Ball Mill Synthesis of Bulk Quaternary Cu<Subscript>2</Subscript>ZnSnSe<Subscript>4</Subscript> and Thermoelectric Studies

In this work, quaternary chalcogenide Cu₂ZnSnSe₄ (CZTSe) was synthesized using a mechanochemical ball milling process and its thermoelectric properties were studied by electrical resistivity, Seebeck coefficient, and thermal conductivity measurements. The synthesis process comprises three steps viz., wet ball milling of the elemental precursors, vacuum annealing, and densification by hot pressing. The purpose of this is to evaluate the feasibility of introducing wet milling in place of vacuum melting in solid state synthesis for the reaction of starting elements. We report the structural characterization and thermoelectric studies conducted on samples that were milled at 300 rpm and 500 rpm. X-ray diffraction (XRD) analysis showed the existence of multiple phases in the as-milled samples, indicating the requirement for heat treatment. Therefore, the ball milled powders were cold pressed and vacuum annealed to eliminate the secondary phases. Annealed samples were hot pressed and made into dense pellets for further investigations. In addition to XRD, energy dispersive spectroscopy (EDS) studies were performed on hot pressed samples to study the composition. XRD and EDS studies confirm CZTSe phase formation along with ZnSe secondary phase. Electrical resistivity and Seebeck coefficient measurements were done on the hot pressed samples in the temperature range 340–670 K to understand the thermoelectric behaviour. Thermal conductivity was calculated from the specific heat capacity and thermal diffusivity values. The thermoelectric figure of merit zT values for samples milled at 300 rpm and 500 rpm are ～0.15 and ～0.16, respectively, at 630 K, which is in good agreement with the values reported for solid state synthesized compounds.     

Effect of Transition-Metal Additives on Thermoelectric Properties of YB<Subscript>22</Subscript>C<Subscript>2</Subscript>N

The higher boride compound YB₂₂C₂N has been reported as a promising n-type high-temperature thermoelectric material and possible counterpart to boron carbide. To investigate the influence of transition-metal additives on the thermoelectric properties of YB₂₂C₂N, a series of Rh, Co, Cu, and Ni samples were prepared. The resistivity and Seebeck coefficient of the samples were measured in the temperature range of 323 K to 1073 K. Samples with Rh and Co additives showed a considerable reduction of resistivity in comparison with pure YB₂₂C₂N and maintained their semiconducting properties at high temperatures. A sample with Co, obtained using long-term ball milling, showed the highest absolute value of Seebeck coefficient among all previously studied YB₂₂C₂N-based materials. Analyses of the influence of transition-metal additives and processing methods such as ball milling on the thermoelectric properties of YB₂₂C₂N are presented.     

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 Chevrel-Phase Sulfides M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mo<Subscript>6</Subscript>S<Subscript>8</Subscript> (M: Cr,Mn, Fe,Ni)

Chevrel-phase sulfides M _x Mo₆S₈ (M, Cr, Mn, Fe, Ni; x: 1.3, 2.0) were prepared by reacting appropriate amounts of M, Mo, and MoS₂ powders. The samples were then consolidated by pressure-assisted sintering to fabricate dense compacts. While Cr_1.3Mo₆S₈ crystallized in a triclinic structure, Mn_1.3Mo₆S₈, Fe_1.3Mo₆S₈, and Ni_2.0Mo₆S₈ crystallized in a hexagonal structure. The Seebeck coefficient, electrical resistivity, and thermal conductivity of the sintered samples were measured over the temperature range of 300 K to 973 K. All the samples exhibited a positive Seebeck coefficient. The Seebeck coefficient, electrical resistivity, and thermal conductivity of M_1.3Mo₆S₈ (M: Cr, Mn, Fe) were almost identical and increased with temperature. However, the corresponding values and temperature dependent behavior of Ni_2.0Mo₆S₈ were different from those of M_1.3Mo₆S₈ (M: Cr, Mn, Fe). For Ni_2.0Mo₆S₈, as temperature increased, the Seebeck coefficient and thermal conductivity increased while the electrical resistivity decreased. The highest value of the thermoelectric figure of merit (0.17) was observed in Cr_1.3Mo₆S₈ at 973 K.     

