WO2013054219A1

WO2013054219A1 - Diffusive ceramic envelope

Info

Publication number: WO2013054219A1
Application number: PCT/IB2012/055089
Authority: WO
Inventors: Johannes Petrus Maria Ansems; Susanne Lina GOLDBACH; Peter Cornelis Marie GERARDTS; Antonius Adrianus Maria Marinus
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2011-10-11
Filing date: 2012-09-25
Publication date: 2013-04-18
Anticipated expiration: 2014-04-11

Abstract

There is provided a ceramic envelope for a light emitting device, a light emitting device comprising such a ceramic envelope, and a method for manufacturing a ceramic envelope. The ceramic envelope comprises a first translucent ceramic material forming a first body, and a second material having a different refractive index than the first material. The first material has a solubility limit of the second material and it comprises a concentration of the second material that is higher than the solubility limit. As a consequence, the second material forms a plurality of second bodies within the first body. Light that passes through the ceramic envelope is refracted at the plurality of second bodies. Thereby the diffusivity properties of the ceramic envelope are improved.

Description

DIFFUSIVE CERAMIC ENVELOPE

FIELD OF THE INVENTION

The present invention relates to a ceramic envelope for a light emitting device. The ceramic envelope is of the type that comprises a first and a second material.

BACKGROUND OF THE INVENTION

Recent environmental regulations have increased the need for new types of lighting techniques. In particular, conventional incandescent lamps are replaced by more energy efficient semiconductor lighting devices, such as light emitting diodes (LEDs).

Existing LED replacement lamps have a heat sink for cooling the LED. The heat sink, usually being arranged beneath the LED lamp, is usually made of a metal and is thus non-transparent with respect to light radiation. Typically the size of the heat sink is large in comparison to the light emitting part of the lamp. As a result, it may be difficult to transmit light to the lower hemisphere of the lamp.

Several attempts have been made to overcome this shortcoming of LED lamps. For example, the diameter of the heat sink has been made as small as possible to allow light to be transmitted to the lower hemisphere. Another approach has been to provide the LED lamp with an outer bulb. By varying the size of the outer bulb the size of the light emitting area can be chosen. In this way, the outer bulb affects the luminous distribution of the lighting device.

However, existing outer bulbs still have shortcomings with respect to the luminous distribution. For example, existing outer bulbs do not provide an angular luminous distribution which is homogeneous. Thus, there is a need for improvement of existing outer bulbs.

SUMMARY OF THE INVENTION

In view of the above, an object of the invention is to provide a ceramic envelope for a light emitting device which has an improved luminous distribution. In particular, it is an object of the invention to provide a ceramic envelope having a high total light transmittance and still a good diffusivity.

According to a first aspect, the above objects are achieved by a ceramic envelope for a light emitting device, comprising a first translucent ceramic material forming a first body, and a second material having a different refractive index than the first material, the first material having a solubility limit of the second material, and the first material comprising a concentration of the second material, wherein the concentration is higher than the solubility limit such that the second material forms a plurality of second bodies within the first body.

By solubility limit is meant the maximum concentration of the second material which may be solved by the first material.

With the arrangement according to the invention, the diffusivity properties of the ceramic envelope may be tuned, thereby affecting the luminous distribution. More specifically, the envelope comprises a first material forming a first body. Within the first body, there are "islands", or second bodies, of a second material having a different refractive index than the first material. Consequently, light that falls on and passes through the ceramic envelope will be refracted inside the envelope when it passes from the first body to one of the second bodies. In this way, the "islands" of the second material work as scattering centers for light. Thus, the provision of second bodies of the second material in first body of the first material, enables tuning of the diffusivity properties of the material. Hence, by such tuning it is possible to achieve an optimal trade-off between light transmission and diffusivity.

The first translucent ceramic material may further be birefringent, meaning that the first translucent ceramic material is anisotropic with respect to light. For example, an unpolarized light beam may be inclined when it passes through a birefringent material on the interface of two crystals due to different refractive indices in two directions. For example, the first translucent ceramic material may be Polycrystalline Alumina AI₂O₃ (PC A) or Aluminium nitride A1N.

Preferably, the second material has a high melting point such that it does not evaporate during the manufacturing of the ceramic envelope by a sintering process. Further, the second material does preferably not absorb light in the visible wavelength range. For example, the second material may be zirconia Zr0₂.

