1524 Journal of the American Ceramic Society-Kriven and Lee Vol. 88. No 6 β: B-cristobalt a: a-cristobalite p(11)(101) P(220)a1(200) P(222) 1450°C 1350c 500m Fig. 5. Scanning electron photograph of fragile cristobalite sample hot- 200°C essed at 1200C and annealed at 1300C for 50 h The sample was not polished and etched. 1100°C lite with the(101)and (200) peaks from a-cristobalite, which are the high intensity peaks of the cristobalite system, because the 000°C peaks were located at almost the same 20 value(Fig. 3). There- fore, the relative amounts of a- and B-cristobalite phase was compared from the relative intensity of (102) and(222)B peaks. In general, the relative volume ratios of a- and B-cristobalit 45 50 phases and the average grain size increased with increasing an- 20(CuKa) lealing time. The increase continued up to an annealing time of 50 h. The rate of grain growth decreased gradually from 12 h ically stabilized, amorphous, silica powder after heating at various tem- annealing time of 50 h resulted in fragile samples with extensive peratures. The holding time at each temperature was I h. cracks. This was because the thermally-induced transformation occurred spontaneously, primarily due to the critical particle size effect. The sample annealed for 50 h showed almost 80 vol% lite sintered at 1350C, a-cristobalite was present in the B- a-cristobalite. Figure 5 is an SEM micrograph of the fragile cristobalite matrix, whereas, for the polycrystalline cristobalite cristobalite sample which was hot-pressed and annealed for sintered at 1450.C, the a-cristobalite was the matrix phase 50 h. Extensive "macrocracks were observed over the whole (Fig. 3). The amorphous-type, cordierite powder crystallized sample surface. to a-cordierite at 1250C The polished and etched surface micrographs of cristobalite annealed at 1300oC for various times are shown in Fig. 6. Some macrocracks were detected in the microstructure having an (2) Critical Grain Size average grain size of 5 um, at annealing times of 12 h. The hot- The formation of a-cristobalite on cooling was affected by va ressed cristobalite annealed for 30 h consisted of about 72%a- to a phase occurred Spontaneous transformation from B phase cristobalite as shown in Fig. 4. It also exhibited macrocracks in cristobalite grain size stabilized the p phase. The grain size of the hemically doped B-cristobalite was controlled by annealing the influence of stress. Shear stress-induced阝→∝- cristobalite Figure 4 shows the plot of volume fraction of a-cristobalite conversion for annealed and ground specimens at various an- erage grain sizes for hot-pressed cristobalite samples as a nealing times are compared with the unground specimens. It is tion of annealing time at 1300C. It was impossible to com recognized that grinding involves a complex stress state. Fur- pare the intensity of the(111)and (220) peaks from B-cristoba thermore, displacive transformations can be induced by shear deformation that can be induced by grinding. This has been widely demonstrated in ceramics such as density of as hot-pressed cristobalite specim The mineral rhombohedral calcite(CaCO3), for ex- (98% relative density) ample, can be cyclically transformed to orthorhombic aragonite by repeated grinding in a planetary mill. Figure 7 displays the esult for hand-ground cristobalite. In view of the Gaussian dis- tribution of grain sizes around the critical particle size for shear- induced transformation the two effects of shear-induced and thermally-induced transformation are superimposed. However the tendency exists for an increase in stress-induced a-phase wit increasing annealing time up to maximum. Specifically, a max oRatio of a/B-cristobalite phase imum increase of about 17 vol% a-cristobalite was calculated in the polycrystalline sample annealed at 1300C for 10 h. This in- dicates that the optimum range of critical grain size for shear Annealing Time at 1300C(h) stress-induced transformation was approximately 4-5 um. The increase in the amount of a-cristobalite in the ground specimens Ime fraction of a-cristobalite and average grain sizes of ho over that in the only annealed specimens, decreased with in pressed alite samples as a function g time at I300°C. creasing annealing time above 10 h. This may be attributed tolite sintered at 13501C, a-cristobalite was present in the b￾cristobalite matrix, whereas, for