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.     

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).     

Significant ZT enhancement in p-type Ti(Co,Fe)Sb–InSb nanocomposites via a synergistic high-mobility electron injection,energy-filtering and boundary-scattering approach

It has been demonstrated that InSb nanoinclusions, which are formed in situ, can simultaneously improve all three individual thermoelectric properties of the n-type half-Heusler compound (Ti,Zr,Hf)(Co,Ni)Sb (Xie WJ, He J, Zhu S, Su XL, Wang SY, Holgate T, et al. Acta Mater 2010;58:4795). In the present work, the same approach is adopted to the p-type half-Heusler compound Ti(Co,Fe)Sb. The results of resistivity, Seebeck coefficient, thermal conductivity and Hall coefficient measurements indicate that the combined high-mobility electron injection, low energy electron filtering and boundary scattering, again, lead to a simultaneous improvement in all three individual thermoelectric properties: enhanced Seebeck coefficient and electrical conductivity as well as reduced lattice thermal conductivity. A figure of merit of ZT ≈ 0.33 was attained at 900 K for the sample containing 1.0 at.% InSb nanoinclusions, a ～450% improvement over the nanoinclusion-free sample. This represents a rare case that the same nanostructuring approach works successfully for both p-type and n-type thermoelectric materials of the same class, hence pointing to a promising materials design route for higher-performance half-Heusler materials in the future and hopefully will realize similar improvement in thermoelectric devices based on such half-Heusler alloys.     

Magnetic property and magnetocaloric effect in TmCoAl compound

A large reversible magnetocaloric effect accompanied by a second order magnetic phase transition from paramagnetic (PM) to ferromagnetic (FM) has been observed in TmCoAl intermetallic compound. For the magnetic field change of 5 T, the maximum value of magnetic entropy change (−ΔS_M^max) and the value of refrigerant capacity (RC) are evaluated to be 18.2 J/kg K and 211 J/kg, respectively. In particular, a large −ΔS_M^max (10.2 J/kg K) is achieved at 7.5 K under a low magnetic field change from 0 to 2 T with no thermal hysteresis and magnetic hysteresis loss. The large reversible magnetocaloric effect (both the large −ΔS_M and the high RC) indicates that TmCoAl is one of a promising material for magnetic refrigeration in low temperature.     

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.     

Thermoelectric performance of polycrystalline Sn1-xCuxSe (x = 0–0.03) prepared by high pressure method

P-type Sn_1-xCu_xSe (x = 0–0.03) polycrystal was prepared through melting synthesis and high pressure (6.0 GPa) sintering (HPS) method. The composition and microstructure of the samples was analyzed, and the thermoelectric transport properties were investigated in the temperature range of 303 K–823 K. The results indicate that the electrical conductivity increases as Cu content increases. An observable improvement is found for the Seebeck coefficient when x is 0.01. In addition, the total thermal conductivities (κ_tot) of all samples decrease with rising temperature, and reach its minimum values at 773 K. As a result, the maximum power factor (PF) and ZT_max value are 378 μW m⁻¹ K⁻² and 0.79 for Sn_0.97Cu_0.03Se at 823 K, respectively.     

Microstructure and thermoelectric properties of Sb doped Hf0.25Zr0.75NiSn Half-Heusler compounds with improved carrier mobility

Half-Heusler (HH) semiconductor alloys are being widely investigated due to their promising potential for thermoelectric (TE) power generation applications. Sb is an effective doping element for n-type ZrNiSn half-Heuslers alloys. HH thermoelectric materials Hf_0.25Zr_0.75NiSn_1−xSb_x (0 ≤ x ≤ 0.03) were synthesized by induction melting combined with plasma activated sintering (PAS) technique. X-ray diffraction concluded that single-phase HH compounds without compositional segregations were obtained. Presence of bended lamellar structures was revealed by the FESEM. Sb doping significantly enhanced the electrical conductivity, power factor and carrier concentration of the alloys. An increase in the carrier mobility was also observed. Consequently, optimum values of 4.36 × 10⁻³ W/mK² and 4.7 × 10²⁰ cm⁻³ were achieved for power factor and carrier concentration, respectively. As a result, a ZT value of 0.83 at 923 K was obtained which is about 67% improvement compared to the un-doped sample.     

