Higher Power Factor Values Research Articles

Magnèli phase TinO2n-1 bulks prepared by SPS followed by carbon reduction and their thermoelectric performance

We fabricated Magnèli phase TinO2n-1 compacts from nano-sized rutile TiO2 powder by spark plasma sintering (SPS) followed by carbon reduction at elevated temperatures. These compacts were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD results showed that the n value in Magnèli phase TinO2n-1 mainly depended on the reduction temperature and holding time. Higher reduction temperature or longer holding time favored to smaller n value through withdrawing more oxygen atoms from the lattice of TiO2. Meanwhile, the thermoelectric properties including electrical resistivity, thermal conductivity, Seebeck coefficient, power factor and ZT value of these compacts were measured or calculated. The electrical conductivity was greatly improved while Seebeck coefficient still kept rather large values, which resulted in an extremely high value of power factor at elevated temperature. The highest power factor reached 6.5 × 10−4 W m−1 K−2 at 973 K and the greatest ZT value ran up to 0.1 at 873 K.

Effect of synthesis process on the densification, microstructure, and electrical properties of Ca0.9Yb0.1MnO3 ceramics

AbstractCa0.9Yb0.1MnO3 thermoelectric materials have been prepared, through a classical solid‐state sintering method, from attrition‐ and ball‐milled precursors. After calcination step, microstructural observations have shown that attrition‐milled precursors possess much smaller particle sizes than the obtained by ball milling. Smaller precursors sizes lead to higher reactivity, producing higher density, hardness, and thermoelectric phase content in the sintered materials. The thermoelectric properties reflect the microstructural features, decreasing electrical resistivity in the attrition milling prepared samples without a drastic decrease in the Seebeck coefficient. As a consequence, power factor values are higher than the obtained in the classical solid‐state method samples. Moreover, the highest power factor values at 800°C are much higher than the best results obtained in this CaMnO3 family. As a result, it has been found that it is possible to tailor the thermoelectric properties of Ca0.9Yb0.1MnO3 ceramics by designing the appropriate preparation procedure while keeping in mind its industrial scalability.

Effect of precursors on the microstructure and electrical properties of Bi2Ba2Co2O x

Bi2Ba2Co2O x thermoelectric materials have been prepared through three different synthesis methods. The corresponding precursors were obtained from a coprecipitation method using oxalic acid, attrition milling, and classical solid state method (used as reference). Microstructural studies of precursors have shown that the ones produced by coprecipitation and attrition milling led to smaller grain sizes than the ones obtained through ball milling. Infrared spectroscopy has demonstrated that a thermal treatment at 450 °C totally decomposes the metallic oxalates, producing a mixture of Bi and Co oxides, and Ba carbonate, the same average composition of the other precursors. After sintering procedure, it has been determined that the small grain size precursors produce high-density samples, reflected in a decrease of electrical resistivity without significant modification of the Seebeck coefficient. As a consequence, power factor values are more than two times higher than the measured in samples prepared by the classical solid state method. Moreover, the highest power factor values are in the order of the best reported in textured materials, demonstrating the critical role of precursors in the final properties of these materials.

Investigation of energy-efficiency improvement in an electrical drive system based on multi-winding machines

This paper presents an investigation of energy-efficiency improvement of an electrical drive system based on multi-winding machines. An improved power factor (PF) control scheme is proposed allowing energy-efficiency enhancement and optimization in high-power variable-speed drives based on multi-winding machines. Thus, the double-star induction machine is taken, as an example of multi-winding machines, to introduce the principle of the study presented in this paper. In fact, the purpose of this paper is to maintain the PF of the power winding, of the double-star induction machine, in vicinity of unity whatever the drive operating point, allowing discontinuous PWM (DPWM) techniques selection. Therefore, the switching losses can be reduced significantly using DPWM techniques at high PF values and this contributes strongly to energy-efficiency improvement. Moreover, in high-power applications, having low converter losses is essential to maintain thermal limits. In addition, this control scheme can be generalized for several kinds of multiphase machines.

