WO2007010462A2

WO2007010462A2 - High-refractive optical material and electric lamp with interference film

Info

Publication number: WO2007010462A2
Application number: PCT/IB2006/052412
Authority: WO
Inventors: Hans Van Sprang; Margot L. Van Grootel
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-07-20
Filing date: 2006-07-14
Publication date: 2007-01-25
Also published as: WO2007010462A3

Abstract

The electric lamp has a light-transmitting lamp vessel (1) accommodating a light source (2) provided with an interference film (5) comprising a plurality of high- refractive and low-refractive optical layers. The high-refractive optical layer comprises a high-refractive optical material with a rutile structure and a mixed oxide of 96-99 mole% titanium oxide and 1-4 mole% niobium oxide. The mixed oxide is preferably constituted by 97-98 mole% titanium oxide and 2-3 mole% niobium oxide. The invention also relates to a high-refractive optical material having a rutile structure formed by a mixed oxide of 96-99 mole% titanium oxide and 1-4 mole% niobium oxide. In the high-refractive optical material according to the invention, the grain size growth is substantially hampered during life and the phase transition from anatase to rutile is prevented.

Description

High-refractive optical material and electric lamp with interference film

FIELD OF THE INVENTION

The invention relates to a high-refractive optical material having a rutile structure.

The invention also relates to a substrate provided with an optical layer of such a high-refractive optical material.

The invention further relates to an electric lamp comprising a light- transmitting lamp vessel in which a light source is arranged, and an interference film comprising a plurality of high-refractive optical layers and low-refractive optical layers, wherein the high-refractive optical layer comprises such a high-refractive optical material.

BACKGROUND OF THE INVENTION

Such high-refractive optical materials having a rutile structure are known per se. These materials are used, inter alia, in high-refractive optical layers in optical thin- film optical interference coatings. Such a (layer of a) high-refractive optical material can be used, for instance, as part of a further refractive or diffractive optical structure.

Thin- film optical interference coatings, also known as interference filters, comprising a plurality of (alternating) layers of two or more materials of different refractive indices are well known in the art. Optical interference filters are used, for example, in laser technology. Optical filters are also used with incoherent light sources such as gas discharge lamps and halogen lamps for increasing the luminous efficacy of the lamps, as color filters or color-correction filters and also as reflectors. In addition, such interference films or coatings are employed to selectively reflect and/or transmit light radiation from various portions of the electromagnetic spectrum such as ultraviolet, visible and infrared (IR) radiation. The lighting industry employs these interference films to coat reflectors and lamp envelopes. One application in which these thin- film optical coatings have been found to be useful is improvement of the illumination efficiency or efficacy of incandescent and arc lamps by reflecting infrared (IR) radiation emitted by a filament or arc back to the filament or arc while transmitting the visible light portion of the electromagnetic spectrum emitted by the filament or arc. This lowers the amount of electric energy required to be supplied to the filament or arc so as to maintain its operating temperature. In other lamp applications, in which it is desired to transmit IR radiation, such filters can reflect the shorter wavelength portions of the spectrum, such as ultraviolet and visible light portions emitted by a filament or arc and transmit primarily the infrared portion in order to provide heat radiation with little or no visible light radiation.

Optical interference films, also referred to as optical coatings or optical (interference) filters, used for applications in which the interference film will be exposed to high temperatures in excess of 500°C, are made of alternating layers of refractory metal oxides such as titania (titanium dioxide, TiO₂, n=2.7 for rutile TiO₂), niobia (niobium pentoxide, Nb₂O₅, n=2.35), zirconia (zirconium oxide, ZrO₂, n=2.3), tantala (tantalum pentoxide, Ta₂O₅, n=2.2) and silica (silicon oxide, SiO₂, n=1.45), wherein silica is the low- refractive index material and the titania, niobia, zirkonia or tantala is the high-refractive index material (the values of the respective refractive indices are given at a wavelength λ = 550 nm). In halogen lamp applications, these interference films are applied on the outside surface of the quartz lamp vessel containing the light source (filament or arc). The outside surface, and thus the interference film, can reach operating temperatures ranging from 800°C to 900°C.

