Solid state thermochromic device, and method for producing said device

Abstract

A solid-state thermochromic device and method for producing the device, the device including: a stack successively including, from a rear face to a front face exposed to solar radiation: a) a solid substrate of an inorganic material resistant up to a temperature of 550° C.; b) an infrared-reflective layer of an electronically conductive material; c) electronically insulating interface layers; d) an electronically insulating inorganic dielectric layer transparent to infrared radiation, of cerium oxide CeO2, with a thickness between 400 and 900 nm; e) electronically insulating interface layers; f) a layer of an infrared-active thermochromic material, an n-doped VO.sub.2 vanadium oxide, and crystallized in a monoclinic or rutile phase, with a thickness between 30 and 50 nm; and g) a solar-protective coating, transparent to infrared radiation.

Claims

1. A solid-state thermochromic device comprising a stack, wherein the stack comprises, successively, from a rear face to a front face exposed to solar radiation: a) a solid substrate made of an inorganic material resistant up to a temperature of 550° C.; b) an infrared-reflective layer made of an electronically-conductive material; c) electronically insulating interface layers; d) an electronically insulating inorganic dielectric layer transparent to infrared radiation, made of cerium oxide CeO.sub.2, with a thickness of 400 to 900 nm; e) electronically insulating interface layers; f) a layer of an infrared-active thermochromic material which is an n-doped VO.sub.2 vanadium IV oxide, and crystallized in a monoclinic or rutile phase, having a thickness of 30 to 50 nm; and g) a solar protective coating, solar reflective coating, transparent to infrared radiation.

2. A device according to claim 1, wherein the solid substrate is made of a material selected from: aluminum, silicon, borosilicate glasses.

3. A device according to claim 1, wherein the solid substrate is in a form of a layer.

4. A device according to claim 1, wherein the electronically-conductive material of infrared-reflective layer b) is selected from metals, noble metals, gold, silver or platinum; aluminum, and chromium; metal alloys; and electronically-conductive metal oxides transparent in visible range, tin-doped indium oxide (ITO), fluorine-doped tin oxide (FTO), antimony-doped tin oxide (ATO), or aluminum-doped zinc oxide (AZO).

5. A device according to claim 1, wherein the reflective layer b) has a thickness of 80 to 150 nm.

6. A device according to claim 1, wherein the interface layers c) are 2 in number.

7. A device according to claim 1, wherein the interface layers c) have a total thickness of 10 to 30 nm.

8. A device according to claim 1, wherein the interface layers c) comprise, starting from the reflective layer, a first layer of Si.sub.3N.sub.4 or of AlN, then a second layer of SiO.sub.2 or Al.sub.2O.sub.3.

9. A device according to claim 8, wherein the first layer of Si.sub.3N.sub.4 or AlN has a thickness of 5 to 15 nm, and the second layer of SiO.sub.2 or Al.sub.2O.sub.3 has a thickness of 5 to 15 nm.

10. A device according to claim 1, wherein the interface layers e) are 2 in number.

11. A device according to claim 1, wherein the interface layers e) have a total thickness of 105 to 155 nm.

12. A device according to claim 1, wherein the interface layers e) comprise, starting from the reflective layer, a first layer of SiO.sub.2, then a second layer of Si.sub.3N.sub.4.

13. A device according to claim 12, wherein the first layer of SiO.sub.2 has a thickness of 5 to 15 nm, and the second layer of Si.sub.3N.sub.4 has a thickness of 100 to 140 nm.

14. A device according to claim 1, wherein the vanadium IV oxide, VO.sub.2, is n-doped with oxygen vacancies and/or by substitution of the V.sup.4+cations using Z.sup.n+metal cations, having a valency n greater than 4.

15. A device according to claim 14, wherein the vanadium IV oxide, VO.sub.2, is n-doped only by oxygen vacancies, and corresponds to the formula VO.sub.2, wherein x is from 0 exclusive to 0.25 inclusive.

16. A device according to claim 14, wherein the vanadium IV oxide, VO.sub.2, is n-doped only by substitution of the V.sup.4+cations using Z.sup.m+metal cations, having a valency n greater than 4, and corresponds to the formula V.sub.1−yZ.sub.yO.sub.2, wherein y ranges from 0.01 to 0.03.

17. A device according to claim 14, wherein n is equal to 5 or 6, and Z is selected from: Nb, Ta, Mo, or W.

18. A device according to claim 14, wherein the vanadium IV oxide, VO.sub.2, is n-doped at a same time by oxygen vacancies and by substitution of the V.sup.4+cations using Z.sup.n1 metal cations, having a valency n greater than 4, and corresponds to the formula V.sub.1−yZ.sub.yO.sub.2−x.

19. A device according to claim 1, wherein the solar protective coating, solar reflective coating, transparent to infrared g), includes a Bragg mirror.

20. A device according to claim 19, wherein the Bragg mirror consists of an alternation of a layer of a metal oxide of high refractive index (n of 2 to 2.5) and of a layer of a metal oxide of low refractive index (n of 1.3 to 1.8).

21. A device according to claim 1, wherein the stack consists of, successively, from the rear face to the front face, elements a) to g).

22. An object including the device of claim 1.

23. The object according to claim 22, wherein the object comprises one of a satellite, building, and passenger compartment of a vehicle.

