Growth of magnesium diboride films on 2 inch diameter copper discs by hybrid physical–chemical vapor deposition

Abstract

Magnesium diboride (MgB2) coating is a potential candidate to replace bulk niobium (Nb) for superconducting radio frequency cavities due to the appealing superconducting properties of MgB2. MgB2 coating on copper may allow cavity operation near 20–25 K as a result of the high transition temperature (Tc) of MgB2 and excellent thermal conductivity of Cu. We have grown MgB2 films on 2 inch diameter Cu discs by hybrid physical–chemical vapor deposition for radio frequency characterization. Structural and elemental analyses showed a uniform MgB2 coating on top of a Mg–Cu alloy layer with occasional intrusion of Mg–Cu alloy regions. High Tc values of around 37 K and high critical current density (Jc) on the order of 107 A cm−2 at zero field were observed. Radio frequency measurements at 11.4 GHz confirmed a high Tc and showed a quality factor (Q0) much higher than for Cu and close to that of Nb.

Magnesium diboride on inner wall of copper tube: A test case for superconducting radio frequency cavities

“…5(a)] shows an MgB 2 layer intruded by a large number of bright flecks. According to our previous work of MgB 2 coating on Cu disks [8], these flecks are a Mg-Cu alloy appearing in areas where Mg vapor is insufficient during the deposition. It confirms that the top of the tube suffers from Mg deficiency due to the specific gas dynamics in the area.…”

“…This has been observed in Nb coated Cu cavities [7]. MgB 2 coated Cu cavities working at 20-25 K will eliminate the need for liquid He refrigeration [8].…”

“…As discussed in Ref. [8], a Mg-Cu alloy formed on the Cu surface at this stage before the B 2 H 6 gas mixture was introduced, which initiated the MgB 2 deposition. The flow rate of the 5% B 2 H 6 in H 2 gas mixture was 20 standard cubic centimeters per minute (sccm).…”

“…Growth of MgB 2 on Cu is challenging because Mg and Cu form alloys at elevated temperatures. This problem has been solved by lowering the deposition temperature to around 470°C [8], where the formation of Mg 2 Cu promotes the growth of MgB 2 [9,10]. The films showed high T c , high critical current density (J c ) and high Q when measured in a cryogenic high-Q hemispheric cavity at 11.4 GHz [8].…”

“…This problem has been solved by lowering the deposition temperature to around 470°C [8], where the formation of Mg 2 Cu promotes the growth of MgB 2 [9,10]. The films showed high T c , high critical current density (J c ) and high Q when measured in a cryogenic high-Q hemispheric cavity at 11.4 GHz [8]. Coating of stainless steel dummy 6 and 3 GHz SRF cavities with MgB 2 using HPCVD has been previously demonstrated [11,12].…”