Phosphor, method for producing a phosphor and use of a phosphor
09725646 · 2017-08-08
Assignee
Inventors
- Tim Fiedler (München, DE)
- Daniel Bichler (Augsburg, DE)
- Stefan Lange (Augsburg, DE)
- Rebecca Römer (Regensburg, DE)
- Frank Jermann (Königsbrunn, DE)
- Frauke Thienel (München, DE)
- Barbara HUCKENBECK (Augsburg, DE)
- Alexander Baumgartner (Donaustauf, DE)
- Vera Stöppelkamp (Augsburg, DE)
- Norbert Bönisch (Königsbrunn, DE)
- Hailing Cui (Regensburg, DE)
Cpc classification
H10K59/38
ELECTRICITY
H01L2924/00014
ELECTRICITY
C09K11/77348
CHEMISTRY; METALLURGY
H01L2924/00014
ELECTRICITY
International classification
H01S5/028
ELECTRICITY
Abstract
A phosphor is disclosed. In an embodiment a phosphor includes an inorganic substance which includes, in its composition, at least an element D, an element A1, an element AX, an element SX and an element NX where D includes one, two or more elements selected from the group consisting of Mn, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, alkali metals and Yb, A1 includes one, two or more elements selected from the group consisting of divalent metals not included in D, SX includes one, two or more elements selected from the group consisting of tetravalent metals, AX includes one, two or more elements selected from the group consisting of trivalent metals, and NX includes one, two or more elements selected from the group consisting of O, N, S, C, Cl, and F, wherein the inorganic substance has the same crystal structure as Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.61.
Claims
1. A phosphor for emission of red light, wherein the phosphor has the formula:
Sr.sub.xCa.sub.1-xAlSiN.sub.3:Eu, wherein x is: 0.85<x0.95, wherein between 0.35% and 2.2% inclusive of Sr, Ca and/or Sr/Ca lattice sites are replaced by Eu, and wherein, in a x-ray structure analysis, the phosphor in orthorhombic description exhibits a reflection (R) having the Miller indices 1
2. The phosphor according to claim 1, wherein the phosphor exhibits, in a powder diffractogram on monochromatic irradiation with Cu-K.sub.1 radiation, the reflection (R) having the Miller indices 1
3. The phosphor according to claim 2, wherein the reflection (R), based on a main reflection, exhibits an intensity between 0.3% and 8% inclusive.
4. The phosphor according to claim 1, wherein the phosphor has a dominant wavelength between 596 nm and 606 nm inclusive, wherein a breadth of a radiation spectrum emitted by the phosphor at half a maximum height is between 75 nm and 87 nm inclusive, and wherein the phosphor has a relative absorption maximum in a wavelength range between 410 nm and 450 nm inclusive and, when induced, is configured to emit radiation with blue light.
5. A phosphor for emission of red light, wherein the phosphor has the formula:
Sr.sub.xCa.sub.1-xAlSiN.sub.3:Eu, wherein x is: 0.8<x1, wherein between 0.1% and 5% inclusive of Sr, Ca and/or Sr/Ca lattice sites are replaced by Eu, and wherein, in an x-ray structure analysis, the phosphor in orthorhombic description exhibits a reflection (R) having the Miller indices 1
6. A phosphor for emission of red light, wherein the phosphor has the formula:
Sr.sub.xCa.sub.1-xAlSiN.sub.3:Eu, wherein x is: 0.8<x1, wherein between 0.1% and 5% inclusive of Sr, Ca and/or Sr/Ca lattice sites are replaced by Eu, and wherein, in an x-ray structure analysis, the phosphor in orthorhombic description exhibits a reflection (R) having the Miller indices 1
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) A phosphor described here is elucidated in detail hereinafter by embodiments with reference to drawings. Identical reference numerals indicate identical elements in the individual figures. However, the drawings are not to scale; instead, individual elements may be shown in excessively large size for better understanding.
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(66) A phosphor described here is elucidated in detail hereinafter by embodiments with reference to the drawing. Identical reference numerals indicate identical elements in the individual figures. However, the drawings are not to scale; instead, individual elements may be shown in excessively large size for better understanding.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
(67) One embodiment of a phosphor described here can be produced as follows: reactants used for the synthesis of the phosphor of the general empirical formula Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6:Eu are the binary nitrides of the constituent elements, i.e. Ca.sub.3N.sub.2, Sr.sub.3N.sub.2, AlN and Si.sub.3N.sub.4. Since these are highly oxidation-sensitive and hydrolysis-sensitive substances, what is called a glovebox is employed, under an N.sub.2 atmosphere with O.sub.2<1 ppm and H.sub.2O<1 ppm. In addition, doping with Eu.sup.2+ is accomplished using Eu.sub.2O.sub.3. The reactants are weighed out such that the following atomic ratio is effectively present, in a simplifying representation: Sr:Ca:Si:Al:Eu=(1+a):(1a):2:2:y, where y corresponds to the degree of doping, i.e. the proportion of divalent lattice sites which are substituted by Eu. In addition, various fluxes are added; see the above explanation. A reactant mixture is scaled up, for example, to a total starting weight of 50-100 g with retention of the atomic ratios described above. It is also possible to use other total starting weights.
(68) The reactant mixture is introduced, for example, into a PET mixing vessel together with ZrO.sub.2 balls and mixed on a roller table in a glovebox for 6 h. Subsequently, the balls are removed from the mixture and the powder is transferred into a closed molybdenum crucible. This crucible is placed into an outer tungsten crucible, a semicircular open tungsten tube, and transferred into a tube furnace. There is a flow of 3 l/min of forming gas with 92.5% N.sub.2 and 7.5% H.sub.2 through the tubular furnace during the run time. In the tubular furnace, the mixture is heated at a rate of 250 K/h to 1650 C., kept at this temperature for 4 h and then cooled down at 250 K/h to 50 C. The calcined cake formed is taken out after the furnace has cooled, comminuted with a mortar mill and sieved through a sieve having a mesh size of 31 m. The sieve fraction <31 m is the phosphor used.
(69) The sieving may optionally be followed by a further calcining, tempering and/or washing operation and/or a coating operation.
(70) Illustrative starting weights m in g and resulting color loci CIE x, CIE y, also referred to as chromaticity coordinates, of the emission spectrum of the particular phosphor in the CIE standard chromaticity diagram on excitation with blue light at 460 nm and on complete absorption of the blue light are listed in tabular form in
(71)
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(73) For the phosphor according to the prior art, a very small change in the FWHM with rising x is observed (dotted line). The novel phosphor of the invention, in contrast, at an Sr content of a>0.6, which would correspond in formal terms to an x>0.8 in the case of the known phosphor, exhibits a very significant change in the half-height width FWHM with rising a. Moreover, the half-height width of the novel phosphor is significantly lower than in the case of the phosphor according to the prior art. The parameter a can therefore readily be chosen between 0.6 and 1.0, or between 0.8 and 1.0; boundary values are preferably excluded. Very good properties are exhibited especially by phosphors having a value of a between 0.64 and 0.96, or between 0.82 and 0.98, including the boundary values. Particular preference is given to a range for a between 0.68 and 0.92, or between 0.84 and 0.96, including the boundary values.
(74) In the case of use of constituents other than Sr, Ca, the value of a may also be much lower.
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(76) What is shown here is a dependence of a spectral half-height width FWHM of the emission on the Sr content for embodiments of the novel phosphor Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6:Eu of the invention (open symbols) compared to known phosphors composed of the Sr.sub.xCa.sub.1-xAlSiN.sub.3:Eu system (solid symbols). The Sr content corresponds to the parameter a for Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6:Eu or x for Sr.sub.xCa.sub.1-xAlSiN.sub.3:Eu, where a=2x1. In addition, a dominant wavelength ldom of the spectrum emitted by the phosphor and the Eu content are specified.
(77) Surprisingly, a phosphor Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6:Eu of the invention with a=0.8 (which would correspond to x=0.9), with a comparable dominant wavelength ldom, shows a much smaller half-height width FWHM compared to conventional phosphors of the Sr.sub.xCa.sub.1xAlSiN.sub.3:Eu type. The much smaller half-height width FWHM is virtually independent of the Eu content of the sample used.
(78)
(79) Phosphors of the novel type feature a small half-height width FWHM of the emission compared to the previously known phosphor (within the range of 79 to 81 compared to 86 to 88 for the prior art) and a very high luminous efficiency LER (110 to 111% compared to 100 to 101% for the prior art) combined with simultaneously high internal quantum efficiency QI and external quantum efficiency QE (about 113% compared to 100% to 110% for the prior art); see the table in
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(81) All the phosphor mixtures were adjusted such that, in the CIE standard chromaticity diagram, a color locus close to Planck with a correlated color temperature CCT of about 2700 K is attained. The color rendering index CRI of all the LEDs measured is 801. All the red phosphors R used show a comparable dominant wavelength of about 600.5 nm1 nm.
(82) Further details of the phosphor mixtures as shown in
(83)
(84) Further data relating to the LED measurements from
(85) Red phosphors composed of the novel material system were subjected to a hydrolysis test in order to assess aging stability of the phosphor with respect to air humidity; see
(86) In
(87) To improve the stability of the novel phosphor of the invention to hydrolysis, specimens of the phosphor were coated with an inert material (SiO.sub.2) after the synthesis.
(88) Untreated and subsequently coated samples were subjected to a hydrolysis test, in order to assess the aging stability of the phosphor to air humidity. For this purpose, the phosphor powders were stored at 130 C. and 100% rel. air humidity for 48-56 h. The quantum efficiency and the absorption of the phosphors in the blue spectral region (450-470 nm) were measured both before and after this treatment. A measure of the stability of a phosphor with respect to hydrolysis (decomposition of the phosphor in the presence of water) is considered to be the change in the relative conversion efficiency (calculated from the quantum efficiency and absorption in the spectral range of 450-470 nm) before and after the degradation test. The coating distinctly improves the stability.
(89) Figures boa and bob show SEM images of various phosphors.
(90) The figures show SEM images of the uncoated phosphor before and after the degradation process in different magnifications. What are shown are embodiments of the novel phosphor having the composition Sr(Sr.sub.0.8Ca.sub.0.2)Si.sub.2Al.sub.2N.sub.6:1.2% Eu.
(91) Formation of cracks in the individual phosphor grains is apparent in the SEM images of the sample after the degradation test.
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(93) The figures show SEM images of the coated phosphor before and after the degradation process in different magnifications.
