Search

Magnesium Diboride has a great potential for magnet applications in many areas such as high energy physics, Mill, and space-related projects. The main goal of our ongoing research effort is to study magnet design, winding process, and magnet testing. Last year, in a 14 cm bore solenoid we reached a central magnetic flux density of 1 tesla at 16K with almost 500 m of commercial multi-filament tape. In order to design appropriate quench detection and protection systems, it is important to define the quench propagation characteristics in solenoids made with this superconductor. In this work we show our experimental activity, from winding to testing, with a 14 cm bore solenoid made with commercial multi-filament MgB2 tape. In particular we address a quench propagation study on a coil made with 2 m of the same tape. [ABSTRACT FROM AUTHOR]

Due to their excellent shape memory and superelasticity properties, NiTi compounds are considered among the most important shape memory alloys. The present work presents a synthesis method for obtaining dense NiTi from elemental powders using a modified spark plasma sintering (SPS) setup characterized by considerably higher current density through the sample at the same total current, as well as drastically decreased heat losses. This provides the possibility to maximize the use of the strong effect of current density on enhanced interdiffusion in metal-metal systems reported in the literature. In addition, the heat loss reduction results in significantly lower total power consumption for synthesis compared to the commonly used SPS setups. The new setup is based on a metallic die/plunger set, combined with a graphite heat generating foil and a thermoelectrical insulation layer. The effect of the processing time on the homogenization of NiTi and the evolution of the intermetallic compounds with time is studied and compared to conventional SPS. Blending for 5 minutes and processing for 30 minutes (with neither preheating, nor postheating, nor quench) were enough to get almost pure dense cubic NiTi (p = 6.10 gcm[sup-3], dominant NiTi phase ≈ 97 pct). A small fraction of both Ti and NiTi[sub2], but neither Ni nor Ni[sub3]Ti, was detected after 20 minutes of processing. Ni[sub4]Ti[sub3] was detected but not quantified. [ABSTRACT FROM AUTHOR]

Sharp cube textured Ni-9%atW alloy substrates were successfully achieved using the powder metallurgy process and gave promise for coated conductor with reduced AC losses and improved mechanical properties. X-ray studies indicated that the newly developed Ni-9%atW alloy substrates have a good out-of-plane and in-plane orientation with FWHM values about 3.7° and 7.05° respectively. Smooth surface as well as fine grains were obtained in the newly developed textured substrate. Curie temperature is suppressed to be 30 K and magnetic hysteretic losses are zero for Ni-9%atW alloy substrates at 77 K. [ABSTRACT FROM AUTHOR]

Many efforts have been made to improve the current carrying capability of Y1Ba2Cu3O7 (YBCO) films by introducing pinning centers using a variety of techniques. In this study, we have succeeded in producing Y2O3, BaZrO3 and Sm doped YBCO films for the development of coated conductors by a metal-organic deposition method using trifluoroacetates (TFA-MOD). Jc exceeding 5 MA/cm2 at 77 K and self field was obtained in Y2O3 doped YBCO film. Also it was found that YBCO films with Sm substituting for Y have shown an improved critical current density characteristic over a wide range of magnetic fields. A Jc of 0.25 MA/cm2 at 3 T and 77 K was obtained which is more than 10% of the zero-field Jc. Similar results were obtained in BaZrO3 doped YBCO films. Additionally, TEM analysis was performed to study the microstructures and the pinning mechanisms. The results indicate that significant enhancement of self-field and in-field Jc of Y2O3, BaZrO3 and Sm doped YBCO films may be due to introducing nano-scaled Y203particles, nano-scaled 90° rotated Y1/3Sm2/3Ba2Cu3O7 domains and BaZrO3 nanodots by different chemical doping, respectively. This indicates that chemical doping is a promising approach for enhancement of current carrying capability of YBCO coated conductors. [ABSTRACT FROM AUTHOR]

The properties of MgB2 have the potential to make this material a viable solution for applications in which temperature, costs or weight are considered relevant constraints. In order to realize large scale applications, it is important to investigate the material, but also the winding process for MgB2 wires and tapes. In the literature small coils have already demonstrated central magnetic flux density above 2 Tesla, overcoming common winding problems related to MgB2 wires. In this current research, efforts are being made in order to improve the performance of solenoid coils, which are of particular interest for many applications, e.g. for space propulsion systems such as the VASIMR engine. A number of coils with MgB2 tapes are being built. In this paper we present results of the test of a 14 cm bore solenoid wound with 400 meters of multifilament, copper stabilized tape. The magnet was tested in a cryocooled vacuum chamber and it reached 175 A at 16 K with a central B0 of 1 Tesla. [ABSTRACT FROM AUTHOR]

