High light efficiency, long life and low price are the three levels that white LEDs must enter into general illumination. Improving the internal and external quantum efficiency of the chip, design and process of high-tech package structure, and improving the light conversion efficiency of YAG:Ce3+ are the current three issues. The phase structure, particle morphology and luminescence properties of the powder are carefully studied. The synthesis of pure phase and spherical phosphor with uniform size is the basic work of the phosphor engineer. The concentration of stable Ce3+ must be paid attention to in the packaging process.
Phase structure
In the Y2O3-AL2O3 system, three different phases can be found by XRD, among which Y3AL5O12 (YAG) yttrium aluminum garnet phase, YALO3 (YAP) yttrium aluminum perovskite phase and Y4AL2O9 (YAM) yttrium aluminum single slant phase. The latter two types of YAP and YAM are mesophases. By brightness measurement, only the pure YAG phase has the highest luminance. In the traditional hard chemical synthesis, due to the difficulty of uniform mixing of raw materials, the reaction needs to be converted to YAG pure at a higher temperature (1600 ° C) and a longer time (such as several hours) due to the influence of solid phase chemical reaction kinetics. phase. In soft chemical synthesis, there is also a multi-stage conversion of the YAP-YAM-YAG mesophase, except for the difference in temperature (eg 1350-1450 ° C) and time (1.5-2 hours), so the chemical reaction temperature and reaction time are obtained. The basic conditions of pure phase.
铈-activated yttrium aluminum garnet YAG:Ce3+ is a cubic crystal, AL is located in the tetrahedral and octahedral lattice of the crystal, Y is in the dodecahedral position, and the trivalent ions Sc3+, La3+, Gd3+ and Lu3+ of the IIIB group have The same as the Y3+ saturated bare outer electron layer, the atomic radius is similar, it is possible to form a dodecahedral lattice. Since the price of Sc and Lu is high and the sensitization effect of Gd is better than that of La, Gd is substituted for part Y as a matrix material in the synthesis of such crystals. Similarly, the trivalent ions Ga3+ and In3+ of the same group IIIA have Al3+. The same exposed outer electron layer can enter the octahedral and tetrahedral lattice positions, and also serves as the matrix material, except that the ionic radii of Ga3+ and In3+ are larger than Al3+, and the emission wavelength has a blue shift. Therefore, the composition of the actual synthesized YAG is (Y1-a-bCeaGdb)3(AL1-cGac)5O12. The LED white light is generated by the 460 nm blue light emitted by the InGaN-based chip and the yellow-green light mixed by the YAG phosphor absorbing part of the blue light of about 555 nm. Since the red light wave is insufficient, a co-activator such as 612 nm Pr3+ and Sm3+ which enhances the emission effect is incorporated in the synthesis of the low color temperature YAG.
Ce3+ as the main activator, its luminescence intensity, photoluminescence spectrum and color coordinate value are closely related to Ce3+ concentration, and Ce3+ has the best concentration. Insufficient content, low luminous efficiency, high concentration of concentration quenching, and factors affecting Ce3+ concentration have a direct impact on light conversion efficiency. For example, chemical composition, purity of raw materials, reduction conditions from Ce4+ to Ce3+, degree of reduction, and how to avoid the oxidation of Ce3+ to Ce4+, which is a key factor affecting the luminous efficiency and stability of YAG:Ce3+.
The choice of flux should not only facilitate the entry of matrix Y, Gd, Al, Ga, etc., but also facilitate the entry of activators Ce3+, Pr3+, Sm3+, etc., and also facilitate easy removal in the post-treatment process to avoid mixing. Residues of atoms other than impurities, reducing miscellaneous phases. B2O3, AlF3, AlCl3, BaF2, NH4F, NH4Cl, etc. with the same element of AL can be selected, and single or several kinds of composites can be selected, and the actual effect is preferably several kinds of composites.
The purity of the raw materials directly affects the pure phase, and the selected raw materials are all pure in the spectrum even 5N-6N.
The morphology of grain morphology includes crystallization behavior, large filial distribution of morphology particles, morphology and its regularity.
