Explain the effect of physical properties of phosphor on the light-to-heat ratio of white LED light output

1 Introduction

White LED (WLED) is a new generation of solid-state green light source, which has many advantages such as energy saving, environmental protection, small volume, high luminous efficiency and stable performance.

At present, there are three paths for WLED to realize white light in PC/MC mode: 1) blue LED chip + yellow phosphor; 2) violet LED chip + red + green + blue trichromatic phosphor; 3) blue LED chip + green LED chip + red LED chip. Among the three ways to realize white light, the most economical and practical way to achieve industrialization is to apply a yellow phosphor chip to a blue LED chip, and the WLED using this method has a luminous efficacy of 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 fixtures becomes worse and worse, LED light sources must have better thermal characteristics to meet the needs of the market. The thermal characteristics of an LED source are typically characterized by a light output cold to heat ratio. The light output cold-heat ratio of WLED, that is, the ratio of the photoelectric parameter (light flux) of the LED light source at high temperature to the photoelectric parameter (light flux) at normal temperature, can be used to verify the thermal stability of the LED light source.

In WLED sources, phosphors play a crucial role in the realization of white light. The phosphor is generally an inorganic luminescent material with an ordered crystal structure, and the stability of its physicochemical properties is related to the following factors: material system, dispersion coefficient, powder compatibility, and powder morphology. The influencing factors of the WLED light output cold-heat ratio are related to the WLED device material, which is a key material in the aforementioned devices. The physical properties of the phosphor (material system, dispersion coefficient, powder compatibility, powder morphology) have not been reported on the effect of WLED light output cooling and heating ratio, and the problem of solving the thermal characteristics of LED light source is also Therefore, it is important to explore the relationship between the physical properties of phosphors and the heat-to-heat ratio of WLED light output, and it has a certain guiding role for subsequent product design.

2. Experimental part

This article uses SMD 2835 package form, blue chip, emission band is 450-455nm, each LED light source has 3 LED chips in series, the phosphor scheme is composed of YAG yellow fluorescent material, nitride red fluorescent material and Ga-YAG/LuAG Yellow-green fluorescent material. Each set of experiments only changed the type of yellow-green powder and fixed the amount of glue and the other two phosphors, and each LED source had the same amount of dispense. The yellow, red and yellow green phosphors and glues are yellow: red: yellowish green: glue=0.50:0.15:1.5:1. Five samples of the same phosphor are selected for testing. The test conditions are pulse. The current was 100 mA, the test temperature was 25 ° C, 50 ° C, 75 ° C, 85 ° C, 95 ° C, 105 ° C, taking the average of the luminous flux. Powder parameter test equipment: particle size is measured by laser particle size analyzer, thermal quenching performance, excitation emission spectrum is tested by Fluoromax-4; particle SEM morphology is tested by scanning electron microscope; packaging equipment: ASM solid crystal machine, ASM wire Machine, vacuum deaerator, Musashi dispensing machine. Packaged finished photoelectric parameter test equipment: remote integrating sphere tester.

3. Results and discussion

Phosphors are generally inorganic materials. According to their matrix classification, commonly used systems are aluminates, nitrides/nitrogen oxides, silicates, fluorides, and the like. Figure 1.1 shows the thermal quenching performance of phosphors of different systems. It can be seen that the thermal stability of aluminate in the powders of several systems is the best, the thermal stability of fluorides and silicates is poor, and the heat of nitrides The stability is worse than that of aluminates but better than fluorides and silicates.

Figure 1.1 Thermal quenching performance of different system phosphors

Fig.1.1 The heat quenching properties of different system

Therefore, the aluminate system is the research object. The typical representative of the aluminate system is YAG, the chemical formula is Y3Al5O12:Ce, the crystal structure belongs to the cubic system, the lattice constant is 1.2002nm, and the crystal structure of YAG is shown in Figure 1.2. It can be seen from the crystal structure that there are three polyhedra in the space composed of Y, Al and O, namely: dodecahedron (Fig. 1.2a), octahedron (Fig. 1.2b), tetrahedron (Fig. 1.2c), The coordination number of the oxygen atom is (Y33+) octagonal, (Al23+) hexacoordinate, and (Al33+) tetracoordinate.

