Mg-Ni基合金的微结构、吸放氢行为及其催化改性

发布时间：2018-04-28 23:15

本文选题：镁基储氢合金 + 合金化　；参考：《西北工业大学》2016年博士论文

【摘要】：镁基合金以其储氢密度大、重量轻、储量丰富、可逆性好等优点受到广泛关注,被认为是最有可能应用于车载储氢装置的材料之一,但活化困难、放氢温度高、吸/放氢动力学缓慢成为其应用的主要障碍。镁基储氢合金改性主要围绕合金化、组织调控、表面改性等展开,方式单一,并未整体考虑吸/放氢反应的完整过程。基于整体调控思路开发镁基储氢合金的手段很少涉及,且合金微结构及性能之间的关系、催化改性合金活化机制及吸/放氢行为不够明确。上述问题研究不仅能深入理解微结构与储氢性能之间的相关性,而且可以阐明催化剂在活化及后续吸/放氢中的作用,同时为综合性能优异镁基合金体系的开发提供新思路。鉴于吸/放氢过程复杂性,本文基于整体调控思路,即高能球磨/熔体快淬优化内部组织基础之上的表面催化改性获得综合性能优良的储氢合金。考虑到氢能源汽车领域对大容量储氢材料的迫切需求,基于微合金化思路,设计理论吸氢量高的富镁合金Mg-10wt%Ni(Mg10Ni)进行研究,并对传统Mg_2Ni合金进行挖潜。对基于整体调控思路制备的催化改性样品,探讨了微结构与储氢性能之间的构效关系,揭示了催化改性Mg-Ni基合金的活化机制,研究了催化吸/放氢热动力学并探讨了催化剂的去稳定化作用。本文主要研究内容及取得的创新结果如下:采用SEM、TEM、EDS、XRD等手段研究了高能球磨及表面催化前后传统Mg_2Ni合金微结构及相组成,结果表明:熔铸Mg_2Ni合金主要由包晶Mg_2Ni相构成,还含有极少量残余MgNi2相和Mg相。铸态组织粗大,不利于快速完全吸/放氢,高能球磨可显著细化合金组织,球磨不同时间可获得不同组织状态Mg_2Ni合金,球料比20:1时,球磨38h后Mg_2Ni合金基本呈现非晶态。短时高能球磨引入MWCNTs,改善了表面特性而保留了组织调控后的非晶结构,实现了Mg_2Ni合金表面活性化与内部精细化调控。非晶态Mg_2Ni合金的吸氢性能较优,30min内可吸氢3.5wt%,且初始吸氢速率快,催化改性后最终吸氢量达到3.8wt%。微合金化Mg10Ni合金由先析出Mg枝晶相和层片共晶Mg-Mg_2Ni构成,微观组织粗大,比表面小,缺乏H扩散通道;熔体快淬使基体Mg相及第二相Mg_2Ni纳米化,合金组织有效细化,晶界/相界增多;Mg_2Ni纳米化同时引入微孪晶,提高了富镁合金的自催化效率。短时高能球磨使催化剂在合金颗粒表面均匀分布,为吸/放氢过程引入高活性质点,改善了表面特性。添加MWCNTs或TiF3可改善Mg10Ni合金活化特性;Mg10Ni-MWCNTs-Ti F3活化性能优异,不存在孕育期,首次即可快速吸氢并达到理想吸氢量。XPS结果表明富镁合金颗粒表面MgO和Mg(OH)2构成的钝化膜,降低了表面活性,阻碍了H_2吸附解离,造成合金活化困难。催化剂提高Mg10Ni活化性能不仅在于借助改善吸/放氢过程引起的晶格应变促使表面已形成钝化膜破碎并剥落,裸露出新鲜合金表面,还表现在阻碍表面钝化膜形成。TiF3为H_2吸附解离及MgH_2异质形核提供低能垒界面,加速吸氢反应;MWCNTs可促使H_2吸附及H快速扩散;Mg10Ni-MWCNTs-TiF3活化过程中,MWCNTs对H的辅助扩散与TiF3对H_2的吸附解离及对MgH_2的异质形核相互协同作用而表现出优异的活化性能。不同次数吸氢动力学拟合指数m不同,尤其是催化改性前后Mg10Ni的首次吸氢动力学拟合指数m差别较大,表明改性处理Mg10Ni合金首次吸氢过程的控速步骤不同,吸氢机制不同。在250℃,2.5MPa下,活化后Mg10Ni、Mg10Ni-MWCNTs、Mg10Ni-TiF3的吸氢量分别为5.39wt%、6.52wt%、5.06wt%;Mg10Ni-MWCNTs-TiF3,吸氢性能更优异,前1min和5min可分别吸氢5.93wt%及5.99wt%;活化后Mg10Ni、Mg10Ni-MWCNTs、Mg10Ni-TiF3及Mg10Ni-MWCNTs-TiF3在300℃、2.5MPa下吸氢量分别达到6.01wt%、6.64wt%、5.89wt%和5.92wt%,300s内吸氢量均超过5.0wt%。TEM及SEM相关结果及吸氢P-t曲线表明,TiF3促使H_2解离为活性H;TiF3自由表面及TiF3与基体界面为MgH_2异质形核提供基底,导致吸氢初始阶段合金表面形成大量氢化物核心,有限长大后碰撞,形成MgH_2包覆Mg的核-壳结构,后续吸氢过程受控于H在MgH_2层中的缓慢扩散,导致Mg10Ni-TiF3吸氢量较Mg10Ni降低;MWCNTs比表面活性高且具有管状结构,有效促使H_2解离及H扩散,可使H转移到材料表面其它部位或次表面,降低了MgH_2初始形核率并加快H扩散,使Mg10Ni在较低形核率下充分长大,达到较高吸氢量;复合添加MWCNTs及TiF3后,TiF3解离的大量H可通过MWCNTs扩散传输至合金表面其它部位或次表面,降低了MgH_2的异质形核率,实现了饱和吸氢。温度升高,吸/放氢热力学P-C-T曲线之间的压差逐渐减小,滞后效应明显降低,吸/放氢平台更加平坦;催化改性Mg10Ni样品滞后性存在显著差异:Mg10Ni-MWCNTs滞后性较小,Mg10Ni-TiF3滞后性较大,Mg10Ni-MWCNTs-TiF3滞后性最小;通过Van’t Hoff方程拟合不同温度条件下放氢热力学P-C-T曲线,计算改性Mg10Ni体系氢化物生成焓和生成熵,从热力学角度评价催化剂作用。相比Mg10Ni而言,添加MWCNTs和TiF3后,Mg10Ni体系氢化物生成焓从-77.36kJ/mol H_2分别降低至-73.75kJ/mol和-75.51kJ/mol,复合添加MWCNTs和TiF3后,氢化物形成焓降低至-73.37kJ/mol。富镁合金体系氢化物形成焓降低表明催化剂添加可降低Mg-Ni体系氢化物稳定性,改善合金放氢性能。
[Abstract]:Magnesium based alloys are widely concerned because of their high density of hydrogen storage, light weight, rich reserves and good reversibility. It is considered to be one of the most likely materials to be used in vehicle hydrogen storage devices, but it is difficult to activate, high hydrogen storage temperature and slow absorption / desorption kinetics as the main obstacle. The modification of magnesium based hydrogen storage alloys is mainly around alloying. The methods of developing magnesium based hydrogen storage alloys are rarely involved in the development of magnesium based hydrogen storage alloys, and the relationship between the microstructure and properties of the alloys, the activation mechanism of the modified alloys and the behavior of absorption / desorption are not clear enough. The above problems are not only studied. The relationship between microstructures and hydrogen storage properties can be understood in depth, and the role of catalysts in activation and subsequent desorption / desorption can be clarified, and a new idea is provided for the development of excellent comprehensive magnesium based alloy systems. In view of the complexity of the absorption / dehydrogenation process, this paper is based on the overall adjustment control idea, that is, the interior of high energy ball milling / melt quenching. On the basis of the surface catalytic modification, the hydrogen storage alloys with excellent comprehensive properties are obtained. Considering the urgent demand for large capacity hydrogen storage materials in the field of hydrogen energy vehicles, based on the micro alloying idea, the design of the magnesium rich alloy Mg-10wt%Ni (Mg10Ni) with high hydrogen absorption capacity is studied, and the traditional Mg_2Ni alloy is tapping the potential. The structure-activity relationship between the microstructure and the hydrogen storage properties was discussed. The activation mechanism of the catalytic modified Mg-Ni based alloy was revealed. The catalytic kinetics of catalytic desorption / desorption of hydrogen and the destabilization of the catalyst were studied. The main contents and the innovation results were as follows: using SEM, TEM, EDS, XRD The microstructure and phase composition of the traditional Mg_2Ni alloy before and after high energy ball milling and surface catalysis are studied. The results show that the melting and casting of Mg_2Ni alloy is mainly composed of peritectic Mg_2Ni phase, and contains a very small amount of residual MgNi2 and Mg phases. Mg_2Ni alloy with different tissue state can be obtained at the time. When the ball is compared to 20:1, the Mg_2Ni alloy is basically amorphous after ball milling 38h. Short time high energy ball milling is introduced to MWCNTs. The surface characteristics are improved and the amorphous structure controlled by the tissue is retained. The surface activity of the Mg_2Ni alloy and the regulation of the internal fine refinement are realized. The hydrogen absorption property of the amorphous Mg_2Ni alloy is achieved. It can be better, 30min can absorb hydrogen 3.5wt%, and the initial hydrogen absorption rate is fast. After catalytic modification, the final hydrogen absorption capacity of 3.8wt%. microalloyed Mg10Ni alloy is composed of Mg dendrite phase and lamellar eutectic Mg-Mg_2Ni. The microstructure is coarse, smaller than the surface and lack of H diffusion channel; melt rapid quenching makes the matrix Mg phase and the second phase Mg_2Ni nanoscale, alloy microstructure The increase of grain boundary / phase boundary and the introduction of micro twins in Mg_2Ni nanocrystalline can improve the autocatalytic efficiency of the magnesium rich alloy. Short time high-energy ball milling makes the catalyst distributed evenly on the surface of the alloy particles, introducing high active particles for the absorption / desorption process and improving the surface properties. The addition of MWCNTs or TiF3 can improve the activation properties of the Mg10Ni alloy; Mg10Ni-MW The activation performance of CNTs-Ti F3 is excellent, and there is no incubation period. It is the first time to quickly absorb hydrogen and reach the ideal hydrogen absorption capacity.XPS. The result shows that the passivation film composed of MgO and Mg (OH) 2 on the surface of the magnesium rich alloy particles reduces the surface activity, hinders the H_2 adsorption and dissociation and causes the alloy activation difficult. The catalyst improves the activation performance of Mg10Ni not only with the help of the improvement of the Mg10Ni. The lattice strain caused by the absorption / desorption process causes the surface to form the passivating film to break and exfoliate, expose the surface of the fresh alloy, and also show that the formation of.TiF3 is H_2 adsorption dissociation and the MgH_2 heterostructure nucleus provides the low energy barrier interface and accelerates the hydrogen