PhD Dissertations

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Browsing by Institution Author "TAMDOĞAN, Enes"

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    Immersion cooling of suspended and coated nano-phosphor particles for extending the limits of optical extraction of light emitting diodes
    (2017-08) Tamdoğan, Enes; Arık, Mehmet; Arık, Mehmet; Yaralıoğlu, Göksenin; Başol, Altuğ; Budaklı, M.; Graham, S.; Mechanical Engineering; Department of Mechanical Engineering; TAMDOĞAN, Enes; Tamdoğan, Enes
    Energy efficiency, long life, exceptional color and performance of solid-state light (SSL) sources have resulted in a rapidly increasing trend in a number of practical applications especially for general lighting after a long history of incandescent lamps. However, LEDs, as a solid state lighting technology, have some limitations as typical electronics so thermal management is vital for LEDs. Moreover, predicting the life and the light quality is essential to assess and enhance the performance of LED performance. Specifically, to improve the heat dissipation one major parameter used to evaluate the LED performance is thermal resistance. The major obstacle in estimating the thermal resistance for LEDs is the accuracy of determining the junction temperature especially for high power LEDs. Ideally, the junction temperature is determined reliably by monitoring the device temperature at a position close to the junction. That could be achieved with small temperature sensors that are placed very close to the junction. But there are still physical limitations to this method due to the sensor itself would be larger than the junction, which would result in an additional error to the measurement and will not be very useful in most applications. On the other hand, to convert blue to white light, GaN LEDs are usually encapsulated with a phosphor-epoxy mixture that cures into a soft material at high temperatures. During LED operation, significant self-heating occurs causing the glass-like epoxy to undergo large displacements due to its high thermal expansion coefficient at a critical temperature. This enhanced displacement inside the LED package may fracture the gold wire bonds and ultimately lead to device failure. Thus, the inclusion of phosphor into a high brightness LED package is another complex task. In a typical 450 nm or 470 nm blue LED, YAG phosphor is introduced and combined with the blue emission to create what appears to the eye is white light. While, the common practice for phosphor carrier medium is typically a silicone with high refraction index, the geometry of the phosphor is a primary design variable and can be classified as dispersed (dispersed inside the liquid coolant as particles), remote (remote coated under the dome), and local (coated over chip). In each case the geometry greatly affects the ultimate optical output of the LED color qualities and conversion efficiency. Since there is limited information about individual losses in an LED system and there is no available correlation to predict the total losses, this study is for filling the gap for both of these fundamental problems; estimating the thermal resistance and increasing the ultimate optical output of an LED with different coating technologies. Thus, the junction temperatures are first measured in the current work with Raman Spectroscopy, Infrared (IR) imaging as well as Forward Voltage method (FVM) for a 455 nm bare blue LED chip (without any phosphor coating). Then, the same samples have been coated with a phosphor-epoxy (13%, 4300 CCT) mixture to convert blue light into white light. After that junction temperatures were measured experimentally with the previously mentioned three methods and compared to each other. While IR imaging shows better capability on capturing the possible hotspots over the surface, Raman and Forward Voltage methods were in reasonably good agreement on measuring the junction temperature for 455 nm blue (uncoated) LED chip. However, the measurements performed after coating have shown slightly different results with Infrared (IR) imaging and Raman method, while Forward Voltage method has still shown meaningful results for coated chips. To be able to have a reasonable comparison, all cases have been measured with FVM simultaneously with one other mentioned method as in couple. As the second phase of the work, after identifying the junction temperatures accurately, LED effective liquid cooling is examined since it is tackling a major challenge "hot phosphor losses" that provides unique information for both fundamental nano-fluid (phosphor based) heat transfer and improved light conversion efficiency. Thus, topside liquid cooling with optically-transparent liquids is utilized to reduce average chip temperatures and to improve the uniformity of chip and phosphor temperature, leading to higher light extraction efficiencies. Furthermore, computational models and experimental studies of heat transfer and optical behavior to validate modeling results were performed for three proposed coating configurations (coated over chip, dispersed inside the liquid coolant as particles and remote coated under the dome) with di-electric liquid cooling. While, the phosphor is in direct contact with the LED chip in the current coating applications, it has shown higher junction temperatures beside of lower conversion efficiency and possible color shifts. The dispersed phosphor idea inside the liquid coolant as particles has resulted with lower conversion efficiency beside of any important thermal enhancements on the LED junction. However, the phosphor in the remote coating system is not affected by the LED temperature and thus maintained a consistent conversion rate and overall color point. Moreover, the remote phosphor with immersion cooling system has extended the Lumen Extraction Limits of White LEDs in excess of 53%, as long as the remote-phosphor system is well designed.