Magnesium ferrite MgFe2O4 was synthesized with two different methods, spark plasma sintering (SPS) and conventional solid-state reaction sintering (SSRS), and thermoelectric properties were investigated. SPS processing was found to yield two attractive features: SPS at 900 °C enabled retaining the submicron particle size of 0.3–0.5 µm from ball-milling, leading to lower thermal conductivity, 3 W/mK@300 K. 1200 °C SPS sintering led to the same sample grain size of 1.0–3.0 µm as SSRS, but still exhibited significantly lower thermal conductivity of 4.3 W/mK@300 K compared to the SSRS sample with 14 W/mK@300 K, which exhibited neck formation between particles. Furthermore, while the finer microstructuring led to a reduction in the thermal conductivity, the resistivity of SPS MgFe2O4 showed little dependence on the particle size at expected thermoelectric working temperatures above 523 K, which indicates success to some degree of phonon selective scattering due to differences in mean-free-paths of electrons and phonons. As a process, SPS samples are found to exhibit four- to sevenfold enhancement of ZT compared to the conventional SSRS sample. While the maximum ZT in the present samples is relatively low, taking a value of 0.07 for the SPS 1200 °C sintered sample, the processing insights may be utilized for similar systems.

Boron carbide/hafnium diboride composites were prepared by spark plasma sintering of a mixture of hafnium diboride and boron carbide powders. Boron carbide was prepared with a 13.3 at.% composition of carbon, known as the ideal carbon content to maximize the dimensionless figure of merit. The hafnium diboride content was varied between 0 and 20% by weight, and the effect on the thermoelectric properties was studied. Addition of HfB2 generally yielded an increase in the electrical conductivity and simultaneously a reduction in thermal conductivity, indicating it has potential as an enhancer of thermoelectric properties. However, the increase in electrical conductivity was not as large as observed in some composite systems, since HfB2 turned out to be a poor sintering additive leading to lower relative densities, and was furthermore offset by a moderate decrease in Seebeck coefficient. For future composite design, the sintering characteristics of the additives can be concluded as an important additional parameter to be taken into account. The optimal hafnium diboride content for relatively dense samples was found to be 10 wt%, resulting in an improvement in the maximum figure of merit, up to ZT = 0.20 at 730 °C.

Chalcogenide compounds of composition ABX2 (with A, B, representing metal elements, X as the chalcogenide) have recently been attracting attention as thermoelectric materials. Possible magnetic enhancement of the thermoelectric properties has been proposed for electron-doped chalcopyrite CuFeS2 which exhibits large power factors at room temperature. Promising ZT values close to 1 have been reported for TmAgTe2 or YCuTe2 due to very low thermal conductivities (< 0.5 W m−1 K−1 at 800 K) resulting from a disordered sublattice of Cu atoms. To further explore the possibilities of the ABX2 compounds, we study GdCu1+xTe2 which is a monoclinic layered compound that crystallizes in the C2/m space group, and has the f-electron Gd as a magnetic element, versus d-electron Fe in CuFeS2. Although the stoichiometric compound could not be stabilized, we report on the properties of this series of compounds sintered by spark plasma sintering (SPS). As for the other materials of ABX2 type, compositions with x > 0.25 show very low thermal conductivities (0.64 W m−1 K−1 above 500 K for x = 0.25), which, together with a Seebeck coefficient above 0.2 mV K−1, allows to reach a moderate ZT value near 0.2 at 540 K. We also discuss the dependence with the Cu excess content of the transport properties and the lack of effect of the magnetic moment of Gd on the Seebeck coefficient.View of the crystal structure of GdCuTe2 where Cu atom positions are half filled in the stoichiometric compound. Comprehensive investigation into thermoelectric transport properties and magnetic properties was made.Download high-res image (249KB)Download full-size image

We devised an alternative, faster and simple method to prepare hierarchically nanoporous ZnO powders (NZnO) via a quick interfacial reaction (double emulsion) from the mixtures of zinc nitrate hexahydrate (solution, Zn(NO3)2·6H2O), n-hexane (C6H14), ammonium bicarbonate (NH4HCO3), Tween 80 (C64H124O26) and Span 80 (C24H44O6), which was calcined at 450 °C. Due to the micro/nanopore structures, the obtained NZnO powders had a larger surface area (∼69.7 m2 g−1) than the commercial ZnO powders. For the thermoelectric evaluation, the textured NZnO pellets (T-NZnOP) were prepared by low pressure spark plasma sintering (SPS) using the newly synthesized NZnO powders (NZnO). In this study, the synthesized NZnO powders that were made into T-NZnOP (sample pellets) showed a significantly lower thermal conductivity with distinctive electrical properties when compared to the bulk commercial ZnO powders. The thermoelectric enhancement can be attributed to the nanopore distribution found in the porous T-NZnOP material which demonstrates the potential usefulness of this method for other porous oxide thermoelectric (TE) materials.