(94) What are shown are embodiments of the novel phosphor having the composition Sr(Sr.sub.0.8Ca.sub.0.2)Si.sub.2Al.sub.2N.sub.6:1.2% Eu.
(95) No formation of cracks in the phosphor grains is apparent in the SEM images of the sample after the degradation test.
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(98) What is reported therein about these phosphors composed of the CaAlSiN.sub.3:Eu system (referred to hereinafter as calsin) is that the conversion efficiency stagnates with rising activator content (>0.8% Eu).
(99) Similar behavior is also known for SCASN. The described phosphor composed of the (Sr,Ca)AlSiN.sub.3:Eu system with Sr content 80% shows strong dependence of the relative emission intensity of the luminescence signal on the activator content. This behavior is described, for example, in H. Watanabe et al., J. Electrochem. Soc., 2008, 155 (3), F31-F36. The behavior is shown in
(100) In contrast to pure calsin (cf.
(101) Similar behavior is also described in U.S. Pat. No. 8,274,215.
(102) In the case of an Eu content of at least 1% (x=0.01), the luminescence signal decreases or virtually stagnates (Sr content: 80%).
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(105) In order to determine the structure of the novel phosphor of the invention, crystals of the novel phosphor were chosen under a light microscope and prepared for a diffractometry study. The measurement was effected on a Bruker D8 Venture with rotating anode and CCD detector. A summary of the results (important goodness factors and the underlying refined parameters) can be found in the table in
(106) The diffraction patterns collected were examined in great detail for their quenching conditions. A basic pattern discernible is a structure derived from AlN (wurtzite structure type) which can be described in the P21 space group.
(107) The solution and refinement of the data set were effected with the JANA2006 software package (Petricek, V., Dusek, M. & Palatinus, L. (2006). Jana2006. The crystallographic computing system. Institute of Physics, Prague, Czech Republic.).
(108) The refinement proceeds very efficiently with the following restrictions: since Si and Al are indistinguishable by x-ray methods, all Si and Al positions were refined with the population of Si:Al=1:1 as weighed out and a single thermal displacement factor for Si and Al.
(109) In addition, all nitrogen atoms together and all alkaline earth metal atoms together were each described by one thermal displacement factor. All further parameters (e.g. the atomic position parameters) were freely refined.
(110) Results of these single-crystal studies are discussed in detail hereinafter.
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(112) A whole series of known nitrides used as phosphors can be derived from AlN with wurtzite structure. Because of this fundamental structural relationship, the diffractograms (particularly x-ray powder diffractograms) of these compounds also often appear to be similar at first glance. However, differences are found in clearly pronounced details. The structures can differ significantly, as shown in
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(114) According to
(115) The pseudohexagonal base structure is clearly apparent.
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(117) Finally,
(118) The circled reflections are examples of reflections which cannot exist in the Cmc21 space group. The clearly apparent occurrence thereof rules out description in the Cmc21 space group. The novel phosphor thus cannot have the same structure as, for example, (Ca,Sr)AlSiN.sub.3, LiSi.sub.2N.sub.3, NaSi.sub.2N.sub.3.
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(120) The single-crystal diffractometry solution of the novel structure Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6 is shown in detail in
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(125) This ordering into a position having a mixed Sr/Ca population and a position fully populated by Sr alone in the novel phosphor Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6:Eu of the invention is advantageous, for example over the structure of SCASN (cf.
(126) Proceeding from
(127) In
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(134) The novel phosphor described here offers the following advantages in particular: lower half-height width of emission, associated with higher luminous efficiency at the same dominant wavelength, the possibility of achieving higher activator concentrations of Eu at >0.8% with simultaneously high quantum efficiency and conversion efficiency, associated with a smaller phosphor demand in LED applications and simplified processibility, improved aging stability with respect to moisture compared to conventional (Sr,Ca)AlSiN.sub.3:Eu having low Sr content, and improvement of thermal stability.
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(136) For use in a white LED together with a GaInN chip, for example, a construction similar to that described in U.S. Pat. No. 5,998,925 is used. The structure of such a light source for white light is shown explicitly in
(137) Generally, in the case of an LED chip that emits UV radiation as radiation source, it is possible to use a phosphor mixture composed of at least three different phosphors (blue-emitting phosphor, for example BaMgAl.sub.10O.sub.7:Eu.sup.2+ or (Ba,Sr,Ca).sub.5(PO.sub.4).sub.3Cl:Eu.sup.2+, together with a green/yellow-emitting phosphor, for example one of the garnet phosphors described here, and an orange/red-emitting phosphor, for example one of the invention), and, in the case of a blue light-emitting LED chip as radiation source, to use a phosphor mixture composed of at least two different phosphors (a green/yellow phosphor, for example one of the garnet phosphors described here, and an orange/red-emitting phosphor, for example one of the invention).
(138)
(139) In a number of further embodiments of the present invention, conventional blue light-emitting InGaN LEDs were provided with a conventional silicone potting material with different amounts of various phosphors of the invention or conventional phosphors as comparative examples embedded therein. In this case, phosphors of the invention in particular are to be used partly together with other phosphors for color conversion of blue primary light to the red or yellow or yellow-orange wavelength range. These phosphors may also be used, inter alia, for full conversion of the primary light from the primary radiation source to the red or yellow or yellow-orange wavelength range.
(140) Alternatively, the primary radiation source used, rather than an LED such as an InGaN LED, may also be an organic light-emitting device (OLED) comprising a layer stack of organic semiconductive layers disposed between an anode and a cathode. In this case, at least one of the electrodes has to be transparent to the radiation produced by the OLED, in which case the phosphors of the invention can be disposed in the beam path of the radiation above the transparent electrode.
(141) Embodiments for full conversion of red: the table in
(142) It can be inferred unambiguously from this table in
(143) In the table in
(144) The second phosphors described in EP patent application EP 1 696 016 A1 are especially phosphors including the elements M, A, D, E, and X, where M is one or more elements selected from the group consisting of: Mn, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb, A is one or more elements selected from the group consisting of divalent metal elements other than M, D is one or more elements selected from the group consisting of tetravalent metal elements, E is one or more elements selected from the group consisting of trivalent metal elements, X is one or more elements selected from the group consisting of: O, N, and F, and where the phosphors have the same crystal structure as CaAlSiN.sub.3.
(145) In addition, second phosphors used may also be phosphors of the general formula (M.sub.1-xEu)x(Al,Q)(Si,Y)N.sub.3 where M=Ca, Sr, alone or in combination or in combination with other divalent and/or monovalent elements, for example Li, Q=trivalent cation other than Al.sup.3+, Y=other tetravalent cations other than Si.sup.4+, N.sup.3 may be partly replaced by O.sup.2, F.sup., Cl.sup., Br.sup., C.sup.4.
(146) In the table in
(147) Analogously to inventive example 3 in inventive example 4 as well, which is compared with a comparative example 4 in the table in
(148) Two inventive examples 5 and 6 are compared in the table in
(149) Further lighting devices of the invention are to be described hereinafter, where different amounts of phosphors of the invention having high europium concentrations are present as dopant in the potting material of the radiation source that emits the primary radiation, for example a blue LED. Lighting devices of this kind can also be used, for example, for full conversion of the primary radiation from the radiation source to a secondary radiation, for example in a red or yellow or yellow-orange wavelength range. More particularly, it is possible for lighting devices of the invention, in particular embodiments of the present invention, to have, as a radiation source that emits primary light, a blue LED having a dominant wavelength of 300-500 nm, preferably 400-500 nm, further preferably 420-470 nm, and a phosphor of the invention having the general formula Sr(Sr.sub.aM.sub.1-a)Si.sub.2Al.sub.2N.sub.6:D where M is selected from the group of Ca, Ba, Zn, Mg, preferably Ca, and D is preferably Eu, and where the europium concentrations may be 6 mol %, further preferably 8 mol %. The radiation emitted by these radiation sources may have a half-height width FWHM of 90 nm, preferably 85 nm, and a dominant wavelength of 607 nm, preferably 609 nm.
(150) The table in
(151) The phosphors of the invention may be used with a multitude of different garnet phosphors as second phosphors. These may especially have the general structural formula:
(Gd,Lu,Tb,Y).sub.3(Al,Ga,D).sub.5(O,X).sub.12:RE
(152) with X=halide, N or divalent element, D=tri- or tetravalent element and RE=rare earth metals as activator, especially cerium with an optional co-dopant, for example lanthanoids, e.g. Pr, Sm, Nd.
(153) The garnets may additionally also have the following general formula:
(Gd,Lu,Tb,Y).sub.3(Al,Ga).sub.5(O).sub.12:RE
(154) with RE=rare earth metals as activator, especially cerium with an optional co-dopant, for example lanthanoids, e.g. Pr, Sm, Nd.
(155) Embodiments of Streetlighting Applications:
(156) In a further embodiment of the present invention, it is possible to provide lighting devices of the invention which can especially also be used for general lighting applications, for example streetlighting, with a CRI70 and high color temperatures (5000 K).
(157) More particularly, these lighting devices may have, as radiation source, a blue LED having a dominant wavelength of 300-500 nm, preferably 400-500 nm, further preferably 420-470 nm, with at least one phosphor of the invention having the general formula Sr(Sr.sub.aM.sub.1-a)Si.sub.2Al.sub.2N.sub.6:D where M is selected from the group of Ca, Ba, Zn, Mg, with D=Eu, present in the beam path of the radiation source as first phosphor and at least one yellow/green-emitting garnet phosphor of the general formula (Y,Lu,Gd).sub.3(Al,Ga).sub.5O.sub.12:Ce present as second phosphor.
(158) The first phosphor used here may be a phosphor of the invention having the general formula Sr(Sr.sub.aM.sub.1-a)Si.sub.2Al.sub.2N.sub.6:D where M is selected from the group of Ca, Ba, Zn, Mg, preferably Ca, and D is preferably Eu, with a0.8, preferably a0.82, further preferably a0.85, and a europium content of 0.1-5 mol %, preferably 0.1-3 mol %, further preferably 0.1 to 2 mol %. The phosphor of the invention has a peak emission wavelength of about 600-640 nm, preferably 605-625 nm, and a half-height width FWHM of <85 nm, preferably <80 nm, further preferably <78 nm. Such a phosphor of the invention together with the garnet phosphor as second phosphor can give a lighting device in which a wide range of correlated color temperature CCT within the range of at least 6500-4000 K, preferably 6500-3000 K, is possible, where the CRI is at least 70.
