【作者】 斯庭智；

【导师】 刘宁； 张庆安；

【作者基本信息】 合肥工业大学 ， 材料学， 2010， 博士

【摘要】 随着世界能源匮乏和环境恶化,氢能作为一种新型的清洁能源受到人们的广泛关注。高贮氢容量和低成本贮氢材料的开发是实现氢能规模应用的关键。本文在对国内外贮氢合金研究进展全面综述的基础上,选择新型R-Mg-Ni(R = Ca和La)层状结构合金为研究对象,并采用感应熔炼或激光烧结等方法制备合金。通过粉末X射线衍射Rietveld分析方法精确确定合金相结构,并且结合EXPO程序对新型化合物晶体结构进行解析;通过高分辨透射电镜(HTEM)分析层状堆垛化合物亚结构特征;采用扫描电镜(SEM)和电子背散射衍射(EBSD)观察合金组织结构,并结合能谱(EDS)分析确定合金相化学成分。采用Sieverts型装置测定合金的压力-组分-温度(P-C-T)曲线;结合范特-霍夫(van’t Hoff)曲线计算合金氢化物形成焓(ΔH )和熵(ΔS);并通过差热分析(DSC-TGA)进一步研究合金贮氢热力学性能;此外,采用兰电(Land)电池测试系统对部分合金进行了电化学性能测试。本文探索了新型层状合金的微结构特征及其贮氢性能,系统研究了合金成分-结构-性能之间的内在联系。本文首次在Ca-Mg-Ni合金系中发现了新型三元化合物Ca3Mg2Ni13。该化合物的空间群被确定为R-3m (No. 166),晶格常数a和c分别为4.9783和36.180 ?,Z = 3。Ca3Mg2Ni13化合物的层状堆垛结构是由三个块层(block)形成一个排列周期沿c轴堆垛而成。单个块层由两个亚块层(sub-block)构成;其中,一个亚块层是由一个[CaMgNi4]单元层(unit)和一个[CaNi5]单元层构成,而另一个则由单个[CaMgNi4]单元层构成。Mg在Ca3Mg2Ni13化合物中的理论固溶度很高(22.22 at.%),并且其实际固溶度接近理论固溶度。因此,Ca3-xMg2+xNi13合金的贮氢性能可在很宽的成分范围进行调节。Mg含量的增加导致Ca3Mg2Ni13型相晶胞减小,这有效地改善了合金的吸放氢热力学性能。Ca2.0Mg3.0Ni13合金的吸放氢焓变分别为–28和30 kJ/mol H2。而且,由于吸放氢循环时不发生氢致分解和非晶化,该合金具有良好的循环稳定性。对Ca3?xLaxMg2Ni13 (x = 0, 0.75, 1.5, 2.25, 3)的研究发现,La替代Ca不利于Ca3Mg2Ni13型相的形成,La在Ca3Mg2Ni13型相中的最大固溶度大约为x = 0.124。其中Ca1.5La1.5Mg2Ni13合金具有最高的吸放氢平台压和最大的可逆放氢容量(1.34 wt.%)。La替代的合金循环稳定性得到较大的改善,当x由0增加到3,合金放电容量循环衰减率(S30)从13.7增加到67.6%。本文通过熔炼制备了(Ca1.0-xMgx)Ni3 (x = 0.16, 0.33, 0.5, 0.67)合金,合金由PuNi3型(Ca, Mg)Ni3主相和少量杂相组成。随Mg含量的增加,(Ca, Mg)Ni3主相的点阵常数和晶胞体积减小。Mg原子只能占据PuNi3型(Ca, Mg)Ni3主相中Ca原子的6c位置,随Mg含量的增加,Mg在该位置的占位因子增大。(Ca0.33Mg0.67)Ni3合金中的Mg含量稍低于其在PuNi3型相中的最大固溶度16.67 at.%。通过Mg的替代,(Ca0.5Mg0.5)Ni3和(Ca0.67Mg0.33)Ni3合金氢化物形成焓和熵接近于LaNi5合金,它们具有一定的应用前景。本文,首次将激光烧结技术引入到La-Mg-Ni贮氢合金的制备。研究了激光烧结的(La0.67Mg0.33)Ni3合金相结构和电化学性能。(La0.67Mg0.33)Ni3合金由LaNi5主相、PuNi3型(La, Mg)Ni3和少量的LaMgNi4相组成。烧结的合金具有完全不同于熔炼合金的新颖的网状结构特征。随烧结功率的升高,PuNi3型(La, Mg)Ni3相含量增加,并且(La, Mg)Ni3相成分发生改变。1000和1400 W烧结合金中的(La, Mg)Ni3相分别具有(La0.6Mg0.4)Ni3和(La0.67Mg0.33)Ni3的化学成分。PuNi3型(La, Mg)Ni3相的形成导致激光烧结合金具有较高放电容量,1000和1400 W烧结的试样分别具有321.8和344.8 mAh/g的放电容量。为了确定文献[5]报道的A7B23型La5Mg2Ni23化合物的晶体结构,本文感应熔炼制备了La5Mg2Ni23合金。该合金由Ce2Ni7型、Gd2Co7型、LaNi5、Pr5Co19型、Ce5Co19型和LaMgNi4六相组成。进一步研究发现,Ca5Mg2Ni23合金由PuNi3型(Ca, Mg)Ni3、Gd2Co7型(Ca, Mg)2Ni7、CaNi5和少量的Ni组成。此外,La5?xCaxMg2Ni23 (x = 1, 2, 3)合金主要由PuNi3型、Gd2Co7型和CaCu5型相组成。在La-Mg-Ni、Ca-Mg-Ni和La-Ca-Mg-Ni系中都未发现A7B23型化合物的存在。La5?xCaxMg2Ni23 (x = 0, 1, 2, 3)合金易于活化(Na≤4),合金电极的腐蚀电位Ecorr随Ca添加量的增加而减小。La5?xCaxMg2Ni23的电化学循环稳定性由合金电极的腐蚀抗力和相组成共同作用。Ca替代的合金具有高的放电容量。得益于合金中合适的Ca含量与最高的PuNi3型和Gd2Co7型相含量,La3Ca2Mg2Ni23合金具有最高的放电容量(404.2 mAh/g)和最优的高倍率放电能力(HRD600 = 61.6%)。A2B7型三元Ca-Mg-Ni和二元Ca-Ni化合物相同,只有一种Gd2Co7型结构。Ca3MgNi14化合物的空间群为R-3m (No. 166);晶胞参数为a = 4.9702(2) ?, c = 35.1111(1) ?;Z = 3。其晶胞参数与Ca3Mg2Ni13的十分相近,但它们的原子占位不同。该化合物的层状堆垛结构是由三个块层沿c轴排列形成一个周期,每个块层由一个[CaMgNi4]单元和两个[CaNi5]单元组成。研究了Mg在Ca2Ni7中的固溶度和(Ca2-xMgx)Ni7合金的贮氢性能。研究发现,Mg在Ca2Ni7合金中最大固溶度大约为x = 0.5。(Ca1.75Mg0.25)Ni7合金中(Ca, Mg)2Ni7相中存在大量的“块层间”层错,然而,(Ca1.5Mg0.5)Ni7合金中(Ca, Mg)2Ni7相中层错较少。