Thermoelectric properties of p-type (Bi,Sb)2Te3 alloys fabricated by the hot pressing method

P-type (Bi_0.25Sb_0.75)₂Te₃ powders were fabricated by melting/grinding and mechanical alloying processes. Thermoelectric properties of the hot-pressed (Bi_0.25Sb_0.75)₂Te₃ were characterized with the powder processing method, powder size, hot pressing temperature, and the amount of excess-Te dopant. Specimens fabricated by melting/grinding exhibited lower Seebeck coefficient, lower electrical resistivity and higher thermal conductivity, compared to the specimens prepared by mechanical alloying. 3 wt.% excess Te-doped (Bi_0.25Sb_0.75)₂ Te₃, fabricated by melting/grinding and hot pressing at 550°C, exhibited a figure-merit of 3.2 x 10^-3/K. For 1 wt.% excess Te-doped specimen prepared by mechanical alloying and hot pressing at 550°C, a figure-merit of 3.05 x 10^-3/K was obtained.     

Thermoelectric properties of the hot-pressed Bi2(Te0.85Se0.15)3 alloy with addition of BN powders

Thermoelectric properties of the hot-pressed Bi₂(Te_0.85Se_0.15)₃ alloy were investigated with the addition of BN powders as phonon scattering centers. The Seebeck coefficient and electrical resistivity of the alloy increased as the volume fraction of BN increased. Although the thermal conductivity of the alloy decreased as the volume fraction of BN increased due to the reduction of K_el, the lattice thermal conductivity varied little. The figure-of-merit of the alloys, 1.6×l0^-3/K without the addition of BN, decreased as the volume fraction of BN increased because the increment of the electrical resistivity was much larger than the decrement of the thermal conductivity due to grain refinement.     

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.     

机械合金化及热压烧结制备Fe-Cr-Mn基超细晶奥氏体不锈钢研究

采用高能研磨诱导的机械合金化方法制备了Fe-Cr-Mn基不锈钢合金粉末;对机械合金化粉末分别进行了退火和热压烧结,分析了退火过程中的相变规律,并对热压烧结获得的奥氏体不锈钢进行了组织和耐蚀性能研究.结果表明:机械合金化获得的不锈钢合金粉由亚稳态的纳米晶铁素体构成;退火/热压烧结处理后,铁素体逐渐转变为热力学上更加稳定的...     

Effect of preliminary plastic deformation on the structure and physicomechanical properties of aging invar alloy N30K10T3

The invar alloy N30K10T3 after water quenching from 1150°C (austenite, γ phase) has the temperature of the start of martensitic transformation M _s ≈ ?80°C and the Curie temperature T _C ≈ 200°C. The effect of aging-induced phase decomposition in a deformed supersaturated solid solution on its hardness HV, electrical conductivity σ, magnetic permeability μ, and linear expansion coefficient β has been studied. It has been shown that cold plastic deformation of the alloy (at 20°C) to 30–50% increases its hardness, virtually does not change the conductivity, and decreases permeability. Aging of the deformed invar results in increasing HV and σ and decreasing μ. At room temperature, the deformed invar has a low linear expansion coefficient; its magnitude grows the faster, the greater the aging temperature T _a. Plastic deformation increases the density of dislocations, which form a banded substructure in austenitic grains. Besides, a metastable martensitic phase has been observed, which undergoes a reverse martensitic transformation into austenite upon heating in the temperature range from 550°C to 650°C. This transformation causes a decrease in the linear expansion coefficient β(T) of the deformed material. In samples aged at T _a = 700°C (after deformation), an athermal aging-induced martensite (α_a phase) appears after cooling them to 20°C. The appearance of the α_a phase is due to an increase in the temperature of the start of the martensitic transformation to above the room temperature caused by aging. In the samples containing the α_a phase, there is observed a decrease in β in the temperature range from 350 to 670°C, which is due to the reverse transformation of the aging-induced martensite into austenite (α_a → γ).     

The influence of the transformation of electronic structure and micro-structure on improving the thermoelectric properties of zinc antimonide thin films

Measurements of electronic structure, microstructure and thermoelectric properties of zinc antimonide thin films prepared by direct current magnetron co-sputtering were carried out. The as-deposited zinc antimonide thin film had a very high resistivity similar to insulating ceramics, which was due to a low binding energy of both zinc and antimony, with the electron scattering increases and impedes the current transport. With the increase in annealing temperature, the films became more crystalline and the thermoelectric properties were also improved. The resistivity of the film decreased rapidly with its crystallinity when the annealing temperature was above 350 °C. The Seebeck coefficients of the thin films were positive, indicating that the films were P-type. The Seebeck coefficient of those samples increased with increasing annealing temperature. The thin film annealed at 400 °C has an optimal power factor of 1.87 × 10⁻³ Wm⁻¹ K⁻² with a Seebeck coefficient of 300 μVK⁻¹ and a resistivity of 4.82 × 10⁻⁵ Ωm at 573 K.     

