In this research ceramic pigments have been synthesized with crystalline spinel structure and chromium based with a stoichiometry ACr 2O 4. A was an element with +2 valence metal, in this case, metals were zinc and iron, these pigments have been synthesized by non-conventional methods like the co-precipitation assisted by ultrasound and high milling energy. Pigments were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), spectrophotometry, and colorimetric CIELab method. Results showed that it was possible to obtain a crystallin desired structure at temperatures below 900°C by non-conventional methods. These results showed the advantages of ceramic pigments obtained by alternative routes, because it was possible to have a better control over stoichiometry and colorimetric structure properties. Furthermore, they were obtained at temperatures lower than those used by the traditional ceramic route.
Ceramic pigments have been used in the ceramic industry to decorate plates, tiles, floors, and walls, for this reason they are very important in the construction industry. Ceramic pigments are inorganic chemical compounds, which provide color and are insoluble in a medium with which they do not interact physically or chemically [
Ceramic pigments have different crystal structures, such as the spinel structure. Spinel type structures feature high strength, high thermal stability, low sintering temperature and easy incorporation of chromophore ions in the lattice, obtaining different colored pigments [
Traditionally, the synthesis of ceramic pigments has been led by the conventional ceramic way, but in recent years there has been a rise in the implementation of non-conventional methods. One reason for this is the importance of decreasing the temperature and time of the heat treatment with the corresponding reduction in gas emissions. Most synthesis methods employed are mainly solid-state reaction [
In this work, the methods employed were ultrasound-assisted co-precipitation and high-energy milling.
Co-precipitation is important to avoid many environmental problems related to water resources, concentrations of metals in aqueous systems and technology for wastewater treatment. Co-precipitation varies in each case and the partition in the solid phase can be accomplished by surface adsorption, ion exchange, precipitation of the surface, and formation of occlusion solid solution [
Mechanochemical synthesis involves high energy milling of the reactants powders; in this case they are oxides. Its effect is to induce or directly activate the chemical reaction in a subsequent heat treatment at a relatively low temperature. Types of high-energy grinding are distinguished by the nature of the reagents and chemicals and the structural changes during the process [
For this synthesis of ceramic pigments, the structure is defined as ACr2O4, where A is a metal with valence +2, for this case metals are Fe and Zn.
The raw materials used were: FeCl2∙4H2O PANREAC, ZnCl2 CARLOERBA, CrCl3∙6H2O MERCK. Reagent grade were used. With these salts, aqueous solution was prepared using distilled water, at a temperature of 70˚C. This solution was added drop wise to an aqueous solution containing NaOH MERCK (reagent grade), for pH adjustment, (pH was between 11 and 12), KNO3 as an oxidizing agent, (reagent grade, MALLINKRODT), at the same conditions. This solution was placed in ultrasound equipment and left for one hour, then allowed to stand for a few minutes and was vacuum filtered. The precipitate was washed with water and ethanol to remove soluble salts and again was filtered. The product was dried in an electric stove at 100˚C for 12 hours, and then the sample was subjected to a heat treatment for 8 hours at 700˚C. The heating rate was 5˚C per minute.
The number of MERCK raw materials were two; NaOH and CrCl3∙6H2O.
The raw materials were industrial grade Fe2O3, ZnO and Cr2O3. These oxides were ground in a suspension with a ratio of 40% of solids. This suspension was added to the grinding vessel containing alumina balls and placed in a planetary mill. The rotational speed was 250 rpm during 2 hours. After that, the suspension was dried at 100˚C for 24 hours, then, the powders were de-agglomerated and sieved by sieve #100. Finally, the sample was subjected to a heat treatment for 8 hours at 700˚C. The heating rate was 5˚C per minute.
Crystalline phases were determined by X-ray diffraction (XRD) in a PANalytical diffractometer, model XPert- Pro, using CuKα radiation of 1.54060 Å, from 4˚ to 75˚, at a rate of 0.02˚/min and 400 cps, and analyzed with the software X’Pert Highscore Plus.
