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Abstract: Very thick, ∼40μm, clean, and highly textured MgB2 film was effectively grown on an Al2O3 substrate. The fabrication technique was by the hybrid physical-chemical vapor deposition (HPCVD) using B2H6 gas and Mg ingot as the sources. The X-ray diffraction (XRD) analysis shows a highly (101)-oriented MgB2 crystal structure without any impurity detected. There is no signal from the substrate in the XRD spectrum, indicating that the film thickness has exceeded the X-ray penetration length. Scanning electron microscopy (SEM) reveals that the film is composed of highly packed MgB2 micro-crystals with a uniform size distribution of about 2μm in diameter and 0.2μm in thickness. According to the compositional analysis of energy-dispersive X-ray spectroscopy (EDX), no oxygen, hence no MgO, exists in the textured film, consistent with the XRD result. Also, the transport properties are similar to those of a single crystal, indicating a clean film of good crystallite. The zero field transition temperatures are determined as TC(onset)=39.2K and TC(zero)=38.4K, giving a sharp transition typical of a clean sample. The residual resistivity ratio (RRR) is determined as 6.4 and the magnetoresistance (MR) is about 28% at 40K under the applied field of 9T, which are similar to those of a single crystal. The zero temperature upper critical field, HC2(0), is extrapolated as 19 T from the TC(onset) at applied field up to 9T. [Copyright &y& Elsevier]

Careful investigations on a series of the MgB2 phases quenched at different sintering temperatures during fabrication process have been carried out, and important information is obtained, by the X-ray diffraction (XRD), the scanning electron microscopy (SEM) images, and the temperature dependent magnetization (M–T) measurements. The particle sizes of the raw magnesium and boron powders are on the order of 0.1 mm and 1 μm, respectively. Three stages in the formation of the polycrystalline MgB2 phase are identified in different sintering temperature ranges according to the corresponding superconducting properties and the crystallization conditions. The MgB2 phase first appears in a microscopic scale at about 530 °C according to the M–T measurement. Then, the MgB2 phase forms macroscopically within a narrow temperature range from 653 to 660 °C, while the residual raw materials of the magnesium and boron components persist to the temperature of 700 °C. The optimum sintering temperature range for the MgB2 phase to form is determined from 750 to 900 °C. [Copyright &y& Elsevier]

Abstract: It is well-known that there is a relatively small temperature window for the growth of epitaxial and c-axis oriented YBCO thin films by various methods. In this study, this window was more precisely examined for YBCO film growths by photo-assisted MOCVD. At specified total reactor pressure of 4.6torr, and specified respective partial pressures for the oxidizing agents, O2 and N2O at 1.7torr and 1.1torr, the film growths were tested at several substrate temperature (T s) points from 780°C to 864°C. The film growth time for each test was 3min. As to compositional purity, purity in c-axis orientation, and crystal structural quality of these tested films by using photo-assisted MOCVD technique, it was found that for the growth of purely c-axis oriented YBCO thin films with high crystalline quality, the appropriate growth temperature range (or “T s window”) was from about 800°C to 830°C, at relevant pressures as specified above. T c and J c of a typical YBCO film sample grown within this “T s window”, i.e., 810°C, were found as 90K and 1.32MA/cm2 (77K, 0Oe) respectively by magnetization measurements. It is noticeable that this film, and other purely c-axis oriented YBCO films are all single-crystal like and dense, with no SEM visible grain boundary or void. The causes of precipitates found on top surface of these c-axis oriented YBCO films are also discussed. [Copyright &y& Elsevier]

Abstract: Superconducting YBa2Cu3O7− x thin films were deposited on LaAlO3 (100) substrates by a photo-assisted metal-organic chemical vapor deposition (MOCVD) technique. In our experiment, growth temperature, chamber pressure and growth time were all fixed; the only process variable was the susceptor inclination angle (Φ) between the axis of gas inlet and the susceptor surface. It was found that the growth rate of YBCO films increases monotonically as Φ increases, whereas the superconducting critical current density (J c) has a maximum (1.2×106 A/cm2) near Φ =22.5°. The experimental results were found to be well correlated with the three-dimensional simulations of gas flow inside the MOCVD reactor, with Φ being the only process variable. [Copyright &y& Elsevier]

Abstract: We have made a progress in fabricating MgB2 films, ∼25μm, on the stainless steel substrate by hybrid physical–chemical vapor deposition. The superconducting transition temperature is, T C (onset)=39.6K with a transition width, ΔT =0.5K. The characterization by scanning electron microscope and X-ray diffraction indicates that its structure is polycrystalline. At T =0K, the upper critical field H C2 is determined as 15.2T by extrapolation from a polynomial fitting to the transition temperatures under various applied fields, T C(H). In the self field, the critical current density J C is determined as 3.74MA/cm2 at T =15K by a magnetic measurement according to the Bean model. [Copyright &y& Elsevier]