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Summary High phonon velocities, i.e., as measured by the speed of sound (vs) lead to high lattice thermal conductivity (κlat), which is detrimental to thermoelectric performance. Conventional wisdom associates vs exclusively with structural features such as average atomic mass but not the number of conducting electrons. Here, we demonstrate vs reduction from electronic doping in eight well-known thermoelectric semiconductors and establish carrier density nH as the main cause for the observed lattice softening by ruling out alternative factors such as changes in density, average atomic mass, and defect formation. In p-type SnTe and n-type La3–xTe4, we find respective decreases of 16% and ∼20% in vs when raising the nH from ∼1019 to 1021 cm–3, which is sufficient to decrease κlat by nearly 50%. Such giant softening effects can account for 25% of the optimized thermoelectric figure of merit (zTmax) in high-performing materials (zTmax > 1) by suppressing total thermal conductivity.

Accurate density functional theory calculations of the interrelated properties of thermoelectric materials entail high computational cost, especially as crystal structures increase in complexity and size. New methods involving ab initio scattering and transport (AMSET) and compressive sensing lattice dynamics are used to compute the transport properties of quaternary CaAl2Si2-type rare-earth phosphides RECuZnP2 (RE = Pr, Nd, Er), which were identified to be promising thermoelectrics from high-throughput screening of 20 000 disordered compounds. Experimental measurements of the transport properties agree well with the computed values. Compounds with stiff bulk moduli (>80 GPa) and high speeds of sound (>3500 m s−1) such as RECuZnP2 are typically dismissed as thermoelectric materials because they are expected to exhibit high lattice thermal conductivity. However, RECuZnP2 exhibits not only low electrical resistivity, but also low lattice thermal conductivity (∼1 W m−1 K−1). Contrary to prior assumptions, polar-optical phonon scattering was revealed by AMSET to be the primary mechanism limiting the electronic mobility of these compounds, raising questions about existing assumptions of scattering mechanisms in this class of thermoelectric materials. The resulting thermoelectric performance (zT of 0.5 for ErCuZnP2 at 800 K) is among the best observed in phosphides and can likely be improved with further optimization.

Yb14MnSb11 is a p-type high-temperature thermoelectric material with operational temperatures as high as 1273 K. Rare-earth (RE) substitution into this phase has been shown to increase the melting ...

The Zintl compound Eu2ZnSb2 was recently shown to have a promising thermoelectric figure of merit, zT ∼ 1 at 823 K, due to its low lattice thermal conductivity and high electronic mobility. In the current study, we show that further increases to the electronic mobility and simultaneous reductions to the lattice thermal conductivity can be achieved by isovalent alloying with Bi on the Sb site in the Eu2ZnSb2−xBix series (x = 0, 0.25, 1, 2). Upon alloying with Bi, the effective mass decreases and the mobility linearly increases, showing no signs of reduction due to alloy scattering. Analysis of the pair distribution functions obtained from synchrotron X-ray diffraction revealed significant local structural distortions caused by the half-occupied Zn site in this structure type. It is all the more surprising, therefore, to find that Eu2ZnBi2 possesses high electronic mobility (∼100 cm2 V−1 s−1) comparable to that of AM2X2 Zintl compounds. The enormous degree of disorder in this series gives rise to exceptionally low lattice thermal conductivity, which is further reduced by Bi substitution due to the decreased speed of sound. Increasing the Bi content was also found to decrease the band gap while increasing the carrier concentration by two orders of magnitude. Applying a single parabolic band model suggests that Bi-rich compositions of Eu2ZnSb2−xBix have the potential for significantly improved zT; however, further optimization is necessary through reduction of the carrier concentration to realize high zT.

Thermoelectric materials are a unique class of compounds that can recycle energy through conversion of heat into electrical energy. A new 21–4–18 Zintl phase has been discovered in the Yb–Mn–Sb sys...

