Key Takeaways & Executive Findings
- •• • Critical transition at 1400 °C: Below this temperature, structural ordering occurs (ID/IG decreases, sp2 content increases), while above it, disorder increases (ID/IG increases, diffraction peak broadens). This demarcation is essential for predicting PPyC behavior in reactor operation. • • Lattice parameter shifts: At 1400 °C, a=2.4193 Å and c=3.4911 Å (most ordered); at 1600 °C, a=2.4347 Å (more disordered in a-axis) and c=3.4853 Å (more ordered in c-axis). These anisotropic changes indicate differential atomic migration along crystallographic axes, affecting dimensional stability. • • Porosity evolution: Below 1400 °C, closed porosity increases while mesopore volume decreases, indicating open-to-closed pore conversion. Above 1400 °C, closed porosity decreases and mesopore surface area increases, suggesting partial reopening of pores. This directly impacts fission gas retention and mechanical integrity. • • Onion-like carbon (OLC) with amorphous-core/crystalline-shell growth mode discovered, with three transformation stages. This nanostructural feature explains the anomalous volume expansion vs. thickness shrinkage contradiction, providing a mechanistic basis for optimizing TRISO fuel performance.
Abstract
Porous pyrolytic carbon (PPyC) serves as the buffer layer in TRi-structural ISOtropic (TRISO) fuel particles, providing storage for fission gases, preventing damage to outer layers, and absorbing stresses caused by fuel-kernel swelling. However, the changes of PPyC micro- and meso-structure at high temperatures remain insufficiently understood. In this study, PPyC fabricated by chemical vapor deposition was heat-treated from 1200 to 1600 °C and characterized across atomic-to-mesoscopic scales. Results show that the structure changes with temperature with a transition at approximately 1400 °C. Below 1400 °C, a decrease in Raman ID/IG ratio, narrowing of the graphite diffraction peak, and increased sp2 hybridization indicate progressive ordering associated with defect redistribution. Concurrent decreases in true density and mesopore volume, together with increased closed porosity, are consistent with partial conversion of open pores into closed pores. Above 1400 °C, increased ID/IG ratio, broadening of the diffraction peak near the rhombohedral graphite (101) reflection, and transition regions between crystalline and amorphous material observed by TEM indicate increasing structural disorder. Meanwhile, initially distinct PPyC particle boundaries blur and merge into broad, plate-like domains. Subsequent decrease in closed porosity and increase in mesopore surface area are consistent with partial connection of closed pores to the open-pore network. This work shows that intrinsic coupling between atomic-scale structural change and mesoscale pore connectivity provides a basis for assessing high-temperature structural stability of PPyC in TRISO fuel particles.
1. Introduction
TRISO-coated fuel particles are a cornerstone of fourth-generation high-temperature gas-cooled reactors (HTGRs), where the porous pyrolytic carbon (PPyC) buffer layer must accommodate fission-gas pressure and fuel-kernel swelling while maintaining structural integrity. However, post-irradiation examinations have consistently revealed microstructural degradation in the buffer layer, including densification, fracture, and interfacial separation. The as-fabricated PPyC exhibits pronounced heterogeneity with porosity gradients and complex pore networks, which critically governs its thermomechanical response. Despite progress in understanding thermal evolution of various carbon materials, the specific behavior of low-density PPyC under high-temperature treatment remains inadequately characterized, particularly the coupling between atomic-scale structural changes and mesoscale pore connectivity.
This study addresses that gap by systematically heat-treating PPyC from 1200 to 1600 °C and characterizing it across atomic-to-mesoscopic scales. The identification of 1400 °C as a critical transition temperature, where the dominant mechanism shifts from defect redistribution to disorder-induced vacancy migration, provides a quantitative basis for predicting structural stability. Furthermore, the discovery of an onion-like carbon growth mode and a microstructure-mesopore coupling mechanism resolves the contradiction between theoretical volume expansion and experimental thickness shrinkage. These findings not only deepen fundamental understanding but also offer practical guidance for optimizing TRISO fuel fabrication and performance under extreme conditions.
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Lei Jinhong, Yang Xu, Cheng Xing, Yang Hui, Zhang Kaihong, Yu Hao, Liu Xiaoxue, Zhao Hongsheng, Liu Bing (2026). Revealing Abnormal Micro- and Meso-Structure Evolution Mechanism of Porous Pyrolytic Carbon in TRISO Coated Fuel Particles under High-Temperature Treatment. New Carbon Materials. https://doi.org/10.1016/S1872-5805(26)61113-4
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Frequently Asked Questions
What is the critical temperature threshold for structural transition in PPyC, and how does it affect the material's performance in TRISO fuel?
The critical temperature is approximately 1400 °C. Below this, PPyC undergoes ordering with decreased ID/IG ratio and increased sp2 content, leading to densification and conversion of open pores to closed pores. Above 1400 °C, structural disorder increases, with broadening of the (101) diffraction peak and blurring of particle boundaries, accompanied by partial reopening of closed pores. This transition is crucial because it dictates the buffer layer's ability to retain fission gases and accommodate swelling; operating above 1400 °C may compromise its protective function.
How do lattice parameters evolve with temperature, and what are the implications for dimensional stability?
At 1400 °C, the lattice parameters are a=2.4193 Å and c=3.4911 Å, representing the most ordered state. At 1600 °C, a increases to 2.4347 Å (more disordered in a-axis) while c decreases to 3.4853 Å (more ordered in c-axis). This anisotropic change indicates that atomic migration along the a-axis becomes more significant at higher temperatures, potentially leading to anisotropic dimensional changes. This must be accounted for in fuel particle design to avoid stress-induced failure.
What is the onion-like carbon (OLC) structure discovered, and how does it form?
OLC with an amorphous-core/crystalline-shell growth mode was discovered. It forms through three transformation stages during heat treatment. This structure arises from the interplay between defect migration and atomic rearrangement, leading to a core-shell morphology. The OLC structure helps explain the anomalous volume expansion vs. thickness shrinkage behavior, as the amorphous core may accommodate strain while the crystalline shell provides structural integrity.
How does the pore structure evolve, and what are the implications for fission gas retention?
Below 1400 °C, closed porosity increases while mesopore volume decreases, indicating that open pores are being sealed off. This is beneficial for trapping fission gases. Above 1400 °C, closed porosity decreases and mesopore surface area increases, suggesting that closed pores reconnect to the open network, potentially allowing fission gas release. This transition is critical for predicting fuel performance under accident conditions.
What is the microstructure-mesopore coupling mechanism, and how does it resolve the volume expansion vs. thickness shrinkage contradiction?
The coupling mechanism links atomic-scale structural changes (e.g., lattice parameter shifts, defect migration) to mesoscale pore connectivity. The contradiction arises because theoretical volume expansion from atomic rearrangements is offset by pore collapse and densification, leading to net shrinkage. This mechanism provides a framework for modeling the thermomechanical behavior of PPyC, enabling more accurate predictions of TRISO fuel performance.
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