Thermoelectric properties of Zn4Sb3 intermetallic compound doped with Aluminum and Silver

The β-phase Zn₄Sb₃ has attracted much attention because of its high thermoelectric performance in the intermediate temperature range thanks to disorder in the Zn lattice site. In this work are presented structural, thermal, electric and thermoelectric characterization of Zn₄Sb₃ pure and Ag, Al doped, prepared by a simple synthesis. Structural and microstructural analyses reveal homogeneous one-phases having compositions in agreement with the nominal ones. After thermoelectric characterization, Ag doping results mostly effective in lowering the resistivity and Seebeck coefficient value, by introducing holes in the system. On the other hand, the Al substitution yields a very small decrease of the Seebeck coefficient but, at the same time, a significant decrease of the thermal conductivity mainly due to the depressed phonon contribution. The thermal conductivity behavior is the main responsible for the good thermoelectric performances of (Zn_0.99Al_0.01)₄Sb₃, whose thermoelectric figure of merit reaches the encouraging value of 0.23 at 260 K.     

Synthesis and thermoelectric properties of the quaternary type-I Si clathrates K8-δGaxAlySi46-x-y

The effects of substituting Ga atoms with Al on the thermoelectric properties of the ternary Si clathrate K₈Ga₈Si₃₈ were assessed over the temperature range of 10–700 K. For this purpose, the series of quaternary Si clathrates K_8-δGa_xAl_ySi_46-x-y (x + y ≈ 8; 0 ≤ y < 8) was synthesized by heating mixtures of K, Ga, Al and Si at temperatures between 1223 and 1353 K. The lattice constant of K_8-δGa_xSi_46-x was found to increase with the Al content, y, and Al substitution also significantly changed the transport properties of the clathrate, even though Al and Ga are isovalent. Electrical resistivity measurements showed that the conduction mechanism changed from variable range hopping conduction to metallic conduction with increasing y. The Seebeck coefficient, S, was negative for all of the samples, indicating that the dominant carriers were electrons, while the absolute value of S decreased with y. All specimens had approximately the same value of thermal conductivity, κ, on the order of 1.4 W/Km at 300 K, except for K_7.7Al_7.5Si_38.8. K_7.5Ga_4.8Al_3.0Si_38.7 exhibited the largest power factor, and therefore the highest thermoelectric dimensionless figure of merit, ZT, with a value of 0.066 at 550 K. This value is approximately six times that of the ternary Si clathrate K_7.4Ga_7.7Si_38.9.     

Thermal conductivity of Ni3V–Ni3Al pseudo-binary alloys

The effect of changes in the composition and microstructure of the Ni₃V–Ni₃Al pseudo-binary alloys on their thermal conductivity has been investigated. For Ni₃V and Ni₃Al-based single-phase alloys, the thermal conductivity shows a maximum value at the stoichiometric compositions, and it decreases as the V (or Al) content of the Ni₃Al (or Ni₃V) alloy increases, following the Nordheim rule. For Ni₃V–Ni₃Al two-phase alloys, the thermal conductivity of the constituent Ni₃Al phase exhibits a smaller value than that of the Ni₃V phase. Eventually, the thermal conductivity of the two-phase alloys decreases as the Al content increases because of the increase in the volume fraction of the Ni₃Al phase with low conductivity. As the temperature increases from 293 K to 1073 K, the conductivity increases for all of the alloys but not for stoichiometric Ni₃V. However, the dependence of the thermal conductivity on the alloy composition between 293 K and 1073 K is similar. Hence, it is confirmed that the thermal conductivity of the Ni₃V–Ni₃Al pseudo-binary alloys is controlled by the composition and volume fraction of the constituent phase.     

Thermoelectric properties of Indium doped Cu2GeSe3

Cu₂Ge_1−xIn_xSe₃ (x = 0, 0.05, 0.1, 0.15) compounds were prepared by a solid state synthesis. The powder X-ray diffraction pattern of the undoped sample revealed an orthorhombic phase. The increase in doping content led to the appearance of additional peaks related to cubic and tetragonal phases along with the orthorhombic phase. This may be due to the substitutional disorder created by Indium doping. Scanning Electron Microscopy micrographs showed a continuous large grain growth with low porosity, which confirms the compaction of the samples after hot pressing. Elemental composition was measured by Electron Probe Micro Analyzer and confirmed that all the samples are in the stoichiometric ratio. The electrical resistivity (ρ) systematically decreased with an increase in doping content, but increased with the temperature indicating a heavily doped semiconductor behavior. A positive Seebeck coefficient (S) of all samples in the entire temperature range reveal holes as predominant charge carriers. Positive Hall coefficient data for the compounds Cu₂In_xGe_1−xSe₃ (x = 0, 0.1) at room temperature (RT) confirm the sign of Seebeck coefficient. The trend of ρ as a function of doping content for the samples Cu₂In_xGe_1−xSe₃ with x = 0 and 0.1 agrees with the measured charge carrier density calculated from Hall data. The total thermal conductivity increased with rising doping content, attributed to an increase in carrier thermal conductivity. The thermal conductivity revealed 1/T dependence, which indicates the dominance of Umklapp phonon scattering at elevated temperatures. The maximum thermoelectric figure of merit (ZT) = 0.23 at 723 K was obtained for Cu₂In_0.1Ge_0.9Se₃.     