Yb14MnSb11 is a member of a remarkable structural family of compounds that are classified according to the concept of Zintl. This structure type, of which the prototype is Ca14AlSb11, provides a flexible framework for tuning structure-property relationships and hence the physical and chemical properties of compounds. Compounds within this family show exceptional high temperature thermoelectric performance at temperatures above 300 K and unique magnetic and transport behavior at temperatures below 300 K. This review provides an overview of the structure variants, the magnetic properties, and the thermoelectric properties. Suggestions for directions of future research are provided.

The Zintl phases, Yb14 MSb11 (M = Mn, Mg, Al, Zn), are now some of the highest thermoelectric efficiency p-type materials with stability above 873 K. Yb14MnSb11 gained prominence as the first p-type thermoelectric material to double the efficiency of SiGe alloy, the heritage material in radioisotope thermoelectric generators used to power NASA's deep space exploration. This study investigates the solid solution of Yb14Mg1-x Al x Sb11 (0 ≤ x ≤ 1), which enables a full mapping of the metal-to-semiconductor transition. Using a combined theoretical and experimental approach, we show that a second, high valley degeneracy (N v = 8) band is responsible for the groundbreaking performance of Yb14 MSb11 This multiband understanding of the properties provides insight into other thermoelectric systems (La3-x Te4, SnTe, Ag9AlSe6, and Eu9CdSb9), and the model predicts that an increase in carrier concentration can lead to zT > 1.5 in Yb14 MSb11 systems.

The thermoelectric figure of merit reported for n-type Mg3(Sb,Bi)2 compounds has made these materials of great engineering significance, increasing the need for accurate evaluations of their thermal conductivity. Thermal conductivity is typically derived from measurements of thermal diffusivity and determination of the specific heat capacity. The uncertainty in this method (often 10% or more) is frequently attributed to measurement of heat capacity such that estimated values are often more accurate. Inconsistencies between reported thermal conductivity of Mg3(Sb,Bi)2 compounds may be attributed to the different values of heat capacity measured or used to calculate thermal conductivity. The high anharmonicity of these materials can lead to significant deviations at high temperatures from the Dulong-Petit heat capacity, which is often a reasonable substitute for measurements at high temperatures. Herein, a physics-based model is used to assess the magnitude of the heat capacity over the entire temperature range up to 800 K. The model agrees in magnitude with experimental low-temperature values and reproduces the linear slope observed in high-temperature data. Owing to the large scatter in experimental values of high-temperature heat capacity, the model is likely more accurate (within ±3%) than a measurement of a new sample even for doped or alloyed materials. It is found that heat capacity for the solid solution series can be simply described (for temperatures: 200 K ≤ T ≤ 800 K ) by the polynomial equation: c p [ Jg − 1 K − 1 ] = 3 N R M W ( 1 + 1.3 × 10 − 4 T − 4 × 10 3 T − 2 ) , where 3 N R = 124.71 J mol − 1 K − 1 , M W is the molecular weight [ g mol − 1 ] of the formula unit being considered, and T is temperature in K. This heat capacity is recommended to be a standard value for reporting and comparing the thermal conductivity of Mg3(Sb,Bi)2 including doped or alloyed derivatives. A general form of the equation is given which can be used for other material systems.

Since the report of high zT in Si95Ge5 there has been significant interest in low Ge alloy compositions for thermoelectric applications. The application of YbP was explored as a means to lower thermal conductivity. A series of 3% phosphorus (P) doped n-type Si95Ge5 (SiGe) alloy was reacted with YbH2 (0, 1, 2%). YbP was formed in the SiGe alloy matrix from the reaction between YbH2 and P during the Spark Plasma Sintering (SPS) process. Thermoelectric property measurements were performed on sintered pellets from room temperature to 1273 K. X-ray diffraction patterns were collected from the ground powder samples and confirmed the main phase possessed diamond structured Si95Ge5 (space group: Fd 3 ¯ m) as well as the presence of YbP (space group: Fm 3 ¯ m). The carrier concentration of the sample was controlled by the amount of YbH2 added, removing some of the phosphorus to form YbP. n-type Si95Ge5 alloy samples with higher YbP amounts showed higher electrical resistivity and lower thermal conductivity attributed to loss of the P dopant. Another composite series of n-type Si95Ge5 with YbP were synthesized with additional P compositions (1, 2%). The thermoelectric properties were characterized from room temperature to 1273 K, and the samples possess electrical resistivity, carrier concentrations, and thermal conductivity as expected from the additional P dopant. The presence of YbP lowered lattice thermal conductivity when the sample was appropriately doped. The Seebeck coefficients were measured with both off-axis and uniaxial axis experimental configurations. These results show that the off-axis measurements overestimate the Seebeck coefficients of the Si95Ge5 alloy samples. This is attributed to a cold finger effect and therefore only the uniaxial data are combined for zT calculations. The n-type Si95Ge5 samples with YbP and less than 3% P dopant show similar zT compared with the Si95Ge5 sample with no YbP inclusions and 3% P dopant with a peak zT of 0.6 at 1200 K.

