18 research outputs found
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/cm^2 in low field at 5K. These values fall by a factor of about 100 as the
magnetic field increases to 3T. 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 coated SiC substrate and reacted
at 1100 degrees C for 15 min, then Tc is suppressed only a couple of degrees
Kelvin and critical current densities are found to be approximately 2-5 x 10^6
A/cm^2 for superconducting layers ranging from 4-10 micrometers thick. These
materials show Jc values over 10,000 A/cm^2 at 25K and 1.3 T.Comment: 10 pages, 6 figure
The Effect of Excess Carbon on the Crystallographic, Microstructural, and Mechanical Properties of CVD Silicon Carbide Fibers
Superconductivity in MgB_2 doped with Ti and C
Measurements of the superconducting upper critical field, H_{c2}, and
critical current density, J_c, have been carried out for MgB_2 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 in the MgB_2
lattice and apparently shortens the electronic mean free path thereby raising
H_c2. Titanium forms precipitates of either TiB or TiB_2 that enhance the flux
pinning and raise J_c. Most of these precipitates are intra-granular in the
MgB_2 phase. 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 MgB_2, the resulting superconductor has H_{c2}(T=0) ~ 25 T and
J_c ~ 10,000 A/cm**2 at 5 K and 2.2 T.Comment: 11 pages, 10 figure
Systematic effects of carbon doping on the superconducting properties of Mg(BC)
The upper critical field, , of Mg(BC) 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 C and rises at about C. This means that 3.5% C will depress from to and
raise from to . Higher fields are probably
attainable in the region of 5% C to 7% C. These rises in are
accompanied by a rise in resistivity at from about
to about . Given that the samples are polycrystalline wire
segments, the experimentally determined curves represent the upper
manifold associated with
Effect of pressure on the superconducting transition temperature of doped and neutron-damaged MgB2
Measurements of the superconducting transition temperatures for Al-doped,
C-doped and neutron-damaged-annealed MgB2 samples under pressure up to ~8 kbar
are presented. The dT_c/dP values change systematically with the decrease of
the ambient pressure T_c in a regular fashion. The evolution of the pressure
derivatives can be understood assuming that the change in phonon spectrum is a
dominant contribution to dT_c/dP
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(BC) 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(T=0), approximately scales with T starting with an
undamaged T near 37 K and H(T=0) near 32 T. Up to an annealing
temperature of 400 C the recovery of T 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 order along the c- direction
coincides with an increase in resistivity, but no apparent change in the
evolution of T and H. To first order, it appears that carbon doping
and neutron damaging effect the superconducting properties of MgB
independently
Formation of MgB2 at low temperatures by reaction of Mg with B6Si
Formation of MgB2 by reactions of Mg with B6Si and Mg with B were compared,
the former also producing Mg2Si as a major product. Compared to the binary
system, the ternary reactions for identical time and temperature were more
complete at 750 C and below, as indicated by higher diamagnetic shielding and
larger x-ray diffraction peak intensities relative to those of Mg. MgB2 could
be produced at temperatures as low as 450 C by the ternary reaction. Analyses
by electron microscopy, x-ray diffraction, and of the upper critical field show
that Si does not enter the MgB2 phase.Comment: Submitted to Supercond. Sci. Techno
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Continuous Fiber Ceramic Composite (CFCC) Program: Gaseous Nitridation
Textron has developed a mature process for the fabrication of continuous fiber ceramic composite (CFCC) tubes for application in the aluminum processing and casting industry. The major milestones in this project are System Composition; Matrix Formulation; Preform Fabrication; Nitridation; Material Characterization; Component Evaluatio
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