Chalcopyrite CuGaTe2 is under research for its high thermoelectric performance. Different routes have been investigated recently for enhancing its thermoelectric parameters. In this work we report the synthesis of chalcopyrite CuGa1−xMnxTe2 (x = 0.0, 0.01, 0.02, and 0.03) by a solid state method and through which an enhanced power factor was obtained. The samples were characterized for electrical, thermal and thermoelectric transport properties in the temperature range 325–870 K after performing stability analysis using TG-DTA data. XRD patterns confirm a phase pure tetragonal structure for all nominal compositions with the space group I2d. The electrical conductivity σ increases drastically by Mn2+ doping which increases the hole carriers, while the Seebeck coefficient S still retains large positive values. As a result, the power factor of CuGa0.99Mn0.01Te2 reaches 1.55 mW K−2 m−1 at 718 K. Calculations using the relationship of S and ln σ suggest that the power factor observed for Mn-doped samples is higher than that expected for CuGaTe2 with optimized carrier concentration, suggesting that the Mn-doping brings additional effects other than simple carrier tuning. The total thermal conductivity is reduced by Mn doping, with a minimum thermal conductivity of 1.6 W m−1 K−1 for the x = 0.01 sample. The maximum value for ZT reached at 870 K was 0.83, which is more than 40% enhancement as compared to that of pure CuGaTe2. Strong interactions between the magnetic moments of Mn and charge carriers are inferred by the large negative Weiss temperature in the magnetic susceptibility and distinct anomalous Hall effect, the latter of which develops in accordance with the increase of magnetization at low temperature. These results suggest that the carrier–magnetic moment interaction plays an essential role in the enhancement of the thermoelectric properties of CuGa1−xMnxTe2.

•Micro/nanostructures in thermoelectric skutterudite were induced through evaporation.•These micro and nano pores strongly achieve phonon selective scattering.•Unique design gives rise to unprecedented high ZT=1.6 for filler-free skutterudites.Increasing energy demands require new materials, e.g., thermoelectrics, for efficient energy conversion of fossil fuels. However, their low figure of merit (ZT) limits widespread applications. Nanostructuring has been an effective way of lowering the thermal conductivity. However, grain growth at elevated temperature is still a big concern, for otherwise expected to be long-lasting thermoelectric generators. Here, we report a porous architecture containing nano- to micrometer size irregularly shaped and randomly oriented pores, scattering a wide spectrum of phonons without employing the conventional rattling phenomenon. Lattice thermal conductivity reaches the phonon glass limit. This design yields >100% enhancement in ZT, as compared to the pristine sample. An unprecedented and very promising ZT of 1.6 is obtained for Co23.4Sb69.1Si1.5Te6.0 alloy, by far the highest ZT ever reported for un-filled skutterudites, with further benefits, i.e. rare-earth-free and improved oxidation resistance enabling simple processing.Figure optionsDownload full-size imageDownload high-quality image (137 K)Download as PowerPoint slide

•Surprisingly metal-rich YB48 single crystals with YB66 type structure were grown.•The lower limit of homogeneity range of YB66 was extended from YB56 to YB48.•Power factor of YB48 was much higher than that of YB66.•ZT was 30 times that of YB66, making it a promising high temperature thermoelectric.It was discovered that the well-known higher boride YB66, one of the first reported phonon glass electron crystals (PGEC), could be obtained in a much more metal-rich composition than previously thought possible. Using the floating zone growth method, YB48 single crystals with YB66 crystal structure could be obtained, and their thermoelectric properties measured. This expansion of the homogeneity range of the well-known YB66 compound is surprising and a new Y atomic site was discovered. YB48 exhibits much higher power factors than YB66 which increase rapidly with increasing temperature. The obtained dimensionless figure of merit of this compound at 990 K is approximately 30 times higher than that of previously reported YB66 samples, and higher than any other pristine higher boride. This discovery reveals YB48 as a promising high temperature thermoelectric material.