Summary Refractory rare-earth tellurides with the Th 3 P 4 structure type have attracted considerable interest as high-performance thermoelectric materials since the 1980s due to their high dimensionless figure of merit ( ZT ). Extensive work has been conducted on La 3−x Te 4 with peak ZT values greater than 1.1 at 1,273 K. The high ZT of La 3-x Te 4 is in part due to a large peak in the density of states near the Fermi level from the La 5d states. Here, we revisit Pr 3−x Te 4 , for which our electronic structure calculations predict a favorable modification of the density of states by the introduction of praseodymium's 4f electrons. This was experimentally verified by preparing Pr 3−x Te 4 samples with varying Pr vacancy concentrations using a mechanochemical synthesis approach. The thermoelectric properties were measured and a ZT of 1.7 at 1,200 K was achieved with Pr 2.74 Te 4 . The 50% improvement in peak ZT compared with La 3−x Te 4 resulted from an increased effective mass, improved Seebeck coefficient, and lower thermal conductivity.

The Zintl phase EuGa_2Sb_2 was synthesized via ball milling followed by hot pressing. The crystal structure of EuGa_2Sb_2 is comprised of a 3-D network of polyanionic [Ga_2Sb_2]^(2−) tunnels filled with Eu cations that provide charge balance (Eu^(2+)[Ga_2Sb_2]^(2−)). Here we report the temperature-dependent resistivity, Hall Effect, Seebeck coefficient and thermal conductivity for EuGa_(2−x)In_xSb_2 (x = 0, 0.05, 0.1) from 300 K to 775 K. Experimental results demonstrate that the material is a p-type semiconductor. However, a small band gap (∼0.1 eV) prevents EuGa_2Sb_2 from having high zT at higher temperatures. Isoelectronic substitution of In on the Ga site leads to point defect scattering of holes and phonons, thus reducing thermal conductivity and resulting in a slight improvement in zT.

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 (La_(3–x)Te_4), 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 (Nd_(3–x)Te_4), another rare-earth telluride with a structure similar to La_(3–x)Te_4. Density functional theory (DFT) calculations predicted a significant increase in the Seebeck coefficient over La_(3–x)Te_4 at equivalent vacancy concentrations because of 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 Nd_(3–x)Te_4 at various carrier concentrations. These measurements were compared to La_(3–x)Te_4 in order to elucidate the impact of the four 4f electrons of Nd on the transport properties of Nd_(3–x)Te_4. A zT of 1.2 was achieved at 1273 K for Nd_(2.78)Te_4, which is a 10% improvement over that of La_(2.74)Te_4.

