Eddy current simulation in thick cylinders of finite length induced by coils of arbitrary geometry

Eddy currents are inevitably induced when time-varying magnetic field gradients interact with the metallic structures of a magnetic resonance imaging (MRI) scanner. The secondary magnetic field produced by this induced current degrades the spatial and temporal performance of the primary field generated by the gradient coils. Although this undesired effect can be minimized by using actively and/or passively shielded gradient coils and current pre-emphasis techniques, a residual eddy current still remains in the MRI scanner structure. Accurate simulation of these eddy currents is important in the successful design of gradient coils and magnet cryostat vessels. Efficient methods for simulating eddy currents are currently restricted to cylindrical-symmetry. The approach presented in this paper divides thick conducting cylinders into thin layers (thinner than the skin depth) and expresses the current density on each as a Fourier series. The coupling between each mode of the Fourier series with every other is modeled with an inductive network method. In this way, the eddy currents induced in realistic cryostat surfaces by coils of arbitrary geometry can be simulated. The new method was validated by simulating a canonical problem and comparing the results against a commercially available software package. An accurate skin depth of 2.76 mm was calculated in 6 min with the new method. The currents induced by an actively shielded x-gradient coil were simulated assuming a finite length cylindrical cryostat consisting of three different conducting materials. Details of the temporal-spatial induced current diffusion process were simulated through all cryostat layers, which could not be efficiently simulated with any other method. With this data, all quantities that depend on the current density, such as the secondary magnetic field, are simply evaluated.     

Spring constant modulation in a zone plate tweezer using linear polarization

At large NAs a micro-Fresnel zone plate produces a focal spot that is more elliptical than that produced by an objective lens with the same NA. Using this anisotropy we demonstrate a method for modulating the spring constant of an optical trap by rotating the linear input polarization. The focal spot ellipticity is enhanced by the apodization factor of the zone plate and its extremely high NA. By measuring the positions of trapped particles we obtain two-dimensional histograms of particle position. These indicate that the trap spring constant is 2.75 times larger perpendicular to the incident polarization than along it. The elliptical focal spot distribution can be rotated by rotating the incident polarization, allowing the spring constant along a given direction to be modulated.     

Characterization of cell mismatch in a multi-crystalline silicon photovoltaic module

In this study the causes and effects of cell mismatch were identified in a multi-crystalline silicon photovoltaic module. Different techniques were used to identify the causes of the mismatch, including Electroluminescence (EL) imaging, Infrared (IR) imaging, current–voltage (I–V) characteristics, worst-case cell determination and Large Area Laser Beam Induced Current (LA-LBIC) scans. In EL images the cracked cells, broken fingers and material defects are visible. The presence of poorly contacted cells results in the formation of hot-spots. LA-LBIC line scans give the relative photoresponse of the cells in the module. However, this technique is limited due to the penetration depth of the laser beam. The worst case cell determination compares the I–V curves of the whole module with the I–V curve of the module with one cell covered, allowing the evaluation of the performance of each cell in a series-connected string. These methods allowed detection of the poorly performing cells in the module. Using all these techniques an overall view of the photoresponse in the cells and their performance is obtained.     

Compressibility and sound velocity of some liquid rare earths

We report the first measurements of the sound velocity in liquid La, Ce, Pr and Yb and deduce the adiabatic and isothermal compressibilities. The temperature coefficient of the sound velocity in liquid Ce is positive, which we interpret as support for the suggestion that there is increasing delocalization of the 4f electrons in the liquid state.     

Signal-to-noise ratio improvements in in vivo high resolution micro-volume selected spectroscopy

A system capable of in vivo volume selected 1H NMR spectroscopy of voxels as small as 0.2 cm3 is described. Signal-to-noise ratio improvements with probe design and a novel signal steering device are detailed. A high-resolution, image-directed proton spectrum from 0.2 cm3 of a rat's brain at 200 MHz obtained using the SUBMERGE/SPACE pulse sequence is presented. Single-scan voxel shimming was implemented to improve spectral resolution.     

In situ electron microscopy studies of the sintering of palladium nanoparticles on alumina during catalyst regeneration processes.

