Systematic effects of carbon doping on the superconducting properties of Mg(B1−x_{1-x}Cx_x)2_2

The upper critical field, $H_{c2}$, of Mg(B$_{1-x}$C$_x$)$_2$ has been measured in order to probe the maximum magnetic field range for superconductivity that can be attained by C doping. Carbon doped boron filaments are prepared by CVD techniques, and then these fibers are then exposed to Mg vapor to form the superconducting compound. The transition temperatures are depressed about $1 K/$ C and $H_{c2}(T=0)$ rises at about $5 T/$ C. This means that 3.5% C will depress $T_c$ from $39.2 K$ to $36.2 K$ and raise $H_{c2}(T=0)$ from $16.0 T$ to $32.5 T$. Higher fields are probably attainable in the region of 5% C to 7% C. These rises in $H_{c2}$ are accompanied by a rise in resistivity at $40 K$ from about $0.5 \mu \Omega cm$ to about $10 \mu \Omega cm$. Given that the samples are polycrystalline wire segments, the experimentally determined $H_{c2}(T)$ curves represent the upper $H_{c2}(T)$ manifold associated with $H\perp c$

Effects of Neutron Irradiation on Carbon Doped MgB2 Wire Segments

We have studied the evolution of superconducting and normal state properties of neutron irradiated Mg(B$_{.962}$C$_{.038}$)$_2$ wire segments as a function of post exposure annealing time and temperature. The initial fluence fully suppressed superconductivity and resulted in an anisotropic expansion of the unit cell. Superconductivity was restored by post-exposure annealing. The upper critical field, H$_{c2}$(T=0), approximately scales with T$_c$ starting with an undamaged T$_c$ near 37 K and H$_{c2}$(T=0) near 32 T. Up to an annealing temperature of 400 $^ o$C the recovery of T$_c$ tends to coincide with a decrease in the normal state resistivity and a systematic recovery of the lattice parameters. Above 400 $^ o$C a decrease in order along the c- direction coincides with an increase in resistivity, but no apparent change in the evolution of T$_c$ and H$_{c2}$. To first order, it appears that carbon doping and neutron damaging effect the superconducting properties of MgB$_2$ independently

A study of the deposition of carbide coatings on graphite fibers

The chemical vapor deposition of boron carbide and silicon carbide on graphite fibers to increase their electrical resistance was studied. Silicon carbide coatings were applied without degradation of the mechanical properties of the filaments. These coatings typically added 1000 ohms to the resistance of a filament as measured between two mercury pools. When SiC-coated filaments were oxidized by refluxing in boiling phosphoric acid, average resistance increased by an additional 1000 ohms; in addition resistance increases as high as 150 K ohms and breakdown voltages as high as 17 volts were noted. Data on boron carbide coatings indicated that such coatings would not be effective in increasing resistance, and would degrade the mechanical properties

Systematic effects of carbon doping on the superconducting properties of Mg(B1-xCx)(2)

The upper critical field, H-c2, of Mg(B1-xCx)(2) has been measured in order to probe the maximum magnetic field range for superconductivity that can be attained by C doping. Carbon doped MgB2 filaments were prepared, and for carbon levels below 4% the transition temperatures are depressed by about 1 K/% C and H-c2(T=0) rises by about 5 T/% C. This means that 3.8% C substitution will depress T-c from 39.2 to 36.2 K and raise H-c2(T=0) from 16.0 to 32.5 T. These rises in H-c2 are accompanied by a rise in resistivity at 40 K from about 0.5 to about 10 muOmega cm.This article is published as Wilke, R. H. T., S. L. Bud’ko, P. C. Canfield, D. K. Finnemore, Raymond J. Suplinskas, and S. T. Hannahs. "Systematic effects of carbon doping on the superconducting properties of Mg (B 1− x C x) 2." Physical Review Letters 92, no. 21 (2004): 217003. DOI: 10.1103/PhysRevLett.92.217003. Copyright 2004 American Physical Society. Posted with permission

