The thermoelectric properties of W-substituted CaMn1-xWxO3-δ (x = 0.01, 0.03; 0.05) samples, prepared by soft chemistry, were investigated from 300 K to 1000 K and compared to Nb-substituted CaMn0.98Nb0.02O3-δ. All compositions exhibit both an increase in absolute Seebeck coefficient and electrical resistivity with temperature. Moreover, compared to the Nb-substituted sample, the thermal conductivity of the W-substituted samples was strongly reduced. This reduction is attributed to the nearly two times greater mass of tungsten. Consequently, a ZT of 0.19 was found in CaMn0.97W0.03O3-δ at 1000 K, which was larger than ZT exhibited by the 2% Nb-doped sample.

Filled skutterudites are one of the most promising materials for thermoelectric (TE) power generation applications at intermediate temperatures due to their superior TE and thermomechanical performance as compared to other materials. In the past, we have demonstrated that n-type skutterudites can be optimized so that their maximum TE figure of merit reaches 1.7 at 850 K. TE performance of the p-type, however, is lagging behind, which hinders the optimization of skutterudites-based TE module development. In this paper we reveal that the underlying reasons for inferior TE properties of the p-type root in their electronic band structures, which result in higher thermal conductivity at elevated temperatures due to bipolar lattice thermal conduction and lower power factor because of heavy valance bands induced strong electron-phonon interactions. We also identify means of improving the power factor and reducing bipolar effect.

On the search for cost-competitive thermoelectric clathrates we have investigated the influence of Sn substitutions for Ge on the structural and thermoelectric properties of the type-I clathrate Ba8Cu5Si6Ge35. The solid solubility of Sn was found to be limited to 0.6 atoms per unit cell. A series of compounds with the nominal compositions Ba8Cu5Si6Ge35-xSnx (x = 0.2, 0.4, 0.6) was synthesized in a high-frequency furnace. The samples were annealed, and subsequently ball milled and hot pressed. The hot pressed samples were characterized by X-ray powder diffraction, energy-dispersive X-ray spectroscopy and transport property measurements. Our results show that the substitution of Ge by Sn introduces vacancies at the 6d site of the type-I clathrate structure and shifts the highest dimensionless thermoelectric figure of merit ZT from 570 °C for the Sn free sample to lower temperatures. The highest figure of merit ZT = 0.42 is reached at about 320 °C for the Sn-substituted sample Ba8Cu5Si6Ge35Sn0.6.

Nanostructuring has been the foremost approach to the manufacture of high-performance thermoelectric materials for nearly a decade. This study explores a novel nanostructuring technique, attrition-enhanced nanocomposite synthesis, in maximum indium-filled, iron-substituted cobalt antimonide skutterudites. In0.3Fe0.8Co3.2Sb12 was synthesized and subjected to varying degrees of mechanical attrition (via ball milling). These samples exhibited increased indium precipitation coincident with the duration of mechanical attrition. Indium readily diffused through the skutterudite crystal structure and rapidly precipitated forming 20-50 nm-sized indium-rich inclusions during sintering.

Ti-Ni-Sn type half-Heusler alloys which have the versatility to be either p- or n-type depending on the type of substitution, have been synthesized and investigated in the present work. The added advantage of doping them with multiple elements is that they will be amenable to bulk amorphous phase formation. The hole doped alloys were predominantly single phase with a cubic structure, while the electron doped alloys were found to have minor additional phases. All the alloys exhibit extremely weak metallic-like or degenerate semiconductor transport behaviour in the temperature range 20 K to 1000 K. The resistivity of p-type alloys exhibits semi-metallic-to-semiconducting transition at ∼ 500 K while the n-type alloys exhibit a weak metallic-like behaviour in the complete temperature range. The Seebeck coefficient has strong temperature dependence with a maximum of 45 μV K−1 in the temperature range 600-700 K in the p-type alloys. The n-type alloys however exhibit a linear variation of the Seebeck coefficient with temperature. The total thermal conductivity of the alloys increases with increasing temperature without any peak at low temperatures indicating significant disorder induced scattering. The p-type alloys have the lowest thermal conductivity compared to the n-type alloys. These alloys become amorphous after pulsed laser deposition except one alloy which exhibits compensated transport behaviour.

