SYNTHESIS OF Fe x Zn 1-x Cr 2 O 4 BROWN CERAMIC PIGMENT BY STARCH ASSISTED SOL-GEL PROCESS

. In the present paper, the ceramic pigments Fe x Zn 1-x Cr 2 O 4 (x = 0  1) were synthesized with the starch-assisted sol-gel method. The resulting pigments were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and CIE L*a*b* color measurement. The results show that Fe x Zn 1-x Cr 2 O 4 pigments form at sintering temperature of 1100 O C for 60 minutes. The ACr 2 O 4 spinel (A: Zn, Fe) and FeCrO 3 perovskite phase with excellent crystallinity appears. The brown color intensity increases gradually with the increase of the number of substituted Fe 2+ cations. The pigments meet industrial requirements in terms of physicochemical


Introduction
Pigments play an important role in the fabrication of enamel used for ceramic and glass. One of the important physico-mechanical properties of inorganic pigment is the colour fastness at high temperature. Therefore, inorganic pigments are usually synthesized on a stable substrate such as spinel, mullite, cordierite, perovskite, etc. AB2O4 spinel has a face-centered cubic lattice formed from O 2-anions, where, A 2+ and B 3+ cations locates in tetrahedron and octahedron holes, respectively [1]. With a stable crystal structure, spinel has many wonderful physicochemical properties such as high mechanical stability, thermal endurance and chemical durability. ZnCr2O4 brown pigment is stable at high temperatures, under light and in chemicals such as acid and base. Therefore, it is usually used to produce: magnetic material [2], ceramic pigments [3,4], luminescent material [5,6], gas sensor [7], humidity sensor [6], photocatalyst [8], etc.
ZnCr2O4 spinel is mainly synthesized with the traditional ceramic method from ZnO and Cr2O3 oxides. In this method, raw materials are mechanically mixed and sintered at high temperature. The product has an unidentical and large size and many phases [9]. To reduce the sintering temperature, ZnCr2O4 spinel is currently synthesized with wet methods such as sol-gel [2,10], precursor method [11], burning [5,12], coprecipitation [13], ultrasonic and high energy ball milling combined co-precipitation [14], microemulsion [7], hydrothermal method [15], hydrothermal combined co-precipitation [16]. The starch-assisted sol-gel process exhibits many advantages such as (i) easy to control the regular proportion of product; (ii) metal cations equally disperse in starch to form a metal-starch complex; (iii) the size of raw material particles are reduced; (iv) contact surface area between reactants is enhanced. This facilitate the solid-phase reaction at low temperature [17].
In the present study, the synthesis of In the starch-assisted sol-gel process, the where λ is the X-ray wavelength (Å); θ is the diffraction angle (rad) of the (311)      The XRD patterns of ST1100 and GT1100 are shown in Fig. 4. With the GT1100 sample, the main crystal phase is Cr2O3, and the spinel phase appears with low intensity. The solid-phase reaction in the traditional method mostly occurs at more than 1200 O C, which is very high. This demonstrates that the starch-assisted sol-gel method facilitated solid-phase reaction because of the identical distribution of metal oxides [8,17]. Fig. 4. XRD pattern of ST1100 and GT1100 samples

Preparation of FexZn1-xCr2O4 pigment
The composition and regular formula of pigment samples, image and color intensity of tile samples after enameling are shown in Table 2   The XRD pattern of FexZn1-xCr2O4 pigment  When isomorphous Zn 2+ in the spinel lattice is replaced by Fe 2+ , the crystal phase composition of the product is multi-phase including spinel and Cr2O3 can form both spinel and perovskite.
The morphology and particle size of Fe0.5Zn0.5Cr2O4 pigment prepared at 1100 O C for 60 min (Fig. 7) show that the product has a uniform particle size with a diameter varying from 280 to 420 nm. The particles do not agglomerate into a large bulk and the boundary between particles is clear. The nanometer-sized pigment synthesized with the starch-assisted sol-gel method can be used in the pigment inkjet technology onto the surface of the product that is a current hot topic in ceramic production [10].

Conclusion
FexZn1-xCr2O4 pigments (x varying from 0 to 1) were successfully synthesized with the starchassisted sol-gel method. The suitable sintering temperature is 1100 O C which is lower than that of the traditional method. The obtained product exhibits the main crystal phase of spinel, where x is 0 and x is 1. When 0 < x < 1, the perovskite phase appears beside spinel. The enamel on tiles is brown, smooth with no air bubbles nor disabilities, such as enamel shrink and enamel rift.
The color depends on the amount of iron. The pigment is suitable for the production of enamel and meets the industrial requirements of ceramic production.