Development of effective thermal management strategies for LED luminaires

The efficacy, reliability and versatility of the light emitting diode (LED) can outcompete most established light source technologies. However, they are particularly sensitive to high temperatures, which compromises their efficacy and reliability, undermining some of the technology s key benefits. Consequently, effective thermal management is essential to exploit the technology to its full potential. Thermal management is a well-established subject but its application in the relatively new LED lighting industry, with its specific constraints, is currently poorly defined. The question this thesis aims to answer is how can LED thermal management be achieved most effectively? This thesis starts with a review of the current state of the art, relevant thermal management technologies and market trends. This establishes current and future thermal management constraints in a commercial context. Methods to test and evaluate the thermal management performance of a luminaire system follow. The defined test methods, simulation benchmarks and operational constraints provide the foundation to develop effective thermal management strategies. Finally this work explores how the findings can be implemented in the development and comparison of multiple thermal management designs. These are optimised to assess the potential performance enhancement available when applied to a typical commercial system. The outcomes of this research showed that thermal management of LEDs can be expected to remain a key requirement but there are hints it is becoming less critical. The impacts of some common operating environments were studied, but appeared to have no significant effect on the thermal behaviour of a typical system. There are some active thermal management devices that warrant further attention, but passive systems are inherently well suited to LED luminaires and are readily adopted so were selected as the focus of this research. Using the techniques discussed in this thesis the performance of a commercially available component was evaluated. By optimising its geometry, a 5 % decrease in absolute thermal resistance or a 20 % increase in average heat transfer coefficient and 10 % reduction in heatsink mass can potentially be achieved . While greater lifecycle energy consumption savings were offered by minimising heatsink thermal resistance the most effective design was considered to be one optimised for maximum average heat transfer coefficient. Some more radical concepts were also considered. While these demonstrate the feasibility of passively manipulating fluid flow they had a detrimental impact on performance. Further analysis would be needed to conclusively dismiss these concepts but this work indicates there is very little potential in pursuing them further

Heat Pump-Based Novel Energy System for High-Power LED Lamp Cooling and Waste Heat Recovery

Unlike incandescent light bulb, which radiates heat into the surroundings by infrared rays, light emitting diode (LED) traps heat inside the lamp. This fact increases the difficulty of cooling LED lamps, while it facilitates the recovery of the generated heat. We propose a novel energy system that merges high-power LED lamp cooling with the heat pump use; the heat pump can cool the LED lamp and at the same time recover the waste heat. In this way, a high percentage of the energy consumed by the LED lamp can be utilized. In this work, we developed a prototype of this energy system and conducted a series of experimental studies to determine the effect of several parameters, such as cooling water flow rate and LED power, on the LED leadframe temperature, compressor power consumption, and system performance. The experimental results clearly indicate that the energy system can lead to substantial energy savings

Linear laser diode arrays for improvement in optical disk recording for space stations

The design and fabrication of individually addressable laser diode arrays for high performance magneto-optic recording systems are presented. Ten diode arrays with 30 mW cW light output, linear light vs. current characteristics and single longitudinal mode spectrum were fabricated using channel substrate planar (CSP) structures. Preliminary results on the inverse CSP structure, whose fabrication is less critically dependent on device parameters than the CSP, are also presented. The impact of systems parameters and requirements, in particular, the effect of feedback on laser design is assessed, and techniques to reduce feedback or minimize its effect on systems performance, including mode-stabilized structures, are evaluated

Improving the Reliability and Modal Stability of High Power 870 nm AlGaAs CSP Laser Diodes for Applications to Free Space Communication Systems

The operating characteristics (power-current, beam divergence, etc.) and reliability assessment of high-power CSP lasers is discussed. The emission wavelength of these lasers was optimized at 860 to 880 nm. The operational characteristics of a new laser, the inverse channel substrate planar (ICSP) laser, grown by metalorganic chemical vapor deposition (MOCVD), is discussed and the reliability assessment of this laser is reported. The highlights of this study include a reduction in the threshold current value for the laser to 15 mA and a degradation rate of less than 2 kW/hr for the lasers operating at 60 mW of peak output power

