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We have successfully synthesized MgB2 thick films on 4H-SiC substrate by hybrid physical-chemical deposition (HPCVD). The films have transition temperature TC above 40 K. X-ray diffraction (XRD) shows the c-axis oriented structure of MgB2, with Mg and small MgO impurities. The critical current density JC, estimated using the measured magnetic hysteresis loop and the Bean model, is 6 MA/cm2 in self field at 10 K., Comment: 6 pages, 4 figures,Supercond. Sci. Technol. IN PRESS

We have successfully deposited thick MgB2 film on the (0001) crystalline surface of sapphire by the method of hybrid physical-chemical vapor deposition (HPCVD). The film thickness is about 1.3 micron. It has a dense and interlaced structure. The film surface, shown by SEM, is stacked with MgB2 microcrystals. Transport measurements by the 4-probe technique have demonstrated that its critical temperature is about 40.2 K, with a sharp transition width of 0.15 K. The residual resistivity ratio (RRR) is about 11. By extrapolation, HC2(0) is determined as 13.7 T from the magneto-transport measurement. Also by hysteresis measurement and applying the Bean model, the critical current density is estimated as 5*1010 A/m2 in zero magnetic field. The present work has demonstrated that HPCVD is an effective technique to fabricate the MgB2 thick film with decent superconducting properties. Hence, it is important for the future superconducting application, in particular, as a crucial preliminary stage to fabricate superconducting tape., Comment: 7 pages with 4 figures included, Phys. Stat. Sol. (a) In press

The size effect of the raw B powder on the MgB2 phase formation has been studied by the technique of in-situ high temperature resistivity (HT-rT) measurement. The onset temperature, Tonset, and the completion temperature, TPF, of the phase formation are determined directly during the ongoing thermal process. These two temperatures, Tonset and TPF of the sample synthesized using nanometer B and Mg powders (NanoB-MgB2) are 440 C and 490 C, respectively, the same as those of the sample using micrometer B and nanometer Mg powders (MicroB-MgB2). This indicates that the phase formation temperature of MgB2 do not depend on the B powder size. On the other hand, the upper limit of the sintering temperature, TN, above which the sample loses superconductivity, is below 750 C for NanoB-MgB2, much lower than 980 C for the MgB2 prepared using micron-sized B powder and millimeter sized Mg powder (DM-MgB2). In comparison with the sample directly sintered at 650 C < TN, an interesting, irreversible transformation in the crystal structure of the MgB2 phase was observed with the sample going through the stages of initial sintering at 750 C, then re-sintering at 650 C in an Mg-rich environment after the processes of regrinding and pressing. Possible explanation of the observed properties is discussed., Comment: 7 pages with 4 figures included

MgB2 thick film was synthesized on the stainless steel substrate by the technique of hybrid physical-chemical vapor deposition (HPCVD), using Mg ingot and B2H6 as the raw materials. The film thickness is about 10 microns. The scanning electron microscope (SEM) images reveal that it is consisting of highly dense MgB2 crystals with the size ranging from 0.2 to 3 microns. The superconducting transition occurs at 38 K (Tc, onset) and ends at 27 K (Tc, zero), giving the transition width of 11 K. The fabricated film exhibits high ductility and remains attached to the substrate after it was bent to a curvature of about 200 microns., Comment: 4 pages, Proceedings of SPIE, the 5th International Conference on Thin Film Physics and Applications 2004

The MgB2 superconductor synthesized in a flowing argon atmosphere using nanometer magnesium powder as the raw materials, denoted as Nano-MgB2, has been studied by the technique of in-situ high temperature resistance measurement (HT-RT measurement). The MgB2 phase is identified to form within the temperature range of 430 to 490 C, which is much lower than that with the MgB2 sample fabricated in the same gas environment using the micron-sized magnesium powder, denoted as Micro-MgB2, reported previously. The sample density of the Nano-MgB2 reaches 1.7 g/cm3 with a crystal porosity structure less than a micrometer, as determined by the scanning electron microscope (SEM) images, while the Micro-MgB2 has a much more porous structure with corresponding density of 1.0 g/cm3. This indicates that the Mg raw particle size, besides the sintering temperature, is a crucial factor for the formation of high density MgB2 sample, even at the temperature much lower than that of the Mg melting, 650 C. The X-ray diffraction (XRD) pattern shows a good MgB2 phase with small amount of MgO and Mg and the transition temperature, TC, of the Nano-MgB2 was determined as 39 K by the temperature dependent magnetization measurement (M-T), indicating the existence of a good superconducting property., Comment: 10 pages, 4 figure, Solid State Communication

Very thick, ~ 40 $\mu$m, clean, and highly textured MgB2 film was effectively grown on an Al2O3 substrate. The fabrication technique is 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 exceeds 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 2 $\mu$m in diameter and 0.2 $\mu$m 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) = 39.2 K and TC(zero) = 38.4 K, giving a sharp transition typical of a clean sample. The residual resistivity ratio (RRR) is determined as 6.4 and the magnetoreisitance (MR) is about 28 % at 40 K under the applied field of 9 T, 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 9 T., Comment: 10 pages, 4 figures

We report the study of the polycrystalline MgB2 phase formation during the fabrication processes. Three major phases are identified in different sintering temperature ranges according to the corresponding superconducting properties and the crystalization conditions. The optimum fabrication parameters are obtained for the MgB2 superconducting phase formation., Comment: 10 pages, figures included