Suresh Perumal

The origin of low thermal conductivity in Sn1−xSbxTe: phonon scattering via layered intergrowth nanostructures

Inorganic solids with low thermal conductivity are of great interest for thermoelectric applications. The formation of synthetic nanostructures by matrix encapsulation is one of the important strategies for thermal conductivity reduction through phonon scattering. Here, we report the reduction of lattice thermal conductivity near the theoretical minimum limit, κmin, in SnTe via spontaneous formation of nanodomains of the Sb-rich layered intergrowth SnmSb2nTe3n+m compounds, which are natural heterostructures. High-resolution transmission electron microscopy of Sn1−xSbxTe samples reveals the formation of endotaxial Sb rich nanoprecipitates (2–10 nm) along with super-structured intergrowth nanodomains (10–30 nm), which are the key features responsible for the significant reduction of lattice thermal conductivity in SnTe. This mechanism suggests a new avenue for the nanoscale engineering in SnTe to achieve low lattice thermal conductivities. Moreover, the presence of Sb improves the electronic transport properties by aliovalent cation doping which optimizes the hole concentration in SnTe. As a result, an enhanced thermoelectric figure of merit, zT, of ∼1 has been achieved for the composition of Sn0.85Sb0.15Te at 800 K. The high zT sample exhibits the Vickers microhardness value of ∼136 HV which is double that of pristine SnTe and is significantly higher than those of the present state-of-the-art thermoelectric materials.

High performance thermoelectric materials and devices based on GeTe

Thermoelectric materials have received recent attention due to their ability to convert waste heat to electrical energy directly and reversibly. Inorganic materials, especially Bi2Te3, PbTe and Si–Ge based alloys, have been investigated in the temperature range of 300–1000 K, among which PbTe based materials have been extensively studied, and reported to be the leading thermoelectric materials for mid-temperature power generation. However, environmental concern limits their large scale production due to the toxic nature of Pb. As an alternative, GeTe-rich alloys such as TAGS (GeTe–AgSbTe2) have been largely investigated since the 1960s. Most recently, some of the new materials in the GeTe family have been introduced such as Ge0.87Pb0.13Te, the homologous series of Sb2Te3(GeTe)n and Ge0.9Sb0.1Te, and are reported to exhibit high thermoelectric performance, inherently formed nano and microstructure modulations, and high thermal and mechanical stability. These collective enhanced properties of GeTe-rich alloys have generated great interest in investigating further new GeTe based alloys for intermediate temperature thermoelectric applications. In order to provide the fundamental understanding, technological insights, and to further promote the GeTe based alloys, we hereby present a review on (i) the crystal structure, nano/microstructure, phase transition, electronic structure, and thermoelectric properties of GeTe, (ii) correlation of compositional and microstructure modulations and thermoelectric properties of doped GeTe, TAGS based alloys, Ge–Pb–Te materials, and Ge–Sb–Te materials, (iii) mechanical properties, (iv) past and present devices based on GeTe materials and (v) future directions.

Reduction of thermal conductivity through nanostructuring enhances the thermoelectric figure of merit in Ge1−xBixTe

A promising thermoelectric figure of merit, zT, of ∼1.3 at 725 K was obtained in high quality crystalline ingots of Ge1−xBixTe. The substitution of Bi3+ in a Ge2+ sublattice of GeTe significantly reduces the excess hole concentration due to the aliovalent donor dopant nature of Bi3+. Reduction in carrier density optimizes electrical conductivity, and subsequently enhances the Seebeck coefficient in Ge1−xBixTe. More importantly, a low lattice thermal conductivity of ∼1.1 W m−1 K−1 for Ge0.90Bi0.10Te was achieved, which is due to the collective phonon scattering from meso-structured grain boundaries, nano-structured precipitates, nano-scale defect layers, and solid solution point defects. We have obtained a reasonably high mechanical stability for the Ge1−xBixTe samples. The measured Vickers microhardness value of the high performance sample is ∼165 HV, which is comparatively higher than that of state-of-the-art thermoelectric materials, such as PbTe, Bi2Te3, and Cu2Se.

Effect of co-substitution of Mn and Al on thermoelectric properties of chromium disilicide

CrSi2 was earlier reported to be an interesting thermoelectric material for high temperature applications because of its high oxidation resistance and good mechanical properties. In order to enhance its figure of merit, Mn at Cr site and Al at Si site were substituted into CrSi2. Our results indicate that Cr1−xMnxSi2−xAlx solid solutions exhibit significantly lower thermal conductivity and a higher figure of merit than CrSi2.

Ultrahigh Average Thermoelectric Figure of Merit, Low Lattice Thermal Conductivity and Enhanced Microhardness in Nanostructured (GeTe)x(AgSbSe2)100-x

Waste heat sources are generally diffused and provide a range of temperatures rather than a particular temperature. Thus, thermoelectric waste heat to electricity conversion requires a high average thermoelectric figure of merit (ZTavg ) of materials over the entire working temperature along with a high peak thermoelectric figure of merit (ZTmax ). Herein an ultrahigh ZTavg of 1.4 for (GeTe)80 (AgSbSe2 )20 [TAGSSe-80, T=tellurium, A=antimony, G=germanium, S=silver, Se=selenium] is reported in the temperature range of 300-700 K, which is one of the highest values measured amongst the state-of-the-art Pb-free polycrystalline thermoelectric materials. Moreover, TAGSSe-80 exhibits a high ZTmax of 1.9 at 660 K, which is reversible and reproducible with respect to several heating-cooling cycles. The high thermoelectric performance of TAGSSe-x is attributed to extremely low lattice thermal conductivity (κlat ), which mainly arises due to extensive phonon scattering by hierarchical nano/meso-structures in the TAGSSe-x matrix. Addition of AgSbSe2 in GeTe results in κlat of ≈0.4 W mK-1 in the 300-700 K range, approaching to the theoretical minimum limit of lattice thermal conductivity (κmin ) of GeTe. Additionally, (GeTe)80 (AgSbSe2 )20 exhibits a higher Vickers microhardness (mechanical stability) value of ≈209 kgf mm-2 compared to the other state-of-the-art metal chalcogenides, making it an important material for thermoelectrics.