SmB62 single crystals were successfully grown by the floating zone (FZ) method. The high-temperature thermoelectric properties were investigated, together with magnetic properties and specific heat at low-temperature. The electrical resistivity, ρ, shows variable-range-hopping (VRH) behavior with significantly lower values than other rare-earth RB62 (RB66) compounds. An effective magnetic moment, μeff, of 0.42 μB/Sm was estimated, which if straightforwardly taken indicates a mixed valency for SmB62 with Sm2+:Sm3+ = 1:1, which is the first ever indicated for RB66-type compounds. Localization length of the VRH at the Fermi level, ξ, was estimated to be 3.33 Å indicating that carriers in SmB62 are much less localized than in YB66 which has 0.56 Å. The thermoelectric behavior of SmB62 is striking, with ρ reduced by two orders of magnitude while maintaining large Seebeck coefficients, and as a result the power factor is ∼30 times higher than other rare-earth phases. Overall the figure of merit ZT amounts to ∼0.13 at 1050 K, with an extrapolated value of ∼0.4 at 1500 K, an expected working temperature for topping cycles in thermal power plants; that gives a ∼40 times enhancement for Sm. Since there are few thermoelectric materials applicable for very-high temperature applications, this discovery gives new interest in the samarium higher borides.RB62 (RB66) compounds can be considered to be the first actual phonon glass electron crystal (PGEC) system. SmB62 single crystals were successfully grown by the floating zone (FZ) method. Mixed valency of Sm was indicated, which is the first instance in RB66-type compounds. The thermoelectric behavior of SmB62 is striking, with ρ reduced by two orders of magnitude while maintaining large Seebeck coefficients, and as a result the power factor is ∼30 times higher than other rare-earth phases. Overall the figure of merit ZT amounts to ∼0.13 at 1050 K, with an extrapolated value of ∼0.4 at 1500 K, the expected working temperature for topping cycles in thermal power plants; that gives a ∼40 times enhancement for Sm.Figure optionsDownload full-size imageDownload as PowerPoint slide

Developing reliable synthetic methods for the fabrication of nanostructures of prospective materials is vital for emerging technologies like thermoelectrics that are in need of a breakthrough. In thermoelectrics nanostructuring can potentially enhance phonon scattering while preserving electrical conductivity due to the different length scales of phonons and electrical carriers. A solvothermal reaction between a colloidal dispersion of dodecylsulfate intercalated copper hydroxide layers in ethylene glycol and an alkaline solution of TeO2 was found to successfully yield single crystalline 2D nanosheets of copper telluride, Cu1.75Te. Ethylene glycol reduces TeO2 to Te2− and Cu2+ to Cu+ leading to the formation of Cu1.75Te. The solvated copper hydroxide layers act as templates to facilitate the formation of nanosheets. The nanosheets are a few nanometers in thickness and their lateral dimensions are in the order of micrometers. Thermoelectric measurements suggest that the nanosheet fabrication helps in dramatically decreasing the lattice thermal conductivity thereby increasing ZT.

Synthesis conditions, morphology, and thermoelectric properties of Y1−xB28.5C4 were investigated. Y1−xB28.5C4 is the compound with the lowest metal content in a series of homologous rare earth borocarbonitrides, which have been attracting interest as high temperature thermoelectric materials because they can embody the long-awaited counterpart to boron carbide, one of the few thermoelectric materials with a history of commercialization. It was revealed that the presence of boron carbide inclusions was the origin of the p-type behavior previously observed for Y1−xB28.5C4 in contrast to Y1−xB15.5CN and Y1−xB22C2N. In comparison with that of previous small flux-grown single crystals, a metal-poor composition of YB40C6 (Y0.71B28.5C4) in the synthesis successfully yielded sintered bulk Y1−xB28.5C4 samples apparently free of boron carbide inclusions. “Pure” Y1−xB28.5C4 was found to exhibit the same attractive n-type behavior as the other rare earth borocarbonitrides even though it is the most metal-poor compound among the series. Calculations of the electronic structure were carried out for Y1−xB28.5C4 as a representative of the series of homologous compounds and reveal a pseudo gap-like electronic density of states near the Fermi level mainly originating from the covalent borocarbonitride network.

