Effect of Pre-treatment on the Bi-2223/Ag Tapes Fabricated with Different Powders

Samples of Bi-2223/Ag tapes fabricated with powders contained different lead oxide compounds were annealed at a temperature lower than the formation temperature of Bi-2223 phase before traditional heat treatment, which was called pretreatment. In pre-treatment stage, samples were heat treated at 780 ℃ in 8.5% O2, and the dwelling time is different. Experimental results show that pre-treatment had different influence on the formation and size of the Bi-2223 grains for tapes fabricated with powders contained ...     

Microstructure and properties of Bi-2223/Ag/Ni tapes

To reduce the production cost of Bi-2223 superconducting tapes, Bi-2223/Ag/Ni tapes were fabricated by the powder-in-tube process. The effect of heat treatment on the microstructure and critical current of the Bi-2223/Ag/Ni tapes were studied. The phase compositions of the samples were characterized using XRD. The microstructure was observed using SEM. Experimental results indicate that higher temperature is more conducive to the formation of Bi-2223 phase at an atmosphere of 8.5% O 2 . After the two-step heat treatment, the critical current of samples is about 67.5 A.     

Bi-based superconductors fabricated in high magnetic fields

The microstructure and superconducting properties of Bi-2223 superconductor fabricated in high magnetic fields were investigated. The results shows that the Bi-2212 grains with their c-axis parallel to the magnetic field were formed after the partial-melting and solidification in 8 T magnetic field, and transformed into the Bi-2223 grains with c-axis alignment during the further sintering process at 840 ℃ without magnetic field. The conversion of Bi-2212 grains to Bi-2223 grains has the heredity in grain alignment. The mixed structures of the Bi-2223 and the Bi-2212 grains with their c-axis parallel to the magnetic field are formed in samples sintered at 850-855 ℃ in 10 T magnetic field. When sintered in 10 T below 845 ℃, a high proportion of Bi-2223 phase is obtained, however no preferred orientation is observed. The Bi-2223 grains with their c-axis parallel to the axial direction of the vertical tube furnace are formed not only on the surface, but also in the center of the sample sintered at 850 ℃ for 120 h in a 15 ℃/cm temperature gradient without magnetic field. Moreover, the samples sintered in the temperature gradient and in a 10 T magnetic field have a stronger c-axis alignment ofbi-2223 phase.     

Amorphization and magnetic properties of Fe62Nb38 mechanically alloyed powders

The amorphization and magnetic properties of Fe62Nb38 mechanically alloyed powders were investigated. In the initial mechanical alloying processes, the lattice structure of pure Fe is destroyed due to the cold-welding and fracturing, accompanying the reduction of ferromagnetic properties. The Ms value of Fe62Nb38 powders with ball-milling time t=6 h is only 48.1 A.m^2/kg. With prolongating of mechanical alloying processes, a solid state amorphization reaction (SSAR) takes place and the Fe-Nb ferromagnetic amorphous phase is formed. With the milling time increasing from 6 to 18 h, the saturation magnetization of Fe62Nb38 powders increases with enhancement of the proportion of ferromagnetic amorphous phase in milled powders. The Ms value of the Fe62Nb38 amorphous powders is 98 A.m^2/kg, which is very close to the value estimated from dilute model. However, the Curie temperature of the Fe62Nb38 amorphous phase is only 206℃, which is much smaller than that of the pure Fe. This implies that the exchange interaction between Fe atoms in amorphous alloyed Fe62Nb38 becomes weak due to the Nb dilution. Investigation shows that the variation of magnetic properties of milled powders is one of important tools for describing the amorphization by mechanical alloying.     

Structure evolution of Cu-based shape memory powder during mechanical alloying

The microstructures of shape memory powders of Cu-AI-Ni prepared from pure powders of Cu, AI and Ni using the mechanical alloying（MA） technique were studied by means of hardness measurement, metallograph observation, XRD and SEM. The hardness reaches the peak as the increase in hardness due to plastic deformation and the decrease in hardness due to kinetic annealing reach a balance. The process of MA leads to the formation of a laminated structure, and the layer becomes thinner with an increase in milling time. The pre-alloyed shape memory powder can be formed by milling at 300 r/min for 50 h using a planetary ball mill.     

MODELING OF STRAIN-INDUCED PRECIPITATION KINETICS IN Nb MICROALLOYED STEELS

On the basis of the thermodynamic calculation of precipitation and considering the effect of strain on the precipitation behavior and chemical composition （Si and Mn）, the kinetics of precipitation from austenite has been investigated for different temperatures and strains. Nucleation theory and the solubility product of niobium, carbon, and nitrogen in austenite have been used to derive equations for the start time of precipitation as a function of temperature and composition. The value of n in Avrami equation was determined using the available experimental data from the published reports, which indicated that n is a constant independent of temperature and the end time of precipitation is a function of n and the start time of precipitation. The values of the start time and end time of precipitation predicted by the new model are compared with the experimental values and a good agreement was obtained between both.     

