SUBSTRATE PROVIDED WITH A LAYER OF THERMOCHROMIC OXIDE

Abstract: The invention relates to a substrate comprising a thermochromic oxide, said substrate being a particle having a diameter less than 200 mu m, while said particle-type substrate is provided with a coating of crystalline thermochromic oxide having a thickness of less than 150 nm and a phase transition at a temperature lower than 150 DEG C.

Description: Substrate provided with a layer of thermochromic oxide. The invention relates to a substrate provided with a layer of thermochromic oxide. The invention further relates to special applications of such a substrate, in particular as an optical temperature indicator and as a temperature-dependent reflector for IR radiation. The substrate mentioned in the opening paragraph is known per se from US patent 3,834,793. According to this US patent, switchable optical components are obtained in that very thin films of thermochromic materials are provided on a reflecting substrate. A suitable substrate which is mentioned is aluminum on a quartz substrate or stainless steel. The substrate provided with a layer of thermochromic oxide is immediately applicable as such as a temperature-dependent mirror. Such thermochromic materials exhibit a change in their reflection characteristics, and thus in their color as a result of a temperature change. Such changes can be very considerable with certain thermochromic materials, such as from red to black, from yellow to red, and from yellow to a dark brown, a wide-ranging summary of thermochromic oxides and their characteristic color changes being given in US patent 3,323,241. This latter US patent also mentions vanadium oxide, of which it is said that the reflection of bulk vanadium oxide is very bad, and that the contrast upon a phase transition between high and low temperature is also small. Although the characteristic reflection of V0 2 is not excellent, an important reason for the use of V0 2 is given as being that the transition temperature is low, which facilitates its use for practical purposes. Another reason is that V0 2 is comparatively stable compared with other thermochromic oxides. It is an object of the present invention, therefore, to develop a substrate which can be provided in the form of small particles in a matrix, which is subsequently applied as a coating layer on any basic surface, as desired, of any dimensions whatsoever. Another object of the present invention is to provide a substrate which is insensitive to corrosion and which is provided with a layer of thermochromic oxide. Another object of the present invention, furthermore, is to provide a substrate which can be used as an optical temperature indicator in a convenient manner. In addition, the present invention has for its object to provide a substrate which can be used as a temperature-dependent reflector for IR radiation in a simple manner. The substrate as mentioned in the opening paragraph, according to the invention, is characterized in that the substrate is a particle with a diameter of < 200 μm, which particle-type substrate is provided with a layer of crystalline thermochromic oxide with a thickness of < 150 nm and a phase transition at a temperature below 150 °C. The word "particle" should be understood to mean a plate-shaped particle or flake within the scope of the present invention. Such particles are characterized by an aspect ratio > 10. Furthermore, it is important for the present invention that the orientation of the particles should be parallel to the base surface on which the particles are provided. A diameter of < 200 μm is desirable in particular for the purpose of providing the substrate as a dispersed particle in a transparent matrix which can be provided on a base surface, for example the sole of an electric iron, by known methods. If a particle with a diameter outside the range mentioned above is used, it is not possible in practice to provide the substrate in a thin layer on a base surface without the particles exhibiting the desired degree of reflection. In addition, the ease of processing of, for example, a dispersion in which a particle is present having a diameter outside the above range will be unsatisfactory, in particular in the case of spraying of such a dispersion. The thermochromic oxide must be crystalline because the amorphous state has no phase transition. Moreover, the phase transition must take place at a temperature below 150 °C in view of the practical application, the layer of crystalline thermochromic oxide being provided in a thickness of < 150 nm so as to achieve the desired optical effects. In a special embodiment of the invention, it is preferred that the layer of crystalline thermochromic oxide with a thickness of < 100 nm has a phase transition at a temperature of < 100 °C. Such a transition temperature is desired especially for applications in the domestic sphere, for example for the sole of an electric iron, where especially the risk of physical contact with a hot surface, which causes injuries, is to be prevented. Suitable substrates which may be mentioned are zinc, copper, aluminum, alloys thereof, and mica. Aluminum with a diameter of < 75 μm, in particular preferably from 7 to 45 μm, is especially preferred as a substrate because of its favorable optical effects, while also a greater dynamic range is observed. It was also shown in experiments that aluminum is a very stable substrate if a number of layers are to be provided thereon. In another embodiment of the present invention, furthermore, it is preferred that mica with a diameter of < 150 μm is