A photo-darkening multilayer coating structure

Abstract

A photo-darkening multilayer coating structure is disclosed, comprising a substrate, a hydrogen-rich yttrium oxy-hydride (YHxOy) layer, and an oxygen-deficient molybdenum trioxide (MoO3-z) layer deposited sequentially. Upon exposure to ultraviolet or visible light, the YHxOy layer releases hydrogen, which is absorbed by the overlying MoO3-z layer to form a hydrogen molybdenum bronze (HxMoO3) phase. This light-induced hydrogen transfer causes a significant reduction in optical transmittance within the 400–900 nm spectral range by at least 10% to 60%. The resulting darkened state is stable for at least one week under ambient conditions without the need for continuous light exposure or external power input. The structure enables passive and durable optical modulation suitable for use in light exposure indicators, smart coatings, or solar management applications.

Description

[0001] A photo-darkening multilayer coating structure

[0002] Field of the Invention

[0003]

[0001] The present invention relates to photo-darkening coatings produced by physical vapour deposition, specifically by magnetron sputtering.

[0004] Background of the Invention:

[0005]

[0002] The photochromic activity of oxygen-containing rare-earth hydride coatings, such as yttrium oxy-hydride [YHO] [1], is influenced by several factors: composition, thickness , and illumination conditions. These coatings are believed to form an oxyhydride phase, in which both oxygen [0] and hydrogen [H] act as anions. YHO is notable for its ability to exhibit a colour-neutral, reversible photochromic effect. Specifically, YHO films with an H:Y atomic ratio ranging from approximately 0.5 to 3.0 can achieve a contrast of up to «40% when illuminated by a solar simulator at room temperature and ambient pressure [2]. The darkened state of these coatings reverts to its original transparent condition after a few hours or days in the absence of light.

[0006]

[0003] Yttrium oxy-hydride coatings have undergone extensive research in recent years, however, the exact mechanisms behind their photochromism and the specific phases of the oxygen-containing rare-earth hydrides are not yet fully understood. Various hypotheses have been proposed to explain the photochromic behavior of YHO films [3]. Hydrogen release has been observed in YHO coatings during initial coloration cycles [4]. Photochromic YHO films are created by exposing P-YH2- 8 coatings to an oxygen-rich atmosphere. Due to their high reactivity with oxygen, this exposure leads to the oxidation of the films, producing coatings that are both transparent and photochromic.

[0007]

[0004] Hydrogen, due to its small atomic diameter and unique electronic configuration, can intercalate into oxides and serve as an electron donor or acceptor. This dual behaviour is responsible for a range of optical and electrical phenomena, broadening its application spectrum. Notably, these effects are particularly strong in tungsten oxide [WO3] and molybdenum oxide MoO3], materials recognized for their gasochromic properties and their utility in hydrogen and ammonia sensors [5]. Upon dissociation, hydrogen atoms from molecular hydrogen or hydrogen-containing compounds can intercalate into the oxide structure, resulting in the formation of molybdenum and tungsten bronzes.

[0008]

[0005] There is known a photoelectric light-modulating intelligent window structure [6], comprising a multilayer arrangement including a yttrium oxyhydrogen (YOxHy) dimming layer and an oxygen-deficient tungsten oxide (WOz) functional layer formed sequentially on an electrothermal layer. The structure is enclosed between two transparent substrates - optionally PVB-laminated glass - separated by a sealed hollow layer filled with inert gas and is powered by a DC source to induce dimming through light-induced and / or thermally assisted modulation.

[0009]

[0006] There is known a photochromic coating structure [7], comprising a bilayer film composed of oxygen-containing yttrium hydride [YHX:O] coated with tungsten oxide WO3]. The structure demonstrates enhanced and reversible photochromic response under UV illumination, with improved optical contrast and switching speed compared to YHX:O alone.