(159) In this case, the garnet phosphor as second phosphor may especially have the general formula Lu.sub.3(Al,Ga).sub.5O.sub.12:Ce or (Y,Lu).sub.3(Al,Ga).sub.5O.sub.12:Ce, in which case the maximum excitability is preferably between 440-455 nm, further preferably between 454-450 nm. The yellow/green-emitting garnet phosphor is selected such that it has a high conversion efficiency and high thermal stability. A preferred yellow/green-emitting phosphor is a (Y,Lu).sub.3(Al,Ga).sub.5O.sub.12:Ce having a cerium content of 0.5-5 mol %, preferably 1-3 mol %, and a Y content of 0-50 mol %, preferably 0-30 mol %, so as to result in a phosphor of the general formula (Lu.sub.1-xY.sub.x).sub.3(Al,Ga).sub.5O.sub.12:Ce with x=0 to 0.5, preferably x=0 to 0.3. Other variants of the garnet phosphor are also possible, having similar spectral characteristics, especially variants with (Y,Lu).sub.3(Al,Ga).sub.5O.sub.12:Ce where at least some of the Al has been exchanged for Ga.
(160) In a further embodiment, a garnet phosphor of the following general formula Y.sub.3(Al,Ga).sub.5O.sub.12:Ce is used, having a maximum excitability in the range of 440-455 nm, preferably 445-450 nm. The preferred yellow/green-emitting phosphor is selected such that it has a high conversion efficiency and high thermal stability. The preferred yellow/green-emitting phosphor is a phosphor of the general formula Y.sub.3(Al,Ga).sub.5O.sub.12:Ce having a cerium content of 1.5-5 mol %, preferably 2-5 mol %, and a gallium content of 0-50 mol %, preferably 0-30 mol %. Within this system, however, other element combinations are also possible, which result in similar spectral properties.
(161) Hereinafter the optical properties of two comparative examples 1 and 2 where a garnet phosphor has been combined with a conventional 2-5-8 phosphor in the standard silicone potting material of a conventional InGaN LED having the dominant wavelength of 444.5 or 444.6 nm at 350 mA are to be compared with three embodiments of the present invention. The area of each of the LED chips is 1 mm.sup.2.
(162) The compositions of the various phosphors and the respective concentrations of the phosphors in the standard potting material and the relative proportions of the two phosphors are given in
(163) It can be inferred from the table in
(164)
(165) In summary, it can be stated that neither of comparative examples 1 and 2 attains the necessary CRI of 70 at 3000 K for streetlighting applications. For this reason, comparative examples 1 and 2 can be used either only in lighting devices having a correlated color temperature CCT between 6500-4000 K and not within a range of 6500-3000 K, or an additional, third phosphor has to be used to improve the color temperature, but this complicates the manufacturing process for the lighting devices. Thus, lighting devices of the invention, because of the phosphor of the invention, can have a sufficient CRI>70 within a broad color temperature range of 6500-3000 K for streetlighting applications. In contrast to conventional phosphor mixtures, no additional, third phosphor is needed for the achievement of a broad color temperature range.
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(167) In further embodiments of the present invention, the efficiency of the lighting devices, for example for streetlighting, at a particular color temperature CCT and a particular CRI, can be improved by adding a further, third phosphor to the phosphor mixtures. More particularly, a lighting device in this embodiment of the present invention may have a radiation source having a blue light-emitting LED having a dominant wavelength between 440-455 nm, and also a red-emitting phosphor of the invention having a peak wavelength between 605-620 nm, preferably 605-616 nm, and a half-height width FWHM80 nm, preferably 78 nm, as the first phosphor, a green/yellow-emitting phosphor having a peak wavelength between 540-565 nm, preferably 546-556 nm, and a half-height width FWHM100 nm, preferably 105 nm, as the second phosphor, and a yellow/orange-emitting phosphor having a peak wavelength of 580-590 nm, preferably 582-588 nm, and a half-height width FWHM80 nm, preferably 78 nm, as the third phosphor.
(168) The first phosphor used may especially be a red-emitting phosphor of the general formula Sr(Sr.sub.aM.sub.1-a)Si.sub.2Al.sub.2N.sub.6:D where M is selected from the group of Ca, Ba, Zn, Mg, preferably Ca, having a peak emission between 605-620 nm, preferably 605-616 nm, and a half-height width FWHM below or equal to 80 nm, preferably below or equal to 78 nm. It may especially be the case here that the value a0.8, preferably a0.84, and the europium content is between 0.1-5 mol %, preferably 0.1-3 mol %, further preferably between 0.1-2 mol %. These red-emitting phosphors feature a high thermal stability and a high conversion efficiency under operating conditions typical for streetlighting.
(169) More particularly, the green/yellow-emitting phosphor as the second phosphor may be a garnet phosphor of the general formula (Y,Lu,Gd,Tb).sub.3(Al,Ga).sub.5O.sub.12:Ce which is matched to the emission wavelengths of the blue LED and the two other phosphors. More particularly, the phosphor may be a green-emitting garnet phosphor of the general formula (Y,Lu).sub.3(Al,Ga).sub.5O.sub.12:Ce, especially a yellow/green-emitting phosphor of the general formula (Y,Lu).sub.3(Al,Ga).sub.5O.sub.12:Ce having a cerium content of 1-5 mol %, preferably 2-4 mol %, and an yttrium content of 0-50 mol %, preferably 0-30 mol %, further preferably 0-20 mol %, and a gallium content of 0-50 mol %, preferably 0-30 mol %, further preferably 0-15 mol %, and so the result is a garnet phosphor of the following general formula: (Lu.sub.1-xY.sub.x).sub.3(Al.sub.1yGa.sub.y).sub.5O.sub.12:Ce with x=0 to 0.5, preferably x=0 to 0.3, further preferably x=0 to 0.15 and y=0 to 0.5, preferably y=0 to 0.2, further preferably y=0 to 0.15. Other element combinations within the general formula (Y,Lu,Gd,Tb).sub.3(Al,Ga).sub.5O.sub.12:RE with RE=rare earth metal, e.g. Ce, are likewise possible.
(170) The third phosphor used may especially be a phosphor of the general formula (Ca,Sr,Ba).sub.2(Si,Al).sub.5(N,O).sub.8:Eu which, together with the primary light source, for example a blue LED, and the green/yellow-emitting phosphor and the red-emitting phosphor of the invention, achieves a CRI65, preferably 70, over a wide range of correlated color temperature CCT (at least of 6500-4000 K, further preferably of 6500-3000 K). In order to achieve the above-described spectral properties for the yellow/red- or yellow/orange-emitting phosphor as the third phosphor, this has a europium content of 0.1-5 mol %, preferably of 0.1-3 mol %, further preferably of 0.1-2 mol %, and a barium content of 50-100 mol %, preferably 70-100 mol %, further preferably 80-100 mol %, and a calcium content of 0-20 mol %, preferably 0-10 mol %, where the proportion of strontium is chosen such that the alkaline earth metals barium, strontium and calcium together with the europium dopant add up to 100%.
(171) There follows a description of one embodiment of a lighting device of the invention having three different first to third phosphors in relation to a comparative example. Comparative example 1 is a lighting device having a blue LED chip (dominant wavelength 445 nm) having a chip area of 1 mm.sup.2, in which 14% by weight of a phosphor mixture composed of two different phosphors is present in the standard silicone potting material of the LED, where the ratio of the green to the red phosphor is 4.7:1.
(172) The green-emitting phosphor here is a phosphor of the formula (Lu.sub.0.85Y.sub.0.15).sub.3Al.sub.5O.sub.12:Ce (3 mol %), and the red-emitting phosphor a phosphor of the formula (Sr.sub.0.5Ba.sub.0.5).sub.2Si.sub.5N.sub.8:Eu (i mol %), the LED being operated at a driver current of 350 mA. In inventive example 1, present in the standard silicone potting material of a blue LED having a dominant wavelength of 445 nm at a driver current of 350 mA is 14% by weight of a phosphor mixture comprising the following three first to third phosphors: Sr(Sr.sub.0.86Ca.sub.0.14)Si.sub.2Al.sub.2N.sub.6:Eu (0.8 mol %) as the first phosphor, (Lu.sub.0.85Y.sub.0.15).sub.3Al.sub.5O.sub.12:Ce (3 mol %) as the second phosphor and (Sr.sub.0.1Ba.sub.0.9).sub.2Si.sub.5N.sub.8:Eu (1 mol %) as the third phosphor, where the ratio of first phosphor:second phosphor:third phosphor is 0.67:5.3:0.33. The area of the LED chip is again 1 mm.sup.2.
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(179) Embodiments of backlighting applications: a further embodiment of the present invention is directed to phosphor mixtures for backlighting applications. For backlighting applications, a broad color space has to be achieved with narrow-band red- and green-emitting phosphors, the phosphor mixture determining the optical properties of the LED, such as brightness, efficiency and robustness.
(180) For the sRGB/Rec709 color space, lighting devices which can be used as backlighting devices for LCDs, for example, are especially those having, as primary radiation source, a blue LED having a dominant emission wavelength of 430-470 nm, preferably 440-460 nm, further preferably 445-455 nm, and also containing a green/yellow-emitting garnet phosphor of the general formula (Lu,Y,Gd,Tb).sub.3(Al,Ga).sub.5O.sub.12:Ce and also a red-emitting phosphor, especially an inventive phosphor of the general formula Sr(Sr.sub.aM.sub.1-a)Si.sub.2Al.sub.2N.sub.6:D where M is selected from the group of Ca, Ba, Zn, Mg, preferably Ca, and D is preferably Eu. It is optionally also possible for further converters or phosphors or non-converting materials such as diffusers to be present in the phosphor mixture.
(181) The garnet phosphor may especially have the general composition (Lu,Y).sub.3(Al,Ga).sub.5O.sub.12:Ce and may also take the form of yttrium aluminum gallium garnet having a gallium content of 20 mol %x60 mol %, further preferably 30 mol %x50 mol %, further preferably 30 mol %x45 mol %, so as to result in the general formula: Y.sub.3(Al.sub.1-xGa.sub.x).sub.5O.sub.12:Ce with 0.2x0.6, further preferably 0.3x0.5, further preferably 0.3x0.45. In addition, the garnet phosphor may also take the form of lutetium aluminum gallium garnet having the following general formula: Lu.sub.3(Al.sub.1-xGa.sub.x).sub.5O.sub.12:Ce with 0x0.6, further preferably 0x0.4, further preferably 0x0.25, with a cerium content of 0.5-5 mol %, preferably 0.5-3 mol %, further preferably 0.5-2.5 mol %, based in each case on the rare earth metals.
