Electrical Properties Of Medium Research Articles

Measurement of Bulk Electrical Properties Using GPR with a Variable Reflector

Bulk electrical properties of media are important inherently for ground penetrating radar (GPR) applications and for providing a means to determine indirectly other physical properties such as moisture content. We have developed a reflector whose reflectivity can be controlled electronically. This variable reflector controlled by a GPR provides an effective method to measure bulk electrical properties of media. For sample measurements, the GPR is placed on one side of a sample and the variable reflector on the opposite side. GPR trace data are then acquired with the reflector in an on-state and in the off-state. By differencing these measurements, we improve the ability to detect the specific reflection event from the variable reflector. This process removes both the direct wave and clutter from the trace data, improving the quality of the refection event and our ability to accurately pick its arrival time and amplitude. We describe the variable reflector, a prototype instrument based on the reflector and numerical modeling performed to understand its response. We also show the results of testing applications to the measurement of wood chip moisture content and monitoring of the electrical properties of concrete during the curing process.

Estimating the Efficiency of Antennas Used as Sensors in Microwave Tissue Measurements

A measurement-based method to evaluate the efficiency of antennas used in microwave tissue imaging is proposed. The method is versatile and, in principle, applicable to antennas operating in free space as well. Two identical antennas are employed, one transmitting and the other one receiving. A signal flow graph (SFG) is developed for the two-port network formed by the antennas and the medium between them. The medium's electrical properties mimic those of tissue and can vary according to the application. Several measurements are performed with one of the antennas being loaded with known loads. A system of equations is constructed from the acquired data. The solution of this system produces the efficiency of the antenna in addition to other unknown parameters of the SFG. The efficiencies of two antennas are estimated from measured and simulated data and are verified through comparisons.

Ultrasonic waves in crystals with charged dislocations

A system of equations for charged dislocations, where the quadratic nonlinear terms are taken into account, is derived using the variational principle. This system describes the propagation of ultrasonic (US) waves in crystals with charged dislocations. From the linearized system of equations a linear dispersion equation is derived. Formulas for the phase linear velocity of the wave and the absorption coefficient are obtained, which show essential influence of charged dislocations and electrical properties of media on the mentioned quantities. For a nonlinear US wave an equation for the amplitude of the first harmonic is derived and, as a consequence, expressions are obtained for the nonlinear velocity of the US wave, for the attenuation of the first harmonic's amplitude, and for phase variation.

The performance of interventional loopless MRI antennae at higher magnetic field strengths

Interventional, "loopless antenna" MRI detectors are currently limited to 1.5 T. This study investigates whether loopless antennae offer signal-to-noise ratio (SNR) and field-of-view (FOV) advantages at higher fields, and whether device heating can be controlled within safe limits. The absolute SNR performance of loopless antennae from 0.5 to 5 T is investigated both analytically, using electromagnetic (EM) dipole antenna theory, and numerically with the EM method of moments, and found to vary almost quadratically with field strength depending on the medium's electrical properties, the noise being dominated by direct sample conduction losses. The prediction is confirmed by measurements of the absolute SNR of low-loss loopless antennae fabricated for 1.5, 3, and 4.7 T, immersed in physiologically comparable saline. Gains of 3.8 +/- 0.2- and 9.7 +/- 0.3-fold in SNR, and approximately 10- and 50-fold gains in the useful FOV area are observed at 3 and 4.7 T, respectively, compared to 1.5 T. Heat testing of a 3 T biocompatible nitinol-antenna fabricated with a redesigned decoupling circuit shows maximum heating of approximately 1 degrees C for MRI operating at high MRI exposure levels. Experiments in the rabbit aorta confirm the SNR and FOV advantages of the 3 T antenna versus an equivalent commercial 1.5 T device in vivo. This work is the first to study the performance of experimental internal MRI detectors above 1.5 T. The large SNR and FOV gains realized present a major opportunity for high-resolution imaging of vascular pathology and MRI-guided intervention.

Ultramicrotomy in the Analysis of Magnetic Recording Media

Particulate magnetic recording media is widely used to store information in computer memory applications. The media consists of submicron-size gamma iron oxide and alumina particles held together by a binder resin onto a substrate (mylar for floppy disks and aluminum for rigid disks). The electrical properties of media depend on the physical characteristics (like size and shape), dispersion and orientation of iron oxide. Well- dispersed and optimum-sized alumina improvies wear resistance of the relatively soft media. Transmission electron microscopy (TEM), with its excellent spatial resolution, is found to be very useful in analyzing media. Ultramicrotomy can be conveniently used to prepare cross-sections of the media for TEM study The major advantage of ultramicrotomy is that it provides a large amount of uniformly thin area relatively quickly.