【作者】 李谦；

【导师】 周国治； 林勤； 蒋利军；

【作者基本信息】 北京科技大学 ， 冶金物理化学， 2004， 博士

【摘要】 氢的储运是氢能得以推广应用的关键环节,由于镁基储氢合金的容量高,资源丰富以及价格低廉而备受世界各国关注。但目前它仍存在吸放氢温度高、氢化反应速度慢、多数合金需要活化或热处理等问题。能够满足实际需要的是在低温下可快速吸放大量氢气、循环寿命长的储氢合金。围绕此目的,本文总共研究了五个体系17种镁基储氢合金在不同温度和氢气压下氢化反应的物理化学性质。首先采用正交实验设计方法优化了氢化燃烧法（Hydriding Combustion Synthesis,HCS）制备Mg-La-Ni三元系储氢合金的工艺,同时用HCS一段保温法制备出Mg2Ni、Mg-Ag-Ni和Mg-Al-Ni储氢合金。此外,优化了机械合金化方法（Mechanical Alloying,MA）制备Mg-8at.%LaNi_0.5、Mg-6at.%LaNi及Mg-Mm（NiCoMnAl）₅复合材料的工艺,并用MA合成了Mg₂Ni和Mg_1.9Al_0.1Ni合金。值得注意的是Mg-LaNi_x系列新型复合合金是目前最具潜力的储氢材料之一。应用近代分析方法（等温定容法、常温和高温XRD、DSC、TG、SEM和TEM等）,研究了上述17种镁基储氢合金吸放氢过程的热力学和动力学性质、物相结构、形貌组织以及它们之间的相互关系。考察了Mg_2-xM_xNi（M=Ag,Al;x=0,0.05,0.1,0.2,0.5）合金的储氢性能,并发现适量的Ag（x<0.2）和Al（x≤0.1）取代Mg后,合金仍为六方晶系,但合金的吸放氢平台压升高,吸放氢温度降低,吸放氢速率加快。当添加组元过量后（x≥0.2）,改变了Mg₂Ni的晶体结构,出现非储氢相,导致合金储氢容量降低。若以等量Ag和Al取代Mg,在553K和等压下,Mg_1.9Ag_0.1Ni和Mg1.9Al0.1Ni合金的储氢量分别为3.32和2.79w[H],说明添加Ag对储氢量的影响小于添加Al对储氢量的影响。对比HCS和MA制备的Mg_2-xAl_xNi（x=0,0.1）合金氢化性质,MA制备的合金氢化反应物理化学性质优于前者。系统地研究了用HCS和MA合成的Mg-8at.%LaNi0.5、Mg-6at.%LaNi和Mg-4at.%LaNi1.5系列合金氢化反应的物理化学性质,优化的合金组分为Mg-8at.%LaNi0.5。在3.0MPaH2和553K下,Mg-8at.%LaNi0.5（HCS）合金在15min内可吸氢4.80w[H],在相同的时间内,0.0133MPaH2下放氢4.50w[H]。同样压力下,MA合成的Mg-8at.%LaNi0.5合金在423⁵73K,10min之内吸氢量达到3.86⁶.29 w[H];同样的时间和温度,0.0133MPaH2下放氢量为3.14⁶.18 w[H]。显然,MA制备的Mg-8at.%LaNi0.5合金氢化性能好于HCS制备的合金,它不需要活化过程,吸放氢容量高,氢化反应速率快,是最具希望的储氢合金之一。XRD和TEM的分析结果表明:在MA制备Mg-8at.%LaNi_0.5合金的过程中,形成的纳米晶可以增大氢原子的溶解度和氢原子的扩散速率,使得合金的实际吸氢量（MgH_x,x>2）可以超过通常粒度下的理论极限。此外,合金粉中物相LaH₃和Mg₂Ni的催化作用有利于改善合金氢化反应的动力学性质。热力学性质是用以分析储氢合金氢化反应进行方向、限度和最大产出率的重要依据,尽管近年来实验技术有了突飞猛进的发展,但要想完全依靠实验去获取这许多数据是不现实的。尤其对于多元体系更是如此,它根本不可能完全通过实验解决。最为可行的办法就是在有限的实验基础上从理论上去估算。目前对多元镁基储氢合金氢化物热力学性质的理论计算模型很少,并存在参数不全、误差大和计算复杂等缺陷。对此,本文利用逐步回归分析法和微观结构参数法首次建立了预报Mg_2-xM_xNi_1-yMl_y多元系合金氢化反应热效应△H⁰和储氢量C的半经验数学模型:该模型考察了微观结构参数之间的交互作用,很好地阐释了Mg_2-xM_xNi_1-yMl_y合金组元的微观结构参数与合金氢化反应宏观性能之间的内在联系。指出了影响镁基储氢合金热力学性质和储氢量的主要因素,它们是:合金的电负性差△X²、电子浓度（e/a）2/3、电子密度△n^2/3、电荷-半径比Z/R和温度T等,这都是一些容易获得的参数。当合金的电负性差△X₂减小,电子浓度（e/a）^2/3和温度T升高时,合金吸氢平台压升高,氢化物形成焓变的负值减小。而当合金的电负性差△X₂和温度T增高,电子浓度（e/a）^2/3和电荷-半径比Z/R减小时,合金的吸氢量增加。采用此模型预报Mg_2-xM_xNi_1-yMl_y多元系合金的氢化物生成焓和吸氢量时,合金氢化物生成焓变的绝对误差一般小于±5 kJ/mol,合金理论吸氢量和实验值的最大绝对误差为±0.3w[H],理论计算值和实验值符合得很好。吸放氢过程的动力学是对储氢材料研究的重点,长期以来它一直落后于实验研究。目前有两种方法:一是半经验模型法,它通过实验数据与各种模型的数学表达式相比较来确定反应的动力学机构类型,这种方法的缺点是只能描述单一曲线,无法将温度,压力诸因数同时考虑在内,因而它只是一种单因素的数值拟合而不是一种具有物理意义的模型;另一种是解微分方程组的方法。它动用了复杂的数值计算,还无法进行理论上的分析和讨论。以至迄今为止,很多实验工作者尚无法采用,而仍以实验点的逐点描述给出动力学机制的结果。针对此种情况,本研究工作还提出了一种全新的处理方法。从简化假设入手,推导了一个新的模型,它是一个简单的解析表达式,将体系的吸放氢百分数表达为时间、温度、压力和粒子半径的显函数:本模型不但简化了计算,而且还可以从理论上对各种物理参数如温度、压力和粉末粒度等因素的影响进行理论上的讨论。我们还引进了一个“特征时间”的新概念,它将在储氢材料的研究中发挥重要的作用。通过与实验结果的对比,本模型能十分精确地描述我们的实验结果,并预报了在不同温度和压力下一些新的结果。将它用到其他作者的实验中也得到很好的结果。更多还原

【Abstract】 Hydrogen storage and transportation is a key issue in the application of hydrogen energy. The Mg-based alloys as hydrogen storage material are attractive for researchers in the world because of its high capacity, abundant resource and low cost. However, their practical application has been limited due to its high working temperature, relatively poor hydriding/dehydriding (H/D) kinetics and some special requirement in activation and heating treatment. In order to obtain some proper hydrogen storage materials with a large amount of hydrogen capacity and fast absorbing and desorbing rate at a low temperature environment, various attempts have been made in our lab to overcome these difficulties mentioned above.In my thesis, the physicochemical characteristics in H/D reaction for seventeen kinds of alloys in five systems were carefully investigated by means of several neoteric