Phase characterization, thermal stability, high-temperature transport properties, and electronic structure of rare-earth Zintl phosphides Eu3M2P4 (M = Ga, In).

Two rare-earth-containing ternary phosphides, Eu3Ga2P4 and Eu3In2P4, were synthesized by a two-step solid-state method with stoichiometric amounts of the constitutional elements. Refinements of the powder X-ray diffraction are consistent with the reported single-crystal structure with space group C2/c for Eu3Ga2P4 and Pnnm for Eu3In2P4. Thermal gravimetry and differential scanning calorimetry (TG-DSC) measurements reveal high thermal stability up to 1273 K. Thermal diffusivity measurements from room temperature to 800 K demonstrate thermal conductivity as low as 0.6 W/m·K for both compounds. Seebeck coefficient measurements from room temperature to 800 K indicate that both compounds are small band gap semiconductors. Eu3Ga2P4 shows p-type conductivity and Eu3In2P4 p-type conductivity in the temperature range 300-700 K and n-type conductivity above 700 K. Electronic structure calculations result in band gaps of 0.60 and 0.29 eV for Eu3Ga2P4 and Eu3In2P4, respectively. As expected for a valence precise Zintl phase, electrical resistivity is large, approximately 2600 and 560 mΩ·cm for Eu3Ga2P4 and Eu3In2P4 at room temperature, respectively. Measurements of transport properties suggest that these Zintl phosphides have potential for being good high-temperature thermoelectric materials with optimization of the charge carrier concentration by appropriate extrinsic dopants.

Impact of Anionic Substitution in Yb14MgSb11-x As x Compounds on the Electronic and Thermoelectric Properties.

The effects of anionic site substitution on the electronic transport properties of Yb14MgSb11-x As x compounds were investigated using density functional theory (DFT) with on-site Coulomb interaction correction (PBE+U). By replacing the Sb atoms at the four symmetry sites in Yb14MgSb11 with As, we found that the electronic and thermoelectric properties of the compound can be altered substantially. For most of the cases, the thermoelectric properties improve compared to the base compound Yb14MgSb11. Substitution at the tetrahedral site (Sb2) in particular yields the highest improvement in the thermoelectric properties. Detailed insight into the electronic and structural changes caused by the selective site substitutions is also discussed.

Thermoelectric Properties of Scandium Sesquitelluride.

Rare-earth (RE) tellurides have been studied extensively for use in high-temperature thermoelectric applications. Specifically, lanthanum and praseodymium-based compounds with the Th3P4 structure type have demonstrated dimensionless thermoelectric figures of merit (zT) up to 1.7 at 1200 K. Scandium, while not part of the lanthanide series, is considered a RE element due to its chemical similarity. However, little is known about the thermoelectric properties of the tellurides of scandium. Here, we synthesized scandium sesquitelluride (Sc2Te3) using a mechanochemical approach and formed sintered compacts through spark plasma sintering (SPS). Temperature-dependent thermoelectric properties were measured from 300⁻1100 K. Sc2Te3 exhibited a peak zT = 0.3 over the broad range of 500⁻750 K due to an appreciable power factor and low-lattice thermal conductivity in the mid-temperature range.

Synthesis and characterization of vacancy-doped neodymium telluride for thermoelectric applications.

Thermoelectric materials exhibit a voltage under an applied thermal gradient and are the heart of radioisotope thermoelectric generators (RTGs), which are the main power system for space missions such as Voyager I, Voyager II, and the Mars Curiosity rover. However, materials currently in use enable only modest thermal-to-electrical conversion efficiencies near 6.5% at the system level, warranting the development of material systems with improved thermoelectric performance. Previous work has demonstrated large thermoelectric figures of merit for lanthanum telluride (La3-x Te4), a high-temperature n-type material, achieving a peak zT value of 1.1 at 1275 K at an optimum cation vacancy concentration. Here we present an investigation of the thermoelectric properties of neodymium telluride (Nd3-x Te4), another rare-earth telluride with a similar structure to La3-x Te4. Density functional theory (DFT) calculations predicted a significant increase in the Seebeck coefficient over La3-x Te4 at equivalent vacancy concentrations due to an increased density of states (DOS) near the Fermi level from the 4f electrons of Nd. The high temperature electrical resistivity, Seebeck coefficient, and thermal conductivity were measured for Nd3-x Te4 at various carrier concentrations. These measurements were compared to La3-x Te4 in order to elucidate the impact of the four 4f electrons of Nd on the transport properties of Nd3-x Te4. A zT of 1.2 was achieved at 1273 K for Nd2.78Te4, which is a 10% improvement over that of La2.74Te4.

