Harini Sosiati

MgB2 films with very high critical current densities due to strong grain boundary pinning

MgB2 superconductor has a great potential for applications because of its high Tc and Bc2, exceeding those of any Nb-base superconductors at any temperature. It is now important to understand its flux pinning so as to raise Jc to high values over a wide field range. We show that nanometer-sized columnar-grain structure can produce Jc exceeding 5×106A∕cm2. The angular dependence of Jc indicates that the strongest pinning occurs when the field is aligned parallel to the grain boundaries. Our results confirm earlier deductions that grain boundaries in MgB2 act as effective pinning centers like those in Nb3Sn.

Microstructures of MgB2/Fe tapes fabricated by an in situ powder-in-tube method using MgH2 as a precursor powder

Microstructures of MgB2/Fe tapes fabricated by an in situ powder-in-tube method using MgH2 as a precursor powder have been studied by means of x-ray diffraction and analytical transmission electron microscopy combined with a focused ion beam microsampling technique. The overall microstructures in the tapes are characterized as densely crystallized MgB2 areas with 10–200 nm grain size, uncrystallized areas mainly containing MgO and amorphous phases enriched with B, and a number of holes and cracks. The crystallized MgB2 areas increase upon doping with SiC nanoparticles. Si and C atoms decomposed from SiC nanoparticles during heat treatment exhibit different spatial distributions: the Si atoms are inhomogeneously distributed, forming silicides such as Mg2Si with grain size of 5–20 nm, while the C atoms tend to be uniformly distributed in the MgB2 matrix. A significant difference in distribution of O atoms between the SiC-doped and non-doped specimens was observed. The processes of formation of these microstructures and their relationships with the critical current density under magnetic fields have been discussed.

Relationship between microstructure and Jc property in MgB2/α-Al2O3 film fabricated by in situ electron beam evaporation

A transmission electron microscopy (TEM) study has been carried out on an MgB2/?-Al2O3 film that exhibits the typical property of critical current density (Jc) under magnetic fields. The MgB2 layer of 300?nm in thickness was grown on a (001)?-Al2O3 substrate using an in situ electron beam evaporation method. Jc of the film takes significantly high values when the applied magnetic field is perpendicular to the film surface. The MgB2 layer consists of fine columnar MgB2 crystals 20?30?nm in size. The columnar MgB2 crystals grow almost perpendicular to the substrate surface and have no crystallographic orientation relationship with the ?-Al2O3 substrate because of an amorphous layer formed first on the substrate. A high density of columnar grain boundaries within the MgB2 layer may be effective for the enhancement of the flux-pinning under the perpendicular magnetic field.

Properties of MgB/sub 2/ films with very high transport critical current densities

Magnesium diboride, MgB/sub 2/, thin films are fabricated through two different kinds of process. One is an in-situ process by using electron beam deposition and the other is an ex-situ process by using the combination of pulsed laser deposition and heat treatment. The critical current density, J/sub c/, is investigated as a function of external magnetic field in the range of 0-7 T and/or temperatures ranging from 4.2 K to the critical temperature by using dc 4-probe transport method. The in-situ processed film shows very high J/sub c/, e.g., 7.1/spl times/10/sup 6/, 1.2/spl times/10/sup 6/, and 1.4/spl times/10/sup 5/ A/cm/sup 2/ in 0, 4, and 7 T (perpendicular fields), respectively. Angular dependence of J/sub c/ is much different between the two kinds of films. J/sub c/-angle (magnetic field) curves of the in-situ processed film show two peaks; one is around the field perpendicular to the film surface and the other is around the parallel field. On the contrary, the curves of the ex-situ processed film have only one peak around the parallel field. Microstructure analyses show that the in-situ film has columnar grains aligned perpendicular to the film surface and that the ex-situ one has granular grains with random orientations. These results indicate that the grain boundaries between columnar grains act as effective pinning centers and enhance J/sub c/ in the perpendicular fields.

Effects of heat treatments on microstructure formation in MgB/sub 2//YSZ/Hastelloy film

Annealing processes in MgB/sub 2/ films deposited on yttria-stabilized zirconia (YSZ)-buffered Hastelloy substrates were studied by transmission electron microscopy. The MgB/sub 2/ films with Mg-rich compositions were fabricated at room temperature using a pulsed laser deposition (PLD) method. The as-deposited films do not exhibit clear superconducting transitions, and have inhomogeneous microstructures composed of amorphous regions and a small amount of crystalline MgB/sub 2/ and MgO with several tens of nm in size. Various sizes of voids are also incorporated within the amorphous regions. After the annealing at 600 or 680/spl deg/C, the films exhibit clear superconducting transitions. The amorphous regions crystallize into fine grains of MgB/sub 2/ or MgO 5-50 nm in size. The voids remain in the annealed films and their sizes depend on the annealing condition. Magnesium-and oxygen-rich layers are formed near the MgB/sub 2//YSZ interface and the film surface. It is suggested that the finer grains of MgB/sub 2/ and MgO obtained by the lower-temperature annealing give a larger critical current density under a high magnetic field.

