The preparation of composite materials is a promising methodology for concurrent optimization of electrical and thermal transport properties for improved thermoelectric (TE) performance. This study demonstrates how the acoustic impedance mismatch (AIM) and the work function of components decouple the TE parameters to achieve enhanced TE performance of the (1-z)Ge_0.87Mn_0.05Sb_0.08Te-(z)WC composite. The simultaneous increase in the electrical conductivity (σ) and Seebeck coefficient (α) with WC (tungsten carbide) volume fraction (z) results in an enhanced power factor (α²σ) in the composite. The rise in σ is attributed to the creation of favorable current paths through the WC phase located between grains of Ge_0.87Mn_0.05Sb_0.08Te, which leads to increased carrier mobility in the composite. Detailed analysis of the obtained electrical properties was performed via Kelvin probe force microscopy (work function measurement) and atomic force microscopy techniques (spatial current distribution map and current-voltage (I-V) characteristics), which are further supported by density functional theory (DFT) calculations. Furthermore, the difference in elastic properties (i.e., sound velocity) between Ge_0.87Mn_0.05Sb_0.08Te and WC results in a high AIM, and hence, a large interface thermal resistance (R_int) between the phases is achieved. The correlation between R_int and the Kapitza radius depicts a reduced phonon thermal conductivity (κ_ph) of the composite, which is explained using the Bruggeman asymmetrical model. Moreover, the decrease in κ_ph is further validated by phonon dispersion calculations that indicate the decrease in phonon group velocity in the composite. The simultaneous effect of enhanced α²σ and reduced κ_ph results in a maximum figure of merit (zT) of 1.93 at 773 K for (1-z)Ge_0.87Mn_0.05Sb_0.08Te-(z)WC composite for z = 0.010. It results in an average thermoelectric figure of merit (zT_av) of 1.02 for a temperature difference (ΔT) of 473 K. This study shows promise to achieve higher zT_av across a wide range of composite materials.

Because of the large Seebeck coefficient, low thermal conductivity, and earth-abundant nature of components, tetrahedrites are promising thermoelectric materials. DFT calculations reveal that the additional copper atoms in Cu-rich Cu₁₄Sb₄S₁₃ tetrahedrite can effectively engineer the chemical potential towards high thermoelectric performance. Here, the Cu-rich tetrahedrite phase was prepared using a novel approach, which is based on the solvothermal method and piperazine serving both as solvent and reagent. As only pure elements were used for the synthesis, the offered method allows us to avoid the typically observed inorganic salt contaminations in products. Prepared in such a way, Cu₁₄Sb₄S₁₃ tetrahedrite materials possess a very high Seebeck coefficient (above 400 μVK⁻¹) and low thermal conductivity (below 0.3 Wm⁻¹K⁻¹), yielding to an excellent dimensionless thermoelectric figure of merit ZT ≈ 0.65 at 723 K. The further enhancement of the thermoelectric performance is expected after attuning the carrier concentration to the optimal value for achieving the highest possible power factor in this system.

The influence of grain size and interface thermal resistance on thermal conductivity of PbTe–CoSb₃ polycrystalline composite.

Highly porous, In-filled CoSb₃ skutterudite materials with an attractive thermoelectric figure of merit (ZT ~ 1) and corresponding dense samples were fabricated through the cost-effective method of reduction in oxides in dry hydrogen and the pulsed electric current sintering (PECS) method, respectively. The reduction process was described in detail using in situ thermogravimetric analysis of Co₂O₃, Sb₂O₃ and In(NO₃)₃·5H₂O separately and in a mixture. Two methods to synthesise the same material were examined: (a) free sintering of an initially reduced powder and (b) PECS. The free-sintered materials with higher porosities (up to ~ 40%) exhibited lower values of electrical conductivity than the dense PECS samples (porosity up to ~ 5%), but the benefit of an even sixfold reduction in thermal conductivity resulted in higher ZT values. The theoretical values of thermal conductivity for various effective media models considering randomly oriented spheroid pores are in good agreement with the experimental thermal conductivity data. The assumed distribution and shape of the pores correlated well with the scanning electron microscope analysis of the microstructure. The lowest value of thermal conductivity, equal to 0.5 W/m K, was measured at 523 K for In_0.1Co₄Sb₁₂ with 41% porosity. The highest value of ZT_max = 1.0 at 673 K was found for the In_0.2Co₄Sb₁₂ sample in which the porosity was 36%. Graphic