Isotherms pressure composition

Figure 1. Ideal pressure-composition isotherms showing the hydrogen solid-solution phase, a, and the hydride phase, j3. The plateau marks the region of coexistence of the a and fl phases. As the temperature is increased the plateau narrows and eventually disappears at some consolule temperature...

There are few systematic guidelines which can be used to predict the properties of AB2 metal hydride electrodes. Alloy formulation is primarily an empirical process where the composition is designed to provide a bulk hydride-forming phase (or phases) which form, in situ, a corrosion— resistance surface of semipassivating oxide (hydroxide) layers. Lattice expansion is usually reduced relative to the ABS hydrides because of a lower VH. Pressure-composition isotherms of complex AB2 electrode materials indicate nonideal behaviour. 

A range of different methods measures the solubility of hydrogen in metals and alloys. Manometric methods [98] and gas volumetric methods [99] have been used to determine pressure-composition isotherms at selected temperatures for a range of alloys [100-103],... 

Fig. 6. Pressure-composition isotherms at 300°C for MgH2 with ceramic catalyst addition.

The Li-Mg-B-N-H structure possesses storage capacity of more than 10 wt.% at around 150-200 "C. However, the reversibility of the hydrogen sorption characteristics was determined using pressure-composition isotherms as shown in Figure 6. From this study, we found reversible hydrogen absorption-desorption behavior (-3-4 wt.%) of the new complex hydride Li-Mg-B-N-H. The improvement in temperature and reversible hydrogen storage capacity were unaffected even after ten... 

Figure 6. Pressure-composition isotherms of Li-Mg-B-N-Fl prepared by different synthesis procedures...

Figure 5.23 Pressure composition isotherms for critical temperature 7. The construction of the hydrogen absorption in atypical metal (left). The van t Hoff plot is shown on the right. The slope of solid solution (a-phase), the hydride phase the line is equal to the enthalpy of formation (p-phase) and the region ofthe coexistence ofthe divided by the gas constant and the intercept with two phases. The coexistence region is the axis is equal to the entropy of formation...

The thermodynamic aspects of hydride formation from gaseous hydrogen are described by means of pressure-composition isotherms in equilibrium (AG = 0). While the solid solution and hydride phase coexist, the isotherms show a flat plateau, the length of which determines the amount of H2 stored. In the pure P-phase, the H2 pressure rises steeply vfith increase in concentration. The two-phase region ends in a critical point T, above which the transition from the a- to the P-phase is continuous. The equilibrium pressure peq as a function of temperature is related to the changes AH° and AS° of enthalpy and entropy ... 

Figure 5.36 Pressure-composition isotherms for NaAIH4 and Na3AIH6,2-...

Figure 5.55 Examples of bcc alloys and corresponding pressure-composition isotherms. The bcc structure offers three times more interstitial sites than the fee and hep structures.

Absorption pressure-composition isotherms were established by metering fixed amounts of hydrogen into the system and determining the pressure after equilibrium. In each case, the sample chamber was isolated before hydrogen... 

Figure 1. Pressure-composition isotherms for the Dy2Co7-H system. Hydrogen concentration is expressed as atoms of H per formula unit of Dy2Co7. X designates absorption O designates desorption.

Typical pressure-composition isotherms for the LaNi5-H2 system are shown in Figure 2 (14). From plots of In Pplateau vs- 1 / T, several workers determined the experimental heats and entropies of Reaction 1. The enthalpies are the heats of formation for the /J-phase hydride from the a-phase hydrogen-saturated alloy (see Table I). 

The experimental solubility data for solid naphthalene in supercritical carbon dioxide, given as moles naphthalene dissolved per liter, are shown in Figure 6. Qualitatively the three pressure-composition isotherms show characteristic behavior for a solid-supercritical fluid system. Each isotherm initially shows a large increase in solubility with increasing pressure, and then a limiting value is reached at higher pressures. 

What determines whether nonstoichiometric behavior can readily be observed is largely the form of the dissociation pressure-composition isotherms in the equilibrium diagram. If the free energy rises too steeply with excess of either component, it may not be practicable to detect stoichiometric changes which, in principle, are taking place. The explicit relation between the stoichiometric defect (= x for the composition MOx+a.) and the free energy is complicated and awkward to work... 

In a thermodynamic treatment of nonstoichiometry in hydrides, assuming the solid solution of metal in the metal hydride, Messer (34) obtained theoretical pressure-composition isotherms using relations between the activity coefficients and mole fractions of the components of the solution. By extending Messer s... 

Figure 3. Pressure-composition isotherms for uranium-hydrogen system...

Figure 12 Hydrogen pressure-composition isotherms for the FeTi-hydrogen system. (Reprinted with permission from Ref. 58. 2003 American Chemical Society)...

Complex metal hydride solid-state compound containing homoleptic, anionic metal-hydrogen complexes d, d, d etc electron configuration of transition element h.H enthalpy of hydride formation as determined from pressure-composition isotherms during desorption by using van t Hoff s method... 

From elements by sintering at 753 773 K under 40-50 bar hydrogen pressure black powder, stable in air nonmetal-lic pressure-composition isotherms show two plateaus of... 

Figure 4.1 (a) Schematicof a pressure-composition isotherm, a isthe solid solution of hydrogen... 

Figure 7.4 Pressure-composition isotherms of annealed (Mgo.esC o.ttlNit-H [37].

As discussed above, due to the absence of any stable compound in the dehydrogenated state and the formation of a rather stable hydride the desorption enthalpy is not decreased compared to the original single alkali composed hydrides. Fossdal et al. [58] measured pressure-composition isotherms for TiF3-enhanced material in the temperature range 170-250 °C. From the van t Floff plot they determined a dissociation enthalpy of 56.4 0.4kJ (mol Fl2) and a corresponding entropy of 137.9 0.7 (mol H2). The respective value of the reaction enthalpy of reaction (7.4) is 47 kj (mol H2) [19]. 

Figure 7.12 Pressure-composition isotherms for the system, IVIgjAl...

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