Electron scattering by acceptor centers in <Emphasis Type="Italic">p</Emphasis>-Ag<Subscript>2</Subscript>Te at low temperatures

Resonant electron scattering in p-Ag₂Te at acceptor concentrations N _a < 4.2 × 10¹⁶ cm⁻³ has been observed in the temperature range of 50–80 K. The contribution of the resonant scattering to the temperature dependences of the conductivity σ(T) and thermopower α₀(T) has been calculated. It is shown that this contribution exceeds that of charge carrier scattering by acoustic phonons.     

High Power Factor of HPHT-Sintered GeTe-AgSbTe<Subscript>2</Subscript> Alloys

The thermopower and electrical resistivity of alloys of GeTe and AgSbTe₂ (TAGS) sintered at high pressure (up to 4.5 GPa) and high temperature (HPHT) have been studied from 300 K to 750 K. An apparatus for measuring thermopower and electrical resistivity at temperatures >300 K is described. The linear temperature dependence of thermopower and electrical conductivity indicates that these materials are likely to be degenerate semiconductors. At a sintering pressure of 4.0 GPa, the calculated power factor shows a steady progression, reaching a maximum at a sintering temperature of 800°C, with a subsequent decrease at the highest sintering temperature of 850°C. The maximum power factor of 4.32 × 10⁻³ W m⁻¹ K⁻² at ~675 K is ~25% higher than reported values. These results illustrate that HPHT processing is a feasible and controllable way of tuning the properties of thermoelectric materials.     

Effect of copper doping on kinetic coefficients and their anisotropy in PbSb<Subscript>2</Subscript>Te<Subscript>4</Subscript>

In anisotropic PbSb₂Te₄ and PbSb₂Te₄:Cu single crystals, nine main independent components of the Hall, electrical-conductivity, thermopower, and Nernst-Ettingshausen effects and their anisotropy in the range 77–450 K have been studied. PbSb₂Te₄ single crystals exhibit a high hole concentration (p ≈ 3 × 10²⁰ cm⁻³). Copper exhibits a donor effect and significantly (approximately by a factor of 2) reduces the hole concentration in PbSb₂Te₄. The temperature dependences of the kinetic coefficients, except for the Hall effect, have a form typical of the one-band model. The significant anisotropy of the Hall coefficient R ₁₂₃/R ₃₂₁ ≈ 2 at low temperatures corresponds to the multi-ellipsoid model of the energy spectrum of holes in PbSb₂Te₄. An important feature of the data on transport phenomena is the high thermopower anisotropy (ΔS ≈ 60–75 μV/K) in the mixed conductivity region caused by the mixed scattering mechanism. Data on the anisotropy of the transverse Nernst-Ettingshausen effect confirm the mixed mechanism of hole scattering; in the cleavage plane, scattering at acoustic phonons dominates, while in the trigonal axis direction, impurity scattering appears significant. Doping with copper enhances the role of impurity scattering in the direction of the trigonal axis c ₃; as a result, two components of the Nernst-Ettingshausen tensor Q ₃₂₁ and Q ₁₃₂ in the PbSb₂Te₄:Cu single crystal are positive at low temperatures, whereas, in the undoped crystal, only the Q ₃₂₁ component is positive.     

Thermal Conductivity and Other Transport Properties of Mg<Subscript>2</Subscript>Sn:Ag Crystals

The temperature dependence of the thermal conductivity κ(T), electrical resistivity ρ(T), and Seebeck coefficient S(T) of Mg₂Sn:Ag crystals with 0 at.% to 1 at.% Ag content were measured at T = 2 K to 400 K. The crystals were cut from ingots that were prepared by the vertical Bridgman method. Undoped samples show a dramatic κ ∝ T ³ rise at low temperatures to a peak value κ _15K = 477 W m⁻¹ K⁻¹. This leads to exceptionally large phonon drag effects causing giant thermopower with S rising sharply to a peak value S _20K = 3000 μV K⁻¹. At higher temperatures S decreases and changes sign to intrinsic values S ≈ −60 μV K⁻¹. The addition of Ag changes the transport properties as follows: (a) κ decreases systematically, the peak shifts to 30 K and falls to 7 W m⁻¹ K⁻¹; (b) ρ changes from high to low values; (c) S(T) changes to a linear dependence with S _300K ≈ 150 μV K⁻¹ to 200 μV K⁻¹.     