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.     

Synthesis and thermoelectric properties of CoSb3/WO3 thermoelectric composites

In this study, nano-sized WO₃ powder was dispersed into CoSb₃ powder by ball milling and CoSb₃/WO₃ thermoelectric composites were fabricated using hot-pressing sintering. The results showed that the WO₃ phase distributed uniformly in the form of clusters and the average size of cluster was lower than 4 μm. As the content of WO₃ increased, the electrical conductivity and Seebeck coefficient of CoSb₃/WO₃ composites decreased. The thermal conductivity of composites decreased obviously which resulted from the phonon scattering by the WO₃ inclusions locating on the grain boundaries of CoSb₃ matrix. The highest thermoelectric figure of merit ZT = 0.40 was achieved at 650 K for CoSb₃/2%WO₃ composite.     

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.     

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.     

Effect of Fe additions on microstructure and mechanical properties of a multi-component Nb–16Si–22Ti–2Hf–2Al–2Cr alloy at room and high temperatures

The phase constitutions, microstructural evolutions, and mechanical properties of Nb–16Si–22Ti–2Hf–2Al–2Cr–xFe alloys (where x = 1, 2, 4, 6 at.%, hereafter referred to as 1Fe, 2Fe, 4Fe and 6Fe alloys, respectively) prepared by arc-melting were investigated. It was observed that the nominal Fe content affected the solidification path of the multi-component alloy. The as-cast 1Fe alloy primarily consisted of a dendritic-like Nb_SS phase and (α+γ)-Nb₅Si₃ silicide, and the as-cast 2Fe and 4Fe alloys primarily consisted of an Nb_SS phase, (α+γ)-Nb₅Si₃ silicide and (Fe + Ti)-rich region. In addition to the Nb_SS phase, a multi-component Nb₄FeSi silicide was present in the as-cast 6Fe alloy. When heat-treated at 1350 °C for 100 h, the 1Fe and 6Fe alloys almost exhibited the same microstructures as the corresponding as-cast samples; for the 2Fe and 4Fe alloys, the (Fe + Ti)-rich region decomposed, and Nb₄FeSi silicide formed. The fracture toughness of the as-cast and heat-treated Nb–16Si–22Ti–2Hf–2Al–2Cr–xFe samples monolithically decreased with the nominal Fe contents. It is interesting that at room temperature, the strength of the heat-treated samples was improved by the Fe additions, whereas at 1250 °C and above, the strength decreased, suggesting the weakening role of the Nb₄FeSi silicide on the high-temperature strength. As the nominal Fe content increased from 1 at.% to 6 at.%, for example, the 0.2% yield strength increased from 1675 MPa to 1820 MPa at room temperature; also, the strength decreased from 183 MPa to 78 MPa at 1350 °C.     

Structural and mechanical properties of Fe–Al compounds: An atomistic study by EAM simulation

The structural properties, formation enthalpies, and mechanical properties of Fe–Al compounds (FeAl, Fe₂Al, Fe₃Al, FeAl₂, FeAl₃ and Fe₂Al₅) are studied by using embedded-atom method (EAM) which is acquired by Mobius lattice inversion. The potential is transferrable and therefore does well for studying different Fe–Al compounds. The calculated lattice parameters and cohesive energies of Fe–Al compounds agree with the experimental and some EAM results. According to elastic constants restrictions, all the six Fe–Al compounds are mechanically stable. The calculated bulk moduli of the compounds increase with the increasing Fe concentration. Furthermore, results showed that FeAl, Fe₃Al, FeAl₃, FeAl₂, Fe₂Al₅ have lower ratios of shear modulus to bulk modulus and Fe₂Al has higher ratio.     

