Development of a high-throughput thermoelectric screening tool for combinatorial thin film libraries

We have developed a high-throughput thermoelectric screening tool for the study of combinatorial thin films. This tool consists of a probe to measure resistance and Seebeck coefficient on an automated translation stage. A thin film library of the (Ca_1−x−ySr_xLa_y)₃Co₄O₉ ternary system has been fabricated on a Si (1 0 0) substrate, using combinatorial pulsed laser deposition by the natural-composition-spread method. We have demonstrated successful mapping of the resistance and Seebeck coefficient of this film library. The mapping indicates that the substitution of La for Ca results in an increase of both resistance and Seebeck coefficient, and that of Sr results in a decrease of resistance. The screening tool allows us to measure 1080 data points in 6 h.     

Modified texture and high temperature transport properties of doubly substituted BaxAgyCa2.8Co4O9 thermoelectric oxide

Doubly substituted polycrystalline compound bulk samples of Ba_xAg_yCa_2.8Co₄O₉ were prepared via citrate acid sol-gel method followed by spark plasma sintering. The phase composition, orientation, texture and high temperature electrical properties were systematically investigated. The results showed that the orientation and the texture could be modified by altering ratio of Ba to Ag. The resistivity and the Seebeck coefficient of substituted samples were decreased by decreasing Ba/Ag ratio except for that of Ba_0.1Ag_0.1Ca_2.8Co₄O₉ sample with lowest electrical resistivity (7.2 mΩ cm at 973 K), moderately high Seebeck coefficient (172 μV/K at 973 K) and improved power factor (0.42 mW/mK² at 973 K).     

Effect of chromium and tellurium impurities on the thermoelectric parameters of galena

Concentration dependences of the Seebeck coefficient, resistivity, and thermal conductivity of thermoelectric PbS crystals with chromium (0 < x ≤ 0.01) and tellurium (0 < y ≤ 0.03) impurities are examined in the temperature region of 300–800 K. It is shown that the introduction of chromium increases the number of free electrons in PbCr _x S _1–x crystals and reduces the Seebeck coefficient. However, an increase in the concentration of tellurium in PbCr _x S _1–x–y Te _y alloys raises the Seebeck coefficient while simultaneously reducing the thermal conductivity. As a result, the thermoelectric efficiency of PbCr _x S _1–x–y Te _y crystals increases. The reasons for the observed effects are discussed.     

Seebeck and magnetoresistive effects of Ga-doped ZnO thin films prepared by RF magnetron sputtering

In this paper, Ga-doped ZnO (GZO) films were deposited on glass substrates at different substrate temperatures by RF magnetron sputtering. The effect of substrate temperature on the structural, surface morphological properties, Seebeck and magnetoresistive effects of GZO films was investigated. It is found that the GZO films are polycrystalline and preferentially in the [0 0 2] orientation, and the film deposited at 300 °C has an optimal crystal quality. Seebeck and magnetoresistive effects are apparently observed in GZO films. The thermoelectromotive forces are negative. Decreasing substrate temperature and annealing in N₂ flow can decrease carrier concentration. The absolute value of the Seebeck coefficient increases with decreasing carrier concentration. The maximal absolute value of Seebeck coefficient is 101.54 μV/K for the annealed samples deposited at the substrate temperature of 200 °C. The transverse magnetoresistance of GZO films is related to both the magnetic field intensity and the Hall mobility. The magnetoresistance increases almost linearly with magnetic field intensity, and the films deposited at higher substrate temperature have a stronger magnetoresistance under the same magnetic field, due to the larger Hall mobility.     

Effects of double filling of Pb and Ba on thermoelectric properties of PbxBayCo4Sb11.5Te0.5 compounds by HPHT

Skutterudite compounds Pb_xBa_yCo₄Sb_11.5Te_0.5 (x≤0.23,y≤0.27) with bcc crystal structure have been prepared by the high pressure and high temperature (HPHT) method. The study explored a chemical method for filling Pb and Ba atoms into the voids of CoSb₃ to optimize the thermoelectric figure of merit ZT in the system of Pb_yBa_xCo₄Sb_11.5Te_0.5. The structure of Pb_xBa_yCo₄Sb_11.5Te_0.5 skutterudites was evaluated by means of X-ray diffraction. The Seebeck coefficient, electrical resistivity and power factor were performed from room temperature to 710 K. Compared with Co₄Sb_11.5Te_0.5, the thermal conductivity of Pb and Ba double-filled samples was reduced evidently. Among all filled samples, Pb_0.03Ba_0.27Co₄Sb_11.5Te_0.5 showed the highest power factor of 31.64 μW cm⁻¹ K⁻² at 663 K. Pb_0.05Ba_0.25Co₄Sb_11.5Te_0.5 showed the lowest thermal conductivity of 2.73 W m⁻¹ K⁻¹ at 663 K, and its maximum ZT value reached 0.63 at 673 K.     