First-Principles and Experimental Studies of Y-Doped Mg<Subscript>2</Subscript>Si Prepared Using Field-Activated Pressure-Assisted Synthesis

The thermoelectric properties of Y-doped (1000 ppm, 2000 ppm, 3000 ppm) Mg₂Si fabricated using field-activated pressure-assisted synthesis (FAPAS) have been characterized using measurements of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) at temperatures ranging from 285 K to 810 K. The Y-doped Mg₂Si samples were n-type in the measured temperature range. A first-principles calculation revealed that the Y atoms were expected to be primarily located at Mg sites. In sample doped with 2000 ppm Y, which exhibited the best electrical and thermal conductivity, the absolute value of the Seebeck coefficient increased in the temperature range of 320 K to 680 K, being higher than that of undoped Mg₂Si. Moreover, this sample exhibited a higher level of electrical conductivity and a higher power factor. In addition, introduction of Y decreased the thermal conductivity appreciably, indicating that Y dopants are favorable for improving the properties of Mg₂Si.     

Effect of pH on Structural and Electrical Properties of Electrodeposited Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Thin Films

Bi₂Te₃ thin films were electrodeposited at various pH values of a bismuth nitrate and tellurium oxide plating solution. Enhancement in pH results in a decrease in grain size. Transmission electron microscopy reveals the transformation of the film morphology from dispersed nanoparticles to connected chain-like nanostructures of Bi₂Te₃ as pH is increased. Electrical characterization for samples deposited in the temperature range of 300 K to 425 K shows a fourfold increase in Seebeck coefficient, S, between its maximum and minimum value as the solution pH changes from 1 to 3.5. Such enhancement of S is attributed to the increased connectivity of the nanostructures at higher pH.     

Thermoelectric Characterization of Zone-Melted and Quenched Zn<Subscript>4</Subscript>Sb<Subscript>3</Subscript>

Because of its complex structure, Zn₄Sb₃ exhibits relatively low thermal conductivity. This, in combination with large values of the Seebeck coefficient and moderate to high electrical conductivity, makes the material especially interesting for thermoelectric application in temperatures up to 400°C. The phase purity and thermal stability of Zn₄Sb₃ are major issues for its thermoelectric performance and are strongly dependent on the synthesis method, atmosphere, density, and grain size. Therefore, Zn₄Sb₃ was prepared by both zone melting and quenching in this study, and pressed samples from crushed powders of three different grain sizes were compared. The effect of thermal cycling was studied, along with repeated structural analysis and Seebeck mapping. It was found that zone melting leads to improved thermal stability regarding decomposition via Zn loss, which finally may result in the formation of ZnSb. Larger grain size seems to reduce the degradation, because of lower concentration of grain boundaries, thus hindering diffusion inside the material.     

Synthesis and Characterization of <Emphasis Type="Italic">p</Emphasis>-Type Pb<Subscript>0.5</Subscript>Sn<Subscript>0.5</Subscript>Te Thermoelectric Power Generation Elements by Mechanical Alloying

A mechanical alloying (MA) process to transform elemental powders into solid Pb_0.5Sn_0.5Te with thermoelectric functionality comparable to melt-alloyed material is described. The room-temperature doping level and mobility as well as temperature-dependent electrical conductivity, Seebeck coefficient, and thermal conductivity are reported. Estimated values of lattice thermal conductivity (0.7 W m⁻¹ K⁻¹) are lower than some reports of functional melt-alloyed PbSnTe-based material, providing evidence that MA can engender the combination of properties resulting in highly functional thermoelectric material. Though doping level and Sn composition have not been optimized, this material exhibits a ZT value >0.5 at 550 K.     

Effect of Heat Treatment on the Thermoelectric Properties of Bismuth–Antimony–Telluride Prepared by Mechanical Deformation and Mechanical Alloying

In this work, p-type 20%Bi₂Te₃–80%Sb₂Te₃ bulk thermoelectric (TE) materials were prepared by mechanical deformation (MD) of pre-melted ingot and by mechanical alloying (MA) of elemental Bi, Sb, and Te granules followed by cold-pressing. The dependence on annealing time of changes of microstructure and TE properties of the prepared samples, including Seebeck coefficient, electrical resistivity, thermal conductivity, and figure-of-merit, was investigated. For both samples, saturation of the Seebeck coefficient and electrical resistivity were observed after annealing for 1 h at 380°C. It is suggested that energy stored in samples prepared by both MA and MD facilitated their recrystallization within short annealing times. The 20%Bi₂Te₃–80%Sb₂Te₃ sample prepared by MA followed by heat treatment had higher a Seebeck coefficient and electrical resistivity than specimens fabricated by MD. Maximum figures-of-merit of 3.00 × 10^?3/K and 2.85 × 10^?3/K were achieved for samples prepared by MA and MD, respectively.     

