Composite ceramic which comprises a conversion phosphor and a material having a negative coefficient of thermal expansion

Abstract

The present invention relates to a composite ceramic which comprises a conversion phosphor and a further material, characterized in that the further material has a negative coefficient of thermal expansion, and to a process for the preparation thereof. Furthermore, the present invention also relates to the use of the composite ceramic according to the invention as emission-converting material, preferably in a white light source, and to a light source, a lighting unit and a display device.

Claims

1. Composite ceramic which comprises a conversion phosphor and a further material, wherein the further material has a negative coefficient of thermal expansion which is in the range from 1*10.sup.6 to 12*10.sup.6K.sup.1 in the case of a temperature change in the range from 20 C. to 200 C.

2. Composite ceramic according to claim 1, in which the conversion phosphor is a Ce-, Eu- and/or Mn-containing material.

3. Composite ceramic according to claim 1, in which the conversion phosphor is a Ce-containing material.

4. Composite ceramic according to claim 3, in which the Ce-containing material is a Ce-containing garnet which has the formula E.sub.3G.sub.2(TO.sub.4).sub.3:Ce, where E is selected from the group consisting of Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu and mixtures thereof, and G and T are each selected, independently of one another, from the group consisting of Al, Sc, Ga and mixtures thereof, or G and T together stand for the combination Mg/Si or the combination Mg/Ge, where Mg and Si or Mg and Ge respectively are present in the same molar proportions.

5. Composite material according to claim 1, in which the further material is a tungstate or molybdate.

6. Composite ceramic according to claim 1, in which the molar ratio of the conversion phosphor to the further material is in the range from 1:0.5 to 10:1.

7. Process for the preparation of a composite ceramic according to claim 1, which comprises: a) providing a conversion phosphor; b) providing a material having a negative coefficient of thermal expansion; c) mixing of the two components provided in steps a) and b) to give a mixture; and d) sintering of the mixture.

8. Process according to claim 7 wherein the sintering is carried out at a temperature in the range from to of the melting temperature of the material having a negative coefficient of thermal expansion.

9. Process for the preparation of a composite ceramic according to claim 1, which comprises: a) coating of a conversion phosphor with an oxide of aluminium; b) mixing of the coated conversion phosphor obtained in step a) with a W- or Mo-containing component to give a mixture; and c) sintering of the mixture obtained in step b) at a temperature in the range from 1000 to 1600 C., to obtain a composite ceramic which comprises a conversion phosphor and an aluminium tungstate or aluminium molybdate material which has a negative coefficient of thermal expansion in the range from 1*10.sup.6 to 12*10.sup.6K.sup.1 in the case of a temperature change in the range from 20 C. to 200 C.

10. An emission-converting material comprising a composite ceramic according to claim 1.

11. A light source comprising an emission-converting material according to claim 10.

12. Light source which comprises a composite ceramic according to claim 1 and a primary light source.

13. Lighting unit which comprises at least one light source according to claim 12.

14. Composite ceramic according to claim 1, in which the conversion phosphor is a Ce-, Eu- and/or containing material, which comprises 0.1 to 5 atom-% of Ce, Eu and/or Mn, based on the total number of atoms at the lattice sites which are replaced by the Ce, Eu and/or Mn in the Ce-, Eu- and/or Mn-containing material.

15. Composite ceramic according to claim 1, in which the conversion phosphor is a Ce-containing garnet.

16. Composite material according to claim 1, in which the further material is a tungstate or molybdate, which is selected from the group consisting of Al.sub.2W.sub.3O.sub.12, Y.sub.2W.sub.3O.sub.12, YAlW.sub.3O.sub.12, ZrW.sub.2O.sub.8, Al.sub.2Mo.sub.3O.sub.12, Y.sub.2Mo.sub.3O.sub.12, YAlMo.sub.3O.sub.12, ZrMo.sub.2O.sub.8, Al.sub.2WMo.sub.2O.sub.12, Y.sub.2WMo.sub.2O.sub.12, YAlWMo.sub.2O.sub.12, ZrWMoO.sub.8, Al.sub.2MoW.sub.2O.sub.12, Y.sub.2MoW.sub.2O.sub.12, YAlMoW.sub.2O.sub.12 and mixtures thereof.

