Effects of Y211 phase contents on the critical current density Jc and microstructural analysis in YBCO bulk superconductors

YBCO bulk superconductors were prepared by the solid state reaction and top-seed-melt-textured growth(TSMTG) process. By using the AC susceptibility measurement, the critical transition temperature Tc of samples is 91.5 Kfor the highest value, and the transition width △TC is less than 1 K. The highest magnetization critical current densities Jcachieved 106 A/cm2 under 5 T at 10 K and 1.35×104 A/cm2 under 2 T at 70 K (H//c), respectively. The results combining theSEM observation indicate that doping of Y211 particles is more effective in improving the growth quality of melt-texturedYBCO superconductor and in reducing the micro-cracks of specimens. Doping of Y2O3 powder forms the rod-shaped Y211particles, but doping of Y211 particles directly to matrix materials forms the spherical Y211 particles mainly. Combiningthe microstructures with Jc measurements shows that the interfaces are most important on flux bundle pinning, in which thegradient of free energy is larger than that of other place between the Y     

籽晶定向浸渗生长法制备YBCO块材的微观组织特征研究

采用籽晶浸渗生长工艺和定向凝固技术(DSIG)可以制备具有良好取向的单畴YBCO块材.利用Y-211颗粒和液相预制体作为初始材料以浸渗生长工艺可以控制Y-123基体上的Y-211颗粒的尺寸和分布并且可以制备出近终形的块材.讨论了籽晶定向浸渗生长工艺的主要参数(浸渗结构、温度梯度和最高加工温度).另外,当液相为035时,Y-211颗粒掺杂的尺寸小于其它2种液相成分所得到的Y-211颗粒掺杂尺寸.     

液相烧结制备织构（123＋211）的YBa2Cu3O7—x超导体

用液相烧结法研究了制备名义组成（１２３＋２１１）织构的ＹＢａ２Ｃｕ３Ｏ７－ｘ超导体。在１２３（ＹＢａ２Ｃｕ３Ｏ７－ｘ）粉中掺适量的２１１（Ｙ２ＢａＣｕＯ５）粉消除ＢａＣｕＯ２在晶界上沉积。在包晶转变温度附近降低冷却速率允许有足够时间沿ａ－ｂ面进行质量转移，ａ－ｂ面是最快的生长面，晶粒长度受质量转移速率控制。ＸＲＤ，ＳＥＭ，ＥＤＸＡ分析指出，晶粒是长条形，取向垂直于ｃ轴，平行于表面。样品１２３具有强     

Single-crystal star-like zinc oxide: synthesis, characterization and growth mechanism

A novel star-like single-crystal ZnO structure was synthesized by a simple solvothermal method.The as-prepared products were characterized by XRD, SEM and TEM.The star-like ZnO, which shows sixfold symmetry, was constructed by six uniform arms distributing symmetrically around the [0001] zone axis.It is also found that the arms are not perpendicular to the [0001] zone axis and each arm is grown at a certain angle with the [0001] direction.CBED pattern and corresponding simulation demonstrate that the convex part of the star-like ZnO is O-terminated (000-1) plane and the concave part is Zn-terminated (0001) plane.H2S plays a crucial role in the synthesis process.The anisotropic growth habit along [0001] and [000-1] results in the formation of star-like structure.     

Synthesis and superconductivity of Y2Ba1Cu1O5 and Pt added Nd1Ba2Cu3O7?y bulk superconductor

Nd₁Ba₂Cu₃O_7−y (Ndl23) bulk superconductor with T_c>77K was prepared by a melt process under 1 atm pure oxygen and reduced oxygen partial pressures. The addition of Y₂Ba₁Cu₁O₅ (Y211) particles suppressed the generation of microcracks during oxygenation in resulting in the dispersion of Y211 fine particles. The addition of a small amount of Pt resulted in the reduction of the size of Y211 particles. The oxygen partial pressure (Po₂) of 0.01 atm was the optimum atmosphere for the synthesis of Ndl23 in which Y211 particles were dispersed finely and homogeneously. The addition of Y211 and Pt enhanced the flux pinning force in a melt-processed Ndl23 specimen because the presence of Y atoms in a melt phase suppressed, to some extent, the substitution of Nd on Ba site in an Ndl23 crystal prepared by a melt process.     