得益于Mg替代合金的晶胞的减小和(Ca, Mg)2Ni7相中层错的减少,(Ca1.5Mg0.5)Ni7合金吸放氢热力学性能得到改善。为了研究Mg在La2Ni7中的固溶度,激光烧结制备了(La1.4Mg0.6)Ni7合金。该合金具有复相结构,由La3MgNi14主相、LaNi5相和少量的LaMgNi4相组成。Mg在Ce2Ni7型La2Ni7相中的最大固溶度与其在Gd2Co7型Ca2Ni7相中的相同。激光烧结合金中La3MgNi14主相的含量都高于普通熔炼的合金。由于激光功率1200 W烧结(La1.4Mg0.6)Ni7合金具有最多的La3MgNi14相,因此,其具有最高的放电容量、最佳的循环稳定性和最优的高倍率放电性能。La4MgNi19化合物具有两种多型性结构:Pr5Co19和Ce5Co19型结构。La4MgNi19相单个块层(block)由一个[LaMgNi4]单元和三个[LaNi5]单元组成。Pr5Co19型La4MgNi19相的层状堆垛结构由两个块层形成一个排列周期(2H结构)。而Ce5Co19型La4MgNi19相的层状堆垛结构由三个块层形成一个排列周期(3R结构)。Ca替代的La4?xCaxMgNi19合金中La4MgNi19相随Ca替代量的增加而减少。Ca4MgNi19合金由CaNi5、Gd2Co7型和PuNi3型相组成,不存在对应于La4MgNi19结构的Ca4MgNi19化合物。通过熔炼合金粉末冶金高压气体保护烧结和淬火的方法可制备由Pr5Co19和Ce5Co19型相组成的高纯La4MgNi19合金。合金P-C-T曲线显示Pr5Co19和Ce5Co19型相吸放氢性能相同,然而,合金吸放氢不同阶段的XRD图表明Pr5Co19型相吸放氢平台稍低于Ce5Co19型相的平台。研究发现,Pr5Co19型相为高温相,Ce5Co19型相为低温相;并且它们之间的多型性转变十分缓慢。La4MgNi19相稳定存在温度区间大约在840~960 oC之间。930和870 oC淬火合金的贮氢容量分别为1.53和1.51 wt.%,因此,Pr5Co19和Ce5Co19型La4MgNi19相的贮氢能力相当。但Ce5Co19型La4MgNi19相贮氢平台斜率比Pr5Co19型的大,从而导致可逆电化学贮氢能力下降。930和870 oC淬火La4MgNi19合金的放电容量分别为374.9和358.2 mAh/g。更多还原

【Abstract】 With the deficiency of fossil fuel resources and the deterioration of environment, hydrogen has attracted great attentions as a kind of clean energy with great development potential. The development of the hydrogen storage materials with high storage capacity and low cost is key break for the application of hydrogen energy. Based on the review of the research and development of hydrogen storage alloys, the novel R-Mg-Ni(R = Ca and La)alloys with layered structure were selected as the object of the study in this paper. The alloys were prepared by conduction melting or green compact laser sintering. The phase structure of the alloys was analyzed with the Rietveld refinement program RIETAN-2000. The structure of the new compound was determined with the EXPO program. The sub-structure of the compounds with layered structure was observed by high resolution transmission electron microscope (HRTEM). The microstructures and phase compositions were examined using a scanning electron microscope (SEM) or electron backscattered diffraction (EBSD) with an energy dispersive X-ray spectrometer (EDS). The pressure-composition-temperature (P-C-T) isotherms were measured using a Sieverts-type apparatus, and the enthalpy (ΔH ) and entropy (ΔS) of hydride formation were calculated according to the curves of van’t Hoff. The thermodynamics of hydrogen storage were further studied by by combined differential scanning calorimetry and thermogravimetric analysis (DSC-TGA). For same alloys, moreover,the electrochemical properties were carried out by using a Land battery testing system. The structure and hydrogen storage properties of the novel R-Mg-Ni(R = Ca and La)alloys with layered structure were systematically studied, and the composition-structure-property correlation was understood in