【作者】 汪海斌；

【导师】 董闯； 陈充林； 王清；

【作者基本信息】 大连理工大学 ， 凝聚态物理， 2013， 博士

【摘要】 对于能源材料中涉及氢与氧的吸放、固溶和相变的材料,由于氢与氧含量多变及位置存在一些无序分布,材料结构复杂多变,导致基于晶体学的结构处理方式不易有效分析材料结构与相关物理化学性能。V基储氢合金和钙钛矿均属于能源材料,在使用过程中都涉及气固反应,且晶体结构上具有相似性,它们的结构中金属元素呈简单的体心立方排布,而掺杂会带来化学无序,氢原子或氧原子位于金属的间隙位置,在固体结构中都存在化学含量变化大和一些位置无序分布的特点,形成了结构复杂的化合物。本文引入源自准晶非晶的团簇加连接原子模型来分析这些结构复杂的氢、氧化合物。首先优化了具有低V含量的Ti-Cr-V和Ti-Fe-V固溶体储氢合金成分,采用团簇加连接原子模型分析了相应的吸放氢性能与成分和团簇结构的关系；进而研究了固体氧化物燃料电池中氧空位复杂多变的双钙钛矿和钙钛矿结构与氧的吸放行为,使用团簇模型分析了在钙钛矿氧化还原反应过程中氧空位含量的变化对应的物态变化和局域团簇构型之间的关系。钒基固溶体储氢合金具有储氢含量高的优势。为进一步优化合金成分,减少昂贵的V用量以降低合金成本,本文首先系统研究了铜模吸铸制备工艺对于Ti-Cr-V和Ti-Fe-V体系储氢合金晶体结构与储氢性能的影响。在晶体结构表征与性能测试结果的基础上,使用固溶体结构的团簇模型分析了合金成分的微结构与储氢性能。具有相同成分的低V含量合金(Ti0.46Cr0.54)100-xVx (x=2.5,5.0,7.1),电弧熔炼随炉冷却得到的是以Laves相为主的结构,而铜模吸铸能够获得单相BCC结构。PCT吸放氢测试发现,随炉冷却方法制备的以Laves相为主Ti-Cr-V合金吸氢量最大只能达到2.0wt.%,而铜模吸铸得到的具有BCC结构的合金随V含量增加,吸氢量分别达到2.7wt.%,3.14wt.%和3.15wt.%。对(Ti0.46Cr0.54)95V5合金进行DTA热分析测试表明,其采用铜模吸铸得到的BCC结构在650℃以下结构稳定,750℃才有第二相析出。Ti/Cr比对铜模吸铸工艺制备的低V含量合金(TiyCr1-y)95V5(y=0.38-0.54)性能影响显著,Cr含量较高而使Ti/Cr比接近TiCr2时,合金(y=0.38和y=0.42)通过铜模吸铸也不能获得单一的BCC结构,储氢性能降低,Ti含量较高时,合金(y=0.50,0.54)首次吸氢量大而放氢量低,只有Ti/Cr比在7/8附近(y=0.44,0.46,0.48)时,铜模吸铸的低V合金不仅具有BCC结构,而且可逆的有效储氢量趋于理论值(2.0wt.%)。通过采用团簇加连接原子模型为低V含量的Ti-Cr-V合金构建了三元固溶体模型,发现Ti/Cr比的改变造成局域结构中的间隙类型发生重大变化,从而导致合金吸放氢性能随之改变。Ti/Cr比接近团簇[Ti7Cr8]的合金成分较高的可逆吸氢量可能与相应合金中大量存在Ti2Cr2四面体间隙和一部分富Cr型间隙有关。在Ti-Fe-V三元合金体系中,吸铸工艺不能有效抑制(Tio.46Feo.54)100·xVx (x=5-60)中低V含量的合金中Ti(Fe1-xVx)三元Laves相以及AB型TiFe相的形成,而(Ti0.46Fe0.54)100-xVx (x=5-60)中高V含量的合金能够获得BCC结构,但吸氢量低,没有第二平台区,可能与较小的间隙位有关。富钛合金(Ti0.69Fe0.31)100-xVx (x=10-50)中V含量为50at.%的(Ti0.69Fe0.31)50V50合金在80℃C的最大吸氢量达到3.5wt.%,有效储氢量达到1.7wt.%。通过Fe/VFe比例对储氢性能的影响发现,只有Fe含量在15～25at.%范围的VFe合金用作Ti-Fe-V储氢合金原材料,才可能获得较优异的储氢性能。所制备的合金中具有较高吸放氢性能的Ti-Fe-V合金成分,可表示为[Fe-Ti8-Fe2V4]和[Ti-Fe4V4-Ti6]两种团簇的组合加上连接原子V,团簇个数与V的比例约为1：7-1：8,从两种团簇结构可发现结构中以Ti2FeV四面体间隙占主导地位,说明在Ti-Fe-V体系中Ti2FeV四面体间隙可能最有利于提高合金的可逆吸放氢量。考虑到吸放氧循环会破坏钙钛矿块材晶界导致块体碎裂甚至结构分解,且氧平衡压低,因此,对钙钛矿吸放氧的研究没有使用块材和PCT方法,而采用薄膜样品和原位电阻法检测其相变进行的实验方案。另一方面,固体氧化物燃料电池将氢气等气体燃料的化学能直接转化为电能,能量转化效率高,但过高的工作温度使其应用不便,需要开发用于中低温薄膜型固体氧化物燃料电池的电极材料。因此本文进一步对钙钛矿薄膜样品采用气固反应过程中原位电阻测量方法进行了中低温(260-700℃)钙钛矿吸放氧的基础实验研究。采用脉冲激光沉积设备在LaAlO3基片上生长了具有双钙钛矿结构的LaBaCo2O5+δ和多晶立方钙钛矿结构的(CaBa)Co2O5+δ薄膜。通过电阻检测薄膜电阻变化研究薄膜在还原性气氛和氧化性气氛中的相变规律。研究发现,H-N混合气体可以在中温(400℃以上)使得LaBaCo2O5+δ和(CaBa)Co2O5+δ薄膜中的Co离子价态从+4还原为+3价,薄膜发生从半导体向绝缘体的转变,而02使薄膜状态发生逆向转变。氧化还原反应的平衡关系随温度发生变化,在更低的温度范围还原反应进行的更加彻底,薄膜发生半导体-绝缘体-半导体的三态转变,对应于在氧化还原过程中钴离子价态在Co4+-Co3+-Co2+之间变化。两种材料获得+2价钴离子的温度范围分别是260～350℃和350-500℃。将两种钙钛矿表示为钴氧团簇加连接原子(La、 Ba、 Ca)的形式,分析发现氧化还原过程中局域结构的变化具有一定的规律性,即钴氧团簇结构在C006八面体、C005四棱锥、C004平面四边形以及C004四面体等不同构型的团簇之间演变,吸放氧过程中钙钛矿具有较高的结构稳定性和化学稳定性。含有Co2+价离子的LaBaCo2O5+δ和(CaBa)Co2O5+δ薄膜对02具有很高的化学活性和超快氧化速率,电阻变化率在106Ω/s以上,有望用于开发高灵敏度气体传感器、化工催化剂,而LaBaCo2O5+δ较高的电子电导率更利于开发SOFC电极材料。更多还原

【Abstract】 For energy materials involving hydrogen or oxygen absorption and desorption actions, solid solution and multiple solid-state phase transformations, owing to variational hydrogen or oxygen content and some disorded space distribution, it is very hard to study the relation between the material structures and the related physical or chemical properties. Both V-based hydrogen-storage alloys and perovskites were studied as energy materials related with gas-solid reactions in the process of using. They also have some structural similarity such as body-centered cubic arrangements of metallic elements while interstitial site of hydrogen or oxygen atoms to form some complex structures with some disorded space distributions and variations in hydrogen or