In the last years, the development of pigments for the production of tiles, ceramic coatings or cosmetics has become a need, because the aesthetic aspect and the color frequently represent the parameters of interest [1]. Color is an optical property that takes to countless applications. A good ceramic pigment has indispensable
requirements, such as high temperature stability, reproducibility and chemical inertia [2]. The final color of each pigment is due to the addition of a chromophore ion (usually transition metals) into an inert matrix, or this ion may be part of the own matrix, as in the case of ferrite [3].
Among the pigment classes, one of the most important is the spinel group, AB2O4, due to its capacity of accommodating different cations, leading to a variety of colors and tonalities. Among spinels, this work evaluated the behavior of barium
monoferrite, BaFe2O4. Spinel ferrites combine interesting soft magnetic properties with rather high electrical resistivities. Some ferrites have also been applied as brown pigments, as catalytic materials, magnetic materials and wave absorption
materials [4].
Numerous studies have been done on the phase relations in Ba–Fe–O ternary system. Three stable phases were reported, namely, Ba2Fe2O5, BaFe2O4 and the hexagonal BaFe12O19 [5,6].
In spite of this, many apparently contradictory results have been found, with the hexagonal BaFe2O4 phase usually being reported as coexisting with BaFe12O19 and Fe2O3, along with other metastable phases. BaFe12O19 and a-BaFe2O4 are
mutually insoluble in each other as solids, and both coexist up to 1000 8C, after which point a third phase, the metastable hexagonal Ba2Fe6O11, can also develop until the ternary mixture reaches its liquid point at 1175 8C, reverting to
BaFe12O19 and a-BaFe2O4 on cooling [7]. Fine-particle spinel ferrites, such as BaFe2O4, are useful for the low temperature preparation of high-density ferrites and as
suspension materials for ferromagnetic liquids. Nanoparticles of BaFe2O4 demonstrate a resonance anomaly near 125 K that could indicate the presence of a magnetic phase. On the other hand, hexagonal magnetic hard ferrites such as BaFe12O19 are
magnetic materials of great scientific and technological interest due to their relatively strong anisotropy and moderate, but still interesting magnetization. They are applied as permanent magnets, in microwave devices or in perpendicular magnetic recording. Another application is in catalysis area [4,8–11].
Different synthesis methods have been evaluated, such as coprecipitation [12,13], aerosol [14,15] or sol–gel [16,17]. In this work, the polymeric precursor method (Pechini) [17] was used in BaFe2O4 synthesis, for application as ceramic
pigment.
2. Experimental procedure
The polymeric precursor was prepared by the Pechini method, which has been used to synthesize polycationic powders. Precursors used were citric acid (Vetec), iron III
nitrate (Vetec) and barium acetate (Reagen), to synthesize the metallic citrate, which was polymerized using ethylene glycol (Synth).
Fig. 1 schematically presents the BaFe2O4 synthesis. After the primary calcination, the polymeric precursor was obtained, which was calcined between 500 and 1100 8C, with a heating rate of 10 8C min 1 in air atmosphere.
The determination of the crystalline phases was carried out by X-ray diffraction (XRD) with Siemens D-5000 Diffractometer with Cu Ka radiation (l = 1.5406 A ° and 2u = 208–708), at room temperature. Cell volume was calculated using the
Rede 93 program, based on the least square method, developed at the Chemistry Institute of Unesp, at Araraquara, SP, Brazil [18]. Quartz was used as an external standard.
Infrared spectra were obtained using KBr pellets, in the range of 2000 to 400 cm 1 (spectrophotometer BOMEM, model MB–102).
The surface area measurements of the pigments were accomplished by a Micromeritics ASAP 2000 equipment, using N2 as the adsorption/desorption gas. The particle average
diameter was calculated using the BET method, dBET. Scanning electron microscopy (ZEISS DSM, 940) was used to characterize the pigment morphology. In the laboratory test, pigments were applied on ceramic pieces. A mixture of glaze (commercial glaze—GERBI, Brazil) and 3% of sieved pigment was used (mass ratio). The mixture
was homogenized in a ball mill during 10 min. The slip was poured on the ceramic biscuits obtaining an uniform glaze layer, which was then heat treated up to 500 8C with heating rate of 10 8C min 1, which was then increased to 15 8C min 1
up to 1000 8C for 1 h. Than, the furnace was cooled back to room temperature at 10 8C min 1.