SOLUBILITY CHARACTERISTICS OF GaAs IN Bi AND THEIR PHASE DIAGRAM

A modified liquid phase epitaxy apparatus for semiconductor materials was used to measurethe solubility of GaAs in Bi.Two phase diagrams rich in Bi under H_2 and N_2 atmosphereswere obtained according to the results of measurement.A new phenomenon,in which theparameter Q value(quantity of GaAs dissolved in Bi in fixed time/saturation quantitu,ofGaAs in Bi)was different from each other at various temperatures and there existed a maxi-mum Q value at definite temperature,was observed.This phenomenon may be regarded as acommon feature of a simple binary metallic system which has the phase diagram similar tothat of Bi-GaAs.The difference observed from the dependence of Q values on temperature inboth H_2 and N_2 atmospheres was discussed.     

SYNTHESIS OF NANO-ZnO PARTICLES FOR ALUMINUM METALLURGY AS INERT ANODE MATERIAL

Nano-ZnO particle was produced by evaporating zinc powders in air at air flow-rate from 0.2 to 0.6m^3/h.Nano-ZnO particles was formed by the oxidation of the evaporated zinc vapor.X-ray diffraction shows the powders to be ZnO with lattice parameters of α=0.3249nm and c=0.5205nm.The particle size is dependent upon the transit time from the source to the collection area.The size of particles was ranged between 81 to 103nm.The average density resulted was 4.865g/cm^3.Normal ZnO and nano-ZnO were investigated to use them in aluminum metallurgy as an inert anode material.A certain amount of both oxides were molded subsequently inserted to the molten cryolite-aluminum oxide to investigate the corrosive behavior of both oxides.When the sintering temperature increased up to 1300℃,the weight loss ratio rose to 5.01%-7.33% and up to 7.67%-10.18% for nano-ZnO and normal ZnO,respectively.However, when the samples in the molten cryolite aluminum oxide were put for long time,the corrosive rate was found to be higher.It was found that the corrosive loss weight ratio of nano-ZnO anode was much lower than the normal one made from ordinary-ZnO providing that the nano-ZnO is more possible to be use inert anode material.     

CALCULATION OF THE VACANCY VOLUME OF METALS BY MODIFIED BROOKS'''' MODEL

1.IntroductionVacancies are very important point defects, which have remarkable influence on the mechanical, optical, and electrical properties of materials[1]. Besides the vacancy formation energy, the vacancy volume is also an important quantity to understand of many processes in metals. The vacancy volume can be measured by experiments[2-5] or calculated by theories[6-13]. Due to different experimental methods, the experimental values are different from each other, for example, the exper…     

IMPROVING THE SURFACE PROPERTY OF TC4 ALLOY BY LASER NITRIDING AND ITS MECHANISM

The mixing technology of laser and heated nitrogen was applied to improve the surface hardness of titanium alloy (TC4). The samples were nitrided with laser power density 0f 6.5×105W·cm-2, the scanning speed various from 100 to 500mm · mm-1. The nitrogen gas was pre-heated to 300℃ to accelerate the nitriding process. Some interested samples were tested with XRD method (X-ray diffraction) to analyze the composition of nitrides, and the surface hardness of HV was measured. The results show that TiN and Ti2N were formed on the surface of Ti alloy with proper nitriding parameters, but TiN is the main composition. The surface hardness increased by three times, which is from the original value of 269 to 794kg· mm -2. The mechanism of the mixing technology is considered mainly of the activation of nitrogen by laser power and the pre-heated process which accelerated the nitriding process. The nitridation process can be considered as six steps given in detail. The result by analyzing the mechanism of improving the surface property of TiAl alloy shows the improvement of surface property due to two factors: the first reason is the result of laser annealing, and the second one is the formation of TiN.     

铋系高温超导粉体的制备

首先采用喷雾热分解法制备了铋系超导粉体，然后对得到的粉体进行焙烧处理，焙烧时采用不同的工艺参数，包括温度、保护气体的氧分压等。利用XRD，SEM等分析手段，研究了焙烧后粉体的粒度分布，形貌，相组成及其均匀性等。最后，采用粉末套管法将焙烧后的超导粉体制备成单芯超导带材，在形变热处理后，带材样品获得了超过30A的临界电流。     

Ca含量对Bi-2223超导带材的结构及电流传输性能的影响

采用喷雾干燥法制备了4种不同Ca含量的BSCCO 2223初始粉末，并研究了Ca掺杂对Bi-2223银包套超导带材的相转化、微观结构和电流传输性能的影响．实验表明，增加Ca含量促进了2223相的转化和提高了带材的临界电流，但同时也在超导芯中引入了较多的富Ca的非超导杂相．分析认为，较高的Ca含量增加了体系烧结过程中液相的含量，进而促进了相转化和晶粒生长及晶间连接，因而改善了电流传输能力。     

