Thesis or Dissertation Metal-Hydride Formation Mechanism Measured by Acoustic Emission Methods

Piao, Jiaxiang  ,  朴, 甲相

pp.1 - 55 , 2015-09-24 , 新潟大学
学位の種類: 博士(工学). 報告番号: 甲第4080号. 学位記番号: 新大院博(工)甲第431号. 学位授与年月日: 平成27年9月24日
Hydrogen storage alloys are important in the energy storage area because of that these alloys have the capacity to absorb and desorb amount of hydrogen. There are a lot of interests in the formation mechanism of metallic hydride. It has been known that the change of lattice energy of a transition metal by hydrogenation is interpreted in the terms of the lattice expansion and the change of Fermi level by our research group. In this thesis, we focus on the phenomenon of that metal transformed by slight stress in the case of hydrogenation easily and want to clarify the formation mechanism of metallic hydride. We will propose acoustic emission measurement methods by using gas pressure technique and electrochemical charging technique in order to obtain the dynamical information of hydrogenation process of palladium. The analysis with the experimental results was discussed by the dislocation theory. This thesis includes 4 chapters. In the first chapter, the motive and purpose of this research was stated. In the second chapter, a hydrogenation process of palladium has been studied by acoustic emission (AE) methods with a new gas pressure cell. By using noble Ar gas, the AE events were observed intermittently for the stepwise pressurization. On the repetition of the re-pressurization, the AE events showed the Kaiser effect, i.e., a dislocation motion induced effect. The typical power spectrum of AE signal by noble gas demonstrated the fundamental signal modes of 250 and 550 kHz. By the hydrogen gas re-pressurization, the characteristic AE behavior was recognized as following: 1) The continuous AE events were observed by the stepwise pressurization, 2) in the early stage of primary H solid solution of Pd, the fundamental AE modes were measured as 250 and 550 kHz, 3) in the hydride formation stage, the continuous AE events were also observed, and the typical AE mode was changed to 250 kHz mainly. The hydride formation mechanism was discussed on the basis of the dislocation theory. From these results, when a hydrogen atom is dissolved into an fcc metallic lattice, it occupies an interstitial octahedral site and pushes aside metal atoms around its site as is recognized from the lattice expansion induced by hydrogenation. In the third chapter, a hydrogenation process of Pd was studied by AE method with a new electrochemical hydrogen charging technique. The AE measurement method with the electrochemical separated AE cell by removing coexisted AE waves with generated H_2/O_2 gas on the counter electrode. Characteristic AE power spectra caused by cavitation and /or bubble burst have been appraised by using an electrochemical separated AE cell. The characteristic of the AE power spectrum due to Pd hydride formation was around 250 kHz. In the second chapter, the spectrum was evaluated as 250 kHz obtained using gas pressure AE cell. This means that the AE method with the electrochemical AE cell will be a powerful way to investigate the hydride formation mechanism because the high H_2 gas pressure condition can be easily provided by hydrogen over voltage. In the forth chapter, we make a conclusion about this thesis. We established the validity of the AE measurement methods by using gas pressure technique and electrochemical charging one. And, we propose that the partial dislocation of Pd lattice will play an important role in hydride formation. Finally we will propose that this research could contribute to the research and development of, for instance, aluminum based hydride alloys which crystal structure will drastically change by hydrogenation.

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