The Preparation And High-temperature Oxidation Resistance Of Encapsulated Carbon Black Pigment

The cobalt-black pigment has been widely used in the ceramics. But due todisadvantages of the traditional pigment such as limited in resources, high-costing and the riskof heavy-metal ion releasing, people are expecting a new one which is bothenvironmental-friendly and cheaper. Carbon black(CB) performs the strong ability in blackcoloring in many areas but the high-temperature circumstance for its easy conversion intocarbon dioxide and depigments. In this research, we tried to encapsulate the CB with a layerof silicon dioxide or zircon in order to enhance the high-temperature oxidation resistance andprevent CB from burning out in glaze.CB dispersion liquid with the particle size below100nm was prepared using PVP as thepolymer surface. The three methods of acid-alkali two steps method, microemulsion methodand emulsion gelation method were applied to coat the CB with a shell of silicondioxide(CB@SiO2). The experimental results show that the third composites exhibit the betterhigh-temperature stability and the emulsion gelation method is the best one. The existence ofchemical bonds of O-Si-C on the interface between of SiO2and CB were identified by XPSand FTIR which was beneficial for the bonding strength. Heat treatment for vitrificationenforced the shelldensification and sealed the pores for the oxygen transportation which wasplaying a key role in improving the high temperature oxidation resistance performance. Wealso found that the amorphous SiO2layer would not crystallize in the heating process whichwould be beneficial for the volume stability.There was an exothermic peak in the range of1100℃to1200℃from TG-DSC curves ofthe CB@SiO2composites, it could be attributed to the melting down of the SiO2layer,andthus, the pre-coated CB were exposed again in the air and oxidated. In the study about theeffect of vitrification process on the high temperature stability of CB@SiO2, we found that,the purity and thickness of the SiO2shell varied while the vitrification time and vitrificationtemperature were adjusted. When the composites were treated at a lower temperature or for ashort term, the shell was thick but impure, and as a result, it represented the good oxidationresistance in the lower temperature-treatment period, but it performed not so good at hightemperature. In the reversed case, it exhibited the opposite result. We also found that when thecomposites were vitrificated in the protection of Ar2rather than in air, there were much moreCB left which led to a lower L value. But, because of the incomplete release of the organicstuffs, the impurities in the coating layer increased which resulted in poor oxidation resistanceat high temperature. The CB powder coated by zircon(CB@ZrSiO4)were prepared by the method of solco-precipitation. The effect of mineralizers and temperature schedule on the performance ofthe composites. The composites synthesized with the combined mineralizer of NaF and CeOhad perfect crystalloid of zircon of square pyramid shape, but it performed poorly to protectethe CB from burning out. For the composites mineralized by NaF or LiF respectively, thezircon crystals were sphere stone-like shape. It was speculated that CB were embedded in thecore of zircon in the forming process. The conversion of the ZrO2to ZrSiO4was improvedwhen the calcining temperature increased, and there were not ZrO2left while it was treated at1200℃for one hour, but it made only a slight improvement in the oxidation resistance thanthat at1150℃。The heating rate of10℃/min is reasonable for the composites synthesis. Thelower heating rate would reduce the defects of the ZrO2crystals that would lead to the weakreaction activity and slow down the formation of ZrSiO4.In order to test the performance of the composites in glaze, CB@SiO2and CB@ZrSiO4were fired with the low temperature frit in the proportion of10wt%respectively. The blackglazed ceramics both showed the lower L values when heated to950℃at the rate of40℃/min.The higher rate would destroy the SiO2shell drastically because of the mismatch in thermalcoefficient between SiO2and CB. The shell of SiO2was eroded more fiercely by meltingglass when the temperature increased to1000℃, as a result, the protection function of theSiO2layer of was destroyed. In contrast, the composites of CB@ZrSiO4had better stability,therefore it performed well at high temperature.