Electrical Properties of (110)-Oriented Nondoped Mg2Si Films with p-Type Conduction Prepared by RF Magnetron Sputtering Method

Magnesium silicide (Mg₂Si) thick films with (110) orientation were fabricated on (001) sapphire substrate using radiofrequency magnetron sputtering. Stoichiometric Mg₂Si films with composition Si/(Mg + Si) = 0.33 were achieved over a range of vacuum from 10 mTorr to 140 mTorr and 300°C. On postannealing the film at 500°C, the out-of-plane lattice parameter shifted to lower values and the electrical conductivity increased by two orders of magnitude. A room-temperature Seebeck coefficient of 517 μV K^?1 was observed and found to decrease with increasing temperature; the Seebeck coefficient remained at a constant positive value of 212 μV K^?1 at 500°C. This can be related to the possibility of p-type conduction in this temperature range.     

Theoretical and experimental investigations of the thermoelectric properties of Al-, Bi- and Sn-doped ZnO

In this paper, the thermoelectric properties of ZnO doped with Al, Bi and Sn were investigated by combining experimental and theoretical methods. The average Seebeck coefficient of Bi doped ZnO over the measured temperature range is improved from −90 to −497 μV/K. However, segregation of Bi₂O₃ in ZnO:Bi sample, confirmed by FESEM, lead to enormous grain growth and low electrical conductivity, which makes Bi is not a good dopant to improve ZT value of ZnO. As a 4+ valence cation, Sn doping actually show an increase in carrier concentration to 10²⁰ cm⁻³, further enhancing the electrical conductivity. Unfortunately, the Seebeck coefficient of ZnO:Sn samples is even lower than pure ZnO sample, which lead to a low ZT value. As for ZnO:Al sample, with nearly no change in lattice thermal conductivity, electrical conductivity and Seebeck coefficient were both enhanced. Threefold enhancement in ZT value has been achieved in ZnO:Al sample at 760 °C compared with pure ZnO.     

Hopping transport in doped (Pb0.78Sn0.22)1−x InxTe solid solutions

Transport coefficients in (Pb_0.78Sn_0.22)_1?x In_xTe solid solutions with indium content x=0.03 and 0.05 additionally doped with acceptors (Tl) or donors (Cl) have been measured and analyzed. The Seebeck coefficient S is positive for x=0.05 in the low temperature range 77–200 K; its sign changes to negative when the Tl acceptor is added. This unusual behavior of the thermoelectric power can be attributed to hopping conduction at a nonmonotonic energy dependence of the density of localized states. The density-of-states function has been determined at x=0.03 and 0.05 from experimental data on the thermopower. Theoretical estimates of the Nernst-Ettingshausen coefficient are made for x=0.03 additionally doped with Cl. The estimates are based on taking into account, along with the hopping conduction, the contribution from electrons with energies above the mobility edge and on using the critical electrical conductivity exponent obtained in the percolation theory. The activation energies characterizing the temperature dependences of conductivity and Hall and Nernst-Ettingshausen coefficients are discussed and compared.     

Synthesis and Thermoelectric Properties of Yb-doped Ca0.9−x Yb x La0.1MnO3 Ceramics

The microstructure and thermoelectric properties of Yb-doped Ca_0.9?x Yb _x La_0.1 MnO₃ (0 ≤ x ≤ 0.05) ceramics prepared by using the Pechini method derived powders have been investigated. X-ray diffraction analysis has shown that all samples exhibit single phase with orthorhombic perovskite structure. All ceramic samples possess high relative densities, ranging from 97.04% to 98.65%. The Seebeck coefficient is negative, indicating n-type conduction in all samples. The substitution of Yb for Ca leads to a marked decrease in the electrical resistivity, along with a moderate decrease in the absolute value of the Seebeck coefficient. The highest power factor is obtained for the sample with x = 0.05. The electrical conduction in these compounds is due to electrons hopping between Mn³⁺ and Mn⁴⁺, which is enhanced by increasing Yb content.     

