Effects of nano-TiO<Subscript>2</Subscript> dispersion on thermoelectric properties of Co<Subscript>4</Subscript>Sb<Subscript>11.7</Subscript>Te<Subscript>0.3</Subscript> composites

Nano-TiO2/Co4Sb11.7Te0.3 composites were prepared by mechanical alloying (MA) and cold isostatic pressing (CIP) process.The phase composition,microstructure,and thermoelectric properties were characterized.The diffraction spectra of all samples well corresponds to CoSb3 skutterudite diffraction plane.TiO2 agglomerates into irregular clusters.They locate at the grain boundaries or some are distributed on the surface of Co4Sb11.7Te0.3 particles.For composites with high TiO2 content (0.6% and 1.0% TiO2),the phonon scattering by TiO2 particle,pores,and small size grains can result in a remarkable reduction in thermal conductivity.The maximum value of ZT is 0.79 for sample with 0.6 wt.% TiO2 at 700 K,which is 11% higher than that of non-dispersed sample.     

Thermoelectric properties of In<Subscript>z</Subscript>Co<Subscript>4</Subscript>Sb<Subscript>12</Subscript> skutterudites

In this study, indium-filled CoSb₃ skutterudite is synthesized via encapsulated induction melting and subsequent annealing at 823 K for six days, and the crystal structure, lattice constant, filler position, phase homogeneity and stability were investigated. All of the In-filled CoSb₃ samples were n-type conducting samples. The temperature dependence of the electrical resistivity showed In_zCo₄Sb₁₂ is a highly degenerate semiconducting material. The thermal conductivity was reduced considerably by In filling. The highest thermoelectric figure of merit value was achieved when the In filling fraction is 0.25. It was found that the ZT of the In-filled CoSb₃ (In_zCo₄Sb₁₂) was higher than that of the In-substituted CoSb₃ (Co_3.75In_0.25Sb₁₂ and Co₄Sb_11.75In_0.25). This is mainly due to the lower thermal conductivity and higher Seebeck coefficient.     

Thermoelectric Properties of Cu-doped Bi<Subscript>2−x</Subscript>Sb<Subscript>x</Subscript>Te<Subscript>3</Subscript> Prepared by Encapsulated Melting and Hot Pressing

P-type Bi_2?xSb_xTe₃:Cu_m (x = 1.5–1.7 and m = 0.002–0.003) solid solutions were synthesized using encapsulated melting and were consolidated using hot pressing. The effects of Sb substitution and Cu doping on the charge transport and thermoelectric properties were examined. The lattice constants decreased with increasing Sb and Cu contents. As the amount of Sb substitution and Cu doping was increased, the electrical conductivity increased, and the Seebeck coefficient decreased owing to the increase in the carrier concentration. All specimens exhibited degenerate semiconductor characteristics and positive Hall and Seebeck coefficients, indicating p-type conduction. The increased Sb substitution caused a shift in the onset temperature of the intrinsic transition and bipolar conduction to higher temperatures. The electronic thermal conductivity increased with increasing Sb and Cu contents owing to the increase in the carrier concentration, while the lattice thermal conductivity slightly decreased due to alloy scattering. A maximum figure of merit, ZT_max = 1.25, was achieved at 373 K for Bi_0.4Sb_1.6Te₃:Cu_0.003.     

Microstructure and Thermal Properties of Atmospheric Plasma-Sprayed Yb<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> Coating

In the present work, Yb₂Si₂O₇ powder was synthesized by solid-state reaction using Yb₂O₃ and SiO₂ powders as starting materials. Atmospheric plasma spray technique was applied to fabricate Yb₂Si₂O₇ coating. The phase composition and microstructure of the coating were characterized. The density, open porosity and Vickers hardness of the coating were investigated. Its thermal stability was evaluated by thermogravimetry and differential thermal analysis (TG-DTA). The thermal diffusivity and thermal conductivity of the coating were measured. The results showed that the as-sprayed coating was mainly composed of crystalline Yb₂Si₂O₇ with amorphous phase. The coating had a dense structure containing defects, such as pores, interfaces and microcracks. The TG-DTA results showed that there was almost no mass change from room temperature to 1200 °C, while a sharp exothermic peak appeared at around 1038 °C in DTA curve, which indicated that the amorphous phase crystallized. The thermal conductivity of the coating decreased with rise in temperature up to 600 °C and then followed by an increase at higher temperatures. The minimum value of the thermal conductivity of the Yb₂Si₂O₇ coating was about 0.68 W/(m K).     