Carbon Ion Implanted Silicon for Schottky Light-Emitting Diodes

Research in the field of Photonics is in part, directed at the application of light-emitting materials based on silicon platforms. In this work silicon wafers are modified by carbon ion implantation to incorporate silicon carbide, a known light-emitting material. Ion beam synthesis treatments are applied with implant energy of 20 keV, and ion fluences of 3, 5 and 10 × 1016 ions/cm2 at both ambient temperature and high temperature (400 °C). The samples are annealed at 1000 °C, after implantation. The carbon ion implanted silicon is characterized using Raman and Fourier transform infrared spectroscopic techniques, grazing-incidence X-ray diffraction, transmission electron microscopy and electron energy loss spectroscopy. The materials are observed to have a multilayer structure, where the ambient temperature implanted materials have an amorphous silicon layer, and an amorphous silicon layer with carbon-rich, nanoscale inclusions. The high temperature implanted materials have the same layers, with an additional polycrystalline Si layer at the interface between the implanted layer and the target substrate and the amorphous Si layer with SiC inclusions is reduced in thickness compared to the ambient temperature samples. The carbon-rich inclusions are confirmed to be SiC, with no evidence of carbon clusters in the materials observed using Raman spectroscopy. The carbon ion-implanted material is used to fabricate Schottky diodes having a semitransparent gold contact at the implanted surface, and an aluminum contact on the opposite side. The diodes are tested using current-voltage measurements between -12 and +15 V. No reverse breakdown is observed for any of the diodes. The turn-on voltages for the ambient temperature implanted samples are 2.6±0.1 V, 2.8±0.6 V and 3.9±0.1 V for the 3, 5 and 10 × 1016 ions/cm2 samples, respectively. For the high temperature implanted samples, the turn-on voltages are 3.2±0.1 V, 2.7±0.1 V, and 2.9±0.4 V for the implanted samples with same fluences. The diode curves are modeled using the Shockley equation, and estimates are made of the ideality factor of the diodes. These are 188±16, 224.5±5.8, and 185.4±9.2 for the ambient temperature samples, and 163.6±6.3, 124.3±5.3, and 333±12 for the high temperature samples. The high ideality factor is associated with the native oxide layer on the silicon substrate and with the non-uniform, defect-rich implanted region of the carbon ion implanted silicon. Red-orange visible light emission from the diodes is observed with voltage greater than the turn-on voltage applied across the diodes. The luminescence for the ambient temperature samples is attributed to porous silicon, and amorphous silicon. The high temperature implanted samples show luminescence associated with porous silicon, nanocrystalline silicon carbide, and defects in silicon related to ion implantation. The luminescent intensity observed for the ambient temperature samples is higher than for the high temperature samples. The dominant luminescence feature in the carbon ion-implanted silicon material is porous silicon, which is described by quantum confinement of excitons in silicon

Synthesis and Characterization of Polymer Composites Containing Aligned Conducting Polymers and Carbon Nanotubes

Miniaturization of electronics and impending demand for bendable electronic gadgets creates a dire need for a thin and flexible film technology that would not only provide spot cooling for the crammed transistors, but also tap into the waste heat generated to produce a portable power source. Thin film thermoelectrics offer a viable solution and have several structural, chemical and economical advantages over inorganic thermoelectric materials. However, their low power factor compared to that of inorganic materials prevents them from being used for practical applications. The tradeoff between Seebeck coefficient and electrical conductivity restricts the improvement of power factor through increase in number of charge carriers. However, controlled modulation of the mobility of charge carriers has a potential to increase the electrical conductivity without adversely affecting the Seebeck coefficient. This research involves investigating a novel way to fabricate organic thermoelectric thin films with high power factor by modulating the morphology of the conducting polymer poly-(3,4-ethylenedioxythiophene) (PEDOT) and creating a composite with carbon nanotubes to control the mobility and hence the electrical conductivity of the thin films. Aligned PEDOT-carbon nanotube composite thin films were fabricated and characterized to study both the alignment of the polymer chains and change in their electrical conductivity. This research utilized the bottom up self organized molecular system templates to control the nano structure and ordering of the polymer–carbon nanotube composite. Liquid crystal template was used to capture all the monomer (3,4-ethylenedioxythiophene) EDOT molecules within the cylindrical cores of hexagonal mesophase oriented in effective net direction within domains, and the monomers were electro-polymerized to obtain aligned polymer chains. This aligned structure renders better anisotropic electrical conductivity along the polymer chain direction. A non percolated dispersion of carbon nanotubes and dopants was incorporated into the aligned PEDOT thin film by spraying as well as internally dispersing within the liquid crystal network before polymerization. The carbon nanotube and dopant incorporation into the aligned PEDOT thin films increased the electrical conductivity by about two orders of magnitude

