Thermophysical properties of FeAl (Fe-40 at.%Al)

The thermophysical properties — electrical resistivity, thermal conductivity, thermal expansion, and specific heat, of a B2 iron-aluminide (Fe-40 at.% Al) alloy are measured. The measured values of electrical resistivity indicate three distinct regions. An initial sharp rise below 400°C is followed by a gradual increase to near saturation around 900°C. Resistivity above this temperature exhibits an anomalous negative temperature dependence. The thermal conductivity displays a continuous rise as a function of temperature for T<800°C, beyond which it saturates to a value of 0.17 W/cm-°C. The relation between electrical resistivity and thermal conductivity obeys the Wiedemann-Franz law signifying the dominance of electrons in the heat transport. The measurements of specific heat indicate a complex behavior suggesting inseparable contributions of various temperature dependent phenomena arising from phonons, conduction electrons and magnons. Both the thermal expansion and mean coefficient of thermal expansion (MCT) exhibit a rising trend with temperature. The temperature dependence of the various modes of lattice, electronic, and magnetic excitations is invoked to explain the observed variations in properties. The role of the inherent electronic and magnetic structure on physical properties is highlighted.     

Thermoelectric properties of FeVSb half-Heusler compounds by levitation melting and spark plasma sintering

Half-Heusler compound FeVSb has been successfully synthesized by levitation melting followed by spark plasma sintering and annealing process. The phase transformation of FeVSb in the fabricating process has been studied, and pure FeVSb half-Heusler phase was obtained. Thermoelectric properties of pure FeVSb with different relative densities have been investigated, and the porosity dependence of thermal conductivity of FeVSb at 300 K follows Maxwell-Eucken equation well. A maximum power factor of 48 × 10⁻⁴ W m⁻¹ K⁻² was obtained at 350 K from the sample with the highest relative density of 97%. A state-of-the-art ZT value of ∼0.25 has been attained at 550 K for pure FeVSb, which is one of the highest reported ZT value for FeVSb half-Heusler compound.     

Processing and characterization of porous Ti2AlC with controlled porosity and pore size

In this work, we demonstrate a simple and inexpensive way to fabricate porous Ti₂AlC, one of the best studied materials from the MAX phase family, with controlled porosity and pore size. This was achieved by using NaCl as the pore former, which was dissolved after cold pressing but before pressureless sintering at 1400 °C. Porous Ti₂AlC with samples a volume fraction of porosity ranging from ～10 to ～71 vol.% and different pore size ranges, i.e. 42–83, 77–276 and 167–545 μm, were successfully fabricated. Fabricated samples were systematically characterized to determine their phase composition, morphology and porosity. Room temperature elastic moduli, compressive strength and thermal conductivity were determined as a function of porosity and/or pore size. For comparison, several samples pressureless-sintered without NaCl pore former, or fabricated by spark plasma sintering, were also characterized. The effects of porosity and/or pore size on the room temperature elastic moduli, compressive strength and thermal conductivity of porous Ti₂AlC are reported and discussed in this work. It follows that porosity can be a useful microstructural parameter to tune mechanical and thermal properties of Ti₂AlC.     

A new semiconductor Al2Fe3Si3 with complex crystal structure

Al₂Fe₃Si₃, a new semiconductor with complex triclinic structure was synthesized by arc melting and spark plasma sintering, followed by heat treatment. The nominal compositions of samples have been changed to compensate Al evaporation during synthesis process, and single Al₂Fe₃Si₃ phase has been obtained with the nominal composition of Al: Fe: Si = 26: 37: 37 (6 at.% Al excess against stoichiometry). In this study, we measured the sound velocity, thermal expansion coefficient, Vickers hardness, fracture toughness, electrical conductivity, Seebeck coefficient, and thermal conductivity of the new semiconductor Al₂Fe₃Si₃. The Al₂Fe₃Si₃ sample displayed positive Seebeck coefficient from 300 to 850 K, with a maximum Seebeck coefficient of 110 μV/K at 430 K. The Debye temperature of Al₂Fe₃Si₃ was 640 K, which was similar to or higher than those of other Al, Fe, Si based thermoelectric materials, but the lattice thermal conductivity was lower, 4–5 W/mK, due to the complex crystal structure of Al₂Fe₃Si₃. The maximum ZT value was 0.06 at 580 K.     

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.     

