Synthesis reproducibility of mixed spinels Fe 1-Ψ Co Ψ Cr 2-2Ψ Al 2Ψ O 4 (0≤Ψ≤1), obtained by Solution Combustion Synthesis using urea as fuel, has been studied. Pigments with spinel structure Fd-3m have been obtained for all the compositional range analyzed. Characteristics such as crystallinity, cell parameter, crystal size and specific surface area show a noticeable dependence with Ψ, but some of them present a low reproducibility, indicating a pronounced dependency with process conditions in each batch.Colouring power of synthesized pigments is highly significant, so they can be directly integrated in ceramic glasses without introducing a second thermal treatment. However, the generated colour also suffers from a limited reproducibility.
IntroductionSolution combustion synthesis (SCS) is an innovative method to synthesize ceramic pigments, which allows obtaining materials in a direct pathway, with a low particle size, using short reaction times and moderate temperatures [1].Experimental results reported on this topic show that the traditional methodology for preparation of spinels via ceramic method involves annealing at temperatures up to 1400ºC and long soaking times [2]. During this thermal treatment, solid state reactions take place and the chromophore cations are incorporated into the newly generated host lattice. However, due to the elevated temperature reached during the process, the resulting pigment presents low specific surface area and high particle size, which is reflected in a low colouring power. For this reason, in the industrial practice a milling operation of the pigment is mandatory in order to obtain a marketable product. In addition, the ink-jet technology applied to ceramic decoration requires pigments with a particle size around 500 nm, which supposes an increase in the cost of the milling stage and a change in the milling technology. As a consequence, there is a growing demand for alternative routes for synthesizing pigments that give directly particle sizes near to the interval recommended for ink-jet machines. SCS is a method suitable to overcome some important disadvantages of ceramic pigment traditional processing because it could generate pigments as low cohesive masses formed of nanograins, easier to mill up to the submicrometer interval.3 SCS method begins with a concentrated aqueous solution of precursors of the desired product (being the nitrates the most frequently used), and a suitable fuel (glycine, urea, hexamethylentetraamine, hydrazine and their derivates, etc.). The solution is heated at high rate in order to evaporate all water and, afterwards, to ignite the fuel present in the solid residue, which supplies enough energy to carry out the synthesis reaction. The reaction product is frequently obtained as a spongy mass.SCS has been used to synthesize large number of inorganic materials, either simple Most of the spinel-type ceramic industrial pigments include more than three metals in their composition to adjust the final colour. As a consequence, it was investi...