Yb14ZnSb11 has been of interest for its intermediate valency and possible Kondo designation. It is one of the few transition metal compounds of the Ca14AlSb11 structure type that show metallic behavior. While the solid solution of Yb14Mn1-xZnxSb11 shows an improvement in the high temperature figure of merit of about 10% over Yb14MnSb11, there has been no investigation of optimization of the Zn containing phase. In an effort to expand the possible high temperature p-type thermoelectric materials with this structure type, the rare earth (RE) containing solid solution Yb14-xRExZnSb11 (RE = Y, La) was investigated. The substitution of a small amount of 3+ rare earth (RE) for Yb2+ was employed as a means of optimizing Yb14MnSb11 for use as a thermoelectric material. Yb14ZnSb11 is considered an intermediate valence Kondo system where some percentage of the Yb is formally 3+ and undergoes a reduction to 2+ at ~85 K. The substitution of a 3+ RE element could either replace the Yb3+ or add to the total amount of 3+ RE and provides changes to the electronic states. RE = Y, La were chosen as they represent the two extremes in size as substitutions for Yb: a similar and much larger size RE, respectively, compared with Yb3+. The composition x = 0.5 was chosen as that is the typical amount of RE element that can be substituted into Yb14MnSb11. These two new RE containing compositions show a significant improvement in Seebeck while decreasing thermal conductivity. The addition of RE increases the melting point of Yb14ZnSb11 so that the transport data from 300 K to 1275 K can be collected. The figure of merit is increased five times over that of Yb14ZnSb11 and provides a zT ~0.7 at 1275 K.

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&ndash, 1100 K. Sc2Te3 exhibited a peak zT = 0.3 over the broad range of 500&ndash, 750 K due to an appreciable power factor and low-lattice thermal conductivity in the mid-temperature range.

Within the field of thermoelectric materials for energy conversion magnesium silicide, Mg2Si, is an outstanding candidate due to its low density, abundant constituents and low toxicity. However electronic and thermal tuning of the material is a required necessity to improve its Figure of Merit, zT. Doping of Yb via reactive YbH2 into the structure is performed with the goal of reducing the thermal conductivity. Hydrogen is released as a by-product at high temperatures allowing for facile incorporation of Yb into the structure. We report on the properties of Yb- and Bi-doped Mg2Si prepared with MgH2 and YbH2 with the focus on the synthetic conditions, and samples’ microstructure, investigated by various electron microscopy techniques. Yb is found in the form of both Yb3Si5 inclusions and Yb dopant segregated at the grain boundary substituting for Mg. The addition of 1 at% Yb concentration reduced the thermal conductivity, providing a value of 30 mW/cm K at 800 K. In order to adjust carrier concentration, the sample is additionally doped with Bi. The impact of the microstructure on the transport properties of the obtained material is studied. Idealy, the reduction of the thermal conductivity is achieved by doping with Yb and the electronic transport is adjusted by doping with Bi. Large grain microstructure facilitates the electronic transport. However, the synthetic conditions that provide the optimized microstructure for electrical transport do not facilitate the additional Yb dopant incorporation. Therefore, the Yb and Bi containing sample with the optimized microstructure provides a zT=0.46 at 800 K.

Yb14MnSb11 is the most efficient bulk p-type thermoelectric material for high temperature applications. Materials with Y and Sc substitutions in Yb14MnSb11 were made both in Sn-flux and by ball milling. These small 3+ rare earth (RE) cations were introduced with the goal of providing chemical pressure on the structure. The RE3+ cation is smaller than Yb2+ and also donates one additional electron to this p-type semiconductor. In Yb14−xRExMnSb11 (RE = Sc, Y) the maximum x was about 0.5. X-ray diffraction experiments on the single crystals obtained from Sn-flux showed that Sc preferentially substitutes for Yb(1) and Yb(3), and decreases the size of the unit cell by about 0.3%. Y substitutes on all Yb sites and increases the size of the unit cell by about 0.2%. Samples with Yb14−xRExMnSb11 (x~0.3) were prepared via powder metallurgy and spark plasma sintering for transport and thermal conductivity measurements. Electron microprobe of the Sc-substituted sample showed small regions (≤1 μ m) containing greater amounts of Sc, and X-ray powder diffraction of the ball milled Sc sample could be fitted as phase pure Yb14−xScxMnSb11. Y-substituted samples showed larger regions of excess Y in electron microprobe, and small amounts of Yb4Sb3 in X-ray powder diffraction. The Sc sample has slightly reduced carrier concentration over optimized Yb14MnSb11, while the Y samples have even lower carrier concentrations. These carrier concentrations lead to comparable resistivity to Yb14MnSb11 in the Sc-substituted material, and higher resistivities in the Y-substituted material. All materials had similar Seebeck coefficients that slightly exceed Yb14MnSb11 at high temperatures, with the Sc-substituted sample having the highest despite having a higher carrier concentration. Sc-substituted samples also had a slightly higher thermal conductivity over the Y-substituted samples, which had comparable thermal conductivity to Yb14MnSb11. The zT values of the Sc and Y substituted samples are similar (zT1000 K~0.8), however below that of Yb14MnSb11 due to the compensation of Seebeck and resistivity.