experimental analytical instruments including isovolumetric method, pressure-composition isotherm (PCI), X-ray diffraction (XRD), differential scanning calorimeter (DSC), thermogravimetric analysis (TG), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Firstly, the method of orthogonal test designing has been used to optimize the technique of hydriding combustion synthesis (HCS) for Mg-La-Ni ternary alloys, then the Mg2Ni, Mg-Ag-Ni and Mg-Al-Ni systems were prepared by using the technique of keeping temperature constant in one-step. Besides, Mg-La-Ni ternary alloys and Mg-Mm(NiCoMnAl)5 composite were also obtained by the optimized mechanical alloying method (MA) and the Mg2Ni and Mg1.9Al0.1Ni alloys were produced by the same way either. In the present communication, our attention was paid to the thermodynamics, kinetics, structure and morphology for clarifying the relationships between these characteristics. It is worth mentioning that the new Mg-LaNix composite is one of the most promising hydrogen storage materials recently.Comparing the characteristics of hydrogenation of Mg2Ni and those of Mg2-xMxNi(M=Ag,Al;x=0.05,0.1,0.2,0.5)alloys, it can be seen that after the suitable substitution of Ag(x<0.2) or Al(≤0.1) for Mg in Mg2Ni alloy, the equilibrium plateaus of the alloys increase, the temperature of hydrogen absorption and desorption decreases, the H/D rates become faster but the structure is still hexagonal. But excessive additive in Mg2Ni (x>0.2) will result in the change of the structure and the decrease of the hydrogen storage capacity due to the non-absorbing phase. The same amount of silver or aluminum was added to substitute Mg in Mg2Ni prepared by HCS, e.g. Mg1.9Ag0.1Ni and Mg1.9Al0.1Ni, they can absorb 3.32 and 2.79 w[H] at 553K and 3MPaH2. Compared with their H/D properties, it can be concluded that the properties of the alloy prepared by MA is better than that by HCS.A serial of Mg-8at.%LaNi0.5, Mg-6at.%LaNi and Mg-4at.%LaNi1.5 alloys prepared by MA or HCS were investigated and it is found that the mechanically alloyed Mg-8at.%LaNi0.5 composite material is the best one among the five systems in my research, that can absorb 3.86~6.29 w[H] under 3MPa hydrogen pressure and desorb 3.14~6.18 w[H] under 0.0133 MPa in 10 minutes above 423K without any activation. However, the same constituent alloy obtained by HCS can only reach the amount of 4.80 w[H] under the 3MPaH2 and desorb 4.50 w[H] under the 0.0133MPa in 15 minutes at 553K. The difference of the H/D characteristics between Mg-8at.%LaNi0.5 prepared by HCS and MA can be explained through the results obtained from XRD and TEM. The presence of the nanocrystalline produced in the mechanical alloying process is beneficial to the solubility of hydrogen atom in alloy and the diffusion of hydrogen atom through hydrides that results in the increase of the real hydrogen storage capacity being larger than the theoretic values, e.g. MgHx(x>2). Moreover, the multiphase structure(nano-/amorphous) and a catalytic effect of LaH3 and Mg2Ni formed in the ball-milling process can also accelerate the H/D rate for the composite material of mechanically alloyed Mg-8at.