Hybrid material for optoelectronic applications

Abstract

A hybrid material for light emitting diodes, comprising a) an organopolysilazane material, comprising repeating units of formulae (I) and (II)
[SiR.sup.1R.sup.2NR.sup.3].sub.x(I)
[SiHR.sup.4NR.sup.5].sub.y(II)
wherein the symbols and indices have the following meanings: R.sup.1 is C.sub.2-C.sub.6-alkenyl or C.sub.4-C.sub.6-alkadienyl; R.sup.2 is H or an organic group; R.sup.3 is H or an organic group; R.sup.4 is H or an organic group; R.sup.5 is H or an organic group; x is 0.001 to 0.2; and y is 2x to (1x), with the proviso that x+y1 and that y can be 0 if R.sup.2 is H, and b) inorganic nanoparticles having a mean diameter in the range of from 1 to 30 nm, which are surface modified with a capping agent comprising a C.sub.1-C.sub.18-alkyl and/or C.sub.1-C.sub.18-alkenyl group,
is useful as encapsulation material for LEDs.

Claims

1. A hybrid material for light emitting diodes, comprising a) an organopolysilazane material, comprising repeating units of formulae (I) and (II) ##STR00007## wherein the symbols and indices have the following meanings: R.sup.1 is C.sub.2-C.sub.6-alkenyl or C.sub.4-C.sub.6-alkadienyl; R.sup.2 is H or an organic group; R.sup.3 is H or an organic group; R.sup.4 is H or an organic group; R.sup.5 is H or an organic group; x is 0.001 to 0.2; and y is 2x to (1x), with the proviso that x+y1 and that y can be 0 if R.sup.2 is H, and (b) inorganic nanoparticles having a mean diameter in the range of from 1 to 30 nm, which are surface modified with a capping agent comprising a C.sub.1-C.sub.18-alkyl and/or C.sub.1-C.sub.18-alkenyl group.

2. The material according to claim 1, wherein R.sup.1 is (C.sub.2-C.sub.6)-alkenyl or (C.sub.4-C.sub.6)-alkadienyl; R.sup.2 is (C.sub.1-C.sub.8)-alkyl, (C.sub.2-C.sub.6)-alkenyl, (C.sub.3-C.sub.6)-cycloalkyl, (C.sub.6-C.sub.10)-aryl or H; R.sup.3 is H or (C.sub.1-C.sub.8)-alkyl, (C.sub.2-C.sub.6)-alkenyl, (C.sub.3-C.sub.6)-cycloalkyl or (C.sub.6-C.sub.10)-aryl; R.sup.4 is H or (C.sub.1-C.sub.8)-alkyl, (C.sub.2-C.sub.6)-alkenyl, (C.sub.3-C.sub.6)-cycloalkyl or (C.sub.6-C.sub.10)-aryl; R.sup.5 is H or (C.sub.1-C.sub.8)-alkyl, (C.sub.2-C.sub.6)-alkenyl, (C.sub.3-C.sub.6)-cycloalkyl or (C.sub.8-C.sub.10)-aryl; x is 0.02 to 0.1 and y is preferably 2*x to 0.98.

3. The material according to claim 1 wherein R.sup.1 is vinyl or allyl; R.sup.2 is (C.sub.1-C.sub.4)-alkyl, phenyl or H; R.sup.3 is H; R.sup.4 is (C.sub.1-C.sub.4)-alkyl, phenyl or H; R.sup.5 is H; x is 0.03 to 0.075 and y is 2*x to 0.97.

4. The material according to claim 1, wherein R.sup.1 is vinyl; R.sup.2 is methyl, ethyl, propyl or phenyl; R.sup.3 is H and R.sup.4 is methyl, ethyl, propyl or phenyl; R.sup.5 is H; x is 0.03 to 0.06 and y is 2*x to 0.97.

5. The material according to claim 1, wherein the organopolysilazane comprises one or more organopolysilazane comprising a repeating unit of formula (I) and/or formula (II) and one or more repeating units of formula (III) and/or (IV), ##STR00008## wherein R.sup.6, R.sup.7, and R.sup.9 are independently an organic group; R.sup.10 is H or an organic group, and R.sup.8 and R.sup.11 are independently H or an organic group.