Synthesis and electrical properties of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-based thermoelectric materials doped with Er,Tm, Yb,and Lu

Nanopowders of Bi₂Te₃ and R_0.1Bi_1.9Te₃ (where R = Er, Tm, Yb, Lu) are obtained by microwave solvothermal synthesis. The powder-like materials are compacted by cold isostatic compression followed by annealing in argon. The influence of the doping agent on the structure and characteristics of the derived materials are investigated. It is demonstrated that the introduction of rare-earth elements (2 at %) into the bismuth-telluride lattice leads to a decrease in the electrical resistivity and to an increase in the Seebeck coefficient. The best thermoelectric properties are obtained for the sample of bismuth telluride doped with thulium.     

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.     

Strain-Controlled Transport Mechanism in Strongly Correlated LaNiO<Subscript>3</Subscript>

A density functional theory + Hubbard U (DFT + U) method is employed to investigate the effect of strain on the electronic and transport properties of the correlated metal LaNiO₃. LaNiO₃ without strain is characterized by a low temperature Fermi liquid behaviour of resistivity, a negative Seebeck coefficient and a positive Hall coefficient. Density of states, resistivity, thermopower and Hall coefficient obtained within the DFT + U approach reveal that LaNiO₃ under both compressive and tensile strain is more metallic compared to the unstrained system. However, LaNiO₃ under tensile strain is found to be more strongly correlated than that under compressive strain. Electron localization function calculation shows that there is a substantial increase in the covalent part of the chemical bonding, which corroborates an increase in the resistivity for LaNiO₃ under tensile strain. Our first-principle–based calculation clearly demonstrates that the transport properties of LaNiO₃ can be tuned by applying suitable strain.     

Investigation of the Dielectric Permittivity and Electrical Conductivity of Ce<Subscript>2</Subscript>S<Subscript>3</Subscript>

The rare-earth semiconductor β-Ce₂S₃ compound samples were synthesized and their dielectric permittivity and electrical conductivity were measured in the temperature range 90–400 K. The energy-band structure has been determined. It is shown that the long-known large electrical parameter spread of semiconductor compounds close in composition to Ce₂S₃ is explained by the structure of impurity donor levels formed by cerium atoms and ions with different ionization degrees.     

Thermoelectric Properties of Layer-Antiferromagnet CuCrS<Subscript>2</Subscript>

We have performed a detailed study of the electrical and thermal conductivities and thermoelectric power behavior of an antiferromagnetic-layer compound of chromium, CuCrS₂, from 15 K to 300 K. Unlike previous studies, we find noninsulating properties and sensitive dependence on the preparation method, the microstructure, and the flaky texture formed in polycrystalline samples after extended sintering at high temperatures. Flakes are found to be metallic, with strong localization effects in the conductivity on cooling to low temperatures. The antiferromagnetic transition temperature T _N (=40 K) remains essentially unaffected. The Seebeck coefficient is found to be in the range of 150 μV/K to 450 μV/K, which is exceptionally large, and becomes temperature independent at high temperatures, even for specimens with low resistivity values of 5 mΩ cm to 200 mΩ cm. We find the thermal conductivity κ to be low, viz. 5 mW/K cm to 30 mW/K cm. This can be attributed mostly to the dominance of lattice conduction over electronic conduction. The value of κ is further reduced by disorder in Cu occupancy in the quenched phase. We also observe an unusually strong dip in κ at T _N, which is probably due to strong magnetocrystalline coupling in these compounds. Finally we discuss the properties of CuCrS₂ as a heavily doped Kondo-like insulator in its paramagnetic phase. The combination of the electronic properties observed in CuCrS₂ makes it a potential candidate for various thermoelectric applications.     