Interference films or coatings are applied by means of evaporation or (reactive) sputtering techniques and also by chemical vapor deposition (CVD) and low- pressure chemical vapor deposition (LPCVD) processes. These deposition techniques generally produce relatively thick layers which tend to crack and severely limit the filter design.

The phase stability, oxidation state, and thermal expansion mismatch of the high-refractive index layer materials with the (quartz glass) substrate at higher temperatures is a matter of concern. Changes herein may cause delamination of the interference film, for instance, due to thermal mismatch, or may introduce an undesirable degree of light scattering and/or light absorption in the interference film. The high-refractive index materials are normally deposited at temperatures relatively close to room temperature (typically below 250°C) and are deposited as amorphous or microcrystalline layers. Generally, most high- refractive index layers undergo crystallization at temperatures above 550°C, for instance, during life of the electric lamp (typically several thousands of hours). Crystallization involves crystal grain growth, which may disturb the optical transparency of the coating through light scattering. In addition, care has to be taken that the high-refractive index layer material should not become oxygen-deficient during the (physical) layer deposition process and during lamp operation at high temperatures, because this deficiency generally leads to undesirable light absorption.

Optical multilayer interference films comprising titanium oxide as a material for high-refractive optical layers and silicon oxide as a material for low-refractive optical layers are currently used by various companies, in particular, on so-termed cold-mirror reflectors and on small, low-wattage halogen lamps with an operation temperature below approximately 650°C. It is known that these interference films tend to become cloudy (scattering) at temperatures above 700°C. The use of infrared (IR) reflecting interference films based on titanium oxide as high-refractive optical layer and silicon oxide as low- refractive optical layer is preferred for cost-saving reasons, because the relatively large difference between the refractive indices of the respective layer materials allows use of a relatively small number of layers in the filter design and an overall thinner film stack for realizing adequate IR reflection, requiring less time during deposition of the interference film. Nevertheless, although TiO₂ with a refractive index n=2.3 (at a wavelength λ = 550 nm) is commonly used for low-temperature halogen lamps, no high- index TiO₂ZSiO₂ IR-reflecting multilayer interference films on high-temperature (e.g. halogen) electric lamps have been commercialized until now because of the above-mentioned problems with scattering, absorption and/or coating cracking/delamination phenomena when the TiO₂ZSiO₂ interference film is exposed to temperatures exceeding 700°C. Around and above this temperature range, there are internal phase transitions from amorphous to crystalline and/or between different crystalline phases, particularly in the well-known anatase and rutile types of crystallites, creating scattering crystallites and inducing volume changes. In addition, these transitions affect the temperature-dependent mechanical stresses to which the multilayer stack is exposed, which may subsequently induce layer cracking and/or delamination.

US patent publication US-A 4,940,636 discloses an optical interference film assembled from alternating (amorphous) low-refractive optical silicon dioxide (SiO₂) layers and high-refractive optical layers made of mixed oxides chosen from a group consisting of 88-95 mole.% TiO₂ and 5-12 mole.% ZrO₂, 88-95 mole.% TiO₂ and 5-12 mole.% of an oxide selected from the group HiO₂, TiO₂-ZrO₂, TiO₂-HiO₂, TiO₂-Nb₂O₅, TiO₂Ta₂O₅ and

Ta₂O₅.2TiO₂ and mixtures and combinations of these materials. The mixed oxides have a crystal structure which corresponds to the crystal structure obtained after a heat treatment between 700°C and HOO⁰C. The optical interference filter is stable, also after a long period at elevated temperatures.

A drawback of the known high-refractive optical layer materials is that grain growth in such layers at temperatures above 550°C is not sufficiently suppressed.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to wholly or partly eliminate the above- mentioned drawback. According to a first aspect of the invention, this object is achieved by a high-refractive optical material having a rutile structure, the high-refractive optical material comprising a mixed oxide of 96-99 mole% titanium oxide and 1-4 mole% niobium oxide.

