Temperature responsive optical limiter, composition and device

Abstract

The present invention relates to optical power-limiting devices, and more particularly, to an optical power-limiting passive (self-adaptive) device and to a method for limiting solar power transmission in devices such as windows, using scattering level changes in a novel thermotropic composition that contains salt nano or microparticles embedded in a solid transparent host layer, where temperature change induces change in the refraction index of the matrix as well as of the embedded particles, creating a scattering layer, substantially reflecting the incident light thus limiting the amount of light passing through the window, green house covers, car sun roofs, solar panel windows and protection layers on housing roofs and walls, as a function of ambient temperature.

Claims

1. A thermotropic coating, limiting transmission of solar energy there-through, comprising: salt nanoparticles or salt microparticles, embedded in a solid transparent host matrix; said coating being reversible between: a translucent and non-transmittant state; and a transparent and transmittant state; said state depending on the ambient temperature; wherein said host and said salt particles are selected such that a change in ambient temperature induces a change in the refraction index of said matrix at a first rate, and induces a change in the refraction index of said embedded salt particles at a second rate different from the first rate, to produce scattering and reflection of solar light and thus limit transmission of light passing there through, as a function of the ambient temperature.

2. The thermotropic coating of claim 1, wherein said coating is formulated such that at a pre-determined temperature the refractive indices of said host matrix layer and of said embedded salt particles are similar, yielding minimal light scattering and relatively high transmittance.

3. The thermotropic coating of claim 1 wherein said coating is formulated to be transparent at low ambient temperature, thus allowing light and heat to enter a window coated with said coating, for maximal savings of lighting and heating expenses.

4. The thermotropic coating of claim 1 wherein said coating is formulated to be translucent when the ambient temperature reaches a predefined high temperature, said coating back-scattering the majority of the solar heat and light impinging on it, for maximal savings of lighting and cooling expenses.

5. The thermotropic coating of claim 1 wherein said matrix is formed of an organic polymer having a refractive index which varies as a function of the temperature.

6. The thermotropic coating of claim 1 wherein said salt particles are made of an inorganic material, having a refractive index which varies at a slower rate as a function of the temperature, than the rate of variation of said refractive index of said matrix.

7. The thermotropic coating of claim 1, wherein said coating is formulated to be transparent at a predefined temperature, by selecting two compatible polymeric matrices, wherein a first matrix has a refractive index higher than that of the salt particles, and a second matrix has a lower refractive index than that of the salt particles, and said two polymers are mixed at a predetermined ratio, to select the exact temperature at which said coating is transparent.

8. The thermotropic coating of claim 1, wherein said coating is sandwiched or laminated between two transparent layers of glass, polymer or crystal panes.

9. The thermotropic coating of claim 1 wherein said coating is present as a laminating layer adhered or glued to transparent glass or polymer window-panes.

10. The thermotropic coating of claim 1, wherein said coating is comprised of environmentally friendly materials.

11. The thermotropic coating of claim 1 where said host matrix is organic and selected from: a polymer film, a polymerizable composition, or a transparent adhesive.

12. The thermotropic coating of claim 1 where said host matrix is inorganic and selected from: mineral glass, sol-gel.

13. The thermotropic coating of claim 1 where said host matrix is an inorganic-organic composite.

14. The thermotropic coating of claim 1 where the host is selected from: a UV cured transparent adhesive; an epoxy based transparent adhesive; a silicone based transparent adhesive; a polymer host matrix selected from: Polyvinyl butyral (PVB), Cellulose acetate butyrate (CAB), and Cellulose acetate (CA); Acrylic polymers; and combinations thereof.

15. The thermotropic coating of claim 1 where said salt microparticles or nanoparticles are selected from: LiF, LiCl, LiBr, LiI, NaF, NaCl, NaBr, NaI, KF, KCl, KBr, KI, RbF, RbCl, RbBr, RbI, CsF, CsCl, CsBr, CsI, BaF.sub.2, CaF.sub.2, MgF.sub.2, ZnSe, ZnS; and mixtures and combinations thereof.

16. The thermotropic coating of claim 1, further comprising a stabilizer selected from: a hindered amine light stabilizer (HALS), a UV absorber, a thermal stabilizer, a singlet oxygen quencher, an antioxidant, and any combination thereof.

17. The thermotropic coating of claim 1 wherein said salt particles are encapsulated to prevent degradation and enhance the dispersability of said particles.

18. The thermotropic coating of claim 1 further comprising light absorbing materials embedded in the host material to increase the modulation of light attenuation between said two states of said coating.

19. The thermotropic coating of claim 1 further comprising thermochromic nano-particles to increase the modulation of light attenuation between said two states of said coating.

20. The thermotropic coating of claim 19, wherein said thermochromic nano-particles are formed of vanadium dioxide (VO.sub.2).

21. The thermotropic coating of claim 1 wherein said coating is positioned between two polarizers oriented in the same linear polarization, to increase the modulation of light attenuation between said two states of said coating.

22. The thermotropic coating of claim 1, wherein said host matrix is present in a layer of maximal thickness of 500 microns.

