Thermoelectric properties of Co substituted synthetic tetrahedrite

Transition metal atom (Co) substituted synthetic tetrahedrite compounds Cu12-xCoxSb4S13 (x = 0, 0.5, 1.0, 1.5, 2.0) were prepared by solid state synthesis. X-Ray Diffraction (XRD) patterns revealed tetrahedrite as the main phase, whereas for the compounds with x = 0, 0.5 a trace of impurity phase Cu3SbS4 was observed. The surface morphology showed a large grain size with low porosity, which indicated appropriate compaction for the hot pressed samples. The phase purity, as monitored by Electron Probe Micro Analysis (EPMA) is in good agreement with the XRD data. The elemental composition for all the compounds almost matched with the nominal composition. The X-ray Photoelectron Spectroscopy (XPS) data showed that Cu existed in both +1 and +2 states, while Sb exhibited +3 oxidation states. Elastic modulus and hardness showed a systematic variation with increasing Co content. The electrical resistivity and Seebeck coefficient increased with increase in the doping content due to the decrease in the number of carriers caused by the substitution of Co2+ on the Cu1+ site. The positive Seebeck coefficient for all samples indicates that the dominant carriers are holes. A combined effect of resistivity and Seebeck coefficient leads to the maximum power factor of 1.76 mW m(-1) K-2 at 673 K for Cu11.5Co0.5Sb4S13. This could be due to the optimization in the carrier concentration by the partial substitution of Co2+ on both the Cu1+ as well as Cu2+ site at the same doping levels, which is also supported by the XPS data. The total thermal conductivity systematically decreased with increase of doping content as it is mainly influenced by the decrease of carrier thermal conductivity. The maximum thermoelectric figure of merit zT = 0.98 was obtained at 673 K for Cu11.5Co0.5Sb4S13. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Thermoelectric Properties of In-Doped Cu2ZnGeSe4

Recently, much research has been focused on finding new thermoelectric materials. Cu-based quaternary chalcogenides that belong to A(2)BCD(4) (A = Cu; B = Zn, Cd; C = Sn, Ge; D = S, Se, Te) are wide band gap materials and one of the potential thermoelectric materials due to their complex crystal structures. In this study, In-doped quaternary compounds Cu2ZnGe1-xInxSe4 (x = 0, 0.025, 0.05, 0.075, 0.1) were prepared by a solid state synthesis method. Powder x-ray diffraction patterns of all the samples showed a tetragonal crystal structure (space group I-42m) of the main phase with a trace amount of impurity phases, which was further confirmed by Rietveld analysis. The elemental composition of all the samples showed a slight deviation from the nominal composition with the presence of secondary phases. All the transport properties were measured in the temperature range 373-673 K. The electrical resistivity of all the samples initially decreased up to similar to 470 K and then increased with increase in temperature upto 673 K, indicating the transition from semiconducting to metallic behavior. Positive Seebeck coefficients for all the samples revealed that holes are the majority carriers in the entire temperature range. The substitution of In3+ on Ge4+ introduces holes and results in the decrease of resistivity as well as the Seebeck coefficient, thereby leading to the optimization of the power factor. The lattice thermal conductivity of all the samples decreased with increasing temperature, indicating the presence of phonon-phonon scattering. As a result, the thermoelectric figure of merit (zT) of the doped sample showed an increase as compared to the undoped compound.

Thermoelectric properties of indium doped Cu2CdSnSe4

Recently, research in copper-based quaternary chalcogenide materials has been found to be interesting for the study of thermoelectric properties because of their low thermal conductivity due to complex crystal structures. In the present work, stoichiometric quaternary chalcogenide compounds Cu2CdSn1-xInxSe4(x = 0, 0.025, 0.05, 0.1) were prepared by solid state synthesis. The powder X-ray diffraction patterns of all the samples showed a tetragonal crystal structure with the space group I (4) over bar 2m of the main phase. In addition to this phase, a small amount of impurity phase CdSe was present in all the samples, as confirmed by Rietveld analysis. The elemental composition of all the samples characterized by an Electron Probe Micro Analyzer showed a slight deviation from the nominal composition. The transport properties were measured in the temperature range of 350 K-723 K. The positive Seebeck coefficient of all the compounds indicate that the majority carriers are holes. The Seebeck coefficient and electrical resistivity did not follow the trend in the expected manner with In doping, which could be influenced by the presence of the impurity phases. The total thermal conductivity of all the samples was dominated by the lattice thermal conductivity, while the electronic contribution was very small due to the low carrier contribution. A lattice thermal conductivity decrease with an increase of temperature indicates the dominance of phonon-phonon scattering at higher temperatures. The maximum figure of merit zT = 0.30 at 723 K was obtained for the compound Cu2CdSn0.9In0.1Se4. (C) 2016 Elsevier Ltd. All rights reserved.

