Na掺杂对Bi-2212高温超导体性能影响研究

本文采用放电等离子烧结(SPS)工艺制备了Na掺杂的Bi_2.1Sr_1.96Ca_1-xNa_xCu_2.0O_8+d超导材料。Bi-2212前驱体粉末采用改进的共沉淀工艺制备,并在烧结前将前驱体粉末与NaOH混合。本文系统研究了Na离子掺杂对Bi-2212体系晶格结构、相组成、微结构和相关超导性能的影响。Na离子掺杂导致Bi-2212体系相转变温度的显著降低。因此在SPS过程中,烧结温度相应发生了变化。但Na掺杂仍然导致大量Bi₂Sr₂CuO₆ (Bi-2201)相生成,进而导致了超导相含量的降低和织构度的破坏,所以给体系的超导电性带来了负面的影响。另一方面,Na掺杂相Bi-2212中掺入了大量的点缺陷,这些点缺陷可以作为有效钉扎中心。因此,通过优化Na掺杂Bi-2212的烧结工艺,有可能在高场条件下获得体系性能的改善。     

Mechanical and Thermal Transport Properties of Suspension Thermal-Sprayed Alumina-Zirconia Composite Coatings

Micro-laminates and nanocomposites of Al₂O₃ and ZrO₂ can potentially exhibit higher hardness and fracture toughness and lower thermal conductivity than alumina or zirconia alone. The potential of these improvements for abrasion protection and thermal barrier coatings is generating considerable interest in developing techniques for producing these functional coatings with optimized microstructures. Al₂O₃-ZrO₂ composite coatings were deposited by suspension thermal spraying (APS and HVOF) of submicron feedstock powders. The liquid carrier employed in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to unique and novel fine-scaled microstructures. The suspensions were injected internally using a Mettech Axial III plasma torch and a Sulzer-Metco DJ-2700 HVOF gun. The different spray processes induced a variety of structures ranging from finely segregated ceramic laminates to highly alloyed amorphous composites. Mechanisms leading to these structures are related to the feedstock size and in-flight particle states upon their impact. Mechanical and thermal transport properties of the coatings were compared. Compositionally segregated crystalline coatings, obtained by plasma spraying, showed the highest hardness of up to 1125 VHN_{3 N}, as well as the highest abrasion wear resistance (following ASTM G65). The HVOF coating exhibited the highest erosion wear resistance (following ASTM G75), which was related to the toughening effect of small dispersed zirconia particles in the alumina-zirconia-alloyed matrix. This microstructure also exhibited the lowest thermal diffusivity, which is explained by the amorphous phase content and limited particle bonding, generating local thermal resistances within the structure.     

Perovskite Type Compounds and High Tc Superconductors

Recently, it has been found that copper-containing oxides have superconducting transition temperatures as high as 90 K. Further, T_c values up to room temperature have been rumored. The discovery that these oxides have the perovskite and related structure has renewed interest in these structures. An examination of the structure and properties of perovskite-type compounds may lead to a better understanding of these high T_c materials. In addition, this information may also help in learning how to prepare these materials reproducibly and aid in the search for still higher T_c superconductors. This paper provides some background information on perovskite compounds to describe what is known about the physics of new superconducting compounds.     

A metallurgical approach to high-Tc microcomposites

A metallurgical approach has been developed to produce high-critical-temperature (high-T_c) superconducting oxide/silver microcomposites. Metallic precursor alloys, which contain the metallic constituents of the superconducting oxides and silver, were made by melt spinning, melt dipping or melt writing, and then converted into superconducting microcomposites by atmospherecontrolled oxidation and annealing. The Bi-Pb-Sr-Ca-Cu-Ag alloy system was chosen as the system upon which to apply these methods, and superconducting composites with zero resistance at 104–110K and engineering critical current densities of 400–700 A/cm² at 77K were produced in this way with excellent reproducibility. The analysis of the process includes a variant of a temperature-time-transformation diagram. The processing methods discussed in this article have advantages in producing superconducting materials with improved mechanical properties and flexible shapes, including long ribbons, long wires and coils; they also can be used to join superconductors to either one another or normal metals.     

Microstructure and Properties of High-Tc Superconductors

The recent advances in high-critical-temperature surperconducting materials have precipitated an abundance of research into this exciting technology. Such research includes significant discoveries related to sperconducting properties, including the role of copper-oxygen layers in perovskite-based structures and the effects of 45° twins, 90° domains and subgrain boundaries. The identification of superconducting oxides has also led to an in-depth exploration of practical forming methods. It is anticipated that the formation of superconducting thin films with textured microstructures by using in-situ processing methods to reduce thermal budgets will play a critical role in the fabrication and integration of advanced superconductor and semiconductor devices.     

