Effect of sintering temperature on the superconducting properties of MgB2 superconductor co-added with a high concentration of Si and C

In this study, as much as 10 and 15 wt.% nanosized silicon and carbon (Si+C) were reacted with (Mg+2B) at 650°C and 850°C, respectively, for 1 hour. The phase formation, surface morphology and superconducting properties of these samples were evaluated. The relative peak intensity as calculated from the XRD patterns indicates the formation of large Mg2Si volume fraction at low sintering temperature. MgB4 phase was detected in the samples sintered at high temperature as a result of Mg deficiency. The C substitution level as estimated from the lattice parameters, was shown to increase in the samples reacted with a higher amount of (Si+C) at high temperature. Scanning electron micrograph showed that (Si+C) co-addition had refined the grain size and improved the grain coupling of MgB2. The superconducting transition temperature was found to decrease with increasing addition level. The superconducting transition width was also broadened because of a large volume fraction of secondary phases. The improved field dependent critical current density at both 5 K and 20 K is accounted to enhanced scattering by C substitution and grain boundary pinning

X-ray powder diffraction study on the MgB2 superconductor reacted with nano-SiC: the effects of sintering temperature

SiC added MgB2 polycrystalline samples were synthesized at low (650°C) and high (850°C) temperatures in order to study the sintering effect on the phase formation and superconducting properties. The MgB2 bulks with additions of 0wt%, 1wt%, 3wt% and 5wt% SiC were studied with powder X-ray diffraction technique. We observed that MgB2 remained as the primary phase for both sintering temperatures in all samples with the presence of MgO and Mg2Si as the main impurities. Some diffraction peaks associated with unreacted SiC is also noticeable. The relative intensity of the Mg2Si peaks was found to decrease in samples sintered at higher temperature. Temperature dependent magnetic moment measurements showed that the superconducting transition temperature, Tc decreases as the SiC addition level increases while lower sintering temperature degrades Tc to a greater extent. The changes in the physical properties is discussed based on the results of phase formation, full width half maximum (FWHM), lattice parameter and crystallite size

Effects of nano-SiC addition on the superconducting properties of magnesium diboride

In this study, we report the results on phase formation, microstructures, and superconducting properties of a series of MgB2 samples with different level of SiC additions. The polycrystalline samples were prepared via solid state reaction by mixing magnesium, boron and silicone carbide powders according to the ratio of Mg:B:SiC = 1:2:x. XRD spectra showed that MgB2 is the primary phase while Mg2Si, MgO and MgB4, together with some unreacted SiC are the secondary phases as the addition increases. The presence of Mg2Si became more significant as the addition level increased. SEM images showed smaller grains as the addition level increases indicating more grain boundaries were formed. The Tc was as low as 30.5K for x=15wt%. The field dependence of Jc showed that x=1wt% sample gave the best performance at both 5K and 20K

Reaction method control of impurity scattering in C-doped MgB2: proving the role of defects besides C substitution level

In this study, Si and C were incorporated into polycrystalline MgB 2 via in situ reaction of Mg and B with either SiC or with separate Si and C (Si+C). The electrical transport and magnetic properties of the two series of samples were compared. The corrected resistivity at 40 K, ρA(40 K), is higher for the samples reacted with SiC regardless of the carbon (C) substitution level, indicating larger intragrain scattering because of the simultaneous reaction between Mg and SiC and carbon substitution during the formation of MgB2. In addition, because of the cleaner reaction route for the samples reacted with SiC, the calculated active area that carries current, AF, is twice that of the (Si+C) samples. On the other hand, the upper critical field, Hc2, was similar for both sets of samples despite their different C substitution levels, which proves the importance of defect scattering in addition to C substitution level. Hence, the form of the precursor reactants is critical for tuning the form of Hc2(T)

Superconducting properties of MgB2 after reaction with silicon and carbon-containing additives

