White LED (WLED) is a new generation of solid-state green light source. It has many advantages such as energy saving, environmental protection, small volume, high light efficiency, stable performance and so on.
At present, there are three ways for WLED to achieve white light in PC/MC mode: 1) blue LED chip + yellow phosphor; 2) purple LED chip + Red + Green + blue tribasic phosphor; 3) blue LED chip + green LED chip + Red LED chip. Among the three ways to achieve white light, the most economical and practical way to achieve industrialization is to coat blue LED chips with yellow phosphor. The light efficiency of WLED using this way is up to 250 lm/W. As the market competition of lighting terminal products becomes more and more fierce and the heat dissipation environment of lighting lamps becomes worse and worse, the LED light source should have better thermal characteristics to meet the market demand. The thermal characteristics of LED light source are usually characterized by the cold-heat ratio of light output. The cold-heat ratio of light output of WLED, i.e. the ratio of photoelectric parameters (light flux) of LED light source at high temperature to photoelectric parameters (light flux) at room temperature, can be used to verify the thermal stability of LED light source.
In WLED light source, phosphor plays a vital role in the realization of white light. Phosphors are generally inorganic luminescent materials with orderly crystal structure. The stability of their physical and chemical properties is related to the following factors: material system, dispersion coefficient, powder compatibility, powder morphology. The influencing factors of the cold-heat ratio of WLED light output are related to the materials of WLED devices. Fluorescent materials are the key materials in the devices mentioned above. The physical properties of phosphors (material system, dispersion coefficient, powder compatibility and powder morphology) have not been reported. It is also very important to solve the thermal characteristics of LED light source. Therefore, it is of practical significance to discuss the relationship between the physical properties of phosphors and the cold-heat ratio of WLED light output. At the same time, it has certain significance for the subsequent product design. Guiding role.
In this paper, SMD 2835 package, blue light chip, emission band 450-455 nm, each LED light source has three series LED chips, phosphor scheme consists of YAG yellow fluorescent material, nitride red fluorescent material and Ga-YAG/LuAG yellow green fluorescent material. Each group only changed the type of yellow and green powder and fixed the amount of glue and the other two phosphors, and each LED light source had the same amount of dispensing. The ratio of yellow, red and yellow-green phosphors to glue is yellow:red:yellow-green:glue=0.50:0.15:1.5:1. Five samples with the same ratio of phosphors are selected for the test. The test conditions are pulse current 100 mA, the test temperature is 25, 50, 75, 85, 95, 105, and the average value of light flux is obtained. Powder parameters testing equipment: particle size is tested by laser particle size analyzer, thermal quenching performance and excitation emission spectrum are tested by Fluoromax-4; particle SEM morphology is tested by scanning electron microscopy; packaging equipment: ASM crystallization machine, ASM welding wire machine, vacuum defoamer, Wuzang dispensing machine. Photoelectric parameter testing equipment for packaging products: remote integrating sphere tester.
3. RESULTS AND DISCUSSIONS
Phosphors are generally inorganic materials. According to their matrix classification, the commonly used systems are aluminates, nitrides/nitrogen oxides, silicates, fluorides and so on. Fig. 1.1 is the thermal quenching performance of fluorescent powders in different systems. It can be seen that the thermal stability of aluminates in several systems is the best. The thermal stability of fluorides and silicates is poor. The thermal stability of nitrides is worse than that of aluminates, but better than that of fluorides and silicates.
Fig. 1.1 Thermal quenching properties of phosphors in different systems
Therefore, this paper takes aluminate system as the research object. The typical representative of aluminate system is YAG. Its chemical formula is Y3Al5O12:Ce. The crystal structure belongs to cubic system. The lattice constant is 1.2002 nm. The crystal structure of YAG is shown in Figure 1.2. From the crystal structure, it can be seen that there are three kinds of polyhedrons in the space composed of Y, Al and O: dodecahedron (Fig. 1.2a), octahedron (Fig. 1.2b), tetrahedron (Fig. 1.2c). The coordination numbers of oxygen atoms are (Y33+) octahedron, (Al23+) hexahedron, (Al33+) tetrahedron, respectively.
