P-type CoSb3 Research Articles

Valence Bands Convergence in p-Type CoSb3 through Electronegative Fluorine Filling

Abstract Band convergence is considered to be a strategy with clear benefits for thermoelectric performance, generally favoring the co-optimization of conductivity and Seebeck coefficients, and the conventional means include elemental filling to regulate the band. However, the influence of the most electronegative fluorine on the CoSb3 band remains unclear. In this paper, DFT calculations show that the valence band maximum gradually shifts downward with the increase of fluorine filling, lastly the valence band maximum converges to the highly degenerated secondary valence bands in F-filled skutterudites. The effective degeneracy near the secondary valence band promotes more valleys to participate in electric transport, leading to a more than threefold carrier mobility and nearly twofold effective mass for F0.1Co4Sb12 compared to Co4Sb12. This work provides a new and promising route to boost the thermoelectric properties of p-type skutterudites.

Enhancing thermoelectric properties of p-type CoSb3 skutterudite by Fe doping

In this report, we have successfully synthesized thin films of Co1-xFexSb3 (0 ≤ x ≤ 0.1) using electrochemical deposition technique. The electrical and thermoelectric properties of CoSb3 thin films have been investigated by varying the concentration of Fe. X-ray diffraction analysis confirms formation of single phase of Co1-xFexSb3 (0 ≤ x ≤ 0.05) with increase in lattice parameter indicating incorporation of Fe at the Co site. The X-ray photoelectron spectroscopy study reveals that Fe2+ substitutes Co3+ thus generating excess hole carriers, further confirmed by the positive signs of Seebeck and Hall coefficients. It is noted that the thermoelectric properties are mainly controlled by Fe substitution in terms of increasing carrier effective mass as well as phonon scattering. Fe substitution does not significantly alter the carrier mobility and thus confining the transport properties to be mainly leveraged by carrier concentration. Scanning thermal microscopy is used to study the effect of Fe incorporation on the thermal conductivity of nanostructured Co1-xFexSb3 thin films. Enhanced value of power factor and relatively lower value of average thermal conductivity for Co1-xFexSb3 sample is achieved at an optimized value of 5% Fe, as a result of enhanced electron transport and phonon scattering. Furthermore, the improved thermoelectric performance makes this natural-abundant, non-toxic and cheap CoSb3:Fe material a very promising candidate for low cost thermoelectric energy generation.

Effect of hot pressing on the microstructure and thermoelectric properties of TGZM-grown YbFe-doped CoSb3 skutterudite

Skutterudites based on CoSb3 are considered potential candidates in thermoelectric applications in the field of mid-temperature power generation. The annealing process necessitates more than a week to complete the peritectic phase transition for traditional melting-quenching-annealing-sintering methods. Herein, a new strategy combining the temperature gradient zone melting process with hot pressing (TGZM-HP) was successfully employed to synthesize YbFe-doped p-type CoSb3 in a shorter time. The resulting samples show enhanced thermoelectric properties compared to TGZM-grown ones, especially in the case of 1 at.% YbFe-doped CoSb3. This compound exhibits a maximal Seebeck coefficient of 166.3 μVK−1 and a power factor of 1.8 mWm−1K−2 at 723 K, showing an increase in these values of 25% and 51%, respectively. Moreover, phonon scattering appears to be enhanced due to the significant reduction in the grain size of the TGZM-HP samples, resulting in a minimum thermal conductivity of 2.71 Wm−1K−1. A dimensionless figure of merit (ZT) of 0.48 was obtained for 1 at.% YbFe-doped CoSb3. This value is 1.6 times larger than that measured for TGZM-grown CoSb3. This work provides an alternative strategy to fabricate skutterudite materials and foster their development.

Theoretical analysis of thermoelectric performance in p-type CoSb3 based skutterudite by simultaneous partially void filling and Sn substitution

The investigation of CoSb3-based skutterudites is an important research area. Filling their structural voids and/or doping them upon substitution either at the metal or at the pnicogen sites improves the point defect scattering mechanism and enhance the thermoelectric figure of merit (ZT), thus resulting in a higher Seebeck coefficient and electrical conductivity and a lower thermal conductivity. In this work we analyze the effect of Sn substitution on the structural, electronic and thermoelectric properties of doped and partially filled CoSb3 (Ba0.5Co8Sb24-xSnx), where (x = 0, 1, 2, 3 and 4). Our calculations are performed within the fully potential-linearized augmented plane wave method (FP-LAPW) using a semi-classical Boltzmann transport theory. The theoretical analysis show that the charge balance of Ba0.5Co8Sb24-xSnx strongly depends on the substitution configuration. The Ba0.5Co8Sb23Sn composition is the most stable in terms of energy. When we examine the effect of Sn substitution on the electronic and thermoelectric properties, we find that the electronic properties are moderate and thermoelectric performance is improved. That is, the estimated dimensionless electronic figure of merit (ZTe) is approximately 0.67 at T = 500K for Ba0.5Co8Sb23Sn; ranking this material as a promising candidate for thermoelectric applications.