this paper.The novel Ca3Mg2Ni13 was found in the Ca-Mg-Ni system. Ca3Mg2Ni13 crystallizes in space group R-3m (No. 166); cell parameters: a = 4.9783(2) ? and c = 36.180(2) ?; Z = 3. The Ca3Mg2Ni13 structure has three blocks stacked along the c axis in one period, and each block is composed of two sub-blocks. One only contains one layer of [CaMgNi4] unit, and the other consists of one layer of [CaMgNi4] unit and one layer of [CaNi5] unit. Theoretically, the solid solubility of Mg in Ca3Mg2Ni13-type phase is as high as 22.22 at.%. Moreover, the maximum solid solubility is close to the theoretic value. Therefore, the hydrogen storage properties of the Ca3-xMg2+xNi13 alloys can be adjusted within a wide range of Mg content. The increase of Mg content leads to the decrease in the lattice parameters of Ca3Mg2Ni13–type compound, which effectively improves the thermodynamics of hydrogen absorption–desorption. The enthalpy changes for the hydrogen absorption and desorption of Ca2.0Mg3.0Ni13 are–28 and 30 kJ/mol H2, respectively. Moreover, Ca2Mg3Ni13 shows good cycling stability because the hydrogen–induced amorphization and decomposition do not occur during hydrogen absorption–desorption cycles. For Ca3?xLaxMg2Ni13 alloys, it was found that the La substitution is unfavorable for the formation of the Ca3Mg2Ni13-type phase. The maximum solid solubility of La in the Ca3Mg2Ni13-type phase is around x = 0.124 in the present study. Among the Ca3?xLaxMg2Ni13 alloys, the Ca1.5La1.5Mg2Ni13 alloy has highest equilibrium pressures of hydrogen absorption-desorption and possesses a highest hydrogen desorption capacity of 1.34 wt.% at 318 K. The electrochemical measurements indicated that the cyclic stability of the alloys is improved, and S30 increases from 13.7 to 67.6% when x increases from 0 to 3.In this paper, the (Ca1.0-xMgx)Ni3 (x = 0.16, 0.33, 0.5, 0.67) alloys were prepared by conduction melting. All alloys contained a PuNi3-type main phase (Ca, Mg)Ni3 and a small amount of impure phases. The lattice parameters and unit cell volume of the PuNi3-type (Ca, Mg)Ni3 phase decreased with increasing Mg content. Mg atoms only occupied the 6c (Ca2) sites of PuNi3-type structure. Moreover, the occupation factor of Mg on the 6c site increased with increasing Mg content. The maximum solid solubility of Mg in the (Ca0.33Mg0.67)Ni3 alloy was slightly smaller than the theoretic solid solubility of 16.67 at.