The temperature dependences of both heat capacity and thermal conductivity in nanolamellar Cr₂AlC single crystals are measured using modulated photothermal radiometry and compared to first-principles calculations. The electronic contribution to the thermal conductivity of Cr₂AlC single crystals is computed ab initio by determining the electronic transport coefficients using density functional theory and by solving the Bloch–Boltzmann transport equation with a temperature-dependent relaxation time. The lattice thermal conductivity is predicted by going beyond the quasi-harmonic approximation and considering renormalized second- and third-order force constant matrices, with anharmonic three-phonon scattering, isotopic scattering, and scattering by carbon vacancies. Isotopic scattering does not modify the lattice thermal conductivity. In contrast, even a small concentration of carbon vacancies induces a substantial decrease of the in-plane lattice thermal conductivity. The anisotropy measured in the thermal conductivity, with a ratio of ∼2 over the whole temperature range, is confirmed theoretically. This anisotropy seems to mainly arise from lattice contributions. A similar anisotropy is expected for other MAX phases with identical layered structures.

中文翻译：

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高温下Cr ₂ AlC单晶的热容和各向异性导热系数

纳米层状Cr ₂ AlC单晶中热容量和热导率的温度依赖性是使用调制光热辐射法测量的，并与第一性原理计算进行了比较。从头计算电子对Cr ₂ AlC单晶导热性的贡献通过使用密度泛函理论确定电子输运系数，并通过求解与温度相关的弛豫时间的布洛赫-玻尔兹曼输运方程。通过超越准谐近似，并考虑重新归一化的二阶和三阶力常数矩阵，非谐三声子散射，同位素散射和碳空位散射，可以预测晶格热导率。同位素散射不会改变晶格的热导率。相反，即使浓度很小的碳空位也会引起面内晶格热导率的大幅降低。理论上证实了在整个温度范围内，导热系数的各向异性约为2。这种各向异性似乎主要来自晶格贡献。对于具有相同分层结构的其他MAX相，预计会有类似的各向异性。