Microstructure control and iodine doping of bismuth telluride

On the materials scale, thermoelectric efficiency is defined by the dimensionless figure of merit zT. This value is made up of three material components in the form zT = Tα²/ρκ, where α is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the total thermal conductivity. Therefore, in order to improve zT would require the reduction of κ and ρ while increasing α. However due to the inter-relation of the electrical and thermal properties of materials, typical routes to thermoelectric enhancement come in one of two forms. The first is to isolate the electronic properties and increase α without negatively affecting ρ. Techniques like electron filtering, quantum confinement, and density of states distortions have been proposed to enhance the Seebeck coefficient in thermoelectric materials. However, it has been difficult to prove the efficacy of these techniques. More recently efforts to manipulate the band degeneracy in semiconductors has been explored as a means to enhance α.

The other route to thermoelectric enhancement is through minimizing the thermal conductivity, κ. More specifically, thermal conductivity can be broken into two parts, an electronic and lattice term, κ_e and κ_l respectively. From a functional materials standpoint, the reduction in lattice thermal conductivity should have a minimal effect on the electronic properties. Most routes incorporate techniques that focus on the reduction of the lattice thermal conductivity. The components that make up κ_l (κ_l = 1/3Cνl) are the heat capacity (C), phonon group velocity (ν), and phonon mean free path (l). Since the difficulty is extreme in altering the heat capacity and group velocity, the phonon mean free path is most often the source of reduction.

Past routes to decreasing the phonon mean free path has been by alloying and grain size reduction. However, in these techniques the electron mobility is often negatively affected because in alloying any perturbation to the periodic potential can cause additional adverse carrier scattering. Grain size reduction has been another successful route to enhancing zT because of the significant difference in electron and phonon mean free paths. However, grain size reduction is erratic in anisotropic materials due to the orientation dependent transport properties. However, microstructure formation in both equilibrium and nonequilibrium processing routines can be used to effectively reduce the phonon mean free path as a route to enhance the figure of merit.

This work starts with a discussion of several different deliberate microstructure varieties. Control of the morphology and finally structure size and spacing is discussed at length. Since the material example used throughout this thesis is anisotropic a short primer on zone melting is presented as an effective route to growing homogeneous and oriented polycrystalline material. The resulting microstructure formation and control is presented specifically in the case of In₂Te₃-Bi₂Te₃ composites and the transport properties pertinent to thermoelectric materials is presented. Finally, the transport and discussion of iodine doped Bi₂Te₃ is presented as a re-evaluation of the literature data and what is known today.

Enhanced Thermoelectric Performance at the Superionic Phase Transitions of Mixed Ion-Electron Conducting Materials

The quality of a thermoelectric material is judged by the size of its temperature de- pendent thermoeletric-figure-of-merit (zT ). Superionic materials, particularly Zn₄Sb₃ and Cu₂Se, are of current interest for the high zT and low thermal conductivity of their disordered, superionic phase. In this work it is reported that the super-ionic materials Ag₂Se, Cu₂Se and Cu_1.97Ag_0.03Se show enhanced zT in their ordered, normal ion-conducting phases. The zT of Ag₂Se is increased by 30% in its ordered phase as compared to its disordered phase, as measured just below and above its first order phase transition. The zT ’s of Cu₂Se and Cu_1.97Ag_0.03Se both increase by more than 100% over a 30 K temperatures range just below their super-ionic phase transitions. The peak zT of Cu₂Se is 0.7 at 406 K and of Cu_1.97Ag_0.03Se is 1.0 at 400 K. In all three materials these enhancements are due to anomalous increases in their Seebeck coefficients, beyond that predicted by carrier concentration measurements and band structure modeling. As the Seebeck coefficient is the entropy transported per carrier, this suggests that there is an additional quantity of entropy co-transported with charge carriers. Such co-transport has been previously observed via co-transport of vibrational entropy in bipolaron conductors and spin-state entropy in Na_xCo₂O₄. The correlation of the temperature profile of the increases in each material with the nature of their phase transitions indicates that the entropy is associated with the thermodynamcis of ion-ordering. This suggests a new mechanism by which high thermoelectric performance may be understood and engineered.

