Increase In Thermoelectric Figure Research Articles

Substitutional effects of Ni on thermoelectric properties of cobalt spinels to yield more than 10-fold increase in the thermoelectric figure of merit

The low thermoelectric figure of merit, ZT, is severely limiting the wide practical applications of thermoelectric materials. In this research theoretical calculations were performed on Ni substituted cobalt oxide (Co3O4) semiconductor materials to identify the qualitative trend of thermoelectric properties due to substitutions. Ni substitutions were made in the tetrahedral sites of 56 atom Co3O4 super cell. With the increasing Ni substitution, the band gap near the Fermi region started to close, from 0.337 eV for pure Co3O4 to 0 eV with Ni+2 substitutions in the tetrahedral sites of Co3O4, thereby showing an increasing metallic behavior which is also evident from the electrical conductance calculations. To calculate band structure, Seebeck coefficient, electrical conductance and thermal conductance due to electrons, first principles based density functional theory, DFT, combined with non-equilibrium Green's function method for two-probe systems were used. To calculate the lattice thermal conductance, reverse non-equilibrium molecular dynamics was used. The calculated Seebeck coefficients were 720 μV/K for pure Co3O4 and decreased exponentially to 35 μV/K for Co2NiO4, where the qualitative trend aligned very well with the available experimental data for Ni substituted cobalt oxide materials. These calculated properties assisted in calculating the theoretical thermoelectric figure of merit, and to qualitatively assess the effect of doping on the overall performance. Improved thermoelectric properties such as low thermal conductance and high Seebeck coefficient for low Nickel substitutions (12.5% of tetrahedral sites represented with Ni0·125Co2·875O4), was observed which yielded a 10-fold increase in thermoelectric figure of merit, ZT, in comparison to pure Co3O4 at 300 K. Additionally, the effect of Nickel substitutions on the p-type behavior was presented and discussed in this research article.

The effects of alkaline-earth-metal-element doping on the thermoelectric properties of β-Zn4Sb3

The effects of alkaline-earth metal elements Ca, Sr, and Ba on the electronic structure and thermoelectric properties of β-Zn4Sb3 were investigated by performing self-consistent ab initio electronic structure calculations within density functional theory and solving the Boltzmann transport equations within the relaxation time approximation. The results demonstrate that these alkaline-earth metal elements with s orbitals could introduce giant sharp resonant peaks in the electronic density of states (DOS) near the host valence band maximum in energy. And these deliberately engineered DOS peaks result in a sharp increase of the room-temperature Seebeck coefficient of β-Zn4Sb3 by a factor of nearly 8/9/19, respectively. Additionally, with the simultaneous increase of conductivity and decline of carrier thermal conductivity upon Ca/Sr/Ba doping, potentially, at least, 10/4/2-fold increase in optimizing power factor, and 14/12/8-fold increase in thermoelectric figure of merit of β-Zn4Sb3 at room temperature are achieved. And their corresponding optimal Fermi levels are all located near the host valence band maximum.

Дефекты межслоевой поверхности и термоэлектрические свойства в слоистых пленках топологических изоляторов n-Bi-=SUB=-2-=/SUB=-Te-=SUB=-2.7-=/SUB=-Se-=SUB=-0.15-=/SUB=-S-=SUB=-0.15-=/SUB=-

Interlayer surface defects and thermoelectric properties in layered films of n-Bi2Te2.7Se0.15S0.15 topological insulators L. N. Lukyanova*, O. A. Usov, M. P. Volkov, V. A. Rusakov Ioffe Institute Russian academy of science, 194021 St.Petersburg, Russia *E-mail: lidia.lukyanova@mail.ioffe.ru Abstract In layered films of n-Bi2Te2.7Se0.15S0.15 topological insulators optimized for temperatures below room temperature, the morphology of the (0001) interlayer surface and thermoelectric properties were studied. On the profiles of the (0001) surface, we identified neutral impurity defects arising from the substitution of Se and S atoms for Te atoms and donor antisite defects of tellurium at bismuth sites, which affect the thermoelectric properties. The average value of thermoelectric figure of merit in n-Bi2Te2.7Se0.15S0.15 films increases to <Z> ≈ 3.0•10-3 K-1 in the range 80 – 215 K, while in bulk solid solution <Z> ≈ 2.0•10-3 K-1. An increase in the thermoelectric figure of merit in films is associated with an increase in the energy dependence of the relaxation time due to an increase in the effective scattering parameter reff. It is shown that in films the Seebeck coefficient, the density of states effective mass m/m0, and the material parameter proportional to the power factor increase, while the lattice κL and electronic thermal conductivity κe decrease, which determines the increase in thermoelectric figure of merit.