The effect of pulsed magnetron sputtering on the structure and mechanical properties of CrB2 coatings

CrB₂ thin films possess desirable combinations of properties (high hardness, wear resistance, chemical inertness, high thermal and electrical conductivity), which are attractive for a wide range of potential applications. Pulsed magnetron sputtering (PMS) of loosely-packed blended powder targets has allowed the deposition of stoichiometric chromium diboride coatings. The structure and properties of these coatings were found to be strongly dependent on the deposition process parameters; therefore investigation of the coating structures could explain certain differences between them and provide important information about the characteristics of the deposition process. In this study, characterization of the CrB₂ films was performed by scanning and transmission electron microscopy (SEM and TEM) techniques. The microstructures and properties of coatings deposited with different parameters are compared and changes that resulted from the variation of these parameters (particularly the pulsing duty cycle and the substrate biasing conditions) are discussed. The results show that besides the pulsing frequency, the target pulsing duty cycle is an important parameter of the PMS process, which is able to affect such coating properties as hardness, thickness and stress. Coating thickness measurement results suggest more intense bombardment of a growing film by energetic ions at lower values of duty cycle. Structural TEM analysis revealed two extremely different types of coating microstructures, obtained at quite similar substrate biasing conditions, i.e. floating (∼ − 15 V) and negatively biased (− 30 V). It appears that the structures of the coatings deposited at the negatively biased substrate are significantly affected by high-energy ion bombardment, which is a peculiarity of PMS that can modify film growth conditions. These conditions are not present when the substrate is allowed to float.     

Characteristics of Ceramic Coatings Made by Thin Film Low Pressure Plasma Spraying (LPPS-TF)

The thin film low pressure plasma spray process (LPPS-TF) has been developed with the aim of efficient depositing uniform and thin coatings with large area coverage by plasma spraying. At high power input (~150?kW) and very low pressure (~100?Pa) the plasma jet properties change considerably and it is even possible to evaporate the powder feedstock material providing advanced microstructures of the deposits. This relatively new technique bridges the gap between conventional plasma spraying and physical vapor deposition. In addition, the resulting microstructures are unique and can hardly be obtained by other processes. In this paper, microstructures made by LPPS-TF are shown and the columnar layer growth by vapor deposition is demonstrated. In addition to the ceramic materials TiO₂, Al₂O₃ or MgAl₂O₄, the focus of the research was placed on partially yttria-stabilized zirconia. Variations of the microstructures are shown and discussed concerning potential coating applications.     

金属氧化物表面与界面的第一性原理热力学:一个研究实例

通过第一性原理计算研究,能够揭示和阐明材料相关表面与界面热力学性质,这是通向纳米和界面材料设计自由的重要一步,即以理想性能为目标,科学设计制备工艺参数,针对性地调控材料微观结构.以SnO2纳米颗粒表面和原位内氧化制备获得的Ag-SnO2界面计算研究为例,系统介绍这一计算研究策略.基于缺陷热力学模型,构建不同环境温度和氧分压条件下所对应的热力学平衡状态的表面和界面结构模型,计算评估其相应性能.通过一系列的第一性原理能量学计算,进一步构建出平衡状态的表面与界面相图.这些相图能够充分描述制备和服役过程中,工况对材料表面和界面微观结构和相应性能的影响和作用关系.介绍和讨论如何应用这些相图理解和科学设计相应材料的制备工艺.     

Thermal stability and magnetic saturation of annealed nickel-tungsten and tungsten milled powders

Crystal structures, microstructures and magnetic saturation of annealed pure W powder along with W-40 wt.% Ni powder mixtures processed by high-energy ball milling were investigated using XRD, DTA, SEM and saturation magnetization techniques. Thermally induced transformations occurred at low temperature annealing. Supersaturated metastable Ni(W) solid solution formed during mechanical milling decomposed during annealing treatment into FCC Ni-rich, FCC W-rich phases and an eta-type phase which was constituted of BCC lattice of W enveloped by two FCC lattices of Ni and W. The structures of the major annealing products were close to Ni₁₀W and W₃Ni₂. The magnetic saturation of the milled W powder and W-Ni mixtures decreased with the increase in annealing temperature. Milling time was more influential on the magnetic properties of the annealed pure W powders.     