Power factor enhancement via simultaneous improvement of electrical conductivity and Seebeck coefficient in tellurium nanowires/reduced graphene oxide flexible thermoelectric films

The simultaneous improvement of electrical conductivity (σ) and Seebeck coefficient (S) is essential to realize the thermoelectric materials with high power factor value. Here, we report the discovery that the redox reaction happened between the reduced graphene oxide (RGO) sheets and tellurium (Te) nanowires can lead to a simultaneous improvement of σ and S in the RGO/Te nanowires hybrid films. With optimization, the electrical conductivity and Seebeck coefficient can reach 633S/m and 382μV/K, respectively, pushing the power factor value up to 68.4μW/(mK2), approximately 25 times larger than the reported pure Te nanowires film. Based on all of the characterizations, the redox reaction between the RGO sheets and Te nanowires have been confirmed and the transportation characteristics of the hybrid films are predicted. We demonstrate the new approach that via turning the redox reaction, the interfacial interaction can be optimized, enabling a novel flexible thermoelectric films with superior performance. To our knowledge, this is the first time that the TE properties of RGO/inorganic hybrid films are reported.

Investigation of Preparation and Thermoelectric Properties of Ca<sub>2.5</sub>La<sub>0.5</sub>Co<sub>4</sub>O<sub>9</sub> Porous Ceramics

CO(NH2)2 as pore-former used to prepare porous Ca2.5La0.5Co4O9 thermoelectric materialsby solid-state reaction technique.Microstructure,density and thermoelectric properties (temperature dependence of electrical resistivity,Seebeck coefficient) of the samples were studied in details.The results have shown that the porosity increases and the pore structure is improved with the pore-former content increasing,and electrical resistivity and density decrease while Seebeck coefficient raise.The improvement in electrical resistivity and Seebeck coefficient leads to higher power factor values at 913 K (around 1.32 mW/cm·K2) than undoped samples.

Enhanced thermoelectric performance of n-type Bi2Se3 doped with Cu

The thermoelectric properties of Cu-doped CuxBi2Se3 (x=0, 0.005, 0.010, 0.015 and 0.020) were investigated in the temperature range of 300K to 590K. The results indicate that, at 300K, the thermal conductivity of moderately doped CuxBi2Se3 (x=0.010 and 0.015) is by 30–50% lower than that of un-doped Bi2Se3, which is attributed to enhanced phonon scattering by the dopant. Though the absolute values of the thermopower are reduced by the doping, the electrical resistivity of the doped samples is strongly reduced (by ∼30% at 500K, for instance), thus maintaining high values of power factor at both 300K and 590K, especially for samples with x⩾0.01. As a result, the lightly doped compound Cu0.01Bi2Se3 exhibits the best thermoelectric performance, with a figure of merit, ZT, of 0.54 at 590K. This is more than twice larger than that of the un-doped Bi2Se3 studied here, suggesting that proper Cu-doping in Bi2Se3 is a promising way to improve its thermoelectric performance.

Transport properties of the system Bi2Te2.7Se0.3−xPbx fabricated by a conventional melting technique

N-type Bi2Te2.7Se0.3−xPbx (x = 0, 0.02, 0.04, 0.06 and 0.08) thermoelectric compounds have been prepared by a conventional melting technique. Microstructure analyses have been carried out for all samples using x-ray diffraction and a scanning electron microscope. The data reveal that the samples are crystallized in the polycrystalline rhombohedral Bi2Te3 phase and the grains have irregular shapes and sizes. The chemical stoichiometry has been examined by means of energy dispersive x-ray spectroscopy (EDAX). The stoichiometries match with the chemical formula (Bi)2(Te-Se-Pb)3. However, superstoichiometry of Pb at the expense of Se content has been observed. The thermoelectric measurements prove that the dominant charge carriers in all materials are electrons over the temperature range of measurements (200–400 K). The temperature dependence of the electrical conductivity shows metallic behavior for all the nominal compositions and over the whole range of temperature. The highest power factor value was recorded for the undoped sample at T = 330 K and was found to be 15.51 μWK−2 cm−1.

