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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.

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.