6. The material according to claim 5, wherein the symbols in formulae (III) and (IV) have the following meanings: R.sup.6, R.sup.7 and R.sup.9 are independently (C.sub.1-C.sub.8)-alkyl, (C.sub.3-C.sub.6)-cycloalkyl or (C.sub.6-C.sub.10)-aryl; R.sup.10 is independently (C.sub.1-C.sub.8)-alkyl, (C.sub.3-C.sub.6)-cycloalkyl or (C.sub.6-C.sub.10)-aryl, (C.sub.2-C.sub.6)-alkenyl, (C.sub.4-C.sub.6)-alkadienyl or H and R.sup.8 and R.sup.11 are H, (C.sub.1-C.sub.8)-alkyl, (C.sub.3-C.sub.6)-cycloalkyl or (C.sub.8-C.sub.10)-aryl.

7. The material according to claim 1, wherein the molecular weight M.sub.w of the organopolysilazanes is in the range of 2000-150.000.

8. The material of claim 1, wherein the organopolysilazane material has a viscosity of 100-100.000 mPas at 25 C.

9. The material according to claim 1, wherein the amount of low molecular material with M.sub.w<500 g/mol in the organopolysilazane material is below 15 wt.-%.

10. The material according to claim 1, wherein the inorganic nanoparticles have an average mean diameter in the range of from 3 to 20 nm.

11. The material according to claim 1, wherein the inorganic nanoparticles are selected from the group consisting of semiconductor nanoparticles, metal nanoparticles, metal oxide nanoparticles and a combination comprising at least one of the foregoing.

12. The material according to claim 11, wherein the inorganic nanoparticles are selected from the group consisting of ZrO.sub.2, BaTiO.sub.3 and TiO.sub.2.

13. The material according to claim 1, wherein the inorganic nanoparticles are surface modified with a alkoxysilane or chlorosilane capping agent.

14. The material according to claim 13, wherein the capping agent is selected from methyltrimethoxysilane, trimethylmethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, n-octyltrimethoxysilane, n-octyltriethoysilane, phenyltrimethoxysilane, 2-[methoxy(polyethyleneoxy)propyl]-trimethoxy-silane, methoxy(triethyleneoxy)propyltrimethoxysilane, 3-aminopropyltri-methoxysilane, 3-mercaptopropyltrimethoxysilane, vinyltrimethoxysilane, allyltrimethoxysilane, 3-isocyanatopropyltriethoxysilane, 3-isocyanatopropyl-trimethoxysilane and glycidoxypropyltrimethoxysilane.

15. The material according to claim 13, wherein the capping agent is selected from alkoxysilanes of formula (V)
R.sub.nSi(OR).sub.m (V) wherein n is 1, 2 or 3 and m is 4-n; R is methyl, ethyl, linear, branched or cyclic alkyl with 3-8 carbon atoms, phenyl, (C.sub.2-C.sub.6)-alkenyl; and R is methyl, ethyl, n- or iso-propyl, n- or isobutyl.

16. The material according to claim 1, wherein the amount of inorganic nanoparticles is in the range of from 1 to 85% by weight, based on the whole material.

17. The material according to claim 1, wherein the refractive index of the material is in the range of from 1.35 to 2.50.

18. A process for producing an LED, comprising the steps of I) encapsulating the light emitting material of the LED with a hybrid material according to claim 1, by dispensing the material and II) curing the organopolysilazane of the invention optionally adding curing catalyst for 1 min to 6 h at a temperature of from 80 C. to 220 C. in an inert atmosphere or air.

19. The process of claim 18, comprising the steps Ia) providing a material according to claim 1 by mixing the organopolysilazane in solution with a nanoparticle dispersion, removing the solvent and Ib) applying the material to the LED as an encapsulation material.

20. An encapsulation material for LEDs, obtainable by a) providing a material according to claim 1, b) optionally subjecting the material to crosslinking by treatment with a base, and c) curing the material optionally adding curing catalyst by treating the material to a temperature in the range of from 80 to 220 C. for a period of from 1 min to 6 h in an inert atmosphere or air.

21. An LED, comprising the material according to claim 1 as an encapsulation material.

Description

EXAMPLES

Synthesis Examples

(1) The following example shows the synthesis of an organopolysilazane material of the invention by base catalyzed crosslinking of the organosilazanes ML-33 and HTT-1800 available from AZ Electronic Materials Germany GmbH, Wiesbaden, Germany.

