Characterization of Porosity Development in Oxidized Graphite using Automated Image Analysis Techniques

Contescu, Cristian I¹  Burchell, Timothy D¹ 

[1]ORNL

关键词: GRAPHITE;    ORNL;    OXIDATION;    PORE STRUCTURE;    POROSITY;    IMAGES;    DATA ANALYSIS;    AUTOMATION;    MORPHOLOGICAL CHANGES;    TEMPERATURE DEPENDENCE graphite oxidation;    NGNP;    HGTR;   

DOI  :  10.2172/970899
RP-ID  :  ORNL/TM-2009/192
PID  :  OSTI ID: 970899
Others  :  TRN: US1001067

学科分类：材料科学（综合）

美国|英语

来源: SciTech Connect

【 摘 要 】

This document reports on initial activities at ORNL aimed at quantitative characterization of porosity development in oxidized graphite specimens using automated image analysis (AIA) techniques. A series of cylindrical shape specimens were machined from nuclear-grade graphite (type PCEA, from GrafTech International). The specimens were oxidized in air to various levels of weight loss (between 5 and 20 %) and at three oxidation temperatures (between 600 and 750 oC). The procedure used for specimen preparation and oxidation was based on ASTM D-7542-09. Oxidized specimens were sectioned, resin-mounted and polished for optical microscopy examination. Mosaic pictures of rectangular stripes (25 mm x 0.4 mm) along a diameter of sectioned specimens were recorded. A commercial software (ImagePro) was evaluated for automated analysis of images. Because oxidized zones in graphite are less reflective in visible light than the pristine, unoxidized material, the microstructural changes induced by oxidation can easily be identified and analyzed. Oxidation at low temperatures contributes to development of numerous fine pores (< 100 m2) distributed more or less uniformly over a certain depth (5-6 mm) from the surface of graphite specimens, while causing no apparent external damage to the specimens. In contrast, oxidation at high temperatures causes dimensional changes and substantial surface damage within a narrow band (< 1 mm) near the exposed graphite surface, but leaves the interior of specimens with little or no changes in the pore structure. Based on these results it appears that weakening and degradation of mechanical properties of graphite materials produced by uniform oxidation at low temperatures is related to the massive development of fine pores in the oxidized zone. It was demonstrated that optical microscopy enhanced by AIA techniques allows accurate determination of oxidant penetration depth and of distribution of porosity in oxidized graphite materials.