Thermoelectric Properties of Ag-doped Mg<Subscript>2</Subscript>Ge Thin Films Prepared by Magnetron Sputtering

Silver doped p-type Mg₂Ge thin films were grown in situ at 773 K using magnetron co-sputtering from individual high-purity Mg and Ge targets. A sacrificial base layer of silver of various thicknesses from 4 nm to 20 nm was initially deposited onto the substrate to supply Ag atoms, which entered the growing Mg₂Ge films by thermal diffusion. The addition of silver during film growth led to increased grain size and surface microroughness. The carrier concentration increased from 1.9 × 10¹⁸ cm⁻³ for undoped films to 8.8 × 10¹⁸ cm⁻³ for the most heavily doped films, but it did not reach saturation. Measurements in the temperature range of T = 200–650 K showed a positive Seebeck coefficient for all the films, with maximum values at temperatures between 400 K and 500 K. The highest Seebeck coefficient of the undoped film was 400 μV K⁻¹, while it was 280 μV K⁻¹ for the most heavily doped film at ∼400 K. The electrical conductivity increased with silver doping by a factor of approximately 10. The temperature effects on power factors for the undoped and lightly doped films were very limited, while the effects for the heavily doped films were substantial. The power factor of the heavily doped films reached a non-optimum value of ∼10⁻⁵ W cm⁻¹ K⁻² at 700 K.     

On the nature of charge carrier scattering in Ag<Subscript>2</Subscript>Se at low temperatures

The electric and thermoelectric properties of silver selenide in the temperature range of 4.2–300 K have been studied. The data obtained are interpreted within the theory of one-type carriers and Kane dispersion relation, with allowance for the character of electron-electron interaction. It is established that, for the concentrations n ≤ 7.8 × 10¹⁸ cm⁻³, charge carriers are scattered by impurity ions at T ≤ 30 K and by acoustic and optical phonons and point defects at T ≥ 30 K. Electron-electron interactions are found to be elastic at T < 30 K.     

Thermoelectric Properties of Co<Subscript>4</Subscript>Sb<Subscript>12</Subscript> Skutterudite Materials with Partial In Filling and Excess In Additions

The properties of Co₄Sb₁₂ with various In additions were studied. X-ray diffraction revealed the presence of the pure δ-phase of In_0.16Co₄Sb₁₂, whereas impurity phases (γ-CoSb₂ and InSb) appeared for x = 0.25, 0.40, 0.80, and 1.20. The homogeneity and morphology of the samples were observed by Seebeck microprobe and scanning electron microscopy, respectively. All the quenched ingots from which the studied samples were cut were inhomogeneous in the axial direction. The temperature dependence of the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) was measured from room temperature up to 673 K. The Seebeck coefficient of all In-added Co₄Sb₁₂ materials was negative. When the filler concentration increases, the Seebeck coefficient decreases. The samples with In additions above the filling limit (x = 0.22) show an even lower Seebeck coefficient due to the formation of secondary phases: InSb and CoSb₂. The temperature variation of the electrical conductivity is semiconductor-like. The thermal conductivity of all the samples decreases with temperature. The central region of the In_0.4Co₄Sb₁₂ ingot shows the lowest thermal conductivity, probably due to the combined effect of (a) rattling due to maximum filling and (b) the presence of a small amount of fine-dispersed secondary phases at the grain boundaries. Thus, regardless of the non-single-phase morphology, a promising ZT (S ² σT/κ) value of 0.96 at 673 K has been obtained with an In addition above the filling limit.     