Synthesis and magnetocaloric characterization of rapidly solidified ErMn2 melt-spun ribbons

We have fabricated melt-spun ribbons of nominal composition ErMn₂ and studied their phase constitution and magnetocaloric (MC) properties by X-ray diffraction, scanning electron microscopy (SEM), magnetization and heat capacity measurements. The major phase formed shows the MgZn₂-type hexagonal structure (C14-type); however, both XRD and SEM analyses revealed the formation of impurity phases (i.e., Er₆Mn₂₃ and ErMn₁₂). Ribbons exhibit a saturation magnetization of 149 Am²kg⁻¹ at 2 K and a Curie temperature of T_C = 15 K. A field-induced metamagnetic transition at very low critical magnetic fields was observed below 8 K that leads to a change of sign in the magnetic entropy change ΔS_M below this temperature (ΔS_M^peak = 2.5 Jkg⁻¹K⁻¹ at 2 K and 5 T). For a magnetic field change of 5 T (2 T) applied along the ribbon length, the samples show a large peak value of the magnetic entropy change ΔS_M^peak of −20.5 (−10.8) Jkg⁻¹K⁻¹, a full-width at half-maximum δT_FWHM for the ΔS_M(T) curve of 20 (12) K, and a maximum adiabatic temperature change ΔT_ad^max of 7.4 (3.6) K. The obtained results are compared with the reported in literature by other authors for bulk alloys.     

Large magnetocaloric effect in a wide temperature range induced by two successive magnetic phase transitions in Ho2Cu2Cd compound

The magnetic and magnetocaloric properties of Ho₂Cu₂Cd compound have been studied. The compound reveals two successive magnetic phase transitions at 30 K and 15 K which are corresponding to the paramagnetic to ferromagnetic transition and a spin reorientation, respectively. Two successive magnetic transitions in Ho₂Cu₂Cd resulting two peaks in the temperature dependence of magnetic entropy change curves, −ΔS_M (T). Two peaks are partly overlapped and induced a large MCE in a wide temperature range, i. e., large refrigerant capacity (RC) and relative cooling power (RCP). For a magnetic field change of 0–7 T, the values of maximal −ΔS_M, RC and RCP are 25.1 J/kg K, 527 J/kg and 732 J/kg for Ho₂Cu₂Cd, respectively.     

The effect of a novel organic compound chiral macrocyclic tetraamide-I interfacial layer on the calculation of electrical characteristics of an Al/tetraamide-I/p-Si contact

The Al/tetraamide-I/p-Si Schottky barrier diode (SBD) has been prepared by adding a solution of a novel nonpolymeric organic compound chiral macrocylic tetraamide-I in chloroform on top of a p-Si substrate and then evaporating the solvent. It has been seen that the forward-bias current–voltage (I–V) characteristics of Al/tetraamide-I/p-Si SBD with a barrier height value of 0.75 eV and an ideality factor value of 1.77 showed rectifying behaviour. The energy distribution of the interface state density determined from I–V characteristics increases exponentially with bias from 5.81 × 10¹² cm⁻² eV⁻¹ at (0.59-E_v) eV to 1.02 × 10¹³ cm⁻² eV⁻¹ at (0.40-E_v) eV. It has showed that space charge limited current (SCLC) and trap charge limited current (TCLC) are the dominant transport mechanisms at large forward-bias voltages.     

Current–voltage characteristics of Al/Rhodamine-101/n-GaAs and Cu/Rhodamine-101/n-GaAs rectifier contacts

The organic compound Rhodamine-101 (Rh101) film on an n-type GaAs substrate with carrier concentration of 7.3 × 10¹⁵ cm⁻³ has been formed by means of the evaporation process, and thus Al/Rh101/n-GaAs and Cu/Rh101/n-GaAs contacts have been fabricated. Our aim is to realize a modification of Schottky barrier height (SBH) of the devices using a thin non-polymeric organic compound layer. The Al/Rh101/n-GaAs and Cu/Rh101/n-GaAs contacts have behaved like rectifying contact with the SBH values of 0.68 eV and 0.72 eV, and with ideality factor values of 2.61 and 2.60 obtained from their forward bias current–voltage (I–V) characteristics at the room temperature, respectively. It has seen that the SBH values obtained for these devices are significantly different from those obtained for the conventional Al/n-GaAs or Cu/n-GaAs Schottky diodes. Furthermore, it has been demonstrated that the trapped-charge-limited current is the dominant transport mechanism at large forward bias voltage.     