Optimization of electrical and thermal transport properties of Fe0.25Co0.75Sb3 Skutterudite employing the isoelectronic Bi-doping

The practical applications of unfilled CoSb3-based Skutterudites for thermoelectric (TE) devices have been limited primarily owing to their bipolar conduction behaviour, which is detrimental for the TE performance. In the present studies, a peak TE figure-of-merit (ZT)max ~ 0.7 at 860 K has been realized with a pioneering (ZT)average ~ 0.34 in p-type CoSb3 co-doped by Fe and Bi at an optimal composition of Fe0.25Co0.75Sb2.995Bi0.005. The synthesis route for the alloy was the combination of arc melting and the spark plasma sintering. The TE properties measurement results suggest that the Fe doping at the Co site in CoSb3 lead to the unipolar p-type conduction while Bi substitution at Sb site helps in reducing the thermal conductivity via enhanced point defect induced phonon scattering. The synthesized samples were characterised for phase, morphology and structure using X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy, based on which some TE properties of the synthesized samples have also been discussed. In addition, the effect of magnetic entropy on the TE performance has also been studied experimentally as well as theoretically. The first-principles based density functional theory has been employed for the theoretical calculation of the electronic band-structure, electrical transport properties and the magnetic moment, which are found to be in fair agreement with the experimental observations.

Incorporation of MoS2 nanosheets in CoSb3 matrix as an efficient novel strategy to enhance its thermoelectric performance

The nanocomposite approach is considered as an effective way to improve the thermoelectric properties of bulk materials and we have exploited it by simultaneous though independent tackling of the electron and phonon transports. In the present study, through the strategy of anchoring the CoSb3 nanoparticles on the 2-dimensional nanosheets of MoS2, we demonstrate a controlled interplay of the newly created CoSb3/MoS2 interfaces in nanocomposites of varying concentration of MoS2 via significant enhancement of the phonon scattering without deterioration of electron transport. A concurrent occurrence of low energy carrier filtering on account of the interfacial potential barrier helps in beneficial manipulation of grain to grain carrier transport. The dimensionless figure of merit ZT maximizes to 0.53 at 600 K for the CoSb3/MoS2 nanocomposite containing 3 wt% of MoS2, 4-fold increase over the pristine CoSb3 in the 300–600 K range. This study paves the way towards improvement of the thermoelectric performance of p-type CoSb3 using 2D MoS2 as an interfacial additive.

Microstructure and thermoelectric properties of doped p-type CoSb3 under TGZM effect

The Co-96.9wt% Sb hypoeutectic alloy doped by 0.12wt% YbFe was solidified in a Bridgman-type furnace based on temperature gradient zone melting (TGZM) effect. A mushy zone was observed between the complete liquid zone and the solid zone at different thermal stabilization time ranging from 15min to 40h. The mushy-zone solidified microstructures of the alloy only consist of CoSb3 and Sb phase. After 40h thermal stabilization time, the volume fraction of CoSb3 in the mushy zone increases significantly up to 99.6% close to the solid-liquid interface. The hardness and fracture toughness of doped CoSb3 can reach 7.01±0.69GPa and 0.78±0.08MPa·m1/2, respectively. Meanwhile, the thermoelectric properties of the alloy were measured ranging from room temperature (RT) to 850K. The Seebeck coefficient of the specimen prepared by TGZM effect after 40h could reach 155μV/K and the ZT value is 0.47 at 660K, showing that it is feasible to prepare CoSb3 bulk material via TGZM effect. As a simple and one-step solidification method, the TGZM technique could be applied in the preparation of skutterudite compounds.

