Phonon Bridge Effect in Superlattices of Thermoelectric TiNiSn/HfNiSn With Controlled Interface Intermixing

The implementation of thermal barriers in thermoelectric materials improves their power conversion rates effectively. For this purpose, material boundaries are utilized and manipulated to affect phonon transmissivity. Specifically, interface intermixing and topography represents a useful but complex parameter for thermal transport modification. This study investigates epitaxial thin film multilayers, so called superlattices (SL), of TiNiSn/HfNiSn, both with pristine and purposefully deteriorated interfaces. High-resolution transmission electron microscopy and X-ray diffractometry are used to characterize their structural properties in detail. A differential 3 ω -method probes their thermal resistivity. The thermal resistivity reaches a maximum for an intermediate interface quality and decreases again for higher boundary layer intermixing. For boundaries with the lowest interface quality, the interface thermal resistance is reduced by 23% compared to a pristine SL. While an uptake of diffuse scattering likely explains the initial deterioration of thermal transport, we propose a phonon bridge interpretation for the lowered thermal resistivity of the interfaces beyond a critical intermixing. In this picture, the locally reduced acoustic contrast of the less defined boundary acts as a mediator that promotes phonon transition.

No-Phonon and Phonon-Assisted Indirect Transitions in Si1−xGex Single Crystals

ABSTRACTThe absorption properties of high quality bulk Si1−xGex single crystals (0 ≤ × ≤ 0.16) are studied. Transmission measurements were performed at the indirect absorption edge. The shape of the absorption edge is characterized by structures attributed to the onset of no-phonon and phonon-assisted indirect optical transitions. Threshold energies have been determined using a differential method. A corrected calibration function for the dependence of the excitonic energy gap on composition is presented for the compositions, where x ranges from 0 to 0.16. The presented calibration function can be used for the composition analysis by an optical method. The analysis is independent of absolute transmission values.A simulation of the absorption spectra by calculations based on second order perturbation theory was used to fit the experimental data This procedure allows to estimate the oscillator strength of the no-phonon transition in dependence on x.

Nickel Related Absorption Lines in High-Pressure Synthetic Diamond

ABSTRACTDiamonds grown by the temperature gradient method using a nickel catalyst show an optical absorption band, extending from 2.5 eV to 2.56 eV, with a zero-phonon line at 2.51 eV. At 4 K this line exhibits fine structure; it is made up of 3 components at 2.509 eV, 2.510 eV and 2.511 eV. In absorption the fine structure components have the same relative intensity at different temperatures. We report on the effects of uniaxial stress on the zero-phonon transition at 77 K. We propose an interpretation of the results according to which the optical defect has a tetrahedral symmetry, and the 2.51 eV line occurs between two T2 states.