Ac electrical properties and dielectric relaxation of the new mixed crystal (Na0.8Ag0.2)2PbP2O7

The effect of Mn substitution on transport and thermoelectric properties of Cr_1?xMn_xSb₂ (x = 0, 0.01, 0.03, 0.05) have been investigated at the temperatures from 7 K to 313 K. The results show that similar to CrSb₂ all the doped samples are semiconducting. However, electrical resistivity of Cr_1?xMn_xSb₂ is found to decrease greatly with increasing doping content of Mn. Besides, Mn substitution leads to suppression of the plateau appearing in the plot of resistivity vs. temperature for CrSb₂ and makes the resistivity anomaly originating from antiferromagnetic transition shift to higher temperatures. Moreover, corresponding to the plateau a large peak of thermopower (?431 μV K^?1) was observed at ～60 K, which was also found to be suppressed by Mn doping. In addition, lattice thermal conductivity of Cr_1?xMn_xSb₂ was found to decrease substantially with increasing Mn content, though Mn doping also results in slow reduction in absolute value of thermopower at the temperatures T > ～200 K. As a result, at T > ～260 K the figure of merit ZT of heavily doped sample Cr_1?xMn_xSb₂ (x = 0.03 and 0.05) was improved. Specifically, at 313 K ZT value of Cr_0.95Mn_0.05Sb₂ is ～14% larger than that of CrSb₂, indicating that thermoelectric properties of CrSb₂ can be improved by proper substitution of Mn for Cr.     

High temperature thermoelectric properties of TiNiSn-based half-Heusler compounds

A class of intermetallics of TiNiSn half-Heusler compound with MgAgAs structure type is currently of interest as a potential high temperature thermoelectric material. The ternary TiNiSn compound has showed promising thermoelectric properties, a high Seebeck coefficient and low electrical resistivity. The present study reports the effect of Hf alloying on Ti site, Pt and Pd alloying on Ni site, and Sb doping on Sn site for the optimization of thermoelectric properties of TiNiSn-based compounds. Also, to achieve a low thermal conductivity, a powder metallurgy technique is used for the fabrication of the compounds. These efforts result in the dimensionless figure of merit, ZT as 0.78 for the hot-pressed (Ti_0.95Hf_0.05)Ni(Sn_0.99Sb_0.01) sample at 770 K with a large power factor (4.1 mW/mK²), which makes these materials very attractive for potential power generation applications.     

Preparation and thermoelectric properties of p-type (Ga2Te3)x–(Bi0.5Sb1.5Te3)1−x (x = 0–0.2) alloys prepared by spark plasma sintering

p-Type (Ga₂Te₃)_x–(Bi_0.5Sb_1.5Te₃)_1−x (x = 0–0.2) alloys were prepared by spark plasma sintering technique, and the effect of gallium telluride (Ga₂Te₃) on the thermoelectric properties was experimentally determined. Measurements have shown that the electrical conductivities of (Ga₂Te₃)_x–(Bi_0.5Sb_1.5Te₃)_1−x are improved and thermal conductivities reduced after the introduction of Ga₂Te₃ in the Bi_0.5Sb_1.5Te₃ alloy without noticeable loss of Seebeck coefficient. The maximum thermoelectric figure of merit ZT of 1.0 is obtained with molar fraction x = 0.1 at 335.5 K, being approximately two times that of the Bi_0.5Sb_1.5Te₃ at the corresponding temperature, thus proving that (Ga₂Te₃)_x–(Bi_0.5Sb_1.5Te₃)_1−x (x = 0–0.2) alloys are to be a promising material for application.     

Thermoelectric power and electrical resistivity of Ag-doped Na1.5Co2O4

Polycrystalline samples of Na_1.5Co_2−xAg_xO₄ (x = 0, 0.1 and 0.2) were synthesized from powder precursors prepared by a polymerized complex (PC) method. The thermoelectric power (S) and electrical resistivity (ρ) of Na_1.5Co_2−xAg_xO₄ (x = 0, 0.1, 0.2) were measured simultaneously in a helium atmosphere, in the temperature range from room temperature to 973 K. The thermoelectric power and power factor of Na_1.5Co_1.8Ag_0.2O₄ are higher than that of Na_1.5Co₂O₄ over a wide range of temperatures.     

