  Two different hot isostatic pressing cycles (HIPing) were investigated to
synthesize bulk MgB2 samples: a "standard" cycle where a low vessel pressure is
maintained while heating to the process temperature with a subsequent
simultaneous pressure and temperature decrease and a new method - dense
material cooling under pressure (DMCUP). The latter method allowed the
synthesis of dense samples with diameters up to 20 mm and thicknesses up to 10
mm from commercial MgB2 powder. Optimal conditions for the DMCUP method with
glass encapsulation (maximum pressure 200 MPa, maximum temperature 1000 C over
200 min, and cooling under pressure) resulted in a dense material with a sharp
superconducting transition at 38.5 K. This method employs a pressure which is
one order of magnitude less than previously reported for pressure assisted
sintering of dense material and can be scaled to larger sample sizes and
complex shapes. The data for density, microhardness, fracture toughness and
sound speed as well as superconducting properties for bulk magnesium diboride
are presented. Ball milling the powder enhances sintering and results in a more
homogeneous final microstructure.
