Sputter-Deposited Low-Emissivity Coatings on Glass
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MRS BULLETIN/SEPTEMBER 1997
the other hand, in a hot desert climate the rejection of solar energy by the window is most important. The overall coefficient of heat transfer of a window, or its U-value, is commonly used to describe the rate of heat transfer due to the indoor-outdoor temperature difference per units of area and temperature. 6 Heat transfer to and between glass plates occurs by conduction through the gas and the solid edges of the unit, convection,
and radiation.6 We will not consider the edge effects in this article. Thus Figure 1 shows the values pertinent to the centerof-glass performance of the glazing.7 In comparison with a single glass pane, a double pane IGU allows less than 50% of rate of heat transfer due to the indooroutdoor temperature difference just described. More than 94% of the thermal energy from bodies at 21°C (a typical indoor temperature in winter) is emitted in the infrared wavelength range of 5-40 /xm, with a peak intensity at about 10 fim. In this wavelength range, soda-limesilicate glasses are opaque and act nearly as black bodies: They absorb more than 80% of the thermal radiation, and at radiative thermal equilibrium, re-emit it. Consequently window glass is a relatively poor barrier to radiative heat transfer. This deficiency may be substantially reduced by applying a coating with a selectively high reflectance in the 5 40-/j,m spectral range. Glass with this type of coating remains opaque in the thermal-radiation range. However due to its highly reflective surface, it will have a low emissive power relative to a black body—hence the designation loiu-
8 0.9
J2
0.8
6
LT
+ 0.7 S o E 4
0.6
§3
0.5
U-value
°~
0.4
2
3
0.3
o o
2
0.2
1 -
0.1
Glazing System Figure 1. Center-of-glass winter U-value (coefficient of heat transfer of a window), solar transmittance (Tsoi), and luminous transmittance (LT)13 of (A) a single glass pane, (B) two glass panes, (C) single silver-layer low-E coating on glass (hemispherical emissivityeh = 0.10), and (D) double silver-layer low-E coating on glass (hemispherical emissivityeh = 0.04). In C and D, the coated glass and uncoated lights are on the outside-facing and inside-facing positions of the insulated glass unit (IGU), respectively. Systems B, C, and D were evaluated with 1/2-in. spacing and argon gas fill; 3-mm-thick clear float glass was used in all cases. (In the United States, U-values are commonly expressed in terms of Btu/h ft2 °F; for this conversion, the reader can multiply the m.k.s. values by 0.1/6.)
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Sputter-Deposited Low-Emissivity Coatings on Glass
emissivity (low-E) glass. Emissivity is thus defined as the ratio of the emissive power of a body to that of a black body at the same temperature. The emittance of an opaque material equals its absorptance and is a wavelength-dependent parameter; it also varies with temperature and surface finish of an object.8 The parameter quoted for window glass is emissivity, integrated over the appropriate spectral and angular ranges.9 Figure 1 demonstrates the effect of low-E
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