Example A (Synthesis of Organopolysilazane)

(2) A 250 ml flask was purged with dry nitrogen and charged with 16.7 g HTT-1800, 33.3 g ML-33 and 100 g 1.4-Dioxane. After cooling down to 0 C. 0.5 g of potassium-hexamethyldisilazane were added. After addition of the catalyst, gas formation could be observed. The mixture was stirred for 2 h at 0 C. and for additional 2 h at 20 C. Then 0.5 g chlorotrimethylsilane were added. The precipitate was removed by filtration and all of the solvent was removed by evaporation under reduced pressure.

(3) Yield: 47 g of a colorless viscous oil.

Examples B-E (Synthesis of Dispersions of Inorganic Nanoparticles)

Example B (Synthesis of ZrO2 Dispersions)

(4) The zirconium dioxide dispersion in toluene with a mean particle size of <10 nm was made of watery ZrO.sub.2 dispersions available from NYACOL. For this aim, the watery ZrO.sub.2 dispersions were first diluted with methanol, then methyltrimethoxysilane was added, then they were stirred at 60 C. for 6 h and finally the solvent was replaced with toluene by distillation.

Example C (Synthesis of TiO2 Dispersions)

(5) The titanium dioxide dispersions in toluene with a mean particle size of ca. 10 nm and ca. 25 nm were made from isopropanolic TiO.sub.2 dispersions available from Lotus-Synthesis or Sigma-Aldrich, respectively. For this aim, first methyltrimethoxysilane was added to the isopropanolic TiO.sub.2 dispersions, then these were stirred at 60 C. for 6 h and subsequently the solvent was replaced with toluene by distillation.

Example D (Synthesis of SiO2 Dispersions)

(6) The silicon dioxide dispersions in toluene with a mean particle size of from 15 nm to 85 nm were made from aqueous SiO.sub.2 dispersions (KLEBOSOL), available from AZ Electronic Materials. For this aim, first the aqueous SiO2 dispersions were diluted with n-propanol, the methyl-trimethyloxysilane was added, then the dispersions were stirred at 60 C. for 6 h and subsequently the solvent was replaced with toluene by distillation.

Example E (Synthesis of ZrO2 Dispersion by Nonaqueous Process)

(7) ZrO2 nanoparticles were synthesized according to the nonaqueous process described in S. Zhou, G. Garnweitner, M. Niederberger and M. Antonietti, Langmuir, 2007, 23, pages 9178-9187 and T. A. Cheema and G. Garnweitner published in Chemie-Ingenieur-Technik, 2008, 84/3, pages 301-308.

(8) 80 ml zirconium(IV)-n-propoxide and 500 ml benzyl alcohol were placed in an Teflon coated steel autoclave and heated to 220 C. for 4 days. After cooling down to room temperature, the solid ZrO2 material was separated from the supernatant liquid by centrifugation. The isolated material was washed 3 times with 1 L of ethanol. 5 g of the wet solid material was then suspended in 200 ml of THF and 1.7 g n-propyl(trimethoxy)silane were added. The mixture was sonicated for 10 min and stirred at room temperature for 3 days. The transparent dispersion was then concentrated by evaporation of a part of the THF to adjust a solid content of 20 weight %. The average particle size was characterized by light scattering and was found to be 7 nm.

Examples 1-9 (Synthesis of Hybrid Material)

(9) All nanoparticle dispersions from Examples B-E were adjusted to a solids content of 20 weight percent.

Example 1

(10) In a 250 ml flask 50 g of polysilazane was dissolved in 50 g of THF in a nitrogen atmosphere and under careful exclusion of water. While stirring, 175 g of a zirconium dioxide dispersion in toluene with a mean particle size of approx. <10 nm and a solids content of 20 weight percent were dropped in. The THF and the majority of the toluene were distilled off at the rotary evaporator at 50 C. bath temperature and a reduced pressure of approx. 15 mbar. The last remaining toluene was removed from the residue in a high vacuum of <0.1 mbar at 40 C. for less than 24 h.

(11) 83 g of a colorless transparent oil remained.

Example 2

(12) Example 2 was carried out analogously to example 1, the only difference being that the amount of zirconium oxide dispersion in toluene was increased to 250 g.

(13) The final product was 97 g of a colorless transparent oil.

Example 3

(14) Example 3 was carried out analogously to example 1, the only difference being that the amount of zirconium oxide dispersion in toluene was increased to 750 g.

(15) 194 g of a colorless transparent oil remained as the final product.

