恆溫蒸發熔煉鑄造製程合成鎂基介金屬化合物及其氫化特性之研究

Study on hydrogenation properties of Mg-based intermetallic compounds produced by Isothermal Evaporation Casting Process

許哲瑋(Che-wei Hsu)

指導教授 ： 李勝隆

國立中央大學/工學院/機械工程學系/博士(2011年)

摘要

鎂基儲氫合金因具有儲氫量高、質量輕、原料成本低廉之優點而被視為相當有潛力之儲氫合金系統，但由於鎂蒸氣壓大，且鎂（649℃）與在儲氫應用上所需搭配之元素如鎳（1455℃）、銅（1085℃）等元素其熔點差異甚大，因此傳統熔煉法無法成功熔配出大量高純度鎂基儲氫合金，如Mg2Ni、Mg2Cu等合金。因此本研究重點在於開發恆溫蒸發熔煉鑄造製程，進行鎂基介金屬化合物量產製程開發，於800℃下進行鎂與鎳熔湯攪拌，使其形成均質鎂鎳熔湯，其後進行降溫至兩相區持溫，持溫過程因鎂蒸氣壓高之特性，使其自行蒸發，導致液相組成往富鎳方向偏移，其後提高溫度至720℃，加速其蒸發速率，最後達到Mg2Ni包晶反應線，僅殘留均質Mg2Ni固體。所生產之Mg2Ni合金經由XRD與ICP分析，確認為高純度Mg2Ni合金。其後粉碎並過篩所生產之Mg2Ni塊材，進行不同初始粒徑對活化性質之測試，而其充分氫化後之Mg2Ni合金於300℃下可達3.58wt.%之儲放氫量，接近其理論值3.6wt.%，並施以極限吸放氫循環測試發現Mg2Ni合金吸氫極限溫度約在140℃時仍可吸收3wt.%之氫氣，而放氫極限溫度約為217℃，尚有0.55wt.%之放氫量。
本研究進一步利用銅元素取代鎳元素，合成Mg-Cu-Ni三元合金，除驗證恆溫蒸發熔煉鑄造製程於熔配過程添加第三元素之穩定度外，亦進行氫化平台壓力改質研究及三元Mg-Cu-Ni合金吸放氫過程相變化之觀察，提出固溶侷限現象，解釋固溶元素對氫化平台壓力提升階段之模型，並利用此模型之概念進一步觀察Mg-Ni-Ag三元合金之微結構，由銀元素固溶量進行PCI平台區壓力變化階段之預測與驗證，嘗試建立合金微結構與PCI氫化平台壓之關係。
最後，則綜合鎂基儲氫合金之特性，透過回收利用恆溫蒸發熔煉鑄造製程所釋放之鎂蒸氣，鍍附於鎳網結構之表面，合成新型鎳鎂織構吸氫材，其結果顯示此特殊結構之鎳鎂織構可有效提升氫原子之擴散，降低吸放氫溫度，其反應機制可做為後續新型態儲氫母材開發基礎，深具鎂基低溫儲氫材開發潛力。

關鍵字

鎂基介金屬 ； 金屬氫化物 ； 儲氫合金 ； 恆溫蒸發熔煉鑄造製程

並列摘要

Mg-based hydrogen storage alloys are attractive materials for hydrogen storage because due to their high hydrogen storage capacity, light density and low cost. Unfortunately, most studies indicate that it is difficult to produce Mg2Ni alloy with the accurately desirable composition by conventional melting methods because of the large differences in melting points and vapor pressures between Mg（649℃）and Ni（1455℃）. Therefore, an innovative method, Isothermal Evaporation Casting Process (IECP), is developed to produce Mg2Ni alloy for mass production in this study. In the past, high vapor pressure of Mg was considered as a disadvantage for producing pure Mg2Ni alloy. However, this characteristic was used to develop a refinement procedure to separate primary Mg2Ni alloy from Mg/Mg2Ni eutectic matrix. Characteristics of as-cast specimens measured by X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and electron probe X-ray microanalyzer (EPMA) reveal that mass production of Mg2Ni alloy was successfully fabricated by IECP.
A series experiments in hydrogenation properties of as-prepared Mg2Ni are also investigated. It is found that he well-activated Mg2Ni alloy achieves 3.58wt.% at 300℃ corresponding to the theoretical hydrogen storage capacity of Mg2NiH4 hydride. In addition, to research the modification of ternary Mg-Ni based alloys, the Mg2Cu1-xNix (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) alloys are also fabricated by IECP. The XRD analysis results showed that the cell volume decreases with increasing Ni concentration, and crystal structure transforms Mg2Cu with face-centered-orthorhombic into Ni-containing alloys with hexagonal structure. The Ni-substitution effects on the hydriding reaction indicated that absorption kinetics and hydrogen storage capacity increase in proportion to the concentration of the substitutional Ni. The activated Mg2Cu and Mg2Ni alloys absorbed 2.54 and 3.58 wt% H, respectively, at 300 ℃ under 50 atm H2. After a combined high temperature and pressure activation cycle, the charged samples were composed of MgH2, MgCu2 and Mg2NiH4 while the discharged samples contained ternary alloys of Mg-Cu-Ni system with the helpful effect of rising the desorption plateau pressures compared with binary Mg-Cu and Mg-Ni alloys.
The model of constriction phenomenon induced by 3rd additives is proposed to explain that the hydrogen diffusion process may have on the lattice expansion behavior, as opposed to that of the reactions taking place on the solute atom and multi-hydrides, is an important factor to consider in determining the kinetic parameters of hydrogen movement in metal lattice. Following this model, the correlation of the plateau in PCI and the microstructure of hydrogen storage alloy may be identified clearly.
Finally, this study demonstrated the feasibility of a novel Mg vapor deposition treatment on Ni foam to synthesize a Ni-Mg texture-like structure as a new type of hydrogen absorber. Energy dispersive spectrometry (EDS) yielded an estimative value of the weight percent ratio of Ni and Mg of 71.8 and 20.5 in as-prepared Ni-Mg texture-like structure. The microstructural changes were also characterized by XRD and the formed hydride tetragonal-MgH2 was confirmed. The unique combination of large surface area of catalyst (Ni) and hydrogen acceptor (Mg) reduced the hydrogenation and dehydrogenation temperatures and performed the capability of reversible hydrogen storage capacity up to 0.72 wt.% H2 at 25℃. Ni-Mg texture-like structure achieved significant hydriding-dehydriding performances at lower temperature than traditional Mg-based hydrogen storage alloys. A possible hydrogen storage mechanism was also discussed where the catalytic Ni foam with large surface area was shown to be a vital factor in improving hydriding and dehydriding kinetics.

並列關鍵字

Mg-based intermetallic compound ； metal hydride ； Hydrogen storage alloys ； Isothermal Evaporation Casting Process

參考文獻

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