Characterizing thermal-oxidation behaviors of nuclear graphite by combining O2 supply and micro surface area of graphite

Abstract

The effects of different parameters on oxidation rate are non-linear, interactive and diversified in which the change of adequacy of O2 supply is an important indicator. The influence of microstructure on oxidation rate became stronger worsening the fitting linearity to calculate the activation energy based on present method with the decreased adequacy of O2 supply due to the increase of temperature, the decrease of gas flow rate, etc. Here, we proposed a method to characterize thermal-oxidation behaviors of nuclear graphite by combining O2 supply and micro surface area of graphite. The proposed method improved the linearity and reduced the standard error of Arrhenius plots of oxidized graphite IG-110 (10 L/min reactant gas) and ET-10 (0.2 L/min reactant gas). The value of activation energy of graphite IG-110 oxidized under ASTM D7542 condition is calculated as 220 kJ/mol by this method echoing the results of previous studies with sufficient O2 supply. For the conditions with less O2 supply at low gas flow rate and/or high temperature, the change of microstructure of oxidized graphite should be obtained as an important factor influencing oxidation rate of graphite.

Oxidation Experiments and Kinetics Analysis of Nuclear Graphite ET-10 by Gas Analysis and Microstructure Observation

“…The ratio of oxygen supply to oxygen consumed by graphite oxidation is often considered as an indicator of whether the oxidation is in the kinetic-controlled regime [14,29]. Here, we calculated the minimum value of this ratio in an oxidation experiment obtained by Equation ( 8) as follows:…”

“…The specimen provided by IBIDEN Co. Ltd. is a right cylinder measuring 25.4 mm in height and 25.4 mm in diameter. Graphite ET-10 has the properties of low impurities and fine grains [29].…”

“…In the chemical kinetics-controlled regime, the active surface area (ASA) is usually determined by the initial ASA and the mass loss, which is independent of the oxidation temperature. Therefore, when the mass loss of the specimen is limited in a certain range, e.g., 5-10%, the changes of the oxidation rates at different temperatures can indicate the evolution of the oxidation behavior between the three regimes [14,29,31]. In this manner, the variations of the oxidation rates and the microstructures of the specimens can help to observe the evolution of the oxidation behavior.…”

“…The oxidation of graphite ET-10 started when the temperature reaches 700 • C [19], which is higher than the predicted temperature by ASTM D7542. Studies on other graphite showed that the differences in gas flow rates and the oxidation temperatures resulted in diversified oxidation behaviors [19][20][21], which can be interpreted by the change of oxygen supply and the specimen's microstructure [14,29].…”

Graphite-based solid lubricant for high-temperature lubrication

“…Besides, its high thermal conductivity further facilitates its widespread application in hightemperature processing procedures [12]. In spite of the aforementioned advantages in the lubricating field, however, graphite is inferior in oxygen resistance at the elevated temperature in the ambient atmosphere [13][14][15]. Specifically, graphite is easily oxidized to carbon monoxide (CO) beyond 400 ℃ and carbon dioxide (CO 2 ) above 500 ℃.…”