[0010] Brief Description of the Invention

[0011]

[0007] The aim of the invention is to provide a passive, multilayer photo-darkening coating structure with enhanced and durable optical modulation properties under ambient conditions, by enabling light-induced and persistent hydrogen transfer between a hydrogen-rich yttrium oxyhydride layer and an oxygen-deficient molybdenum trioxide layer.

[0012]

[0008] The aim set is achieved by a photo-darkening multilayer coating structure comprising: a substrate; a photochromic yttrium oxy-hydride YHxOy] layer deposited on the substrate; the YHxOylayer having a hydrogen-rich composition capable of releasing hydrogen upon exposure to ultraviolet or visible light; and an oxygendeficient molybdenum trioxide MoOs-z] layer deposited on the YHxOylayer; the MoO3_z layer having a stoichiometry and morphology that enables hydrogen uptake from the YHxOylayer under the ultraviolet or visible light exposure, thereby forming a hydrogen molybdenum bronze HxMoO3] phase within the MoO3_z layer.

[0013]

[0009] According to the preferred embodiment, the chemical composition of yttrium oxy-hydride, YHxOy, is within the range 1 < x < 4 and 1 < y < 10 and the chemical composition of molybdenum trioxide, Moth-z, is within the range 0 < z < 0.3. According to yet another embodiment, the YHxOylayer preferably has a thickness between 50 and 2500 nm, and the MoOs-z layer preferably has a thickness between 10 and 1000 nm.

[0014]

[0010] The claimed multilayer photo-darkening coating structure can be used in an optical indicator, a disposable photo-exposure tag, or a passive solar shading element for indicating cumulative light exposure, passive light modulation, or solar heat gain control.

[0015] [Oil] The invention provides also a method of fabricating the coating structure, comprising: [i] depositing a YH2-8 [0 < 8 < 0.3] layer on a substrate by physical vapour deposition of yttrium in an argon / hydrogen atmosphere, [ii] oxidizing the YH2-8 layer in an oxygen-containing environment to form a transparent and photochromic YHxOy phase, and [hi] depositing a MoOs-z layer on the YHxOy layer by physical vapour deposition of molybdenum in an argon / oxygen atmosphere, such that the resulting multilayer coating exhibits an optical contrast AT between 10% and 60% in the 400- 900 nm spectral range upon exposure to ultraviolet or visible light under ambient conditions, where optical contrast is defined as the absolute change in transmittance between the initial and photo-darkened states.

[0016] Brief Description of the Drawings

[0017]

[0012] The invention is further described in the accompanying drawings, in which: Fig. 1. shows the principal structure of the YHxOy / MoOs-z coating on a substrate.

[0018] Fig. 2a - a diagram of light transmittance [250 - 2500 nm] of the YHxOy coating before and after 20 hours of UV light irradiation, where the indicated 40% represent the absolute change in light transmittance;

[0019] Fig. 2b - a diagram of light transmittance [250 - 2500 nm] of the YHxOy / MoOs-z coating before and after 20 hours of UV light irradiation, where the indicated 55% represent the absolute change in light transmittance;

[0020] Fig. 3 - a diagram of darkening measurements of YHxOy and YHxOy / MoOs-z samples in the 500-700 nm range;

[0021] Fig. 4 - a diagram of X-ray diffractograms for YHxOy and YHxOy / MoOs-z samples;

[0022] Fig. 5a - a surface SEM image of a YHxOy / MoOs-z sample with a 136 nm thick MoOs-z layer;

[0023] Fig. 5b - a cross-sectional SEM image of a YHxOy / MoOs-z sample with a 136 nm thick MoOs-z layer. Detailed description of the invention

[0024]

[0013] The multilayer photo-darkening coating structure comprising: a substrate; a photochromic yttrium oxy-hydride (YHxOy) layer deposited on the substrate; the YHxOylayer having a hydrogen-rich composition capable of releasing hydrogen upon exposure to ultraviolet or visible light; and an oxygen-deficient molybdenum trioxide MoOs-z] layer deposited on the YHxOy layer; the MoO3_z layer having a stoichiometry and morphology that enables hydrogen uptake from the YHxOy layer under the ultraviolet or visible light exposure, thereby forming a hydrogen molybdenum bronze (HxMoOs) phase within the MoO3_z layer.

