SLM成形IN718合金高温持久各向异性影响因素

为研究激光选区熔化(selective laser melting, SLM)成形IN718持久各向异性的影响因素,对其打印态分别进行固溶时效(solution treatment and aging, SA)和直接时效(direct aging, DA)处理,采用X射线衍射、扫描电子显微镜及电子背散射衍射对两种状态试样xOy面及yOz物相、显微组织和织构进行表征,并在690 MPa,650 ℃下对两种状态的横向/纵向试样进行持久性能测试,测试后对断口和截面裂纹进行了表征分析量化研究.研究表明,DA态试样很大程度保持了打印态的显微组织,有明显熔池痕迹. 晶粒尺寸几乎没有变化. 显微组织中有大量偏析,XRD显示有衍射强度微弱的MC相的峰.而SA态试样晶粒尺寸及分布与DA态试样类似,同时存在大范围的δ相析出.横向/纵向试样的高温持久性能的差异随着加载时裂纹萌生点数量差异的减小而减小.垂直于应力加载轴向的熔池结构及晶界结构的差异是影响其高温持久各向异性的关键因素.

     

Improvement of thermal conductivity of WC-FeAl hard materials by elimination of oxide materials

The dependence of thermal conductivity in tungsten carbide- iron aluminide (WC-FeAl) hard materials on oxygen contained in the pre-sintered powders was investigated. The control of oxygen concentration in the hard materials was done by changing fabrication processes including heat treatment of raw WC powder. Although thermal conductivity of the hard materials strongly depended on the average WC grain size, it was apparently improved by reduction of oxygen Concentration. The maximum thermal conductivity was 169 W (mK)⁻¹ with low volume fraction of FeAl binder (10 vol%Fe_0.6Al_0.4) to total WC-FeAl. The hardness and transverse rupture strength had negative relationship with the average WC grain size, indicating that they also had negative relationship with thermal conductivity. The improving technique such as uniform powder mixing with suppressing powder oxidation is required to fabricate WC-FeAl hard materials of high transverse rupture strength and thermal conductivity.     

Silver oxalate-based solders: New materials for high thermal conductivity microjoining

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Low lattice thermal conductivity and enhanced thermoelectric performance of SnTe via chemical electroless plating of Ag

Interface engineering has been regarded as an effective strategy to manipulate the thermoelectric performance of materials.Here,we use a facile chemical electroless plating and a spark plasma sintering process to fabricate Ag-plated SnTe bulk.After sintering,a small amount of plated Ag can be doped into SnTe to suppress the Sn vacancies and the others form Ag precipitates with a size distribution from nanoscale to microscale,which introduces Ag/SnTe interfaces to enhance the Seebeck coefficient via energy filtering effect.Simultaneously,these structures result in strong scattering to reach a low lattice thermal conductivity of-0.62 W·m^–1·K^–1.Consequently,a maximum figure of merit(zT)of-0.67 at 823 K is achieved in 2 wt%Ag-plated SnTe,which is-60%higher than that of pristine SnTe.Moreover,the microhardness indentation test results show that the mean microhardness of 2 wt%Ag-plated SnTe is HV 64.26,which is much higher than that of pristine SnTe,indicating that Ag electroless plating can improve the mechanical properties of SnTe.This work has provided a facile and eco-friendly method to realize the interface engineering for manipulating the thermoelectric and mechanical properties of SnTe.     

Research Progress on AgSbTe2-based Thermoelectric Materials

Thermoelectric power generation represents a class of energy conversion technology,which has been used in power supply of aeronautic and astronautic exploring missions,now showing notable advantages to harvest the widely distributed waste heat and convert the abundant solar energy into electricity at lower cost than Si based photovoltaic technology.Thermoelectric dimensionless figure of merit ZT plays a key role in the conversion efficiency from thermal to electrical energy.Low thermal conductivity and large Seebeck coefficient make the AgSbTe 2 compound a very promising candidate for high efficiency p-type thermoelectric applications.The AgSbTe2-based thermoelectric system has been repeatedly studied as prospective thermoelectric materials.In this review,we firstly clarify some fundamental tradeoffs dictating the ZT value through the relationship ZT =S 2 σT/κ.We also pay special attentions to the recent advances in AgSbTe2 based thermoelectric materials.Finally,we provide an outlook of new directions in this filed.     

Microstructure and thermal conductivity of porous ZrO2 ceramics

ZrO₂ ceramics usually have nonuniform microstructures due to the agglomeration of fine ZrO₂ powders. In this paper, the thermal conductivity of porous ZrO₂ ceramics with different microstructures was investigated by the laser-flash method. It was found that the thermal conductivity of porous ZrO₂ ceramics with uniform microstructure was clearly lower than that of ceramics with nonuniform microstructure. Scanning electron microscopy observations showed that porous ceramics with nonuniform microstructures have wider pore and grain size distributions, relative to those with uniform microstructures. Theoretical analyses were done to model the thermal conductivity of porous ceramics using an effective medium approach. These analyses revealed that matrix grain size distribution has a significant impact on the thermal conductivity of porous ceramics, while the effect of pore size distribution is negligible.     

