Patent Application
20180335651
The present invention relates to window coatings and, in particular, to a thermochromic low-emissivity film that dynamically adjusts reflectivity and emissivity with temperature.
The emissivity of a surface measures its effectiveness in emitting energy as thermal radiation. Window glass is by nature highly thermally emissive. To improve thermal control (insulation and solar optical properties), thin film coatings can be applied to the window. Low-e coatings reduce the emission of radiant infrared energy, thus tending to keep heat on the side of the glass where it originated, while letting visible light pass. There are two types of transparent low-e coatings in today's market—semiconductive coatings, such as indium tin oxide (ITO), and metallic coatings, such as silver. Such coatings can be applied by physical vapor deposition, chemical vapor deposition, sol-gel methods, etc. Sputter-deposited silver-based coatings, with emissivities of 2-8%, represent the majority of the current low-e market. Silver-based low-e coatings are available as single-silver, double-silver, and triple-silver products. Triple-silver stacks have the highest selectivity of visible light transparency (VLT) and low infrared (IR) emissivity, with an emissivity of about 0.022, but are also the most expensive.
Single-pane windows still make up about 40% of all window glass in the southern states, and nearly 30% in the midwest and northern states. These high percentages account for significant energy loss when heating energy is considered, and even more when air-conditioning is considered. The dominant technology at present for energy efficient windows is double-pane “insulated” glass with low-e coatings. Unfortunately, the return on investment (ROI) to replace single-pane windows with double-pane windows, in terms of energy savings, averages over 20 years. This is generally not considered a good investment and, as a result, the single-pane window stock is only diminishing by about 2% per year. A low-cost retrofit system could produce significant savings for consumers (˜$12 billion/year) and significantly reduce our national energy consumption and CO2 production.
The present invention is directed to a thermochromic low-emissivity film comprising a layer comprising vanadium dioxide (VO2), wherein the VO2 undergoes a metal-insulator transition at a thermochromic transition temperature, such that the layer transmits infrared radiation below the thermochromic transition temperature and reflects infrared radiation above the thermochromic transition temperature, and a layer comprising a transparent conductive oxide (TCO), wherein the thermochromic low emissivity film has an emissivity of less than 0.4 at a wavelength of 10 microns. The layer comprising VO2 can comprise a thin film of VO2 having a film thickness less than 300 nm or a film comprising a plurality of less than 300 nm VO2 nanoparticles dispersed in a first transparent polymer matrix. The VO2 nanoparticles can be doped. For example, the dopant can comprise tungsten, niobium, tantalum, molybdenum, titanium, zirconium, hafnium, magnesium, copper, nickel, cobalt, chromium, aluminum, hydrogen, lithium, scandium, yttrium, germanium, or silicon. The doping can change the thermochromic transition temperature from between about −15° C. to 80° C. The layer comprising a TCO can comprise a thin film of the TCO or a film comprising a plurality of TCO nanoparticles dispersed in a second transparent polymer matrix. For example, the TCO can comprise In2O3, (In,Sn)2O3 (ITO), fluorine-doped SnO2, SnO2, ZnO, CdO, Ga2O3, or (Ga,In,Zn)2O3, with a plasma wavelength longer than 800 nm. The invention can further comprise a thermally insulating polymer layer between the VO2 and TCO layer. The polymer matrices/layer can be transparent from 0.4 μm to 2.5 μm wavelength. For example, the polymer matrices/layer can comprise polyester, polyether, polyimide, polystyrene, or polyurethane.
The thermochromic low-emissivity films of the present invention can combine a low U-Value film with dynamic IR transmission in winter for residential heating and IR rejection in summer to reduce cooling loads, at a price point similar to low-e coatings. In particular, the invention can: 1) reduce energy loss through existing windows, 2) be easily applied by existing window film installers, 3) open new markets for window films that do not exist today, 4) guarantee a usable lifespan of more than 10 years, 5) deliver undistorted transparent views from the inside-out, 6) offer customers a realistic ROI through energy savings with increased comfort, 7) minimize internal condensation associated with static low-e films, and 8) reduce overall CO2 production. In addition to windows, the thermochromic films can have applications for paint, shingles, and textiles, for example.
