Alloy heat transport properties

The summary of the structural, thermodynamic and transport properties determined by various authors (as discussed above) has been shown in Tables 12.3,12.4 and 12.5, respectively. The lattice constant values calculated by different authors for GaN have been observed to be in good agreanent. The same has been found to be true for the AIN alloy also. However, the room temperature linear thermal expansion coefficient values for GaN vary widely. Unlike GaN, the expansivities for the AIN alloy, as determined by different studies, are in good agreement and the values for the other binary alloys are of similar order. The isochoric heat capacity, Cy, values show that these alloys follow the Dulong-Petit law for solids whereby at high tanperatures, C 3/ . It should also be noted that the diffusion coefficients for the binary nitrides have similar values at room temperature. 

This resonance at ks K provides a partial explanation for the surprising temperature dependence of equilibrium and transport properties of dilute alloys and mixed-valent systems. The important conclusion emerging from the relevant theory (Bickers et al. 1985, Allen et al. 1986) is that the low-temperature susceptibility and specific heat reflect a greatly enhanced density of states proportional to IjT, not l/ p. Since these arguments are based on solution of the single-impurity Anderson model, they cannot be expected to apply a priori to concentrated systems. Nonetheless, for temperatures T > the single-impurity results, scaled by the number of impurities, seem to work for most properties. The most prominent deviation at low temperatures occurs for the resistivity, which Bloch s theorem forces to zero whereas the single-impurity result saturates (Lee et al. 1986). 

Loop Tests Loop test installations vary widely in size and complexity, but they may be divided into two major categories (c) thermal-convection loops and (b) forced-convection loops. In both types, the liquid medium flows through a continuous loop or harp mounted vertically, one leg being heated whilst the other is cooled to maintain a constant temperature across the system. In the former type, flow is induced by thermal convection, and the flow rate is dependent on the relative heights of the heated and cooled sections, on the temperature gradient and on the physical properties of the liquid. The principle of the thermal convective loop is illustrated in Fig. 19.26. This method was used by De Van and Sessions to study mass transfer of niobium-based alloys in flowing lithium, and by De Van and Jansen to determine the transport rates of nitrogen and carbon between vanadium alloys and stainless steels in liquid sodium. 

The Al-6Cu-Mn alloy differs from other aluminum alloys in that it has relatively good mechanical properties at low temperatures and, therefore, can be successfully used for low-temperature structures. Unlike the duralumin-type alloys, the Al-6Cu-Mn heat-treatable alloy contains more copper (5.8 to 6.8%) and no magnesium [ ]. It is similar to the USA alloy 2219. It has satisfactory weldability and mechanical properties over a wide range of temperatures from 20 K up to 523 K, making it suitable for welded vessels for liquefied gas transportation and storage. 

Knowing the excitation spectrum one can compute the thermodynamic properties. In the local-moment regime they exhibit low-temperature T 7 ) Kondo anomalies that are due to the resonance states. For example, the static magnetic susceptibilty x(T), the specific heat, various transport coefficients and also dynamical quantities (photoemission spectra, dynamical structure function for neutron scattering) have been calculated (Bickers et al. 1985, Cox et al. 1986). An excellent model system for comparison with experimental data are the dilute (La, Ce)Bg alloys because of a fourfold degenerate Fg ground state of cerium (Zirngiebl et al. 1984). 

This reaction is currently unavoidable and appears to be favored at hot and dry operating conditions of the fuel cell. The peroxide decomposition forms reactive radials such as hydroxyl, OH, and peroxyl, OOH, that cause oxidative degradation of both the fuel cell membrane and catalyst support [67]. Both electrodes currently use Pt or Pt alloys to catalyze both the HOR and ORR reactions. The catalyst particles are typically supported on a high surface area, heat-treated carbon to both increase the effectiveness of the catalyst and to provide a path for the electrons to pass through to the external circuit via the gas diffusion media (which is typically also made of carbon) and the current collecting bipolar plates. In addition, the catalyst particles are coated in ionomer to facilitate proton transport however, the electrode structure must also be porous to facilitate reactant gas transport. A schematic of a typical PEM MEA is shown in Fig. 17.1. A boundary condition exists at the catalyst particle where protons from the ionomer, electrons from the electrically conducting Pt and carbon, and reactant gases meet. This is usually referred to as the three-phase boundary. The transport of reactants, electrons, and protons must be carefully balanced in terms of the properties, volume, and distribution of each media in order to optimize operation of the fuel cell. 

---