WO1998026926A1

WO1998026926A1 - Selective solar radiation control coatings for windows and plastic films characterized by an absence of silver

Info

Publication number: WO1998026926A1
Application number: PCT/US1997/023542
Authority: WO
Inventors: Timothy Ellison; Masatsugu Izu
Original assignee: Energy Conversion Devices, Inc.
Priority date: 1996-12-19
Filing date: 1997-12-19
Publication date: 1998-06-25
Also published as: AU5802698A

Abstract

A selective solar radiation control coating for optical substrates characterized by the absence of free metal such as silver and by the absence of moisture sensitive dielectrics such as ZnO. The selective solar radiation control coating is preferably a multilayer coating formed from moisture resistant dielectric and/or semiconductor materials.

Description

SELECTIVE SOLAR RADIATION CONTROL COATINGS FOR WINDOWS AND PLASTIC FILMS CHARACTERIZED BY AN ABSENCE OF SILVER

FIELD OF THE INVENTION

The instant invention relates to selective solar radiation control coatings in general and specifically to selective solar radiation control coatings characterized by a lack of any free metal, particularly a lack of silver. The coatings are formed from an optical stack of dielectric and/or semiconductor layers.

BACKGROUND OF THE INVENTION

Low emissivity (low-E) coatings on glass and plastics for window applications is presently a billion dollar industry. Today, good quality insulating glass (IG), low-E windows can outperform standard insulated walls in terms of their heat loads, even on north facing walls. About 33% of all new windows today have low-E coatings of some sort; the DOE's goal is to increase the level of market penetration during the next decade to over 66%.

IG windows were developed to reduce heat loss through windows in northern climates where the primary house and building energy usage is for heating. IG windows are sealed dual-pane window units filled with low conductivity argon gas, with the gap between the windows optimized to minimize heat loss through the combined effects of conduction through the glass and the insulating gas and convection in the gas. For a high quality IG unit, radiation of long wavelength infrared (10 ^»m, far IR) radiation from the warm house and window to the outside can become a significant source of heat loss. Such heat loss can be about one to two hundred W/m² for very low outside temperatures. To reduce these losses, low-E windows were developed. Such windows have a thin layer of silver, one of the lowest emissivity metals, which minimizes far IR radiation from the window and reflects the far IR in the house back into the house.

Such windows, however, are not appropriate for southern climates where the predominant house and building energy usage is for cooling. In these regions, with relatively low (compared to northern climates) inside to outside temperature differences, the heat gain due to far IR is typically much less than 50 W/m²; the heat gain due to direct solar radiation (from about 0.3 to 1.3 » m wavelengths), however, can be over an order of magnitude higher (« 1 ,000 W/m²). Neither IG units nor Low E coatings have much effect on this major heat load. Summer heat gain through windows accounts for 1 - 1 1/2% of the total energy used for cooling in the United States -- approximately $10 billion per year as estimated by the Lawrence Berkeley Laboratories, which is equivalent to about 0.7 million barrels of oil per day. Solar control windows would save a typical homeowner in Phoenix AZ between $300-$600 per year depending upon the type of windows they are replacing.

For use in southern climates the silver-based low-E coatings have recently been adapted to make selective low-E coatings. Such coatings, besides reflecting the far IR, have additional silver and dielectric layers to increase reflectance or absorbance of the UV spectrum, and to increase reflectance or absorbance of the near infrared (about 1 • m, near IR) or "heat" portions of the solar spectrum. These coatings must be protected from the elements and, thus, are always enclosed in the inert air gap of an insulating glass (IG) unit, or incorporated into the sealed plastic interlayer of a laminated unit since the very thin silver coatings are quickly oxidized by the atmosphere, and the ZnO coatings are damaged by moisture. (More durable low-E coatings can be applied to single glazing; these hard coats (pyrolytic coatings), however, have not yet been developed with selective characteristics).