% in the PuNi3-type phase. The enthalpy and entropy of hydride formation for the (Ca0.5Mg0.5)Ni3 and (Ca0.67Mg0.33)Ni3 alloys are close to those of the practically applied LaNi5 alloy. La-Mg-Ni alloys were prepared by green compact laser sintering for the first time in this paper. The structure and electrochemical properties of the laser sintered (La0.67Mg0.33)Ni3 alloys were investigated. Except for small amount of LaMgNi4, the alloys consisted of a main phase LaNi5 and a secondary phase with PuNi3 structure. The novel network microstructure for the laser sintered alloys is quite different to that of the alloys prepared by conduction melting. With increasing of sintering power, ternary La-Mg-Ni phase with PuNi3 structure increased. Moreover, the composition of ternary La-Mg-Ni phase with PuNi3 structure changed from (La0.6Mg0.4)Ni3 to (La0.67Mg0.33)Ni3. The discharge capacities of the alloys prepared by laser sintering at 1000 and 1400 W are 321.8 and 344.8 mAh/g, respectively.In order to determine the crystal structure of the A7B23-type La5Mg2Ni23 compound reported by literature [5], the La5Mg2Ni23 alloy was prepared by conduction melting. It was found that the La5Mg2Ni23 alloy consists of Ce2Ni7-type, Gd2Co7-type, LaNi5, Pr5Co19-type, Ce5Co19-type and LaMgNi4 phases. The Ca5Mg2Ni23 alloy contain PuNi3-type (Ca, Mg)Ni3, Gd2Co7-type (Ca, Mg)2Ni7, CaNi5 and a small amount of Ni. Moreover, the La5?xCaxMg2Ni23 (x = 1, 2, 3) alloys consist of PuNi3-type, Gd2Co7-type and CaCu5-type phases. In a word, A7B23-type compound has not been found in La-Mg-Ni, Ca-Mg-Ni and La-Ca-Mg-Ni system. The La5?xCaxMg2Ni23 (x = 0, 1, 2 and 3) alloys can be activated to their maximum discharge capacities within four cycles. Ecorr of the alloys decreased with increasing of Ca content. The cyclic stabilities of the La5?xCaxMg2Ni23 alloys are related to both phase abundance and corrosion potential. The discharge capacities of the Ca-substituted alloys are higher than that of the La5Mg2Ni23 alloy. Among these alloys, the La3Ca2Mg2Ni23 alloy has a highest discharge capacity (404.2 mAh/g) and a best high-rate dischargeability (HRD600 = 61.6%) due to the optimum Ca content and the highest abundance of the PuNi3-type and Gd2Co7-type phases.Similar to the binary Ca-Ni compound, the A2B7-type Ca-Mg-Ni compound has the structure of Gd2Co7. Ca3MgNi14 compound crystallizes in space group R-3m (No. 166); cell parameters: a = 4.9702(2) ?, c = 35.1111(1) ?; Z = 3. These parameters are similar to those of Ca3MgNi14, but the atomic coordinates are quite different.The Ca3MgNi14 structure has three blocks stacked along the c axis in one period, and each block is composed of one layer of [CaMgNi4] unit and two layers of [CaNi5] unit. The solid solubility of Mg in Ca2Ni7 and hydrogen storage properties of the (Ca2-xMgx)Ni7 alloys were investigated in this paper. It was found that the maximum solid solubility of Mg in the (Ca2-xMgx)Ni7 phase is about x = 0.5. The stacking faults in the (Ca, Mg)2Ni7 phase for the (Ca1.75Mg0.25)Ni7 alloy are of‘inter-block-layer’type. However, few stacking faults were observed in the (Ca, Mg)2Ni7 phase for the (Ca1.5Mg0.5)Ni7 alloy. Owing to smaller lattice