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Chem Soc Rev 1 1 Chem Soc Rev 1 5 TUTORIAL REVIEW 5 Crystallisation in oxide glasses – a tutorial review Q1 Q2 10 a a b 10 Cite this: DOI: 10.1039/c3cs60305a N. Karpukhina,* R. G. Hill and R. V. Law Glasses and glass-ceramics have had a tremendous impact upon society and continue to have profound industrial, commercial and domestic importance. A remarkable number of materials, with exceptional 15 optical and mechanical properties, have been developed and enhanced using the glass-ceramic method 15 over many years. In order to develop glass-ceramics, glass is initially prepared via high temperature synthesis and subsequently heat treated, following a carefully designed and controlled process. A glass- ceramic system comprises crystalline and non-crystalline phases; in multicomponent systems these phases are significantly different from the initial glass composition. The properties of glass-ceramics are 20 defined by microstructure, crystal morphology as well as the final chemical composition and physical 20 properties of the residual glass. Knowing the mechanism of glass crystallisation, it is possible to predict and design a glass-ceramic system with near-ideal properties that exactly fulfil the requirements for a Received 18th August 2013 particular application. This tutorial review is a basic introduction to the crystallisation in glasses and DOI: 10.1039/c3cs60305a mainly focuses on silicate and closely related oxide glasses. The review describes and discusses key 25 learning points in five different sections, which facilitate the understanding of glass crystallisation and 25 www.rsc.org/csr development of glass-ceramics. Key learning points 30 (1) Nucleation and crystal growth are two fundamental stages of crystallisation in glass. 30 (2) The structure and unique properties of glass-ceramics can be controlled and specifically tailored for an application through the manipulation of the initial glass composition. (3) At the pre-nucleation stage the processes occurring in a glass dictate the subsequent mechanism of crystallisation. (4) Nanocrystallisation can be achieved in the glass if certain glass design rules are followed. (5) A combination of the structural techniques available in modern science rather than a single method is more likely to characterise accurately the mechanism 35 of glass crystallisation. 35 Introduction upon rapid cooling or quenching the melt to the temperatures 40 below the glass transition to prevent crystallisation. Fig. 1 40 The process of crystallisation is widely applicable to many schematically demonstrates the path difference for a specific materials that can crystallise from the amorphous state or the physical property change, e.g. molar volume, enthalpy or melt; this includes materials as diverse as oxide glasses, organic density etc., when the liquid state is supercooled and the glass polymers, metals, chalcogenides and molecular systems e.g. water. transition range is achieved compared to slow cooling followed by 45 In glass-ceramic (GC) systems, major advances in understanding crystallisation. Thus, the quenching stops the melt from following 45 the process of crystallisation, in particular the role of nucleating the thermodynamically favoured path towards crystalline matter agents, have significantly extended the nano-level control of crystal- and traps the system in a metastable amorphous state. lisation for a vast variety of inorganic materials. The nature of the glass transition still remains one of the Glasses are amorphous solids that exhibit characteristic most problematic issues within the field of amorphous solids. 50 glass transition temperature (Tg) and are typically obtained It has been described variously as a quasi-thermodynamic 50 transition or a kinetic process. It is the process in which a fundamental change takes place in the physical properties of a Q3 Dental Physical Sciences, Institute of Dentistry, Barts and The London School of the materials e.g. specific heat capacity, volume, enthalpy, etc. Medicine and Dentistry, Queen Mary University of London, Mile End Road, London E1 4NS, UK The truth is that both thermodynamics and kinetics play an 55 b Department of Chemistry, Imperial College London, Exhibition Road, important part in this transition and its exact nature lies 55 1–4 London SW7 2AZ, UK beyond the scope of this review. This journal is c The Royal Society of Chemistry 2013 Chem. Soc. Rev., 2013, 00,1À13 | 1 Tutorial Review Chem Soc Rev 1 crystalline and non-crystalline phases. The nature of crystalline 1 regions in GCs can vary significantly from the poorly crystalline species with the high number of defects and/or imperfections up to well developed and defined crystals. This has a funda- 5 mental influence on the physical and chemical properties of 5 the GC material. It is advantageous to induce crystallisation in glass in a carefully controlled manner. The rate, extent and degree of crystal formation in glass are therefore of great interest. 