Synthesis of nanostructured powders by new chemical processes

New manufacturing processes, such as thermochemical, mechanochemical and chemical vapor condensation processes, have been developed to obtain nanostructured materials. Nanoscale size tungsten (W) base composite powders of less than 60 nm particle size such as WC/Co and W/Cu systems can be synthesized by thermochemical and mechanochemical processes using metallic salt precursors as starting materials. Nanostructured W base composite materials showed better sinterability, microstructural uniformity with ultrafine microstructures and mechanical properties than did commercial W base composite materials. Non-agglomerated nanoscale size TiO₂ powder can be synthesized by the Combustion Flame Chemical Vapor Condensation (CF-CVC) process using metallorganic precursors as a starting material. In this paper, scientific and technical issues on the synthesis of nanostructured materials by the new chemical processes are considered.     

High Steady Magnetic Field Processing of Functional Magnetic Materials

The materials science community has been enriched for some decades now by the “magneto-science” approach, which consists of applying a magnetic field during material processing. The development of anisotropic properties by applying a steady magnetic field is now a well-established effect in the material processing of magnetic substances, which benefits from the unidirectional and static nature of the field delivered by superconducting magnets. Among other effects, magnetic anisotropy in functional magnetic materials, which arises from the alignment of magnetic moments under external field, can be developed at various structural scales. Magnetic ordering, magnetic patterning, and texturation are at the origin of this anisotropy development. Texture is developed in materials from magnetic orientation due to magnetic forces and torques or from stored energy. In metals and alloys, for instance, this effect can occur either in their liquid state or during solid-state thermomagnetic treatments and can thus impact significantly the material functional magnetic properties. Today’s improved superconducting magnet technology allows higher field intensities to be delivered more easily (1 T up to several tens of Teslas) and enables researchers to gather evidence on magnetic field effects that were formerly thought to be negligible. The magneto-thermodynamic effect is one of them and involves the magnetization energy as an additional parameter to tailor microstructures. Control of functional properties can thus result from magnetic monitoring of the phase transformation, and kinetics can be impacted by the magnetic energy contribution.     

Application of Suspension Plasma Spraying (SPS) for Manufacture of Ceramic Coatings

Conventional thermal spray processes as atmospheric plasma spraying (APS) have to use easily flowable powders with a size up to 100 μm. This leads to certain limitations in the achievable microstructural features. Suspension plasma spraying (SPS) is a new promising processing method which employs suspensions of sub-micrometer particles as feedstock. Therefore much finer grain and pore sizes as well as dense and also thin ceramic coatings can be achieved. Highly porous coatings with fine pore sizes are needed as electrodes in solid-oxide fuel cells. Cathodes made of LaSrMn perovskites have been produced by the SPS process. Their microstructural and electrochemical properties will be presented. Another interesting application is thermal barrier coating (TBC). SPS allows the manufacture of high-segmented TBCs with still relatively high porosity levels. In addition to these specific applications also the manufactures of new microstructures like nano-multilayers and columnar structures are presented.     

Electrochemical noise characterisation of base metal and weld metal of an ASTM 516 steel in H2S containing environment

The inspection of steel structures exposed to sour environments has revealed a higher incidence of cracks at welded joints. This tendency is associated to several factors, including residual stresses, different microstructures and dissimilar electrochemical behaviour between base metal and weld metal. The crack incidence in general is attributed to hydrogen absorption due to the cathodic reduction of this species. In this work, the electrochemical noise behaviour of weld and base metals of an ASTM 516, C‐Mn steel exposed to a H₂S containing environment is studied. Current and voltage fluctuations are measured and the spectral noise impedance (R_sn) is determined. The current fluctuations are measured by means of a Zero Resistance Ammeter and voltage fluctuations with respect to a calomel reference electrode. In this case, residual stresses and microstructures of both materials can be assumed as secondary factors to hydrogen cracking. However, electrochemical noise measurements indicated the existence of different electrochemical processes for base metal and weld metal in the environment considered.     

Effect of neutron irradiation on the properties of the FeSe compound in the superconducting and normal states

The influence of atomic disorder induced by irradiation with fast neutrons on the properties of normal and superconducting states of polycrystalline samples of FeSe has been studied. The irradiation with fluences of fast neutrons up to 1.25 × 10²⁰ cm^?2 at the irradiation temperature T _irr ?? 50°C leads to relatively small changes in the temperature T _c of the superconducting transition and in the electrical resistivity ??₂₅ at 25 K. This behavior is related to the relatively low concentration of radiation defects arising at a given irradiation temperature, which is a consequence of a specific crystal structure of FeSe, which is more simple as compared to other layered compounds of this class.     

