The effect of Cu addition on the thermoelectric properties of Cu2CdGeSe4

Recently, research in copper based quaternary chalcogenide materials has focused on the study of thermoelectric properties due to the complexity in the crystal structure. In the present work, stoichiometric quaternary chalcogenide compounds Cu_2+xCd_1−xGeSe₄ (x = 0, 0.025, 0.05, 0.075, 0.1, 0.125) were prepared by solid state synthesis. The powder X-ray diffraction patterns of all the samples showed a tetragonal crystal structure with the space group I-42m of the main phase, whereas the samples with x = 0 and x = 0.025 revealed the presence of an orthorhombic phase in addition to the main phase as confirmed by Rietveld analysis. The elemental composition of all the samples characterized by Electron Probe Micro Analyzer showed a slight deviation from the nominal composition. The transport properties were measured in the temperature range of 300 K–723 K. The electrical conductivity of all the samples increased with increasing Cu content due to the enhancement of the hole concentration caused by the substitution of Cd (divalent) by Cu (monovalent). The positive Seebeck coefficient of all the samples in the entire temperature ranges indicates that holes are the majority carriers. The Seebeck coefficient of all the samples decreased with increasing Cu content and showed a reverse trend to the electrical conductivity. The total thermal conductivity of all the samples decreased with increasing temperature which was dominated by the lattice contribution. The maximum figure of merit ZT = 0.42 at 723 K was obtained for the compound Cu_2.1Cd_0.9GeSe₄.     

Phases and thermoelectric properties of Ge1−x(Pb0.9Yb0.1)xTe alloys

A series of Ge_1−x(Pb_0.9Yb_0.1)_xTe alloys with x = 0.05, 0.10, 0.15, 0.20 and 0.30 were prepared by a conventional melting and a spark plasma sintering (SPS) techniques. The phases and thermoelectric properties for the alloys were investigated. The alloys consist of the GeTe-based rhombohedral single phase for x = 0.05, while both GeTe-based rhombohedral and PbTe-based rock-salt phases due to spinodal decomposition for the higher Pb content (x ≥ 0.10). The amount of the PbTe-based phase increases with the Pb content x increasing. All samples show p-type conduction. As Pb content x increases, the thermal conductivity reduces obviously, while the Seebeck coefficient and the electrical resistivity increases slightly. The maximum ZT of 1.4 at 723 K was eventually achieved in the sample with x = 0.15 due to its rather low thermal conductivity, from 3.7 W m⁻¹K⁻¹ at room temperature to 1.4 W m⁻¹K⁻¹ at 723 K (3.7–1.4 W m⁻¹K⁻¹), relative high Seebeck coefficient (46.5–141 μV K⁻¹) and relative low electrical resistivity (3.0–7.36 μΩ m).     

Electronic properties of CeRh1−xGexIn; evolution from an intermediate-valence to a localized 4f-state

We examined how electronic properties of the intermediate-valent CeRhIn are influenced by gradual substitution of rhodium with germanium. Results of specific-heat, magnetic susceptibility, electrical resistivity, thermopower and X-ray photoelectron spectroscopy (XPS) measurements performed in wide range of temperature and magnetic field on polycrystalline samples of CeRh_1−xGe_xIn, for x = 0.1, 0.2 and 0.3, are presented and compared to corresponding data reported earlier for CeRhIn, its hydrides and CePd_xRh_1−x In solutions. A systematic shift from the intermediate-valence to a localized 4f-state with increasing content of germanium is evident from all obtained results. Non-Fermi liquid state is formed at low temperatures for the solution with the lowest Ge content, but it is destroyed by magnetic field and/or further doping with Ge.     

