Civil Engineering Reference
In-Depth Information
0.7
µ
m
8.8
Cross section through a 'penniform' TiO
2
thin fi lm made by
sputter deposition and simultaneous substrate rotation at 50 rpm.
From Rodríguez
et al.
(2000).
containing the species to be incorporated in the fi lm, withdrawing at a con-
trolled rate, and heating to remove volatile components in the solution; an
alternative technique suitable for coating small objects is to apply the solu-
tion in the form of drops, then spin the substrate at a controlled speed in
order to make an even layer, and fi nally heat treat. In either case the deposi-
tion can be repeated over and over in order to make a thicker fi lm. As an
alternative to dipping and spinning, the chemical solution can be applied
by spraying. Film deposition processes of dipping, spinning and spraying are
often referred to jointly as 'sol-gel deposition' (Klein, 1994; Frenzer and
Maier, 2006). Figure 8.9 illustrates a cross section of a multi-layer coating
made by dip coating (Boström
et al.
, 2011). It has two layers of nickel par-
ticles in alumina, with different compositions, and a top layer of silica. The
coating is backed by metallic aluminium. This coating is an example of a
solar-absorbing surface that avoids thermal re-emission and is thus of the
kind mentioned as the second example in Section 8.1.
Chemical vapor deposition (CVD) uses heat to decompose a vapour of
a 'precursor' substance in order to make a thin fi lm of a desired composition
(Morosanu, 1990; Pierson, 1999). This deposition technique can be made
more effi cient by combining it with plasma treatment in what is denoted
'plasma enhanced CVD' or 'PECVD'. A variety of the CVD technique is
referred to as 'spray pyrolysis'; a fl uid containing the precursor is then
sprayed onto a hot substrate.
Electrochemical techniques embrace cathodic electroplating from a
chemical solution (Lowenheim, 1978) and anodic conversion of a metallic
surface to form a porous oxide. Anodization is most common in the case of
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