Laser-induced voltage characteristics of Bi2Sr2Co2Oy thin films on LaAlO3 substrates

We report the laser-induced voltage (LIV) effects in c-axis oriented Bi₂Sr₂Co₂O_y thin films grown on (0 0 1) LaAlO₃ substrates with the title angle α of 0°, 3°, 5° and 10° by a simple chemical solution deposition method. A large open-circuit voltage with the sensitivity of 300 mV/mJ is observed for the film on 10° tilting LaAlO₃ under a 308 nm irradiation with the pulse duration of 25 ns. When the film surface is irradiated by a 355 nm pulsed laser of 25 ps duration, a fast response with the rise time of 700 ps and the full width at half maximum of 1.5 ns is achieved. In addition, the experimental results reveal that the amplitude of the voltage signal is approximately proportional to sin 2α and the signal polarity is reversed when the film is irradiated from the substrate side rather than the film side, which suggests the LIV effects in Bi₂Sr₂Co₂O_y thin films originate from the anisotropic Seebeck coefficient of this material.     

Thermoelectric properties of SnzCo8Sb24 skutterudites

Sn-filled CoSb₃ skutterudite compounds were synthesized by the induction melting process. Formation of a single δ-phase of the synthesized materials was confirmed by X-ray diffraction analysis. The temperature dependences of the Seebeck coefficient, electrical resistivity and thermal conductivity were examined in the temperature range of 300-700 K. Positive Seebeck and Hall coefficients confirmed p-type conductivity. Electrical resistivity increased with increasing temperature, which shows that the Sn-filled CoSb₃ skutterudite is a degenerate semiconductor. The thermal conductivity was reduced by Sn-filling because the filler atoms acted as phonon scattering centers in the skutterudite lattice. The lowest thermal conductivity was achieved in the composition of Sn_0.25Co₈Sb₂₄.     

Low temperature thermoelectric properties of Pb- or Sn-doped Bi-Sb alloys

Pb- or Sn-doped Bi₈₈Sb₁₂ alloys were prepared by direct melting, quenching, and annealing. The Bi-Sb alloy phase was predominant in all samples. Pb or Sn atoms were distributed almost uniformly in Bi₈₈Sb_12, while some segregation was confirmed at the grain boundaries when Pb or Sn was involved heavily. The thermoelectric properties of these doped materials were investigated by measuring the Hall coefficient, electrical resistivity, and Seebeck coefficient between 20 K and 300 K. The Hall and Seebeck coefficients of Pb- or Sn-doped samples were positive at low temperatures, indicating that the doping element acted as an acceptor. Temperatures resulting in positive Hall and Seebeck coefficients further increased with increasing doping amount and with respect to the annealing process. As a result, a large power factor of 1.2 W/mK² could be obtained in the 3-at% Sn-doped sample at 220 K, with a large positive Seebeck coefficient.     

Thermoelectric properties of Ge-doped Bi85Sb15 alloys at low temperatures

Bulk polycrystalline Bi₈₅Sb_15−xGe_x (x=0, 0.5, 1, 1.5, 2) composites were prepared by mechanical alloying followed by pressureless sintering. The thermoelectric properties were studied in the temperature range of 77–300 K. The results indicate that increasing the Ge concentration causes the Seebeck coefficient to change sign from negative to positive. Moreover, it is found that the maximum value of the Seebeck coefficient can be precisely controlled with the Ge concentration. The maximum dimensionless figure of merit reaches 0.07 at 140 K. These results suggest that the preparation of p-type Bi–Sb alloys is possible by using the Ge-doping approach.     

Power factor enhancement in Pr-substituted alloys prepared by high-pressure sintering

Bi₈₅Sb_15−xPr_x (x=0,1,2,3) alloys with partial substitution of Pr for Sb were synthesized by mechanical alloying followed by high-pressure sintering. The crystal structure was characterized by X-ray diffraction. The electrical conductivity and Seebeck coefficient were measured in the temperature range of 80–300 K. The results show that the electrical conductivity and Seebeck coefficient of Pr-substituted samples are both larger than those of the reference sample, Bi₈₅Sb₁₅, in the whole measurement temperature range. The power factor of Bi₈₅Sb₁₃Pr₂ reaches a maximum value of 3.83×10⁻³ W K⁻² m⁻¹ at 235 K, which is about four times larger than that of the reference sample, Bi₈₅Sb₁₅, at the same temperature.     