Investigation of thermoelectric properties of Cu2GaxSn1 − xSe3 diamond-like compounds by hot pressing and spark plasma sintering

P-type compounds Cu₂Ga_xSn_1 ? xSe₃ (x = 0.025, 0.05, 0.075) with a diamond-like structure were consolidated using hot pressing sintering (HP) and spark plasma sintering (SPS) techniques. High-temperature thermoelectric properties as well as low-temperature Hall data are reported. Microstructural analysis shows that the distribution of Ga is homogeneous in the samples sintered by HP but inhomogeneous in the samples sintered by SPS, even with an electrically isolating and thermally conducting BN layer during the sintering. The Seebeck coefficients of the samples sintered by HP and SPS show similar dependence on the carrier concentration and are insensitive to the composition inhomogeneity. In contrast, the composition inhomogeneity results in lower carrier mobility and thus lower electrical conductivity in the samples sintered by SPS than those sintered by HP. Lattice thermal conductivity is further reduced through Ga doping; however, this effect is weakened by the inhomogeneous distribution of Ga. Due to their larger carrier mobility and lower lattice thermal conductivity, the samples sintered by HP exhibit 15–35% higher thermoelectric figure of merits (ZT) than those SPS samples with a high Ga doping level and without the coated BN layer, in which the composition homogeneity is worse. A ZT value of 0.43 is obtained for the HP Cu₂Ga_0.075Sn_0.925Se₃ sample at 700 K.     

Thermoelectric properties of Cd doped tetrahedrite: Cu12−xCdxSb4S13

The effect of Cd doping on the thermoelectric properties of synthetic tetrahedrites Cu_12−xCd_xSb₄S₁₃ (x = 0, 0.25, 0.5, 0.75, 1, 1.25 and 1.5) was studied. Powder X-Ray Diffraction and electron probe micro analysis confirm the presence of tetrahedrite as main phase with a trace of impurity phases. The influence of Cd substitution on Cu was confirmed by a systematic increase of electrical resistivity and Seebeck coefficient with doping content. Power factor decreased with doping content mainly affected by the significant increase in electrical resistivity. The total thermal conductivity was found to be reduced from 1.32 W/m-K for the sample with x = 0 to 0.71 W/m-K for the sample x = 0.75 at 623 K but showed an abrupt increment for x = 1 and decreased for the subsequent samples with x = 1.25 and x = 1.5. A combined result of power factor and thermal conductivity leads to thermoelectric figure of merit, zT = 0.9 at 623 K for Cu_11.25Cd_0.75Sb₄S₁₃.     

Crystal structure and thermoelectric properties of Cu2Cd1−xZnxSnSe4 solid solutions

Cu₂Cd_1–xZn_xSnSe₄ solid solutions were synthesized, and their phase constitutions and thermoelectric properties were investigated. The solid solutions crystallized in the stannite-type structure for Zn contents x up to 0.65 and in the kesterite-type structure for 0.7 ≤ x ≤ 1.0. The lattice parameter a and cell volume V of the compounds decreased linearly with increasing x for both the stannite-type (0 ≤ x ≤ 0.65) and the kesterite-type (0.7 ≤ x ≤ 1) structures. The lattice parameter c decreased with increasing x for the compounds with the kesterite-type structure but increased for the compounds with the stannite-type structure. The c/a ratio increased with increasing Zn content, which indicated an weakening of the lattice distortion. The Seebeck coefficient tended to decrease with increasing Zn content, whereas the electrical conductivity and thermal conductivity increased. The figure of merit ZT increased with increasing x over the composition range of 0 ≤ x ≤ 0.60 and then fluctuated with a further increase in x. A maximum ZT of 0.23 was achieved for Cu₂Cd_0.4Zn_0.6SnSe₄ at 720 K.     