SiC is one of the promising dopants that effectively improves the critical current density (Jc) of MgB2 by substituting C into B-site and enhances electron scattering. However, the roles of Si and C in influencing the superconducting properties of MgB2 are not fully understood. Furthermore, systematic study on the optimum dopant addition level and effect of sintering temperature are required in order to provide further insight into how SiC or both Si and C enhance Jc. In this study, nano-SiC and combination of nano-Si and nano-C (Si+C) that made of similar ratio to that of SiC [up to 15 weight percentage (wt.%)] was reacted with Mg+B powder by in situ solid state method. These bulks were sintered at 650°C and 850°C, respectively. Characterizations are performed by using Xray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Magnetic Property Measurement System (MPMS). These samples were compared in terms of phase formation, lattice properties, microstructure and superconducting properties. At 650°C, samples reacted with SiC show smaller a-axis and more vigorous lattice distortion because of higher C substitution at B site as compared to same amount of (Si+C) addition. This is due to the reactive form of C atoms released from Mg-SiC reaction with lower Gibbs free energy. Higher C substitution in SiC reacted sample results in more severe degradation in superconducting transition temperature (Tc) arising from more severe lattice distortion. Samples reacted with SiC (up to 5 wt.%) show stronger improvement in Jc at both 5 K and 20 K mainly because of smaller grains that enhance grain boundary pinning and degraded crystallinity due to lattice defect. At 850°C, (Si+C) reacted samples have greater extent of a-axis contraction and more severe lattice distortion because of higher C substitution than those samples reacted with SiC. Such phenomenon is probably due to the availability of more C in which separate Si and C particles are used or effect of higher sintering temperature. Higher level of C substitution in (Si+C) reacted samples leads to more severe Tc suppression because of more severe lattice distortion. On the other hand, (Si+C) reacted samples show stronger Jc improvement at both 5 K and 20 K due to higher C substitution at B-site that further enhances electron scattering. As a conclusion, for sintering at 650°C, reaction of SiC with MgB2 is preferred while sintering at 850°C, reaction of (Si+C) with MgB2 is favored as higher C substitution and more lattice defects occur in these samples which effectively enhance the Jc. Among all samples, the Jc of 5 wt.% SiC reacted MgB2 at 650°C, as compared to the pure sample,is improved by a factor of two at 5 K for the applied field of 6 T and 60 % more at 20 K for the applied field of 4 T, respectively. This improvement is due to degraded crystallinity and higher C substitution that further scatter the electrons and smaller grains that enhance grain boundary pinning

The superconducting properties of MgB2 synthesized by reaction of Mg and higher boride phase

In this work, the synthesis of MgB2 by reaction between MgB4 and Mg has been investigated. The MgB4 powders were initially acid-washed in order to eliminate mainly as well as other impurities. Later, an appropriate amount of Mg was added for reaction at temperature range from 650°C to 950°C for 4 hours. The samples were characterized by x-ray diffraction (XRD), scanning electron microscope (SEM) and SQUID magnetometer (MPMS). Results from XRD showed the dominant phase of MgB2 with some impurities of MgB4 and MgO in all samples. The scanning electron micrographs showed denser microstructures compared to samples obtained by direct in-situ reaction of Mg and B. However, the measured physical density was decreased upon increasing the sintering temperature attributed to the loss of Mg. The superconducting transition temperature, Tc at 37.2K was found to improve at lower sintering temperature

Electrical transport and magnetic properties of MgB2 reacted with Si and C

SiC is among the promising dopants which can be used to increase the critical current density of MgB2 superconductor up to the level required for producing high magnetic field. In this work, we are motivated to study how Si and C influence the electromagnetic properties of bulk MgB2 prepared by reacting the material with SiC or combination of Si and C elements (Si+C). While there is no significant change in the e-axis lattice parameter, the a-axis is smaller compared with that of the pure sample indicating substitution of C on the B site. The C substitution level was estimated based on the relative change of c with respect to a (c1a). By varying the sintering temperature, the C substitution level is different between the samples reacted with SiC and {Si+C) despite their superconducting transition temperature is close to each other. The upper critical fields were determined from the temperature and magnetic field dependence of resistive transition. These results together with the inductive critical current density were correlated to the resistivity properties and C substitution