Field of the Invention

This invention is related to Composite Oxynitride Ceramic Converter and Light Source Having Same.

Background of the Invention

[0001] It is well known to use phosphors to convert blue light emitted from light emitting diodes (LEDs) into longer wavelengths ("wavelength conversion") in order to affect the overall color of the light emitted from the LED package. Such LEDs are generally referred to as phosphor-conversion LEDs (pc-LEDs). Depending on the application and desired light output, the phosphor may convert only a portion of the light emitted by the LED, as in the case of white LEDs, or the phosphor may effect a full conversion of the blue LED light to another color such as green.

[0002] The phosphor may be embedded in a silicone resin applied over the LED or in some advantageous applications it may be applied to the LED as a solid, sintered ceramic converter. Such solid, monolithic converters have at least two distinct advantages over phosphor powders. First, luminescent ceramic converters can be made in defined shapes and uniform thicknesses to provide better color consistency and control in manufacturing. Second, they can be made translucent which can reduce scattering losses and improve extraction efficiency. Examples of luminescent ceramic converters are described in U.S. Patent No. 7,554,258, U.S. Patent Application Publication 2007/0126017 and International Patent Application Publication No. WO 2006/087660.

[0003] Oxynitride-based green-emitting phosphors such as p-SiAION:Eu and SrSi2O2N2:Eu provide many advantages in color gamut, thermal and chemical stability, and quenching effects. However, it is difficult to achieve dense ceramic parts with these materials even at high temperatures or long dwell times because they typically have a very low bulk diffusion coefficient, either elongated grains (β-SiAION) or plate-like grains (SrSi2O2N2), and large particle sizes. Hence, oxynitride ceramic converters normally have lower conversion efficiencies compared to their powdered form because of the severe scattering caused by the porous microstructure.

Summary of the Invention

[0004] The present invention is a composite oxynitride ceramic converter that combines a first phase of a triclinic SrSi202N2:Eu phosphor with a second phase of a hexagonal Sr3Si6N ₄ 0g:Eu phosphor to achieve a dense composite ceramic converter with a high quantum efficiency (QE) which may be used for green LED applications. In particular, the composite oxynitride ceramic converter preferably emits a green light with a peak wavelength from 540 to 570nm, and more preferably from 550 to 560nm.

[0005] In another aspect, the composite oxynitride ceramic converter has a relative molar proportion of x moles of the SrSi202N2:Eu phosphor to (1-x) moles of the Sr3Si6N ₄ 0g:Eu phosphor where x is from 0.01 to 0.99, more preferably, from 0.20 to 0.80, and even more preferably from 0.30 to 0.60.

[0006] In a further aspect, the composite oxynitride ceramic converter is paired with an LED to form a light source. More particularly, the ceramic converter is preferably combined with a blue-emitting LED to substantially convert the blue light emitted by the LED to a green light thereby providing a green-emitting LED light source.

[0007] According to one embodiment the composite oxynitride ceramic converter having a first phase of a triclinic SrSi202N2:Eu phosphor and a second phase of a hexagonal Sr3Si6N ₄ 0g:Eu phosphor.

[0008] According to one embodiment the SrSi202N2:Eu phosphor and the

Sr3Si6N ₄ 0g:Eu phosphor contain from 0.1 mole percent Eu to 5 mole percent Eu.

[0009] According to one embodiment the SrSi202N2:Eu phosphor and the Sr3Si6N ₄ 0g:Eu phosphor contain from 0.1 mole percent Eu to 3 mole percent Eu. [0010] According to one embodiment the SrSi202N2:Eu phosphor and the Sr3Si6N ₄ 0g:Eu phosphor contain from 0.3 mole percent Eu to 1.5 mole percent Eu.

[0011] According to one embodiment the converter has a relative molar proportion of x moles of the SrSi202N2:Eu phosphor to (1 -x) moles of the Sr3Si6N ₄ 09:Eu phosphor where x is from 0.01 to 0.99.

[0012] According to one embodiment the converter has a relative molar proportion of x moles of the SrSi202N2:Eu phosphor to (1 -x) moles of the Sr3Si6N ₄ 0g:Eu phosphor where x is from 0.20 to 0.80.

[0013] According to one embodiment the converter has a relative molar proportion of x moles of the SrSi202N2:Eu phosphor to (1 -x) moles of the Sr3Si6N ₄ 0g:Eu phosphor where x is from 0.30 to 0.60.

[0014] According to one embodiment the average grain size in the composite oxynitride ceramic converter is 0.1 to 10 μηη.

[0015] According to one embodiment the average grain size the average grain size in the composite oxynitride ceramic converter is 2 to 10 μηη.