Thermal and transport properties of as-grown Ni-rich TiNi shape memory alloys

Electrical resistivity, Seebeck coefficient, specific heat and thermal conductivity measurements on the Ti_50−xNi_50+x (x = 0.0–1.6 at.%) shape memory alloys are performed to investigate their thermal and transport properties. In this study, anomalous features are observed in both cooling and heating cycles in all measured physical properties of the slightly Ni-rich TiNi alloys (x ≤ 1.0), corresponds to the transformation between the B19′ martensite and B2 austenite phases. Besides, the transition temperature is found to decrease gradually with increasing Ni content, and the driving force for the transition is also found to diminish slowly with the addition of excess Ni, as revealed by specific heat measurements. While the signature of martensitic transformation vanishes for the Ni-rich TiNi alloys with x ≥ 1.3, the characteristics of strain glass transition start to appear. The Seebeck coefficients of these TiNi alloys were found to be positive, suggesting the hole-type carriers dominate the thermoelectric transport. From the high-temperature Seebeck coefficients, the estimated value of Fermi energy ranges from ∼1.5 eV (Ti_48.4Ni_51.6) to ∼2.1 eV (Ti₅₀Ni₅₀), indicating the metallic nature of these alloys. In addition, the thermal conductivity of the slightly Ni-rich TiNi alloys with x ≤ 1.0 shows a distinct anomalous feature at the B19′ → B2 transition, likely due to the variation in lattice thermal conductivity.     

Enhanced thermoelectric cooling properties of Bi2Te3−xSex alloys fabricated by combining casting,milling and spark plasma sintering

Bi₂Te_3−xSe_x alloys are extensively used for thermoelectric cooling around room temperature, but, previous studies have reported peak thermoelectric efficiency of the material at higher temperature around 450 K. This study presents the casting followed by high energy ball milling and spark plasma sintering as a thriving methodology to produce efficient and well-built Bi₂Te_3−xSe_x material for the thermoelectric cooling around room temperature. In addition, changes in electrical and thermal transport properties brought up by amount of Se in the Bi₂Te_3−xSe_x material for this methodology are measured and discussed. Although Seebeck coefficient and electrical conductivity showed irregular trend, power factor, thermal conductivity and figure of merit ZT gradually decreased with the increase in amount of Se. A maximum ZT value of 0.875 at 323 K was obtained for x = 0.15 sample owing to its higher power factor. This value is 17% and 38% greater than for x = 0.3 and x = 0.6 samples respectively. At 323 K, herein reported ZT value of 0.875 is higher than the state of art n-type Bi₂Te₃ based thermoelectric materials produced by the time consuming and expensive methodologies.     

Thermoelectric properties and electronic structure of p-type Mg2Si and Mg2Si0.6Ge0.4 compounds doped with Ga

Mg₂Si:Ga_x and Mg₂Si_0.6Ge_0.4:Ga_x (x = 0.4% and 0.8%) solid solutions have been synthesized by direct melting in tantalum crucibles and hot pressing. The effect of Ga doping on the thermoelectric properties has also been investigated by measurements of thermopower, electrical resistivity, Hall coefficient and thermal conductivity in temperature range from 300 to 850 K. All samples exhibit a p-type conductivity evidenced by positive sign of both thermopower and Hall coefficient in the investigated temperatures. The maximum value of the dimensionless figure of merit ZT was reached for the Mg₂Si_0.6Ge_0.4:Ga(0.8%) compound at 625 K (ZT ∼ 0.36). The p-type character of thermoelectric behaviours of Ga-doped Mg₂Si and Mg₂Si_0.6Ge_0.4 compounds well corroborates with the results of electronic structure calculations performed by the Korringa-Kohn-Rostoker method and the coherent potential approximation (KKR-CPA), since Ga diluted in Mg₂Si and Mg₂Si_0.6Ge_0.4 (on Si/Ge site) behaves as hole donor due to the Fermi level shifted to the valence band edge. The onset of large peak of DOS from Ga impurity at the valence band edge, well corroborates with high Seebeck coefficient measured in Ga-doped samples.     

First-principle calculations of the electronic,elastic and thermoelectric properties of Ag doped CuGaTe2

To improve the performance of a thermoelectric material CuGaTe₂, element Ag is doped to replace element Ga and we investigate the electronic structure, phase stability, elastic and thermoelectric properties of CuGa_1−xAg_xTe₂ (x = 0, 0.25 and 0.5) via first-principles method. The phase stability of CuGa_1−xAg_xTe₂ is discussed by analyzing the formation energy, cohesive energy and elastic constants. The calculated sound velocities decrease with the increase of Ag content, which is favorable for reducing the lattice thermal conductivity. The analysis of band structures shows that the replacement of Ga by Ag makes CuGaTe₂ undergo a direct-indirect semiconductor transition. The Ag doping induces steep density of states in valence band edge, which is beneficial for increasing the carrier concentration and improving thermoelectric performance of CuGaTe₂.     

