| 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.
|
PDF Full Text Request