Thermoelectric properties of tin-filled skutterudites prepared by mechanical alloying process

Sn-filled (Fe, Co)Sb₃ skutterudites of the form of Sn_yFe₃Co₅Sb₂₄ (0≤y≤1.5) were synthesized by the mechanical alloying of elemental powders followed by vacuum hot pressing. The phase transformations that occur during both mechanical alloying and vacuum hot pressing were examined by X-ray diffraction. Single-phase Sn-filled skutterudite was successfully produced by vacuum hot pressing using as-milled powders without subsequent annealing. The thermoelectric properties of the hot-pressed specimens were evaluated as a function of temperature and tin content. The void filling of tin (up to y=1.0) in Fe₃Co₅Sb₂₄ appeared to increase the thermoelectric figure of merit.     

机械合金化-热压制备Nb/NbCr2复合材料的组织与性能

采用机械合金化与热压工艺制备以Nb固溶体为软第二相的Laves相Nb/NbCr2复合材料。研究不同成分的Nb、Cr元素粉经20 h球磨后在1 250℃,0.5 h热压工艺下所获得的Nb/NbCr2复合材料的组织和性能。结果表明：随着偏离Laves相化学配比的Nb含量的增大,材料的致密度、抗压强度、塑性应变均增加,而维氏硬度减小。Laves相含量为29%的Cr-77.5Nb合金的组织均匀,Nb固溶体与Laves相间隔分布,晶粒尺寸达到亚微米级,屈服强度为2 790 MPa,抗压强度为3 174 MPa,塑性应变达到5.44%,充分实现了细晶和软第二相综合增韧的效果。     

Synthesis of TiB_2 TiC by mechanical alloying

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｉＢ2 ａｎｄＴｉＣａｒｅｂｏｔｈｓｔａｂｌｅｃｏｍｐｏｕｎｄｓｗｉｔｈｃｅｒｔａｉｎｏｆｔｈｅｃｈａｒａｃｔｅｒｓｏｆｍｅｔａｌｓ (ｌｕｓｔｅｒ,ｃｏｎｄｕｃｔｉｖｉ ｔｙ ,ｅｔｃ) .Ｔｈｅｙｈａｖｅｅｘｔｒａｏｒｄｉｎａｒｙｈａｒｄｎｅｓｓ ,ｅｘｃｅｌ ｌｅｎｔｒｅｓｉｓｔａｎｃｅｔｏａｂｒａｓｉｏｎａｎｄｅｒｏｓｉｏｎ ,ｓｏｔｈｅｙａｒｅｖｅｒｙｕｓｅｆｕｌｎｏｎ ｏｘｉｄｅｃｅｒａｍｉｃ .Ｔｈｅｙａｒｅｏｆｔｅｎｕｔｉｌｉｚｅｄｔｏｆａｂｒｉｃａｔｅｃｅｒａｍｉｃ …     

Synthesis of TiB2 nanocrystalline powder by mechanical alloying

1 Introduction TiB2 has been widely used in some industrial fields owing to its high melting temperature, hardness, elastic modulus, electro-conductibility and thermal diffusivity, and excellent refractory properties and chemical inertness. Usually, TiB2…     

机械活化热压制备CuCr50触头材料的组织与性能

采用机械合金化（MA）活化CuCr50粉末，然后对MA粉进行真空热压制备出CuCr50触头材料。结果表明，CuCr50MA粉为亚稳态过饱和固溶体，这种过饱和固溶体在随后的热压过程中发生脱溶现象。随脱溶程度的不同，CuCr50块体材料的组织与性能也发生相应的变化。由于MA活化作用，使得CuCr50MA粉在较低的温度保压较短的时间内便获得了致密度高的块体材料，并且第二相cr分布均匀，尺寸细小，其综合性能优于其它工艺方法获得的CuCr50触头材料。     

Microstructural and mechanical evaluation of Al-TiB2 nanostructured composite fabricated by mechanical alloying

Production of bulk Al-TiB₂ nanocomposite from mechanically alloyed powder was studied. Al-20 wt.% TiB₂ metal matrix nanocomposite powder was obtained by mechanical alloying (MA) of pure Ti, B and Al powder mixture. A double step process was used to prevent the formation of undesirable phases like Al₃Ti intermetallic compound, which has been described in our previous papers. The resultant powder was consolidated by spark plasma sintering (SPS) followed up by hot extrusion. The structural characteristics of powder particles and sintered samples were studied by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Hardness measurements were conducted on the cross section of powder particles and sintered sample and the tensile behavior of extruded samples was evaluated. The results showed that the prepared Al-20 wt.% TiB₂ nanocomposite has good thermal stability against grain growth and particle coarsening. Extruded Al-20 wt.% TiB₂ showed a hardness value of 180 VHN and yield and tensile strength of 480 and 540 MPa, respectively, which are much higher than those reported for similar composites made by other processes.     

