Search

Mg(BH4)2 was previously studied as a promising hydrogen storage material, because of its high gravimetric storage capacities for hydrogen and suitable thermodynamic properties. Mg(BH4)2 began to decompose at about 300 ℃, and formed MgB2 at the end of hydrogen desorption process with the weight content of 14.9% of hydrogen lost. Aside from the prominent hydrogen storage property, the decomposition process from Mg(BH4)2 to MgB2 can be a potential method for fabricating superconducting MgB2 at a low sintering temperature. In this paper, MgB2 bulk was prepared by an in-situ reaction, using the Mg(BH4)2 pressed block as a precursor. The resistance change of the sample was monitored during the Mg(BH4)2 decomposition process and the resistance-temperature (R-T) curve of this process was recorded. Phase of MgH2, Mg and B were formed as the block slowly release its hydrogen before MgB2 occurred. According to the R-T curve, the phase formation of MgB2 started in a relatively low temperature of 410 ℃. Because MgB2 was critically formed by Mg and B derived from Mg(BH4)2, we can compare our formation temperature with previous study on MgB2 prepared by Mg and B in different particle size. The fitting result indicated that the particle size of Mg and B harvest from Mg(BH4)2 decomposition was only 3.4 nm on average. The nearly atomic level mixture of Mg and B resulted in a high chemical reactivity, which was the main reason for low sintering temperature. X-ray diffraction results showed that the purity of MgB2 was 95.2%, and the size of MgB2 grains was 10–18 nm. SEM images showed that the MgB2 bulk had a porous structure and poor connectivity, which was caused by large amount the hydrogen release during the decomposition. MgB2 nanofibers can also be observed inside the bulk. In the superconductivity test, the superconducting transition temperature of the bulk was 35 K. After all, such in situ method to fabricate MgB2 showed a great advantage in some aspects, as its low-cost precursors, low sintering temperature, small grain-size and high superconducting transition temperature in the formed MgB2, which have the potential in industrial scale fabrication of MgB2 bulks and wires.

The relationship of resistivity versus synthesizing temperature of sol gel YBa2Cu3Oy samples was studied when prepared under flowing oxygen conditions. A set of high-temperature ρ–T curves was obtained for the whole process. After the sample finished the test measuring, its resistivity was ρ300=9.83 ×10−3Ω.cm at room temperature. The ρ–T curve also showed that the orthorhombic–tetragonal phase transformation of sol-gel YBa2Cu3Oy sample occurred at 581 degrees C for the sample in the rising temperature process, but at 613 degrees C in the cooling process, lower than that of the samples made by using the conventional powder metallurgy methods.

By a method of hybrid physical-chemical vapour deposition (HPCVD) on three metal substrates of stainless steel, copper and niobium, we deposit MgB2 superconducting films over 1 μm thickness. All of them have zero resistance temperatures TC(0)>36 K and critical current densities JC (10 K, 0 T) >106 A/cm2. Meanwhile, in the bending test, all the MgB2 superconducting films adhere strongly to the metal substrates without peeling off. Therefore, the MgB2 superconducting films supplied by the HPCVD method exhibit preferable electrical, magnetic and mechanical properties, and have potential applications in future.

Thick MgB2 (magnesium diborate) films, ∼10 μm, with Tc (onset) = 39.4 K and Tc (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.

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., 7 pages with 4 figures included

This report is focused on the MgB2 nanosheets which have a hexagonal single crystal structure with variable thickness and different radial dimensions. The nanosheets are fabricated by hybird physical-chemical vapor deposition for the first time, as far as we know, and meanwhile they can be site-specific transferred so as to make physical properties measurement. Results of electrical and magnetic measurements indicate that the nanosheets is superconductive with a Tconset=38 K, Tc (0)=33 K. The images of scanning electrical microscope show that the nanosheets have a nanoscale thickness and have not only a large scale in wildth from several microns to hundreds of microns but also a flat cleaning surface. The selected area electrical diffraction data is consistent with the early report of MgB2 diffraction. According to those results, the single crystal nanosheets with high quality can be surely indentified as MgB2. It suggests a new technique for MgB2 single crystal fabrication, and a zero electrical behavior is observed in nanoscale single crystal MgB2. This could be a new opportunity to make a right material for the afterward research such as flux vortices, nanoscale mechanical properties.

High-quality MgB2 films are fabricated via hybrid physical-chemical vapor deposition (HPCVD) and irradiated by Ti ions. Compared with the unirradiated film, the Ti-irradiated MgB2 film shows a high critical current density (Jc) in magnetic field and also a high upper critical field (Hc2), while the superconducting transition temperature (Tc) does not decrease significantly. The Ti-irradiated film with a best fluence at 1 1013/cm2 shows a high Jc of 1.72 105 A/cm2 in 4 T perpendicular field at a temperature of 5 K and a moderately decreased Tc at 39.9 K.

We fabricate MgB 2 ultra-thin films via hybrid physical-chemical vapor deposition technique. Under the same background pressure, the same H 2 flow rate, by changing B 2 H 6 flow rate and deposition time, we fabricate a series of ultra-thin films with thickness ranging from 5 nm to 80 nm. These films grow on SiC substrate, and are all c -axis epitaxial. We study the Volmer-Weber mode in the film formation. As the thickness increases, critical transition temperature T c (0) also increases and the residual resistivity decreases. Especially, a very high T c (0) ≈ 32.8 K for the 7.5 nm film, and T c (0) ≈ 36.5 K, low residual resistivity ρ (42 K)≈ 17.7 μΩcm, and extremely high critical current density J c (0 T,4 K) ≈ 10 7 A/cm 2 , upper critical field H c2 (0) for 10 nm film are achieved. Moreover, by optimizing the H 2 flow rate, we obtain relatively smooth surface of the 10 nm epitaxial film, with a root-mean-square roughness of 0.731 nm, which makes them well qualified for device applications.