Papers by Keyword: Light Elements

Abstract: Lithium hydride LiH is one of the attractive hydrogen storage materials, because it stores 12.7 mass% of H2. However, H2 desorption reaction occurs over 600 °C due to the large enthalpy change of H2 desorption Ho = 181 kJ/mol H2. The purpose of this work is to control the enthalpy change of LiH to much lower value by a mechanical alloying with Si, where the Li-Si alloy is thermodynamically more stable than Li. The alloy was synthesized from Li granule and Si powder by a mechanical alloying method. The H2 absorption and desorption properties were characterized by High-Pressure Differential Scanning Calorimetry and Thermogravimetry - Differential Thermal Analysis - Mass Spectroscopy, and X-ray diffraction measurement. Pressure - Composition - Isotherm measurements were performed at 400, 450, and 500 °C to estimate the enthalpy change. From the results, it was confirmed that reversible H2 ab/desorption reactions of the Li-Si alloy were expressed as 7LiH + 3Si ↔ (3/7)Li12Si7 + (13/7)LiH + (18/7)H2 ↔ Li7Si3 + (7/2)H2 (theoretically 5.0 mass% H2) at 400 °C. From van’t Hoff plot obtained by the results of PCI measurements, the enthalpy change of the former reaction was estimated to be Ho = 103 kJ/mol H2, which is lower than that of LiH.

Abstract: The mechanisms of formation of activated states upon diffusion of impurity interstitial atoms in metals have been identified, which allows formulation of the theoretical criterion for reliability of corresponding experimental data. This criterion is valid for almost all experimental results gained using precise techniques for bcc, fcc, and hcp metals. In case of hydrogen, oxygen, and nitrogen, most adequate are the models of potential diffusion barrier that are based on the elasticity theory. For carbon and boron, a less energy-taking mechanism is realized which involves the electron system rearrangement in a crystal during the diffusion jump. The accuracy of the theoretical criterion is in line with the capacities of modern precision techniques.