The detailed description will refer to the following drawings, wherein like elements are referred to by like numbers.
The present invention is directed to low cost, thermally dynamic, low emissivity bilayer films that can be incorporated into flexible window coatings to modify solar heat gain, solar reflectivity and thermal resistance in windows. The bilayer films can comprise a layer comprising a vanadium dioxide (VO2) film or VO2 nanoparticles dispersed in a transparent polymer matrix and a transparent conductive oxide (TCO) film or TCO nanoparticles dispersed in a transparent polymer matrix. The VO2 film or nanoparticles can transition from IR reflective when warm, to IR transparent when cool. This dynamic reflectivity is passive by nature, and requires no electronics or power source to shift. In addition, this dynamic transition can occur at any design temperature, and when the nanoparticles are dispersed, they remain transparent in the visible spectrum during both phases. The thermochromic low emissivity film can reduce the U-Value (inverse of the total thermal resistance) of single-pane windows and take advantage of solar gain by automatically reflecting heat away when it's hot outside, and allowing heat in when it's cold outside. Although the invention is described as a thermochromic low emissivity coating for windows, the invention also has applications for thermochromic paints, shingles, and textiles (clothing), for example.
As shown in
Thermochromic Vanadium Dioxide Films and Nanoparticles
The thermochromic nanoparticles can be smaller than the effective medium limit for scattering (˜λ/3n, or 50 nm for 450 nm light, and VO2 refractive index of 3), to enable scattering/haze-free aftermarket thermochromic window films.
Vanadium dioxide is potentially a low-e, high transparency alternative to silver. Further, when the low emissivity VO2 film/nanoparticle layer is combined with a TCO film/nanoparticle layer, a very low emissivity film with a tunable solar heat gain coefficient can be realized. For example, the most desirable solar region to tune is the solar infrared portion between 800 nm to 2.5 microns, while keeping emissivity high from 5-15 microns. Therefore, the low emissivity VO2 nanoparticles can be combined with a TCO film/nanoparticles that have a plasma wavelength between 800 nm and 2.5 microns to make a very low emissivity film with a large tunable solar heat gain coefficient. (The plasma wavelength occurs at the crossover in the infrared from high transmission at shorter wavelengths to high reflection at longer wavelengths). The tunable emissivity bilayer can have an emissivity less than 0.4 to greater than 0.5-0.8 for emissivity-based cooling.
As an example,
Alternatively, the low emissivity coating can comprise a composite film comprising a mixture of VO2 and TCO nanoparticles dispersed in a transparent polymer matrix, as shown in
The tunability of the emissivity can be further aided by a multilayer construction. For example, a temperature-dependent VO2 film can be coated on a transparent, thermally insulating layer on top of a TCO layer, facilitating switching of emissivity from low to high in response to changes in the environmental temperature. For example, the thermally insulating layer can be a transparent polymer layer between an outer layer comprising vanadium dioxide and the bottom layer comprising a transparent conductive oxide layer, as shown in
Thermochromic Nanoparticle Synthesis, Milling and Film Integration
Monolithic films of doped VO2 can be grown directly on glass by sputtering or chemical vapor deposition. However, these methods may be impractical for the aftermarket window film market. For these markets, nanoparticle fillers (SiO2, TiO2, silver) are widely used to provide UV absorption or other passive optical properties to commercially available polymer-based window films. However, no commercial nanoparticle VO2/polymer window films are known to be currently available. While thermochromic VO2 research has been an area of interest since the 1970s, the difficulty in preparing phase-pure VO2, and not alternate phases such as V2O5 (opaque) or V6O11/V2O3 (semiconducting, gray), has contributed to difficulty with commercialization and quality control. In addition, very fine particle sizes ˜50 nm that are well-dispersed are required to limit optical scattering/haze in composites.