(182) The red-emitting phosphor of the invention may have an activator content of 2 mol %, preferably an activator content of 3 mol %, further preferably a content of 4 mol %, where the divalent metals, which are preferably strontium and calcium, have a calcium content of 15 mol %, preferably 10 mol %, further preferably 8 mol %, so as to result in the following general formula: Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6:Eu with a0.7, preferably a0.8, further preferably a0.84.
(183) Both the abovementioned garnet phosphors may be used, the use of the yttrium aluminum gallium garnet rather than the lutetium aluminum gallium garnet bringing the advantage that the yttrium garnet has a lower specific density, such that less phosphor is required for the phosphor mixtures and, at the same time, a lower percentage of rare earth metals has to be used for the production of the phosphors, and so they can be produced less expensively.
(184) For backlighting applications having elevated demands on the color space (for example Adobe RGB, NTSC or DCI-P3), phosphor mixtures having very narrow-band-emitting green-yellow phosphors are required. Preferably, lighting devices of this kind have, as radiation source, a blue LED having a dominant wavelength between 430-470 nm, preferably 440-460 nm, further preferably 445-455 nm.
(185) The green/yellow-emitting phosphors used may be nitridoorthosilicates which have the general composition AE.sub.2xRE.sub.xSiO.sub.4xN.sub.x:Eu with AE=Sr, Ca, Ba, Mg, RE=rare earth metals and/or AE.sub.2xRE.sub.xSi.sub.1yO.sub.4x2yN.sub.x:Eu, and AE and RE as defined in the previous example, and which are more particularly described in the patent application WO 2011/160944. It is likewise possible to use orthosilicates of the general formula AE.sub.2SiO.sub.4:Eu with AE=Ca, Ba, Mg, Sr. Both the nitridoorthosilicates and orthosilicates having the abovementioned empirical formulae preferably include a combination of at least two alkaline earth metals, further preferably a combination of strontium and barium having the ratio of 0.5Ba:Sr2, further preferably 0.75Ba:Sr1.25. The nitridoorthosilicates may also be described by the general formula AE.sub.2xL.sub.xSiO.sub.4xN.sub.x:RE where AE contains one or more elements selected from Mg, Ca, Sr, Ba and RE contains one or more elements selected from rare earth metals, preferably at least Eu, and L contains one or more elements selected from rare earth metals other than RE, with 0<x0.1, preferably 0.003x0.02. A further general composition is AE.sub.2xL.sub.xSi.sub.1yO.sub.4x2yN.sub.x:RE where AE contains one or more elements selected from Mg, Ca, Sr, Ba and RE contains one or more elements selected from rare earth metals, preferably at least Eu, and L contains one or more elements selected from rare earth metals other than RE, with 0<x0.1, preferably 0.003x0.02, and 0<y0.1, preferably 0.002y0.02.
(186) It is also possible to use yellow/green-emitting beta-SiAlONs of the general formula Si.sub.6zAl.sub.zO.sub.zN.sub.8z:Eu with 0<z4. The beta-SiAlONs may also have the general formula Si.sub.6xAl.sub.zO.sub.yN.sub.8y:RE.sub.z where 0<x4, 0<y4, 0<z<1 and RE is one or more elements selected from rare earth metals, preferably at least Eu and/or Yb.
(187) It is also possible to use yellow- to green-emitting nano-semiconductor materials, called quantum dots, containing at least one compound selected from: a group II-VI compound, a group IV-VI compound or metal nanocrystals.
(188) Inventive red-emitting phosphors may especially phosphors of the following general formula: Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6:D having an activator content of 4 mol %, preferably 8 mol %, further preferably 10 mol %, further preferably mol %, where the divalent metals are preferably strontium and calcium with a calcium content of mol %, preferably 10 mol %, further preferably 8 mol %, so as to result in the general formula Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6:D with a0.7, preferably a0.8, further preferably a0.84.
(189) There follows a discussion of some embodiments of lighting devices discussed here in comparison with conventional lighting devices. The phosphors of inventive examples 1 and 2 shown in
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(191) White light-emitting LED lighting devices were constructed with the aid of various combinations of embodiments of the present invention and comparative examples. A white point having the CIE coordinates CIE-x=0.285 and CIE-y=0.275 was chosen here. The resulting LED emission spectra were analyzed and compared, and the coverage of the color space was determined by employing a standard set of LCD filter absorption curves and determining the resulting filtered color points for the blue, green and red channels. It can be inferred from
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(193) Second phosphors used may also be other phosphors, for example from the group of the SiONs, SiAlONs, silicates and quantum dots.
(194) Embodiments of flash applications: the phosphors of the invention, especially the phosphors of the general structural formula Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2(N,O).sub.6, can also be used for flash applications together with the garnets activated by means of an activator, especially the above-described cerium-activated garnets. For this purpose, the radiation source used, which emits a primary radiation, is a blue-emitting LED, for example an InGaN LED, having a dominant wavelength of 300-500 nm, preferably 400-500 nm, further preferably 420-470 nm. A particularly suitable spectrum for a flash application, for example in mobile phone cameras, has an intensity, based on the maximum of the spectrum in the cyan color range (about 450-500 nm), of at least 12.5%. The intensity of the spectrum in the wavelength range of >650 nm may at the same time be comparatively small, since typical sensors of cameras have a high sensitivity in this range and this spectral region is frequently filtered out by special IR filters in order to avoid disruptive influences of radiation from this radiation range on the sensor and the image quality.
(195) For flash applications, it is possible here to use lighting devices having only one radiation source wherein the blue-emitting and/or UV radiation-emitting LED chip contains a phosphor mixture comprising at least one of the phosphors of the invention, for example together with a yellow/green-emitting garnet phosphor, in the beam path thereof. LED devices for flash applications may additionally at least also have two different LED modules, in which case one module emits a comparatively cold white light (correlated color temperature CCT between 4000-6000 K) and the further module a comparatively warm white light (correlated color temperature CCT between about 1500-3000 K). By controlled feeding of current to the two LED modules, even better variability of lighting situations of the environment in the subject of the photo is possible, for example in artificial light or in daylight.
(196) For a cold white color locus, the first phosphor used may be an inventive orange/red-emitting phosphor of the general formula:
Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2(N,O).sub.6:Eu
(197) with a0.8, preferably a0.82. The europium content is between 0.1-20 mol %, or between 1-10 mol %, further between 0.1-5 mol %, preferably between 0.1-3 mol %, further preferably between 0.1-2 mol %, based on the alkaline earth metals.
(198) The emission peak of the phosphors of the invention may be between 600-640 nm, preferably between 605-625 nm, and the spectral half-height width at half the maximum height (FWHM) should be <85 nm, preferably <80 nm, additionally preferably <78 nm. The emission intensity at wavelengths greater than 650 nm should be very low, since typical sensors of cameras have a high sensitivity in this range.
(199) Second phosphors used for a cold white application may then be the above-described garnets of the general formula:
(Gd,Lu,Tb,Y).sub.3(Al,Ga).sub.5(O).sub.12:RE
(200) with RE=rare earth metals, especially Ce.
(201) The garnets are especially blue/green- to yellow-emitting phosphors of the formulae Lu.sub.3(Al,Ga).sub.5(O).sub.12:Ce and (Lu,Y).sub.3(Al,Ga).sub.5(O).sub.12:Ce, which have particularly good excitability at a wavelength in the range of 425-455 nm, preferably 430-450 nm. Particular preference is given to a blue/green-emitting phosphor having very good stability and conversion efficiency at high temperatures and high radiation intensities which are typical for flash applications, having the formula Lu.sub.3(Al.sub.1-xGa.sub.x).sub.5(O).sub.12:Ce with a cerium content of 0.5-5 mol %, preferably 0.5-2 mol %, based in each case on the rare earth metals, and a gallium content x of 0 to 0.5, preferably 0.15 to 0.3. Other garnets having other element combinations are likewise possible, especially variants in which some or all of the lutetium has been replaced by yttrium in the formula of the garnet Lu.sub.3(Al,Ga).sub.5(O).sub.12:Ce. These combinations of a first and a second phosphor, compared to conventional combinations of phosphors where the above-described garnet phosphor has been mixed with another red-emitting phosphor from the class of the 2-5-8 phosphors of the general formula (Ca,Sr,Ba).sub.2(Si,Al).sub.5(N,O).sub.8:Eu with optional co-dopants, for example lanthanoids such as Mn, Nd, Dy, Sm, Tm and alkali metals such as Li, Na, K, have better stability in relation to the color point and higher LED efficiencies at elevated currents. Hereinafter, phosphors of the general formula (Ca,Sr,Ba).sub.2(Si,Al).sub.5(N,O).sub.8:Eu having optional co-dopants are referred to as 2-5-8 phosphors. In addition, phosphor mixtures of the invention exhibit reduced intensity of emission at wavelengths>650 nm, but the intensity, based on the maximum of the spectrum in the cyan color region, of at least 12.5% is satisfied as an important condition for flash applications.
(202) For warm white flash applications, it is preferable to use a yellow-emitting garnet phosphor (Gd,Lu,Tb,Y).sub.3(Al,Ga).sub.5(O).sub.12:RE, preferably of the formula (Gd,Y).sub.3(Al,Ga).sub.5(O).sub.12:Ce or (Tb,Y).sub.3(Al,Ga).sub.5(O).sub.12:Ce, having maximum excitation in the range of 435-470 nm, preferably 440-465 nm. The preferred yellow-emitting phosphor has a very high stability and conversion efficiency at high temperatures and high radiation intensities (high currents) which are typical of flash applications. A particularly preferred yellow/green-emitting phosphor is Y.sub.3(Al.sub.1-xGa.sub.x).sub.5(O).sub.12:Ce with a cerium content of 1.5-5 mol %, preferably 2.5-5 mol %, and a gallium content x of 0 to 0.5, preferably x of 0 to 0.1. Other element combinations within the (Gd,Lu,Tb,Y).sub.3(Al,Ga).sub.5(O).sub.12:Ce system having similar spectral characteristics are likewise possible.