Excellent control in p- and n-type transport characteristics was previously obtained for the thermoelectric YxAlyB14 compounds through Al flux method. In this study, new attempts were made to reduce their grain sizes to obtain dense samples and to possibly lower the thermal conductivity. Introducing the reduction of grain sizes into YxAlyB14 samples was attempted by two methods; one was through mechanical grinding, and the other was by synthesizing YxAlyB14 via Y0.56B14 (denoted as “vYB-YAlB14”). Mechanical grinding using ball milling with Si3N4 pots and balls was found not to be an efficient way to decrease the grain size because of contamination of Si3N4. In contrast, vYB-YAlB14 samples were successfully synthesized. Through the synthesis of Y0.56B14, the boron network structure was first formed. Afterward, YxAlyB14 was obtained by adding Al in the boron network structure through a heat treatment. Due to shorter heating time at lower temperature, the grain sizes were discovered to be smaller than that of Al flux method. The decrease of grain size was found to be beneficial for the densification of YxAlyB14 and the decrease of its thermal conductivity.

•We investigated B/C substitution in YB22C2N which is potentially the long awaited n-type counterpart to p-type boron carbide.•The B/C substitution range of YB22C2N was indicated to be very small compared to boron carbide, and the substitution was found to occur in the –C–B–C– chains.•Small B/C ratio was indicated to significantly improve the thermoelectric properties of YB22C2N.Structural and thermoelectric characteristics for a series of samples Y1−xB22+yC2−yN have been studied. The compound crystallizes with a Y1−xB22C2N-type structure, R-3m space group. The Y1−xB22C2N compound has previously been revealed to exhibit n-type behavior and is potentially the long awaited counterpart of p-type boron carbide. Rietveld refinement revealed a variable boron and carbon composition ranging within 0.08≤y≤0.25 at 0.26≤x≤0.30. The changes in stoichiometry in Y1−xB22+yC2−yN are realized via B/C substitution in one of the C sites of –C–B–C– chains. A small B/C ratio, namely increase of carbon, was revealed to positively affect the thermoelectric properties. The presence of YB28.5C4 as an additive phase was also shown to be beneficial for the thermoelectric properties.

High-quality one dimensional amorphous SiO2 nanostructures with different morphologies (nanowires and starfish-like nanostructures) are synthesized through a simple catalysis-free approach and effective thermal evaporation process. The morphologies, microstructures, and compositions of the products are investigated by X-ray powder diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). A promising optical property (cathodoluminescence (CL) in a strong ultraviolet (UV) emission and a weak blue emission at room temperature) was detected in the as-synthesized nanostructures. Field-emission measurements show that the SiO2 nanostructures may also be a promising FE emitter candidate if we can improve the conductivity and decrease the density of the nanostructures.Highlights► A simple catalysis-free approach and effective thermal evaporation process ► High-quality one dimensional amorphous SiO2 nanostructures ► A strong ultraviolet (UV) emission and a weak blue emission at room temperature ► A promising FE emitter's candidate

High quality CeB6 thin films have been obtained through direct evaporation of raw micron-sized CeB6 powders at a pressure of 70 Pa. The X-ray diffraction (XRD), Raman spectrum, scanning electron microscope (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and the field-emission equipment were used to characterize the morphology, structure, composition and FE properties of the samples. The XRD and Raman spectrum analysis results show the as-prepared product is cubic phase CeB6. The TEM, SAED and HRTEM analysis reveal that the samples are mixtures of thin films (polycrystalline) and small crystals (single crystallines aligned preferentially in the [1 1 0] direction). Compared to oxide nanostructures, field-emission measurements show that the CeB6 films have better FE performance with turn-on field and threshold field of 12.93 V/μm and 14.86 V/μm, respectively.Highlights► High quality CeB6 thin films. ► Direct evaporation of raw micron-sized CeB6 powders. ► Better FE performance.

