Increases in the irreversibility field and the upper critical field of bulk MgB2 by ZrB2 addition

Abstract

In a study of the influence of ZrB 2 additions on the irreversibility field, µ ο H irr and the upper critical field B c2 , bulk samples with 7.5 at. % ZrB 2 additions were made by a powder milling and compaction technique. These samples were then heated to 700-900 o C for 0.5 hours. Resistive transitions were measured at 4.2 K and µ ο H irr and B c2 values were determined. An increase in B c2 from 20.5 T to 28.6 T and enhancement of µ ο H irr from 16 T to 24 T were observed in the ZrB 2 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 ο H irr were also determined by M-H loop extrapolation and closure. Values of µ ο H irr which were enhanced with ZrB 2 doping (as compared to the binary) were seen at temperatures up to 34 K, with µ ο H irr values larger than those for SiC doped samples at higher temperatures. The transition temperature, T c , was then measured using DC susceptibility and a 2.5 K drop of the midpoint of T c 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 ZrB 2 doping. by an in-situ reaction of a stoichiometric mixture of 99.9 % pure Mg and amorphous B powders with a typical size of 1-2 µm. Powders were mixed in SPEX mill for 48 mins. KeywordsDoping was achieved by adding 7.5 mol% of ZrB 2 (from Alfa Aesar) prior to the mixing.Powders with SiC were doping were also fabricated for comparison. The milled powder was then compacted in the form of a cylindrical pellet in a steel die. These pellets were heat-treated in a steel holder, encapsulated in a quartz tube under 200 torr of Ar. Details of the sample composition, and heat-treatment are given in Table I. After sintering at temperatures of from 700-900 deg C for 0.5 hours, the cylindrical pellets were reshaped into cuboids for property measurements.Magnetization measurements were performed from 4.2 K to 40 K on these samples using a vibrating sample magnetometer (VSM) with field sweep amplitude of 1.7 T and a sweep rate of 0.07 T/s. Susceptibility, χ dc, vs. temperature, T measurements were performed using a 50 mT field sweep amplitude after initial zero field cooling.Magnetically determined critical current density, J c, at various temperatures was extracted from the magnetization loop using the Bean model 15 .At 4.2 K, µ o H irr , and B c2 were determined by resistive transitions with applied field, with the measurements being performed at the National High Magnetic Field Laboratory (NHMFL) Tallahassee. A four point resistance measurement was performed, using silver paste to connect the leads, and the distance between the voltage taps was 5mm. The applied current was 10 mA, and current reversal was used. All measurements were made at 4.2 K in applied fields ranging from 0 to 33 T. The samples were placed perpendicular to the applied field, values of µ 0 H irr and B c2 being obtained taking the 10%and 90%...

Improved critical current densities in MgB2 tapes with ZrB2 doping

“…Contrast to Bhatia's experiments [10], a peak shift is not identified clearly in the XRD pattern for our tapes.…”

Prospects for improving the intrinsic and extrinsic properties of magnesium diboride superconducting strands

“…Thus, we might expect the dopant to be effective primarily at lower temperatures (assuming π band scattering). It is interesting to note that the increase for ZrB 2 [25] is not very different from that contributed by SiC doping [22]. Additionally, even with substitutional doping the possibility of unreacted residues cannot be ignored.…”

“…Additionally, even with substitutional doping the possibility of unreacted residues cannot be ignored. Although they were not present as a result of ZrB 2 doping, they were observed with NbB 2 doping [25] and are known to be present after SiC doping [22].Of course, the unreacted residue is not necessarily inert, and may contribute flux pinning centers, or indirectly increase either π and σ scattering via lattice distortion and in that way increase H irr . However, although numerous dopants (including metallic elements, borides, carbides, oxides, and organic compounds) have been added to MgB 2 little direct attention has been paid to the possible existences of such dual doping mechanisms.…”

Drawing induced texture and the evolution of superconductive properties with heat treatment time in powder-in-tubein situprocessed MgB2strands

“…These can be accomplished through: (i) site-substitution into the Mg sublattice by Zr [21,22], Na [23], or similar-sized cations [24][25][26][27], (ii) site-substitution into the B sublattice by C [28][29][30][31][32] (with regard to which we note that it has been difficult to distinguish t-he influence of substitution from effect of possible C-induced lattice strain [33][34]), and (iii) increasing the connectivity between MgB 2 grains (or grain clusters) to increase the efficiency of supercurrent flow [35][36][37][38].…”