Petroleum cokes as pore generators in ceramic electrodes: Limitations for their use

The use of pore generators is a classical way to adjust the pore size distribution of porous ceramics. However, they must be selected to alter the processing of the ceramic as little as possible. Petroleum coke is an attractive pore generator for some ceramic compositions hard to process with other pore generators as starches. The analysis of the behaviour of the two main varieties, green coke and calcined coke, as pore generators in ceramic electrodes of Sb-doped SnO 2 have shown that they allow the shaping of the electrodes by dry-pressing, but proportions higher than 10 wt% require great care in the process. The design of the sintering cycle is restricted by their thermal behaviour to a heating rate ≤ 2 °C·min −1 in the 150–600 °C interval to avoid defects in the sintered electrodes. Green coke undergoes a sudden expansion around 270 °C that is absent in calcined coke, but calcined coke oxidises at lower rate and its remainder can reduce SnO 2 to Sn at higher temperatures. Calcined coke generates lower porosity in the sintered specimens than green coke at equal proportion in the raw materials mixture and requires higher proportions to obtain a bimodal pore size distribution. In addition, the type of coke has no definite effect on the properties of the sintered ceramic. In this work, the critical property, electrical resistivity of the electrodes, is mainly related to the generated porosity. The described limitations must be considered prior to select any of two cokes as pore generator in ceramics.