Q.A Zhang

Annealing treatment of AB2-type hydrogen storage alloys: I. crystal structures

Abstract As part of a study on annealing effects on AB 2 -type hydrogen storage alloys, the crystal structures of two multi-component alloys have been studied. There exist three, the C15 and C14 Laves phases and the Zr 7 Ni 10 phase in the as-cast alloys. Annealing treatment leads to a transition from a multiphase structure to a single stable Laves phase structure. The stable Laves phase type (C15 or C14) is dependent on the electron-atom ratio. It was also found that annealing treatment results in the increase of the lattice parameters of the C15 phase and the decrease of that of the C14 phase.

Phase structures and electrochemical properties of Cr-added V3TiNi0.56Hf0.24Mn0.15 alloys

The phase structures and the electrochemical properties of the V3TiNi0.56Hf0.24Mn0.15Crx (x=0, 0.1, 0.2 and 0.3) alloys have been studied. It is found that the alloys consist of a main phase with b.c.c structure and a Laves-type secondary phase with hexagonal structure. Increase on Cr content leads to change in the chemical composition and lattice parameters of each phase as well as in the molar fraction ratio of the main phase to the secondary phase. In V3TiNi0.56Hf0.24Mn0.15Crx alloys, increasing Cr content can improve the durability of the alloy electrode in KOH electrolyte despite the fact that its maximum hydrogen desorption capacity decreases slightly and high rate capability worsens.

Annealing treatment of AB2-type hydrogen storage alloys : II. Electrochemical properties

Abstract As the second part of a study on the effects of annealing treatment on AB 2 -type hydrogen storage alloys, the effect of annealing treatment on electrochemical properties of two multi-component alloys is reported in this paper. The effects on the electrochemical properties are closely related to the stable Laves phase structure. When the stable phase is C15, annealing treatment leads to an increase of the stability of the alloy hydride, a great decrease of the electrocatalytic activity and a significant decrease of the discharge capacity. When the stable phase is C14 after annealing treatment, the stability of its hydride decreases, the electrocatalytic activity decreases slightly, and the discharge capacity remains almost unchanged. For all AB 2 alloys, annealing treatment also leads to deterioration in the activation property and improvement in the cycle life.

Phase structures and electrochemical properties of the Laves phase hydrogen storage alloys Zr1−xTix(Ni0.6Mn0.3V0.1Cr0.05)2

Abstract The effects of the partial substitution of Ti for Zr in Zr1−xTix(Ni0.6Mn0.3V0.1Cr0.05)2 alloys are reported in this paper. The main phases C15 and C14 Laves phases, and secondary phase Zr7Ni10 were found. With increasing Ti content, abundance of C15 phase decreases and that of C14 phase increases. Increase of Ti content leads to decrease of the lattice parameters of both C15 and C14 phases. Pressure-composition isotherms show a decrease of the stability of the alloy hydride. Ti substitution for Zr in the alloys is effective to improve the activation and high-rate dischargeability. A critical substitution content of Ti is found at x=0.2. The cycling stability and the high rate dischargeability are deteriorated for the alloy electrodes with high Ti content (x>0.3).

Effects of annealing treatment on phase structures, hydrogen absorption–desorption characteristics and electrochemical properties of a V3TiNi0.56Hf0.24Mn0.15Cr0.1 alloy

Abstract The effects of annealing treatment on the phase structures, hydrogen absorption–desorption characteristics and electrochemical properties of the V3TiNi0.56Hf0.24Mn0.15Cr0.1 alloy have been studied. It is found that both the as-cast and annealed alloys consist of a main phase with b.c.c. structure and a C14-type secondary phase with hexagonal structure. With increasing annealing temperature, the lattice parameters of two phases decrease and the volume fraction of the main phase increases slightly. The present results indicate that annealing treatment decreases the maximum hydrogen desorption capacity, which could be attributed to a decrease in stability of alloy hydride due to the decrease in cell volume of the alloy phases. Furthermore, increasing annealing temperature decreases the high rate capability and deteriorates the durability of the alloy electrode in the alkaline electrolyte during charging–discharging cycles, which would be associated with the change in composition of the secondary phase and the grain growth of the main phase.