Morphology was observed with a scanning electron microscope (SEM), JEOL model 5410 by backscattered electrons and secondary electrons mode, BE and SE respectively.
The spectral reflectance and the colorimetric coordinates were measured in a spectrophotometer UV-VIS-NIR, Glacier TM X with a CCD linear array, ranging from 200 nm to 1025 nm, the illuminant was D65 CIE and ASTM E1349-06 [
In
Sample | Coprec-Asist Ultrasound | High milling energy | Time |
---|---|---|---|
ZnCr2O4 | ZCO3 | ZCO25278 | 8 h/700˚C |
FeCr2O4 | FCO3 | FCO25278 | 8 h/700˚C |
exhibit big agglomerates but instead each particle can be easily distinguished, also one can observe a homogeneous particle size. b) This picture was obtained at 5000× magnification, the homogeneity of the sample can be seen, since the image taken was in BE mode, and the particles do not have differences in shades, this indicates the uniformity of the pigment’s composition.
In
In
In
In
L* | a* | b* |
---|---|---|
64.105 | −9.087 | 22.866 |
L* | a* | b* |
---|---|---|
15.754 | 17.447 | 19.523 |
15 showing that the sample is dark, a* and b* are positive, corresponding to red and yellow respectively, yielding a brown-black color.
In
In
In
L* | a* | b* |
---|---|---|
78.134 | −3.683 | 16.632 |
presence of yellow.
In
In
In
be observed, because no differences are observed in the hues, the image was taken at 5000× magnification.
In
In
In
L* | a* | b* |
---|---|---|
42.405 | 24.219 | 38.282 |
The evaluation of the two unconventional synthesis routes was carried out; the method that yielded better results is based on ultrasound-assisted co-precipitation, because the expected color was achieved, in each case, green- gray for ZnCr2O4 and black-brown for FeCr2O4, the desired crystalline phases, spinels, a homogeneous particle size less than 1 μm, with the correct morphology and homogeneous chemical composition. The method of high energy milling also obtained a spinel phase and homogeneity of particle sizes, however, there was an incomplete reaction, since oxides were present in a secondary phase and also the hue in the case of FeCr2O4 was not as expected. This is because the co-precipitation method from salt solutions which contain scattered ions enables the more efficient thermal treatment of the reaction, while high energy milling needs to break the bonds of the oxides first which then split and form new links to the desired crystalline phase and quite possibly the applied energy has not been sufficient to achieve what was intended, although it was noted that the sizes are small which helps to increase the reactivity in the process, which may also be required for another body grinder to achieve greater energy to form the spinel phase.
According to the results presented, pigments obtained via ultrasound assisted co-precipitation and high-energy milling showed that very good properties were obtained, since all obtained the spinel crystal phase, the tones of pigments were within expectations with small deviations and homogeneity in particle size and the composition were good. The co-precipitation method assisted by ultrasound showed the best result in every way.
In the ultrasound assisted co-precipitation method, ceramic pigments were obtained with the desired stoichiometry and structure. Generally the pigments obtained by this method showed particle sizes of about 1 micron or less, good homogeneity in morphology and chemical composition. The reflectance factor and colorimetric coordinate’s characteristics exhibited by the pigments were comparable to products obtained by traditional routes in the industry which are obtained at temperatures above 1000˚C. Therefore this method achieved good quality pigments obtained at lower temperatures and in less time than the heat treatment in the conventional ceramic method.
Regarding the method of high-energy milling, although all samples exhibited spinel phase pigments, they also exhibited secondary phases of oxides. This may be because the energy applied, or the grinding time was not enough for all the materials to react as expected and this was evident in the color of FCO25278, because a darker tone was expected between brown and black, however, a dark red was observed. The results of the experiments also show that the reaction between iron and chromium required more energy for the ion migration compared to zinc and chromium. But the high-energy milling method cannot be ruled out, because good results may be achieved by optimizing some parameters, such as time of grinding, the mill speed, the ratio of solids and also the type of balls, and then a pigment with the right features for this type of structure may be obtained. Also the method could also be used for other systems where it could provide better pigmenting results.