[0016] According to one embodiment the converter emits a green light having a peak wavelength from 540 to 570nm. [0017] According to one embodiment the converter emits a green light having a peak wavelength from 550 to 560nm.

[0018] According to one embodiment the converter comprises first grains having a plate-like shape and second grains having an elongated shape with a hexagonal cross section. [0019] According to one embodiment the second grains having the elongated shape have an aspect ratio in a range of greater than 1 to less than about 12.

[0020] According to one embodiment the light source comprising a light emitting diode (LED) and a composite oxynitride ceramic converter, the composite oxynitride ceramic converter having a first phase of a triclinic SrSi202N2:Eu phosphor and a second phase of a hexagonal Sr3Si6N ₄ 0g:Eu phosphor.

[0021] According to one embodiment of the light source the SrSi202N2:Eu phosphor and the Sr3Si6N ₄ 0g:Eu phosphor contain from 0.1 mole percent Eu to 5 mole percent Eu.

[0022] According to one embodiment of the light source the converter has a relative molar proportion of x moles of the SrSi202N2:Eu phosphor to (1-x) moles of the Sr3Si6N ₄ 0g:Eu phosphor where x is from 0.20 to 0.80.

[0023] According to one embodiment of the light source the composite oxynitride ceramic converter emits a green light having a peak wavelength from 540 to 570nm.

[0024] According to one embodiment of the light source the LED emits a primary light and the composite oxynitride ceramic converter converts substantially all of the primary light to the green light.

[0025] According to one embodiment of the light source the primary light has a peak wavelength in the range of 420 to 490nm.

[0026] According to one embodiment of the light source the green light has a peak wavelength from 550 to 560nm.

Brief Description of the Drawings

[0027] Figure 1 is a photomicrograph of the microstructure of a composite oxynitride ceramic converter according to this invention. [0028] Figures 2A and 2B show the x-ray diffraction pattern for a composite oxynitride ceramic converter according to this invention overlaid with diffraction patterns for SrSi202N2 and Sr3Si6N ₄ 0g phases, respectively.

[0029] Figures 3A and 3B are illustrations of the crystal structures for the SrSi202N2 and Sr3Si6N ₄ 0g phases, respectively.

[0030] Figure 4A is a graphical comparison of the emission spectra of composite oxynitride ceramic converters formed by pressureless sintering and spark plasma sintering (SPS) techniques. Figure 4B shows the color points of composite oxynitride ceramic converters of Figure 4A.

[0031] Figure 5 is a cross-sectional illustration of a light source according to this invention.

[0032] Figures 6A to 6E show x-ray diffraction data.

Detailed Description of the Invention

[0033] For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.

[0034] References to the color of a phosphor, LED, laser or conversion material refer generally to its emission color unless otherwise specified. Thus, a blue LED emits a blue light, a yellow phosphor emits a yellow light and so on.

[0035] As used herein the term "ceramic converter" refers to a solid monolithic piece comprised of at least one sintered, polycrystalline luminescent material, such as an inorganic phosphor, that converts at least a portion of the light from an excitation source to light of a different wavelength. The ceramic converter has a density that is preferably at least about 90% of the theoretical density of the luminescent material that comprises it. A preferred density of the inventive composite oxynitride ceramic converter is at least about 95% of theoretical density. More preferably, the composite ceramic converter has a density that is at least about 98%, or even 99%, of theoretical density.

[0036] As described above, it is difficult to achieve a properly dense ceramic converter with green oxynitride phosphors such as p-SiAION:Eu and SrSi202N2:Eu. Methods including hot isostatic pressing (HIP), spark plasma sintering (SPS) or high pressure (HP) may be employed to produce ceramic converters, but with these phosphors the resulting parts are generally unsatisfactory because of the large amount of pores present in the ceramic bulk. For example, without addition of sintering aids, single-phase, elongated p-SiAION:Eu does not densify even at 1725°C by SPS; and single-phase, plate-like SrSi202N2:Eu parts generally have a lower QE, especially round about 30 % lower QE, either by SPS or pressureless sintering methods.