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.     

Effect of Al-doping on suppression of thermal conductivity in Si dispersed β-FeSi2

Silicon dispersed β-FeSi₂ with different aluminium concentrations are synthesized using eutectoid decomposition of α-Fe₂Si_5−xAl_x (0 ≤ x ≤ 0.1). Phase fractions, microstructure and thermoelectric properties of the above compositions have been investigated. Al-doping in Si dispersed β-FeSi₂ results in increased hole-carrier concentration thereby enhancing the electrical conductivity without compromising the Seebeck coefficient. This results in maximum power factor value of 4.7 μWcm⁻¹ K⁻² at 773 K for the sample with x = 0.1 which is significantly higher than that of an undoped sample. The thermal conductivity of the samples was fitted with the Debye-Callaway model to understand the various scattering processes involved. The analysis shows that an increased point defect scattering of phonons with Al-doping in addition to scattering by Si/β-FeSi₂ interface lowers the thermal conductivity significantly.     

Influence of Sb doping on thermoelectric properties of Mg2Ge materials

The Sb-doped Mg₂Ge compounds were successfully synthesized by tantalum-tube weld melting method followed by hot pressing and the thermoelectric properties were examined. The effects of Sb doping on the electrical conductivity, Seebeck coefficient, and thermal conductivity have been investigated in the temperature range of 300–740 K. It was found that the Sb doping with sufficient Mg excess increased the electrical conductivity dramatically, leading to enhancement of the power factors. The thermal conductivity was also reduced upon Sb doping, mainly due to mass fluctuation scattering and strain field effects. Mg_2.2Ge_0.095Sb_0.005 showed a maximum thermoelectric figure of merit of ≈0.2 at 740 K.     

Magnetic and magnetocaloric properties in Cu-doped high Mn content Mn50Ni40−xCuxSn10 Heusler alloys

The effects of Cu substitution on the phase transitions and magnetocaloric effect of Mn₅₀Ni_40−xCu_xSn₁₀ Heusler alloys were investigated. With the increase of Cu content, the martensitic transformation (MT) temperature shifts substantially towards lower temperature, while the Curie temperature of austenite remains almost unchanged. The reverse MT temperature decreases from 180 to 171 K for Mn₅₀Ni₃₉Cu₁Sn₁₀ alloy as the magnetic field increases from 1 to 30 kOe. Under an applied magnetic field of 30 kOe, the maximum values of magnetic field induced entropy changes are 19.6, 28.9, and 14.2 J/kg K for x = 0, 1, and 2, respectively. The effective refrigerant capacities and hysteresis losses for these alloys were discussed in this paper.     

An investigation on the crystal structures of Ti50Ni50−xCux shape memory alloys based on density functional theory calculations

The present research has investigated the martensite crystal structures and electronic structures of Ti₅₀Ni_50−xCu_x (x = 0, 5, 12.5, 15, 18.75, 20, 25) shape memory alloys using density functional theory (DFT). The computational results are compared with the reported data and it is found that the equilibrium lattice constants are in good agreement with reported values. It is also found that with Cu addition to NiTi, the lattice parameters (a and c) and the monoclinic angle decrease, whereas the lattice parameter b increases. With increasing Cu content, fewer electrons were transferred from Ti to Ni in comparison with that in binary NiTi alloys, and the NiTi monoclinic structure becomes unstable. When Cu content is increased to around 20 at%, an orthorhombic crystal structure is formed which agrees well with reported experimental observations.     