Electrical measurements of dielectric properties of molybdenum-doped zinc oxide thin films

Effects of molybdenum element content on electrical conductivity of ZnO sprayed thin films were investigated using the impedance spectroscopy method in the frequency ranging from 5 Hz to 13 MHz for temperature lying in 300–475 °C domain. It is observed that AC conductivity is a power law. The values of dielectric constants ε₁ and ε₂ were found to decrease with frequency and increase with temperature. The activation energy determined from the plot of both DC conductivity and the hopping frequency with 1000/T shows that the hopping conduction is the dominant mechanism. Also, experimental data of DC conductivity were analyzed using the small polaron hopping model. The impedance analysis of undoped ZnO and Mo-doped ZnO (1% and 2%) shows only one semicircle implying the response originated from a single capacitive element corresponding to the bulk grains. However, the same analysis for ZnO:Mo (3% ) shows two semicircles which proves the existence of grain boundaries. Finally, analyses of polaron hopping mechanism and Urbach tailing allow some explanations of these transport phenomena. This study shows an effective variation of electrical measurements of Mo-doped ZnO films in terms of temperature leading to possible use of such films as gas sensors.     

Low-Temperature Synthesis and Thermoelectric Properties of n-Type PbTe

n-Type PbTe compounds were synthesized at temperatures as low as 430°C. After synthesis, the materials were ground, cold pressed, and sintered at 600°C. The effect of this low-temperature synthesis on the structural features and thermoelectric properties of as-prepared and PbI₂-doped materials was investigated for the first time. The Seebeck coefficient, and electrical and thermal conductivity were measured in the temperature range 2 K ≤ T ≤  610 K. The results show that all materials exhibit n-type conduction and the thermoelectric properties are improved by doping. ZT values reach 0.5 at 610 K, and the discrepancies with the literature are discussed.     

Thermoelectric Properties of Al-Doped ZnO Thin Films

We have prepared 2 % Al-doped ZnO (AZO) thin films on SrTiO₃ substrates by a pulsed laser deposition technique at various deposition temperatures (T _dep = 300–600 °C). The thermoelectric properties of AZO thin films were studied in a low temperature range (300–600 K). Thin film deposited at 300 °C is fully c-axis-oriented and presents electrical conductivity 310 S/cm with Seebeck coefficient ?65 μV/K and power factor 0.13 × 10^?3 Wm^?1 K^?2 at 300 K. The performance of thin films increases with temperature. For instance, the power factor is enhanced up to 0.55 × 10^?3 Wm^?1 K^?2 at 600 K, surpassing the best AZO film previously reported in the literature.     

Thermoelectric Properties of Polyaniline Films with Different Doping Concentrations of (±)-10-Camphorsulfonic Acid

The temperature dependence of the thermoelectric properties was investigated for polyaniline (PANI) films doped with different concentrations of (±)-10-camphorsulfonic acid (CSA) with molar ratio x of CSA to two phenyl-nitrogen units of x = 1 to 0.2. All PANI-CSA films exhibit p-type conduction. The temperature dependence of the electrical conductivity of the films with low CSA concentrations is consistent with a transport mechanism of variable-range hopping. On the other hand, the Seebeck coefficient above room temperature shows a linear increase with temperature, attributed to the metallic nature of PANI-CSA. As the CSA concentration decreases, the absolute value of the Seebeck coefficient increases while the electrical conductivity extremely decreases, probably due to the changes not only in the carrier concentration but also in the degree of structural disorder. The power factor increases monotonically with increasing CSA concentration toward x = 1 (the maximum limit). The thermal conductivity value of CSA-PANI film with x = 1 is as low as about 0.20 W m^?1 K^?1 in the through-plane direction and about 0.67 W m^?1 K^?1 in the in-plane direction. The thermoelectric figure of merit ZT in the in-plane direction is estimated to be approximately 1 × 10^?3 for x = 1.     