Surface Engineering Solutions for Immersion Phase Change Cooling of Electronics

Micro- and nano-scale surface modifications have been a subject of great interest for enhancing the pool boiling heat transfer performance of immersion cooling systems due to their ability to augment surface area, improve wickability, and increase nucleation site density. However, many of the surface modification technologies that have been previously demonstrated show a lack of evidence concerning scalability for use at an industrial level. In this work, the pool boiling heat transfer performance of nanoporous anodic aluminum oxide (AAO) films, copper oxide (CuO) nanostructure coatings, and 1D roll-molded microfin arrays has been studied. Each of these technologies possess scalability in production, thus making them a subject of great interest to industry. To evaluate each surface modification technology, a custom pool boiling setup filled with 3MTM NovecTM HFE-7100 dielectric fluid was utilized. The pool boiling setup was autonomously operated by computer control using a custom LabVIEWTM program. Compared to natively oxidized aluminum samples, AAO samples showed improvements in surface area, but not in wickability or nucleation site density, allowing for the isolated study of the influence of increased surface area on pool boiling performance. Serving as an inverse analogue to nanoporous AAO films, protrusive CuO nanostructure coatings were shown to offer improvements in critical heat flux (CHF), wettability, and nucleation activity over their natively oxidized copper counterparts. At the micro-scale, 1D roll- molded microfin arrays were shown to have improved CHF and nucleation activity over their planar counterparts. Following the initial pool boiling evaluation of each surface finish, the practical applicability of 1D roll-molded microfin arrays was demonstrated through a comparative study of cooling solutions for a field-programmable gate array (FPGA). In this study, the junction-to-ambient thermal resistance for an immersion cooling configuration that utilized a mounted 1D roll-molded microfin array surface was found to be lower than that of both a conventional forced-air cooling system and an immersion cooling configuration with no mounted surface. This finding highlights the significance of 1D roll-molded microfin array surfaces as an industrially acceptable means of improving the capabilities of immersion cooling systems

Fluorescence microscopy: Established and emerging methods, experimental strategies, and applications in immunology

Cutting-edge biophysical technologies including total internal reflection fluorescence microscopy, single molecule fluorescence, single channel opening events, fluorescence resonance energy transfer, high-speed exposures, two-photon imaging, fluorescence lifetime imaging, and other tools are becoming increasingly important in immunology as they link molecular events to cellular physiology, a key goal of modern immunology. The primary concern in all forms of microscopy is the generation of contrast; for fluorescence microscopy contrast can be thought of as the difference in intensity between the cell and background, the signal-to-noise ratio. High information-content images can be formed by enhancing the signal, suppressing the noise, or both. As improved tools, such as ICCD and EMCCD cameras, become available for fluorescence imaging in molecular and cellular immunology, it is important to optimize other aspects of the imaging system. Numerous practical strategies to enhance fluorescence microscopy experiments are reviewed. The use of instrumentation such as light traps, cameras, objectives, improved fluorescent labels, and image filtration routines applicable to low light level experiments are discussed. New methodologies providing resolution well beyond that given by the Rayleigh criterion are outlined. Ongoing and future developments in fluorescence microscopy instrumentation and technique are reviewed. This review is intended to address situations where the signal is weak, which is important for emerging techniques stressing super-resolution or live cell dynamics, but is less important for conventional applications such as indirect immunofluorescence. This review provides a broad integrative discussion of fluorescence microscopy with selected applications in immunology. Microsc. Res. Tech., 2007. © 2007 Wiley-Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/56150/1/20455_ftp.pd