Baoli Du

Enhanced thermoelectric performance of porous magnesium tin silicide prepared using pressure-less spark plasma sintering

Magnesium tin silicide based thermoelectrics contain earth abundant and non-toxic elements, and have the potential to replace established commercial thermoelectrics for energy conversion applications. In this work, porosity was used as a means to improve their thermoelectric properties. Compared to dense samples of Sb doped Mg2Si0.5Sn0.5 with a maximum zT of 1.39 at 663 K, porous samples (37% porosity) prepared by a pressure-less spark plasma sintering technique showed significantly lower thermal conductivity and higher Seebeck coefficient, resulting in an increased maximum zT of 1.63 at 615 K. The possible origins of the enhanced Seebeck coefficient can be attributed to a change of carrier concentration and modification of the band structure, produced by microstructural engineering of the surface composition and particle–particle contacts.

The impact of lone-pair electrons on the lattice thermal conductivity of the thermoelectric compound CuSbS2

The discovery and design of compounds with intrinsically low thermal conductivity, especially compounds with a special bonding nature and stable crystal structure, is a new direction to broaden the scope of potential thermoelectric (TE) materials. This study revealed unambiguously the origin of the impact of the lone pair electrons on lattice thermal conductivity in Cu–Sb–S compounds by correlating the special bonding on the Sb site with the phonon dispersion spectrum and density of states. By substitution of Sb with the transition metal Fe and group IIIA element Ga without s2 electrons, lone-pair electrons on some of the Sb sites were removed, which created a scenario with opposite influences on lattice thermal conductivity from the loss of lone-pair electrons and gain of alloy scattering. We investigated the competition between the alloy phonon scattering and the extra phonon scattering mechanism linked to lone-pair electrons on trivalent Sb3+ sites in chalcostibite CuSbS2, which is a model system for benchmarking and quantifying the impact of lone-pair electrons on the lattice thermal conductivity of Cu–Sb–S compounds. A significant deviation from the classic alloy model was observed. Along with the impact of the lone-pair electrons on the bonding arrangement and crystal structure, the role of lone-pair electrons in the phonon transport of the TE compound CuSbS2 was well demonstrated and quantified. Two Sb-related quasi-single-frequency vibration modes behaving like localised Einstein harmonic oscillators were discovered and correlated with the bonding circumstance around Sb sites. These results give unequivocal evidence that the trivalent VA atom creates special bonding and vibration modes because of its nonbonding 5s lone-pair electrons.

Flash spark plasma sintering of magnesium silicide stannide with improved thermoelectric properties

Spark plasma sintering has become a routine method for the densification of thermoelectric (TE) materials. However, the impacts and details of direct Joule heating within TE materials have not been fully quantified and clarified. Here we investigated the feasibility of flash-sintering (high heating rate Joule heating) magnesium silicide stannide (MSS) using a spark plasma sintering furnace. A Mg2.1Si0.487Sn0.5Sb0.013 (MSS) green compact was sandwiched between two graphite punches without a die. Then a DC pulse voltage was applied between the punches and the current passed completely though the compact, without any of the current bypassing through a graphite die as occurs with a convectional SPS die–punch system. The direct heating was so efficient that a heating rate of ∼1000 °C was achieved and the sample was fully sintered in less than 45 s. Due to the high local Joule heating at the contacts of the particles, the MgO distribution pattern was modified and optimised, which broke the coated passivation layer on the MSS aggregates. The onset densification temperature was 170 to 350 °C lower than that in convectional SPS (750 °C). Importantly, it was possible to produce dense samples in a wide sintering window of ∼6 s, and the flash-sintering was controllable and repeatable. Flash sintering could open a new way for rapid densification of dense nanostructured and/or textured TE materials with low electrical resistivity by optimising the distribution or removal of the surface oxidation of the powder grains.

Enhanced thermoelectric performance of Sn-doped Cu3SbS4

Cu3SbS4 is an earth-abundant and low-cost alternative thermoelectric material for medium temperature applications. Tin doping into Cu3SbS4 yields materials with high thermoelectric performance. The electronic structure of Sn-doped Cu3SbS4 was studied using both hybrid density functional theory (DFT) and the quasi-particle self-consistent GW (QSGW) approach. A synthesis method involving mechanical alloying (MA) and spark plasma sintering (SPS) was employed to produce dense and single phase Cu3SbS4 samples with very fine grain size. Previously unreported nano-scale twins on {112} planes were observed by transmission electron microscopy (TEM). All of the samples showed very low lattice thermal conductivity, which is attributed to their microstructures. Sn was found to substitute Sb successfully in Cu3SbS4 and work effectively as an acceptor dopant, leading to an enhanced power factor. A maximum zT value of 0.72 at 623 K was achieved in Cu3Sb1−xSnxS4 (x = 0.05), which is comparable to the Se analogue Cu3SbSe4.