Microwave-assisted synthesis of Pr–ZrSiO4, V–ZrSiO4 and Cr–YAlO3 ceramic pigments

An innovative two-step route was successfully applied to the microwave-assisted synthesis of Pr–ZrSiO4, V–ZrSiO4 and Cr–YAlO3 ceramic pigments. It is able to reduce the total synthesis time from many hours down to a few minutes. Industrial batches were pelletized and underwent a short time pre-heating in conventional electric furnace (from 300 up to 1000 °C) prior to dielectric heating in microwave oven (2.45 GHz, 800 W continuous or paused). Pre-heating is necessary to reach a thermal level at which pigment precursors are activated under microwave irradiation (high enough tan δ of dielectric properties). Activation kinetics was followed by measuring (optical pyrometer) the external temperature of crucible during MW irradiation and the internal temperature at the end of heating process. Pigments were characterized by colourimetry (CIE Lab) and XRPD with Rietveld refinement. The V–ZrSiO4 system is easily activated, already at pre-heating of 300 °C, with good reaction yield and colour performance; nevertheless it is difficult to keep under control, as temperatures exceeding 1200 °C are rapidly reached. The Pr–ZrSiO4 system is activated after pre-heating over 600 °C and exhibits a slow kinetics, resulting in a low reaction yield and unsatisfactory colour. The Cr–YAlO3 pigment is formed with colour and reaction yield close to those of the correspondent industrial product, even though after pre-heating at 800 °C or more. A peculiar behaviour of silica phases was observed in zircon systems, particularly after lower pre-heating temperatures. It consists in an early transformation of quartz (present as precursor) into cristobalite and abundant amorphous phase, before the SiO2 + ZrO2 → ZrSiO4 transformation occurred.