used, which material is particularly insensitive to corrosion. VO 2 is preferably used as the crystalline thermochromic oxide, in particular V0 2 with a monoclinic structure, which material has a high refractive index and is semi- transparent. Monoclinic V0 2 is particularly suitable for use as a thermochromic oxide, having a phase transition to a tetragonal structure at a temperature of approximately 68 °C, at which phase transition the refractive index changes significantly. Although US patent 5,364,467 discloses a particle-type metal substrate provided with a first colorless layer of metal oxide and a non-selectively absorbing layer of vanadium oxide disposed thereon, it is not known from this US patent to use such particles as a thermochromic material. The application disclosed therein relates to paints and varnishes, in particular for the automobile industry. In addition, this US patent does not disclose that the thermochromic oxide must be crystalline. It is furthermore preferred in certain embodiments that a layer of a transparent or selectively absorbing metal oxide is provided between the substrate and the layer of thermochromic oxide provided thereon, which additional layer is preferably provided to a thickness of < 1 μm, more in particular to a thickness of < 500 nm. Preferably, a metal oxide chosen from a group comprising titanium dioxide, zinc oxide, silicon dioxide, aluminum oxide, tin oxide, vanadium pentoxide, chromium oxide, iron (LU) oxide, and mixtures thereof is preferably used as a suitable transparent or selectively absorbing metal oxide, silicon dioxide and aluminum oxide being particularly preferred on account of their low refractive index values and their high chemical inertness. Furthermore, silicon nitride and magnesium fluoride may be mentioned as suitable transparent or selectively absorbing materials for the intermediate layer. It is desirable in certain embodiments of the present invention that the phase transition of the layer of crystalline thermochromic oxide should be lowered, which effect may be achieved in that such a layer is doped with one or several transition metals having a valence of > 5, for example chosen from a group comprising titanium, niobium, molybdenum, iridium, tantalum, and tungsten, and mixtures thereof. If it should be desirable in a certain embodiment of the present invention for the phase transition of the layer of crystalline thermochromic oxide to be raised, it is preferred that this layer of thermochromic oxide is doped with one or several transition metals with a valence of < 4, for example chosen from a group comprising aluminum, chromium, and iron, or mixtures thereof. It should be noted, however, that it is somewhat unpredictable whether the phase transition will be raised or lowered in the case of a valence of 4. The quantity of the dopants mentioned above to be added depends on the desired temperature shift and is given by the general formula V i-x M x 0 2 , in which M is the dopant element and x the stoichiometric quantity, for which it is true that 0 < x <0.1, preferably 0 < x < 0.05. In certain embodiments of the present invention, furthermore, it is preferred that an additional layer of a transparent or selectively absorbing metal oxide is provided over the layer of crystalline thermochromic oxide. Such an additional layer not only has an optical function, but also a protective function, for example in the incorporation of pigment in a matrix or during the use of the substrate in question. It is also possible in such an embodiment that a layer of transparent or selectively absorbing metal oxide is also present between the layer of crystalline thermochromic oxide and the substrate, although the present invention is not limited to the presence of such an intermediate layer. It is thus possible in accordance with the invention to provide a substrate, for example aluminum, with a layer of V0 2 and, provided thereon, a layer of, for example, silicon dioxide. It is alternatively possible, however, to provide, for example, aluminum with, for example, a silicon dioxide layer and subsequently with a V0 2 layer, which V0 2 layer may possibly be provided with, for example, a silicon dioxide layer. A suitable metal oxide which may be provided over the layer of crystalline thermochromic oxide may be chosen from a group comprising titanium dioxide, zinc oxide, silicon dioxide, aluminum oxide, tin oxide, chromium oxide, iron oxide, and mixtures thereof, silicon dioxide and aluminum oxide being particularly preferred on account of their low refractive index values and their chemical inertness. In addition, silicon nitride and magnesium fluoride may be mentioned as suitable transparent or selectively absorbing materials for such a covering layer. The thickness of such a transparent or selectively absorbing metal oxide is < 1 μm, in particular preferably < 500 nm. The substrate according to the invention is especially suitable for use as an optical temperature indicator. The particle-type substrate is thus provided in a transparent matrix on a base surface which is subject to temperature changes. Possible applications which may be mentioned: the sole plate of an electric iron, the wall of a water kettle, an electric hot plate, and the like, wherever it is particularly desirable for the color of the substrate to provide an indication on the prevailing temperature. Another application of the substrate according to the invention is as a temperature-dependent reflector for LR radiation, in particular for objects whose temperature is to be controlled, for example satellite panels.