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.     

Processing map and microstructure evaluation of AA6061/Al2O3 nanocomposite at different temperatures

Hot compression behavior of Al6061/Al₂O₃ nanocomposite was investigated in the temperature range of 350–500 °C and the strain rate range of 0.0005–0.5 s^?1, in order to determine the optimum conditions for the hot workability of nanocomposite. The activation energy of 285 kJ/mol for the hot compression test is obtained by using hyperbolic sine function. By means of dynamic material model (DMM) and the corresponding processing map, safe zone for the hot workability of AA6061/Al₂O₃ is recognized at temperature of 450 °C and strain rate of 0.0005 s^?1 and at temperature of 500 °C and the strain rate range of 0.0005–0.5 s^?1, with the maximum power dissipation efficiency of 38%. Elongated and kinked grains are observed at 400 °C and strain rate of 0.5 s^?1 due to the severe deformation.     

Tensile properties of hot rolled Mg_(97)Zn_1Y_2 alloy sheets at elevated temperatures

The elevated temperature tensile properties of Mg97Zn1Y2 magnesium alloy sheets, hot rolled at 390, 420 and 450 ℃ respectively, were tested in a temperature range from room temperature to 250 ℃ with a strain rate of 1.0×10-3 s-1. The results show that the variations in yield strength for Mg97Zn1Y2 magnesium alloy sheets hot rolled at 390 ℃ and 420 ℃ with temperature resemble each other due to their similar morphology of the chain-shaped strengthening phase. The yield strength maintains at a high level of 283 MPa before 200 ℃ and decreases significantly at 250 ℃. Despite of the fine lamellar structure of Mg97Zn1Y2 magnesium alloy sheet hot rolled at 450 ℃, its yield strength decreases linearly owing to occurrence of the coarse grain, and drops to 239 MPa at 250 ℃. The elongation for all hot rolled Mg97Zn1Y2 magnesium alloy sheets increases slightly with increasing testing temperature.     

Electronic transport properties of Te-doped CoSb<Subscript>3</Subscript> skutterudites prepared by hot pressing

Te-doped CoSb₃ (CoSb_3−yTe_y) skutterudites were prepared by hot pressing and their electronic transport properties examined. A single δ-phase was successfully obtained. The Seebeck and Hall coefficients confirmed that all the Te-doped CoSb₃ showed n-type conduction. The Te atoms successfully acted as electron donors by substitution of the Sb atoms. The carrier concentration increased an order of 10²⁰ cm⁻³ by Te doping, whereas the carrier mobility decreased as the doping content increased. The Seebeck coefficient and electrical resistivity decreased with an increase in the Te content. The doping considerably reduced the thermal conductivity due to electron-phonon scattering. The lattice contribution was dominant over the electronic contribution.     

The Sliding Wear and Friction Behavior of M50-Graphene Self-Lubricating Composites Prepared by Laser Additive Manufacturing at Elevated Temperature

M50 steel is widely applied to manufacture aircraft bearings where service lives are mainly determined by the friction and wear behaviors. The main purpose of this study is to investigate the tribological behaviors and wear mechanisms of M50-1.5 wt.% graphene composites (MGC) prepared by laser additive manufacturing (LAM) (MGC-LAM) sliding against Si₃N₄ ball from 25 to 550 °C at 18 N–0.2 m/s. XRD, EPMA, FESEM, and EDS mapping were conducted to understand the major mechanisms leading to the improvement in the sliding behavior of MGC-LAM. The results indicated that MGC-LAM showed the excellent friction and wear performance at 25-550 °C for the lower friction coefficient of 0.16-0.52 and less wear rate of 6.1-9.5 × 10^?7 mm³ N^?1 m^?1. Especially at 350 °C, MGC-LAM obtained the best tribological performance (0.16, 6.1 × 10^?7mm³ N^?1 m^?1). It was attributed to the dense coral-like microstructure, as well as the formed surface lubricating structure which is composed of the upper uniform lubricating film with massive graphene and the underneath compacted layer.     