Facile Preparation and Thermoelectric Properties of Bi2Te3 Based Alloy Nanosheet/PEDOT:PSS Composite Films

Bi2Te3 based alloy nanosheet (NS)/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) composite films were prepared separately by spin coating and drop casting techniques. The drop cast composite film containing 4.10 wt % Bi2Te3 based alloy NSs showed electrical conductivity as high as 1295.21 S/cm, which is higher than that (753.8 S/cm) of a dimethyl sulfoxide doped PEDOT:PSS film prepared under the same condition and that (850-1250 S/cm) of the Bi2Te3 based alloy bulk material. The composite film also showed a very high power factor value, ∼32.26 μWm(-1) K(-2). With the content of Bi2Te3 based alloy NSs increasing from 0 to 4.10 wt %, the electrical conductivity and Seebeck coefficient of the composite films increase simultaneously.

Comparative aging characteristics between a high oleic natural ester dielectric liquid and mineral oil

This paper presents a comparative evaluation of the aging characteristics between a high oleic natural ester dielectric liquid (HONE) and mineral oil based on an accelerated single-temperature thermal aging experiment at 150°C for a period of 3000 hours. The important physical (e.g., viscosity, moisture content, acidity) and dielectric (e.g., power factor, alternating current (AC) dielectric breakdown) properties of both the oils were measured and analyzed at different aging intervals. As expected, results show differences in aging behaviors between mineral oil and the natural ester liquid. The relative moisture content in HONE had stable and lower values during the aging duration as compared to the case of mineral oil, where the relative moisture content increased with the aging duration. Further, unlike the observations in mineral oil, the power factor (PF) and acidity of HONE increased drastically with increasing aging times. The most relevant observation made from this study was that in spite of this significant increase in the PF and acidity of HONE with aging, its AC dielectric breakdown voltage was quite stable, which is not observed in the case of mineral oil. This indicates that for natural ester liquids, higher values of PF (or dielectric dissipation factor) and/or acidity due to aging may not necessarily give a true picture on the dielectric performance of the ester. It is therefore important to note here that the threshold limits of some of the diagnostic parameters which are already established for mineral oil do not apply for ester liquids.

Thermopower Peculiarities of Uniaxial-Strained Bismuth Nanowires

We present the results of the investigations of the thermopower and electrical resistance of Pyrex-coated Bi nanowires. Here we show that there is a critical thickness of wire below which structural defects can strongly influence the charge transport in Bi nanowires, fabricated by the melt spinning method. At low temperature the contribution of holes to the thermopower dominates the electron contribution. Thermoelectric parameters of Bi nanowires may be controlled by applying a thermal treatment and/or a uniaxial pressure. A fairly high value of thermoelectric power factor was found under strain in the temperature range of 80–300 K, where the dominant mechanism contributing to the thermopower is diffusive thermoelectric generation with electrons as the majority carrier.

High temperature thermoelectric properties of the Ca3−xBxCo4O9 system

The Ca3−xBxCo4O9 system (x = 0.0, 0.05, 0.75 and 1.0) has been prepared using the solid-state reaction technique. Structural, microstructural, transport properties (temperature dependence of resistivity, thermoelectric power) of the samples fabricated were investigated in details. The structural/microstructural investigations showed that complex and multiphase crystallographic structure was formed with the B-substitution. A decrease in the grain size was observed in the B-substituted samples. The resistivity increased with the substitution due to decrease in the grain size and weak-link among the grains. It is found that that the thermoelectric power is insensitive to the B-substitution. The highest power factor value was obtained in the x = 0.0 sample, however, it decreased in the B-substituted samples.

