Rapid Deposition of Buffer Layers for YBCO Coated Conductors on Biaxially-Textured Ni Tapes

Epitaxial growth of CeO₂ buffer layers on biaxially textured (001)Ni tapes was studied using pulsed laser deposition. Relationships between microstructure and deposition parameters were systematically studied in order to develop reliable long tape coating processes. It was found that orientation and texture of CeO₂ buffer layers were sensitive to deposition parameters. X-ray diffraction analyses showed that CeO₂ buffer layers had pure (001) orientation at 860 °C. Under optimized deposition conditions, highly (001) oriented CeO₂ buffer layers have been achieved at a high deposition rate of 1.5 nm/s in-plane texture and out-plane texture of CeO₂ buffer layers were 4.25 degrees and 5.85 degrees, respectively.     

Effect of microstructure on the oxidation resistance of porous carbon materials in air

The oxidation resistance of porous carbon materials in air was studied by thermal analysis. The effects that the size, ordering, and orientation of carbon microcrystallites and the curvature of the surface of primary particles have on the temperatures of oxidation onset and peak weight loss were examined. The extent and orientation of surface graphite-like layers were shown to have a crucial effect on the oxidation resistance of porous carbon materials, which may remain stable to oxidation up to 700°C, the oxidation temperature of graphite.     

中间相沥青基石墨纤维的微观结构

本文用透射电镜、X 射线衍射分析等方法观察和分析了辐射状中间相沥青基石墨纤维的显微结构。结果表明,中间相沥青基石墨纤维中同时存在着有序石墨和乱层石墨两种相。在有序石墨相中,石墨片层堆叠的有序程度较高,且在纵向沿纤维轴高度择优取向,在横向则基本上沿径向排列。有序石墨微晶一般呈略有弯曲的片状,并具有分枝结构,微晶在纵向沿纤维轴交错排列,而在横向则沿径向从芯部向表层不断延伸、分叉和交织。在乱层石墨相中,石墨片层堆叠的有序程度较差,且沿纤维轴的择优取向程度较低。乱层石墨相一般镶嵌在有序石墨微晶之间。此外,在某些地方还存在着严重的晶格畸变。     

Phase and Texture Evolution of Ag/Ni Composite-Sheathed Bi-2223 Tapes with Different Thickness

The influence of green tape thickness on the Bi-2223 phase formation and texture evolution in Ag/Ni composite-sheathed tapes fabricated by the “powder-in-tube” technique has been studied. Microstructural observations by SEM as well as critical current density (J _c) measurements at 77 K, 0 T have been performed to analyze the performance of the tapes. The results show an important influence of the green tape thickness on the critical current depending on the content and texture of Bi-2223 phase. The J _c increases with decreasing thickness. Moreover, texture measured by omega scans shows that the texture of the Bi-2223 phase is significantly influenced by the thickness of the green tape after the first and final sintering processes. Alignment of Bi-2223 grains in the thin tapes is much better. Higher performance of Ag/Ni composite-sheathed Bi-2223 tapes can be obtained by controlling the thickness of the green tapes.     

碳质中间相形成机理研究

碳质中间相是制备高性能炭材料制品的优质前驱体，其产品已在高性能复合材料、超高功率石墨电极和二次锂离子电池用负极材料等方面得到了成功应用。碳质中间相产品的结构复杂、形态各异，造成了生产这些产品涉及的原料和处理过程的多样化。为了分析和预测一定条件下获得的碳质中间相产品的性能，进而确定其具体的用途，需要弄清碳质中间相的形成机理。通过系统地探讨碳质中间相的形成过程，对以往文献中的解释给予了归纳和分析。分析认为传统解释中存在许多不合理的地方，不能对一些现象给予合理的解释；“微域构筑”理论在传统解释的基础上有了很大进步，但由于该理论中引入了实际上并不存在的片层微晶单元而使其存在缺陷；“颗粒基本单元构筑”理论摒弃了以上两种理论中的不合理成分，能够对碳质中间相的形成过程进行很好的解释。此外，对“颗粒基本单元构筑”理论的一些应用也进行了讨论。     

单向C/C复合材料基体中间相在磁场中的取向

本工作利用扫描电镜、X射线衍射仪及正交偏光显微镜研究了沥青中间相以及单向C／C复合材料基体中间相在800mT磁场中处理后的取向,证实了沥青中间相的层片平行于磁场方向;在本工艺条件下,制造的单向C／C复合材料基体中间相层片垂直于碳纤维.     

