Bulk samples of MgB 2 were prepared with 5, 10, and 15 wt % Y 2 O 3 nanoparticles, added using a simple solid-state reaction route. Transmission electron microscopy showed a fine nanostructure consisting of ϳ3-5 nm YB 4 nanoparticles embedded within MgB 2 grains of ϳ400 nm size. Compared to an undoped control sample, an improvement in the in-field critical current density J C was observed, most notably for 10% doping. At 4.2 K, the lower bound J C value was ϳ2 ϫ10 5 A cm Ϫ2 at 2 T. At 20 K, the corresponding value was ϳ8ϫ10 4 A cm Ϫ2 . Irreversibility fields were 11.5 T at 4.2 K and 5.5 T at 20 K. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1506184͔In slightly more than one year after the discovery of superconductivity in magnesium diboride, there is now a wide body of evidence indicating that MgB 2 does not contain intrinsic obstacles to current flow between grains, unlike the high-temperature superconducting cuprates. Evidence for strongly coupled grains has been found even in randomly aligned, porous, and impure samples, 1,2 suggesting that dense forms of MgB 2 will be attractive in high-current applications at 20-30 K and perhaps 4.2 K. So far, however, bulk samples have demonstrated modest values of the irreversibility field 0 H*(T) reaching about 4 T at 20 K and 8 T at 4.2 K.3 For comparison, established low-temperature superconductors, e.g., NbTi ͑10 T͒ and Nb 3 Sn ͑20 T͒, have significantly higher irreversibility fields at 4.2 K, while Bi 2 Sr 2 Ca 2 Cu 3 O 10 ͑3 T͒ is becoming established at 20 K. 4 MgB 2 tape results are somewhat more promising, with 0 H* values of above 5 at 20 K, 5-8 where partial orientation of crystallites parallel to the field is playing a role. Since the irreversibility field is the practical limit to magnet applications, it is desirable to make 0 H* values as high as possible.A central question is how to further increase the irreversibility field in addition to introducing crystallographic texture. Alloying additions, such as atomic substitution for Mg or B or added interstitial atoms, increase electron scattering and decrease the coherence length, producing higher upper critical and irreversibility fields.9,10 Adding nanometer-scale defects can produce similar effects. For example, proton irradiation studies showed that 0 H* increased significantly from ϳ3.5 to ϳ6 T at 20 K with only moderate damage, corresponding to atomic displacements of a few %, due to either vacancies or interstitials.11 Mechanical processing also produces structural defects, and similar increases in the irreversibility field have been reported. 6,8,12 These increases were steeper than the concomitant reductions in the critical temperature T c , suggesting it is viable to improve the accessible field range without sacrificing other superconducting properties too much.To explore more practical and scaleable routes to defect incorporation in bulk MgB 2 , the present study explores chemical and nanostructural changes via addition of nanoparticles. Coherently ordered Mg-B-O precipitates are known to ...

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This paper will describe a procedure for modelling the complete macroscopic response (including strain hardening and tension softening) of two short ®bre reinforced cementitious composites and show how their microstructural parameters in¯uence this response. From a mathematical point of view it is necessary to examine how bridging forces imposed by the ®bres alter the opening of multiple cracks in elastic solids under unidirectional tensile loading.The strain hardening is essentially due to elastic bridging forces which are proportional to crack opening displacements. After a certain critical crack opening displacement is reached, some ®bres progressively debond from the elastic matrix and thereafter provide a residual bridging force by frictional pull-out, while others continue to provide full bridging. This results in a kind of elastoplastic bridging law which governs the initial tension softening response of the composite.Besides the usual square-root singularity at crack tips, the elasto-plastic bridging law introduces a logarithmic singularity at the point of discontinuity in the bridging force. These singularities have been analytically isolated, so that only regular functions are subjected to numerical integration. Unbridged multiple crack problems have in the past been solved using double in®nite series which have been found to be divergent. In this paper a superposition procedure will be described that eliminates the use of double in®nite series and thus the problem of divergence. It is applicable to both unbridged and bridged multiple cracks. The paper will end by showing how the model of multiple bridged cracks can accurately predict the prolonged nonlinear strain hardening and the initial tension softening response of two cementitious composites.