The inventors have observed that the grains in known (pure) rutile TiO₂ high- refractive optical materials tend to grow substantially when such layers are subjected to temperatures well above 700°C. The separate grains of known rutile TiO₂ grow at the expense of each other and, as a result, the heated optical layers tend to show a milky appearance (diffuse scattering) when the TiO₂ grains exceed sizes of above approximately 100 nm.

The inventors have had the insight that the growth of the grain size is substantially hampered during the lifetime of the high-refractive optical material when relatively small amounts of niobium oxide are added to the high-refractive optical titanium oxide layer. According to the invention, the rutile titanium oxide is doped with 1-4 mole% niobium oxide. The resulting high-refractive optical material has a composition Ti^ .X)Nb_xO₂, wherein 0.01 < x < 0.04.

Doping TiO₂ films with Nb₂O₅ is found to delay the grain growth. The presence of a dopant or a compound in a nano-grained structure has its effect on the rate of grain growth and the density of the rutile nucleation. When small amounts of niobium are incorporated into the high-refractive optical TiO₂ layer, it is found that niobium is substitutional^ present in the TiO₂ lattice.

Different methods are available for doping a TiO₂ layer. A suitable method of doping TiO₂ is to deposit the titanium and the dopant simultaneously. An example of such a method is (reactive) simultaneously sputtering from both a titanium target and from a target provided with the dopant material. This method is also referred to as co-sputtering.

In addition, the inventors have had the insight that the phase transition from anatase to rutile TiO₂ has to be prevented. This phase transition generally occurs in a known temperature range above 700°C and is regarded as being responsible for the change of appearance and performance of the high-refractive optical layer material, in particular when employed in an interference film provided on a lamp vessel operating at temperatures well above 700°C. It was found that a co-sputtered high-refractive optical layer comprising 96-99 mole% titanium oxide doped with 1-4 mole% niobium oxide already exhibited a rutile structure as deposited. When such an as-deposited layer is subjected to temperatures above 700°C, the anatase-rutile transition is prevented. By preventing the anatase-rutile transition in the Ti(i -X)Nb_xO₂ layers (0.01 < x < 0.04) according to the invention, a major breakthrough is established in providing high-refractive optical layer materials suitable for a variety of purposes. When deposited as a single Ti(_1-X)Nb_xθ2 layer, the high-refractive optical material according to the invention finds application as part of a further refractive or diffractive optical structure. For instance, by forming a 2D periodic grid of holes or lines in the high-refractive optical layer comprising a high-refractive optical material according to the invention, a 2D photonic band gap can be created for radiation in the plane of the high- refractive optical layer. In these types of diffractive, sub- wavelength structures, the performance is critically dependent on the index ratio between high and low- index materials.

A dopant concentration of less than 1 mole% niobium oxide does not have sufficient influence on the TiO₂ rutile layer structure to realize the desired effects. A dopant concentration of more than 4% niobium oxide does not result in all the niobium being incorporated into the TiO₂ crystal structure. In addition, if amounts larger than 4 mole% of niobium oxide are incorporated into the TiO₂ layer, the optical properties will change and the refractive index will become smaller, which is undesirable. Also higher amounts of niobium oxide require longer times to reach an equilibrium distribution and hence might cause undesirable grain growth in the rutile layer during the annealing step used for stress removal. The inventors have found that relatively small amounts of niobium dope are required and sufficient to obtain the desired effects. The high-refractive optical material preferably comprises a mixed oxide of 97-98 mole% titanium oxide and 2-3 mole% niobium oxide. In this preferred range of doping titanium oxide with niobium, the high-refractive optical material has a rutile crystal structure with the structural formula Ti(_1-x)Nb_xO₂, wherein 0.02 < x < 0.03).

