Thermoelectric Properties of Ca-Filled CoSb<Subscript>3</Subscript>-Based Skutterudites Synthesized by Mechanical Alloying

Ca _z Co_4−x (Fe/Mn) _x Sb₁₂ skutterudites were prepared by mechanical alloying and hot pressing. The phases of mechanically alloyed powders were identified as γ-CoSb₂ and Sb, but they were transformed to δ-CoSb₃ by annealing at 873 K for 100 h. All specimens had a positive Hall coefficient and Seebeck coefficient, indicating p-type conduction by holes as majority carriers. For the binary CoSb₃, the electrical conductivity behaved like a nondegenerate semiconductor, but Ca-filled and Fe/Mn-doped CoSb₃ showed a temperature dependence of a degenerate semiconductor. While the Seebeck coefficient of intrinsic CoSb₃ increased with temperature and reached a maximum at 623 K, the Seebeck coefficient increased with increasing temperature for the Ca-filled and Fe/Mn-doped specimens. Relatively low thermal conductivity was obtained because fine particles prepared by mechanical alloying lead to phonon scattering. The thermal conductivity was reduced by Ca filling and Fe/Mn doping. The electronic thermal conductivity was increased by Fe/Mn doping, but the lattice thermal conductivity was decreased by Ca filling. Reasonable thermoelectric figure-of-merit values were obtained for Ca-filled Co-rich p-type skutterudites.     

Effect of Heat Treatment on the Thermoelectric Properties of Bismuth–Antimony–Telluride Prepared by Mechanical Deformation and Mechanical Alloying

In this work, p-type 20%Bi₂Te₃–80%Sb₂Te₃ bulk thermoelectric (TE) materials were prepared by mechanical deformation (MD) of pre-melted ingot and by mechanical alloying (MA) of elemental Bi, Sb, and Te granules followed by cold-pressing. The dependence on annealing time of changes of microstructure and TE properties of the prepared samples, including Seebeck coefficient, electrical resistivity, thermal conductivity, and figure-of-merit, was investigated. For both samples, saturation of the Seebeck coefficient and electrical resistivity were observed after annealing for 1 h at 380°C. It is suggested that energy stored in samples prepared by both MA and MD facilitated their recrystallization within short annealing times. The 20%Bi₂Te₃–80%Sb₂Te₃ sample prepared by MA followed by heat treatment had higher a Seebeck coefficient and electrical resistivity than specimens fabricated by MD. Maximum figures-of-merit of 3.00 × 10^?3/K and 2.85 × 10^?3/K were achieved for samples prepared by MA and MD, respectively.     

Design of Ball-Milling Experiments on Bi2Te3 Thermoelectric Material

In this work, factorial ball-milling experiments have been applied to Bi₂Te₃ material, for the first time, aiming to investigate the effect of the main process parameters on the structural features and thermoelectric properties of the ball-milled materials. The selected main parameters were the duration of milling, the speed, and the ball-to-material ratio. Analysis suggests a strong effect of the speed and duration of processing, whereas the ball-to-material ratio is of minor importance. This approach is advantageous for better understanding of the milling mechanism and the importance of the role of each independent parameter as well as their interaction. All experiments led to nanocrystalline Bi₂Te₃, whose structural features were studied. The nanocrystalline size was estimated based on x-ray diffraction analysis, while transmission electron microscopy (TEM) studies were also performed to confirm the presence of nanoscale crystals. A mathematical model was developed based on statistical analysis for prediction of the crystalline size and the Seebeck coefficient of the nanopowders. The thermoelectric properties were also investigated on selected, highly dense pellets fabricated via hot-pressing of the nanopowders.     

