Enhanced thermoelectric and mechanical performance of polycrystalline p-type Bi0.5Sb1.5Te3 by a traditional physical metallurgical strategy

In this paper, p-type Bi_0.5Sb_1.5Te₃ polycrystalline materials have been fabricated by a traditional vacuum melting method, and the effects of cooling rate and MoSi₂ addition on the microstructure, thermoelectric and mechanical performance of the polycrystalline materials have been studied detailedly. It shows that the amount of Te-rich eutectic phase increases and the lamellar microstructure has been refined with the increase of the cooling rate. Due to the combined effect of cooling rate on the carrier concentration and mobility, the air cooled sample has higher figure of merit than the furnace cooled, water cooled and liquid nitrogen cooled samples, and a maximal ZT of 1.02 at 50 °C was obtained for the air cooled polycrystalline sample. Under the same air cooling condition, the inhomogeneous nucleation sites increase with increasing the amount of MoSi₂ particles, therefore the amount of Te-rich eutectic phase increases and the lamellar microstructure get refined, and the thermal conductivity of the sample decreases significantly due to the extra phonon scattering by the refined microstructure and MoSi₂ particles. The resulted figure of merit ZT increases with increasing the amount of MoSi₂ particles, and it decreases with further increasing the MoSi₂ content after attaining the vertex of ZT = 1.33 at 100 °C at a content of 0.2 wt.% MoSi₂. The flexural strength of the air cooled polycrystalline sample also increases with the amount of MoSi₂ increasing from 0 to 0.3 wt.%, and a nearly 56% enhancement was achieved for the 0.2 wt.% MoSi₂ sample (28.0 MPa) compared with the MoSi₂ free sample. The improvement of flexural strength is in agreement with the Hall–Petch strengthening mechanism due to the lamellar microstructure refinement induced by MoSi₂.     

Anomalous temperature-dependent Young’s modulus of a cast LAST (Pb–Sb–Ag–Te) thermoelectric material

Thermomechanical characterization is important to material evaluation and device design in the development of thermoelectric technology. In this study, we utilize the resonant ultrasound spectroscopy (RUS) technique to examine the elastic behavior of a cast LAST (Pb–Sb–Ag–Te) material with a composition of Ag_0.86Pb₁₉Sb_1.0Te₂₀ between room temperature and 823 K. The temperature-dependent Young’s modulus exhibits a monotonically decreasing trend with increasing temperature. However, an abnormal slope change in the Young’s modulus–temperature curve around 500 K is observed. In addition, hysteresis between heating and cooling data in the temperature range of 450–550 K is observed, which appears to be dependent on the heating/cooling rate during the RUS experiments such that the hysteresis disappears when the heating/cooling rate was decreased from 5 to 2 K min^–1. In this study we propose an order–disorder transition model for the anomalous temperature-dependent Young’s modulus behavior observed in this study.     

Effects of hot pressing on electric performances of Bi0.5Sb1.5Te3

The effects of hot pressing on electric performance and mechanical strength of Bi0.5Sb1.5Te3 thermoelectric material prepared through vacuum melting and milling were studied. The phase constituent and microstructure were analyzed by X-ray Diffraction and cold field emission Scanning Electric Microscope. Aeolotropisms of the material on microstructure and electric performances are approved. With the rise of hot pressing temperature (from 300-500 ℃) and pressure (30-70 Mpa), electric conductivity and power factor are improved. Moreover, Bi0.5Sb1.5Te3 material can gain ideal thermoelectric performances and increased mechanical strength by hot pressing.     

The critical cooling rate and microstructure evolution of Zr41.2Ti13.8Cu12.5Ni10Be22.5 composites by Bridgman solidification

The critical cooling rate and microstructure evolution were experimentally studied at cooling rates between 0.85 and 15.34 K/s by a Bridgman technique in Vit-1 (Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5). Our results show that the sample solidified at the rate above 10.03 K/s was fully amorphous whereas the samples solidified between 1.70 and 10.03 K/s were partially amorphous with crystalline phases, and samples solidified below 1.70 K/s were essentially crystalline. As the cooling rate decreased from 15.34 K/s to 1.82 K/s, the values of T_g and T_x were around 628 K and 702 K, respectively, but the heat of crystallization (ΔH_x) decreased with decreasing cooling rate.     