The concentration of zirconia may be higher than 1000 ppm, and preferably between 3000 ppm and 10000 ppm. Depending on other additives, Zirconia may be dissolved in alumina up to an amount of 200 ppm. By increasing the concentration of zirconia, the diffusivity of the ceramic envelope will increase. However, the concentration of zirconia will also affect the light transmittance of the envelope negatively. The inventors have found that the above concentration levels give an optimal trade of between light transmittance and diffusivity of the envelope. The ceramic envelope may be porous, meaning that the ceramic envelope comprises small pores. As light passes the envelope, it will be scattered at the pores. Thus, the presence of pores are advantageous in that they increase the scattering (and diffusivity) of the ceramic envelope and thereby affect the luminous distribution.

The ceramic envelope may further have a surface arranged to receive light emitted by the light emitting device, the surface being rough. By the surface being rough is meant that the surface may comprise a plurality of irregularities such as recesses and protrusions. For example, the inner surface of the envelope may be rough. In this way, an incident light beam will be partly reflected as it encounters the surface of the envelope. This may be a particularly favorable way of introducing additional scattering in case the first material is a highly transparent material.

The first material may further be an aluminium-based garnet material which does not absorb light in the visible range, preferably Yttrium Aluminium Garnet (YAG) or Ytterbium Aluminium Garnet (YbAG). An advantage with these materials is that they are highly transparent and thus have a high transmittance of light.

The first material may further be a spinel, such as MgAl₂0₄. Such materials are available as transparent materials, at least on a lab-scale.

The first material may further be an oxide base on rare earth metals which does not absorb light in the visible range. The first material may further be a white oxide, such as ZnO or MgO. By choosing different materials for the first translucent ceramic material, different light transmitting properties may be obtained for the ceramic envelope.

According to a second aspect, the above objects are achieved by a light emitting device comprising a light emitting element and a ceramic envelope according to the above, the ceramic envelope being arranged to receive light emitted by the light emitting element.

According to a third aspect, the above objects are achieved by a method for producing a ceramic envelope for a light emitting device. The method comprises: providing a first ceramic material, providing a second material having a different refractive index than the first ceramic material, forming a mixture from the first ceramic material and the second material, wherein the first ceramic material has a solubility limit of the second material, and wherein the concentration of the second material in the mixture is higher than the solubility limit, forming a body from the mixture, and thermally treating the body. It is noted that the invention relates to all possible combinations of features recited in the claims. Thus, all features and advantages of the first aspect likewise apply to the second aspect and third aspect, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The various aspects of the invention, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which:

Fig. la illustrates a light emitting device comprising a ceramic envelope

according to embodiments;

Fig. lb illustrates an exploded view of the light emitting device of Fig. la. Fig. 2 illustrates schematically a cross-section of a portion of a ceramic

envelope according to embodiments;

Figs. 3a-b show reflectivity as a function of wavelength for different

concentrations of zirconia in a ceramic envelope and transmittance as a function of zirconia concentration, respectively;

Fig. 4 is a flowchart of a method for manufacturing a ceramic envelope

according to embodiments;

Figs. 5-6 schematically illustrate cross-sections of a part of a ceramic envelope according to embodiments;

Figs. 7a-b show reflectivity as a function of wavelength for a ceramic

envelope for different sintering temperatures and transmittance as a function of sintering temperature, respectively;

Figs. 8a-b illustrate reflectivity as a function of wavelength for samples of a ceramic envelope having a uniform distribution of porosity and transmittance for these samples, respectively.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled addressee. Like reference numerals/symbols refer to like elements throughout.

Figs, la-b illustrate a light emitting device 1. For example, the light emitting device 1 may be a replacement lamp for a general lighting service. The light emitting device 1 comprises one or several light emitting elements 5 and an envelope 3 which at least partly encloses the light emitting element 5 such that light emitted by the light emitting element 5 passes through the envelope 3 when the light emitting element device 1 is in use.

The light emitting element 5 may be or comprise one or more light emitting diodes (LEDs). The light emitting element 5 may further comprise a separate heat conducting element (not shown) to conduct and exchange heat with the surroundings in order to cool the light emitting element 5. Such a heat conducting element, sometimes referred to as a heat sink, is typically non-transparent and made of metal. Thus light emitted by the light emitting element 5 cannot pass through the heat conducting element. Typically, the size of the heat conducting element is large in comparison to the light emitting part of the light emitting element 5. For this reason, it may be difficult to transmit light to the part of hemisphere of the envelope 3 where the heat conducting element is located. Usually, for a light emitting device 1 oriented as in Fig. 1, this concerns the lower part of the hemisphere of the envelope 3. In order to obtain a homogeneous luminous intensity distribution over a wide angular range it is thus favorable to make the diameter of the heat conducting element small. Additionally, or alternatively, the envelope 3 may be used to affect the luminous intensity distribution as described next.

