Molten Borates and Silicates

Physicochemical data of borate and silicate melts having stoichiometric compositions are shown in Table 4.2. Although a large amount of information regarding mixtures of molten oxides or molten silicates and borates can be found in the literature, data regarding pure, stoichiometric single compounds is relatively scarce. 

Some information beyond what is shown in Table 4.2 has been reported. The adiabatic compressibility, obtained from sound velocity and density data (Sect. 3.3.3) was reported [55] as xs/GPa at 1473 K for Na2Si03 (0.588), Na2Si205 (0.643), K2Si03 (1.27), K2Si20s (0.791), and at 1173 K for UBO2 (1.205) and 026407 (1.29). The isothermal compressibility Xx/GBa (the reciprocal of the bulk 

Molecular dynamics simulation of Mg2Si04 [63] yielded values for the isothermal compressibility Kt = 0.025 GPa at 2100 K, the density at 2110 K and at 2 GPa 

Angell CA (1966) The importance of the metastable liquid state and glass transition phenomaion to transport and structure studies in ionic liquids. I. Transport properties. J Phys Chem 70 2793-2803 

Barin I, Knacke O (1973) Thermochemical properties of inorganic substances. Springer, Berlin 

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The study of absorption spectra has demonstrated the unusual invariance of octahedral chromophores, Cr(III)X6, in glasses and minerals (4, 77). Empirical evidence from glass-making 124) has been extended, and one can study e.g. the equilibria between blue Co(II)04 and pink Co(II)06 or between purple Ni(II)04 and yellow-green Ni(II)06 in borate and silicate glasses 65). Similar observations can be made on molten chlorides 27) and sulfates 64). Relations between the crystal structure and absorption spectra of mixed oxides 61y 87, 90, 103, 104y 105) and fluorides 91) have been carefully evaluated. In this connection, Clark 11) discussed the invariance of the chromophores, M(II)Cl6 and M(III)Cl6, in solid halides and halide complexes. 

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

One can quote exceptions to these generalizations. The tetraalkylammonium salts as a class are liquid at temperatures below 300 K. There are liquid electrolytes— produced from dissolving AICI3 into some complex organics—which are liquid at room temperature (Tables 5.3 and 5.4). Above the normal range of 300-1300 K is another set of molten electrolytes, the molten silicates, borates, and phosphates, for which the characteristic temperature range is 1300-2300 K (Tables 5.5 and 5.6). 

More generally, Valez et al. have reviewed the corrosion behaviour of silicate and borate glasses in contact with alkali metals and molten salts, as well as in aqueous conditions. 

The history of Molten Carbonate Fuel Cell (MCFC) can be traced back to the late nineteenth century when W.W. Jacques had produced his carbon-air fuel cell, a device for producing electricity from coal. This device used an electrolyte of molten potassium hydroxide at 400-5(X) °C in an iron pot [94]. Jacques suggested to replace molten alkali electrolytes with molten salts such as carbonates, silicates, and borates. 

Antiwetting additives admixtures of barium sulfate, aluminium fluoride, aluminium borate, aluminium titanate, calcium silicate (woUastonite), aluminium nitride, silicon carbide and its combinadmis to alumina silica refractories for decreasing the wetting ability of molten aluminium to these refractories. In combination with decreasing the pore dimensions, they give good results. 

Glasses are usually non-stoichiometric mixtures (excepting molten NaP03) of differing oxides forming borates, silicates, phosphates,. .. or, under more special circumstances, mixed fluorides The narrow absorption and luminescence... 

Table 4.2 The molar masses, M, melting points, r , molar enthalpies of melting, A, //, constant pressure molar heat capacities, Cp, (from [2]), and the molar volumes, V, isobaiie expansivities, ap, and viscosities, rj, at l.ir , of molten silicates and borates...

Bockris JO M, Kojonen E (1960) The compressibilities of eertain molten alkali silicates and borates. J Am Chem Soc 82 4493-4497... 

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