Thermochromic nanoparticle preparation and dispersion involves (1) precipitation of a spherical precursor compound, (2) collection and purification of these precursor materials, (3) calcination in a controlled atmosphere to form V2O3, and (4) a thermal anneal in controlled atmosphere to oxidize the powder to the desired VO2—R rutile structure, phase-pure thermochromic pigment. Materials processing for product engineering entails (5) milling for particle size control and optimal optical scattering performance with (6) surface modification for effective dispersion within the film matrix, and (7) film production operations to mass produce the product composite material. Performance characterization and validation can be conducted through these processing stages including particle size determination, XRD crystalline phase identification, TGA/DTA for thermochromic quality, FTIR for surface modification, and visible to IR optical property characterization vs. temperature for the final film to optimize engineering variables such as pigment loading, transmittance/reflectance, solar modulation, and thermal gain.
The process currently being used for producing the vanadium organic precursor (VOP) is a facile approach based on the mixing of alkoxide precursors (V and dopants) in pyridine to an acetone solution with controlled water content, to initiate the precipitation of round particles of the nominal composition V2O5xPyyH2O (where x≈0.8 and y≈0.9). See Y. Li et al., ACS Nano 4, 3325 (2010); J. Leonard et al., Macromolecules 45, 671 (2012); and D. Kunz et al., ACS Nano 7(5), 4275 (2013). Particle size is controlled by nucleation rate, and is directly influenced by the water content used for destabilization. The reaction of alkoxide precursors with wet solution is rapid. Scale-up operations are possible with a continuous flow system, controlling addition rates of volumetric feedstock for reaction and collection by a high-volume filter press for recovery of VOP material. Once recovered and dried, the VOP material is calcined under reducing conditions to form V2O3 particles, and further oxidized under low pO2 conditions to transform the material to the desired active VO2—R (rutile, metallic) high temperature phase, with the thermal phase transition to VO2-M (monoclinic, transparent) at room temperature. Mass transport in VO2 at 500° C. is expected to result due to surface diffusion during annealing. See C. D. Landon et al., Appl. Phys. Lett. 107, 023108 (2015). This leads to morphology variation and sintering in films and aggregates of particles. Processing operations, including milling, are required to prepare the pigment for composite film manufacture. See S. Yamamoto et al., Proc. Mater. Res. Soc. Symp. 879E (2005); S. Wang et al., J. Mater. Chem. 2 (17), 6365 (2011); S. Yamamoto et al., Chem. Mater. 21, 198 (2009); Y. Gao et al., In Nanofabrication, Masuda, Y., Ed. (2011); and K. Sato et al., J. Am. Ceram. Soc. 91(8), 2481 (2008). Two methods to mitigate these issues are processing atmosphere control to enable significantly decreased annealing times. Examples of particles formed using this processing route are shown in
Effective dispersion of the nanoparticles is necessary for control of optical properties in the final film. Li et al. calculated that VO2 nanoparticles incorporated into composite films can provide improvements in luminous transmittance and enhanced transmittance modulation of solar energy. VO2—R exhibits a strong plasmon resonance in the near infrared, whereas VO2-M has no resonance. An assumption in these calculations is that the size of the particles is much smaller than the wavelength of interest, meaning that for IR spectral response, the particles should be dispersed below 100 nm effective sizes. See J. Zheng et al., Powder Technol. 91(3), 173 (1997). Mie theory provides an adequate approximation for the scattering efficiencies of VO2—R particles, and analytical solutions are available from the utility, ‘Mieplot’. See M. Z. He et al., Powder Technol. 161(1), 10 (2006). The particle size and optical properties of the VO2—R rutile phase most strongly affect the optical transmission of a nanoparticle film. For small nanoparticles, absorption plays a dominant role in transmission. As particle size increases above 100 nm, scattering properties become more dominant. Bai et al. calculated optical properties for 200 nm spheres of either solid or aggregated nanoparticles; to first approximation, the VO2-M phase scatters more strongly for particle-size distribution (PSD)<300 nm, and VO2—R particles scatter more for PSD>300 nm. See H. Bai et al., Nanotechnology 20(8), 085607 (2009).
