and sintered at 673 K in oxygen pressure of 0. 1 - 11.6 MPa were examined in order to obtain infor- mation regarding the path through which the ceramic ~mposite could be synthesized. The results are shown in Fig. 3. It is suggested that the main constituents of the samples were AgSPbz06 [3 J, SrO, and CuO in each case, and that Ag,PbzOh is a precursor material for synthesizing SrPbOj and Ag constituents in the composite finally sintered at 1148 K. It is also sug- gested that the annealing process under high oxygen pressure is very effective for the crystal growth of

Ag,Pb206. The resistivities of SrPbO> and CuO sintered at

1148 K in oxygen pressure of 12.55 MPa were also measured. The resistivities of SrPbOs decreased with decreasing temperature starting at about 300 K; how- ever, at about 100 K these values slightly increased with decreasing temperature, as if they were semi- conductors. The resistivity of CuO behaves as a

hf. Tunas et al. /Materials Letters 20 (1994) 275-278 277

2 20 40 60 68

26 (‘1

Fig. 3. X-ray diffraction patterns (a) of the well ground raw ma- terial powder, (b) of the powder calcined at 603 K in an 0, pres- sure of 0.1 MPa, (c) of the powder calcined at 673 K for 252 in an O2 pressure of 0.1 MPa, (d) of the sample (c) annealed at 673 K for 1 I h in an O2 pressure of 11.62 MPa. Circles, triangles and x show the Bragg peaks of Ag,Pb,O,, CuO, and Au which are used for standard materials in diffraction data.

semiconductor, and the resistivities of the synthe- sized SrPbOJ and CuO ceramic samples were respec- tively 3.98 X lOa and 0.74 G m at 300 K.

The resistivity of the CuO-SrPb03-Ag ceramic composite. sample was estimated to be 0.17 $2 m us- ing Landauer’s formula [4] with the following as- sumptions: (1) The composite consists of CuO, SrPb03 and Ag spherical particles with nominal frac- tions of 73.08%, 21.81% and 5.1 I%, which were es- timated using 2 number, densities and lattice con- stants; (2) Ag particles do not directly affect the resistivity, namely the resistivity was assumed to be the same as that of SrPb03. As shown in Fig. 4 the estimated resistivities are very consistent with those for samples sintered in an oxygen pressure less than 9.8 MPa.

On the other hand, in the samples sintered at higher oxygen pressures, the resistivities are much smaller than the estimated value of 0.17 G m and also show the metal-like temperature dependence. It is sug- gested that the structure of this ceramic composite strongly depends on the oxygen pressure during the sintering process and that percolation channels [ 51 are formed in the samples sintered in oxygen pres- sure higher than 9.8 MPa. Furthermore, the resistiv- ities of the sintered samples were smaller than the re- sistivity of the SrPbO, bulk sample sintered under an oxygen ambient of 12.55 MPa. Therefore, it is sug- gested that the percolation channels are rather formed with Ag and SrPbO, particles.

In some oxides the following are well known: dif-

I q cue

0 A Sr Pb 0,

-I ; -1

_-_!!____(_JL__

d

log P (MPa)

Fig. 4. Resistivities at 300 K versus ambient O2 pressures in the sintering process. The dotted line shows the value estimated by Landauer’s theory.

fusion coefficients of metal ions, Dmetal, are much larger than that of O-ion, and D,,,,,[ depends on the ambient oxygen pressure PO, and a positive correla- tion, In D,,,xln PO holds; the diffusion coefficient of a single-charged cation is larger than that of a higher-charged cation; and furthermore, diffusion constants in the grain boundary region are generally larger than those in the bulk region [ 61. It is there- fore very consistent with the general character of the diffusion in the grain structure of the oxide compos- ite that the diffused Ag atoms are distributed in the surroundings of the SrPbO, and CuO particles and form a grain near the point of intersection of some grain boundaries.

The critical volume fractions in some percolation systems were obtained, for example, 0.135 in the PrBazCu30,-Ag system [ 71 and 0.15 in the A1203- Sic system [ 8 1. In our composite system, the volume fraction of conductive particles seems to be at the point at which the percolation channels just start to grow, so that the conductivity of this composite is very sensitive to the change of grain structure.

In summary, CuO-SrPbO,-Ag homogeneous ce- ramic composites were synthesized at 1148 K. Sr- Pb03 and Ag in the composites were synthesized from raw materials and Ag,Pb20e. Ag particles in the com- posite were very stable in the oxygen pressure range of 0.1-10 MPa at 1148 K. The lattice constants of CuO and SrPbO, were little influenced by the oxygen pressure during the sintering process. The grain

278 M. Tanaka et al. /Materials Letters 20 (1994) 275-278

structure was strongly affected by the oxygen pres- sure during the sintering process. The electrical con- ductivity of the composites sintered at lower oxygen pressure was consistent with that estimated by the Landauer formula. On the other hand, those sintered at higher oxygen pressure behaved as if the samples included some percolation channels.

References

[ I] Swanson and Tage, NBC Circular 539, Vol. 1 (1953) 49. [ 21 R.D. Shannon, J. Solid State Chem. 3 ( 1971) 184. [ 31 Bystrom and Evers, Acta Chem. Stand. 4 ( 1959) 6 13. [4] R. Landauer, J. Appl. Phys. 23 ( 1952) 779. [5] S. Kirkpatrick, Rev. Mod. Phys. 45 (1973) 574. [6] W.D. Kingery, H.K. Bowen and D.R. Uhlmann, Introduction

to ceramics 2nd Ed. f Wiley, New York, 1975) ch. 6. [7] J.J. Lin, Phys. Rev. B 44 (1991) 789. [ 81 A. Sawaguchi, K. Toda and K. Niihara, J. Ceram. Sot. Japan

99 (1991) 523 (in Japanese).