Increases in the irreversibility field and the upper critical field of bulk MgB2 by ZrB2 addition -: art. no. 042505

In a study of the influence of ZrB2 additions on the irreversibility field, mu(0)H(irr) and the upper critical field B-c2, bulk samples with 7.5 at % ZrB2 additions were made by a powder milling and compaction technique. These samples were then heated to 700-900 degrees C for 0.5 h. Resistive transitions were measured at 4.2 K and mu(0)H(irr) and B-c2 values were determined. An increase in B-c2 from 20.5 T to 28.6 T and enhancement of mu(0)H(irr) from 16 T to 24 T were observed in the ZrB2 doped sample as compared to the binary sample at 4.2 K. Critical field increases similar to those found with SiC doping were seen at 4.2 K. At higher temperatures, increases in mu(0)H(irr) were also determined by M-H loop extrapolation and closure. Values of mu(0)H(irr) which were enhanced with ZrB2 doping (as compared to the binary) were seen at temperatures up to 34 K, with mu(0)H(irr) values larger than those for SiC doped samples at higher temperatures. The transition temperature, T-c, was then measured using dc susceptibility and a 2.5 K drop of the midpoint of T-c was observed. The critical current density was determined using magnetic measurements and was found to increase at all temperatures between 4.2 K and 35 K with ZrB2 doping. (c) 2005 American Institute of Physics.