oxygen contents. So this thesis introduced a cluster-plus-glue-atom model from the structure studies of qusicrystal and amorphous metals to study some energy materials involving hydrogen and oxygen absorption and desorption. First, the compositions of Ti-Cr-V and Ti-Fe-V solid solution hydrogen storage alloys with low V content were optimized, then the relations between these compositions and related cluster structures and their hydrogen storage properties were analyzed using the cluster-plus-glue-atom model. Second, the oxygen absorption and desorption actions of perovskites usually used in SOFCs were studied, and based on the same cluster model, the relations between the changing states related with oxygen vacancies and the local cluster configurations in the processes of oxidation/redox reactions of perovskites were analyzed.V-based BCC solid solution alloys possess large H-storage capacities. For optimizing their compositions and reducing the use of expensive pure V, this paper investigates the Ti-Cr-V and Ti-Fe-V systems. Copper mould suction-casting method was used to prepare alloy samples. The local structure as well as the hydrogen storage properties of the alloys were analyzed by our cluster-plus-glue-atom model. XRD measurements found that the structures of as-cast (Tio.46Cro.54)100-xVx (x=2.5,5.0,7.1) alloy ingots evolve with V contents from pure Laves phase (x=2.5), to dual-phase TiCr2-BCC structures (5.0,7.1) while the suction-cast (Ti0.46Cr0.54)100-xVx (x=2.5,5.0,7.1) alloys only contain single BCC phase. PCT measurements found that, for the alloy ingots and rods with the same composition, the hydrogen-absorption contents of the rapidly solidified alloy rods, are much larger than those of the slowly cooled alloy ingots. The maximum hydrogen contents of the suction-cast alloy rods (Tio.46Cro.54)100-xVx (x=2.5,5.0,7.1) are respectively2.7wt.%,3.14wt.%and3.15wt.%. Furthermore, the hydrogen-storage properties of the suction-cast low-V alloys (TiyCr1-y)95V5(y=0.38-0.54) are sensitive to Ti/Cr ratios and only those alloys with Ti/Cr ratios close to the CN14cluster [Ti7Cr8] have good hydrogen-storage properties, probably due to the existence of abundant Ti2Cr2and Cr-rich tetrahedral such as Cr2TiV and Cr3Ti as indicated by the cluster-plus-glue-atom model.In Ti-Fe-V system, neither the ternary Laves phase Ti(Fe1-xVx) nor the AB-type TiFe intermetallic compound can be prohibited effectively by suction-casting method for the alloys with low V content of the (Ti0.46Fe0.54)100-xVx (x=5-60) alloys. While high V content alloys of the (Tio.46Feo.54)100-xVx (x=5-60) alloys have BCC structure but low hydrogen storage capacities, probably due to their small average size of their interstitial sites. For high Ti content alloys (Ti0.69Fe0.31)100-xVx (x=10-50), only those alloys with V contents larger than40. at.%possess excellent hydrogen storage properties. The hydrogen absorption and desorption measurements indicate that the suction-cast alloy (Ti9/13Fe4/13)50V50has high hydrogen absorption capacity with the maximum hydrogen content of3.48wt.%and effective hydrogen content of1.7wt.%at353K. The effect of Fe content in VFe alloys on Ti-Fe-V hydrogen storage properties was studied. It is found that only those VFe alloys with Fe content between15. at.%and25. at.