Fabrication of Bulk Glass from the Eutectic Melt in Y2O3–CaO–Al2O3 System

Tokyo Institute of Technology, Edo, Tōkyō, Japan
Journal of Electroceramics (Impact Factor: 1.74). 12/2006; 17(2):1075-1078. DOI: 10.1007/s10832-006-9010-5


We have studied glass formable region in the Y2O3-CaO-Al2O3 system. An optimal composition for the amorphous phase formation was found in the pseudo-eutectic 12CaO⋅7Al2O3-CaYAlO4 system. An amorphous bulk plate of 10 × 25 × 1.8 mm was successfully fabricated using a simple molding technique. The fabricated
glass plate showed an IR absorption edge at around 5.6 μm. The crystallization of the glass sample was observed above 890∘C by an X-ray diffraction (XRD) and Thermogravimetry and Differential thermal analysis (TG-DTA).

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    ABSTRACT: Crack formation in the C12A7 nano-composite during crystallization was successfully avoided by using the eutectic glass in the C12A7–CaYAlO4 system. The crystal phases from the eutectic glass were identified to be C12A7 (major phase) and CaYAlO4 (minor phase) by X-ray diffraction and high-resolution transmission electron microscope. It was indicated that origin of cracks upon crystallization of C12A7 glass was the volume expansion caused by crystallization of C12A7 in the glass and the cracking could be avoided by the volume shrinkage by crystallization of CaYAlO4. By using a conventional molding technique, we have also succeeded to fabricate a bulk C12A7 composite with arbitrary shapes.
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    ABSTRACT: Crystallization behavior during annealing of eutectic glass in the C12A7–CaYAlO4 system was investigated by SEM, STEM, XRD, and TG–DTA. At 900°C, phase separation of Y-poor and Y-rich phases appeared without crystallization. The phase separation might lead bulk crystallization of C12A7 by subsequent crystallization. At 950°C, crystallization of unknown metastable phase was observed together with C12A7 crystallization. Above 1000°C, crystallization of CaYAlO4 and C12A7 was observed. Crystallization behavior was schematically explained and bulk crystallization of C12A7 was explained by the phase separation.
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    ABSTRACT: Eutectic melt solidification is shown to avoid cracking during solidification by quenching an amorphous phase. Subsequent annealing results in micro/nanostructure ceramics. This strategy has been applied to HfO2–Al2O3–GdAlO3 and Y2O3–CaO–Al2O3 ternary systems that have deep eutectics. In most cases, ceramic materials cracks when they are solidified from melt due to the thermal stress accumulated on the grain boundaries, the large specific volume difference between the melt and the crystalline solid, etc. The main reason why this strategy works is that a eutectic composition yields an amorphous phase from the melts by rapid cooling and the amorphous phase enables to design crystallization without cracking by postannealing. Appropriate postannealing for the quenched amorphous enables to control the crystallization behavior from the amorphous phase, which yields nanostructured composites without cracking. For the HfO2–Al2O3–GdAlO3 case, the melt solidification and postannealing yields a nanocomposite with high transparency due to reduced scattering of 5–10 nm crystallites. For the Y2O3–CaO–Al2O3 case, a plate-shaped bulk composite is obtained without cracking by molding the melt and postannealing.
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