parameters and lower density of stacking faults, the reversibility of hydrogen absorption-desorption of the (Ca1.5Mg0.5)Ni7 alloy can be improved by increasing Mg content. In order to investigate the solid solubility of Mg in La2Ni7, the (La1.4Mg0.6)Ni7 alloys were prepared by laser sintering. It is found that all alloys consist of multiple phases, which are Ce2Ni7-type phase, LaNi5 and LaMgNi4. The solid solubility of Mg in La2Ni7 is same as that in Ca2Ni7. The amount of the La3MgNi14 phase in the laser sintered alloys is higher than that in the alloys prepared by conduction melting. Among these alloys, the alloy sintered at 1200 W has a highest discharge capacity, best cyclic stability and best high-rate dischargeability due to the highest amount of the main phase La3MgNi14.La4MgNi19 compound have two types of structure: a hexagonal structure of the Pr5Co19-type and a rhombohedral structure of the Ce5Co19-type. Each block of La4MgNi19 compound is composed of one layer of [CaMgNi4] unit and three layers of [CaNi5] unit. The Pr5Co19-type structure has two blocks stacked along the c axis in one period (2H), but the Ce5Co19-type has three blocks stacked along the c axis in one period (3R). The La4MgNi19 phase in the Ca-substituted La4?xCaxMgNi19 alloys decreased with increasing of Ca content. It is interesting that the Ca4MgNi19 alloy consists of CaNi5, Gd2Co7-type and PuNi3-type phases. That is, the Ca4MgNi19 compound with La4MgNi19 structure was not found in this paper. The La4MgNi19 alloys with the Pr5Co19-type and Ce5Co19-type phases were prepared by melted alloy-powder metallurgy sintering at high pressure Ar atmosphere and succedent quenching. P-C-T curves indicated the Pr5Co19-type and Ce5Co19-type phases have same hydrogen storage properties. However, X-ray diffraction patterns for the La4MgNi19 alloy at different absorption and desorption hydrogen stage revealed the hydrogen absorption and desorption plateaus of the Pr5Co19-type phase is lower than those of the Ce5Co19-type phase. It is found that the Pr5Co19-type structure is high temperature phase and the Ce5Co19-type is low temperature phase. Furthermore, the phase transformation between the Pr5Co19-type and Ce5Co19-type was very sluggish. The La4MgNi19 phase can steadily exist at about the temperature range of 840~960 oC. The hydrogen storage capacities of the La4MgNi19 alloys quenched at 930 and 870 oC are 1.53 and 1.51 wt.%, respectively. Therefore, the hydrogen storage content of the Pr5Co19-type is comparatively with that of the Ce5Co19-type La4MgNi19 phase. Owing to larger slope of hydrogen storage plateaus for the Ce5Co19-type La4MgNi19 phase, the dischargeability of this phase reduced. The discharge capacities of the alloys quenched at 930 and 870 oC are 374.9 and 358.2 mAh/g, respectively.更多还原