Update on MgB2 Front from Temple university Wenura Withanage Advisor – Prof. Xiaoxing Xi

MgB2 deposition on 2” Cu discs Mg vapor reacts and forms alloy with Cu starting around 450 C at 10 Torr ( Copper surface color changes to silverish) Mg vapor pressure is low at low temperatures Coating whole 2” area uniformly challenging Scaling Up for 2” Cu discs and Reducing the growth Temperature HPCVD system is modified to accommodate thick 2” Cu disc Deposition conditions were optimized for Cu substrates Figure 1: (a) Schematic diagram of the modified HPCVD setup to accommodate 2-inch diameter Cu discs. (b) Photograph of the sample holder containing 2-inch Cu disc and Mg pellets placed on the heating element. (c) Photograph of the sample holder containing the Mg pellets and Cu disc with the Mo cap. (d) Photograph of a regular HPCVD setup for small substrates. Withanage W. K. et al Supercond. Sci. Technol. (submitted)

Q vs T measurements carried out using Cryogenic RF measurement system at SLAC 11.4 GHz Figure 9: (a) Quality factor vs. temperature curve of a MgB2 coated Cu disc measured in a Cu cavity. (b) Quality factor vs. temperature curves for a MgB2 coated Cu disc, a Nb reference sample, and a Cu reference sample, measured in a Nb cavity. Withanage W. K. et al Supercond. Sci. Technol.(submitted) Films showed Tc close to 37.5 K MgB2 samples on Cu discs shows comparable Q with Nb reference samples and higher Q than copper reference sample.

XRD analysis Withanage W. K. et al Supercond. Sci. Technol. (submitted) Figure 2: (a) XRD θ - 2θ scans for Cu, Mg on Cu and MgB2 film on Cu for 2θ from 34° to 41°. (b) XRD θ - 2θ scan for a MgB2 film on Cu for 2θ from 20° to 60°. Mg2Cu promotes low temperature ( ~ 450 C) MgB2 growth on Cu Poor XRD signal from MgB2 due to the polycrystalline growth

Cross sectional analysis Purple color characteristic of MgB2 can be seen across the 2-inch diameter Three different color regions are visible Uniform MgB2 layer thickness through out the all three regions Incursion of bright color areas were visible in the area 3 Figure 3: (a) Optical image of a MgB2 film on a Cu disc. (b), (c) and (d) SEM images of the FIB fabricated cross sections from three different areas. The area inside the yellow color box in (d) was used for the EDS elemental mapping shown in figure 4. Withanage W. K. et al Supercond. Sci. Technol. (submitted)

EDS analysis White color regions inside the MgB2 layer was mainly composed of Cu and Mg Figure 4: (a) SEM image of the EDS elemental mapping area; (b) EDS signal from the Pt M⍺1,2 line; (c) EDS signal from the Mg K⍺1,2 line; and (d) EDS signal from the Cu L⍺1,2 line. Withanage W. K. et al Supercond. Sci. Technol. (submitted)

Cross section of a MgB2 on unpolished surface Uniform MgB2 coating around 650 nm through out the cross section including the hills and valleys in the Cu surface Figure 5: SEM image of the FIB fabricated cross section of a MgB2 film grown on a Cu disc with Pt layer, MgB2 layer, Mg - Cu alloy layer and bulk Cu. The black lines were drawn along the interfaces for illustration purposes. Withanage W. K. et al Supercond. Sci. Technol. (submitted)

Surface morphology Dense coverage No crystal structure visible Figure 6: (a) SEM image of the MgB2 film surface on a Cu disc. (b) Zoomed in SEM image. Withanage W. K. et al Supercond. Sci. Technol. (submitted) Dense coverage No crystal structure visible

Figure 8: (a) Magnetic moment vs Figure 8: (a) Magnetic moment vs. magnetic field curves of a 650 nm thick MgB2 film on a Cu disc at 5 K and 20 K. (b) Critical current density vs. applied magnetic field curves of a 650 nm thick MgB2 film on a Cu disc at 5 K and 20 K. The magnetic field was applied perpendicular to the film surface. Withanage W. K. et al Supercond. Sci. Technol. (submitted) High zero field critical field (above 107 A/cm2), similar to the reported critical field values of MgB2 on ceramic and Cu substrates Critical field rapidly suppress with increasing field characteristic of clean MgB2 films

Summary Clean MgB2 films were successfully coated on 2-inch diameter Cu discs with uniform thickness through out the diameter Films showed high Tc ~ 37 K nd high zero field Jc above 107 A/cm2 RF measurements showed Q comparable to reference bulk Nb sample indicating similar Rs for the MgB2 coated Cu discs

Acknowledgement Dr. Xiaoxing Xi Dr. Alireza Nassiri (ANL) Dr. Namhoon Lee Dr. Paul Welander (SLAC) Dr. Grigory Eremeev (JLAB) Dr. Anne – Marie (JLAB)

Jc vs. H curves for C doped and un-doped MgB2 films Zhuang C G et al 2008 Supercond. Sci. & Technol. 21 5

SLAC system H Sample being tested 11.4 GHz E Back to SCLAC results