Brian B. Schwartz

Magnetic Properties of Magnetotactic Bacteria

Abstract This paper reports on the magnetic properties of magnetosomes in the freshwater magnetotactic bacterium Aquaspirillum magnetotacticum . The magnetosomes are well crystallized particles of magnetite with dimensions of 40 to 50 nm, which are arranged within the cells in a single linear chain and are within the single-magnetic-domain (SD) size range for magnetite. A variety of magnetic properties have been measured for two samples of dispersions of freeze-dried cells consisting of (1) whole cells (M-1) and (2) magnetosomes chains separated from cells (M-2). An important result is that the acquisition and demagnetization of various type of remanent magnetizations are markedly different for the two samples and suggest that remanence is substantially affected by magnetostatic interactions. Interactions are likely to be much more important in M-2 because the extracted magnetosome chains are no longer separated from one another by the cell membrane and cytoplasm. Other experimental data for whole cells agree with predictions based on the chain of spheres model for magnetization reversal. This model is consistent with the unique linear arrangement of equidimensional particles in A. magnetotacticum . The magnetic properties of bacterial and synthetic magnetites are compared and the paleomagnetic implications are discussed.

Improved high field performance of Nb-Al powder metallurgy processed superconducting wires

Improved overall critical current densities Jc’s were achieved with powder metallurgy processed Nb‐A1 which combined reduced powder sizes and increased nominal areal reductions R. Increased Jc values were obtained for a variety of different heat treatments. For a Nb‐8 wt. % A1 wire with R=3.4×105 a very short treatment at 1100 °C followed by a 750 °C treatment gave (at 4.2 K)Jc>104 A/cm2 at 19 T; a 900 °C treatment followed by a 750 °C treatment gave Jc=104 A/cm2 at 18 T and, at 2 K, Jc was greater than 104 A/cm2 at 20 T; and a treatment of 800 °C for 8 h gave (at 4.2 K)Jc=104 A/cm2 at 17.5 T.

Improvements of ’’insitu’’ multifilamentary Nb3Sn superconducting wires

’’In situ’’ multifilamentary Nb3Sn wires with improved high‐field properties are reported. An overall critical current density greater than 104 A/cm3 is achieved at 14 T for a Cu–36 wt%–Nb–20 wt% Sn material. The results are comparable to the best reported commercial multifilamentary Nb3Sn materials.

Mechanical properties of in situ multifilamentary Nb3Sn superconducting wires

The mechanical properties of in situ Cu–36 wt% Nb–x wt% Sn superconducting wires are presented where 5<x<20 wt% Sn. For high x, no degradation of critical current Jc is observed for stresses σ up to 700 MPa (∼100 ksi) and a maximum Jc occurs at σ≃450 MPa. Strains of ∼0.6–0.9% are measured at 300 and 77 K for 450 MPa, and ∼1.5–2% for 700 MPa. Cyclic stress data are consistent with a prestress model for composite materials.

Mechanical properties of in situ multifilamentary Nb/sub 3/Sn superconducting wires

The mechanical properties of in situ Cu–36 wt% Nb–x wt% Sn superconducting wires are presented where 5<x<20 wt% Sn. For high x, no degradation of critical current Jc is observed for stresses σ up to 700 MPa (∼100 ksi) and a maximum Jc occurs at σ≃450 MPa. Strains of ∼0.6–0.9% are measured at 300 and 77 K for 450 MPa, and ∼1.5–2% for 700 MPa. Cyclic stress data are consistent with a prestress model for composite materials.

Fabrication on a laboratory scale and mechanical properties of Cu-Nb-Sn multifilamentary superconducting composite wires produced by cold powder metallurgy processing

Superconducting Cu‐Nb‐Sn multifilamentary composites are fabricated inexpensively on a laboratory scale. Small (40 μm) particles of Cu and Nb are compacted, placed in a suitable external jacket for containment, then elongated at room temperature to form a multifilamentary circular wire. Processing yields a multifilamentary Cu‐Nb‐Sn superconductor with high overall critical current densities Jc at high magnetic fields. Measurements of the mechanical properties show no degradation of Jc for strains greater than 1% for composite made with a large areal reduction ratio.

