Glass-ceramics are ceramic materials produced through controlled crystallisation (nucleation and crystal growth) of a parent glass. The great variety of compositions and the possibility of developing special microstructures with specific technological properties have allowed glassceramic materials to be used in a wide range of applications. One field for which glass-ceramics have been developed over the past two decades is that of glazes for ceramic tiles. Ceramic tiles are the most common building material for floor and wall coverings in Mediterranean countries. Glazed tiles are produced from frits (glasses quenched in water) applied on the surface of green tiles and subjected to a firing process. In the 1990s, there was growing interest in the development of frits that are able to crystallise on firing because of the need for improvement in the mechanical and chemical properties of glazed tiles. This review offers an extensive evaluation of the research carried out on glass-ceramic glazes used for covering and pavement ceramic tile is accomplished. The main crystalline phases (silicates and oxides) developed in glass-ceramic glazes have been considered. In addition, a section focused on glazes with specific functionality (photocatalytic, antibacterial and antifungal activity, or aesthetic superficial effects) is also included.

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This paper provides the results of a study that examines the effect of fluorine content on glass stability (GS) and the crystallisation mechanism for a series of glasses in the SiO 2-CaO-K 2 OF system. Four glass compositions, with fluorine contents ranging from 2.51 to 5.63 wt. %, were analysed by differential scanning calorimetry (DSC). The evaluation of GS was based on various parameters derived from characteristic temperatures of non-isothermal DSC curves, namely, the working range (T TS), reduced glass transition temperature (T gr), Weinberg (K w), Hrubÿ (K H) and Lu-Liu (K LL) parameters. The predominant crystallisation mechanism for each glass was assessed via the Thakur and Thiagarajan method by determining the variation in crystallisation temperature between powder and bulk glass samples (T p). The evaluation of glass forming ability (GFA) was based on the critical cooling rate (q c), which is determined from the Weinberg, Hrubÿ and Lu-Liu parameters. The results indicate that an increase in fluorine content leads to a change in the crystallisation process of these glasses, from a mechanism of surface crystallisation that is predominant in glasses with lower fluoride content (MIS-E, MIS-6 and MIS-8 with 2.51, 3.39 and 4.27 wt. % of fluorine, respectively) to a prevailing volume crystallisation in MIS-10 glass (5.63 wt.% of fluorine). An increase in fluorine content also leads to a decrease in GFA of the corresponding melts, which results in the segregation of CaF 2 crystals during cooling. The results demonstrated by DSC analyses are supported by the results obtained from X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM).

Devitrification behavior and preferred crystallization mechanism of glasses based on fluorrichterite (Na 2 CaMg 5 Si 8 O 22 F 2) composition.

This paper presents the results of a study that analyses the effect of fluorine content on glass forming ability (GFA), glass stability (GS) and preferred crystallisation mechanism for a series of glasses in the SiO 2 -Al 2 O 3 -MgO-K 2 O-F system. Three glass compositions, with fluorine contents ranging from 4.50 to 5.70 wt. %, were investigated by differential scanning calorimetry (DSC). The GS was established by estimating different parameters derived from characteristic temperatures of non-isothermal DSC curves, namely, the working range ( T TS ), reduced glass transition temperature (T gr ), Weinberg (K w ), Hrubÿ (K H ) and Lu-Liu (K LL ) parameters. The prevalent crystallisation mechanism for each glass was assessed by determining the dissimilarity in crystallisation temperature ( T p ) between fine (<63µm powder) and coarse glass samples.The estimation of glass forming ability (GFA) was based on the critical cooling rate (q c ), which is determined from the Weinberg, Hrubÿ and Lu-Liu parameters. The results point out that the compositions of these glasses result in melts with a high tendency to crystallize during cooling (q c > 120°C/min) and obtaining amorphous glasses is only possible by fast cooling of the melt.In a subsequent thermal treatment, a volume crystallization mechanism will be prevalent in the process of devitrification of these F-phlogopite based glasses. Nevertheless, the increasing on the fluorine content in the glass composition leads to a variation in the location of the first developed crystals from the internal volume of the glass particle to surface sites. The results established by DSC analyses are verified by the results obtained from field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). R. Casasola, J.M. Pérez, M. Romero, Glass forming ability and thermal stability of F-phlogopite based glasses.

The present paper shows the results of a nucleation and crystallisation study of a Na-fluorrichterite glass carried out by dynamic scanning calorimetry (DSC). The kinetic study was performed using different procedures (Kissinger, Matusita-Sakka and Kissinger-Akahira-Sunose (KAS) methods), and the Avrami parameter was determined from the Ozawa and Malek approximations and the Malek equation. The results have indicated the coexistence of surface and bulk crystallisation in the devitrification process of the studied glass. The kinetic study has shown that the activation energy of the crystallisation process is over 400 kJ/mol and that the mechanism proposed is a Johnson-Mehl-Avrami mechanism with n equal to 3, which implies that the crystallisation develops through the three-dimensional growth of crystals. The study of the variation of the activation energy with crystallisation using the KAS method has shown that the crystallisation process undergoes a multiple step mechanism, where the main part of the whole process corresponds to the three-dimensional growth of crystals. The mechanism proposed was confirmed by applying the Pérez-Maqueda et al. criterion.

F-richterite based glasses in the SiO 2-CaO-MgO-Na 2 OF system were investigated by X-ray diffraction, differential scanning calorimetry and field emission scanning electron microscopy to ascertain the influence of fluorine content on preferential crystallization mechanism and on the spatial position of crystals developed in the initial steps of glass devitrification. The results highlight that both surface and volume mechanisms occur in the crystallization of fluorrichterite glasses. However, these glasses preferentially devitrify by a surface crystallization mechanism, although an increase of fluorine content leads to a modification in the location of the first developed crystals.

A study on the devitrification of fluorophyllosilicate glass precursors is presented. The research has been focused on the early stages of the crystallization process and shows the variation in the crystallization mechanism with increasing the fluorine content. The devitrification process has been studied by means of differential scanning calorimetry (DSC) and fieldemission scanning electron microscopy (FESEM). These complementary techniques established that both surface (heterogeneous nucleation) and volume (internal homogeneous nucleation) mechanisms are present in the crystallization process of fluorophlogopite-based glasses, the latter being predominant. By increasing the percentage of fluorine in the parent glass, a variation in the location of the first crystals developed from the internal volume of the glass toward the external surface was observed. Such an alteration in the crystallization mechanism was also checked by examining the microstructure of crystallized samples prepared under short-time treatments.