Structure and thermoelectric performance of β-Cu2Se doped with Fe,Ni, Mn,In, Zn or Sm

Cu_1.99A_0.01Se (A = Fe, Ni, Mn, In, Zn or Sm) alloys with high thermoelectric performance were prepared through a conventional melting, ball milling and quenching route, followed by a spark plasma sintering technique. Elemental doping did not change the structure type of Cu₂Se. All the samples showed p-type conduction. All the doping elements except Indium reduced the electric resistivity and modified the carrier concentration, leading to a significant increase in the power factor. The lattice distortion and point defects due to the substitution of Cu became new phonon scattering centers, leading to a significant decrease in thermal conductivity. All the samples except the In-doped sample obtained better thermoelectric properties compared with the undoped Cu₂Se sample. The values of the figure of merit ZT of the samples doped with Zn, Mn, Ni, Fe and Sm were 1.25, 1.28, 1.51, 1.07 and 1.07 at 823 K, respectively. In Cu_2−xNi_xSe system, High ZT value of 1.51 is obtained for the sample of x = 0.0075 and 0.010 at 823 K.     

Effects of Co substitution for Fe on the glass forming ability and properties of Fe80P13C7 bulk metallic glasses

Bulk magnetic Fe_80−xCo_xP₁₃C₇ (x = 0, 5, 10, 15, 20 at.%) glassy alloy rods were prepared by the combination method of fluxing treatment and J-quenching technique, and the attainable maximum diameter for fully glass formation gets to 2.5 mm for x = 5. The effects of Co substitution for Fe on the glass formation ability (GFA), thermal stability, mechanical properties and magnetic properties have been investigated systematically. It was found that the partially substitution of Co for Fe can enhance the GFA of Fe₈₀P₁₃C₇ alloy, while excessive substitution will lead to the degradation of GFA. The compressive test shows that the substitution of Co for Fe results in the decease of fracture strength, and then significantly enhance the room temperature plastic strain of the present Fe-based BMGs, which can be identified that the plastic strain at room temperature gets to 2.5% and 3.0% for x = 5 and 10, respectively. The saturation magnetization of Fe_80−xCo_xP₁₃C₇ (x = 0, 5, 10, 15, 20 at.%) BMGs firstly increases from 1.477 T to 1.550 T with increasing Co content from x = 0 to 5, and then deceases from 1.549 T to 1.519 T with increasing Co content from x = 5 to 20. The Curie temperature of the present FeCoPC BMGs quickly increases with the substitution of Co for Fe.     

Magnetic properties and tuneable magnetocaloric effect with large temperature span in GdCd1−xRux solid solutions

The magnetic properties and the magnetocaloric effect (MCE) in the GdCd_1−xRu_x (x = 0.1, 0.15, and 0.2) solid solutions have been systematically investigated. A large reversible MCE has been observed in GdCd_1−xRu_x accompanied by a second order magnetic phase transition from paramagnetic to ferromagnetic at T_C ∼ 149, 108, and 73 K for x = 0.1, 0.15, and 0.2, respectively. Under a field change from 0 to 7 T, the maximum values of magnetic entropy change (–ΔS_M^max) are 5.6, 7.8, and 11.0 J/kg K for x = 0.1, 0.15, and 0.2, respectively, the corresponding values of the relative cooling power (RCP) are 889, 852, and 828 J/kg. The considerable reversible MCE and large RCP values together with the tuneable T_M in a wide temperature range make the GdCd_1−xRu_x solid solutions considerable for active magnetic-refrigeration.     

Crystal structure,electrical, and magnetic properties of the Sm2Cu0.8Ge3 compound

The crystal structure, electrical, and magnetic properties of the new ternary compound Sm₂Cu_0.8Ge₃ have been investigated using powder X-ray diffraction, electrical resistivity, and magnetic susceptibility measurements. X-ray diffraction patterns reveal that this compound crystallizes in a tetragonal α-ThSi₂ structure (space group I4₁/amd) with lattice parameters a = 0.413(1) nm and c = 1.420(3) nm. The irreversibility of the zero field cooled and field cooled dc magnetization data reveals the occurrence of spin glass transition with the spin freezing temperature T_f ∼ 8.5 K in this compound. Ac susceptibility and isothermal remanent magnetization data also confirm the existence of the spin glass phase. In addition, a broad maximum is observed in the ρ(T) curve near T_f. The spin glass phase is ascribed to the formation of random interaction between the Sm ions due to the irregular distribution of Cu and Ge atoms as well as the deficiency of the occupancy on the 8e crystallographic sites of the sample.