Figure 1.2YAG crystal structure diagram
3.1 Effect of Phosphor Material on Cold/Heat Ratio of WLED Light Output
In this experiment, Ga-YAG and LuAG yellow-green powders were used as the research objects. The crystal structure of Ga-YAG and LuAG belong to Yttrium aluminium garnet as shown in Figure 1.1. The general chemical formula of Yttrium aluminium garnet is as follows:
(RE1-rSmr) 3 (Al1-sGas) O12:Ce (1) formula (1), RE = La, Lu, Y, Gd, Sc, 0 < R < 1, 0 < s < 1. Generally speaking, Ga-YAG and LuAG belong to the cubic system, but their cell parameters are different. Ga-YAG is the partial substitution of Ga3+ for Al3+, while LuAG is the complete substitution of Lu3+ for Y3+. The ion radii of LuAG are rGa3+ (eight coordinates) = 0.69, rY3+ (eight coordinates) = 1.04, rAl3+ (six coordinates) = 0.62, r3+ (six coordinates) = 1.001. Combining with the matching degree of ion radius, the thermal stability of crystal structure formed by complete substitution is better than that by partial substitution in theory. From the material point of view, the thermal stability of the material itself can be characterized by thermal quenching performance.
The relationship between the thermal quenching properties of GRF-G and GRF-L is shown in Fig. 1.3. It can be seen that the brightness attenuation of GRF-L decreases with the increase of temperature, and the thermal quenching performance of GRF-L is better than that of GRF-G.
Figure 1.3 Thermal quenching properties of GRF-G and GRF-L
In the experiment, Ga-YAG and LuAG are GRF-G and GRF-L, respectively. The morphology of Ga-YAG and LuAG under electron microscope is shown in Figure 1.4. It can be concluded that the morphologies of GRF-G and GRF-L are spherical and smooth.
Figure 1.4 SEM morphologies of GRF-G and GRF-L, respectively
GRF-G and GRF-L were used as yellow and green powder to encapsulate 2835 finished lamp beads. The change between the luminous flux of the finished lamp beads and the test temperature was shown in Fig. 1.5. It can be concluded that the heat/cold ratio of the luminous flux decreases gradually with the increase of temperature. The output heat/cold ratio of WLED at 85 C is better than that of GRF-G.
Fig. 1.5 Cold-heat ratio of GRF-G and GRF-L WLED light output
GRF-G and GRF-L WLED light output cold-heat ratio, GRF-L is better than GRF-G, which is related to the thermal quenching performance of fluorescent materials and the structure of the powder itself. Therefore, different materials of optical conversion materials (partial and complete substitution) have an impact on the light output cold-heat ratio of WLED.
Effect of 3.2 Fluorescent Powder's Dispersion Coefficient on the Cold/Heat Ratio of WLED Light Output
Discrete coefficient refers to the measurement of relative width or inhomogeneity of particle size distribution of phosphor sample. It is defined as the ratio of the distribution width to the central particle size, in which the distribution width is the difference of a group of characteristic particle sizes of the boundary particle size. Generally, the dispersion coefficient is expressed as follows:
Formula (2) S denotes the dispersion coefficient, and d10, D50 and D90 denote the particle size of 10%, 50% and 90% corresponding to the cumulative volume distribution of the powder, in unit um. D50 denotes the median particle size of the powder. Generally speaking, the smaller the S value is, the more concentrated the size distribution of the powder particles is. The number of defects on the surface of the particles in a unit volume is almost the same. There is no difference in the heating performance and the better the thermal stability. In this experiment, GRF-S, GRF-M and GRF-B were used as yellow-green powder, which were packaged in the same scheme with yellow powder and red powder respectively. The dispersion coefficients of GRF-S, GRF-M and GRF-B were 0.925, 1.125 and 1.325, respectively. Figure 1.6 shows the thermal quenching performance of GRF-S, GRF-M and GRF-B with different discrete coefficients. It can be seen that the brightness of the fluorescent materials decreases continuously with the increase of temperature. Among them, GRF-B has the greatest attenuation, followed by GRF-M, and GRF-S has the smallest thermal quenching performance. Among them, GRF-S has the best thermal quenching performance. Therefore, from the powder point of view, the thermal quenching performance of the powder with small dispersion coefficient is better, which is consistent with the above analysis conclusion.
