Papers by Keyword: MgB₂

Abstract: The MgB2 PITs, prepared by filling Fe tubes with MgB2 as a core, were processed through different number of passes at room temperature via Routes A, BA, C and BC. The mechanical properties of the PITs were measured in terms of density and hardness. The extent of the compaction was also studied through shear punch test. The shape of the compacts remains circular even after four passes via Route C & BC. Higher density and hardness along with higher USS is observed from the PITs of Route C compared to Route BC. The PITs processed through Route A and Route BA did not show sufficient compaction to carryout the density measurements. Thus, the current study shows that Route C is the optimal route; to attain good mechanical properties in ECAE processed MgB2 PITs with Fe tube.

Abstract: We have successfully synthesized MgB2 thick films on 4H-SiC substrate by hybrid physical-chemical deposition (HPCVD). They have TC above 40 K, X-ray diffraction (XRD) shows the polycrystal bulk like structure. And JC estimated using magnetic hysteresis loops (MHLs) and Bean model is 7.4 MA/cm2 in self field, at 5 K.

Abstract: Pulsed magnetic field was first employed in the study on MgB2 superconductors. Superconductivity properties and microstructures of Zn and SiC doped MgB2 were discussed in this paper. The superconductors showed different superconductivities after magnetic sintering. Critical current density and flux pinning force were relevant to the dopants properties in MgB2. Grains in the pulsed magnetic processed MgB2 matrix were smaller than those in the normal processed one. The refinement microstructures have caused the critical current density (Jc) anisotropy in both the Zn and SiC doped MgB2.

Abstract: Fe-sheathed MgB2 tapes were prepared by the in situ powder-in-tube (PIT) technique using nanoscale Si/N/C and SiC whisker as doping materials, respectively. It is found that the doped tapes exhibited superior field performance and higher critical current (JC) values than the undoped tapes in the magnetic field up to 12 T. Moreover, the improvement of JC for the SiC whisker doped samples was more significantly than that in Si/N/C doped tapes, while flux pinning ability enhancements were similar. It is proposed that the difference of impurity content in MgB2 matrix, which affects the grain connectivity, is responsible for JC value difference between Si/N/C and SiC whisker doped samples.

Abstract: The discovery of superconductivity at 39 K in magnesium diboride (MgB2) has attracted much attention from theoretical and experimental points of view because it has the highest superconducting transition temperature in the conventional intermetallic superconductors reported so far. After the discovery of superconductivity in MgB2, many attempts have been made to synthesize this material with an aim of improving superconducting properties. In this study, MgB2 was synthesized from anhydrous B2O3. To provide homogeneous mixing of magnesium and boron oxide powders and to make easy sintering, powders were mechanically milled in an attritor under argon gas atmosphere. The mixture was treated with ethylenediamine solution to separete MgB2 from the product. The suspension solution was filtrated under vacuum and the solid particulates of MgO were removed. The filtrate was evaporated in a vacuum oven at 80 °C and sintered at 900 °C for 2 h under flowing argon gas atmosphere. XRD results showed that the final product was MgB2 with a small amount of MgO. Mechanical milling decreased the synthesis time and temperature of MgB2. The critical temperature of the specimens was determined as 37.4 K by an A.C susceptometer.

Abstract: After the emergence of AC NbTi strands, superconducting transformers were successfully built. But the very high cost of 4 K cryogenics made these transformers economically not attractive. The high Tc superconductors (HTS), operating at much higher temperatures, change these conclusions with low cost HTS conductors. The high cost of PIT tapes and the relatively large AC losses remain issues. The second generation HTS wires, the REBCO coated conductors, are under development and achieved substantial progress recently. They operate at higher temperatures and intrinsically show lower AC losses especially for transformers. MgB2 is the third option. The magnetic flux density conditions make possible the operation at 27 K and they show low costs. This paper provides a preliminary design for an on-board 40 MVA transformer using YBCO coated conductors and MgB2 wires. Both superconducting transformers show similar volume and weight. The power density per unit mass and volume is improved by a factor about two, cryogenic included, compared to resistive systems. This makes them very attractive for on-board mobile systems. The economical point of view will be discussed based on some targets price/performance for superconductors and cryocoolers. MgB2 is penalized by its operation at lower temperature (27 K / 77 K), which makes cryogenics very expensive. The advantage of the low cost of MgB2 compared to REBCO may be lost except with very low AC loss MgB2 tapes.

Abstract: Bulk MgB2 samples were synthesized by hot isostatic pressing under pressures up to 200MPa at 950°C. In these conditions, full densification of samples was obtained (~98% of theoretical density). SEM, EDS and XRD analysis on final dense bodies were used to evaluate samples, and show increasingly better control over the amounts of secondary MgO (down to ~10%) and complete prevention of formation of MgB4 by using simple glass encapsulation techniques and addition of Mg(s) to the capsule. The samples display superconducting properties, including a narrow critical transition in electrical properties (Tc ~36-38K). Magnetic studies were performed, allowing the determination of the superconducting fraction and critical current density Jc of the materials. Contrary to the Tc, the Jc is quite sensitive to the processing and microstructure and values from 0.3 to 0.6x106 A/cm2 are obtained at 10K. The reduction of Jc with the applied magnetic field requires further improvements to reduce weak links.

Abstract: We successfully developed the potential parameters for simulation of MgB2. With these potential parameters, we calculate the lattice parameters and volume variations with pressure up to 240GPa. All these results agree well with experimental data under 40GPa and provide reasonable tendencies from 40GPa to 240GPa. By employing the McMillan expression, it is found that the lattice stiffening dominants the behavior of Tc under pressure in the scope of BCS theory. Using our calculated Grüneisen parameter G g , the simulated pressure effect on Tc accords well with experimental results. Our result shows that the Tc of MgB2 can be destroyed by high pressure.