ZL270LF铝合金的热变形行为与组织演变

通过热压缩实验研究了ZL270LF铝合金在变形量为70%,温度为300~550 ℃,应变速率为 0.01~10 s^-1范围的热变形行为,建立了流变应力本构方程模型,绘制出了二维热加工图,确定了最佳热加工区域,采用电子背散射衍射（EBSD）和透射电子显微镜（TEM）技术研究了该合金的组织演变规律。结果表明：ZL270LF铝合金的流变应力随变形温度的升高和应变速率的降低而降低,热变形激活能为309.05 kJ/mol,最优热加工区为温度470~530 ℃、应变速率为0.01~1 s^-1。该合金在热变形过程中存在3种不同的DRX机制,即连续动态再结晶（CDRX）、不连续动态再结晶（DDRX）和几何动态再结晶（GDRX）,其中CDRX是ZL270LF铝合金动态再结晶的主要机制。     

Transient Oxidation of Cu-5at.%Ni(001): Temperature Dependent Sequential Oxide Formation

The transient oxidation stage of a model metal alloy thin film was characterized with in situ ultra-high vacuum (UHV) transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and analytical high-resolution TEM. We observed the formations of nanosized NiO and Cu₂O islands when Cu-5at.%Ni(001) was exposed to oxygen partial pressure, $ {\text{pO}}_{ 2} = 1 \times 10^{ - 4} \,{\text{Torr}} $ and various temperatures in situ. At 350 °C epitaxial Cu₂O islands formed initially and then NiO islands appeared on the surface of the Cu₂O island, whereas at 550 °C NiO appeared first. XPS and TEM revealed a sequential formation of NiO and then Cu₂O islands at 550 °C. The temperature-dependent oxide selection may be due to an increase of the diffusivity of Ni in Cu with increasing temperature.     

Beneficial Effects of Ce Implantation into Preformed Cr2O3 Scales on the Subsequent Oxidation of Ni–20Cr Alloy

The influence of Ce implantation into preformed Cr₂O₃ scales with a dose of 1 × 10¹⁷ ions/cm² on the subsequent oxidation behavior of Ni–20Cr alloy at 1050°C in air has been investigated. The pre-oxidation was carried out at 1050°C in air for 0.5 and 1 hr respectively Cr₂O₃ and NiCr₂O₄ formed on Ni–20Cr alloy. The oxidation rate was decreased remarkably due to Ce implantation regardless of whether it was implanted into the alloy or into the pre-formed oxide scales, and the beneficial effect decreased with increasing pre-oxidation time, the alloy implanted directly with Ce had the lowest oxidation rate constant. During cyclic oxidation (350 cycles) Ce implantation played a similar benefical effect on the oxide-spallation resistance for blank and pretreated alloys. The result indicates that Ce incorporated into the oxide scale affected the diffusion of the reaction species and also the spallation resistance of the oxide scales. The change of the oxidation process is attributed to the segregation of Ce at the oxide grain boundaries     

铸态粗晶Ti-5553钛合金高温变形行为与机理研究

本文借助Gleeble-3800热模拟试验机系统地研究了铸态粗晶Ti-5553合金在温度700 ℃~1100 ℃、应变速率为0.001 s_^-1~10 s_^-1条件下的高温变形行为。研究结果表明合金的流变应力对变形温度和速率都有强敏感性,流变软化过程也随变形参数的改变呈现出不同的模式。通过经典的动力学模型,建立了合金高温变形的本构关系和激活能分布图,进一步基于动态材料模型构建了合金的热加工图并实现了对不同加工区间变形机制的识别。合金在低温区(700 ℃)和高速率区( 1 s_^-1)均展现出失稳变形的特征,包括外部开裂、绝热剪切带、局部流变等机制,在实际加工中应对这些加工区域进行规避。合金在800 ℃及中低速率( 0.1 s_^-1)变形下的主导机制为α相的动态析出,在中高温(900 ℃-1100 ℃)及中低速率变形下的主导机制为动态回复与动态再结晶的结合。此外,合金在高温较低应变速率(1100 ℃/0.01 s_^-1)条件的变形中表现出大范围动态再结晶的行为特点并伴随稳定的流变软化,因此此条件附近的参数区间被认定为该合金的最优加工窗口,应在实际加工中给予优先考虑。     

Flow Stress and Processing Map of a PM 8009Al/SiC Particle Reinforced Composite During Hot Compression

Hot compression tests of 8009Al alloy reinforced with 15% SiC particles (8009Al/15%SiC_p composites) prepared by powder metallurgy (direct hot extrusion methods) were performed on Gleeble-3500 system in the temperature range of 400-550 °C and strain rate range of 0.001-1 s^?1. The processing map based on the dynamic material model was established to evaluate the flow instability regime and optimize processing parameters; the associated microstructural changes were studied by the observations of optical metallographic and scanning electron microscopy. The results showed that the flow stress increased initially and reached a plateau after peak stress value with increasing strain. The peak stress increased as the strain rate increased and deformation temperature decreased. The optimum parameters were identified to be deformation temperature range of 500-550 °C and strain rate range of 0.001-0.02 s^?1 by combining the processing map with microstructural observation.     