Thermoelectric Properties of Zr<Subscript>3</Subscript>Mn<Subscript>4</Subscript>Si<Subscript>6</Subscript> and TiMnSi<Subscript>2</Subscript>

The Seebeck coefficient, electrical resistivity, and thermal conductivity of Zr₃Mn₄Si₆ and TiMnSi₂ were studied. The crystal lattices of these compounds contain relatively large open spaces, and, therefore, they have fairly low thermal conductivities (8.26 Wm⁻¹ K⁻¹ and 6.63 Wm⁻¹ K⁻¹, respectively) at room temperature. Their dimensionless figures of merit ZT were found to be 1.92 × 10⁻³ (at 1200 K) and 2.76 × 10⁻³ (at 900 K), respectively. The good electrical conductivities and low Seebeck coefficients might possibly be due to the fact that the distance between silicon atoms in these compounds is shorter than that in pure semiconductive silicon.     

The effect of irradiation with electrons on the electrical parameters of Hg<Subscript>3</Subscript>In<Subscript>2</Subscript>Te<Subscript>6</Subscript>

The results of studying the electrical properties of Hg₃In₂Te₆ crystals irradiated with electrons with the energy E _e = 18 MeV and the dose D = 4 × 10¹⁶ cm⁻² are reported. It is shown that, irrespective of the charge-carrier concentration in the initial material, the Hg₃In₂Te₆ samples acquire the charge-carrier concentration (1.6–1.8) × 10¹³ cm⁻³ after irradiation. The phenomenon of Fermi level pinning in an irradiated material is discussed. The initial charge-carrier concentration, which remains virtually unchanged after irradiation and which ensures the high radiation resistance of Hg₃In₂Te₆ crystals, corresponds to a compensated material, similar to an intrinsic semiconductor at T > 260 K.     

Thermoelectric Properties of the One-Dimensional Cobalt Oxide CaCo<Subscript>2</Subscript>O<Subscript>4</Subscript>

In this work we studied the crystal structure and physical properties of the new one-dimensional cobalt oxide CaCo₂O_4+δ . The CaCo₂O_4+δ phase crystallizes as a calcium-ferrite-type structure, which consists of a corner- and edge-shared CoO₆ octahedron network including one-dimensional double chains. The specific-heat Sommerfeld constant γ was found to be 4.48(7) mJ/mol K². This result suggests that the CaCo₂O_4+δ phase has a finite density of states at the Fermi level. Metallic temperature dependence of the Seebeck coefficient S with a large thermoelectric power (S = 151 μV/K at 387 K) was observed. The origin of the large thermoelectric power may be attributed to the quasi one-dimensional character of the energy band near the valence band maximum in CaCo₂O_4+δ .     

Thermoelectric Performance of Zn and Nd Co-doped In<Subscript>2</Subscript>O<Subscript>3</Subscript> Ceramics

Polycrystalline In₂O₃ ceramics co-doped with Zn and Nd were prepared by the spark plasma sintering (SPS) process, and microstructure and thermoelectric (TE) transport properties of the ceramics were investigated. Our results indicate that co-doping with Zn²⁺ and Nd³⁺ shows a remarkable effect on the transport properties of In₂O₃-based ceramics. Large electrical conductivity (~130 S cm⁻¹) and thermopower (~220 μV K⁻¹) can be observed in these In₂O₃-based ceramic samples. The maximum power factor (PF) reaches 5.3 × 10⁻⁴ W m⁻¹ K⁻² at 973 K in the In_1.92Nd_0.04Zn_0.04O₃ sample, with a highest ZT of ~0.25.     

Effect of temperature on electron spectra in the region of the intrinsic-absorption edge of CdGa<Subscript>2</Subscript>Se<Subscript>4</Subscript>

The results of investigation of the temperature dependence of CdGa₂Se₄ absorption coefficient in the polarized radiation at 5–300 K and that of the emission intensity at 4.2–77 K are presented. The changes observed for the polarization dependence of absorption coefficient and the radiation intensity with decreasing temperature are attributed to a different velocity of motion of states of the valence-band top Γ₃ + Γ₄ and Γ₂ with changing tetragonal compression.     

