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Structural Characteristics and Hydrogen Storage Properties of Mg-Ni Alloy Powder Synthesized by Electrical Discharge Alloying Process
|關鍵字:||electrical discharge alloying process|
Mg-Ni alloy powders
|摘要:||本研究以放電合金化法（electrical discharge alloying, EDA）藉由固定電極材料（純鎳）及加工件（純鎂）製備Mg-Ni儲氫合金粉末，探討不同極性對合金粉末結構及儲氫性質之影響。實驗結果顯示以鎳為負極所製作的儲氫合金粉末，粉末形貌多數為球狀，只有少部分為破裂不規則狀；而以鎂為負極所製作的儲氫合金粉末則多為破裂不規則狀。所有合金粉末表面有EDA製程所產生的裂紋與孔洞。由ICP與EDS分析結果中，以鎳及鎂為負極儲氫合金粉末均含有Mg及Ni，但以鎂為負極的儲氫合金粉末之Ni含量相當低。此外，由XRD分析結果可知，未進行吸氫前，以鎳為負極所製作的儲氫合金粉末主要含有Mg_2Ni、MgNiO_2、MgNi_2、Mg_6Ni、Mg及Ni相，而以鎂為負極所製作的儲氫合金粉末則含有Mg_2Ni、MgNiO_2、MgNi_2、Mg_6Ni及Mg相。進行吸氫反應之後，所有儲氫合金粉末主要的相有Mg_2NiH_4、Ni_2H、MgNiO_2及Mg_4NiO相，由XRD結果可以確認放電合金化法製備的儲氫合金粉末具有吸氫的能力。由儲氫曲線中發現，粉末會在280˚C之間進行吸氫，而以鎳為負極之合金粉末的儲氫能力比以鎂為負極之合金粉末還高，推測與粉末中鎳的含量有直接的關係。|
Mg-based alloys are considered to have the same potential as hydrogen-storage materials because of their light weight, high hydrogen capacity and low cost. Mg-Ni alloy is one of the most promising materials for hydrogen storage due to its lower specific weight, higher hydrogen capacity and larger selection of raw materials compared with other types of hydrogen storage alloys. The alloy Mg_2Ni is more stable in the air, with a compelling hydrogen desorption temperature of 250˚C, but a lower hydrogen capacity of 3.6 wt%. On the other hand, the stable ionic Mg-H bonding (i.e., the MgH_2 formation energy at -74.4 kJ/mol) makes it difficult for Mg to desorb hydrogen. Nevertheless, the addition of Ni into pure Mg is known to be an effective way to induce catalysis. The electrical discharge machining (EDM) is a widely used non-conventional machining method for manufacture of dies for forging/extrusion, machining of super alloys, composite and advanced ceramics. In EDM, the mechanism for removing materials primarily turns electrical energy into thermal energy through a series of successive sparks between the electrode and the workpiece in a dielectric fluid. The thermal energy is consumed to generate high temperature plasma that erodes workpiece materials. The traditional machining technique is often based on material removal using tooling materials that are harder than work-piece materials and is unable to machine them economically. During the EDM process, the melted workpiece reacts with the electrode and then is rapidly solidified in a dielectric liquid to form powdery debris. The study aims to investigate the effects of different polarities on the structural and hydrogen storage properties of the Mg-Ni alloy powders prepared for hydrogen storage using single types of materials for the fixed electrode (pure nickel) and the workpiece (pure magnesium) in the electrical discharge alloying (EDA) process. Experiment results reveal that the powder prepared using nickel as the cathode mostly contains spheroidal particles, of which only a small part appears to be broken and irregular while that prepared using magnesium as the cathode mostly contains broken and irregular particles. Both powders have cracks and pores that have been generated during the EDA process on the surface. Results of the ICP and EDS analyses show that the powders prepared using both nickel and magnesium as the cathode both contain Mg and Ni while that prepared using only magnesium as the cathode contains considerably less Ni. Results of the XRD analysis further reveal that the powder prepared using nickel as the cathode mainly contains Mg_2Ni, MgNiO_2, MgNi_2, Mg_6Ni, Mg and Ni phases while that prepared using magnesium as the cathode contains Mg_2Ni, MgNiO_2, MgNi_2, Mg_6Ni and Mg phases prior to hydrogen absorption. The main phases of both powders are Mg_2NiH_4, Ni_2H, MgNiO_2 and Mg_4NiO following hydrogen absorption. The foregoing results confirm that powders prepared using the EDA process are capable of absorbing hydrogen. The hydrogen absorption temperature of the Mg-Ni alloy powder synthesized by the EDA process is 280˚C on the hydrogen absorption curve. Results of the hydrogen storage testing indicate that the powder prepared using nickel as the cathode is significantly more capable of absorbing hydrogen compared with that prepared using magnesium as the cathode. The hydrogenation property of the Mg-Ni alloy depends on the behavior of nickel components.
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