We have examined the upper critical field of a large and representative set of present multifilamentary Nb 3 Sn wires and one bulk sample over a temperature range from 1.4 K up to the zero-field critical temperature. Since all present wires use a solid-state diffusion reaction to form the A15 layers, inhomogeneities with respect to Sn content are inevitable, in contrast to some previously studied homogeneous samples. Our study emphasizes the effects that these inevitable inhomogeneities have on the field-temperature phase boundary. The property inhomogeneities are extracted from field-dependent resistive transitions which we find broaden with increasing inhomogeneity. The upper 90%-99% of the transitions clearly separates alloyed and binary wires but a pure, Cu-free binary bulk sample also exhibits a zero-temperature critical field that is comparable to the ternary wires. The highest 0 H c2 detected in the ternary wires are remarkably constant: The highest zero-temperature upper critical fields and zero-field critical temperatures fall within 29.5± 0.3 and 17.8± 0.3 K, respectively, independent of the wire layout. The complete field-temperature phase boundary can be described very well with the relatively simple Maki-DeGennes model using a two-parameter fit, independent of composition, strain state, sample layout, or applied critical state criterion.

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Ќ transition is more appropriate for untextured samples than the bottom or 10% point on the small-current-density, resistive H c2 transition which corresponds to H c2 ʈ . However, the resistive H c2 transition is still useful for measuring the breadth of the parallel H c2 transition ⌬H, which may be indicative of inhomogeneity in composition in the sample. Hopes for expanding the useful range of MgB 2 are encouraged by earlier work that has shown that H c2 ʈ ͑0͒ can exceed 70 T in C-doped MgB 2 thin films, 2 but so far the highest H c2 ͑0͒ of C-or SiC-doped wires or bulks is ϳ35 T, 3,4,9,10 only half this value. Since H c2 and H irr enhancement is crucial for magnet applications, we have here systematically studied the H c2 transition and J c ͑H , T͒ behavior of pure and SiC-doped bulks. Irrespective of this high-field perspective on MgB 2 , we should also point out that J c ͑H͒ falls off only slowly in the 10-30 K range, making MgB 2 useful for lower field applications without liquid He.Our previous reports 6,7 showed that higher J c values were obtained in tapes using MgH 2 rather than Mg powder. Nano-SiC addition improved the high-field J c at low temperatures and produced a measured H c2 value of 23 T at 4.2 K. Here we present a more detailed study of MgB 2 samples cut from this same tape measured without any extraneous sheath material.MgB 2 bulk samples were prepared by conventional in situ powder-in-tube method with commercial MgH 2 and amorphous B powders which were mixed and packed into a pure Fe tube in air.7 5 or 10 mol % of ϳ30 nm SiC powder 5 was added for the doped samples. The filled tubes were groove rolled into 2 mm square rods and then flat rolled into 0.5 mm thick by 4 mm wide tapes. 50 mm long samples were heat treated at 600, 700, 800, and 900°C for 1 h under Ar atmosphere making the 12-sample set. 7 After peeling away the Fe sheath, resistivity curves were measured with 5 mA transport currents in a 9 T Quantum Design physical properties measurement system, the 33 T Bitter magnet at the National High Magnetic Field Laboratory ͑NHMFL͒ in Tallahassee, and the 60 T short pulse magnet at the NHMFL in Los Alamos National Laboratory. The 10% and 90% points on the resistive transition curves were used to define a transition breadth ⌬H and H c2 ʈ . Magnetization properties were measured in an Oxford Instruments vibrating sample magnetometer, from which the critical current density J c ͑H , T͒ was calculated assuming fully connected samples using the expression J c ͑H , T͒ = 0.5⌬M12b / ͑3bd − d 2 ͒, where b and d are the width and thickness of the rectangular section bar. Extrapolation of J c ͑H͒ to zero allowed extraction of H irr . However, following Rowell, 11 we believe that the connected cross section 1 / F of our samples is much less than unity, based on calculations of 1 / F using the relation ͑T͒ = F͓⌬ sc ͑T͒ + ͑0͔͒, where n is the measured normal state resistivity and ⌬ sc ͑300-50 K͒ = 7.3 ⍀ cm ͑Table I͒. Table I provides an overview of the properties of the four samples. SiC additions depres...

Presented herein are the preparation and crystallographic/microanalytical/magnetic/spectroscopic characterization of the Pt-centered four-shell 165-atom Pd-Pt cluster, (mu(12)-Pt)Pd(164-x)Pt(x)(CO)(72)(PPh(3))(20) (x approximately 7), 1, that replaces the geometrically related capped three-shell icosahedral Pd(145) cluster, Pd(145)(CO)(x)(PEt(3))(30) (x approximately 60), 2, as the largest crystallographically determined discrete transition metal cluster with direct metal-metal bonding. A detailed comparison of their shell-growth patterns gives rise to important stereochemical implications concerning completely unexpected structural dissimilarities as well as similarities and provides new insight concerning possible synthetic approaches for generation of multi-shell metal clusters. 1 was reproducibly prepared in small yields (<10%) from the reaction of Pd(10)(CO)(12)(PPh(3))(6) with Pt(CO)(2)(PPh(3))(2). Its 165-atom metal-core geometry and 20 PPh(3) and 72 CO ligands were established from a low-temperature (100 K) CCD X-ray diffraction study. The well-determined crystal structure is attributed largely to 1 possessing cubic T(h) (2/m3) site symmetry, which is the highest crystallographic subgroup of the noncrystallographic pseudo-icosahedral I(h) (2/m35) symmetry. The "full" four-shell Pd-Pt anatomy of 1 consists of: (a) shell 1 with the centered (mu(12)-Pt) atom encapsulated by the 12-atom icosahedral Pt(x)Pd(12-x) cage, x = 1.2(3); (b) shell 2 with the 42-atom nu(2) icosahedral Pt(x)Pd(42-x) cage, x = 3.5(5); (c) shell 3 with the anti-Mackay 60-atom semi-regular rhombicosidodecahedral Pt(x)Pd(60-x) cage, x = 2.2(6); (d) shell 4 with the 50-atom nu(2) pentagonal dodecahedral Pd(50) cage. The total number of crystallographically estimated Pt atoms, 8 +/- 3, which was obtained from least-squares (Pt(x)/Pd(1-x))-occupancy analysis of the X-ray data that conclusively revealed the central atom to be pure Pt (occupancy factor, x = 1.00(3)), is fortuitously in agreement with that of 7.6(7) found from an X-ray Pt/Pd microanalysis (WDS spectrometer) on three crystals of 1. Our utilization of this site-occupancy (Pt(x)Pd(1-x))-analysis for shells 1-3 originated from the microanalytical results; otherwise, the presumed metal-core composition would have been (mu(12)-Pt)Pd(164). [Alternatively, the (mu(12)-Pt)M(164) core-geometry of 1 may be viewed as a pseudo-Ih Pt-centered six-shell successive nu(1) polyhedral system, each with radially equivalent vertex atoms: Pt@M(12)(icosahedron)@M(30)(icosidodecahedron)@M(12)(icosahedron)@M(60)(rhombicosidodecahedron)@M(30)(icosidodecahedron)@M(20)(pentagonal dodecahedron)]. Completely surprising structural dissimilarities between 1 and 2 are: (1) to date 1 is only reproducibly isolated as a heterometallic Pd-Pt cluster with a central Pt instead of Pd atom; (2) the 50 atoms comprising the outer fourth nu(2) pentagonal dodecahedral shell in 1 are less than the 60 atoms of the inner third shell in 1, in contradistinction to shell-by-shell growth processes in all other known shell-based s...