Effects of neutron irradiation on carbon doped MgB2 wire segments

We have studied the evolution of superconducting and normal state properties of neutron irradiated Mg(B0.962C0.038)2 wire segments as a function of post-exposure annealing time and temperature. The initial fluence fully suppressed superconductivity and resulted in an anisotropic expansion of the unit cell. Superconductivity was restored by post-exposure annealing. The upper critical field, Hc2(T = 0), approximately scales with Tc, starting with an undamaged Tc near 37 K and Hc2(T = 0) near 32 T. Up to an annealing temperature of 400 °C the recovery of Tc tends to coincide with a decrease in the normal state resistivity and a systematic recovery of the lattice parameters. Above 400 °C a decrease in ordering along the c-direction coincides with an increase in resistivity, but no apparent change in the evolution of Tc and Hc2. To a first order approximation, it appears that carbon doping and neutron damage affect the superconducting properties of MgB2 independently.This is the version of the article before peer review or editing, as submitted by an author to Superconductor Science and Technology. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at DOI: 10.1088/0953-2048/19/6/024. Copyright 2006 IOP Publishing Ltd. Posted with permission

Titanium additions to MgB2 conductors

A series of doping experiments are reported for MgB2 conductors that have been synthesized using doped boron fibers prepared by chemical vapor deposition (CVD) methods. Undoped MgB2 samples prepared from CVD prepared fibers consistently give critical current densities, Jc, in the range of 500,000 A/cm2 in low field at 5 K. These values fall by a factor of about 100 as the magnetic field increases to 3 T. For heavily Ti-doped boron fibers where the B/Ti ratio is comparable to 1, there is a substantial suppression of both Tc, superconducting volume fraction, and Jc values. If, however, a sample with a few percent Ti in B is deposited on a carbon substrate and reacted at 1100 °C, then Tc is suppressed only a couple of degrees and critical current densities are found to be Jc ∼ 2–5 × 106 A/cm2 for superconducting layers ranging from 4 to 10 μm thick. These materials show Jc values over 10,000 A/cm2 at 25 K and 1.3 T.This is a manuscript of an article published as Anderson Jr, N. E., W. E. Straszheim, S. L. Bud’ko, P. C. Canfield, D. K. Finnemore, and Raymond J. Suplinskas. "Titanium additions to MgB2 conductors." Physica C: Superconductivity 390, no. 1 (2003): 11-15. DOI: 10.1016/S0921-4534(03)00863-3 Copyright 2003 Elsevier Science B.V. Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0). Posted with permission

Superconductivity in MgB2 doped with Ti and C

Measurements of the superconducting upper critical field, Hc2, and critical current density, Jc, have been carried out for MgB2 doped with Ti and/or C in order to explore the problems encountered if these dopants are used to enhance the superconducting performance. Carbon replaces boron on the MgB2 lattice and apparently shortens the electronic mean free path of MgB2 and raising Hc2. Titanium forms precipitates of either TiB or TiB2 that enhance the flux pinning and raise Jc. Most of these precipitates are intra-granular in the MgB2 phase. For samples containing both C and Ti doping, the C appears to still replace B in the MgB2 lattice and the Ti precipitates out as a boride. If approximately 0.5%Ti and approximately 2%C are co-deposited with B to form doped boron fibers and these fibers are in turn reacted in Mg vapor to form doped MgB2, the resulting superconductor has μ0Hc2(T = 0) ∼ 25 T and Jc ∼ 10,000 A/cm2 at 5 K and 2.2 T.This is a manuscript of an article published as Wilke, R. H. T., S. L. Bud’ko, P. C. Canfield, M. J. Kramer, Y. Q. Wu, D. K. Finnemore, R. J. Suplinskas, J. V. Marzik, and S. T. Hannahs. "Superconductivity in MgB2 doped with Ti and C." Physica C: Superconductivity 418, no. 3-4 (2005): 160-167. DOI: 10.1016/j.physc.2004.11.022. Copyright 2004 Elsevier B.V. Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0). Posted with permission