【作者】 刘静；

【导师】 周国治； 李谦；

【作者基本信息】 上海大学 ， 钢铁冶金， 2009， 博士

【摘要】 本文提出了一种磁场结合氢化燃烧合成或烧结合成的新方法(MACS或MASS法)制备了Mg-Ni合金氢化物、La2Mg17体系复合材料和La-Mg-Ni基AB3合金,通过等温定容法、HPDSC、XRD、SEM、EDS和粒度分布等方法系统地研究了它们的储放氢性能、相结构和形貌特征。对MACS法制备的Mg-Ni合金,强磁场能促进Mg2NiH4相的形成,显著降低合成温度。经吸放氢循环后都能得到以Mg2NiH4为主相的化合物,其中4 T磁场制备的Mg-Ni合金杂相含量最低,它在573 K下的吸氢量为3.586 wt.%,具有最优的热力学性能。DSC结果显示磁场明显降低材料的放氢吸热峰和吸氢放热峰。MACS工艺合成的La-Mg材料具有复相结构,强磁场促进添加相Ni在La-Mg合金表面形成Mg2NiH4相。其PCT曲线出现两个平台,最大吸氢量为4.162 wt.%,Mg2NiH4相起到储氢相和催化相的双重作用。Ni能使材料初始放氢温度从625 K降低到520 K。MASS法制备的La-Mg-Ni合金均由PuNi3型结构的(La, Mg)Ni3主相所组成。合金氢化后转变为以La2Ni7、MgNi2和LaNi3结构为主相的产物。PCT测试表明,1073 K的烧结温度下,经1 T磁场制得的合金具有最小的吸放氢滞后系数(0.480)和最大的放氢量(1.307 wt.%)。而在4 T磁场下,1023 K烧结的合金具有最小的滞后系数(0.519)、最高的吸氢量(1.525 wt.%)、最高的放氢量(1.474 wt.%)以及最高的放氢率(0.967)。采用La2Mg中间合金为原料也可获得La2MgNi9合金,用Co部分替代Ni后,合金晶胞参数随磁场增大,表明磁场有利于Co的取代。4 T下制备的La2MgNi7.5Co1.5合金具有最小的滞后系数(0.427)和最高的吸氢量(1.538 wt.%)。Co能同时提高合金的热力学和动力学性能。在考虑温度对储氢材料体系平衡氢压影响的基础上建立了新的动力学模型,发现对一般合金的氢化过程而言,在一定氢气压力下存在最优氢化温度(Topt)和最少特征氢化时间(tc-min)。这两个重要的参数在工程应用领域具有理论指导意义。采用本文推导的新模型对MACS法制备的Mg-Ni合金吸氢动力学进行研究后发现:在523 ~ 623 K下,合金吸氢动力学均受氢原子扩散所控制。当吸氢温度较低时,影响反应速率的最主要因素是活化能的大小,而在相对高的温度下则是合金的热力学本性,总体说来,4 T磁场制备的合金吸氢动力学最优。在553 ~ 623 K范围内,Mg-Ni合金放氢动力学也都受氢原子扩散所控制,其中2 T磁场制备的合金特征放氢时间仅是无磁场制备样品的1/4,放氢性能最优。无论是单纯的La2Mg17合金还是添加Fe3O4、Nb2O5或Ni作为催化相的复合材料,磁场的引入都会降低材料的吸氢动力学性能,其中对La2Mg17-Ni材料影响最明显,而La2Mg17-Nb2O5反之。在所有样品中,添加Ni作为催化相的样品具有最优的吸氢活化能(约5.3 kJ·mol-1 H2)、最优的特征吸氢时间(tc-473 = 84 s)、最低的Topt (502 K)和最少的tc-min (82 s)。与吸氢不同,材料的放氢均受表面渗透过程所控制,磁场能略微提高La2Mg17-Fe3O4体系材料的放氢速率,而对该系列的其他体系材料则有不利影响。对MASS法制备的La-Mg-Ni基AB3合金,当磁场强度低于4 T时,其吸氢过程受氢原子扩散所控制,而达到8 T时,受氢原子的表面渗透所控制。其中,1 T磁场制备的样品具有最小的特征吸氢时间(132 s),氢化动力学最优。Co部分取代Ni后合金吸氢速率提高近5倍。而磁场(4 T)则可以提高La2MgNi7.5Co1.5合金的动力学性能近4倍。所有MASS法制备的AB3合金在300 ~ 333 K下的放氢过程均受氢原子的扩散所控制。对La2MgNi9合金,1 T的磁场能得到最优放氢动力学。Co部分替代Ni可大幅度提高合金放氢性能,其中1 T磁场下制备的La2MgNi7.5Co1.5合金性能最佳,其活化能比其他合金低1个数量级,仅2.7 kJ·mol-1 H2,特征放氢时间也最短。更多还原