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.     

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.     

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.     

Assessing Coating Reliability Through Pore Architecture Evaluation

Plasma-sprayed thermal barrier coatings (TBCs) exhibit many interlamellar pores, voids, and microcracks. These microstructural features are primarily responsible for the low global stiffness and the low thermal conductivity commonly exhibited by such coatings. The pore architecture thus has an important influence on such thermophysical properties. In the present work, the effect of heat treatment (at temperatures up to 1400 °C, for times of up to 20 h) on the pore architecture of detached YSZ top coats with different impurity levels have been characterized by mercury intrusion porosimetry and gas-sorption techniques. Stiffness and thermal conductivity were also monitored to assess the effect of change in pore architecture on properties. While the overall porosity level remained relatively unaffected (at around 10-12%) after the heat treatments concerned, there were substantial changes in the pore size distribution and the (surface-connected) specific surface area. Fine pores (<~50 nm) rapidly disappeared, while the specific surface area dropped dramatically, particularly at high-treatment temperatures (~1400 °C). These changes are thought to be associated with intrasplat microcrack healing, improved intersplat bonding and increased contact area, leading to disappearance of much of the fine porosity. These microstructural changes are reflected in sharply increased stiffness and thermal conductivity. Increase in thermal conductivity and stiffness were found to be more pronounced for coatings with higher impurity content (particularly alumina and silica). Reliability issues surrounding such increase in thermal conductivity and stiffness are discussed along with a brief note on the effect of impurities on TBC life.     

Effect of high-temperature aging on the thermal conductivity of nanocrystalline tetragonal yttria-stabilized zirconia

The thermal conductivity of yttria-stabilized zirconia (YSZ) thermal barrier coatings increases with high-temperature aging. This common observation has been attributed to the densification of the coatings as porosity sinters out and pores and cracks spheroidize to minimize their surface energy. We show that the thermal conductivity of fully-dense 3 mol.% Y₂O₃ stabilized zirconia (3YSZ) also increases with high-temperature aging, indicating that densification and pore shape changes alone are not responsible for all the observed increase in thermal conductivity of coatings. Instead, there are also increases due to a combination of phase separation and grain growth. The increase in thermal conductivity can be described by a Larson–Miller parameter. It is also found that the increase in thermal conductivity with aging is greatest when measured at room temperature and decreases with increasing measurement temperature. Measured at 1000 °C, the thermal conductivity of zirconia is almost temperature independent and the changes in thermal conductivity with aging are less than 15%, even after aging for 50 h at 1400 °C.     

Thermal and mechanical properties of perovskite-type barium hafnate

Polycrystalline samples of the barium perovskite-type oxide, BaHfO₃ were prepared by solid-state reactions from HfO₂ and BaCO₃ powders. The thermal expansion coefficient, heat capacity, thermal diffusivity, thermal conductivity, elastic modulus, Debye temperature, and micro-Vickers hardness were measured. The crystal structure of BaHfO₃ is of the cubic perovskite type with the lattice parameter 0.4171 nm at room temperature. The sample bulk density is 91% of the theoretical density. The average linear thermal expansion coefficient is 6.93 × 10⁻⁶ K⁻¹ in the temperature range between 300 and 1500 K. The Young's modulus equals 194 GPa. The thermal conductivity at room temperature is 10.4 Wm⁻¹K⁻¹.     

Thermophysical properties of lanthanum zirconate coating prepared by plasma spraying and the influence of post-annealing

Nano-scaled lanthanum zirconate powder prepared by co-precipitation–calcination method was plasma-sprayed into a thick coating on an alloy substrate. We investigated the thermophysical properties of the free-standing coating, including thermal conductivity and thermal expansion coefficients (TECs). Minimum value of the thermal conductivity (at 900 °C) of the coating was about 0.73 W m⁻¹ K⁻¹, and the average TEC in the measurement range was about 9.45 × 10⁻⁶ K⁻¹. Although the TEC value was similar to that of the bulk material, the change tendency versus temperature was different. After annealing at 1300 or 1400 °C for 50 h, we found that the heat insulation performance of the coating decreased with the heat treatment temperature, while the hardness and fracture toughness increased. A peak of sudden decrease in TEC value can be observed in the curve, and with increasing temperature, the peak shifted to high-temperature direction.     