Ga、K双掺杂对N型Bi_2Te_(2.7)Se_(0.3)材料热电性能的影响

采用真空熔炼及热压方法制备了Ga和K双掺杂N型Bi2Te2.7Se0.3热电材料。XRD分析结果表明,Ga和K已经完全固溶到Bi2Te2.7Se0.3晶体结构中,形成了单相固溶体合金。SEM分析表明,材料组织致密且有层状结构特征。通过Ga和K部分替代Bi,在300~500 K的大部分温度范围内,Ga和K双掺杂对提高Bi2Te2.7Se0.3的Seebeck系数产生了积极的作用,同时双掺杂样品的电导率也得到明显的提高。Ga和K双掺杂样品的热导率都大于未掺杂的Bi2Te2.7Se0.3,Ga0.02Bi1.94K0.04Te2.7Se0.3合金在500 K获得ZT最大值为1.05。     

Effect of alloying elements on structure and mechanical properties of semi-solid processed cast iron

Semi-solid metal processing of alloys is one of the key technologies for producing advanced materials. Through semi-solid processing, it is possible to produce high quality cast components from grey cast iron. A series of experiments were carried out to clarify the effect of the alloying elements copper, chromium, molybdenum, and nickel on the properties of cast iron. A comparison was made of the microstructure and mechanical properties in semi-solid processed cast iron and ordinary cast iron. This showed that an increase in the level of alloying elements in cast iron gave a higher level of hardness. However, the tensile strength of alloyed semi-solid cast iron did not exceed that of grey iron, for every composition, as a result of the characteristic microstructure produced by semi-solid processing. Thus, the alloying elements had little effect on the tensile strength of semi-solid cast iron, but did have an effect on elongation. The tensile strength of semi-solid cast iron, however, still depends on the cooling rate.     

Phase analysis of the mechanically alloyed Fe−Si powder by differential dissolution technique

The binary Fe?Si elemental powders mixture (1∶2 in atomic proportion) has been milled for different milling times in an attrition mill. The phase characterization of mechanically alloyed powder was investigated using the chemical method of differential dissolution (DD) and the X-ray diffraction (XRD) method. In powder specimens milled for more than 15 hr, ∈-FeSi and unreacted Si were observed. The formation of a supersaturated solid solution of Si in ∈-FeSi induced by mechanical alloying (MA) was also verified. The lattice parameter of the ∈-FeSi of as-milled powders changed from 4.4876 Å to 4.4668 Å according to the increase of MA time. Based on the results of the DD analysis, unreacted Si could be classified as (1) crystalline Si, (2) Si supersaturated in ∈-FeSi, or (3) amorphous Si. Therefore formation of the β-FeSi₂ after annealing could be explained by the reaction between the ∈-FeSi and the Si classified into types (1) and (2). It seemed that the amorphous Si induced by MA did not react with the ∈-FeSi during annealing at 700°C.     

Characterization and sinterability of oxide-dispersion strengthened nickel powder produced by mechanical alloying

Among the main requirements for the Ni/8% yttria stabilized zirconia (Ni/8YSZ) material, currently used for manufacturing solid oxide fuel cell (SOFC) anodes, fine homogeneous microstructure, considerable structural and mechanical stability, and sufficient gas permeability are of primary concern. In the present investigation, oxide-dispersion strengthened composite Ni powders containing 2, 5, and 10 vol.% 8YSZ were produced by mechanical alloying (MA) in air using a planetary milling machine and ZrO₂ milling media. The progress of the MA process was followed by particle size analysis, optical metallography, and x-ray diffraction (XRD) techniques. Results showed that dispersion of the oxide particles and structural refinement reached a significant point after milling for 180 h. The crystallite size and lattice distortion showed considerable dependence on the processing parameters. The mechanically alloyed powders were sintered at 1100° to 1350 °C. The mechanically alloyed powder containing 10 vol.% 8YSZ exhibited maximum densification. The minimum sintered density was observed for the composite powder containing 2 vol.% 8YSZ.     

