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S. Soltanian

Publications and source records attributed to S. Soltanian.

18 recordsLinked to original sources
arXiv2007

Mechanism of enhancement of electromagnetic properties of MgB2 by nano-SiC doping

A comparative study of pure, SiC, and C doped MgB2 wires has revealed that the SiC doping allowed C substitution and MgB2 formation to take place simultaneously at low temperatures. C substitution enhances Hc2, while the defects, small grain size and nanoinclusions induced by C incorporation and low temperature processing are responsible for the improvement in Jc. The irreversibility field (Hirr) for the SiC doped sample reached the benchmarking value of 10 T at 20 K, exceeding that of NbTi at 4.2 K. This dual reaction model also enables us to predict desirable dopants for enhancing the performance properties of MgB2.

cond-mat.supr-con↗
arXiv2004

Large Upper Critical Field and Irreversibility Field in MgB2 wires with SiC additions

Resistive transition measurements are reported for Magnesium diboride strands with SiC dopants. The SiC was added as 10 mol of SiC to 90 mol of binary magnesium diboride. Three different SiC powders were used, the average particle sizes were 200 nm, 30 nm, and 15 nm. The strands were heat treated for times ranging from 5 to 30 minutes at temperatures from 675 to 900. Strands with 200 nm size SiC additions had Hirr and Bc2 which maximized at 25.4 T and 29.7 T after heating at 800C for 30 minutes. The highest values were seen for a strand with 15 nm SiC heated at 725C for 30 minutes which had a Hirr of 29 T and a Bc2 higher than 33 T

cond-mat.supr-con↗
arXiv2004

Effect of Nano Scale Fe Doping on Superconducting Properties of MgB2

Iron is an important sheath material for fabrication of MgB2 wires. However, the effect of Fe doping on the superconducting properties of MgB2 remains controversial. In this work, we present results of nano-scale Fe particle doping in to MgB2. The Fe doping experiments were performed using both bulk and thin film form. It was found that Fe doping did not affect the lattice parameters of MgB2, as evidenced by the lack of change in the XRD peak positions for MgB2. Because of the high reactivity of nano-scale Fe particles, Fe doping is largely in the form of FeB at low doping level while Fe2B was detected at 10wt% doping by both XRD and TEM. There is no evidence for Fe substitution for Mg. The transition temperature decreased modestly with increasing Fe doping levels. The Jc(H) performance was severely depressed at above 3wt% doping level. The detrimental effect of nano-scale Fe doping on both Tc and Jc(H) is attributable to the grain decoupling as a result of magnetic scattering of Fe-containing dopants at grain boundaries.

cond-mat.supr-con↗
arXiv2004

The effect of sample size on field dependence of Jc for MgB2 superconductor

A strong effect of sample size on magnetic Jc(H) was observed for bulk MgB2 when Jc is obtained directly from the critical state model. Thus obtained zero-field Jc (Jc0) decreases strongly with the sample size, attaining a constant value for the samples larger than a few millimetres. On the other hand, the irreversibility field (Hirr) defined at Jc = 100 A/cm2 increases with the sample size. The decrease of Jc0 is described in terms of voids in the bulk MgB2 samples and superconducting screening around the cells of superconducting material between these voids (35 micro-m), because of concentration of the current in the narrow bridges connecting the cells. For samples larger than a few millimetres, the value of magnetic Jc is in agreement with the transport Jc and it is restricted by the voids. The critical state model is not suitable for obtaining Jc for small bulk MgB2. The increase of Hirr with the sample size is an artefact of defining Hirr by the value of Jc at which an additional superconducting screening on 1mm scale dominates Dm.

cond-mat.supr-con↗
arXiv2004

Superconducting screening on different length scales in high-quality bulk MgB2 superconductor

High quality bulk MgB2 exhibit a structure of voids and agglomeration of crystals on different length-scales. Because of this, the superconducting currents percolate between the voids in the ensuing structure. Magnetic measurements reveal that the superconducting currents circulate on at least three different length-scales, of ~1 micrometre, ~10 micrometre and whole of the sample (~millimetre). Each of these screenings contributes to the measured irreversible magnetic moment (Dm). The analysis of the field dependence of Dm for samples of subsequently decreasing size showed that the critical current obtained using the simple critical state model is erroneous. This leads to the artefact of the sample size-dependent critical current and irreversibility field. Our data analysis enables the separation of the contribution of each of the screening currents to Dm. The field dependence of each of the currents follows a stretched exponential form. The currents flowing around whole of the sample give a dominant contribution to Dm in the intermediate fields (1T < H < 4T at 20K) and they can be used to obtain the value of Jc from critical state model, which corresponds to the transport Jc.

