No abstract

Dislocation-free decoration images containing up to 80,000 vortices have been obtained on high quality Bi2Sr2CaCu2O8+x superconducting single crystals. The observed flux line lattices are in the random manifold regime with a roughening exponent of 0.44 for length scales up to 80-100 lattice constants. At larger length scales, the data exhibit nonequilibrium features that persist for different cooling rates and field histories.

Main magnets for magnetic resonance imaging (MRI) are largely constructed with low temperature superconducting material. Most commonly used superconductors for these magnets are niobium-titanium (NbTi). Such magnets are operated at 4.2 K by being immersed in a liquid helium bath for long time operation. As the cost of liquid helium has increased threefold in the last decade and the market for MRI systems is on average increasing by more than 7% every year, there is a growing demand for an alternative to liquid helium. Superconductors such as magnesium-diboride (MgB 2 ) and niobium-tin (Nb 3 Sn) demonstrate superior current carrying quality at higher critical temperatures than 4.2 K. In this article, electromagnetic designs for conduction cooled main magnets over the range of medium field strengths (1.5 T) to ultrahigh field strengths (7.0 T) are presented. These designs are achieved by an improved functional approach coming from a series of developments by the present research group and using properties of the state-of-the-art second generation MgB 2 wires and Nb 3 Sn wires developed by Hyper Tech Research Inc. The MgB 2 magnet designs operated at different field strengths demonstrate excellent homogeneity and shielding properties at an operating temperature of 10 K. At ultrahigh field, the high current density on Nb 3 Sn allowed by the larger magnetic field on wire helps to reduce the superconductor volume in comparison with high field NbTi magnet designs. This allows for a compact magnet design that can operate at a temperature of 8 K. Overall, the designs created show promise in the development of conduction cooled dry magnets that would reduce dependence on helium.

Purpose:The authors investigate the ability of current models for magnetic nanoparticles immersed in dilute ferrofluids and external sinusoidal magnetic fields to explain recent experiments in which the relaxation effects are dominated by viscous damping. Methods: The Fokker-Planck (FP) equation appropriate for the nanoparticle magnetic moment distribution corresponding to the underlying stochastic Langevin model is numerically studied and solutions compared to experimental results. The FP equation is solved using an expansion in Legendre polynomials. The polydisperse properties of the ferrofluids are incorporated into the analysis. Results: By using a FP approach that includes polydispersion, the authors obtain good agreement with recent experimental results using ferrofluids containing nanoparticles with average hydrodynamic diameters in the 40-120 nm range. Conclusions: For nanoparticles used in recent magnetic spectroscopy experiments, the FP approach can be used to accurately model experimental data in the situation where Brownian relaxation effects are dominant and the ferrofluids are dilute.

No abstract

Phantom and in vivo data demonstrated that additional degrees of freedom in a parallel transmission system can be used to control RF induced heating in long conductors. A novel constrained optimization approach to reduce device heating was also presented that can be run in just few seconds and therefore could be added to an iMRI protocol to improve RF safety.

Aerosol deposition (AD) is a novel ceramic film preparation technique exhibiting the advantages of room-temperature operation and highly efficient film growth. Despite these advantages, AD has not been used for preparing humidity-sensing films. Herein, room-temperature AD was utilized to deposit BaTiO films on a glass substrate with a Pt interdigital capacitor, and their humidity-sensing performances were evaluated in detail, with further optimization performed by postannealing at temperatures of 100, 200, ..., 600 °C. Sensor responses (i.e., capacitance variations) were measured in a humidity chamber for relative humidities (RHs) of 20-90%, with the best sensitivity (461.02) and a balanced performance at both low and high RHs observed for the chip annealed at 500 °C. In addition, its response and recovery were extremely fast, respectively, at 3 and 6 s and it kept a stable recording with the maximum error rate of 0.1% over a 120 h aging test. Compared with other BaTiO-based humidity sensors, the above chip required less thermal energy for its preparation but featured a more than 2-fold higher sensitivity and a superior detection balance at RHs of 20-90%. Cross-sectional transmission electron microscopy imaging revealed that the prepared film featured a transitional variable-density structure, with moisture absorption and desorption being promoted by a specific capillary structure. Finally, a bilayer physical model was developed to explain the mechanism of enhanced humidity sensitivity by the prepared BaTiO film.

No abstract