Fig. 1.6 Thermal quenching properties of GRF-S, GRF-M and GRF-B
In this paper, the influence of discrete coefficients on the cold-heat ratio of WLED light output is studied. The package form of 2835 is adopted. The target parameters are Ra=80-82 and CCT=3000K. The same package scheme is used to verify the relationship between different discrete coefficients and the cold-heat ratio of WLED light output. Figure 1.7 shows the relationship between GRF-S, GRF-M and GRF-B light output with different discrete coefficients. With the increase of temperature, the finished lamp beads are obtained. The cold-to-heat ratio of light flux is constantly smaller, GRF-S, GRF-M and GRF-B have the largest attenuation in finished products, GRF-B takes the second place and GRF-S is the smallest. This shows that the WLED of GRF-S has the best cold-to-heat ratio of light output and the WLED of GRF-B has the worst cold-to-heat ratio of light output. Therefore, different dispersion coefficients have an effect on the cold-to-to-to-heat ratio of light output of WLED. The smaller the dispersion coefficient, the better the cold-to-to-to-heat ratio of light output of WLED.
Figure 1.7 Cold-heat Ratio of WLED Light Output for GRF-S, GRF-M and GRF-B
Effect of 3.3 Powder-Rubber Compatibility on the Cold/Heat Ratio of WLED Light Output
In order to improve the stability of phosphors, some post-treatment processes are usually used, such as secondary quenching, coating, etc. The coating process is mostly used, and the coating material is SiO_2, etc. However, even with this process, the thermal stability of phosphors is often unsatisfactory, especially in the light output cold-heat ratio of WLED. Generally, when phosphor is mixed with encapsulation glue in the encapsulation process, there may be some voids on the contact surface between particle surface and colloid, which may contain unexpired air, so that the thermal stability of finished products will be affected when heated, in order to solve this problem. Relevant manufacturers have proposed a new post-treatment process, which contains a special layer of material on the surface of phosphor particles by certain coating means. After special treatment, phosphor particles will quickly coagulate into a large particle in water, thus preventing water from entering. After this process, the particles will be packaged tightly in the package colloid when combined with the package colloid. There is no void on the surface of particles. Increasing the compatibility of powder and glue can theoretically improve the cold-heat ratio of WLED light output.
Fig. 1.8 Thermal quenching properties of RF-G and CRF-G
In this paper, we use 2835 package form with Ra=80-82 and CCT=3000K as the target parameters. The same package scheme is used to verify the effect of phosphors with improved powder compatibility and without improved powder compatibility on the cold-heat ratio of light output of WLED. The above two are expressed as CRF-G and RF-G, respectively. Fig. 1.8 shows the thermal quenching performance of RF-G and CRF-G. It can be seen that the luminance of phosphors decreases with the increase of temperature. The decreasing range of CRF-G is smaller than that of RF-G, which indicates that the thermal stability of CRF-G is better than that of RF-G.
Figure 1.9 Cold-heat Ratio of WLED Light Output for RF-G and CRF-G
In this paper, the effect of powder-glue compatibility on the cold-heat ratio of WLED light output is studied by using 2835 package. The target parameters are Ra=80-82 and CCT=3000K. The same package scheme is used to verify the effect of powder-glue compatibility on the cold-heat ratio of WLED light output. Figure 1.9 shows the relationship between CRF-G of improved powder-glue compatibility and WLED light-output cold-heat ratio of RF-G without improved powder-glue compatibility. With the increase of temperature, the Cooling-Heat ratio of the pearlescent light output of WLED is smaller, and the attenuation range of CRF-G and RF-G is the largest, followed by CRF-G. It shows that the Cooling-Heat ratio of CRF-G's WLED is better, and that of RF-G's WLED is poorer. Therefore, the compatibility of powdered glue has an effect on the Cooling-Heat ratio of WLED's light output. The compatibility of phosphor is better than that of WLED.