On a previous study on samples of doped-YB44Si2, an improvement of thermoelectric properties has been achieved. Regarding the interesting effect of the doping of transition elements on the thermoelectric properties, a single crystal study has been carried out on Zn doped, Rh doped and Ni doped samples to assess how the transition element doping affects the crystal structure. Refinements were carried out based on the structural model solution of YB44Si2 reported in a previous study. Variations in the silicon contents were found in the doped single crystals. Splitting of partially occupied sites has also been detected for some of the doped samples. In this paper we present differences in the partial occupations of boron and silicon sites. Possibility of transition elements insertions based on the differences in crystal structures will be presented.Graphical AbstractNew transition elements doped YB44Si2 were synthesized and have nominal compositions YB41.1Si1.1Rh0.02 and YB41Si1.3Ni0.06. Insertion of transition elements into the crystal structure of YB44Si2 leads to the transformation of B12 icosahedra into B11 polyhedrons for a few percent of them.Highlights► Differences in the partial occupations of boron and silicon sites→Possibility of transition elements insertions. ► Mixed occupancy of split positions. ► Insertion of transition elements between B12 icosahedra.

Transition-metal (Mo, Mn, Fe, Rh, Ti, Cu, Zn) doping was carried out on the borosilicide compound REB44Si2 (RE = rare earth). REB44Si2 compounds exhibit Seebeck coefficients greater than 200 µV K−1 at high temperatures and unlike most compounds, the figure of merit shows a steep increase at T > 1000 K making them promising high-temperature thermoelectric materials. Although zinc itself does not remain in the final product, zinc doping was found to improve the crystal quality, which has been a long-standing problem for the borosilicides. As a result, a significant increase of the thermoelectric power factor by more than 30% was achieved.

Thermoelectric properties of a series of layered homologous rare-earth boron carbonitrides: HoB17CN, REB22C2N (RE=Y,Er,Lu)(RE=Y,Er,Lu), and YB28.5C4, were investigated. Samples for measurements were prepared in the form of hot pressed or isostatically pressed and annealed single phase polycrystalline powder. This series of compounds has structures where B6 octahedral and rare-earth atomic layers reside between an increasing number of B12 icosahedral and C–B–C chain layers, and has structural analogy to boron carbide. Interestingly, a variation from p-type thermoelectric behavior for YB28.5C4 to n-type for REB22C2N and HoB17CN was observed. This is the first non-doped compound among the boron-rich borides in which n-type thermoelectric behavior has been observed. Similar to other boron cluster compounds low values of the thermal conductivity κκ(κ⩽0.02Wcm-1K) were found. The origins of the low κκ in such compounds has not been fully explained, but comparison among the homologous series shows that the thermal conductivity appears to increase as the number of boron cluster layers increases. This result indicates that the heavy rare-earth atoms residing in the boron matrix may play a role in depressing thermal conductivity in addition to other features common to boron cluster compounds. Although the absolute values of the determined figures of merit ZT are not large for hot pressed samples, the Seebeck coefficients and power factors for both n-type and p-type in this series show an increase at temperatures exceeding 1000 K.Both p-type and n-type thermoelectric behavior are observed in this homologous boron cluster compound series. This is the first unmodified compound among the boron-rich borides in which n-type behaviour has been observed. Comparative thermal conductivity results indicate the heavy rare earth atoms residing in the boron matrix play a role to depress thermal conductivity.

We have been investigating the high-temperature thermoelectric properties of some novel rare earth borides with a structure containing B12 icosahedra. Doping effects on the TE properties in such systems were investigated for the first time. A series of Nb-doped YB66 and C-doped YB66 single crystals were grown by the floating zone method. The Nb-doped compounds have approximate chemical formulas ranging from YNb0.30B66 to YNb0.33B66 while the C-doped compound has a formula of YB66C0.6. The effect of Nb-doping on the thermoelectric properties was not monotonic and appears to be complex. As a result of Nb-doping, the room temperature resistivity and the characteristic temperature T0 were considerably reduced. At room temperature the power factor of the Nb-doped YB66 sample with 89% site occupancy was three times greater than that of non-doped YB66. However, in the important high-temperature region, the non-doped sample actually exhibited the highest power factor for T>550K. Furthermore, owing to a structural feature of YB66, thermal conductivity actually increases with doping of transition metals. Taking into account all the thermoelectric properties, transition metal doping of YB66 is therefore not suitable for our purposes. On the other hand, doping of carbon, which is assumed not to go into the same sites as the transition metals, yielded a lowering of the thermal conductivity. Furthermore, contrary to Nb-doping, carbon doping did not result in a reversal of the relative magnitude of resistivity at extremely high temperatures and therefore, an increase in the figure of merit of factor 2 was realized at 1000 K.View of the structure of Nb-doped YB66 around the doping site of (1/4,1/4,1/4). Boron atoms (green circles), yttrium atom (red circle) and Nb atom (black circle) are displayed. The Nb atom replaces a short B–B dumbbell pair.