This work presents a large-format thermopile focal plane module based on kilo-pixel arrays able to image from UV to far-infrared scenes. The spectral selectivity is achieved by means of butcher block filters integrated into sub-assembly modules as part of the optical telescope of the instrument.

Porous ceramics have been widely used under extreme environments due to their high strength, good thermal shock resistance, and excellent corrosion resistance. Recently, silicon aluminum oxynitride (SiAlON) ceramic, a solid solution of Si3N4 with AlN, SiO2, and Al2O3, attracted our interest because of its superior mechanical and physical properties for applications under extreme environments (i.e., high temperature, high pressure, excellent mechanical wear, and low PH). However, in spite of its many unique properties, porous SiAlON production has not been scaled up sufficiently to meet industrially relevant quantities due to its high synthesis cost and the difficulty of manufacturing articles. Here, we report on a scalable two-step carbothermal reduction and nitridation (CRN) method to synthesize mechanically robust SiAlON ceramic materials with controlled porosity levels. The morphologies and chemical composition of the synthesized porous SiAlON ceramics were characterized using SEM, XRD, EDAX, and microprobe analysis. In addition, the flexural strength of the engineered porous SiAlON ceramics is also reported in this paper.

Silicon nanoparticles produced via a plasma-based technique have been sintered into bulk nanostructured samples. These samples have micron-sized crystalline domains and contain well-dispersed oxide nanoinclusions. We have compared the thermoelectric performance of such structure to that of a control sample produced by sintering ball-milled silicon powders. The control sample has lower precipitate density and is composed of nanograins. Despite the stark difference in nanostructure, both samples have comparable thermal conductivity, and the sample with nanoinclusions has higher power factor and ZT. This result confirms that grain size engineering is not the only promising route to achieving improved thermoelectric performance. By controlling the feedstock powder processing technique, it is possible to obtain well-dispersed nanoinclusions in sintered bulk samples. These are as effective at reducing thermal transport properties as grain boundaries.

Many of the missions proposed and successfully completed by the National Aeronautics and Space Administration seek to scientifically investigate remote locations in our solar system, in particular to better understand the origin, evolution and structure of planetary systems. Long-lived, robust power systems are a fundamental capability for such missions, and radioisotope thermoelectric generators (RTGs) have proven to be a reliable power for exploration missions in deep space for the past 50 years. With increasing power needs for future missions, the improvement of thermoelectric materials’ conversion efficiency is necessary. In this paper, we show how compositing with inert metallic inclusions can be efficiently used to improve the electronic properties of Yb14MnSb11. In this study, we found that the power factor of the p-type high temperature material, Yb14MnSb11, increases from ∼8 to ∼11.5 μW cm−1 K−2 when composited with 5 vol. % W particles. At the same time, the composite samples have a higher thermal conductivity and, therefore, the final zT remains unchanged (∼1.3 at 1273 K). Preliminary hardness tests indicated a qualitative increase in mechanical robustness for the tungsten composite samples. These results can play a relevant role in device design and performance, improving the thermoelectric impedance matching for leg segmentation and helping overcome the intrinsic brittleness of high temperature ceramics such as Yb14MnSb11 for advanced device fabrication.

