June 2005 Transformation Weakening of Interphas Dispers CuKo radiation(40 kv, 40 mA). All XRD data was obtained solvent Solvent: ethanol and at room temperature after furnace cooling. The relative volume ratios of a- and B-cristobalite phases were determined by inte- ball milling for 24h grating the X-ray peak areas of (102)of a-cristobalite and (222) of p-cristobalite by the equation Add plasticizers+ binder Plasticizer: PEG/DP =[(102)2/(22l+1(102)×100 ballmilling for 15h which Va was volume fraction of a-cristobalite, 1(102) and /(222)B were peak intensities of (102) and(222)B respectively about 10000 to 15000 cP (F Flexural Testing: Four-point flexural testing was ageing for 48h erformed using a 10 mm inner span and a 20 mm outer pan, at a crosshead speed of 0.01 mm/min on a universal test- Tape casting and drying Casting rate: 1 cm/s ing machine(model 4502, Instron Corp, Canton, MA). A min- Drying conditio imum number of five bars were tested for each composition. The saturated atmosphere apparent work of fracture was obtained by dividing the area at room temp. under the load versus displacement curve by the cross-sectional of the sample atting, stacking and lamination Lamination condition (G) Indentation Test. A Vickers hardness test was ca 80°Cfr30 min under ar ried out with a micro-hardness tester 32 12. Mark V lab uniaxial compression Inc, East Granby, CT) under a 6 kg tation load in order to study crack propagation profiles and interaction with the Burnet RT400°c(cmin)2 a hold (H) Microstructure Characterization: The microstruc- 400-550°C(0.5°C/min)3 h hold ure of sintered cristobalite and the crack propagation behavior in the laminates after bend testing were observed by optical Cold isostatic pressing CIP condition roscopy(Nikon SMZ-2T, Tokyo, Japan)and sEM(Model 170 MPa for 10 min DS-130. International Scientific Instruments, Santa Clara, CA) To observe grain size and cracking, polished and then annealed ples were chemically etched in boiling phosphoric acid 1200° Cfor lh at28MPa for 30s Fig. 2. Flow chart of tape casting procedure for fabrication of lami- nated composites. IlL. Results and discussion (1) Powder Analysis and Development of Crystalline Phases inates were cut into bend-bars. The cutting direction was along The calcined, amorphous-type silica powder was very soft, with the longitudinal axis of the specimens in the plane of the an agglomerated particle size range of 10-60 um. After ball- lamination. The bend-bars with dimensions of 30 mm milling, the powder had a narrower particle size distribution, x 3.0 mm x 2.5-3.0 mm were polished to a 15 um finish with forming small particles of approximately 0. 1-0. 4 um in size. The diamond paste. The specimens were then annealed at 1300"C for as-calcined powder had a BET specific surface area of 25 m/g The high specific surface area meant that the calcined powder was very porous. The ball-milled powder had a specific surface area of 75 m/g. The average particle size of the amorphous-type (2) characterization cordierite powder, after calcination at 750C for I h and ball- (A) Specific Surface Area Measurement: The specific milling for 12 h, was about 0.3 um, with a specific surface area of surface area of the calcined powders and ball-milled powders 80 m /g were compared by nitrogen gas absorption(Model ASAP 2400, Room temperature XRD spectra following the developmen Micromeritics, Norcross. GA) of crystalline phases of the calcined, amorphous-type silica pow (B) Thermal Expansion Coefficient Measurement: The der after various heating temperatures are shown in Fig 3.An variation of thermal expansion coefficient for polycrystalline amorphous phase was observed at 1000C. Above 1100.C, the cristobalite and mullite/cordierite mixtures were examined using B-cristobalite crystalline phase was detected and B-cristobalite a dilatometer(Netzsch Dilatometer, 402E, Selb, Germany) peaks were developed almost completely at 1200C. With heating up to 1100C at heating rate of 5C/min In the mul increasing temperature the amount of a-cristobalite phase lite/cordierite mixture, to obtain a relative density of above 95% increased gradually, while the amount of B-cristobalite de- in each specimen, the specimens had to be sintered at differer creased. At 1450.C, B-cristobalite remained as a minor phase temperatures. in the a-cristobalite matrix. The a form at high temperature was transformation resulting from the large using distilled water as a displacement liquid. The relative den- lite indicated that the aep transformation occurred at 180C y of each specimen was calculated from the theo density on heating and at 170C on cooling. The transformation tem- of mullite(3. 18 g/cm), cordierite(2.52 g/cm),an nd cristo perature was lower than that of pure cristobalite because of the (a phase: 2.33 g/cmand p phase: 2.26 g/cm) dopant effects. As the graphs show, the thermal expansion D) Average Grain Size Measurement: The average grain Defficient of B-cristobalite was approximately 1.5 x 10/C ize of sintered and annealed cristobalite was analyzed according and tended to decrease on heating. a change in thermal expan- to the Jeffries-Saltykov method sion coefficient was observed at the ae B transformation tem- (E) X-Ray Diffraction(XRD) Analysis: The develop- perature. It is known that the transformation of cristobalite nt of crystallinity in calcined, amorphous-type silica powder results in a large increase in the thermal expansion coefficient. 