Effect of physical properties of the hottest fluor

Effect of physical properties of fluorescent materials on the cold heat ratio of white LED light output

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

at present, there are three ways for WLED to realize white light in pc/mc mode: 1) blue LED chip + yellow phosphor; 2) Purple LED chip + Red + Green + blue 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 as high as 250lm/W. With the increasingly fierce market competition of lighting terminal products and the increasingly poor heat dissipation environment of lighting fixtures, LED light sources need to have better thermal characteristics to meet the needs of the market. The thermal characteristic of LED light source is usually characterized by the light output cold heat ratio. The cold heat ratio of light output of WLED, that is, the ratio of photoelectric parameters (luminous flux) of LED light source at high temperature to photoelectric parameters (luminous 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 arranged crystal structure. The stability of their physical and chemical properties is related to the following factors: material system, dispersion coefficient, powder glue compatibility, powder morphology. The influence factor of the cold heat ratio of WLED optical output is related to the WLED device material, and the fluorescent material is the key material in the above devices. The research on the influence of the physical characteristics of phosphors (material system, dispersion coefficient, powder glue compatibility, powder morphology) on the cold heat ratio of WLED light output has not been reported. At the same time, it is also very important to solve the problem of the thermal characteristics of LED light source. Therefore, it is of practical significance to explore the relationship between the physical characteristics of phosphors and the cold heat ratio of WLED light output, and it has a certain guiding role for subsequent product design

2. The experimental part

this paper adopts the packaging form of SMD 2835. The blue light chip has an emission band of nm. Each LED light source has three 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 group of experiments only changed the type of yellow and green powder, fixed the amount of glue and the content of the other two phosphors, and each LED light source had the same dispensing amount. The ratio of yellow, red and yellow green phosphors and 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 testing. The test condition is pulse current 100 Ma, the test temperature point is 25 ℃, 50 ℃, 75 ℃, 85 ℃, 95 ℃, 105 ℃, and the average value of light flux is taken. Powder parameter testing equipment: the particle size is tested by laser particle size analyzer, and the thermal quenching performance and excitation emission spectrum are tested by fluoromax-4; The SEM morphology of particles was tested by scanning electron microscope; Packaging equipment: ASM solid crystal machine, ASM wire bonding machine, vacuum defoaming machine, Musashi dispensing machine. Photoelectric parameter testing equipment for packaged products: remote integrating sphere tester

3. Results and discussion

phosphors are generally inorganic materials. According to their matrix classification, commonly used systems include aluminates, nitrides/nitrogen oxides, silicates, fluorides, etc. Figure 1.1 shows the thermal quenching performance of phosphors in different systems. It can be seen that aluminate has the best thermal stability among the powders of several systems, fluoride and silicate have poor thermal stability, and nitride has worse thermal stability than aluminate but better than fluoride and silicate

Figure 1.1 thermal quenching performance of phosphors in different systems

therefore, aluminate system is taken as the research object in this paper. The typical representative of aluminate system is YAG, whose chemical formula is y3al5o12:ce. The crystal structure belongs to cubic crystal system, and the lattice constant is 1.2002nm. The crystal structure of YAG is shown in Figure 1.2. It can be seen from the crystal structure that there are three kinds of polyhedrons in the space composed of Y, Al and O, which are dodecahedron (Fig. 1.2A), octahedron (Fig. 1.2b) and tetrahedron (Fig. 1.2C), in which the coordination numbers of oxygen atoms are (y33+) octa coordination, (al23+) Hexa coordination and (al33+) tetra coordination respectively

Figure 1.2 schematic diagram of crystal structure of YAG

3.1 effect of phosphor material on the cold heat ratio of WLED light output

in this experiment, GA YAG and luag yellow green powder are used as the research objects. GA YAG and luag both belong to the crystal structure of yttrium aluminum garnet, as shown in Figure 1.1. The general chemical formula of yttrium aluminum garnet is:

(RE1 rsmr) 3 (al1 SGAs) o12:ce (1) in formula (1), re=la, Lu, y, GG D, SC, 0 ≤ R 1, 0 ≤ s ≤ 1. Generally speaking, GA YAG and luag belong to cubic crystal 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+. Their ion radii are: rga3+ (eight coordination) =0.69, ry3+ (eight coordination) =1.04, ral3+ (six coordination) =0.62, rlu3+ (six coordination) =1.001 [4]. Combined with the matching degree of ion radius, theoretically, the thermal stability of the crystal structure formed by complete substitution is better than that of partial substitution. From the material point of view, the thermal stability of the material itself can be characterized by thermal quenching performance

as shown in Figure 1.3, the relationship between the thermal quenching performance of grf-g and G, including the filling and reinforcing material rf-l, is shown. It can be seen that the brightness attenuation of the powder shows a downward trend with the increase of temperature, and the thermal quenching performance of grf-l is better than that of grf-g

Fig. 1.3 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 the electron microscope is shown in Fig. 1.4. It can be concluded that the particle morphology of grf-g and grf-l is approximately spherical, and their surface is smooth

Figure 1.4 the left and right figures show the SEM morphology of grf-g and grf-l respectively

grf-g and grf-l are used as yellow and green powder to package 2835 finished lamp beads. The change between the luminous flux of the finished lamp beads and the test temperature is shown in Figure 1.5. It can be concluded that the cold heat ratio of luminous flux gradually decreases with the increase of temperature, and the cold heat ratio of WLED light output grf-l at 85 ℃ is better than grf-g

