Major drawback of studying diffusion in multi-component systems is the lack of suitable techniques to estimate the diffusion parameters. In this study, a generalized treatment to determine the intrinsic diffusion coefficients in multi-component systems is developed utilizing the concept of pseudo-binary approach. This is explained with the help of experimentally developed diffusion profiles in the Cu(Sn, Ga) and Cu(Sn, Si) solid solutions.Estimation of the diffusion coefficients following the conventional approach in a binary system implementing the diffusion couple technique is rather straightforward. It is a stringent process in a ternary system and impossible in multi-component systems [1]. As an alternate approach, the concept of the average effective interdiffusion coefficient was introduced by Dayananda and Sohn [2]. This is a kind of average interdiffusion coefficient of the main and cross interdiffusion coefficients over a wide composition range. Much recently, Chen et al. [3] proposed a method to 2 1 * * * *

Magnesium diboride (MgB2) bulk superconductors may have practical applications as permanent magnets owing to their ability to trap larger fields than conventional ferromagnets and a transition temperature of 39 K that make them attractive for use in cryogen-free systems. Unlike the cuprate high temperature superconductors, grain boundaries in MgB2 act as pinning sites not weak links, and so show good current carrying ability in polycrystalline samples. This enables the materials to be processed using standard ceramic processing methods which are scalable to large diameters and mass production. The maximum trapped field in bulk superconductors scales with the critical current density (Jc ) of the material as well as the radius of the sample. To obtain the highest possible Jc values in MgB2 at high fields requires the bulk materials to be fully dense but fine-grained material, and possibly with a nano-scale distribution of non-superconducting impurity particles to further enhance pinning. Field assisted sintering technology (FAST) is a rapid process for obtaining dense ceramics from materials like MgB2 which are difficult to sinter with conventional pressure-less techniques. Rapid heat treatments are attractive both from a manufacturing point of view and because the total time that the sample is held at high temperature is short, limiting grain coarsening. In this paper, we report a systematic study of the influence of processing temperature on microstructure and superconducting properties of MgB2 bulks manufactured using FAST. We conclude that processing temperatures above 1000 °C are required to obtain materials that have sufficiently high electrical connectivity to generate large magnetic moments. However, the intrinsic (intragrain) Jc values in MgB2 are better in the samples processed at 900 °C owing to their finer scale microstructures and the MgB2 lattice being more defective.

MgB2 is a promising material for intermediate temperature applications where conventional low temperature superconductors cannot be used, especially if the range of magnetic fields over which is has acceptable current carrying performance can be expanded. However, its applicability is limited by poor properties at elevated magnetic fields. Carbon-based dopants can be used to dramatically improve the high-field performance of MgB2, but at the cost of a reduction in the superconducting transition temperature (Tc) that limits the operation temperature to 20 K or below. Here we report an enhancement of superconducting performance of MgB2 with the addition of cubic and hexagonal boron nitride, without any significant reduction in Tc. Ex-situ bulk samples of MgB2 with two forms of boron nitride addition were manufactured by the field assisted sintering technique (FAST) after high energy ball milling of powder mixtures. We find that hexagonal BN (hBN) nanoparticles mixed homogenously with MgB2 powder react much more easily to produce Mg-N-B impurities than larger cubic BN (cBN) particles (~ 10 µm) under the same processing conditions. The addition of 1 wt% hBN or 5 wt% cBN combined with 6 h of milling has been demonstrated to improve the critical current density (Jc) of MgB2 over the entire magnetic field range. It is proposed that the nano-sized Mg-N-B impurities, that typically reside at MgB2 grain boundaries, increase pinning strength by introducing additional flux pinning centres. In addition, excess Mg may benefit the low-field performance by improving the connectivity. This work shows the significance of microstructural characterization on inhomogeneous superconducting materials to analyse their performance.

Bi-2212/Ag superconducting wires are being considered as key conductor candidates for the development of high-field magnets. This work focuses on the design and testing of various joint structures for multifilamentary Bi-2212 wires using powder-in-tube techniques. The joints are made between the unreacted wires and subsequently heat treated with the wires. The microstructure and transport properties of different joint architectures including step joints, scarf joints and etched joints have been explored. The performance of etched-joints are found to be very sensitive to the etching parameters including etching time, making it difficult to develop a reliable jointing process. Scarf joints and step joints can carry higher currents (320 A and 475 A in self-field at 4 K) than the etched joints (225 A in self-field at 4 K) as a result of connecting a larger area of the exposed filaments without damaging the filaments.