The
fabrication and examination of a porous silica thin film, potentially
for use as an insulating thin film, were investigated. A vertically
aligned carbon nanotube (CNT) forest, created by chemical vapor deposition
(CVD), was used as scaffolding to construct the porous film. Silicon
was deposited on the CNT forest using low-pressure CVD (LPCVD) and
then oxidized to remove the CNTs and convert the silicon to silica
for electrical or thermal passivation (e.g., thermal barrier). Thermal
conductivity was determined using a 1D heat-transfer analysis that
equated radiative heat loss in a vacuum with conduction through the
substrate and thin film stack. A comparison of the surface temperature
differences between a sample film and a reference of comparable thermal
resistance enabled determination of the increase in the thermal resistance
and of the thermal conductivity of the films. For film thicknesses
of approximately 55 μm, the cross-plane thermal conductivity
was found to be 0.054–0.071 W m–1 K–1 over 378–422 K. This thermal conductivity value is in the
range of other silica aerogels and consistent with the low gravimetric
density of 0.15 g cm–3 for the samples. The film
is also relatively smooth and flat, with an average arithmetic mean
roughness of 1.04 μm.
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