Single phase erbium borides ErB2, ErB4, and ErB12 show Seebeck coefficients and power factors with absolute values that are significantly lower than those of a stable Er-B multi phase composite obtained through high temperature solid-solid reaction from the elements (molar ratio Er:B = 1:6). According to quantitative Rietveld analysis the composite consists of erbium diboride (1 %), tetraboride (83 %), and dodecaboride (16 %), and the measurement of the electrical conductivities, Seebeck coefficients, and thermal conductivities leads to ZT values as high as 0.53 at 830 K. Such refractory materials can be used for energy conversion in a range of high temperatures that are otherwise difficult to address.

Applications that produce a large amount of heat, such as combustion engines, can benefit from high temperature thermoelectrics to reduce the amount of energy lost. Superlattice (SL) structures have shown reduced thermal conductivity at room temperature and below, suggesting applicability at high temperatures may be possible. This reduction could greatly increase the thermoelectric figure of merit. The Si/SiGe material system is studied here for high temperature application. Two growth temperatures of 300 C and 500 C are examined. Two superlattice periods were studied (8 nm and 20 nm) to determine the effects of lattice spacing on thermal conductivity. Laser Flash Analysis is applied to determine the thermal diffusivity, hence thermal conductivity, from 100 C to 500 C. Thermal diffusivity was found to be an order of magnitude lower than the constituent alloy at 100 C. Superlattice spacing and growth temperature showed little effect on the diffusivity within the error of this measurement.

In the present study, we report the enhancement in figure-of-merit (ZT) of nanostructured n-type Silicon-Germanium (Si80Ge20) thermoelectric alloy synthesized using high energy ball milling followed by spark plasma sintering (SPS). After 90 h of ball milling of elemental powders of Si, Ge and P (2 at.%), a complete dissolution of Ge in Si matrix has been observed forming the nanostructured n-type Si80Ge20 alloy powder. X-ray diffraction analysis (XRD) confirmed the crystallite size of the host matrix (Si) to be ∼7 nm and also indicated the formation of an additional phase of SiP nano-precipitates after SPS. HR-TEM analysis revealed that the nano-grained network was retained post-sintering with a crystallite size of size of 9 nm and also confirmed the SiP precipitates formation with a size of 4 to 6 nm. As a result, a very low thermal conductivity of ∼2.3W/mK at 900°C has been observed for Si80Ge20 alloy primarily due to scattering of phonons by nanostructured grains and nano-scaled SiP precipitates which further contribute to this scattering mechanism. Electrical conductivity values of SiGe sintered alloy are slightly lower to that of reported values in literature. This was attributed to the formation of SiP which creates a compositional difference between the grain boundary region and the grain region, leading to a chemical potential difference at interface and the grain region. Figure-of-merit (ZT) of n-type Si80Ge20 nanostructured alloy was found to be ≈1.5 at 900°C, which is the highest reported so far at this temperature.

The thermoelectrical properties of α and γ phases of NaxCo2O4 having different amounts of Na were evaluated. The γ NaxCo2O4 samples were synthesized by thermal decomposition in a metal-citric acid compound, and the α NaxCo2O4 samples were synthesized by self-flux processing. Dense bulk ceramics were fabricated using spark plasma sintering (SPS), and the sintered samples were of high density and highly oriented. The thermoelectrical properties showed that γ NaxCo2O4 had higher electrical conductivity and lower thermal conductivity compared with α NaxCo2O4 and that α NaxCo2O4 had a larger Seebeck coefficient. These results show that γ NaxCo2O4 has a larger power factor and dimensionless figure of merit, ZT, than α NaxCo2O4.

