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Research On Thermal Characteristics And Reliability Of The Key Interface Structure Of LED

Posted on:2016-07-09Degree:DoctorType:Dissertation
Country:ChinaCandidate:D J LiuFull Text:PDF
GTID:1108330482959885Subject:Mechanical Manufacturing and Automation
Abstract/Summary:PDF Full Text Request
With the rapid development of electrionic technology, the electronic devices tend to be highly integrated and micro-scale degree. In the micro-system, with more numbers of interface structures which interconnect the device, the energy must be consumed when transferred. So the following thermal design will become significantly in the electronic devices. In the trend of energy conservation and emission reduction, LED which gets a new star leads the illuminating industry by virtue of energy conversation, environmental protection and non-pollution. So far, with about 70%~80% of energy transforming into heat energy, LED only has about 20%~30% of photoelectric conversion efficiency which impedes the widespread application LED. Therefore thermal reliability problem remains an important problem which should be addressed urgently. The development of nanotechnology promotes the new material technology. So much attention has been paid on the graphene material which is low-dimensional nanostructure by virtue of its speciality in thermology, mechanics, photology and electrology. In the thermology, the theoretical research and numerical simulation in graphene nanoribbons and graphene system were studied, the physical mechanism of heat conduction was understood, which made possible for the application of electronic devices,In this paper, the LED was chosen as a research object. In view of thermal reliability problem, firstly the thermal transfer characteristics were analyzed by the theoretical analysis and the experimental method in the macroscopic scale. And then the nonequilibrium molecular dynamics method was preliminary used to explore the high thermal conductive material which is suitable for the LED in nano scale. Last but not least, a multi-scale model of LED was constructed. The important conclusions and innovative achivements of this work are as following:(1) Based on system structure the LED mathematical model in the power type was built and the LED thermal transient test system was established. The thermal transport characteristics of LED on the different location of the cold plate were studied by theoretical calculation and experimental test.The theoretical mathematical model of LED was built by the thermal analysis software Flotherm to study the temperature distribution of LED in the different places of the cool plate. Then the relation of the junction temperature and the substrate were analyzed. After that, thermal transient test system was established to extract the cumulative structure function and differential structure function, then the entire thermal resistances of A, B, C and D was obtained being 26.19K/W,26.45K/W,26.94K/W and 27.06K/W respectively. Comparing the numerical simulation and the experiment, the heat effect of the heat conduction material in the transmission channel was revealed, which provides the theoretical basis for seeking the high thermal interface materials or the new interface structure.(2) The theoretical thermal transport model of graphene nanoribbon was built. The thermal conductivity value was calculated under different conditions and status. The local postion phonon spectra of graphene nanoribbon were calculated and drawed. And the thermal transport mechanism was explored and analyzed. This purpose of this section is searching for new material for high efficient thermal transport interface structure design and manufacture of LED device.The mathematics model of the defect graphene nanoribbon was established by Material Studio software.The molecular dynamic simualtion tool is LAMMPS software. The defect locations which have the effect on the heat transfer of single-layer graphene nanoribbons in the horizontal direction and vertical direction were investigated. When the vertical distance being 1.065nm, the defect location moved from the left to right, the thermal conductivity droped to 73.17W/mK and then it will increase to 80.09W/mK. At the same time, five different vertical distances were analyzed to find the reliable relationship which decreased at the very start and then increased. The tendency is like the bathtub curve. When the horizontal distance being 4.059nm, the defect location moved from down to up, the tendency of thermal conductivity was also decreasing at the very start and then increasing. It is cyclical variation. The phonon spectra of different places were calculated and drawed. By analyzing the phonon overlaps, it was thought that when the temperature was lower, on the defect location in the horizontal direction, the phonon cannot go through the defect. As the temperature and the frequent phonon increasing. The high frequent phonon will cause the tunnel effect. The effect of the defect on the thermal conductivity in the vertical direction was less than that of the horizontal direction. At the same time, it was thought a cyclical variation in the vertical direction had something with the chiral zigzag’s characteristics of graphene nanoribbon.(3) The heterostructure interface thermal transport theoretical model of graphene and silicon was established. The effects of temperature, size and different doped elements on the thermal transport characteristics of graphene heterostructure was investigated. The purpose of this section is to search for a new type heat transport mode and to match rules for highly efficient thermal transport interface structure design and manufacture of LED device.The graphene and silicon heterostructure interface model was built by using the Material Studio software. The thermal transport characteristics of graphene and silicon heterostructure interface were studied by molecular dynamics method. It was found that when the temperature was between 300K-800K, the thermal conductivity of a perfect heterostructure showed a decreasing tendency. However, when the temperature was between 300K-500K, the thermal conductivity showed slowly rising tendency.That is to say, the thermal conductivity of perfect graphene and silicon heterostructure interface showed slowly risng tendency at the start and then downtrend, which showed temperature dependence. When the defect area of heterostructure’s contact was 3%, the characteristic of heat transferring was similar to the perfect heterostructure. With the increase of the defect area of heterostructure’s contact area, the whole showed a decreasing tendency. When the defect area of heterostructure’s contact reached to 35%, it was thought that the main reason for the decrease of thermal conductivity affected by the temperature was the coupling of the graphene phonon ZA mode and silicon wave.In the size, the different width heterstructure model was established. In the simulation size range, it was found that the thermal conductivity of heterostructure was less than that of the single layer, multilayer zigzag and armchair graphene nanoribbons. when the width of heterostructure being 2.71nm, the thermal conductivity of heterostructure surface increase with the increase of the size, which showed the size effect. When it is 100nm, the effect tended to be weak. Then the perfect heterostructure’s exponential function of thermal conductivity was obtained. The characteristics of different defect ratio graphene and silicon heterostructure with boron doping and nitrogen doping were investigated. With the increase of the defect ratio,the thermal conductivity of boron doping and nitrogen doping of heterostructure presents a downward trend. And the thermal transport characteristic effect to heterostructure with boron doping is higher than that of with nitrogen doping. It was analyzed that the difference between the atomic mass casued a certain degree anharmonic lattice vibration.(4) The mathematical theoretical multiscale model of key interface structure for LED was calculated. And the LED’s heat dissipation characteristics with different interface material, convection coefficient and input power were compared.The coupling MD/FE multiscale model of LED lamp was calculated. It was found that when the graphene structure material was the interface material, the steady temperature of the LED lamp droped by 8.4%. When the convection coefficient increased, the effect tendencies of graphene material and tin alloy solder’s on the lamp’s junction temperature were similar, which was much more than the conductive silver. The increase of power will reduce the life of LED rapidly. The result provides a new structure for the design of LED.
Keywords/Search Tags:LED, thermal resistance, interface structure, molecular dynamics, graphene, multiscale, thermal conductivity
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