77 research outputs found
Effects of stoichiometry, purity, etching and distilling on resistance of MgB2 pellets and wire segments
We present a study of the effects of non-stoichiometry, boron purity, wire
diameter and post-synthesis treatment (etching and Mg distilling) on the
temperature dependent resistance and resistivity of sintered MgB2 pellets and
wire segments. Whereas the residual resistivity ratio (RRR) varies between RRR
\~ 4 to RRR > 20 for different boron purity, it is only moderately affected by
non-stoichiometry (from 20% Mg deficiency to 20% Mg excess) and is apparently
independent of wire diameter and presence of Mg metal traces on the wire
surface. The obtained set of data indicates that RRR values in excess of 20 and
residual resistivities as low as rho{0} ~ 0.4 mu Ohm cm are intrinsic material
properties of high purity MgB2
Carbon doping of superconducting magnesium diboride
We present details of synthesis optimization and physical properties of
nearly single phase carbon doped MgB2 with a nominal stoichiometry of
Mg(B{0.8}C{0.2})2 synthesized from magnesium and boron carbide (B4C) as
starting materials. The superconducting transition temperature is ~ 22 K (~ 17
K lower than in pure MgB2). The temperature dependence of the upper critical
field is steeper than in pure MgB2 with Hc2(10K) ~ 9 T. Temperature dependent
specific heat data taken in different applied magnetic fields suggest that the
two-gap nature of superconductivity is still preserved for carbon doped MgB2
even with such a heavily suppressed transition temperature. In addition, the
anisotropy ratio of the upper critical field for T/Tc ~ 2/3 is gamma ~ 2. This
value is distinct from 1 (isotropic) and also distinct from 6 (the value found
for pure MgB2).Comment: 11 pages, 13 figures, submitted to Physica
Effects of Boron Purity, Mg Stoichiometry and Carbon Substitution on Properties of Polycrystalline MgB
By synthesizing MgB using boron of different nominal purity we found
values of the residual resistivity ratio () from 4 to
20, which covers almost all values found in literature. To obtain high values
of , high purity reagents are necessary. With the isotopically pure boron
we obtained the highest 20 for the stoichiometric compound. We also
investigated MgB samples with 0.8 1.2. For the range
MgB up to MgB we found average values
of between 14 and 24. For smaller variations in stoichiometry () . All of our data point to the conclusion that high
() and low () are intrinsic
material properties associated with high purity MgB. In addition we have
performed initial work on optimizing the formation of carbon doped MgB
via the use of BC. Nearly single phase material can be formed by reaction
of nominal Mg(BC) for 24 hours at . The
for this composition is between and (depending on
criterion).Comment: accepted to Physica C, special MgB2 issu
An Overview of the Basic Physical Properties of MgB
The basic physical properties of MgB have been well established over the
past two years of intensive research. At this point there is a general
consensus about the values for the isotope shift, critical fields, most of the
salient length scales, and general anisotropies. In this paper we will review
the determination of these parameters and set the stage for further, more
detailed discussions of specific aspects of the physics of MgB.Comment: accepted to Physica C, special MgB2 iss
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
Anisotropy parameters of superconducting MgB
Data on macroscopic superconducting anisotropy of MgB are reviewed. The
data are described within a weak coupling two-gaps anisotropic s-wave model of
superconductivity. The calculated ratio of the upper critical fields
increases with decreasing temperature in
agreement with available data, whereas the calculated ratio of London
penetration depths decreases to reach
at T=0. Possible macroscopic consequences of
are discussed.Comment: accepted to Physica C, special MgB2 issu
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
Macroscopic anisotropy in superconductors with anisotropic gaps
It is shown within the weak-coupling model that the macroscopic
superconducting anisotropy for materials with the gap varying on the Fermi
surface cannot be characterized by a single number, unlike the case of clean
materials with isotropic gaps. For clean uniaxial materials, the anisotropy
parameter defined as the ratio of London penetration depths,
, is evaluated for all 's. Within the two-gap model
of MgB, is an increasing function of .Comment: 4 pages, 2 figure
Effects of C, Cu and Be substitutions in superconducting MgB2
Density functional calculations are used to investigate the effects of
partial substitutional alloying of the B site in MgB2 with C and Be alone and
combined with alloying of the Mg site with Cu. The effect of such substitutions
on the electronic structure, electron phonon coupling and superconductivity are
discussed. We find that Be substitution for B is unfavorable for
superconductivity as it leads to a softer lattice and weaker electron-phonon
couplings. Replacement of Mg by Cu leads to an increase in the stiffness and
doping level at the same time, while the carrier concentration can be
controlled by partial replacement of B by C. We estimate that with full
replacement of Mg by Cu and fractional substitution of B by C, Tc values of 50K
may be attainable.Comment: 5 pages, 4 figure
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
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