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.     

When thermoelectric materials come across with magnetism

Nowadays,thermoelectric materials have attracted a lot of attention as they can directly convert heat into electricity and vice versa.However,while strenuous efforts have been made,those conventional strategies are still inevitably going to meet their performance optimization limits.For this reason,brand new strategies are badly needed to achieve further enhancement.Here,the roles played by magnetism in recent advances of thermoelectric optimization are concluded.Firstly,magnetic thermoelectric materials can just be treated like other normal materials because the use of universal optimization strategies can still get good results.So,it is not a situation which is all or nothing and the tactics of using magnetism for thermoelectric optimization can coexist with other strategies.Besides,through magnetic doping,we can introduce and adjust magnetism in materials for further optimization.Magnetism provides more possibilities in thermoelectric optimization as it can directly influence the spin states in materials.Furthermore,in the form of magnetic secondphase nanoclusters,magnetism can be introduced to thermoelectric materials to conquer the dilemma that the solid solubility of many magnetic ions in thermoelectric materials is too low to have any significant effect on thermoelectric properties.Finally,when exposed to an external magnetic field,topological materials can rely on its unique band structures to optimize.     

New composite thermoelectric materials for energy harvesting applications

The concept of using nanostructured composite materials to enhance the dimensionless thermoelectric figure of merit ZT relative to that for their counterpart homogeneous alloyed bulk crystalline materials of similar chemical composition is presented in general terms. Specific applications are made to the Si-Ge and Bi_2-−xSb_xTe₃ systems for use in high-temperature power generation and cooling applications. The scientific advantages of the nanocomposite approach for the simultaneous increase in the power factor and decrease in the thermal conductivity are emphasized insofar as their simultaneous occurrence is enabled by the independent control of these physical properties through the special properties of their nanostructures. Also emphasized are the practical advantages of using such bulk samples both for thermoelectric property measurements and for providing a straightforward path to scaling up the materials synthesis and integration of such nanostructured materials into practical thermoelectric powergeneration and cooling modules and devices.     

含Sm、Mn的P型FeSi2基热电材料的电学性能

用悬浮熔炼法制备了含Sm、Mn的P型FeSi2基热电材料。实验结果表明，其电学性能是由掺杂的两种元素共同决定的，Sm对降低样品电阻率的作用较大，而Mn有助于提高样品的热电动势率。要保证有较高功率因子，Mn、Sm掺杂总摩尔分数应小于5％，而Mn的最住掺杂摩尔分数在1．7％左右。     

含Sm、Mn的P型FeSi_2基热电材料的电学性能

用悬浮熔炼法制备了含Sm、Mn的P型FeSi2基热电材料。实验结果表明,其电学性能是由掺杂的两种元素共同决定的,Sm对降低样品电阻率的作用较大,而Mn有助于提高样品的热电动势率。要保证有较高功率因子,Mn、Sm掺杂总摩尔分数应小于5%,而Mn的最佳掺杂摩尔分数在1.7%左右。     

Electrical conductivity and thermoelectric power of polypyrrole with different doping levels

The electrical conductivity and absolute thermoelectric power of the conducting polymer polypyrrole have been measured between approximately 4 K and 350 K. Normally-doped films had a conductivity of 26 Ω⁻¹ cm⁻¹, whilst lightly-doped films had a conductivity of 8 Ω⁻¹ cm⁻¹. The Mott variable-range hopping model for electrical conductivity is obeyed at higher temperatures for both types. The thermoelectric power is approximately linear in temperature, reaching about 5 μV K⁻¹ at 200 K, but is sublinear at higher temperatures and becomes constant above about 300 K. The Kuivalainen model, which explains the reported anomaly between optical and d.c. electrical conductivity, gives a good fit to the experimental data.     

Effect of carbide formers on microstructure and thermal conductivity of diamond-Cu composites for heat sink materials

Diamond-copper composites were prepared by powder metallurgy, in which the diamond particles were pre-coated by magnetic sputtering with copper alloy containing a small amount of carbide forming elements (including B, Cr, Ti, and Si). The influence of the carbide forming element additives on the microstructure and thermal conductivity of diamond composites was investigated. It is found that the composites fabricated with Cu-0.5B coated diamond particles has a relatively higher density and its thermal conductivity approaches 300 W/(m·K). Addition of 0.5%B improves the interfacial bonding and decreases thermal boundary resistance between diamond and Cu, while addition of 1%Cr makes the interfacial layer break away from diamond surface. The actual interfacial thermal conductivity of the composites with Cu-0.5B alloy coated on diamond is much higher than that of the Cu-1Cr layer, which suggests that the intrinsic thermal conductivity of the interfacial layer is an important factor for improving the thermal conductivity of the diamond composites.     