Cooling rate controlled microstructure and magnetic properties of metastable Fe20Nd80 alloys

The microstructures and magnetic properties of as-cast and annealed Fe₂₀Nd₈₀ (hypereutectic) alloys were studied. Depending on the cooling rate the Fe₂₀Nd₈₀ alloys solidify into various metastable eutectic-like structures, which present a wide range of coercivity values varying from 0.2 to 0.48 T measured at room temperature. The alloys cooled at rates faster than 50 K/s display fine eutectic-like regions of Nd+A₁ and are hard magnetic. The TEM studies clearly show that the hard magnetic A₁ regions are microscopically inhomogeneous. The alloys cooled slower than 25 K/s show a discontinuous rod-type eutectic of Nd+Fe₁₇Nd₂ and, consequently, exhibit a soft magnetic behaviour. The correlations between the cooling rate, microstructure, and coercivity of the Fe₂₀Nd₈₀ alloys are discussed.     

Magnetic and related properties of Ce5CoGe2, CeCoGe and CeCo2Ge2

Magnetic susceptibility, magnetization, specific heat, electrical resistivity and thermoelectric power of polycrystalline Ce₅CoGe₂, CeCoGe and CeCo₂Ge₂ were studied at temperatures down to 2 K and in magnetic fields up to 5 T. The novel phase Ce₅CoGe₂ was found to be a ferromagnet with the Curie temperature of about 11 K, which exhibits in the paramagnetic region some features of a dense Kondo system with strong crystal field effect. The re-investigated CeCoGe was found to order antiferromagnetically below 4.6 K, while CeCo₂Ge₂ is an intermediate-valence system, in agreement with previous reports. The thermoelectric power of the latter compound is large (up to 40 μV/K) and positive in the whole temperature range studied, while in the magnetically ordered systems Ce₅CoGe₂ and CeCoGe it is mostly negative and distinctly smaller (down to −1.5 and −11 μV/K, respectively).     

冷却速率对含TiB2的TiAl合金凝固组织和力学性能的影响

通过熔模精密铸造制备不同厚度的Ti-48Al-2Cr-2Nb和Ti-48Al-2Cr-2Nb-0.25TiB2合金铸板,研究冷却速率和TiB2添加对合金凝固组织和力学性能的影响.实验结果表明,当凝固速率从37增加至2×102 K/s时,合金的凝固路径并未发生改变.基体合金的晶粒从650细化至300μm,Ti-48Al-...     

Tungsten–vanadium–yttria alloys for fusion power reactors (II): Mechanical characterization

This is the second part of a study in which the aim is to evaluate the influence of adding 2 or 4 wt.% vanadium (V) and 0.5 wt.% yttria (Y₂O₃) to pure tungsten (W) and the effects on the mechanical properties at high temperature. In the first part, the microstructure at room temperature was analysed. These W-based alloys were processed by powder metallurgy and consolidated by hot isostatic pressing. Smooth and single edge laser-notched beam specimens were tested to determine the flexural strength, yield strength and fracture toughness. Three-point bending tests were performed in both oxidising and vacuum atmospheres at temperatures ranging from 77 K, via immersion in liquid nitrogen, to 1473 K. The hardness and elastic modulus were measured via Vickers microindentation and impulse excitation technique, respectively. The results were compared with nanoindentation tests to determine the possible size effect on both parameters. The fracture surfaces of the tested alloys at different temperatures were analysed with a field emission scanning electron microscope, which was useful for establishing the link between the macroscopic behaviour and the micromechanisms of failure.The results show improvement of the mechanical properties in the entire temperature range from the addition of V and Y₂O₃ to W. The formation of a W-V solid solution benefits the conglomeration of the material and drastically reduces porosity, whereas Y₂O₃ helps to stabilise high-temperature degradation. The combination of both alloying elements creates a synergistic effect that triples the microhardness and flexural strength values in comparison with pure W produced by the same powder metallurgy route.     

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.     

Directional thermoelectric properties of Ru2Si3

The orthorhombic compound Ru₂Si₃ is currently of interest as a high-temperature thermoelectric material. In order to clarify the effects of crystal orientation on the thermoelectric properties of Ru₂Si₃, we have examined the microstructure, Seebeck coefficient, electrical resistivity, and thermal conductivity of Ru₂Si₃ along the three principal axes, using these measured quantities to describe the relative thermoelectric performance as a property of crystal orientation. Ru₂Si₃ undergoes a high temperature (HT)→low temperature (LT) phase change and polycrystalline Si platelet precipitation during cooling, both of which are expected to effect the thermoelectric properties. The HT tetragonal→LT orthorhombic phase transformation results in a [010]//[010], [100]//[001] two-domain structure, while polycrystalline Si precipitation occurs on the (100)_LT and (001)_LT planes. The [010] orientation is found to posses superior thermoelectric properties (with the dimensionless figure of merit, ZT_[010]/ZT_[100]>4 at 900 K), due principally to the larger Seebeck coefficient along the [010] direction. The effect of the domain structure on the thermoelectric properties is discussed.     