Grain growth of the crystals in the high-refractive optical layer material according to the invention is hampered. A preferred embodiment of the high-refractive optical material according to the invention is characterized in that the average crystal size of the high-refractive optical layer is smaller than 100 nm. Such crystal sizes are obtained after a heat treatment in the ambience at a temperature of at least 800°C for at least two hours. The as-deposited high-refractive optical layer materials, i.e. before any heat treatment, were found to be nano-crystalline (average crystal size typically below 10 nm). The average crystal size increased to around 25 nm when the high-refractive optical layer was heated to approximately 900°C. By maintaining the temperature at 900°C for more than 10 hours, the average crystal size gradually increased to approximately 50 nm.

A preferred embodiment of the high-refractive optical material according to the invention is characterized in that the principal lattice distances of the high-refractive optical material are between 1% and 2% larger than those of pure rutile titanium oxide. Such lattice distances are obtained after a heat treatment in the ambience at a temperature of at least 800°C for at least two hours. Evidence of the incorporation of niobium into the TiO₂ crystal lattice can be derived from electron diffraction measurements. The lattice planes are denoted in a well-known manner by [hkl], while principal lattice distances can be derived from the lattice planes. In general, principal lattice planes are [100] [010], [001], [110], [101], [111], [200], [211] and [220]. It was observed that the principal lattice distances of as- deposited high-refractive optical material according to the invention substantially resemble those of the rutile lattice of pure TiO₂, whereas the principal lattice distances of the high- refractive optical material according to the invention after heating to a temperature of approximately 900°C for more than 10 hours are substantially larger than those of the rutile lattice of pure TiO₂. After heating the high-refractive optical material according to the invention at these high temperatures, the principal lattice distances of the high-refractive optical material are between 1% and 2% larger than those of pure rutile titanium oxide, thereby shifting in the direction of the rutile lattice of pure Nb₂O₅. It can be concluded from these results that the niobium is probably incorporated into the rutile lattice OfTiO₂. Substrates that are able to withstand temperatures of at least 600°C can be provided with a layer comprising any of said optical high-refractive materials. Said substrates may be, for example, glass devices such as float glass or packaging such as bottles, or ceramic materials or metal objects such as reflectors. Said substrates can be given desired properties by said layer, for example, a better reflection of a specific part of the UV, visible and/or IR-spectrum. For example, a UV-sensitive chemical compound contained in a glass bottle which is provided with a coating of said optical high-refractive material that reflects specifically in the UV region can be given a longer shelf life due to a delayed photochemical reaction. According to a second aspect the invention, an electric lamp is provided, comprising a light-transmitting lamp vessel accommodating a light source, at least a portion of the lamp vessel being provided with an interference film, the interference film comprising a plurality of high-refractive and low-refractive optical layers, the high-refractive optical layer comprising a high-refractive optical material having a rutile structure, the high- refractive optical material comprising a mixed oxide constituted by 96-99 mole% titanium oxide and 1-4 mole% niobium oxide.

The high-refractive optical material preferably comprises a mixed oxide of 97-98 mole% titanium oxide and 2-3 mole% niobium oxide. The low-refractive optical layer preferably comprises silicon dioxide. In this favorable embodiment, the interference film comprises Ti(_1-x)Nb_xθ2 layers, wherein 0.02 < x < 0.03, as high-refractive index material and silicon oxide as low-refractive index material, said interference film exhibiting an improved performance at elevated temperatures. In the known interference films comprising (pure) titanium oxide, relatively large grains tend to grow at elevated temperatures. The size of these grains is known to be limited in interference films by the thickness of the titanium oxide layer and generally does not exceed twice or three times the thickness of the titanium oxide layer when observed in the plane of the layer. In the known interference films employing titanium oxide as high- refractive index material, grain sizes of over 100 nm are observed, giving rise to visible degradation of the interference film due to light scattering. In addition, in the known interference films with titanium oxide as high-refractive index material, the anatase phase at elevated temperatures (above approximately 550°C) transforms into the rutile phase, leading to an increased density of the titanium oxide layer. Excessive growth of rutile crystals in the known layers of titanium oxide at elevated temperatures (above approximately 700°C) upsets the regular structure of the interference film and induces undesirable light scattering.