(203) LED lighting devices suitable for flash applications may, independently of lighting devices having just one LED, for example, at least also have two or three blue-emitting LED chips as primary radiation-emitting radiation sources, with the phosphors and phosphor mixtures already mentioned above having been incorporated in the beam path thereof. The CIE color gamut of the converted light of the LED lighting device is preferably on the line of a blackbody emitter (Planck) in the range from 6500 K to 2700 K with a deviation of 3 steps of a MacAdam ellipse, more preferably in the range from 5000 K to 3000 K.
(204) In the case of an LED lighting device having two LED chips as radiation sources, in a further embodiment, the radiation from the first LED chips, after conversion, has a CIE color locus within a range enclosed by the following CIE color coordinates (Cx/Cy): (0.21; 0.247), (0.26; 0.24), (0.24; 0.32), (0.28; 0.31). The second LED chip as the second radiation source, after conversion, has a CIE color locus which is enclosed by the following CIE coordinates: (0.45; 0.41), (0.46; 0.39), (0.58; 0.39), and (0.58; 0.42). In such an LED lighting device, the individual radiation sources can be operated with different driver currents, advantageously with mixing of the converted light emitted in an optical element, such as a common lens, to give an overall emission radiation.
(205) In the case of an LED lighting device which is suitable for flash applications and has three LED chips as radiation sources, in a further embodiment, the first two LED chips have the CIE color loci already described above and the third radiation source, the third LED module, after conversion, has a CIE color locus which is enclosed by the following coordinates: (0.40; 0.44), (0.39; 0.51), (0.45; 0.52), and (0.47; 0.46). In this embodiment too, the converted light emitted is mixed by a downstream optical element in the beam path, such as a lens.
(206) The phosphor particles preferably have a median particle size d.sub.50 of 5-30 m, more preferably 7-17 m. The particle size distribution can be determined, for example, via laser diffraction by means of the Fraunhofer approximation which is known to those skilled in the art.
(207) Some inventive examples of LED lighting devices suitable for flash applications are to be elucidated in detail hereinafter. In a comparative example 1, an inventive example 1 and a comparative example 2 and an inventive example 2, different phosphor mixtures according to the prior art and phosphor mixtures comprising phosphors of the invention are incorporated in each case into a standard silicone potting material of a blue-emitting InGaN-LED chip as radiation source. The illumination area of each of the LED chips is 1 mm.sup.2.
(208) In comparative example 1, 11.5 percent by weight of phosphor based on the silicone potting material is incorporated, using Lu.sub.3Al.sub.4GaO.sub.12:Ce having a cerium content of 1.5 mol % based on the rare earth metals as green-emitting phosphor. The red-emitting phosphor used is a conventional 2-5-8 phosphor SrBaSi.sub.5N.sub.8:Eu having an Eu content of 1.5 mol % based on the alkaline earth metals. The dominant wavelength of the LED chip at 350 mA is 447 nm and the ratio of the green/red phosphors is 5.7:1. As inventive example 1, the same green-emitting phosphor as used in comparative example 1 is used, and the red phosphor used is an inventive phosphor Sr(Sr.sub.0.86Ca.sub.0.14)Si.sub.2Al.sub.2N.sub.6:Eu having an Eu content of 0.4 mol %, based on the rare earth metals. The ratio of the green/red phosphors is 3.9:1. The dominant wavelength of the LED chip at 350 mA is again 447 nm.
(209) The table in
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(211) LED spectra of comparative example 1 and of inventive example 1 at currents of 40 and 1000 mA are shown in
(212) The effect observed can be attributed to the different red-emitting phosphors in the conventional and inventive phosphor mixtures. In the case of the 2-5-8 phosphors used in the conventional phosphor mixture, a decrease in the conversion efficiency with increasing current is observed, which is manifested in a decrease in the red emission in relation to the yellow/green emission in the LED spectrum in conventional phosphor mixtures with increasing current. In the case of phosphor mixtures comprising the phosphor of the invention, a distinctly reduced decrease by comparison in the conversion efficiency of the red phosphor with increasing current is observed. The relative decrease in the conversion efficiency with increasing current for a phosphor of the invention, such as for a conventional 2-5-8 phosphor, is shown in
(213)
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(215) The shift in the color point of the LEDs of comparative example 1 and of inventive example 1 with increasing current is shown in
(216) In a further comparison of an inventive example 2 compared with a conventional comparative example 2, a phosphor mixture having an amber color point is used. Comparative example 2 comprises a cerium-activated garnet phosphor (Y.sub.0.957Ce.sub.0.043).sub.3Al.sub.5O.sub.12 together with a conventional 2-5-8 phosphor (Ca.sub.0.1Sr.sub.0.4Ba.sub.0.5).sub.2Si.sub.5N.sub.8:Eu having an Eu content of 3.25 mol % based on the alkaline earth metals used. The concentration of the phosphors is 41% by weight based on the silicone potting material and the dominant wavelength of the blue LED chip at a current of 350 mA is 444.7 nm. The ratio of the yellow/red phosphors is 5.9:1 (% by weight ratios). In inventive example 2, the same garnet phosphor as in comparative example 2 is used, but in that case an Sr(Sr.sub.0.86Ca.sub.0.14)Si.sub.2Al.sub.2N.sub.6:Eu phosphor of the invention having an Eu content of 2 mol % based on the rare earth metals is used in a yellow/red ratio of 5:1. The dominant wavelength of the blue-emitting LED chip at a current of 350 mA is 444.5 nm, with use of 39% by weight of phosphor mixture in relation to the silicone potting material.
(217) It can be inferred from the table in
(218) In the most recent generation of mobile communications devices, it is possible to use true-tone flashes which, as well as a cold white light-emitting LED, also have a second LED which emits either warm white light or yellow light (amber). This second LED is used in situations where the ambient light has a lower color temperature than daylight (cold white), in order to produce a flash which approximates as closely as possible to the color temperature of the ambient light. In order to produce light of various color temperatures, the cold white light and the light of the yellow-emitting LED have to be mixed with one another in various ratios. This can be achieved, for example, by operating the two LEDs with different currents. When the flash has a similar color temperature to the ambient light, the colors in the image have a more natural appearance.
(219) Similarly to inventive example 1, the inventive phosphor mixtures of inventive example 2 are also more stable with increasing current than the conventional phosphor mixtures, which lose a considerable proportion of their red emission compared to the yellow emission components, particularly at relatively high currents.
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(222) The normalized LED spectra for inventive example 2 and comparative example 2 are shown in
(223) In summary, it can be stated that it is therefore possible to achieve an equal light yield with lower operating currents in the case of phosphor mixtures of the invention compared to conventional phosphor mixtures. Since the current consumption in mobile applications, for example mobile phones, is a crucial criterion for operating life, a low consumption is very important. In addition, brighter LEDs are possible with the phosphor mixtures of the invention, which extends the range of color points, especially for true-tone flashes. Should a higher light yield not be desirable, it is possible to use LED chips with weaker emission of radiation compared to conventional LED chips, for example for yellow-emitting LEDs (amber), which reduces rejects during LED chip production and hence also enables the utilization of LED chips that are otherwise too dark.
(224) Embodiments of warm white light with CRI80: in a further embodiment of the present invention, phosphors of the invention are used for production of warm white light, for example for general lighting applications. Warm white light-emitting lighting devices comprising phosphors of the invention can especially achieve a color rendering index (CRI) of 80, preferably 82.
(225) Radiation sources of particularly good usability for lighting devices for production of warm white light may be blue-emitting LEDs, for example InGaN-LEDs, which emit light having a dominant emission wavelength of 430-470 nm, preferably 440-460 nm, further preferably of 445-455 nm, as primary radiation. First phosphors used for conversion of the primary radiation may be inventive phosphors of the general formula Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6:D where 0.7a, preferably 0.8a, further preferably 0.84a, where a<1 and where the proportion of activator D, preferably europium, based on the molar proportion of the alkaline earth metals, is 1 mol %, preferably 2 mol %, further preferably 3 mol %.
(226) Second phosphors used may, for example, be a green/yellow-emitting garnet of the general formula (Gd,Lu,Y,Tb).sub.3(Al,Ga).sub.5(O).sub.12:RE with RE=rare earth metal, preferably Ce. The garnet preferably has the general formula Y.sub.3(Al.sub.1-xGa.sub.x).sub.5(O).sub.12:Ce where the proportion of Ga is 0.2x0.6, preferably 0.3x0.5, further preferably 0.35x0.45.
(227) One advantage of using a garnet phosphor of the formula Y.sub.3(Al.sub.1-xGa.sub.x).sub.5(O).sub.12:Ce rather than a garnet phosphor of the formula Lu.sub.3(Al.sub.1-xGa.sub.x).sub.5(O).sub.12:Ce is that the first garnet phosphor Y.sub.3(Al.sub.1-xGa.sub.x).sub.5(O).sub.12:Ce has a lower density of about 4.5-5 g/cm.sup.3, while the second phosphor Lu.sub.3(Al.sub.1-xGa.sub.x).sub.5(O).sub.12:Ce has a density of about 6.7-7 g/cm.sup.3, and therefore a smaller mass of phosphor is consumed for a given application. In addition, therefore, the cheaper and more widely available Y.sub.2O.sub.3 can be used rather than Lu.sub.2O.sub.3 as starting material for the production of the phosphor, such that the procurement cost for the garnet phosphor is reduced.
(228) The use of a first phosphor of the invention exhibits higher absorptions compared to phosphor mixtures containing conventional 2-5-8 phosphors or CaAlSiN.sub.3 phosphors. Surprisingly, the absorption in the case of phosphors of the invention, given the same activator content, is considerably higher than in the case of conventional 2-5-8 phosphors. This enables a drastic reduction in the amount of red-emitting phosphor compared to conventional solutions and a very high conversion efficiency. At the same time, the excellent optical properties of the red-emitting phosphors of the invention enable a very high light yield and high conversion efficiency with high CRI.
(229) Some embodiments of lighting devices of the invention for general lighting applications having a high CRI are to be described in detail hereinafter.
(230)
(231) In comparative example 1, a cerium-activated yttrium aluminum gallium garnet (abbreviated to YAGaG in the figures which follow) with a conventional (Sr.sub.0.7Ca.sub.0.3)AlSiN.sub.3 phosphor (abbreviated to SCASN in the figures which follow) is incorporated as phosphor mixture in a concentration of 15% by weight relative to the total mass of the silicone potting material of the blue LED, where the ratio of green to red phosphor is 2.0 (% by weight ratio). The absolute concentration of green phosphor is 10% by weight and that of red phosphor 5% by weight. In addition, the correlated color temperature CCT, the CRI, the R9 value for red hues and the relative conversion efficiency relative to comparative example 2 are reported.
