Patent Application
20060226772
Electronic solid-state light sources typically use a population of phosphor particles disposed in a host matrix. Typically, these phosphors are Barium Gallium Sulfide (BGS-green) having a mean particle size of 25 μm and specific gravity of 3.8 and Zinc Selenium Sulfide (ZnSeS-orange) having a mean particle size of 25 μm and specific gravity of 4.6. ZnSeS, because of its higher specific gravity, settles faster than does BGS. This settling causes a drop in the white light conversion efficiency of the device. After settlement the luminous intensity (Iv) is only 1.2 candela (cd) and flux only 0.68 lumen (lm) and thus not sufficient for many light emitting diode applications, such as automotive, mobile, illumination, etc.
One current method of suspending the phosphor (to reduce settling) is by coating the phosphor with a material having an affinity for the host matrix. One example of a coating material is an adhesion promoter. Adhesion promoters are also added to the host matrix to help suspend the phosphor. Another method of suspending the phosphor is by using a high viscosity host matrix to create resistance to the movement of the phosphor particles through the host matrix. This high viscosity reduces the rate of sedimentation of the phosphor particles.
For a variety of reasons these prior art methods of suspending the phosphor are not effective on large particle size and high specific gravity phosphors, especially the ZnSeS phosphor, mainly because the phosphor coating material is not strong enough to hold the phosphor in the host matrix due to the gravitational pull on such high specific gravity phosphors. Similarly, resistance through the high viscosity host matrix does not hold the high specific gravity phosphors in the host matrix over time.
In summary then, in a two-phosphor system, both of the phosphors must work together in a certain ratio to convert the color of the light coming from the solid state light source to a desired color, usually white. The ineffective phosphor suspension method of the prior art allowed the ZnSeS phosphor, particles with higher specific gravity to settle faster than did the BGS phosphor particles with lower specific gravity. Consequently, to achieve the desired intensity of light output a high phosphor loading is needed to counteract the settling phenomena in order to convert a high percentage of the primary light to secondary light.
One embodiment shows a method for producing a light emitting device using two phosphors dispersed homogenously in a host matrix. The host matrix may be a polymer, such as heat and/or UV curable epoxy, silica glass, silica gel, silicone, etc. A dispersant, preferably inorganic, is applied to the host matrix to create a strong and stable three dimensional network within the host matrix. The BGS and ZnSeS phosphors are trapped equally in the three dimensional network, holding them homogenously in place within the host matrix. In one embodiment, the inorganic dispersant can be a hydrophobic formed silica, such as, for example, R812.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.
For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
The phosphor particles are then stirred together with the host until the phosphors are homogeneously dispersed in the host. This mixture is then cured to form the solid host with the homogeneously dispersed phosphors therein. In one embodiment, the mixture is cured at a temperature of 140° C. for a period of 2 hours.
Note that the mixing and/or curing can occur within device 10 and/or can occur separately and formed to fit into reflector 13. Also note that the phosphors can be mixed in different ratios if desired.
In
The homogeneously suspended phosphors improve the optical performance of the light emitting device since more controlled color mixing is achieved than is achieved with previous systems. The brightness of the device will also be improved as all the incident light is captured and converted by the phosphors suspended in the cured matrix to produce the secondary light. In one example, the Iv increased by 15% from 1.2 cd to 1.4 cd and the flux increased by 45% from 0.68 lm to 1 lm and little, if any phosphor settling was observed.
If desired other materials can be added into the host matrix either prior to curing or thereafter to achieve additional advantages. For example, an adhesion promoter can be added to promote adhesion of the host matrix to other surfaces such as to the diffuser surface of the illumination device. Another example would be a UV inhibitor that would be added to increase the resistance of the host matrix to breakdown due to UV light impacting the host matrix. Another additive can be an oxidation stabilizer to enable the host matrix to resist breakdown due to heat of the lamp. Examples of adhesion promoters are Elvaloy from DuPont Polymer Modifiers, Silquest from GE Silicones and Lotader from Arkema, examples of UV inhibitors are Chimassorb and Tinuvin from Ciba Specialty Chemicals and Baerostab from Baerlocher and an example of an oxidation stabilizer is Irgastab from Ciba Specialty Chemicals.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