Spinodal decomposition in (CaxBa1−x)yFe4Sb12

The thermoelectric and structural properties of a double filled skutterudite solid solution (Ca_xBa_1?x)_yFe₄Sb₁₂ were investigated. Using X-ray powder and X-ray micro analyses (EPMA) an immiscibility gap was established with a critical point at x ≈ 0.45 and a critical temperature that depends on the filling level (T_C = 590 ± 5 °C at y = 0.8 and T_C = 610 ± 5 °C at y = 0.9). The thermoelectric properties were measured for samples prepared in four different states: (i) a single phase solid solution (Ca_xBa_1?x)_yFe₄Sb₁₂; (ii) a two phase microcrystalline mixture of Ca_yFe₄Sb₁₂ and Ba_yFe₄Sb₁₂; (iii) a single phase structure obtained after annealing of the latter sample at 600 °C for 200 h; (iv) a spinodally demixed sample after annealing at 400 °C for 672 h. The thermoelectric properties of the phase mixture (ii) are compatible with data reported for the microcrystalline end members (Ca_yFe₄Sb₁₂ and Ba_yFe₄Sb₁₂), whilst the single phase (i and iii) and spinodally decomposed (iv) samples show increased thermopower and decreased thermal conductivity, similarly to those observed for nano-structured Ca_yFe₄Sb₁₂ and Ba_yFe₄Sb₁₂.     

Effects of Ag doping on thermoelectric properties of Zn4Sb3 at low temperatures

The thermoelectric properties of Ag-doped compounds (Zn_1?xAg_x)₄Sb₃ (x = 0, 0.0025, 0.005, 0.01) have been studied at the temperatures from 15 to 300 K. The results indicate that low-temperature (T < 300 K) thermal conductivity of the moderately doped (Zn_1?xAg_x)₄Sb₃ (x = 0.0025 and 0.005) reduced remarkably as compared with that of Zn₄Sb₃ due to enhanced impurity (dopant) scattering of phonons. Electrical resistivity and Seebeck coefficient were found to increase first and then decrease obviously with the increase in the Ag content, which could be ascribed to the change of carrier concentration presumably due to different Zn positions occupied by Ag upon increasing doping content. Moreover, the lightly doped compound (Zn_0.995Ag_0.005)₄Sb₃ exhibited the best thermoelectric performance due to the improvement in both its electrical resistivity and thermal conductivity, whose figure of merit (at 300 K), ZT, is about 1.3 times larger than that of β-Zn₄Sb₃ obtained in the present study. Present results suggest that proper Ag doping in Zn₄Sb₃ is a promising way of improving its thermoelectric properties.     

Thermal and electronic properties of Bi1 − xSbx alloys

Polycrystalline Bi_1 ? xSb_x (x = 0.10, 0.12 and 0.15) semiconducting alloys were synthesized by mechanical alloying in order to achieve homogeneous thermoelectric materials with improved mechanical strength. The homogeneity of the powder samples were repeatedly checked by X-ray diffraction and scanning electron microscopy to standardize the milling conditions. The best possible homogenized material was developed with the milling conditions of BPR 30:1, ball diameter 30 mm, 400 rpm and milling time of 15 h. The electrical resistivity, thermoelectric power and thermal conductivity were measured in the temperature range 300–500 K. It was found through these experiments that the composition with x = 0.12 behaved in a normal semiconducting way, whereas the other two compositions (x = 0.10 and 0.15) showed degenerate semiconductor behaviour. These features have been qualitatively explained from the band structure and interband scattering mechanisms.     

High-performance half-Heusler thermoelectric materials Hf1−x ZrxNiSn1−ySby prepared by levitation melting and spark plasma sintering

Half-Heusler thermoelectric materials Hf_1?xZr_xNiSn_1?ySb_y (x = 0, 0.25, 0.4, 0.5; y = 0.02, 0.04, 0.06) have been prepared by levitation melting followed by spark plasma sintering or hot pressing. X-ray diffraction analysis and scanning electron microscopy observation show that single-phased half-Heusler compounds without compositional segregations have been obtained by levitation melting in a time-efficient manner. A small amount of Sb doping can improve the electrical power factor but undesirably increases the thermal conductivity due to the increased carrier thermal conductivity. The isoelectronic substitution of Zr for Hf substantially decreased the lattice thermal conductivity. A state-of-the-art ZT value of 1.0 has been attained at 1000 K for the levitation-melted and spark-plasma-sintered Hf_0.6Zr_0.4NiSn_0.98Sb_0.02, which is one of the highest achieved ZT values for half-Heusler thermoelectric alloys.     