(232) Comparative example 2 contains a mixture of a lutetium aluminum garnet (abbreviated to LuAGaG in the figures which follow) and a conventional 2-5-8 phosphor (abbreviated to 258 in the figures which follow). In contrast, inventive examples 1 and 2 contain either an yttrium aluminum garnet or a lutetium aluminum garnet together with different phosphors of the invention (abbreviated to 226 in the figures which follow).
(233) It can be inferred from the tabular listing in
(234)
(235) The phosphors of the invention exhibit a very low spectral half-height width at half the maximum height FWHM compared to the conventional red-emitting phosphors, as apparent from
(236)
(237) The thermal quenching of various green/yellow-emitting garnet phosphors and a green-emitting orthosilicate phosphor from room temperature to 125 C. in relation to the absolute brightness at 25 C. is shown in
(238) The adverse effects of the thermal quenching of the various red-emitting phosphors used in the comparative and inventive examples on the absolute brightness are shown in
(239)
(240) In a further embodiment of the present invention, a lighting device for production of a white light having a CRI90 is provided, wherein the radiation source emits a primary radiation in the wavelength range between 430 nm and 470 nm, preferably 440 and 460 nm, further preferably 445 nm and 455 nm, and the second phosphor present is a garnet of the general formula (Gd,Lu,Y,Tb).sub.3(Al,Ga).sub.5(O).sub.12:RE, preferably (Lu,Y).sub.3(Al,Ga).sub.5(O).sub.12:RE with RE=rare earth metal, preferably Ce.
(241) In this lighting device for production of a white light having a CRI90, in the first phosphor, which may have one of the general formulae already described above, especially Sr(Sr.sub.aM.sub.1-a)Si.sub.2Al.sub.2N.sub.6:D, the metal M is Sr and Ca, where the parameter a is as follows: 0.7a, preferably 0.8a, further preferably 0.84a, where the proportion of the activator D is 1.5%, preferably 3.5%, further preferably 4.5% mol %.
(242) Some embodiments of phosphor mixtures of the invention having a high CRI90 are to be presented hereinafter.
(243)
(244) With reference to the preceding
(245)
(246) A graph comparison of the temperature-dependent changes in the LED color locus of two embodiments of the present invention compared to comparative example 1 from room temperature to 85 C. is shown in
(247)
(248) The emission spectra of various green-emitting garnet phosphors at an excitation wavelength of 460 nm, which are used in phosphor mixtures of the invention for warm white light applications, are shown in
(249)
(250) Further embodiments of the present invention are directed to phosphor mixtures or lighting devices in which at least three phosphors are disposed in the beam path of the radiation source, for example of a blue LED. In order to adjust either the CRI or the LED efficiency for a given color locus, it is possible to use phosphor mixtures having more than two phosphors. Especially in the case of use of three phosphors, for example of a green-emitting phosphor, a yellow-emitting phosphor and a red-emitting phosphor, there are several ways of obtaining an LED having a particular color point. However, one problem in the prior art is that many conventional orange/red-emitting phosphors have a broad-band emission and a considerable portion of the red light is emitted within a range to which the human eye is relatively insensitive.
(251) What are proposed, therefore, are phosphor mixtures including at least one phosphor of the invention. Such lighting devices of the invention therefore have, as radiation source for the primary radiation, a blue LED having a dominant wavelength of 300-500 nm, preferably 400-500 nm, further preferably 420-470 nm. The phosphor mixture contains at least one red-emitting phosphor of the invention as the first phosphor having the general formula Sr(Sr.sub.aM.sub.1-a)Si.sub.2Al.sub.2N.sub.6:D where M is selected from the group of Ca, Ba, Zn, Mg, preferably Ca, a yellow/green-emitting garnet phosphor of the general formula (Y,Lu,Gd,Tb).sub.3(Al,Ga).sub.5O.sub.12:Ce having a peak wavelength of 500-570 nm, preferably of 510-560 nm, further preferably of 520-550 nm, as the second phosphor, and, as the third phosphor, either an orange/red-emitting inventive phosphor of the general formula Sr(Sr.sub.aM.sub.1a)Si.sub.2Al.sub.2N.sub.6:D where M is selected from the group of Ca, Ba, Zn, Mg, preferably Ca, or an orange/red-emitting 2-5-8 phosphor of the general formula M.sub.2(Si,Al).sub.5(N,O).sub.8:Eu with M=Ca, Sr, Ba or a yellow-emitting garnet phosphor of the general formula (Y,Lu,Gd,Tb).sub.3(Al,Ga).sub.5O.sub.12:Ce having a peak emission wavelength of 580-650 nm, preferably of 590-640 nm, further preferably of 600-625 nm for the phosphor of the invention or the 2-5-8 phosphor, and of 500 to 600 nm, preferably 525 to 575 nm, further preferably of 535 to 565 nm, for the garnet phosphor.
(252) The red-emitting phosphor of the invention as the first phosphor is preferably selected such that it gives, in combination with the blue LED and the garnet phosphor and the yellow/red phosphor, a CRI of 75, preferably 80, further preferably 85 and more preferably 90 for a wide range of correlated color temperature CCT of at least 4000 to 2700 K, further preferably of 5000 to 2700 K, additionally preferably of 6500 to 2400 K. This is best achieved by using a phosphor of the invention having a half-height width FWHM of <85 nm, preferably <82 nm, further preferably <80 nm, having the general formula Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6:Eu with a 0.8 and a europium content of 0.1 to 10 mol %, preferably 2-5 mol %. The preferred phosphors of the invention have a high thermal stability and a high conversion efficiency under typical operating conditions for warm white lighting devices.
(253) The green/yellow-emitting garnet phosphor used as the second phosphor may, for example, have the general formula Lu.sub.3(Al,Ga).sub.5O.sub.12:Ce, and it is selected so as to result in a high conversion efficiency and thermal stability. This can be done, for example, by selecting a garnet phosphor of the general formula Lu.sub.3(Al,Ga).sub.5O.sub.12:Ce having a cerium content of 1-5 mol %, preferably 1-3 mol %, and a gallium content of 0-50 mol %, preferably 0-30 mol %, so as to result in the general formula Lu.sub.3(Al.sub.1-xGa.sub.x).sub.5O.sub.12:Ce with 0x0.5, preferably 0x0.3. Other element combinations within the general system of the garnet phosphors are likewise possible, especially variants in which at least some of the lutetium is replaced by yttrium in the general formula.
(254) The third phosphor used may preferably be a phosphor which, in combination with the blue LED and the garnet phosphor, and also the red-emitting phosphor of the invention, gives a CRI of 75, preferably 80, further preferably 85 and most preferably 90 over a wide CCT range, for example of 4000-2700 K, further preferably of 5000-2700 K, most preferably of 6500-2400 K. For example, the third phosphor used may be a nitridosilicate phosphor of the general formula (Ca,Sr,Ba).sub.2(Si,Al).sub.5(N,O).sub.8:Eu, where the peak wavelength is 580-650 nm, preferably 590-640 nm, further preferably 600-625 nm, with a europium content of 0.1 to 10 mol %, preferably of 0.1 to 5 mol %, additionally preferably of 0.5-3 mol %, and a barium content of 30-100 mol %, preferably of 40-75 mol %, further preferably of 45-55 mol %, and a calcium content of 0-20 mol %, preferably 0-10 mol %, additionally preferably 0-5 mol %, where the strontium content is selected such that it adds up to 100 together with the alkaline earth metals and the europium.
(255) Alternatively, the third phosphor used may also be an inventive phosphor of the general formula Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6:Eu having a half-height width FWHM of <85 nm, preferably <80 nm, additionally preferably <78 nm, where the peak wavelength is 580-650 nm, preferably 590-640 nm, further preferably 600-625 nm. In order to achieve these spectral properties, an inventive phosphor of the general formula Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6:Eu with a0.8, preferably a0.82, further preferably a0.85, with a europium content of 0.1 to 5 mol %, preferably 0.1 to 3 mol %, most preferably 0.1 to 2 mol %, is used.
(256) The third phosphor used may additionally be a yellow-emitting garnet phosphor (Lu,Gd,Tb,Y).sub.3(Al,Ga).sub.5O.sub.12:Ce having a peak emission wavelength of 500-600 nm, preferably 525-575 nm, further preferably of 535-565 nm. This can especially be accomplished by a garnet phosphor of the general formula Y.sub.3(Al,Ga).sub.5O.sub.12:Ce having a high conversion efficiency and thermal stability. This can be accomplished, for example, by using a garnet phosphor of the general formula having a cerium content of 1 to 6 mol %, preferably 1 to 4 mol %, and a gallium content of 0-50 mol %, preferably 0-25 mol %, so as to result in the general formula Y.sub.3(Al.sub.1-xGa.sub.x).sub.5O.sub.12:Ce with 0x0.5, preferably 0x0.25, and other element combinations are additionally also possible within this system, for example at least partial replacement of yttrium by lutetium.
(257) Particular technical advantages of such lighting devices of the invention according to inventive examples 1 and 2 compared to comparative examples 1 and 2 are to be elucidated hereinafter.
(258)
(259)
(260) Embodiments of LARP applications: in a further embodiment of the present invention, the radiation source used which emits the primary light may also be a laser, for example a laser diode. In this case, it is advantageous when the first phosphor of the invention is spaced apart from the laser radiation source (LARP; laser activated remote phosphor). LARP applications of this kind are known, for example, from PCT patent applications WO 2012/076296 A2, WO 2011/098164 A1 and WO 2013/110495 A2, and the further patent applications DE 10 2012 209 172 A1, DE 10 2010 062 465 A1, DE 10 2011 002 961 A1 and DE 10 2012 201 790 A1, which are hereby fully incorporated by reference. With lighting devices of this kind, for example projectors, it is possible to achieve significantly higher luminances than with conventional radiation sources.
(261) In the embodiments which follow, a blue LED laser diode having a radiation density of 8.9 W/mm.sup.2 and a peak wavelength of 446 nm is used, the beam of which is directed onto either conventional or inventive phosphor grains present at the base of an Ulbricht sphere having a reflective inner surface. The converted light reflected is subsequently collected and analyzed. Such a test setup simulates the conditions in an LARP lighting device.
