Crystallization behavior and microstructure of lithium-calcium aluminogermanate glasses
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Crystallization behavior and microstructure of lithium-calcium aluminogermanate glasses Moo-Chin Wang Department of Mechanical Engineering, National Kaohsiung Institute of Technology, 415 Chien-Kung Road, Kaohsiung, 80782, Taiwan, Republic of China (Received 25 April 1996; accepted 26 November 1996)
The crystallization behavior and microstructure of lithium-calcium aluminogermanate (LCAG) glasses have been studied by using differential thermal analysis (DTA), x-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and electron diffraction (ED). Uniform crystallization of the LCAG glass was found to result from two stages of the heating process. The kinetics of crystallization of the LCAG glasses was studied by DTA using the nonisothermal method. The activation energy for 3CaO ? Al2 O3 ? 3GeO2 crystal growth was 693 kJymol. The precipitated crystals determined by XRD analysis were mainly 3CaO ? Al2 O3 ? 3GeO2 , and minor phases of 2CaO ? Al2 O3 ? GeO2 and Li2 O ? Al2 O3 ? 2GeO2 . Morphology and microstructure of the glasses after heat treatment determined by SEM and STEM techniques are presented. Crystallization starts at the surface of the glass sample and then proceeds toward the interior of glass matrix. The morphology of 2CaO ? Al2 O3 ? GeO2 is that of a subangular bell-shaped single crystal growing in a preferred orientation through the segregated phase matrix of fine dispersion of 3CaO ? Al2 O3 ? 3GeO2 crystals. The Li2 O ? Al2 O3 ? 2GeO2 phase grows anisotropically in the fine fibrillar morphology ¯ and parallel to the [331].
I. INTRODUCTION
Glass-ceramics is a polycrystalline material formed from the melt as a glass followed by controlled crystallization at a temperature between the glass transition temperature and the melting temperature of the major crystalline phase(s). The glass-ceramic materials having a submicron grain size of the crystalline phase generally possesses much higher mechanical strength, impact strength, greater refractoriness, or lower thermal expansion coefficient than many ordinary glasses. Low thermal expansion glass-ceramics are becoming established in several applications such as hot plate tops, cooking ware, and laser envelops, where the dimensional stability and/or ability to resist thermal shock are necessary. The Li2 O–Al2 O3 –SiO2 (LAS) system is of particular importance from a technological standpoint in that it encompasses a relatively low thermal expansion coefficient. The phase relations and properties of this material have received a great deal of attention during the past three decades.1–8 Furthermore, the addition of MgO to the LAS, forming a Li2 O–MgO– Al2 O3 –SiO2 (LMAS) glass-ceramics system, has been studied.9–13 Details of the crystallization of b-spodumene related to the addition of CaO to the Li2 O–Al2 O3 –SiO2 –TiO2 system have been presented.14–19 Ge and Si belong to IV B group in the periodic table, and electronic configurations of Ge41 and Si41 are similar to
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