Superionic Noble Metal Chalcogenide Thermoelectrics

Measurements and modeling of Cu₂Se, Ag₂Se, and Cu₂S show that superionic conductors have great potential as thermoelectric materials. Cu₂Se and Ag₂Se are predicted to reach a zT of 1.2 at room temperature if their carrier concentrations can be reduced, and Cu-vacancy doped Cu₂S reaches a maximum zT of 1.7 at 1000 K. Te-doped Ag₂Se achieves a zT of 1.2 at 520 K, and could reach a zT of 1.7 if its carrier concentration could be reduced. However, superionic conductors tend to have high carrier concentrations due to the presence of metal defects. The carrier concentration has been found to be difficult to reduce by altering the defect concentration, therefore materials that are underdoped relative to the optimum carrier concentration are easier to optimize. The results of Te-doping of Ag₂Se show that reducing the carrier concentration is possible by reducing the maximum Fermi level in the material.

Two new methods for analyzing thermoelectric transport data were developed. The first involves scaling the temperature-dependent transport data according to the temperature dependences expected of a single parabolic band model and using all of the scaled data to perform a single parabolic band analysis, instead of being restricted to using one data point per sample at a fixed temperature. This allows for a more efficient use of the transport data. The second involves scaling only the Seebeck coefficient and electrical conductivity. This allows for an estimate of the quality factor (and therefore the maximum zT in the material) without using Hall effect data, which are not always available due to time and budget constraints and are difficult to obtain in high-resistivity materials. Methods for solving the coherent potential approximation effective medium equations were developed in conjunction with measurements of the resistivity tensor elements of composite materials. This allows the electrical conductivity and mobility of each phase in the composite to be determined from measurements of the bulk. This points out a new method for measuring the pure-phase electrical properties in impure materials, for measuring the electrical properties of unknown phases in composites, and for quantifying the effects of quantum interactions in composites.

Does the orbital degeneracy play any role in the high thermopower of lamellar cobaltites?

This article investigates the role of the CoO6 octahedron distortion on the electronic properties and more particularly on the high value of the Seebeck coefficient in the BiCaCoO lamellar cobaltites. Our measurements provide clues indicating that the t2g orbital degeneracy lifting has to be considered to account for the observed high temperature limit of the thermopower. They also provide experimental arguments for locating the a1g and eg′ orbitals levels on the energy scale, through the compression of the octahedron. These results are in agreement with recent ab initio calculation including the electronic correlations and concluding for the inversion of these levels as compared to the expectation from the crystal field theory. © 2007 American Institute of Physics.

Hydrothermal synthesis and thermoelectric transport properties of impurity-free antimony telluride hexagonal nanoplates

Impurity-free single-crystalline antimony telluride hexagonal nanoplates (see figure) are synthesized by a facile and quick hydrothermal treatment without any organic additives or templates. The inherent crystal structure is the driving force for the growth of these Sb2Te3 hexagonal nanoplates. Films of these nanoplates shows p-type behavior, and exhibit a promisingly high Seebeck coefficient of 425 mu V K-1 at room temperature.

Synthesis of polycrystalline nanotubular Bi2Te3

Polycrystalline nanotubular Bi2Te3 could be prepared via a high-temperature solution process using nanoscale tellurium, decomposed from trioctylphosphine oxide (TOPO) extracted tellurium species (Te-TOPO), as sacrificial template. The formation of such tubular structure is believed to be the result of outward diffusion of Te during the alloying process. The electrical properties (Seebeck coefficient and electrical conductivity) of the polycrystalline nanotubular Bi2Te3 have been studied and the experimental results show that the electrical conductivity is approximately three orders of magnitude smaller than bulk bismuth telluride materials mainly due to the much larger resistance brought by the insufficient contact between the nanotubular structures.

Preparation and transport properties of a bulk icosahedral quasicrystalline Ti45Zr35Ni17Cu3 alloy

A bulk alloy which consists of the single icosahedral quasicrystalline phase (I-phase) in Ti45Zr35Ni17CU3 alloy has been fabricated by mechanical alloying and subsequent pulse discharge sintering technique. Crystallographic structure analyses show that the bulk alloy is an I-phase. The transport properties of the bulk alloy are examined, and the results show that the room-temperature thermal conductivity is 5.347 W K-(1) m(-1), and the electrical conductivity decreases with increasing the temperature from 300 to 450K. The Seebeck coefficient is negative at the temperature range from 300 to 360K, and changes to positive from 370 to 450K. Hall effect measurements indicate the bulk I-phase alloy has a high carrier concentration. The specific heat capacity increases when the temperature increases from 280 to 324 K.

