The amorphous Mg0.9Ti0.1Ni1?xPdx (x=0, 0.05, 0.1, 0.15) hydrogen storage alloys were prepared by mechanical alloying. The hydrogen desorption kinetics of the electrode alloys were studied by potentiostatic discharge experiments and linear polarization measurements. The experimental results show that the three-dimensional diffusion model dominates the hydrogen desorption process of the electrode alloys. The rate constants of hydrogen desorption reaction, which are obtained from a linear fitting of the model equation, increase with temperature. The activation energies of hydrogen desorption were calculated according to the Arrhenius equation. The calculated values were 46.2, 24.29, 33.4 and 34.95 kJ/mol for x=0, 0.05, 0.1 and 0.15 of Mg0.9Ti0.1Ni1?xPdx (x=0, 0.05, 0.1, 0.15) electrode alloys, respectively. The exchange current densities were determined by the linear polarization experiments. The variation of exchange current densities with Pd content in the alloy electrodes agrees with that of activation energies with Pd content.
Disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O) is an attractive candidate for phase change materials. The main problem for its practical use comes from incongruent melting character during thermal cycling. Experimentally, heat of fusion of the pure salt decreased from 200 to 25 jog 1 in a four-run freeze-thaw cycling. Additives such as thickening agent or in-situ synthesized polyacrylate sodium in the molten salt can prevent its phase separation to some extent. In the test, sodium alginate 3.0%-5.0% (w/w) thickened mixture containing Na2HPOn·12H2O and some water showed constant heat storage capacities. Polyacrylate sodium gelled salt was synthesized through polymerizing sodium acrylate in the melt of Na2HPOn·12H2O and some extra water at 50 ℃. Optimum conditions composed of sodium acrylate 3.0%-5.0% (w/w), cross-linking agent N,N-methylenebis-acrylamide 0.10%-0.20% (w/w), K2S208 and Na2SO3 (mass ratio 1 ; 1) 0.06%-0.12% (w/w). As opposed to normal large crystals of pure Na2HPOn·12H2O in solid state, the gelled salt existed in a large number of tiny particles dispersed in the gel network at room temperature, commonly less than 2 mm. But only those sample particles with sizes less than 0.2 mm may have relatively stable thermal storage property. A problem encountered was the poor reproducibility of the synthesis method: heat storage capacity of the product was often very different even though the synthesis was carried out in the same conditions. An alternative gelling method by sodium alginate grafted sodium acrylate was tried and it showed a fairly good effect. Heat capacities and heat of fusion of Na2HPO4·12H2O were measured by an adiabatic calorimeter.