(262) In the table in
(263) In the table in
(264) More particularly, in all flash applications and also in other conversion applications of the phosphors of the invention, for example general lighting applications, and provided that the applications are not full-conversion applications, it is also possible for proportions of unconverted primary radiation from the radiation source to be present, even if this is not mentioned explicitly. Mixing of this unconverted primary radiation with the converted secondary radiation results in an overall emission of the lighting device. As already described further up, for example, it is possible to achieve warm white light-emitting lighting applications with phosphors of the invention by converting the blue primary radiation from InGaN LEDs by means of the phosphors of the invention to a red component, and by means of green/yellow-emitting phosphors to a green-yellow component, in which case additive color mixing of the blue primary radiation and the two converted secondary radiations gives warm white light as the overall emission of the lighting device.
(265) Embodiments with different Eu dopant concentrations:
(266) Embodiments with further co-dopants or dopants other than Eu: Further embodiments of phosphors of the invention having the general formula Sr.sub.(1-x-h)(Sr.sub.aM.sub.1-a).sub.(1yi)A.sub.(x-y)B.sub.(h+i)/2E.sub.(h+i)/2Si.sub.(2-z)G.sub.zAl.sub.2N.sub.6:D and Sr.sub.(1-x)(Sr.sub.aM.sub.1-a).sub.(1-y)B.sub.(x-y)Si.sub.2(x+y)Al.sub.2+(x+y)N.sub.6:D are to be described in detail hereinafter.
(267)
(268)
(269)
(270) The table in
(271)
(272)
(273) The thermal quenching of two phosphors of the invention is shown in
(274)
(275) An overview of the most important spectral properties, especially the color locus in the CIE color space, the luminous efficiency LER, the dominant wavelength dom and the half-height width of the emission FWHM is given in tabular form in
(276) The x-ray diffractograms of the phosphors Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6:Eu (bottom) and Sr(Sr.sub.aCa.sub.1-a)Si.sub.2Al.sub.2N.sub.6:Eu,Li (top) are shown in
(277)
(278)
(279) The first matrix material may comprise a multitude of materials that are transparent both to the primary radiation and to the secondary radiation that has arisen through conversion. More particularly, the first matrix material may be selected from a group of materials consisting of: glass, silicone, epoxy resin, polysilazane, polymethacrylate and polycarbonate, and combinations thereof. The polymethacrylate used may especially be polymethylmethacrylate (PMMA). In addition, the phosphor may also be in the form of a ceramic converter element.
(280)
(281) The lighting device in
(282) However, another alternative option is an embodiment according to
(283) In addition, it is possible that the first phosphor 40 of the invention is disposed directly upon the radiation source 35, for example in the form of a silicone, phosphor-in-glass converter element, or ceramic converter element (see
(284)
(285)
(286)
(287)
(288) There follows a description of embodiments of phosphors of the invention by an alternative characterization compared to the above disclosure, but one which is consistent with the above disclosure.
(289) A phosphor is specified. Additionally specified are a process for producing such a phosphor and a use of such a phosphor.
(290) One problem to be addressed is that of specifying a phosphor which has comparatively narrow-band spectral emission in the red spectral region.
(291) This problem is solved inter alia by a phosphor, by a process and by a use having the features of the independent claims. Preferred developments are the subject of the dependent claims.
(292) In at least one embodiment, the phosphor is set up to emit red light. Red light means that the phosphor emits radiation having a dominant wavelength between 585 nm and 640 nm inclusive, especially between 590 nm and 615 nm inclusive.
(293) The dominant wavelength is especially that wavelength which is found to be the point of intersection of the spectral color line of the CIE standard chromaticity diagram with a straight line, this straight line proceeding from the white point in the CIE standard chromaticity diagram and running through the actual color locus of the radiation. In general, the dominant wavelength differs from a wavelength of maximum intensity. More particularly, the dominant wavelength in the red spectral region is at smaller wavelengths than the wavelength of maximum intensity.
(294) In at least one embodiment, the phosphor has the general empirical formula Sr.sub.xCa.sub.1xAlSiN.sub.3:Eu. It is possible that the phosphor includes further elements, for instance in the form of impurities, in which case these impurities taken together preferably have a maximum proportion by weight in the phosphor of not more than 0.1 permille or 10 ppm, parts per million.
(295) In at least one embodiment of the phosphor, x>0.8 or x0.82 or x0.85 or x0.89. Alternatively or additionally, x1 or x<1 or x0.98 or x0.95 or x0.92.
(296) In at least one embodiment, the proportion of the Sr lattice sites which have been replaced by Eu is at least 0.01% or 0.1% or 0.35% or 0.5%. Alternatively or additionally, this proportion is at most 10% or 5% or 3% or 2.2% or 1.8%.
(297) In at least one embodiment, the phosphor, in an x-ray structure analysis, has a reflection having the Miller indices 1
(298) In at least one embodiment, the phosphor is set up to emit red light and preferably to be excited by blue light and has the general empirical formula Sr.sub.xCa.sub.1-xAlSiN.sub.3:Eu with 0.8<x1. A proportion of the Sr lattice sites between 0.1% and 5% inclusive has been replaced by europium. In an x-ray structure analysis, the phosphor in orthorhombic description has a reflection having the Miller indices 1
(299) Phosphors which can be excited in the ultraviolet spectral range up to the blue/green spectral range and which emit red light are of great significance for the production of white light-emitting diodes. Specifically in the case of light-emitting diodes having low color temperatures, called warm white light-emitting diodes, and/or in the case of light-emitting diodes having a high color rendering index, phosphors of this kind are required. Phosphors of this kind are also required in a multitude of other applications, for instance for display backlighting, what are called color-on-demand applications or else for orange and red full conversion light-emitting diodes. Use in combination with an organic light-emitting diode, OLED for short, is likewise possible. The phosphor described here is usable for such applications.
(300) In at least one embodiment, the phosphor in a powder diffractogram on irradiation with monochromatic Cu-K.sub.1 radiation has a reflection at an angle 2 between 36.7 and 37.0, according to the composition of the phosphor. The exact position of this reflection depends on the parameter x in the general empirical formula of the phosphor. An intensity of this reflection, especially based on a main reflection, is preferably at least 0.3% or 0.5% and/or at most 10% or 8% or 5% or 4%.
(301) In at least one embodiment, the dominant wavelength of the phosphor is at least 596 nm or 598 nm. Alternatively or additionally, the dominant wavelength is at most 606 nm or 604 nm. The wavelength of maximum intensity is, for example, at least 605 nm or 610 nm and/or at most 630 nm or 625 nm.
(302) In at least one embodiment, the phosphor has a spectral half-height width at half the maximum height, FWHM or full-width at half maximum for short, of at least 70 nm or 75 nm or 78 nm. The maximum of this spectral range is preferably 90 nm or 87 nm or 84 nm or 82 nm.
(303) In at least one embodiment, the phosphor has an absorption maximum in the blue spectral region, especially a relative absorption maximum. The blue spectral region especially refers to wavelengths of at least 400 nm and/or of at most 480 nm. For example, the absorption maximum is at at least 410 nm or 420 nm and/or at at most 450 nm or 440 nm.
(304) The abovementioned values relating to the spectral properties of the phosphor especially apply at room temperature, i.e. at about 300 K.
(305) Additionally specified is a process for producing such a phosphor. Features of the phosphor are therefore also disclosed for the process, and vice versa.
(306) In at least one embodiment, the process has at least the following steps, preferably in the sequence specified: A) providing reactants in the solid state for Sr, Al, Si and Eu and optionally for Ca, B) mixing the reactants, C) heating the reactants under a forming gas atmosphere to at least 1500 C. and forming a calcined cake, and D) comminuting the calcined cake to give the phosphor.
(307) In at least one embodiment of the process, at least step C) or else all steps are effected at atmospheric pressure. More particularly, the process in that case is not effected under high pressure conditions. Preferably, the atmospheric pressure and/or a total pressure is between 0.9 bar and 1.5 bar or 0.95 bar and 1.05 bar inclusive.
(308) Reactants and sources used for strontium, aluminum and/or calcium may be the respective pure metals or else metal alloys with the appropriate metals. Reactants used may likewise be silicides, nitrides, oxynitrides, halides and/or oxides of these metals. In addition, it is possible to use mixtures of these compounds.
(309) Reactants or sources used for silicon for the production of the phosphor may be a silicon-metal compound, a silicon nitride, an alkaline earth metal silicide, silicon diimide, or a mixture of these compounds. Preference is given to using silicon nitrides and/or silicon metals.
(310) Reactants or sources used for Eu may be metallic europium, a europium alloy, a europium oxide, a europium nitride or a europium halide. It is likewise possible to use mixtures of these compounds. Preference is given to using europium oxide as reactant for europium.
(311) In at least one embodiment, a melting agent and/or a flux is used for the improvement of crystallinity and/or to promote crystal growth. For this purpose, preferably chlorides, fluorides, halides and/or boron compounds of the alkaline earth metals used are employed. Combinations of two or more melting agents or fluxes may also be used. Melting agents and/or fluxes used are especially at least one of the following substances: LiF, LiCl, NaF, NaCl, SrCl.sub.2, SrF.sub.2, CaCl.sub.2, CaF.sub.2, BaCl.sub.2, BaF.sub.2, NH.sub.4Cl, NH.sub.4F, KF, KCl, MgF.sub.2, MgCl.sub.2, AlF.sub.3, H.sub.3BO.sub.3, B.sub.2O.sub.3, Li.sub.2B.sub.4O.sub.7, NaBO.sub.2, Na.sub.2B.sub.4O.sub.7, LiBF.sub.4.
(312) In at least one embodiment, the starting substances, especially for Sr, Ca, Al and/or Si and also Eu, are weighed out according to the general empirical formula of the phosphor. It is possible that the alkaline earth metal components are also weighed out with an excess, in order to compensate for any evaporation losses that occur during the synthesis.
(313) In at least one embodiment, step D) is followed by a step E). In step E), the phosphor is calcined further, which can also be referred to as heat treatment. The calcination is especially effected at a temperature of at least 1500 C. and preferably under a nitrogen atmosphere or forming gas atmosphere. Forming gas refers to a mixture of N.sub.2 and H.sub.2. The temperature of at least 1500 C. in steps C) and/or E) is preferably employed for at least four hours or six hours. For example, in each of steps C) and E), a temperature of 1650 C.50 C. is employed.
