CHAPTER ONE

1.0INTRODUCTION

Modern science and technology require new materials with special properties for diverse applications in our day to day activities. Among these new materials, one group plays a very special role and that is glass-ceramics material (Manfredini and Rinco, 1997).  It occurred to Reamur in 1739 that dense ceramics could be made through crystallization of a base glass, but the invention of glass-ceramics through phase separation took place in the 1950s by the inventor S.D. Stookey (Holand and Beall, 2002).When the general theorieson nucleation and crystal growth in glasses were well established, glass-ceramics have played vital role in a wide range of scientific and technological developments. Inaddition, there are great deal of text books, journal publications, conference proceedings describing the basis, characteristics, production, properties, advantages as well as wide variety of applications of these materials(Casasola and Rincon, 2012). Glass ceramics are polycrystalline materials produced from controlled nucleation and crystallization of carefully formulated base glasses (Shackleford and Doremus, 2008). They are inorganic materials and their microstructureis characterized by fine-grained randomly oriented crystals with some residual glass, without voids, micro crack or porosity and with two or more phases assemblage dispersed in the matrix of residual glassy phase(Danielaet al., 2010 and

Rezvani, 2011).

The chemical composition and microstructure of glass-ceramics are derived from primary base glasses, which are subjected to controlled heat treatment to nucleate crystals in the matrix of residual glassy phase (Mahdavi et al., 2011). Nucleation is vital in glass-ceramics production as it promotes the development of high density growth and crystallization (Alekseev et al.,2010 and Jing et al., 2013).

Glass-ceramics material is not a fully crystal because the microstructural configuration is 50vol% to 95vol% crystalline with the remainder being residual glassy phase (Salamaet al., 2002;Ali, 2008; Rezvani, 2011 and Bahman &Behzad, 2012b). Two or more phases assemblage may precipitate during controlled thermal treatment and their composition is completely different from the corresponding base glass, it follows that the composition of the residual glassy phase differs from the corresponding base glass(Carland Hoche,2002; Ali, 2008; Hussainet al., 2010; Daniela et al., 2010 and Khater et al., 2013).

In the transformation of base glass to glass ceramics, nucleation is attainable heterogeneously or homogeneously. Bulk crystallization otherwise termed as heterogeneous nucleation involves utilization of various nucleating agents to promote the development of micro-heterogeneities and crystallization which result in the transformation of metastable base glass to stable microcrystalline glass-ceramics (Francis et al., 2004; Ren-Pinget al.,2007 andBahman & Behzad 2012b).  A wide variety of nucleating agents promote the crystallization process of base glass and enhance the formation of fine-grained microstructure.

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