The fact that the highly selective silver-based low-E coatings are so susceptible to corrosion is a severe impediment to expanding their market, especially in the south where these coatings would have the greatest impact in reducing energy loads for cooling. To prevent corrosion, the coated glass must be immediately sealed within IG units. Furthermore, special care and procedures (e.g. cleaning methods, tooling to grind the coatings off of the edges of the glass, etc.) are necessary. These handling procedures are not available to the large majority of small window companies which predominate the retail markets, especially in the south (the 25 largest window companies serve about 30% of the market; the next 100 companies another 30%; and the next 1200 companies another 30%). Consequently, these glass coating companies can sell their glass to only a few of the largest window manufacturers, principally in the northern portion of the U.S.

In summary, the present selective Low-E coatings, initially developed for use in northern climates with large indoor/outdoor temperature differences, are not well suited as solar radiation control devices in southern climates. Specifically, while the expensive double-wall insulated glass units are only of marginal benefit in these southern regions, the present corrosion-prone selective Low-E coatings require such units for protection from the environment. Furthermore, the present Low-E coatings are only of marginal benefit, since the Low-E attributes pertain only to the far IR spectrum. It should noted that the present selective low-E coatings are based upon 20 year old sputtering technology. Present selective low-E coatings are made using thick (several hundred A) zinc oxide dielectric coatings and thin (tens of A) silver coatings which are deposited by the use of magnetron sputtering. These materials were chosen because they were most adaptable to this coating technology. Besides the silver oxidation problems, the ZnO coatings are very susceptible to moisture damage and can only be sputtered at rates of about 30 A/s. Therefore, an easily deposited, durable, metallic metal (silver) free, solar-control coating which can be deposited on single pane glass or a flexible polymer substrate is very desirable and would be very commercially lucrative.

SUMMARY OF THE INVENTION

The instant invention relates to selective solar radiation control coatings for optical substrates characterized by the absence of free metal such as silver and the absence of moisture sensitive dielectrics such as ZnO. The selective solar radiation control coating is preferably a multilayer coating formed from durable scratch and crack resistant, moisture resistant dielectric and/or semiconductor materials such as silicon nitride and silicon carbide. The selective solar radiation control coating preferably absorbs substantially all of the incident UV, reflects at least about 40% of portions of the incident IR and transmits at least 85% of the incident visible light.

Generically, the selective solar radiation control coating is formed by alternating layers of a dielectric material having a higher index of refraction than

that of the substrate and layers of a high band gap material (i.e. band gaps of about 2.0 or higher).

One example of a selective solar radiation control coating of the instant invention is one formed from a silicon carbide layer disposed between a first and a second silicon nitride layer. The silicon carbide layer has a band gap of about 2.0 eV or higher and is between about 500 and 700 Angstroms thick. This silicon carbide layer absorbs essentially all of the UV solar radiation. The first and second silicon nitride layers each have a refractive index of about 1.97 or higher and are between about 100 and 300, and 300 and 500 Angstroms thick, respectively. The coatings are useful for forming coated optical articles which include an optical substrate having at least one surface, and at least one selective solar radiation control coating deposited onto the optical substrate.

The optical coatings are preferably deposited by a microwave plasma enhanced chemical vapor deposition (microwave PECVD) process which can deposit the instant coatings at several hundred A/s leading to lower capital costs and, consequently, lower coating costs.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 depicts the optical performance of a silicon nitride/silicon carbide SSRC coating and plots the wavelength of light on the abscissa versus the fraction of the light transmitted or reflected on the ordinate;

Figure 2 depicts the optical performance of a diamond-like- carbon(DLC)/siiicon carbide SSRC coating and plots the wavelength of light on the abscissa versus the fraction of the light transmitted or reflected on the ordinate;

and Figures 3, 4 and 5 depict the optical performance of 1 , 2 and 3 dual layered silicon oxide/silicon nitride SSRC coatings, respectively, and plot the wavelength of light on the abscissa versus the fraction of the light transmitted or reflected on the ordinate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a durable non silver-based selective solar radiation control (SSRC) coating which can be used to address the full potential market for SSRC coated glass, especially in southern climates. Furthermore, the instant coatings, which are made practical by low pressure high deposition rate microwave plasma-enhanced chemical vapor deposition (microwave PECVD) processing, are more economical than present coatings. Present low-E coatings are deposited using magnetron sputtering which deposits the thick dielectric layers at about 30 A/s. In contradistinction, the microwave PECVD process used in the instant invention can deposit the instant coatings at several hundred A/s leading to lower capital costs and, consequently, lower coating costs.

