Developments in Ceramic Materials Research

Coloured ZrSiO4 Ceramic Pigments 265
The peak shape was fitted using a modified Pearson VII function. The background of
each profile was modelled using a six-parameters polynomial in 2θ m , where m is a value from
0 to 5 with six refined coefficients.
Specific surface areas were determined by adsorption of N2 at subcritical temperatures
(classical BET procedure) using a Coulter SA 3100 apparatus.
The particle morphology was examined by scanning electron microscopy using a
Cambridge 150 Stereoscan. SEM analyses were performed using a Hitachi 2400 scanning
electronic microscope. Powder samples were coated with a thin gold layer deposited by
means of a sputter coater.
Diffuse reflectance spectra were acquired in the Vis-NIR range from 350 to 1200 nm
using a JASCO/UV/Vis/NIR spectrophotometer model V-570 equipped with a barium
sulphate integrating sphere. A block of mylar was used as reference sample following a
previously reported procedure [29]. The colour of the fired samples was assessed on the
grounds of L*, a* and b* parameters, calculated from the diffuse reflectance spectra, through
the method recommended by the Commission Internationale de l’Eclairage (CIE) [30]. By
this method the parameter L* represents the brightness of a sample; a positive L* value stays
for a light colour while a negative one corresponds to a dark colour; a* represents the green
(-) → red (+) axis and b* the blue (-) → yellow (+) axis.
XPS spectra were obtained using an M-probe apparatus (Surface Science Instruments).
The source was monochromatic Al Kα radiation (1486.6 eV). A spot size of 200 x 750 μm
and a pass energy of 25 eV were used. The energy scale was calibrated with reference to the
4f7/2 level of freshly evaporated gold sample, at 84.00 ± 0.1 eV, and with reference to 2p3/2
and 3s levels of copper at 932.47 ± 0.1 eV and 122.39 ± 0.15, respectively; 1s level
hydrocarbon-contaminant carbon was taken as the internal reference at 284.6 eV. The
accuracy of the reported binding energies (BE) can be estimated to be ± 0.2 eV.
Pr-, V-, Fe-Doped Pigments
RESULTS
Structural and Morphological Aspects
The structural features of the three families of pigments (Pr-, V- and Fe-doped zircon) are
first compared by considering samples prepared at an intermediate calcination temperature
(1000°C). Figure 2 reports the comparison between the X-ray pattern of four samples
obtained in the same experimental conditions with a Me/Zr molar ratio of 0.02. In order to
obtain an accurate quantitative estimation of the phases present in the powder, the spectra of
all samples have been fitted with the Rietveld program QUANTO; the results are reported as
insets to the diffractograms. In the absence of the guest ion (Figure 2a) the sample shows a
large quantity of amorphous phase and the only crystalline phases are monoclinic and
tetragonal zirconia. No crystalline phases related to silica can be appreciated. Upon addition
of praseodymium (Figure 2b), instead, tetragonal zirconia is the major component and the
amorphous amount is reduced. No monoclinic ZrO2 is appreciable. The presence of vanadium
(Figure 2c) further promotes the crystal growth of the sample: the desired phase zircon
(ZrSiO4) is formed, although to a limited extent (about 10%) and ZrO2 in the tetragonal form