Large reversible magnetocaloric effect in the RECoC2 (RE=Ho and Er) compounds

The crystal structure, magnetic properties and magnetocaloric effect (MCE) of HoCoC₂ and ErCoC₂ have been investigated. They both crystallize in orthorhombic CeNiC₂-type structure with Amm2 space group and a second-order paramagnetic to ferromagnetic phase transition around the Curie temperature T_C ∼11 K and ∼14 K occurred in them. Under the magnetic field change (ΔH) of 0–5 T, the maximal values of magnetic entropy change, refrigerant capacity and relative cooling power are 15.6 J/kg K, 183 J/kg, 242 J/kg for HoCoC₂ and 17.2 J/kg K, 243 J/kg, 375 J/kg for ErCoC₂, respectively.     

Effect of Austenitizing Temperature on Microstructure and Mechanical Properties of Semi-High-Speed Steel Cold-Forged Rolls

The effect of austenitizing temperature on the microstructure and mechanical properties of semi-high-speed steel (S-HSS) cold-forged rolls was investigated. Low-temperature austenitizing below 1313 K induced carbide coarsening during subsequent tempering at 973 K due to the nucleation effect of undissolved M₇C₃. On the other hand, the heavy dissolution of M₇C₃ above 1353 K caused the fine carbide formation on lath and plate boundaries, which retarded the subgrain growth during tempering. The increase in strength with increasing austenitizing temperature was attributed to the fine carbide distribution and the high dislocation density. Furthermore, as the austenitizing temperature increased, the impact energy markedly reduced, due to the large prior austenite grain size and the high strength. Finally, based on the microstructure and mechanical properties, an optimal austenitizing temperature range between 1313 and 1333 K was determined.     

Thermoelectric properties of p-type (Bi2Te3)x(Sb2Te3)1−x prepared by spark plasma sintering

High performance (Bi₂Te₃)_x(Sb₂Te₃)_1?x bulk materials have been prepared by combining fusion technique with spark plasma sintering, and their thermoelectric properties have been investigated. With the increase of Bi₂Te₃ content, the electrical resistivity and Seebeck coefficient increase greatly and the thermal conductivity decreases signi?cantly, which lead to a great improvement in the thermoelectric ?gure of merit ZT. The maximum ZT value reaches 1.33 at 398 K for the composition of 20% Bi₂Te₃–80%Sb₂Te₃ with 3 wt% excess Te.     

Influence of Cooling Rate on Transformation Behavior of 0.15% V Microalloyed Steel

Dilation study was carried out in a thermo-mechanical simulator to understand the phase transformation behavior of a 0.15% vanadium microalloyed steel at three different cooling rates, i.e., 2, 5, and 10 °C/s. Results of the study showed that the phase transformation temperatures, namely Ar₃, Ar₁, and B_s of 0.15% vanadium microalloyed steel increased with increase in the cooling rate from 2 to 10 °C/s. It was also observed that a cooling rate of 5 °C/s was sufficient to form bainite microstructure in the plates while both bainite and martensite were formed when cooling rate was increased to 10 °C/s. The findings of dilation study were used to design a rolling programme for commercial production of high strength steel plates of 0.15% vanadium steel chemistry.     

A new generation of p-type didymium skutterudites with high ZT

This work evaluates the influence of single, double and triple filling of didymium, Ca and Ba in Fe₄Sb₁₂ as well as in Fe₃CoSb₁₂ on the thermoelectric performance. Various filling levels, as well as various preparation methods and nanostructuring were used to improve the thermoelectric performance. It is shown that samples prepared via ball milling have a higher ZT (ZT = 1.1) than their hand milled counterparts (ZT ≈ 0.8). Co/Fe-substituted samples have ZT > 1.2 i.e. 25% higher than samples without Co, an average ZT up to 0.93 and an efficiency up to 14% for the temperature gradient of 300–800 K. With this good thermoelectric performance in such a wide temperature range these materials are hitherto the best p-type skutterudites for thermoelectric devices.     

Thermoelectric properties of semi-conducting compound CoSb3 doped with Pd and Te

Hot-pressed samples of the semi-conducting compound CoSb₃-doped Pd and Te were prepared and characterized by X-ray and microprobe analysis. Thermoelectric characterization was done through measurements of the electrical and thermal conductivities as well as the Seebeck coefficient between room temperature and 900 K. All samples had n-type conductivity. The dimensionless thermoelectric figure of merit ZT increases with increasing temperature and reaches a maximum value of 1 at 873 K.     