The new SSRC coating described here uses only moisture resistant dielectric and/or semiconducting coatings. These materials are deposited in layers upon whatever optical substrate is desired, typically glass or polymer, to form an optical stack. The optical stack is designed to absorb as much UV radiation as possible, reflect as much near IR radiation as possible and transmit as much visible light as possible.

One particularly good combination of materials for production of the optical stack is a Si₃N₄ (silicon nitride) dielectric and an amorphous silicon semiconductor with added carbon to increase the bandgap (SiC, silicon carbide). Unfortunately, these materials can only be sputtered at rates of about 10 and <5 A/s, respectively, and consequently are impractical using present sputtering technology. Besides requiring a machine approximately an order of magnitude longer, and thus more expensive than is necessary for the ZnO/Ag coatings, the sputtering material costs are also considerably more expensive. However, the microwave PECVD process makes these coatings economical; in fact, the microwave PECVD process can deposit these coatings at several hundred A/s, making this process more economical than the present sputtering processes. Furthermore, these coatings can be applied to either glass (rigid) or thin plastic (flexible) substrates.

Of great importance is the fact that these SSRC coatings contain no moisture sensitive dielectrics (such as ZnO) or free metals which are susceptible to oxidation (such as silver). Consequently these coatings do not need to be placed in an IG unit. This results in additional cost savings in southern climates were the indoor to outdoor temperature difference is not large enough to justify the cost of IG units.

There is an additional market that these coatings can address which the present silver-based coatings cannot, the automotive market. Automotive engineers are very interested in such coatings today to lower the heat load in cars, especially in the future electric vehicles. Selective low-E coatings are presently in use on the windshields of some of the higher end models; these coatings are sandwiched between two layers of glass to provide the necessary environmental protection, an expense that can be avoided by use of a durable non- silver based coating. Furthermore, the instant non-silver based coating could also be used on the side windows where weight restrictions preclude the use of two- layer glass laminates.

We note that in northern climates the optimal SSRC device would have different properties in the winter than in the summer. In these climates it is desirable to reflect (or absorb) the UV and reflect the far IR in both the winter and the summer. However, in the winter it is desirable to transmit the near IR to reduce heating costs, while in the summer, it is desirable to reflect the near IR to reduce cooling costs. The durability and low cost of the instant SSRC devices would enable the design of windows where this coating, mounted on a clear, flexible plastic substrate (possibly mounted inside an IG unit) could be raised or opened in the winter and lowered or closed in the summer like a shade or blinds. In essence the coated shade or blind could be closed whenever it is desirable to block outside heat, but be easily opened when it is desirable to allow outside heat in.

We note that the low-cost easily-marketed SSRC coating will also have beneficial effects on the environment due to lower world wide energy usage for heating and cooling, which will reduce CO₂ emissions.

One embodiment of a selective solar radiation control coating of the instant invention is one formed from a silicon carbide layer disposed between a first and a second silicon nitride layer. The silicon carbide layer has a band gap of about 2.0 eV or higher and is between about 500 and 700 Angstroms thick. This silicon carbide layer absorbs essentially all of the UV solar radiation. The first and second silicon nitride layers each have a refractive index of about 1.9 or higher and are between about 100 and 300, and 300 and 500 Angstroms thick, respectively. Optical performance of the silicon nitride/silicon carbide SSRC coating is shown in Fig. 1 (the additional IR and UV absorption in the glass substrate are not included in this figure). The wavelength of light is plotted on the abscissa and the fraction of the light transmitted or reflected is plotted on the ordinate. Specifically illustrated are: (a) the solar radiation spectrum, (b) the average human eye sensitivity, (c & d) transmission spectra for single and dual SSRC coatings, respectively, of a typical SSRC coating of the instant invention, and (e & f) reflectivity spectra for typical single and dual SSRC coatings, respectively, of a typical SSRC coating of the instant invention.