Preparation and dielectric properties of (Ca0.61,Nd0.26)TiO3nanoparticles by a sol–gel method

Ultrafine (Ca0.61,Nd0.26)TiO3 powder was synthesized by a sol–gel method. The X-ray diffraction result of the powder showed the presence of an orthorhombic GdFeO3-type perovskite phase with minute amounts of CaCO3. As the heat-treatment temperature of the xerogel was increased to 800 °C, CaCO3 disappeared completely. The average particle size calculated from the transmission electron microscopy micrograph of the powder was 5–10 nm. Compared with conventional solid-state method (1400 °C for 4 h), the ultrafine (Ca0.61,Nd0.26)TiO3 powder can be sintered compactly at relatively low temperature (1200 °C for 2 h). Microwave dielectric characteristics of (Ca0.61,Nd0.26)TiO3 ceramics fabricated from nanoparticles prepared via the sol–gel process were improved. At 1200 °C, very dense ceramics with uniform grains and good dielectric properties of dielectric constant (e) = 90.2, Q × f value (the product of quality factor Q and frequency f) = 25200 GHz, and a temperature coefficient of resonant frequency (τf) = +243ppm °C−1 were achieved.

Preparation and thermoelectric properties of Mg2Si1?xSnx

Mg2Si1−xSnx (x = 0.2, 0.4, 0.6 and 0.8) thermoelectric materials have been prepared by vacuum melting and melting/hot-pressing methods. The phase structures and electrical transport properties were measured. The relationships between electronic structures and electronic properties are discussed. X-ray diffraction (XRD) showed that the hot-pressed compounds were better formed. The electrical conductivity and Seebeck coefficient measurements showed that the properties of the compounds were dramatically affected by solid solubility. When x= 0.2, the highest electrical conductivity resulted in the highest power factor of 1.32×10−3 W m−1 K−2 at about 500 K, while the highest value of Seebeck coefficient of −459 μV K−1 was obtained at about 450 K for x= 0.6.

Phase structures and electrochemical properties of V3TiNi0.56Hf0.24Cox alloys

The phase structures and electrochemical properties of the V3TiNi0.56Hf0.24Cox alloys have been studied. It is found that the addition of Co into the V3TiNi0.56Hf0.24 alloy decreases the amount of the main phase and increases the amount of the secondary phase. The main phase is a V-based solid solution with b.c.c. structure and the secondary phase is an under-stoichiometric C14-type Laves phase of (Ti, Hf)(V, Co, Ni)m (m<2) with hexagonal structure. With increasing Co content, the lattice parameters of both the main phase and the secondary phase decrease. This leads to a decreasing stability of the alloy hydride and deterioration in discharge capacity of the alloy electrode. However, increasing Co content improves the durability of the electrodes in KOH solution. In addition, because of the poor reaction kinetics of the Laves phase due to inadequate Ni content, the V3TiNi0.56Hf0.24Cox alloys show poorer high rate capabilities.

Influence of Mn on phase structures and electrochemical properties of V3TiNi0.56Hf0.24 alloy

Abstract The influence of the addition of Mn on the phase structures and the electrochemical properties of V 3 TiNi 0.56 Hf 0.24 alloy has been studied. It is found that adding Mn does not change the structures of the main phase and the secondary phase but changes the lattice parameters, composition and molar fraction of each phase in different degrees. In V 3 TiNi 0.56 Hf 0.24 Mn x , the maximum hydrogen desorption capacity decreases while the cycle life of the alloy electrode does not change significantly with increasing Mn content. However, it is interesting to note that with x =0.15 the best high rate capability is obtained.

Contribution of rare-earths to activation property of Zr-based hydride electrodes

Abstract The effect of rare earth RE alloying (RE=Ce, Ce-rich mischmetal Mm or La-rich mischmetal Ml) on the crystalline characteristics and electrochemical performances of AB2 type (ZrRE)(MnVCrNi)2 hydride electrodes was investigated. It is found that RE-alloyed electrodes are activated at the first charge/discharge cycle in comparison with 15 cycles needed to activate RE-free alloys. The electrochemical capacity of the RE modified alloy series studied is also found to increase by about 15%. The RE modification effect on activation has been discussed on the basis of electrochemical impedance spectroscopy (EIS) measurement and XRD analysis.