Papers by Keyword: CoSb₃

Abstract: Encapsulated induction melting was attempted to prepare binary skutterudite CoSb3 and its thermoelectric properties were investigated. Single phase δ-CoSb3 was successfully obtained by the subsequent isothermal heat treatment at 773K-873K for 24 hours in vacuum. Thermoelectric properties were changed with constituent phases because β-CoSb and Sb are metallic/semimetallic phases, while γ-CoSb2 and δ-CoSb3 are semiconducting phases. Thermoelectric properties were remarkably improved by the proper heat treatment and they were closely related to phase transitions.

Abstract: Single-phase polycrystalline CoSb3 skutterudite was prepared through a new combination of mechanical alloying (MA) and spark plasma sintering (SPS). In order to investigate the influence of MA conditions on the microstructure and thermoelectric properties, MA synthesis were carried out under various conditions with different milling times. The powder sample MAed for 6h still consisted of metal Sb, and then transformed to CoSb3 with a little amount of metal Sb and CoSb2 phases after MA for 15h. Further prolonging the MA time resulted in the decomposition of CoSb3 to CoSb2 phase. The average grain size of the SPSed samples decreased from 650nm to 250nm as MA-time was prolonged from 6 to 24h. Lattice parameters estimated form XRD patterns increase with the increasing MA time. All samples SPSed at 600°C for holding 5 min show an n-type conduction. The electrical resistivity was 1030, 895, 410, 260 μm for the samples from the MA-derived powders with MA-time of 6, 15, 24 and 33h at room temperature, respectively, then reduced to 60 μm at 400°C for all samples. An optimum MA time is 24 h in which the sample shows the highest power factor 612μW/m*K2 at 150°C.

Abstract: Undoped CoSb3 powders were synthesized by mechanical alloying of elemental powders using a nominal stoichiometric composition. Nanostructured, single-phase skutterudite CoSb3 was successfully produced by vacuum hot pressing using as-milled powders without subsequent annealing. Phase transformations during synthesis were investigated using XRD, and microstructure was observed using SEM and TEM. Thermoelectric properties in terms of Seebeck coefficient, electrical conductivity, thermal conductivity and figure of merit were systematically measured and compared with the results of analogous studies. Lattice thermal conductivity was reduced owing to increasing phonon scattering in nanostructured CoSb3, leading to enhancement in the thermoelectric figure of merit. Mechanical Alloying associated with vacuum hot pressing technique offers an alternative potential processing route for the production of skutterudites.

Abstract: Fe doped skutterudite CoSb3 with a nominal composition of FexCo1-xSb12 (0≤x≤2.5) have been synthesized by mechanical alloying (MA) of elemental powders, followed by vacuum hot pressing. Phase transformations during mechanical alloying and vacuum hot pressing were systematically investigated using XRD. Single phase skutterudite was successfully produced by vacuum hot pressing using as-milled powders without subsequent annealing. However, second phase in the form of marcasite structure FeSb2 was found to exist in case of x≥2, suggesting the solubility limit of Fe with Co in this system. Thermoelectric properties as functions of temperature and Fe contents were evaluated for the hot pressed specimens. Fe doping up to x=1.5 with Co in FexCo4-xSb12 appeared to increase thermoelectric figure of merit (ZT) and the maximum ZT was found to be 0.78 at 525K in this study.

Abstract: Te doping effect on the thermoelectric properties of solid solution was investigated. 2at% Te doping was found to be most effective to improve the dimensionless figure of merit, ZT, of pseudo-binary CoSb3-4mol%Zn4Sb3. Maximum ZT of 0.69 was obtained at 450K in CoSb3- 4mol%Zn4Sb3 doped with 2at% Te. Heavy doping of Te above 3at% caused thermal conductivity to increase very high because of the formation of metallic CoSb2 phases and thus did not improve ZT.

Abstract: Induction melting was attempted to prepare the undoped and Nb-doped CoSb3 compounds, and their thermoelectric properties were investigated. Single phase d-CoSb3 was successfully obtained by induction melting and subsequent annealing at 400°C for 2 hours in vacuum. The positive signs of Seebeck coefficients for all the specimens revealed that Nb atoms acted as p-type dopants by substituting Co atoms. Electrical conductivity decreased and then increased with increasing temperature, indicating mixed behaviors of metallic and semiconducting conductions. Electrical conductivity increased by Nb doping, and it was saturated at high temperature. Maximum value of the thermoelectric power factor was shifted to higher temperature with the increasing amount of Nb doping, mainly originated from the Seebeck coefficient variation.

Abstract: In an effort to synthesize homogenized single phase d-CoSb3, this study considers the mechanical alloying (MA) of elemental Co and Sb powders using a nominal stoichiometric composition followed by hot pressing. Single phase, undoped CoSb3 skutterudites were successfully produced by vacuum hot pressing using MA powders without subsequent annealing. Phase transformations during mechanical alloying, powder annealing, and hot pressing were systematically investigated using XRD and SEM. Thermoelectric properties were measured and compared with the results of similar studies. Temperature dependences were also evaluated, and their correlations to phase transformation were examined.

Abstract: Induction melting was attempted to prepare the undoped and Nb-doped CoSb3 compounds, and their thermoelectric properties were investigated. Single phase d-CoSb3 was successfully obtained by induction melting and subsequent annealing at 400°C for 2 hours in vacuum. The positive signs of Seebeck coefficients for all the specimens revealed that Nb atoms acted as p-type dopants by substituting Co atoms. Electrical conductivity decreased and then increased withincreasing temperature, indicating mixed behaviors of metallic and semiconducting conductions. Electrical conductivity increased by Nb doping, and it was saturated at high temperature. Maximum value of the thermoelectric power factor was shifted to higher temperature with the increasing amount of Nb doping, mainly originated from the Seebeck coefficient variation.

Abstract: CoSb₃ compounds were prepared by the arc melting and their thermoelectric properties were investigated at 300K-600K. Annealing effects were examined and they were correlated to phase transformation and homogenization. Undoped CoSb₃ showed p-type conduction and intrinsic semiconducting behavior at all temperatures examined. Thermoelectric properties were changed with constituent phases because α-CoSb₂, β-CoSb and Sb are metallic or semimetallic phases while δ-CoSb₃ is semiconducting phase. Thermoelectric properties were remarkably improved by annealing in vacuum and they were closely related to phase transitions. Single phase δ-CoSb3 was successfully obtained by annealing at 400°C for 24hrs.