24. The object according to claim 23, wherein the vehicle comprises one of an automobile, aircraft, train or vessel.

25. A method for producing the device according to claim 1, comprising: a) deposition of the infrared-reflective layer made of the electronically-conductive material on the solid substrate of inorganic material resistant up to a temperature of 550° C.; b) deposition of electronically insulating interface layers on the infrared-reflective layer deposited during a); c) deposition of the electronically insulating inorganic dielectric layer transparent to infrared radiation, made of cerium oxide CeO.sub.2, having the thickness of 400 to 900 nm, on the interface layers deposited during b); d) deposition of electronically insulating interface layers on the electronically insulating dielectric layer, transparent to infrared radiation, made of cerium oxide CeO.sub.2, and deposited during c); e) deposition of a layer of an infrared-active thermochromic material, which is an undoped vanadium oxide VO.sub.2, or a vanadium oxide doped with oxygen vacancies VO.sub.2−x, in a monoclinic phase, on the interface layers deposited during d), and intercalation of ultrafine layers, with a thickness of 0.1 to 0.5 nm of a metal Z, in the layer of undoped vanadium oxide VO.sub.2; f) annealing of the substrate, and of the layers deposited during a) to e) at a temperature of more than 450° C. to less than 550° C., to crystallize the thermochromic material; and g) deposition of the solar protective coating, solar reflective coating, transparent in the infrared, on the layer of thermochromic material.

26. A method according to claim 25, wherein annealing of the substrate, and of the layers deposited during a) to e), is carried out under an argon and oxygen atmosphere containing at least 96% by volume of argon.

27. A method according to claim 25, wherein the layers and the solar protective coating are deposited by a physical vapor deposition method (PVD) selected from: magnetron cathode sputtering, laser ablation, and evaporation.

28. A method according to claim 27, wherein all the layers and the solar protective coating, are deposited under vacuum by a same physical vapor deposition method.

29. A method according to claim 25, wherein a)-g) are carried out, in a same vacuum chamber, continuously, without opening a chamber between each of a)-g).

30. A method according to claim 25, wherein e) comprises deposition of vanadium oxide doped with oxygen vacancies VO.sub.2-x, in the monoclinic phase, on the interface layers deposited during d), and intercalation of ultrafine layers, with a thickness of 0.1 to 0.5 nm of a metal Z, in the layer of vanadium oxide doped with oxygen vacancies VO.sub.2-x.

31. A method according to claim 25, wherein Z comprises tungsten.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic view of the device according to the invention.

(2) FIG. 2 shows a graph showing the infrared reflectivity of the reflective metal background of the device according to the invention, prepared in step 1 of the example.

(3) On the ordinate is the reflectivity, and on the abscissa is the wavelength (in μm).

(4) FIG. 3 is a graph which represents the variation of the emissivity produced by a device obtained at the end of step 5 of the example with a 2% tungsten doping, of active thickness 40 nm and with a surface area of 2.5×2.5 cm.sup.2, integrated from 2.5 to 25 μm, as a function of the temperature, compared to the emissivity of a black body heated to the same temperature.

(5) On the ordinate is the emissivity E, and on the abscissa is the temperature T (in ° C.).

(6) It appears that Δε=0.3 with Tc=15° C. over a switching range of 5 to 25° C.

(7) FIGS. 4A and 4B are graphs which show the optical indices (refractive index n and extinction coefficient k in the infrared of the constituents of the solar mirror prepared in step 6 of the example, namely of CeO.sub.2 oxide (FIG. 4A) and of SiO.sub.2 oxide (FIG. 4B), obtained from the reflection and transmission spectra of model layers of CeO.sub.2 and SiO.sub.2 with a thickness of 200 nm deposited on silicon.

(8) On the ordinate are the optical indices n and k, and on the abscissa is the wavelength (in nm). It appears that the CeO.sub.2 material is completely transparent to IR radiation from 2.5 to 16 μm (k=0). On the other hand, the SiO.sub.2 material has an absorption peak at 9 μm (phonon band) that tarnishes its IR transparency.

(9) FIG. 5 is a graph which represents the variation of the emissivity produced by a device, obtained at the end of step 5 of the example with a 1% tungsten doping, of active thickness 50 nm and 5×5 cm.sup.2 surface area, integrated from 2.5 to 25 μm, as a function of the temperature, compared to the emissivity of a black body heated to the same temperature. This device is not covered with solar protection.

(10) On the ordinate is the emissivity, and on the abscissa is the temperature T (in ° C.). It appears that Δε=0.4 with Tc=35° C. over a switching range of 20 to 50° C.

(11) FIG. 6 is a graph which represents the variation of the emissivity produced by a device, which is obtained at the end of step 5 of the example with a 1% tungsten doping, of active thickness 50 nm and 5×5 cm.sup.2 surface area, integrated from 2.5 to 25 μm, as a function of the temperature, compared to the emissivity of a black body heated to the same temperature. This device which is in accordance with the invention is furthermore covered with a solar protection prepared as in step 6.

(12) On the ordinate is the emissivity, and on the abscissa is the temperature T (in ° C.). It appears that Δε=0.3 with Tc=35° C. over a switching range of 20 to 50° C.

(13) FIG. 7 is a graph which represents the variation of solar reflection R (this is the reflection coefficient R measured in the solar radiation range from 0.28 to 2.5 μm, here truncated to 1.1 μm) (in %) at 22° C., as a function of the wavelength (in nm) for:

(14) a device, obtained at the end of step 5 of the example, comprising a layer with a thickness of 40 nm of thermochromic active material (VO.sub.2 doped with 2% tungsten) (Curve A).

(15) a device obtained at the end of step 5 of the example, comprising a layer of a thickness of 40 nm of thermochromic active material (VO.sub.2 doped with tungsten) and covered, in addition, by a solar protection constituted by a double centering Bragg mirror [λ.sub.0 (1)=550 nm and λ.sub.0 (2)=825 nm] prepared as in step 6 of the example (Curve B).