Microstructure and Thermomechanical Properties of Atmospheric Plasma-Sprayed Yb<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating

In this study, a Yb₂O₃ coating was fabricated by the atmospheric plasma spray technique. The phase composition, microstructure, and thermal stability of the coating were examined. The thermal conductivity and thermal expansion behavior were also investigated. Some of the mechanical properties (elastic modulus, hardness, fracture toughness, and flexural strength) were characterized. The results reveal that the Yb₂O₃ coating is predominantly composed of the cubic Yb₂O₃ phase, and it has a dense lamellar microstructure containing defects. No mass change and exothermic phenomena are observed in the thermogravimetry and differential thermal analysis curves. The high-temperature x-ray diffraction results indicate that no phase transformation occurs from room temperature to 1500 °C, revealing the good phase stability of the Yb₂O₃ coating. The coefficient of thermal expansion of the Yb₂O₃ coating is (7.50-8.67)?×?10^?6 K^?1 in the range of 200-1400 °C. The thermal conductivity is about 1.5 W m^?1 K^?1 at 1200 °C. The Yb₂O₃ coating has excellent mechanical properties and good damage tolerant. The unique combination of these properties implies that the Yb₂O₃ coating might be a promising candidate for T/EBCs applications.     

High thermoelectric performance of Yb0.26Co4Sb12/yGaSb nanocomposites originating from scattering electrons of low energy

n-type filled skutterudite Yb_0.26Co₄Sb₁₂ composites in which p-type GaSb nanostructured inclusions (5–20 nm) were dispersed were fabricated by an in situ method involving the introduction of metastable void-filling impurity Ga and enrichment of Sb in the synthesis procedure. With the homogeneously dispersed GaSb nanoinclusions, which probably result from the scattering of low-energy electrons by the GaSb-related boundary energy barriers, the power factor is enhanced due to the significant enhancement of the Seebeck coefficient. The total thermal conductivity was decreased with the depression of electronic thermal conductivity. As a result, the dimensionless thermoelectric figure of merit ZT value was improved over a wide working temperature range of 300–850 K, and the expected optimal thermal-electric conversion efficiency η_opt 15.0% was obtained for the Yb_0.26Co₄Sb₁₂/0.2GaSb nanocomposite.     

Solvothermal synthesis and thermoelectric properties of skutterudite compound Fe（0.25）Ni（0.25）Co（0.5）Sb3

Nanostructured skutterudite-related compound Fe_0.25Ni_0.25Co_0.5Sb₃ was synthesized by a solvothermal method using FeCl₃, NiCl₂, CoCl₂, and SbCl₃ as the precursors and NaBH₄ as the reductant. The solvothermally synthesized powders consisted of fine granules with an average particle size of tens of nanometers. The bulk material was prepared by hot pressing the powders. Transport property measurements indicated a heavily doped semiconductor behavior with n-type conduction. The thermal conductivity is about 1.83 W·m⁻¹·K⁻¹ at room temperature and decreases to 1.57 W·m⁻¹·K⁻¹ at 673 K. The low thermal conductivity is attributed to small grain size and high porosity. A maximum dimensionless figure of merit of 0.15 is obtained at 673 K.     

Synthesis and thermoelectric properties of double-filled skutterudites CeyYb0.5−yFe1.5Co2.5Sb12

Ce–Yb double-filled skutterudites Ce_0.5?yYb_yFe_1.5Co_2.5Sb₁₂ (y = 0.1, 0.2, 0.4 and 0.5) were synthesized by a melting method with subsequent annealing. The thermal conductivity, electrical conductivity and Seebeck coefficient were measured from room temperature up to 773 K. The thermal conductivities of all the double-filled skutterudites were found to be lower than the Yb single-filled skutterudites. An enhancement in the dimensionless thermoelectric figure of merit ZT was also observed in all the double-filled skutterudites as compared to the Yb single-filled skutterudite. Ce_0.3Yb_0.2Fe_1.5Co_2.5Sb₁₂ has the highest dimensionless figure of merit ZT of 0.32 at 723 K, which is 55% higher than the Yb single-filled skutterudite at the same temperature.     