%are possible to be used to produce Ti-Fe-V alloys with excellent hydrogen storage properties. The compositions of Ti-Fe-V alloys with high hydrogen storage properties can be expressed as [Fe3V4Ti8]x[Ti7Fe4V4]1-xVy, y=7～8. The cluster model shows that Ti2FeV tetrahedral interstices are dominant in these alloys, which may be the reason to increase the hydrogen storage capacities of Ti-Fe-V system.In consideration of that the oxidation reduction cycles may destroy the grain boundary of perovskites sintered body or even disintegrate them, and the ultra-low equilibrium pressure, sintered samples PCT method were not used to study the oxygen absorption and desorption of perovskites but by thin film samples and in situ electric resistance measurements. On the other side, Solid Oxide Fuel Cells (SOFCs) have high energy transformation efficiency to convert the chemical energy of fuel gas like H2into electric energy, but their high temperature working environment causes a lot of inconveniences to the customers. So we need to develop low-medium temperature SOFC electrode with thin film structures. Therefore, we studied the oxygen absorption and desorption actions of perovskites thin films by in situ electric resistance measurements. Pulsed laser depositon system (PLD) was used to prepare the LaBaCo2O5+δ (LBCO) and (CaBa)Co2O5+δ (CBCO) thin films. Oxidation/redox chemical dynamics on highly epitaxial LaBaCo2O5+δ (LBCO) thin films and polycrystalline (CaBa)Co2O5+δ (CBCO) have been systematically studied by precise ac bridge measurement systems. Microstructural studies from x-ray diffraction and electron microscopy reveal that the LBCO thin films have excellent epitaxial nature with c-axis oriented and highly single crystallinity structures. Electrical conductivity measurements indicate that the as-grown LBCO films have ultrahigh electrical conductivity. Especially, the chemical dynamic studies discovered that the LBCO and CBCO thin films are extremely sensitive to reducing/oxidizing environments in O2and H2at various temperatures (260～700℃) with superfast oxygen surface exchange dynamics. H-N mixture gas is capable to reduce the LBCO and CBCO thin films from semiconductor to insulator above400℃, which are identified with variation between Co-valence states of Co4+and Co3+, while the reversible reactions occur in O2.Owing to the varation of the equilibrium relation of the oxidation/redox reactions, the reduction reactions are more thoroughly at lower temperatures and include conductor-insulator-semimetal transition corresponding to the variation of valence state Co4+-Co3+-Co2+. The oxidation reactions of Co2+to Co3+at low temperatures have faster reaction rates than that of Co3+to Co4+at temperature of260-350℃for LBCO and350-500℃for CBCO. The structures of these two perovskites were expressed as [Co-0cluster] plus glue atoms (La, Ba, Ca). In the processes of oxidation/redox reactions, the local structures of different states of perovskites changed from CoO6octahedron to CoO5rectangular pyramid, CoO4quadrilateral and CoO4tetrahedron. Both LBCO and CBCO showed stable perovskite structures and chemical activity. The extremely short response time, giant resistance change above106Ω/s, and excellent chemical stability in a broad temperature range varying from260℃to700℃suggests that the LBCO and CBCO thin films are excellent candidates for, catalyst, and high temperature ultra-sensitive chemical sensor applications, while the LBCO thin film is better to be used as the SOFC electrode materials owing to its high electoral conductivity.更多还原