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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]

The relation of the resistivity vs. synthesizing temperature of YBa2Cu3Oy samples was studied while the material being was prepared under atmospheric conditions. A set of ρ–T curves was obtained for the entire process. At the end of the last sintering step the resistivity values of the YBa2Cu3Oy sample was ρ1233=5.64×10−2 Ω cm at 960 °C, and ρ300=4.32×10−3 Ω cm at room temperature. The ρ–T curves for the heating stages also show that the formation temperature of YBa2Cu3Oy crystal grains and the re-crystallization temperature are in the range of 480–870 °C. When a YBa2Cu3Oy sample was reheated, the ρ–T curve also shown that the orthorhombic–tetragonal phase transformation of YBa2Cu3Oy sample occurred at 690 °C. [Copyright &y& Elsevier]

In situ resistance measurement of MgB2 sample was performed in flowing argon atmosphere. This set of measuring curve was compared with the in situ resistance measurement result of MgB2 sample performed in vacuum. This measurement result presents a resistance decreasing taken as a whole as increasing temperature from room temperature to 800 °C, and it indicated that the Mg powder began to melt at about 530 °C. At this temperature the resistance values of MgB2 sample presented a sharp down, and it is lower than MgB2 sample prepared in vacuum conditions that appeared at 638 °C. This indicated that the phase formation of MgB2 in argon atmosphere was at lower temperature than in vacuum. [Copyright &y& Elsevier]

Superconducting MgB2 bulk samples were simply sintered by heating stoichiometric mixture of Mg and B in vacuum. In situ resistance measurements were performed carefully in order to find the best annealing process. The in situ resistance measurements exhibit a decrease in resistance when the heating temperature is increased. The measurements suggested that the Mg powder melting temperature was below 650 °C and the temperature of 800 °C was higher than required. A MgB2 bulk sample sintered at 750 °C has a superconducting transition temperature of Tc(R=0)=40.3 K, higher than that sintered at 680 or 810 °C. [Copyright &y& Elsevier]

We report the solution fabrication of a MgB2 coated conductor on a stainless steel substrate. The precursor solution of Mg(BH4)2 diethyl ether is initially synthesized by refluxing the milled mixture of NaBH4 and MgCl2 in diethyl ether. Then the Mg(BH4)2 diethyl ether is spin coated on a stainless steel substrate and annealed in Mg vapor, which yields a homogeneous MgB2 coated conductor. X-ray diffraction indicates that the grown MgB2 coated conductor is polycrystalline. It has a superconducting transition temperature of 34-37 K. The slope of the upper critical field Hc2 increases with decreasing temperature, and the extrapolated value of HC2(0) reaches ∼28 T. The critical current density estimated by the Bean model is JC (25 K, 0 T)∼106 A·cm-2. These parameters indicate that the solution method is potentially able to produce MgB2 coated conductors that can satisfy application purposes. [ABSTRACT FROM AUTHOR]

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]

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]

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 430–490 °C, which is much lower than that with the MgB2 sample fabricated in the same gas environment using the micron size magnesium powder, denoted as Micro-MgB2 and reported previously. The sample density of the Nano-MgB2 reaches 1.7 g/cm3 with a crystal porosity structure less than a μm, 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, beside 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 to be 39 K by temperature dependent magnetization measurements (M–T), indicating good superconducting properties. [Copyright &y& Elsevier]

MgB2 ultrathin films have potential to make sensitive superconducting devices such as superconducting single-photon detectors working at relatively high temperatures. We have grown epitaxial MgB2 films in thicknesses ranging from about 40 nm to 6 nm by using the hybrid physical-chemical vapor deposition method and performed electrical transport measurements to study the thickness dependence of the superconducting critical temperature Tc. With reducing film thickness d, although a weak depression of the Tc has been observed, which could be attributed to an increase of disorder (interband impurity scattering) in the film, the Tc retains close to the bulk value of MgB2 (39 K), being about 35 K in the film of 6 nm thick. We show that this result, beneficial to the application of MgB2 ultrathin films and in accordance with recent theoretical calculations, is in contrast to previous findings in MgB2 films prepared by other methods such as co-evaporation and molecular-beam epitaxy, where a severe Tc suppression has been observed with Tc about one third of the bulk value in films of ∼5 nm thick. We discuss this apparent discrepancy in experiments and suggest that, towards the ultrathin limit, the different degrees of Tc suppression displayed in currently obtained MgB2 films by various techniques may arise from the different levels of disorder present in the film or different extents of proximity effect at the film surface or film-substrate interface. [ABSTRACT FROM AUTHOR]