The temperature dependence of effective thermal conductivity of open-celled steel alloy foams

The effective thermal conductivity of steel alloy FeCrAlY (Fe-20 wt.% Cr-5 wt.% Al-2 wt.% Y-20 wt.%) foams with a range of pore sizes and porosities was measured between 300 and 800 K, under both vacuum and atmospheric conditions. The results show that the effective thermal conductivity increases rapidly as temperature is increased, particularly in the higher temperature range (500-800 K) where the transport of heat is dominated by thermal radiation. The effective conductivity at temperature 800 K can be three times higher than that at room temperature (300 K). Results obtained under vacuum conditions reveal that the effective conductivity increases with increasing pore size or decreasing porosity. The contribution of natural convection to heat conduction was found to be significant, with the effective thermal conductivity at ambient pressure twice the value of vacuum condition. The results also show that natural convection in metal foams is strongly dependent upon porosity. © 2003 Elsevier B.V. All rights reserved.

Investigation into IGBT dV/dt during turn-off and its temperature dependence

In many power converter applications, particularly those with high variable loads, such as traction and wind power, condition monitoring of the power semiconductor devices in the converter is considered desirable. Monitoring the device junction temperature in such converters is an essential part of this process. In this paper, a method for measuring the insulated gate bipolar transistor (IGBT) junction temperature using the collector voltage dV/dt at turn-OFF is outlined. A theoretical closed-form expression for the dV/dt at turn-OFF is derived, closely agreeing with experimental measurements. The role of dV/dt in dynamic avalanche in high-voltage IGBTs is also discussed. Finally, the implications of the temperature dependence of the dV/dt are discussed, including implementation of such a temperature measurement technique. © 2006 IEEE.

Concentration dependence of the Er3+ visible and infrared luminescence in Y2-x Erx O3 thin films on Si

Y2-x Erx O3 thin films, with x varying between 0 and 0.72, have been successfully grown on crystalline silicon (c-Si) substrates by radio-frequency magnetron cosputtering of Y2 O 3 and Er2 O3 targets. As-deposited films are polycrystalline, showing the body-centered cubic structure of Y2 O3, and show only a slight lattice parameter contraction when x is increased, owing to the insertion of Er ions. All the films exhibit intense Er-related optical emission at room temperature both in the visible and infrared regions. By studying the optical properties for different excitation conditions and for different Er contents, all the mechanisms (i.e., cross relaxations, up-conversions, and energy transfers to impurities) responsible for the photoluminescence (PL) emission have been identified, and the existence of two different well-defined Er concentration regimes has been demonstrated. In the low concentration regime (x up to 0.05, Er-doped regime), the visible PL emission reaches its highest intensity, owing to the influence of up-conversions, thus giving the possibility of using Y2-x Er x O3 films as an up-converting layer in the rear of silicon solar cells. However, most of the excited Er ions populate the first two excited levels 4I11/2 and 4I13/2, and above a certain excitation flux a population inversion condition between the former and the latter is achieved, opening the route for the realization of amplifiers at 2.75 μm. Instead, in the high concentration regime (Er-compound regime), an increase in the nonradiative decay rates is observed, owing to the occurrence of cross relaxations or energy transfers to impurities. As a consequence, the PL emission at 1.54 μm becomes the most intense, thus determining possible applications for Y2-x Erx O 3 as an infrared emitting material. © 2009 American Institute of Physics.

InGaN/GaN LEDs grown on Si(111): Dependence of device performance on threading dislocation density and emission wavelength

We demonstrate the growth of crack-free blue and greenemitting LED structures grown on 2-inch and 6-inch Si(111) substrates by metalorganic vapour phase epitaxy (MOVPE), using AlN nucleation layers and AlGaN buffer layers for stress management. LED device performance and its dependence on threading dislocation (TD) density and emission wavelength were studied. Despite the inherently low light extraction efficiency, an output power of 1.2 mW at 50 mA was measured from a 500 μm square planar device, emitting at 455 nm. The light output decreases dramatically as the emission wavelength increases from 455 nm to 510 nm. For LED devices emitting at similar wavelength, the light output was more than doubled when the TD density was reduced from 5×1 09 cm-2 to 2×109 cm-2. Our results clearly show that high TD density is detrimental to the overall light output, highlighting the need for further TD reduction for structures grown on Si. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.