For large-scale electric power applications of MgB2 wire, especially at AC environments, development of nonmagnetic sheath material is essential. In this study, Cu-sheathed MgB2 wire was fabricated using the powder-in-tube method and ultra-fine Mg and B precursor prepared by high-energy ball milling. Lowering the annealing temperature and shortening the annealing time effectively limit the reaction between Mg and Cu. Critical current density of the wire annealed at 700° C for 5 minutes is 4.3 × 104 A/cm² at 20 K and self-field. Adding excess Mg in precursor further enhances critical current to 250 A at 20 K and self-field. These results demonstrate the potential of Cu-sheathed MgB2 wires for large-scale applications. [ABSTRACT FROM AUTHOR]

Superconducting coils with different dimensions were manufactured via a wind-reaction process by using Fe-sheathed MgB2 wires, which were fabricated using the powder-in-tube method and ultra-fine starting precursors. Very good grain connectivity as well as high core density was obtained in Fe-sheathed wires and tapes. At 20 K and self-field, the critical current of the coil with ID 12 mm and 12 turns is 360 A, which is equivalent to the Jc of 1.8 × 105 A/cm². At 6 Tesla external magnetic field and 4.2 K, the coil with ID 14 cm and 10 turns possesses the Jc of 1 × 104 A/cm². In-field current carrying capability of MgB2 wires and tapes were further enhanced by Ti additive. These results demonstrate the potential of MgB2 superconducting wires and tapes for large-scale applications. [ABSTRACT FROM AUTHOR]

Epitaxial films of rare-earth (RE) niobates, RE3NbO7 with pyrochlore structures, were grown on biaxially textured nickel-3 at.% tungsten (Ni-W) substrates using a chemical solution deposition process. A precursor solution of 0.3-0.50 M concentration of total cations was spin coated on to short samples of Ni-W substrates, and the films were crystallized at 1050-1100 °C in a gas mixture of Ar-4% H2 for 15 min. Detailed studies revealed that RE-niobates with ionic radius ratio RRE/RNb (R = ionic radius) from 1.23 to 1.40 (i.e., Sm, Eu, Gd, Ho, Y, and Yb) grow epitaxially with the pyrochlore structure. X-ray studies showed that the films of pyrochlore RE niobate films were highly textured with cube-on-cube epitaxy. Scanning electron and atomic force microscopy investigations of RE3NbO7 films revealed a fairly dense and smooth microstructure without cracks and porosity. The rare-earth niobate layers may be potentially used as buffer layers for YBa2Cu3O7-δ coated conductors. [ABSTRACT FROM AUTHOR]

A new series of rare earth-niobate, RE3NbO7 (RE = Y,Gd,Eu), buffer layers were developed for the growth of superconducting Yba2Cu3O7-8 (YBCO) films on biaxially textured Ni-W (3 at. %) substrates. Using chemical solution deposition, smooth, crack-free, and epitaxial RE3NbO7 (RE = Y,Gd,Eu) films were grown on cube-textured Ni-W substrate. YBCO film with a critical current density of 1.1 x 106 A/cm² in self-field at 77 K was grown directly on a single Gd³NbO7-buffered Ni-W substrate using pulsed laser deposition. [ABSTRACT FROM AUTHOR]

The outcome of hepatitis C virus (HCV) infection acquired in childhood is uncertain because of the diversity of the epidemiological and clinical features of infection and disease. The aim of this study was to determine the outcome of HCV infection in 105 Egyptian children who tested positive for HCV antibody (anti-HCV). The data of 105 anti-HCV-positive children presenting to the Pediatric Hepatology Unit, Cairo University Children's Hospital, between 1995 and 2002, were retrospectively analysed for risk factors. Seventy-four children with available polymerase chain reaction results were further analysed clinically, serologically and histologically. The age range was 1.3–22 years, with a mean of 11.2 ± 4.9 years. History of blood transfusion was found in 81 children (77%). HCV RNA was detected in 58.1% of 74 children. Persistently elevated alanine aminotransferase (ALT) levels were present in 40 patients (54.1%). Hepatitis B virus markers (HBsAg and/or anti-HBc) were detected in 18 patients (24.3%). Twenty-six of the 43 HCV RNA-positive children underwent a diagnostic liver biopsy that showed chronic hepatitis in 19 patients (73.1%), cirrhosis in one case only (3.8%), and normal biopsy findings in seven children (26.9%). Blood transfusion remains a major risk of HCV transmission among Egyptian children. HCV infection is not always benign in the childhood period. ALT levels remain elevated in half of the children and histological abnormalities are detected in three quarters of HCV RNA-positive cases. [ABSTRACT FROM AUTHOR]

