金属硼氢化物和氨硼烷的第一原理结构与性能研究

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	金属硼氢化物和氨硼烷的第一原理结构与性能研究
	刘超仁
学位类型	硕士
导师	王平 ; 胡青苗
	2011
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	储氢材料第一原理硼氢化物氨硼烷 Hydrogen Storage Material First Principle Calculations Borohydride Ammonia Borane
摘要	"高效储氢被公认为氢能产业化进程中的“瓶颈”环节。近年来，富氢含硼材料日渐成为储氢材料领域突出的研究热点。本工作采用基于密度泛函理论的第一原理方法，研究了数种金属硼氢化物的晶体结构与热力学性质，以及氨硼烷（NH3BH3）的低温及室温结构。金属硼氢化物大多具有高储氢密度，但放氢温度过高或过低、反应可逆性差等问题严重制约其实际储氢应用。本工作计算研究了LiMg(BH4)3、NaZn2(BH4)5、LiZn2(BH4)5等双阳离子硼氢化物的晶体结构与热力学性质。研究结果表明，LiMg(BH4)3放氢反应焓变为55.8 KJ/molH2，是一种热力学性质优异的新型高容量储氢材料。LiZn2(BH4)5和NaZn2(BH4)5的放氢反应焓变分别为27.6与23.0 KJ/molH2，材料热稳定性显著优于Zn(BH4)2。该部分结果从理论计算角度说明，双金属阳离子组合是调控硼氢化物热力学性质的有效手段。 NH3BH3具有高达19.6 wt%的质量氢密度，是最具代表性的化学氢化物。文献结果表明，NH3BH3于~225K发生正交®四方结构转变；其低温正交相结构已基本确定，而室温四方相结构尚存在争议。作为放氢机制研究的基础，解析NH3BH3的室温相结构具有重要意义。本文采用第一性原理平面波赝势方法研究了两种氨硼烷结构（低温相Pmn21、室温相P42cm）的晶格参数、电子结构与动力学性质。计算结果表明，Pmn21结构的能量低于P42cm结构，符合实验观测结果；Pmn21到P42cm相变所引起的结构变化主要体现为NH3BH3分子间双氢键键长显著增加，而分子内部化学键键长变化不大。上述研究结果对于丰富和发展金属硼氢化物和氨硼烷的储氢基础理论具有一定的意义，同时也将为开展相关材料的实验研究提供借鉴和指导。"
其他摘要	"Development of high-performance hydrogen storage materials is generally regarded as the “bottle neck” for the widespread application of hydrogen. Recently, hydrogen-rich B-containing hydrides have garnered considerable attention as potential hydrogen storage media, among which complex borohydrides and ammonia borane (NH3BH3) are the focuses. In this work, we investigate several bimetallic borohydrides’ crystal structures and thermodynamic properties and NH3BH3’s low temperature and room temperature structures using first principle calculation method. Monometallic borohydrides typically possess high hydrogen capacities, but problems like too high or too low temperature for hydrogen release and poor reversibility have severely hindered their practical applications. In the present study, we focus on their bimetallic counterparts. Several bimetallic borohydrides, including LiMg(BH4)3、NaZn2(BH4)5、LiZn2(BH4)5, are investigated on their crystal structures and thermodynamic properties aspects. It was found that the enthalpy of hydrogen desorption from LiMg(BH4)3 is about 55.8 kJ/mol H2, suggesting that it is a promising hydrogen storage candidate from thermodynamic perspective. While for LiZn2(BH4)5 and NaZn2(BH4)5, the enthalpies for hydrogen desorption are about 27.6 and 23.0 kJ/mol H2, respectively, larger than that for Zn(BH4)2 (~ 10 kJ/mol H2), indicating the two borohydrides have better thermal stability with respect to Zn(BH4)2. These calculation results suggest that manipulation of mixed-metal combinations would be an effective approach to tailoring the thermodynamics of borohydrides. NH3BH3, a representative chemical hydride, contains an extremely high hydrogen density (19.6 wt%). Literatures show that its crystal structure undergoes a phase transition from an orthorhombic phase to a tetragonal phase as the temperature is raised above ~225K. The crystal structure of the low temperature orthorhombic phase has been well determined, but the room temperature tetragonal phase still lacks of a generally accepted crystal structure. A well resolved room temperature structure of NH3BH3 would be of significance for furthering our understanding on its hydrogen release mechanism. In this sense, we calculated the lattice parameters, electronic structures, and dynamics for both the orthorhombic phase (Pmn21) and the tetragonal phase (P42cm) using a first principle pseudo-potential plane-wave method. The obtained results show that the low temperature structure (Pmn21) is energetically more stable than the room temperature structure (P42cm).This is consistent with the experimental observation that lower temperature phase is the Pmn21 structure whereas the room temperature phase is the P42cm structure. Additionally, it was found that the phase transition from orthorhombic (Pmn21) phase to tetragonal (P42cm) phase results in a remarkable increase in the intermolecular dihydrogen bond length but meanwhile little variation is observed for the intramolecular bond length. These theoretical findings have enriched our knowledge regarding metallic borohydrides and NH3BH3, and thereby may act as a guide for subsequent experimental efforts on the two systems."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64386
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	刘超仁. 金属硼氢化物和氨硼烷的第一原理结构与性能研究[D]. 北京. 中国科学院金属研究所,2011.