Hierarchical thermoelectrics: crystal grain boundaries as scalable phonon scatterers

Hierarchical Phonon Scattering from Nano to Macro Scale in Ag-Nano/TiO2-Micro Particle-Decorated p-type Bismuth Telluride Bulk Composites

We study the thermoelectric properties of a p-type Bi0.4Sb1.6Te3.4 (BST) composite with Ag nanoparticle-decorated TiO2 microparticles (US-Ag/TiO2). The dispersion of US-Ag/TiO2 particles, synthesized by an ultrasonication (US) method, into the matrix effectively decreases lattice and bipolar thermal conductivity, attributed to the scattering centers formed at nano and micro scales. The electron backscattering diffraction (EBSD) measurements revealed smaller grain sizes within the BST composite when paired with the US-Ag/TiO2 particle dispersion. These reduced grain sizes, alongside nanoparticle-decorated microparticles dispersed throughout the matrix, scatter phonons effectively from long- to short-wavelength phonons and subsequently decrease lattice thermal conductivity. While the power factors of the composites are reduced, significant suppression of lattice and bipolar thermal conductivity has led to an increase in the maximum zT value (1.4 at 325 K) for a 0.9 wt % US-Ag/TiO2 particle dispersion within the BST matrix. This particle dispersion in the BST composite consistently demonstrates a high zT value across an extensive temperature spectrum, leading to an exceptionally high average zTavg value (1.38 up to 400 K), which is superior to the other values from reported BST composites. Thus, this research indicates that the dispersion of nanoparticle-decorated microparticles within a thermoelectric material matrix can significantly improve thermoelectric performance, which has promising implications for practical applications in thermoelectric cooling and sustainable and economical energy harvesting technologies.

Synergistic Approach Toward a Reproducible High zT in n-Type and p-Type Superionic Thermoelectric Ag2Te

Recently, superionic thermoelectrics have attracted enormous attention due to their ultralow thermal conductivity and high figure-of-merit (zT). However, their high zT is generally obtained deep inside the superionic phase, e.g., near 1000 K in Cu₂X (X: chalcogen atom) family despite a relatively low superionic transition temperature of ∼400 K. At such high temperatures, the liquid-like flow of the metal ions results in material's degradation. Here, we present thermoelectric properties of superionic Ag₂Te synthesized by various methods. The sintered Ag₂Te samples are shown to exhibit an unpredictable behavior with respect to the sign of thermopower (S) in the superionic phase and the magnitude of electrical conductivity (σ). We overcome this issue using an all-room-temperature fabrication technique leading to an excellent reproducibility from one sample to another. To improve the zT of Ag₂Te beyond the phonon-liquid electron-crystal limit (∼0.64 at 575 K in the ingot samples), we adopted a heirarchical nanostructuring technique, which effectively suppressed the thermal conductivity, leading to a significant improvement in the zT values for both n-type and p-type samples. We obtained zT of 1.2 in the n-type and 0.64 in the p-type Ag₂Te at 570 K. These values supersede the zT of any Ag₂Te previously reported. At 570 K, for our ball-milled/cold-pressed samples, the critical current density for metal-ion migration exceeds 15 A cm^-2, which further confirms that Ag₂Te is a promising thermoelectric material. Our results are supported by first-principles density functional theory calculations of the electronic and thermal properties.

Layered Tin Chalcogenides SnS and SnSe: Lattice Thermal Conductivity Benchmarks and Thermoelectric Figure of Merit

Tin sulfide (SnS) and tin selenide (SnSe) are attractive materials for thermoelectric conversion applications. Favorable small band gap, high carrier mobility, large Seebeck coefficient, and remarkably low lattice thermal conductivity are a consequence of their anisotropic crystal structure of symmetry Pnma , made of corrugated, black phosphorus-like layers. Their internal lattice dynamics combined with chemical bond softening in going from SnS to SnSe make for subtle effects on lattice thermal conductivity. Reliable prediction of phonon transport for these materials must therefore include many-body effects. Using first principles methods and a transferable tight-binding potential for frozen phonon calculations, here, we investigate the evolution of thermal lattice conductivity and thermoelectric figure of merit in Pnma -SnS and -SnSe, also including the high-temperature Cmcm -SnS phase. We show how thermal conductivity lowering in SnS at higher temperatures is largely due to dynamic phonon softening ahead of the Pnma - Cmcm structural phase transition. SnS becomes more similar to SnSe in its lifetime and mean free path profiles as it approaches its high-temperature Cmcm phase. The latter nonetheless intrinsically constraints phonon group velocity modules, preventing SnS to overtake SnSe. Our analysis provides important insights and computational benchmarks for optimization of thermoelectric materials via a more efficient computational strategy compared to previous ab initio attempts, one that can be easily transferred to larger systems for further thermoelectric materials nanoengineering. The good description of anharmonicity at higher temperatures inherent to the tight-binding potential yields calculated lattice conductivity values that are in very good agreement with experiments.