(16) FIG. 7 also shows the solar spectrum (Curve C).

(17) It appears that the apposition of the sun protection reduces the solar absorption coefficient α from 0.49 to 0.34.

(18) FIG. 8 shows a graph which represents the variation of the solar reflection R (R is the reflection coefficient measured in the solar radiation range from 0.28 to 2.5 μm, here truncated to 1.1 μm) (in %) at 22° C., as a function of the wavelength (in nm) for:

(19) a device obtained at the end of step 5 of the example, comprising a layer with a thickness of 50 nm of thermochromic active material (VO.sub.2 doped with 2% tungsten) (Curve A).

(20) a device obtained at the end of step 5 of the example, comprising a layer with a thickness of 50 nm of thermochromic active material (VO.sub.2 doped with 2% of tungsten) and covered, in addition, with a solar protection consisting of a double centering Bragg mirror [λ.sub.0 (1)=550 nm and λ.sub.0 (2)=825 nm] prepared as in step 6 of the example (Curve B).

(21) FIG. 8 also shows the solar spectrum (Curve C).

(22) It appears that the apposition of the sun protection reduces the solar absorption coefficient α from 0.59 to 0.38.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(23) FIG. 1 shows a solid state thermochromic device active in the infrared according to the invention, which comprises a stack of inorganic layers with a layer made of a thermochromic material based on vanadium IV oxide, with doped VO.sub.2.

(24) It should be noted that the indications shown in FIG. 1, in particular with regard to the material constituting each of the layers, and the protective coating, are only given as examples, illustrations, and in no way need to be considered as constituting any limitation.

(25) This device comprises a front face (1) directly exposed to solar radiation (2), and a rear face (3) bonded to the wall (or glazing) which conducts the heat towards the front face in order to re-emit it towards the side of the satellite (or building or vehicle) by infrared emissivity when the temperature T of the device (and therefore, in particular, the temperature of the thermochromic material whose temperature T is greater than Tc. In fact it may be considered that T wall=T device=T thermochromic material because the device consists of a stack of thin layers) is greater than Tc in hot phase, or which retains the heat inside the satellite (or building or vehicle) when the temperature T of the device is lower than Tc in the cold phase, with a low emissivity (without sunshine in the case of a satellite or with a winter sun in the case of a building or a passenger compartment of a vehicle such as an automobile).

(26) The device according to the invention firstly comprises a substrate or support (4) that essentially plays the role of mechanical support of the device.

(27) The substrate or support (4) generally does not have transparency to infrared radiations.

(28) The solid substrate or support (4) is made of an inorganic material that is resistant up to a temperature of 550° C., especially up to a temperature of 540° C., for example up to a temperature of 500° C. in particular in an argon and oxygen atmosphere containing at least 96% by volume of argon.

(29) The term “inorganic material resistant up to a temperature of 550° C., in particular up to a temperature of 540° C., for example up to a temperature of 500° C.”, is generally understood to mean that this material is not mechanically, physically or chemically degraded when exposed to such a temperature.

(30) Materials that are particularly suitable as a material of the solid substrate are aluminum, silicon, and borosilicate glasses.

(31) Advantageously, the substrate (4) is in the form of a layer, or sheet, preferably a layer or sheet having a thickness of 0.3 to 1 mm, for example a thickness of 0.5 mm. It may be, for example, a silicon substrate having a thickness of 0.5 mm, whose face that is intended to receive the subsequent layers, is polished.

(32) On the substrate or support (4) is disposed a reflective layer (5), also referred to as a reflective background. This layer is made of an electronically-conductive material.

(33) Preferably, this electronically-conductive material is selected from metals, metal alloys, and electronically-conductive metal oxides, such as tin-doped indium oxide (ITO), fluorine-doped tin oxide (FTO), antimony-doped tin oxide (ATO), and aluminum-doped zinc oxide (AZO), more preferably the reflective layer is silver.

(34) The reflective layer generally has between 60% and 100% reflectivity in the mid infrared, preferably 100%.

(35) The reflective layer (5) generally has a thickness of 80 to 150 nm, preferably 100 nm.

(36) The metals that may constitute the reflective layer (5) may be chosen, for example, from noble metals such as gold, silver or platinum; aluminum, chromium, and their alloys.

(37) A preferred metal is silver, and in this case the thickness of the reflective layer (5) is preferably 100 nm.

(38) Electronically-conductive metal oxides are well known to the man skilled in the art.

(39) Examples of such conductive oxides are tin-doped indium oxide (ITO), fluorine-doped tin oxide (FTO), antimony-doped tin oxide (ATO), or aluminum-doped zinc oxide (AZO), that should preferably be combined with aluminum-based interface layers to avoid the formation of the vacant (lacunar) compound □ (vacant sites of the spinel structure) AlSiO.sub.4. It is the same with a reflective aluminum background.

(40) The electronically-conductive material of the reflective layer (5), especially when it is an electronically-conductive metal oxide, is generally chosen from materials that may be deposited in a thin layer by a PVD process, such as cathode sputtering, laser ablation or evaporation, and preferably among the materials that may be deposited in a thin layer by cathode sputtering.

(41) In FIG. 1, the reflective layer (5) is a silver layer with a thickness of 100 nm.

(42) Interface layers (6) are arranged on the reflective layer (5).

(43) These interface layers (6) make it possible to ensure the adhesion of the dielectric layer (9) on the reflective layer (5).

(44) The interface layers may, preferably, should be 2 in number.