Colorimetric parameters (L*, a* and b*) and diffuse reflectance of powders and glazed samples were measured with the Gretac Macbeth Color-eye spectrophotometer 2180/2180 UV, from 300 to 800 nm, using the D65 illuminant withmeasurement at 88. The CIE-L*a*b* colorimetric system,recommended by the CIE (Commission Internationale de l’Eclairage) [19] was followed. In this system, L* is the lightness axis (where black is equal to 0 and white to 100), b*represents the color varying from blue (negative axis) to yellow (positive axis), a* represents the color varying from green
(negative axis) to red (positive axis).
3. Results and discussion
Fig. 2 illustrates the XRD patterns of the materials synthesized by the Pechini method, calcined at different temperatures.
The samples present a single phase above 700 8C, identified as an orthorhombic spinel like phase space group-Bb21m (36), according to the index card JCPDS 46–0113, whose lattice parameters are: a = 19.042 A ° , b = 5.3838 A ° and c = 8.4445 A ° .
At 500 8C, a non-identified intermediate phase is also observed, which disappears at 700 8C. Castro et al. synthesized barium monoferrite, using the combustion method. Fe3O4, Ba(NO3)2 and BaCO3 were found as intermediate phases. After calcination at 700 8C, Fe3O4 and BaCO3 can still be found [5].
BaFe2O4 shows a tunnel structure with iron having a tetrahedral coordination. Corners of the FeO4-tetrahedra change their directions blockwise in one layer parallel to the a/b-plane. As a result, large and long tunnels are created by 12
FeO4-tetrahedra in the tetrahedra network of orthorhombic- BaFe2O4. Each of the large tunnels contains two Ba2+ (Ba(1)) atoms (double tunnels). In addition, smaller compressed quadrangular tunnels exist containing exclusively Ba2+
(Ba(2)) (single tunnels) and those tunnels which are too small for an intercalation of Ba2+ (vacant tunnels). The sequence double tunnel–single tunnel–vacant tunnel–single tunnel repeats along the a-axis. Ba(1) shows a monocapped trigonal
prismatic oxygen surrounding in the large tunnels. Along the tunnel direction there are no connections between these BaOpolyhedra.
The surrounding of Ba(2) is different, as soon as it shows edge-sharing BaO-polyhedra resulting in [BaO]-chains along the tunnels [20].
The BaFe2O4 structure leads to a different ligand field for
iron, when compared to usual hematite pigments. For hematite,
iron is surrounded by six oxygens in octahedral coordination. In
barium monoferrite, iron is in tetrahedral coordination. As a
consequence a different splitting of the five d orbitals is
observed, leading to different colors [21].
Table 1 presents the values of the lattice parameters and the
unit cell volume of the system in study. It is observed that the
unit cell volume increases with temperature in a more effective
way from 700 to 800 8C, getting almost constant above this
temperature.
The absorption vibrations of the system BaFe2O4, Fig. 3,
consist of well-defined bands, as follows:
Bands at 1460 and 850 cm 1, which decrease their intensity
with temperature increase, while bands at 1753, 1060 and
700 cm 1 disappear at 1000 8C. According to literature
results, BaCO3 presents bands at 1750, 1460, 1060, 860 and
700 cm 1 [22], while FeCO3 presents bands at 1523, 876 and
736 cm 1 [23,24]. These results indicate the presence of
carbonates in the present material. These carbonate bands
were not observed in XRD patterns, due to their low
resolution.
Bands at 600 and 450 cm 1 get well defined at higher
temperatures. These bands are assigned to v1 and v2
vibrations of spinels [25–27], which belong to the same
T1u representation [28,29]. Similar results were obtained by
Gonza´lez-Carren˜o et al. [14], which found bands at 586 and
434 cm 1, for hexaferrite BaFe12O19.