Fabrication of Ti3AIC2 ceramic material by mechanical alloying

Ti3A1C2 has the properties of ceramics and metals. These excellent properties indicate that Ti3A1C2 is a very promising material to extensive applications. Ti3A1C2 ceramic material was prepared by mechanical alloying. The effects of milling time and sintering temperature on the fracture, microstructure and mechanical properties of Ti3A1C2 ceramic material were analyzed by laser particle analyzer, X-ray diffraction,and scanning electron microscopy. The experimental results showed that Ti3A1C2 had the best comprehensive properties after the composite powder was milled for 3 h and sintered at 1630℃ for 2 h. The relative density, bending strength, and hardness of the sample reached 92.23%,345.2 MPa, and HRA 34.1, respectively. The fracture surface indicated that the fracture of the material belonged to ductile rapture.     

机械合金化联合真空热压烧结工艺制备CoCrFeNi高熵合金的力学性能和耐腐蚀性能

采用机械合金化和真空热压烧结工艺制备了CoCrFeNi高熵合金。采用X射线衍射仪、扫描电子显微镜、电感耦合等离子体发射光谱和光学显微镜对产物的相结构和微观结构进行了表征,并采用万能试验机、维氏硬度计和电化学工作站对其力学性能和耐腐蚀性能进行了研究。结果表明：与电化学还原联合热压烧结工艺以及电弧熔炼法制备的CoCrFeNi高熵合金性能相比,机械合金化联合真空热压烧结工艺制备的CoCrFeNi高熵合金具有良好的抗拉伸强度和断裂伸长率,其合金硬度是电弧熔炼法制备合金的2倍,在0.5 mol/L H₂SO₄、1 mol/L KOH和3.5%（质量分数）NaCl水溶液中,该合金具有与304不锈钢及电化学还原联合热压烧结工艺或电弧熔炼法制备的合金相当的耐腐蚀性能。     

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.     

机械合金化对Mg_2Ni相形成的影响

用两步法 (即由机械合金化和压制烧结两个步骤组成 )制备了Mg2 Ni合金。实验证明 :混合粉经机械合金化后 ,晶粒细化 ,增加了固态扩散的能力 ,有利于固相反应进行 ,使Mg2 Ni产率明显提高。不同温度烧结处理的结果表明 :烧结温度是影响Mg2 Ni相形成的重要因素 ,烧结温度达到 843K ,Mg Ni粉基本能完全转变为Mg2 Ni相。     

机械合金化制备Cu-0.5Wt%Nb合金组织与性能的研究

采用机械合金化法制备了Cu-0.5wt％Nb纳米弥散强化铜合金。通过金相、透射电镜观察了该合金粉末态、冷轧态及不同温度退火后合金的组织结构变化。通过测量该合金不同状态的硬度及相对电导率,实验结果表明,其冷轧态硬度较高(可达160kg/mm2),随退火温度升高硬度呈缓慢下降趋势;900℃退火后,硬度仍可达91kg/mm2,表明此合金抗高温软化性能较好.此外该合金相对电导率最高可达89% IACS,这进一步说明利用机械合金化法制备的Cu-0.5wt%Nb合金具有优越的综合性能.     

Complete flux jumping in nano-structured MgB2 superconductors prepared by mechanical alloying

High density nano-crystalline MgB₂ bulk superconductors with induced pinning centres were prepared from elemental precursors by a sequence of ball milling, heat treatment, and final pressing. The XRD results revealed the main phase was MgB₂ with a minor component of MgO. The magnetic moment versus temperature indicated that the transition temperature of MgB₂ samples was around 34 K, which is less than the typical transition temperature of commercial powders and also the transition temperature strongly depended on the milling time. It is well known that introduction of defects, grain boundaries and impurities act as effective flux pinning centres in MgB₂ and results in increased critical current density, J_c and decreased the transition temperature, T_c. The magnetization measurements were performed using VSM at 10 K, 20 K and 30 K, and the M–H curves indicated a complete flux jump effect, which is a severe problem for the application of superconductors. It was determined that a noticeable amount of heating (0.3 K jumps at 10 K) occurs at these jumps. In addition, it was found that the sweeping rate of magnetic field and the size of bulk sample are very effective to promote the flux jumping and whereas a low sweeping rate (12 Oe/s) avoids these “avalanches”, consistent with a kind of supercritical phenomenon: going slower allows the field gradients to stay close enough to equilibrium so that the avalanche effect is not triggered. In contrast, the sweeping rate of 100 Oe/s leads to numerous jumps.     

