Summary. Carbon in solid form exhibit diverse structure and physical properties. It is this diversity

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1 Summary Carbon in solid form exhibit diverse structure and physical properties. It is this diversity coupled with the ability to prepare carbon products in complete random to perfectly aligned torm that imparts the end products with a wide range of contrasting and useful properties These carbon materials are called engineering carbons. The examples are graphite electrodes, carbon brushes, mechanical carbon, glassy carbons, carbon black, adsorbent carbon, carbon fibers and composites etc. Carbon is mainly derived through pyrolysis of carbonaceous organic materials such as wood, pitches or thermosetting resins. For most of the applications, pitches are often used as precursor for processing of the carbonaceous materials. Pitches are different low molecular and high molecular weight carbonaceous materials derived from organic precursors by relatively low temperature process. The carbon yield of pitch is low (~ 35-45%), This results in ultimately porous carbon. In order to increases the carbon yield, pitches have quite often to be modified by different routes such as heat treatment, chemical treatment etc. Novel methods have been developed to transform the pitches to more useful products. The controlled treatment in air and nitrogen allows to control the viscosity and softening point of the pitch. This procedure results in materials with an excellent spinnability that can be easily carbonized with intermediate step of stabilization. Pitch modified by heat treatment undergoes polymerization, whereas chemical treatment results in dehydrogenation and condensation. These treatments raise the softening point and increase the yield of carbon. Heat treatment in nitrogen blowing leads to polymerization of pitch. On fbrther heat treatment at C, these macromolecules result in the formation of mesophase structure in pitch. Modification by air blowing promotes the crosslinking betw'een adjacent aromatic molecules, suppress the evaporation of light 1

2 fraction and stacking of large aromatic molecules, thus retarding the formation of liquid crystalline mesophase. Carbon-metal complexes in which metals are dispersed in carbon matrix form an important category of materials called "Advanced carbon based materials". By incorporating metals into matrix of carbon, adsorption and catalytic properties of carbon can be enhanced. These carbon metal complexes find application such as unique catalyst for oxidation-reduction in electrical -magnetic devices such as chemical sensors, magnetic wave absorbers, antibacterial agents, adsorbents etc. Carbon can be used as adsorbent for the drug or bacteria or virus and metal acts as driving force. Modification of pitch by incorporation of metal such as silver, copper, iron and platinum has been carried out by several researchers to develop new types of stable metal carbon composites and metal loaded carbon fibers. Fine silver particle have strong antibacterial activity against stapplococcus and E. Colie and carbon fibers loaded with silver has been used as biocidal materials. Active carbon impregnated with chromium, copper, silver has been used for protection of respiratory tract. The electrical resistivity and mechanical properties of silver containing carbon fibers have been used for various applications Pitch based carbon fibers containing Fs show magnetic anisotropy of Fe species in carbon matrix. The present studies were undertaken to study the modification of pitches through airblowing, N2-blowing and various metal loaded carbon materials such as silver, iron. Further to use these pitches to make carbon products such as carbon fibers, composites etc. Fibers have been drawn using thermally treated and metal incorporated pitches. These fibers were stabilized i using different routes followed by carbonization and activation. Metal loaded carbon