Electronic structure and transport properties of SrAl2Si2: Effect of yttrium substitution

We report the results of yttrium substitution on the electrical resistivity (ρ), the thermal conductivity (κ), as well as the Seebeck coefficient (S) of the Sr_1?xY_xAl₂Si₂ alloys with 0 ≤ x ≤ 0.20. Both ρ(T) and S(T) data suggest that SrAl₂Si₂ is a semimetallic, low charge carrier density system with a pseudogap at the Fermi level density of states (DOS). Upon substituting Y onto the Sr sites, the electrical resistivity and the absolute value of the Seebeck coefficient decrease significantly. Such an observation can be associated with the modification of the electronic band structure due to electron doping via Y substitution. Analysis of the thermal conductivity reveals the contribution of various thermal scattering mechanisms through chemical substitution. Theoretical studies with density functional theory are also employed to investigate the electronic band structure of Sr_1?xY_xAl₂Si₂. It is revealed that SrAl₂Si₂ possesses a shallow DOS at the Fermi level with both n-type and p-type charge carriers. Upon Y substitution a shift in the Femi level occurs such that the Sr_1?xY_xAl₂Si₂ system becomes more metallic with increasing x, being consistent with the experimental findings.     

Directional thermoelectric properties of Ru2Si3

The orthorhombic compound Ru₂Si₃ is currently of interest as a high-temperature thermoelectric material. In order to clarify the effects of crystal orientation on the thermoelectric properties of Ru₂Si₃, we have examined the microstructure, Seebeck coefficient, electrical resistivity, and thermal conductivity of Ru₂Si₃ along the three principal axes, using these measured quantities to describe the relative thermoelectric performance as a property of crystal orientation. Ru₂Si₃ undergoes a high temperature (HT)→low temperature (LT) phase change and polycrystalline Si platelet precipitation during cooling, both of which are expected to effect the thermoelectric properties. The HT tetragonal→LT orthorhombic phase transformation results in a [010]//[010], [100]//[001] two-domain structure, while polycrystalline Si precipitation occurs on the (100)_LT and (001)_LT planes. The [010] orientation is found to posses superior thermoelectric properties (with the dimensionless figure of merit, ZT_[010]/ZT_[100]>4 at 900 K), due principally to the larger Seebeck coefficient along the [010] direction. The effect of the domain structure on the thermoelectric properties is discussed.     

Crystal, electric transport properties and electronic structures of the Ti5Me1−xSb2+x series of compounds (MeCr, Mn, Fe, Co, Ni, Cu)

The Ti₅Me_1−xSb_2+x compounds where MeCr, Mn, Fe, Co, Ni, Cu, were synthesized and their crystal structure was determined (W₅Si₃ structure type, space group I4/mcm). The transport properties were investigated by means of electrical resistivity and Seebeck coefficient measurements in the temperature range 80–380 K. All the investigated compounds exhibit metallic-like type of conductivity confirmed by electronic structure calculations based on Density Functional Theory.     

Magnetocaloric effect of nonstoichiometric La1−xFe11.4+xSi1.6 alloys with first-order and second-order magnetic transitions

Variation from a first-order magnetoelastic transition (FOMT) to a second-order magnetic transition (SOMT) could be controlled by finely adjusting the Fe/La ratio in the non-stoichiometric La_1−xFe_11.4+xSi_1.6 (x = 0, 0.05, 0.10, 0.15 and 0.20) alloys. Particularly, a nearly stoichiometric ratio of NaZn₁₃-type phase was obtained with an addition of excess 0.15 at% Fe. When x value increased from 0 to 0.20, the Curie temperature (T_C) increased from 198.6 to 216.6 K due to lattice shrinkage and the magnetic entropy change (−ΔS_M) decreased from 18.4 to 8.0 J/(kg·K) (at 0−2 T) and from 22.5 to 13.8 J/(kg·K) (at 0−5 T), which may be ascribed to the change from the itinerant-electron metamagnetism (IEM) to second order magnetic transition. Thus their effective refrigeration capacities declined from 347.6 to 245.9 J kg⁻¹ under 0–5 T, which values were comparable to the Gd₅Si₂Ge_1.9Fe_0.1. The Fe/La ratio in NaZn₁₃-phase was possibly dominant to induce the transition from first to second order among La_1−xFe_11.4+xSi_1.6 alloys. The thermal hysteresis decreased by 87% to 0.3 K and magnetic hysteresis losses reduced by 90% to 1.4 J kg⁻¹ under 0–5 T. Near the transition border, the alloys with x = 0.15 produced a giant magnetocaloric effect of about 11.8 J/(kg·K) under 0–2 T at T_C = 205.6 K. The magnetic hysteresis was 3.01 J kg⁻¹ and the thermal hysteresis was 1.9 K. The material near the border of transition could be suitable for future magnetic refrigeration application.     