Investigation of the Y2Te3–Cu2Te–PbTe system at 870 K and crystal structures of the Y7Cu3Te12 and YCu0.264Te2 compounds

The structural and magnetic properties of doubly substituted Nd₂Fe_17−x−yTi_xGa_y (0 ≤ x ≤ 1.0, 0 ≤ y ≤ 3) compounds have been investigated by a combined technique of X-ray diffraction, neutron diffraction and magnetic measurements. Rietveld refinements of the diffraction data indicate that all the samples crystallize in the rhombohedral Th₂Zn₁₇-type structure. For a given Ti content (x), the lattice parameters a and c, and unit cell volume V of Nd₂Fe_17−x−yTi_xGa_y all increase linearly with increasing Ga content. The addition of Ti initially has a considerably positive effect on the increasing rates of a, c, and unit cell volume V, but later the effect becomes very slight and even negative with the further increase of Ti content. The site occupancies of Ti and Ga in the crystallographic sites change a little compared to what is observed in the corresponding singly substituted compounds. The effects of Ti and Ga on the bond lengths of the doubly substituted compounds are quite different from that of the singly substituted compounds. Magnetic measurements show the T_C of Nd₂Fe_17−x−yTi_xGa_y increases with increasing Ti content for a low Ga content while it behaves conversely for a higher Ga content. The T_C of Nd₂Fe_17−x−yTi_xGa_y increases with increasing Ga content for a particular Ti content, while the addition of Ti results in a slower increase of T_C on the Ga content (y ≤ 3). For a given Ti content, the M_s of Nd₂Fe_17−x−yTi_xGa_y first increases a little and then decreases with the increase of Ga content, while for a given Ga content the M_s of Nd₂Fe_17−x−yTi_xGa_y decreases with the increase of Ti content.     

Enhanced thermoelectric performance of dual-element-filled skutterudites BaxCeyCo4Sb12

Single-phase polycrystalline dual-element-filled skutterudites Ba_xCe_yCo₄Sb₁₂ (0 < x < 0.4, 0 < y < 0.1) are synthesized by the melting–quenching–annealing and spark plasma sintering methods. The electrical conductivity, Seebeck coefficient, thermal conductivity and low-temperature Hall data of these compounds are reported. Our results suggest that there is essentially no difference in electrical transport properties between the dual-element-filled Ba_xCe_yCo₄Sb₁₂ and single-element-filled Ba_yCo₄Sb₁₂ systems. The Ba–Ce co-filling is more effective in lattice thermal conductivity reduction than Ba single filling in the temperature range of 300–850 K. Very low lattice thermal conductivity values less than 2.0 W m^?1 K^?1 are obtained at room temperature. Consequently, enhanced thermoelectric figure of merits (ZT) for these dual-element-filled CoSb₃ skutterudites are achieved at elevated temperatures, in particular ZT = 1.26 at 850 K for Ba_0.18Ce_0.05Co₄Sb_12.02.     

Thermoelectric properties of Fe0.2Co3.8Sb12−xTex skutterudites

Skutterudites Fe_0.2Co_3.8Sb_12?xTe_x (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) were synthesized by induction melting at 1273 K, followed by annealing at 923 K for 144 h. X-ray powder diffraction and electron microprobe analysis confirmed the presence of the skutterudite phase as the main phase. The temperature-dependent transport properties were measured for all the samples from 300 to 818 K. A positive Seebeck coefficient (holes are majority carriers) was obtained in Fe_0.2Co_3.8Sb₁₂ in the whole temperature range. Thermally excited carriers changed from n-type to p-type in Fe_0.2Co_3.8Sb_11.9Te_0.1 at 570 K, while in all the other samples, Fe_0.2Co_3.8Sb_12?xTe_x (x = 0.2, 0.3, 0.4, 0.5, 0.6) exhibited negative Seebeck coefficients in the entire temperature range measured. Whereas for the alloys up to x = 0.2 (Fe_0.2Co_3.8Sb_11.8Te_0.2) the electrical resistivity decreased by charge compensation, it increased for x > 0.2 with an increase in Te content as a result of an increase in the electron concentration. The thermal conductivity decreased with Te substitution owing to carrier–phonon scattering and point defect scattering. The maximum dimensionless thermoelectric figure of merit, ZT = 1.04 at 818 K, was obtained with an optimized Te content for Fe_0.2Co_3.8Sb_11.5Te_0.5 and a carrier concentration of ～n = 3.0 × 10²⁰ cm^?3 at room temperature. Thermal expansion (α = 8.8 × 10^?6 K^?1), as measured for Fe_0.2Co_3.8Sb_11.5Te_0.5, compared well with that of undoped Co₄Sb₁₂. A further increase in the thermoelectric figure of merit up to ZT = 1.3 at 820 K was achieved for Fe_0.2Co_3.8Sb_11.5Te_0.5, applying severe plastic deformation in terms of a high-pressure torsion process.     