The calculation of electronic parameters of an Ag/chitin/n-Si Schottky barrier diode

We have formed polymeric organic compound chitin film on n-Si substrate by adding a solution of polymeric compound chitin in N,N-dimethylacetamide and lithium chloride on top of an n-Si substrate and then evaporating solvent. It has been seen that the chitin/n-Si contact has demonstrated clearly rectifying behavior and the reverse curves exhibit a weak bias voltage dependence by the current–voltage (I–V) curves studied at room temperature. The barrier height and ideality factor values of 0.959 eV and 1.553, respectively, for this structure have been obtained from the forward bias I–V characteristics. Furthermore, the energy distribution of the interface state density located in the semiconductor band gap at the chitin/n-Si substrate in the energy range from (E_c − 0.897) to (E_c − 0.574) eV have been determined from the I–V characteristics. The interface state density, N_ss, ranges from 5.965 × 10¹² cm⁻² eV⁻¹ in (E_c − 0.897) eV to 1.706 × 10¹³ cm⁻² eV⁻¹ in (E_c − 0.574) eV and has an exponential rise with bias this energy range.     

Low-temperature X-ray single-crystal analysis of a conductive mixed-valence MMX-chain system,Pt2(dta)4I (dta = CH3CS2−)

The one-dimensional (1-D) chain structure of the title compound (Pt₂(dta)₄I) (dta=CH₃CS₂⁻) has been investigated by X-ray single-crystal analysis at low temperatures. An anomalous behavior was observed for the temperature dependence of the lattice constant a (⊥ chain). Below 90 K, a slight increase of the β value was observed, which might relate to the expected spin-Peierls distortion associated with dta-ligand twisting. A little difference in Pt–I distances was observed at 16 K. No obvious superstructure reflections were observed at low temperatures.     

Electronic and transport properties of Mg2Si under isotropic strains

Density functional theory and Boltzmann theory calculations of the electronic and thermoelectric properties of Mg₂Si subjected to isotropic strains have been performed. The electrical conductivity, the Seebeck coefficient and the power factor have been evaluated at two temperatures (300 K and 900 K) and two charge carrier concentrations (10¹⁸ cm⁻³ and 1.2 × 10²⁰ cm⁻³ electrons and holes). Up to 3% of both compressive and tensile strains have been applied to the material. From our results, we can highlight that a significant improvement of both the Seebeck coefficient (S) and the power factor (PF) is obtained at low temperature and moderate n-doping. The increase in S and PF amounts to 40% and 100%, respectively, compared to the unstrained Mg₂Si.     

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.     

High hydrogen permeability in the Nb-rich Nb–Ti–Ni alloy

The microstructure and the hydrogen permeability of the Nb-rich Nb–Ti–Ni alloy, i.e., the Nb₅₆Ti₂₃Ni₂₁ alloy were investigated and compared with those of the Nb₄₀Ti₃₀Ni₃₀ alloy. The Nb₅₆Ti₂₃Ni₂₁ alloy consisted of a combination of the primary phase bcc- (Nb, Ti) solid solution with the eutectic phase {bcc- (Nb, Ti) + B2-TiNi}. The volume fraction of the former and the latter phases were 62 and 38 vol.%, respectively. The Nb₅₆Ti₂₃Ni₂₁ alloy showed the higher Φ value of 3.47 × 10⁻⁸ (mol H₂ m⁻¹ s⁻¹ Pa^−0.5) at 673 K, which is 1.8 times higher than that of the Nb₄₀Ti₃₀Ni₃₀ alloy, which has been reported to be highest in the Nb–Ti–Ni system. The present work demonstrated that the Nb-rich Nb–Ti–Ni alloys consisting of only the primary phase bcc- (Nb, Ti) and the eutectic phase {bcc- (Nb, Ti) + B2-TiNi} are promising for the hydrogen permeation membrane.     