Improved thermoelectric properties of doped A0.5B0.5NiSn (A, B = Ti, Zr, Hf) with a special quasirandom structure

MNiSn (Ti, Hf, Zr) is used as the parent material in constructing special quasirandom structure-based half-Heusler compounds that experimentally mimic atomic occupation disorders. We randomly doped Ti, Hf, and Zr with an occupation probability of 50% to obtain Ti0.5Zr0.5NiSn, Ti0.5Hf0.5NiSn, and Zr0.5Hf0.5NiSn, respectively. We systematically studied the electronic structure and thermoelectric transport properties of these compounds by referring to density functional theory and the semiclassical Boltzmann transport equation. Compared with the parent compound MNiSn (M = Ti, Zr, Hf), the substituted compounds retain the characteristic of semiconductivity. The lattice thermal conductivity of the three compounds is about 2.5 Wm−1 K−1 at about 1000 K, which is lower than that of their corresponding parent materials (5.0–6.0 Wm−1 K−1). The maximum thermoelectric figures of merit for Ti0.5Zr0.5NiSn (0.57), Ti0.5Hf0.5NiSn (0.55), and Zr0.5Hf0.5NiSn (0.58) are higher than those of their parent compounds TiNiSn (0.35), ZrNiSn (0.496), and HfNiSn (0.473). The increase in thermoelectric figure of merit is due to low lattice thermal conductivity and high power factor. Therefore, the three predicted compounds can be considered candidate thermoelectric materials that can be used at a carrier concentration of 1020–1022 cm−3 and a temperature of 900–1200 K.

The effects of transition element doping on the thermoelectric properties of β-Zn4Sb3

The effects of transition elements Fe, Co, and Ni on the electronic structure and thermoelectric properties of β-Zn4Sb3 were investigated by performing self-consistent ab initio electronic structure calculations within density functional theory and solving the Boltzmann transport equations within the relaxation time approximation. The results demonstrate that these transition elements with 3d orbitals could introduce giant sharp resonant peaks in the electronic density of states (DOS) near the host valence band maximum or conduction band minimum in energy. And these deliberately engineered DOS peaks result in a sharp increase of the room-temperature Seebeck coefficient of β-Zn4Sb3 by a factor of nearly 60, 80 and 130, respectively. Additionally, with the simultaneous decline of carrier thermal conductivity upon Co/Ni doping, potentially, at least, 1.21/1.13-fold increase in thermoelectric figure of merit of β-Zn4Sb3 at room temperature are achieved, indicating that the substitution of Co and Ni for Zn can effectively elevate thermoelectric performance of β-Zn4Sb3.

Modulation of thermal conductivity and thermoelectric figure of merit by anharmonic lattice vibration in Sb2Te3 thermoelectrics

Anharmonic lattice vibration in metallic and semiconducting Sb2Te3 is investigated using temperature dependent Raman spectroscopic studies, synchrotron powder diffraction, and heat-capacity measurements. Thermal variation of structural parameters divulges temperature dependence of structural anisotropy in both the samples. It is revealed that semiconducting Sb2Te3, having higher defect density and larger phonon anharmonicity, possesses lower thermal conductivity. As compared to its metallic counterpart, significant increase in thermoelectric figure of merit, ZT is reported for the semiconducting Sb2Te3. Correlation among structural anisotropy, phonon anharmonicity and ZT is established.

Increase in thermoelectric figure of merit in Sm x Pb1−x Te solid solutions through substitution of lead atoms by samarium atoms

On the basis of temperature relationships of electric conduction (σ), thermo-emf (α), heat conduction (K tot), thermoelectric power α 2 σ and figure of merit Z in Sm x Pb1−x Te solid solutions are calculated. It has been shown that the growth in the number of samarium atoms, which replace lead atoms, leads to an increase in Z. It has been found out that relatively high values of Z in Sm x Pb1−x Te solid solutions as compared to PbTe are mainly due to scattering of phonons and holes on defects as well as due to an increase in an effective mass in them during substitution of lead by samarium.

Effect of Sn Substitution on the Thermoelectric Properties of Nanostructured Bulk Bi2-xSbxTe3 Alloy.

ABSTRACTThe carrier concentration and electronic transport properties in Bi2-xSbxTe3 alloy can be tuned by varying the Bi to Sb ratio, for high thermoelectric figure of merit. The concentration of intrinsic antisite defects in these alloys is also known to change with Bi to Sb ratio. Here we report the thermoelectric figure of merit of Sn doped Bi0.5Sb1.5Te3 alloy. Different atomic percentages of Sn was substituted at Bi/Sb site in Bi0.5Sb1.5Te3 alloy, synthesized by planetary ball milling. The electrical conductivity decreases with increasing Sn doping but for higher Sn content the electrical conductivity increases compared to undoped alloy. The Seebeck coefficient changes in accordance to electrical conductivity, resulting in small decrease in power factor for highest Sn doping. The lattice thermal conductivity shows a systematic decrease, with increasing Sn concentration resulting in a significant thermal conductivity reduction. Hence an increase in thermoelectric figure of merit could be achieved for the highest Sn (3at%) doping in Bi0.5Sb1.5Te3 alloy as compared to undoped alloy.