Thermoelectric Properties of CdTe1−x Cl x Material Prepared by Spark Plasma Sintering Method

CdTe compound is a prospective thermoelectric material due to its high Seebeck coefficient and low thermal conductivity. In the present study, we optimized its carrier concentration by substituting Cl on the Te site in order to improve the electrical conductivity and decrease the lattice thermal conductivity. The polycrystalline CdTe_1?x Cl _x (x = 0.005, 0.01, 0.03, 0.05) samples were fabricated by solid state reaction followed with spark plasma sintering, and the relative densities of the sintered samples were higher than 98%. Thermoelectric properties, including Seebeck coefficient (α), electrical conductivity (σ). and thermal conductivity (κ), were measured in the temperature range of 300–700 K. The increase of Cl content (x) caused an increase of σ, and the maximum ZT value of 0.2 was obtained at about 630 K for the CdTe_0.97Cl_0.03 sample.     

Structure and Transport Properties of Bulk Nanothermoelectrics Based on Bi x Sb2−x Te3 Fabricated by SPS Method

Ball milling with subsequent spark plasma sintering (SPS) was used to fabricate bulk nanothermoelectrics based on Bi _x Sb_2?x Te₃. The SPS technique enables reduced size of grains in comparison with the hot-pressing method. The electrical and thermal conductivities, Seebeck coefficient, and thermoelectric figure of merit as functions of temperature and alloy composition were measured for different sintering temperatures. The greatest value of the figure of merit ZT = 1.25 was reached at the temperature of 90°C to 100°C in Bi_0.4Sb_1.6Te₃ for sintering temperature of 450°C to 500°C. The volume and quantitative distributions of size of coherent dispersion areas (CDA) were calculated for different sintering temperatures. The phonon thermal conductivity of nanostructured Bi _x Sb_2?x Te₃ was investigated theoretically taking into account phonon scattering on grain boundaries and nanoprecipitates.     

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.     

Variable-range-hopping conduction via indium impurity states in Pb0.78Sn0.22Te solid solution

The Seebeck coefficient S was measured in a wide temperature range T<100 K in Pb_0.78Sn_0.22Te solid solutions doped with 3 at. % of In, with additional Cl doping of up to 3 at. %. The temperature derivative ?|S|/?T changes its sign from negative to positive below 100 K. Theoretical estimations in terms of hopping conduction via highly localized indium-related states show that the transition to variable-range-hopping conduction must occur at temperatures of about 50–100 K, in agreement with the obtained experimental data.     

Thermoelectric Properties of Cu-doped Bi<Subscript>0.4</Subscript>Sb<Subscript>1.6</Subscript>Te<Subscript>3</Subscript> Prepared by Hot Extrusion

Cu_0.003Bi_0.4Sb_1.6Te₃ alloys were prepared by using encapsulated melting and hot extrusion (HE). The hot-extruded specimens had the relative average density of 98%. The (00l) planes were preferentially oriented parallel to the extrusion direction, but the specimens showed low crystallographic anisotropy with low orientation factors. The specimens were hot-extruded at 698 K, and they showed excellent mechanical properties with a Vickers hardness of 76 Hv and a bending strength of 59 MPa. However, as the HE temperature increased, the mechanical properties degraded due to grain growth. The hot-extruded specimens showed positive Seebeck coefficients, indicating that the specimens have p-type conduction. These specimens exhibited negative temperature dependences of electrical conductivity, and thus behaved as degenerate semiconductors. The Seebeck coefficient reached the maximum value at 373 K and then decreased with increasing temperature due to intrinsic conduction. Cu-doped specimens exhibited high power factors due to relatively higher electrical conductivities and Seebeck coefficients than those of undoped specimens. A thermal conductivity of 1.00 Wm^?1 K^?1 was obtained at 373 K for Cu_0.003Bi_0.4Sb_1.6Te₃ hot-extruded at 723 K. A maximum dimensionless figure of merit, ZT _max = 1.05, and an average dimensionless figure of merit, ZT _ave = 0.98, were achieved at 373 K.     