热处理对片状Fe-Si-Al磁微粉微波电磁性能的影响

以气体雾化Fe-Si-Al(Fe5.5Si9.5Al)合金粉末为原料,采用退火热处理和机械球磨相结合的方法制备出了片状Fe-Si-Al磁微粉,并研究了退火热处理对片状磁微粉微波电磁性能的影响.研究表明:通过对原料进行机械球磨可以获得片状形貌优良、磁性能良好的Fe-Si-Al磁微粉.机械球磨使Fe-Si-Al粉末的磁导率...     

Solid-State Synthesis and Thermoelectric Properties of Mg2Si1−x Sn x

Mg₂Si_1?x Sn _x (0 ≤ x ≤ 1) solid solutions have been successfully prepared by mechanical alloying and hot pressing as a solid-state synthesis route. All specimens were identified as phases with antifluorite structure. The electrical conduction changed from n-type to p-type at room temperature for x ≥ 0.5 due to the intrinsic properties of Mg₂Sn. The absolute value of the Seebeck coefficient decreased with increasing temperature, and the electrical conductivity increased with increasing temperature; this is indicative of nondegenerate semiconducting behavior. The thermal conductivity was reduced by Mg₂Si-Mg₂Sn solid solution due to phonon scattering by the alloying effect.     

Preparation of Conducting Polyaniline–Bismuth Nanoparticle Composites by Planetary Ball Milling

Conducting polyaniline (PANi)/Bi nanoparticle composites have been prepared using a planetary ball-milling technique. PANi powder and (±)-10-camphorsulfonic acid (CSA) as a charge carrier dopant were dissolved in m-cresol. The solution and Bi powder (under 90 μm) with different values of the molar ratio R = aniline/Bi, R = 0.1 to 1, were encased in a zirconia pot with zirconia balls, then milled at a rotating velocity of 600 rpm for 6 h to 12 h. X-ray diffraction, transmission electron microscopy, Fourier-transform infrared spectroscopy, ultraviolet-visible (UV-Vis) transmission spectroscopy, thermal gravimetric analysis, and differential thermal analysis were used to characterize the CSA-doped PANi/Bi nanoparticle composites. Bi nanoparticles ranging in size from several tens of nanometers to a few hundred nanometers were dispersed in the CSA-doped PANi matrix. PANi also acted as a protection agent for Bi particles from aggregation and perhaps oxidation. The CSA-doped PANi/Bi nanocomposite films were obtained by casting the dispersed solution onto a quartz substrate and drying. The Seebeck coefficient and the electrical conductivity of the films were measured in the temperature range from room temperature to ∼400 K.     

Thermoelectric Properties of Higher Manganese Silicides Prepared by Mechanical Alloying and Hot Pressing

Higher manganese silicides (HMS), MnSi_1.75–δ , were synthesized by mechanical alloying and consolidated by hot pressing. The optimum condition of mechanical alloying was ball milling at 400 rpm for 6 h, and sound sintered compacts could be obtained by hot pressing at temperature higher than 1073 K. The phase fraction of HMS showed no significant difference with compositional (δ) variation, but the MnSi_1.75 specimen had the lowest fraction of MnSi of approximately 3%. The lattice constants of HMS with compositional variation were similar to values reported in the literature. All specimens showed Nowotny phase with tetragonal structure, and exhibited i-type conduction at measuring temperatures between 323 K and 823 K. HMS behaved as degenerate semiconductors in that the absolute values of the Seebeck coefficient increased and the electrical conductivity slightly decreased with increasing temperature. MnSi_1.73 showed the highest figure of merit of 0.28 at 823 K.     

Numerical Study of Effects of Scattering Processes on Transport Properties of Bi Nanowires

In this study, we investigated the effects of scattering on the transport properties of Bi nanowires. The electrical conductivities and Seebeck coefficients of Bi nanowires were calculated using the Boltzmann equation, with an energy-dependent relaxation time corresponding to the scattering process. Decreasing the wire diameter increased the Seebeck coefficient for all of the scattering processes examined, because a semimetal–semiconductor transition occurred. In 80-nm-diameter nanowires, the Seebeck coefficient for ionized impurity scattering was larger than that of the acoustic deformation potential. On the other hand, in 20-nm-diameter nanowires, the dependence of the Seebeck coefficient on the scattering process was negligible, compared with the influence of wire diameter.     

