1. Field of the Invention
The present invention relates to low emissivity coatings. More specifically, the present invention relates to multilayer coatings for controlling thermal radiation from substrates transparent to visible light.
2. Discussion of the Background
Solar control coatings on transparent panels or substrates are designed to permit the passage of visible light while blocking infrared (IR) radiation. High visible transmittance, low emissivity coatings on, e.g., architectural glass and automobile windows can lead to substantial savings in costs associated with environmental control, such as heating and cooling costs.
Generally speaking, coatings that provide for high visible transmittance and low emissivity are made up of a stack of films. The stack includes one or more thin metallic films, with high IR reflectance and low transmissivity, disposed between anti-reflective dielectric layers. The IR reflective metallic films may be virtually any reflective metal, such as silver, copper, or gold. Silver (Ag) is most frequently used for this application due to its relatively neutral color. The anti-reflective dielectric layers are generally metal oxides selected to minimize visible reflectance and enhance visible transmittance.
Conventional low emissivity coatings generally strive to maintain reflection relatively constant throughout the visible spectrum so that the coating has a “neutral” color; i.e., is essentially colorless. However, conventional low-emissivity coatings fail to provide the extremes of reflected color required for aesthetic and other reasons by certain applications.
To achieve the desired properties in a coated substrate, the composition and thickness of each of the layers of a multilayer coating must be chosen carefully. For example, the thickness of an IR reflective layer such as Ag must be chosen carefully. It is well known that the emissivity of a Ag film tends to decrease with decreasing Ag sheet resistance. Thus, to obtain a low emissivity Ag film, the sheet resistance of the Ag film should be as low as possible. Because film surfaces and pinholes in very thin Ag films contribute to sheet resistance, increasing Ag film thickness to separate film surfaces and eliminate pinholes can decrease sheet resistance. However, increasing Ag film thickness will also cause visible transmission to decrease. It would be desirable to be able to increase visible transmission by decreasing Ag film thickness without increasing sheet resistance and emissivity.
Thin, transparent metal films of Ag are susceptible to corrosion (e.g., staining) when they are brought into contact, under moist or wet conditions, with various staining agents, such as atmosphere-carried chlorides, sulfides, sulfur dioxide and the like. To protect the Ag layers, various barrier layers can be deposited on the Ag. However, the protection provided by conventional barrier layers is frequently inadequate.
Coated glass is used in a number of applications where the coating is exposed to elevated temperatures. For example, coatings on glass windows in self-cleaning kitchen ovens are repeatedly raised to cooking temperatures of 120-230° C., with frequent excursions to, e.g., 480° C. during cleaning cycles. In addition, when coated glass is tempered or bent, the coating is heated along with the glass to temperatures on the order of 600° C. and above for periods of time up to several minutes. These thermal treatments can cause the optical properties of Ag coatings to deteriorate irreversibly. This deterioration can result from oxidation of the Ag by oxygen diffusing across layers above and below the Ag. The deterioration can also result from reaction of the Ag with alkaline ions, such as sodium (Na+), migrating from the glass. The diffusion of the oxygen or alkaline ions can be facilitated and amplified by the deterioration or structural modification of the dielectric layers above and below the Ag. Coatings must be able to withstand these elevated temperatures. However, multilayer coatings employing Ag as an infrared reflective film frequently cannot withstand such temperatures without some deterioration of the Ag film.
It would be desirable to provide low emissivity, multilayer coatings exhibiting any of a wide range of colors, along with improved chemical, thermal and mechanical stability.