Five-fold branched Si particles in laser clad AlSi functionally graded materials

Many five-fold branched Si particles (Si_p) were observed in Al–40 wt% Si functionally graded materials produced by a single-step laser cladding process on cast Al-alloy substrate. In this paper the five-fold twinning and growth features of Si_p are scrutinized with orientation imaging microscopy and electron microscopic examination. It is a more in depth study of formation of the Si particles in functionally graded materials as published in our previous paper [Pei, Y. T. and De Hosson, J. Th. M., Acta mater., 2000, 48, 2617]. These Si particles have grown from twinned decahedron nuclei consisting of five tetrahedrons that share a common 110 axis. The twin plane re-entrant edge (TPRE) mechanism explains both the branch growth in the radial direction and the elongation of Si_p along their common 110 axis. Subsequent twinning within the twinned tetrahedrons provides additional re-entrant grooves on their top faces, which are important for the rapid elongation and consequently for the continuous growth of the branched particle. The 7.5° mismatch that arises by putting together five tetrahedrons around a common 110 axis is accommodated by small-angle grain boundaries (SAGBs). The SAGBs may disturb the progress of growth steps, which causes the particles to branch. The most remarkable facts of the study are that the five-fold branched silicon particles are much bigger (25∼40 μm) than the nanometer sizes previously reported in the literature and the 7.5° mismatch is accommodated mainly by multiple SAGBs. The examples of a single SAGB reported before are just a special case of the SAGB mechanism.     

不同方法制备的Gd2BaCuO5粉体性能比较

采用低温燃烧合成法(LCS)、共沉淀法和固相法合成了超细Gd2BaCuO5(Gd211)粉体,并对3种粉体的形貌、化学纯度、比表面积和粒度进行了比较。结果表明,LCS工艺制备的Gd211粉体粒度最小,团聚最轻。将以上3种粉体作为掺杂剂制备GdBa2Cu3O5(Gd123)超导块材,块材的微观形貌说明微细Gd211粉体的添加能有效减小Gd123块体中Gd211颗粒粒度,临界电流密度的比较表明LCS法所制粉体的添加比另外两种粉体更能有效提高临界电流密度。     

ZrMn2合金及其氢化物的电子结构与成键特性的第一原理研究

采用基于密度泛函的赝势平面波方法，对ZrMn2贮氢合金及其氢化物的电子结构进行计算，并分析成键特性，结果表明，在ZrMn2合金中，Mn（2）原子之间的相互作用强于Mn原子与Zr原子之间的相互作用，Mn（2）-Mn（2）原子间存在明显的共价作用，但在吸氢（H）后，Mn（2）-Mn（2）相互作用明显减弱．H与Zr之间在c轴方向的键合力较弱，当H原子进入吸氢四面体后所引起的晶格膨胀导致Zr原子易于向外移动，使得c轴的膨胀大于α轴的膨胀．     

Synthesis of yttrium barium copper oxide-0.325Ag superconductors via intermediate precursor with overall composition Y:Ba:Cu:0=l:2:3:Y, Y>7 produced by high-energy attrition milling

A highly homogeneous composite precursor containing nano-scale particles was synthesized from the highenergy attrition milling of a mixture of metallic Y, metallic Cu and barium nitrate (Ba(N0₃)₂) containing 5 wt.% silver. The particle size of the 20 hour attrition-milled precursor was in the range of 30~80 nm as estimated by the XRD technique and direct TEM observation. With the heat treatment of the attrition-milled precursor, an intermediate precursor with the overall composition Y:Ba:Cu:O=l:2:3:y, y>7 was synthesized. The analyzed Y:Ba:Cu:Ag:O molar ratio of the intermediate precursor by AES/AAS and iodimetric titration was 1:1.972:3.022:0.323:7.41, which was very close to the estimated composition: 0.5Y₂O₃+2BaCuO_2.5+CuO +0.325Ag. Y123-Ag superconductors in powder or bulk forms were prepared using heat treatment or thermomechanical processing of the intermediate precursor containing BaCu0_2.5. The high oxygen orthorhombic Y123-Ag phase (y>7) was obtained in quenched samples which were held for 30 min or less at 950°C in ambient air. As the holding time extended to 40 min, the sample was still orthorhombic Y123-Ag (6.7<y <7). The transport critical current density (J_c) at 77 K and zero magnetic field for the quenched sample which was held for 40 min at 950°C was ~1.3xl0³ A/cm².     

The role of dislocations in the growth of nanosized voids in ductile failure of metals

Dislocations are the most important element in our understanding of the mechanical response of metals. Their postulation in 1934 led to revolutionary advances in our ability to predict the mechanical behavior of materials. The authors recently advanced a dislocation mechanism for void growth in ductile metals. This paper reviews the analytical and atomistic calculations carried out in support of this model. The emission of shear dislocation loops, nucleated at the surface of nanosized voids, is responsible for the outward flux of matter, promoting void growth. This is a new paradigm in the initiation of void growth, which was attributed to convergent vacancy diffusion or to prismatic loops by others. The analytical treatment is based on the emission of a dislocation from a void in the plane along which the shear stresses are maximum. Molecular dynamics calculations performed for different orientations of the tensile axis show how the loops generate and expand outward. These loops involve the emission of partial dislocations and are the counterpart for voids of the Ashby geometrically necessary shear loops postulated for rigid particles. This process is demonstrated for bicrystalline and nanocrystalline copper.     

