Application of ceramic components in LED lighting
Time: 2019-03-05 Writer: mingrui
Application of ceramic components in LED lighting
Led is a new type of solid state light source, which has attracted great attention since its appearance.Its luminescence mechanism is generated by the transition of electrons in the PN junction between energy bands.Under the action of the external electric field, the radiation recombination of the electrons and the holes has an electroinduced effect, and some energy is converted into light energy, while the lattice vibration generated by the non-radiation recombination converts the rest energy into heat energy.
Ceramic materials prepared by modern technology have the characteristics of good insulation, high thermal conductivity, high infrared emissivity and low expansion coefficient, which can be a new material for LED lighting.At present, ceramic materials are mainly used in LED packaging chip heat sink materials, circuit substrate materials and lamp radiator materials.
Heat transfer mechanism of ceramic materials
Ceramic is a nonmetallic material with no free electrons in the crystal structure and excellent insulating properties.Its heat transfer belongs to the phonon thermal conductivity mechanism, when the lattice is complete and without defects, the greater the average free path of phonon, the higher the thermal conductivity.The theory indicates that the maximum thermal conductivity of ceramic crystal materials can be up to 320W/mK.
It is generally believed that the structural defects are the main factors affecting the thermal conductivity of ceramic materials.In the sintering process, oxygen impurities enter into the ceramic lattice, accompanied by vacancy, dislocation, antiphase domain boundary and other structure defects, significantly reduce the phonon average free path, resulting in the reduction of thermal conductivity.Modern ceramic technology by generating the second phase, the oxygen on the grain boundary, reduced the possibility of oxygen impurity into the lattice, as the grain boundary of oxygen concentration is greatly reduced, grain internal spontaneous diffusion of oxygen to the grain boundary to reduce grain of oxygen content in the matrix, the amount and type of defects, thereby reducing the chances of phonon scattering and increase the phonon mean free path.Due to the different preparation technologies, the thermal conductivity of ceramic materials is also different. The thermal conductivity of common ceramic materials is shown in table 1.
The thermal conductivity of ceramic materials is also closely related to the content of additives.Liang guangchuan et al. of hebei university of technology also conducted an experimental study on the relationship between the content of rare earth oxide Y2O3 and the density and thermal conductivity.They used an aluminum nitride (AlN) ceramic powder with an average particle size of 3m, an oxygen impurity content of 0.97wt%, and an additive purity of 99.95% Y2O3.
The relationship between the content of Y2O3 and the thermal conductivity of the powder is shown in figure 1 after sintering and polishing (0.25m) in the atmospheric pressure nitrogen environment.It can be seen from figure 1 that adding a proper amount of rare earth oxide Y2O3 can make the thermal conductivity of aluminum nitrite ceramics reach around 160W/mK, which has exceeded the thermal conductivity of die-cast aluminum ADC12 (the thermal conductivity of ADC12 is 96.2w /mK) and can be used as the manufacturing material of heat diffuser.
Aluminum nitride ceramic has low expansion coefficient and high thermal conductivity, which is often used as heat sink of chip package.A major bottleneck of LED heat dissipation is the circuit substrate. The thermal conductivity of ordinary aluminum substrate is only 1.0~ 2.5w /mK, which is less than 20% of the ceramic substrate (as shown in figure 2). The use of ceramic substrate can significantly reduce the PN junction temperature of LED (hereinafter referred to as junction temperature).
Ceramic substrates can be made by casting or eutectic sintering, but the price is higher and large-scale application is premature.Ceramic is used as heat sink component in chip packaging. Due to its simple geometry, some LED packaging manufacturers have started to use it.Afore-mentioned two basically are to use the conductivity of material conduct heat to go up to radiator, need not consider how to send out heat almost in the air, what care when the design is its conduct heat coefficient.
The radiators of LED lamps are used to dissipate heat into the surrounding space, often using alumina (Al2O3) ceramic materials (as shown in figure 3).Alumina ceramics are cheap, mature technology, die casting sintering technology, design freedom, low price, at this stage has been a certain scale of application, the following will be analyzed in detail.
Thermal radiation mechanism of ceramic materials
We know that the basic ways of heat exchange are conduction, convection and radiation.In order to dissipate heat effectively, people often reduce the thermal resistance of heat flow path and strengthen convection coefficient, but ignore heat radiation.LED lamp is provided use natural convection to come loose heat commonly, radiator delivers the heat that LED produces to radiator surface quickly, because convection coefficient is lower, quantity of heat cannot send out in time in the air around, bring about surface temperature to rise tall, the working environment of LED deteriorates.Improving the emissivity can effectively take the heat of radiator surface away through the form of thermal radiation. The general aluminum radiator can improve the surface emissivity through anodic oxidation. The ceramic material itself can have the characteristics of high emissivity without the need for complicated follow-up treatment.