(45) The nature of these interface layers is so chosen in order to ensure the best adhesion of the dielectric layer (9) on the reflective layer (5).

(46) Thus, in FIG. 1, the device comprises a nitrided first layer (7) to passivate silver, and a second oxidized layer (8) to bond the cerium oxide.

(47) The interface layers (6) may have a total thickness of 10 to 30 nm, while each layer may have a thickness of 5 to 15 nm.

(48) Generally both layers have the same thickness.

(49) The materials of the interface layers (6) are chosen in order to ensure the best adhesion of the dielectric layer (9) on the reflective layer (5).

(50) The materials of the interface layers (6) are also generally chosen from materials that may be deposited in a thin layer by a PVD process, such as cathode sputtering, laser ablation or evaporation, and preferably from the materials that may be deposited in a thin layer by cathode sputtering.

(51) Suitable materials are for example SiO.sub.2 and Si.sub.3N.sub.4, and the interface layers may then comprise an Si.sub.3N.sub.4 layer (7), and an SiO.sub.2 layer (8).

(52) In the embodiment shown in FIG. 1, the interface layers (6) comprise, starting from the reflective layer, a first layer consisting of Si.sub.3N.sub.4 or AlN (7), then a second layer consisting of SiO.sub.2 or Al.sub.2O.sub.3 (8).

(53) The nitrided layer protects silver against oxygen introduced during the deposition of CeO.sub.2, and the oxidized layer, bonded to the nitrided layer by Si—Si or Al—Al bonds, creates O—O bridges between it and the layer of CeO.sub.2.

(54) The first layer consisting of Si.sub.3N.sub.4 (7) may have a thickness of 5 to 15 nm, for example 10 nm, and the second layer consisting of SiO.sub.2 (8) may have a thickness of 5 to 15 nm, for example 10 nm.

(55) In FIG. 1, the first layer consisting of Si.sub.3N.sub.4 (7) has a thickness of 10 nm, and the second layer consisting of SiO.sub.2 (8) has a thickness of 10 nm.

(56) On the interface layers (6), or rather on the interface layer (8) that is deposited last on the reflective layer, which is, for example, in the embodiment shown in FIG. 1, a layer of SiO.sub.2 of a thickness of 10 nm, is disposed an electronically insulating dielectric layer and transparent to infrared radiation (9) over a wavelength range of 2.5 to 25 μm (mid IR).

(57) This electronically insulating dielectric layer and transparent to infrared radiation has a thickness of 400 to 900 nm, preferably 700 to 900 nm.

(58) This electronic insulating dielectric layer and transparent to infrared radiation (9) is made of cerium oxide CeO.sub.2.

(59) On the electronically insulating dielectric layer and transparent to the infrared radiation (9) are arranged interface layers (10).

(60) These interface layers (10) make it possible to ensure the adhesion and the integrity of the thermochromic layer (13) deposited on the electronically insulating dielectric layer and transparent to the infrared radiation (9), by preventing the formation of the CeVO.sub.4 compound with IR absorption between 9 and 13 μm.

(61) The interface layers (10) must be 2 in number and are generally silicon-based, non-reactive with CeO.sub.2 and with VO.sub.2.

(62) Thus, in FIG. 1, the device comprises two interface layers (11, 12).

(63) After bonding the first layer of SiO.sub.2 (11) with CeO.sub.2 via O—O bridges, the second layer of Si.sub.3N.sub.4 (12), referred to as the buffer layer, which is much thicker and adheres to SiO.sub.2 with the aid of Si—Si bonds, prevents any reactivity of VO.sub.2 with CeO.sub.2.

(64) The nature of the interface layers (10) is chosen in order to ensure the best adhesion and the best integrity of the thermochromic layer (13) on the electronically insulating dielectric layer transparent to infrared radiation (9).

(65) The interface layers (10) may have a total thickness of 105 to 155 nm. Two consecutive interface layers are generally of different materials.

(66) The materials of the interface layers (10) are also selected to ensure the best adhesion and integrity of the thermochromic layer (13) on the electronically insulating dielectric layer transparent to infrared radiation (9).

(67) The materials of the interface layers (10) are also generally selected from materials that may be deposited in a thin layer by a PVD method, such as cathode sputtering, laser ablation or evaporation, and preferably from among materials that may be deposited in a thin layer by cathode sputtering.

(68) In the embodiment shown in FIG. 1, the interface layers (10) comprise, from the electronic insulating dielectric layer transparent to infrared radiation (9), a first layer consisting of SiO.sub.2 (11) and then a second buffer layer consisting of Si.sub.3N.sub.4 (12).

(69) The first layer consisting of SiO.sub.2 (11) may have a thickness of 5 to 15 nm, for example 10 nm, and the second layer consisting of Si.sub.3N.sub.4 (12) may have a thickness of 100 to 140 nm, for example 120 nm.

(70) In FIG. 1, the first layer, consisting of SiO.sub.2 (11) has a thickness of 10 nm, and the second layer consisting of Si.sub.3N.sub.4 layer (12) has a thickness of 120 nm.

(71) FIG. 1 shows an advantageous embodiment in which the layers (11), (12) and (7), (8) are arranged symmetrically on either side of the layer (9), wherein the SiO.sub.2 layers are disposed in contact with each of the faces of the layer (9).

(72) On the interface layers (10), or rather on the interface layer (12) deposited last on the reflective layer, which is, for example, in the embodiment shown in FIG. 1, a buffer layer of Si.sub.3N.sub.4 (12) having a thickness of 120 nm, is disposed a layer (13) of a thermochromic material that is active in the infrared and that is a doped and crystallized vanadium oxide VO.sub.2, in a monoclinic or rutile phase, with a thickness of 30 to 50 nm, preferably 30 to 40 nm.