3 composites have been made by using metal loaded fibers as well as cokes. Also developed carbon products using industrial thermally cracked cokes. The work incorporated in the thesis has been divided in to eight chapters. First chapter gives introduction about modification of pitches and its characteristics for value added carbon products. It also contains literature survey and elucidates the necessity to undertake the work. Various experimental techniques used in the present studies for modification of pitches and its characteristics for value added carbon products have been illustrated in chapter II. The modification of pitch (Coal-tar pitch and Coal-tar based Lean QI pitch) have been carried out by heat treatment of these pitches at different temperature range ( C) for different times (l-4hrs) in air and nitrogen. The pitches as such and after modification have been characterized. Softening point was measured by Ring and Ball Method, QI content was measured by Soxhlet apparatus. C/H ratio has been measured using C, H, N, S and 0 analyzer whereas coke yield was measured by pyrolysis technique. The metal ions (iron/silver) were incorporated by solution route in pitch as such as well as in modified pitches using suitable solvent. Thermally treated modified pitches and metal incorporated pitches were melt spun into fibers at different temperatures. The resulting fibers were stabilized in air and carbonized upto 1000 C. These fibers were characterized for metal content measured using Shimadzu UV-1601 UV-VIS spectrophotometer. Surface characteristic i.e. surface area was measured by Micromeritics Gemini 2375 BET instrument with nitrogen (N2) as the adsorbate at liquid nitrogen temperature (77K). Microstructural examination was observed by Optical Microscope (Laborlux 12 POLS) and SEM (Hitachi S-3000N). Magnetic property was measured using Vibrating Sample Magnetometer (Lakeshore 7300 Series). Experiment has also been made to incorporate silver into PAN and PAN-ox fibers through solution route. Metal loaded fibers as

4 well as cokes were used to develop metal loaded carbon composite. The carbon silver coke and composite were characterized for physical properties. Electrical properties were measured by Hewlett Packard 4284A Precision LCR meter. Microstructural analysis was done using Optical Microscope (Laborlux 12 POLS) and SEM (Hitachi S-3000N). Carbon products were also made by using industrial thermally cracked cokes with suitable binders. These products were heat treated at different temperature range ( C) in presence of inert atmosphere, and finally characterized for density using Mettler Toledo AG204 Densitometer, porosity, specific electrical properties. Mechanical properties were measured by Instron Universal Testing Machine Studies on thermal treatment of pitches and their characterization have been compiled in chapter III Coal-tar pitch and Lean QI pitch have been modified by heat treatment in air and nitrogen. Pitches have been modified by heat treatment in air at different temperature range ( C) for 4 hrs. under continuous bubbling of air in liquid mass. Experiments have also been performed in N2 at different temperature range ( C) for 4 hrs under continuous stirring. The pitch as such and in modified form has been characterized for yield, softening point, coke yield, QI, C/H ratio etc. The yield of pitch after treatment is found to be dependent on processing temperature and time. However, at higher treatment temperature or for higher treatment times, the yield is found to decrease. This may be due to volatilization of unreacted low molecular weight species. Further, Yield is found to be lower when treated in air than when treated in nitrogen Softening point is also found to increase with treatment temperature and time. Nitrogen blowing results in small increase of softening point only after treatment temperature of 350 C, while heating in air exhibits this effects even at lower temperature of IV

5 250 C. The treatment in blowing air at 275 C for 4 hrs produces pitch with sufficiently high softening point C. Pitches as such have low coke yield of the order of 30-56%. In order to increase the coke yield, pitches are modified by air blowing. After air blowing coke yield of pitches have been found to increase to 60-69%. The QI content is found to increase with treatment temperature and is related to the softening point of the samples. The larger the amount of QI, higher the softening point. Modification of coal-tar pitch by air blowing increases the amount of aromatic group compared to the aliphatic groups. So C/H ratio is found to be higher in air blown pitches than for N2 blown pitches. Development of carbon fibers from modified pitches and their characteristics are discussed in chapter IV. Thermally modified pitches were spun into continuous pitch fibers. These fibers have low softening point corresponding to that of the parent pitch. These pitch fibers were stabilized in air at 230 C for 4 hrs. in longer period of total stabilization time in the range of hrs High softening point pitch fibers were stabilized in air at 300 C for 2 hrs. in shorter period of total stabilization time of around 7 hrs. All these fibers were carbonized to 1000 C in nitrogen atmosphere. These fibers showed about 3-7 % shrinkage during stabilization and about 13-18% shrinkage during carbonization. The yield after carbonization was found to be about 70 %. These fibers were characterized for physical properties, surface characteristics, magnetic properties and microstructural analysis. Microstructure examination of the fibers w'as observed using optical microscope and SEM. Pitch fibers as such exhibit isotropic character in optical microscope. The carbonized fibers were subsequently activated in steam at C. These fibers exhibited % burn off after steam activation. Magnetic properties of the pitches as such as well as after modification, of spun fibers, stabilized fibers v

