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The flash methods, which are the most popular transient methods for measuring the thermal diffusivity of solid materials, have evolved into ultrafast laser flash methods by using picosecond or nanosecond pulse lasers as a heating source and a thermo-reflectance technique such as high-speed thermometry. In conventional ultrafast laser flash methods, thermal diffusivity is determined by fitting an analytical equation after single pulse heating to observe thermo-reflectance signals, although actual thermo-reflectance signals are observed after periodic pulse heating. This paper presents an exact analytical solution of the temperature response expressed by Fourier series for one-dimensional heat diffusion after periodic pulse heating. These Fourier coefficients are directly related to the Laplace transformation of the temperature response after single pulse heating. The signal observed for a 100 nm thick platinum thin film on a fused quartz substrate was analyzed by this Fourier expansion analysis and fitted by analytical equations with three parameters: heat diffusion time across thin film, the ratio of heat effusion of the substrate to thin film, and the amplitude of the signal over the entire range of pulse interval in the time domain. Robustness in determining the thermal diffusivity of the thin film by the ultrafast laser flash method can be improved by this new analysis approach. [ABSTRACT FROM AUTHOR]

Understanding the electronic origin of good thermoelectric (TE) transport properties is a crucial step to design and discover new TE materials. Mg3Sb2 is an intensely researched thermoelectric material, the high TE performance of which comes from six-fold degenerate conduction band minimum at low symmetry k points, leading simultaneously to a high Seebeck coefficient and a good carrier mobility. Furthermore, upon alloying, its low-lying conduction band structure changes, which improves or deteriorates the TE transport properties. Although extensively studied experimentally, the electronic origin of these changes is not well understood. Based on the first-principles derived Wannier function tight-binding analysis of both pristine Mg3Sb2 and its Bi/Ca alloyed derivatives, we reveal that the simple Mg s–Sb p orbital interactions are not adequate and sufficient to describe the electronic structure of Mg3Sb2. Inclusion of both higher energy Mg p orbitals and lower energy Sb s orbitals is demonstrated to be important to understand the shape of the lowest conduction band and its change upon alloying. The systematic chemical understanding developed in this work could enable us to better control the conduction band dispersion by manipulating chemical interactions in Mg3Sb2. [ABSTRACT FROM AUTHOR]

Zintl phase Mg3(Bi,Sb)2-based materials have garnered significant interest in recent times due to their potential in room-temperature thermoelectric applications. Recently, bulk n-type Mg3(Bi,Sb)2 alloy materials with high thermoelectric performance have been extensively studied. However, achieving reliable n-type conduction in thin films of these materials remains a challenge due to the formation of negatively charged Mg vacancies. Herein, we report the successful growth of n-type Mg3Bi2−xSbx based composite films by incorporating additional Mg through co-sputtering. The impact of sputtered power and deposition temperature on the thermoelectric properties and conduction behavior of single-sputtered thin films was thoroughly investigated. Experimental results, including X-ray diffraction analysis and transport properties characterization, reveal that increasing sputtering power and deposition temperature yield p-type Bi-rich Mg3(Bi,Sb)2 thin films. Besides, n-type conduction is achieved by co-sputtering with Mg by forming a Mg-rich region within the Mg3Bi2−xSbx thin film. The resulting Mg3Bi2−xSbx composite film demonstrates a high Seebeck coefficient of −375 μV K−1 at 373 K, leading to a maximum power factor of 132 μW m−1 K−2 at the same temperature. Our research examines the key role of Bi or Mg concentration in finely tuning the conduction properties of Mg3Bi2−xSbx-based thin films. This progressive-edge investigation promises to unlock new avenues for the utilization of Mg3(Bi,Sb)2 thin films in developing low temperature thermoelectric applications. [ABSTRACT FROM AUTHOR]

Developing thermoelectric (TE) performance is impeded by the compromise of TE parameters, resulting in inadequate conversion efficiency of heat to electricity. Herein, this work reports that Mo is a particularly effective additive in Mg3Sb2‐based alloys with significantly improved electronic transport via grain‐boundary engineering and band‐structure regulation synergy. In addition, phonon transport is simultaneously suppressed by employing multiple effects, lattice imperfection scattering, reduced phonon group velocity, and enhanced atomic disorder, leading to a minimum κlat = 0.52 W m−1 K−1 at 723 K. As a result, an outstanding ZT peak of ≈1.84 at 723 K and ZTav = 1.34 within 323–723 K are achieved in Mg3Sb2‐based alloys, and the corresponding fabricated single‐leg TE module shows an exceptionally high conversion efficiency of ≈12% under a hot‐side temperature of 450 °C. These results demonstrate the great potential for advancing mid‐temperature heat harvesting in Mg3Sb2‐based materials. [ABSTRACT FROM AUTHOR]