(73) Because of its very small thickness, this layer (13) of a thermochromic material may be described as very thin layer. This very small thickness of the layer of thermochromic material is one of the essential characteristics of the device according to the invention.

(74) The vanadium oxide may be deposited in a very thin, very fine layer by a PVD process such as cathode sputtering, laser ablation or evaporation, and preferably by sputtering.

(75) In FIG. 1, the layer of thermochromic material (13) is a layer of V.sub.0.98W.sub.0.02O.sub.2-x with a thickness of 30 to 50 nm.

(76) On the layer of a thermochromic material that is active in the infrared and is a crystallized and doped vanadium oxide VO.sub.2 (13), is arranged a solar protective coating (15) reflecting the solar radiation (2) and transparent to infrared rays (14) (over all mid IR, from 2.5 to 25 μm with an absorption peak at 9 μm).

(77) The term “transparent to infrared radiation” is generally understood to mean that this coating (15) is transparent to infrared rays of wavelengths between 2.5 μm and 25 μm, preferably between 2.5 μm and 16 μm.

(78) This coating (15) is generally made of a non-toxic inorganic material.

(79) Preferably, this coating (15) is made of a material selected from metal and metalloid oxides, and mixtures of two or more of these metal oxides and metalloid oxides.

(80) This, or these, oxide(s) of metals or metalloids, preferably amorphous, is/are preferably chosen from oxides which may be easily deposited by PVD, in particular by magnetron cathode sputtering, from an oxide or metal target, such as cerium oxide CeO.sub.2, yttrium oxide Y.sub.2O.sub.3, or SiO.sub.2.

(81) The coating (15) generally has a thickness of 0.5 to 1.25 μm, preferably 1 μm to 1.25 μm, more preferably 1.25 μm.

(82) This coating (15) is generally constituted by a Bragg mirror.

(83) Preferably, this Bragg mirror comprises an alternation of a layer of a metal oxide of high refractive index (n of 2 to 2.5, for example 2.2) such as CeO.sub.2, and of a layer of a metal oxide of low refractive index (n of 1.3 to 1.8, for example 1.5), such as SiO.sub.2.

(84) In FIG. 1, the coating (15) comprises alternating CeO.sub.2 layers and SiO.sub.2 layers and may have a total thickness of from 0.5 to 1.25 μm, preferably from 1 μm to 1.25 μm, more preferably 1.25 μm. However, by way of example, it is indicated in FIG. 1 that the coating (15) represented in this figure has a thickness of 1 μm. It is composed of 3 pairs of CeO.sub.2 and SiO.sub.2 layers centered on 550 nm of respective 55 and 95 nm thicknesses, then of 3 pairs of CeO.sub.2 and SiO.sub.2 layers centered on 825 nm of respective 80 and 150 nm thicknesses, and of a terminal layer of CeO.sub.2 of 80 nm.

(85) The operation of the device of FIG. 1, when used, for example, to equip a satellite, may be described in the following manner.

(86) In the solar exposure phase, the satellite's internal electronics need to be protected from the high temperature, which can reach 100° C. at the wall. First, the Bragg mirror located on the front face of the device makes it possible to reject solar radiation by reflection to more than 70%, over the range of 0.28 to 2.5 μm.

(87) This results in a decrease in the solar absorptivity of devices comprising 40 and 50 nm of 2% doped IR active thermochromic material whose a coefficients decrease respectively from 0.49 to 0.34 (FIG. 7) and from 0.59 to 0.36 (FIG. 8) after apposition of the solar protection.

(88) On the other hand, the IR active thermochromic material composed of VO.sub.2 doped with 2 atomic % tungsten (y=0.02) has a metallic character (rutile phase) obtained at T>Tc with Tc=15° C. (FIG. 3). Its high emissivity at high temperature (ε.sub.HT=0.75) will thus produce an evacuation of the heat stored inside the satellite by infrared radiation towards the outside, over the range of 2.5 to 25 μm.

(89) The solar protection located in the front face therefore needs therefore to be transparent throughout the entire infrared range. It is the same when the device of FIG. 1 is used for glazing for buildings or automobiles. When the doping rate of the thermochromic material decreases (y=0.01), Tc rises to 35° C. (FIG. 5), and the apposition of the Bragg mirror has the effect of increasing the value of ε.sub.HT to 0.85. However, this improvement of ε.sub.HT is effected to the detriment of the low value of the ε.sub.BT emissivity which rises from 0.35 to 0.55. In the absence of solar radiation, the conservation of heat inside the satellite will then be less effective. There is therefore a compromise between the best rejection of solar gains (obtained with the highest thickness of the Bragg mirror, i.e. 1.25 μm), and the best energy efficiency at low temperature (obtained without solar protection with the lowest doping rate, i.e. y=0.01).

(90) The device according to the invention is prepared by the method described above.

(91) The apparatus used to implement the method according to the invention for preparing the device according to the invention may be, for example, a physical vapor deposition (PVD) frame comprising: a vacuum chamber of a volume of, for example, 0.1 m.sup.3 in which an initial pressure prevails, for example of approximately 5 10.sup.−7 mbar, wherein the maximum pumping speed for producing the vacuum in the chamber is 900 L/s with a chamber initially filled with nitrogen; at most 6 cathodes with a diameter of 3 inches (or 76 mm), or 2 cathodes with a diameter of 6 inches (or 152 mm) and 2 cathodes with a diameter of 3 inches (or 76 mm); each deposit is produced by magnetron cathode sputtering from a metal or metal oxide target, for example made of Ag, Si, CeO.sub.2, V, or W, in radio frequency RF mode, with an applied power of 1 to 10 W/cm.sup.2 of the target, or in DC mode, with an applied power of 0.6 to 2 W/cm.sup.2, in order to obtain slow or high deposition rates as a function of the material to be deposited and the fragility of the target, for example of 3 to 120 nm/minute for CeO.sub.2 and Ag respectively.