Strain induced peculiarities in transport properties of Bi nanowires

We report results on the effect of strain on the thermopower and electrical resistance of glass-coated individual Bi nanowires. Here, we show that there is a critical diameter of wires below which the contribution of holes to the charge transport in pure Bi nanowires is more significant than that of electrons. The properties of Bi nanowires are examined in the light of a strain induced electronic topological transition. At low temperatures, the thermopower dependences on strain exhibit a non-monotonic behavior inherent in thinner wires, where the thermopower is dominated by the diffusion transport mechanism of holes. The hole-dominated transport can be transformed into electron-dominated transport through a smooth manipulation with the phonon spectrum and Fermi surface by applying a uniaxial strain. A fairly high value of the thermoelectric power factor (S2/ρ = 89 μW cm−1 K−2) was found in the temperature range of 80–300 K, where the dominant mechanism contributing to the thermopower is diffusive thermoelectric generation with electrons as the majority carrier.

Structural, electrical, and thermoelectrical properties of (Bi1 − x Sb x )2Se3 alloys prepared by a conventional melting technique

Polycrystalline solid solutions of (Bi1 − xSbx)2Se3 (x = 0, 0.025, 0.050, 0.075, 0.100) were prepared using a facile method based on the conventional melting technique followed by annealing process. X-ray analysis and Raman spectroscopical measurements revealed formation of Bi2Se3 in single phase. The electrical and thermoelectric properties have been studied on the bulk samples in the temperature range 100–420 K. The electrical conductivity measurements show that the activation energy and room-temperature electrical conductivity dependences on the Sb content respectively exhibit minimum and maximum values at x = 0.05. The thermoelectric power exhibited a maximum value near the room temperature suggesting promising materials for room-temperature applications. The highest power factor value was found to be 13.53 μW K−2 cm−1 and recorded for the x = 0.05 compound.

Preparation of p-type CuCr1−x Mg x O2 bulk with improved thermoelectric properties by sol–gel method

Fine powders of p-type CuCr1−xMgxO2 (0 ≤ x ≤ 0.15) with improved thermoelectric properties were synthesized by sol–gel method through nitrate-citrate route. The sintered CuCr1−xMgxO2 (0 ≤ x ≤ 0.15) bulk contained the CuCr1−xMgxO2 phase with a rhombohedral structure, R3 m, along with trace second phase CuO, MgCr2O4. With the doping amount of Mg increasing, the grain size enlarged with oppositely decreased of the porosity. The substitution of Mg could give four orders improvement on electrical conductivity up to 10.34 S/cm at room temperature compared with the undoped samples. The positive Seebeck coefficients of doping oxides implied the typical p-type conducting oxides obtained by sol–gel method. The highest value of power factor has been achieved to be 1.37 × 10−4 Wm−1 K−2 for CuCr0.90Mg0.10O2 at 470 K. As a conclusion, we realized that the sol–gel method is a very effective and feasible shortcut on transparent oxide synthesis with desirable thermoelectric properties.

High thermoelectric performance of In, Yb, Ce multiple filled CoSb3 based skutterudite compounds

Filling voids with rare earth atoms is an effective way to lowering thermal conductivity which necessarily enhances thermoelectric properties of skutterudite compounds. Yb atom is one of the most effective species among the rare earth atoms for filling the voids in the skutterudite structure due to a large atomic mass, radius and it is intermediate valence state. In this work, we aim to find the best filling partners for Yb using different combinations of Ce and In as well as to optimize actual filling fraction in order to achieve high values of ZT. The traditional method of synthesis relying on melting–annealing and followed by spark plasma sintering was used to prepare all samples. The thermoelectric properties of four samples of Yb0.2In0.2Co4Sb12, Yb0.2Ce0.15Co4Sb12, Yb0.2Ce0.15In0.2Co4Sb12, and Yb0.3Ce0.15In0.2Co4Sb12 (nominal) were examined based on the Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient. Hall coefficient and Seebeck coefficient signs confirm that all samples are n-type skutterudite compounds. Carrier density increases with the increasing Yb+Ce content. A high power factor value of 57.7μW/K2/cm for Yb0.2Ce0.15Co4Sb12 and a lower thermal conductivity value of 2.82W/m/K for Yb0.2Ce0.15In0.2Co4Sb12 indicate that small quantities of Ce with In may be a good partner to Yb to reduce the thermal conductivity further and thus enhance the thermoelectric performance of skutterudites. The highest ZT value of 1.43 was achieved for Yb0.2Ce0.15In0.2Co4Sb12 triple-filled skutterudite at 800K.

