Magnetization loops (Fig. 4) for OT and CT specimens show interesting differences.For OT, flux-jumps (at T<10 K) occur on the ascending and descending branches, while forCT sample flux-jumps are on the descending branch, only. The result can be understood if OTsample is less thermally stable and/or it has a stronger pinning. We have seen in section 3.1that the core in OT has a less uniform/continuous contact with the Fe-sheath meaning that apoor Fe-MgB2 contact is available in this sample when compared with sample CT. Fe-sheathplays the role of the thermal stabilizer balancing thermal and magnetic energies towardselimination of the macro flux jumps formation. A poor contact between MgB2 core and Fesheath for OT sample when compared to CT sample is one reason for more flux-jumps in thefirst sample.Curves of Jc(H), Jc0(T), Hirrmag(T), (Jc0 x Hirrmag)(T), Fpmax(T) are presented in Fig. 5.Pinning-force-related parameters are in Fig. 6. A higher Jc0(T) is found for sample OT, whileHirrmag(T) is similar for all samples. Thus, a higher product Jc0 x Hirrmag is estimated for sampleOT. The product shows an average quality between low and high magnetic fields and is usedonly for a comparative analysis. Sample OT shows the highest Fpmax (T) and a balancedcontribution of PP and GPB pinning mechanisms since the curves p(T), q(T), h0(T) and kn(T)are between those for CT (that has curves located in the region with lower values where GBPis stronger) and for the bulk sample (that has the curves pushed to higher values where the PPis dominant). We shall remind that sample OT has the smallest MgB2 crystallites meaning thatmore boundaries are present in this sample and they can act as pinning centers enhancing Jcand indeed we have observed a higher Jc0 and Fpmax for this sample. If more boundaries areavailable, one would also expect a stronger contribution of GBP for the OT sample, but this isnot obvious and apparently a stronger GBP is for the CT sample. Results seem to be incontradiction, but this promotes the following understanding: the intermediate relativeposition of the pinning-force-related parameters and the highest Fpmax for OT sample lead toideea that both GBP and PP are stronger than for the other samples and their weight ratioreflected by the curves position in the pinning-parameter – temperature space (Fig. 6) is betterbalanced. Nevertheless, all samples are located in the PP region and the dependence of thepinning parameters vs. temperature is similar. Namely, all samples show an increase of p, h0and kn with the temperature increase, indicating on a stronger GBP contribution at lowertemperatures. This observation is not supported by q(T) variation which is approximatelyconstant for OT and CT samples and it is descendant for the bulk sample. If we assume thatOT sample provides, in a first approximation. the optimum conditions for removal of the Mgfreefrom the raw powder and we combine it with the observation that this sample has ahigher Jc and Fpmax, the result is aligned with conclusions for the MgB2 in-situ tapes of ref.2. Authors found that the residual Mg left in the reacted wires decreases Jc.A final understanding on the interpretation of the pinning-force-related parameters
All Research Proposal
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