17. Composite ceramic according to claim 1, in which the molar ratio of the conversion phosphor to the further material is in the range from 1:1 to 5:1.

18. Composite ceramic which comprises a conversion phosphor and a further material, wherein the further material is a tungstate or molybdate and has a negative coefficient of thermal expansion.

19. Composite material according to claim 18, in which the further material is a tungstate or molybdate, which is selected from the group consisting of Al.sub.2W.sub.3O.sub.12, Y.sub.2W.sub.3O.sub.12, YAlW.sub.3O.sub.12, ZrW.sub.2O.sub.8, Al.sub.2Mo.sub.3O.sub.12, Y.sub.2Mo.sub.3O.sub.12, YAlMo.sub.3O.sub.12, ZrMo.sub.2O.sub.8, Al.sub.2WMo.sub.2O.sub.12, Y.sub.2WMo.sub.2O.sub.12, YAlWMo.sub.2O.sub.12, ZrWMoO.sub.8, Al.sub.2MoW.sub.2O.sub.12, Y.sub.2MoW.sub.2O.sub.12, YAlMoW.sub.2O.sub.12 and mixtures thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the emission spectrum of a comparative material YAG:Ce and a composite ceramic which consists of Y.sub.3Al.sub.5O.sub.12:Ce and Al.sub.2W.sub.3O.sub.12 (3:1), as prepared in Example 3a. The spectrum was recorded with an Edinburgh Instruments FL900 fluorescence spectrometer, using a Xe medium-pressure lamp (Osram) as excitation source.

(2) FIG. 2 shows the emission spectrum of a comparative material YAG:Ce and a composite ceramic consisting of Y.sub.3Al.sub.5O.sub.12:Ce and YAlW.sub.3O.sub.12 (3:1), as prepared in Example 3b. The spectrum was recorded with an Edinburgh Instruments FL900 fluorescence spectrometer, using an Xe medium-pressure lamp (Osram) as excitation source.

EXAMPLES

Example 1

Preparation of a Composite Ceramic which Consists of Y3(Al1-aSia/2Mga/2)5O12:Ce and Al2W3O12

(3) The powder components for the composite ceramic, i.e. yttrium aluminate and aluminium tungstate, are prepared separately. The starting materials used are metal nitrates Y(NO.sub.3).sub.3, Al(NO.sub.3).sub.3, Ce(NO.sub.3).sub.3 for the yttrium aluminate and Al(NO.sub.3).sub.3 with WO.sub.3 in ammonia for aluminium tungstate, which are in each case mixed to give homogeneous solutions. The metal cations in the solutions are stabilised by the addition of a complexing agent (for example trisamines) and then evaporated to give a solid residue. Further heating of the dry residue results in ignition and the formation of a sponge-like precursor structure. The precursors are calcined at temperatures of 800 to 1000 C. and converted into the compounds Y.sub.3-xCe.sub.xAl.sub.5O.sub.12 and Al.sub.2W.sub.3O.sub.12 in the form of soft agglomerates. The Al.sub.2W.sub.3O.sub.12 powder is coated with Al.sub.2O.sub.3 by wet-chemical means, where the coating process is achieved by the hydrolysis of aluminium isopropylate in alcoholic medium with addition of ammonia as catalyst in a mixing reactor.

(4) In a subsequent step, the two powders are mixed, where the proportion by volume of the second component is in the range from 10 to 60% by vol. and is determined taking into account the dilatometric measurements of the finished ceramics.

(5) The mixed powder is pressed uniaxially and isostatically in two steps in the form of thin discs, where the pressing pressure is in the range from 100 to 300 MPa. The subsequent sintering is achieved in air as a multistep process, where the temperatures are in the range from 1000 C. to 1600 C. The sintered ceramics are ground using a diamond suspension and cut to dimensions matched to the excitation source by means of a picolaser.

