CHARACTERIZATIONS OF THE CORE-SHELL STRUCTUREDMgB2/CARBON FIBER SYNTHESIS BY RF-SPUTTERING AND THERMAL EVAPORATION

MgB2/carbon fibers have been synthesized by the combination of RF-sputtering ofBand thermal evaporation ofMg, followed by co-evaporation. First, boron layer was deposited by RF-sputtering on the carbon fiber with average diameter of 7.1 μm. Later this coated layer ofBwas reacted withMgvapor to transform intoMgB2. Since theMgB2reaction proceed withMgdiffusion into the boron layer,Mgvapor pressure and the diffusion time had to be controlled precisely to secure the complete reaction. Also the deposition rate of each element was controlled separately to obtain stoichiometricMgB2, sinceMgwas evaporated by thermal heating andBby sputtering system. The sinteredBtarget was magnetron sputtered at the RF-power of ~200 W, which corresponded to the deposition rate of ~3.6 Å/s. With the deposition rate ofBfixed, the vapor pressure ofMgwas controlled by varying the temperature of tungsten boat with heating element control unit between 100 and 900°C. TheMgB2layers with the thickness of 200–950 nm could be obtained and occasionallyMgOappeared as a second phase. Superconducting transition temperatures were measured around ~38 K depending on the deposition condition.