Figure 1.2 Schematic diagram of the crystal structure of YAG

Fig.1.2 The schematic of YAG crystal structure

3.1 The effect of the material of the phosphor on the cooling and heating ratio of the WLED light output

In this experiment, Ga-YAG and LuAG yellow green powder were used as the research object. The crystal structure of Ga-YAG and LuAG belonging to yttrium aluminum garnet is shown in Figure 1.1. The chemical formula of yttrium aluminum garnet is:

(RE1-rSmr)3(Al1-sGas)O12:Ce(1)

In the formula (1), RE = La, Lu, Y, Gd, Sc, 0 ≤ r < 1, 0 ≤ s ≤ 1. In general, Ga-YAG and LuAG belong to the cubic system, but their unit cell parameters are different. Ga-YAG is a partial substitution of Ga3+ for Al3+, while LuAG is a complete substitution of Lu3+ for Y3+. The ionic radius is: rGa3+ (eight coordination) = 0.69 ?, rY3 + (octa coordination) = 1.04 ?, rAl3 + (six-coordinate) = 0.62 ?, rLu3 + (six-coordinate) = 1.001 ? [4]. Combined with the matching degree of the ionic radius, the theoretical complete replacement of the crystal structure formed by partial substitution is better. From the material point of view, the thermal stability of the material itself can be characterized by thermal quenching properties.

As shown in Figure 1.3, the relationship between the powder quenching performance between GRF-G and GRF-L shows that the temperature decay of the powder shows a downward trend with the increase of temperature, and the heat of GRF-L The quenching performance is superior to the thermal quenching performance of GRF-G.

Figure 1.3 Thermal quenching performance of GRF-G and GRF-L

Fig.1.3 The heat quenching properties of GRF-G and GRF-L

In the experiment, Ga-YAG and LuAG are GRF-G and GRF-L, respectively. The morphology under electron microscope is shown in Figure 1.4. It can be concluded that the particle morphology of GRF-G and GRF-L is approximately spherical and the surface is smooth.

Effect of Phosphor Physical Properties on the Cooling Ratio of White LED Light Output The Effect of Phosphor Physical Properties on the Cooling Ratio of White LED Light Output

The left and right images of Figure 1.4 show the SEM features of GRF-G and GRF-L, respectively.

Fig.1.4 The SEM morphology of GRF-G (left) and GRF-L (right)

GRF-G and GRF-L are used as yellow-green powder to package 2835 finished lamp beads. The change between the luminous flux and the test temperature of the finished lamp bead is shown in Figure 1.5. It can be concluded that the heat-to-heat ratio of the luminous flux gradually increases with the increase of temperature. Falling, the WLED light output at 85 ° C is better than GRF-L than GRF-G.

图1.5 GRF-G和GRF-L的WLED光输出冷热比

Fig.1.5 specific value of photoelectric parameterson ordinary and high temperature of light output with WLED of GRF-G and GRF-L

GRF-G和GRF-L的WLED光输出冷热比，GRF-L比GRF-G要好，这与荧光材料的热淬灭性能以及粉体本身的结构有关，因此不同材质的光转换材料(部分取代与完全取代)对WLED光输出冷热比存在影响。

3.2 Influence of the dispersion coefficient of phosphor on the cooling and heating ratio of WLED light output

The discrete coefficient refers to a measure of the relative width or unevenness of the particle size distribution of the phosphor sample. It is defined as the ratio of the distribution width to the center particle diameter, wherein the distribution width is the difference of a set of characteristic particle diameters of the boundary particle diameter, and the dispersion coefficient generally adopts the following expression:

S=(d90-d10)/d50 (2)[5]

In the formula (2), S represents a dispersion coefficient, and d10, d50, and d90 are particle diameters of the phosphors corresponding to 10%, 50%, and 90% of the volume cumulative distribution of the powder, respectively, and the unit is um, wherein d50 represents powder particles. The median particle size. In general, the smaller the S value, the more concentrated the powder particle size distribution, and the number of defects on the surface of the particle per unit volume is substantially the same, and the thermal performance is not different, and the thermal stability is better. 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 S of GRF-S, GRF-M and GRF-B were respectively For: 0.925, 1.125, 1.325. Figure 1.6 shows the thermal quenching properties of GRF-S, GRF-M, and GRF-B with different discrete coefficients. It can be seen that as the temperature increases, the brightness of the fluorescent material is continuously attenuated, and the attenuation of GRF-B is the largest. GRF-M is the second, GRF-S is the smallest, and the heat quenching performance of GRF-S is the best among the three. Therefore, from the powder point of view, the thermal quenching performance of the dispersion coefficient is small, which is consistent with the above analysis.