The envelope 3 forms an outer bulb with respect to the light emitting element 1. When the light emitting device 1 is in use, light emitted by the light emitting element 5 reaches and passes through the envelope 3. In this way, the envelope 3 functions as a light emitting and light scattering area of the light emitting device 1. By varying the size and shape of the envelope, 3 the size and shape of the light emitting area is thus varied. In particular, the angular luminous intensity distribution of the light emitting device 1 may be varied in this way. By having an envelope 3, the drawbacks of the heat conducting element of the light emitting element 5 may thus be reduced and compensated for.

The envelope 3 is translucent and should preferably be made of a material having a high thermal conductivity and low electrical conductivity. Thus, glass and plastic materials are of limited use due to their thermal properties. Similarly, pure metallic materials cannot be used due to their bad transmission properties and high electrical conductivity. Suitable materials for the envelope are translucent ceramics with low electrical conductivity and high thermal conductivity. Some advantages with ceramic lamps are that they comprise a low number of components and without the need to comprise plastic parts for isolation.

Further, the envelope 3 is preferably made of a scattering material, such that incident light is scattered in different directions upon passage of the envelope 3, thereby influencing the angular luminous intensity distribution. For example the material may be birefringent, thus having anisotropic properties with respect to light refraction. To achieve a desired luminous intensity distribution it is favorable to tune the diffusivity properties of the envelope 3. For efficiency reasons, it is preferable to give the material of the envelope 3 a low reflectivity and low light absorption, preferably below 10%, and thereby a high total light transmittance (TLT), preferably higher than 90%, and still keeping good diffusivity, preferably larger than 35%.

Fig. 2 illustrates a cross-section of a portion 3a of the ceramic envelope 3. The envelope comprises a first translucent and ceramic material 7. The first material 7 forms a first body 1 1 (the portions illustrated in white). Further, the ceramic envelope 3 comprises a second material 9. The second material 9 forms a plurality of second bodies 13 (the portions illustrated by black and white stripes), or islands, within the first body 11. The plurality of second bodies 13 may be essentially uniformly distributed within the first body 11. The second material 9 has a different refractive index than the first material 7 such that light passing from the first material 9 to the second material 7 is refracted. In this way, the second bodies 13 may be thought of as light scattering centers within the first material 7. Thus, the presence of second bodies 13 in the first body 11 introduces additional scattering in the material, thereby increasing the diffusivity of the material.

The first material 7 comprises a concentration of the second material 9. More precisely, during manufacturing, an amount of the second material 9 is added to the first material 7 resulting in a concentration of the second material 9 in the first material 7.

However, the first material 7 may only chemically solve a predetermined concentration, referred in the art to as a solubility limit, of the second material 9. When adding an amount of the second material 9 to the first material 7 corresponding to a concentration that is higher than the solubility limit, a portion of the added second material 9 will not be solved in the first material 7. As a result, and as illustrated in Figure 2, the excess amount of second material 9 forms a plurality of second bodies 9 within the first body 1 1 of first material 7. The first and second bodies 11 and 13 may be resembled to a first and a second phase, respectively, the first phase comprising the first material 7 and the second phase comprising the second material 9. The illustrated first material 7 is according to embodiments in a crystalline form, comprising a plurality of grains. For example, the first material 7 may be

polycrystalline alumina (PCA) A1₂0₃ which is a birefringent material.

The second material 7 typically has a high melting point so that it does not evaporate during manufacturing. Thus, the melting point of the second material 7 may be higher than the temperature applied during manufacturing. Further, the second material 9 should preferably not absorb light in the visible wavelength range resulting in colored or greyish material having undesired optical properties. For example, the second material 9 may be zirconia Zr0₂. The concentration of zirconia should preferably be higher than 1000 ppm and preferably between 3000 ppm and 10000 ppm, at least if the first material is PCA. These concentrations correspond to concentrations that are higher than the solubility limit for zirconia in PCA. Depending on the presence of additives, the solubility limit of zirconia in PCA is about 200 ppm.

