The invention proceeds from a red-emitting luminescent material belonging to the class of nitridosilicates according to the preamble of claim 1. The invention relates further to a light source having a luminescent material of such kind and to a method for producing the luminescent material. The light source is in particular a conversion LED. Luminescent materials of such kind are intended particularly for use in white LEDs.
WO 01/40403 shows a conversion LED that employs a red-emitting luminescent material belonging to the class of nitridosilicates. The material concerned is the luminescent material MxSiyNz:Eu, with M being represented by Ca, Sr, Ba, and/or Zn. What therein applies is that z=2/3x+4/3y. The foremost representatives are M2Si5N8:Eu and MSi7N3:Eu.
The object of the present invention is to disclose a red-emitting luminescent material that is characterized by a high degree of stability so is well suited for use also in thermally stressed environments. A further object is to disclose a light source that has a luminescent material possessing those properties and a method for producing the luminescent material.
Said objects are achieved by means of the characterizing features of claim 1 or, as the case may be, 6 or, as the case may be, 14.
Particularly advantageous embodiments can be found in the claims dependent thereon.
The novel luminescent material is a modified, preferably Eu2+-doped, alkaline earth nitridosilicate M2Si5N6, where M=one or more elements belonging to the group Sr, Ca, Ba, with the nitridosilicate having been stabilized by an oxidic—in partitular alkaline earth-phase. The luminescent material will thereby ensure the provision of an efficient, stabilized emitting red luminescent material that can be excited by blue or ultraviolet light and has a dominant wavelength in the 600-nm range.
The Eu2+-doped alkaline earth nitridosilicates M2Si5N8, where M=one or more elements belonging to the group Sr, Ca, Ba, are a red-emitting luminescent-material system that has been known for quite a long time. However, Sr2Si5N8 in particular, which is extremely interesting on account of its dominant wavelength of approximately 600 nm-610 nm, is limited in its applications owing to its chemical instability at higher temperatures (>100° C.) and when oxidized and exposed to high radiation.
While exhibiting no weaknesses in terms of emission stability, a competing system such as that of CaAlSiN3 is precluded from many applications on account of its narrow useful range of emission wavelengths from 615 nm to 620 nm.
Thus to date no totally satisfactory solution is known for a red-emitting luminescent material system that exhibits a high degree of stability even when subjected to high temperatures.
M3N2, Si3N4, and Eu2O3 are needed as starting substances for preparing known M2Si5N8. A novel stabilized alkaline earth nitridosilicate is obtained by extending the educt side at least to include SiO2. The result is an initial mixture that stoichiometrically no longer yields M2Si5N8 but is instead a modified Eu2+-doped alkaline earth nitridosilicate M2Si5N8, where M=one or more elements belonging to the group Sr, Ca, Ba, with the nitridosilicate having been stabilized at least by a second oxidic phase, in particular by SiO2. In particular the educt side has been extended to include SiO2 and additional M3N2.
In a specific embodiment variant the resulting product conforms to stoichiometry (1-a)(M2Si5N8:Eu)*a(SiO2), with its preferably being the case that 0<a<0.25.
In another specific embodiment variant the resulting product conforms to stoichiometry (1-a-b)(M2Si5N8:Eu)*a(SiO2)*b(M′3N2), with its preferably being the case that 0<a<0.25 and 0≦b≦0.30. M′ is in particular therein different from the M used, for example M=Sr and M′=Ca. In particular, a and b are therein selected such as ultimately to yield a product in the form (M2Si5N8:Eu)*(W3Si2O4N2:Eu) (M′=one or more elements belonging to the group Sr, Ca, Ba).
Generally, though, it is not simple stoichiometry that results as the product; rather it is the case that for describing the product a phase diagram has to be used that is based on the three partial components SiN4/3, SiO2, and MN2/3.
M is preferably therein represented by Sr at over 50 mol. %
The percentage share of the entire doping means, here in particular Eu, of M ought generally to be in the 0.1-to-15-mol.-% range. Another or an additional doping agent such as, for example, Ce or Mn is not precluded. In particular, interesting systems are Eu, Mn on the one hand and Ce, Li (with or without Eu) on the other. Zi serves in the latter instance to provide charge compensation.
The luminescent material is suitable in particular for photonic excitation by a light source. Such sources are, for example, lamps such as fluorescent lamps or high-pressure discharge lamps, but in particular also conversion LEDs. The luminescent material can here be used in particular for generating white light. The RGB principle is therein usually applied. The inventive luminescent material is therein used for the red emission. Another luminescent material such as, for instance, a sion, in particular Ba sion as known per se, is used for the green emission. For the blue emission, what is best suited is the primary radiation of a blue-emitting LED; a peak wavelength of 410 to 500 nm is preferred.
A blue luminescent material such as BAM can additionally be used in the case of a peak wavelength of 410 to 430 nm. Excitation can in the case of long-wave excitation in the 470-to-500-nm range be provided by another luminescent material's secondary radiation.