The radiation mechanism of ceramic materials is produced by the disresonance effect of random vibration.High radiation ceramic materials such as silicon carbide, metal oxide, boride infrared active polarity are very strong in vibration, the vibration due to the polarity has the greatly strengthened the anharmonic effect, the absorption coefficient of the double frequency and frequency area, generally with 100 ~ 100 cm - 1 order of magnitude, absorb the equivalent of moderate intensity area in the area of residual reflection with low reflectivity, therefore, is beneficial to form a relatively flat strong radiation belts.
Generally speaking, the radiation band with high thermal radiation efficiency is generally extended from the strong resonance wavelength to the short-wave entire two-phonon combination and frequency region, including part of the multi-phonon combination region, which is the common characteristic of most high-radiation ceramic material radiation bands. It can be said that the strong radiation band mainly originates from the two-phonon combination radiation of this band.With a few exceptions, the radiation bands of general radiating ceramics are concentrated in the diphonon and triphonon regions larger than 5m.Therefore, for infrared radiation ceramics, the radiation in the band 1-5m mainly comes from the in-band transition of free carriers or the direct transition of electrons from the impurity level to the conduction band, and the radiation in the band larger than 5m is mainly attributed to the combined radiation of diphonons.
Liu Weiliang, LuoSuMing made research for room temperature infrared radiation ceramics, ceramic sample testing infrared radiation rate is about 0.82 ~ 0.94, the different surface quality of the far-infrared ceramic glaze were tested, the emissivity is about 0.6 ~ 0.88, and the fracture surface SEM photos of far-infrared ceramic powder is added in the glaze of 10 wt % of radiation performance, the glaze quality, color, and the cost is better, the emissivity is 0.83, other performance meet the national porcelain standards requirements.Cui wanqiu and wu chunyun tested the low-temperature far-infrared ceramic block samples, and the infrared emissivity was 0.78~0.94.Li hongtao and liu jianxue found that the radiation rate of far-infrared ceramics at room temperature is generally up to 0.85, and the highest radiation rate of Enecoat glaze coating abroad is up to 0.93~0.94.Many studies show that ceramic material or glaze itself has a high infrared emissivity, which is an important parameter to replace the traditional aluminum radiator.
Study on LED lighting of alumina ceramic materials
3.1 experimental test of ceramic LED lamps
The thermal conductivity of alumina ceramics is strongly related to the composition (purity) of alumina (see table 2).The commonly used Nom.95% alumina ceramics (referred to as 95 ceramics for short) has a thermal conductivity of about 22.4W/mK and a pressure resistance of 10kV/mm. The sample made of this LED luminaire is shown in figure 4.
3.2 computer simulation of ceramic LED lamps and aluminum die-casting LED lamps
In order to study and design ceramic LED lamps, we use computer software for simulation analysis.The flow field analysis software used in this study is flo-efd (EFD for short), which is the flagship product of NIKA and mainly used in automobile, aerospace, machinery, ship, electronic communication, medical equipment, energy and chemical industry, hvac, fluid control equipment, LED semiconductor industry, etc..The software can be used for thermal analysis of various LED packaging products, aerospace lights, various energy-saving lights, LED light emitting tubes, vehicle lamps, display screens, etc.
To facilitate comparing with experimental test, the computer simulation analysis, the ambient temperature of 15 ℃, the temperature distribution as shown in figure 5 (for easy viewing, hide the lens and the fixed part).In order to compare the thermal performance of 95 ceramic lamps with that of aluminum die-casting lamps, the temperature distribution obtained by computer simulation is shown in figure 6 (the radiator material of the lamps is DC12 aluminum alloy, the lamp holder is PBT plastic, and the remaining parameters remain unchanged).
3.3 result analysis
The lamp holder of ceramic lamps and lanterns is 95 ceramic material (the lamp holder of aluminum die casting lamps and lanterns is PBT plastic), each component has been fully utilized.Experimental tests, 1.0 h basic heat balance, the environment temperature of arithmetic average is about 14.4 ℃, the experimental test and computer simulation analysis was carried out on the temperature distribution in the value of comparison, the results are shown in table 4.
The results of computer analysis show that under natural convection, the thermal performance of 95 ceramic lamps and lanterns is no less than that of aluminum die-casting lamps and lanterns, ceramic lamps and lanterns can make full use of the geometric characteristics of each part, so the overall temperature of the lamps and lanterns reduced to a lower level.
Ceramic materials used in LED lighting prospects
The use of ceramics has a long history. Ceramic materials prepared by modern technology have a high thermal conductivity, and the air flows down naturally, which can be used as the cooling material of LED lighting lamps.Aluminum nitride ceramics can be directly used as sealing crystal frame or line layer;Alumina ceramics is cheap, sintering technology is mature, can be glazed into different colors, due to its excellent electrical insulation performance, and acid resistance, by many customers.However, ceramic material is not perfect, ceramic radiator fin can not be too thin (thickness 1.5mm), slightly higher density (about 1.5 times of aluminum), in high stress will produce cracks, unglazed surface pollution.
In general, ceramic materials are promising for LED applications, especially for smaller lighting fixtures.
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