Patent Application
20120181919
The present invention is directed to a converter for a light emitting element that converts a wavelength of light from the light emitting element (e.g., a blue light emitting diode) to a different wavelength (e.g., a yellow, red, or green light), and to method of making the converter.
Ceramic elements for converting a wavelength of light from a light source are known. See, for example, U.S. Patent Publication Nos. 2003/0025449, 2004/0145308 and 2007/0126017 and International Patent Publication No. WO 2006/097876. As described therein, a composite of phosphor particles is embedded in a ceramic or glass matrix; for example cerium-activated yttrium aluminum garnet (YAG:Ce) phosphor particles embedded in polycrystalline alumina. The latter document notes that porosity of the finished composite matrix is at most about 1%. The color of the converted light is adjusted by changing the mix of the phosphor particles in the matrix.
A sol-gel process is a known wet-chemical technique that can be used to make a metal oxide starting from either a chemical solution or colloidal particles (a sol) to produce an integrated network (a gel). A precursor, such as a metal alkoxide or metal chloride, undergoes hydrolysis and polycondensation reactions and then evolves to form an inorganic continuous network containing a liquid phase (the gel). Removal of the liquid in the wet gel under certain conditions results in a porous, low density material. For example, a transparent, porous, sol-gel alumina is known. See, e.g., Yoldas, A Transparent Porous Alumina, Ceramic Bulletin, 54(3), 286-288, (1975).
An object of the present invention is to provide a novel luminescent converter for a light emitting element.
A further object of the present invention is to provide a novel method of making this luminescent converter that uses a sol-gel process with a low temperature drying step to avoid damage to the embedded phosphor particles.
A yet further object of the present invention is to provide a novel a luminescent converter that includes a transparent, sol-gel-derived ceramic matrix having at least one type of phosphor embedded therein that changes a wavelength of the input light to light that has a different wavelength, where the ceramic matrix is 20-80% porous with a majority of the pores having a diameter in a range of 2-20 nm, and in particular, 2-10 nm. More preferably, the ceramic matrix is 40-60% porous.
Another object of the present invention is to provide a novel method of making this converter that includes the steps of preparing a sol-gel ceramic matrix embedded with at least one type of phosphor in the matrix, and drying the matrix at no more than about 600° C., preferably less than 500° C., and in some embodiments less than 175° C., to form the converter.
These and other objects and advantages of the invention will be apparent to those of skill in the art of the present invention after consideration of the following drawings and description of preferred embodiments.
With reference now to
The ceramic matrix of the luminescent converter is 20-80% porous with a majority of the pores (schematically shown as dots 20 in
The ceramic matrix 16 is preferably alumina, but also may be silica, yttria, zirconia, hafnia, or indium tin oxide (ITO). The pores 20 may be filled with one of air, oxygen, and helium. The ITO matrix and pores filed with air or oxygen may be used to provide an electrically conducting matrix, if needed. The helium filled pores may be used to increase the thermal conductivity. Preferably, the ceramic matrix contains particles of plural different types of phosphors 18 that may include YAG:Ce phosphor particles and at least one of nitride, sulfide, oxynitride and oxysulfide phosphors. Other phosphors may also be used, depending on the color of the output light. The plural different types of phosphors may be embedded homogeneously throughout the ceramic matrix, or may be unevenly distributed to provide a particular color effect.
The porous ceramic matrix 16 is made with a sol-gel process in which the shaped ceramic is dried at a low temperature, preferably less than 600° C., more preferably less than 500° C., and in some embodiments less than 175° C.
The method of making the converter 10 for a light emitting element may include the steps of forming a sol-gel ceramic matrix 16 embedded with at least one type of phosphor particles 18 in the matrix where, preferably, the phosphors are selected to change a wavelength of input light so that the overall output light from the converter is a white light, and drying the phosphor-embedded, sol-gel matrix at no more than about 600° C. to form a converter that is preferably 20-80% porous with a majority of the pores 20 having a diameter in a range of 2-20 nm.
An example of the light converting element comprising a sol-gel alumina and a YAG:Ce phosphor was made by adding 12 cc de-ionized water to 1 g of YAG:Ce and 4.1 g Catapal B (boehmite) or Dispal 23N4-80 (alumina hydroxide oxide); stirring for three minutes; adding 1 cc of 50% HNO3 acid to age and gel at room temperature for six hours; and drying in an oven at 90° C. for 16 hours. The sol-gel composite was placed on a blue-light LED and yielded 31 lumens of a white light compared to 45 lumens of white light produced by a monolithic, sintered YAG:0.5% Ce ceramic converter.
A second example was made by adding 12 cc of 20 wt % alumina sol (50 nm) (Nyacol AL-20) to 0.6 g of YAG:4% Ce and 0.3 g of Ca2Si5N8:Eu2+; stifling for three minutes; adding 0.2 cc of 30% NH4NO3 to age and gel at room temperature for six hours; and drying in an oven at 90° C. for 16 hours and then at 175° C. for 16 h. The sol-gel composite having a thickness of about 0.1 mm thick was placed on a blue LED and produced 144 lumens per watt of a white light (x=0.2904, y=0.2147) whose spectrum is shown in
A third example was made by adding 20 cc of 20 wt % ZrO2 (Y2O3-doped) sol (100 nm) (Nyacol DRYS4-20) to 1.12 g of YAG:4% Ce; stifling for three minutes; adding 0.4 cc of 30% NH4NO3 to age and gel at room temperature for six hours; and drying in an oven at 90° C. for 16 hours and then at 175° C. for 16 h.
A fourth example was made by adding 12 cc of 30 wt % silica sol (20 nm) (Nyacol DP5820) to 1.2 g of YAG:4% Ce and 0.5 g of Ca2Si5N8:Eu2+; stirring for three minutes; adding 0.2 cc of 30% NH4NO3 to age; exchanging with ethanol at room temperature; and drying in an oven at 90° C. for 16 hours and then at 175° C. for 16 h.
The drying step may involve removing liquid from the sol-gel under a supercritical condition. This helps reduce shrinkage to a negligible level and provides a reasonable surface finish so that surface grinding and polishing can be reduced or eliminated.
Further, the phosphor-embedded sol-gel ceramic matrix 16, 18 may be placed directly on the light emitting element (no adhesive necessary), where the drying step occurs with the matrix directly on the light emitting element 14. That is, the sols can be cast directly on an LED die, followed by drying at less than 175° C., a temperature that is tolerated by the die. This forms a direct bond between the converter body and the die and improves the refractive index match with the die without the need for the commonly used silicone resin glue. In addition, the phosphor-embedded, sol-gel matrix may be readily shaped into an optically active shape such as the lens shown in
The low temperature processing of the present invention affords a particular advantage in that the phase stability of certain phosphors is maintained. For example, a number of the red-emitting LED phosphors suffer significant brightness loss or even decompose at the higher temperatures need to form sintered ceramics.
The low temperature processing also allows impure YAG:Ce to be used. Commercial YAG:Ce typically contains small amounts (0.01-1 wt %) of unreacted impurities such as Y2O3, alumina, perovskite, monoclinic, and Ce aluminate that can cause problems for high temperature (>1700° C.) sintered transparent YAG:Ce ceramics. This problem does not arise with the lower temperatures of the present invention.
If the phosphors are nano-sized (less than about 200 nm), the entire sol-gel composite may be transparent.
While embodiments of the present invention have been described in the foregoing specification and drawing, it is to be understood that the present invention is defined by the following claims when read in light of the specification and drawing.