Oriented growth of silicide and carbon in SiC-based sandwich structures with nickel

We report on the formation kinetics and the metal-mediated structuring in nanoregions of silicide and carbon containing interlayers in SiC-based materials. The silicide formation and the graphite texturisation are determined by complex reactive diffusion processes. High resolution and analytical electron microscopy evidenced a δ-Ni₂Si growth with a 5 0 6 fibre texture in parallel orientation to the 0 0 0 1 direction of the SiC substrate. The oriented growth of graphitic regions in the silicide hints to a diffusion controlled carbon precipitation from the silicide supersaturated with carbon, explaining the observed orientation relationships between graphite and silicon carbide: perpendicular and parallel to the {0 0 0 1} silicon carbide surfaces.     

Structure of mesophase pitch-based carbon fibres

The structure of five samples of commercially available carbon fibres with ultra-high modulus produced from mesophase pitch was studied by the complementary techniques of high resolution electronmicroscopy, X-ray diffraction and transverse magnetoresistance effect. The fibres with high strength and elongation to failure were found to be composed of turbostratic carbon structure, which was different from the three-dimensional graphite structure in ultra-high modulus carbon fibres. Transmission electron microscope examination revealed that the mesophase pitch-based fibres with high strength have a basic structure unit with folded sheets arranged nearly parallel to the fibre axis similar to those of high modulus carbon fibres produced from PAN. The present fold structure was suggested to contribute consequently to the lower graphitizability of the fibres and to the strong effects on the fibre strength. By controlling the microstructure, it is expected that the crystallographic as well as the mechanical properties could be improved significantly even from the same kind of precursor materials such as mesophase pitch.     

Analysis of current distributions in a multi-laminated HTS tape conductor for solenoid coils

A multi-laminated HTS tape conductor has been recently developed for large coils. If the HTS tapes are simply laminated to form the conductor, the current distribution in the laminated tape conductor of the coil is imbalanced because of the differences among inductances of tapes. Transposition of the tapes in the conductor is effective for homogeneous current distribution, but the tape may be damaged due to the lateral bending. The solenoid coil has enough space to transpose the tapes at both ends. However, a proposed theory so far requires a restriction in the number of coil layers for homogeneous current distribution in the laminated tape conductor. It is very important to analyze current distributions in the multi-laminated tape conductor for the solenoid coil with arbitrary layers. In this paper, we apply the Maxwell integral equation to the region contoured by adjacent laminated tapes to analyze the current distributions of the tapes in an infinite solenoid coil, and demonstrate that the flux across the region is conserved as long as the tapes are not saturated, and finally induce the fundamental equations as functions of coil construction parameters, such as layer radii, laminated tape spaces, and winding pitches. We use the fundamental equations for 2-layer and 4-layer coils to verify the homogeneous current distribution of the laminated tape conductor for an arbitrary layer number. Since the flux between the tapes in the inner layer of a 2-layer coil is contributed from the outer layers, the tape space in the outer layer must be larger than that in the inner layer because of the balance between the two fluxes. Moreover, we have developed an analysis method for a finite solenoid coil.     

高定向石墨块的控制制备及其导热性能影响因素研究

以廉价易得的高结晶度天然鳞片石墨(NG)和中间相沥青为原料, 采用中温热模压一次成型再高温炭化、石墨化处理可以制备高密度、高定向、高导热石墨块体材料。XRD、SEM和PLM分析表明该石墨块具有高度择优取向结构, 其内部石墨片垂直热压方向有序堆积排列。原料中鳞片石墨和沥青粘结剂的组成和配比以及制备工艺参数等对所制石墨材料的面向导热性能有显著影响。采用86wt%+32目鳞片石墨和14wt%AR中间相沥青在500℃、10 MPa下热模压成型的炭块经1000℃炭化、2800℃石墨化后样品的热物理综合性能较好, 其体积密度达到1.91 g/m³以上, 室温面向热导率为550 W/(m·K), 3000℃石墨化室温面向热导率高达620 W/(m·K)。     