High critical currents in cold‐powder‐metallurgy‐processed superconducting Cu‐Nb‐Sn composites

Cold‐powder‐metallurgy‐processed superconducting Cu‐Nb‐Sn (discontinuous) multifilamentary composites have been fabricated. Overall critical current Jc comparable to the best in situ and commercial multifilamentary Nb3Sn (scaled for the same Nb content) have been achieved. Values of Jc approximately 105 A/cm2 at 12 T, 5×104 A/cm2 at 14 T, and 2×103 A/cm2 at 18 T are observed for a material with Cu–40 wt.% Nb–20 wt.% Sn with respect to Cu. The physical characteristics of the starting materials and some advantages of the cold‐powder‐metallurgy process are discussed.

Magnetization and magnetic suspension of YBa2Cu3Ox-AgO ceramic superconductors

Abstract The magnetic force which accounts for the newly-discovered suspension of a superconductor below a permanent magnet is determined by the magnetization of the superconductor and the magnetic-field gradient. Magnetization measurements were carried out on a series of YBa 2 Cu 3 O x -AgO ceramic superconductors, with T c ≈93 K. The samples were from the set of samples in which the magnetic-suspension phenomenon was first discovered. Magnetization data were taken at 4.2 and 77 K in magnetic fields up to 180 kOe. Hysteresis loops at low fields, up to 1.2 kOe, were also studied at 4.2, 77 and 87 to 88 K. The magnetization and hysteresis in most of the samples are among the largest observed to date in ceramic high- T c superconductors. In most of our samples, the remanent moment at 4.2 K is about 80 emu/g, and about 3 emu/g at 77 K. The large magnetization and hysteresis indicate the presence of strong pinning forces. The strong hysteresis at 77 K results in an appreciable positive magnetization, parallel to the field, when the field H is decreased from a finite value (above≈0.5 kOe). This positive magnetization increases with decreasing H . The positive magnetization can be produced by bringing a permanent magnet close to the superconductor, and then withdrawing it slowly. This leads to an attractive magnetic force between the superconductor and the permanent magnet. Calculations, based on a realistic model, show that at 77 K this magnetic attraction can be sufficiently strong to balance the gravitational force. As a result, the superconductor can be suspended below a permanent magnet. The expected damped oscillatory motion near the suspension point, following the application of a vertical impulse to the superconductor, is discussed. This motion is more complicated than that near the bottom of a conventional potential well. Some remaining problems associated with the magnetic-suspension phenomenon are outlined.

Large‐scale applications of superconductivity

Ever since its discovery by Kamerlingh Onnes in 1911, superconductivity has contained the promise of important applications. The ability of superconductors to carry current without resistance was expected to revolutionize the field of electrical engineering. The fulfillment of this promise, however, was deferred for half a century; it is only since the mid 1960s that there has been much progress in applied superconductivity. Superconductivity, with its ability to generate an intense, large‐volume magnetic field economically, can now offer alternatives in the fields of energy generation, storage and distribution as well as in transportation. Some prototype quasi‐commercial devices are already in use, and approximately 30 million dollars are being spent annually to develop this technology further.

In situ multifilamentary superconducting wires fabricated using a controlled high‐temperature gradient

A 0.64‐cm‐diam. Cu‐Nb composite rod was continuously melted at 1800 °C, then resolidified in a controlled large temperature gradient (∼ 400 °C/cm) to produce a microstructure of aligned [directionally solidified, (DS)] dendrites in a copper matrix. Primary dendrite arm sizes of ∼50, 20, and 10 μm were measured for growth rates of 5.6×10−4, 1.7×10−2, and 0.11 cm/sec, respectively. Higher growth rates produced an equiaxed structure typical of chill casting. After processing, the overall Jc of the DS wires and the effect of stress on Jc were comparable to those of chill‐cast specimens. The atomic fraction of Nb converted to Nb3Sn increased with decreasing filament size, and Tc was approximately 17.6 K for the DS alloys.