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ABSTRACT: The measured transport critical current densities, J<sub>c</sub> , of MgB <sub>2</sub> superconductors fall short of their intrinsic J<sub>c</sub> s on account of the grain boundary blockage, sausaging, and porosity seen in most powder-processed wire samples. Hence, it becomes important to understand and to be able to measure the degree of what can be referred to as “connectivity” in order to be able to assess the highest attainable J<sub>c</sub> in a given class of samples. In this paper connectivity is determined with the aid of normal state resistivity in an extension of the model originally proposed by Rowell. The normal-state resistivity temperature dependence is fitted to a standard Bloch–Grüneisen (B-G) equation in the range 50–300 K. Such an approach leads not only to a connectivity parameter but also to other useful data: the actual intragrain residual resistivity (indirectly related to the upper critical field) and a resistively determined Debye temperature, θ<sub>R</sub> . The latter quantity, coupled to the transition temperature, T<sub>c</sub> , provides a measure (by way of the McMillan formula) of the electron-phonon coupling constant, usually designated λ . The B-G-based connectivity model was applied to our own experimental data on binary and heavily doped MgB <sub>2</sub> samples as well as published resistivity data. To complete the study, low temperature specific heat measurements, performed on binary and doped bulk samples provided calorimetrically determined Debye temperatures, θ<sub>D</sub> , for comparison to the resistively determined values and excellent agreement was found. Calorimetric measurements also probed the homogeneity of the doped samples in terms of the roundness of t-
he electronic specific heat jump near T<sub>c</sub> .
Journal of Applied Physics 06/2009; · 2.17 Impact Factor
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ABSTRACT: The magnetic and transport properties of magnesium diboride films represent performance goals yet to be attained by powder-processed bulk samples and conductors. Such performance limits are still out of the reach of even the best magnesium diboride magnet wire. In discussing the present status and prospects for improving the performance of powder-based wire we focus attention on (1) the intrinsic (intragrain) superconducting properties of magnesium diboride, Hc2 and flux pinning, (2) factors that control the efficiency with which current is transported from grain-to-grain in the conductor, an extrinsic (intergrain) property. With regard to Item-(1), the role of dopants in Hc2 enhancement is discussed and examples presented. On the other hand their roles in increasing Jc, both via Hc2 enhancement as well as direct fluxoid/pining-center interaction, are discussed and a comprehensive survey of Hc2 dopants and flux-pinning additives is presented. Current transport through the powder-processed wire (an extrinsic property) is partially blocked by the inherent granularity of the material itself and the chemical or other properties of the intergrain surfaces. These and other such results indicate that in many cases less than 15% of the conductor's cross sectional area is able to carry transport current. It is pointed out that densification in association with the elimination of grain-boundary blocking phases would yield five-to ten-fold increases in Jc in relevant regimes, enabling the performance of magnesium diboride in selected applications to compete with that of Nb-Sn.
05/2008;
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R. Zeng,
L. Lu,
W. X. Li,
J. L. Wang,
D. Q. Shi,
J. Horvat,
S. X. Dou, M. Bhatia,
M. Sumption,
E. W. Collings,
J. M. Yoo,
M. Tomsic,
M. Rindfleisch
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ABSTRACT: MgB2/Fe wires with 10 at. % excess Mg produced by in situ powder-in-tube processing were compared to normal stoichiometric MgB2/Fe wires prepared by the same method. It was found that the critical current density (Jc) and the irreversibility field (Hirr) were significantly enhanced for MgB2/Fe wires with excess Mg. The transport Jc for 10 at. % Mg excess samples sintered at 800 °C, measured at a field of up to 14 T, increased by a factor of 2 compared to that for the normal MgB2 wires. The best Jc results for the 10 at. % Mg excess sample were obtained by heating the sample at 600 °C for 1 h; the Jc at a field of 8 T and at temperature of 10 K reached 3×104 A/cm2. The detailed analysis of the effect of excess Mg on the microstructures, the Jc, and the Hirr, of MgB2/Fe wires are presented in this paper.
Journal of Applied Physics 04/2008; 103(8):083911-083911-6. · 2.17 Impact Factor
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ABSTRACT: In attempts to increase density and improve connectivity, powder‐in‐tube MgB2 monofilamentary strands were prepared and then subjected to various pressure schedules. The strands were subjected to cold deformation processes such as cold uniaxial pressing and cold rolling, as well as hot uniaxial pressing at various pressures. It was seen that porosity was reduced in the hot pressed samples, although cracking due to the deformation procedure could be seen. Increases in transport properties were noted, with the 5 T, 4.2 K transport Jc of a sample deformed under 458 MPa (HP3) achieving a value of 1.81×105 A/cm2, compared to a value 0.86×105 A/cm2 for the undeformed strand. SiC doped strands were also tested and analyzed. The transport critical current, Jc, values of all the samples were intercompared to analyze the effect of various pressing conditions. Microstructural changes occurring due to different pressure and temperature schedules were also investigated using scanning electron microscopy.