Transport properties on Sn-filled and Te-doped CoSb<Subscript>3</Subscript> skutterudites

Sn-filled and Te-doped CoSb₃ skutterudites (Sn_xCo₈Sb_23.25Te_0.75) were synthesized by the encapsulated induction melting process. Single δ-phase was successfully obtained by subsequent heat treatment at 823 K for 6 days. Structural characterizations were carried out through X-ray diffraction studies. Transport properties such as the Seebeck coefficient, electrical resistivity, thermal conductivity, carrier concentration and mobility were measured and analyzed. The unfilled Co₈Sb_23.25Te_0.75 sample showed n-type conductivity from 300 K to 700K. However, the Sn-filled Sn_xCo₈Sb_23.25Te_0.75 showed n-type conductivity for z=0.25 and 0.5, and p-type conductivity for z=1.0 and 1.5 from 300 K to 700 K. Thermal conductivity was reduced by the impurity-phonon scattering. The dimensionless figure of merit (ZT) was remarkably improved over that of untreated CoSb₃. However, the ZT value decreased when filling with z≥1.0 because the conductivity type was changed from n-type to p-type, thereby allowing bipolar conduction. The details are discussed in terms of the two-band model and the bipolar thermoelectric effect.     

Microstructure and Thermophysical Properties of SrZrO<Subscript>3</Subscript> Thermal Barrier Coating Prepared by Solution Precursor Plasma Spray

Strontium zirconate (SrZrO₃) thermal barrier coatings were deposited by solution precursor plasma spray (SPPS) using an aqueous precursor solution. The phase transition of the SrZrO₃ coating and the influence of the aging time at 1400 °C on the microstructure, phase stability, thermal expansion coefficient, and thermal conductivity of the coating were investigated. The unique features of SPPS coatings, such as interpass boundary (IPB) structures, nano- and micrometer porosity, and through-thickness vertical cracks, were clearly observed evidently in the coatings. The vertical cracks of the coatings remained substantially unchanged while the IPB structures gradually diminished with prolonged heat treatment time. t-ZrO₂ developed in the coatings transformed completely to m-ZrO₂ phase after heat treatment for 100 h. Meanwhile, the SrZrO₃ phase in the coatings exhibited good phase stability upon heat treatment. Three phase transitions in the SrZrO₃ coatings were revealed by thermal expansion measurements. The thermal conductivity of the as-sprayed SrZrO₃ coating was ~1.25 W m^?1 K^?1 at 1000 °C and remained stable after heat treatment at 1400 °C for 360 h, revealing good sintering resistance.     

Thermoelectric properties of P-type Yb-filled skutterudite YbxFeyCo4-ySb12

While intensive work has been done on n-type Yb filled skutterudites in the past, very little is known about their p-type counterparts for potential applications as thermoelectric materials. In this paper, we report a systematic study of high temperature thermoelectric transport properties of p-type Yb-filled Fe-compensated skutterudites Yb_xFe_yCo_4-ySb₁₂ with the aim to complement the knowledge base for the Yb-filled skutterudite family. The highest ZT_max = 0.6 was found in Yb_0.6Fe₂Co₂Sb₁₂ at 782 K. YbFe₄Sb₁₂ exhibits the second highest ZT_max = 0.57 at 780 K, which is much higher than the previous estimate of 0.4 for the same composition.     

Effect of Titanium Additions on the Thermophysical Properties of Glass-Forming Cu<Subscript>50</Subscript>Zr<Subscript>50</Subscript> Alloy

Differential scanning calorimetry, laser flash method, and dilatometry were used to study the thermophysical properties of quenched Cu₅₀Zr_50–xTi_x (x = 0, 2, 4, 6, 8) alloys in the temperature range from room temperature to 1100 K. Data obtained on the heat capacity, thermal diffusivity, and density have been used to calculate the coefficient of thermal conductivity. Temperatures corresponding to the stability of martensite CuZr phase, its eutectoid decomposition, and formation in Cu₅₀Zr_50–xTi_x alloys with different Ti contents upon heating have been determined. It has been found that the thermal diffusivity and thermal conductivity of the studied alloys are low and a typical of metallic systems. As the titanium content increases, the coefficients of thermal conductivity and thermal diffusivity vary slightly. It has been shown that the low values of thermophysical characteristics correspond to the better capability of amorphization and can be a criterion for the glass-forming ability of Cu–Zr-based alloys.     