For example, a light beam 14 that falls on the envelope portion 3a will be refracted and scattered during its passage through the envelope portion 3 a. More precisely, the light beam 14 will be refracted as it encounters the surface of the envelope portion 3a due to the difference in refractive indices between the air and the ceramic material of the envelope portion 3a. Further, the light beam 14 will be scattered at the interfaces between grains or crystals inside of the envelope portion 3 a due to birefringency of the first material 7. Still further, as the light beam 14 encounters a surface between the first body 11 and the second body 13 it will be scattered due to the difference in refractive indices between the first and second materials 7 and 9.

In order to introduce even more light scattering at least one of the surfaces 12 of the ceramic envelope 3 may be rough, that is, it may comprise a plurality or irregularities such as recesses and/or protrusions. For example, the inner surface facing the light emitting element 5 of the envelope 3 may be rough. In this way, an incident light beam 14 will be partly reflected as it encounters the surface 12 of the envelope 3. This may be particularly favorable if the first material 7 is a highly transparent material.

Figs. 3a-b are graphs showing the results of experiments performed by the inventors. Fig. 3a shows the reflectivity of different samples of a ceramic envelope 3 for wavelengths in the interval 300 nm to about 800 nm. The samples are of the type disclosed with respect to Fig. 2. The samples comprise PCA with zirconia Zr0₂ in different

concentrations, namely 1000 ppm, 2000 ppm, 3000 ppm, and 7000 ppm. The reflectivity, or the total forward transmission, is a measure of the diffusivity of the ceramic envelope. More specifically, the total forward transmission is the amount of light that passes through a body without being reflected or absorbed. This value should preferably be larger than 35% , and more preferably larger than 50%. From Fig. 3a it can be seen that as the concentration of zirconia increases, the reflectivity increases over the whole wavelength range. For a zirconia concentration of 1000 ppm the reflectivity is well above 50% over the whole wavelength range, whereas for a zirconia concentration of 7000 ppm the reflectivity is above 56% for the whole wavelength range. Thus, by increasing the concentration of zirconia the diffusivity of the material increases.

Fig. 3b shows the total light transmittance (TLT) for the samples described with reference to Fig. 3b. Preferably, the total light transmittance should be above 90%, more preferably above 92%, and most preferably above 95%. As the amount of zirconia in the samples increases, the transmittance decreases in an essentially linear fashion. However, even for the sample with the highest zirconia concentration, that is, the sample with concentration 7000 ppm, the transmittance is about 92% which is well above the desired 90%. For the samples with concentrations 1000 ppm, 2000 ppm and 3000 ppm the transmittance is comparable to that of the reference samples, that is, samples without any concentration of zirconia.

Figs. 3a-b when considered in combination shows that ceramic envelopes of the type provided by the present invention may be used to obtain an optimal trade off in light transmission and light reflection. In particular, the diffusivity properties of the envelope 3 may be tuned by varying the concentration of second material 9 in the first material 7 while still keeping the light transmission at a high level. By tuning the diffusivity properties, a desired luminous intensity distribution may be obtained for the light emitting device 1 comprising the envelope 3.

A method for manufacturing a ceramic envelope according to an embodiment of the present invention will now be described with reference to Fig. 4 and Fig. 2. In particular, the manufacturing method may be used to manufacture a ceramic envelope as disclosed above with reference to Fig. 2.

In step 400 a first ceramic material 7 is provided. The first ceramic material 7 may for example be AI₂O₃.

In step 402, a second material 9 is provided. The second material 9, which for example is zirconia Zr0₂, has a different refractive index than the first ceramic material 7.

In step 404 a mixture is formed from the first ceramic material 7 and the second material 9. The mixture may for example be in the form of a slurry or a paste. Further, the concentration of the second material 9 in the mixture is higher than maximum

concentration that may be solved by the first material 7. In other words, the concentration of the second material 9 is present in a concentration that is higher than the solubility limit. Thus, a portion of the added second material 9 will not be solved by the first material 7.

In step 406, a body is formed from the mixture. More specifically, a body may be formed or shaped from the mixture by all common shaping processes such as injection moulding, slip or gel casting, and pressing.

In step 408, the body is subject to thermal treatment. More specifically, the body may be subject to a sintering process. The sintering may be performed at a temperature in the range of about 1400°C to 1900°C. The sintering may take place in reducing or inert atmosphere like ¾, vacuum, Ar or N₂, or combinations thereof. Further, the sintering can be pressureless or under hot isostatic pressure conditions up to 2000 bar. Alternatively, microwave sintering could be applied.