(92) Such deposition conditions allow industrialization of the process, described as reactive, for the manufacture of thin-layer oxides from metal targets, except for CeO.sub.2 whose target is an oxidized ceramic of the same composition (fragile target).

(93) The plasma-forming gas atmosphere in which the deposit is produced is chosen as a function of the material which constitutes the deposited layer.

(94) Thus, this atmosphere may consist of: argon, with a pressure of, for example, 9 10.sup.−3 mbar, for depositing a reflective layer made of metal, for example made of silver; a mixture of argon and nitrogen, for example in the proportions of 70 to 75% and 25 to 30% by volume respectively, with a total pressure of 2.1 to 2.3 10.sup.−2 mbar, for example of 2.2 10.sup.−2 mbar, for the deposition of an interface layer made of Si.sub.3N.sub.4; a mixture of argon and oxygen, for example in the proportions of 83 to 96% and 4 to 17% by volume respectively, with a total pressure of 1.7 to 3.3, for example 3.2 10.sup.−2 mbar or 1.2 10.sup.−2 mbar, or 1.75 10.sup.−2 mbar for the deposition of an interface layer, or of a layer of the SiO.sub.2 solar protection coating, for the deposition of a dielectric layer or of a layer of the CeO.sub.2 solar protection coating, or for the deposition of a VO.sub.2 thermochromic layer that is active in the infrared.

(95) The annealing of the stack obtained at the end of step e) is carried out in the same vacuum chamber where the deposits are made.

(96) Generally, this annealing is carried out under the same conditions (in an atmosphere similar to that used for the deposition of VO.sub.2) as the conditions in which the deposition of VO.sub.2 was carried out, namely under the same partial pressure of argon/oxygen at 500° C. for at least 1 hour.

(97) The device according to the invention that is inorganic, robust and with a simplified design for industrialization, and which may, in particular, operate in the mid infrared, finds in particular its application in the thermal protection of satellites.

(98) For example, it is possible to use “tiles” (“patches”) for satellites composed of several “solid state” thermochromic devices according to the invention to replace high energy-consuming mechanical shutters.

(99) The invention will now be described with reference to the following example, given by way of nonlimiting illustration, which describes the manufacture of a thermochromic device according to the invention, such as that represented in FIG. 1, by the method according to the invention, implementing the magnetron cathode sputtering technique to prepare all the layers.

EXAMPLE

(100) All the layers of the device according to the invention, and prepared by the device according to the invention, are prepared by implementing the same technique, namely the magnetron cathode sputtering technique, in the same magnetron sputtering chamber under vacuum (namely, at a residual pressure lower than 10.sup.−6 mbar (high vacumm, secondary vacuum) before introduction of the plasma-forming gases that are used during the deposition phases of the materials in thin layers which are carried out under primary vacuum (rough vacuum)), under a pressure of argon, or of a mixture of argon and nitrogen, or of a mixture of argon and oxygen in reactive mode, while maintaining the vacuum between the deposition of two successive layers and without opening the chamber before the last layer of the device is deposited.

(101) A different target must however be used for the deposition of each layer containing a different metal. Thus, the interface layers use the same target (Si), while the solar protection layer (or Bragg mirror) also uses this same target (Si), as well as the target used for the dielectric layer (CeO.sub.2). Five targets are therefore necessary to produce the device according to the invention by the method according to the invention, i.e. the magnetron cathode sputtering technique, namely: a silver target for producing the reflective background, a silicon target for producing the interface layers and the low refractive index layer of the Bragg mirror, a cerium oxide target for producing the dielectric layer and the high refractive index layer of the Bragg mirror, a vanadium target for producing the thermochromic material, a tungsten target for doping the thermochromic material.

(102) 1. Preparation of the Reflective Background on the Substrate.

(103) The metal layer which acts as a reflective background is a layer of silver with a thickness of 100 nm deposited by magnetron cathode sputtering.

(104) This silver layer is deposited on the polished side of a silicon substrate with a thickness of 0.5 mm, and a surface of 2.5×2.5 cm.sup.2 or 5×5 cm.sup.2.

(105) The deposition power is 90 W in DC mode (V=380V, I=0.24A), while the diameter of the silver target is 75 mm.

(106) The argon pressure in the chamber is 10.sup.−3 mbar, while the target-substrate distance is 8 cm.

(107) The duration of the deposit is 50 s.

(108) This silver layer has 100% reflectivity in the infrared as shown in FIG. 2 which represents the infrared reflectivity of the metal (background) base of the device according to the invention, and is prepared by the method according to the invention.

(109) 2. Elaboration of the Interface Layers Between the Reflective Background (Base) and the Main Dielectric Layer to Ensure the Adhesion of the Main Dielectric Layer on the Reflective Background (Base).

(110) The first interface layer is composed of Si.sub.3N.sub.4 and has a thickness of 10 nm.

(111) It is deposited on the silver layer which acts as a reflective background by reactive magnetron cathode sputtering in RF radio frequency mode, with a power of 250 W applied to a silicon target 75 mm in diameter, under an argon and nitrogen atmosphere, respectively injected at 60 and 20 sccm, wherein the total pressure in the chamber is 2.2 10.sup.−2 mbar.