AIP Conference Proceedings. 03/2008; 986(1):375-381.
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ABSTRACT: The fundamental reaction of Mg + B to MgB<sub>2</sub> formation was investigated in order to improve the connectivity of the reacted strands, dopant diffusion, and ultimately the transport properties. Initially, differential scanning calorimetry (DSC), studies were performed to determine the thermodynamics of the solid-state reaction. It was consistently evident from the DSC scans that the formation of the MgB<sub>2</sub> phase was completed below the Mg melting point (~ 655degC). Efforts were made to characterize and understand the differences between the microstructures resulting from the high temperature (above 655degC) and the low temperature (below 655degC) heat-treatments. Transport properties (4.2 K, mostly) of the MgB<sub>2</sub> strands, synthesized by the in-situ reaction between mixed Mg and B powders with 5% or 10% of SiC, both above and below the Mg melting point were measured. The results were correlated with the reaction temperatures. Transport J<sub>c</sub>s of the order of 4.8 times 10<sup>4</sup> A/cm<sup>2</sup> at 8 T and a B<sub>c2</sub> of 22 T (both at 4.2 K) were obtained for the lower temperature HTed samples, these J<sub>c</sub>s are higher than those measured on samples reacted above the Mg melting temperature.
IEEE Transactions on Appiled Superconductivity 07/2007; · 1.04 Impact Factor
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ABSTRACT: Two standard internal-Sn type rod-in-tube (RIT) and two newly developed tube type conductors were investigated. Both types of strand were doped with Ta. A series of heat-treatments with varying temperature and duration time were performed to study their influence on the microstructure and properties. In this work, the fully reacted strands were analysed. Non-Cu J<sub>c</sub> was measured and B<sub>c2</sub> values were estimated via Kramer plot extrapolation. B<sub>c2</sub> values were also measured by resistive transitions at high magnetic fields, and compared with the values from Kramer plot extrapolations. High resolution scanning electron microscopy (SEM) was used to observe the cross-section and the composition across the strands' cross-section was measured by using energy dispersive spectroscopy (EDS). scanning transmission electron microscopy (STEM) was used to measure the composition variation in the individual filaments in the RIT strands. Superconducting properties were correlated to strand composition.
IEEE Transactions on Appiled Superconductivity 07/2007; · 1.04 Impact Factor
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ABSTRACT: Magnesium diboride multifilamentary strands consisting of 7, 19, 37, and 61 filaments were fabricated using a Nb chemical barrier, 101 Cu as an internal stabilizer, and either monel or oxide-dispersion-strengthened Cu (ODS Cu) as an outer sheath. The various strands were given single-step heat treatments of 700degC for 20-40 minutes. Transport critical current density, J<sub>c</sub>, measurements were performed at 4.2 K on 1-meter lengths of strand on ITER barrels in fields of up to 15 T. Higher temperature transport values at 0-12 T were subsequently measured on short samples. Typical J<sub>c</sub> values were at 1times10<sup>5</sup> A cm<sup>-2</sup> at 5 T and 4.2 K and similar values at 2 T and 20 K. The use of ODS Cu as the outer sheath material improved the ductility and stability of the strand without much sacrifice in transport J<sub>c</sub>. Optimization of transport properties through variation of the Mg/B stoichiometry was also studied. Pinning force curves at various temperatures were analysed.
