June 2005 Transformation Weakening of Interphas (a) c 5.0 (d) 5.0um Fig. 6. Scanning electron micrographs of the polished and etched surfaces for hot pressed cristobalite according to different annealing times of (a)Oh, b)10 h, (c)12 h, and(d)30 h(where arrows indicate"macrocracks") the fact that the amount of B-cristobalite, which can be trans- calcined powder (powder A)was attrition milled for I h. The formed by shear stress, was decreased by the spontaneous, ther- attrition-milled powder (powder B), in which the particle size mally-induced transformation which occurred in the oversized was reduced to less than I um, was also annealed at the sam rains temperature and same times as for the as-calcined powder A The critical grain size in the chemically doped B-cristobalite The as-calcined powder consisted of agglomerates composed of can be confirmed by the effect of initial powder size. The as- tiny crystallites in the B-f The volume fraction of d-cristoba- lite was measured for the two annealed powders. Powder A showed about 40 vol a-cristobalite. In contrast, powder B showed less than 10 vol a-cristobalite. The results of the effect of powder size, therefore, were consistent with the grain size trition milling the agglomerates were milled into tiny crystallites At higher annealing temperatures, sintering and grain growth ould be favored in agglomerates, but would be difficult in in- dividually separated crystallites. The volume fraction of a- 二 stobalite was measured for the two annealed powders. Pov der a showed about 40 vol% a-cristobalite. In contrast, powder B showed less than 10 vol% a-cristobalite. The results of the consistent with the grain ● Before grinding O After grinding (3) Composition of laminate Table I shows the variation of thermal expansion coefficient and flexural strength for the mullite/cordierite mixtures as a function Annealing Time at 1300C (h) ficient and flexural strength decreased as cordierite content was Fig. 7. Grinding effect on the relative volume ratio of l- and B- creased. To match the thermal expansion coefficient to the cristobalite phases for hot-pressed nemically doped B-cristobalite (1. 5 x 10/C), the mullite/cor Ing time at1300°C dierite layer should also have a low thermal expansion coeffithe fact that the amount of b-cristobalite, which can be trans￾formed by shear stress, was decreased by the spontaneous, ther￾mally-induced transformation which occurred in the oversized grains. The critical grain size in the chemically doped b-cristobalite can be confirmed by the effect of initial powder size. The as￾calcined powder (powder A) was attrition milled for 1 h. The attrition-milled powder (powder B), in which the particle size was reduced to less than 1 mm, was also annealed at the same temperature and same times as for the as-calcined powder A. The as-calcined powder consisted of agglomerates composed of tiny crystallites in the b-form. The volume fraction of a-cristoba￾lite was measured for the two annealed powders. Powder A showed about 40 vol % a-cristobalite. In contrast, powder B showed less than 10 vol % a-cristobalite. The results of the effect of powder size, therefore, were consistent with the grain size effect controlling thermally-induced transformation. During at￾trition milling the agglomerates were milled into tiny crystallites. At higher annealing temperatures, sintering and grain growth would be favored in agglomerates, but would be difficult in in￾dividually separated crystallites. The volume fraction of a￾cristobalite was measured for the two annealed powders. Pow￾der A showed about 40 vol% a-cristobalite. In contrast, powder B showed less than 10 vol% a-cristobalite. The results of the effect of powder size therefore, were consistent with the grain size effect controlling thermally-induced transformation. (3) Composition of Laminates Table I shows the variation of thermal expansion coefficient and flexural strength for the mullite/cordierite mixtures as a function of cordierite content. As expected, the thermal expansion coef- ficient and flexural strength decreased as cordierite content was increased. To match the thermal expansion coefficient to the chemically doped b-cristobalite (1.5 10
6 /1C), the mullite/cor￾dierite layer should also have a low thermal expansion coeffi- Fig. 6. Scanning electron micrographs of the polished and etched surfaces for hot pressed cristobalite according to different annealing times of (a) 0 h, (b) 10 h, (c) 12 h, and (d) 30 h (where arrows indicate ‘‘macrocracks’’). Fig. 7. Grinding effect on the relative volume ratio of a- and b￾cristobalite phases for hot-pressed cristobalite as a function of anneal￾ing time at 13001C. June 2005 Transformation Weakening of Interphases 1525