Analysis of the Optical and Thermal Properties of Transparent Insulating Materials Containing Gas Bubbles

Transparent insulating materials combine high visible light transmission and excellent thermal insulation, and have potential applications in solar energy utilization, building energy conservation and commercial freezers. As a medium of low absorption and low thermal conduction, introducing gas bubbles into transparent mediums such as glass and polycarbonate (PC) may improve simultaneously their light transmission and thermal insulation performances through decreasing the absorption and thermal conduction in the materials. However, gas bubbles can also enhance the scattering which is a competition to the effect of the absorption decrease. Moreover, the material design should also consider the balance between the visible light transmittance and effective thermal conductivity. Therefore, a radiative transfer model for the transparent medium containing large gas bubbles (with a diameter much larger than the wavelengths concerned) with the assumption of independent scattering and the Maxwell-Eucken thermal conduction model were adopted to calculate the transmittance, reflectance and effective thermal conductivity. Subsequently, the effects of the volume fraction of gas bubbles (f(v)) and bubble radius (r) were discussed, and the two balances mentioned above were analyzed. The results showed that the transmittance always decreases when f(v) increases with fixed r or when r decreases with fixed f(v). The transmittance includes two components, named as the collimated transmittance and bulk transmittance due to the forward scattering. The collimated transmittance depends on the effects of absorption decrease and scattering increase, whereas in the weak absorption region, the effect of the scattering increase dominates, making the collimated transmittance decrease, and the decreasing rate is larger than the increasing rate of the bulk transmittance as only the forward scattering contributes to the bulk transmittance. Therefore, the transmittance decreases when f(v) increases with fixed r or when r decreases with fixed f(v). In addition, as f(v), increases from 0 to 0.5, the effective thermal conductivity (k(e)) of the glass decreases from 1.4 to 0.58 W/(m.K), and k(e) of the PC decreases from 0.236 to 0.113 W/(m.K). At the same tithe, the transmittances of both materials at 0.55 mu m can be kept larger than 50% for f(v)=0.5 as long as the bubble radius is larger than 0.7 mm. To elucidate the application performance, a heat transfer model of a freezer adopting glass or PC as a cover was analyzed. Although the decrease percentage of k(e) for glass is higher than that of PC, the effect of the energy saving is more significant for PC, as the cooling load can be saved by 9.6% when f(v) increases from 0 to 0.5, while the corresponding value for glass is only 2.7% because that the decreasing rate of the cooling load with k(e) is higher at a lower k(e).