The relative percentage shares of the phase triangle SiN4/3, SiO2, MN2/3 are preferably in a range having the following key points, with the sum in each case adding to 100 mol. %:
SiO2:SiN4/3:MN2/3=7.5%:97.5%:25% (1)
SiO2:SiN4/3:MN2/317.5%:57.5%:25% (2)
SiO2:SiN4/5:MN2/3=7.5%:47.5%:45% (3)
SiO2:SiN4/3:MN2/317.5%:37.5%:45%. (4)
What applies particularly preferably is:
SiO2:SiN4/3:MN2/310%:65%:25% (1)
SiO2:SiN4/3:MN2/315%:60%:25% (2)
SiO2:SiN4/5:MN2/310%:45%:45% (3)
SiO2:SiN4/3:MN2/315%:40%:45%. (4)
The invention is described in more detail below with the aid of several exemplary embodiments:
Exemplary embodiments of the novel red-emitting luminescent material are as follows. The resulting dominant emission wave-length is therein in the 595-to-6.0-nm range.
a) Initial mixture 1, normal nitridosilicate
8.7 g Sr3N2, 10.9 g Si3N4, and 0.3 g Eu2O3 are weighed-in in a protective-gas atmosphere and homogenized. The educt mixture is then roasted in the tube or chamber furnace for several hours in a reducing atmosphere at temperatures of between 1,200° C. and 1,800° C. That can be followed by a second roasting, likewise in a reducing atmosphere, at between 1,200° C. and 1,800° C.
b) Initial mixture 2, stabilized nitridosilicate
10.9 g Sr3N2, 1.7 g SiO2, 6.8 g Si3N4, and 0.4 g Eu2O3 are weighed-in in a protective-gas atmosphere and homogenized. The educt mixture is then roasted in the tube or chamber furnace for several hours in a reducing atmosphere at temperatures of between 1,200° C. and 1,800° C. That can be followed by a second roasting, likewise in a reducing atmosphere, at between 1,200° C. and 1,800*C.
c) Initial mixture 3, stabilized nitridosilicate
11.5 g Sr3N2, 0.9 g SiO2, 7.2 g Si3N4, and 0.4 g Eu2O3 are weighed-in in a protective-gas atmosphere and homogenized. The educt mixture is then roasted in the tube or chamber furnace for several hours in a reducing atmosphere at temperatures of between 1,200° C. and 1,800° C. That can be followed by a second roasting, likewise in a reducing atmosphere, at between 1,200° C. and 1,800° C.
d) Initial mixture 4, stabilized nitridosilicate
10.7 g Sr3N2, 2.2 g SiO2, 6.7 g Si3N4, and 0.4 g Eu2O3 are weighed-in in a protective-gas atmosphere and homogenized. The educt mixture is then roasted in the tube or chamber furnace for several hours in a reducing atmosphere at temperatures of between 1,200° C. and 1,800° C. That can be followed by a second roasting, likewise in a reducing atmosphere, at between 1,200° C. and 1,800° C.
e) Initial mixture 5, stabilized nitridosilicate
8.8 g Sr3N2, 1.2 g Ca3N2, 2.7 g SiO2, 6.9 g Si3N4, and 0.4 g Eu2O3 are weighed-in in a protective-gas atmosphere and homogenized. The educt mixture is then roasted in the tube or chamber furnace for several hours in a reducing atmosphere at temperatures of between 1,200° C. and 1,800° C. That can be followed by a second roasting, likewise in a reducing atmosphere, at between 1,200° C. and 1,800° C.
Proceeding generally from the M2Si5N8 point toward the dashed line, we find the quantum efficiency will first drop dramatically before climbing again at the marked region's boundary. That is illustrated by way of example with M=Sr. M is here preferably predominantly more than 50 mol. %, particularly preferably M=Sr. The percentage share of Eu in M is preferably 0.1 to 15 mol. %. The exemplary region having the key points—which region is identified in the phase triangle by the dashed line—is therefore preferably (the associated stoichiometry is indicated in parentheses):
Having quantum efficiencies of more than 70%, the region identified by the unbroken line is to be given particular preference. The rectangle is here described by the following key points:
Adding SiO2 to the initial mixture results in a second phase that significantly improves the luminescent material M2Si5N8:D—with D being represented preferably by Eu—in terms of all its limiting characteristics while not weakening it in any of its characteristics. The resulting ancillary phase is a hitherto unknown, probably oxinitridic phase, which according to current knowledge crystallizes in the structure of Ba3Al2O6 (space group Pa
Sr2Si5N8, which is unstable in all respects and whose dominant emission wavelength of approximately 600-610 nm is favorable for numerous applications, can be stabilized in all the above aspects (temperature quenching, laser stability, oxidation stability) without detriment to its optical properties.
According to
The chip is connected to electric terminals 3 and a bond wire 14. It is surrounded by a housing 8 acting as a reflector 17. Mounted on the chip is a potting 5 containing a luminescent-material mixture 6 in dispersion form. The luminescent materials are a red-emitting luminescent material of the modified nitridosilicate type and a yellow-green luminescent material such as, for instance, YAG:Ce or a sion such as BaSi2O2N2:Eu.
Number | Date | Country | Kind |
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10 2008 058 295.6 | Nov 2008 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP09/64072 | 10/26/2009 | WO | 00 | 8/23/2011 |