%LaNi0.5.Thermodynamic properties are the most important parameters for predicting the direction, limitation and maximal output ratio of H/D reactions. Although a great progress of the experimental techniques have been made, it is still impossible to obtain all relevant thermodynamic data in this field only relying on the experimental method, especially for the multicomponent systems. The unique feasible solution is that, calculating thermodynamic properties with theoretical model based on some limited experimental data. At present there are a few models used to calculate the thermodynamic properties for Mg-based multicomponent systems, however, these models possess many defects such as difficult to collect the value for parameters, very complicated expression and introducing large errors. In my thesis, based on the method of stepwise regression and considering the parameters of microcosmic atomic structure, a new semi empirical mathematical model has been established for predicting the heat of formation(?H0) and the amount of hydrogen absorbed(C) of multicomponent Mg2-xMxNi1-yMly alloys for the first time,they are, The proposed models have investigated the interaction of the individual microcosmic parameters and revealed the inner relationship between the macroscopic properties of hydrogenation and the microcosmic parameters of structure for Mg2-xMxNi1-yMly alloys, which indicates that the most important factors affecting the thermodynamic properties of Mg-based alloys should be the difference among electronegatives of constituent elements ?X2、the electron concentration (e/a)2/3、the electron density ?n2/3、the ratio of the number of valance electron to the radius of metallic atom Z/R as well as temperature. These models have successfully been applied to predict the enthalpy of formation of hydrides and the hydrogen content for Mg2-xMxNi1-yMly alloy with the maximum absolute error of±5kJ/mol and±0.3 w[H], respectively.The kinetics of hydrogen absorption/desorption (A/D) is one of key points in the field of hydrogen storage materials, in which the theoretical study is far behind the experiment research. At present there are two major methods to describe the kinetic behavior for hydrogenation, namely, semiempirical models and the method of solving a group of differential equations. The former one is using a series of mathematic formula to fit experimental data, from which to select the best one. This method is actually a kind of data fitting and cannot give a clear physical meaning for many parameters. Though the latter one is a kind of rigorous solution, that is too complicated only relying on numerical calculation and difficult to perform a theoretical analysis for the practical system. As a result, most of experimental researchers couldn’t follow it and prefer to do a point-by-point data description rather than doing this kind of complicated calculation.In my thesis, a new model has been proposed after introducing an approximation assumption for the mechanism of absorption/desorption of hydrogen. This new formulae is an analytic solution expressing the reacted fractionξas a function of time t, temperature T and particle size R0.Since it is an explicit function that is easy to use and perform a theoretical analysis. Besides, a new physical conception, namely, the“characteristic absorption/desorption time of reacted fraction of hydrogen”has been introduced to the derivation of this new model, which not only simplifies the expression of formulae but also offers some significant physical meanings that will be useful for future theoretical discussion. The application of this new model to the Mg-based hydrogen storage alloy shows that this new model works very well. When this model is used to other systems offered by other authors, a good agreement has also been obtained.更多还原