This silicon nitride/silicon carbide SSRC coating absorbs nearly all the UV, reflects about 40% of the IR, and transmits over 86% of the visible light. The transmission peak is centered around the portion of the spectrum to which the human eye is sensitive, consequently making the coating appear colorless; the thickness of the coatings and the bandgap of the SiC coating can be adjusted to fine tune the position and shape of the transmission curve, which adjusts the coating for color neutrality. Also shown in Fig. 1 are the transmission and reflection curves for a two-layer silicon nitride/silicon carbide SSRC coating (i.e. a coating on either side of the substrate, or on one surface of each pane of glass in an IG unit). Such a compound coating would allow transmission of over 75% of the visible spectrum and would reflect about 60% of the IR spectrum. These coatings compare very well with the present selective silver-based low-E coatings on the market today.

Another embodiment of a selective solar radiation control coating of the instant invention is one formed from a silicon carbide layer disposed between a first and a second diamond-like carbon layer. The silicon carbide layer has a band gap of about 2.0 eV or higher and is about between about 300 and 450 Angstroms thick. The first and second diamond-like carbon layers each have a refractive index of about 2.3 or higher and are between about 200 and 350, and about 400 and 500 Angstroms thick, respectively.

Optical performance of the diamond-like-carbon/silicon carbide SSRC coating is shown in Fig. 2. The wavelength of light is plotted on the abscissa and the fraction of the light transmitted or reflected is plotted on the ordinate. Specifically illustrated are: (a) the solar radiation spectrum, (b) the average human eye sensitivity, (c & d) transmission spectra for single and dual SSRC coatings, respectively, of a typical SSRC coating of the instant invention, and (e & f) reflectivity spectra for typical single and dual SSRC coatings, respectively, of a typical SSRC coating of the instant invention.

This diamond-like-carbon/silicon carbide SSRC coating absorbs nearly all the UV, reflects about 40% of the IR, and transmits about 94% of the visible light. The transmission peak is centered around the portion of the spectrum to which the human eye is sensitive, consequently making the coating appear colorless; the thickness of the coatings and the bandgap of the SiC coating can be adjusted to fine tune the position and shape of the transmission curve, which adjusts the coating for color neutrality. Also shown in Fig. 2 are the transmission and reflection curves for a two-layer coating. Such a compound coating would allow transmission of over 88% of the visible spectrum and would reflect about 54% of the IR spectrum. These coatings also compare very well with the present selective silver-based low-E coatings on the market today. Yet another embodiment of a selective solar radiation control coating of the instant invention is one formed from one or more dual layered coatings of silicon oxide and silicon nitride deposited onto the substrate. The silicon nitride layer is deposited adjacent the substrate. If more than one dual-layer coating is applied, the silicon oxide and silicon nitride layers alternate. The silicon oxide layers have a refractive index of about 1.48 and are typically between about 1100 and 1900 Angstroms thick, and the silicon nitride layers have an index of about 1.97 and are typically between about 1000 and 1500 Angstroms thick.

The optical performance of 1 , 2 and 3 dual layered silicon oxide/silicon nitride SSRC coatings deposited onto an optical substrate is shown in Figs. 3, 4 and 5, respectively. The wavelength of light is plotted on the abscissa and the fraction of the light transmitted or reflected is plotted on the ordinate. Specifically illustrated are: (a) the solar radiation spectrum, (b) the average human eye sensitivity, (c) the transmission spectrum for the dual layer SSRC coating, (1 , 2 and 3 dual-layers, respectively for figures 3, 4, and 5), and (d) the reflectivity spectrum for the dual layer SSRC coating, (1 , 2 and 3 dual-layers, respectively for figures 3, 4, and 5).

