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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]

Systematic studies of synthesizing behaviors of sol-gel YBa2Cu3O7− x samples in flowing oxygen atmosphere and their superconductivity have been performed. A set of high temperature ρ-T curves has been obtained for the whole synthesizing process. After four rounds of synthesizing, the resistivity of the sample was around ρ=1.00×10−3Ω · cm at room temperature. The ρ-T curve of the fourth round shows that the orthorhombic to tetragonal phase transformation of the sample occurs around 600 °C, which is lower than that of the YBa2Cu3O7− x sample prepared by conventional solid-state reaction method. Other measurements, such as X-ray diffraction, SEM measurement and low temperature R-T and M-T measurement, were also performed. And the R-T and M-T measurement results suggest that during the synthesizing process, there exist some state at which the sample has better superconductivity than the other states. Moreover, we found screw dislocations presenting on the sample broken surface from the SEM images. This will change the concept that the screw dislocations can only grow on the surface of the YBCO thin films and single crystals. [ABSTRACT FROM AUTHOR]

High-density MgB2 (HD-MgB2) superconducting samples ( D ⩾ 2.2 g/cm3), using different sources of magnesium powder as raw material, were synthesized in ambient pressure in a rich Mg environment. The magnesium powders used in the fabrication process include nanometer-sized magnesium particles, powders from Alfa Aesar, ordinary off-the-shelf powder, and magnesium chip. The fabrication procedure involved a double-sintering process in a rich-Mg environment. A transition temperature T c of 39 K was observed. Samples with the equally high density and matching superconducting properties were obtained as well by a triple sintering process of the MgB2 powder directly from Alfa Aesar. [ABSTRACT FROM AUTHOR]

Superconducting MgB2 thick film has been prepared via hybrid physical-chemical vapor deposition method on Al2O3 (0001) substrate by using B2H6 and magnesium ingot as raw materials reacted from 730 to 830°C for 40 min under 20 to 30 kPa. Its thickness is about 40 µm. The MgB2 thick film shows Tc (onset) = 39.0 K and Tc (0) = 37.2 K. X-ray diffraction pattern shows that the film grown along (101) direction has small amount of impurities of Mg and MgO. Scanning electron microscopy and energy dispersive X-ray spectroscopy indicated that these impurities existed indeed and were Mg rich. The MgO film was formed on the surface of the MgB2 thick film to further protect the sample from oxidation. We presented a new mechanism for the formation of the thick film. [ABSTRACT FROM AUTHOR]

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]

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]

Thick MgB2 (magnesium diborate) films, ∼10 μm, with T c (onset) = 39.4 K and T c (zero) = 39.2 K have been successfully grown on a stainless steel substrate using a technique called hybrid physical-chemical deposition (HPCVD). The deposition rate is high, ∼6.7 nm/s. The X-ray diffraction (XRD) indicates that it is highly (101) and c-axis oriented. The scanning electron microscope (SEM) images demonstrate that the film grown is in “island-mode”. The uniform superconducting phase in the film is shown by the M-T measurement. [ABSTRACT FROM AUTHOR]

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]

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]