10 The inorganic oxide glasses and GCs, e.g. silicates, phosphates, 10 borates, aluminosilicates etc., have tremendous industrial, com- mercial and domestic importance. They have an immense number of applications from optical and electrical materials to bioactive and machinable GCs.9 The crystallisation processes within 15 these systems have been extensively studied in the literature.9,11 15 In particular, two classic examples of the studied mechanism of Fig. 1 Glass formation via supercooling and glass transition behaviour of a crystal nucleation in glass, the lithium disilicate and calcium melt cooled down with the temperature schematically demonstrated via sodium silicate systems, have received substantial attention the molar volume change as a function of temperature. Here two alter- over a number of decades.12,13 The lithium disilicate has had 20 native pathways are clearly shown which are contingent upon the cooling considerable commercial application especially in the area of 20 rate. Rapid cooling (top pathway) allows the formation of a glass going optical and dental GCs. through a Tg region. Slower cooling (bottom pathway) allows crystal- lisation at the melting temperature, Tm, to form a crystal. This review serves as an introduction to the crystallisation in glasses and formation of GCs. Here, we will focus on glass forming melt-derived silicates and aluminosilicates and their 25 Although some materials readily form a glass and others do not closely associated derivatives and will discuss the basics of the 25 do this so easily, it is now understood that nearly all materials can be effects of heat treatment upon these glasses. forced into the amorphous state provided a high enough quenching rate is achieved. Thus, it is the quenching technology that may be a limitation for the glass formation, the latter is also termed vitrifica- 1 Nucleation and crystal growth are 30 tion. Currently the quenching rate up to 104–106 KsÀ1 is possible to two fundamental stages of 30 achieve for silicate (SiO2-based) glasses using the laser melting and crystallisation in glass hammer- or roller-quenching.5–7 A critical cooling rate, i.e. the minimum quenching rate Thermodynamically metastable glasses can undergo crystal- needed for a successful vitrification of material, was introduced lisation upon heat treatment when the amorphous component 35 to quantify the glass forming ability. If the critical cooling rate is partially converts into crystalline material. Glass crystallisation 35 not achieved upon quenching the obtained glass would contain is an exothermic event and can be investigated by thermal a fraction of crystals, i.e. glass would crystallise or ‘‘devitrify’’. analysis, such as differential scanning calorimetry (DSC) or the The presence of crystals in the glass can fundamentally alter the closely related differential thermal analysis (DTA) (cf. Section 5). properties and structure of the glass and therefore is not desired On a typical DSC trace for a given glass composition the 40 when it occurs in an uncontrolled manner upon quenching. crystallisation will be displayed as an exothermic event follow- 40 Another way of assessing how readily a glass of a given ing the glass transition as demonstrated in Fig. 2. The glass composition forms is to look at glass stability (GS) against crystallisation temperature, Tc, corresponds to the exothermic crystallisation when it undergoes heat treatment. Most of the peak temperature on a DSC trace. introduced GS parameters can be calculated from the relationships The solid obtained after heat treatment (if complete re-melting 8 45 between Tg, Tm and crystallisation temperature Tc, the latter will be into a glass melt has not been attained) contains both the crystalline 45 defined in the text below. Over decades a number of theories have and residual amorphous phases and the final material is frequently been put in place to establish the relationship between the referred to as a GC. The physical and chemical properties of the GCs composition of glasses and the crystallisation mechanism, in are radically altered compared to the properties of the glass before particular for silicate glasses. This resulted in significant progress devitrification including the physical appearance as seen from 50 in glass science and also enabled development of GC materials with Fig. 3. 50 desired microstructure and properties.9 In glasses crystallisation occurs in two steps, the first, Heat treatment of inorganic oxide glasses10 produces GCs. nucleation, is the formation of small nuclei often on a nano- The term ‘‘ceramic’’,
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