Example 4

(16) Example 4 was carried out analogously to example 1, the only difference being that the zirconium oxide dispersion was replaced by 250 g of a titanium dioxide dispersion in toluene with a mean particle size of approx. 10 nm and a solids content of 20 weight percent.

(17) 96 g of a colorless nearly transparent oil remained as the final product.

Example 5

(18) Example 5 was carried out analogously to example 4, the only difference being that the titanium dioxide dispersion in toluene was coated with Vinyltrimethoxysilane instead of methyltrimethoxysilane.

(19) 95 g of a colorless nearly transparent oil remained as the final product.

Example 6

(20) Example 6 was carried out analogously to example 1, the only difference being that the zirconium dioxide dispersion was replaced by 250 g of a titanium dioxide dispersion in toluene with a mean particle size of approx. 25 nm and a solids content of 20 weight percent.

(21) 95 g of a colorless opalescent oil remained as the final product.

Example 7

(22) Example 7 was carried out analogously to example 1, the only difference being that the zirconium dioxide dispersion was replaced by 250 g of a silicon dioxide dispersion in toluene with a mean particle size of approx. 15 nm and a solids content of 20 weight percent.

(23) 94 g of a colorless nearly transparent oil remained as the final product.

Example 8

(24) Example 8 was carried out analogously to example 1, the only difference being that the zirconium dioxide dispersion was replaced by 250 g of a silicon dioxide dispersion in toluene with a mean particle size of approx. 85 nm and a solids content of 20 weight percent.

(25) 96 g of a colorless slightly cloudy oil remained as the final product.

Example 9

(26) Example 9 was carried out analogously to example 1, the only difference being that 250 g of the zirconium dioxide dispersion of Example E in THF with a mean particle size of approximately 7 nm and a solids content of 20 weight percent was used.

(27) 94 g of a colorless transparent oil remained as the final product.

Reference Example 10

(28) The polysilazane as it is was used as a reference example 9 for a polysilazane without addition of any nanoparticles.

(29) The examples are shown in comparison in Table 1.

(30) The viscosity of the oil-like products is determined with a R/S Plus Rheometer by the company Brookfield. The transmission of the liquid samples was measured with a UV-VIS spectrophotometer Lambda 850 from Perkin Elmer before curing. The products were cured in order to determine the refractive index. For this aim, the oil-like products were each mixed with 0.5 wt.-% di-t.butyl-peroxide, poured into a 3 cm2 cm PTFE receptacle to a height of 3 mm and cured for 3 h at 180 C. in a nitrogen atmosphere. Subsequently, the hardened molded piece was removed from the PTFE receptacle and the refractive index was measured with a Prism Coupler Model 2010/M by the company Metricon working at 594 nm.

(31) TABLE-US-00001 TABLE 1 Listing of the Examples Raw Raw Ratio 1:2 Viscosity RI* Example material 1 material 2 [m:m] [mPas at 25 C.] [594 nm] Transmittance** 1. Poly- ZrO.sub.2, <10 nm 1.0:0.7 39000 1.57 99% silazane 2. Poly- ZrO.sub.2, <10 nm 1.0:1.0 44000 1.61 99% silazane 3. Poly- ZrO.sub.2, <10 nm 1.0:3.0 55000 1.74 98% silazane 4. Poly- TiO.sub.2, 10 nm 1.0:1.0 43000 1.65 91% silazane 5. Poly- TiO.sub.2, 10 nm 1.0:1.0 43000 1.65 90% silazane 6. Poly- TiO.sub.2, 25 nm 1.0:1.0 38500 1.66 <50% silazane 7. Poly- SiO.sub.2, 15 nm 1.0:1.0 43500 1.49 68% silazane 8. Poly- SiO.sub.2, 85 nm 1.0:1.0 37500 1.49 <50% silazane 9. Poly- ZrO.sub.2, <10 nm 1.0:1.0 48000 1.60 99% silazane 10. Poly- 37000 1.50 100***% Reference silazane 11. HRI- 1.55 100% Reference **** Silicone 12. LRI- 1.41 100% Reference **** Silicone *Refractive Index of the cured sample **Transmittance of the liquid sample at 450 nm and thickness of 3 mm relative to example 9 Since the organopolysilazane and the nanoparticles do not absorb light of 450 nm wavelength, the reduced transmittance is ascribed to scattering ***Transmittance of the Reference Example was normalized to be 100% at all wavelength **** HRI-Silicone and LRI-Silicone are available from Dow Corning

(32) The comparison of Reference Example 10 with Example 1, Example 2 and Example 3 shows that increasing the proportion of highly refractive zirconium dioxide increases the refractive index. Due to the nanoparticles, a slight increase of viscosity can be observed. As the nanoparticles are smaller than 10 nm, no or a very small degree of scattering of light occurs and the optical transparency is just as high or only slightly lower than in Reference Example 10.