Figure 1.6 Thermal quenching performance of GRF-S, GRF-M and GRF-B

Fig.1.6 The heat quenching properties of GRF-S, GRF-M and GRF-B

In this paper, the effect of discrete coefficient on the cooling and heating ratio of WLED light output is studied. The 2835 package form is adopted, the target parameters are Ra=80-82, CCT=3000K, and the same packaging scheme is used to verify that different discrete coefficients are cold for WLED light output. The relationship between the heat ratio, Figure 1.7 shows the relationship between the cold-heat ratio of the WLED light output of GRF-S, GRF-M, and GRF-B with different discrete coefficients. With the increase of temperature, the ratio of the hot and cold state of the finished lamp flux is constantly increasing. Small, GRF-S, GRF-M, GRF-B attenuation amplitude in the finished product GRF-B is the largest, GRF-M is second, GRF-S is the smallest, indicating that GRF-S WLED light output is the best cold-heat ratio, GRF -B's WLED light output has the worst cold-heat ratio, so different dispersion coefficients have an effect on the WLED light output cold-heat ratio. The smaller the dispersion coefficient, the better the WLED light output cold-heat ratio.

1.7 GRF-S, GRF-M, GRF-B WLED light output cold-heat ratio relationship

Fig.1.7 specific value of photoelectric parameterson ordinary and high temperature of light output with WLED of GRF-S, GRF-M and GRF-B

3.3 Influence of compatibility of powder rubber on the heat-cooling ratio of WLED light output

In order to improve the stability of the product after the phosphor synthesis, a certain post-treatment process, such as secondary quenching treatment, coating process, etc., is often used. The coating process is mostly used, and the packaging material used is SiO2 or the like, but Even with such a process, its thermal stability is often reflected in the WLED light output cold-heat ratio. Generally, when the phosphor is mixed with the encapsulant during the encapsulation process, there may be a certain gap between the surface of the particle and the colloid, and the inside may contain unexhausted air, so that the thermal stability of the product may be affected when it is heated. In order to solve this problem. A related manufacturer has proposed a new post-treatment process, which contains a special layer of material on the surface of the phosphor particles by a certain coating method. After the special treatment, the phosphors will quickly condense into a large particle after being placed in the water. Thereby preventing moisture from entering, the particles processed by the process, when combined with the encapsulant, the encapsulant is tightly wrapped on the surface of the particles, there is no problem of voids, and the compatibility of the powder is increased. Theoretically, Can improve the WLED light output cold-heat ratio [6].

Figure 1.8 Thermal quenching performance of RF-G and CRF-G

Fig.1.8 The heat quenching properties of RF-G and CRF-G

In this paper, the 2835 package is adopted, the target parameters are Ra=80-82, CCT=3000K, and the same packaging scheme is used to verify the improvement of the compatibility of the powder and the improvement of the compatibility of the powder. The effects of the ratios are expressed as CRF-G and RF-G, respectively. Figure 1.8 shows the thermal quenching performance of RF-G and CRF-G. It can be seen that the luminescence brightness of phosphors tends to decrease with increasing temperature, and the decreasing range of CRF-G is smaller than that of RF-G. It shows that the thermal stability of CRF-G is better than RF-G in terms of phosphor itself.

Figure 1.9 RF-G and CRF-G WLED light output cold-heat ratio relationship

Fig.1.7 specific value of photoelectric parameterson ordinary and high temperature of light output with WLED of RF-G and CRF-G

In this paper, the effect of compatibility of powder rubber on the heat-cooling ratio of WLED light output is in the form of 2835 package. The target parameters are Ra=80-82, CCT=3000K, and the same packaging scheme is used to verify the compatibility of the powder. The effect of powder on the cooling and heating ratio of WLED light output, Figure 1.9 shows the relationship between the improved powder compatibility CRF-G and the unmodified powder compatibility RF-G WLED light output cold-heat ratio, with temperature The WLED light output of the finished lamp bead flux is colder and hotter, the attenuation amplitude of CRF-G and RF-G in the finished product is the largest, and the CRF-G is the second, indicating the WRF of CRF-G. The output is hot and cold, and the RF-G WLED light output is relatively cold and hot. Therefore, the compatibility of the powder has an effect on the heat-cooling ratio of the WLED light output. The phosphor with improved compatibility of the powder has no improved powder. The compatibility of the phosphor is better at the WLED light output.