the polycrystalline cristobalite sintered at 14501C, the a-cristobalite was the matrix phase (Fig. 3). The amorphous-type, cordierite powder crystallized to a-cordierite at 12501C.48 (2) Critical Grain Size The formation of a-cristobalite on cooling was affected by var￾ying the grain size.24 Spontaneous transformation from b phase to a phase occurred at larger grain sizes. In contrast, a small b￾cristobalite grain size stabilized the b phase. The grain size of the chemically doped b-cristobalite was controlled by annealing time. Figure 4 shows the plot of volume fraction of a-cristobalite and average grain sizes for hot-pressed cristobalite samples as a function of annealing time at 13001C. It was impossible to com￾pare the intensity of the (111) and (220) peaks from b-cristoba￾lite with the (101) and (200) peaks from a-cristobalite, which are the high intensity peaks of the cristobalite system, because the peaks were located at almost the same 2y value (Fig. 3). There￾fore, the relative amounts of a- and b-cristobalite phase was compared from the relative intensity of (102)a and (222)b peaks. In general, the relative volume ratios of a- and b-cristobalite phases and the average grain size increased with increasing an￾nealing time. The increase continued up to an annealing time of 50 h. The rate of grain growth decreased gradually from 12 h annealing time onwards. Large grain sizes of about 8.5 mm at an annealing time of 50 h resulted in fragile samples with extensive cracks. This was because the thermally-induced transformation occurred spontaneously, primarily due to the critical particle size effect.24,51 The sample annealed for 50 h showed almost 80 vol% a-cristobalite. Figure 5 is an SEM micrograph of the fragile cristobalite sample which was hot-pressed and annealed for 50 h. Extensive ‘‘macrocracks’’ were observed over the whole sample surface. The polished and etched surface micrographs of cristobalite annealed at 13001C for various times are shown in Fig. 6. Some macrocracks were detected in the microstructure having an average grain size of 5 mm, at annealing times of 12 h. The hot￾pressed cristobalite annealed for 30 h consisted of about 72% a￾cristobalite as shown in Fig. 4. It also exhibited macrocracks in the microstructure having an average grain size of 7 mm. The transformation of b to a cristobalite was susceptible to the influence of stress. Shear stress-induced b-a-cristobalite conversion for annealed and ground specimens at various an￾nealing times are compared with the unground specimens. It is recognized that grinding involves a complex stress state. Fur￾thermore, displacive transformations can be induced by shear deformation that can be induced by grinding. This has been widely demonstrated in ceramics such as zirconia and in ensta￾tite.26,27,53 The mineral rhombohedral calcite (CaCO3), for ex￾ample, can be cyclically transformed to orthorhombic aragonite by repeated grinding in a planetary mill.54 Figure 7 displays the result for hand-ground cristobalite. In view of the Gaussian dis￾tribution of grain sizes around the critical particle size for shear￾induced transformation, the two effects of shear-induced and thermally-induced transformation are superimposed. However, the tendency exists for an increase in stress-induced a-phase with increasing annealing time up to maximum. Specifically, a max￾imum increase of about 17 vol% a-cristobalite was calculated in the polycrystalline sample annealed at 13001C for 10 h. This in￾dicates that the optimum range of critical grain size for shear stress-induced transformation was approximately 4–5 mm. The increase in the amount of a-cristobalite in the ground specimens over that in the only annealed specimens, decreased with in￾creasing annealing time above 10 h. This may be attributed to Fig. 4. Volume fraction of a-cristobalite and average grain sizes of hot￾pressed cristobalite samples as a function of annealing time at 13001C. Fig. 3. Room temperature X-ray diffraction spectra of calcined, chem￾ically stabilized, amorphous, silica powder after heating at various tem￾peratures. The holding time at each temperature was 1 h. Fig. 5. Scanning electron photograph of fragile cristobalite sample hot￾pressed at 12001C and annealed at 13001C for 50 h. The sample was not polished and etched. 1524 Journal of the American Ceramic Society—Kriven and Lee Vol. 88, No. 6