Results of crystallite size and particle diameter (BET), as a
function of temperature, are also presented in Table 1. An
increase of both parameters with temperature is observed, but
while crystallite size increases 70%, particle size increases
400%. This result is evidenced by the number of crystallites in
each particle, which increases from 4.8 to 14.5, indicating the
particle sintering. This is confirmed by SEM analysis (Fig. 4),
whose photomicrographs indicate the presence of aggregates,
with particle coalescence.
The color analysis of BaFe2O4, calcined at different
temperatures, was obtained correlating the results of reflectance
in the visible region (reflected wavelength) with colorimetric
coordinates (tonality variation, brightness and saturation).
Fig. 5 illustrates the diffuse reflectance of the pigments
before and after laboratory test. All curves present higher
reflection from 650 to 750 nm. It may be observed that
reflectance increases from 600 to 800 8C, probably due to
carbonate amount decrease, as observed by infrared spectroscopy
(Fig. 3). The decrease in reflectance observed at higher
temperatures is probably due to the sintering among particles,
observed in BET (Table 1) and SEM (Fig. 4) results.
It should be observed that curve profile is different from
other ferrite results. Ferrite reflectance usually presents a band
between 650 and 780 nm [3,30]. For BaFe2O4, there is no high
reflectance plateau. Otherwise, reflectance increases continuously
up to 750 nm. This may be due to the iron ligand field. For
the other ferrites, Fe2+ and Fe3+ are present in octahedral and
tetrahedral coordination, while, in barium ferrite, iron is only in
tetrahedral coordination. It should be emphasized that
octahedral ligand field leads to a higher splitting of d orbitals
than tetrahedral one. Consequently, electron transition occurs
with a higher energy absorption (or a smaller wavelength) [21].
As a consequence, the resulting color is a direct evidence of
Fe3+ or Fe2+ ions coordinated by oxygen [31,32]. This result
may also be observed in Fig. 6, which presents the colorimetric
coordinates of pigments before and after laboratory test. After
laboratory test, the ceramic pieces presented a good aesthetic
aspect, without defects as bubbles, superficial texture, among
others.
Results presented in Fig. 6 indicate that L* and b*
coordinates increase up to 700 8C, decreasing at higher
temperatures. As stated before, for diffuse reflectance results,
the increase may be due to carbonate elimination, while the
decrease may be due to sintering among particles. A continuous
decrease with temperature was observed for a* parameter.
According to Garcı´a et al. [33], the iron (III) oxides suffer a
reduction to Fe2+ ion, promoted by the emission of molecular
oxygen according to Eq. (1), when fired at high temperature or
when in the presence of unoxidized organic material. In the
present case, this unoxidized organic material is in the form of
carbonate and may lead to Fe3+ reduction, changing the
pigment color.
3Fe2O3!2Fe3O4 þ 1
2O2 (1)
The laboratory test, done with pigment calcined at 1000 8C
(Figs. 5 and 6), indicates that the pigment is chemically and
thermally stable, as no surface defects were observed.
Comparing the colorimetric coordinates of the pigment
calcined at 1000 8C before and after laboratory test, a small
change in tonality may be observed (DH* = 2.8) while the
saturation decrease (DC* = 4.4) and the lightness increase
(DL* = 5.3) are more important.
4. Conclusion
The pigment BaFe2O4 was obtained by the polymeric
precursor method, with single phase and brown color. Diffuse
reflectance and chromatic coordinates results indicate that
carbonate presence as well as sintering among particles change
the color, leading to its variation as a function of the heat
treatment of the pigment precursor. Differences between UV–
vis spectra of BaFe2O4 and other ferrites are probably due to the
iron ligand field—while the former presents iron in tetrahedral
sites, the latter present iron in octahedral and tetrahedral sites.
The pigment presents a suitable technological behavior without
reactions between glaze and pigment, indicating that powders
are chemically and thermally inert up to 1000 8C.