[0025]

[0014] The coating structure has an optical transmittance in the 400-900 nm spectral range that is reducible by at least 10% to 60% upon exposure to ultraviolet or visible light under ambient conditions, the darkened state remaining stable for at least 1 week after cessation of light exposure.

[0026]

[0015] The light induced movement of hydrogen from YHxOy to the MoOs-z coating causes reduction, leading to the formation of molybdenum bronze. This material absorbs light in the red part of the spectrum [above 630 nm], thereby enhancing the photochromic properties of the YHxOy / MoOs-z structure.

[0027]

[0016] According to the invention the substrate can be a rigid or flexible solid material selected from the group consisting of glass, polymers, metals, and ceramics.

[0028]

[0017] The chemical composition of the yttrium oxy-hydride coating, YHxOy, can be within the range 1 < x < 4 and 1 < y < 10; and the layer thickness - from 50 nm to 2500 nm.

[0029]

[0018] The chemical composition of the molybdenum trioxide, MoOs-z, can be within the range 0 < z < 0.3, and the layer thickness - from 10 nm to 1000 nm.

[0030]

[0019] The study conducted by the inventors has confirmed a synergistic effect of combining YHxOy and MoOs-z coatings into a layered YHxOy / MoOs-z structure. The YHxOy / MoO3-z coatings produced during this study demonstrated an enhanced darkening rate and improved contrast compared to standalone YHxOy coatings.

[0031]

[0020] The resulting photo-darkened state induced by light exposure is stable under ambient temperature and pressure for at least 1 week and is not reversible in the absence of external thermal or electrical activation.

[0021] The photo-darkening is induced solely by exposure to light and does not require external electrical bias, thermal input, or chemical activation.

[0032]

[0022] The optical transmittance of the obtained coating in the 400-900 nm spectral range decreases by an amount between 10% and 60% after 1 to 24 hours of continuous exposure to ultraviolet or visible light under ambient conditions.

[0033]

[0023] The HxMoth optical absorption peak is within the 700-800 nm range.

[0034]

[0024] The MoO3_z layer has an oxygen deficiency level determined by the oxygen partial pressure or by the sputtering pressure during deposition, such that the resulting optical contrast of the coating structure is adjustable by the oxygen partial pressure or by controlling the sputtering pressure.

[0035]

[0025] The darkening speed and contrast of the double-layer coating can be further controlled by varying the amount of oxygen vacancies and the density of the Moth-z layer. This can be achieved by adjusting the total pressure and / or the partial pressure of oxygen during the fabrication of the Moth-z layer.

[0036]

[0026] The Moth-z layer, when deposited independently under identical conditions, exhibits less then 5% photochromic contrast, and the darkening effect is mostly enabled through photo-induced hydrogen migration from the YHxOylayer.

[0037]

[0027] The method of fabricating the coating structure according to the invention comprises: [i] depositing a YH2-8 layer on a substrate by physical vapour deposition of yttrium in an argon / hydrogen atmosphere, [ii] oxidizing the YH2-8 layer in an oxygencontaining environment to form a transparent and photochromic YHxOyphase, and [hi] depositing a MoOs-z layer on the YHxOy layer by physical vapour deposition of molybdenum in an argon / oxygen atmosphere, such that the resulting multilayer coating exhibits an optical contrast AT between 10% and 60% in the 400-900 nm spectral range upon exposure to ultraviolet or visible light under ambient conditions, where optical contrast is defined as the absolute change in transmittance between the initial and photo-darkened states.

