Developments in Ceramic Materials Research

264
S. Ardizzone, C. L. Bianchi, G. Cappelletti et al.
Results from the different characterisations, obtained on the three classes of pigments, are
discussed and cross compared, with specific reference to the promoting effect played by the
guest metal on the structural features of the material and to the optical and colour
characteristics of the powders. In the case of V-doped pigments the role played by different
mineralizers in promoting both the structure and the colour is discussed.
EXPERIMENTAL
All the chemicals were of reagent grade purity and were used without further
purification; doubly distilled water passed through a Milli-Q apparatus was used to prepare
solutions and suspensions.
Sample Preparation
The zircon samples were prepared by a previously reported sol-gel procedure consisting
of two different hydrolysis steps [16]. At the beginning Si(OC2H5)4 (0.20 mol) was added
drop by drop to an aqueous solution containing variable amounts of the metal salt (Fe, Pr or
V) previously stirred at 60°C for about 20 minutes in order to achieve the complete
dissolution of the dopant ion. Then 0.35 mol of ethanol and 0.10 mol of reaction catalyst
(HNO3 or HCl depending on the counterion of the metal) were added to the mixture which
was left under rapid stirring for 30 minutes (i.e. the time of the first hydrolysis step).
Subsequently a solution of Zr(OC3H7)4 (0.20 mol) and water was added to the mixture. The
final suspension was kept under stirring at 60°C for 2 hours (i.e. the time of the second
hydrolysis step). The product, cooled at room temperature, was dried in oven at 60°C and
then the obtained xerogel was thermally treated at the selected temperature in the range 600–
1200°C for 6 hours.
A first series of samples was prepared by using a variable Me/Zr molar ratio in the range
0.005-0.10.
A second series of samples was obtained, only in the case of V-doped pigments, by using
a constant V/Zr molar ratio of 0.1 but adding different mineralizers (m) (LiF, LiCl, NaF,
NaCl, KF, KCl) before the second hydrolysis step. The m/Zr molar ratio was kept constant at
0.26 for all the syntheses.
Sample Characterisation
Structural characterisation of the powders was performed by X-ray diffraction, using a
Siemens D500 diffractometer, using Cu Kα radiation in the 10-80° 2θ angle range. The fitting
program of the peaks was a particular Rietveld program [26,27], named QUANTO [28],
devoted to the automatic estimation of the weight fraction of each crystalline phase in a
mixture.