Tin whisker formation on a lead-free solder alloy studied by transmission electron microscopy

Tin whiskers grown on a Sn/Cu-plated polyimide (PI)-flexible substrate are a serious problem in electronic industrial application, because the whiskers lead to catastrophic electrical short circuit failures. Here, we report characterization of microstructures in the whiskers grown on a surface of Sn/Cu-plated PI-flexible substrate by cross-sectional transmission electron microscopy (TEM) to analyze the behavior and the formation mechanism of whiskers. The whisker was found to be monocrystalline beta-Sn and grown with the preferred directions of [110] and [101]. The whiskers formed on this tin surface are nucleated and grown by the compressive stress, that is induced externally by insertion of Sn/Cu-plated PI-flexible substrate into the connector.

Study of parallel oriented electrospun polyvinyl alcohol (PVA) nanofibers using modified electrospinning method

Parallel orientedpolyvinyl alcohol (PVA) nanofibershasbeen successfully prepared by using modified electrospinning method. This method uses two pairs of copper (Cu) electrodes which are set apart at a certain distance and applied voltage of 15 kV. The concentrations of PVA were varied from 11%, 13%, 15%, 17%, and 19%. The width of gap collector were varied from 5 mm, 10 mm, 15 mm, and 20 mm. The diameter of nanofibers increase as increasing concentration of PVA. As the width of gap collector increase, first diameter of nanofibers decrease and reach a minimum value at 355 ± 7nm in 15 mm of gap, then the diameters increase again. We also calculated the alignment parameter (S) for given aligned nanofiber. The result showed that alignment parameters (S) were on values around 0,9-1.

Synthesis of copper nanorods by aqueous solution method without heating external

So far, synthesizing Cu nanorods (CuNRs) are complicated, expensive, and needs a long time. In this study, We report a simple way to synthesized CuNRs by the aqueous solution method without external heating. Instead of external heating, we used internal heating from the exothermic reaction of NaOH and water. Copper (II) nitrate trihydrate Cu (NO3)2.3H2O, sodium hydroxide (NaOH), ethylenediamine (EDA), and hydrazine (N2H4) were used in our work. The morphology and formation structure of CuNRs have been investigated using Scanning Electron Microscope-Energy Dispersive X-ray Spectroscopy (SEM-EDXS), and X-ray Diffraction (XRD). The SEM analysis confirmed the formation of CuNRs with a diameter of 200-300 nm and length of 3-7 µm.

Fabricating Polyvinyl Alcohol Fibers with Evenly Distributed Nano Silver

Fabrication of polyvinyl alcohol (PVA) fibers loaded with evenly distributed nanosilver has been sucessfully done using electrospinner. The electrospinner is set at 15 kV DC voltage with distance between electrodes 13 cm, using a syringe hole diameter 0,5 mm. The feeding solution for the electrospinning was prepared by directly mixing the solution of PVA in water with a stable colloid of nanosilver at room temperature. The fibers morphology was characterized using scanning electron microscope. The optical property was tested using spectrometer. The distribution of silver nanoparticles in the fibers was tested using transmission electron microscope. The result indicates that the fibers still showing plasmonic property of silver having peak at 410 nm with no crystaline changes. The diameter of fibers loaded with nanosilver are smaller compared to that of the ones without nanosilver.The distribution of nanosilver in fibers made of PVA and the ones made of PVA and chitosan are compared and discussed.

Properties of the Treated Kenaf/Polypropylene (PP) Composites

Untreated and treated kenaf fibers were used in the fabrication of kenaf/polypropylene (PP) composites. The ratio of the fibers as fillers and PP as matrix was 40:60 in wt%. Small amounts of maleic anhydride grafted polypropylene (MAPP) were added as a coupling agent. Fourier transform infrared (FTIR) spectroscopy was used to estimate the cellulose contained in the treated kenaf fibers. A flexural test of the composites was conducted by referring to JIS (A 5905, 1994, Fiberboards). A composite with alkali-treated fiber had better properties (modulus of rupture (MOR) and elasticity (MOE), absorbed water and thickness in swelling) than that with a combination of steam-alkali and alkali-bleaching treated fibers, and untreated fibers. The dispersion characteristic of the fibers within the polymer matrix has been suggested as one of the significant factors affecting the composite properties. These composites are classified as medium density fiberboard (MDF, JIS, type 30 and 25).