Effect of oxidative stabilization on the sintering of mesocarbon microbeads and a study of their carbonization
Introduction
High-density isotropic graphite has been used in a wide range of industries such as semiconductor, metallurgy, electric discharge machining, and energy fields. The material has an isotropic structure and property usually obtained through cold isostatic press (CIP) [1], [2], [3], and it can be manufactured by two typical procedures.
The first is the industrial process wherein filler coke and binder pitch are used as raw materials. The process starts with pulverizing and kneading followed by isostatic pressing, carbonization (baking), pitch impregnation, and graphitization. Carbonization and impregnation are usually repeated several times to attain an adequate bulk density.
The other process involves a self-sintering carbonaceous mesophase. Several attempts have been made to prepare high-density carbon from carbonaceous mesophase [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], which is a discotic liquid crystal generated as an intermediate during liquid phase carbonization. Mesocarbon microbeads (MCMBs) have been recognized as attractive precursors for high-density isotropic graphite, and their use has several advantages. First, kneading is no longer necessary due to MCMBs’ self-sintering capability. Second, volume shrinkage during carbonization is remarkable so bulk density largely increases, and impregnation is no longer necessary. By applying the self-sintering precursor, manufacturing can be substantially simplified. Third, MCMBs are spherical in shape, thereby allowing them to be compacted with a random orientation for the creation of isotropic materials.
The self-sintering capability of MCMBs is attributed to β-resin (toluene insoluble and quinoline soluble fraction) which transfers into liquid and makes MCMBs adhere to each other by a liquid-phase sintering mechanism. However, the relatively high thermoplasticity of the mesophase results in the distortion and swelling of green artifacts during carbonization [14]. Oxidative pretreatment is an effective method to reduce plasticity to an appropriate level [15], [16], [17].
Understanding how oxidative pretreatment affects the sintering mechanism and subsequent mechanical properties is essential. In this paper, two types of MCMBs with different oxidation extents were compared in their pyrolysis process, sintering mechanism and physical properties of their artifacts. Compaction by CIP was done without binder, followed by carbonization and graphitization. The final physical properties were then presented. Hoffmann and Huttinger [18], [19], [20] proposed a sintering mechanism performed in two steps: a liquid-phase sintering step in the temperature range of 300–500 °C and a subsequent final or solid-phase sintering step up to 1300 °C. In this study, in situ scanning electron microscopy (in situ SEM) was used to understand sintering mechanism in the temperature range of 900–1300 °C further. The pyrolysis behavior of both original and oxidized MCMBs was examined by characterization of weight variation through thermal gravimetric analysis, evolved gas analysis by thermogravimetry–mass spectrometer (TG–MS), thermogravimetry–infrared spectroscopy (TG–FT-IR), and chemical composition variation by elemental analysis.
Section snippets
Raw materials
Two types of commercially available MCMBs were provided by China Steel Chemical Corporation, Taiwan. MCMBs were generated from coal-tar pitch followed by oxidation pretreatment in air at around 200 °C to different extents. The two types of MCMBs were prepared from the same process except for oxidation pretreatment. One was lightly oxidized and thus had better self-sintering capability (M-LO), whereas the other was highly oxidized for comparison (M-HO). The general characteristics of the raw
Physical properties of green, carbonized, and graphitized artifacts
The results show that physical properties were greatly influenced by oxidation pretreatment. Several important physical properties of the artifacts are summarized in Table 2, Table 3. Carbonized artifacts prepared from M-LO show flexural strength of up to 55.88 MPa in transverse direction and 56.50 MPa in vertical direction. After graphitization treatment at 2900 °C, graphite artifact with flexural strength of about 34 MPa was obtained. However, the flexural strength obtained from M-HO is much
Concluding remarks
High-density isotropic graphite was successfully prepared from MCMBs. A graphite artifact with excellent physical properties and isotropic ratio can be obtained when MCMBs is oxidized to an appropriate extent. Oxidation pretreatment influences the pyrolysis behavior and sintering capability of MCMBs substantially. If MCMBs are well-sintered during liquid-phase sintering, cracks do not generate on the particle boundary during shrinkage, and a continuous microstructure can be obtained; this is
Acknowledgments
This study was supported by China Steel Chemical Corporation, Taiwan and the Science and Technology Commission of Shanghai Municipality under Grant No. 09JC1411200. The authors also acknowledge the Program for New Century Excellent Talents in University (NCET-10-0496).
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