IEEE Transactions on Appiled Superconductivity 07/2007; · 1.04 Impact Factor
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ABSTRACT: The U.S. Dept. of Energy's goal for multifilamentary (MF) strand intended for accelerator applications includes current densities, J<sub>c</sub>, of up to 1200 A/mm<sup>2</sup> in fields of 12-16 T. Although MgB<sub>2</sub> films possess B<sub>c2</sub>s at 0 K as high as 50 T, and J<sub>c</sub>s (self field) as high as 10<sup>7</sup> A/cm<sup>2</sup> have been reported, it may be several years before the above goal is attained in round MF MgB<sub>2</sub> wire. But when its electrical properties become suitable MgB<sub>2</sub>'s critical temperature of 39 K will be an added advantage in the accelerator magnets' high radiation environment. In the meantime, some near-term applications have been identified. Continually improving properties may enable MgB<sub>2</sub> to be used in the windings of: (i) undulator magnet upgrades and replacements, (ii) wiggler magnets, (iii) light source bending magnet replacements, or (iv) some of the lower field solenoids of a proposed muon collider. Active research-and-development programs are yielding MgB<sub>2</sub> strands with some remarkable properties: critical fields B<sub>c2</sub> and B<sub>irr</sub> in the ranges 28-33 T and 24-29 T, respectively, and J<sub>c</sub> (4.2 K)s in the range 1.1-5.5times10<sup>5</sup> A/cm<sup>2</sup>, 5-2 T, respectively. Recent superconducting undulator (SCU) magnets are calling for J<sub>c</sub>, B<sub>W</sub> (winding-field), B<sub>0</sub> (beam-line field) combinations such as 1.1times10<sup>5</sup> A/cm<sup>2</sup>, 3.3 T, 1.0 T. These specifications, in the light of its existing performance levels, indicate MgB's potential suitability for the windings of SCU's. A model MgB<sub>2</sub> coil-set is described
IEEE Transactions on Appiled Superconductivity 07/2006; · 1.04 Impact Factor
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ABSTRACT: The transport critical current density (Jc) was measured at 4.2 K for MgB2 monofilamentary and 7-, 19-, and 37-stack multifilamentary strands. Simple, one-step heat treatments (HT) were used, with temperatures of 675 and 700 °C, and times from 10–40 min. Most measurements were performed on 1 m segments of strands wound onto barrel holders. Transport properties of monofilament, 7-, 19-, and 37-stack strands were compared, and the influence of CuNi and monel outer sheaths was investigated. HT optimization studies were performed on various strands. Transport Jcs of 0.8 mm OD strands reached 2 × 105 A cm−2 at 4 T and 4.2 K (1 µV cm−1), and 1.15 × 106 A cm−2 at 4.2 K and zero field. Smaller 10-filament wires with ODs as small as 0.25 mm (40 µm filaments) exhibited good performance in some cases. The temperature and field dependences of the transport Jc were also measured; a typical example was 2 × 104 A cm−2 at 4 K, 20 T.
Superconductor Science and Technology 01/2006; 19(2):155. · 2.66 Impact Factor
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ABSTRACT: In a study of the influence of Zr B <sub>2</sub> additions on the irreversibility field, μ<sub>0</sub>H<sub> irr </sub> and the upper critical field B<sub>c2</sub> , bulk samples with 7.5 at % Zr B <sub>2</sub> additions were made by a powder milling and compaction technique. These samples were then heated to 700–900 ° C for 0.5 h . Resistive transitions were measured at 4.2 K and μ<sub>0</sub>H<sub> irr </sub> and B<sub>c2</sub> values were determined. An increase in B<sub>c2</sub> from 20.5 T to 28.6 T and enhancement of μ<sub>0</sub>H<sub> irr </sub> from 16 T to 24 T were observed in the Zr B <sub>2</sub> doped sample as compared to the binary sample at 4.2 K . Critical field increases similar to those found with SiC doping were seen at 4.2 K . At higher temperatures, increases in μ<sub>0</sub>H<sub> irr </sub> were also determined by M-H loop extrapolation and closure. Values of μ<sub>0</sub>H<sub> irr </sub> which were enhanced with Zr B <sub>2</sub> doping (as compared to the binary) were seen at temperatures up to 34 K </form-
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ula>, with μ<sub>0</sub>H<sub> irr </sub> values larger than those for SiC doped samples at higher temperatures. The transition temperature, T<sub>c</sub> , was then measured using dc susceptibility and a 2.5 K drop of the midpoint of T<sub>c</sub> was observed. The critical current density was determined using magnetic measurements and was found to increase at all temperatures between 4.2 K and 35 K with Zr B <sub>2</sub> doping.
Applied Physics Letters 08/2005; · 3.84 Impact Factor
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ABSTRACT: A racetrack coil using MgB<sub>2</sub>/Cu strand has been fabricated and tested for transport current density at 4.2 K in self field. The monofilamentary strand was 1.0 mm OD and insulated with S-glass braid. Eighty turns of strand (42 m) were wound onto a stainless steel former with outer dimensions 25 cm×10 cm×0.86 cm. The resulting racetrack coil was heat treated at 675°C for 30 minutes in flowing Ar. The strand, with a superconducting fraction of 26%, occupied 49% of the total coil pack cross sectional area. The coil I<sub>c</sub> at 4.2 K and self field was 120 A (using a 1 μV/cm criterion). This led to a J<sub>c,sc</sub> (across the whole coil) of 6.12×10<sup>4</sup> A/cm<sup>2</sup>, a J<sub>e</sub> in the wire of 1.59×10<sup>4</sup> A/cm<sup>2</sup>, and an overall winding J<sub>e</sub> of 7.9×10<sup>3</sup> A/cm<sup>2</sup> at 4.2 K in self field. The n-values ranged from 17 to 56.