Thermoelectric Properties of Co-Doped TiS<Subscript>2</Subscript>

The thermoelectric properties of cobalt-doped compounds Co _x Ti_1−x S₂ (0 ≤ x ≤ 0.3) prepared by solid-state reaction were investigated from 5 K to 310 K. It was found that the electric resistivity ρ and absolute thermopower |S| for all the doped compounds decreased significantly with increasing Co content over the whole temperature range investigated. The increased lattice thermal conductivity of the doped compounds would imply enhancement of the acoustic velocity. Moreover, the ZT value of the doped compounds was improved over the whole temperature range investigated, and specifically reached 0.03 at 310 K for Co_0.3Ti_0.7S₂, being about 66% larger than that of TiS₂.     

Negative magnetoresistance in silicon with manganese-atom complexes [Mn]<Subscript>4</Subscript>

It is experimentally shown that an anomalously high negative magnetoresistance is observed in silicon with manganese-atom complexes [Mn]₄ at room temperature. It is established that the negative magnetoresistance has the largest value at T = 230–240 K, while its value decreases with temperature, and the inversion of the magnetoresistance sign takes place at T < 170 K; i.e., the positive magnetoresistance is observed. It is established that the negative magnetoresistance and its temperature dependence are substantially affected by the intensity of both integrated and monochromatic light.     

Thermoelectric Properties of <Emphasis Type="Italic">In Situ</Emphasis> Formed Bi<Subscript>0.85</Subscript>Sb<Subscript>0.15</Subscript>/Bi-Rich Particles Composite

A Bi-15 at.%Sb alloy, homogenized by equal channel angular extrusion (ECAE) at T = 523 K, has been treated just above its solidus temperature, causing segregation of a secondary Bi-rich phase at the grain boundaries. This process results in an in situ composite. The thermoelectric properties of the composite have been measured in the range of 5 K < T < 300 K. The results are compared with those of the homogeneous alloy. The presence of a Bi-rich phase improves the Seebeck coefficient at T < 50 K, and enhances the electrical conductivity by a factor of 1.4 at T = 300 K up to a factor of 3.4 at T = 50 K; unfortunately, the thermal conductivity also increases by about 50% in the same temperature range. As a result, the figure of merit, Z, is slightly suppressed above T = 110 K, but increases at lower temperatures, reaching a peak value of 4.2 × 10⁻³ K⁻¹ at T = 90 K. The power factor considerably increases over the whole temperature range, rendering this material suitable as the n-type leg of a cryogenic thermoelectric generator for cold energy recovery in a liquefied natural gas plant.     

Thermoelectric Properties of NdGd<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript> Prepared by CS<Subscript>2</Subscript> Sulfurization

Ternary rare-earth sulfides NdGd_1+x S₃, where 0 ≤ x ≤ 0.08, were prepared by sulfurizing Ln₂O₃ (Ln = Nd, Gd) with CS₂ gas, followed by reaction sintering. The sintered samples have full density and homogeneous compositions. The Seebeck coefficient, electrical resistivity, and thermal conductivity were measured over the temperature range of 300 K to 950 K. All the sintered samples exhibit a negative Seebeck coefficient. The magnitude of the Seebeck coefficient and the electrical resistivity decrease systematically with increasing Gd content. The thermal conductivity of all the sintered samples is less than 1.9 W K⁻¹ m⁻¹. The highest figure of merit ZT of 0.51 was found in NdGd_1.02S₃ at 950 K.     