Yb14MnSb11 is a p-type high temperature thermoelectric material that has been shown to have a peak zT of 1.3 at 1273 K and stable lifetime testing at that temperature for over 1500 h by NASA. Yb14MgSb11 is a structural analog, but the highest temperature thermoelectric properties have not yet been reported. Yb14MgSb11 has been prepared in an environmentally friendly route employing metal hydrides to provide phase pure samples with excellent control of stoichiometry. We present a comparative study employing either MgH2 or YbH2 as a reactive precursor that also facilitates milling of the elements. High purity compositions are synthesized, and their high temperature thermoelectric properties were measured on dense pellets. Temperature-dependent thermoelectric properties were measured from 300 to 1273 K. Yb14MgSb11 exhibited a peak zT = 1.2 at 1273 K due to an appreciable power factor and low-lattice thermal conductivity. Carrier concentration and hall mobility were also measured from 300 to 1275 K and ranged from 5.3 × 1020 to 1.3 × 1021 cm−3 and from 4.7 to 0.7 cm2 V−1 S−1, respectively.

International audience; A new type-I clathrate, Ba8Cu14Ge6P26, was synthesized by solid-state methods as a polycrystalline powder and grown as a cm-sized single crystal via the vertical Bridgman method. Single-crystal and powder X-ray diffraction show that Ba8Cu14Ge6P26 crystallizes in the cubic space group Pm (3) over barn (no. 223). Ba8Cu14Ge6P26 is the first representative of anionic clathrates whose framework is composed of three atom types of very different chemical natures a transition metal, tetrel element, and pnicogen. Uniform distribution of the Cu, Ge, and P atoms over the framework sites and the absence of any superstructural or local ordering in Ba8Cu14Ge6P26 were confirmed by synchrotron X-ray diffraction, electron diffraction and high-angle annular dark field scanning transmission electron microscopy, and neutron and X-ray pair distribution function analyses. Characterization of the transport properties demonstrate that Ba8Cu14Ge6P26 is a p-type semiconductor with an intrinsically low thermal conductivity of 0.72 W m(-1) K-1 at 812 K. The thermoelectric figure of merit, ZT, for a slice of the Bridgman-grown crystal of Ba8Cu14Ge6P26 approaches 0.63 at 812 K due to a high power factor of 5.62 mu W cm(-1) K-2. The thermoelectric efficiency of Ba8Cu14Ge6P26 is on par with the best optimized p-type Ge-based clathrates and outperforms the majority of clathrates in the 700-850 K temperature region, including all tetrel-free clathrates. Ba8Cu14Ge6P26 expands clathrate chemistry by bridging conventional tetrel-based and tetrel-free clathrates. Advanced transport properties, in combination with earth-abundant framework elements and congruent melting make Ba8Cu14Ge6P26 a strong candidate as a novel and efficient thermoelectric material.

Na- or K-doped samples of Ba1-x(Na, K)(x)Ga2Sb2 were prepared by ball-milling followed by hot-pressing. The topological analysis of the electron density of BaGa2Sb2 implies a polar covalent nature of the Sb-Ga bonds in which the Sb atoms receive the electrons transferred from Ba rather than the Ga atoms. Successful doping of BaGa2Sb2 with Na or K was confirmed with combined microprobe and X-ray diffraction analysis. Alkali metal doping of BaGa2Sb2 increased the p-type charge carrier concentration to almost the predicted optimum values (similar to 10(20) h(+) cm(-3)) needed to achieve high thermoelectric performance. With increasing alkali metal concentration, electronic transport was shifted from non-degenerate semiconducting behaviour observed for BaGa2Sb2 to degenerate one for Na- or K-doped compounds. Overall, the thermoelectric figure of merit, zT, values reached up to similar to 0.65 at 750 K, considerably higher than the undoped sample (zT similar to 0.1 at 600 K), and a slight improvement relative to previously reported Zn-doped samples (similar to 0.6 at 800 K).