As a result of the thermal treatment, the particles in the body adhere to each other. In particular, the first ceramic material 7 having a portion of the second material 9 solved therein, adheres to each other to form a first body 1 1. By an appropriate choice of the first material 7, the first ceramic material 7 becomes translucent upon thermal treatment. Moreover, the portion of the added second material 9 which has not been solved by the first material 7 forms a plurality of second bodies 13 within the first body 11. In this way, the plurality of second bodies 13 forms a second phase within the resulting ceramic envelope 3.

There are alternative ways of varying the diffusivity properties of a ceramic envelope. Such alternatives will be described with reference to Figs. 5-6. These alternative embodiments may either be used separately or in combination with the embodiment disclosed with reference to Fig. 2.

Fig. 5 illustrates a cross-section of a portion 3b of a ceramic envelope 3. The material of the portion 3b is in a crystalline form. Thereby the material comprises a plurality of grains 15, each having a surface 17. In comparison to the portion 3a illustrated in Fig. 2, portion 3b comprises smaller grains 15. Since the grains 15 are smaller, a light beam that passes through the sample will pass a higher number of grain surfaces 17. At each grain surface 17 the light will be scattered, and hence, the total scattering of the light during the passage through the portion 3b increases. In this way the diffusivity of the ceramic envelope 3 is affected.

A ceramic envelope 3 of this type may be manufactured by a sintering process. In order to obtain a ceramic envelope 3 having smaller grains 15, the standard thermal processing conditions may be varied. More precisely, smaller grains 15 may be obtained by decreasing the sintering temperature. This can be achieved by either pressureless high thermal sintering or under hot isostatic pressing conditions up to 2000 bar at somewhat lower temperatures. The exact choice of sintering temperature depends for example on the composition of the material, such as the concentration of Zr0₂, or on the specific product application.

Figs. 7a-b are graphs showing the results of experiments performed by the inventors. Fig. 7a shows the reflectivity of different samples of a ceramic envelope 3 for wavelengths in the interval 300 nm to about 800 nm. The reflectance is a measure of the diffusivity of the sample. The samples comprise PC A and are of the type disclosed with respect to Fig. 5. The samples have been thermally processed at different sintering temperatures during their manufacture. More precisely, the samples have been thermally processed at temperatures 1800°C, 1775°C, 1750°C, 1725°C, 1700°C, 1675°C, 1650°C, and 1625°C. In Fig. 5 it can be seen that the reflectivity increases for decreasing temperature, reflecting the fact that the grains become smaller as the sintering temperature decreases.

Further, for a fixed sintering temperature, the reflectivity is approximately constant over the wavelength interval. For sintering temperatures of approximately 1725°C and below, the reflectivity is above 35%.

Turning to Fig. 7b, the total light transmittance is shown for the samples discussed with respect to Fig. 7a. The total light transmittance increases as a function of sintering temperature in an essentially linear fashion. However, even for the lowest sintering temperature 1625°C the total light transmittance is well above 90%.

In conclusion, the diffusivity of a ceramic envelope 3 may be tuned by varying the sintering temperature at the same time as the total light transmittance is kept at a high level. Thus an optimal trade-off between diffusivity and transmission of a ceramic envelope may be achieved. The variation of the sintering temperature, and thereby the grain size may be used in combination with the approach disclosed with respect to Fig. 2 where a second material 9 is added in a concentration that is higher than the solubility limit to form a plurality of second bodiesl3.

Fig. 6 illustrates a cross-section of a portion of a ceramic envelope 3c. The portion 3 c comprises a first material 7 forming a first body 11. The first material 7 may have the same properties as the first material 7 disclosed with respect to Fig. 2. Further, the portion 3c comprises a plurality of pores 19 formed within the first body 11. In other words, the first material 7 is porous. As a light beam enters and passes through the portion 3 c, it will be scattered at surfaces 17 formed within the first material 7. In addition, as the light beam encounters a pore 19 in the material it will be scattered. In this way, the pores 19 function as scattering centers. By having pores 19 which are uniformly distributed in the ceramic portion 3c, a uniform scattering behavior in the ceramic envelope 3 may be obtained.