Thermoelectric Properties of In<Subscript>0.3</Subscript>Ga<Subscript>0.7</Subscript>N Alloys

We report on the experimental investigation of the potential of InGaN alloys as thermoelectric (TE) materials. We have grown undoped and Si-doped In_0.3Ga_0.7N alloys by metalorganic chemical vapor deposition and measured the Seebeck coefficient and electrical conductivity of the grown films with the aim of maximizing the power factor (P). It was found that P decreases as electron concentration (n) increases. The maximum value for P was found to be 7.3 × 10⁻⁴ W/m K² at 750 K in an undoped sample with corresponding values of Seebeck coefficient and electrical conductivity of 280 μV/K and 93␣(Ω cm)⁻¹, respectively. Further enhancement in P is expected by improving the InGaN material quality and conductivity control by reducing background electron concentration.     

Thermoelectric Properties of the Narrow-Gap Intermetallic Compound Ga<Subscript>2</Subscript>Ru: Effect of Re Substitution for Ru Atoms

In this paper, the effect of hole doping on the thermoelectric properties of the binary narrow-gap semiconducting intermetallic compound Ga₂Ru in the temperature range from 373 K to 973 K was investigated. We synthesized sintered pellets by spark plasma sintering (SPS) after arc-melting and succeeded in preparing crack-free samples. The maximum dimensionless figure of merit ZT _max was 0.50 at 773 K for the sintered Ga₂Ru alloy. The temperature dependence of the electrical resistivity and its magnitude at 373 K dramatically changed from negative (~11,000 μΩcm) to positive (~200 μΩcm) upon hole doping by the substitution of Re for Ru atoms. Also, the Seebeck coefficient at 373 K changed from 300 μV/K to 75 μV/K. These changes were identified by the increase in carrier concentrations observed by Hall- effect measurements. In particular, large power factors (2.0 mW/m K² to 3.0 mW/m K²) were obtained over a wide temperature range from 373 K to 973 K upon Re substitution. The lattice thermal conductivity beneficially decreased with increasing Re concentration as a result of an alloying effect.     

Improvement of Thermoelectric Power Factor of Hydrothermally Prepared Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Compared with its Solvothermally Prepared Counterpart

We report on the successful hydrothermal synthesis of Bi_0.5Sb_1.5Te₃, using water as the solvent. The products of the hydrothermally prepared Bi_0.5 Sb_1.5Te₃ were hexagonal platelets with edges of 200–1500 nm and thicknesses of 30–50 nm. Both the Seebeck coefficient and electrical conductivity of the hydrothermally prepared Bi_0.5Sb_1.5Te₃ were larger than those of the solvothermally prepared counterpart. Hall measurements of Bi_0.5Sb_1.5Te₃ at room temperature indicated that the charge carrier was p-type, with a carrier concentration of 9.47 × 10¹⁸ cm⁻³ and 1.42 × 10¹⁹ cm⁻³ for the hydrothermally prepared Bi_0.5Sb_1.5Te₃ and solvothermally prepared sample, respectively. The thermoelectric power factor at 290 K was 10.4 μW/cm K² and 2.9 μW/cm K² for the hydrothermally prepared Bi_0.5Sb_1.5Te₃ and solvothermally prepared sample, respectively.     

Thermoelectric Properties of Half-Heusler Bismuthides ZrCo<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Bi (<Emphasis Type="Italic">x</Emphasis> = 0.0 to 0.1)

The usefulness of half-Heusler (HH) alloys as thermoelectrics has been mainly limited by their relatively large thermal conductivity, which is a key issue despite their high thermoelectric power factors. In this regard, Bi-containing half-Heusler alloys are particularly appealing, because they are, potentially, of low thermal conductivity. One such a material is ZrCoBi. We prepared pure and Ni-doped ZrCoBi by a solid-state reaction. To evaluate thermoelectric potential we measured electrical resistivity (ρ = 1/σ) and thermopower (σ) up to 1000 K and thermal conductivity (κ) up to 300 K. Our measurements indicate that for these alloys resistivity of approximately a few mΩ cm and thermopower larger than a hundred μV K⁻¹ are possible. Low κ values are also possible. On the basis of these data we conclude that this system has a potential to be optimized further, despite the low power factors (α ² σT) we have currently measured.