Effect of the sintering temperature on the microstructure and superconducting properties of MgB₂ bulks manufactured by the field assisted sintering technique

Abstract

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.

Improved pulsed field magnetisation in MgB2 trapped-field magnets

“…The bulk was sintered at 1300 C and 50 MPa for 5 minutes. Further details of the sintering process can be found in [15]. The absolute trapped field potential of the sample was characterised by FCM utilising a 12T large bore magnet system with precise control of the bulk temperature.…”

Improving the connectivity of MgB2 bulk superconductors by a novel liquid phase sintering process

“…∆T c decreased from 1.7 K in unmodified MgB 2 to 0.6 K in the sample containing 10 wt.% Mg, as shown in Table 2. Matthews et al [11] reported a similar reduction in ∆T c in unmodified MgB 2 samples sintered at higher temperature. The authors attributed this to heterogeneities (composition and/or disorder) initially present in the as-received powder being annealed out at high temperature.…”

“…Sintering or Direct Current Sintering is a popular technique that has been used successfully to produce dense MgB 2 specimens by different research groups [7][8][9][10][11]. This technique consists in driving a high current (usually at a low voltage) through an assembly composed of a graphite die in which the powder is enclosed between two graphite punches.…”

“…To estimate the effective cross-sectional area for macroscopic current transport in the bulk samples, four-point transport measurements and analysis of connectivity have been performed to obtain the normal state electrical resistivity as a function of temperature. Following the analysis of Collings et al [28], values of connectivity were extracted from the data using the procedure described in a previous article by Matthews et al [11]. The Materials constant K used in the connectivity analysis was estimated by fitting reference single crystal data from Eltsev et al [29].…”

Dense MgB2 Ceramics by Ultrahigh Pressure Field-Assisted Sintering

“…Figure 4a shows the benefit of identical temperature for 3 and 5 GPa where a threshold of resistivity appeared from 950 • C. The benefit of pressure in Figure 4b at 1.7 GPa probably derived from the high-density ceramics that were reached in this range of pressure. Resistivity values decreased systematically with increasing processing temperature, suggesting the MgB 2 lattice contains fewer defects and likely results in the superconductor having no parasitic phases [20,21]. Up to now, the majority of dense samples have been made by high-temperature and high-pressure (HT-HP) processes.…”