| There are many kinds of borates. Some of them have complex structures, which provide rich resources for searching useful borates, such as new nonlinear optic crystal materials.Some of hydrated calcium borates are useful chemical industrial materials which are used in glass, pottery and porcelain enamel industry, especially in unalkali glass industry. In order to look for the convenient and feasible methods to synthesis useful borates, in this paper, we have synthesized 11 Ca-borates by the different methods.Five hydrated calcium borates, α-4CaO·5B2O3·7H2O, CaO3B2O3·4H2O, CaO3B2O3·5H2O,2CaO·7B2O3·8H2O and 3CaO·10B2O3·12H2O, have been synthesized by the method of phase transformation of NaCa[BsO6(OH)6]-5H2O in different boric acid solution and different temperatures. The mechanisms of dissolution and phase transformation have been proposed and discussed.Using the active CaO, H3BO3 as materials, we prepared 7 hydrated Ca-borates, 2CaO·B2O3·H2O, 2CaO·B2O31.5H2O, 2CaO·2B2O3·H2O, 2CaO·5B2O3·5H2O, 2CaO·3B2O3·H2O, 3CaO·10B2O3·12H2O and β-4CaO·5B2O3·7H2O at different temperatures and different compositions by the methods of hydrothermal reaction and molten H3BO3 as solvent, which were characterized by XRD, FT-ER as well as TG.The enthalpy of solution of seven synthesized pure hydrated Ca-borates in 1mol·L-1 HCl (aq) were determined by microcalorimeter. The standard molar enthalpies of formation of these Ca-borates were obtained through designing thermochemicai cycles. Thermodynamic properties of four Ca-borates were also calculated by group contribution method.The conclusions can been gotten through the above experimental results as follows:(1) Higher boron concentration of solution is helpful in the formation of higher polymerization degree borate.(2) Under hydrothermal condition, the solid phase is affected by the temperature strongly. The synthesized borates contain more crystal water molecules at low temperature, and contain less crystal water molecules at high temperature. |
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