Fast fabrication and enhancement of thermoelectric power factor of p-type nanostructured CoSb3(1+δ) (δ=0.00, 0.01 and 0.02) using solvothermal synthesis and evacuating-and-encapsulating sintering

Nanostructured skutterudite p-type CoSb3(1+δ) (δ=0.00, 0.01 and 0.02) samples are synthesized via a combination of solvothermal synthesis and evacuating-and-encapsulating sintering. All the samples are of single phase with cubic structure. The thermopower of all the samples is positive, indicating that the predominant carriers are hole in the temperature range of 300–700K. Both the electrical resistivity and thermopower follow the variable range hopping process for 300≤T≤425K. The power factor of all samples exhibits a monotonic increase with temperature up to 700K. The highest power factor is obtained for CoSb3(1+δ) with δ=0.01, which represents 145% increase when compared to CoSb3 with δ=0.00. The ZT value is 0.07 at 550K for CoSb3(1+δ) with δ=0.01, representing an improvement of about 95% compared to CoSb3 with δ=0.00. These results suggest that excess antimony provides an effective avenue of improving the thermoelectric properties of CoSb3.

Strong temperature dependence of the Hall factor of p-type CoSb3: A re-analysis incorporating band nonparabolicity

The Hall factor has been calculated as a function of temperature for p-type CoSb3 samples reported by Hui et al. [J. Appl. Phys. 115, 103704 (2014)] in addition to those reported by Caillat et al. [J. Appl. Phys. 80, 4442 (1996)], incorporating the valence band nonparabolicity. For the nominally undoped p-type samples, reported by Caillat et al., the Hall factor was calculated to increase with temperature from about 1.6 around room temperature to about as large as 5 at 900 K. Owing to the incorporation of the Hall factor, the net acceptor concentration has been corrected to be larger by about 1.6 times. Furthermore, due to the strong temperature dependence of the Hall factor, the band-gap energy deduced through fitting the temperature dependence of the Hall coefficient has been corrected to be smaller by about 0.03 eV than the values deduced when assuming a constant Hall factor of unity. It has also been shown that, even when the temperature dependence of the Hall factor is considered, the incorporation of the second valence band into the calculation is necessary to explain the anomalous increase of the Hall coefficient with temperature in the Sn-doped p-type CoSb3 sample reported by Hui et al.

Effects of impurity-band conduction on thermoelectric properties of lightly doped p-type CoSb3

Experimental data on the thermoelectric properties of lightly doped p-type CoSb3 reported by Mandrus et al. [Phys. Rev. B 52, 4926 (1995)], Arushanov et al. [Phys. Rev. B 56, 1911 (1997)], and Dyck et al. [Phys. Rev. B 65, 115204 (2002)] have been analyzed, incorporating both the impurity-band conduction and the nonparabolicity of the valence band. The concentrations of a resonant acceptor, a deep acceptor, and a compensating donor level have been considerably corrected from those obtained in the original studies by Arushanov et al. and Dyck et al. The impurity-band conduction in the deep acceptor level has been proved to be the dominant conduction mechanism at low temperatures in the sample of Dyck et al. while that in the resonant acceptor level has been proved to be the dominant one in the other samples. Especially, the impurity-band conduction has been proved to be predominant up to as high as 400 K in the sample of Mandrus et al. As a result of incorporating the effects of both the impurity-band conduction and the nonparabolicity, the agreement of the simulated Seebeck coefficient with the experimental one has been excellently improved.

Analysis of high-temperature thermoelectric properties of p-type CoSb3 within a two-valence-band and two-conduction-band model

Experimental data on the thermoelectric properties of p-type CoSb3 reported by Caillat et al. [J. Appl. Phys. 80, 4442 (1996)] have been analyzed, assuming not only a pair of the first valence (v1) and the first conduction (c1) bands but also the second valence (v2) and the second conduction (c2) bands. By taking into account the excitation of carriers into the v2 and the c2 bands, the behavior of the Hall coefficient as well as that of the Seebeck coefficient at high temperatures is well explained. By taking into account the nonparabolicity of the v1 band, the temperature dependence of mobility is well explained with assuming scattering due to acoustic phonons, nonpolar and polar optical phonons, and ionized impurities. Furthermore, various material parameters of CoSb3, such as the band-gap energy, effective masses, and deformation potentials, have been deduced from fitting the calculation to the experimental data on the temperature dependences of the Hall coefficient, the mobility, and the Seebeck coefficient. Among them, the band-gap energy and the effective mass of the v1 band have been corrected from the original values estimated by Caillat et al. In addition, it is shown that the experimental data on the hole-concentration dependences of both the room-temperature Seebeck coefficient and the cyclotron mass are well reproduced by the theoretical calculation using the deduced values for the nonparabolic v1 band.

