High-temperature thermoelectric properties of p-type skutterudites Ba0.15Yb x Co3FeSb12 and Yb y Co3FeSb9As3

Two series of p-type polycrystalline skutterudites, Ba_0.15Yb _x Co₃FeSb₁₂ and Yb _y Co₃FeSb₉As₃ with varying Yb concentrations, were synthesized by solid-state reaction and then densified by hot pressing. The phase and stoichiometries of the resulting materials were characterized by powder X-ray diffraction and energy dispersive spectroscopy, while their high-temperature transport properties were investigated from 300 to 800 K. The Seebeck coefficients and electrical resistivities increased linearly with increasing temperature for the double-filled specimens. The Seebeck coefficients and electrical resistivities did not change very much for the As-substituted specimens. The thermal conductivity for all specimens decreased with increasing temperature up to 700 K, corresponding to the plateau in the Seebeck coefficient, and then increased again due to bipolar diffusion. We find that double filling is a more feasible approach to thermoelectric property optimization than single filling with As substitution.     

Breakdown of phonon glass paradigm in La- and Ce-filled Fe4Sb12 skutterudites

The material class of skutterudites is believed to have strong potential for thermoelectric application due to the very low thermal conductivity of the filled structures. It is generally assumed that the atoms filling the skutterudite cages act as 'rattlers' and essentially induce a disordered lattice dynamics referred to as 'phonon glass'. Here, we present neutron spectroscopy experiments and ab initio computational work on phonons in LaFe(4)Sb(12) and CeFe(4)Sb(12). Our results give unequivocal evidence of essentially temperature-independent lattice dynamics with well-defined phase relations between guest and host dynamics, indicative of a quasi-harmonic coupling between the guests and the host lattice. These conclusions are in disagreement with the 'phonon glass' paradigm based on individual 'rattling' of the guest atoms. These findings should have an essential impact on the design and improvement of thermoelectric materials and on the development of microscopic models needed for these efforts.     

Yb0.3Co4Sb12/Mo-Cu热电元件的界面结构与界面电阻

通过放电等离子烧结(SPS)实现阻挡层Ti-Al、过渡焊接层Ni与热电臂Yb_0.3Co₄Sb₁₂的一体化烧结, 使用Ag-Cu-Zn共晶合金完成热电元件Yb_0.3Co₄Sb₁₂/Ti-Al/Ni与Mo-Cu电极的钎焊连接。扫描电镜(SEM)显示出Yb_0.3Co₄Sb₁₂/Ti-Al/Ni/Ag-Cu-Zn/Mo-Cu接头中各界面结合良好, 无裂纹, 成分分析发现Yb_0.3Co₄Sb₁₂/Ti-Al界面存在AlCo、TiCoSb及TiSb₂等金属间化合物(IMC)。500℃下等温时效30 d后, Yb_0.3Co₄Sb₁₂/Ti-Al界面处的金属间化合物厚度无明显变化; Ag-Cu-Zn/Ni界面处Cu、Zn扩散趋于稳定, Cu-Zn扩散层厚度达到约40 μm。界面接触电阻测试结果表明, 等温时效前后Yb_0.3Co₄Sb₁₂/Ti-Al/Ni/Ag-Cu-Zn/Mo-Cu元件的界面接触电阻率均低于10 μΩ·cm²。     

Effect of copper nanoparticle addition on the electrical and optical properties of thin films prepared from silver nanoparticles

The fabrication of transparent and conductive silver (Ag) and copper (Cu)-doped Ag films using simple spin-coating method with Ag and Cu nanoparticles (NPs) as starting material is described in this study. The aggregation of Ag NP and the grain formation caused by heat treatment were hindered by the addition of small amount Cu NP, and a continuous film was obtained even though the thickness was in the order of 10 nm. When the total metal concentration of NP solution precursor was 5 wt% with the ratio between Ag and Cu being 95:5, the surface resistivity (ρ _s) of Ag–Cu film was 3.17 Ω/sq; and when the concentration was reduced to 3.5 wt%, the ρ _s was 16.3 Ω/sq. The transmission of latter was more than 60 % with the maximum value 82.1 % at 328 nm in the near-UV region (300–400 nm), however, decreased to about 38 % in the visible region (400–700 nm) and near-IR region. The inhomogeneity of the film increased leading to the decrease of the conductivity with the time extension during the heat treatment.     