International audience; The structure of Ba0.5Cr5Se8 has been recently resolved, and its thermoelectric and magnetic properties have been studied. A ZT of 0.12 was found at around 800 K. Here, we report a study on the pseudo-hollandite BaxCr5Se8 solid-solution with 0.5 ≤ x ≤ 0.55 and its thermoelectric and magnetic properties. There is no significant impact either on the cell parameters depending on the cation content or on the magnetic properties. However, thermoelectric properties are radically changed depending on x content. While the low thermal conductivity, around 0.8 W m-1 K-1, remains similar for all samples, a respective increase and decrease of the resistivity and the Seebeck coefficient are observed with increasing Ba content. The maximum Seebeck coefficient is found with Ba0.5Cr5Se8 at around 635 K with 315 μV K-1, and the Seebeck coefficient then decreases and is correlated with an activation of minority charge carriers confirmed by Hall measurements. A similar but steeper behavior is observed for the Ba0.55Cr5Se8 temperature dependence plot at around 573 K. Finally, the best thermoelectric performances are found using the lowest content of Ba, unlike when x tends to 0.55, ZT approaches a tenth of the initial best value. BaxCr5Se8 compounds are antiferromagnetic with TN = 58 K. A large peak in thermal conductivity is observed around the antiferromagnetic transition for all stoichiometry. © The Royal Society of Chemistry 2016.

Complex multinary compounds (ternary, quaternary, and higher) offer countless opportunities for discovering new semiconductors for applications such as photovoltaics and thermoelectrics. However, controlling doping has been a major challenge in complex semiconductors as there are many possibilities for charged intrinsic defects (e.g., vacancies, interstitials, antisite defects) whose energy depends on competing impurity phases. Even in compounds with no apparent deviation from a stoichiometric nominal composition, such defects commonly lead to free carrier concentrations in excess of 10^(20) cm^(−3). Nevertheless, by slightly altering the nominal composition, these defect concentrations can be tuned with small variation of the chemical potentials (composition) of each element. While the variation of chemical composition is undetectable, it is shown that the changes can be inferred by mapping (in nominal composition space) the boundaries where different competing impurity phases form. In the inexpensive Zintl compound Ca_9Zn_(4+x)Sb_9, the carrier concentrations can be finely tuned within three different three-phase regions by altering the nominal composition (x = 0.2–0.8), enabling the doubling of thermoelectric performance (zT). Because of the low thermal conductivity, the zT can reach as high as 1.1 at 875 K, which is one of the highest among the earth abundant p-type thermoelectrics with no ion conducting.

Silicon (Si) nanoparticles embedded in a Mg2Si matrix (Mg2Si/xSi) have been successfully synthesized at 623 K from MgH2 and Bi containing Si nanoparticle powders. The use of MgH2 in this synthetic route avoids the formation of oxides through the generation of hydrogen and provides a route to homogeneously mixed Si nanoparticles within a doped Mg2Si matrix. The samples were characterized by powder X-ray diffraction, thermogravimetry/differential scanning calorimetry (TG/DSC), electron microprobe analysis (EMPA), and scanning transmission electron microscopy (STEM). The final crystallite size of Mg2Si obtained from the XRD patterns is about 50 nm for all the samples and the crystallite size of Si inclusions is approximately 17 nm. Theoretical calculations indicate that ∼5 mol% concentrations of Si nanoparticles with diameters in the 5–50 nm range could decrease the lattice thermal conductivity of Mg2Si by about 1–10% below the matrix value. Reduction in thermal conductivity was observed with the smallest amount of Si, 2.5 mol%. Larger amounts, x = 10 mol%, did not provide any further reduction in thermal conductivity. Analysis of the microstructure of the Bi doped Mg2Si/xSi nanocomposites showed that the Bi dopant has a higher concentration at grain boundaries than within the grains and Bi preferentially substitutes the Mg site at the boundaries. The nanocomposite carrier concentration and mobility depend on the amount of Bi and Si inclusions in a complex fashion. Agglomerations of Si start to show up clearly in the Bi doped 5 mol% nanocomposite. While the inclusions result in a lower thermal conductivity, electrical resistivity and Seebeck are negatively affected as the presence of Si inclusions influences the amount of Bi dopant and therefore the carrier concentration. The x = 2.5 mol% nanocomposite shows a consistently higher zT throughout the measured temperature range until the highest temperatures where a dimensionless figure of merit zT ∼ 0.7 was obtained at 775 K for Mg2Si/xSi with x = 0 and 2.5 mol%. With optimization of the electronic states of the matrix and nanoparticle, further enhancement of the figure of merit may be achieved.