Short Notes K9

phys. stat. sol. (a) - 61, K9 (1980) Subject classification: 1.4 and 14.3.3; 22.8.2 Zentralinstitut fiir Festkarperphysik und Werkstofforschung der Akademie der Wissenschaften der DDR, Dresden11 Spinel Phases in ZnO Varistors

K.-H. BATHER, D. HINZ, N. MATTERN, M. BITTERLICH, and BY

w. BRUCKNER

Metal oxide varistors a r e ceramic semiconducting devices characterized by an extremely nonlinear current-voltage characteristic /l/. Phase-analytical in- vestigations of such multicomponent varistors (cf. for instance /2, 3/) showed, beside doped ZnO and various Bi203-rich phases, always the existence of spinel phases. For a detailed analysis of the spinel phases found in the ceramics two different types of varistors were prepared (according to conventional ceramic techniques). For low voltage varistors (with a voltage per unit thickness in the order of Vc w 40 Vmm” a t a current density of 1 mA cm’2) a mixture of 97.5 mol% ZnO, 0.5 mol% B503, 0.5 mol% Co203, 0.5 mol% NiO, 0.5 mol% Ti02, and 0.5 mol% MnCO was used, whereas for medium voltage varistors (with Vc a 150 Vmm-l) 95.95 mol% calcined ZnO was mixed with 1 .0 mol% B5O3, 1 .0 mol% Sb203 a s well as 0.8 mol% NiO, 0.5 mol% MnC03, 0.4 mol% Cr203, and 0.35 mol% Co304. The pressed discs were sintered at about 1500 K in air for 2 h.

3

The X-ray phase analysis was carried out according to the goniometer step by step technique, where by means of a readjusted monochromator the detection sensibility of small phase portions could be increased. In order to obtain an en- richment of the phases formed by the additive oxides the samples were appro- priately prepared (the ZnO grains were preferentially etched away using 10 n NaOH, according to /2/).

The spinel phases, localized especially a s inclusions at three-grain junctions, were quantitatively analyzed by means of a scanning electron microprobe quanto- meter. The various analyzed regions were carefully chosen by means of the picture of the backscattered electrons. The intensities of the selected X-ray lines were measured with wavelength-dispersive spectrometers. The correcture

1) Postfach, DDR-8027 Dresden, DDR.

K l O physica status solidi (a) 61

programme MAGIC IV was used for the determination of the element concen- t ration.

The lattice constant of the spinel phases determined by X-ray diffraction amounts to a. = (8.45 2 0.01 )xl0-lo m for low voltage varistors (1.v. ) and a. = (8.56 2 O . O l ) x l O - l o m for medium voltage varistors (m.v. ). From the results of the quantitative microanalysis (Table 1 the following compositions of the spinels were determined:

'"" Z~.54Ni0.16C00.10Mn0.42Ti0. 78'4 '

m*v.: Znl. 91Ni0.15C00. 08'"0. 07cr0.23Sb0. 57'4

Taking into account the relative and absolute size of ions /4/, the Madelung energy of the structure,and the influence of the ligand field stabilization energy /5, 6/ the si te preference enthalpies of the ions were estimated for the tetra- hedrally coordinated lattice si tes as well as the octahedrally ones in the spinel structure /5/. Considering these results and the necessity of charge neutrality when ions with different ionization degrees a r e substituted one obtains for the structure the following completely inverse spinels: a Zn2Ti04-spinel, doped with Co, Mn,and Ni

and a Zn7/3Sb2,304-spinel, doped with Co, Mn, Ni, and C r r

Here the brackets characterize the lattice coordination: { } tetrahedral, [ 1 octahedral ones. With the experimental results of Table 1 the parameters x

and y were fitted to

1.v.: x ~ 0 . 2 5 y 0.20 , m.v.: x ~ 0 . 2 3 y * 0.10 .

Thereby concentration deviations of < 1 at% result, they lie within the range of e r ro r s of the EPM analyses. This shows the correctness of the sum formulas given above,

For confirmation of these results the spinel phases were separately pre- pared, too. The basic spinels Zn2Ti0 and Zn+3b2Ol2 a s well as the doped 4

Short Notes Kl1

T a b l e 1

Quantitative analysis of the spinel phase in low voltage varistors (1.v.) and medium voltage varistors (m. v. ) in at% of cations

element

Ti Zn Sb C r Mn c o N i

*I measured concentration

51 .'2

14.1 3.2 5.2

7.5 2.2 2.6 5.1

$) Mean values resulting from measurements of 1 0 measuring points at various sites.

spinels occurring in the ceramics were synthesized. In each case the spinels could be prepared in a single phase form. The lattice parameters of the basic types determined to be a. = (8.469 2 0.002)xl 0-1 m and a. = (8.59 7 2 0 .002)~ xl0-l' m agree with the corresponding data from literature /6 to 9/. For the doped spinels smaller values of the lattice parameters (compared with those

ones of the basic types) must be expected /2/. The lattice constants of the doped spinels synthesized with a composition according to 1.v. o r m.v., respectively, were found out to be indeed smaller:

a. = (8,46 2 O . O l ) x l O - l o m o r a = (8.56 2 O . O l ) x l O - l o m, respectively. 0

They agree with those ones determined in the real ceramics.

References /1/ M. MATSUOKA, T. MASUYAMA, and Y. IDA, Japan. J. appl. Phys. - 8,

/2/ M. INADA, Japan. J. appl. Phys. - 17, 1 (1978). 1275 (1969h J. Japan. SOC. appl. Phys., Suppl. - 39, 94 (1970).

/3/ D. HINZ, K. -H. BATHER, w. BRUCKNER, D. BEHLING, F. LANGE, and M. WOLF, Proc. 5th Internat. Symp. High Purity Materials Science Technology, Dresden, May 5 to 9, 1980, Vol. 3 (p. 50).

/4/ N. N. GREENWOOD, Ionenkristalle, Gitterdefekte und nichtstochiometrische Verbindungen, Verlag Chemie, Weinheim 1973.

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/5/ A. NAVROTSKY and 0. J. KLEPPA, J. inorg. nuclear Chem. - 29, 2701 (1967). /6/ A. NAVROTSKY and A. MUAN, J. inorg. nuclear Chem. - 33, 35 (1970). /7/ R.C. LINARES and A.D. MILLS, Acta cryst. 1 15 1048 (1962). /8/ G. BAYER, Naturwissenschaften - 48, 46 (1961). /9/ T. WATANABE and I. NAKADA, Japan. J . appl. Phys. - 17, 1745 (1978).

(ReceivedrJuly 8, 1980)

physica status solidi (a) 61