Sintering of a palladium catalyst supported on alumina (Al2O3) in an oxidizing environment was studied by in situ transmission electron microscopy (TEM). In the case of a fresh catalyst, sintering of Pd particles on an alumina surface in a 500 mTorr steam environment happened via traditional ripening or migration and coalescence mechanisms and was not significant unless heating above 500 degrees C. After the catalyst was used for the hydrogenation of alkynes, TEM coupled with convergent beam electron diffraction and electron energy loss spectroscopy analysis revealed that most of the Pd particles were lifted from the alumina surface by hydrocarbon buildup. This dramatically different morphology totally changed the sintering mechanism of Pd particles during the regeneration process. Catalytic gasification of hydrocarbon around these particles in an oxidizing environment allowed the Pd particles to move around and coalesce with each other at temperatures as low as 350 degrees C. For catalysts heating under 500 mTorr steam at 350 degrees C, steam stripped hydrocarbon catalytically at the beginning, but the reaction stopped after 4 h. Heating in air resulted in both catalytic and noncatalytic stripping of hydrocarbon.     

Substructured molecular dynamics using multibody dynamics algorithms

This paper reports a new research effort aimed at using efficient multibody dynamics methods to simulate coarse-grained molecular systems. Various molecular systems are studied and the results of nanosecond-long simulations are analyzed to validate the method. The systems studied include bulk water, alkane chains, alanine dipeptide and carboxyl terminal fragments of calmodulin, ribosomal L7/L12 and rhodopsin proteins. The stability and validity of the simulations are studied through conservation of energy, thermodynamics properties and conformational analysis. In these simulations, a speed up of an order of magnitude is realized for conservative error bounds with a fixed timestep integration scheme. A discussion is presented on the open-source software developed to facilitate future research using multibody dynamics with molecular dynamics.     

In situ environmental transmission electron microscopy to determine transformation pathways in supported Ni nanoparticles

We have applied in situ environmental transmission electron microscopy (ETEM) to follow the dynamic phase transformations that take place in SiO(2) supported Ni nanoparticles during oxidation and reduction processes. The gas environments used for in situ ETEM studies were relevant to partial oxidation of methane (POM) reaction. In the presence of the CH(4)+O(2) gas mixture (in 2:1 ratio) at 400°C, Ni transforms to NiO due to the high O-chemisorption energy. NiO void structures were formed during the oxidation because of the Kirkendall type process where diffusion of Ni cations along NiO grain boundaries is eight orders of magnitude greater than the diffusion of O anions. Reduction was performed under a CO+H(2) mixture at 400°C (in 1:2 ratio) and also in the presence of CH(4) at 500°C. Particle reduction processes also takes place via the diffusion of Ni cations along the NiO grain boundaries leaving NiO on the surface of the nanoparticle. NiO is the phase that is present on the surface of the nanoparticle during the intermediate stage of reduction.     

Cacao seeds are a "Super Fruit": A comparative analysis of various fruit powders and products

Background  

Numerous popular media sources have developed lists of "Super Foods" and, more recently, "Super Fruits". Such distinctions often are based on the antioxidant capacity and content of naturally occurring compounds such as polyphenols within those whole fruits or juices of the fruit which may be linked to potential health benefits. Cocoa powder and chocolate are made from an extract of the seeds of the fruit of the Theobroma cacao tree. In this study, we compared cocoa powder and cocoa products to powders and juices derived from fruits commonly considered "Super Fruits".     

X-ray absorption fine structure study of amorphous metal oxide thin films prepared by photochemical metalorganic deposition

The oxidation state and local geometry of the metal centers in amorphous thin films of Fe₂O₃ (Fe³⁺ oxidation state), CoFe₂O₄ (Co²⁺/Fe³⁺ oxidation states), and Cr₂O₃ (Cr³⁺ oxidation state) are determined using K edge X-ray absorption near-edge structure (XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. The metal oxide thin films were prepared by the solid-state photochemical decomposition of the relevant metal 2-ethylhexanoates, spin cast as thin films. No peaks are observed in the X-ray diffraction patterns, indicating the metal oxides are X-ray amorphous. The oxidation state of the metals is determined from the edge position of the K absorption edges, and in the case of iron-containing samples, an analysis of the pre-edge peaks. In all cases, the EXAFS analysis indicates the first coordination shell consists of oxygen atoms in an octahedral geometry, with a second shell consisting of metals. No higher shells are observed beyond 3.5 Å for all samples, indicating the metal oxides are truly amorphous, consistent with X-ray diffraction results.