Neutron diffraction of the B12 cluster compound Tb11B44Si2 was measured to investigate the magnetic structure. TbB50 and isostructural TbB44Si2 exhibit the first magnetic transitions ever observed in the magnetically dilute higher boride compounds. An antiferromagnetic transition was observed in the magnetic properties measurement of Tb11B44Si2 at TN=16.4 K. Due to the boron-rich nature of the system, a modified synthesis technique was used in order to obtain a sample as free of 10B as possible. Neutron diffraction patterns at 300 and 4 K were successfully obtained. No significant additional peaks due to magnetic ordering were observed at 4 K and it is indicated that the magnetic transitions in these REB50-type systems are of non-long range order character.

Physical properties of a series of homologous RE–B–C(N) B12 cluster compounds REB17CN, REB22C2N, and were investigated. The structures of the compounds are layer-like along the c-axis, with rare earth and B6 octahedral layers separated by B12 icosahedral and C–B–C chain layers whose number increases successively from two B12 layers for the REB17CN compound to four for the REB28.5C4 compound. The rare earth atoms are configured in two triangular flat layers which are stacked on top of one another in AB stacking where the nearest-neighbor rare earth directions are the three atoms forming a triangle in the adjacent layer. The series of homologous compounds exhibit a spin glass transition with Tf shifting in correspondence with variations of the basal plane lattice constants, consistent with the magnetic interaction being effective in the basal planes. The isothermal remanent magnetization shows a stretched exponential decay . Exponents determined for the different homologous compounds were scaled as a function of Tr=T/Tf and found to follow the empirical dependency determined for typical spin glasses. It is indicated that a mixture of disorder originating from the partial occupancy of the rare earth sites and frustration of interactions due to the unique configuration is responsible for the manifestation of spin glass transitions in these homologous systems.

The gadolinium phase of the YB50 structure-type compounds was successfully obtained for the first time. In the case of the REB50 compounds, which were recently discovered, the rare-earth atom series smaller than gadolinium (terbium Tb to lutethium Lu) were found to form compounds. This system is interesting, because it displays the first magnetic transition ever observed in the B12 borides. There is a high incentive to synthesize the gadolinium phase of these compounds, because the mechanism of the transition is not completely clear, and data available up to now have shown that the Curie-Weiss temperatures roughly follow a dependence on the deGennes factor. By adding silicon, we were successful in synthesizing gadolinium borosilicide GdB41Si1.2, which is isostructural to YB50. Magnetic susceptibility measurements show that the Curie-Weiss temperature of GdB41Si1.2 is −7.2 K and lower than the terbium borosilicide analog, thus showing a deviation from the deGennes factor dependence. Although a sizable paramagnetic tail obscured the behavior of the low temperature susceptibility, measurement of the specific heat of GdB41Si1.2 revealed a peak, unambiguously indicating a magnetic transition with a transition temperature of 4 K.

•Successful growth of strontium hexaboride thin films by a CVD process.•Growth achieved using elemental metal sources together with decaborane.•Remarkably fast growth achieved.•Demonstrates an effective way to deposit highly crystalline boride thin films.Thin films of SrB6 were deposited on sapphire substrates using a chemical vapor deposition method, with elemental strontium and decaborane, B10H14, used as the strontium and boron sources, respectively. The formation of highly crystalline, phase-pure SrB6 films was confirmed with X-ray diffraction and reflection high energy diffraction (RHEED) analysis, and the films’ thermoelectric transport properties were measured. A relatively high deposition temperature of 850–950 °C was found to be optimal for obtaining well-crystallized films at an extremely high deposition rate. The thermoelectric transport properties of the SrB6 thin films were observed to be comparable to those reported for bulk materials, but an unexpectedly high electrical resistivity led to a reduced power factor value for the thin films.