Evaluation of Temperature-Dependent Effective Material Properties and Performance of a Thermoelectric Module

We devised a novel method to evaluate the temperature-dependent effective properties of a thermoelectric module (TEM): Seebeck coefficient (S _m), internal electrical resistance (R _m), and thermal conductance (K _m). After calculation, the effective properties of the module are converted to the average material properties of a p–n thermoelectric pillar pair inside the module: Seebeck coefficient (S _TE), electrical resistivity (ρ _TE), and thermal conductivity (k _TE). For a commercial thermoelectric module (Altec 1091) chosen to verify the novel method, the measured S _TE has a maximum value at bath temperature of 110°C; ρ _TE shows a positive linear trend dependent on the bath temperature, and k _TE increases slightly with increasing bath temperature. The results show the method to have satisfactory measurement performance in terms of practicability and reliability; the data for tests near 23°C agree with published values.     

Effect of Composition on Thermoelectric Properties of Polycrystalline CrSi2

Ingots with compositions CrSi_2?x (with 0 < x < 0.1) were synthesized by vacuum arc melting followed by uniaxial hot pressing for densification. This paper reports the temperature and composition dependence of the electrical resistivity, Seebeck coefficient, and thermal conductivity of CrSi_2?x samples in the temperature range of 300 K to 800 K. The silicon-deficient samples exhibited substantial reductions in resistivity and Seebeck coefficient over the measured temperature range due to the formation of metallic secondary CrSi phase embedded in the CrSi₂ matrix phase. The thermal conductivity was seen to exhibit a U-shaped curve with respect to x, exhibiting a minimum value at the composition of x = 0.04. However, the limit of the homogeneity range of CrSi₂ suppresses any further decrease of the lattice thermal conductivity. As a consequence, the maximum figure of merit of ZT = 0.1 is obtained at 650 K for CrSi_1.98.     

Post-annealing Effect on Microstructures and Thermoelectric Properties of Bi0.45Sb1.55Te3 Thin Films Deposited by Co-sputtering

p-Type Bi_0.45Sb_1.55Te₃ thermoelectric (TE) thin films have been prepared at room temperature by a magnetron cosputtering process. The effect of postannealing on the microstructure and TE properties of Bi_0.45Sb_1.55Te₃ films has been investigated in the temperature range from room temperature to 350°C. x-Ray diffraction analysis shows that the annealed films have polycrystalline rhombohedral crystal structure, and the average grain size increases from 36?nm to 64?nm with increasing annealing temperature from room temperature to 350°C. Electron probe microanalysis shows that annealing above 250°C can cause Te reevaporation, which induces porous thin films and dramatically affects electrical transport properties of the thin films. TE properties of the films have been investigated at room temperature. The hole concentration shows a trend from descent to ascent and has a minimum value at the annealing temperature of 200°C, while the Seebeck coefficient shows an opposite trend and a maximum value of 245?μV?K^?1. The electrical resistivity monotonically decreases from 19.8?mΩ?cm to 1.4?mΩ?cm with increasing annealing temperature. Correspondingly, a maximum value of power factor, 27.4?μW?K^?2?cm^?1, was obtained at the annealing temperature of 250°C.     

Enhanced Thermoelectric Properties of Antimony Telluride Thin Films with Preferred Orientation Prepared by Sputtering a Fan-Shaped Binary Composite Target

p-Type antimony telluride (Sb₂Te₃) thermoelectric thin films were deposited on BK7 glass substrates by ion beam sputter deposition using a fan-shaped binary composite target. The deposition temperature was varied from 100°C to 300°C in increments of 50°C. The influence of the deposition temperature on the microstructure, surface morphology, and thermoelectric properties of the thin films was systematically investigated. x-Ray diffraction results show that various alloy composition phases of the Sb₂Te₃ materials are grown when the deposition temperature is lower than 200°C. Preferred c-axis orientation of the Sb₂Te₃ thin film became obvious when the deposition temperature was above 200°C, and thin film with single-phase Sb₂Te₃ was obtained when the deposition temperature was 250°C. Scanning electron microscopy reveals that the average grain size of the films increases with increasing deposition temperature and that the thin film deposited at 250°C shows rhombohedral shape corresponding to the original Sb₂Te₃ structure. The room-temperature Seebeck coefficient and electrical conductivity range from 101 μV K^?1 to 161 μV K^?1 and 0.81 × 10³ S cm^?1 to 3.91 × 10³ S cm^?1, respectively, as the deposition temperature is increased from 100°C to 300°C. An optimal power factor of 6.12 × 10^?3 W m^?1 K^?2 is obtained for deposition temperature of 250°C. The thermoelectric properties of Sb₂Te₃ thin films have been found to be strongly enhanced when prepared using the fan-shaped binary composite target method with an appropriate substrate temperature.     