Influence of the synthesis method on the porosity,microstructure and electrical properties of La0.7Sr0.3MnO3 cathode materials

Lanthanum strontium manganite (La_1 ? xSr_xMnO₃, LSM) has been studied as a promising material for application as a cathode in solid oxide fuel cells. In the present work La_0.7Sr_0.3MnO₃ nanopowders were synthesized by three different methods (combustion, citrate and solid-state) and characterized by thermal analysis, X-ray diffraction, physical adsorption of N₂ and scanning electron microscopy. All powders exhibited single LSM phase formation with crystallite sizes in the range of 12–20 nm. Nanopowders were sintered at 1100 °C to produce porous pellets. The porosity, particle size and microstructure of LSM sintered bodies are strongly dependent on the preparation methodology. The samples synthesized by combustion and citrate methods presented smaller particle sizes and higher porosity after sintering than that derived from solid-state synthesis. However, the electrical conductivity, measured by two-probe technique, was very similar for all three samples.     

Influence of processing variables on the mechanical properties of extruded maize

Maize was extrusion cooked under different conditions of feed moisture and extruder screw speed to produce foams of different bulk density. The foams were tested in tension, com-pression and flexure according to British Standards for plastic materials. A power law corre-lation between mechanical properties and bulk density was used with a density power law index in the range 1.5 to 2.3. This represents a special case of Ashby's treatments of foams where the foam wall properties have not been included. These pore wall properties were a function of processing conditions. A pin identation test was used to obtain localized mech-anical properties and indicated the different deformation behaviour of low- and high-density foams together with the greater strength of the foam surface.     

Relationships between processing conditions and mechanical properties of PA12 tubes

In polyamide 12 (PA12) tube extrusion, calibration is the key step of the process that affects the subsequent mechanical properties. In previous work it has been shown that according to the calibration conditions, a very oriented skin layer may be created, which has been correlated to an important decrease of elongation at break. In this paper, we present new results showing a good correlation between molecular orientation and fracture toughness, as evaluated by the EWF (Essential Work of Fracture) approach. They concern notched specimens and confirm the results obtained in classical tensile testing. EWF is very sensitive to processing conditions, and especially to induced orientation: it decreases from the external to the inner regions of the tube, and increases with skin orientation.     

Influence of glass addition and sintering temperature on the structure,mechanical properties and dielectric strength of high-voltage insulators

Soda-lime glass as a substituent for the feldspar was used to prepare high-tension electrical porcelain by standard chemical solid reaction technique. The effect of glass substitution and sintering temperature on the physical properties, microstructure, hardness, modulus of rupture, flexural strength and Dielectric breakdown strength were examined. Zero water absorption (WA %) and apparent porosity (AP %) were achieved for the samples with glass content >15 wt.% sintered at 1100 °C. The apparent density was found to increase with sintering temperature. The Vicker’s micro-hardness increased with both glass addition and sintering temperature. Both of the modulus of rupture (MOR) and flexural strength (σ_f) had maxima values at 15 wt.% glass addition. The structure and morphology were characterized by X-ray diffraction and scanning electron microscope (SEM). It showed the formation of mullite needles at sintering temperature of 1100 °C, which enhanced the mechanical and electrical properties of the porcelain. The dielectric breakdown strength increased with sintering temperature and glass addition. The highest dielectric strength was found at 10 wt.% of glass addition depending on the Na₂O and Fe₂O₃ content.     

Influence of boron addition on microstructure and mechanical properties of dental cast titanium alloys

As-cast titanium alloys prepared using a dental cast machine with a series of boron additions have been studied using an optical microscope, XRD, SEM, and hardness and tensile testing. It has been shown that a small amount of boron addition induces a significant refinement of as-cast structure and improvement of mechanical properties. Titanium borides, TiB particles, are observed in Ti matrix. Tensile ductility of cast Ti–B and Ti–0.5Si–B alloys is improved obviously when boron content is about 0.086–0.14 mass%. This is primarily due to the role of borides precipitated at the prior β boundary and refinement of the prior β grains. Cast Ti–B alloys with a good combination of greater tensile ductility and strength can be obtained with very low boron addition.     