(314) In at least one embodiment, the reactants are mixed in a ball mill or in a tumbling mixer. In the mixing operation, it may be advantageous to choose the conditions such that a large amount of energy is introduced into the mixture, which results in grinding of the reactants. The resultant increase in homogeneity and reactivity of the mixture can have a positive influence on the properties of the resulting phosphor.
(315) By controlled alteration of the bulk density or by modification of the agglomeration of the reactant mixture, it is possible to reduce the formation of secondary phases. In addition, a particle size distribution, a particle morphology and/or a yield of the resulting phosphor can be affected. Techniques of particular suitability for the purpose are sieving and pelletizing operations, including use of suitable additives.
(316) In at least one embodiment, a tempering is effected, especially in a crucible made from tungsten, molybdenum or boron nitride. The tempering is preferably effected in a gas-tight oven in a nitrogen atmosphere or in a nitrogen/hydrogen atmosphere. The atmosphere may be flowing or stationary. It is additionally possible for carbon in finely divided form to be present in the oven space. Also possible are multiple tempering steps of the phosphor, in order to improve the crystallinity or particle size distribution. Further advantages may be a lower defect density, associated with improved optical properties of the phosphor and/or a higher stability of the phosphor. Between the tempering steps, the phosphor may be treated in a wide variety of different ways, or it is possible to add substances such as melting agents to the phosphor.
(317) For grinding of the phosphor, it is possible, for instance, to use a mortar mill, a fluidized bed mill or a ball mill. In the grinding operation, it is to be ensured that the proportion of splintered grains produced is kept to a minimum, since these can worsen the optical properties of the phosphor.
(318) The phosphor can additionally be washed. For this purpose, the phosphor can be washed in water or in aqueous acids such as hydrochloric acid, nitric acid, hydrofluoric acid, sulfuric acid, organic acids or a mixture of these. The phosphor may alternatively or additionally be washed in an alkali such as sodium hydroxide solution, potassium hydroxide solution, an aqueous ammonia solution or mixtures thereof. Alternatively or additionally, washing in organic solvents such as acetone, propanol and/or phenol is possible. The washing preferably follows after the grinding.
(319) In at least one embodiment, for instance, the tempering, further calcining, grinding, sieving and/or washing result in removal of secondary phases, glass phases or other contamination and hence an improvement in the optical properties of the phosphor. It is also possible by this treatment to selectively remove or dissolve small phosphor particles and to optimize the particle size distribution for the application. In addition, such a treatment can alter a surface of the phosphor particles in a controlled manner, for example the removal of particular constituents from the particle surface. This treatment can, also in conjunction with a downstream treatment, lead to improved stability of the phosphor.
(320) Additionally specified is the use of such a phosphor. Features relating to use are therefore also disclosed for the process and the phosphor, and vice versa.
(321) In at least one embodiment, the phosphor is used in a light-emitting diode. The light-emitting diode comprises at least one semiconductor chip that emits in the blue spectral region in operation. The phosphor is arranged downstream of the semiconductor chip along a beam path.
(322) The blue light produced by the semiconductor chip is partly or fully absorbed by the phosphor and converted to red light. It is possible that further phosphors are present, especially for generation of green and/or yellow light. In addition, the light-emitting diode preferably emits mixed radiation including blue light from the semiconductor chip and converted radiation from the phosphor, and also green and/or yellow light from the further phosphor.
(323) One embodiment of a phosphor described here can be prepared as follows: reactants used for the synthesis of the phosphor of the general empirical formula Sr.sub.xCa.sub.1-xAlSiN.sub.3:Eu are the binary nitrides of the constituent elements, i.e. Ca.sub.3N.sub.2, Sr.sub.3N.sub.2, AlN and Si.sub.3N.sub.4. Since these are highly oxidation-sensitive and hydrolysis-sensitive substances, what is called a glovebox is employed, under an N.sub.2 atmosphere with O.sub.2<1 ppm and H.sub.2O<1 ppm. In addition, doping with Eu.sup.2+ is accomplished using Eu.sub.2O.sub.3. The reactants are weighted out such that the following atomic ratio is present:
(324) Ca:Sr:Al:Si:Eu=(1x):x:1:1:y, where y corresponds to the degree of doping, i.e. the proportion of Sr lattice sites which are substituted by Eu. In addition, various fluxes are added; see the table in
(325) The reactant mixture is introduced into a PET mixing vessel together with ZrO.sub.2 balls and mixed on a roller table in a glovebox for 6 h. Subsequently, the balls are removed from the mixture and the powder is transferred into a closed molybdenum crucible. This crucible is placed into an outer tungsten crucible, a semicircular open tungsten tube, and transferred into a tube furnace. There is a flow of 3 L/min of forming gas with 92.5% N.sub.2 and 7.5% H.sub.2 through the tubular furnace during the run time. In the tubular furnace, the mixture is heated at a rate of 250 K/h to 1650 C., kept at this temperature for 4 h and then cooled down at 250 K/h to 50 C. The calcined cake formed is taken out after the furnace has cooled, comminuted with a mortar mill and sieved through a sieve having a mesh size of 31 m. The sieve fraction <31 m is the phosphor used.
(326) The sieving may optionally be followed by a further calcining, tempering and/or washing operation.
(327) Illustrative starting weights m in g and resulting color loci CIE x, CIE y, also referred to as chromaticity coordinates, of the emission spectrum of the particular phosphor in the CIE standard chromaticity diagram on excitation with blue light at 460 nm and on complete absorption of the blue light are also listed in the table in
(328)
(329)
(330)
(331)
(332) The phosphor with x0.8 thus features a small half-height width FWHM of the emission and a very high luminous efficiency LER with simultaneously high internal quantum efficiency QI and external quantum efficiency QE; see the table in
(333)
(334) All phosphor mixtures were adjusted so as to achieve, in the CIE standard chromaticity diagram, a color locus close to Planck with a correlated color temperature CCT of about 2700 K. The color rendering index CRI of all LEDs measured is 801. All red phosphors R used show a comparable dominant wavelength of about 600.5 nm1 nm.
(335) Further details of the phosphor mixtures as shown in
(336)
(337) Further data relating to the LED measurements from
(338) Red phosphors composed of the Sr.sub.xCa.sub.1-xAlSiN.sub.3:Eu material system were subjected to a hydrolysis test, in order to assess an aging stability of the phosphor with respect to air humidity; see
(339)
(340)
(341) With regard to phosphors composed of the CaAlSiN.sub.3:Eu system, the literature reports that, with rising activator content, especially >0.8% Eu, a conversion efficiency stagnates; see table 1 in EP 2 135 920 A1.
(342) Surprisingly, the novel phosphor with a high Sr content shows different behavior. With rising Eu content, the emission intensity I, even in the case of an Eu content>1%, continues to increase in an approximately linear manner. This property offers various technical advantages for application, especially lower phosphor demand and the possibility of achieving color loci with greater CIE x.
(343)
(344)
(345)
(346) In orthorhombic description, this reflection R originates from the 1
(347) In order to describe the structure of the novel phosphor described here, the following steps were conducted in order. A summary of the results, important R values and the fundamental refined parameters can be found in the table in
(348) 1) A Rietveld refinement was conducted with the known phases Sr.sub.0.99Eu.sub.0.01AlSiN.sub.3 from ICSD 98-041-9410, AlN from ICSD 98-060-8626 and SrF.sub.2 from ICSD 98-004-1402. The crystal structure data of Sr.sub.0.99Eu.sub.0.01AlSiN.sub.3 were fitted as Ca.sub.0.1Sr.sub.0.89Eu.sub.0.01AlSiN.sub.3.
(349) 2) All reflections were fitted by means of a profile parameter fit, with equal FWHM for all reflections. Any reflections which were assigned to extraneous phases, for instance SrF.sub.2 and AlN, were eliminated from the search. The other reflections were used for a lattice parameter search. This lattice parameter search found that almost all reflections can be described with the original cell, but without quenching conditions. For this reason, in the next step, a Rietveld refinement was conducted with the original structure, but transferred to the P1 space group.
(350) 3) A trial refinement of the experimental data was conducted on the basis of the structure model for Sr.sub.xCa.sub.1-xAlSiN.sub.3 known from the literature, except that it had been transferred to the lower-symmetry P1 space group; see also the preceding step 2. This refinement likewise converged, but does not explain the reflection R observed.
(351) 4) In order to explain the additional reflections observed, a new structure model different than the known CaAlSiN.sub.3 structure was set up. The new structure model of the phosphor described here is distinctly different than the CaAlSiN.sub.3 structure already known. In the crystallographic sense, this is a superstructure variant. This structure can be derived formally from that of CaAlSiN.sub.3 by a reduction in symmetry. In the structure model thus derived for the novel phosphor, there is a good explanation and description of the at least one reflection R additionally observed.
(352) This structure model of the novel phosphor differs from the structure model from the above-elucidated step 3). In step 3), the known structure model of CaAlSiN.sub.3 was merely described in an alternative, lower-symmetry space group. Only through the actual introduction of a new structure model different than CaAlSiN.sub.3 is a good description of the experimentally observed reflections possible, especially of the new reflection R.
(353) For this purpose, it is necessary in particular to split up the position having a mixed Sr/Ca population, which has a mixed population in the original Cmc2.sub.1 space group and describes four alkaline earth metal atoms simultaneously for reasons of symmetry, into four individual positions. In the model of the conventional phosphor, all four positions have mixed populations of Sr and Ca. In the model of the novel phosphor, three of the positions are populated only by Sr, and just one of the positions has mixed population both with Sr and with Ca.
(354) The new reflection R shown is thus a superstructure reflection which can be described in P1, but not in Cmc2.sub.1, since it infringes the quenching conditions for this space group.
(355) The refinement of the powder x-ray data observed, based on the known structure model in space group Cmc2.sub.1, leads to the goodness factors in the first column in
(356)
(357) In comparison, the perspective diagram according to
(358) The novel phosphor described here offers the following particular advantages: lower half-height width of emission, associated with higher luminous efficiency at the same dominant wavelength, the possibility of achieving higher activator concentrations of Eu at >0.8% with simultaneously high quantum efficiency and conversion efficiency, associated with a lower phosphor requirement in LED applications and simplified processibility, improved aging stability with respect to moisture compared to conventional (Sr,Ca)AlSiN.sub.3:Eu having a low Sr content, and improvement in the thermal stability.
(359) The invention described here is not restricted by the description with reference to the embodiments. Instead, the invention encompasses every new feature and every combination of features, which especially includes every combination of features in the claims, even if this feature or this combination itself is not explicitly specified in the claims or embodiments.