Thermoelectric properties of Spark Plasma Sintered In xYb yLa 0.3- x- yCo 4Sb 12 skutterudite system

A thermoelectric material with sufficient performance at high temperatures is a key factor for thermoelectric generation from solar-thermal energy (I. R. radiation). In this study, we analyzed CoSb 3 skutterudite compounds as a thermoelectric material at high temperatures and synthesized filled skutterudites with two types of filler atoms: 1) Yb- and La-fillers with a different electronegativity ( X) from the Sb atoms and 2) the In-filler with nearly the same electronegativity ( X) as the Sb atoms. All the samples were sintered with the spark plasma sintering method, and their thermoelectric properties were investigated. From the results, the Yb atoms were the most effective in decreasing the thermal conductivity of this system due to their very different electronegativity ( X) value from the Sb atoms and lower melting temperature than that of the La-filler. In addition, In-filled compositions had high power factor values while the In-free composition had the lowest power factor value due to its lower melting temperature. The addition of La atoms for filling was not effective in reducing the thermal conductivity of this filled skutterudite system due to its higher melting point. A maximum ZT value of 0.9 at 782 K was obtained in the In 0.1Yb 0.2Co 4Sb 12 specimen.

Dramatic enhancement of thermoelectric power factor in PbTe:Cr co-doped with iodine

We report thermoelectric properties of PbTe doubly doped with chromium (Cr) and iodine (I). Cr is found to create a local enhancement of density of states (DOS) in the conduction band of PbTe, and I is found to tune the position of the Fermi level. The coincidence of the Fermi level with the enhanced DOS is found to produce up to 135% enhancement in thermopower leading to a high power factor (PF) of 56.16 × 10−4 Wm−1K−2 at 500 K. Such high value of power factor can produce thermoelectric figure of merit (ZT) above 2.

Thermoelectric properties of Cu 1− xPt xFeO 2 (0.0 ≤ x ≤ 0.05) delafossite-type transition oxide

The samples of Cu 1− x Pt x FeO 2 (0 ≤ x ≤ 0.05) delafossite were synthesized by solid state reaction method for studying thermoelectric properties. The properties of Seebeck coefficient, electrical conductivity and thermal conductivity were measured in the high temperature ranging from 300 to 960 K. The results of Seebeck coefficient, electrical conductivity and power factor were increased with increasing Pt substitution and temperature. The thermal conductivity was decreased from 5.8 to 3.5 W/mK with increasing the temperature from 300 to 960 K. An important results, the highest value of power factor and ZT is 2.0 × 10 −4 W/mK 2 and 0.05, respectively, for x = 0.05 at 960 K.

Thermoelectric properties of Ca 2.8Cu 0.2Co 4O 9 ceramics fabricated by solution combustion method

Nano-sized Ca 2.8Cu 0.2Co 4O 9 powders were synthesized by the solution combustion process, using five different fuels, i.e., aspartic acid, alanine, glutamic acid, glycine, and urea. The combustion process lowered both the calcination and sintering temperatures. The sintered Ca 2.8Cu 0.2Co 4O 9 consisted of plate-like grains that were favorable for high energy conversion efficiency. In the comparative study of various different fuels, aspartic acid-processed Ca 2.8Cu 0.2Co 4O 9 showed an extremely high Seebeck coefficient, thus resulting in an enhanced power factor. The highest power factor value (8.05 × 10 −4 Wm −1 K −2) was attained for the aspartic acid-processed sample at 800 °C.