Additional measures can be taken to further improve the stability of the interference film at higher temperatures. A preferred embodiment of the electric lamp according to the invention is characterized in that the lamp vessel is provided with an adhesion layer between the lamp vessel and the interference film having a geometrical thickness of at least 50 nm. This measure counteracts (sudden) cracking of the interference film and/or its delamination from the lamp vessel. Another preferred embodiment of the electric lamp according to the invention is characterized in that the interference film at a side facing away from the lamp vessel is provided with a layer of silicon oxide having a geometrical thickness of at least 50 nm. Such a capping layer limits the deterioration of the interference film. The silicon oxide "capping" layer on the air side of the interference film provides protection of the interference film, in particular at elevated temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

In the drawings:

Figure 1 is a cross-sectional view of an electric incandescent lamp provided with an interference film according to the invention; Figure 2 A is a light microscope picture of a TiO₂ layer after heating at 700°C for ten minutes;

Figure 2B is a light microscope picture of a Ti(_1-x)Nb_xθ2 layer after heating at 700°C for ten minutes;

Figure 3 A is a TEM picture of a stack of Ti^ .X)Nb_xO₂ layers as-deposited by means of co-sputtering;

Figure 3B is a TEM picture of a stack of Ti(_1-x)Nb_xθ2 layers after heating to 900°C;

Figure 3 C is a TEM picture of a stack of Ti(_1-x)Nb_xθ2 layers after heating at 900°C for two hours; Figure 3D is a TEM picture of a stack of Ti(^_x)Nb_xO₂ layers after heating at

900°C for three hours, and

Figure 3 E is a TEM picture of a stack of Ti(_1-x)Nb_xθ2 layers after heating at 900°C for ten hours.

DESCRIPTION OF EMBODIMENTS

In Figure 1, the electric lamp comprises a lamp vessel 1 of quartz glass accommodating an incandescent body as the light source 2. Figure 1 is purely diagrammatic and not drawn to scale. Notably, some dimensions are shown in a strongly exaggerated form for the sake of clarity. Current conductors 3 issuing from the lamp vessel 1 to the exterior are connected to the light source 2. The lamp vessel 1 is filled with a gas containing halogen, for example, hydrogen bromide. At least a part of the lamp vessel 1 is coated with an interference film 5 comprising a plurality of layers of at least silicon oxide and titanium oxide. In the example of Figure 1, the suitable part of the lamp vessel 1 functions as substrate for depositing the interference film 5. The interference film 5 allows passage of visible radiation and reflects infrared (IR) radiation. In the example of Figure 1, the lamp vessel 1 is mounted in an outer bulb 4, which is supported by a lamp cap 6 with which the current conductors 3 are electrically connected. The electric lamp shown in Figure 1 is a 60 W mains-operated lamp having a service life of at least 2500 hours.

As a first step, pure TiO₂ was deposited on a substrate by reactive sputtering in an Ar/θ2 atmosphere. The TiO₂ layers as deposited by means of this well-known standard deposition method are in the anatase phase. In addition, a small amount of rutile TiO₂ was detected. By suitably changing the sputter conditions, the ratio of anatase and rutile TiO₂ could be influenced. In particular, by decreasing the target-substrate distance and changing the Ar/O₂ ratio to less oxygen and more argon, it was possible to deposit TiO₂ layers in the rutile phase. When such a rutile TiO₂ layer was heated to approximately 800°C for 10 hours, the grains in the rutile TiO₂ were observed to grow to dimensions of the order of the layer thickness. Both anatase and rutile are bifringent, rutile TiO₂ even more than anatase TiO₂. The optical anisotropy results in light scattering in the layer, resulting in an increased diffuse transmission and a hazy appearance of the pure TiO₂ layer. The relatively big grains disturb the optical properties of TiO₂.