Bi系高温超导体中非公度调制结构相的空间群测定

采用会聚束电子衍射 (CBED) 对Bi--2212相和Bi--2223相中具有非公度调制结构的超胞结构进行了测定. 测定结果表明: 具有 A型调制结构Bi--2212相超胞的点群是2mm, 空间群是I2mm; 具有B型调制结构Bi--2223相超胞的点群是mmm, 空间群是Ibmm. 结合实验结果讨论了摇摆曲线对CBED图对称性的影响, 并指出: 采用CBED测定晶体结构的对称性时, 应以高阶Laue线为依据, 而不应依 靠0层盘内摇摆曲线的对称性. CBED图模拟结果表明: Bi--2212相CBED图显示的是调制结构超胞的对称性, 而不是亚胞的对称性.     

Microstructure and properties of Nb<Subscript>3</Subscript>Sn superconductors for the magnet system of the international thermonuclear experimental reactor

Results of a study of promising Nb₃Sn superconductors fabricated with the use of a “bronze” method and a method of “internal source of tin” for toroidal coils of the magnet system of the international thermonuclear experimental reactor (ITER) are presented. The main structural parameters and factors of the process of manufacturing technical superconductors based on Nb₃Sn, which determine the superconducting properties, are analyzed. The current-carrying capacity and the losses to hysteresis are determined for conductors with various diameters. The structure of the layer of superconducting Nb₃Sn compound is studied, and the relation between the structural parameters and the superconducting properties of the developed conductors is described. Examples of Nb₃Sn superconductors with a reinforcing element from a nanocomposite alloy Cu - 18% Nb are presented. Strength characteristics of the reinforced conductors are studied. Prospects of further enhancement of superconducting properties of Nb₃Sn superconductors obtained by both methods are estimated.     

On the microstructure-polarization behavior correlation of a 9Cr-1Mo steel weld joint

The use of 9Cr-1Mo ferritic steel necessitates its fabrication by the process of welding. The heat-affected zone (HAZ) of 9Cr-1Mo ferritic steel is a combination of many microstructures. In the present study, the corrosion properties of the base metal, weld metal, and the various regions of the HAZ are assessed with respect to their microstructures. The various microstructures in the HAZ were simulated by heat treatment of the normalized and tempered base metal at 1463, 1200, and 1138 K for 5 min followed by oil quenching. The microstructure of the base metal in the normalized and tempered condition revealed martensite laths with M₂₃C₆ carbides at lath boundaries, and uniform dispersion of fine, acicular M₂C. The weld metal showed predominantly martensitic structure with dispersion of carbides. Simulation of the microstructures of the HAZ by heat treatment resulted in the following microstructures: coarse-grained martensite of 75 μm size at 1463 K, fine-grained martensite at 1200 K, and martensite + proeutectoid α-ferrite at 1138 K. In all cases, carbide precipitation was observed in the martensitic matrix. Microhardness measurements of HAZ-simulated base metal showed increasing hardness with increasing heat treatment temperature. The hardness values obtained corresponded very well with the regions of the actual HAZ in the weld joint. Electrochemical polarization studies were carried out on the base metal, weld metal, weldment (base metal + weld metal + HAZ), and the simulated HAZ structures in 0.5 M sulfuric acid solution. Critical current densities (i _crit1 and i _crit2), passive current densities (i _pass and i _sec-pass), and transpassive potential (E _tp) were the parameters considered for evaluating the corrosion resistance. The HAZ structures simulated at 1463 and 1200 K, corresponding to coarse- and fine-grained martensitic regions of an actual HAZ, had corrosion properties as good as the normalized and tempered base metal. Of the various simulated HAZ structures, the intercritical region, which was simulated at 1138 K, possessed the worst corrosion resistance. The weld metal possessed the worst corrosion resistance of the various microstructural regions in the weld joint. The weldment adopted the degraded corrosion properties of the weld metal.     

Development of Ti-6Al-4V and Ti-1Al-8V-5Fe Alloys Using Low-Cost TiH2 Powder Feedstock

Thermo-mechanical processing was performed on two titanium alloy billets, a beta-titanium alloy (Ti1Al8V5Fe) and an alpha-beta titanium alloy (Ti6Al4V), which had been produced using a novel low-cost powder metallurgy process that relies on the use of TiH₂ powder as a feedstock material. The thermomechanical processing was performed in the beta region of the respective alloys to form 16-mm diameter bars. The hot working followed by the heat treatment processes not only eliminated the porosity within the materials but also developed the preferred microstructures. Tensile testing and rotating beam fatigue tests were conducted on the as-rolled and heat-treated materials to evaluate their mechanical properties. The mechanical properties of these alloys matched well with those produced by the conventional ingot processing route.