[0038]

[0028] The physical vapour deposition steps are carried out in a vacuum chamber evacuated to a base pressure below 8 x 10“6Torr. During deposition of the YH2-8 layer, argon and hydrogen gases are introduced to maintain a working pressure of 3 to 10 mTorr, and the yttrium target is sputtered using pulsed direct current magnetron sputtering. After reaching the desired layer thickness, the hydrogen flow is stopped, and an oxygen-containing atmosphere is introduced to oxidize the layer for a duration sufficient to obtain a quasi-stable YHxOycomposition. The chamber is subsequently reevacuated, and the MoO3_z layer is deposited by sputtering a molybdenum target in an argon / oxygen atmosphere, maintaining a working pressure of 1 to 10 mTorr. The thicknesses and gas flow rates in both steps are selected to achieve optimal photodarkening performance and hydrogen transfer properties.

[0039] Examples of Implementation of the Invention

[0040]

[0029] The following steps were performed for obtaining the YHxOy / Moth-z coating. A substrate [e.g., glass, silicon, or transparent polymer) was placed in a vacuum chamber. The magnetrons were positioned with Y and Mo targets, so that they are directed towards the substrate. The vacuum chamber was closed and evacuated to a pressure of less than 8 x IO-6Torr. 30 seem of Ar and 3-16 seem of H2 gas flows were introduced into the chamber. Using a throttle valve, a total working pressure of 3-10 mTorr was achieved. The power supply was turned on, providing the Y target with a pulsed direct current of 200 W average power, 80 kHz pulse frequency, and a 2.5 ps pulse interval. Once the plasma discharge has stabilized, a shutter installed before the substrate was opened and a 50-2500 nm thick YH2-8 coating was deposited. Once the desired thickness of the YH2-8 layer was achieved, the shutter was closed, the power supply and both gas flows were turned off. An oxygen or an oxygen-containing atmosphere was introduced into the vacuum chamber and allowed to oxidize the YH2-8 coating for about 30 minutes. Once the coating was oxidized [the layer has achieved a quasi-stable chemical composition), the vacuum chamber was evacuated again to a pressure of less than 8 x 10“6Torr. 30 seem of Ar and 1-10 seem of O2 gas flows were introduced into the chamber. Using the throttle valve, a total working pressure of 1-10 mTorr was achieved in the chamber. The power supply was turned on, providing the Mo target with a pulsed direct current of 200 W average power, 80 kHz pulse frequency, and a 2.5 ps pulse interval. Once the plasma discharge has stabilized, the shutter installed before the substrate was opened and a 10-1000 nm thick MoOs-z coating was deposited. Once the desired thickness of the MoOs-z layer was achieved, the shutter was closed, the power supply was turned off, and both gas flows were stopped. The air was introduced into the vacuum chamber, the chamber opened, and the substrate with the applied coating was removed.

[0041]

[0030] Under the particular embodiments, the multilayer photo-darkening coatings were deposited using the physical vapor deposition (PVD) device "Sidrabe G500M". Before the process, the vacuum chamber [«0.1 m3) was evacuated to a pressure of pbase < 8 x 10“6Torr, using the turbomolecular high-vacuum pump "Pfeiffer Vacuum HiPace 1800" and the rotary fore-vacuum pump "Pfeiffer Vacuum DUO 65M". Yttrium [99.9% purity) and molybdenum [99.95% purity) metal targets, placed on planar balanced rectangular magnetrons installed in the equipment, were used for the deposition of the layers. The corresponding target size for the magnetrons was 150 mm x 75 mm x 3 mm, and they were water-cooled. Argon [99.9999% purity) was used as the working gas. Oxygen [99.999% purity) served as the reactive gas for the MoOs-z layer deposition, while hydrogen [99.999% purity) was used for the YHxOylayer deposition. Gas dosing was provided by MKS 2179B flow controllers and the MKS 6470 multichannel control unit. The required working pressure was maintained with the help of a VAT 64246-PE52 throttle valve and a VAT PM-5 control unit.