Example 2

Preparation of a Composite Ceramic which Consists of Y3(Al1-aSia/2Mga/2)5O12:Ce and YAlW3O12 (3:1)

(6) The powder components for the composite ceramics, i.e. magnesium- and silicon-doped yttrium aluminate and yttrium aluminium tungstate, are prepared separately by a ceramic method. The metal oxides in the form of fine powders are mixed, calcined and synthesised at temperatures in the range from 800 C. to 1200 C. The YAlW.sub.3O.sub.12 powder is coated with Al.sub.2O.sub.3 by wet-chemical means, where the coating process is achieved by the hydrolysis of aluminium isopropylate in alcohol medium with addition of ammonia as catalyst in a mixing reactor. The Y.sub.3(Al.sub.1-aSi.sub.a/2Mg.sub.a/2).sub.5O.sub.12:Ce powder is finely ground and mixed with Al.sub.2O.sub.3-coated YAlW.sub.3O.sub.12 powder. The mixture was shaped in the form of thin discs by two-step uniaxial and isostatic pressing, where the pressing pressure is in the range from 100 to 300 MPa. The subsequent sintering is achieved in air as a multistep process, where the temperature is in the range from 1000 C. to 1600 C. The sintered ceramics are ground using a diamond suspension and cut to dimensions matched to the excitation source by means of a picolaser.

Example 3

Specific Experimental Procedure for the Preparation of the Composite Ceramics from YAG:Ce and a Tungstate (Example 3a: Al2W3O12; Example 3b: AlYW3O12)

(7) Step 1.

(8) YAG:Ce powder was prepared by a self-combustion method, where an aqueous nitrate solution of the metals was mixed with tris(hydroxymethyl)-aminomethane (TRIS: M=121.14 g/mol), dried and then ignited. The black precursor was calcined at 1000 C. and in the process converted into a finely particulate colourless powder.

(9) 8.4594 g (0.02524 mol) of Y.sub.2O.sub.3 were dissolved in 10 ml of HNO.sub.3 and made up to about 250 ml with H.sub.2O. 46.8913 g (0.08421 mol) of Al(NO.sub.3).sub.3*9H.sub.2O and 0.03256 g (0.00005 mol) of Ce(NO.sub.3).sub.3*9 H.sub.2O were then added and dissolved.

(10) TRIS was added to the homogeneous solution and made up to about 500 ml with H.sub.2O. After warming, the H.sub.2O was evaporated, and the residues were ignited. The precursor was dried for 12 h in a drying cabinet, ground in a mortar and then calcined at 1000 C. in an oven for 1 h.

(11) Step 2.

(12) Al.sub.2W.sub.3O.sub.12 powder was prepared by a ceramic method, where the finely particulate oxides Al.sub.2O.sub.3 and WO.sub.3 were mixed and treated in two steps, firstly at a temperature of 1000 C. and then at 1100 C. (with intermediate grinding in a mortar).

(13) 1.019 g (0.01 mol) of Al.sub.2O.sub.3 (nano) was ground with 6.955 g (0.003 mol) of WO.sub.3 with ethanol (agate mortar). The suspension was dried and homogenised in the mortar, then sintered at 1000 C. for 12 h, ground in a mortar and calcined at 1100 C. in air for 12 h.

(14) Step 3.

(15) AlYW.sub.3O.sub.12 powder was prepared by a ceramic method from the oxides Al.sub.2O.sub.3, WO.sub.3 and Y.sub.2O.sub.3, where all oxide powders were ground together in a mortar and treated in two steps at the temperatures 1000 C. (6 h) and at 1100 C. (12 h) in air.

(16) 0.5098 g (0.005 mol) of Al.sub.2O.sub.3 (nanoscale) were ground with 6.955 g (0.003 mol) of WO.sub.3 and 1.129 g (0.005 mol) of Y.sub.2O.sub.3 in ethanol. The suspension was dried and homogenised in the mortar, then sintered at 1000 C. for 12 h, ground in the mortar and calcined at 1100 C. in air for 12 h.

(17) Step 4.