Thermodynamic Analysis and Experimental Study on the Oxidation of PbX (X = S, Se) Nanostructured Layers

Heat treatment in an oxygen-containing medium is a necessary procedure in the technology of forming photodetectors and emitters based on lead chalcogenides. Lead chalcogenide layers (PbS, PbSe) were prepared via a chemical bath deposition method. Surface oxidation of lead chalcogenide layers was analyzed using X-ray diffraction and Raman spectroscopy methods, and thermodynamic analysis of the oxidation of PbSe and PbS layers was also performed. The calculated phase diagrams from 20 °C to 500 °C showed good agreement with the experimental results. According to the thermodynamic analysis, the oxidation products depend on the initial composition of the layers and temperature of the annealing. In some cases, the formation of a separate metallic phase Pb is possible along with the formation of lead oxide PbO and other oxides. The performed thermodynamic analysis makes it possible to substantiate the two-stage annealing temperature regimes which ensure an increase in the speed of photodetectors.

The impact of tilt grain boundaries on the thermal transport in perovskite SrTiO3 layered nanostructures. A computational study

Thermal management at solid interfaces presents a technological challenge for modern thermoelectric power generation. Here, we define a computational protocol to identify nanoscale structural features that can facilitate thermal transport in technologically important nanostructured materials. We consider the highly promising thermoelectric material, SrTiO3, where tilt grain boundaries lower thermal conductivity. The magnitude of the reduction is shown to depend on compositional and structural arrangements at the solid interface. Quantitative analysis indicates that layered nanostructures less than 10 nm will be required to significantly reduce the thermal conductivity below the bulk value, and it will be virtually independent of temperature for films less than 2 nm depending on the orientation with a reduction of thermal transport up to 75%. At the nanoscale, the vibrational response of nanostructures shows concerted vibrations between the grain boundary and inter-boundary regions. As the grain boundary acts markedly as a phonon quencher, we predict that any manipulation of nanostructures to further reduce thermal conductivity will be more beneficial if applied to the inter-boundary region. Our findings may be applied more widely to benefit other technological applications where efficient thermal transport is important.

Thickness Effects for Thermoelectric Property of Antimony Telluride Nanoplatelets via Solvothermal Method

Nanostructures have the potential to exhibit good thermoelectric properties by tuning and controlling their size and thickness, and the competing electrical and thermal properties can be decoupled by engineering the interface and grain boundary. In the present study, Sb2Te3 nanoplatelets with different sizes were fabricated using a practical solvothermal method. The thickness of the platelets were regulated between sizes of 10 nm and 100 nm, and the opposite edge length was varied between 1 and 10 μm by altering chemical conditions. Consequently, manipulating the grain size made it suitable to benefit the carrier transport and also block phonons for the thin platelets, resulting in a significant decrease in thermal conductivity and simultaneous increase in electrical conductivity. The results showed that the optimized figure of merit ZT, increased from 0.2 to 1.0 for thin samples, providing a comprehensive understanding of size-dependent thermoelectric performance.

Ab Initio-Based Bond Order Potential to Investigate Low Thermal Conductivity of Stanene Nanostructures

We introduce a bond order potential (BOP) for stanene based on an ab initio derived training data set. The potential is optimized to accurately describe the energetics, as well as thermal and mechanical properties of a free-standing sheet, and used to study diverse nanostructures of stanene, including tubes and ribbons. As a representative case study, using the potential, we perform molecular dynamics simulations to study stanene's structure and temperature-dependent thermal conductivity. We find that the structure of stanene is highly rippled, far in excess of other 2-D materials (e.g., graphene), owing to its low in-plane stiffness (stanene: ∼ 25 N/m; graphene: ∼ 480 N/m). The extent of stanene's rippling also shows stronger temperature dependence compared to that in graphene. Furthermore, we find that stanene based nanostructures have significantly lower thermal conductivity compared to graphene based structures owing to their softness (i.e., low phonon group velocities) and high anharmonic response. Our newly developed BOP will facilitate the exploration of stanene based low dimensional heterostructures for thermoelectric and thermal management applications.