Electrical Properties and Defect Structure of Y2O3

Abstract

Guarded measurements of the electrical conductivity of high-purity, polycrystalline YZO3 in thermodynamic equilibrium with the gas phase were made under controlled temperature and oxygen partial pressure conditions. Data are presented as isobars from 1200O to 16OO0C, and as isotherms from oxygen partial pressures of 10-1 to 10-1' atm. The ionic contribution to the total conductivity, determined by the blocking electrode polarization technique, was less than 1% over the entire range of temperatures and oxygen partial pressures studied. Yttria is shown to be an amphoteric semiconductor with the region of predominant hole conduction shifting to higher pressures at higher temperatures. In the region of $-type conduction, the conductivity is represented by the expressionThe observed pressure dependence is attributed to the predominance of fully ionized yttrium vacancies. Yttria is shown to be a mixed conductor below 900°C. samples, fabricated by the methods described in the following, were prepared from Lindsay Code 1118 yttrium oxide powder having a spectrographic analysis (ppm) of: Fe < 5, Si 20, Mg 1, A1 < 10, Ca 2, and rare earths < l .( A ) Sam@le Y-2: This sample was hot-pressed in a graphite die a t 3000 psi and 125OoC for 2 hr, and then sintered in air at 18OO0C for G hr.This sample was hot-pressed in a graphite die at 3000 psi and 1250°C for 2 hr, reoxidized in air at 8OO0C for 12 hr, sintered in vacuum at 1800°C for 2 hr, and finally reoxidized in air at 8OO0C for 12 hr.(C) SumpZe Y-4: This sample was cold-pressed at 20,000 psi, sintered in vacuum at 180Ooc for 2 hr, and reoxidized in air a t 800°C for 12 hr.( D ) Samples Y-9 and Y-10: These samples were coldpressed a t 20,000 psi, sintered in vacuum a t 2300°C for 2 hr, and reoxidized in air at 1300°C for 48 hr. The density was 98% of theoretical X-ray density.Electrodes were applied to the disk faces of all samples using platinum paste (Engelhard No. 6082). One face was completely coated; a center electrode and guard ring were painted on the opposite face. The samples were then heated in air to 800°C to remove the organic binder in the paste.( B ) Sample Y-3:

Mixed Conduction in Solid Oxide Electrolytes.

“…The ionic contribution -to the conductivity was found to be less than one percent under these conditions although at temperatures less than 900'C, mixed conduction was reported. The differences between the results of the investigations of Schmalzried and Tare [57] and Tallan and Vest [58] is rationalized by the higher purity of the materials for the latter study. The conductivity measurements of Rudolph [36] suggest an ionic plateau at intermediate oxygen pressures for A-type La203.…”

“…Tallan and Vest [58] have more recently established that Y203 of 99. 99% purity is an amphoteric semiconductor over the P02 range from 10-1 to 10-17 atm at 1200 to 1600'C.…”

P-Type and N-Type Semiconductivities of Solid Yttrium Sulfide

“…When the density of positive holes is denoted by p, the equilibrium constant for reaction (7) can be expressed by (8) Taking the condition of electrical neutrality, 3[VY]=p, into account, the above equation becomes …”

Electrolytic Behavior of Yttria

“…More carefully controlled conductivity measurements on yttria have been made by Tallan and Vest (12). The electrical conductivity of reportedly very pure, polycrystalline Y203 in controlled Po2 atmospheres and temperatures from 12000 to 1600~ was obtained from guarded (3-probe) measurements with an a-c bridge.…”

“…Tallan and Vest (12) cannot be quantitatively converted into log Pe and log Pe domain boundaries without considerable additional assumptions regarding the temperature dependence of the ionic conductivity of Y203. However, the p-type to n -t y p e conductivity transition line was deduced from their conductivity m i n i m a (12) and is shown on Fig. 1 …”