Thermal properties of MoSi2 with minor aluminum substitutions

Mo(Si_1−xAl_x)₂ compositions (x = 0–0.1) have been prepared by a modified SHS route under uniaxial hydrostatic pressure. Oxidation studies carried out by thermal analysis and sheet resistivity indicate an improvement in the low temperature (700–900 K) oxidation resistance with increasing aluminum addition. Dilatometric results show a decrease in the α value up to x = 0.05 substitution. With the aluminum substitution, both thermal expansion coefficient and thermal conductivity show decrease in their values except in the biphasic region. The x = 0.05 composition containing both C11_b and C40 phases is a promising material for high temperature thermal barrier coating as it shows higher oxidation resistance and a similar K/α value as compared to pure MoSi₂.     

Optical constants of new amorphous As–Ge–Se–Sb thin films

The present paper reports the effect of replacement of selenium by antimony on the optical constants of new quaternary chalcogenide As₁₄Ge₁₄Se_72−xSb_x (where x = 3, 6, 9, 12 and 15 at.%) thin films. Films of As₁₄Ge₁₄Se_72−xSb_x glasses were prepared by thermal evaporation of the bulk samples. The transmission spectra, T(λ), of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. A straightforward analysis proposed by Swanepoel, based on the use of the maxima and minima of the interference fringes, has been applied to derive the real and imaginary parts of the complex index of refraction and also the film thickness. Increasing antimony content is found to affect the refractive index and the extinction coefficient of the As₁₄Ge₁₄Se_72−xSb_x films. Optical absorption measurements show that the fundamental absorption edge is a function of composition. With increasing antimony content the refractive index increases while the optical band gap decreases.     

Fabrication and thermoelectric properties of highly textured NaCo2O4 ceramic

Highly textured NaCo₂O₄ polycrystalline sample was fabricated by means of the cold high-pressure compacting followed by the solid-state reaction. X-ray diffraction and scanning electron microscope were employed to show that the plate-like grains within the sample are aligned along the pressing direction. The resistivity ρ and thermoelectric power S along the preferred {0 0 1} plane were measured in the whole temperature range from 15 to 973 K in air and the correlation between thermoelectric properties and texture was investigated. It was found that both ρ and S exhibit metallic behavior in the whole temperature range and the above sample exhibits lower ρ and higher S due to high texture and density. The power factor exhibits a steep rise above 400 K and reaches 761 μW m⁻¹ K⁻² at 973 K, suggesting a promising candidate for thermoelectric application at higher temperature. The change of slope in both resistivity and thermoelectric power curves at about 450 K might arise from the spin-state transition of Co ions in the CoO₂ blocks.     

Phase separation and thermoelectric properties of Ag2Te-doped PbTe0.9S0.1

Spinodal decomposition is an ideal mechanism for producing bulk nanostructured materials with promising thermoelectric (TE) performance. In this contribution, the phase separation and TE properties of PbTe-PbS samples are investigated. Phase separation driven by spinodal decomposition is observed in PbTe_0.4S_0.6, PbTe_0.5S_0.5, PbTe_0.6S_0.4 and (PbTe_0.9S_0.1)_1?x(Ag₂Te)_x with x = 0, 0.01 and 0.03. The addition of Ag₂Te leads to a deterioration in electrical transport properties at low temperature but to a significantly enhanced higher-temperature power factor of the Ag₂Te-doped PbTe_0.9S_0.1 sample. The very low thermal conductivity of the Ag₂Te-doped sample is attributed to the doping effect of Ag₂Te, the precipitated Ag₂Te, and the nanoscale phase segregation driven by spinodal decomposition. In particular, the spinodal decomposition produces finely dispersed PbTe-rich and PbS-rich phases with solute atoms, coherent or semicoherent interfaces, lattice bending, and other lattice defects, which contribute to the phonon scattering and minimize the thermal conductivity. The highest TE figure of merit, ZT, is ～1.2 at 773 K for the sample with x = 0.03, and even larger ZT values at higher temperature might be expected based on its tendency to increase with the temperature.     