Thermoelectric properties and electronic structure of the Zintl phase Sr5In2Sb6 and the Ca(5-x)Sr(x)In2Sb6 solid solution.

The Zintl phase Sr5In2Sb6 is isostructural with Ca5In2Sb6-a promising thermoelectric material with a peak zT of 0.7 when the carrier concentration is optimized by doping. Density functional calculations for Sr5In2Sb6 reveal a decreased energy gap and decreased valence band effective mass relative to the Ca analog. Chemical bonding analysis using the electron localizability indicator was found to support the Zintl bonding scheme for this structure type. High temperature transport measurements of the complete Ca(5-x)Sr(x)In2Sb6 solid solution were used to investigate the influence of the cation site on the electronic and thermal properties of A5In2Sb6 compounds. Sr was shown to be fully miscible on the Ca site. The higher density of the Sr analog leads to a slight reduction in lattice thermal conductivity relative to Ca5In2Sb6, and, as expected, the solid solution samples have significantly reduced lattice thermal conductivities relative to the end member compounds.

Mechanochemical Synthesis and High Temperature Thermoelectric Properties of Calcium-Doped Lanthanum Telluride La3-xCaxTe4.

The thermoelectric properties from 300 - 1275 K of calcium-doped La3-xTe4 are reported. La3-xTe4 is a high temperature n-type thermoelectric material with a previously reported zTmax ~ 1.1 at 1273 K and x = 0.23. Computational modeling suggests the La atoms define the density of states of the conduction band for La3-xTe4. Doping with Ca2+ on the La3+ site is explored as a means of modifying the density of states to improve the power factor and to achieve a finer control over the carrier concentration. High purity, oxide-free samples are produced by ball milling of the elements and consolidation by spark plasma sintering. Calcium substitution upon the lanthanum site was confirmed by a combination of Rietveld refinements of powder X-ray diffraction data and wave dispersive spectroscopy. A zTmax ~ 1.2 is reached at 1273 K for the composition La2.2Ca0.78Te4 and the relative increase compared to La3-xTe4 is attributed to the finer carrier concentration.

Measurement of the electrical resistivity and Hall coefficient at high temperatures.

The implementation of the van der Pauw (VDP) technique for combined high temperature measurement of the electrical resistivity and Hall coefficient is described. The VDP method is convenient for use since it accepts sample geometries compatible with other measurements. The technique is simple to use and can be used with samples showing a broad range of shapes and physical properties, from near insulators to metals. Three instruments utilizing the VDP method for measurement of heavily doped semiconductors, such as thermoelectrics, are discussed.

Nanostructured materials for thermoelectric applications.

Recent studies indicate that nanostructuring can be an effective method for increasing the dimensionless thermoelectric figure of merit (ZT) in materials. Most of the enhancement in ZT can be attributed to large reductions in the lattice thermal conductivity due to increased phonon scattering at interfaces. Although significant gains have been reported, much higher ZTs in practical, cost-effective and environmentally benign materials are needed in order for thermoelectrics to become effective for large-scale, wide-spread power and thermal management applications. This review discusses the various synthetic techniques that can be used in the production of bulk scale nanostructured materials. The advantages and disadvantages of each synthetic method are evaluated along with guidelines and goals presented for an ideal thermoelectric material. With proper optimization, some of these techniques hold promise for producing high efficiency devices.

Thermoelectric microdevice fabricated by a MEMS-like electrochemical process.

Microelectromechanical systems (MEMS) are the basis of many rapidly growing technologies, because they combine miniature sensors and actuators with communications and electronics at low cost. Commercial MEMS fabrication processes are limited to silicon-based materials or two-dimensional structures. Here we show an inexpensive, electrochemical technique to build MEMS-like structures that contain several different metals and semiconductors with three-dimensional bridging structures. We demonstrate this technique by building a working microthermoelectric device. Using repeated exposure and development of multiple photoresist layers, several different metals and thermoelectric materials are fabricated in a three-dimensional structure. A device containing 126 n-type and p-type (Bi, Sb)2Te3 thermoelectric elements, 20 microm tall and 60 microm in diameter with bridging metal interconnects, was fabricated and cooling demonstrated. Such a device should be of technological importance for precise thermal control when operating as a cooler, and for portable power when operating as a micro power generator.