When the blue light emitted in the GaN chip is irradiated on the YAG; Ce3+ phosphor layer, a part of the blue light penetrates the pores of the powder and is directly transmitted. A part of the blue light is irradiated to the disordered minute crystal through numerous times of diffuse reflection, and then returns to the original powder after being refracted. Surface as. A part of the blue light is scattered to the same color in the phosphor. If the blue light of a certain wavelength emitted by the chip is just absorbed by the phosphor, it matches and radiates yellow-green light of about 555 nm. If the phosphor layer does not match the blue light, the absorbed light is absorbed. It does not convert to yellow-green light, but only transfers energy in the form of heat. It can effectively absorb and realize the conversion of light radiation, which is the light conversion of phosphor YAG:Ce3+. The combination of yellow-green light and residual blue light produces white light, so it is obviously important to study the amount of wavelength absorption of the absorbed light. The light emitted by the 465nm chip is matched with the 465nm phosphor. The 450nm chip is matched with the 450nm phosphor. Therefore, the chip supplier's chip wavelength stability and consistency are required. The phosphor supplier should adapt to different composition configurations. Phosphors of different chips.
From the above analysis, it is known that in order to obtain high light conversion rate, it is necessary to reduce the porosity, reflectance, refractive index, increase of absorption rate and conversion rate of the phosphor layer, size of the phosphor particles, particle size distribution, particle morphology, particle surface state, etc. Powder particles fill the structure and filling characteristics. The void ratio of the phosphor layer is related to the filling type, particle shape and particle size distribution. Granular studies have shown that the particles have a sphericity (sphericality defined as: the ratio of the surface area of ​​the sphere to the surface area of ​​the irregular particles of the same volume), the higher the surface roughness and the angular particles, whether the particles are loosely packed or closely packed. Its stacked porosity is higher, and the smaller the particles, the higher the porosity due to the cohesion of small particles. The way to reduce the porosity is reasonable particle distribution and good particle morphology.
The spherical luminescent particles can obtain a higher bulk density, thereby reducing the scattering of the illuminant. Since the bulk density of the spherical luminescent particles is high, the porosity is reduced, the loss of transmitted light is small, and the optimal particle morphology for the illuminant is obtained. It is a sphere.
In the traditional high-temperature solid-phase method, the grain formation of the product is gradually grown, and it must have sufficient temperature and time. Due to severe sintering, the morphology of the particles is very irregular, it is difficult to obtain spherical particles, the particles are easy to agglomerate, and depolymerization is required. , reduce the particle size. Using the ball milling process, the larger granules tend to be finely ground, while the smaller granules have been ground too fine and the lattice structure is destroyed. The ball milling reduces the crystallinity of the phosphor, the morphology is incomplete, and the size is not uniform. High, angular particles, high powder bulk porosity, resulting in high light transmission and reduced conversion efficiency.
Many experts and scholars are working on the study of crystal morphology, hoping to obtain non-agglomerated spherical grains with finer size (such as 2-3 μm) and narrower size distribution. In order to obtain spherical crystals, it is necessary to start from the raw materials and process. Huang Jinggen pointed out that spherical BAM is obtained by spherical Al2O3 and flat BAM is obtained by flat Al2O3. Lin Jun uses spray pyrolysis to prepare a series of spherical rare earth luminescent materials in chemical coprecipitation process. In the use of complexing agent, PH can be controlled to obtain a spherical body of almost the same size. Wang Zhenchuan obtained 85% of the uniform sphere in the sol-gel preparation precursor, and Li Qiang obtained the YAG micropowder by polymer network gel method. Comprehensive utilization of the different characteristics of soft chemistry and hard chemistry, the synthesis size consistent with the requirements of pure phase spherical crystal is the future direction.
YAG: Luminescence characteristics of Ce3+
YAG: Ce3+ phosphor is partially absorbed by the phosphor under the excitation of blue light and long-wave ultraviolet light. The phosphor produces high-efficiency yellow visible light emission, which has high conversion efficiency and high lumen efficiency, and is a typical down-conversion photoluminescence. The most effective blue light excitation spectrum of the yttrium-activated yttrium aluminum garnet phosphor is matched with the luminescence spectrum of the InGaN chip, and converted into yellow light emission required for white light. In order to improve color rendering, the red band emission rare earth Pr3+ and Sm3+ may be incorporated into the composition or a high-efficiency red phosphor may be added. The amount of rare earth Gd added in the composition is favorable for low color temperature modulation, and the converted color is very sensitive to the thickness of the phosphor layer. Adjust the YAG: Ce3+ variety and dosage to obtain white light of different color temperatures.
The yttrium-activated yttrium aluminum garnet has good physical and chemical stability, is resistant to electron radiation, and has excellent temperature quenching characteristics. The phosphor lumens are maintained by a variety of factors such as the purity of the raw materials, whether there is a miscellaneous phase after burning, crystal morphology, whether water and oxidizing gases are absorbed, affecting the stability of Ce3+, in the storage and storage of the phosphors. In order to avoid the oxidative environment, the occurrence of Ce3+→Ce4+ is limited, and efforts are made to improve the stability of Ce3+. The soft spherical chemical synthesis of approximately spherical uniform uniform phosphor products has high luminous brightness and good coating performance, and the economic benefits of the enterprise are obviously improved.