Heat capacity of the AuZr compound obtained by high-temperature drop calorimetry

The enthalpy increments of the Zr_0.50Au_0.50 phase have been measured by high-temperature drop calorimetry in the temperature range 372–1245 K. From these results, heat capacities of ZrAu have been derived and two changes of the C_p values have been observed, in agreement with the existence of three crystalline forms of ZrAu. No latent heat was observed for the low-temperature (564 K) transition, whereas the enthalpy change at high temperature (841 K) was estimated at 1.55 kJ·mol^?1, from enthalpy increments obtained by drop calorimetry. These results are discussed in relation to the crystal modifications already reported for the ZrAu compound.     

Structure and magnetic behaviors of Gd6FeBi2 compound

Gd-based compounds have attracted a great deal of interest as materials with large magnetocaloric effect and as materials for spintronic applications. Fe₂P-type Gd₆FeBi₂ alloy was synthesized successfully, and the compound shows good soft magnetic behavior at room temperature with a Curie temperature ∼350 K. The compound exhibits non Curie–Weiss behavior in a large temperature range above Curie temperature, and slightly enhanced Gd moment at low temperature. The elevated Curie temperature and enhanced Gd magnetic moment were discussed based on measured electronic structure and the results from another isostructural compound Gd₆CoTe₂. The magnetocaloric effect was also measured in terms of the maximum magnetic entropy change of −4.3 J kg⁻¹ K⁻¹ at 50 kOe and −2.3 J kg⁻¹ K⁻¹ at 20 kOe, respectively.     

Phase compositions,nanoscale microstructures and thermoelectric properties in Ag2−ySbyTe1+y alloys with precipitated Sb2Te3 plates

Nonstoichiometric ternary thermoelectric materials Ag_2?ySb_yTe_1+y (y = 1.26, 1.29, 1.32, 1.35, and 1.38) were prepared by a direct melt-quench and hot press process. In situ composites of AgSbTe₂ and Sb₂Te₃ were obtained over the entire composition range with a typical Widmanstätten pattern. Thermoelectric properties were measured from 300 K to 673 K, which changed systematically with Sb₂Te₃ ratio. The phase transition occurring at about 633 K, forming the single phased AgSbTe₂, can significantly influence the electrical transport properties. Various crystallographic defects in different scales, such as atomic ordering, nanodomains, dislocations and stacking faults, have been observed by high-resolution transmission electron microscopy and their influences on lattice thermal conductivity have been discussed. Due to the extremely low thermal conductivity (about 0.6 W m^?1 K^?1) and large positive Seebeck coefficient of ～250 μV K^?1 detected in Ag_0.71Sb_1.29Te_2.29, the maximum dimensionless figure of merit ZT of 1.37 was obtained at 600 K.     

Infrared and Raman studies of the phase transition in the organic conductor (TTM-TTP)I3

We report the polarized IR reflectance as well as Raman scattering investigations of the organic charge transfer salt (TTM-TTP)I₃ as a function of temperature, below and above the metal–insulator phase transition at T = 160 K. The IR reflectance was measured in the frequency region from 600 to 10,000 cm⁻¹, for the electrical vector of the polarized light parallel and perpendicular to the TTM-TTP stacking axis. For the polarization parallel to stacks the IR spectra are typical for semiconducting charge transfer salts. The electronic part of IR spectra was analysed in terms of a Lorentz model and temperature dependence of the optical transport parameters was determined. For the polarization perpendicular to the stacks we observed two electronic bands at about 5000 and 8000 cm⁻¹. The phase transition at 160 K has nearly no influence on the IR spectrum. The Raman scattering for different excitations (λ = 514.5, 632.6 and 785 nm) was mainly studied within the region of CC stretching vibrations. In this frequency range, three Raman lines at 1426, 1453 and 1486 cm⁻¹ attributed to TTM-TTP molecules are observed. Below 160 K a splitting of the band 1486 cm⁻¹ into two peaks at about 1488 and 1498 cm⁻¹ is found. The intensity and temperature behaviour of the split band at 1498 cm⁻¹ is strongly dependent on sample. The observed spectral modifications are related to an asymmetric deformation of TTM-TTP. Taking into account temperature dependence of bands attributed to the CH stretching and SCH₃ bending vibrations, we suggest that the TTM-TTP deformation can exist also above the phase transition temperature. Above 160 K molecules fluctuate between distorted and symmetrical state forming non-stable domains (pre-transitional effects), but below 160 K the molecular distortion and domains are stable. The existence of electronic band at 5000 cm⁻¹ for the polarization perpendicular to TTM-TTP supports this picture.