A ceramic envelope 3 of this type may be manufactured by a sintering process. The sintering process comprises a sintering step wherein the material is sintered at a temperature in the range 1400°C to 1900°C. The temperature to be chosen depends on other sintering parameters. Optionally, the duration of the sintering may be prolonged with respect to standard sintering by a factor of about 1.5 to 4. The material is sintered in a reducing or inert atmosphere like ¾, vacuum, Ar or N₂, or combinations thereof. The sintering can be pressureless or under hot isostatic pressure conditions up to 2000 bar. Alternatively, microwave sintering may be performed.

One way of controlling the porosity in the material is to avoid (i.e. to exclude) any thermal debinding step in the process. Thermal debinding means removing, by thermal heating, a residual binding agent such that the material reaches an acceptable product strength. Typically, thermal debinding is carried out at temperatures between 900°C to 1400°C.

Figs. 8a-b are graphs showing the results of experiments performed by the inventors. Fig. 8a shows the reflectivity of different samples of a ceramic envelope 3 for wavelengths in the interval 300 nm to about 800 nm. The reflectivity is a measure of the diffusivity of the sample. The samples comprise porous PCA and are of the type disclosed with respect to Fig. 6. In Fig. 6 it can be seen that the reflectivity of the samples having pores (sample 1-4 of Fig. 8a) is much higher compared to the reflectivity for a reference sample. The reflectivity for all porous samples is well above 55% whereas the reflectivity for the reference sample is below 20%. Further, it can be seen that the reflectivity is approximately constant over the wavelength interval for all the samples.

Turning to Fig. 8b, the total light transmittance is shown for the ten samples of porous PCA. The total light transmittance is well above 90% for all the samples.

In conclusion, the diffusivity of a ceramic envelope 3 may be tuned by introducing pores at the same time as the total light transmittance is kept at a high level. Thus an optimal trade-off between reflectivity and transmission of a ceramic envelope may be achieved. The approach with introduction of pores may be used in combination with the approaches disclosed with respect to Figs. 2 and 5. The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the first material may be aluminium based materials such as A1N, which, similarly to Α1₂0₃ is a birefringent and thus scattering material. Aluminium based garnet materials which do not absorb light in the visible range, for example YAG or YbAG, can also be used. Also oxides based on rare earth metals might be considered as long as they are not absorbing light in the visible range. A further group which may be considered for the first material 7 are the spinels, such as MgAl₂0₄ which are available as transparent materials, at least on a lab scale. A still further group which may be considered are the white oxides like ZnO or MgO, although it has not proven yet to sinter them to optically transparent materials.

Claims

CLAIMS:

1. Ceramic envelope for a light emitting device, comprising

a first translucent ceramic material forming a first body, and

a second material having a different refractive index than the first material, the first material having a solubility limit of the second material, and the first material comprising a concentration of the second material, wherein the concentration is higher than the solubility limit such that the second material forms a plurality of second bodies within the first body.

2. Ceramic envelope according to claim 1, wherein the first translucent ceramic material is birefringent.

3. Ceramic envelope according to claim 2, wherein the first translucent ceramic material is polycrystalline alumina.

4. Ceramic envelope according to any one of the preceding claims, wherein the second material is zirconia.

5. Ceramic envelope according to claim 4 when dependent on claim 3, wherein the concentration of zirconia is higher than 1000 ppm, and preferably between 3000 ppm and 10000 ppm.

6. Ceramic envelope according to any one of the preceding claims, wherein the first translucent ceramic material is porous.

7. Ceramic envelope according to any one of the preceding claims, wherein the ceramic envelope has a surface arranged to receive light emitted by the light emitting device, the surface being rough.

8. Ceramic envelope according to any one of the preceding claims, wherein the first translucent ceramic material is A1N.

9. Ceramic envelope according to any one of the preceding claims, wherein the first material is an aluminium-based garnet material which does not absorb light in the visible range, preferably YAG or YbAG.

10. Ceramic envelope according to any one of the preceding claims, wherein the first translucent ceramic material is a spinel.

11. Ceramic envelope according to any one of the preceding claims, wherein the first translucent ceramic material is a white oxide, preferably ZnO or MgO.

12. Light emitting device comprising a light emitting element and a ceramic envelope according to claim 1 , the ceramic envelope being arranged to receive light emitted by the light emitting element.

13. Method for producing a ceramic envelope for a light emitting device, comprising

providing a first ceramic material;

providing a second material having a different refractive index than the first ceramic material;

forming a mixture from the first ceramic material and the second material, wherein the first ceramic material has a solubility limit of the second material, and wherein the concentration of the second material in the mixture is higher than the solubility limit;

forming a body from the mixture; and

thermally treating the body.