Solid-State Synthesis of Te-Doped Mg<Subscript>2</Subscript>Si

Te-doped Mg₂Si (Mg₂Si:Te _m , m = 0, 0.01, 0.02, 0.03, 0.05) alloys were synthesized by a solid-state reaction and mechanical alloying. The electronic transport properties (Hall coefficient, carrier concentration, and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, thermal conductivity, and figure of merit) were examined. Mg₂Si was synthesized successfully by a solid-state reaction at 673 K for 6 h, and Te-doped Mg₂Si powders were obtained by mechanical alloying for 24 h. The alloys were fully consolidated by hot-pressing at 1073 K for 1 h. All the Mg₂Si:Te _m samples showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased and the absolute value of the Seebeck coefficient decreased with increasing Te content, because Te doping increased the electron concentration considerably from 10¹⁶ cm⁻³ to 10¹⁸ cm⁻³. The thermal conductivity did not change significantly on Te doping, due to the much larger contribution of lattice thermal conductivity over the electronic thermal conductivity. Thermal conduction in Te-doped Mg₂Si was due primarily to lattice vibrations (phonons). The thermoelectric figure of merit of intrinsic Mg₂Si was improved by Te doping.     

Thermoelectric Properties of Cold-Pressed Higher Manganese Silicides for Waste Heat Recovery

Polycrystalline higher manganese silicide (HMS) samples with different grain sizes have been obtained by cold-pressing HMS powder under high pressure of about 3?GPa and postprocessing annealing. It was found that the cold-pressing process can reduce the grain size of HMS to 120?nm. The cold-pressed pellets were then annealed at different temperatures to obtain a series of samples with different grain sizes. For comparison, an additional sample was prepared in a regular die under low pressure of 300?MPa, which resulted in lower density and higher porosity than the high-pressure process. For these samples, the effect of grain size and porosity on Seebeck coefficient was not as apparent as that on electrical conductivity and thermal conductivity. The electrical conductivity of the cold-pressed samples increases as the grains grow, and the grain boundary connection is improved during the postprocessing annealing. A significant reduction in the thermal conductivity of the cold-pressed samples was observed. The sample prepared with the low-pressure pressing shows the lowest thermal conductivity of 1.2?W?m^?1?K^?1 at 300?K, which can be attributed to its high porosity of 34% and low phonon transmission coefficient through the grain boundaries. The low-temperature thermal conductivity data of all samples were analyzed to obtain the phonon transmission coefficient and the Kapitza resistance at the grain boundaries.     

High-pressure preparation and thermoelectric properties of Bi0.85Sb0.15

Nanocrystalline Bi_0.85Sb_0.15 powders were prepared by a novel mechanical alloying method. The bulk samples were formed by applying a pressure of 6 GPa at different pressing temperatures and times. Electrical conductivity, Seebeck coefficients, and thermal conductivity were measured in the temperature range 80–300 K. The Seebeck coefficient reaches a maximum value of −173 μV/K at 150 K. The largest figure of merit, 3.46 × 10⁻³ K⁻¹, achieved in this experiment is 50% higher than that of its single-crystal counterpart at 200 K.     

Thermoelectric Properties of Sn-S Bulk Materials Prepared by Mechanical Alloying and Spark Plasma Sintering

To study the possibility of SnS as an earth-abundant and environmentally friendly thermoelectric material, the electrical and thermal transport properties of bulk materials prepared by combining mechanical alloying and spark plasma sintering were investigated. It was revealed that SnS has potential as a good thermoelectric material, benefiting from its intrinsically low thermal conductivity below 1.0 W/m/K above 400 K and its high Seebeck coefficient over 500 μV/K. Although the highest ZT value was 0.16 at 823 K in the pristine sample, further enhancement can be expected through chemical doping to increase the electrical conductivity. It was also revealed that changing the stoichiometric ratio and sintering temperature had less apparent influence on the microstructure and thermoelectric properties of SnS because redundant S in the powders decomposed during the sintering process.     