Phase structure
In the Y2O3-AL2O3 system, three different phases can be found by XRD, among which Y3AL5O12 (YAG) yttrium aluminum garnet phase, YALO3 (YAP) yttrium aluminum perovskite phase and Y4AL2O9 (YAM) yttrium aluminum single slant phase. The latter two types of YAP and YAM are mesophases. By brightness measurement, only the pure YAG phase has the highest luminance. In the traditional hard chemical synthesis, due to the difficulty of uniform mixing of raw materials, the reaction needs to be converted to YAG pure at a higher temperature (1600 ° C) and a longer time (such as several hours) due to the influence of solid phase chemical reaction kinetics. phase. In soft chemical synthesis, there is also a multi-stage conversion of the YAP-YAM-YAG mesophase, except for the difference in temperature (eg 1350-1450 ° C) and time (1.5-2 hours), so the chemical reaction temperature and reaction time are obtained. The basic conditions of pure phase.
铈-activated yttrium aluminum garnet YAG:Ce3+ is a cubic crystal, AL is located in the tetrahedral and octahedral lattice of the crystal, Y is in the dodecahedral position, and the trivalent ions Sc3+, La3+, Gd3+ and Lu3+ of the IIIB group have The same as the Y3+ saturated bare outer electron layer, the atomic radius is similar, it is possible to form a dodecahedral lattice. Since the price of Sc and Lu is high and the sensitization effect of Gd is better than that of La, Gd is substituted for part Y as a matrix material in the synthesis of such crystals. Similarly, the trivalent ions Ga3+ and In3+ of the same group IIIA have Al3+. The same exposed outer electron layer can enter the octahedral and tetrahedral lattice positions, and also serves as the matrix material, except that the ionic radii of Ga3+ and In3+ are larger than Al3+, and the emission wavelength has a blue shift. Therefore, the composition of the actual synthesized YAG is (Y1-a-bCeaGdb)3(AL1-cGac)5O12. The LED white light is generated by the 460 nm blue light emitted by the InGaN-based chip and the yellow-green light mixed by the YAG phosphor absorbing part of the blue light of about 555 nm. Since the red light wave is insufficient, a co-activator such as 612 nm Pr3+ and Sm3+ which enhances the emission effect is incorporated in the synthesis of the low color temperature YAG.
Ce3+ as the main activator, its luminescence intensity, photoluminescence spectrum and color coordinate value are closely related to Ce3+ concentration, and Ce3+ has the best concentration. Insufficient content, low luminous efficiency, high concentration of concentration quenching, and factors affecting Ce3+ concentration have a direct impact on light conversion efficiency. For example, chemical composition, purity of raw materials, reduction conditions from Ce4+ to Ce3+, degree of reduction, and how to avoid the oxidation of Ce3+ to Ce4+, which is a key factor affecting the luminous efficiency and stability of YAG:Ce3+.
The choice of flux should not only facilitate the entry of matrix Y, Gd, Al, Ga, etc., but also facilitate the entry of activators Ce3+, Pr3+, Sm3+, etc., and also facilitate easy removal in the post-treatment process to avoid mixing. Residues of atoms other than impurities, reducing miscellaneous phases. B2O3, AlF3, AlCl3, BaF2, NH4F, NH4Cl, etc. with the same element of AL can be selected, and single or several kinds of composites can be selected, and the actual effect is preferably several kinds of composites.
The purity of the raw materials directly affects the pure phase, and the selected raw materials are all pure in the spectrum even 5N-6N.
The morphology of grain morphology includes crystallization behavior, large filial distribution of morphology particles, morphology and its regularity.
When the blue light emitted in the GaN chip is irradiated on the YAG; Ce3+ phosphor layer, a part of the blue light penetrates the pores of the powder and is directly transmitted. A part of the blue light is irradiated to the disordered minute crystal through numerous times of diffuse reflection, and then returns to the original powder after being refracted. Surface as. A part of the blue light is scattered to the same color in the phosphor. If the blue light of a certain wavelength emitted by the chip is just absorbed by the phosphor, it matches and radiates yellow-green light of about 555 nm. If the phosphor layer does not match the blue light, the absorbed light is absorbed. It does not convert to yellow-green light, but only transfers energy in the form of heat. It can effectively absorb and realize the conversion of light radiation, which is the light conversion of phosphor YAG:Ce3+. The combination of yellow-green light and residual blue light produces white light, so it is obviously important to study the amount of wavelength absorption of the absorbed light. The light emitted by the 465nm chip is matched with the 465nm phosphor. The 450nm chip is matched with the 450nm phosphor. Therefore, the chip supplier's chip wavelength stability and consistency are required. The phosphor supplier should adapt to different composition configurations. Phosphors of different chips.