High temperature thermoelectric properties of skutterudite-Bi2Te3 nanocomposites

Nanocomposite engineering has been proved effective in diverse regimes of material research to attain a performance beyond each constituent phase. In this work, Yb-filled CoSb₃ (bulk matrix/host)-Bi_0.4Sb_1.6Te₃ (secondary inclusion) thermoelectric nanocomposites have been synthesized via an ex situ process. Bi_0.4Sb_1.6Te₃ inclusions are mainly distributed at the grain boundaries of Yb_0.2Co₄Sb₁₂ matrix in the composites. In particular, Te diffuses in situ from Bi_0.4Sb_1.6Te₃ through Yb_0.2Co₄Sb₁₂ matrix during the hot pressing process. This, combined with the grain boundary effect, results in favorable changes in the carrier concentration, carrier mobility, electrical resistivity, Seebeck coefficient, and thermal conductivity. Such synergistic changes are notably absent in the stand-alone Te-doped Yb-filled CoSb₃, suggesting the key role of diffusion and grain boundaries. As a result, a maximum ZT value of 0.96 has been attained for Yb_0.2Co₄Sb₁₂-2 wt% Bi_0.4Sb_1.6Te₃ at 650 K. The present work opens a new avenue towards high performance thermoelectric composites via controlled inter-constituent diffusion and grain boundary effect.     

Magnetoresistive properties of Ni-doped La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganites

La_0.7Sr_0.3Mn_1?x Ni _x O₃ (x = 0, 0.025, 0.050 and 0.075) ceramics were prepared by the conventional solid-state reaction method. The partial substitution of Mn by Ni²⁺ leads to a decrease in cell volume as well as a structural transition from the rhombohedral to the orthorhombic structure. Ni²⁺ doping increases the electrical resistivity, decreases the semiconductor–metal transition temperature (T _ms) and relatively enhances the room temperature magnetoresistance (MR), especially in x = 0.025 and around T _ms. With respect to conduction mechanism, the small polaron hopping (SPH) and the variable range hopping (VRH) models were used to examine conduction in the semiconducting region.     

Properties of the Ti<Subscript>40</Subscript>Zr<Subscript>10</Subscript>Cu<Subscript>36</Subscript>Pd<Subscript>14</Subscript> BMG Modified by Sn and Nb Additions

The results of investigation of the influence of additions of 2 and 3 at.% of Sn and simultaneously of Sn and 3 at.% Nb on microstructure and properties of the bulk metallic glasses of composition (Ti₄₀Cu_36?x Zr₁₀Pd₁₄Sn _x )_100?y Nb _y are reported. It was found that the additions of Sn increased the temperatures of glass transition (T _g), primary crystallization (T _x ), melting, and liquidus as well as supercooled liquid range (ΔT) and glass forming ability (GFA). The nanohardness and elastic modulus decreased in alloys with 2 and 3 at.% Sn additions, revealing similar values. The 3 at.% Nb addition to the Sn-containing amorphous phase decreased as well all the T _g, T _x , T _L, and T _m temperatures as ΔT and GFA; however, relatively larger values of this parameters in alloys containing larger Sn content were preserved. In difference to the previously published results, in the case of the amorphous alloys containing small Nb and Sn additions, a noticeable amount of the quenched-in crystalline phases was not confirmed, at least of the micrometric sizes. In the case of the alloys containing Sn or both Sn and Nb, two slightly different amorphous phase compositions were detected, suggesting separation in the liquid phase. Phase composition of the alloys determined after amorphous phase crystallization was similar for all compositions. The phases Cu₈Zr_3, CuTiZr, and Pd₃Zr were mainly identified in the proportions dependent on the alloy compositions.     

A comparison of the local structure in ball-milled and hand ground skutterudite samples using EXAFS

Skutterudites are considered to be good thermoelectrics with high figures of merit, ZT. After synthesis, electrodes are created by grinding and hot pressing the resulting powder. Materials such as NdFe₄Sb₁₂ exhibit a significantly greater figure of merit, ZT, (about 43%) when it is ball milled to produce fine powders (inital powder 160 nm; after hot pressing ∼330 nm). This enhancement is typically attributed to the reduced particle size, which in turn decreases the mean free path of phonons, and consequently decreases the thermal conductivity. This work aims to investigate whether there is any damage to the crystal structure in the particles formed by ball milling, which could also affect its thermal conductivity. Using a temperature dependent, Extended X-ray Absorption Fine Structure (EXAFS) analysis of a hand ground and ball milled sample of the skutterudite Nd_yFe₄Sb₁₂, we have determined that ball milling causes no significant damage to the local structure around any site. Consequently further improvements in ZT may be possible with smaller particles.     