Electron microscopic study on graphitization of bulk mesophases

Two kinds of bulk mesophases separated by quinoline from Polyvinylchloride (PVC) pitch and from 3, 5-xylenol-formaldehyde resin (XF) pitch were studied by high resolution electron microscopy. The ease of graphitization of both bulk mesophases has been examined by heat-treating them up to 1080 and 2600° C. PVC bulk mesophase mostly shows preferred long range orientation (aromatic carbon layers roughly parallel to each other), but also contains minor components where the molecular orientation occurs only over small distances, or not at all. When heat-treated at high temperature PVC bulk mesophase changes mainly into highly graphitized lamellar crystals. Similarly treated XF bulk mesophase does not show any preferred orientation and leads to porous nongraphitizable material.     

On-line characterization of YBCO coated conductors using Raman spectroscopy methods

The use of Raman spectroscopy for on-line monitoring of the production of superconducting YBa2Cu3O6+X (YBCO) thin films on long-length metal tapes coated with textured buffer layers is reported for the first time. A methodology is described for obtaining Raman spectra of YBCO on moving tape exiting a metal-organic-chemical-vapor-deposition (MOCVD) enclosure. After baseline correction, the spectra recorded in this way show the expected phonons of the specific YBCO crystal orientation required for high supercurrent transport, as well as phonons of non-superconducting second-phase impurities when present. It is also possible to distinguish YBCO films that are properly textured from films having domains of misoriented YBCO grains. An investigation of the need for focus control on moving tape indicated that focusing of the laser on the surface of the highly reflective YBCO films exiting the MOCVD enclosure tends to produce aberrant photon bursts that swamp the Raman spectrum. These photon bursts are very likely a consequence of optical speckle effects induced by a combination of surface roughness, crystallographic texture, and/or local strain within the small grain microstructure of the YBCO film. Maintaining a slightly out-of-focus condition provides the best signal-to-noise ratio in terms of the obtained Raman spectra. In addition to examining moving tape at the post-MOCVD stage, Raman spectra of the film surface can also be recorded after the oxygen anneal performed to bring the YBCO to the optimum superconducting state. Consideration is given to data processing methods that could be adapted to the on-line Raman spectra to allow the tagging of out-of-specification tape segments and, at a more advanced level, feedback control to the MOCVD process.     

Protective coatings for carbon fibers

Protective refractory nanocoatings on carbon bundles and tapes have been investigated. Processes have been developed for growing thin layers of refractory oxides—alumina, zirconia, and silica—on continuous carbon fibers and tapes using sol-gel processing. ZrC/ZrO₂ bilayer coatings have been produced by chemical vapor transport. The surface morphology, phase composition, and elemental composition of the coatings have been studied by x-ray diffraction, high-resolution scanning electron microscopy, and qualitative energy-dispersive x-ray microanalysis. The results demonstrate that the refractory oxide coatings are homogeneous along the length and perimeter of individual fibers and adhere well to the fiber surface, with no peeling. Their thickness is within 200–300 nm. The effect of the nature of the coating on the oxidation resistance of carbon materials is analyzed.     

Crack formation in mesophase pitch-basedcarbon fibres: Part I Some influential factors for crack formation

The crack observable in the transverse section of mesophase pitch-based graphitized fibres was studied to clarify its development on spinning, stabilization, carbonization and graphitization, and to find the factors influential on the development on spinning. The crack was already observed as an embryo in the as-spun fibre of radial texture, which increased the opening angle according to the extent of shrinkage due to carbonization and graphitization. The spinning temperature, length/diameter, diameter and wall material of the spinning nozzle and the kind of mesophase pitch were found influential in crack development, defining the transverse texture and the extent of longitudinal alignment along the fibre axis. Although the radial texture defined under an optical microscope tends to give a crack, not all of such textures carried the crack, suggesting the necessity of more detailed classification of the texture in terms of alignment, shape and size of domains observed under high resolution scanning electron microscopy (HR-SEM). The factors revealed as being influential on the textures appear to be related to the apparent viscosity, the perfectness of the radial orientation and die-swelling at the outlet of the nozzle to modify the texture defined through the nozzle. The interaction of the mesophase pitch with the nozzle wall and flow properties should be emphasized as factors controlling the texture on spinning. This revised version was published online in November 2006 with corrections to the Cover Date.     

