Enhanced Thermoelectric Performance in Oxide Composites of La and Nb Codoped SrTiO3 by Using Graphite as the Electron Mobility Booster

Enhanced Thermoelectric Performance in the SrTi0.85Nb0.15O3 Oxide Nanocomposite with Fe2O3-Functionalized Graphene

Doped SrTiO3 is considered one of the potential thermoelectric (TE) candidates but its TE figure of merit, ZT needs to be improved for practical application of electricity generation from high-grade waste-heat. In the present work, enhanced TE performance has been realized for SrTi0.85Nb0.15O3 (STN) perovskite adopting the strategy of composite formation with Fe2O3-functionalized graphene (FGR). We have achieved a maximum electrical conductivity of 1.4 × 105 S m-1 for 1 wt % FGR added to STN, which is around 1185% larger than that of pristine STN. The presence of FGR in the STN matrix acts as a mobility booster of electrons, overcoming the effect of Anderson localization of electrons, which impedes the electron transport in STN. This is evident from the order of magnitude increase in weighted mobility of STN after FGR addition. Furthermore, the incorporation of FGR causes about a 34% decrease in the lattice thermal conductivity. The Debye-Callaway model demonstrates that the phonon-phonon Umklapp scattering is primarily responsible for reduced thermal conductivity. The presence of FGR sheets along the grain boundaries of STN, Fe2O3 nanoparticles, and lattice imperfections gives rise to the glass-like temperature-independent phonon mean-free-path, especially above Debye temperature. The maximum ZT ∼ 0.57 has been obtained at 947 K for the 1 wt % FGR sample, which is around 420% higher than that of pristine STN. Furthermore, we have fabricated a prototype of a four-legged n-type TE module, demonstrating one of the highest power outputs of 18 mW among reported oxide thermoelectrics.

Enhanced Thermoelectric Performance of Rare-Earth-Free n-Type Oxide Perovskite Composite with Graphene Analogous 2D MXene

Here, the first experimental demonstration on the effect of incorporating new generation 2D material, MXene, on the thermoelectric performance of rare-earth-free oxide perovskite is reported. The charge localization phenomenon is predominant in the electron transport of doped SrTiO₃ perovskites, which deters from achieving a higher thermoelectric power factor in these oxides. In this work, it is shown that incorporating Ti₃ C₂ T_x MXene in a matrix of SrTi_0.85 Nb_0.15 O₃ (STN) facilitates the delocalization of electrons resulting in better than single-crystal-like electron mobility in polycrystalline composites. A 1851% increase in electrical conductivity and a 1000% enhancement in power factor are attained. Besides, anharmonicity caused by MXene in the STN matrix has led to enhanced Umklapp scattering giving rise to lower lattice thermal conductivity. Hence, 700% ZT enhancement is achieved in this composite. Further, a prototype of thermoelectric generator (TEG) using only n-type STN + MXene is fabricated and a power output of 38 mW is obtained, which is higher than the reported values for oxide TEG.

Designing rare earth-free high entropy oxides with a tungsten bronze structure for thermoelectric applications

In recent years, forming high entropy oxides has emerged as one of the promising approaches to designing oxide thermoelectrics. Entropy engineering is an excellent strategy to improve thermoelectric performance by minimizing the thermal conductivity arising from enhanced multi-phonon scattering. In the present work, we have successfully synthesized a rare-earth-free single phase solid solution of novel high entropy niobate (Sr_0.2Ba_0.2Li_0.2K_0.2Na_0.2)Nb₂O₆, with a tungsten bronze structure. This is the first report on the thermoelectric properties of high entropy tungsten bronze-type structures. We have obtained a maximum Seebeck coefficient of -370 μV K^-1 at 1150 K, which is the highest among tungsten bronze-type oxide thermoelectrics. The minimum thermal conductivity of 0.8 W m^-1 K^-1 is obtained at 330 K, which is so far the lowest reported value among rare-earth-free high entropy oxide thermoelectrics. This synergistic combination of large Seebeck and record low thermal conductivity gives rise to a maximum ZT of 0.23 which is so far the highest among rare-earth free high entropy oxide-based thermoelectrics.

Suppression of intrinsic thermal conductivity in Sr1-x Gd x TiO3 ceramics via phonon-point defect scattering for enhanced thermoelectric application

A substantial reduction in the thermal conductivity for strontium titanate (ABO₃) perovskite structure was realized for the A-site substitution of gadolinium (rare earth element) in SrTiO₃ ceramics. The effect of Gd³⁺ substitution on the structure, composition, and thermoelectric properties of SrTiO₃ was investigated. The substitution of Gd³⁺ in the SrTiO₃ matrix resulted in the minimalization of thermal conductivity. The thermal conductivity followed a similar trend as that of thermal diffusivity, but specific heat capacity exhibited a non-monotonic trend. The thermal conductivity is reduced to 1.05 W m^-1 K^-1 for the minimal substitutional composition (Sr_0.99Gd_0.01TiO₃) which is 30% less than that of SrTiO₃ at 303 K. The variation in the ionic radii and atomic mass of the heavier rare earth Gd³⁺ substituted over Sr²⁺ resulted in the reduction of thermal conductivity of SGTO ceramics caused by the corresponding boundary scattering at low temperatures and temperature-independent phonon-impurity scattering at high temperatures.

A Review of Key Properties of Thermoelectric Composites of Polymers and Inorganic Materials

This review focusses on the development of thermoelectric composites made of oxide or conventional inorganic materials, and polymers, with specific emphasis on those containing oxides. Discussion of the current state-of-the-art thermoelectric materials, including the individual constituent materials, i.e., conventional materials, oxides and polymers, is firstly presented to provide the reader with a comparison of the top-performing thermoelectric materials. Then, individual materials used in the inorganic/polymer composites are discussed to provide a comparison of the performance of the composites themselves. Finally, the addition of carbon-based compounds is discussed as a route to improving the thermoelectric performance. For each topic discussed, key thermoelectric properties are tabulated and comparative figures are presented for a wide array of materials.