Grain boundary diffusion and precipitates in B2 Ti−50.2 at.% Ni alloy

Grain boundary diffusion of ⁴⁴Ti and ⁶³Ni in the B2 Ti−50.2 at.% Ni polycrystalline alloy was measured in Harrison's B regime (573−923 K) using the radiotracer technique. The triple product P = sδD_gb (s is the segregation factor, δ the grain boundary width, and D_gb the corresponding grain diffusion coefficient) for Ti and Ni was determined. Although the absolute values of the triple products P are typical for the B2-ordered alloys, both Ti and Ni GB diffusion in NiTi reveals a unique behavior with significant deviations from a linear Arrhenius-type temperature dependence. Transmission electron microscopy analysis of GB structures at 673 K and 923 K substantiated the occurrence of different interface types which may provide the slower and faster grain boundary diffusion paths in agreement with the experimental data. The influence of different types of precipitates on grain boundary diffusion and possible diffusion mechanisms in the different regimes are discussed.     

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.     

Influence of microstructure on the thermophysical properties of sintered SiC ceramics

The specific heat, thermal diffusivity and thermal conductivity of porous SiC ceramics sintered using two kinds of SiC powders (fine and coarse) have been investigated for sintering temperatures in the range 1700–2000 °C. Sintered SiC has a porous structure with approximately 30–40 vol.% porosity. Thermal diffusivity was measured by the laser flash method. The thermal diffusivities and thermal conductivities of sintered SiC ceramics increased with increasing sintering temperature. The specific heat decreased slightly with increasing sintering temperature. The thermal diffusivities and thermal conductivities of SiC sintered from coarse powder were higher than those of SiC sintered from fine powder. The thermal conductivity of samples increased markedly with increasing grain size.     

Polycrystalline elastic moduli of a high-entropy alloy at cryogenic temperatures

CrMnCoFeNi is a FCC high-entropy alloy (HEA) that exhibits strong temperature dependence of strength at low homologous temperatures in sharp contrast to pure FCC metals like Ni that show weak temperature dependence. To understand this behavior, elastic constants were determined as a function of temperature. From 300 K down to 55 K, the shear modulus (G) of the HEA changes by only 8%, increasing from 80 to 86 GPa. This temperature dependence is weaker than that of FCC Ni, whose G increases by 12% (81–91 GPa). Therefore, the uncharacteristic temperature-dependence of the strength of the HEA is not due to the temperature dependence of its shear modulus.     

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.     

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).     

多孔铂电极的蓄热传热性能及热稳定性研究

本研究应用固体物理理论和方法,研究了氧传感器多孔Pt电极材料的热容量、热导率等蓄热传热性能及其热稳定性随温度、时间和晶粒半径的变化规律,探讨了原子非简谐振动对电极材料蓄热传热性能及热稳定性的影响。研究表明,多孔Pt电极的定容热容量随温度的升高先增大后趋于恒定,随晶粒半径和时间的增大而减小;多孔Pt电极的蓄热性能热稳定性系数随温度的升高先急剧增大后迅速减小,最后趋于恒定,在温度约60 K时,其蓄热性能热稳定性最差;多孔Pt电极的热导率随温度的升高先急剧减小后趋于恒定,随晶粒半径的增加而增大,随时间的增长而减小;表面层对热导率的贡献随温度升高先急剧减小后趋于0;多孔Pt电极原子振动的非简谐效应使其热容量有所减小,而使蓄热性能热稳定性系数和热导率有所增大。本研究所得结果与其他文献的结果基本一致,其结论可为固体电解质氧传感器的稳定性问题提供理论指导。     

Effects of LaB6 addition on arc-jet convectively heated SiC-containing ZrB2-based ultra-high temperature ceramics in high enthalpy supersonic airflows

The performances of a ZrB₂–SiC–LaB₆ ultra-high temperature ceramic (UHTC) was investigated in high enthalpy supersonic air flow. The UHTC material reached and maintained steady-state radiative surface temperature of 1973 K (monitored by pyrometer) for 5 min, and survived the arc-jet plasma exposure without any optical evidence of mechanical damage. The oxide scale covering externally the sample evolved into different textures. Transient thermal analysis processed via CFD provided a good agreement between the numerical steady-state temperature and the temperature distribution obtained experimentally. The contemporary addition of LaB₆ and SiC to ZrB₂ had a detrimental effect on the overall oxidation resistance.