**3. History and definition of glass-ceramics**

Glass-ceramics are classified as ceramic materials. They are polycrystalline materials that are formed by controlling the crystallisation of an amorphous glass. These materials are an important type of electroceramic and were successfully developed during the 1940 and 1950s. During this period, S. D. Stookey (Corning, USA) successfully used glass–ceramics as electrical insulators in electronics technology [5, 20]. The fundamental patent of Stookey was based on the concept that the TiO2 works as a nucleating agent in a glass system. Additionally, ZrO2 was used by Tasiro and Wada [5], in 1963 as a nucleating agent. Another discovery was made in the 1950s by Hummel, who discovered the crystal arrangement of the Li2O-Al2O3-2SiO2 system [42].

**Figure 6** shows three types of an atomic structure with different atomic arrangements. A crystalline solid is one which has a long-range order in its atomic structure; an amorphous solid is one in which there is no long-range order in its atomic structure. Crystalline solids have two subdivisions, single crystal and polycrystalline. A single crystal has a periodic atomic arrangement. In this case, there are no grain boundaries. A polycrystalline solid contains many grain boundaries in the structure due to the differences in the orientations of the grains (that have a short-range order) [43].

It is difficult to specifically define a glass since the behaviour of a glass alters with changes in temperature. There are two points at which a glass can be defined; the

**Figure 6.** *The structure of ceramics [43].*

*Ferroelectric Glass-Ceramic Systems for Energy Storage Applications DOI: http://dx.doi.org/10.5772/intechopen.93855*

first is at high temperature, when the glass is a liquid, while the second is at a lower temperature when the glass is considered as a supercooled liquid. Although there are important structural differences between glasses and polycrystalline ceramics, their mechanical and functional properties at room temperature can be similar.

The traditional definition of glass is that it is a supercooled liquid. According to the American Society for Testing and Materials (ASTM), the definition of glass is that it is an inorganic product of fusion which has cooled to a solid state without being crystallized. However, there are alternate definitions for glass, one being that glass is a type of amorphous solid material that lacks long-range order (not a random arrangement) in its atomic structure. Another definition, also put forward by ASTM, it that a glass is a liquid that has lost its ability to flow [5, 16].

### **3.1 Heat treatment of glass-ceramics**

The heat treatment of glass leads to the occurrence of many transitions. Differential scanning calorimetry (DSC) is a form of thermal analysis that depends upon the change in a material's physical properties [42]. In DSC, there is a difference in temperature (ΔT) that is seen between the sample and the reference. Here ΔT represents differences in heat flow as ΔQ. The two quantities, ΔT and ΔQ, are functions of thermal resistance (R), as shown in equation below.

$$
\Delta \mathbf{Q} = \frac{\Delta \mathbf{T}}{\mathbf{R}} \tag{5}
$$

In the first step of the glass transition, some of the physical properties change for amorphous materials. This change occurs in the heat capacity, which can be measured by DSC as an endothermic change in the sample. The transitions in glass due to the effects of temperature occur in the range of temperature which is known as the glass transition temperature (Tg). Therefore, below Tg, materials display a rigid glassy structure. When the temperature is increased above Tg, these materials display a flexible structure.

Another transition which occurs due to changing temperature is crystallisation. In this case, the amorphous materials are transformed into a crystalline structure. With an increase in temperature, the next conversion is melting. At this point, the crystalline structure converts to a viscous amorphous structure. The melting point is dependent upon the chemical impurity of the materials. After the melting stage, a reaction inside the material causes an increase in the density of the material [5, 42].

### **3.2 Crystallisation of glass-ceramics**

Generally, since 1960, there has been much research undertaken regarding glass systems in the field of glass–ceramics. Glass–ceramics are very important in many fields of application. They have demonstrated many desirable thermal, optical, biological, chemical, and electrical properties. Some of these properties provide advantages to glass–ceramics over more traditional materials. A glass–ceramic is a polycrystalline material formed by controlling the crystallization of glass. Therefore, in order to make glass–ceramics from glass, the main manufacturing process needs to be a thermal one. **Figure 7** shows the steps of glass transforming into glass–ceramic. These steps begin at a low temperature with the formation of nuclei, then at higher temperatures crystallisation occurs by growth of the nuclei; this continues to produce the polycrystalline a glass–ceramic microstructure [5, 43].

### *Advanced Ceramic Materials*

Microstructural control is said to be easier when the temperature required for crystallisation lies between but is significantly different from both the glass transition temperature and that of matrix devitrification. In such a case, the desired crystalline phase can be induced to form without devitrification of the glass matrix. The crystallite size generally increases with increasing temperature, as shown by the micrographs in **Figure 8** [44].

In **Figure 9**, the typical thermal preparation of glass–ceramic can be seen. In this case, the raw materials, Li2CO3 and SiO2, are used to create lithium disilicate. There are two main stages in obtaining glass–ceramics: glass formation and glass crystallisation. In each stage, there are many steps which depend upon both temperature and time. The first stage begins by melting the components and then quickly cooling them. The nucleation and crystal growth occurs in the second stage. During this stage, controlled crystallisation of the glass produces nanoscale crystals [5].