2 nd the phase change between a-and B-cristobalite hot-pressed This change was much less for the polycrystalline cristobalite es were studied as a function of he sintered at 1350 C than for the one at 1450C. The difference in annealing time using a Rigaku spectrometer(DMax automated the change of thermal expansion coefficient was attributed to the powder diffractor u/U.SA, Danvers, MA) with a-cristobalite content. In the case of the polycrystalline cristobalinates were cut into bend-bars. The cutting direction was along the longitudinal axis of the specimens in the plane of the lamination. The bend-bars with dimensions of 30 mm 3.0 mm 2.5–3.0 mm were polished to a 15 mm finish with diamond paste. The specimens were then annealed at 13001C for different times. (2) Characterization (A) Specific Surface Area Measurement: The specific surface area of the calcined powders and ball-milled powders were compared by nitrogen gas absorption (Model ASAP 2400, Micromeritics, Norcross, GA). (B) Thermal Expansion Coefficient Measurement: The variation of thermal expansion coefficient for polycrystalline cristobalite and mullite/cordierite mixtures were examined using a dilatometer (Netzsch Dilatometer, 402E, Selb, Germany), heating up to 11001C at heating rate of 51C/min. In the mul￾lite/cordierite mixture, to obtain a relative density of above 95% in each specimen, the specimens had to be sintered at different temperatures. (C) Relative Density Measurement: The density of the sintered specimens was estimated by the Archimedes method using distilled water as a displacement liquid. The relative den￾sity of each specimen was calculated from the theoretical density of mullite (3.18 g/cm3 ), cordierite (2.52 g/cm3 ), and cristobalite (a phase: 2.33 g/cm3 and b phase: 2.26 g/cm3 ). (D) Average Grain Size Measurement: The average grain size of sintered and annealed cristobalite was analyzed according to the Jeffries–Saltykov method.49 (E) X-Ray Diffraction (XRD) Analysis: The develop￾ment of crystallinity in calcined, amorphous-type silica powder and the phase change between a- and b-cristobalite hot-pressed samples were studied as a function of heating temperature and annealing time using a Rigaku spectrometer (DMax automated powder diffractometer, Rigaku/U.S.A., Danvers, MA) with CuKa radiation (40 kV, 40 mA). All XRD data was obtained at room temperature after furnace cooling. The relative volume ratios of a- and b-cristobalite phases were determined by inte￾grating the X-ray peak areas of (102) of a-cristobalite and (222) of b-cristobalite by the equation:24 Va ¼ ½Ið102Þa=Ið222Þb þ Ið102Þa 100 in which Va was volume fraction of a-cristobalite, I(102)a and I(222)b were peak intensities of (102)a and (222)b respectively. (F) Flexural Testing: Four-point flexural testing was performed using a 10 mm inner span and a 20 mm outer span, at a crosshead speed of 0.01 mm/min on a universal test￾ing machine (model 4502, Instron Corp., Canton, MA). A min￾imum number of five bars were tested for each composition. The apparent work of fracture was obtained by dividing the area under the load versus displacement curve by the cross-sectional area of the sample.50 (G) Indentation Test: A Vickers hardness test was car￾ried out with a micro-hardness tester (Zwick 3212, Mark V Lab. Inc., East Granby, CT) under a 6 kg indentation load, in order to study crack propagation profiles and interaction with the microstructure. (H) Microstructure Characterization: The microstruc￾ture of sintered cristobalite and the crack propagation behavior in the laminates after bend testing were observed by optical mi￾croscopy (Nikon SMZ-2T, Tokyo, Japan) and SEM (Model DS-130, International Scientific Instruments, Santa Clara, CA). To observe grain size and cracking, polished and then annealed samples were chemically etched in boiling phosphoric acid for 30 s. III. Results and Discussion (1) Powder Analysis and Development of Crystalline Phases The calcined, amorphous-type silica powder was very soft, with an agglomerated particle size range of 10–60 mm. After ball￾milling, the powder had a narrower particle size distribution, forming small particles of approximately 0.1–0.4 mm in size. The as-calcined powder had a BET specific surface area of 25 m2 /g. The high specific surface area meant that the calcined powder was very porous. The ball-milled powder had a specific surface area of 75 m2 /g. The average particle size of the amorphous-type cordierite powder, after calcination at 7501C for 1 h and ball￾milling for 12 h, was about 0.3 mm, with a specific surface area of 80 m2 /g. Room temperature XRD spectra following the development of crystalline phases of the calcined, amorphous-type silica pow￾der after various heating temperatures are shown in Fig. 3. An amorphous phase was observed at 10001C. Above 11001C, the b-cristobalite crystalline phase was detected and b-cristobalite peaks were developed almost completely at 12001C. With increasing temperature the amount of a-cristobalite phase increased gradually, while the amount of b-cristobalite de￾creased. At 14501C, b-cristobalite remained as a minor phase in the a-cristobalite matrix. The a form at high temperature was due to the spontaneous transformation resulting from the large particle size.22,51 Dilatometry curves for the stabilized cristoba￾lite indicated that the a3b transformation occurred at 1801C on heating and at 1701C on cooling. The transformation tem￾perature was lower than that of pure cristobalite because of the dopant effects.36 As the graphs show, the thermal expansion coefficient of b-cristobalite was approximately 1.5 10
6 /1C and tended to decrease on heating. A change in thermal expan￾sion coefficient was observed at the a3b transformation tem￾perature. It is known that the transformation of cristobalite results in a large increase in the thermal expansion coefficient.52 This change was much less for the polycrystalline cristobalite sintered at 13501C than for the one at 14501C. The difference in the change of thermal expansion coefficient was attributed to the a-cristobalite content. In the case of the polycrystalline cristoba￾Fig. 2. Flow chart of tape casting procedure for fabrication of lami￾nated composites. June 2005 Transformation Weakening of Interphases 1523