Heat dissipation is one of the most important issues for the reliability of electronic equipment. Boiling can be a very efficient heat transfer mechanism when used to face with the electronic technology needs of efficient and compact heat sinks. Recently, cellular structured materials both stochastic and periodic, particularly open cell metal foams, have been proposed as possible enhanced surfaces to lower the junction temperatures at high heat fluxes. Up today, most of the research on metal foams only regards single phase flow, whereas the two phase flow is still almost unexplored. This paper presents an experimental study on the heat transfer of R134a during flow boiling inside a 5 PPI (Pores Per linear Inch) copper foam, which is 5 mm high, 10 mm wide and 200 mm long, and it is brazed on a 10 mm thick copper plate. The experimental measurements were carried out by imposing three different heat fluxes (50, 75, and 100 kW m -2 ) and by varying the refrigerant mass velocity between 50 and 200 kg m -2 s -1 and the vapour quality from 0.2 to 0.90, at constant saturation temperature (30°C). The effects of the refrigerant mass flow rate, heat flux and vapour quality on the heat transfer coefficient, dry out phenomenon, and pressure drop are studied.

The thermoelectric properties of Mo and Ge co-substituted CrSi 2 have been investigated. Polycrystalline samples of (Cr 1−x Mo x )Si 2 (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.10) and (Cr 0.92 Mo 0.08 )(Si 1−y Ge y ) 2 (y = 0, 0.01, and 0.02) were prepared by the two-step arc-melting method followed by spark plasma sintering and annealing. Powder X-ray diffraction confirmed that single-phase (Cr 1−x Mo x )(Si 1−y Ge y ) 2 samples were obtained. Mo and Ge co-substitution in CrSi 2 resulted in both an increase in the power factor and a large decrease in the thermal conductivity, which resulted in a high figure-of-merit (zT) value for the (Cr 0.92 Mo 0.08 )(Si 0.99 Ge 0.01 ) 2 sample (zT = 0.29 at 700 K). This zT value is the highest reported value for CrSi 2 -based thermoelectric materials.

Efficiently decreasing the lattice thermal conductivity, κL, is one of the main concerns in the field of thermoelectrics (TE). Herein, we theoretically investigate κL for single-crystal and polycrystalline CrSi2 using first-principles and the phonon Boltzmann transport equation. Though CrSi2 is known as a potential TE material because of its reasonable power factor, controlling its κL remains as a challenge to be solved. In this study, we discuss how to decrease κL efficiently on the basis of the calculation. The phonon band structure and density of states are computed via harmonic calculation. In addition, the achievable lowest lattice thermal conductivity, κL0, and cumulative lattice thermal conductivity, κcum, are estimated using the Cahill model and anharmonic calculation, respectively. We predict κL0 for CrSi2 to be around 2.2Wm−1K−1 at 650 K, which suggests that CrSi2 is a potential TE material with high zT over 0.39 at 650 K. The phonon mean-free path dependence of κcum indicates that the critical crystallite size for decreasing κL for polycrystalline CrSi2 is 70 nm at 600 K. In addition, it is revealed that the crystallite size should be as small as 7 nm to decrease κL to half. These calculational findings offer useful insights into how to control κL for CrSi2.

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