Clean epitaxial MgB₂films fabricated by theex situannealing of chemical vapour deposition-grown B films in Mg vapour

Abstract

Low residual resistivity in superconducting thin films is required for their applications in radio frequency (RF) cavities. Here we report on clean epitaxial MgB 2 films fabricated by the ex situ annealing of B films, grown by chemical vapour deposition, in Mg vapour. The films show sharp superconducting transitions at about 40 K and a high residual resistivity ratio of about 10. The result indicates that a clean precursor B film and a contamination-free annealing procedure are important for a viable MgB 2 film fabrication process for RF cavity applications.

Superconducting magnesium diboride coatings for radio frequency cavities fabricated by hybrid physical-chemical vapor deposition

“…Table III shows a comparison of the main characteristics of the three samples at different locations. The films have very similar critical temperatures and the values are slightly lower than the ones obtained for films fabricated with the two-step process as reported in the literature [37,38]. This could be attributed to Mg vapor loss during the annealing step as no tight enclosure was used at the bottom of the dummy cavity, the lower annealing temperature, and the shorter annealing time.…”

“…As the cap at the top had a lower temperature than the dummy cavity because it was subjected to the hydrogen gas flow from the diborane line, the Mg vapor condensed on the inside of the cap rather than on the cavity wall, leaving the MgB 2 films free from the Mg condensate on the surface. The process of annealing boron film in Mg vapor at elevated temperatures to produce MgB 2 film has been reported previously for flat substrates [37,38].…”

Enhancing the Superconducting Properties of Magnesium Diboride Without Doping

“…From the resistivity measurements, the area fractions were found to be 0.79 and 0.38 for Pure A and Pure B, respectively, as presented in Table . The area fraction for the Pure A sample is almost equal to that for well‐connected MgB 2 thin films . The resistivity difference between 40 K and 300 K (Δρ 300K−40K ) for a clean thin film fabricated by the HPCVD technique is reported to be ~8.0 μΩ cm, whereas Δρ 300K−40K for Pure A is 9.2 μΩ cm.…”

“…A more direct way to eliminate the void formation, the magnesium diffusion method, has been proposed, and denser wires and bulks can be fabricated . Even so, the grain connectivity is much lower compared to clean MgB 2 thin films grown, for example, by the hybrid physical–chemical vapor deposition (HPCVD) technique, where high‐density structure and very high self‐field critical current density are routinely reported . Is there a way to further improve the grain connectivity of dense polycrystalline MgB 2 and, at the same time, increase the lattice strain so as to enhance the high‐ and low‐field critical current density together?…”

Effects influencing the grain connectivity in ex-situ MgB2 wires

“…One can see apparent differences in the values of normal state resistivities (q MgB2 ). Room temperature q MgB2 of presented cores is ranging from 75 to 300 lX cm, which is considerably higher than for MgB 2 films $10 lX cm [11] and for in situ bulk samples 20-50 lX cm [12]. The highest q MgB2 corresponds to sample A1 reflecting the lowest core purity.…”