[0042]

[0031] The substrates were cleaned in an ultrasonic bath at 50°C for 30 minutes in acetone and then for 30 minutes in isopropanol. Immediately after being removed from the isopropanol, the substrates were dried with nitrogen to prevent unwanted spots from forming.

[0043]

[0032] The distance from the magnetrons to the substrate was 11 cm. The coatings were deposited using pulsed-DC magnetron sputtering, with an average power of 200 W, a pulse frequency of 80 kHz, and a pulse duration of 2.5 ps. The samples were sputtered with a constant argon flow of 30 seem. YH2-8 coatings were deposited with a hydrogen flow in the range of 3-16 seem and a total working pressure of 3-10 mTorr. MoOs-z coatings were deposited with an oxygen flow in the range of 1-10 seem and a total working pressure of 1-20 mTorr.

[0044]

[0033] To obtain photochromic YHxOy films, the YH2-8 coatings were oxidized after deposition by introducing oxygen or an oxygen-containing atmosphere into the vacuum chamber. A quasi-stable composition of the layer is achieved after an oxidation process of approximately 30 minutes.

[0034] The structure of the coatings was studied using a "Rigaku - MiniFlex 600" X-ray diffractometer with Cu Ka radiation [0.154 nm) and a "D / teX Ultra" detector. The maximum power was 600 W, the anode voltage was 15 kV, the precision was ±0.02°, and the instrumental FWHM was 0.005°. For the identification of the cubic p-YH2phase, a diffraction pattern [1CDD 04-002-6939) from the International Centre for Diffraction Data database was used. For the identification of the cubic YHxOyphase, calculated values from the literature [8] were used.

[0045]

[0035] A stylus profilometer "Dektak 150" was used to determine the thickness of the layers.

[0046]

[0036] SEM images of the surface and cross-section of the coatings, with magnifications ranging from 1 to 200 thousand times, were obtained using the "Helios 5 UX" equipment with an electron acceleration voltage of 2 kV.

[0047]

[0037] The light transmittance and reflectance of the coatings in the range of 250 to 2500 nm were determined using an "Agilent Cary 7000" spectrophotometer. During the measurements, the sample was placed at a 6° angle to the incident beam, and the detector was positioned at a 180° angle behind the sample to measure transmittance and at a 12° angle to the sample to measure specular reflectance.

[0048]

[0038] An X-ray photoelectron spectrometer "ThermoFisher - ESCALAB Xi" was used to determine the composition and stoichiometry of the coatings.

[0049]

[0039] The photochromic properties of the coatings were measured using a custom- built device. A UVA lamp with a power of 15 W [« 2.4 mW / cm2) and a wavelength of 385 nm was used for sample irradiation. The samples shown in Figures 2a and 2b were irradiated for 20 hours, and their darkening rate towards the end of the irradiation was <0.5% / h.

[0050] References

[0051] 1. Patent No. US11525180B2. Transparent photochromic device. Publication date: July 27 , 2017.

[0052] 2. Moldarev D., Moro M.V., You C.C., Baba E.M., Karazhanov S.Z., Wolff M., Primetzhofer D. Yttrium oxyhydrides for photochromic applications: Correlating composition and optical response. Physical Review Materials, 2018, 2.11: 115203.

[0053] 3. Dam, Bernard, et al. "Perspective on the photochromic and photoconductive properties of Rare-Earth Oxyhydride thin films." Solar Energy Materials and Solar Cells 273 (2024): 112921.

[0054] 4. D. Moldarev, L. Stolz, M.V. Moro, S.M. Adalsteinsson, I.A. Chioar, S.Z. Karazhanov, D. Primetzhofer, M. Wolff, Environmental dependence of the photochromic effect of oxygen-containing rare-earth metal hydrides, J. Appl. Phys. 129 (2021) 153101.

[0055] 5. He T., Jiannian Y. Photochromism of molybdenum oxide. Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 2003, 4.2: 125-143.