Figure 2A shows a light microscope picture of a TiO₂ layer after heating at 700°C for ten minutes. It is observed that the layer is cracked and delaminates. Doping TiO₂ films with Nb₂O₅ is found to delay the grain growth. The presence of a dopant or a compound in a nano-grained structure has its effect on the rate of grain growth and the density of the rutile nucleation. Not wishing to be held to any particular theory, two basic mechanisms are involved: (1) segregation at the grain boundary of a phase takes place, lowering the surface energy of the grains; and (2) occupation of bulk lattice sites hampers surface ionic mobility. When small amounts of niobium are incorporated into the high-refractive optical TiO₂ layer, it is found that niobium is substitutional^ present in the TiO₂ lattice.

Co-sputtered samples of Nb-doped TiO₂ on a substrate were prepared. The Nb-dope was introduced by simultaneously reactive sputtering from a Ti target and a Nb target in an Ar/O₂ atmosphere. To this end, Nb target material was attached to the Ti target. By way of example, a layer of approximately 200 nm was deposited on a SiO₂ substrate for a period of 3 hours. During reactive sputtering, it was observed that the temperature of the substrate increased to values around 250-300°C. X-Ray Fluorescence spectroscopy (XRF) was used to establish the amount of Nb in the layer. The experiments showed that the amount of niobium in the TiO₂ layer ranged from 2-3 mole% niobium oxide, and that the high- refractive optical layer material further constituted 97-98 mole% titanium oxide. The as- deposited layer had a refractive index of 2.6 (at a wavelength λ = 550 nm), which refractive index is relatively close to the refractive index of known pure rutile TiO2 (typically n = 2.7 at 550 nm). As a subsequent step, the Ti(_1-x)Nbχθ2 layers, wherein 0.02 < x < 0.03, were annealed at high temperatures.

Figure 2B shows a light microscope picture of a Ti^ .X)Nb_xO₂ layer (x=0.03) after heating at 700°C for ten minutes. It is observed that the layer is free from cracks. In addition, the high-refractive optical layer is transparent and free from diffuse scattering. In order to study changes in the high-refractive optical Nb-doped TiO₂ layers, under heating conditions, in-situ heating experiments were performed in a Transmission Electron Microscope (TEM). Figure 3 A shows a TEM picture of a stack of Ti(^_x)Nb_xO₂ layers (0.02 < x < 0.03) as-deposited by means of co-sputtering. The (white) bar in the lower left corner of Figure 3 A depicts a distance of 250 nm. The as-deposited layers were found to be nano-crystalline with an average crystal size of approximately 20 nm.

Figure 3B shows a TEM picture of a stack of Ti^ .X)Nb_xO₂ layers immediately after heating to approximately 900°C. The average crystal size has increased from approximately 20 nm to approximately 30 nm.

Figure 3 C shows a TEM picture of a stack Of Ti(^_x)Nb_xO₂ layers after heating at 900°C for two hours. Needle-like elongated crystals have become visible in this Figure 3C. The (black) arrows in the (black) rectangular box in Figure 3C depict a number of needle-like elongated crystals. Many more needle-like elongated crystals are visible in Figure 3C.

Figure 3D shows a TEM picture of a stack Of Ti(^_x)Nb_xO₂ layers after heating at 900°C for three hours. The (black) rectangular box in Figure 3D indicates the same area as in Figure 3C and was taken within 1 hour from the TEM image of Figure 3C. The rectangular box in Figure 3D indicates the same part of the sample as in Figure 3C; the sample has slightly moved in the TEM. It can be observed from Figure 3D that the amount of the elongated needle-like crystals has substantially decreased. In addition, the average crystal size has increased to approximately 40 nm. Figure 3 E shows a TEM picture of a stack of Ti(^_x)Nb_xO₂ layers after heating at 900°C for 10 hours. It can be observed from Figure 3D that the elongated crystals have disappeared completely. In addition, the average crystal size has increased to approximately 50 nm. Longer heating of the high-refractive optical layer at 900°C and cycling the layer between room temperature and 900°C did not result in a further increase of the average grain size of the crystals.

