Characteristics of Metal Hydrides and Their Applications in Hydrogen Energy and Microstructural Modification

• Main Authors: Chia-Chieh Shen, 沈家傑
• Other Authors: Tsong-Pyng Perng
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/95573531320497603822

博士 === 國立清華大學 === 材料科學工程學系 === 94 === Two kinds of metal hydride system: LmNi5-based intermetallic compounds and Ti-6Al-4V alloy (Ti64) were studied. Cyclic hydrogenation of LmNi5 and LmNi4.8Al0.2 alloys with different hydrogen loadings up to 3000 cycles at room temperature was conducted and compared. The hydrogen loadings at H/M = 0.25, 0.50, 0.75 and 1.0 were studied. After 3000 cycles, for LmNi5, it was observed that the maximum hydrogenation capacities were reduced to 0.95, 0.92, 0.82, and 0.74 for the loadings of 0.25, 0.50, 0.75, and 1.0, respectively, while those in LmNi4.8Al0.2 were only reduced to 0.89 and 0.87 for the loadings of 0.75 and 1.0, respectively. The plateaus in LmNi5 at T=30℃ for the loadings of 0.25 and 0.50 did not change much, but were lowered and became more sloped for the loadings of 0.75 and 1.0, indicating that the formation of an intermediate phase b-LmNi5H3 took place. On the contrary, the plateaus in LmNi4.8Al0.2 did not change much for T=30℃ and 50℃ but became slightly sloped with no observable split at 70℃, indicating that the b-LmNi5H3 might be suppressed by substitution of Al for Ni. Moreover, the X-ray diffraction patterns of LmNi5 showed anisotropic broadening of the peaks for all samples, and even the presence of some second phase for the loading of 1.0H/M. For LmNi4.8Al0.2, the peak broadenings were isotropic and relatively smaller. It was concluded that partial substitution of Ni with Al substantially improved the cyclic hydrogenation stability of LmNi4.8Al0.2 and the process of degradation might be described in terms of formation of an intermediate b-hydride phase followed by phase separation. For application of metal hydrides in hydrogen energy, a portable LmNi5–based hydride cartridge was developed, and the hydrogen energy was successfully converted to electrical power. A prototype metal hydride refrigerator driven by solar heater was developed and tested. The performance showed that (1) it had ability to refrigerate room temperature water (25℃) down to 19℃; (2) its value of COP was approximately 0.1; and (3) its refrigeration power was approximately 18W. In the second part, pressure-composition (P-C) isotherms from 550℃ to 700℃ in Ti and Ti64 were established and compared. Both Ti-H and Ti64-H systems exhibited two pressure plateaus in the P-C isotherms as the indicator of phase transformation. Phase transformations took place at lower hydrogen content and higher hydrogen pressure in Ti64 than in Ti due to the effect of substitutional elements Al and V. Upon hydrogenation, the Ti-H system exhibited the sequence of phase transformation a«aH«aH+bH«bH«bH+d«d, whereas the Ti64-H system showed a sequence of a+b«aH+bH«bH«bH+d«d because of the presence of original b phase. Partial phase diagrams for Ti-H and Ti64-H were established based on their P-C isotherms. Furthermore, grain refinement of Ti64 by repeated isothermal hydrogenation (RIH) was studied. Refined nanostructure in the a matrix resulted mainly from the formation of triangular dendritic bH and d by RIH treatment with a hydrogen loading of 0.7H/M at 600℃. Increasing the cycle number of RIH further refined the structure and increased the hardness. On the contrary, for RIH at 750℃ with a hydrogen loading of 0.5H/M, there was no grain refinement because of annihilation of the interface microdefects during RIH at higher temperature. Coarsening of the a2 precipitate due to increasing the cycle number further led to lower hardness.