(112) The deposition period is 60 s.

(113) The second interface layer is composed of SiO.sub.2 and has a thickness of 10 nm.

(114) It is deposited on the first interface layer, composed of Si.sub.3N.sub.4, by radiofrequency RF reactive magnetron cathode sputtering, with a power of 250 W applied to a silicon target 75 mm in diameter, under an argon and oxygen atmosphere injected respectively at 100 and 20 sccm, wherein the total pressure in the chamber is 3.2 10.sup.−2 mbar.

(115) The deposition time is 60 s.

(116) 3. Preparation of the Main Dielectric Layer.

(117) This layer is composed of CeO.sub.2 and has a thickness of 400 nm.

(118) It is deposited on the second interface layer, composed of SiO.sub.2, by radiofrequency RF reactive magnetron cathode sputtering, with a power of 250 W applied to a CeO.sub.2 target of 175 mm in diameter, under an argon and oxygen atmosphere injected respectively at 40 and 3 sccm, wherein the total pressure in the chamber is 1.2 10.sup.−2 mbar.

(119) The deposition time is 120 min.

(120) 4. Preparation of the Interface Layers Between the Main Dielectric Layer and the Thermochromic Layer.

(121) The first interface layer between the main dielectric layer and the thermochromic layer is composed of SiO.sub.2 and has a thickness of 10 nm.

(122) It is deposited on the main dielectric layer, composed of CeO.sub.2, by reactive magnetron cathode sputtering in RF radio frequency mode, with a power of 250 W applied to a silicon target 75 mm in diameter, under an argon and oxygen atmosphere injected respectively at 100 and 20 sccm, wherein the total pressure in the chamber is 3.2 10.sup.−2 mbar.

(123) The deposition time is 60 s.

(124) The second interface layer between the main dielectric layer and the thermochromic layer is composed of Si.sub.3N.sub.4 and has a thickness of 120 nm in order to prevent any reactivity between the CeO.sub.2 and VO.sub.2 materials and to thus act as a buffer layer.

(125) It is deposited on the first interface layer between the main dielectric layer and the thermochromic layer, composed of SiO.sub.2, by reactive magnetron cathode sputtering in radio frequency RF mode, with a power of 250 W applied to a silicon target of 75 mm diameter, under an atmosphere of argon and nitrogen respectively injected at 60 and 20 sccm, wherein the total pressure in the chamber is 2.2 10.sup.−2 mbar.

(126) The deposition time is 12 minutes.

(127) 5. Preparation of the Active Thermochromic Layer in the Infrared.

(128) This layer is mainly composed of VO.sub.2 and has a thickness of 40 to 50 nm.

(129) It is deposited on the second interface layer between the main dielectric layer and the thermochromic layer, composed of Si.sub.3N.sub.4, by reactive magnetron cathode sputtering in radio frequency RF mode, with a power of 450 W applied to a silicon target of 75 mm diameter, under an atmosphere of argon and oxygen respectively injected at 60 and 2.6 sccm, wherein the total pressure in the chamber is 1.75 10.sup.−2 mbar.

(130) The total duration of the deposition is from 45 s for a layer of 40 nm, to 60 s for a layer of 50 nm.

(131) This layer is doped with 3 or 4 ultrafine layers of tungsten metal deposited by magnetron sputtering in pulsed DC mode (50 kHz, 2 μs), with a power of 100 W applied to a target of tungsten of diameter 150 mm.

(132) The tungsten layers are intercalated in the VO.sub.2 layer, by performing, during the deposition of the VO.sub.2 layer, every 15 s, a sweep in front of the tungsten target with a substrate rotation speed of 15 rpm under 20 sccm of argon.

(133) More precisely, every 15 seconds the deposition of VO.sub.2 is stopped, and the sample is passed very rapidly in front of the target of W to deposit an ultrafine layer of W of a few tenths of a nanometer.

(134) The whole of the stack obtained at the end of this step of preparing the thermochromic layer active in the infrared is then annealed at 500° C. under the same partial pressure of argon/oxygen as that used for the VO.sub.2 deposition, i.e. at about 2.9 10.sup.−2 mbar for 1 h.

(135) The stack subjected to annealing thus comprises, successively, starting from the substrate, the silver reflective background, the interface layers making it possible to ensure the adhesion of the dielectric layer to the reflective background, the main dielectric layer, the interface layers between the main dielectric layer and the thermochromic layer, and finally the thermochromic layer active in the infrared.

(136) It should be noted that the solar protection is not annealed. It is deposited after annealing, otherwise the CeO.sub.2 it contains would react with VO.sub.2.

(137) The thermochromic behavior in the infrared of a device prepared as described above, comprising a layer with a thickness of 40 nm of thermochromic active material, and whose surface is 2.5×2.5 cm.sup.2, is presented in FIG. 3.

(138) More precisely, FIG. 3 shows the variation of the emissivity produced by a device of active thickness 40 nm and surface area 2.5×2.5 cm.sup.2, integrated from 2.5 to 25 μm, as a function of the temperature, relative to the emissivity of a black body brought to the same temperature.

(139) The hysteresis of heating/cooling between the state of lowest emissivity and the state of highest emissivity is almost zero (Δε=0.3 where ε.sub.BT=0.45 and ε.sub.HT=0.75) (ε.sub.BT and ε.sub.HT represent the lowest emissivity and the highest emissivity), while the switching temperature is 15° C. over a range of 2.5 to 25° C.

(140) 6. Preparation of the Solar Protection or Bragg Mirror.