[0037] The present invention overcomes this problem by combining two oxynitride phases to form a composite oxynitride ceramic converter having a dense, homogeneous microstructure and a high QE which may be used in green pc-LED applications. In particular, the ceramic converter contains a first phase of a triclinic SrSi202N2:Eu phosphor and a second phase of a hexagonal Sr3Si6N ₄ O9:Eu phosphor. It is believed that the use of two phases instead of a single phase provides a greater driving force for densification. Although these two oxynitride phases are of similar chemical composition, they have a different of ratio of oxygen to nitrogen atoms (viz. the triclinic SrSi202N2 phosphor has an O/N ratio of 1 :1 whereas the hexagonal Sr3Si6N ₄ 0g phosphor has a higher O/N ratio of 2.25:1.) The difference in the composition creates an unbalanced chemical potential which provides an additional driving force for better densification through mass diffusion/transportation during sintering and makes less costly pressureless sintering methods viable. On the optical performance side, the composite oxynitride ceramic converter offers a higher uncorrected conversion efficiency, -41%, than single phase SrSi2O2N2:Eu converters (-33-35%) made by pressureless sintering and provides a greener emission (555.3nm dominant wavelength and 546.5 nm dominant wavelength, respectively) compared to single phase SrSi02lM2:Eu (555.8nm dominant wavelength and 555 nm dominant wavelength, respectively).

[0038] A typical microstructure of the SrSi202N2:Eu/ Sr3Si6N ₄ 09:Eu composite ceramic converter is shown in Figure 1 and consists of triclinic (light white plate-like shape) and hexagonal (dark) phases. The elongated hexagonal-shaped grains preferably have an aspect ratio (length to width) in a range of greater than 1 to less than about 12. The average grain size in the composite oxynitride ceramic converter is preferably 0.1 to 10 μηη, more preferably 0.5 to 12 μηη and even more preferably 2 to 10 μηη. Figures 2A and 2B show an exemplary x-ray diffraction pattern for the composite oxynitride ceramic converter overlaid with diffraction patterns for the SrSi202N2 and Sr3Si6N ₄ 0g phases, respectively. The unit cell parameters were determined by x-ray diffraction (XRD) techniques based on the diffraction pattern for the hexagonal Sr3Si6N ₄ 0g phase. The hexagonal crystal phase belongs to space group P3 (143) with cell dimensions a=7.106, c=6.6182, vol. =289.5. Crystal structural models of the two phases present in the composite are shown in Figures 3A (triclinic SrSi202N2) and 3B (hexagonal Sr3Si6N ₄ 09).

[0039] This invention is made based on green full conversion topic on SrSi202N2 materials after a series of processing steps and conditions explored for optimization as below: (1) DoE of green SrSi202N2 doped with Eu under different processing conditions. (2) Different debinding processing conditions (in air, vacuum at different temperatures). (3) Different sintering methods used e.g SPS and pressureless sintering. Steps (1 ) was designed to optimize the best starting powders, and step (2) to optimize the best debinding conditions, both steps was designed in such a way to modify the surface chemistry of the phosphor powders and even hence final compositions. Step (3) was designed to select and optimize the densification and luminescent performance by using different sintering methods and conditions. Firstly SPS was used for the evaluation of sintering behavior, conditions and optical performance of the DoE powders. It was found that the phosphor powders processed under the condition - milled in ethanol with no acid treatment resulted in the highest conversion efficiency ( ^¾ 32.9% in sphere pinhole) among the DoE powders, even though minor unidentified 2nd phase present in the as received powder. Tape casting followed by debinding and pressureless sintering process was employed using the most promising DoE powder to explore the cost effective way to make ceramic converters. Ceramic converters made from powder were debinded and got partially oxidized, after pressureless sintering in N2, it was found a phase transformation from X SrSi202N2 to Y Sr3Si6N ₄ 0g occurred partially, i.e. the final ceramic converters consist of two major phases, SrSi202N2 and Sr ₃ Si6N ₄ 09 the former is triclinic phase and the latter hexagonal structure. The composite ceramic converter gave a higher conversion efficiency (40.6% in pinhole) than that produced by SPS process (32.9%).

[0040] The composite oxynitride ceramic converters of this invention may be produced using starting powders in different forms. For example, the starting powders may comprise (1 ) oxynitride-based phosphor powders of solid solution composition Sr _a SibOcN _d :Eu, where 0.95<a≤1.05, 1.5<b<2.5, 1.5<c<2.5, 1.5<d<2.5 and preferably containing 0.1 to 5 mole percent europium (mol%Eu); (2) mixed phosphor powders in a relative molar proportion of x moles SrSi202N2:Eu to (1-x) moles Sr ₃ Si6N ₄ 09:Eu where x is between 0.01 to 0.99 and the phosphors are preferably doped with 0.1 to 5 mol%Eu; or (3) raw powders in the form of Sr, Si, Al, and Eu oxides or nitrides (for example SrO, SrCOs, Sr ₃ N ₂ , Si0 ₂ , Si ₃ N ₄ , Al ₂ 0 ₃ , AIN, Eu ₂ 0 ₃ , SrSi ₂ 0 ₂ N ₂ , Sr ₃ Si ₆ 0i ₂ N ₂ , Sr ₃ Si ₂ 0 N ₂ , Sr ₃ Si ₃ 09N ₄ , S^SisOxNe-x etc.) having relative proportions designed such that the final composite ceramic converter consists of the two major phosphor phases SrSi202N2:Eu and Sr ₃ Si6N ₄ 0g:Eu. The composite converters may be produced by either solid state reaction, SPS, pressureless sintering, or other methods such as hot pressing, hot isostatic pressing and over pressure sintering. Preferably, the oxynitride phosphor phases contain 0.1 to 5 mol%Eu, more preferably 0.1 to 3mol%Eu, and even more preferably 0.3 to 1.5 mol%Eu.