Preparation of NiO Buffer Layer by Direct Oxygenation of Ni Tape

Recently, Nickel-based alloys are widely used for textured substrates tapes. However, the deposition of oxide buffer layers on nickel tapes is required to stop the diffusion of nickel from the tape to superconducting layer and to improve the mismatch between the substrate and superconducting film. Biaxially textured NiO buffer layer is easily formed on cubic textured nickel tape by the technique called surface oxidation epitaxy. In this work, we developed a direct oxygenating method to make the NiO buffer layer on nickel tape. The nickel tape was directly oxygenated in different atmospheres. It is found that in inert atmosphere, a high quality NiO layer can be formed on the surface of the tape. The oxygenating conditions, including atmospheres and temperatures, and their influence on the structure of the NiO buffer layer, were studied in detail and discussed.     

AC losses in multilayer superconducting tapes

ac losses have been investigated experimentally as well as theoretically in tapes having on the surface several normal conducting and/or superconducting layers. The superconducting layers under investigation have been Nb₃Sn on a niobium substrate and Nb₃Ge on a stainless steel substrate. It has been proved that the layered structure of the tapes is well reflected by the stepwise character of the ac losses dependence on the amplitude of the surface magnetic field. The magnetic flux passing through a surperconducting layer or surface barrier into the inside of the tape enhances the losses in the passed barrier or layer.     

Observations of preferred orientation in high-Tc oxide superconductor tapes

Thin tapes of YBa₂Cu₃O_7−δ superconductor have been tape cast by the doctor blade technique. X-ray diffraction studies showed that the (00l) planes of these tapes, to a modest degree, became preferentially aligned parallel to surface of the tape. This modest degree of preferential alignment resulted in a significant increase in measured critical current density.     

Forming Simulation of Thick AFP Laminates and Comparison with Live CT Imaging

Automated fiber placement (AFP) process can be used to manufacture laminates by laying up unidirectional slit tapes along a desired path and placing multiple layers on top of each other. Usually, the slit tapes are placed direct onto the tooling to attain the final part geometry. Alternatively, the laminate can be built up on a planar substrate and can be subsequently formed into the final shape. This kind of processing allows manufacturing highly curved parts, which may not be possible with the direct placement. In the present work a forming simulation of thick AFP laminates is developed to predict the tapes’ orientations and delamination as well as transverse tape spread-ups and separations during the forming process. The simulation model is built up through the material characterization experiments. Validation is performed comparing the results of the simulation vs. the experimental forming on two generic geometries. An optical inspection is made on the external layers of the laminates. In a second step, live computer tomography (CT) scans are used to inspect the tapes within an AFP laminate during forming of an L- and a Z-flange. Tapes re-orientation, gaps and tapes widening are observed experimentally and compared to the simulation results. The simulation is capable to predict the tows orientation and provides indicators concerning the tows spread-up and separation.     

Preparation and structure of mesophase pitch-based thin carbon tape

A mesophase pitch of 100 vol % anisotropy prepared from methylnaphthalene using HF/BF₃ was spun through a slit-shaped nozzle, stabilized in air under strain and carbonized at 1300°C into a very thin slit-shaped carbon tape 1.6 m thick and 14 m wide. Better crystalline orientation of the carbon tape always provided a Bacon anisotropic factor higher by 2% than that of the circular carbon fibre prepared from the same pitch. Excellent mechanical properties of the present carbon tape were obtained. Factors influencing the shape and orientation of the carbon tape were examined in terms of properties of mesophase pitches, spinning temperature, and the extent and strain of stabilization.     

Effect of magnetic field, thermal cycling and bending strain on critical current density of multifilamentary Bi-2223/Ag tape and fabrication of a pancake coil

Multifilamentary HTSC tapes are important for their applications in various electrical devices. Powder-in-tube technique with improved optimized synthesis parameters is regarded as one of the most promising ways to prepare long-length multifilamentary Bi-2223/Ag tapes. Nevertheless, usefulness of such tapes depends on their electrical and mechanical properties. Critical current density of a Bi-2223/Ag tape with 37 filaments has been studied at 77 K with field, field orientation, thermal cycling and bending strain as parameters. Results have been discussed in light of various mechanisms and models. A small pancake coil has been fabricated out of the same tape and the test results presented.