Mechanical properties and microstructure of TiB2–TiC composite ceramic cutting tool material

TiB₂–TiC composite ceramic cutting tool material was prepared by sintering during hot-pressing in vacuum. The effects of nano-scale Ni and Mo additives and sintering heating rate on mechanical properties and grain characteristics were investigated. TiB₂ and TiC grains exhibited prismatic and equiaxed shapes respectively. The diameter and aspect ratio of prismatic TiB₂ grains were influenced by nano-scale Ni/Mo additives. A higher heating rate could cause a higher aspect ratio of prismatic TiB₂ grains. The good mechanical properties of TN1((TiB₂–TiC)/Ni composite ceramic sintered at a heating rate of 50 °C/min) were ascribed to a relatively fine and homogenous microstructure. And a brittle B₄MoTi solid solution phase and wider distribution of grain size induced the lower flexural strength of TNM2((TiB₂–TiC)/(Ni,Mo) composite ceramic sintered at heating rate of 100 °C/min), but the higher aspect ratio of TiB₂ grains could prevent cracks from propagating and ameliorated the fracture toughness. The optimum resultant mechanical properties were obtained by (TiB₂–TiC)/Ni composite ceramic sintered at a heating rate of 50 °C/min.     

The influence of the magnetic field configuration on plasma parameters and microstructure of niobium nitride films

Niobium nitride (NbN) coatings have a variety of interesting properties such as high chemical inertness, excellent mechanical properties, high electrical conductivity, high melting point, and a superconducting transition temperature around 16 K. We have investigated the effects of magnetic field configuration on the plasma characteristic (electron temperature, plasma density, the ion-to-metal flux ratio J_i/J_a and energy parameter E_p) and the microstructure of NbN films grown with a variable magnetron system. The coatings were deposited under identical deposition conditions but with varying the configuration of the magnetic field in the magnetron. The plasma characteristics were determined by planar and cylindrical Langmuir probes for the different magnetic field configurations. The film microstructure and composition were analyzed by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. The film hardness and Young's Modulus were measured by Nanoindentation.The variation of the magnetic field with respect to the unbalance state showed that the field changed from a minimum of 3.6 to a maximum of 4.6 mT at the substrate position (5 cm away from target) while in the target center the corresponding values were 49.0 to 98.0 mT, respectively. The lower magnetic field at the target resulted in higher J_i/J_a ratios, plasma densities and potentials. These characteristics resulted in changes in the value of E_p and as this increased the preferred crystalline orientation changed from [200] to [111] and the hardness and Young Modulus increased to 40 GPa and 430 GPa, respectively.     

Magnetic properties and magnetocaloric effect in the aluminide RENiAl2 (RE = Ho and Er) compounds

Two single-phased aluminide RENiAl₂ (RE = Ho and Er) compounds have been synthesized by an arc-melting method. The magnetic properties and magnetocaloric effect (MCE) of HoNiAl₂ and ErNiAl₂ have been studied by magnetization measurements. A second-order paramagnetic (PM) to ferromagnetic (FM) phase transition together with a considerable reversible MCE were observed around the Curie temperature T_C ∼7.5 K and 5.0 K for HoNiAl₂ and ErNiAl₂, respectively. Under the magnetic field change (ΔH) of 0–5 T, the maximal values of magnetic entropy change, refrigerant capacity and relative cooling power are 14.0 J/kg K, 171 J/kg, 213 J/kg for HoNiAl₂ and 21.2 J/kg K, 267 J/kg, 357 J/kg for ErNiAl₂, respectively.     

Structure and magnetic properties of SmxCo72−xFe16Zr7B4Cu1 (x = 8, 12, 15) alloys

Sm_xCo_72−xFe₁₆Zr₇B₄Cu₁ (x = 8, 12, 15) alloys with low level of Sm and mixed addition of several elements (Fe, Zr, B, Cu) were prepared via arc melting and rapid quenching technique. The influences of Sm content and cooling rate on the microstructure and magnetic properties of alloys were investigated. The ribbons melt-spun at 40 m/s present a novel composite microstructure with nanocrystalline phases embedded in amorphous matrix. Such a unique structure endows the x = 12 ribbons a coercivity as high as ∼20,000 Oe. Ferromagnetism of the amorphous matrix, the pinning effect of amorphous phase during the movement of domain-walls and exchange interactions among adjacent grains may contribute to the high coercivity performance.