Properties of Thorium Oxide Ceramics

Included in this report are data concerning the strength, density, and porosity that may be expected of thoria ceramics formed by dry-pressing and isostatic pressing and fired at 1400°, 1600°, and 1800°C. By dry-pressing and firing at 1400°C., a density of 60% of theoretical (10.05) was obtained; at 1800°C. the density was increased to 80% of theoretical. By isostatic pressing a density of 87% of theoretical was obtained at 1800°C. A study of the densification of thoria ceramics by small additions of other inorganic materials showed the very marked effect of CaO (or CaF₂) in proportions of 0.5 to 3%. In proportions of 0.5 to 1%, CaO additions produced densities of 97% of theoretical for ThO₂ when fired at 1800°C. Experimental results intended to explain this phenomenon tended to substantiate the theory that it is due to formation of holes by substitutional solid solution and migration of ions in the altered structure. Thoria ceramics in the form of 12-in. rods, 1.5 in. in diameter, and small (0.1-in.) pellets with CaO additions were made having densities up to 97% of theoretical; evaluation of the abrasion resistance and strength is given. Porous thoria ceramics with densities of only 50 to 60% of theoretical were produced by addition of volatile material. Tests of the solubility of thoria ceramics showed that water at 250°C. had no effect. Ready dissolution was obtained with nitric acid and hydrofluoric acid as a catalyst.