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Crystal Structure of Yttrium and Other Rare-Earth Borates 253( C ) 1400" F Heat Treatment: Maximum crystallinity was developed during the 1400°F heat treatment as well as maximum resistance to thermal shock. An average AT of 925°F was required to cause failure. The phases lithium titanium silicate, sphene, and albite increased to a maximum whereas rutile remained constant. The structure exhibited no crystal orientation, as in the 1600°F heat treatment, and the crystals were much better developed, as shown in Fig. 11. Fracture characteristics were completely nonconchoidal, as shown in Fig. 12. There were also increases in transcrystalline failure and in secondary fracture planes intersecting the principal fracture surface.(D) 1100°F Heat Treatment: Heat treatment a t 1 100°F Fig. 12. Fracture surface of crystallized glass caused by failure in thermal shock at 925OF.structure is somewhat comparable to a titanium-opacified enamel. Thermal-shock failure occurred a t 550°F with the type of failure common to glassy materials. The conchoidal fracture surface resulting from the ineffective crystallization is shown in Fig. 8. ( B ) 1 600°F Heat Treatment: Thermal-shock resistance following the 1GOO"F heat treatment showed little improvement over the non-heat-treated specimen. The size of the rutile crystals increased slightly and traces of lithium titanium silicate, sphene, and albite developed. The typical structure of the specimen which failed a t 600°F is shown in Fig. 9. The orientation of the crystals in the center of the micrograph was caused by coating flow during heat treatment at too high a temperature. The fracture characteristics were similar to those of the non-heat-treated coating with a slight increase in discontinuities in the predominantly planar fracture (Fig. 10).was below the critical crystallization temperature, resulting in the same structure as the non-heat-treated specimen even though the time was extended to 65 hours. Fracture characteristics and thermal-shock resistance were also the same. The sequence of crystallization for the series started with spontaneous crystallization of rutile during firing. High-temperature heat treatment developed only trace amounts of lithium titanium silicate, sphene, and albite. At 1400°F the maximum development of crystals occurred, resulting in the maximum disturbance of the fracture pattern and increase in thermal-shock resistance. V. SummaryThe growth of crystals and their cumulative effect in developing internal stresses in glass coatings can be conveniently studied with the electron microscope. The information gathered in this manner can be related to the physical properties of the coatings. Heat treatments which developed maximum crystallinity with good distribution of the crystalline phase within the matrix resulted in materials with the best resistance to thermal shock. As resistance to thermal shock increased, the fracture paths became increasingly disrupted owing to the internal stresses developed between crystal and matrix. As i t became necessary for fra...

Susceptibility measurements have been carried out on single-crystal LaCoO3 in the temperature range of 4.2° to 300°K. We find an effective magnetic moment squared that agrees with Heikes et al. at 300°K, but which falls more rapidly with decreasing temperature. Paramagnetic resonance measurements at 4.2°, 77°, and 300°K have ruled out the presence of modest quantities of paramagnetic cobalt, leaving only the possibilities of diamagnetic or highly concentrated paramagnetic Co+3. (Large concentrations of other cobalt valencies are ruled out for chemical reasons.) Using a model found successful by Goodenough, we assume that Co+3 is near the crossover point, with the 1A1 state lowest. A calculation including spin-orbit effects, but assuming cubic symmetry for the 300° to 4.2°K range, gives essentially a linear variation for the energy difference between the 1A1 and the center of gravity of the 5T2 states, opposite to that expected for a cubic variation. This effect can be explained by including a lower symmetry distortion (probably trigonal) which increases with decreasing temperature, similar to that found in LaAlO3:Cr.