23. A laminate article comprising the thermotropic coating of claim 1, wherein said coating is present as an internal layer located within an upper window layer and a lower window layer.

24. An optical power-limiting article comprising the coating of claim 1, wherein said article is selected from: a window, a green house cover, a sun roof, a solar panel, and a protection-layer on a roof or walls.

25. The article of claim 24, wherein power limiting is passive and self-adaptive.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood. With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention.

(2) FIG. 1 depicts a cross-sectional view of the thermotropic device in coated layer configuration.

(3) FIG. 2 depicts a cross-sectional view of the thermotropic device in laminated layer configuration.

(4) FIG. 3A depicts a scheme of the thermotropic window in the On state. FIG. 3B depicts a scheme of the thermotropic window in the Off state.

(5) FIG. 4 depicts the change in refraction index of NaCl and an optical adhesive as a function of wavelength at 25 C. and 90 C.

(6) FIG. 5 depicts the tuning of Off state temperature via mixing ratio of 2 optical adhesives with different refractive indices.

(7) FIG. 6 depicts the tuning of On state threshold temperature and attenuation degree via scattering particles concentration.

(8) FIG. 7 shows transmission spectra at various temperatures.

(9) FIG. 8A shows a thermochromic window sample in the cold Off state, and FIG. 8B shows a thermochromic window sample in the hot On state.

(10) FIG. 9 depicts a cross-sectional view of the thermotropic device with additional absorbing nano-particles, dyes or pigments.

(11) FIG. 10 depicts a cross-sectional view of the thermotropic device with additional thermochromic VO.sub.2 nano-particles.

(12) FIG. 11 depicts a cross-sectional view of the thermotropic device positioned in between two polarizers having aligned polarization.

DETAILED DESCRIPTION OF THE INVENTION

(13) FIG. 1 depicts a cross-sectional view of the thermotropic article in coated layer configuration 1.

(14) The thermotropic coating layer 4 is composed of the host matrix 6 and the scattering nano or microparticles 8 and it is deposited on a transparent substrate 2, such as a glass window or PET sheet, by e.g. dip coating or any other wet coating, on a single window or roll to roll method.

(15) Light enters in direction 10.

(16) On cold days, the coating composition is transparent and allows the majority of the light and heat to be transmitted there-through and exit at the far side, in direction 12. The coating of the invention therefore provides the dwelling with energy savings by lowering heating and lighting costs.

(17) On hot days, the coating is translucent, and light and heat is backscattered-reflected in direction 14, so it does not enter a dwelling, and the interior of the dwelling remains cool.

(18) FIG. 2 shows a cross-sectional view of the thermotropic device in a laminated layer configuration 16. The thermotropic layer 4 is composed of the host matrix 6 and the scattering nano or microparticles 8 and it is deposited in between two transparent substrates 2, such as glass or PET sheet.

(19) Light enters in direction 10.

(20) On cold days, the coating composition is transparent and allows the majority of the light and heat to be transmitted there-through and exit at the far side, in direction 12. The coating of the invention therefore provides the dwelling with energy savings by lowering heating and lighting costs.

(21) On hot days, the coating is translucent, and light and heat is backscattered-reflected in direction 14, so it does not enter a dwelling, and the interior of the dwelling remains cool.

(22) FIG. 3 depicts a scheme of the thermotropic window 16, as described in FIG. 2, in Off (a) and On (b) states. The Off state (a) is active when ambient temperature is cold. The window is transparent allowing most of the solar light and heat 10 to penetrate the room, in direction 12, hence saving on lighting and heating energy costs. When ambient temperature is high, such as during summer time, the window (b) turns translucent and is partially opaque. In this case the window blocks most of the heat while allowing some light to penetrate the room in direction 22 hence helping reduce cooling expenses. The transition between On and Off states is governed solely by ambient temperature. In the (a) case, cold ambient temperature, the salt particles 18 in the thermotropic layer 4 have the same index of refraction as the host, thus window (a) is transparent. In the (b) case, hot ambient temperature, the salt particles 20 in the thermotropic layer 4 have different index of refraction than the host, thus window (b) turns scattering and reflective, sending the scattered light mainly in directions of arrows 24.

(23) FIG. 4 depicts the change in refraction index of the table salt NaCl and transparent optical adhesive A as a function of the wavelength at 25 C. and 90 C. The optical adhesive's refractive index changes at a rate of 210.sup.4/ K, while the NaCl rate of change is much smaller.

(24) In the upper graph, optical adhesive A was used as a host matrix for NaCl particles, and a thermotropic coating is obtained with an Off temperature, in which it is transparent, at approximately 100 C. where the two lower refraction indexes curves are shown in the graph to be about identical.

(25) Referring to FIG. 5, in order to make this device suitable for the use in smart windows application the Off temperature should be adjusted to 20 C. and this can be done by mixing optical adhesive A in the right amount with optical adhesive B for which the refractive index at 20 C. is lower than that of NaCl.