Thermoelectric properties of n-type Bi<Subscript>2</Subscript>Te<Subscript>2.7</Subscript>Se<Subscript>0.3</Subscript> compounds prepared by spark plasma sintering

In this study, the thermoelectric properties of 0.1 wt.% Cdl₂-doped n-type Bi₂Te_2.7Sb_0.3 compounds, fabrieated by SPS in a temperature range of 250°C to 350°C, were characterized. The density of the compounds was increased to approximately 100% of the theoretical density by carrying out consolidation at 350°C. The Seebeck coefficient, thermal conductivity, and electrical resistivity were dependent on a hydrogen reduction process and the sintering temperature. The Seebeck coefficient and the electrical resistivity increased with the reduction process. Also, electrical resistivity decreased and thermal conductivity increased with sintering temperature. The results suggest that carrier density and mobility vary according to the reduction process and sintering temperature. The highest figure of merit, 1.93×10⁻³ K⁻¹, was obtained for the compound consolidated at 350°C for 2 min.     

Hydrogen storage properties of as-cast and annealed low-Co La<Subscript>1.8</Subscript>Ti<Subscript>0.2</Subscript>MgNi<Subscript>8.9</Subscript>Co<Subscript>0.1</Subscript> alloys

Low-Co La_1.8Ti_0.2MgNi_8.9Co_0.1 alloys were prepared by magnetic levitation melting followed by annealing treatment. The effect of annealing on the hydrogen storage properties of the alloys was investigated systematically by X-ray diffraction (XRD), pressure-composition isotherm (PCI), and electrochemical measurements. The results show that all samples contain LaNi₅ and LaMg₂Ni₉ phases. LaCo₅ phase appears at 1,000 °C. The enthalpy change of all hydrides is close to ?30.6 kJ·mol^?1 H₂ of LaNi₅ compound. Annealing not only increases hydrogen capacity and improves cycling stability but also decreases plateau pressure at 800 and 900 °C. After annealing, the contraction of cell volume and the increase of hydride stability cause the high rate dischargeability to reduce slightly. The optimum alloy is found to be one annealed at 900 °C, with its hydrogen capacity reaching up to 1.53 wt%, and discharge capacity remaining 225.1 mAh·g^?1 after 140 charge–discharge cycles.     

Structure,magnetic and magnetocaloric properties of nonstoichiometric TbCo<Subscript>2</Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> compounds

The structure and magnetic and magnetocaloric properties of new nonstoichiometric TbCo₂Ni _x compounds (0 ≤ x ≤ 0.4) have been studied. The alloys with х ≤ 0.1 have been shown to be single-phase with the MgCu₂-type structure; in alloys with х > 0.1, an additional phase with a PuNi₃-type structure has been formed. It has been found that the concentration dependences of the Curie temperature and magnetic moment of the 3d-metal sublattice have a maximum at x = 0.025. The magnetocaloric effect magnitude for the TbCo2Nix compounds has been estimated using the results of magnetic and heat-capacity measurements.     

Radiation Stability of Metal Fe<Subscript>0.56</Subscript>Ni<Subscript>0.44</Subscript> Nanowires Exposed to Powerful Pulsed Ion Beams

The resistance of Fe_0.56Ni_0.44 alloy nanowires (fabricated by template synthesis using polymer track membranes) 60 and 100 nm in diameter to radiation with powerful pulsed 85% C⁺ + 15% H⁺ ions (E = 20 keV, j = 100 A/cm², τ = 90 ns) has been investigated. The conclusion that nanosized regions of explosive energy release, so-called thermal spikes, which are thermalized regions of dense cascades of atomic displacements heated to several thousand degrees (in which the thermal pressure can reach several tens of GPa), play an important role in the nanowire structure change is drawn. These are observed as melted nanosized regions on the nanowire surface. Calculations have shown that energy supplied by an ion beam during the action of a single pulse in the used mode (provided that thermal radiation and thermal conductivity serve as energy sinks) can be both sufficient and insufficient to completely melt nanowires depending on their orientation with respect to the ion beam. The bending and failure of nonmelted nanowires is explained by the generation and propagation of post-cascade shock waves.