Influence of Ball-Milling,Nanostructuring, and Ag Inclusions on Thermoelectric Properties of ZnSb

In the last few years much attention has been given to the promising thermoelectric material Zn₄Sb₃. The related ZnSb phase features a high Seebeck coefficient at room temperature. Its thermoelectric conversion efficiency, however, is low due to its relatively high thermal conductivity. ZnSb has potential as a thermoelectric material if this can be reduced. Nanostructuring of bulk materials and introducing extrinsic particles have been shown to lower lattice thermal conductivity. In this study we created the microstructure by ball-milling of bulk ZnSb and added Ag particles which attain sizes in the micrometer range in this milling process. Hot-pressing was used to obtain dense samples. Several techniques were used for structural characterization. Here we report on scanning electron microscopy, transmission electron microscopy, and x-ray diffraction analysis. Thermoelectrical measurements were conducted around room temperature. Thermal conductivity was reduced by up to 40% by the reported nanostructuring. However, the electrical conductivity and the Seebeck coefficient were adversely affected, leading to no overall improvement in performance.     

Variations of Thermoelectric and Mechanical Properties of Large Lead Telluride Samples Produced by a Short-Term Sintering Method

Although lead telluride is a widely used thermoelectric (TE) material for generator applications in the intermediate temperature range, its mechanical properties have not been fully understood yet. Especially sintered PbTe samples have hardly been investigated with regard to their mechanical properties and homogeneity in general. The aim of the present study is to comprehend the TE and mechanical properties of large PbTe samples produced by a short-term sintering process. The potential and Seebeck microprobe technique was used to reveal the spatial distribution of the Seebeck coefficient on the samples’ surfaces at room temperature. Microhardness measurements were performed with a Vickers indenter. The results demonstrate that, despite a homogeneous density, the functional and mechanical properties vary along the samples, which is attributed to varying local parameters during sintering.     

Nanoscale FeS2 (Pyrite) as a Sustainable Thermoelectric Material

We have synthesized undoped, Co-doped (up to 5%), and Se-doped (up to 4%) FeS₂ materials by mechanical alloying in a planetary ball mill and investigated their thermoelectric properties from room temperature (RT) to 600 K. With decreasing particle size, the undoped FeS₂ samples showed higher electrical conductivity, from 0.02 S cm^?1 for particles with 70 nm grain size up to 3.1 S cm^?1 for the sample with grain size of 16 nm. The Seebeck coefficient of the undoped samples showed a decrease with further grinding, from 128 μV K^?1 at RT for the sample with 70-nm grains down to 101 μV K^?1 for the sample with grain size of 16 nm. The thermal conductivity of the 16-nm undoped sample lay within the range from 1.3 W m^?1 K^?1 at RT to a minimal value of 1.2 W m^?1 K^?1 at 600 K. All doped samples showed improved thermoelectric behavior at 600 K compared with the undoped sample with 16 nm particle size. Cobalt doping modified the p-type semiconducting behavior to n-type and increased the thermal conductivity (2.1 W m^?1 K^?1) but improved the electrical conductivity (41 S cm^?1) and Seebeck coefficient (-129 μV K^?1). Isovalent selenium doping led to a slightly higher thermal conductivity (1.7 W m^?1 K^?1) as well as to an improved electrical conductivity (26 S cm^?1) and Seebeck coefficient (110 μV K^?1). The ZT value of FeS₂ was increased by a factor of five by Co doping and by a factor of three by Se doping.     