From the above analysis, it is known that in order to obtain high light conversion rate, it is necessary to reduce the porosity, reflectance, refractive index, increase of absorption rate and conversion rate of the phosphor layer, size of the phosphor particles, particle size distribution, particle morphology, particle surface state, etc. Powder particles fill the structure and filling characteristics. The void ratio of the phosphor layer is related to the filling type, particle shape and particle size distribution. Granular studies have shown that the particles have a sphericity (sphericality defined as: the ratio of the surface area of ​​the sphere to the surface area of ​​the irregular particles of the same volume), the higher the surface roughness and the angular particles, whether the particles are loosely packed or closely packed. Its stacked porosity is higher, and the smaller the particles, the higher the porosity due to the cohesion of small particles. The way to reduce the porosity is reasonable particle distribution and good particle morphology.
The spherical luminescent particles can obtain a higher bulk density, thereby reducing the scattering of the illuminant. Since the bulk density of the spherical luminescent particles is high, the porosity is reduced, the loss of transmitted light is small, and the optimal particle morphology for the illuminant is obtained. It is a sphere.
In the traditional high-temperature solid-phase method, the grain formation of the product is gradually grown, and it must have sufficient temperature and time. Due to severe sintering, the morphology of the particles is very irregular, it is difficult to obtain spherical particles, the particles are easy to agglomerate, and depolymerization is required. , reduce the particle size. Using the ball milling process, the larger granules tend to be finely ground, while the smaller granules have been ground too fine and the lattice structure is destroyed. The ball milling reduces the crystallinity of the phosphor, the morphology is incomplete, and the size is not uniform. High, angular particles, high powder bulk porosity, resulting in high light transmission and reduced conversion efficiency.
Many experts and scholars are working on the study of crystal morphology, hoping to obtain non-agglomerated spherical grains with finer size (such as 2-3 μm) and narrower size distribution. In order to obtain spherical crystals, it is necessary to start from the raw materials and process. Huang Jinggen pointed out that spherical BAM is obtained by spherical Al2O3 and flat BAM is obtained by flat Al2O3. Lin Jun uses spray pyrolysis to prepare a series of spherical rare earth luminescent materials in chemical coprecipitation process. In the use of complexing agent, PH can be controlled to obtain a spherical body of almost the same size. Wang Zhenchuan obtained 85% of the uniform sphere in the sol-gel preparation precursor, and Li Qiang obtained the YAG micropowder by polymer network gel method. Comprehensive utilization of the different characteristics of soft chemistry and hard chemistry, the synthesis size consistent with the requirements of pure phase spherical crystal is the future direction.
YAG: Luminescence characteristics of Ce3+
YAG: Ce3+ phosphor is partially absorbed by the phosphor under the excitation of blue light and long-wave ultraviolet light. The phosphor produces high-efficiency yellow visible light emission, which has high conversion efficiency and high lumen efficiency, and is a typical down-conversion photoluminescence. The most effective blue light excitation spectrum of the yttrium-activated yttrium aluminum garnet phosphor is matched with the luminescence spectrum of the InGaN chip, and converted into yellow light emission required for white light. In order to improve color rendering, the red band emission rare earth Pr3+ and Sm3+ may be incorporated into the composition or a high-efficiency red phosphor may be added. The amount of rare earth Gd added in the composition is favorable for low color temperature modulation, and the converted color is very sensitive to the thickness of the phosphor layer. Adjust the YAG: Ce3+ variety and dosage to obtain white light of different color temperatures.
The yttrium-activated yttrium aluminum garnet has good physical and chemical stability, is resistant to electron radiation, and has excellent temperature quenching characteristics. The phosphor lumens are maintained by a variety of factors such as the purity of the raw materials, whether there is a miscellaneous phase after burning, crystal morphology, whether water and oxidizing gases are absorbed, affecting the stability of Ce3+, in the storage and storage of the phosphors. In order to avoid the oxidative environment, the occurrence of Ce3+→Ce4+ is limited, and efforts are made to improve the stability of Ce3+. The soft spherical chemical synthesis of approximately spherical uniform uniform phosphor products has high luminous brightness and good coating performance, and the economic benefits of the enterprise are obviously improved.
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