Mg2Si bulk was fabricated by spark plasma sintering (SPS) nano-powder, and the thermoelectric characteristics of the bulk sample were evaluated at temperatures up to 873 K. A pre-synthesized all-molten commercial polycrystalline Mg2Si source (un-doped n-type semiconductor) was pulverized into powder of 75 μm or less. To obtain nano-sized fine powder, the powder was milled using planetary ball mill equipment under an inert atmosphere. Fine Mg2Si nano-powder with a mean grain size of about 500 nm was obtained. XRD analysis confirmed that no MgO existed in the nano-powder. The fine powder was put in a graphite die to obtain a sintering body of Mg2Si and treated by SPS under vacuum conditions. The resulting Mg2Si bulk had high density and did not crack. However, the XRD analysis revealed a small amount of MgO in it. The thermoelectric properties (electrical conductivity, Seebeck coefficient, and thermal conductivity) were measured from room temperature to 873 K. The microstructure of the sintered body was observed by scanning electron microscopy. The maximum dimensionless figure of merit of a sample made from Mg2Si nano-powder was ZT = 0.67 at 873 K.

Thermoelectric properties of the Li-doped Cu0.95-xM0.05LixO (M=divalent metal ion; Mn, Ni, Zn) were investigated at the temperature up to 1273 K. In the doped divalent metal ions, Zn2+ ion was the most effective to reduce the thermal conductivity, and the Ni2+ substitution was preferable to decrease the electrical resistivity. For the Cu0.95-xNi0.05LixO sample at x=0.03, the maxima of the dimensionless thermoelectric figure of merit ZT and the power factor P at 1246 K were 4.2×10-2 and 1.6 ×10-4 W/K2m, respectively. The enhancement of the thermoelectric properties of the Li-doped Cu0.95-xM0.05LixO system was discussed.

We investigated the valence band structure of PbSe by a combined study of the optical and transport properties of p-type Pb1-xNaxSe, with Na concentrations ranging from 0 – 4%, yielding carrier densities in a wide range of 1018 – 1020 cm−3. Room temperature infrared reflectivity studies showed that the susceptibility (or conductivity) effective mass m* increases from ∼ 0.06mo to ∼ 0.5mo on increasing Na content from 0.08% to 3%. The Seebeck coefficient scales with doping in the whole temperature range, yielding lower values for higher Na contents, while the Hall coefficient increases on heating from room temperature showing a peak close to 650 K. The room temperature Pisarenko plot is well described by the simple parabolic band model up to ∼ 1·1020 cm−3. In order to describe the behaviour in the whole concentration range, the application of the two band model, i.e. light hole and heavy hole, was used giving density of states effective masses 0.28mo and 2.5mo for the two bands respectively.

La3-xTe4 is a state-of-the-art high temperature n-type thermoelectric material with a previously reported maximum zT∼1.1 at 1273 K. Computational modeling suggests the La atoms play a crucial role in defining the density of states for La3-xTe4 in the conduction band. In addition to controlling charge carrier concentration, substitution with Ca2+ atoms on the La3+ site is explored as a potential means to tune the density of states and result in larger Seebeck coefficients. High purity, oxide-free samples are produced by ball milling of the elements and consolidated by spark plasma sintering. Powder XRD and electron microprobe analysis are used to characterize the material. High temperature thermoelectric properties are reported and compared with La3-xTe4 compositions. A maximum zT of 1.3 is reached at 1273 K for the composition La2.22Ca0.775Te4.

We report on the development and capabilities of two new measurement systems developed at Fraunhofer-IPM. The first measurement system is based on an extension of the Van der Pauw method and is suitable for cube-shaped samples. A mapping of the electrical conductivity tensor of a Skutterudite-SPS samples produced at the Instituto de Microelectrónica de Madrid is presented. The second measurement system is a ZTmeter also developed at the Fraunhofer-IPM. It enables the simultaneous measurement of the electrical conductivity, Seebeck coefficient and thermal conductivity up to 900 K of cubes at least 5x5x5 mm3 in size. The capacity of this measurement system for measuring the anisotropy of the transport properties of a (Bi,Sb)2Te3SPS sample produced by KTH is demonstrated by simply rotating the samples.

Thermoelectric generators (TEG) are capable of transforming waste heat directly into electric power. With higher temperatures the yield of the devices rises which makes high-temperature contact materials important. The formation of titanium disilicide (TiSi2) and its properties were analyzed and optimized for the use in TEG. Depending on a direct or an indirect transformation into the C54 crystal structure the process forms a layer with a resistivity of 20-22 μΩcm. Process gases influence the resistivity and result in difference of 20 %. The growing rate of TiSi2on silicon dioxide was determined; it shows a strong dependence on the used atmosphere and temperature. A maximum overgrowing length of 30 μm was found.

