**6. References**

302 Sintering of Ceramics – New Emerging Techniques

We can thus deduce the activation energy of viscosity between Tg and Tc1 from the curve ln η=f(1/T) which is found to be equal to ~500±100kJ.mol-1 (Fig. 19). This value has been compared to literature on 80GeSe2-20Ga2Se3 synthesized by the common melt-quenching technique instead of mechanical alloying (Roze et al., 2011). From the data viscosity, similar

(a)

(b)

This chapter provides an overview of a new technique to synthesize chalcogenide glasses and glass-ceramics. This technique combines either the synthesis of glass powder by mechanical milling or the grinding and sieving of pre-existing bulk obtained by the conventional melt-quenching technique and the PCES technique (or SPS). Many chalcogenide glasses presenting real potential applications were set aside since the common synthesis in sealed silica tubes under vacuum did not permit to produce samples big

Fig. 19. Plots of the curve ln η= f(1/T) in the case of our study (a) and in [63] (b).

activation energy is found.

**5. Conclusion** 


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**14** 

Vanja Martinac

*Croatia* 

**Effect of TiO2 Addition on the Sintering** 

*University of Split / Faculty of Chemistry and Technology* 

**Process of Magnesium Oxide from Seawater** 

Magnesium oxide is one of the most important materials used in the production of hightemperature-resistant ceramics. Due to its high refractory properties (MgO melts at (2823 ± 40) oC), MgO ceramic is non-toxic and chemically inert in basic environments et elevated temperatures, resistant to the effect of metal melts, acid gases, alkali slag, neutral salts, and react with carbon only above 1800 oC. Today, in large-scale technicall processes, magnesia (MgO) for refractories is produced from two sources: natural and synthetic. Magnesia from natural sources constitutes 82 % of the world's magnesia installed capacity. The dominant source is magnesite (MgCO3) which occurs in both a macro and a cryptocrystalline forms. Less significant are dolomite (CaCO3·MgCO3), hydromagnesite (3MgCO3·Mg(OH)2·3H2O), brucite (Mg(OH)2) and serpentine (Mg3(Si2O5)(OH)4). Synthetic materials are manufactured either from seawater or from magnesia rich brines. Magnesium oxide obtained from sea water is a high-quality refractory material, and its advantages lie not only in the huge reserves of seawater (1 m3 contains 0.945 kg of magnesium), but in the higher purity of the sintered magnesium oxide (≥ 98 % MgO). The production of magnesium oxide from seawater is a well-know industrial process (Bocanegra-Bernal, 2008; Bonney, 1982; Gilpin & Heasman, 1977; Heasman, 1979; Maddan, 2001; Martinac, 1994; Petric & Petric, 1980, Rabadžhieva et al., 1997) and has been studied all over the world for a number of years. For most of the second half of the twentieth century, seawater provided almost 50 % of the magnesium produced in the western world, and today it still remains a major source of magnesium oxide in many countries. The process involves the extraction of dissolved magnesium, which has a concentration of around 1.3 g dm-3 in seawater (Brown et al., 1997), and 3 to 40 times this values for brines, and the reaction of magnesium salts (chloride and sulphate) with lime or dolomite lime to produce a magnesium hydroxide precipitate. The precipitate is washed and calcined to form caustic magnesia. The apparently simple chemistry of the process is unfortunately complicated in practice because seawater is not a pure solution of magnesium salts and dolomite or limestone, although abundant, are never found free of impurities. Boron is a particulary problematic impurity for the magnesia used as a high quality refractory material. Thus, boron can be a problem in refractory magnesia for specialized refractory applications where a high hot strength is required. Taking into consideration that B2O3 is common impurity in seawater derived magnesia, the aim of this study was to examine the possibility of adding TiO2 in quantities of 1, 2 and 5 wt.-% for

**1. Introduction** 