cond-mat.supr-con↗
arXiv2003

Correlated vortex pinning in Si-nanoparticle doped MgB2

The magnetoresistivity and critical current density of well characterized Si-nanoparticle doped and undoped Cu-sheathed MgB$_{2}$ tapes have been measured at temperatures $T\geq 28$ K in magnetic fields $B\leq 0.9$ T. The irreversibility line $B_{irr}(T)$ for doped tape shows a stepwise variation with a kink around 0.3 T. Such $B_{irr}(T)$ variation is typical for high-temperature superconductors with columnar defects (a kink occurs near the matching field $% B_ϕ$) and is very different from a smooth $B_{irr}(T)$ variation in undoped MgB$_{2}$ samples. The microstructure studies of nanoparticle doped MgB$_{2}$ samples show uniformly dispersed nanoprecipitates, which probably act as a correlated disorder. The observed difference between the field variations of the critical current density and pinning force density of the doped and undoped tape supports the above findings.

cond-mat.supr-con↗
arXiv2003

Nanoscale-SiC doping for enhancing Jc and Hc2 in the Superconducting MgB2

The effect of nanoscale-SiC doping of MgB2 was investigated using transport and magnetic measurements. It was found that there is a clear correlation between the critical temperature Tc, the resistivity r, the residual resistivity ratio, RRR = R(300K)/R(40K), the irreversibility field H* and the alloying state in the samples. SiC-doping introduced many nano-scale precipitates, provoking an increase of r(40K) from 1 mW-cm (RRR = 15) for the clean limit sample to 300 mW-cm (RRR = 1.75) for the SiC-doped sample, leading to significant enhancement of Hc2 and H* with only minor effect on Tc. EELS analysis revealed a number of nano-scale impurity phases: Mg2Si, MgO, MgB4, BOx, SixByOz, BC and unreacted SiC in the doped sample. TEM study showed an extensive domain structure of 2-4nm domains induced by SiC doping. The Jc for the 10% nano-SiC doped sample increased substantially at all fields and temperatures compared to the undoped samples, due to the strong increase in Hc2 and H* produced by SiC doping.

cond-mat.supr-con↗
arXiv2003

On the sample size dependence of the critical current density in MgB$_2$ superconductors

Sample size dependent critical current density has been observed in magnesium diboride superconductors. At high fields, larger samples provide higher critical current densities, while at low fields, larger samples give rise to lower critical current densities. The explanation for this surprising result is proposed in this study based on the electric field generated in the superconductors. The dependence of the current density on the sample size has been derived as a power law $j\propto R^{1/n}$ ($n$ is the $n$ factor characterizing $E-j$ curve $E=E_c(j/j_c)^n$). This dependence provides one with a new method to derive the $n$ factor and can also be used to determine the dependence of the activation energy on the current density.

cond-mat.supr-con↗
arXiv2003

Effect of nano-carbon particle doping on the flux pinning properties of MgB2 superconductor

Polycrystalline MgB2-xCx samples with x=0.05, 0.1, 0.2, 0.3, 0.4 nano-particle carbon powder were prepared using an in-situ reaction method under well controlled conditions to limit the extent of C substitution. The phases, lattice parameters, microstructures, superconductivity and flux pinning were characterized by XRD, TEM, and magnetic measurements. It was found that both the a-axis lattice parameter and the Tc decreased monotonically with increasing doping level. For the sample doped with the highest nominal composition of x=0.4 the Tc dropped only 2.7K. The nano-C-doped samples showed an improved field dependence of the Jc compared with the undoped sample over a wide temperature range. The enhancement by C-doping is similar to that of Si-doping but not as strong as for nano-SiC doped MgB2. X-ray diffraction results indicate that C reacted with Mg to form nano-size Mg2C3 and MgB2C2 particles. Nano-particle inclusions and substitution, both observed by transmission electron microscopy, are proposed to be responsible for the enhancement of flux pinning in high fields.