Pressure consolidation of amorphous ZrO2–Al2O3 by plastic deformation of powder particles

Amorphous ZrO₂–Al₂O₃ powders undergo densification at low temperatures (<650°C) and moderate uniaxial pressures (~750 MPa). It is established that large pressure dependent densification and little time dependent densification occur. Viscous sintering is not the dominant densification mechanism. Study of the particle size effect in densification of amorphous ZrO₂–40% Al₂O₃, and comparison with hot pressing of borosilicate glass powder at 500 and 550°C and cold compaction of silver powder, clearly indicate the possibility of compaction of amorphous ZrO₂–Al₂O₃ by plastic deformation. Good agreement was seen between a model for the compaction of ductile metal powders and the observed hot pressing behaviour.     

Effect of enamel coating on oxidation and hot corrosion behaviors of Ti-24Al-14Nb-3V alloy

The effect of enamel coating on the isothermal and cyclic oxidation at 900 °C in air and on the hot corrosion resistance of Ti-24Al-14Nb-3V in both 85% Na₂SO₄+15%K₂SO₄ and 15%NaCl+85% Na₂SO₄ molten mixed salts at 850 °C was investigated. The results indicated that Ti-24Al-14Nb-3V alloy exhibited poor oxidation resistance due to the formation of nonprotective Al₂O₃+TiO₂+AlNbO₄ scales and poor hot corrosion resistance due to the spallation of scales formed in molten Na₂SO₄+K₂SO₄ and NaCl+Na₂SO₄. Enamel coating suppressed the migration of oxygen and corrosive ions into the substrate to improve the oxidation and hot corrosion resistance of Ti-24Al-14Nb-3V alloy. However, the dissolution of oxides components of the coating into the molten salts degraded enamel coating and the degradation of the coating involved a process by which Cl⁻ anion penetrated into the substrate through voids in the coating to accelerate corrosion of Ti-24Al-14Nb-3V alloy.     

Microstructure and tribological properties of low-friction composite MoS2(Ti,W) coating on the oxygen hardened Ti-6Al-4V alloy

Duplex surface treatment, which combines the oxygen diffusion hardening with a deposition of low friction MoS₂(Ti,W) coating, was applied to improve the Ti-6Al-4V alloy load bearing capacity and tribological properties. The coating (3.1 μm thick) was deposited on the oxygen hardened alloy by magnetron sputtering. Microstructure characterisation was performed by scanning- and transmission electron microscopy methods, as well as X-ray diffractometry. The results of micro/nanostructural analyses performed by high-resolution transmission electron microscopy showed that the coatings are composed of MoS₂ nanoclusters embedded in an amorphous matrix. Some Ti α, W, and Ti₂S nanocrystals were also found in the coating microstructure. The wear resistance and friction coefficient of the hardened oxygen, as well as the coated alloy, was investigated at room temperature (RT), 300 °C, and 350 °C. The presence of the MoS₂(Ti,W) coating decreases the friction coefficient from 0.85 for the oxygen hardened alloy to 0.15 (at RT) and 0.09 (at 300 °C and 350 °C) for the coated one. The coating essentially increases the wear resistance of the alloy at RT and 300 °C. It was found that the wear resistance of the coated alloy decreased significantly during the wear test performed at 350 °C.     

Hot Shortness of Al 4.5% Cu Alloy

Synopsis

The effects of the addition of iron, copper, silicon and titanium at two levels on the hot shortness of the aluminium 4.5% copper casting alloy was studied. The compositions used were within the limits of the British Standard Specification BSS L92. The hot shortness was measured using the ring casting die technique¹ and the effect of compositional changes determined using the analysis of variance. Two compositions of high and low level of hot shortness sensitivity were then retested for hot shortness in a die of another design which confirmed the results of the first series of tests. Thermal analysis, optical and scanning electron microscopy were used for examining the freezing behaviour and the microstructure of the two alloys of extreme cracking behaviour. Two arrest temperatures, 644°C and 544 or 550°C indicate freezing of the α solid solution and the Al-Cu eutectic respectively. The 544°C arrest occurred with the hot short alloy and that at 550°C with the less hot short. In addition, the less hot short alloy shows arrests at 612°C and 586°C. The constituents are iron rich phases which separate out at these latter temperatures, freezing before the eutectic, and forming a framework which isolates the eutectic into pockets increasing the ability of the test pieces to resist constraint stresses imposed by the testing die. Tensile tests on chill cast test bars show that the UTS of both alloys are above the specified minimum but the elongation of the less hot short alloy is lower than specified.