(26) FIG. 5 depicts tuning of Off state temperature via mixing ratio of two optical adhesives, A and B, which have different refractive indices; The former has a higher value and the latter has a lower value than that of NaCl at room temperature. By changing the mixing ratio between 50:50 and 60:40 the transparency temperature was shifted from peak at 35 C. to peak at 10 C. In this manner the Off state temperature can be tailored per application, for example in case of windows dedicated to different geographical locations or windows that are exposed or not exposed to direct sunlight that may add to ambient temperature. The Off state can be seen to span along 20 degrees Celsius before the transparency is gradually reduced as the window turns translucent. It should be noted that below 10 C. condensation of water from the air settling on the window interfered with the measurements and artificially reduced the transmittance value. The thermotropic layer used in this measurement was of thickness of 0.3 to 0.4 mm. The NaCl used is commercial sea salt mixed with the optical adhesives.

(27) FIG. 6 depicts thermotropic devices of which the host material is a mixture of optical adhesives A and B at a ratio of 55:45, respectively. The three devices that were measured for specular transmittance at 555 nm wavelength, as a function of ambient temperature are different in the concentration of scattering NaCl particles embedded in them. The concentrations presented are 27, 35 and 45 weight percent. The NaCl used is ground commercial sea salt, mixed with the optical adhesives.

(28) As can be seen in FIG. 6, changing the concentration of NaCl has minor to no effect on the transparent state, while it is very influential on the behavior in the On state and the transition between states. The larger the concentration, the less transmittance is observed in the On state, which in this case can be said to be above 45 C. Also, the transition between Off and On states is more abrupt and specific attenuation can be reached at a lower temperature when concentration is higher.

(29) Thus, varying the concentration of scattering particles in the thermotropic device one can tailor the right amount of light and heat attenuation in the On state and its threshold temperature.

(30) FIG. 7 shows direct transmission spectra at 24 C. and at 80 C. of the thermotropic device in which the host material is a mixture of optical adhesives A and B at a ratio of 55:45, respectively, and the concentrations of NaCl particles is 45 wt %. Transmission was acquired with two different tools for the visible and the infrared part of the spectrum hence the discontinuities between 850 and 950 nm and the slight offset observed for the 24 C. measurement.

(31) As can be seen, in this case visible light direct transmission is attenuated from 60% to 10% with increasing temperature, while the total solar power is attenuated from 70% to 15%.

(32) FIG. 8 shows (a) a photo taken of a thermotropic device in which the host material is a mixture of optical adhesives A and B at a ratio of 60:40, respectively, and the concentrations of NaCl particles is 29 wt %, at room temperature in the transparent Off state. Part (b) shows a photo taken of a thermotropic device in which the host material is a mixture of optical adhesives A and B at a ratio of 60:40, respectively, and the concentrations of NaCl particles is 29 wt %, at a temperature of 80 C. in the translucent On state.

(33) FIG. 9 depicts a cross-sectional view of the thermotropic device 26. Solar light 10 enters the room, in direction 12. The thermotropic layer 4 is composed of the host matrix and the scattering nano or microparticles 8 with additional absorbing nano-particles, dyes or pigments 30. During the Off state, when the device is transparent, the light absorbers provide some attenuation of visible and/or infrared solar light. During the On state, when the device becomes translucent due to scattering of light, the absorption of light penetrating the device by the light absorbing particles, dyes or pigments 30 is enhanced because of the longer path the light is taking through the host material due to the scattering.

(34) FIG. 10 depicts a cross-sectional view of the thermotropic device 32. Solar light 10 enters the room, in direction 12. The thermotropic layer 4 is composed of the host matrix and the scattering nano or microparticles 8 with additional thermochromic VO.sub.2 nano-particles 34. The inclusion of VO.sub.2 in the device increases the modulation of light attenuation between the Off and On states. During the Off state, when the device is transparent, the VO.sub.2 nanoparticles provide little attenuation of visible and infrared solar light that can be controlled during production stage via the VO.sub.2 concentration in the device. During the On state, when the device becomes translucent and scatters light due to higher ambient temperature, the absorption of the thermochromic VO.sub.2 nano-particles 34 increases due to a phase change from insulator to metallic that the VO.sub.2 material is going through. In this case, the absorption of the metallic state VO.sub.2 nano-particles is enhanced two folds; first because of the longer path the light is taking through the host material due to the scattering and second because of its phase transition.

(35) FIG. 11 depicts a cross-sectional view of the thermotropic device 36. Solar light 10 enters the room, in direction 12. The thermotropic layer 4 is composed of the host matrix and the scattering nano or microparticles 8 and is positioned between two linear polarizers 38 with aligned polarization. In the Off state the first polarizer attenuates the light by approximately 50% and linearly polarizes it. The light then propagates through the thermotropic layer and the second polarizer with no significant further attenuation. In the On state, as well, the first polarizer attenuates the light by approximately 50% and linearly polarizes it. However, then the passing light propagates through the scattering medium and due to the scattering it loses its linear polarization. By the time it reaches the other side of the scattering thermotropic medium it is mostly arbitrarily polarized and therefore being further attenuated by the second polarizer 38. In that way, the modulation between the Off and On states attenuation is increased as compared to the stand alone thermotropic device.

(36) It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.