Thermoelectric properties of Sb-doped Mg2Si semiconductors

The thermoelectric properties of Sb-doped Mg₂Si (Mg₂Si:Sb = 1:x(0.001 ≦ x ≦ 0.02)) fabricated by spark plasma sintering have been characterized by Hall effect measurements at 300 K and by measurements of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) between 300 and 900 K. Sb-doped Mg₂Si samples are n-type in the measured temperature range. The electron concentration of Sb-doped Mg₂Si at 300 K ranges from 2.2 × 10¹⁹ for the Sb concentration, where x = 0.001, to 1.5 × 10²⁰ cm⁻³ for x = 0.02. First-principles calculation revealed that Sb atoms are expected to be primarily located at the Si sites in Mg₂Si. The electrical resistivity, Seebeck coefficient, and thermal conductivity are strongly affected by the Sb concentration. The sample x = 0.02 shows a maximum value of the figure of merit ZT, which is 0.56 at 862 K.     

Modeling of phase stability of the fcc phases in the Ni–Ir–Al system using the cluster/site approximation method coupling with first-principles calculations

A combined CSA (cluster/site approximation)/FP (first-principles) calculation approach was employed to investigate the phase stability and thermodynamic properties of the face-centered cubic (fcc) phases of the Ni–Ir–Al system. For the constituent binaries of the Ni–Ir–Al system, enthalpies of formation of the Ni_xIr_1−x, Ni_xAl_1−x and Ir_xAl_1−x fcc compounds were calculated by first-principles approach at x = 0.75, 0.5 and 0.25 at 0 K, respectively. The pair exchange energies of the Ni–Ir and Al–Ir systems in the CSA model were obtained from FP calculated enthalpies of formation, while those for the Ni–Al binary were adopted from previous work. Thermodynamic model parameters of the fcc phases for the Ni–Ir–Al ternary system were then obtained from the constituent binaries via extrapolation. The calculated isothermal section at 1573 K is in good agreement with the experimental data within the uncertainties of the calculations and experiments.     

Microstructure and plastic deformation behavior of Ni3(Ti,X) (X = Nb,Al) single crystals with long-period geometrically closely packed crystal structures

The crystal structure, phase stability and plastic deformation behavior of Ni₃(Ti_1−xNb_x) [x = 0, 0.01, 0.03, 0.05 or 0.10] and Ni₃(Ti_1−yAl_y) [y = 0.16 or 0.28] single crystals were investigated. The substitution of Ti by Nb in Ni₃Ti induced the formation of various long-period stacking ordered (LPSO) structures with 18-fold, 10-fold or 9-fold stacking sequences of closely packed plane (CPP) depending on the Nb content. In compression tests, the yield stress anomaly (YSA) appeared in ternary LPSO crystals as well as in binary Ni₃Ti by slip on the CPP. The change in stacking sequence of CPP in the LPSO phases strongly affects the YSA behavior due to the change in APB energy on the non-closely packed plane.     

High-temperature thermoelectric properties of Ln(Co,Ni)O3 (Ln = La,Pr, Nd,Sm, Gd and Dy) compounds

The high-temperature thermoelectric properties of polycrystalline LnCo_0.95Ni_0.05O_3±δ (Ln = La, Pr, Nd, Sm, Gd and Dy) compounds have been investigated. The substitution of the lanthanide element produces changes in the crystal structure and in the unit cell lattice parameters. A semiconductor to metal transition takes place for all the compounds at 500 K < T < 800 K. The transition temperature, also monitored by differential scanning calorimetry, shifts to higher temperature with the decrease in the size of the lanthanide element. The Seebeck coefficient has positive values up to T = 1240 K. The thermal conductivity is mainly dominated by the lattice contribution. The dimensionless figure of merit ZT can be enhanced at different temperatures depending on which lanthanide element occupies the A-site in the ABO₃ perovskite-type phases.     

Phase stabilization in the Fe1−xMnxSi2 system induced by ball milling

Phase formation in the Mn doped iron disilicide system Fe_1−xMn_xSi₂ with 0.00 ≤ x ≤ 0.24 was studied using X-ray diffraction and Mössbauer spectroscopy. Samples were prepared at room temperature in Ar atmosphere by two different routes. The first one involved the simultaneous mill of the pure elements, and the second one the milling in two steps, first the premixture of metals and then the addition of Si. Both routes produced β-FeSi₂, ɛ-FeSi and α-FeSi₂ phases. In the first case, the segregation of MnSi and Si was also observed. The diffraction results and the obtained set of hyperfine parameters supported the coexistence of β-FeSi₂, α-FeSi₂ and ɛ-FeSi. The relative phase composition depended on the preparation route, being the obtained fraction of the ɛ-FeSi smaller when the second preparation route was followed. It seemed that the previous formation of a Fe_1−xMn_x alloy before to the silicide overcomes the chemical driving forces.