Preparation of Cr 2O 3 crucibles and the defect structure of Cr 2O 3

Abstract

It is shown that Cr 2O 3 crucibles with less than 5 per cent porosity may be prepared by cold pressing and sintering a powdered mixture of Cr 2O 3 with 1 or 2 per cent metallic chromium, to provide a low oxygen potential, and a small amount of salicylic acid as lubricant. A rubber form is used to provide the cavity and to insure a reasonably uniform hydrostatic pressure. Sintering is done in vacuum or in an inert atmosphere at 1500°C. Consideration of available data on electrical conductivity and tracer diffusivities as well as on sintering leads to the conclusion that at 1600°C the principal defects at high p O 2 (about 10 −6 to 1 atm) are electrons and holes (divalent and tetravalent Cr ions), whereas at low p O 2 (10 −12 to about 10 −6 atm) the principle defects are electrons (divalent Cr ions) and vacant oxygen sites. The latter defects give rise to an off-stoichiometry which appears to be observable (about 1 10 per cent of the oxygen sites vacant).