Kumar P. Dharmasena

The structural performance of near-optimized truss core panels

Theoretical studies have indicated that truss core panels with a tetragonal topology support bending and compression loads at lower weight than competing concepts. The goal of this study is to validate this prediction by implementing an experimental protocol that probes the key mechanical characteristics while addressing node eccentricity and structural robustness. For this purpose, panels have been fabricated from a beryllium–copper alloy using a rapid prototyping approach and investment casting. Measurements were performed on these panels in flexure, shear and compression. Numerical simulations were conducted for these same configurations. The measurements reveal complete consistency with the stiffness and limit load predictions, as well as providing a vivid illustration of asymmetric structural responses that arises because the bending behavior of optimized panels is dependent on truss orientation. 2002 Elsevier Science Ltd. All rights reserved.

On the performance of truss panels with Kagomé cores

The performance characteristics of a truss core sandwich panel design based on the 3D Kagome has been measured and compared with earlier simulations. Panels have been fabricated by investment casting and tested in compression, shear and bending. The isotropic nature of this core design has been confirmed. The superior performance relative to truss designs based on the tetrahedron has been demonstrated and attributed to the greater resistance to plastic buckling at the equivalent core density.

Electrical Conductivity of Open-cell Metal Foams

Cellular metal foams are of interest because of the ability to tailor their mechanical, thermal, acoustic, and electrical properties by varying the relative density and cell morphology. Here, a tetrakaidecahedral unit-cell approach is used to represent an open-cell aluminum foam and a simplified electrical resistor network derived to model low frequency current flow through the foam. The analysis indicates that for the range of relative densities studied (4-12%), the conductivity of tetrakaidecahedral foams has a linear dependence upon relative density. The distribution of metal in the cell ligaments was found to significantly affect the conductivity. Increasing the fraction of metal at the ends of the ligaments resulted in a decrease in electrical conductivity at a fixed relative density. Low frequency electrical conductivity measurements of an open-cell aluminum foam (ERG Duocel) confirmed the linear dependence upon density, but the slope was smaller than that predicted by the unit-cell model. The difference between the model and experiment was found to be the result of the presence of a distribution of cell sizes and types in real samples. This effect is due to the varying number of ligaments, ligament lengths, and the cross-sectional areas available for current conduction across the cellular structure.

Simulation of the eddy current sensing of gallium arsenide Czochralski crystal growth

Abstract The feasibility of eddy current sensing of (1) the melt surface position and (2) the liquid-solid interface shape of 3-inch gallium arsenide crystals being grown by the Czochralski technique has been investigated using an axisymmetric finite element method. The results show clearly that differential sensor designs operating at high frequency ( ≈ 1 MHz) are very sensitive to the distance between the sensor and the surface of the melt providing the opportunity to precisely monitor and control this important variable of the growth process. The calculations also show a weaker effect of interface shape upon the imaginary impedance component at lower frequencies (1–10 kHz). Its physical basis is due to the different skin depths of solid and liquid GaAs. We show that a sensors response to this interface effect can be enhanced by appropriate design of the differential sensors pick-up coils.

Eddy current sensor concepts for the Bridgman growth of semiconductors

Abstract Electromagnetic finite element methods have been used to identify eddy current sensor designs for monitoring CdTe vertical Bridgman crystal growth. A model system consisting of pairs of silicon cylinders with electrical conductivities similar to those of solid and liquid CdTe has been used to evaluate the multifrequency response of several sensors designed for locating and characterizing the curvature of liquid-solid interfaces during vertical Bridgman growth. At intermediate frequencies (100–800 kHz), the sensors imaginary impedance monotonically increases as interfacial curvature changes from concave to convex or the interface location moves upwards through the sensor. The experimental data are in excellent agreement with theoretical predictions. At higher test frequencies (∼ 5 MHz), the test circuits parasitics contribute to the sensors response. Even so, the predicted trends with interface location/curvature were found to be still preserved, and the experiments confirm that the sensors high frequency response depends more on interface location and has only a small sensitivity to curvature. Multifrequency data obtained from these types of sensors have the potential to separately discriminate the location and the shape of liquid-solid interfaces during the vertical Bridgman growth of CdTe and other semiconductor materials of higher electrical conductivity.

