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The sodium ion resistivity of lithia-stabilized polycrystalline @'-alumina was measured as a function of temperature for fine-grained and coarse-grained specimens with a chemical composition of 8.80 Naz0-0.75 Li20-90.45 A1203 (~4%). A model is presented which explains the dependence of sodium ion resistivity on grain size. Using the model the activation energy was determined for the transport of sodium ions across a grain boundary in this form of sodium p"-alumina.

An analysis of strains and stresses in four-point bending creep tests in the limit of small beam deflections resulted in a general equation which relates the load-point deflection, the applied load, the creep exponent (N), and the geometrical parameters of the loading system. Measurements of load-point deflection rates, which are experimentally easy to accomplish in ceramic systems, vs the applied load lead to the direct determination of the creep exponent and the creep compliance in a steadystate creep test. The creep compliance is a function of the temperature, grain size, and all other factors except stress. The elastic equation relating the load-point deflection and the outer fiber strain is strictly valid for viscous creep and a p proximately valid for nonviscous creep (i.e. N>1) if the ratio of the distance between the support points to the distance between the load points is not very large.

Journal of The American Ceramic Society -Discussions and Notes the glass during the first and second stages. At about the temperature of the second DTA reaction, the H modification begins to convert to the L form, with reorganization of the spherulites (third stage). The reversible displacive second-class transformation from the stable L form to the stable H modification occurs later, at 1009" (BzS3) or 1086°C (BgS8).5 In this type of transformation the crystallization of H modifications as metastable precursors is not usual. Lowtemperature crystallization of glasses may produce the metastable H modification instead of the stable L modification when the H modification is stabilized by slight solid solution. All crystallites obtained below 950°C were too small to detect solid solution and/or crystallization of untransformed H modifications in the glasses by X-ray methods. However, in thermodynamic equilibrium, these phases are always stoi~hiometric.~ STOICHIOMETRIC mullite powder (3Al2O3. 2Si02) prepared by a metal alkoxide process was hot-pressed to theoretical density by Mazdiyasni and Brown.' The resulting body has a unique microstructure of fine "interlocking needle, acicular, and polygonal grains" that apparently results in excellent strength and thermalshock resistance. To evaluate the creep resistance of this hotpressed mullite (1500°C at 6000 psi for 60 min), specimens were tested in 4-point bending under dead-load conditions in air. Both stress-and temperature-change experiments were conducted.Stress-change experiments yielded creep rates d 1400°C as shown in Fig. 1. The stress exponent found, i= 1 .O, is a strong indication of a creep process controlled by a diffusional mechanism, i.e. either Coble (grain-boundary diffusion) or Nabm-Hemng (bulk diffusion) creep. This process has an activation energy of = 164+-9 kcallmol (Fig. 2). The strain rate, measured from 1350" to 1450"C, decreased slightly when a specimen was tested at 1450°C and then returned to 1400°C. This discrepancy can be accounted for by a small change in grain size during the test. Initial and final grain sizes of 4.5 and 5.7 p m were measured (linear intercept technique). The creep rate was compared with that of pure AI2O3 at an equivalent grain size.' The creep rate at 1450°C of this dense mullite is = 1 order of magnitude less than that of the "pure" (99.995%) A1203, although at higher temperatures the difference would decrease as a result of the difference in creep activation energies (130 kcallmol for A1203 vs 164 kcal/md for mullite). Equal strain rates would be predicted at 2025"C, well above the melting point of mullite (1880°C).The foregoing data indicate that dense, polycrystalline mullite with a grain size of = 5 pm, prepared by the method of Mazdiyasni and Brown,' is significantly more creep-resistant than Alz03 from 1350°C to near the melting point of mullite. Further work is needed to measure the grain-size dependence and relate the creep behavior to a diffusional mechanism. (11-1) 1591. K. S. Mazdiyasni and L. M. Brown, "Synt...