In order to determine the origin of the needle-like elongated crystals observed in Figures 3C and 3D, electron diffraction data were collected during the experiment. By comparing literature data on lattice distances with the data obtained during the in-situ TEM heating experiment, it is possible to give an explanation for the needle-like elongated crystals. Table I shows the lattice distances measured by means of electron diffraction for the as-deposited Ti(_1-X)Nbχθ2 layers (0.02 < x < 0.03) as well as after heating the high-refractive optical layers at 900°C for 10 hours. The lattice distances for pure rutile TiO₂ and pure rutile Nb₂O₅ have been included for the purpose of comparison. The crystal lattice points corresponding to the diffraction from a periodic set of specific crystal lattice planes are indicated, using the standard index triple [hkl], known to the skilled person, wherein h, k, 1 are the Miller indices of the crystal lattice planes.

Table I Lattice distances measured by means of electron diffraction

Based on the results in Table I and the comparison with values known from the literature, the electron diffraction data indicated that the orientation of the as-deposited high-refractive Ti(_1-x)NbχO₂ optical layers (0.02 < x < 0.03) were typically rutile-like. Almost all of the measured values of the as-deposited samples were close to the values found in literature for pure rutile TiO₂. However, the lattice distances of the high-refractive Ti_(1-x)Nb_xθ2 optical layers (0.02 < x < 0.03) after heating at 900°C for 10 hours shifted towards the literature values of rutile NbO₂. The electron diffraction data confirm that, after heating at 900°C for 10 hours, Nb-rich regions are no longer detected or visible. In addition, it is observed that the niobium is incorporated in the rutile TiO₂ crystals. This incorporation of niobium in the rutile TiO₂ crystals is reflected in the increase of the lattice distances by approximately 1-2%. The observed values for the heat-treated Ti(_1-X)NbχO₂ optical layers have shifted towards the known lattice distances of pure rutile NbO₂, which are larger than the lattice distances of pure rutile TiO₂. By adding relatively small amounts of niobium oxide to the high-refractive optical titanium oxide layer, the growth of the grain size is substantially hampered during the lifetime of the high-refractive Ti(_1-x)Nb_xO₂ optical layer material according to the invention. In addition, the phase transition from anatase to rutile in the high-refractive Ti(_1-x)Nb_xO₂ optical material according to the invention is prevented. By preventing the anatase-rutile transition in the high-refractive Ti(_1-x)Nb_xO₂ optical material according to the invention, a major breakthrough has been established in providing high-refractive optical layers for many applications, in particular for producing interference films on lamps operating at temperatures well above 700°C.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Use of the article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A high-refractive optical material having a rutile structure, the high-refractive optical material comprising a mixed oxide of 96-99 mole% titanium oxide and 1-4 mole% niobium oxide.

2. A high-refractive optical material as claimed in claim 1, wherein the high- refractive optical material comprises a mixed oxide of 97-98 mole% titanium oxide and 2-3 mole% niobium oxide.

3. A high-refractive optical material as claimed in claim 1 or 2, wherein the average crystal size of the high-refractive optical material is smaller than 100 nm.

4. A high-refractive optical material as claimed in claim 1 or 2, wherein the principal lattice distances of the high-refractive optical material are between 1% and 2% larger than those of pure rutile titanium oxide.

5 A substrate provided with an optical layer comprising the high-refractive optical material as claimed in any one of claims 1 to 4.

6. An electric lamp comprising a light-transmitting lamp vessel (1) accommodating a light source (2), at least a portion of the lamp vessel (1) being provided with an interference film (5), the interference film (5) comprising a plurality of high-refractive and low- refractive optical layers, the high-refractive optical layers comprising a high-refractive optical material as claimed in any one of claims 1 to 4.

7. An electric lamp as claimed in claim 6, wherein the low-refractive optical layer comprises silicon dioxide.