Figure 1.5 WLED light output cold heat ratio of grf-g and grf-l

WLED light output cold heat ratio of grf-g and grf-l, 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, light conversion materials of different materials (partial substitution and complete substitution) have an impact on the cold heat ratio of WLED light output

3.2 the influence of the dispersion coefficient of phosphor on the cold heat ratio of WLED light output

the dispersion coefficient refers to the measurement of the relative width or nonuniformity of the particle size distribution of phosphor samples. It is defined as the ratio of the distribution width to the central particle size, where the distribution width is the difference of a group of characteristic particle sizes of the boundary particle size. The dispersion coefficient is generally expressed as follows:

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

in formula (2), s represents the dispersion coefficient, and D10, D50, D90 are the particle sizes of phosphors corresponding to 10%, 50%, 90% in the volume cumulative distribution of the powder, and the unit is um, Where D50 represents the median particle size of powder particles. Generally speaking, the smaller the s value is, the more concentrated the particle size distribution of the powder is, and the number of defects on the surface of the particles per unit volume is roughly the same. There is no difference in its heating performance, and the better its thermal stability is. In this experiment, grf-s, grf-m and grf-b are used as yellow and green powder, which are packaged in the same scheme with yellow powder and red powder respectively. The dispersion coefficients s of grf-s, grf-m and grf-b are 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 dispersion coefficients. It can be seen that with the increase of temperature, the brightness of their fluorescent materials continues to decay, among which grf-b has the largest attenuation amplitude, grf-m takes the second place, and grf-s is the smallest. Among the three, grf-s has the best thermal quenching performance. Therefore, from the point of view of powder, the thermal quenching performance of small dispersion coefficient is better, which is consistent with the above analysis conclusion

figure 1.6thermal quenching performance of grf-s, grf-m and grf-b

this paper studies the influence of dispersion coefficient on the cold heat ratio of WLED optical output. The packaging form of 2835 is adopted, and the target parameter is ra=, cct=3000k. The same packaging scheme is adopted to verify the relationship between different dispersion coefficients and the cold heat ratio of WLED optical output. Figure 1.7 shows the relationship between the cold heat ratio of WLED optical output of grf-s, grf-m and grf-b with different dispersion coefficients, With the increase of temperature, the cold and hot state ratio of luminous flux of finished lamp beads is becoming smaller, and the attenuation amplitude of grf-s, grf-m, grf-b in finished products is the largest, grf-m is the second, and grf-s is the smallest, indicating that the cold and heat ratio of WLED light output of grf-s is the best, and the cold and heat ratio of WLED light output of grf-b is the worst. Therefore, different dispersion coefficients have an impact on the cold and heat ratio of WLED light output. The smaller the dispersion coefficient, the better the cold and heat ratio of WLED light output

figure 1.7 the relationship between the cold heat ratio of WLED light output of grf-s, grf-m and grf-b

3.3 the influence of powder glue compatibility on the cold heat ratio of WLED light output

after the synthesis of phosphor, in order to improve the stability of products, certain post-processing tools are usually used. How to deal with the abnormal conditions encountered in the use of electronic tensile testing machines? Technology, such as secondary quenching treatment, coating process, etc., the coating process is used more, and the packaging material used is SiO2 and other materials, but even if such a process is adopted, its thermal stability, especially reflected in the cold heat ratio of WLED light output, will still be unsatisfactory. Generally, when the phosphor is mixed with the packaging adhesive during the packaging process, there may be a certain gap on the contact surface between the particle surface and the colloid, which may contain unreleased air, which may affect the thermal stability of the finished product when heated. In order to solve this problem. Relevant manufacturers have proposed a new treatment process, including enterprise standards and industry standards. A layer of special substances is contained on the surface of phosphor particles through certain coating methods. The phosphor after special treatment will quickly condense into a large particle in water, so as to prevent water from entering. When the particles treated by this process are combined with the packaging colloid, the packaging colloid will be tightly wrapped on the surface of the particles, There is no gap. Increasing the compatibility of powder and glue can theoretically improve the cold heat ratio of WLED light output [6]

Fig. 1.8 thermal quenching performance of rf-g and crf-g

this paper adopts the packaging form of 2835, the target parameter is ra=, cct=3000k, and uses the same packaging scheme to verify the influence of phosphors with improved powder glue compatibility and those without improved powder glue compatibility on the cold heat ratio of WLED light output. The above two 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 with the increase of temperature, the luminous brightness of the phosphor shows a decreasing trend, in which the decreasing range of crf-g is smaller than that of rf-g, indicating that as for the phosphor itself, the thermal stability of crf-g is better than that of rf-g

figure 1.9 relationship between the cold heat ratio of WLED optical output of rf-g and crf-g

in this paper, 2835 packaging form is adopted for the influence of powder glue compatibility on the cold heat ratio of WLED optical output, and the target parameter is ra=, cct=3000k. The same packaging scheme is adopted to verify the influence of powder with improved powder glue compatibility on the cold heat ratio of WLED optical output. Figure 1.9 shows the relationship between the cold heat ratio of crf-g with improved powder glue compatibility and that of rf-g without improved powder glue compatibility, With the increase of temperature, the cold heat ratio of WLED light output of luminous flux of finished lamp beads is becoming smaller, the attenuation amplitude of crf-g and rf-g in finished products is the largest rf-g, followed by crf-g, indicating that the cold heat of WLED light output of crf-g is better, and the cold heat of WLED light output of rf-g is worse, so powder glue