Kurn Cho

The Cycle Life of Ti0.8Zr0.2 V 0.5Mn0.5 − x Cr x Ni0.8 ( x = 0 to 0.5 ) Alloys for Metal Hydride Electrodes of Ni‐Metal Hydride Rechargeable Battery

Ti-Zr-Mn-V-Ni alloys have high discharge capacities in KOH electrolyte. The rapid degradation of their capacities, however, makes them unsuitable for use as the negative electrode material in a Ni-metal hydride battery. In this work, Mn was partially substituted by Cr in the Ti-Zr-Mn-V-Ni alloys to improve their cycle life. The microstructure, hydrogenation properties, and the cycle life of these Cr-substituted alloys were investigated. The Cr-substitution was very effective in improving the cyclic durability of alloys in spite of decreasing the discharge capacity. The surface oxidation and pulverization rate of the alloys decreased with an increasing amount of Cr.

The effects of partial substitution of Mn by Cr on the electrochemical cycle life of Ti-Zr-V-Mn-Ni alloy electrodes of a Ni/MH battery

Ti-Zr-V-Mn-Ni-based multicomponent alloys demonstrate high discharge capacities in KOH electrolyte. However, the rapid degradation of their capacities makes it impossible that they be used practically as the negative electrode material in the Ni-Metal hydride battery. In this work, Mn was partially replaced by Cr in the Ti-Zr-V-Mn-Ni-based alloys to improve their cycle life. The effects of Cr substitution on microstructure, hydrogenation properties, and the electrochemical characteristics of alloys were investigated. It was found that Cr substitution is very effective to improve the cyclic durability of the alloys although the discharge capacity decreases with Cr content. Alloys with high capacity and long cycle life have been obtained after careful design of the alloy microstructure. Based on the results obtained from EIS, SEM and AES analyses, the degradation mechanisms as well as the effects of Cr on the cyclic durability of the alloy electrodes have been discussed.

Particle-size effect on the ferroelectric phase transition in PbSc1/2Ta1/2O3 ceramics

The particle-size dependence on the ferroelectric phase transition in nanocrystalline PbSc1/2Ta1/2O3 ceramics has been investigated for particles in the 0.01–0.16 μm size range. The phase transition was monitored through measurement of the dielectric constant as a function of temperature, variable temperature x-ray diffraction, and differential scanning calorimetry. The c/a tetragonality ratio decreased monotonically with particle size, reducing to unity at 0 °C for particles ⩽0.053 μm in size. The ferroelectric transition temperature Tc decreased gradually and the phase transition became more diffuse in nature as the particle size decreased. Below a mean particle size of 0.02 μm there was no peak in either the dielectric constant or the differential scanning calorimetry curve as a function of temperature, even though variable temperature x-ray diffraction could detect tetragonality indicative of ferroelectric distortions at sufficiently low temperatures down to the smallest crystallite size of 0.01 μm.

Effect of Cu powder as an additive material on the inner pressure of a sealed-type Ni–MH rechargeable battery using a Zr-based alloy as an anode

Abstract Extensive and systematic work on improving the surface catalytic activity of electrodes has been carried out to decrease the internal pressure of a sealed Ni–MH battery in which Zr 0.9 Ti 0.1 (Mn 0.7 V 0.5 Ni 1.2 ) 0.92 alloys are used in the anode. In order to improve the surface catalytic activity of a Zr–Ti–Mn–V–Ni alloy which are closely related to the inner pressure behavior in a sealed cell, the electrode was fabricated by mixing the alloy with Cu powder. By replacing 50% of carbon black with Cu powder the inner cell pressure rarely increased with cycles. This was due to the active gas recombination reaction. After measuring the surface area of the electrode and the surface catalytic activity, it was found that the surface catalytic activity for the oxygen recombination reaction was much more improved by the addition of Cu powder. This phenomenon is due to the thin Cu layer on the Zr–Ti–Mn–V–Ni alloy surface which is formed by alternate dissolution and precipitation reactions of mixed Cu powder during the charge–discharge cycle. This thin Cu layer has the role of preventing the alloy surface from oxidation and enhancing the possibility of the presence of metallic Ni, which is believed to have a high catalytic activity for the oxygen recombination reaction, on the MH surface. The inner pressure of the cell with the Zr–Ti–Mn–V–Ni alloy was lowered to a level equaling that of the commercial AB 5 -type alloys by the addition of Cu powder.

Electrical properties of CdTe films prepared by close-spaced sublimation with screen-printed source layers

CdTe films have been deposited by a close‐spaced sublimation process with screen‐printed CdTe layers as a new source. The source‐layers were fabricated by screen printing and sintering slurries consisting of propylene glycol, CdCl2 and either CdTe powder or (Cd+Te) powder. When CdTe powder was used as a starting material for the source‐layer, the electrical resistivity of the CdTe film deposited in O2 was about one‐tenth lower than that of the film deposited in He. AES, EDS and PIXE analysis showed that the Cd content in the CdTe films deposited in O2 was smaller than that in the CdTe films deposited in He. Especially, no oxygen element was detected in the CdTe films deposited in O2. It turned out that the sublimated Cd(g) and CdTe(g) from the source‐layer formed cadmium oxides in O2 and as a result the overall composition of vapor source became more Cd‐deficient. The CdTe film with less Cd content increased cadmium vacancy defects (VCd2−Cl+ and VCd2−) and hole concentrations. As the Cd/Te ratio in the st...