NiFe alloy particles doping effect of Gd–Ba–Cu–O bulks processed by a new cold-seeding technology

The process of cold seeding melt growth of GdBa₂Cu₃O_y (Gd123) bulk superconductors using NdBa₂Cu₃O_y (Nd123) thin films was reported. In addition, a novel cold seeding concept of combining MgO crystal and buffer pellet was also introduced. The misorientation caused by the lattice mismatch between MgO and Gd123 melt was overcome by choosing suitable heat treatment program and Gd₂BaCuO₅ (Gd211) content of the buffer pellet. The doping effect of soft ferromagnetic NiFe alloy particles was also reported. The bulk sample with 0.4% (mole fraction) doping amount shows the best performance on the flux trapping. The critical current density is largely enhanced under the external field of 1–2 T, which is promising for large-scale applications. This effect is originated from the substitution of Fe and Ni ions for the Cu sites contributing to magnetic flux pinning.     

Crystal structures and thermal expansion properties of Y2-xErxMo4O15 （x=0.0-2.0） solid solutions

A new series of rare earth compounds,Y2-xErxMo4O15 （x=0.0-2.0）,were synthesized and their structures were determined by powder X-ray diffraction （XRD）. Rietveld analysis of XRD patterns reveals that the compounds of this series crystallize in monoclinic system with space group P21/c. The lattice parameters a,b,c,β and the unit cell volume V de-crease linearly with increasing Er content. The thermal expansion properties of these compounds were studied under high-temperature XRD. Positive thermal expansions of compounds Y2-xErxMo4O15 are found to be anisotropic along the three crystallographic directions where a and c axes expand while b axis contracts in the whole temperature range. By analysis of data,the expansion of weak band Mo2-O14 with rising temperature should be responsible for positive thermal expansion.     

Effect of Y_2Ba_4CuWO_y Addition on the Properties of Single Domain YBCO Superconductors by TSIG Technique

Top seeded infiltration and growth technique (TSIG) is an effective method for the preparation of high quality large YBa2Cu3O7-x (YBCO) bulk superconductors. In this paper, single domain YBCO bulk superconductors, with different Y2Ba4CuWOy (YW2411) additions, have been fabricated by TSIG process. the solid phase pellet is made of a mixture of Y2BaCuO5(Y211) + xwt% YW2411 powders, x=0, 1, 3, 5, 7, 9. The growth morphology, microstructure and levitation force of the YBCO bulks have been investigated, it is fo...     

On the development of grain growth resistant tantalum alloys

During exposure of pure Ta to temperatures up to 1800 °C pronounced grain growth combined with embrittlement becomes a major problem. Doping with elements which form nanometer to submicron sized oxide or silicide particles is an appropriate way to prevent, or at least to hamper, uncontrolled grain growth. In the present paper the effects of doping with varying combinations of Si and Y on microstructure and mechanical properties of cold-worked and annealed Ta have been investigated. For these purposes methods including small-angle neutron scattering and transmission electron microscopy as well as light-optical microscopy have been applied. Ta samples doped with Si show a higher hardness and strength than those doped with Y or made from pure Ta powder. However, the grain growth behaviour of all alloy variants is rather similar. The particle size distributions of doped Ta change significantly with varying annealing treatments as coarsening and dissolution of the prevailing particles (oxides and silicides) take place. Therefore, particles do not play a significant role in grain growth kinetics at temperatures as high as 1800 °C. However, this loss in retarding force is partially compensated for by an increased solution drag stemming from elements in solid solution.     

Effect of Y_2Ba_4CuNbO_y on the Properties of Single Domain YBCO Superconductor by TSIG Process

The top-seeded infiltration and growth process (TSIG) is an important method to fabricate YBCO bulk superconductors. In this paper, single domain YBCO superconductors with different Gd2Ba4CuWOx (GdW2411) additions have been fabricated by TSIG process, the amount of GdW2411 added to Y2BaCuO5 (Y211) is in the range from 0wt% to 8 wt%, but there is no other additions to the liquid source. The growth morphology, microstructure and levitation force of the YBCO bulks have been investigated, it is found that the l...     

氮气含量对CrNx薄膜相结构及摩擦磨损性能的影响

采用电弧离子镀技术,在45钢基体上制备了不同氮气含量的CrNx薄膜.采用X射线衍射技术(XRD)、扫描电子显微镜(SEM)、努氏硬度计和UMT型球-盘摩擦试验机,分别测试了薄膜相结构、表面形貌、显微硬度和摩擦磨损性能.结果表明:CrNx薄膜主要由CrN和Cr2N相组成;随着N2含量的增加,薄膜中Cr2N(211)衍射峰强度逐渐减弱,CrN(220)衍射峰强度逐渐增强;薄膜表面颗粒逐渐减少,表面趋于平整,薄膜硬度出现两个峰值,对应于薄膜为单相Cr2N和CrN的相组成处;与钢基体相比,氮气含量为35%时,CrNx薄膜具有良好的表面质量、最佳的硬度、优良的耐磨损性能.     