cond-mat.supr-con↗
arXiv2002

Significant enhancement of flux pinning in MgB2 superconductor through nano-Si addition

Polycrystalline MgB2 samples with 10 wt % silicon powder addition were prepared by an in-situ reaction process. Two different Si powders, one with coarse (44 mm) and the other with nano-size (<100 nm) particles were used for making samples. The phases, microstructures, and flux pinning were characterized by XRD, TEM, and magnetic measurements. It was observed that the samples doped with nano-sized Si powder showed a significantly improved field dependence of the critical current over a wide temperature range compared with both undoped samples and samples with coarse Si added. Jc is as high as 3000 A/cm2 in 8 T at 5 K, one order of magnitude higher than for the undoped MgB2. X-ray diffraction results indicated that Si had reacted with Mg to form Mg2Si. Nano-particle inclusions and substitution, both observed by transmission electron microscopy, are proposed to be responsible for the enhancement of flux pinning in high fields. However, the samples made with the coarse Si powders had a poorer pinning than the undoped MgB2.

cond-mat.supr-con↗
arXiv2002

Magnetic shielding in MgB2/Fe superconducting wires

Transport critical current (Ic) was measured for MgB2/Fe round wires, with magnetic field oriented perpendicular to the wire and parallel to it. Measurements were made on a wire with pure MgB2 core and another wire where MgB2 core was doped with nano-size SiC. This doping strongly improved the vortex pinning in MgB2. The field dependence of Ic was strongly improved due to the presence of the iron sheath. At 30K, Ic did not depend on the field for fields between 0.09 and 0.7T. At lower temperatures, Ic increased with the field, after initial decrease, resembling a "peak effect". This effect was extended to higher fields as the temperature went down: at 10K the peak appeared at 3.5T. This improvement was not due to mere magnetic shielding by iron, but more likely to an interaction between the iron sheath and superconductor. Improvement of vortex pinning did not affect the range of fields within which this effect was observed.

cond-mat.supr-con↗
arXiv2002

Enhancement of the critical current density and flux pinning of MgB2 superconductor by nanoparticle SiC doping

Doping of MgB2 by nano-SiC and its potential for improvement of flux pinning was studied for MgB2-x(SiC)x/2 with x = 0, 0.2 and 0.3 and a 10wt% nano-SiC doped MgB2 samples. Co-substitution of B by Si and C counterbalanced the effects of single-element doping, decreasing Tc by only 1.5K, introducing pinning centres effective at high fields and temperatures and enhancing Jc and Hirr significantly. Compared to the non-doped sample, Jc for the 10wt% doped sample increased by a factor of 32 at 5K and 8T, 42 at 20K and 5T, and 14 at 30K and 2T. At 20K, which is considered to be a benchmark operating temperature for MgB2, the best Jc for the doped sample was 2.4x10^5A/cm2 at 2T, which is comparable to Jc of the best Ag/Bi-2223 tapes. At 20K and 4T, Jc was 36,000A/cm2, which was twice as high as for the best MgB2 thin films and an order of magnitude higher than for the best Fe/MgB2 tapes. Because of such high performance, it is anticipated that the future MgB2 conductors will be made using the formula of MgBxSiyCz instead of the pure MgB2.

cond-mat.supr-con↗
arXiv2002

Transport critical current of Solenoidal MgB2/Cu Coils Fabricated Using a Wind-Reaction In-situ Technique

In this letter, we report the results of transport Jc of solenoid coils upto 100 turns fabricated with Cu-sheathed MgB2 wires using a wind-reaction in-situ technique. Despite the low density of single core and some reaction between Mg and Cu-sheath, our results demonstrate the decrease in transport Jc with increasing length of MgB2 wires is insignificant. Solenoid coils with diameter as small as 10 mm can be readily fabricated using a wind-reaction in-situ technique. The Jc of coils is essentially the same as in the form of straight wires. A Jc of 133,000 A/cm2 and 125,000 A/cm2 at 4 K and self field has been achieved for a small coil wound using Cu-sheathed tape and Cu-sheathed wire respectively. These results indicate that the MgB2 wires have a great potential for lage scale applications

cond-mat.supr-con↗
arXiv2002

Improvement of critical current density in the Cu/MgB2 and Ag/MgB2 superconducting wires using the fast formation method