Fabrication and fracture analysis of MoSi_2 matrix composites by mechanical alloying

1　ＩＮＴＲＯＤＵＣＴＩＯＮＭｏＳｉ2 ｉｓｏｎｅｏｆｔｈｅｉｎｔｅｒｍｅｔａｌｌｉｃｃｏｍｐｏｕｎｄｓｗｈｉｃｈｈａｖｅｔｈｅｐｏｔｅｎｔｉａｌｉｎｂｅｉｎｇｕｓｅｄａｓａｄｖａｎｃｅｄｈｉｇｈｔｅｍｐｅｒａｔｕｒｅｍａｔｅｒｉａｌｓ[1] .Ｉｔｈａｓｈｉｇｈｍｅｌｔｉｎｇｐｏｉｎｔ(2 0 30℃ ) ,ｈｉｇｈｄｕｃｔｉｌｅ/ｂｒｉｔｔｌｅｎｅｓｓｔｒａｎｓｉｔｉｏｎｔｅｍｐｅｒａｔｕｒｅ(10 0 0℃ ) ,ｈｉｇｈｏｘｉｄａｔｉｏｎｔｅｍｐｅｒａｔｕｒｅ(16 0 0℃ ) ,ｒｅｌａｔｉｖｅｌｙｌｏｗｄｅｎｓｉｔｙ(6 .30…     

Effect of C and milling parameters on the synthesis of WC powders by mechanical alloying

In the current study, the amount of carbon and the effects of milling parameters in production of tungsten-carbide (WC) powder were evaluated. Mechanical alloying (MA) of elemental W and C powders at different carbon-rich and carbon-deficient compositions was studied. XRD results showed that the higher the carbon content the longer the milling period for the formation of WC powder. We also report on the effect of milling parameters on the phase formation. In stoichiometric composition, WC was synthesized faster than in compositions with higher carbon amount. Furthermore, W₂C phase was observed in compositions with higher carbon content milled at low speed and ball-to-powder ratio (BPR), as well as in carbon-deficient composition milled for shorter period. The ab initio calculations were performed in attempt to explain the destabilization of W₂C on further milling.     

Thermoelectric properties of mechanically alloyed iron disilicides consolidated by various processes

Mn doped p-type iron disilicide powders have been produced by a mechanical alloying process. As-milled powders were of metastable state and mostly transformed to β-FeSi₂ phase by subsequent isothermal annealing. As-milled powders were consolidated by various processes such as sintering of the cold compact in vacuum, vacuum hot pressing (VHP), and spray drying/atmospheric plasma thermal spraying. Phase transitions during the processes were investigated using XRD, EDS, and SEM. As-consolidated specimens consisted of a mixture of α-Fe₂Si₅ and ε-FeSi phases, which were gradually transformed into a thermoelectric semiconducting α-FeSi₂ phase by subsequent isothermal annealing in the vicinity of 845°C in vacuum. However, some residual α and ε phases remained even after prolonged annealing. Thermoelectric properties were measured as a function of temperature and correlated with phase transformation. They showed optimum values in the vacuum hot pressed specimen due to a higher fraction of β phase and/or higher density.     

Effect of ball milling time on microstructure and properties of Laves phase NbCr2 alloys synthesized by hot pressing

Laves phase NbCr2 alloys with a composition of Nb-66.7Cr (molar fraction, %) were prepared by mechanical alloying and hot pressing. The microstructures and properties of the Laves phase NbCr2 alloys, prepared from elemental niobium and chromium powders under various ball milling time by hot pressing at 1 250 ℃ for 0.5 h, were investigated. The results indicate that if the ball milling time is longer than 40 h, the synthesizing reaction of Laves phase NbCr2 can be accomplished much sufficiently. Then the nearly full-dense Laves phase NbCr2 alloys can be prepared by hot pressing from ball milled powders with more than 40 h. The hot pressing sample with homogeneous and fine microstructure made from 40 h ball milled powders has the optimum microstructure and properties. It has a relative density of 98.1%, Vickers hardness of 11.4 GPa, compress strength of 1 981 MPa and fracture toughness of 4.82 MPa·m1/2. The effect of fine grain toughening is fully realized.