(33) Examples 4, 5 and 6 show that the refractive index can also be increased by admixing highly refractive titanium dioxide. With nanoparticles of small size, such as in Examples 4 and 5, the optical transparency is retained to 90%, whereas use of larger nanoparticles, such as in example 6, can cause opalescence.

(34) Comparison of Example 4 and 5 proofs that the surface of the nanoparticles can be coated either with non-reactive alkyl groups or with vinyl groups which are able to participate in the crosslinking reaction of the organopolysilazane during the curing process. This will not change the transparency of the product.

(35) Examples 7 and 8 show that the refractive index of the mixture is slightly lowered by admixing a silicon dioxide with a refractive index smaller (n amorphous SiO.sub.2=1.46) than that of the polysilazane. With nanoparticles of 15 nm size, such as in Example 7, the optical transparency is noticeable lowered, whereas use of larger nanoparticles with a radius of about 85 nm, such as in Example 8, causes a marked opalescence.

(36) Reference Examples 11) and 12) show commercially available silicone encapsulation materials. Standard methyl-silicones (Example 12) are known to have good thermal stability, however their big drawback is the low RI of <1.45. Commercially available high RI silicones (example 11) have RI up to 1.55, but can not reach RI numbers beyond 1.6.

(37) Thus, it can be shown that by mixing polysilazane with nanoparticles whose refractive index is higher or lower than that of pure polysilazane, the refractive index of the mixture can be increased or lowered. The refractive index can theoretically be adjusted between 1.3 to 2.4. The mixtures containing nanoparticles can be cured in the same way as pure polysilzane. If the nanoparticles are sufficiently small, no light dispersion occurs and the optical transparency is maintained completely (see FIG. 1).

(38) Example for Lower CTE Sample

(39) Another advantage of the presence of nanoparticles is the reduction in thermal expansion of the hybrid material compared to the pure polysilazane. The CTE (Coefficient of Thermal Expansion, analyzed with a Mettler-Toledo TMA/SDTA840 in a temperature range of 50-150 C.) of cured polysilazane is in a range of 150-250 ppm/K. The CTE of cured polysilazane filled with 50% weight % of nanoparticles is in a range of 125-200 ppm/K. The advantage of reduced CTE is less stress in the material by temperature change and therefore a lower tendency of crack formation.

(40) Example for Hydrolysis Stability

(41) An additional advantage of nanoparticles filled organopolysilazane compared to the pure polysilazane is that the refractive index does not change if the cured material is exposed to higher temperature and humidity conditions. The cured materials of Example 3 and of Example 10 were stored in a climate chamber at 80 C. and 85% relative humidity for 82 h. The refractive index of reference material of Example 10 was reduced from 1.50 to 1.48 by partial hydrolysis of the silazane to siloxane. The refractive index of the material of Example 3 remains unchanged at 1.74. The presence of the nanoparticle prevents the change in refractive index under conditions which are able to at least partly hydrolyze the polysilazane.

(42) FIG. 1 shows transmission curves of various examples:

(43) Transmittance of reference Example 10 was normalized to be 100% at all wavelength. Therefore the spectra shown in FIG. 1 show reduction of transmittance caused by addition of Nanoparticles A) transmittance of material of Example 1 B) transmittance of material of Example 3 C) transmittance of material of Example 4 D) transmittance of material of Example 7

(44) Since the organopolysilazane and the nanoparticles do not absorb light of >350 nm wavelength, the reduced transmittance is ascribed to scattering phenomena. The transmittance of reference Example 10 was normalized to be 100% at all wavelength. Therefore the reduction in transmittance shown in FIG. 1 is a result of light scattering caused by the presence of the nanoparticles. The scattering intensity depends of course not only on the particle size, but also on the difference in refractive index of the particles and the surrounding matrix, as can be calculated by the well known equation:
IM.Math.(dn/dc).sup.2.Math..sup.4
I=Intensity of scattered light
M=Molecular weight. For a homogeneous sphere Mradius.sup.3
dn/dcdifference in refractive index of the particles and the surrounding matrix

(45) If the refractive index of the particles and the surrounding matrix is the same, there will be no light scattering, regardless of the size of the particles. Avoiding scattering by this approach is not useful to make high RI material, because in such a situation the refractive index of the hybrid material is not different from the refractive index of the pure matrix.

