Additive Manufacturing of Thermoelectrics: Emerging Trends and Outlook

Additive manufacturing (AM) has progressed rapidly in recent years, thanks to its versatility in printing complex and intricate shapes. Very recently, it has also been making inroads into functional and energy materials. On the other hand, thermoelectrics is a relatively mature field, with well-established understanding and design, especially on the materials level. However, complexities in device fabrication and scalability issues have greatly hindered thermoelectric (TE) applications. In this Focus Review, we discuss the advent of AM as a timely and important tool not only to overcome the scalability issues but also to achieve shape intricacies and conformability for flexible and wearable applications. In particular, direct ink writing (DIW), a subset under materials extrusion methods, holds great promise as a versatile fabrication technique for integrated TE devices. More importantly, we discuss the great promise of “engineered nanostructuring” using DIW as a new paradigm to improve TE performance beyond intrinsic properties

Single-Crystalline Thin Films for Studying Intrinsic Properties of BiFeO₃–SrTiO₃ Solid Solution Photoelectrodes in Solar Energy Conversion

Solid solutions have been widely investigated for solar energy conversion because of the ease to control properties (e.g., band edge positions, charge carrier transport, and chemical stability). In this study, we introduce a new method to investigate intrinsic solar energy conversion properties of solid solutions through fabricating high-quality single-crystalline solid solution films by pulsed laser deposition. This method rules out external factors, such as morphology, crystalline grain size, orientation, density and distribution, surface area, and particle–particle or particle–conducting layer connection, that have plagued previous studies on solid solution photoelectrodes. Perovskite BiFeO₃ (BFO) and SrTiO₃ (STO) were chosen as “end” members of the solid solutions (i.e., (BFO)_<i>x</i>(STO)_1–<i>x</i> (0 ≤ <i>x</i> ≤ 1)). Optical and photoelectrochemical (PEC) properties of the solid solutions significantly varied with changing compositions. Among the six studied compositions, BFO:STO (3:1 molar ratio) exhibited the highest photocurrent density with the photovoltage of 1.08 V. The photoelectrode also produced stable photocurrent for 12 h. Faradaic efficiencies of H₂ and O₂ formation close to 100% were measured

Metal-Free Synthesis of Biobased Polyisoxazolines toward Sustainable Circular Materials

In this study, we report the synthesis of sustainable polyisoxazolines using a vanillin-derived bifunctional nitrile oxide monomer in metal-free nitrile oxide cycloaddition polymerization under mild conditions. The bioderived polyisoxazolines have high biomass contents in high yields with tunable glass transition temperatures. The furan-containing polyisoxazoline could also be cross-linked by using dynamically exchangeable bonds to yield a circular material that can be easily reprocessed for multiple cycles after use. The material’s circularity was demonstrated by encapsulating a thermoelectric generator, which could be recycled easily to obtain the original generator and polymer for device refabrication, showing potential in reducing electronic waste in green electronic applications

Cold-Sintered Bi₂Te₃‑Based Materials for Engineering Nanograined Thermoelectrics

Bi2Te3-based compounds are currently the most commercially relevant thermoelectric materials near room temperature. They are prepared via hot pressing, hot deformation, spark plasma sintering, and other consolidation processes, which are typically performed at 400–500 °C. Such high-temperature processes are energy-intensive and generate unnecessary waste heat, making them undesirable for a large-scale production. In this study, a low-temperature liquid-phase-assisted sintering (or so-called cold-sintering) process was employed to fabricate p-type Bi0.5Sb1.5Te3 bulk materials at temperatures below 150 °C. At the optimal sintering temperature (130 °C), a ZT value as high as 0.56 at 450 K can be achieved, competitive to that of a commercial Bi0.5Sb1.5Te3 ingot (ZT 0.8–1.0). The addition of a small amount of transient liquid facilitates grain reorientation and expedites a mass transfer process under axial compaction and liquid evaporation conditions, thus resulting in nearly fully densified Bi0.5Sb1.5Te3 pellet samples (>97% theoretical density). Furthermore, the low-temperature sintering process results in the reduction of grain size and promotes twin boundaries, resulting in a low lattice thermal conductivity of 0.57 W m–1 K–1 at 380 K due to phonon scattering. The strategy reported in this work can be used not only as a substitute for high-temperature sintering of other thermoelectric materials but also to engineer phonon scattering for high-performance thermoelectrics

3D-Printed Porous Thermoelectrics for <i>In Situ</i> Energy Harvesting

The rapid growth of industrialization has resulted in an tremendous increase in energy demands. The vast amount of untapped waste heat found in factories and power plants can be harnessed to power devices. Thermoelectric materials enable a clean conversion of heat to electrical energy and vice versa, without the need for moving parts. However, existing thermoelectric generators are limited to capturing heat from exterior surfaces. Additive manufacturing offers itself as a cost-effective process that produces complex parts which can recover waste heat from direct heat flows. Herein, we report the first ever in situ energy harvester through porous 3D thermoelectrics. Complex 3D-printed Bi0.5Sb1.5Te3 open cellular structures of high specific surface area are fabricated to allow a high rate of heat transfer throughout the heat pipes with negligible effect on the liquid flow. This work opens up exciting possibilities of energy harvesting from natural self-sustaining thermal gradients found in exhaust pipes and heat exchangers