Successive phase transitions in the highly ordered complex perovskite PbLu1/2Nb1/2O3

Abstract The structural phase transitions and the electrical behaviour of the complex perovskite PbLu1/2Nb1/2O3 have been investigated using X-ray powder diffraction, dielectric constant measurements, differential scanning calorimetry and measurement of the polarisation as a function of applied electric field. The high-temperature paraelectric phase is highly ordered. A first-order paraelectric-antiferroelectric phase transition occurs at 270°C and an antiferroelectric-ferroelectric phase transition, characterised by dispersion in the curves of dielectric constant as a function of temperature, occurs at ≈ 30°C. The antiferroelectric phase is isostructural with the orthorhombic form of PbYb1/2Nb1/2O3. The low-temperature ferroelectric phase also has an orthorhombic crystal structure.

Effect of external pressure and grain size on the phase transition in the Gd-doped BaTiO3 ceramic

Abstract Various particle sizes of starting barium titanate were used to investigate the effect of external pressure and grain size on the phase transition in the Gd-doped BaTiO 3 ceramic. The particle size of starting BaTiO 3 was somewhat proportional to the grain size, while internal stress is inversely related to the grain size. Grain size refinement as well as external pressure shifts T c to a lower temperature, whereas orthorhombic-to-rhombohedral T 2 shifts to a higher temperature. The tetragonal-to-orthorhombic T 1 decreases with increase in pressure, reaches a minimum value, and then increases.

The intrinsic degradation behavior of (V0.53Ti0.47)0.925Fe0.075 alloy during temperature-induced hydrogen absorption-desorption cycling

Abstract The intrinsic degradation behavior of an (V 0.53 Ti 0.47 ) 0.925 Fe 0.075 alloy having a BCC structure and two plateau regions (the low and high plateau region) has been investigated using P–C isotherm measurements, thermal desorption analysis, XRD and TEM analysis during temperature-induced hydrogen absorption–desorption cycling (thermal cycling). After 400 thermal cycles between room temperature and 600°C under 10 atm H 2 , the total reversible hydrogen absorption capacity decreases by about 40%. In the low plateau region the reversible hydrogen absorption capacity decreases by about 24% after 400 cycles. The majority of the capacity loss takes place during the first 243 cycles. In the high plateau region, however the reversible hydrogen absorption capacity decreases by about 16% gradually with cycling. From the result of the thermal desorption analysis, it is found that the peak obtained at high temperature corresponds to the desorption of hydrogen of the low plateau region which shifts to lower temperatures and the peak area decreases after 223 cycles and then the area is maintained without any clear change up to 400 cycles. The low temperature peak corresponds to desorption of hydrogen from the high plateau; this does not shift but its peak area decreases gradually with cycles. From P–C isotherm measurements it is also found that the degradation behavior at each plateau region is different. In addition, XRD analysis shows that the crystal structure of the de-hydrided sample changes from BCC to BCT after degradation; the hydrided sample maintains FCC structure although the peaks are broadened after degradation. From the result of a static isothermal hydrogenation treatment, it is found that the change of crystal structure from BCC to BCT is caused only by temperature effects. The result of TEM analysis shows that peak broadening is caused by the formation of an amorphous phase in the FCC matrix.

Photovoltaic Properties of Sintered CdS / CdTe Solar Cells with an Indium‐Tin‐Oxide Electrode

Indium-tin-oxide (ITO) electrodes were applied to sintered all-polycrystalline CdS/CdTe solar cells for large area cell fabrication. The CdS and CdTe layers were prepared by sequentially sintering (CdS+CdCl 2 ) slurry at 600 o C and (Cd+Te) slurry at 625 o C. The length of the cells was fixed at 10 mm and the width was varied from 3 to 9 mm. While the application of ITO did not change the junction properties after a high temperature annealing process, it significantly reduced the series resistance of the cells. As the cell width increased from 3 to 9 mm, the efficiency of CdS/CdTe solar cells with ITO electrode decreased slightly from 10.5 to 9.4%, while that of cells without ITO decreased from 10.4 to 8.4% under the illumination of 50 mW.cm -2

Electric field-temperature phase diagram of Zr0.98Sn0.02TiO4

With the use of x-ray diffraction, we investigate the phase diagram of Zr0.98Sn0.02TiO4 as a function of temperature and dc electric field. Accurate satellite position measurements confirm the existence of commensurate phases corresponding to the rational values 1/21 and 1/23 for the incommensurate deviation factor. These results are discussed qualitatively in terms of Landau’s and Aubry’s “devil’s-staircase” approach.