Cage-Shaped Mo<Subscript>9</Subscript> Chalcogenides: Promising Thermoelectric Materials with Significantly Low Thermal Conductivity

Thermoelectric properties of molybdenum selenides containing Mo₉ clusters have been investigated between 300 K and 800 K. Ag _x Mo₉Se₁₁ (x = 3.4 and 3.8) have been synthesized by solid-state reaction and spark plasma sintering. X-ray diffraction and scanning electron microscopy reveal high purity and good homogeneity of the samples. The thermoelectric power of the samples is positive over the whole investigated temperature range, indicating that the majority of charge carriers are holes. The Seebeck coefficient increases with temperature, and the temperature coefficient of the resistivity is positive. Significantly low thermal conductivity, comparable to values reported for state-of-the-art thermoelectric materials, is observed in this new system, and this is assumed to be associated with the rattling effect from the Ag filler atoms. It has been demonstrated that the electrical and thermal properties correlate to the Ag concentration. For x = 3.8, a promising dimensionless thermoelectric figure of merit of ∼0.7 is obtained at 800 K.     

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.     

Free-Standing PEDOT-PSS/Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9</Subscript> Composite Films as Novel Thermoelectric Materials

Free-standing poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT- PSS)/Ca₃Co₄O₉ composite films have been successfully prepared by mechanically blending Ca₃Co₄O₉ powder and PEDOT-PSS solution (Baytron P) and casting the mixed solution on polypropylene (PP) film substrates. X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterization indicated that the Ca₃Co₄O₉ particles were in the shape of sheets and composited well together with PEDOT-PSS. Thermoelectric (TE) measurements revealed that the Seebeck coefficient can be improved by increasing the Ca₃Co₄O₉ content in the composite films, with the largest enhancement being 24.8% compared with a free-standing PEDOT-PSS film. However, it is also shown that the power factor of the composite films decreases with increasing Ca₃Co₄O₉ content, mainly due to the decline of electrical conductivity and the limited improvement of the Seebeck coefficient.     

Low-temperature deposition of SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Films in SiH<Subscript>4</Subscript>/Ar + N<Subscript>2</Subscript> inductively coupled plasma under high silane dilution with argon

SiN_x films on silicon are grown in SiH₄/Ar + N₂ inductively coupled plasma under high silane dilution with argon. The dependences of the deposition rate and properties of silicon nitride on the plasmagas composition, pressure, radio-frequency power and inductive power are studied. In some cases, the found dependences differ from published data for undiluted reagents. It is found that the lowest impurity content in films and their best properties are implemented at a nitrogen-to-silane ratio close to 1.4. An increase in the radio-frequency power results in smoother samples due to the polishing effect of argon ions.     

Thermoelectric Properties of the Entire Composition Range in Mg2Si0.9925−x Sn x Sb0.0075

Eleven samples of nominal composition Mg_2.2Si_0.9925?x Sn _x Sb_0.0075 with x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.9925 were prepared by induction melting, ball milling, and spark plasma sintering. Hall effect, resistivity, Seebeck coefficient, and thermal conductivity measurements were conducted from room temperature to 400°C. Six of the samples were investigated for thermal stability by measuring powder x-ray diffraction while heating to 400°C. All samples were stable in air during the ～12-h-long data collection, except for Mg_2.2Sn_0.9925Sb_0.0075, which showed development of an elemental Sn phase after heating. The lattice parameter of each sample was extracted through Rietveld refinement and revealed a linear dependency on nominal composition. Measurement of top and bottom of the pellets exhibited systematic differences in lattice parameter and Seebeck coefficient, indicating that stoichiometry gradients are created during sintering.     

Thermoelectric Properties of NaCo<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

NaCo₂O₄ has one of the highest figures of merit among all ceramic thermoelectric materials. Because of its large thermopower and low resistivity, the ceramic oxide NaCo₂O₄ is a promising candidate for potential thermoelectric applications. NaCo₂O₄ is, moreover, a ceramic compound with high decomposition temperature and chemical stability in air and it does not contain any toxic elements. Like all 3-d transition ions, Co ions have multiple spin and oxidation states. In this investigation, thermopower and electrical conductivity of NaCo₂O₄ as a function of substitution of Co by Fe ions were measured. Fe substitution for Co causes resistivity to increase, whereas the Seebeck coefficient remained nearly invariant, especially above 330 K. An erratum to this article can be found at