(18) Composite ceramics comprising YAG:Ce and one of the above-mentioned metal tungstates were prepared with the aid of a mixture of Al.sub.2W.sub.3O.sub.12 powder or AlYW.sub.3O.sub.12 powder. To this end, Y.sub.2.997Ce.sub.0.003Al.sub.5O.sub.12 powder (90-99 wt-%) and in each case one of the tungstates (1-10 wt-%) was ground in ethanol (in an agate mortar), dried, wetted with a few drops of the pressing aid polyvinylalcohol (in the agate mortar) and then pressed uniaxially (about 100 MPa) and isostatically (about 300 MPa) to give tablets (thickness about 2.5 mm, diameter about 12 mm), dried in air and finally sintered in two steps, firstly at 1000 C. for 12 h and then at 1100 C. for 2 h. Ceramic mouldings are obtained.

Comparative Example A

Preparation of a Composite Ceramic which Comprises No Material Having a Negative Coefficient of Thermal Expansion

(19) For the comparison, the composite material is prepared from YAG:Ce and Al.sub.2O.sub.3, where the YAG:Ce is prepared by the self-combustion method. The second component used is Al.sub.2O.sub.3 nanopowder (Degussa). After powder mixing (favourably in the ratio 1:1 by volume), the ceramic discs are produced in a similar manner to that in the examples described above.

Comparative Example B

(20) As the second comparative material, ceramics comprising YAG:Ce with undoped YAG are prepared. The YAG:Ce powder is prepared by a coprecipitation method, where the starting materials used are the metal nitrates Y(NO.sub.3).sub.3, Al(NO.sub.3).sub.3, Ce(NO.sub.3).sub.3 and the precipitant used is NH.sub.4HCO.sub.3. The precipitate produced is converted into YAG:Ce by calcination at 800 C. and sintering at 1000 C. After intensive grinding, the YAG:Ce becomes finely articulate and suitable for mixing with the YAG prepared in a similar manner. The mixture was shaped in the form of thin discs by two-step uniaxial and isostatic pressing, where the pressing pressure is in the range from 100 to 300 MPa. The subsequent sintering is achieved in air as a multistep process, where the temperatures are in the range from 1000 to 1600 C. The sintered ceramics are ground using a diamond suspension and cut to dimensions matched to the excitation source by means of a picolaser.

Example 4

Production of LEDs with the Composite Ceramics of Examples 3a and 3b and Comparative Examples A and B

Example 4a

Remote Phosphor Arrangement

(21) A platelet having a diameter of 5 mm and a thickness of 0.1 mm, consisting of the composite ceramic according to the invention, is placed on the SMD LED (chip peak wavelength 450 nm, operating current strength 350 mA, cavity opening diameter 5.5 mm) which is filled with liquid silicone OE 6550 (Dow Corning), so that the circular cavity is sealed. The component is then stored in an oven at 150 C. for 1 h, during which the silicone hardens and bonds strongly to the LED and the ceramic platelet.

Example 4b

Chip-Level Conversion Arrangement

(22) A ceramic phosphor platelet having square dimensions of 11 mm and a thickness of 0.1 mm is placed directly so as to fit on the 11 mm LED chip of an SMD flipchip LED (chip peak wavelength 450 nm, operating current strength 350 mA) with the aid of a drop of silicone OE 6550 (Dow Corning). After the silicone has hardened over a period of 1 h at 150 C., the remaining cavity of the LED is cast out with silicone OE 6550 (Dow Corning), and the entire component is stored at 150 C. for 1 h for hardening of the silicone.

Example 5

Intensity of the Emission

(23) In FIGS. 1 and 2, the composite ceramics from Examples 3a and 3b according to the invention exhibit lower intensities compared with the YAG:Ce ceramics from Comparative Examples A and B, which is to be expected since the YAG:Ce phosphor is in dilute form in the composite ceramic. The spectrum was recorded with an Edinburgh Instruments FL900 fluorescence spectrometer, where the excitation source used was an Xe medium-pressure lamp (Osram).

Example 6

Lifetime

(24) The advantages of the composite ceramics according to the invention in accordance with Examples 1, 2 and 3 can be demonstrated with reference to long-term uses in light-emitting diodes. The material according to the invention exhibits reduced cracking in use compared with ceramics from the prior art. Accordingly, the efficiency drops less quickly, and the ceramic can be used longer with good efficiency.