Developments in Ceramic Materials Research

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S. Ardizzone, C. L. Bianchi, G. Cappelletti et al.
of the ion increases progressively thus making the reticular substitution more difficult and
shifted at higher temperatures. Figure 3 reports also, as inset, the photographs of the
pigments, to show their actual colour. Both Fe- and Pr-doped powders show the desired
pigment colour, red and yellow, respectively; V-doped zircon, instead, presents a dark green
shade of colour which is not the desired blue. This argument will be resumed in the following
in the discussing Figure 6 and in the subsequent part.
Figure 4 reports the specific surface areas of the precursors before the calcination step.
The surface areas decrease progressively in passing from the un-doped sample to the metal
doped with a sequence: Pr>Fe>V. This sequence represents, with an inverse trend, the same
effects observed for the structural features. The “structuring” role of the metals, observed in
the X-ray patterns, is mirrored by a change in the morphological features of the gel, which
apparently becomes more compact and intertwined the more structure promoting the metal is.
Consistent informations can be obtained by examining SEM micrographs of the pigments
calcined at 1200°C. In the case of Pr-doped samples (Figure 5a) large shapeless aggregates,
composed by rounded particles can be appreciated.
The most apparent feature in this micrograph is the presence of a covering layer on top of
the particles. It must be recalled that in the case of this sample SiO2 is still amorphous for
about the 80 % of the initial amount. The aspect of the powder changes significantly in the
case of Fe-doped samples (Figure 5b). No more “covering” effects are appreciable, possibly
because in this sample the fraction of amorphous silica is much lower (about 40%). The
particles are well defined (average size in the range of 100-200 nm), with a spheroidal or
prismatic shape. In the case of V-doped pigments (Figure 5c) the morphology is remarkably
different and shows the advanced growth of the V-doped pigment: the particles are much
bigger, possibly aggregates of smaller ones, spheroidal or rounded off twins with a relatively
large size distribution.
Figure 4. Trend of BET surface areas for undoped and doped (V, Pr, Fe) sol-gel precursors at 0.02
metal/Zr molar ratio.