In order to achieve waste heat recovery using thermoelectric systems, thermoelectric materials showing high conversion efficiency over wide temperature range and high resistance against oxidation are indispensable. A silicide material with good n-type thermoelectric properties and oxidation resistance has been discovered. The composition and crystal structure of the silicide are found out Mn3Si4Al2 (abbreviated as 342 phase) and hexagonal CrSi2 structure, respectively. Element substitution of Mn with 3d transition metals is succeeded. Enhancement of Seebeck coefficient is observed in a Cr-substituted sample. The maximum dimensionless thermoelectric figure of merit ZT is 0.3 at 573 K in air for the Mn2.7Cr0.3Si4Al2 sample. Electrical resistivity of the Mn3Si4Al2 bulk sample holds constant value for 48 h at 873 K in air. This is due to formation of oxide passive layer on the surface of the bulk sample. The 342 phase is a promising n-type material with a good oxidation resistance in the middle temperature range of 500-800 K.

The thermoelectric properties of Bi-FeSb2 nanocomposites are reported. The electrical resistivity and the Seebeck coefficient measurements show a significant dependence on bismuth concentration. Our results reveal that the shifting of the Seebeck peak in FeSb2 nanocomposites is purely a grain size-effect. The thermal conductivity data indicates a presence of an electron-phonon interaction. Over all, our analysis of the the thermoelectric properties of Bi-FeSb2 nanocomposites provide additional evidence for phonon-drag in FeSb2.

Skutterudites are known to be efficient thermoelectric (TE) materials in the temperature range from 600 K to 900 K. Dimensionless figure of merit (ZT) for filled skutterudite TE materials have been reported as ca. 1 at 800 K. Novel nano- engineering approaches and filling of the skutterudites crystal can further improve the transport properties and ultimately the ZT. Although classified among the promising TE materials, research on their large-scale production via bottom up synthetic routes is rather limited. In this work, large quantity of cobalt antimonide (CoSb3) based skutterudites nanopowder (NP) was fabricated through a room temperature co-precipitation precursor method. Dried precipitates were process by thermo-chemical treatment steps including calcination (in air) and reduction (in hydrogen). CoSb3 NPs were then mixed with silver (Ag) nanoparticles at different weight percentages (1%, 5% and 10% by wt) to form nanocomposites. Skutterudite NP was then consolidated by Spark Plasma Sintering (SPS) technique to produce highly dense compacts while maintaining the nanostructure. Temperature dependent TE characteristics of SPS’d CoSb3 and Ag containing nanocomposite samples were evaluated for transport properties, including thermal conductivity, electrical conductivity and Seebeck coefficient over the temperature range of 300 - 900 K. Physicochemical, structural and microstructural evaluation results are presented in detail.

Higher manganese silicide (HMS) is a low-cost and eco-friendly thermoelectric material available for recovering waste heat of 500 to 900 K. In this research, we tried to uniformly disperse the alumina nanoparticles (ANPs) in the HMS matrix to reduce the thermal conductivity and to improve the thermoelectric performance. Influence of addition of ANPs on the thermoelectric properties was investigated. It was confirmed that ANPs were uniformly dispersed in the HMS grain boundary. The lattice thermal conductivity was reduced by adding ANPs. As a result, the maximum thermoelectric performance of ZT=0.58 was achieved at about 800 K by adding 1 vol% of ANPs. The performance of ANPs-added HMS was improved about 25 %.

We present theoretical calculations of the phonon-drag contribution to the Nernst thermoelectric power Syx in Bismuth nanowires. We investigate the thermopower Syx with diameters L ranging from 22 to 900 nm at low temperatures (0.1 - 4.0 K) and high magnetic fields (up to 16 T). We find that the peak of thermopower Syx around 14.75 T exhibits the size effect in two different ways: for wires with L≥200 nm, the peak height increases with decreasing L; for wires with L<200 nm, on the other hand, the peak height rapidly decreases with decreasing L. The dependence is accounted for by considering the contributions of discrete quantized phonon modes. We also discuss the temperature dependence of Syx.