The electrochemical performance of AB3-type hydrogen storage alloy as anode material for the nickel metal hydride accumulators

For the purpose of lowering the cost of metal hydride electrode, the La of LaY₂Ni₉ electrode was replaced by Ce. The electrochemical performances of the CeY₂Ni₉ negative electrode, at a room and different temperatures, were compared with the parent alloy LaY₂Ni₉. At room temperature during a long cycling, the evolution of the electrochemical capacity—the diffusivity indicator (\( \frac{D_{\mathrm{H}}}{a^2} \))—the exchange current density, and the equilibrium potential were determined. At different temperatures, the electrochemical characterization of this alloy allowed the estimation of the enthalpy, the entropy, and the activation energy of the hydride formation. The evolution of the high-rate dischargeability was also evaluated at different temperatures. Compared with the parent LaY₂Ni₉ alloy, CeY₂Ni₉ exhibits an easy activation and good reaction reversibility. This alloy also conserves a good lifetime during a long-term cycling. A lower activation energy determined for this alloy corresponds to an easy absorption of hydrogen into this new alloy.     

Numerical analysis of free convective flows in partially open enclosure

 Laminar steady state buoyancy induced flows in a two-dimensional, air filled partial open enclosure with a discrete flush mounted iso-flux heat source on one of its walls is investigated numerically. The transport equations for energy and vorticity are solved with the aid of the ADI finite difference scheme on uniform mesh. Because of the specific application of the present study in the air cooling of electronic equipments, results are obtained only for a Prandtl number of 0.71 with an aspect ratio of 1.0 for a range of Rayleigh numbers, Ra (≤10⁵), heat flux parameter, Q and opening parameter, A ₀ using constant properties and Boussinesq approximation by imposing approximate conditions at the opening. Results of flow and temperature patterns, velocity and temperature profiles shows that the outgoing flow is governed by strong characteristics of the cavity condition whereas the incoming flow influenced by outside conditions. It is observed that Rayleigh number considerably affects the flow and thermal fields within the open enclosure when compared with intensity of heat flux and size of the opening. Received on 22 January 2001 / Published online: 29 November 2001     

Transverse Vibrations of a Centrally Clamped Rotating Circular Disk

The transverse vibrations of a circular disk of uniform thickness rotatingabout its axis with constant angular velocity are analyzed. The resultsspecialized to the linear case of disks clamped at the center and free atthe periphery are in good agreement with those reported in the literature.The natural frequencies of spinning hard and floppy disks are obtained for various nodal diameters and nodal circles. Primary resonance is shown to occur at the critical rotational speed at which, in the linear analysis, the spinning disk is unable to support arbitraryspatially fixed transverse loads. Using the method of multiple scales, wedetermine a set of four nonlinear ordinary-differential equations governingthe modulation of the amplitudes and phases of two interacting modes. Thesymmetry of the system and the loading conditions are reflected in thesymmetry of the modulation equations. They are reduced to an equivalentset of two first-order equations whose equilibrium solutions aredetermined analytically. The stability characteristics of thesesolutions is studied; the qualitative behavior of the response isindependent of the mode being considered.     

Electrochemical properties of LaY2Ni9 hydrogen storage alloy,used as an anode in nickel-metal hydride batteries

The electrochemical properties of LaY₂Ni₉ alloy used as an anode in nickel-metal hydride batteries were investigated at ambient and at different temperatures. Several techniques, such as the galvanostatic charging and discharging, the constant potential discharge, and the potentiodynamic polarization, were applied to characterize these electrochemical properties. The discharge capacity of the LaY₂Ni₉ alloy increases to reach 258 mAh g^?1 after 5 cycles and decreases to 140 mAh g^?1 after 100 cycles then stabilizes around this same value indicating good cycling held. The hydrogen diffusion coefficient D _H in the bulky alloy is estimated to be (1.02?±?0.11)?×?10^?11 cm² s^?1 correlated with the good stability of electrochemical capacity after 100 cycles. The evolution of the ratio \( \frac{D_{\mathrm{H}}}{a^2} \) and the corrosion current density and potential are correlated with the evolution of the electrochemical capacity during the activation and for a long cycling. The enthalpy, the entropy, and the apparent activation energy of the LaY₂Ni₉ hydride formation are evaluated. The calculated results show that the enthalpy change, the entropy change, and the activation energy are (?42.64?±?1.08), (56.85?±?2.11), and (14.84?±?0.35)?kJ mol^?1, respectively.