Dielectric and Impedance Spectroscopy of Barium Orthoniobate Ceramic

Barium orthoniobate (Ba₃Nb₂O₈), a derivative of the perovskite family, was prepared using a high-temperature solid-state reaction technique (calcination temperature = 1425°C and sintering temperature = 1450°C for 4 h). Preliminary x-ray structural analysis with room-temperature x-ray diffraction data confirmed the formation of a single-phase compound with hexagonal crystal structure. Study of the microstructure of a gold-coated pellet by scanning electron microscopy (SEM) showed that the sample has well-defined grains that are distributed uniformly throughout the surface of the sample. Detailed studies showed that the dielectric parameters (ε _r and tan δ) of the compound at three different frequencies (10 kHz, 100 kHz, and 1000 kHz) are almost constant in the low-temperature region (from room temperature to about 200°C). An anomaly in the relative permittivity (ε _r) (～357°C) suggests the possible existence of a ferroelectric–paraelectric phase transition of diffuse type in the material. Detailed studies of impedance and related parameters show that the electrical properties of the material are strongly dependent on temperature, showing good correlation with its microstructure. The bulk resistance (evaluated from impedance studies) is found to decrease with increasing temperature. This shows that the material has negative temperature coefficient of resistance (NTCR), similar to that of semiconductors. Studies of electric modulus indicate the presence of a hopping conduction mechanism in the system with nonexponential-type conductivity relaxation. The nature of the variation of the direct-current (dc) conductivity with temperature confirms the Arrhenius and NTCR behavior in the material. The alternating-current (ac) conductivity spectra show a typical signature of an ionic conducting system and are found to obey Jonscher’s universal power law.     

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.     

Lower Thermal Conductivity and Higher Thermoelectric Performance of Fe-Substituted and Ce, Yb Double-Filled p-Type Skutterudites

Substituting Fe on Co sites is an effective way to produce p-type skutterudite compounds as well as to reduce the thermal conductivity of skutterudites. In this work, we investigated thermoelectric properties of Fe-substituted and Ce + Yb double-filled Ce _x Yb _y Fe _z Co_4?z Sb₁₂ (x = y = 0.5, z = 2.0 to 3.25 nominal) skutterudite compounds by studying the Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient over a broad range of temperatures. All samples were prepared by using the traditional method of melting–annealing and spark plasma sintering. The signs of the Hall coefficient and Seebeck coefficient indicate that all samples are p-type conductors. Electrical conductivity increases with increasing Fe content. The temperature dependence of electrical conductivity indicates that a transition from the extrinsic to the intrinsic regime of conduction depends on the amount of Fe substituted for Co. The temperature dependence of mobility reflects the dominance of acoustic phonon scattering at temperatures above ambient. Except for Ce_0.5Yb_0.5Fe_3.25Co_0.75Sb₁₂, the thermal conductivity increases with increasing Fe content, reaching the maximum value of 2.23 W/m K at room temperature for Ce_0.5Yb_0.5Fe₃CoSb₁₂. A high power factor (27 μW/K² cm) combined with a rather low thermal conductivity for Ce_0.5Yb_0.5Fe_3.25Co_0.75Sb₁₂ (nominal) lead to a dimensionless figure of merit ZT = 1.0 at 750 K for this compound, one of the highest ZT values achieved in p-type skutterudite compounds prepared by the traditional method of melting–annealing and spark plasma sintering.