【Abstract】 A new processing of hydrogen storage materials assisted by a high magnetic field (MACS or MASS), combining the hydriding combustion synthesis or sinter synthesis with the strong magnetic field, was presented in the thesis. The Mg2NiH4 hydrogen storage metal hydride, La2Mg17-based composites and La-Mg-Ni(Co) AB3 alloys were successfully synthesized by this method. The effects of magnetic intensity on the hydriding/dehydriding properties, phase structure and morphology characteristic of the as-prepared materials were investigated through the method of identical volume, HPDSC, XRD, SEM, EDS, size distribution and so on.For the Mg-Ni alloys prepared by MACS, a high magnetic field promotes the formation of Mg2NiH4 and decreases synthesis temperature obviously. After hydriding/dehydriding cycles, Mg2NiH4 become the major phase for all samples. Among them, the sample prepared under 4 T magnetic field has the lowest impurity. Moreover, its hydrogen capacity at 573 K is 3.586 wt.% and it has the optimum thermodynamics properties. DSC measurements indicate that magnetic fields significantly shift the endothermic/exothermic peaks towards lower temperatures.La2Mg17-based composites have multiphase structures and a high magnetic field promotes the reaction of Ni with alloy to Mg2NiH4 phase on the surface of La-Mg particles. The measurement of PCT curves indicates that the reversible hydriding amount for the La2Mg17-Ni composite is 4.162 wt% at 623 K and the PCT curves have two plateaus in the temperature ranges of 523~623 K. Mg2NiH4 play dual role as hydrogen storage and catalytic phases. Moreover, nickel decreases the initial dehydriding temperature of the material from 625 K to 520 K.All the La-Mg-Ni alloys prepared by MASS have the major phase of PuNi3 type (La, Mg)Ni3. After hydriding/dehydriding cycles, the phase structure is changed to La2Ni7, MgNi2 and LaNi3. The measurement of PCT curves indicates that under the sinter temperature of 1073 K, the sample prepared under 1 T magnetic field has the minimum hydriding/dehydriding hysteresis (0.480) and the maximal dehydriding capacity (1.307 wt.%). Under 4 T magnetic field, the sample prepared at 1023 K has the minimum hydriding/dehydriding hysteresis (0.519), the maximal hydriding/dehydriding capacity (1.525 wt.%/ 1.474 wt.%) and the maximal dehydrogenated rate (0.967). After partial substitution of Ni with Co, the unit cell parameters increase with the increase of magnetic field intensity, indicating that the magnetic field is beneficial to the substitution of Ni with Co. The (La, Mg)(Ni, Co)3 alloy prepared under 4 T magnetic field has the minimum hydriding/dehydriding hysteresis (0.427) and the maximal hydriding capacity (1.538 wt.%). Both the thermodynamics and kinetics properties can be improved by element Co.The influence of temperature on the equilibrium hydrogen pressure was considered and a new hydriding/dehydriding kinetics model for hydrogen storage materials had been developed. It was found that for a general hydrogen storage system, there is a optimum hydriding temperature (Topt) at a certain hydrogen pressure and corresponding to the minimal characteristic hydriding time (tc-min). They are two important parameters which have certain guidance functions on engineering application field.The new model proposed in this thesis has been used to investigate the hydriding/dehydriding kinetics mechanism of the above three systems. For the Mg-Ni alloys prepared by MACS, the rate-controlling step for hydriding process is the diffusion of hydrogen in the hydride layer. At relatively low temperature, the main factor to influence the hydriding rate is activation energy, but at relatively high temperature, the main factor is the thermodynamics characteristic. The sample prepared under 4 T magnetic field has the optimum hydriding kinetics properties. Otherwise, the dehydriding process is also controlled by the diffusion of hydrogen. The characteristic dehydriding time (tc) of the sample prepared under 2 T magnetic field is only 1/4 of that without magnetic in the temperature ranges of 523 ~ 623 K and the former has the optimum dehydriding kinetics properties.However, the magnetic fields deteriorate the hydriding kinetics of La2Mg17-based composites whatever the catalytic phase is Fe3O4, Nb2O5 or Ni. Among them, the most sensitive composite to magnetic field is La2Mg17-Ni, conversely, the least one is La2Mg17-Nb2O5. La2Mg17-Ni composite prepared without magnetic field has the minimal activation energy (5.3 kJ·mol-1 H2), the minimal characteristic hydriding time (tc-473 = 84 s), the minimal Topt (502 K) and the minimal tc-min (82 s). Otherwise, the dehydriding processes of the composites are all controlled by the surface penetration of hydrogen atoms. Magnetic fields slightly improve the dehydriding rate of La2Mg17-Fe3O4 composite, but have adverse influence on other La2Mg17-based composites. For the (La, Mg)Ni3 alloys prepared by MASS, the rate-controlling step for hydriding process is the diffusion of hydrogen in the hydride layer when the magnetic field is lower than 4 T, but with the magnetic increase to 8 T, the rate-controlling step is changed to the surface penetration of hydrogen atoms. The sample prepared under 1 T magnetic field has the minimum characteristic hydriding time (tc = 132 s). After partial substitution of Ni with Co, the hydriding rate is improved by nearly 5 times. Otherwise, magnetic field (4 T) improves the hydriding rate of La2MgNi7.5Co1.5 alloy for about 4 times. The rate-controlling steps of the dehydriding processes for all AB3 alloys are the diffusion of hydrogen atoms under the temperature ranges from 300 to 333 K. For dehydriding kinetics of La2MgNi9 alloys, the optimized magnetic intensity is 1 T. The dehydriding kinetics can be improved obviously by partial substitution of Ni with Co. The activation energy of the sample prepared under 1 T magnetic field is only 2.7 kJ·mol-1 H2. At the same time, it has the minimal tc, indicating that this sample has the optimum dehydriding kinetics properties.更多还原