Hall mobility of amorphous Ge2Sb2Te5

The electrical conductivity, Seebeck coefficient, and Hall coefficient of three-micron-thick films of amorphous Ge₂Sb₂Te₅ have been measured as functions of temperature from room temperature down to as low as 200 K. The electrical conductivity manifests an Arrhenius behavior. The Seebeck coefficient is p-type with behavior indicative of multi-band transport. The Hall mobility is n-type and low (near 0.07 cm²/V s at room temperature).     

Effect of varying mixture ratio of raw material powders on the thermoelectric properties of AlMgB14-based materials prepared by spark plasma sintering

Thermoelectric properties of AlMgB₁₄-based materials prepared by spark plasma sintering were investigated. Al, Mg, and B powders were used as raw material powders. The raw powders were mixed using a V-shaped mixer, and then the mixture was sintered at 1673 K or 1773 K. The mixture ratio of raw powders was varied around stoichiometric ratio of AlMgB₁₄. X-ray diffraction patterns of samples showed that all samples consist of AlMgB₁₄ and MgAl₂O₄. The Seebeck coefficient of the samples exhibited significant change depending on the varying mixture ratio and sintering temperature. One sample exhibited a large negative value for the Seebeck coefficient (approximately −500 μV/K) in the temperature range from 573 K to 1073 K, while others showed positive value (250–450 μV/K). Thus n-type AlMgB₁₄-based material has been realized by varying raw material ratio and sintering temperature.     

Thermoelectric performance of ternary Bi-Sb-Ag alloys prepared by mechanical alloying and subsequent pressureless sintering

The alloys with the general formula of Bi₈₅Sb_15−xAg_x (x=0, 1, 3, 5, 7) were prepared by mechanical alloying and subsequent pressureless sintering (Bi₈₅Sb₁₅ alloy was used for comparison). Their transport properties involving electrical conductivity, Seebeck coefficient, and thermal conductivity had been investigated in the temperature range of 80-300 K. The maximum absolute value of Seebeck coefficient (120 μV/K) was found at 160 K in the alloy Bi₈₅Sb_15−xAg_x (x=3). The figure-of-merit of alloy Bi₈₅Sb_15−xAg_x (x=1) reached a maximum value of 2.16×10⁻³ K⁻¹ at 219 K, which is as large again as that of the reference sample Bi₈₅Sb₁₅.     

Thermoelectric properties of Bi-Sb semiconducting alloys prepared by quenching and annealing

Bi_100−xSb_x (x=8-17) alloys were prepared by direct melting of constituent elements, which was followed by quenching and annealing. The synthesis of high-homogeneity alloys was confirmed by X-ray diffraction, differential thermal analyses and electron microprobe analysis. The semiconducting and thermoelectric properties of the samples were investigated by measuring Hall coefficient, electrical resistivity and Seebeck coefficient in the temperature range from 20 to 300 K for both the as-quenched and annealing samples. The properties change gradually with the Sb concentration x, which is attributed to the variation of the energy gap. The Hall mobility was enhanced by annealing, which leads to a small electrical resistivity and a large Seebeck coefficient. Consequently, large values of about 8.5 mW/mK² for the power factor were obtained in the annealed alloys of x=8,12, and 14.     

High temperature thermoelectric properties of Nb-doped ZnO ceramics

Solid-state reaction processing technique was used to prepare Zn_xNb_1−xO (0≤x≤0.02) polycrystalline bulk samples. In the present study, we find that their lattice parameters a and c tend to decrease with increasing amount of Nb additive. The electrical conductivity of all the Zn_1−xNb_xO samples increased with increasing temperature, indicating a semiconducting behavior in the measured temperature range. The addition of Nb₂O₅ to ZnO led to an increase in the electrical conductivity and a decrease in the absolute value of the Seebeck coefficient. The best performance at 1000 K has been observed for nominal 0.5 at% Nb-doped ZnO, with an electrical resistivity of about 73.13 (S cm⁻¹) and Seebeck coefficient of ∼257.36 μV K⁻¹, corresponding to a power factor (S²σ) of 4.84×10⁻⁴ Wm⁻¹ K⁻². The thermal conductivity, κ, of the oxide decreased as compared to pure ZnO. The figure of merit ZT values of ZnO-doped Nb₂O₅ samples are higher than the ZnO pure sample, demonstrating that the Nb₂O₅ addition is fairly effective for enhancing thermoelectric properties.     

