Two different kinds of bulk MgB_2 material were produced by (i) hot deformation of MgB_2 pellets, prepared by the reaction of Mg and B powders, at temperatures around 900 deg C and pressures up to 450 MPa and (ii) by sintering of MgB_2 powder up to 1000 deg C and pressures up to 8 GPa. In both cases the material has full density and sharp superconducting transitions at up to 39 K. The samples were characterized by measurements of x-ray diffraction, ac susceptibility, electrical resistance and magnetization. Material (i) is anisotropic and shows a weak texture verified by x-ray diffraction and the ratio of the upper critical fields H_(c2)~(a,b)/H_(c2)~c approx 1.2. Material (ii) prepared at optimal conditions is macroscopically isotropic and shows improved superconducting parameters including critical current densities of j_c approx 10~5 Acm~2 at 10 K and 1T, upper critical field mu_0H_(c2)(0 K) approx =23 T and irreversibility field mu_0H_(irr)(10 K) approx =12 T. It is shown that the width of the superconducting transition curves of polycrystalline material measured in higher magnetic fields is mainly due to the anisotropy of H_(c2).
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