Study On Hydrogen Or Oxygen Absorption And Desorption Actions Of V-based Alloys And Perovskites Based On Clusters

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.