拉伸应变对Bi-2223银及银合金包套带材临界电流的影响

研究了Bi-2223带材的应力-应变特征，比较了包套材料、芯丝数目和Ag／超比例对Bi-2223带材不可逆拉伸应变的影响。实验发现，采用高强度的AgMn合金包套材料可提高芯丝的残余压应变，并通过提高Ag/超界面的平整性，减少因为界面不均匀而产生的应力集中，芯丝中微裂纹的出现和扩展将被延迟到更高的应变状态，因此可显著提高带材的不可逆拉应变。同时，通过增加芯丝数量和Ag／超比例也可增强带材抗拉伸应变能力。     

CaCuO2颗粒尺寸对双粉法制备Bi系高温超导带材性能的影响

本文通过基于共沉淀工艺的双粉法制备了Bi_1.76Pb_0.34Sr_1.93Ca_2.0Cu_3.06O_8+d (Bi-2223)前驱体粉末。在这一过程中,首先单独制备了Bi_1.76Pb_0.34Sr_1.93CaCu_2.06O_8+d (Bi-2212)和CaCuO₂(实际相组成为Ca₂CuO₃和CuO)粉末,并分别进行了烧结。通过调节共沉淀工艺过程中的pH值,获得了颗粒尺寸不同的CaCuO₂粉末,然后将Bi-2212与其按照相组成相组成为1:1进行混合,并装入Ag包套中,通过一系列的旋锻、拉拔和轧制工艺,获得设计尺寸的Bi-2223带材。比表面积测试表明随着pH值从3.0增加到5.0和6.5,获得CaCuO₂粉末的平均颗粒尺寸从1.1减小到0.75和0.60 mm。通过扫描电镜对不同尺寸CaCuO₂颗粒制备的Bi-2223生带、第一次热处理和后处理之后带材的相组成和分布进行了表征。结果表明,适当尺寸的CaCuO₂颗粒可以避免团聚现象的出现,因此有利于高载流性能带材的获得。最终通过进一步调节带材的尺寸,1#带材的性能最高,达到了12200 Acm^_-2。     

装管密度对Bi-2223／Ag超导带材性能的影响

研究了不同装管密度对Bi-2223／Ag带材(19芯)性能的影响。结果表明：采用压棒装管工艺提高了带材芯丝的填充系统，有利于提高带材的工程电流密度；第一次热处理后带材表面的鼓泡明显比松装带材少，有利于提高电流沿长度方向上的均匀性，更适合制备长带。压棒装管带材与松装带材相比，宽展更大，最终热处理后芯丝致密，孔洞少，而且2223相含量高，最终带材的载流性能好于松装带材。     

中间变形对Bi-2223/Ag超导带材织构的影响

借助于扫描电镜和X射线衍射分析摇摆曲线，研究了Bi-2223／Ag单芯超导带材制备过程中中间变形对Bi-2223超导相织构的影响。结果表明，织构度与中间变形量的关系曲线呈现峰值变化规律。采用轧制进行中间变形，Bi-2223／Ag单芯带材的变形量εopt为14．3％时Bi-2223相织构最佳；采用压制进行中间变形，变形量εopt为10．7％时Bi-2223相织构最佳。中间变形后的带材经过最终热处理，其Bi-2223相的织构得到进一步改善。四引线法临界电流密度测量结果表明，其与带材的织构度成正对应关系。     

降温速率对Bi-2223/AgAu带材微观结构和传输性能的影响

采用PIT工艺制备了单芯Bi-2223/AgAu带材,系统地研究了第二次热处理阶段(HT2)降温速率对带材相组成、微观结构和传输性能的影响。结果表明：随着降温速率的减小,富铅相3321不断增加,CuO颗粒尺寸逐渐增大。当冷却速率从600°C/h减小到1°C/h时,临界电流密度Jc从7 kA/cm₂增加到11.5 kA/cm₂,增加了64%。由于晶间连接性能和磁通钉扎性能的提高,在较低的降温速率下,Bi-2223/AgAu带材在磁场下的临界电流密度也得到了提高。     

铋系高温超导带材内部组织结构分布不均匀的分析

观察了Ｂｉ—２２２３带村内部组织结构的变化。测定了带材内导电性能不均匀和轧制及冷压过程变形不均匀的关系     

Fabrication of non-superconducting and superconducting joints of Ag/Bi-2223 tapes by diffusion bonding

Diffusion bonding of multifilamentary Ag/Bi-2223(Bi_2Sr_2Ca_2Cu_3O_x) superconducting tapes has been performed successfully.Three types of joints,namely,with silver interlayer (type I),bonded directly (type II),with Bi-2212(Bi_2Sr_2Ca_1Cu_2O_x) powder interlayer or with Bi-2223 powder interlayer (type III) were researched,respectively.The critical currents(I_C) of these joints were measured and microstructures of the joints were evaluated by scanning electron microscope (SEM).The result shows that type I is non-superconducting joint with very low resistance,and type II joint and type III (with Bi-2212 or Bi-2223) joints are both of superconducting property.By comparing,type II joint possesses most excellent superconductivity,type III joint with Bi-2223 powder interlayer takes the second place,and type III joint with Bi-2212 powder interlayer is least.