  The effect of doping with Ti, Ta, SiC in complex with synthesis temperature
on the amount and distribution of structural inhomogeneities in MgB2 matrix of
high-pressuresynthesized-materials (2 GPa) which can influence pining: higher
borides (MgB12) and oxygen-enriched Mg-B-O inclusions, was established and a
mechanism of doping effect on jc increase different from the generally accepted
was proposed. Near theoretically dense SiC-doped material exhibited jc= 1000000
A/sq. cm in 1T field and Hirr =8.5 T at 20 K. The highest jc in fields above 9,
6, and 4 T at 10, 20, and 25 K, respectively, was demonstrated by materials
synthesized at 2 GPa, 600 oC from Mg and B without additions (at 20 K jc= 100
A/sq. cm in 10 T field). Materials synthesized from Mg and B taken up to 1:20
ratio were superconductive. The highest jc (60000 A/sq. cm at 20 K in zero
field, Hirr= 5 T) and the amount of SC phase (95.3 percents of shielding
fraction), Tc being 37 K were demonstrated by materials having near MgB12
composition of the matrix. The materials with MgB12 matrix had a doubled
microhardness of that with MgB2 matrix (25 GPa and 13.08 GPa, at a load of 4.9
N, respectively).
