S. Manocha

Carbon/carbon composites with heat-treated pitches: I. Effect of treatment in air on the physical characteristics of coal tar pitches and the carbon matrix derived therefrom

Coal tar pitch has been heat treated at 350°C in the presence of air and nitrogen for different times. The treatment temperatures have been pre-estimated from the behaviour of pitch during heat treatment in the presence of air. The modified pitches so obtained have been characterised by FTIR, TGA, DTG, DSC, elemental analysis, etc. FTIR studies reveal that the aromaticity of the pitches increases with heat treatment in air at 350°C up to 5 h. TGA studies also exhibit characteristic differences in pyrolysis behaviour of these pitches. C/H ratio and the coke yield have been found to initially increase with treatment time but decrease with longer heat treatment. Comparison of thermal analysis data, C/H ratio and FTIR studies show that during heat treatment of pitch in air, polymerization dominates volatilization of low molecular weight compounds in the pitch.

Studies on solution-derived ceramic coatings for oxidation protection of carbon-carbon composites

Abstract Carbon-carbon composites have been coated with silica and zircon prepared by sol-gel technique and with methylsilsesquioxane, a preceramic polymer. The effect of processing parameters for preparation of the sols and coating procedure on coating morphology has been studied. Oxidation behaviour of uncoated and coated carbon-carbon composites has been studied. Coatings with continuous and uniform structure, obtained by controlled processing, have been found to give effective oxidation protection. Zircon coatings are found to have better oxidation resistance than silica coatings. Coating of the composites with methylsilsesquioxane and subsequent pyrolysis results in the formation of silicon-oxy-carbide coatings. These coatings are found to provide very effective oxidation resistance at high temperature up to 1200 °C.

Oxidation behaviour of Carbon/Carbon Composites impregnated with silica and silicon oxycarbide

Abstract Carbon/Carbon Composites made with high strength carbon fibers as reinforcement and pitch as matrix precursor have been impregnated with silica sol and silicon oxycarbide precursor (in sol form). On subsequent heat treatment to 1000°C and 1600°C, these composites have been found to exhibit slag and glassy coatings. Apparent density of the composites at different stages has been measured by water penetration while pore volume and pore size distribution have been measured by mercury porosimeter. The porosity of composites made with surface cleaned fibers and impregnated with silica has been found to increase while of those impregnated with silicon oxycarbide to decrease on heat treatment from 1000°C to 1600°C. Oxidation behavior and inhibition of oxidation of the impregnated composites has been studied using TGA in the temperature range 40–800°C (up to 20% burn off). The effect of impregnation towards oxidation inhibition has been studied through SEM investigation on partially oxidized samples. It has been found that silicon oxycarbide is better oxidation inhibitor than silica.

Sol-gel processing of carbidic glasses

Carbon incorporation into the silicate network results in the formation of rigid carbidic glasses with improved physical, mechanical and thermal properties. This generated great interest in the development of these heteroatom structured materials through different processing routes. In the present studies, sol-gel processing has been used to prepare silicon based glasses, especially oxycarbides through organic-inorganic hybrid gels by hydrolysis-condensation reactions in silicon alkoxides, 1,4-butanediol and furfuryl alcohol with an aim to introduce Si-C linkages in the precursors at sol level. The incorporation of these linkages has been studied using IR and NMR spectroscopy. These bonds, so introduced, are maintained throughout the processing, especially during pyrolysis to high temperatures. In FFA-TEOS system, copolymerization with optimized mol ratio of the two results in resinous mass. This precursor on pyrolysis to 1000°C results in Si-O-C type amorphous solid black mass. XRD studies on the materials heated to 1400°C exhibit presence of crystalline Si-C and cristobalites in amorphous Si-O-C mass. In organic-inorganic gel system, the pyrolysed mass exhibits phase stability up to much higher temperatures. The carbidic materials so produced have been found to exhibit good resistance against oxidation at 1000°C.