Effect of 3.4 Phosphor Morphology on Cold/Heat Ratio of WLED Light Output
The completeness and smoothness of the particle morphology of phosphors have some influence on their stability. In the synthesis process of high temperature solid phase method, solid powder will undergo phase transformation under the protection of high temperature and high pressure gas, and then solid phase reaction will occur. Finally, under the conditions of the optimum synthesis temperature and the optimum synthesis time, a new solid phase crystallization will be formed. This phase will undergo the crushing process to form a certain particle size phosphor. Generally, in ball mill, prolonging crushing time and increasing ball milling speed cause the smallest particle surface to produce crushing marks, adhere to certain crushing debris or particles are directly split into flakes, which makes the morphology integrity and smoothness of powder particles different. Through the abnormal ball milling crushing process, the particle morphology of phosphor powder is irregular or there are cracks on the surface of the particle. As shown in Figure 2.0, the left picture is the particle morphology of phosphor powder crushed by intense ball milling, and the right picture is the particle morphology of phosphor powder crushed by normal ball milling process. Through the above analysis, it can be inferred that the thermal stability of phosphor powder crushed by intense ball milling is better than that of phosphor powder crushed by normal ball
Figure 2.0 SEM morphologies of GRF-N and GRF-V, respectively
In this paper, we use 2835 package form with Ra=80-82 and CCT=3000K as the target parameters. The same package scheme is used to verify the effect of the phosphor after strong crushing treatment and normal crushing treatment on the cold-heat ratio of WLED light output. The above two are expressed as GRF-N and GRF-V, respectively. Figure 2.1 shows the thermal quenching performance of GRF-N and GRF-V. It can be seen that the luminance of GRF-V decreases with the increase of temperature. The decreasing range of GRF-V is smaller than that of GRF-N, which indicates that the thermal stability of GRF-V is better than that of GRF-N.
Figure 2.1 Thermal quenching properties of GRF-N and GRF-V
In this paper, the effect of intense crushing and normal crushing on the cold-heat ratio of WLED light output is studied. The package form of 2835 is adopted. The target parameters are Ra=80-82 and CCT=3000K. The same package scheme is used to verify the effect of intense crushing and normal crushing on the cold-heat ratio of WLED light output. Figure 2.2 shows the relationship between CRF-N and GRF-V of post-processing. With the increase of temperature, the ratio of cold to hot state of pearlescent flux of finished lamp decreases continuously. The attenuation of CRF-N and RF-V in finished lamp is larger than that of CRF-V. It shows that the output of GRF-V WLED is better than that of GRF-V WLED, and the output of GRF-N WLED is worse. Therefore, the intensive crushing process has an effect on the output of WLED, and the phosphor after intensive crushing process is more normal than that after intensive crushing process. The WLED light output cold-heat ratio of the phosphor in the crushing process is worse.
Figure 2.2 The relationship between heat/cold ratio of WLED light output of GRF-V and GRF-N
In this paper, SMD 2835 is used as the packaging form. Different materials of phosphors, phosphors with different dispersion coefficients, phosphors with different compatibility of powder glue and phosphors with different morphologies are used as yellow-green powder for packaging experiments. The following conclusions can be drawn: LuAG material, small dispersion coefficients, better powder glue compatibility, good particle morphology of phosphors packaging LED light output. Cold and heat are better.
Therefore, the physical properties of phosphors have an effect on the cold-heat ratio of WLED light output. This conclusion can be used as a basis for powder control and product optimization. At the same time, it has theoretical guiding significance and practical reference value for the product design of WLED.
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Authors: Xiao Haitao, Pei Xiaoming, Cai Jie
Source: Shenzhen Ruifeng Optoelectronics Co., Ltd.