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Vapor Pressure Measurements of LiBH4, NaBH4 and Ca(BH4)2 using Knudsen Torsion Effusion Gravimetric Method
AuthorDanyan, Mo M.
Chemical and Materials Engineering
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Hydrogen storage is one of the critical technologies needed on the path towards commercialization for mobile applications. In the past few years, a range of new light weight hydrogen containing material has been discovered with good storage properties. Among them, lithium borohydride (LiBH4) sodium borohydride (NaBH4) and calcium borohydride (Ca(BH4)2) have shown promising results to be used as solid state hydrogen storage material. In this work, we have determined equilibrium vapor pressures of LiBH4 NaBH4 and Ca(BH4)2 obtained by Torsion effusion thermogravimetric method. Results for all the three hydrides exhibited that a small fraction of the materials showed congruency, and sublimed as gaseous compound, but the majority of the material showed incongruent vaporization. Two Knudsen cells of 0.3 and 0.6mm orifice size was employed to measure the total vapor pressures. A Whitman-Motzfeldt method is used to extrapolate the measured vapor pressures to zero orifice size to calculate the equilibrium vapor pressures. In the case of LiBH4 we found that ~2% of the material evaporated congruently (LiBH4(s) → LiBH4(g)) according to the equation: log〖P_(LiBH_4 )⁄P_0 =(-3263.5 ±309)/T+ (1.079 ±0.69)〗 and rest as incongruent vaporization to LiH, B, and hydrogen gas according to the equation log〖P_eq⁄P_0 =(-3263.5 ±309)/T+ (2.458 ±0.69)〗 with Hevap.= 62.475.9 kJ/mol of H2, Sevap.= 47.0513 J/mol of H2.K. The NaBH4 also had somewhat similar behavior, with ~9% congruent evaporation and equilibrium vapor pressure equation of 〖logP〗_(NaBH_4 )=(-7700±335)/T+ (6.7±1.5) and 91% incongruent decomposition to Na and Boron metal, and hydrogen gas. The enthalpy of vaporization; Hevap.= 147.26.4kJ/molH2 and Sevap.= 142 28 kJ/molH2.K (550-650K). The Ca(BH4)2 exhibited similar vaporization behavior with congruency of ~ 3.2%. The decomposition products are CaH2 and Boron metal with evolution of hydrogen gas varying with the pressure equation as log〖P_eq⁄P_0 =(-1562.7 ±177)/T+ (2.57 ±0.155)〗. The enthalpy and entropy of vaporization, Hevap.= 29.93.3kJ/molH2, Sevap.=49.14 2.9 J/molH2.K (400-650K) respectively. More details of the properties of the vaporization such partial pressures of effusing gases, Gibbs energies of vaporization, molecular weight of effusing gases, as well as decomposition products, evaporation reactions, thermodynamic modeling of the disproportionation of these three compounds are discussed in this dissertation.