These dual layered silicon oxide/silicon nitride SSRC coatings absorb nearly all the UV, reflects about 21%, 45%, and 63% of the IR, respectively for 1 , 2 and 3 dual-layers, and transmit about 94-95% of the visible light. The transmission peak is centered around the portion of the spectrum to which the human eye is sensitive, consequently making the coating appear colorless.

Specific examples of the instant invention are shown in Table 1 :

The coatings are useful for forming coated optical articles which include an optical substrate having at least one surface, and at least one selective solar radiation control coating deposited onto the optical substrate. The optical substrate may be glass or plastic. Some dielectric materials useful for the SSRC coatings of the instant invention are silicon nitride, silicon oxide, silicon oxynitride, alloys of these materials with carbon, and diamond-like carbon. Additionally while silicon carbide has been disclosed as a semiconductor material, other materials such as silicon (doped or undoped), germanium (doped or undoped), and germanium carbide are also useful semiconductors.

Therefore, it is to be understood that the disclosure set forth herein is presented in the form of detailed embodiments described for the purpose of making a full and complete disclosure of the present invention, and that such details are not to be interpreted as limiting the true scope of this invention as set forth and defined in the appended claims.

Claims

We Claim:

1. A selective solar radiation control coating for optical substrates, said selective solar radiation control coating characterized by the absence of free silver.

2. The selective solar radiation control coating of claim 1 , wherein said coating is a multilayer coating.

3. The selective solar radiation control coating of claim 2, wherein said multilayer coating is formed from moisture resistant dielectric materials and/or semiconductor materials.

4. The selective solar radiation control coating of claim 5, wherein: said dielectric material is one or more compounds selected from the group consisting of silicon nitride, silicon oxide, silicon oxynitride, alloys of these materials with carbon, and diamond-like carbon; and said semiconductor is one or more materials selected from the group consisting of silicon carbide, silicon, doped silicon, germanium, doped germanium, and germanium carbide.

5. The selective solar radiation control coating of claim 4, wherein said multilayer coating is formed from a silicon carbide layer disposed between a first

and a second silicon nitride layer; said silicon carbide layer has a band gap of about 2.0 eV or higher and is about between about 500 and 700 Angstroms thick; said first and second silicon nitride layers each have a refractive index of about 1.9 or higher and said first and second silicon nitride layers are between about 100 and 300, and about 300 and 500 Angstroms thick, respectively.

6. The selective solar radiation control coating of claim 4, wherein said multilayer coating is formed from a silicon carbide layer disposed between a first and a second diamond-like carbon layer; said silicon carbide layer has a band gap of about 2.0 eV or higher and is about between about 300 and 450 Angstroms thick;

said first and second diamond-like carbon layers each have a refractive index of about 2.3 or higher and are between about 200 and 350, and about 400 and 500 Angstroms thick, respectively.

7. The selective solar radiation control coating of claim 4, wherein said multilayer coating is formed from at least one silicon nitride layer and at least one silicon oxide layer; said at least one silicon nitride layer has a refractive index of 1.97 or higher and is between about 1000 and 1500 Angstroms thick; and said at least one silicon oxide layer has a refractive index of about 1.48 or higher and is between about 1100 and 1900 Angstroms thick.

8. The selective solar radiation control coating of claim 7, wherein said multilayer coating is formed from two silicon nitride layers and two silicon oxide layers, said silicon nitride layers and silicon oxide layers disposed alternately.

9. The selective solar radiation control coating of claim 7, wherein said multilayer coating is formed from three silicon nitride layers and three silicon oxide layers, said silicon nitride layers and silicon oxide layers disposed alternately.

10. The selective solar radiation control coating of claim 1 , wherein said multilayer coating is deposited on an optical substrate formed from glass or plastic.