Winter is far from over. You may still be thinking about getting your snow blower tuned up before the next storm, stocking up on ice melt, and piling on more winter sweaters, but have you thought to check your windows?
Window coatings help keep cold, or heat, outside. Low-E coatings are thin metal or metal oxide layers deposited onto a glass substrate in order to control radiative heat flow. Low-E coatings are typically applied to glazing units installed in buildings to assist in controlling room temperatures, reducing the heat loss from the interior in winter months or to control heat entering through the glass in the summer. The principle of operation is the same in both cases, longer wavelength infrared radiation is reflected by the coating while allowing shorter wavelength light through.
The Efficient Windows Collaborative website helps explain how low-E coatings work:
All materials, including windows, emit (or radiate) heat in the form of long-wave, far-infrared energy depending on their temperature. This emission of radiant heat is one of the important components of heat transfer for a window. Thus reducing the window’s emittance can greatly improve its insulating properties.
Standard clear glass has an emittance of 0.84 over the long-wave portion of the spectrum, meaning that it emits 84% of the energy possible for an object at its temperature. It also means that 84% of the long-wave radiation striking the surface of the glass is absorbed and only 16% is reflected. By comparison, low-E glass coatings can have an emittance as low as 0.04. Such glazing would emit only 4% of the energy possible at its temperature, and thus reflect 96% of the incident long-wave, infrared radiation. Low-E coatings can be formulated to have a broad range of solar control characteristics while maintaining a low U-factor.
Generally, the coatings are 100-200 nm thick. The composition and integrity of the coating is crucial to both the efficiency of the glass and its visual appearance. Therefore, the use of a technique to perform both routine analyses and failure investigation is vital.
Using X-ray photoelectron spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA), the real layer structure of a coated sample can be checked against the expected layer structure. The surface sensitivity of the technique enables profiles with excellent depth resolution to be obtained. The chemical state of the individual elements present can also be detected, allowing the analyst to identify if unwanted oxidation of one of the metal components is occurring, for example. In the case of failure analysis, the layer where delamination has occurred can be identified by using the same depth profiling process.
Read K-Alpha: Characterization of Low-Emissivity Glass Coatings using X-ray Photoelectron Spectroscopy for details of a study in which a sample of low-E glass was depth profiled using XPS.