Example 1

[0041] Several different oxynitride phosphor powders (e.g. SrSi202N2) with a Eu content of 0.1 to 10 or 0.1 to 5 mol%Eu were processed under different conditions. The powders had a particle size (d50) of between 0.20-5. Ομηη, with a preferred range of 0.50- 3.0 μηι. The main SrSi2O2N2:Eu phosphor phase was determined to be≥70 weight percent (wt.%) by XRD with a preferred range of 80wt.% - 99.9wt.%. Ceramic converters were formed by spark plasma sintering. The composite oxynitride phosphor powders were loaded in a graphite pressure die, and the die was heated by allowing a pulsed direct current to pass through it. Boron nitride paint was applied to the portions of the die assembly that came into contact with the powder to prevent a reaction between the powder and graphite. Samples with diameters of 15 and 20 mm, and thicknesses of -1.5- 6.5 mm were sintered using a uniaxial pressure between 5-100 MPa with preferred range of 20-80 MPa. The pressure was applied at a temperature of between 500-1350°C, with a preferred range of 800-1300°C, and held constant until the end of the sintering cycle. Initially the samples were heated at a rate of 30-35°C/min in vacuum from room temperature until 950°C. A gas comprising Ar, N2, H2 or a combination thereof (preferably 100% N ₂ or 2%H ₂ /98%N ₂ (forming gas)) was introduced at 950°C and the sintering continued to the peak temperature. Typically, the ceramic converters were sintered at peak temperatures ranging from 1400-1600°C (preferably 1400-1500°C) for 1 to 360 minutes (preferably 5-240min. or 5-120 min.) Once the sintering process was complete, the samples were rapidly cooled to room temperature. The cooling rate from the final sintering temperature down to 500°C was approximately 350°C/min. The sintered ceramic cylinders were ground down to thin disks with a thickness range of about 50 to 200μηη. Uncorrected conversion efficiencies ("pinhole efficiencies") were measured and the color points calculated. The uncorrected conversion efficiency is a parameter indicating how effectively the blue excitation light is converted into green on a photon-per- photon basis. Example 2

[0042] Oxynitride phosphor powders with compositions similar to Example 1 were suspended in aqueous or organic solvents, such as H2O, ethanol, toluene, benzene, hexane, methanol, and isopropyl alcohol. Milling media was added and the slurry milled to obtain a homogeneous dispersion (e.g. 160 rpm for 24 h). A plasticizer and a binder were added to the suspension and the suspension was further milled (e.g. 60 rpm for 24 h). But the rate can be adjusted depending on the jar size and roller machine used. Solid, dispersant, plasticizer, and binder volume contents in the final composition were 26.23%, 2.68%, 2.64% and 6.10%, respectively. The milling media to solid ratio was kept in the range of 2:1 to 10:1 , and preferably 4:1 to 8:1. The produced slurry was tape cast with a blade height of 30 μηη to 1000 μηη depending on the final thickness required. A slurry was cast on a carrier film. The tape on the carrier film was removed, blanked, and punched into different sizes and shapes such as round, rectangular and square. The milled slurry was tape cast to form platelets having thicknesses from 20-500μηη. Binder burn-out was carried out in different atmospheres (air, N2, H2, N2/H2 or vacuum) in a furnace at a temperature between 300-1100°C. After binder burn-out, the ceramic platelets were sintered in a furnace in a non-oxidizing atmosphere. The sintering profile was adjusted to take into account the starting powder characteristics and furnace conditions. In particular, temperatures were increased at a rate from 2°C/min to 20°C/min and the platelets sintered at a temperature from 1400°C to 1600°C depending on the powder batch, sample size and thickness, and the type of furnace used. The thickness of the sintered ceramic converters ranged from about 50 to 200 μηη. Pinhole efficiencies were measured and color points calculated.

