The present invention relates to a method for producing a glass ceramic having a garnet phase and to an advantageous application of such glass ceramic.
Recently, LEDs have been employed to an increasing degree for lighting purposes because they offer a number of advantageous properties including, for example, high efficiency due to direct conversion of electric energy to light energy, and high compactness.
However, until a few years ago LEDs were employed in “low-emitting” applications only, especially for indication purposes. The high potential of LEDs for applications with high light demand was discovered only recently when increased efforts were made to achieve improved energy input coupling and improved heat management.
LEDs produce light in a very narrow spectral band, while in most of the cases white light is required for illumination purposes. Commercially available white LEDs use a III-V semiconductor emitter for stimulating a luminescent material that emits a secondary wavelength in a lower wavelength band (down conversion). One known solution uses a blue InGaN/GaN LED for stimulating YAG:Ce, a broadband yellow luminescent material. With these LEDs, that have been converted using a luminescent material, a given proportion of the emitted blue light passes the luminescent layer covering the LED chip so that the overall spectrum obtained assumes a color very close to white light. Due to the absence of spectral portions in the blue/green band and in the red wavelength band, the resulting color is, however, not satisfactory in most of the cases.
US 2003/0025449 A1 discloses a hybrid LED comprising a radiation-emitting semiconductor body (chip) which is in direct contact, via an optical coupling agent, with a glass ceramic body that serves as a conversion agent. The glass ceramic body contains crystallites of the garnet type doped with rare earths (such as YAG:CE), thiogalate or chlorosilicate as luminescent material. The starting glasses from which such glass ceramics are produced consist of silicate glasses or borate glasses. The luminescent glass ceramic is produced by mixing a glass frit with a suitable proportion of a luminescent material in powder form, and the mixture is molten, then cast and molded to achieve the desired shape. It is possible in this way, from the very beginning, to produce a glass ceramic body in the desired shape, advantageous for the intended application, for example in the form of a lens.
However, that document does not disclose the way in which to produce such a luminescent glass ceramic having a garnet phase, with properties as advantageous as possible. Rather, the document merely relates to the melting technology used for production in a general way.
Another lighting source using an LED that emits blue light has been known from U.S. Pat. No. 6,791,259 B1. For producing white light, a phosphorous that emits yellow light, such as YAG:Ce or ZnS:Mn, is used, distributed in an epoxy resin or silicon layer or in a glass layer. In addition, a layer with radiation-diffusing particles is provided, which contains particles of BaTiO3, Al2O3, SiO2, CaCO3 or BaSO4. Those radiation-diffusing particles are again embedded in a carrier layer consisting of epoxy resin, silicon or glass.
Such a light-conversion material is connected with certain disadvantages due to the luminescent materials used being embedded in anorganic carrier materials. The granulates used give rise to scattering losses. And non-homogeneous distribution of the granulate on the solid-state emitter may lead to variable color perception as a function of angle. In addition, epoxy resins are instable over time in many respects, especially with respect to their optical and mechanical properties. And as a rule, their thermal stability is also unsatisfactory for generating high brightness. To the extent the respective diffusion materials and luminescence materials are embedded in glass carrier layers, the before-mentioned document does not provide any information how these are to be produced. Moreover, the production and application of a plurality of superimposed layers is complex and expensive.
EP 1 642 869 A1 discloses a glass ceramic which preferably is used for down-conversion of excitation radiation in the blue and UV regions of the spectrum. The glass ceramic comprises the following components (on an oxide basis): 5-50% by weight of SiO2, 5-50% by weight of Al2O3, 10-80% by weight of Y2O3, 0-20% by weight of B2O3, 0.1-30% by weight of rare earths, preferably 15-35% by weight of SiO2, 15-35% by weight of Al2O3, 25-60% by weight of Y2O3, 1-15% by weight of B2O3 and 1-15% by weight of rare earths. The glass ceramic contains crystalline phases in which rare-earth ions are taken up at least in part. Crystalline phases containing yttrium ions as a component are replaced by rear-earth ions in this case at least in part. The phases in question may include, for example, Y3Al5O12 (YAG), Y2SiO5, Y2Si2O7, SrAl2O4, BaMgAl10O17, Sr2P2O7, Sr4Al14O25 and YbO3, that serve as host phase for taking up rare-earth ions at least in part.
The respective glasses are produced by a technological melting process and may then be ceramized. Ceramization is effected by initial tempering at a nucleation temperature of between 850° C. and 900° C., for a period of several hours, and then ceramizing at a temperature of between 1050 to 1150° C. for the time of one hour. The crystal phases identified in this case were Y2Si2O7, Y2SiO5, YbO3.
The conversion efficiency of such glass ceramics is, however, not yet sufficient for many applications because the glass ceramic contains a number of non-convertible crystal phases such as Y2Si2O7.
JP(A) H04-119941 further discloses a glass ceramic that contains 5-50% by weight of SiO2, 5-70% by weight of Al2O3, 10-70% by weight of Y2O3 and 0.1 to 30% by weight of a nucleation agent such as MgO, TiO2, ZrO2 or La2O3. For the production process, starting materials (oxides) are mixed with organic solvents and binders, and are heated to then form a shaped glass by solid-state reaction. The glass so produced is then subjected first to a nucleation process, by tempering at temperatures of between 950° C. and 1010° C., and then to ceramization at a temperature of approximately 1100° C.
The production process is relatively complex. And in addition, the glass ceramic has a conversion efficiency not sufficient for all applications.