Application of a three-dimensional random pore model for thermal oxidation of synthetic graphite

Abstract

Thermal oxidation mass loss of synthetic graphite can be life-limiting for high-temperature applications. The complex microstructure of synthetic graphite has made it difficult to predict oxidation-induced changes.Specifically, there are no accepted models for predicting oxidation pore growth, which affects properties and performance.This paper presents the application of a three-dimensional microstructure-based model for synthetic graphite oxidation in the kinetic regime. Pore structure is described with randomly-placed spheres, whose growth rate is dependent on the penetration depth of oxidant. Pore structure, intrinsic reactivity, sample size, geometry, and initially closed porosity are included in the model. Oxidation mass loss occurs faster for finer pore size, lower bulk density, higher reactivity, and/or smaller samples.For model validation, 30 datasets representing ten (mostly nuclear) high-purity graphite grades were selected, covering air oxidation temperatures from 450 to 750 °C and sample masses spanning four orders of magnitude. Agreements between model and experiment were judged reasonable, particularly the effects of microstructure and sample mass. For the temperature effect, aggregated reactivity ratios showed an activation energy of 187 kJ/mol, within expected kinetic range. To the author's knowledge, this model is the first three-dimensional microstructure-based approach for graphite air oxidation in the kinetic regime.