absorption reaction; MWCNTs can promote the H_2 adsorption and the rapid diffusion of H; Mg10Ni-MWCNTs-TiF3 activation. During the process, the auxiliary diffusion of MWCNTs to H shows excellent activation performance with the adsorption and dissociation of TiF3 to H_2 and the synergistic effect of MgH_2 on the heterostructure of MgH_2. The kinetics fitting exponent m of different times of hydrogen absorption is different, especially for the first hydrogen absorption kinetic index m of Mg10Ni before and after the catalytic modification, which indicates that the modified Mg10Ni coincidence is modified. The mechanism of hydrogen absorption in the first hydrogen absorption process is different, and the mechanism of hydrogen absorption is different. At 250, 2.5MPa, the hydrogen absorption of Mg10Ni, Mg10Ni-MWCNTs, Mg10Ni-TiF3 after activation is 5.39wt%, 6.52wt%, 5.06wt%; Mg10Ni-MWCNTs-TiF3, and the hydrogen absorption performance is better, and the former 1min and 5min can absorb hydrogen 5.93wt% and 5.99wt% respectively. The hydrogen absorption of Mg10Ni-MWCNTs-TiF3 at 300 and 2.5MPa reached 6.01wt%, 6.64wt%, 5.89wt% and 5.92wt% respectively. The hydrogen absorption in 300s exceeded 5.0wt%.TEM and SEM related results and the hydrogen absorption P-t curve showed that TiF3 stimulated the dissociation of H_2 to be active, and the interface between the free surface and the matrix provided the substrate for the heterogeneous nucleation, leading to the initial stage of hydrogen absorption. A large number of hydride cores are formed on the surface of the alloy, and the nucleation and shell structure of MgH_2 coated Mg is formed after limited growth. The subsequent hydrogen absorption process is controlled by the slow diffusion of H in the MgH_2 layer, resulting in the reduction of Mg10Ni-TiF3 hydrogen absorption than Mg10Ni; MWCNTs is higher than the surface activity and has a tubular structure, which can effectively promote H_2 dissociation and H diffusion, and can transfer H to the material. On the other surface or subsurface, the initial nucleation rate of MgH_2 is reduced and the diffusion of H is accelerated so that the Mg10Ni is fully grown at a lower nucleation rate and achieves higher hydrogen absorption. After adding MWCNTs and TiF3, a large number of H can be transferred to the other parts or subsurfaces of the alloy surface through MWCNTs diffusion, and the heterogeneous nucleation rate of MgH_2 is reduced. The pressure difference between the thermodynamic P-C-T curves of absorption / desorption / desorption is gradually reduced, the lag effect is obviously reduced, the absorption / desorption platform is even more flat, and the hysteresis of the modified Mg10Ni samples has significant differences: Mg10Ni-MWCNTs hysteresis is smaller, Mg10Ni-TiF3 lagging is larger, Mg10Ni-MWCNTs-TiF3 lagging is the least; Van 't Ho is used. The FF equation fits the thermodynamic P-C-T curve of hydrogen release under different temperature conditions to calculate the hydride generation enthalpy and entropy of the modified Mg10Ni system, and evaluate the effect of the catalyst from the thermodynamic point of view. After adding MWCNTs and TiF3, the enthalpy of hydride generation in the Mg10Ni system decreases from -77.36kJ/mol H_2 to -73.75kJ/mol and -75.51kJ/mol, as compared with Mg10Ni. After adding MWCNTs and TiF3, the enthalpy of hydride formation decreased to the reduction of hydride formation enthalpy in the -73.37kJ/mol. rich magnesium alloy system, indicating that the addition of the catalyst can reduce the stability of the hydride in the Mg-Ni system and improve the hydrogen release properties of the alloy.

【学位授予单位】：西北工业大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG146.22

【相似文献】