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.     

Formation of the ζ phase at an interface between an Fe substrate and a molten 0.2 mass% Al–Zn during galvannealing

An interstitial free (IF) steel was dipped in a bath of molten Zn containing 0.2 mass% Al. The as-dipped (galvanized) steel was then annealed above the melting point of Zn (galvannealed), say at 773 K, for a period ranging from 1 to 30 s. Evolution of the microstructure of the Zn-containing coating was examined by transmission electron microscopy (TEM) and the chemical composition around the Zn(Al)/Fe interface was analyzed by energy dispersion spectroscopy (EDS) in a dedicated scanning transmission electron microscope (STEM). In the as-dipped (galvanized) steel a continuous layer of Fe₂Al₅ existed at the Zn(Al)/Fe interface, but no Fe–Zn intermetallic compounds were observed. Galvannealing at 773 K for 1 s resulted in the formation of the ζ phase at the Zn(Al)/Fe₂Al₅ interface, which grew at the expense of the Zn(Al) coating during the subsequent galvannealing. Possible mechanisms of nucleation of Fe–Zn intermetallic compounds are discussed.     

Enhanced Seebeck coefficient by energy filtering in Bi-Sb-Te based composites with dispersed Y2O3 nanoparticles

The incorporation of ceramic nanoparticles in the bulk thermoelectric matrix is one of the new strategies to boost the Seebeck coefficient. In this research, different weight percentages of Y₂O₃ (2, 4, and 6) nanoparticles (NPs) were incorporated into the pre-alloyed BiSbTe powder for making nanocomposites (NCs) by mechanical milling. The resultant NCs powders were subsequently consolidation by spark plasma sintering (SPS) at 450 °C. The existence of Y₂O₃ nano-inclusions was confirmed by x-ray diffraction and TEM-SAED analysis. The hardness of the nanocomposites was significantly improved (>49%) compared to that of pure BiSbTe, and this was attributed to grain-boundary hardening and to a dispersion strengthening mechanism. The electrical conductivity decreased while the Seebeck coefficient significantly improved (45%) at room temperature for the NCs to which 2 wt% Y₂O₃ was added. This was due to the scattering of carriers through the energy filtering effect. The electronic component of the thermal conductivity greatly contributed to the reduction of total thermal conductivity (22%) in BiSbTe NCs to which 6 wt% Y₂O₃ was added. A peak ZT of 1.24 was achieved for BiSbTe/(2 wt%) Y₂O₃ NCs due to reduction in their thermal conductivity and improved Seebeck coefficient values.     

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.     

Thermodynamic Description of the Al-Mo and Al-Fe-Mo Systems

The Al-Mo and Al-Fe-Mo systems were critically assessed using the CALPHAD technique. The solution phases (liquid, fcc and bcc) were described by a substitutional solution model. The non-stoichiometric compound AlMo₃ was described by a two-sublattice model (Al,Mo)(Al,Mo)₃ in the Al-Mo binary system and (Al,Fe,Mo)(Al,Fe,Mo)₃ in the Al-Fe-Mo ternary system. Other compounds Al₆₃Mo₃₇, Al₈Mo₃, Al₃Mo, Al₄Mo, Al₁₇Mo₄, Al₂₂Mo₅, Al₁₂Mo and Al₅Mo in the Al-Mo system were treated as stoichiometric compounds in the binary system and as line compounds Al _m (Fe,Mo) _n in the Al-Fe-Mo ternary system. The compounds μ and Fe₂Mo in the Fe-Mo system were treated as (Al,Fe)₇Fe₂(Fe,Mo)₄ and (Fe,Mo)₂(Al,Mo) in the Al-Fe-Mo system, respectively. Compounds Al₅Fe₄, Al₂Fe, Al₅Fe₂ and Al₁₃Fe₄ in the Al-Fe system were treated as (Al,Fe,Mo), Al₂(Fe,Mo), (Al,Fe)₅(Al,Fe,Mo)₂ and (Fe,Mo)_0.235Al_0.6275(Al,Va)_0.1375 in the Al-Fe-Mo system, respectively. Ternary compounds τ₁ and τ₂ were treated as Al₈(Al,Fe)Mo₃ and (Al,Fe,Mo)(Va)₃, respectively. A set of self-consistent thermodynamic parameters of the Al-Fe-Mo system was obtained.