Tailoring of the electrical and thermal properties using ultra-short period non-symmetric superlattices
Thermoelectric modules based on half-Heusler compounds offer a cheap and clean way to create eco-friendly electrical energy from waste heat. Here we study the impact of the period composition on the electrical and thermal properties in non-symmetric superlattices, where the ratio of components varies according to (TiNiSn)���:(HfNiSn)���������, and 0 ��� n ��� 6 unit cells. The thermal conductivity (��) showed a strong dependence on the material content achieving a minimum value for n = 3, whereas the highest value of the figure of merit ZT was achieved for n = 4. The measured �� can be well modeled using non-symmetric strain relaxation applied to the model of the series of thermal resistanc…
CADEM: calculate X-ray diffraction of epitaxial multilayers
This article presents a powerful yet simple program, based on the general one-dimensional kinematic X-ray diffraction (XRD) theory, which calculates the XRD patterns of tailor-made multilayers and thus enables quantitative comparison of measured and calculated XRD data. As the multilayers are constructed layer by layer, the final material stack can be entirely arbitrary.
Half-Heusler superlattices as model systems for nanostructured thermoelectrics
The efficiency of thermoelectric materials is directly related to the dimensionless figure of merit , therefore, one of the means to improve ZT is to reduce the thermal conductivity. Our research focuses on half-Heusler superlattices (SLs) and the relationship between the SL period and the thermal conductivity. The cross-plane thermal conductivity of DC-sputtered TiNiSn/HfNiSn SLs was measured by the 3 method at room temperature and a clear reduction of was achieved for all SL periods, in particular for periods smaller than 20 nm. Moreover, the thermal conductivities of TiNiSn and HfNiSn single films display reduced values compared to the literature data for bulk materials. Furthermore, we …
Alloy-like behaviour of the thermal conductivity of non-symmetric superlattices
In this work, we show a phenomenological alloy-like fit of the thermal conductivity of (A)d1:(B)d2 superlattices with d1 /= d2, i.e. non-symmetric structure. The presented method is a generalization of the Norbury rule of the summation of thermal resistivities in alloy compounds. Namely, we show that this approach can be also extended to describe the thermal properties of crystalline and ordered-system composed by two or more elements, and, has a potentially much wider application range. Using this approximation we estimate that the interface thermal resistance depends on the period and the ratio of materials that form the superlattice structure
Subamorphous thermal conductivity of crystalline half-Heusler superlattices
The quest to improve the thermoelectric figure of merit has mainly followed the roadmap of lowering the thermal conductivity while keeping unaltered the power factor of the material. Ideally an electron-crystal phonon-glass system is desired. In this work, we report an extraordinary reduction of the cross-plane thermal conductivity in crystalline (TiNiSn):(HfNiSn) half-Heusler superlattices (SLs). We create SLs with thermal conductivities below the effective amorphous limit, which is kept in a large temperature range (120–300 K). We measured thermal conductivity at room temperature values as low as 0.75 W m−1 K−1, the lowest thermal conductivity value reported so far for half-Heusler compou…