Enhanced Thermoelectric Power and Electronic Correlations in RuSe2_2

We report the electronic structure, electric and thermal transport properties of Ru$_{1-x}$Ir$_{x}$Se$_2$ ($x \leq 0.2$). RuSe$_2$ is a semiconductor that crystallizes in a cubic pyrite unit cell. The Seebeck coefficient of RuSe$_2$ exceeds -200 $\mu$V/K around 730 K. Ir substitution results in the suppression of the resistivity and the Seebeck coefficient, suggesting the removal of the peaks in density of states near the Fermi level. Ru$_{0.8}$Ir$_{0.2}$Se$_{2}$ shows a semiconductor-metal crossover at about 30 K. The magnetic field restores the semiconducting behavior. Our results indicate the importance of the electronic correlations in enhanced thermoelectricity of RuSb$_{2}$.Comment: 6 pages, 4 figure

A data-driven model on the thermal transfer mechanism of composite phase change materials

Phase change materials (PCMs) that are incorporated with highly conductive nanomaterials to fabricate composite phase change materials (CPCMs), received much focus as a promising energy strategy for latent heat storage and conversion systems, due to their excellent thermophysical properties such as oxidation resistance and large enthalpies of fusion. However, the correct prediction of the thermal conductivity of these CPCMs remains deficient, mainly due to the lack of knowledge on the microscopic heat transfer mechanisms between the nanofiller and matrix interphase. Herein, a data-driven, modified Maxwell model is proposed to determine the thermal conductivity of these CPCMs, using milled carbon fiber (MCF)-reinforced PCMs as validation. This new model incorporates the aspect ratio and morphology smoothness of MCFs and introduces compatibility factors for different types of PCM matrices, which are paraffin and polyethylene glycol (PEG) respectively. At filler loadings above 15 wt%, the theoretical model gave poorer forecasts (with an average prediction error of 0.075) due to the random agglomeration of MCF nanoparticles, which can obstruct the phonon pathway. Regardless, this model accurately estimated the thermal conductivities of MCF/PCMs up to 9 wt% and 11 wt% MCF loading, with percentage fit values being 0.983 and 0.996 for PEG and paraffin systems, respectively. This model also eliminates the limitations of existing models, that were only suitable for composites with low filler loadings

Phase Diagram of KxFe2-ySe2-zSz and the Suppression of its Superconducting State by an Fe2-Se/S Tetrahedron Distortion

We report structurally tuned superconductivity in KxFe2-ySe2-zSz (0<=z<=2) phase diagram. Superconducting Tc is suppressed as S is incorporated into the lattice, eventually vanishing at 80% of S. The magnetic and conductivity properties can be related to stoichiometry on poorly occupied Fe1 site and the local environment of nearly fully occupied Fe2 site. The decreasing Tc coincides with the increasing Fe1 occupancy and the overall increase in Fe stoichiometry from z = 0 to z = 2. Our results indicate that the irregularity of Fe2-Se/S tetrahedron is an important controlling parameter that can be used to tune the ground state in the new superconductor family.Comment: 5 pages, 4 figure