Hydrogen Storage Characteristics of Commercial Mg and MgH

In our laboratory, absorption experiments were carried out on an unmiUed but activated ABCR MgHj powder. In the preliminary stage, the powder was heated up to 350°C in a volumetric Sieverts-type apparatus (Sect. 1.4.2) under a preventing pressure of 3.5 MPa (so as not to allow desorption), held for about 30-45 min to stabilize temperatnre, and then snbjected to the following desorption/absorption cycles  

Fernandez and Sanchez [18] investigated the kinetics of hydrogen absorption and desorption by activated magnesium powder (several cycles of hydrogen absorp-tion/desorption at 375°C) using a volumetric technique. They pointed out that for-mation/decomposition of metal hydrides comprises a number of steps taking place in series transport to the surface, dissociation, H chemisorption, surface- 

Karty et al. [21] pointed out that the value of the reaction order r and the dependence of k on pressure and temperature in the JMAK (Johnson-Mehl-Avrami-Kolmogorov) equation (Sect. 1.4.1.2), and perhaps on other variables such as particle size, are what define the rate-limiting process. Table 2.3 shows the summary of the dependence of p on growth dimensionality, rate-limiting process, and nucleation behavior as reported by Karty et al. [21]. 

The absorption curves in Fig. 2.4a were analyzed by a linear fitting to the JMAK equation from which the reaction rate constant, k, and the reaction order,p, can be determined. The values of the reaction order p are listed in Table 2.4. At the lowest absorption temperature of 250°C, the p parameter is close to 2, and then it decreases to about 1, remaining close to this value at all the other temperatures. The different values of the reaction order suggest that different mechanisms are controlling the rates at various temperature ranges of absorption. It can be seen from Table 2.3, that the value of 2 (1.83 in Table 2.4) suggests that the transformation of Mg to 

The apparent activation energy of decomposition estimated from the Arrhenius plot for gave 120 kJ/mol. Conversely, Stander noticed that if the difference between the experimental dissociation pressures (e.g., 384 kPa) and the equilibrium (plateau) dissociation pressure corresponding to T = const (e.g., 404 kPa at T = 335°C for MgH ) is relatively small, then better fits were obtained with the model of random nucleation followed by one-dimensional growth or instantaneous nucleation followed by two-dimensional growth as given by the equation  

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