(141) The same magnetron cathode sputtering deposition technique as that used in steps 1 to 5 described above is also used to achieve a solar protection, i.e. a Bragg mirror or solar mirror, reflector on glass at 78%, on the stack obtained at the end of steps 1 to 5 described above.

(142) More precisely, this solar protection, Bragg mirror, or solar mirror is deposited on the layer of thermochromic material which is, starting from the substrate, the last layer of the stack obtained at the end of steps 1 to 5 described above.

(143) This solar protection, Bragg mirror, or solar mirror comprises alternating layers of CeO.sub.2 (n=2.5) and SiO.sub.2 (n=1.48) with high and low refractive indices measured at 550 and 825 nm, and mean values respectively equal to 2.2 and 1.5 in the range of 0.28 to 2.5 μm.

(144) A first set of 3 pairs of CeO.sub.2 (thickness 55 nm)/SiO.sub.2 (thickness 95 nm) layers, centered in the visible range, on 550 nm, is first deposited on the thermochromic layer that is active in the infrared.

(145) On this first set of 3 pairs of layers, is then deposited a second set of 3 pairs of CeO.sub.2 (80 nm thickness)/SiO.sub.2 (150 nm thickness) layers centered in the near-infrared range on 825 nm.

(146) Then a terminal layer of CeO.sub.2 with a thickness of 80 nm is deposited on this second set of three pairs of layers.

(147) The same deposition conditions are used for the CeO.sub.2 and SiO.sub.2 deposits of the solar mirror, for the CeO.sub.2 dielectric layer, and for the silica interface layers respectively.

(148) The deposition durations (times) have been adapted according to the deposition rates of each material, namely:

(149) For the first set of three pairs of layers, centered at 550 nm, the CeO.sub.2 deposition rate is 3.3 nm/min, and the CeO.sub.2 deposition time is 17 minutes, and the SiO.sub.2 deposition rate is 10 nm/min, and the SiO.sub.2 deposition time is 9.5 minutes.

(150) For the second set of three pairs of layers, centered at 825 nm: the CeO.sub.2 deposition rate is 3.3 nm/min, and the CeO.sub.2 deposition time is 24 minutes, and the deposition rate of SiO.sub.2 is 10 nm/min, and the SiO.sub.2 deposition time is 15 minutes.

(151) Finally, the terminal layer of CeO.sub.2 is deposited in 24 min.

(152) FIGS. 4A and 4B show the optical indices (refractive index n and absorption index k) in the infrared of the constituents of the solar mirror, namely CeO.sub.2 oxide (FIG. 4A) and SiO.sub.2 oxide (FIG. 4B) as obtained from the reflection and transmission spectra of the model layers of CeO.sub.2 and SiO.sub.2 with a thickness of 200 nm deposited on silicon.

(153) It appears from FIGS. 4A and 4B that the choice of the CeO.sub.2 layer is motivated by its high transparency in the infrared (k=0), in order to alter as little as possible the emissivity variation of the device once covered with solar protection. In addition, this layer provides good protection with respect to the active material.

(154) The SiO.sub.2 layer however has a phonons band centered on 9 μm, responsible for an additional average absorption that displaces the low values of emissivity upwards.

(155) The thermochromic behavior in the infrared, of a device prepared as described above (steps 1 to 5), comprising a layer with a thickness of 50 nm of thermochromic active material doped at 1%, and the surface area of which is 2.5×2.5 cm.sup.2, is shown in FIG. 5. This device is not covered with a solar protection.

(156) More precisely, FIG. 5 represents the variation of the emissivity produced by a device of active thickness 50 nm and surface area 5×5 cm.sup.2, integrated from 2.5 to 25 μm, as a function of the temperature, with respect to the emissivity of a black body brought to the same temperature. This device is not covered with a solar protection.

(157) The thermochromic behavior in the infrared, of a device prepared as described above (steps 1 to 5), comprising a layer with a thickness of 50 nm of thermochromic active material doped at 1%, and the surface area of which is 2.5×2.5 cm.sup.2, is shown in FIG. 6. This device is covered with a solar protection prepared as in step 6.

(158) More precisely, FIG. 6 represents the variation of the emissivity produced by a device of active thickness 50 nm and of surface area 5×5 cm.sup.2, integrated from 2.5 to 25 μm, as a function of the temperature, with respect to the emissivity of a black body brought to the same temperature. This device is covered with a solar protection.

(159) FIGS. 5 and 6 show that, for a device comprising 50 nm of active material doped at 1%, the infrared optical contrast Δε decreases from 0.4 (ε.sub.BT=0.35 and ε.sub.HT=0.75) to 0.3 after affixing of the solar protection (ε.sub.BT=0.55 and ε.sub.HT=0.85) over a range of 20 to 50° C., with a switching temperature around 35° C. It should be noted that there is an increase in the high value of the emissivity that is favorable to the dissipation of heat.

(160) One can appreciate in FIGS. 7 and 8, the beneficial effect of a solar protection consisting of a double centering Bragg mirror (λ.sub.0 (1)=550 nm and λ.sub.0 (2)=825 nm) on the reduction, decrease in the solar absorptivity at 22° C. of two thermochromic devices according to the invention.

(161) The first device comprises a layer with a thickness of 40 nm of active material doped with 2% in the state of high infrared emissivity (VO.sub.2 doped with tungsten), wherein a decreases from 0.49 to 0.34 (FIG. 7).

(162) The second 2% doped device comprises a layer with a thickness of 50 nm of active material in the state of low infrared emissivity (VO.sub.2 doped with tungsten), wherein a decreases from 0.59 to 0.38 (FIG. 8).