Direct growth of REBa2Cu3O7-δ (RE=Yb,Er, Y,Dy, Eu,Sm, Nd and Pr) from undercooled melt using aero-acoustic levitator

The authors have previously reported that the NdBa₂Cu₃O_7-δ (Nd123) phase solidifies directly from undercooled melt using an aero-acoustic levitator combined with a CO₂ laser in dry air flow. The reason why the YBa₂Cu₃O_7-δ (Y123) phase never undergoes primary solidification, however, remains unknown. There are systematic differences in the phase diagrams between stoichiometric Y123 and nonstoichiometric Nd123 with respect to the gradient of the liquidus line and the metastable high-temperature phase. In the present study, the containerless solidification of RE123 (RE=Yb, Er, Y, Dy, Eu, Sm, Nd and Pr) was carried out to compare systematically the direct growth of stoichiometric RE123 (sRE123) with that of nonstoichiometric RE123 (nRE123). In sRE123 specimens, the metastable high-temperature phase, the RE₄Ba₃O₉ (RE430) phase, solidified primarily from undercooled melt, while the nRE123 phase solidified primarily in nRE123 specimens. Moreover, in the Cu-enriched RE₃Ba₆Cu₁₁O_x composition, the sRE123 phase solidified directly from the undercooled melt by suppressing the primary solidification of the RE430 phase. The direct growth of RE123, especially Y123, is discussed using the metastable phase diagram proposed on the basis of the experimental results.     

温度对纯Cr涂层表面形貌和抗腐蚀性能的影响

目的探索温度对纯Cr涂层的抗腐蚀性能和表面形貌的影响。方法采用多弧离子镀技术在锆合金表面制备纯Cr涂层,利用扫描电子显微镜(SEM)与Image J图像处理软件综合分析了纯Cr涂层的表面形貌,并对表面大颗粒的数目及其尺寸大小进行了统计和分析。采用X射线衍射仪对涂层的相结构进行了分析。利用划痕仪和电化学工作站,分析了温度对纯Cr涂层的膜基结合力和抗腐蚀性能的影响。结果 380℃时,涂层表面大颗粒的数目及其平均尺寸最大,温度升高到420℃时,其数值最小;当温度进一步提升到450、500℃时,大颗粒的数目、平均尺寸逐渐增大。温度的升高,使纯Cr涂层由(110)晶面择优生长转为(200)晶面择优生长,晶粒尺寸随温度变化的规律为先减小后增大。涂层的结合力和抗腐蚀性能随温度的变化规律与大颗粒数目随温度的变化规律相反,呈先提高后降低的趋势。结论 420℃时,涂层表面大颗粒的数目及其平均尺寸最小,结合力和抗腐蚀性能较强,组织结构致密度高,晶粒细小,表面光滑平整。     

拉拔工艺对TA2管材织构及性能的影响

本实验成功将拉拔工艺应用于TA2管材的制备,同时应用不同锥角组合对TA2管材进行拉拔,对比了不同锥角组合对TA2管材的显微组织及再结晶织构的影响。结果表明:退火态拉拔管材表现为径向织构,且部分晶粒的(0002)基面法相同管材的宏观轴向(AD)平行;微观的晶粒取向对管材宏观的力学性能有重要影响。单道次拉拔工艺制备的TA2管材其晶粒取向同轧制工艺制备的TA2管材存在明显差异。     

固相块中微纳尺度Gd211粒子的控制及其对单畴GdBCO超导块材性能的影响

通过控制Gd211固相块烧结温度的方法,获得了系列具有不同显微组织结构的Gd211先驱固相块;在此基础上,采用顶部籽晶熔渗生长法,制备了系列单畴GdBCO超导块材。并对其熔渗生长前后之宏观形貌、显微组织结构、以及单畴GdBCO超导块材的磁悬浮力进行了研究。结果表明,只有当先驱固相块的微观结构满足：Gd211粒子均为球状或椭球状颗粒、平均粒径较小、气孔率适中的条件时,单畴GdBCO超导块材中的Gd211粒子平均粒径最小,样品的磁通钉扎力和磁悬浮力最大(约38.7 N,样品直径20 mm,测试温度77 K,永磁体直径20 mm,表面磁场0.5 T)。该研究结果对于如何从微观结构上控制固相先驱块和单畴REBCO超导块材中RE211粒子的形状、粒径大小、气孔率、以及提高其超导性能提供了科学的依据和思路。