Epitaxial growth of Ba8Ga16Ge30 clathrate film on Si substrate by RF helicon magnetron sputtering with evaluation on thermoelectric properties

Epitaxial Ba₈Ga₁₆Ge₃₀ clathrate thin films were successfully grown on Si substrate by using helicon magnetron sputtering. The (1 0 0) lattice of Ba₈Ga₁₆Ge₃₀ was identified grown on four Si(2 0 0) lattices in small mismatch (0.1%). Both the color of samples and XRD results suggest 600 °C is the optimal substrate temperature for the growth of high quality Ba-Ga-Ge clathrate film on Si substrates. High Seebeck coefficients and electrical resistivities for the deposited clathrate thin films in comparison with those of bulk are obtained. The high crystal quality and thermionic effects in heterostructures may contribute to the larger Seebeck coefficients, while the increasing of interface scattering for electrons probably is the reason for large electrical resistivities. Although the thermoelectric (TE) results are not ideal as designed, our results are significant due to the first successful work on epitaxial growth of Ba₈Ga₁₆Ge₃₀ clathrate thin films on Si substrate with large Seebeck coefficient.     

Variable temperature EPR spectra of Copper (II) ions in kainite crystals

X-band electron paramagnetic resonance (EPR) studies on divalent copper ions embedded in KMgClSO₄·3H₂O single crystals have been performed at low temperature (123 K). The angular variation of the EPR spectra reveals the presence of two Cu²⁺ sites, which have different orientations. The spin-Hamiltonian parameters of this six-coordinated cupric ion have been evaluated from the EPR spectra at 123 K. The forbidden lines due to Δm_I=±1 transitions are observed in between allowed transitions. The temperature variation EPR studies have also been performed both for a single crystal and a polycrystalline sample. The ground state wavefunction of Cu²⁺ ions has been estimated and is found to be an admixture of d_3z²−r² and d_x²−y². The temperature variation of the EPR spectra reveals that Cu²⁺ ions exhibit dynamic Jahn-Teller effect. From the polycrystalline EPR data, the temperature dependent magnetic susceptibilities are evaluated and discussed.     

Transport coefficients and defect structure of Sb2−xAgxTe3 single crystals

Incorporation of Ag in the crystal lattice of Sb₂Te₃ creates structural defects that have a strong influence on the transport properties. Single crystals of Sb_2−xAg_xTe₃ (x=0.0; 0.014; 0.018 and 0.022) were characterized by measurements of the temperature dependence of the electrical resistivity, Hall coefficient, Seebeck coefficient and thermal conductivity in the temperature range of 5-300 K. With an increasing content of Ag the electrical resistance, the Hall coefficient and the Seebeck coefficient all decrease. This implies that the incorporation of Ag atoms in the Sb₂Te₃ crystal structure results in an increasing concentration of holes. However, the doping efficiency of Ag appears to be only about 50% of the expected value. We explain this discrepancy by a model based on the interaction of Ag impurity with the native defects in the Sb_2−xAg_xTe₃ crystal lattice. Defects have a particularly strong influence on the thermal conductivity. We analyze the temperature dependence of the lattice thermal conductivity in the context of the Debye model. Of the various phonon scattering contributions, the dominant influence of Ag incorporation in the crystal lattice of Sb₂Te₃ is revealed to be point-defect scattering where both the mass defect and elastic strain play a pivotal role.     

Investigation of the optimal annealing temperature for the enhanced thermoelectric properties of MOCVD-grown ZnO films

In this study, we demonstrate the optimization of the annealing temperature for enhanced thermoelectric properties of ZnO. Thin films of ZnO are grown on a sapphire substrate using the metal organic chemical Vapor Deposition (MOCVD) technique. The grown films are annealed in an oxygen environment at 600–1000°C, with a step of 100°C for one hour. Seebeck measurements at room temperature revealed that the Seebeck coefficient of the sample that was not annealed was 152 μV/K, having a carrier concentration of N _D ~ 1.46 × 10¹⁸ cm^–3. The Seebeck coefficient of the annealed films increased from 212 to 415 μV/K up to 900°C and then decreased at 1000°C. The power factor is calculated and found to have an increasing trend with the annealing temperature. This observation is explained by the theory of Johnson and Lark–Horovitz that thermoelectric properties are enhanced by improving the structure of ZnO thin films. The Hall measurements and PL data strongly justify the proposed argument.     

Electrical transport properties of polyacetylene tetrachloroferrate - [CH(FeCl4)y]x]

Conductivity and thermopower measurements of polyacetylene doped with FeCl₄ are reported. The conductivity changes over 10 orders of magnitude and reaches the maximum value of 200 Ω^-1 cm^-1 at y = 0.07. The thermopower reveals the semiconductor to metal transition at y ? 0.002, with high and temperature independent Seebeck coefficient in the dilute limit and a metallic dependence in the heavily doped samples.