Some Issues of Magnum Boride bulk samples-™ Obtaining Process

Magnum boride bulk superconductor samples (targets) with a diameter >50 mm has been obtained in an induction furnace, using hot press and spark-plasma synthesis methods. Those methods enable to obtain small diameter (up to <25 mm) samples without any difficulties by gradual temperature increase 20-10500C during 5-12 minutes. Sintering and consolidation is performed by the abovementioned methods as follows: Mg:B=1:2 compound is preliminary pressed in tablets in a stainless steel die with pressure p=100 mPa, and then the sample is placed in a graphite die. Referring to MgB2 powders consolidation 950-10500C has been performed by fast heating method (10-15 min). X-ray phase analysis confirm formation of MgB2, which also contain MgO up to 1-5 %, within Tc = 38.5-39 0K. Density reaches 2.35-2.41 g/cm3¬. Increased diameter (>50 mm) of the samples reveal a number of problems. Despite the fact that big diameter samples phase content is like small ones, their porosity is increased and 45-50 % of the obtained samples have macro cracks, and in some cases samples crashed wholly. One of the purposes causing samples crashing is preliminary tableting of Mg+B mixture and delivering pressure in nonoptimal regimes in a sintering furnace, i.e. the samples crack before chemical synthesis is performed and MgB2 obtained. The next stage of the research dealt with samples obtaining in a graphite press-form, being taken out tantalum or Graflex foil, in case when Sintering is conducted in an induction furnace. Mg+B mixture was placed in a press-form, heated till 600-650 0C and then pressure was gradually increased (20 min retention). Sharp increase of pressure during this interval is not acceptable, because melt magnum comes out of the press-form and Mg:B ratio is changed, pressing is increased according to temperature ratio increase till 1050 0C during 80-85 minutes and press 5 tones. In this mould the samples are retained during 10 minutes, then in result of gradual pressure lift and cooling in an inert atmosphere we obtain >50mm diameter cylindrical samples.