Synthesis and thermoelectric properties of Ca2.6Nd0.4Co4O9 compound

The layered cobaltite Ca2.6Nd0.4Co4O9 was synthesized by the solid-state reaction. Their crystal structure was determined by the X-ray powder diffraction and CELL program. The prepared Ca2.6Nd0.4Co4O9 compound has the monoclinic symmetry. The electrical conductivity and Seebeck coefficient were measured from room temperature to 700 degreesC in air. Both the properties increase while rising temperature. The thermoelectric power of Ca2.6Nd0.4Co4O9 is about 242.4 muV (.) K-1. The results imply a promising way to enhance the thermoelectric properties of the layered cobaltite oxides by optimizing their composition and microstructure.

Materiais à base de óxidos com estrutura do tipo perovskite e compósitos como ânodos de PCES

This work was focused on the analysis of transport, thermomechanical and electrochemical properties of a series of perovskite-like oxide materials and composites for potential applications as anodes of intermediate-temperature solid oxide fuel cells (SOFCs) with lanthanum gallate and silicate solid electrolytes. The primary attention was centered on A(Mn,Nb)O3-δ (A = Sr, Ca) and (La,Sr)(Mn,Ti)O3-based systems, lanthanum chromite substituted with acceptor-type and variable-valence cations, and various Ni-containing cermets. Emphasis was given to phase stability of the materials, their crystal structure, microstructure of porous electrode layers and dense ceramics, electronic conductivity, Seebeck coefficient, oxygen permeability, thermal and chemical induced expansion, and anodic overpotentials of the electrodes deposited onto (La,Sr)(Ga,Mg)O3- and La10(Si,Al)6O27- based electrolyte membranes. In selected cases, roles of oxygen diffusivity, states of the transition metal cations relevant for the electronic transport, catalytically active additives and doped ceria protective interlayers introduced in the model electrochemical cells were assessed. The correlations between transport properties of the electrode materials and electrochemical behavior of porous electrodes showed that the principal factors governing anode performance include, in particular, electronic conduction of the anode compositions and cation interdiffusion between the electrodes and solid electrolytes. The latter is critically important for the silicatebased electrolyte membranes, leading to substantially worse anode properties compared to the electrochemical cells with lanthanum gallate solid electrolyte. The results made it possible to select several anode compositions exhibiting lower area-specific electrode resistivity compared to known analogues, such as (La,Sr)(Cr,Mn)O3-δ.

Transparent p-AgCoO2/n-ZnO diode heterojunction fabricated by pulsed laser deposition

Transparent diode heterojunction on ITO coated glass substrates was fabricated using p-type AgCoO2 and n-type ZnO films by pulsed laser deposition (PLD). The PLD of AgCoO2 thin films was carried out using the pelletized sintered target of AgCoO2 powder, which was synthesized in-house by the hydrothermal process. The band gap of these thin films was found to be ~3.89 eV and they had transmission of~55% in the visible spectral region. Although Hall measurements could only indicate mixed carrier type conduction but thermoelectric power measurements of Seebeck coefficient confirmed the p-type conductivity of the grown AgCoO2 films. The PLD grown ZnO films showed a band gap of ~3.28 eV, an average optical transmission of ~85% and n-type carrier density of~4.6×1019 cm− 3. The junction between p-AgCoO2 and n-ZnO was found to be rectifying. The ratio of forward current to the reverse current was about 7 at 1.5 V. The diode ideality factor was much greater than 2.

Structural, magnetic, and electronic properties of VxCr2−xS3(0

A new family of vanadium-substituted chromium sulfides (VxCr2-xS3, 0 < x < 2) has been prepared and characterized by powder X-ray and neutron diffraction, SQUID magnetometry, electrical resistivity, and Seebeck coefficient measurements. Vanadium substitution leads to a single-phase region with a rhombohedral Cr2S3 structure over the composition range 0.0 < x e 0.75, while at higher vanadium contents (1.6 e x < 2.0) a second single-phase region, in which materials adopt a cation-deficient Cr3S4 structure, is observed. Materials with the Cr2S3 structure all exhibit semiconducting behavior. However, both transport and magnetic properties indicate an increasing degree of electron delocalization with increasing vanadium content in this compositional region. Materials that adopt a Cr3S4-type structure exhibit metallic behavior. Magnetic susceptibility data reveal that all materials undergo a magnetic ordering transition at temperatures in the range 90–118 K. Low-temperature magnetization data suggest that this involves a transition to a ferrimagnetic state.

A copper-containing oxytelluride as a promising thermoelectric material for waste heat recovery

The new thermoelectric material BiOCuTe exhibits an electrical conductivity of 224 S cm-1 and a Seebeck coefficient of +186 μV K-1 at 373 K, together with an extremely low lattice thermal conductivity of ∼ 0.5 W m-1 K-1. This results in a ZT of 0.42 at 373 K, which increases to 0.66 at the maximum temperature investigated, 673 K.