Chalcopyrite CuGaTe2 is under research for its high thermoelectric performance. Different routes have been investigated recently for enhancing its thermoelectric parameters. In this work we report the synthesis of chalcopyrite CuGa1−xMnxTe2 (x = 0.0, 0.01, 0.02, and 0.03) by a solid state method and through which an enhanced power factor was obtained. The samples were characterized for electrical, thermal and thermoelectric transport properties in the temperature range 325–870 K after performing stability analysis using TG-DTA data. XRD patterns confirm a phase pure tetragonal structure for all nominal compositions with the space group I2d. The electrical conductivity σ increases drastically by Mn2+ doping which increases the hole carriers, while the Seebeck coefficient S still retains large positive values. As a result, the power factor of CuGa0.99Mn0.01Te2 reaches 1.55 mW K−2 m−1 at 718 K. Calculations using the relationship of S and lnσ suggest that the power factor observed for Mn-doped samples is higher than that expected for CuGaTe2 with optimized carrier concentration, suggesting that the Mn-doping brings additional effects other than simple carrier tuning. The total thermal conductivity is reduced by Mn doping, with a minimum thermal conductivity of 1.6 W m−1 K−1 for the x = 0.01 sample. The maximum value for ZT reached at 870 K was 0.83, which is more than 40% enhancement as compared to that of pure CuGaTe2. Strong interactions between the magnetic moments of Mn and charge carriers are inferred by the large negative Weiss temperature in the magnetic susceptibility and distinct anomalous Hall effect, the latter of which develops in accordance with the increase of magnetization at low temperature. These results suggest that the carrier–magnetic moment interaction plays an essential role in the enhancement of the thermoelectric properties of CuGa1−xMnxTe2.

Developing reliable synthetic methods for the fabrication of nanostructures of prospective materials is vital for emerging technologies like thermoelectrics that are in need of a breakthrough. In thermoelectrics nanostructuring can potentially enhance phonon scattering while preserving electrical conductivity due to the different length scales of phonons and electrical carriers. A solvothermal reaction between a colloidal dispersion of dodecylsulfate intercalated copper hydroxide layers in ethylene glycol and an alkaline solution of TeO2 was found to successfully yield single crystalline 2D nanosheets of copper telluride, Cu1.75Te. Ethylene glycol reduces TeO2 to Te2− and Cu2+ to Cu+ leading to the formation of Cu1.75Te. The solvated copper hydroxide layers act as templates to facilitate the formation of nanosheets. The nanosheets are a few nanometers in thickness and their lateral dimensions are in the order of micrometers. Thermoelectric measurements suggest that the nanosheet fabrication helps in dramatically decreasing the lattice thermal conductivity thereby increasing ZT.

Transition-metal (Mo, Mn, Fe, Rh, Ti, Cu, Zn) doping was carried out on the borosilicide compound REB44Si2 (RE = rare earth). REB44Si2 compounds exhibit Seebeck coefficients greater than 200 µV K−1 at high temperatures and unlike most compounds, the figure of merit shows a steep increase at T > 1000 K making them promising high-temperature thermoelectric materials. Although zinc itself does not remain in the final product, zinc doping was found to improve the crystal quality, which has been a long-standing problem for the borosilicides. As a result, a significant increase of the thermoelectric power factor by more than 30% was achieved.

Synthesis conditions, morphology, and thermoelectric properties of Y1−xB28.5C4 were investigated. Y1−xB28.5C4 is the compound with the lowest metal content in a series of homologous rare earth borocarbonitrides, which have been attracting interest as high temperature thermoelectric materials because they can embody the long-awaited counterpart to boron carbide, one of the few thermoelectric materials with a history of commercialization. It was revealed that the presence of boron carbide inclusions was the origin of the p-type behavior previously observed for Y1−xB28.5C4 in contrast to Y1−xB15.5CN and Y1−xB22C2N. In comparison with that of previous small flux-grown single crystals, a metal-poor composition of YB40C6 (Y0.71B28.5C4) in the synthesis successfully yielded sintered bulk Y1−xB28.5C4 samples apparently free of boron carbide inclusions. “Pure” Y1−xB28.5C4 was found to exhibit the same attractive n-type behavior as the other rare earth borocarbonitrides even though it is the most metal-poor compound among the series. Calculations of the electronic structure were carried out for Y1−xB28.5C4 as a representative of the series of homologous compounds and reveal a pseudo gap-like electronic density of states near the Fermi level mainly originating from the covalent borocarbonitride network.