Isothermal melt texturing is currently a well established technique for manufacturing superconducting bulk materials with high trapped magnetic fields and levitation forces. The technique involves the growth of RE-123 compounds in geometries such as discs and short rods using top seeding of high temperature material such as SmBCO. The technique is relatively simple, however it yields nonuniform microstructure and variable superconducting properties throughout the length of the sample. The top half of the sample is well oriented and possesses good properties, while the bottom half suffers from misorientation, porosity and poor superconducting properties. In this work, the effects of using top and bottom seeding on microstructure and crystal orientation were studied. We also measured trapped magnetic fields throughout the thickness of the sample. In this arrangement, we found that both top and bottom parts of the sample are well oriented and the trapped magnetic fields in both halves are almost the same. These results give promise to the use of this technique to improve the superconducting properties in bulk textured YBCO. [ABSTRACT FROM AUTHOR]

A critical step in the successful fabrication of coated conductors on metal substrates is to produce cube texture substrates using the RABiTS approach. Nickel has been used as a substrate for high-current coated YBa[sub 2]Cu[sub 3]O[sub x] superconductors due to the relative ease in achieving sharp cube texture as well as chemical compatibility. High purity nickel (99.995%) samples were rolled to large deformation and annealed in 5% H[sub 2]-95% Ar atmosphere at 700°C-1100°C for various times. The cube texture was promoted by recrystallization after cold rolling. The effects of deformation reduction per pass, amount of deformation, and annealing temperature and time on the recrystallization process were investigated using XRD and EBSD techniques. These parameters have been optimized to obtain the cube texture {100}〈001〉. The sharpest cube texture (>80% of grains within 7° and >97% within 10° cube textured) was obtained for nickel cold rolled by 2% reduction per pass and annealed at 1000°C for 90 minutes. [ABSTRACT FROM AUTHOR]

This study is aimed at elucidating the nature of power-law creep, threshold stress and stress-induced diffusion in SrCo[sub 0.8]Fe[sub 0.2]O[sub 3-δ] samples subjected to creep. A SrCo[sub 0.8]Fe[sub 0.2]O[sub 3-δ] sample deformed in the transition from a diffusion to a power-law creep regime (900°C, 50 MPa and 15% strain) as well as an assintered sample were analysed by transmission electron microscopy and energydispersive X-ray spectroscopy. The dislocation structure has been found to be highly non-uniform with areas of high dislocation density confined only within a set of well-separated grains. The dislocation structure and distribution indicate that this compound deforms by cooperative grain-boundary sliding in the transition regime. In the grains that do deform by dislocation movement, deformation is achieved by slip through the screw {110}<110> system. A planar map of the composition distribution of constituent cations revealed large fluctuations in cation composition over distances of the order of 100-200 nm, as well as a complementary distribution trend of cobalt and iron. This behaviour is found to be due to sequential stress-induced diffusion of cations, which is in accordance with the temperature dependence of the apparent activation energy. The concentration profiles perpendicular to grain boundaries clearly reveal no preferential segregation of cations at neither random nor coincident-site lattice grain boundaries, which excludes dopant segregation as the mechanism responsible for the threshold stress-like behaviour at low stresses. [ABSTRACT FROM AUTHOR]

The effect of microstructure on oxygen permeation in SrCo0.8Fe0.2O3−δ membranes was investigated using disc samples fabricated under different processing conditions of applied pressure and sintering temperature. The average grain size of the samples was found to remain unchanged as a function of applied pressure, but increased considerably when the sintering temperature was increased from 950 to 1200°C. This change in grain size has a strong effect on the oxygen permeation flux, which increased considerably as the grain size was decreased. The density as well as the microhardness of these samples were also measured and found to change slightly as the processing conditions were changed. [ABSTRACT FROM AUTHOR]