Claims: CLAIMS: 1. A substrate provided with a layer of thermochromic oxide, characterized in that the substrate is a particle with a diameter of < 200 μm, which particle-type substrate is provided with a layer of crystalline thermochromic oxide with a thickness of < 150 nm and a phase transition at a temperature below 150 °C. 2. A substrate as claimed in claim 1, characterized in that the layer of crystalline thermochromic oxide with a thickness of < 100 nm has a phase transition at a temperature of < 100 °C. 3. A substrate as claimed in claims 1 and 2, characterized in that zinc, copper, aluminum, alloys thereof, or mica is used as the substrate. 4. A substrate as claimed in claim 3, characterized in that aluminum with a diameter of < 75 μm is used as the substrate. 5. A substrate as claimed in claim 3, characterized in that mica with a diameter of < 150 μm is used as the substrate. 6. A substrate as claimed in claim 1, characterized in that V0 2 is used as the thermochromic oxide. 7. A substrate as claimed in claim 6, characterized in that V0 2 with a monoclinic structure is used. 8. A substrate as claimed in claims 1 to 7, characterized in that a layer of a transparent or selectively absorbing metal oxide is provided between the substrate and the layer of thermochromic oxide. 9. A substrate as claimed in claim 8, characterized in that the layer of transparent or selectively absorbing metal oxide is substantially formed by a metal oxide chosen from a group comprising titanium oxide, zinc oxide, silicon dioxide, aluminum oxide, tin oxide, chromium oxide, vanadium pentoxide, iron oxide, and mixtures thereof. 10. A substrate as claimed in claims 8 and 9, characterized in that the layer of transparent or selectively absorbing metal oxide is provided to a thickness of < 1 μm. 11. A substrate as claimed in claim 10, characterized in that the layer of transparent or selectively absorbing metal oxide is provided to a thickness of < 500 nm. 12. A substrate as claimed in claims 1 to 11, characterized in that one or several dopants are added to the layer of thermochromic oxide, which dopants are chosen from a group comprising titanium, niobium, molybdenum, iridium, tantalum, tungsten, and mixtures thereof. 13. A substrate as claimed in claims 1 to 11, characterized in that one or several dopants are added to the layer of thermochromic oxide, which dopants are chosen from a group comprising aluminum, chromium, iron, and mixtures thereof. 14. A substrate as claimed in claims 12 and 13, characterized in that the stoichiometric quantity x is < 0.1 in the doped vanadium oxide, as represented by the general formula V 1 _ x M x 0 , in which M is the dopant element. 15. A substrate as claimed in claims 1 to 14, characterized in that a transparent or selectively absorbing layer of metal oxide is provided over the layer of crystalline thermochromic oxide. 16. A substrate as claimed in claim 15, characterized in that the layer of transparent or selectively absorbing metal oxide is substantially formed by a metal oxide chosen from a group comprising titanium oxide, zinc oxide, silicon dioxide, aluminum oxide, tin oxide, chromium oxide, vanadium pentoxide, iron oxide, and mixtures thereof. 17. A substrate as claimed in claims 15 and 16, characterized in that the layer of transparent or selectively absorbing metal oxide is provided to a thickness of < 1 μm. 18. A substrate as claimed in claim 17, characterized in that the layer of transparent or selectively absorbing metal oxide is provided to a thickness of < 500 nm. 19. The use of the substrate as claimed in claims 1 to 18 as an optical temperature indicator. 20. The use as claimed in claim 19, characterized in that the particle-type substrate is provided in a transparent matrix on the sole plate of an electric iron. 21. The use of the substrate as claimed in claims 1 to 18 as a temperature- dependent reflector for LR radiation. 22. The use as claimed in claim 21, characterized in that the substrate is provided on a satellite panel.

Inventors: BOONEKAMP ERIK P, BOONEKAMP, ERIK, P.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: C09K9
IPC codes: A47J36, C09K9, G01K11, G09F3