【作者】 范克；

【导师】 孟令鹏； 蔡新华；

【作者基本信息】 河北师范大学 ， 物理化学， 2009， 硕士

【摘要】 当今世界主要使用的是矿物燃料能源,如:煤、石油等,这些都属于非再生能源。随着世界工业的发展和人口的快速增长,这些能源每日都在以惊人的速度消失。而且大量使用这类能源对环境造成了严重的污染。氢,在宇宙中含量丰富,可以再生,燃烧后的生成物只有水,是一种理想的绿色无污染的二次能源。21世纪也被称为氢能源的时代,对氢能源的开发和利用成为了非常重要的课题。但是,氢气在常温下易泄漏,易爆炸,难于储存和运输,因此对于储氢材料的开发便成为科研工作者关注的焦点和研究的热点。本文以A₂B型镁基储氢材料Mg₂Ni为研究对象,采用密度泛函理论（DFT）的第一性原理平面波赝势（PWP）方法。计算中,选取广义梯度近似（GGA）下的PW91泛函来描述交换相关能,平面波截止能选取380eV。研究了Mg₂Ni及其低温相氢化物的晶体结构、电子密度分布、态密度分布和氢化过程的焓变。并且分别研究了使用Al、Ag原子取代合金中的Mg原子对合金改性的方法,讨论了取代前后储氢合金及其氢化物在晶体结构、电子密度分布、态密度分布和氢化过程的焓变等性质上的变化,并以此为依据来评价“取代法”对合金改性的效果。本论文的研究内容主要分为以下三个部分。第一部分,对Mg₂Ni及其氢化物的晶体结构、电子密度分布、态密度分布和氢化过程的焓变等性质的理论研究。发现在Mg₂Ni中沿着晶胞a轴方向Ni-Ni的相互作用最强,在氢化物中H原子与Ni原子的相互作用最强,并形成了[NiH₄]结构单元,导致氢化物脱氢困难,和Jasen的计算结果一致。并计算了Mg₂Ni合金氢化反应生成低温相氢化物的反应焓变为-65.07kJ/mol（H₂）,与实验值-65.5kJ/mol（H₂）的相对误差仅有0.656%。吸氢过程的焓变较高也是导致氢化物稳定性高的原因。第二和第三部分,分别研究了,使用电负性较大的Al和Ag原子对Mg₂Ni合金的取代改性,并讨论了取代前后储氢合金及其氢化物在晶体结构、电子密度分布、态密度分布和氢化过程的焓变等性质上的变化。使用Al或Ag原子取代Mg原子后,Al、Ag原子和Ni之间的相互作用较强,导致氢化物中的Ni-H间的相互作用被削弱。由于Al、Ag原子的电负性较大,使Ni原子外层电子偏移,减弱了Ni原子和H之间的相互作用,形成三角锥型的[NiH₃]结构单元,导致合金储氢容量降低。并且吸放氢过程的能量变化明显改善,使氢化过程的平衡氢压升高,稳定性降低。从理论上阐明了使用电负性较大的原子对Mg₂Ni合金取代改性的可行性。本论文的创新之处:1.对储氢合金Mg₂Ni及其低温相氢化物LT-Mg₂NiH₄的晶体结构、电子结构以及能量进行理论计算,发现了低温相氢化物中存在[NiH₄]结构单元,氢化物结构稳定,从理论上解释了低温相氢化物脱氢困难的原因。2.通过理论计算,说明了使用Al或Ag原子对Mg₂Ni晶体中的Mg原子进行替代后生成的新合金的晶体结构的变化,以及电子结构的变化,并说明了取代后生成的新型合金的特性。3.研究了新型合金Al-Mg₂Ni和Ag-Mg₂Ni的各类型氢化物的晶体结构和电子结构以及各氢化反应的能量变化。从理论上阐明了使用Al、Ag原子对Mg₂Ni储氢合金的取代改性的可行性。更多还原