[0056] 6. CN 109162618 B.

[0057] 7. La Mao et al. Excellent photochromic properties of an oxygen-containing yttrium hydride coated with tungsten oxide (YHx:0 / W03). Scripta Materialia, vol. 142, pages 36-40, XP085203852, ISSN: 1359-6462.

[0058] 8. Pishtshev A., Strugovshchikov E., Karazhanov S.Z. Conceptual design of yttrium oxyhydrides: phase diagram, structure, and properties. Crystal Growth & Design, 2019, 19.5: 2574-2582.

Claims

Claims1. A photo-darkening multilayer coating structure, comprising:(i) a substrate;(ii) a photochromic yttrium oxy-hydride (YHxOy) layer deposited on the substrate; the YHxOy layer having a hydrogen-rich composition capable of releasing hydrogen upon exposure to ultraviolet or visible light; and(hi) an oxygen-deficient molybdenum trioxide (MoOs-z) layer deposited on the YHxOylayer; the MoOs-z layer having a stoichiometry and morphology that enables hydrogen uptake from the YHxOy layer under the ultraviolet or visible light exposure, thereby forming a hydrogen molybdenum bronze (HxMoOs) phase within the MoO3_z layer; wherein the coating structure has an optical transmittance in the 400-900 nm spectral range that is reducible by at least 10% to 60% upon exposure to ultraviolet or visible light under ambient conditions, the darkened state remaining stable for at least 1 week after cessation of light exposure.

2. The coating structure according to claim 1, wherein the chemical composition of yttrium oxy-hydride, YHxOy, is within the range 1 < x < 4 and 1 < y < 10.

3. The coating structure according to claim 1, or 2, wherein the chemical composition of molybdenum trioxide, MoOs-z, is within the range 0 < z < 0.3.

4. The coating structure according to any of the preceding claims, wherein the YHxOy layer has a thickness between 50 and 2500 nm, and the MoOs-z layer has a thickness between 10 and 1000 nm.

5. The coating structure according to any of the preceding claims, wherein the resulting photo-darkened state induced by light exposure is stable under ambient temperature and pressure for at least 1 week and is not reversible in the absence of external thermal or electrical activation.

6. The coating structure according to any of the preceding claims, wherein the optical transmittance in the 400-900 nm spectral range decreases by an amount between 10% and 60% after 1 to 24 hours of continuous exposure to ultraviolet or visible light under ambient conditions.

7. The coating structure according to any of the preceding claims, wherein the HxMoOs optical absorption peak is within the 700-800 nm range.

8. The coating structure according to any of the preceding claims, wherein the MOO3_Z layer has an oxygen deficiency level determined by the oxygen partial pressure or by the sputtering pressure during deposition, such that the resulting optical contrast of the coating structure is adjustable by the oxygen partial pressure or by controlling the sputtering pressure.

9. The coating structure according to any of the preceding claims, wherein the substrate is a rigid or flexible solid material selected from the group consisting of glass, polymers, metals, and ceramics.

10. Use of the coating structure according to any of the preceding claims in an optical indicator, a disposable photo-exposure tag, or a passive solar shading element for indicating cumulative light exposure, passive light modulation, or solar heat gain control.

11. A method of fabricating the coating structure according to any of the preceding claims, comprising: (ij depositing a YH2-8 layer on a substrate by physical vapour deposition of yttrium in an argon / hydrogen atmosphere, (ii) oxidizing the YH2-8 layer in an oxygen-containing environment to form a transparent and photochromic YHxOyphase, and (hi) depositing a MoOs-z layer on the YHxOy layer by physical vapour deposition of molybdenum in an argon / oxygen atmosphere, such that the resulting multilayer coating exhibits an optical contrast AT between 10% and 60% in the 400- 900 nm spectral range upon exposure to ultraviolet or visible light under ambient conditions, where optical contrast is defined as the absolute change in transmittance between the initial and photo-darkened states.

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