Debye model

About: Debye model is a research topic. Over the lifetime, 7462 publications have been published within this topic receiving 133987 citations.

First-principles study of elastic, electronic, thermodynamic, and thermoelectric transport properties of TaCoSn

Abstract: In this paper, we have performed a comprehensive set of first-principles calculations to study elastic, electronic, thermodynamic and thermoelectric properties of TaCoSn using density functional theory (DFT). Half-heusler, TaCoSn has been found to be elastically and thermodynamically stable, ductile and hard material. The Debye temperature of TaCoSn has been found to be 375.39 K. The calculated energy bands indicate that TaCoSn is an indirect band gap semiconductor and the value of gap is 1.107 eV using PBE functional and it is 1.153 eV by TB-mBJ potentials. Such small increase of band gap by TB-mBJ potential has no significant effect on the transport properties of TaCoSn. In TaCoSn, no significant spin-orbit interaction is found. The density of states at the Fermi energy is dominated by Ta-5d and Co-3d orbitals due to strong hybridization between them. We also calculate the relaxation time and lattice thermal conductivity. The lattice thermal conductivity of TaCoSn (4.95 W/mK at 300 K) is relatively small than that of other half-heusler compounds. The maximum Seebeck coefficient at 500 K is 249.41 μV/K. The obtained power factor (S2σ/τ) at 600 K is ∼12.5 × 1011 W/msK2. The calculated maximum figure of merit (ZT) is 0.73 at 600 K indicates that TaCoSn is a promising material for thermoelectric device applications.

Heat capacity and phonon mean free path of wurtzite GaN

Abstract: The authors report lattice specific heat of bulk hexagonal GaN measured by the heat flow method in the temperature range of 20–300K and by the adiabatic method in the range of 5–70K. The best fit with the accuracy of 3% was obtained for the temperature-independent Debye temperature ΘD=365K and Einstein temperature ΘE=880K. The authors relate these temperatures to the function of density of states. Using their results for heat conduction coefficient, they established in the temperature range of 10–100K the explicit dependence of the phonon mean free path on temperature lph∝T−2. Above 100K, there is an evidence of contribution of the Umklapp processes, which limits phonon free path at high temperatures.

Heat capacity of α-GaN : Isotope effects

Abstract: Until recently, the heat capacity of GaN had only been measured for polycrystalline powder samples. Semiempirical as well as first-principles calculations have appeared within the past few years. We present in this article measurements of the heat capacity of hexagonal single crystals of GaN in the $20--1400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ temperature range. We find that our data deviate significantly from the literature values for polycrystalline materials. The dependence of the heat capacity on the isotopic mass has also been investigated recently for monatomic crystals such as diamond, silicon, and germanium. Multiatomic crystals are expected to exhibit a different dependence of these heat capacities on the masses of each of the isotopes present. These effects have not been investigated in the past to our knowledge. We also present first-principles calculations of the dependence of the heat capacities of GaN, as a canonical binary material, on each of the Ga and N masses. We show that they are indeed different, as expected from the fact that the Ga mass affects mainly the acoustic, that of N the optic phonons. It is hoped that these calculations will encourage experimental measurements of the dependence of the heat capacity on isotopic masses in binary and more complex semiconductors.