Influence of various heating procedures on the sintered density of Sr-celsian glass-ceramic

Abstract

Glass-ceramics are crystalline materials that can be formed by controlled crystallization heat treatments of proper parent glasses. Most of them have unique properties such as translucency, high strength and very low and uniform thermal expansion [1]. They also have very fine and randomly oriented crystals with a few percent of the residual glass. The recent applications of glass-ceramics are for matrix materials for ceramic fiber-reinforced composite materials, substrate materials for semiconductor packaging, heat exchangers, telescope mirrors, artificial hard tissues and cook wares. A relatively new method of glass-ceramic preparation is the sintering of glass powders, followed by a nucleation heat treatment [2–5]. This technique gives cost benefits by reducing the processing temperatures. Furthermore, the formation of complex shapes can be obtained by using equipment common to a ceramic factory. When sintering is employed for the production of a glass-ceramic, sintered density becomes a critical factor for its application. That is, its mechanical properties are highly related to the sintered density and in most cases high density is desirable. In order to obtain high sintered density, fine glass powder is required to increase the surface area that can supply the deriving force for sintering. In addition, heating procedures strongly affect the final density of a glass-ceramic. Many people use different heating procedures for sintering of glass powder and subsequent crystallization of the sintered glass body. A strontium aluminosilicate composition forming monocelsian (SrO · Al2O3 · 2SiO2 : SA2S) as a primary crystalline phase, which has been studied mainly for use as a matrix material for high-temperature ceramic composites [6, 7], were used for the present study. It shows high refractory character, low thermal coefficient of expansion (TCE) of 2.5 × 10−6 /◦C, oxidation resistance and phase stability up to its melting temperature at 1650 ◦C [6–8]. Bansal et al. [8] fabricated stoichiometric SA2S glass-ceramics by cold pressing, cold isostatic pressing and sintering. The samples, heated to 900 ◦C that is slightly above glass transition temperature (Tg: 883 ◦C) for 10 h and subsequently heated to 1300 ◦C that is the crystallization completion temperature (Tc) for 10 h, showed 94% of theoretical density. On the other hand, the samples, heated to 1100 ◦C that is between crystallization onset (To: 1086 ◦C) and crystallization