3D-Printed Porous Thermoelectrics for <i>In Situ</i> Energy Harvesting

The rapid growth of industrialization has resulted in an tremendous increase in energy demands. The vast amount of untapped waste heat found in factories and power plants can be harnessed to power devices. Thermoelectric materials enable a clean conversion of heat to electrical energy and vice versa, without the need for moving parts. However, existing thermoelectric generators are limited to capturing heat from exterior surfaces. Additive manufacturing offers itself as a cost-effective process that produces complex parts which can recover waste heat from direct heat flows. Herein, we report the first ever in situ energy harvester through porous 3D thermoelectrics. Complex 3D-printed Bi0.5Sb1.5Te3 open cellular structures of high specific surface area are fabricated to allow a high rate of heat transfer throughout the heat pipes with negligible effect on the liquid flow. This work opens up exciting possibilities of energy harvesting from natural self-sustaining thermal gradients found in exhaust pipes and heat exchangers

3D-Printed Porous Thermoelectrics for <i>In Situ</i> Energy Harvesting

The rapid growth of industrialization has resulted in an tremendous increase in energy demands. The vast amount of untapped waste heat found in factories and power plants can be harnessed to power devices. Thermoelectric materials enable a clean conversion of heat to electrical energy and vice versa, without the need for moving parts. However, existing thermoelectric generators are limited to capturing heat from exterior surfaces. Additive manufacturing offers itself as a cost-effective process that produces complex parts which can recover waste heat from direct heat flows. Herein, we report the first ever in situ energy harvester through porous 3D thermoelectrics. Complex 3D-printed Bi0.5Sb1.5Te3 open cellular structures of high specific surface area are fabricated to allow a high rate of heat transfer throughout the heat pipes with negligible effect on the liquid flow. This work opens up exciting possibilities of energy harvesting from natural self-sustaining thermal gradients found in exhaust pipes and heat exchangers

N‑Type Thermoelectric AgBiPbS₃ with Nanoprecipitates and Low Thermal Conductivity

Thermoelectric sulfide materials are of particular interest due to the earth-abundant and cost-effective nature of sulfur. Here, we report a new n-type degenerate semiconductor sulfide, AgBiPbS3, which adopts a Fm3̅m structure with a narrow band gap of ∼0.32 eV. Despite the homogeneous distribution of elements at the scale of micrometer, Ag2S nanoprecipitates with dimensions of several nanometers were detected throughout the matrix. AgBiPbS3 exhibits a low room-temperature lattice thermal conductivity of 0.88 W m–1 K–1, owing to the intrinsic low lattice thermal conductivity of Ag2S and the effective scattering of phonons at nanoprecipitate boundaries. Moreover, compared to AgBiS2, AgBiPbS3 demonstrates a significantly improved weighted mobility of >16 cm2 V–1 s–1 at 300 K, leading to an enhanced PF of 1.6 μW cm–1 K–2 at 300 K. The superior electrical transport in AgBiPbS3 can be attributed to the high valley degeneracy of the L point (the conduction band minimum), which is contributed by the Pb s and Pb p orbitals. Further, Ga doping is found to be effective in modulating the Fermi levels of AgBiPbS3, leading to further enhancement of PF with a PFave of 2.7 μW cm–1 K–2 in the temperature range of 300–823 K. Consequently, a relatively high ZTave of 0.22 and a peak ZT of ∼0.4 at 823 K have been achieved in 3% Ga-doped AgBiPbS3, highlighting the potential of AgBiPbS3 as an n-type thermoelectric sulfide

Origin of High Thermoelectric Performance in Earth-Abundant Phosphide–Tetrahedrite

Phosphide-based thermoelectrics are a relatively less studied class of compounds, primarily due to the presence of light elements, which result in high thermal conductivity and inherent stability problems. In this work, we present a stable phosphide–tetrahedrite, Ag6Ge10P12, which possesses the highest zT (∼0.7) among all known phosphides at intermediate temperatures (750 K). We examine the intrinsic electronic and thermal transport properties of this compound by expressing the transport properties in terms of weighted mobility (μW), transport coefficient (σE0), and material quality factor (B), from which we are able to elucidate that the origin of its high zT can be attributed to the platelike Fermi surface and high level of band multiplicity related to its complex band structure. Finally, we discuss the origin of the low lattice thermal conductivity observed in this compound using experimental sound velocity, elastic properties, and Debye–Callaway model, thus laying the foundation for similar stable phosphides as potentially earth-abundant and nontoxic intermediate-temperature thermoelectric materials

Gallium-Doped Zinc Oxide Nanostructures for Tunable Transparent Thermoelectric Films

Ga-doped ZnO (GZO) transparent nanostructured thin films were fabricated via the magnetron sputtering process, and the effect of the Ga doping level in GaxZn1–xO on their thermoelectric performance was investigated. Nanostructured composite Ga0.085Zn0.915O could achieve a power factor up to 1428 μW/mK2 at 850 K, which is one of the highest among the reported thin-film GZO and other metal oxides-based thermoelectrics. A corresponding thermoelectric generator module using GZO as n-type legs was fabricated to attain a maximum power output of 230 nW at ΔT = 138 K with an estimated power density of 19.1 mW cm–2. First-principles calculations were performed to study the thermoelectric properties of GZO, showing that the calculated result is perfectly consistent with the experimental observation that the maximum power factor was achieved at around 8.5% Ga doping. In addtion, 10 nanostructured gallium-doped ZnO thin-film legs were fabricated for transparent thermoelectric modules. This work provides an excellent example to foresee the thermoelectric nanostructured thin-film material property through the theoretical simulation of materials, suggesting that modeling plays a big role in designing and formulating high-performance thermoelectric materials