Thermoelectric Properties of Magnesium Silicide Deposited by Use of an Atmospheric Plasma Thermal Spray

The thermoelectric properties of magnesium silicide (Mg₂Si) samples prepared by use of an atmospheric plasma spray (APS) were compared with those of samples prepared from the same feedstock powder by use of the conventional hot-pressing method. The characterization performed included measurement of thermal conductivity, electrical conductivity, Seebeck coefficient, and figure of merit, ZT. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDX) were used to assess how phase and microstructure affected the thermoelectric properties of the samples. Hall effect measurements furnished carrier concentration, and measurement of Hall mobility provided further insight into electrical conductivity and Seebeck coefficient. Low temperature and high velocity APS using an internal-powder distribution system achieved a phase of composition similar to that of the feedstock powder. Thermal spraying was demonstrated in this work to be an effective means of reducing the thermal conductivity of Mg₂Si; this may be because of pores and cracks in the sprayed sample. Vacuum-annealed APS samples were found to have very high Seebeck coefficients. To further improve the figure of merit, carrier concentration must be adjusted and carrier mobility must be enhanced.     

Numerical Study of Effect of Surface Potential on Transport Properties of Bi Nanowires

We numerically investigated the effect of the surface on the transport properties of Bi nanowires. The effect of the surface was modeled using the surface potential. The energy shift in each band due to the surface potential was calculated by a perturbation method. The effect of the surface potential on the transport properties was estimated using the Boltzmann equation with a constant relaxation time. The results reveal that the surface potential dramatically alters the density of states of T-point holes, whereas it has very little effect on the density of states of L-point holes. This is because the wavefunctions at the L- and T-points have different symmetries. The electrical conductivity increases and the Seebeck coefficient decreases with increasing surface potential. The maximum absolute value of the Seebeck coefficient decreases drastically with increasing surface potential. The Seebeck coefficient has a much stronger dependence on the surface potential than on the wire diameter. These results demonstrate that the transport properties of Bi nanowires are very sensitive to the surface potential.     

Effect of Fe Substitution on Thermoelectric Properties of Fe x In4−x Se3 Compounds

Starting from elemental powder mixtures of Fe _x In_4?x Se₃ (x = 0, 0.05, 0.1, 0.15), polycrystalline In₄Se₃-based compounds with homogeneous microstructures were prepared by mechanical alloying (MA) and hot pressing (HP). With the increase of x from 0 to 0.15, the electrical resistivity and the absolute value of the Seebeck coefficient increased, while the thermal conductivity first decreased and then increased. The maximal dimensionless figure of merit ZT of 0.44 was obtained for the Fe _x In_4?x Se₃ (x = 0.05) sample at 723 K.     

Thermoelectric Properties of Indium-Selenium Nanocomposites Prepared by Mechanical Alloying and Spark Plasma Sintering

Indium-selenium-based compounds have received much attention as thermoelectric materials since a high thermoelectric figure of merit of 1.48 at 705?K was observed in In₄Se_2.35. In this study, four different compositions of indium-selenium compounds, In₂Se₃, InSe, In₄Se₃, and In₄Se_2.35, were prepared by mechanical alloying followed by spark plasma sintering. Their thermoelectric properties such as electrical resistivity, Seebeck coefficient, and thermal conductivity were measured in the temperature range of 300?K to 673?K. All the In-Se compounds comprised nanoscaled structures and exhibited n-type conductivity with Seebeck coefficients ranging from ?159???V?K^?1 to ?568???V?K^?1 at room temperature.     

Fe-doped SnO2 obtained by mechanical alloying

In this work, iron-doped SnO₂ powders were prepared by two methods: mechanical alloying and mechanochemical alloying with successive thermal treatment. The influence of different milling conditions such as ball to powder weight ratio, milling time, rotation velocity of supporting disc and the type of iron starting reactive and their Fe concentration on the structural and magnetic properties of the products were investigated. A greater incorporation of Fe in the SnO₂ structure was observed when the samples were prepared by using mechanochemical alloying and successive thermal treatment.