Mechanical milling procedures are needed for many materials produced by chemical precipitation routes and calcination. Surface diffusion and bonding between particles is common for powder bed transformations in ceramics, and necessitate the grinding of powders to equiaxed materials. An attritor mill combines forces of impact, abrasion, and shear between particles during the rotation of media with the stirring arms. Finer particles experience cleavage and abrasion by compressive forces and shear. See K. Sato et al., J. Am. Ceram. Soc. 91(8), 2481 (2008). The specific energy input to the system can be monitored and evaluated with particle size measurements to optimize the size and dispersion of the pigment particles during milling operations. Wet milling is energetically more favorable, and can be promoted by the control of solution conditions and dispersant loading. See S. Yamamoto et al., Mater. Res. Soc. Symp. Proc., 879E (2005); S. Wang et al., J. Mater. Chem. 21(17), 6365 (2011); S. Yamamoto et al., Chem. Mater. 21, 198 (2009); and Y. Gao et al., In Nanofabrication, Masuda, Y., Ed. (2011). Obtaining dispersion of two phases requires the development of repulsive forces between the highly divided phase and the continuous matrix phase. Covalent bonding to the particle surface is readily achieved using silane coupling agents, and a variety of terminal organic structures are available to control particle dispersion. During the milling operation, a silane coupling agent and/or co-dispersant can be added as the particle size is reduced, to coat the increasing surface area and enable dispersion in the polymer matrix in the drawn sheet form. The pigment can be recovered in post-milling operations and stored for compounding operations in the final composite formation stage.
The thermochromic VO2-polymer composite film can comprise fine (20-50 nm) nanoparticles dispersed within a transparent polymer matrix. For example, the VO2 nanoparticles can be dispersed within UV-curable hard coatings. These are one-component (1K) systems that cure by photoinitiated polymerization of the acrylate monomers and oligomers. As they do not contain solvents, dispersion of the hydrophilic particles into the matrix is straightforward and curing is instantaneous. The UV-curable formulations typically contain an acrylated oligomer based on a polyester (polyethers are also occasionally used). Alternatively, the VO2 particles can be dispersed within an acrylic urethane adhesive layer. This resin system has superior versatility, durability, appearance and superior weatherability compared to other resin systems. The most common coating type is two-component (2K), where an acrylic polyol solution is mixed with a polyisocyanate just before use, and applied to the substrate. The coating then cures by chemical crosslinking to form a durable urethane bond. The cure speed and film properties can be tailored to the application by varying the hardness (Tg) and functionality (OH number) of the acrylic polyol; the isocyanate, type of solvents used, accelerators and heat. This requires surface modification/dispersant/surfactant solutions, which may be achieved by silanization, ionic stabilization, or silica passivation steps. The method of deposition can be large format, roll-to-roll coating of the formulation on a flexible polymeric substrate using Meyer rod or slot die deposition.
Modification of the Thermochromic Transition Temperature
Through chemical doping with tungsten and other elements, the transition temperature can be tuned from 68° C. (150° F.) for undoped VO2 to anywhere in the range of −15° C. to 80° C. (5° F. to 175° F.) for doped VO2. Other dopants that can be used include niobium, tantalum, molybdenum, titanium, zirconium, hafnium, magnesium, copper, nickel, cobalt, chromium, aluminum, hydrogen, lithium, scandium, yttrium, germanium, and silicon. Typically, the dopant concentration is less than 15%, preferably between 1 and 10%.
Accordingly, both the absorbance in the near-IR solar tail (700 nm to 2.5 microns) and the emissivity in the far-IR 8-12-micron emission peak for windows/buildings can be designed to be inherently low-e in the VO2 insulator state and have tunable emissivity through use of a designed, environmentally triggered transition to metallic on hot days (e.g. transition at 20° C. to 35° C.) to trigger a high emissivity state.
The present invention has been described as a thermochromic low-emissivity film. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.
This application claims the benefit of U.S. Provisional Application No. 62/507,350, filed May 17, 2017, which is incorporated herein by reference.
This invention was made with Government support under Contract No. DE-NA0003525 awarded by the United States Department of Energy/National Nuclear Security Administration. The Government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 62507350 | May 2017 | US |