IEEE Transactions on Appiled Superconductivity 07/2005; · 1.04 Impact Factor
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ABSTRACT: The effect of silicon carbide, carbon and metal-diboride (NbB<sub>2</sub>, ZrB<sub>2</sub>, and TiB<sub>2</sub>) additions on the irreversibility field, H<sub>irr</sub> and the upper critical field H<sub>c2</sub> of bulk superconducting MgB<sub>2</sub> have been studied. Samples with 10 mole % of SiC and C and 7.5% of above named metal diboride additions were made separately by a powder milling and compaction technique along with the control sample. These samples were heat-treated at various schedules and H<sub>c2</sub> and H<sub>irr</sub> values were measured. An increase in μ<sub>o</sub>H<sub>c2</sub> from 20.5 T for pure sample to more than 33 T and enhancement of μ<sub>o</sub>H<sub>irr</sub> from 16 T to a maximum of 28 T for SiC doped sample was observed at 4.2 K. For a ZrB<sub>2</sub> doped sample 24 T and 28.6 T of μ<sub>o</sub>H<sub>irr</sub> and μ<sub>o</sub>H<sub>c2</sub> respectively we obtained with only 2 K drop in the T<sub>c</sub>.
IEEE Transactions on Appiled Superconductivity 07/2005; · 1.04 Impact Factor
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X. L. Wang,
A. H. Li,
S. Yu,
S. Ooi,
K. Hirata,
C. T. Lin,
E. W. Collings,
M. D. Sumption, M. Bhatia,
S. Y. Ding,
S. X. Dou
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ABSTRACT: Magnetoresisitivity and critical current density J<sub> c </sub> as a function of temperature and field are studied for Bi <sub>2</sub> Sr <sub>2</sub> Ca <sub>2</sub> Cu <sub>3</sub> O <sub>10</sub> single crystals grown using the traveling solvent floating zone technique. Below a characteristic field B<sup>*</sup> , J<sub> c </sub> as a function of field exhibits a field-independent plateau associated with thermally activated pinning of individual vortices. Analysis of resistive transition broadening revealed that thermally activated flux flow is found to be responsible for the resistivity contribution in the vicinity of T<sub> c </sub> . The activation energy U<sub>0</sub> is 800 K in low field, scales as B<sup>-1/6</sup> for B≪2 T and drops to 200 K with B<sup>-1/2</sup> for B≫2 T .
Journal of Applied Physics 06/2005; · 2.17 Impact Factor
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ABSTRACT: We report a systematic study on the effect of sintering temperature on the phase formation, critical current density, upper critical field and irreversibility field of nanoscale SiC doped MgB2. Bulk and Fe sheathed wires doped with different nano-SiC particle sizes have been made and heat treated at temperatures ranging from 580 to 1000 °C. A systematic correlation between the sintering temperature, normal state resistivity, RRR, Jc, Hc2, and Hirr has been found in all samples of each batch. Samples sintered at a lower temperature have a very fine and well consolidated grain structure while samples sintered at a high temperature contain large grains with easily distinguishable grain boundaries. Low temperature sintering resulted in a higher concentration of impurity precipitates, larger resistivity, higher Jc up to 15 T and lower Tc values. These samples show higher Hc2 and Hirr at T near Tc but lower Hc2 near T = 0 than samples sintered at high temperature. It is proposed that huge local strains produced by nano-precipitates and grain boundary structure are the dominant mechanism responsible for higher Hc2 at T near Tc. However, higher impurity scattering due to C substitution is responsible for higher Hc2 in the low temperature regime for samples sintered at a higher temperature. In addition to high Hc2, it is also proposed that the large number of nano-impurities serve as pinning centres and improve the flux pinning, resulting in higher Jc values at high magnetic fields up to 15 T.