The effects of hydrogen on the yielding and the ductility of niobium have been studied in the temperature range between 373 and 78 K. Tensile tests were undertaken on single- and poly-crystal specimens containing hydrogen ranging between 30 and 230 ppm wt. at a strain rate of 1.7 X 10 s. The fracture surfaces of specimens tested were also examined by means of an SEM. At higher hydrogen concentrations, two ductility maxima were observed in the temperature dependence of the uniform strain of both single- and poly-crystals. The positions of these maxima on the temperature scale varied as a function of the amount of hydrogen dissolved. One of the maxima is found to occur at higher temperatures in single crystals than in polycrystals. Analysis of results obtained indicates the presence of two distinct mechanisms responsible for the loss of ductility in noibium with hydrogen. One mechanism is operative at the onset of embrittlement, and is sensitive to the presence of grain boundaries. The second mechanism predominates at temperatures of the second ductile-brittle transition, and is associated with the precipitation of the hydride phase. This mechanism has similar effects on both single and poly-crystals. [ABSTRACT FROM AUTHOR]

The formation of Yb-123 has been studied at different temperatures in air and in reduced oxygen partial pressure. It is found that the stability and/or the formation kinetics of Yb-123 phase is a major hurdle in manufacturing phase-pure Yb-123 in air. However, under reduced oxygen partial pressure, Yb-123 forms rapidly and more than 90% phase-pure Yb-123 is achieved within three sintering steps. Rods made from this powder were melt-processed in air and showed a Tc of 90 K. Kinetic studies performed by interrupting the growth during the directional solidification of these rods revealed a growth mechanism similar to that of Y-123 and a maximum growth rate of 7.2 mm/h for a stable planar interface. EPMA of the interface showed the liquid to be rich in barium cuprates with a Ba:Cu ratio of 1:3. [ABSTRACT FROM AUTHOR]

Processing of YBCO films by decomposition of metal trifluoroacetate precursors and its application to the development of coated conductors is investigated. The technique involves preparation of the solution, deposition, and a two-stage heat treatment. A stoichiometric mixture of the acetates of Y, Ba, and Cu and trifluoroacetic acid are used as the starting materials. The glassy residue of the trifluoroacetates of Y, Ba, and Cu formed by drying the above mixture is redispersed in a fixed amount of methanol to provide the starting solution of a given concentration. This solution is then deposited on the substrate surface by the spin coating technique and heat treated in two stages. X-Ray characterization shows that the transformation to 123 is complete in less than 1 h. Pole figure analysis of films deposited on (100) SrTiO3 and LaAlO3 single crystal substrates show that these films have a very high degreee of alignment with the substrate both out-of-plane (<0.5°) and in-plane (<1.5°). These chemically derived films were also found to have critical current densities well above 5 × 105 at 77 K for 0.5 μm thick films. The applicability of this technique to the production of thick films of YBCO for the development of coated conductors will be discussed. [ABSTRACT FROM AUTHOR]

The powder-in-tube technique consisting of industrial processes such as wire drawing and rolling has been widely used to fabricate superconducting tapes. In the present investigation a novel technique was adopted to fabricate BSCCO 2223 tapes. Instead of wire drawing, the silver billet was reduced in size by groove rolling. Stress conditions during groove rolling were analyzed and appropriate changes were incorporated to optimize the deformation process. Subsequent thermomechanical treatment resulted in tapes with average critical current densities of 18,000 A/cm2. Phase development and microstructural evolution during the thermomechanical treatment were studied using XRD, SEM, and TEM. The electromechanical properties of monofilament and composite BSCCO tapes were evaluated by subjecting them to in situ tensile tests. The strain tolerance of the composite was found to be better than that of the monofilament BSCCO tape. [ABSTRACT FROM AUTHOR]

The effects of hydrogen on near-threshold fatigue crack growth rate have been investigated in niobium at room temperature. Fatigue tests were performed on a hydrogen-free specimen as well as specimens containing hydrogen in solid solution and in the form of hydride. Specimens were tested using a tension-tension loading cycle at a frequency of 4 Hz and two load ratios of 0.05 and 0.4. The results show that the threshold stress-intensity factor ΔK decreases with the addition of hydrogen and reaches a minimum at a hydrogen concentration approximately equal to the solubility limit of hydrogen in niobium. As the hydrogen concentration exceeds the solubility limit, ΔK increases with the increase of the amount of hydrogen dissolved in the specimen. Analysis of this behavior suggests that stress-induced hydride cannot be responsible for the embrittlement of niobium with hydrogen, and dislocation-hydrogen interaction plays an important role in the embrittlement process. SEM metallography studies also confirm the importance of dislocation in the embrittlement of niobium by hydrogen. [ABSTRACT FROM AUTHOR]