[0043] Figure 4A compares the emission spectra of samples exhibiting the highest pinhole efficiencies made by SPS (Example 1 , indicated as 4-1 ) and pressureless sintering (Example 2, indicated as 4-2) methods. It is shown the emission intensity I in arbitrary units (a.U.) as a function of the wavelength λ in nm. The optical parameters of the samples are compared in the Tables below. The data were obtained by using a 443.5nm blue excitation light source. Figure 4B shows the color points of Examples 1 and 2, respectively.

Table - Optical Parameters of SPS and Pressureless Sintering Samples

and/or Table - Optical Parameters of SPS and Pressureless Sintering Samples

[0044] In the Example 2 sample, if it is assumed that the triclinic (020) peak is the 100%, the ratio of the hexagonal to the triclinic phase is 41 % to 59% (in wt.%). The ratio can be changed by changing debinding and sintering temperatures and atmosphere etc.

[0045] Hot pressing, hot isostatic pressing and over pressure sintering are three possible alternative methods for sintering the green composite ceramic converters. These are relatively less cost effective and efficient process compared to the process of tape casting followed by pressureless sintering.

[0046] In robustness tests in which the composite oxynitride ceramic converters were paired with LEDs, it was found that after 1000 hours of operation the composite ceramic converters were exhibiting nearly fully stable light output. A small color shift towards green was observed, but this seemed to saturate and more importantly the intensity was stable demonstrating the potential for green LED applications. Test conditions: T = 125 °C, I = 1000 mA, Duration = 1000h, and T = 85 °C, rel. humidity = 85 %, I = 500 mA, Duration = 1000h.The pc-LED comprises a reflective housing. [0047] Figure 5 illustrates the use of a composite oxynitride ceramic converter in a phosphor-converted LED (pc-LED) configuration. In particular, a light source 100 in the form of a pc-LED having a composite oxynitride ceramic converter 104 is shown. The composite oxynitride ceramic converter is comprised of a triclinic SrSi2O2N2:Eu ²⁺ phase and a hexagonal Sr3Si6N ₄ O9:Eu ²⁺ phase. The ceramic converter 104 generally has a thickness of between 20μηη and 500μηη and preferably between 100μηη and 250μηη. In a preferred embodiment, the ceramic converter has the shape of a flat plate, although it is not limited to such. [0048] Primary light 106 emitted from light-emitting surface 107 of the blue-emitting LED die 102 passes into ceramic converter 104 which converts at least a portion of the blue light into a secondary light 116 having a different peak wavelength, e.g., a green light. Preferably, the blue primary light 106 has a peak wavelength in the range of 420nm to 490nm. The color of the light eventually emitted from the light-emitting surface 120 of ceramic converter 104 will depend on the ratio of the amount of unconverted primary light 106 that passes through the ceramic converter to the amount of primary light that is converted to secondary light 116 within the ceramic converter. In some applications, substantially all of the primary light 106 is converted and only converted light 116 is emitted (full conversion).

[0049] Pursuant to at least one embodiment, the component comprises a semiconductor chip 102. Said semiconductor chip is set up for generating electromagnetic primary radiation at least from the blue spectral range. The ceramic converter 104 can be arranged in a spatially separated manner from the semiconductor chip 102. For example, the interval between ceramic converter 104 and semiconductor chip 102 can be greater than or equal to 200 μηη, preferably greater than or equal to 750 μηη, more preferably greater than or equal to 900 μηη (so-called remote phosphor conversion). Alternatively, the ceramic converter 104 can directly arranged on the semiconductor chip 102. The semiconductor chip can comprises a semiconductor layer sequence. Said semiconductor layer sequence of the semiconductor chip is preferably based on a lll-IV compound semiconductor material. The semiconductor material is preferably a nitride compound semiconductor material such as Alnlni _-n -mGamN. However, for the sake of simplicity, only the essential components of the crystal lattice of the semiconductor layer sequence, i.e. Al, As, Ga, In, N or P are stated, even though the latter can be replaced and/or supplemented by small quantities of further substances.

[0050] Figures 6A to 6E show x-ray diffraction data, which are calculated. The choosen parameters are: Sr ₃ Si ₆ N ₄ 0 ₉ :Eu, Cell: 7.106 x 7.106 x 6.6192 < 90.0 < 90.0 x 120.0 > Vol=2895, Z=1 , Dx=3.6219, l/lc=2.0(v); 218 Reflections in 0.0-76.58 degs >=0.0% (M = multiplicity); hexangonal: P3 (143) [M3], hP22, [Diffractometer LP] [U(lj)] [F'+F"][A=0.5][VS].

[0051] While there have been shown and described what are at present considered to be preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.

[0052] This patent application claims the priority of US patent application 62/172,552, the disclosure content of which is hereby incorporated by reference.