Abstract
Abstract: Due to their rapid, selective, and volumetric heating, microwaves have been widely used in the past to enhance solid-state reactions as well as the synthesis of ceramic pigments. The aim of this work is to present a case study involving the preparation of blue CoAl2O4 pigment using different microwave applicators and generator frequencies, showing the advantages which can derive from a properly designed microwave reactor for the solid-state synthesis of such pigment. The results show that, when using a properly designed microwave applicator, the specific energy consumption can be significantly lowered compared to conventional heating techniques. Consistently with the data on the dielectric properties of precursors found in literature, the microwave processing of cobalt oxide and aluminum hydroxide mixtures resulted more favorable at 2.45 GHz than at 5.8 GHz microwave frequency.
Highlights
The trend of increasing the firing temperature of ceramic products to achieve outstanding mechanical properties and extremely low porosity during sintering requires the use of thermally stable pigments
The as-synthesized pigment is comparable to industrial standards, but it was obtained in much less time, without mineralizers and with a lower specific energy consumption
Heating curves of samples processed in single mode applicators are similar, but at high temperatures a higher microwave power absorption occurs in the case of a 2.45-GHz generator
Summary
The trend of increasing the firing temperature of ceramic products to achieve outstanding mechanical properties and extremely low porosity during sintering requires the use of thermally stable pigments. The main requirements for this class of pigments include high coloring capabilities, even if used in small amounts, and heat stability, accompanied by a resistance to dissolution, chemical agents, and solvent attack [1]. Synthetic crystalline oxide compounds of two or more different metals are candidate materials for such an application, and they have been extensively used during the past few decades. The wide range of colors for these pigments is due to the presence of transition metals (V, Cr, Mn, Fe, Co, Cu) or rare earth metals (Pr, Nd) in oxidic crystalline phases known for their thermal stability [2]. Past research has generally focused on two main directions: an economization and improvement of existing pigments or the discovery and implementation of new compounds with higher performing characteristics. From an environmental point of view, the possible absence of additives used to enhance the solid-state reactivity, known as mineralizers, makes alternative synthetic routes preferable
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