Enhanced thermoelectric properties of p-type CoSb3/graphene nanocomposite

Nanostructuring and second phase incorporation are considered to be promising ways of enhancing the thermoelectric performance of bulk materials. Here, a design principle is proposed which combines these two methods for improving the thermoelectric performance of p-type CoSb3 by fabricating a CoSb3/graphene (CoSb3/G) nanocomposite, where a second phase, graphene, is introduced in the nanostructured CoSb3 matrix via an in situ one-pot solvothermal route. In addition, CoSb3/G bulk materials were prepared by hot pressing the solvothermally synthesized CoSb3/G powder. It was found that addition of a small amount of graphene can drastically enhance the electrical conductivity due to the increase in both carrier concentration and mobility. In addition, the well dispersed graphene in the nanostructured CoSb3 matrix also contributes to the low lattice thermal conductivity. A dimensionless figure of merit ZT = 0.61 at 800 K has been obtained for the CoSb3/G nanocomposite, which is about a 130% improvement over that of graphene-free CoSb3 (∼0.26).

Fast Preparation and Low-Temperature Thermoelectric Properties of CoSb3

Polycrystalline p-type CoSb3 was synthesized by the high-pressure method. The microstructure and temperature-dependent thermoelectric properties of CoSb3 were investigated. X-ray diffraction and scanning electron microscopy showed that single-phase CoSb3 with fine grain size could be quickly synthesized under high pressure. The carrier concentrations of CoSb3 could be tuned by more than a factor of 10 by changing the pressure during synthesis. With the increase of the synthetic pressure, the Seebeck coefficient and resistivity of CoSb3 increase while the thermal conductivity decreases.

Structural and electronic transport properties of polycrystalline p-type CoSb3

The structural and electronic transport properties of polycrystalline p-type CoSb3 with different grain sizes (about 3 and 3×102 μm) were investigated. The magnetic susceptibility was also measured. Samples were characterized by x-ray diffractometry, electron-probe microanalysis, and optical microscope observation. Samples were found to be stoichiometric and homogeneous. The Hall carrier concentration of the samples is of the order of 1018 cm−3 and weakly dependent on the temperature. The temperature dependence of the Hall mobility suggests that the predominant scattering mechanism drastically changes depending on grain size: for large grain size a combination of the neutral impurity scattering and the acoustic phonon scattering, and for small grain size the ionized impurity scattering. The magnetic susceptibility was found to be essentially diamagnetic independently of grain size, and to vary slightly with temperature. The weak temperature dependence of the susceptibility can be explained by taking into account the three contributions of ion cores, conduction electrons, and trace amounts of magnetic impurities. From the analysis of the susceptibility due to conduction electrons, the band gap energy was determined to be about 70–80 meV, consistent with a recent band structure calculation. Although the effects of nonmagnetic impurity phases segregated (Sb, etc.) on the scattering mechanism are not clear, the grain size is one of the key factors determining the transport properties of polycrystalline CoSb3.

熱電変換 （ＩＩＩ） スクッテルダイト系熱電材料の研究状況

A new class of thermoelectric materials, skutterudites and filled skutterudites, has become a topic because the materials have a dimensionless thermoelectric figure of merit ZT of 1 at 800K. Skutterudites are binary semiconducting compounds with the general formula MX3 (where M is Co, Rh, Ir; and X is P, As Sb). Within these materials, the three isothermal antimonides such as IrSb3, RhSb3, and CoSb3 exhibit excellent electronic transport properties, including a high value of the hole mobility (8000cm2/Vs). For example, p-type CoSb3 has a hole mobility value of 3000cm2/Vs at room temperature, and ZT of Pd/Pt-doped CoSb3 is 0.85.Filled skutterudite alloys with the general formula of RM′4X12, where R is La, Ce, Pr, Nd or Eu and M′ is Fe, Ru or Os, are particularly interesting, because formally they are isoelectric (that is, have the same number of valence electrons per unit cell) with the binary skutterudites such as CoSb3. Relative to CoSb3, the filled skutterudites exhibit a remarkable factor-of-ten decrease in the lattice thermal conductivity, which reaches a value at room temperature equal to that of vitreous silica (SiO2). This is believed to be due to the “rattling” effect of the rare earth atoms that scatters the heat-carrying phonons. For example, CeFe3CoSb12 or LaFe3CoSb12 has a low lattice thermal conductivity value of about 0.015W/cmK at near room temperature.