Specific heat hydrides

Whether rotation-vibration transfer occurs, and how important it is, are questions of considerable dispute. The experimental observation by Millikan106,107, that vibrational deactivation of CO in collision with p-H2 is more than twice as efficient as in collision with o-H2, seems to provide some evidence that rotational energy participates in vibrational relaxation. The only significant difference between o- and p-H2 in the context of this experiment would appear to be the difference in rotational energy states, as illustrated by the fact that at 288 °K (the temperature of the experiment) the rotational specific heat of o-H2 is 2.22, while that of p-H2 is 1.80 cal.mole-1.deg-1. Cottrell et a/.108-110 have measured the vibrational relaxation times of a number of hydrides and the corresponding deuterides. On the basis of SSH theory for vibration-translation transfer the relaxation times of the deuterides should be systematically shorter than those of the hydrides. The... 

Procedure of creation of the heat machine based on periodic circulation of hydrogen and increase in the efficiency its operation demands the detailed information on methods of calculation equilibrium P-C-T (pressure - concentration - temperature) of characteristics, thermodynamic, thermalphysic (factors of specific heat conductivity X and heat transfers a depending on temperature and pressure) and kinetic properties of hydrides. Approach to designing HHP as to an individual kind of HHM can be broken on three part [1] ... 

Duration of a cycle of HHP operation is defined as time required for reaction hydrogenation/dehydrogenation in pair hydride system. This time determines heat capacity of HHP. Duration of a cycle depends on kinetics of hydrogenation reactions, a heat transfer between the heated up and cooling environment, heat conductivities of hydride beds. Rates of reactions are proportional to a difference of dynamic pressure of hydrogen in sorbers of HHP and to constants of chemical reaction of hydrogenation. The relation of dynamic pressure is adjusted by characteristics of a heat emission in beds of metal hydride particles (the heat emission of a hydride bed depends on its effective specific heat conductivity) and connected to total factor of a heat transfer of system a sorber-heat exchanger. The modified constant of speed, as function of temperature in isobaric process [1], can characterize kinetics of sorption reactions. In HHP it is not sense to use hydrides with a low kinetics of reactions. The basic condition of an acceptability of hydride for HHP is a condition of forward rate of chemical reactions in relation to rate of a heat transmission. 

Works on increase of an overall performance of HHP were simultaneously carried out. For example, in [2] a number of the factors influencing specific output power of HHP has been considered. Properties of metal hydrides (absorbing ability, speeds of reactions, porosity of a covering, the characteristic of a heat transmission of a hydride bed) were analyzed for optimum selection. It has been shown that in pressings from powder metal hydrides gas permeability and effective specific heat conductivity of a bed Xes should be in common optimized in the certain range of a weight share of an additional heat-conducting material. 

Thermal conductivity and specific heat are properties that should be considered in heat exchanger design of hydride beds. 

Specific heats of metals and hydrides are easily determined and typically fall in the range of 0.1-0.2 cal/g°C. Thermal conductivity is a little more difficult to determine. The conductivity of the metal or hydride phase is not sufficient the effective conductivity of the bed must be determined. This depends on alloy, particle size, packing, void space, etc. Relatively little data of an engineering nature is now available and must be generated for container optimization. Techniques to improve thermal conductivity of hydride beds are needed. As pointed out earlier, good heat exchange is the most important factor in rapid cycling. 

Hydrides. Magnetic susceptibility data for CrH at 1.4—100 K have been presented. Results were compared with specific heat data for the same sample at 2—10 K. CrHo 97 is paramagnetic with a susceptibility higher than that of pure Cr and strongly increasing at low temperatures. ... 

As noted above, analysis of the kinetic energy dependence of reactions (6) to (8) allows the sums of the product heats of formation to be determined. To derive heats of formation for specific silicon hydride radicals and ions, additional information must be employed. Several additional studies have provided information on SiH, SiH+, and SiHj that can be combined with the results for reactions (6) to (8) to provide a complete set of data. In most cases, the thresholds determined in the original work are adjusted to 0-K values, as discussed in Section II.D.3. Conversion between 0- and 298-K values uses the thermodynamic information in Boo and Armentrout (1987). These results are listed in Table 1 and reviewed in the following sections. 

Thermodynamic properties of rare earth hydrides from specific heat measurements at... 

A number of properties of the rare earth hydrides have been studied by determining the temperature variation of their specific heat. Bieganski et al. 

This section is concerned mainly with crystallographic and magnetic properties. In contrast to the vast amount of experimental work done on these properties, relatively little has been published on other properties such as transport properties. The few examples of specific heat measurements are not included in this section as they have already been discussed at more relevant places elsewhere in this review (see section 4.3). Since the NMR investigations performed on the ternary hydride dealt mainly with the motion of the H atoms, these properties were discussed in the section on diffusion (section 3.6). 

---