3.4 Effect of phosphor morphology on the cooling/heat ratio of WLED light output

The completeness and smoothness of the particle morphology of the phosphor have a certain influence on its stability. In the synthesis process of high-temperature solid-phase method, the solid powder undergoes a phase change under the environment of high temperature and high pressure gas protection, and the solid phase reaction occurs from the solid phase to the solid solution state, and finally at the optimum synthesis temperature and most Under the condition of good synthesis time, a new solid phase crystal is formed, and the phase is subjected to a crushing process to form a phosphor of a certain particle size. The crushing process is generally carried out in a ball mill to prolong the crushing time and increase the ball milling speed to cause the smallest particle. The surface has broken marks, some cracked scraps or the particles are directly smashed into flakes, which makes the powder particles have different integrity and smoothness. Through the abnormal ball milling process, the particle shape of the phosphor is irregular or the surface of the particle is cracked. As shown in Fig. 2.0, the left image shows the shape of the phosphor particle after strong ball milling, and the right picture shows the fluorescence of the normal crushing process. The particle morphology of the powder, by the foregoing analysis, can be inferred that the strong fracture is better than the heat stability of the normally broken particles.

The left and right images of Figure 2.0 show the SEM features of GRF-N and GRF-V, respectively.

Fig.2.0 The SEM morphology of GRF-N (left) and GRF-V (right)

In this paper, the package type of 2835 is adopted, the target parameters are Ra=80-82, CCT=3000K, and the same packaging scheme is used to verify the influence of the phosphor powder on the cold-heat ratio of WLED light output through the intense crushing process and the normal crushing process. The two are represented 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 luminescence brightness of phosphors tends to decrease with increasing temperature, and the decreasing range of GRF-V is smaller than that of GRF-N. It shows that the thermal stability of GRF-V is better than that of GRF-N in terms of phosphor itself.

Figure 2.1 Thermal quenching performance of GRF-N and GRF-V

Fig.2.1 The heat quenching properties of GRF-N and CRF-V

In this paper, the effect of strong crushing process and normal crushing process on the cooling and heat ratio of WLED light output is 2835 package, the target parameters are Ra=80-82, CCT=3000K, and the same packaging scheme is used to verify the strong crushing process. The effect of process and normal crushing treatment on the hot and cold state, Figure 2.2 shows the relationship between the cold-heat ratio of the WLED light output of CRF-N and GRF-V in the post-treatment process, and the ratio of the cold and hot state of the finished lamp flux with the increase of temperature In the continuous reduction, the attenuation amplitude of CRF-N and RF-V in the finished product is larger, and CRF-V is second, which indicates that the WLED light output of GRF-V is better, and the WLED light output of GRF-N is better. The cold and heat are relatively poor, so the strong crushing treatment process has an influence on the cold-heat ratio of the WLED light output. The phosphor of the intense crushing process is inferior to the WLED light output of the normal crushing process.

Figure 2.2 Relationship between GRF-V and GRF-N WLED light output cold-heat ratio

Fig.1.7 specific value of photoelectric parameterson ordinary and high temperature of light output with WLED of GRF-V and GRF-N

4 Conclusion

In this paper, SMD 2835 package form, using different materials of phosphors, different dispersion coefficient of phosphors, different powder compatibility of phosphors and phosphors of different morphologies as yellow green powder for packaging experiments, can be obtained as follows Conclusion: The LED light source with LUAG material, small dispersion coefficient, good powder compatibility and good particle morphology is better.

Therefore, the physical properties of the phosphor have an effect on the light-to-heat ratio of the WLED light output. This research conclusion serves as the basis for powder control and optimization of products. At the same time, it has theoretical guiding significance and practical reference value for WLED product design.

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