Ambient and high-temperature thermal conductivity of thermal sprayed coatings

Aside from its importance as a design parameter for thermal barrier coatings, measuring thermal conductivity of thermal sprayed coatings itself provides a unique method to critically characterize the nature, quantity, and anisotropy of the defect morphologies in these splat-based coatings. In this paper, the authors present a systematic assessment of thermal conductivity of wide range using the flash diffusivity technique. For the case of plasma sprayed yttria-stabilized zirconia (YSZ), coatings obtained from wide-ranging initial powder morphologies as well as those fabricated under different particle states were characterized. Both in-plane and through-thickness properties were obtained. Other material systems that were considered include: metallic alloys and semiconductors of interests. Issues such as reproducibility and reliability in measurements were also considered and assessed. Finally, work in collaboration with the Oak Ridge National Laboratory (ORNL) for alternate approaches to characterization of thermal conductivity as well as high-temperature measurements was performed. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

The thermal conductivity of metallic ceramics

Transition metal carbides, nitrides, and borides can be called metallic ceramics because they are electronically conductive and extremely hard. Their various applications include cutting and grinding tools, thermal-barrier coatings, diffusion-resistant thin films, interconnects, and superconductivity devices. In each case, the ability of the material to resist or permit heat flow is important. Because of the high concentration of nonmetal atom vacancies in the carbides and nitrides, the carriers of heat—conduction electrons and phonons (the quanta of lattice waves)—are severely scattered, and the thermal conductivity, K, is strongly affected, although differently in high- and low-temperature regions. Measurements of both the electrical and thermal conductivity of single-crystal metallic ceramics at low temperatures and the application of the Callaway formalism help explain the puzzling temperature dependence of K. The finding of a large peak in K of NbC just below its superconducting transition temperature confirms phonon-electron scattering and could lead to a thermal switch. The single-crystal thermal conductivity behavior of TiC and WC is used to interpret the measured K values for cemented carbides TiC/Ni-Mo and WC/Co through a broad temperature range. Wendell S. Williams earned his Ph.D. in physics at Cornell University in 1956. He was a research physicist with Union Carbide Corporation, a professor of physics and ceramic engineering at the University of Illinois, and department chair of materials science and engineering at Case Western Reserve University. Dr. Williams, now retired, is a member of TMS.     

High-temperature thermal conductivity of ceramic fibers

The ceramic fibers VK-60, ABK, and VK-80 produced by steam blowing and nozzle dissemination methods have been investigated for the effect of nonfibrous material content, pressure, and temperature on the thermal conductivity at ambient and higher temperatures. It was noticed that with an increase in the aluminum content of the ceramic fibers the thermal conductivity of the material decreased while the insulation properties improved. The VK-80 fibers have the lowest and the VK-60 fibers the highest value of thermal conductivity at ambient temperature. At ambient temperature, the value of thermal conductivity increased with an increase in pressure for all analyzed fibers. ABK fibers showed the least increase and VK-80 registered an increase of about 10% in the values of thermal conductivity for pressures ranging from 0.6 to 6.6 kN/². However, beyond a pressure of 6.6 kN/m², the thermal conductivity of all samples increased. To assess the insulation properties of investigated fibers, the thermal conductivity was measured at different temperatures up to 800 °C. From the obtained results, it was concluded that all three types of fibers have a good potential for future applications, showing good performance in the investigated temperature range.     

Glass-like thermal conductivity in ytterbium-doped lanthanum zirconate pyrochlore

Glass-like thermal conductivity is observed in (La_1?xYb_x)₂Zr₂O₇ (1/6 ? x ? 1/3), which exhibits great potential as a high-temperature thermal barrier coating material. In the pyrochlore-type La₂Zr₂O₇, the large 16c-site La³⁺ is weakly bonded by its surrounding 48f-site oxygen ions, and substitution of La³⁺ with smaller and heavier Yb³⁺ gives rise to a large atomic displacement parameter (ADP) of Yb³⁺ which behaves as a “rattler”, as evidenced by the X-ray diffraction refinement. The localized “rattling” of Yb³⁺ in the cation sublattice significantly scatters the heat-carrying phonons and lowers the thermal conductivity close to the amorphous limit in combination with the intrinsic oxygen vacancies in the anion sublattice. In contrast, substituting Yb³⁺ with the larger La³⁺ in Yb₂Zr₂O₇ does not result in as remarkable a decrease in the thermal conductivity as in Yb-doped La₂Zr₂O₇ due to the absence of rattling atoms. In this study, a resonant phonon scattering is proposed as a new approach to reduce the thermal conductivity of oxides which are important in various thermal engineering applications.     

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.