The powder in tube method has been used to fabricate Ag and Cu clad MgB2 wires using an in-situ reaction method. The effects of short time sintering on the critical current densities of Ag and Cu clad MgB2 wires were studied. All the samples were examined using XRD, SEM, and magnetization measurements. For Ag clad wire Jc is improved by more than two times after the short time sintering process. Jc values of 1.2x10^5 A/cm2 in zero field and above 10^4 A/cm2 in 2T at 20 K have been achieved for Ag clad MgB2 wire which is only sintered for 6 minutes at 800oC. However, a remarkable degree of reaction has been found between the superconducting cores and the sheath materials, leading to the formation of Cu2Mg and Ag3Mg for copper and silver clad wires, respectively. The results from Tc, Jc and Hirr convincingly show that the short sintering causes less reaction between the magnesium and the sheath materials and markedly improves the critical current density. Our result shows that Iron is still the best sheath material because of the lack of reaction between Fe and the superconducting MgB2 material.

cond-mat.supr-con↗
arXiv2001

Improvement of critical current in MgB2/Fe wires by a ferromagnetic sheath

Transport critical current (Ic) was measured for Fe-sheathed MgB2 round wires. A critical current density of 5.3 x 10^4 A/cm^2 was obtained at 32K. Strong magnetic shielding by the iron sheath was observed, resulting in a decrease in Ic by only 15% in a field of 0.6T at 32K. In addition to shielding, interaction between the iron sheath and the superconductor resulted in a constant Ic between 0.2 and 0.6T. This was well beyond the maximum field for effective shielding of 0.2T. This effect can be used to substantially improve the field performance of MgB2/Fe wires at fields at least 3 times higher than the range allowed by mere magnetic shielding by the iron sheath. The dependence of Ic on the angle between field and current showed that the transport current does not flow straight across the wire, but meanders between the grains.

cond-mat.supr-con↗
arXiv2001

Very fast formation of superconducting MgB2/Fe wires with high Jc

In this paper we have investigated the effects of sintering time and temperature on the formation and critical current densities of Fe-clad MgB2 wires. MgB2 wires were fabricated using the powder-in-tube process and sintered for different periods of time at predetermined temperatures. All the samples were examined using XRD, SEM and magnetisation measurements. In contrast to the common practice of sintering for several hours, the present results show that there is no need for prolonged heat treatment in the fabrication of Fe-clad MgB2 wires. A total time in the furnace of several minutes is more than enough to form nearly pure MgB2 with high performance characteristics. The results from Tc, Jc and Hirr show convincingly that the samples which were sintered for 3 minutes above 800 oC are as good as those sintered for longer times. In fact, the Jc field performance for the most rapidly sintered sample is slightly better than for all other samples. Jc of 4.5 times 10 ^5 A/cm2 in zero field and above 10 ^5 A/cm2 in 2T at 15 K has been achieved for the best Fe-clad MgB2 wires. As a result of such a short sintering there is no need for using high purity argon protection and it is possible to carry out the heat treatment in a much less protective atmosphere or in air. These findings substantially simplify the fabrication process, making it possible to have a continuous process for fabrication and reducing the costs for large-scale production of MgB2 wires.

cond-mat.supr-con↗
arXiv2001

High transport critical current density above 30 K in pure Fe-clad MgB2 tape

Fe-clad MgB2 long tapes have been fabricated using a powder-in-tube technique. An Mg + 2B mixture was used as the central conductor core and reacted in-situ to form MgB2. The tapes were sintered in pure Ar at 800 ^(o) C for 1 h at ambient pressure. SEM shows a highly dense core with a large grain size of 100 micron. The Fe clad tape shows a sharp transition with transition width of 0.2 K and Tc0 at 37.5 K. We have achieved the highest transport critical current reported so far at 1.6 times 10^(4) A/cm^2 for both 29.5 K in 1 Tesla and 33 K in null field. R-T and critical current were also measured for fields perpendicular and parallel to the tape plane. The iron cladding shielded on the core from the applied external field, with the shielding being less effective for the field in the tape plane. Fe cladding may be advantageous for some applications as it could reduce the effects of both the self-field and external fields.

cond-mat.supr-con↗