Carbon/carbon composites with heat-treated pitches: II. Development of porosity in composites

Abstract C/C composites made with carbon fibres and coal tar pitch as matrix precursor were characterised in relationship with the properties of the pitch. The influence of pitch pretreatment in nitrogen and in air on the development of porosity in the C/C composites during a first impregnation–carbonisation step is studied. The amount of voids present in the final composite as well as the pore size distribution have been determined using quantitative image analysis. Results indicate that the pretreatment in air induces a significant change in the porosity of the C/C composite since the pore size distribution of the intertow and intratow porosity becomes narrower and the fraction of closed voids increases. Thermal annealing at 2000°C of the composite also affects significantly its porosity in a way depending on the pitch properties due to the propagation of microfissures. Thermal programmed desorption of C/C composite and carbon precursors provides informations on the carbonisation behaviour of the composite and therefore confirms the results obtained by image analysis.

Reactivity in wet air of carbon–carbon composites with treated pitches

Abstract Carbon/carbon composites reinforced with carbon fibres (CF) embedded in a pitch-based carbon matrix were submitted to oxidation at 823 K in dry and wet air. The reactivity of the studied material in both oxidizing environments is determined based on the intrinsic rate which, corresponds to the overall reactivity per active site area (ASA). The reactivity of the two types of constituents, fibres and matrix, taken separately is also studied and compared with their behaviour in the composite. Experimental results point out that both CF and coke are oxidised in the composite; CF appear to be more reactive than the coke matrix. It has been shown that the intrinsic rate, which is nearly constant in the whole range of burn-off, depends on the nature of the carbon material and the oxidizing environment. In particular, the pitch treatment in air contributes to an increase of the oxidation resistance. The heat treatment at 2270 K of the composites leads to a significant decrease of the oxidation rate. Furthermore, the reactivity of the composites has been correlated to their morphological properties.

Studies of the pyrolysis behaviour of silica sol copolymerized with furfuryl alcohol

Abstracts are not published in this journal

Carbon-Ceramic Composites for Friction Applications

Carbon-based composites are known to possess excellent tribo properties coupled with high temperature stability. These requirements can be met using carbon-ceramic composites (CCC). CCC have been developed using different types of reinforcements in the form of particulate and fibers with phenolic resin as carbon matrix precursor. In particulate composites, calcined petroleum coke (CPC), fly ash, silicon carbide, and boron carbide were used as reinforcements, whereas in the category of fibrous composites, carbon fibers were used as an additional reinforcement. A comparison of fly ash-based carbon-ceramic composites (FACCC), carbon fiber-based carbon-ceramic composites (FCCC), and other particulate carbon-ceramic composites (OPCCC) has been studied.

Effect of Steam Activation Parameters on Characteristics of Pine Based Activated Carbon

Activated carbons are well known as adsorbents for gases and vapors. Micro porous carbons are used for the sorption/ separation of light gases, whereas, carbon with bigger pore size are applied for removal of large molecules. Therefore, the control of pore size of activated carbon plays a vital role for their use in specific applications. In the present work, steam activation parameters have been varied to control pore size of the resulting activated carbon. It was found that flow rate of steam has profound effect on both surface characteristic and surface morphology. The flow rate of steam was optimized to retain monolith structure as well as higher surface area.

Activated carbon from biomass

Activated carbon are unique and versatile adsorbents having extended surface area, micro porous structure, universal adsorption effect, high adsorption capacity and high degree of surface reactivity. Activated carbons are synthesized from variety of materials. Most commonly used on a commercial scale are cellulosic based precursors such as peat, coal, lignite wood and coconut shell. Variation occurs in precursors in terms of structure and carbon content. Coir having very low bulk density and porous structure is found to be one of the valuable raw materials for the production of highly porous activated carbon and other important factor is its high carbon content. Exploration of good low cost and non conventional adsorbent may contribute to the sustainability of the environment and offer promising benefits for the commercial purpose in future. Carbonization of biomass was carried out in a horizontal muffle furnace. Both carbonization and activation were performed in inert nitrogen atmosphere in one step to ...