Influence of Nb addition on microstructural evolution and compression mechanical properties of Ti-Zr alloys

In order to expand the application horizon of Ti-Zr based alloys,the influence of Nb content on phase composition,microstructural evolution and biomechanical properties is systematically studied.The phase and microstructural characterization of the as-cast alloys is carried out by X-ray diffraction,optical microscopy and transmission electron microscopy.The results reveal that Nb-containing Ti-Zr alloys transformed from single a phase→α+α+β phase single β phase double β phases with increasing Nb content.In the case of β-type alloys,the addition of Nb improves the bonding energy between atoms,reduces the grain size,increases the elastic modulus,improves the yield strength and renders superior work-hardening behavior.Moreover,the current study provides mechanistic insights into microstructural evolution and strengthening of Nb-containing Ti-Zr alloys with increasing Nb content.Herein,the addition of 5 at.% Nb resulted in an abnormal work hardening during compression deformation under the synergistic influence of stress-induced martensite transformation of β phase and stress-induced twinning of α phase.Moreover,the biomechanical properties are evaluated to demonstrate the potential of Nb-containing Ti-Zr alloys in biological applications.     

The effect of Fe addition on processing and mechanical properties of reaction infiltrated boron carbide-based composites

Dense metal-ceramic composites based on boron carbide were fabricated using boron carbide and Fe powders as starting materials. The addition of 3.5–5.5 vol% of Fe leads to enhanced sintering due to the formation of a liquid phase at high temperature. Preforms, with about 20 vol% porosity were obtained by sintering at 2,050 °C even from an initial boron carbide powder with very low sinterability. Successful infiltration of the preforms was carried out under vacuum (10⁻⁴ torr) at 1,480 °C. The infiltrated composite consists of four phases: B₁₂(C, Si, B)₃, SiC, FeSi₂ and residual Si. The decrease of residual Si is due to formation of the FeSi₂ phase and leads to improved mechanical properties of the composites. The hardness value, the Young modulus and the bending strength of the composites fabricated form a powder mixture containing 3.5 vol% Fe are 2,400 HV, 410 GPa and 390 MPa, while these values for the composites prepared form iron free B₄C powder are 1,900 HV, 320 GPa and 300 MPa, respectively. The specific density of the composite was about 2.75 g/cm³. The experimental results regarding the sintering behavior and chemical interaction between B₄C and Fe are well accounted for by a thermodynamic analysis of the Fe–B–C system.

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Influence of CuO addition to BaSm2Ti4O12 microwave ceramics on sintering behavior and dielectric properties

Microwave dielectric ceramics of tungsten–bronze-type BaSm₂Ti₄O₁₂ were prepared by doping CuO (up to 2 wt.%) as the liquid-phase sintering aid. The effects of CuO additive on the densification, micro structure and dielectric properties were investigated. Due to the liquid-phase effect, the sintering temperature of BaSm₂Ti₄O₁₂ ceramics with 1 wt.% CuO addition can be effectively reduced to 1160 °C, about 200 °C lower than that of pure BaSm₂Ti₄O₁₂ ceramics, while good microwave dielectric properties of ɛ_r = 75.8, Q*f = 4914.6 GHz and τ_f = −7.65 ppm/°C were still achieved.     

Influence of silver additions to type 316 stainless steels on bacterial inhibition,mechanical properties,and corrosion resistance

Bacterial contamination is a major concern in many areas. In this study, silver was added to type 316 stainless steels in order to obtain an expected bacteria inhibiting property to reduce the occurrence of bacterial contamination. Silver-bearing 316 stainless steels were prepared by vacuum melting techniques. The microstructure of these 316 stainless steels was examined, and the influences of silver additions to 316 stainless steels on bacterial inhibition, mechanical properties, and corrosion resistance were investigated. This study suggested that silver-bearing 316 stainless steels could be used in areas where hygiene is a major requirement. The possible mechanisms of silver dissolution from the surfaces of silver-bearing 316 stainless steels were also discussed in this report.     

Influence of long-term service on microstructure,mechanical properties,and service life of HCM12A steel

The investigated material was a pipe measuring φ38?×?6.3, made of HCM12A steel used for coils of boiler superheaters at supercritical and ultra-supercritical parameters of steam. The article presents the results of research on mechanical properties and microstructure of the examined pipe material in the as-received condition and after 100?000?h of service at the temperature of 585°C. Creep tests were carried out for the material serviced for 100?000?h to determine the available residual life. The influence of changes on mechanical properties and microstructure degradation on the loss of creep resistance was assessed compared with the as-received condition. Performed research has shown that the examined steel after 100?000?h of service fulfills the requirements set for this steel in the as-received condition within the range of strength properties at room and elevated temperature. The insignificant loss of life is confirmed by the results of creep test.