(46) FIG. 2 shows the particle size distribution of various examples: A) particle size of Nanoparticles used in Example 1, 2 and 3 B) particle size of Nanoparticles used in Example 7 C) particle size of Nanoparticles used in Example 8

(47) Distribution A) is an example of a exceptionally preferred particle distribution used to make transparent hybride material, which in the definition used here has a transmission of 80% at 450 nm. Distribution B) is an example of acceptable particle distribution and distribution C is an example of non preferred particle distribution. B) and C) will form partly transparent and/or opaque hybrid material, which in the definition used here has a transmission of <80% at 450 nm.

(48) FIG. 3 shows further particle sizes and distributions: A) preferred particle size and distribution: small average <10 nm and narrow distribution B) Less preferred particle size and distribution: small average <10 nm but bimodal distribution containing a big amount of particles >10 nm C) Less preferred particle size and distribution: big average >10 nm and narrow distribution D) None preferred particle size and distribution: big average >10 nm and broad distribution containing particles up to 100 nm size.

(49) FIG. 3 shows examples of small and narrow distributions (A), small but bimodal distributions (B)), a medium size and narrow distribution (C) and a medium size but broad distributions (D). Only distributions obeying the two conditions of small average size of <15 nm and narrow distribution with small amount of average size particles and absence of big size particles as shown in distribution A) are useful to make fully transparent hybrid material.

(50) Iffor any reasona certain turbidity is irrelevant or is even intended, bigger particles with broader distribution can be used as well. In case the turbidity should be adjusted to a certain level at constant refractive index, this can be done by keeping the mixing ratio of nanoparticles and matrix constant (which defines the refractive index) and changing the size (average and distribution) of the nanoparticles.

(51) FIG. 4 shows the comparison of the RI numbers of the Examples 1, 2 and 3 and a theoretical calculation of RI of a mixture of polymer and ZrO.sub.2. A) Theoretical calculation .circle-solid. B) experimental numbers of Example 1, 2 and 3

(52) The calculation is based on the assumption that the polymer has a RI of 1.5 and a density of 1.0 g/ml and the ZrO.sub.2 has a RI of 2.2 and a density of 5.0 g/ml. It can be seen that the experimental results nicely follow the theoretical expectations.

(53) To test the thermal stability, some selected solid cured samples of 30203 mm size were prepared and stored on an open hotplate (exposed to air atmosphere) at a temperature of 120 C., 180 C. and 200 C. for 24 h. After the heat treatment the transmittance was measured using a Perkin Elmer UV-VIS Spectrophotometer Lambda 850 in combination with an integration sphere.

(54) TABLE-US-00002 Material Temperature/Time Transmittance at 450 nm* Example 2 120 C./24 h 99% Example 2 180 C./24 h 97% Example 2 200 C./24 h 97% Ref. 10 200 C./24 h 98% Ref. 11, 120 C./24 h 87% HRI Silicone *Transmittance of solid sample at 450 nm and thickness of 3 mm

(55) FIG. 5 shows transmission curves after stability test at elevated temperatures: A) transmittance of cured material of Example 2 after 24 h at 120 B) transmittance of cured material of Example 2 after 24 h at 180 C. C) transmittance of cured material of reference Example 9 after 24 h at 200 C. D) transmittance of cured material of Example 2 after 24 h at 200 C.

(56) There is a slight decrease in transmittance by heating up to 200 C., but only to a minor and tolerable extent. Comparison of Sample 2 and the Ref. Example 9 proves that there is virtually no deterioration of the transmittance by the presence of the nanoparticles up to 200 C.

(57) FIG. 6: Transmission curves after stability test at elevated temperatures A) transmittance of cured material of Example 2 after 24 h at 120 B) transmittance of cured material of Reference Example 10 (HRI Silicone) after 24 h at 120 C.

(58) FIG. 6) shows the transmittance of Example 2 and Reference Example 10) after heating to 120 C. for 24 h. The commercially available HRI silicones are made of aryl-silicones, for example phenyl-silicones. The aromatic residue increases the RI, however the thermal stability is dramatically reduced. Already at temperatures as low as 120 C., the transmittance drops from the initial value of 100% down to unacceptable 87% at 450 nm.