Oxidation Behavior of Matrix Graphite and Its Effect on Compressive Strength

Abstract

Matrix graphite (MG) with incompletely graphitized binder used in high-temperature gas-cooled reactors (HTGRs) is commonly suspected to exhibit lower oxidation resistance in air. In order to reveal the oxidation performance, the oxidation behavior of newly developed A3-3 MG at the temperature range from 500 to 950 ∘ C in air was studied and the effect of oxidation on the compressive strength of oxidized MG specimens was characterized. Results show that temperature has a significant influence on the oxidation behavior of MG. The transition temperature between Regimes I and II is ∼700 ∘ C and the activation energy ( ) in Regime I is around 185 kJ/mol, a little lower than that of nuclear graphite, which indicates MG is more vulnerable to oxidation. Oxidation at 550 ∘ C causes more damage to compressive strength of MG than oxidation at 900 ∘ C. Comparing with the strength of pristine MG specimens, the rate of compressive strength loss is 77.3% after oxidation at 550 ∘ C and only 12.5% for oxidation at 900 ∘ C. Microstructure images of SEM and porosity measurement by Mercury Porosimetry indicate that the significant compressive strength loss of MG oxidized at 550 ∘ C may be attributed to both the uniform pore formation throughout the bulk and the preferential oxidation of the binder.

Effects of microstructure on the oxidation behavior of A3 matrix‐grade graphite

“…This observation can be explained by the incomplete graphitization of the binder at 1800°C. 4,[7][8][9]13 Although this temperature is below temperatures required for complete graphitization, the lower temperature is necessary to prevent uranium from diffusing out of the fuel kernel. 13,17 Indications of the anisotropic nature (ie, the tendency of the graphite flake to align in the same direction) of the stacking of graphite sheets were identified in the bright-field images ( Figures 1A and 2B) and supported by the selected area electron diffraction (SAED) shown in Figure 2D in which the (2000) reflection displays two arcs rather than one full ring, though additional SAED patterns were not acquired.…”

“…The higher oxidation rate of PGC is consistent with the previous work demonstrating that the oxidation of PGC has a lower activation energy than that of graphite. 4,8,13 It could be inferred that the inflection point in Figure 6B around 685°C is a peak in PGC oxidation rate, above which point oxidation of PGC diminishes and is outweighed by Regime I oxidation of graphite. Therefore, these results support the argument that the oxidation of PGC is predominant over that of graphite below ~700°C.…”

“…8 There have been a significant number of studies of oxidation of nuclear graphite in air, [9][10][11][12] however, very limited studies have been conducted on matrix graphite. 4,13 Due to the significant difference in content of PGC, matrix graphite oxidizes at lower temperatures and at higher rates than nuclear grade graphite. 12 Moormann et al studied oxidation of A3-27 matrix graphite and compared it to V4382 nuclear graphite and found that the higher oxidation rate of the A3-27 was due to the lower activation energy of the binder material.…”

The Degradation of Strength under Varying Oxidizing Conditions for Nuclear Graphite

“…Sample trimming has been utilized previously with good results. [25] A representation of samples tested after trimming can be seen in Figure 3. These samples show similar failures to unoxidized graphite, thus demonstrating that the applied stress has been distributed throughout the specimen volume.…”

Emission characteristics of ultrafine particles from bare and Al2O3 coated graphite for high temperature applications

“…Oxidation reaction was observed to be controlled by chemical kinetics at lower temperature and in-pore diffusion at higher temperature 25 – 27 . Arrhenius plots have been used for the estimation of activation energies during the oxidation process 28 , 29 . The transition temperature for in-pore diffusion depends on impurities, density and microstructure of graphite 30 .…”