Methods for liquid-solid interface shape and location discrimination during eddy current sensing of Bridgman growth

Axisymmetric finite element calculations of the multifrequency eddy current sensor response during vertical Bridgman growth have been conducted for GaAs and CdTe. These are representative of materials that are either ideally (the GaAs case) or marginally suited (CdTe) to eddy current sensing during semiconductor growth by a vertical Bridgman process. The simulations reveal two potential strategies for separately discriminating the interface shape and location. One is based upon a comparison of the sensors high and low frequency imaginary impedance components. The former characterizes the interface location and the latter its shape. The second approach exploits the existence of an inflection point (or a peak) in the imaginary impedance response of an absolute (or a differential sensor) as an interface passes through it. This latter approach is less affected by test circuit contributions to the sensors high frequency response. Both strategies with either sensor type lead to reasonably precise location/shape characterization for GaAs. The differential sensor coupled with the peak position method offers the best precision for less favorable material systems like CdTe. Even for this worst case material, the interface location can be determined to ±1 mm and its curvature estimated with sufficient precision to be of use for characterizing vertical Bridgman growth processes.

Eddy current sensing of vertical Bridgman growth of Cd0.96Zn0.04Te

Abstract An encircling two coil eddy current sensor has been integrated into the ceramic liner of a commercial six zone vertical Bridgman furnace and used to monitor the growth of 72 mm diameter Cd0.96Zn0.04Te crystals. The sensor was maintained at a fixed location with respect to the ampoule and monitored melt cooling/composition, detected the position of the liquid-solid interface, measured the interfaces curvature and determined the electrical resistivity of the solid during post-solidification cooling. The study confirmed earlier predictions that a two coil, multifrequency sensor approach can independently recover the liquid-solid interface location and provide insight about its curvature using data collected in the 50 kHz to 5 MHz frequency range. It suggests that the directional solidification of Cd0.96Zn0.04Te most probably initiates from a Cd-depleted melt, occurs in a colder than anticipated region of the furnace and progresses with a convex interface shape.

Modeling multifrequency eddy current sensor interactions during vertical Bridgman growth of semiconductors

Electromagnetic finite element modeling methods have been used to analyze the responses of two (“absolute” and “differential”) eddy current sensor designs for measuring liquid–solid interface location and curvature during the vertical Bridgman growth of a wide variety of semiconducting materials. The multifrequency impedance changes due to perturbations of the interface’s location and shape are shown to increase as the liquid/solid electrical conductivity ratio increases. Of the materials studied, GaAs is found best suited for eddy current sensing. However, the calculations indicate that even for CdTe with the lowest conductivity ratio studied, the impedance changes are still sufficient to detect the interface’s position and curvature. The optimum frequency for eddy current sensing is found to increase as the material system’s conductivity decreases. The analysis reveals that for a given material system, high frequency measurements are more heavily weighted by the interfacial location while lower frequenc...

Eddy Current Characterization of Metal Foams

Cellular materials are characterized by their relative density, pore shape and orientation, the average cell size, and the degree of pore interconnectivity which all depend upon the method and conditions used for processing. This has created an interest in non-invasive sensor techniques to characterize the foam structure. Multifrequency electrical impedance measurements were performed using an eddy current technique on open cell aluminum foam with systematically varied relative densities and pore sizes. The impedance was dominated at all frequencies by the amount of metal contained within a probed volume of foam and the tortuosity of the current path. At low frequency, the impedance data were found to be relatively insensitive to pore size variations enabling an independent measure of the relative density. At high frequency, the data indicated a strong dependence on the cell size.

Factors Affecting the Performance of Eddy Current Densification Sensors

Hot Isostatic Pressing (HIP) is an increasingly important near net shape process for producing fully dense components from powders [1]. It involves filling a preshaped metal canister with alloy powder, followed by evacuation, and sealing. The can is then placed in a HIP (a furnace that can be pressurized to ~200MPa with an inert gas such as argon). The can is subjected to a heating/pressurization cycle that softens and compacts the powder particles to a fully dense mass and a shape determined by the can shape, the powders initial packing and the thermal-mechanical cycle imposed [2]. Today, many metals, alloys and intermetallics are processed this way (including nickel based superalloys, titanium alloys, NiA1, etc.) and it is increasingly used to produce metal matrix composites.