| Since the discovery of its superconductivity in 2001, MgB2 superconductor has aroused great interest and extensive research work of researchers. Compared to high temperature superconductor (HTS), it has the characteristics of low fabrication cost, lack of weak grain connectivity, simple composition and high magnetic Jc, making it a potential candidate for superconducting applications in the temperature range of 20-30K, such as current lead wire, magnetic resonance imaging, transmission cable, magnetism fabrication, geodetic prospecting, environment, military affairs, and magnetic suspension and so on. In this work, fabrication, microstructural evolution and properties of the SiC nanoparticle doped MgB2 superconductors were investigated systematically. A novel nano-sized Fe3O4 doped MgB2 superconductor was proposed and fabricated, and it has superior superconducting properties in the temperature range of 20-30K.MgB2 bulks were fabricated using Mg powder and B powder as starting materials by a hot pressing process with rapid heating rate. The rapid heating rate in the hot pressing process can promote the liquid-solid reaction between Mg and B to form the MgB2 phase, reducing oxidation of Mg during the fabrication process and leading to product of high phase purity and high density. A suitable fabrication process was obtained:under the protection atmosphere of flowing argon, heating rate is about 200℃/min when T<500℃and 140-150℃/min when 500℃<T<950℃, and the hot pressing is at 950℃for 30min with a pressure of 50MPa. The fabricated sample is 2.295g/cm3 in density, and in the temperature range of 5-30K and at self-field, magnetic Jc values are higher than 106A/cm2. The magnetic Jc value at temperatures higher than 20 K is higher than those of the MgB2 system reported by other researchers, especially at 30K, more than an order of magnitude higher.Boron powder purity is believed to have significant effect on properties of MgB2. The feasibility of using a low price low purity B power (88.84%,2000 Yuan/kg) in replacement of high purity B powder (99%,11000 Yuan/kg) in fabricating high property SiC doped MgB2 was studied. The results show that the Jc of the sample using low purity B powder is only a little bit lower than that of the sample using high purity B powder, but the cost of materials can be reduced by 80%, which is of great importance to commercial manufacture.Phase evolution of nano-particle SiC doped MgB2 hot pressed was studied systematically. XRD analysis shows that the sample is composed of MgB2 as the main phase, with some Mg2Si and a small amount of MgO. During the sintering process, Mg and SiC first react to form Mg2Si and C at about 500℃. Mg and B can react to form MgB2 phase at temperatures greater than 550℃In the temperature range of 550-650℃(melting point of Mg), MgB2 is formed between solid Mg and solid B. At temperatures higher than 650℃, MgB2 is formed by the reaction of liquid Mg and solid B. With the increase of hot pressing temperature, density and grain size of the sample increase.The 5wt% SiC doped sample made by hot pressing at 950℃for 30min has good superconductivity. Peak Jc of the sample is higher than 106A/cm2 at 5K, and under a field of 7T Jc is still higher than 105A/cm2, indicating a better superconductivity than the undoped sample and the results of similar reported work. At 20K and a high field, Jc of the 5 wt% SiC doped sample is more than one order of magnitude higher than that of the undoped sample. This is mainly because the sample has relatively high density (2.295 g/cm3), fine MgB2 grain size (-0.9 p.m), small pore size (-0.16μ.m), and homogeneous microstructure. Especially, well distributed Mg2Si nanoparticles (35-230nm) in the MgB2 matrix can act as flux pinning centers. The 5wt% SiC doped sample has higher Jc than the 10wt% SiC doped one, and this may be due to the more non-superconducting phase in the latter. There are two mechanisms of Jc enhancement by SiC doped one is:C substitution for B site, resulting in strong electron scattering center, reducing mean electron free path and enhancing Jc value; the other is the well-dispersed nano-sized Mg2Si particles can act as flux pinning centers.Well dispersed ferromagnetic nano-scaled particles Fe3O4 synthesized by a chemical co-precipitation method were doped into MgB2 superconductor. The result shows that Fe3O4 particles react with neither Mg nor B during fabrication, and the sample is mainly composed of MgB2 and homogeneously distributed Fe3O4 fine particles. The 5wt% Fe304 doped sample shows high superconducting property. In the temperature range of 5-30K, peak Jc is higher than 10 A/cm2, and especially at 30K, peak Jc is 1.02×10 A/cm, one order of magnitude higher than the SiC doped sample, and the highest values at 30K found in literatures. In the temperature range of 25-30K, its Jc decreases more slowly than that of the SiC doped sample with increasing field. One of the mechanisms of Jc enhancement is flux pinning from nano-paritcles as pinning centers, and the other is the attracting force on fluxoids exerted by the magnetic particles. Due to its excellent superconducting properties at temperatures higher than 25 K, the Fe3O4 doped MgB2 has good potential in practical applications.Some preliminary work was carried out on some new dopants into MgB2 superconductor. Fabrication and properties of CoFe2O4 doped MgB2 and Nd-Fe-B doped MgB2 were studied. It was found that for both systems the dopants disappear to form boride products, introducing impurity phases into MgB2 and reducing superconductivity. But interestingly, at high testing temperature and high field, Fp curves of the two samples show strong jumping, and the upper critical field is increased. Doping of magnetostrictive Tb-Dy-Fe into MgB2 was also studied. It was found that most of the Tb-Dy-Fe has transformed into boride products, losing its magnetostrictive and magnetic properties, resulting in a decreased magnetic Jc. |
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