Superconductor Science and Technology 04/2005; 18(5):658. · 2.66 Impact Factor
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ABSTRACT: Three solenoids have been wound and with MgB2 strand and tested for transport properties. One of the coils was wound with Cu-sheathed monofilamentary strand and the other two with a seven filament strand with Nb-reaction barriers, Cu stabilization, and an outer monel sheath. The wires were first S-glass insulated, then wound onto an OFHC Cu former. The coils were then heat treated at 675C/30 min (monofilamentary strand) and 700C/20 min (multifilamentary strand). Smaller (1 m) segments of representative strand were also wound into barrel-form samples and HT along with the coils. After HT the coils were epoxy impregnated. Transport Jc measurements were performed at various taps along the coil lengths. Measurements were made initially in liquid helium, and then as a function of temperature up to 30 K. Homogeneity of response along the coils was investigated and a comparison to the short sample results was made. Each coil contained more than 100 m of 0.84-1.01 mm OD strand. One of the 7 strand coils reached 222 A at 4.2 K, self field, with a Jc of 300 kA/cm2 in the SC and a winding pack Je of 23 kA/cm2. At 20 K these values were 175 kA/cm2 and 13.4 kA/cm2. Magnet bore fields of 1.5 T and 0.87 T were achieved at 4.2 K and 20 K, respectively. The other multifilamentary coil gave similar results. Comment: 22 pages, 8 figures, 2 tables
03/2005;
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ABSTRACT: Resistive transition measurements are reported for MgB <sub>2</sub> strands with SiC dopants. The starting Mg powders were 325 mesh 99.9% pure, and the B powders were amorphous, 99.9% pure, and at a typical size of 1–2 μm. The SiC was added as 10 mol % of SiC to 90 mol % of binary MgB <sub>2</sub> [(MgB2)0.9(SiC)0.1]. 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 min at temperatures from 675 °C to 900 °C. Strands with 200 nm size SiC additions had μ<sub> 0 </sub>H<sub> irr </sub> and B<sub>c 2 </sub> which maximized at 25.4 T and 29.7 T after heating at 800 °C for 30 min. The highest values were seen for a strand with 15 nm SiC heated at 725 °C for 30 min which had a μ<sub> 0 </sub>H<sub> irr </sub> of 29 T and a B<sub>c 2 </sub> higher than 33 T.
Applied Physics Letters 03/2005; · 3.84 Impact Factor
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ABSTRACT: A racetrack coil using MgB2/Cu strand has been fabricated and tested for transport current density at 4.2 K in self field. The monofilamentary strand was 1.0 mm OD and insulated with S-glass braid. Eighty turns of strand (42 m) were wound onto a stainless steel former with outer dimensions 25 cm x 10 cm x 0.86 cm. The resulting racetrack coil was heat treated at 675C for 30 minutes in flowing Ar. The strand, with a superconducting fraction of 26%, occupied 49% of the total coil pack cross sectional area. The coil Ic at 4.2 K and self field was 120 A (using a 1 microvolt/cm criterion). This led to a Jc,sc (across the whole coil) of 6.12 x 10^4 A/cm2, a Je in the wire of 1.59 x 10^4 A/cm2, and an overall winding Je of 7.9 x 10^3 A/cm2 at 4.2 K in self field. The n-values ranged from 17 to 56.
11/2004;
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ABSTRACT: Critical current density was measured at 4.2 K for MgB2 strands with and without SiC additions. In some cases measurements were performed on longer (1 m) samples wound on barrels, and these were compared to magnetic measurements. Most measurements were performed on short samples at higher fields (up to 18 T). It was found that in-situ processed strands with 10% SiC additions HT at 700-800C show improved Hr and Fp values as compared to control samples, with Hr increasing by 1.5 T. At 900C even larger improvements are seen, with Hr reaching 18 T and Fp values maximizing at 20 GN/m3. Comment: 15 pages, 1 table, six figs
10/2004;
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ABSTRACT: The critical current densities of MgB2 monofilamentary strands with and without SiC additions were measured at 4.2 K. Additionally, magnetic Jc at B = 1 T was measured from 4.2 K to 40 K. Various heat treatment times and temperatures were investigated for both short samples and small helical coils. SiC additions were seen to improve high field transport Jc at 4.2 K, but improvements were not evident at 1 T at any temperature. Transport results were relatively insensitive to heat treatment times and temperatures for both short samples and coils in the 700C to 900C range. Comment: 8 text pages, 1 table, 4 figs
09/2004;
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ABSTRACT: Transport critical current densities and n-values were measured at 4.2 K in fields up to 15 T on 7, 19, and 37-stack multifilamentary MgB2 strands made using an in-situ route. Some strands included SiC additions (particle size 30 nm), while in others Mg-rich compositions were used. Two basic multifilamentary variants were measured, the first had Nb filamentary barriers, the second had Fe filamentary barriers. All samples incorporated stabilizer in the form of Cu 101. Simple, one-step heat treatments were used, with temperatures ranging from 700-800C, and times from 10-30 minutes. Transport critical current densities of 1.75 x 105 A/cm2 were seen at 4.2 K and 5 T in 37 stack strands. Comment: 10 pages, 3 figs, 2 tables
09/2004;
