Agglomerate and Particle Size Effects on Sintering Yttria-Stabilized Zirconia 1950 vol% hexagonal phase is formed during sintering, a percentage adequate for retarding grain growth yet not so high that the reversion causes porosity or other problems. Sintering at higher or lower temperatures requires adjusting of the La,O, composition to obtain the proper concentration of hexagonal phases. Sintering at other temperatures was not as successful in the present study but appears to be possible in principle. The La,O,-Y,O, materials fabricated by this process are near enough to theoretical density to have excellent optical transmittance properties. Using the controlled transient second solid phase sintering mechanism requires a particular set of phase relations that result in a high-temperature two-phase field and a lower-temperature singlephase field for the same composition. Further, these fields must be at ~0 . 8 of the melting point for sufficiently rapid diffusion kinetics. Such phase relations occur for several rare earth-oxide combinations, suggesting that many other systems can be sintered by this mechanism. Other systems have been examined and the results will be reported elsewhere.

Compositions of La,O,-Y,O, were sintered to full density by a process which uses a controlled transient solid phase to retard grain growth sufficiently that pores remain on grain boundaries and are eliminated by solid-state diffusion. The temperature is then decreased into a single-phase region and the structural transition yields a pore-free, single-phase body. Optimum compositions are 8.1 to 12.1 mol% LazO, for a sintering temperature of 2170°C and annealing temperatures of 1920" to 2O2O0C, resulting in materials with near-theoretical total transmittances and specular transmittance within 6% of theoretical.

Approximately close+Not close+ , (~~~1 -Tetrahedral edge ------Octahedral edgeFig. 4. Arrangement of oxygen atoms in (310) plane of mullite structure.packed with oxygen atoms; (001) is the oxygen close-packed plane in the corundum structure. The arrangement of oxygen atoms in the (310) plane of rnullite is shown in an idealized form in Fig. 4. Partof this arrangement is near that of corundum, although there are some differences. Thus, it was considered that the relatively close-packed oxygen plane (310) of mullite would be almost preserved when mullite decomposed into corundum and that it would convert into the (001) plane of corundum, which is also a close-packed oxygen plane. The results of the present study, which indicate that the closepacked oxygen plane would be almost preserved when mullite thermally decomposes into corundum, are compatible with the general ideas reported by Taylor.*

Cation self-diffusion was measured in singlecrystal strontium titanate. Nonhomogeneous diffusion kinetics were observed and were interpreted by the Fisher model. The composite diffusion coefficient, P, of the Fisher model is shown to have a dependence on dislocation density, p, in single crystals at 1875OC given by:for strontium, and pTi4+ = (2.92 x 10-10)p(0.08 f 0.06) for titanium. It has previously been shown that oxygen ions have enhanced diffusivities in dislocation pipes ; therefore, in SrTiOs all ions exhibit enhanced dislocation diffusion. Based on an assumed maximum value for lattice diffusion, the temperature dependence of strontium dislocation pipe diffusion is given by : D,hz = 9.62 X exp ( -113.2 + 22.8 kcal/RT) where h Z is the dislocation core area.