【Abstract】 Nowadays,the mineral energy resources are used most widely,such as the coal,the petroleum oil,which is assigned to the non-regenerated energy resources.With the developing of the industry and the increasing of the population,these energy resources are disappearing very fast.And the environment is polluted seriously by using the mineral energy resources.The hydrogen is a green ideal secondary energy resources,which combustion products is just water.So 21^st century is called the times of hydrogen resource.But the hydrogen is easy to leak out and explode,so it is difficult to store and transport.So,many researchers are focus on the development of the hydrogen storage materials.A first-principle based on density functional theory and plane wave pseudo-potential （PWP） method were used to research the A₂B-type Mg-based hydrogen storage materials Mg₂Ni.The PW91 function of the generalized gradient approximation （GGA） was employed for the exchange correlation of electrons and the plane wave energy-cutoff is 380ev in the calculation.The crystal structure,electron density distribution,density of states（DOS） and the enthalpy of hydrides formation of Mg₂Ni and low temperature Mg₂NiH₄ hydrides were studied.We also investigated the Mg₂Ni alloy doped by Al and Ag atoms respectively in order to modify the rate of hydrogen evolution reaction of the alloy,and discussed the changes of crystal structure,electron density distribution,density of states（DOS） and the enthalpy of hydrides formation, that was used to assess the results of the modification.The dissertation consists of three parts as follows.Part one,gives the theoretical studies on the properties of Mg₂Ni and LT-Mg₂NiH₄ regarding the crystal construction,the density of states（DOS）,the enthalpy changes of hydrogenation process.As a result,Ni-Ni shows strong interaction in the direction of axis a in the crystal cell of Mg₂Ni.Moreover,the strong interaction between H and Ni atoms was discovered in LT-Mg₂NiH₄.And due to the strong interaction mentioned above between H and Ni atoms,[NiH₄]structure unit was formed within LT-Mg₂NiH₄, which leads us to Jasen’s calculation result.The enthalpy changes of hydrogenation process from Mg₂Ni to LT-Mg₂NiH₄ were also calculated,the calculation result is -65.07kJ/mol（H₂） which is coincident with the experimental observation perfectly, the relative error is only 0.656%.Part two and part three shows the studies on the modified properties of Al-doped Mg₂Ni and Ag-doped Mg₂Ni respectively.Both of Al and Ag possess relatively high electronegativity.Furthermore,these two chapters compared the properties of both undoped and Al/Ag-doped Mg₂Ni alloy and its undoped and Al/Ag-doped hydrides LT-Mg₂NiH₄ in respect to the crystal construction,the density of state（DOS） and the enthalpy changes of hydrogenation reaction.We fond that both Al-Ni and Ag-Ni showed strong interaction,which leads to the interaction of Ni-H be weakened. Because of the high electronegativity,the outer electrons structure of Ni was changed that weaken the interaction of Ni-H.Then[NiH₃]with tripod-like structure was formed,which led to the decreasing of hydrogen storage capacity.Therefore,the feasibility of modifying the properties of Mg₂Ni alloy was proved by using dopants possessing relatively high electronnegativity.The novel conclusions and ideas of this work are listed as follows:1.The crystal structure,electron density distribution,density of states（DOS） and the enthalpy of hydrides formation of Mg₂Ni and low temperature phase of Mg₂NiH₄ were calculated in theory,and we also found that[NiH₄]structure unit was formed within LT-Mg₂NiH₄ crystal cell,which led to the high stability of LT-Mg₂NiH₄.The reason of difficulty of the dehydrogenation was explained in theory.2.It explained that the properties changes of Al-doped and Ag-doped Mg₂Ni by the calculating in theory.We also shows the new properties of the new alloy of Al-doped and Ag-doped Mg₂Ni in this paper.3.The crystal structure,electron density distribution,density of states（DOS） and the enthalpy change of hydrides formation of Al-dope and Ag-dope Mg₂Ni was calculated, and the possibility,of Al-doping and Ag-doping to modification of Mg₂Ni was proved in theory.更多还原