6 and carbonized fibers were studied at room temperature. It has been observed that the pitches change from diamagnetic to ferromagnetic under certain processing condition. A novel technique of stabilization of pitch fibers was evolved wherein thermally treated low softening point pitch fibers were stabilized shorter period of time. Pitch fibers were first chemically modified with different concentration (15-50%) of phosphoric acid solution. Subsequently, these were stabilized in air at 230 C for shorter period with total stabilization time of about 34 hrs. All these were carbonized at 800 C in inert atmosphere. The yield of these fibers was found to be increased to about 65-70%. Microstructural analysis of these fibers by optical microscopy exhibit mixed isotropic/ anisotropic structure. Studies on development of iron impregnated pitch based carbon fibers and their characteristics are discussed in chapter V. Coal-tar pitch as such has been modified by addition of metal ion. The metal ion (iron) was dispersed in coal-tar pitch by solution route with a suitable solvent like quinoline. Iron impregnated pitches were drawn into fibers of about 50 pm diameter. These developed fibers were stabilized in air at 230 C for 4 hrs. in longer period of time in the range of hrs. Because of low softening point of pitches, these fibers were stabilized for shorter period of time. Fibers were first chemically modified by different concentration (about 10-50%) of phosphoric acid solution followed by stabilization in air at 230 C for 4 hrs. All stabilized fibers were carbonized upto 1000 C in presence of high purity nitrogen. These iron impregnated pitch based carbon fibers were characterized for metal content, ash content, surface characteristics, magnetic property and microstructural examination. Metal content of fibers was varied because of density of iron is higher compare to the density of pitch. The surface area of carbonized fibers showed about 5-13 m /gm. These vi

7 fibers showed about 9% shrinkage during stabilization and 20% shrinkage during carbonization. Microstructural analysis of these fibers by optical microscope and SEM exhibited more or less isotropic character and uniform distribution of metal species. Magnetic properties of iron containing pitch based carbon fibers were studied. The hysterisis loop of the samples suggested the formation of soft magnets, a property characteristic of ferromagnetism. Chemically modified and without chemically modified iron impregnated pitch based carbon fibers showed about 66-72% and 75-78% yield respectively. Silver uptake by modified pitches and its characteristics have been compiled in chapter VI. In this case, the coal-tar pitch was modified by heat treatment in the temperature domain of C either in nitrogen or air to control the polymer crosslinking. In this case, pitch was modified by heating in nitrogen atmosphere at temperature C for 2-5 hrs with continuous stirring. Pitch was modified by heating it at C for different times (2-5hrs.) under continuous bubbling of air through the liquid mass. For mild condition, i.e. at 300 C for 2 hrs. the yield is found to be same when treated in air or nitrogen. Nitrogen blowing results in smaller increase of softening point only after treatment at temperature 400 C, while heating in air exhibits this effect even at lower temperature of 300 C. The treatment in blowing air at 350 C for two hours produces pitch with sufficently high softening point of C Modified pitches were further treated with metal complexes by solution route. Silver intake has been found to increases from 0.5 to 0.8% in nitrogen treated pitch while the uptake is found to decreases for pitches treated in air at 350 C for 5 hrs. Experiments were also made to incorporate silver into PAN and PAN-ox fibers through solution route. The metal intake has been found to be more in PAN-ox fibers than in PAN as such. Pitch as such as well as after vii

8 different modification were further heated to temperature of C in nitrogen atmosphere to obtain cokes. Metal loaded carbon composites were prepared using metal loaded PAN-ox fibers as reinforcement and coke as matrix. The silver impregnated fibers were co-milled with sinterable coke for different interval of time. Elliptical/ spheroid shape beads of 3mm diameter were formed. The beads were pyrolysed to 650 C in presence of nitrogen and activated with steam at different temperature from C. The silver loaded coke and PAN-ox fiber reinforced composite were characterized for metal content surface characteristics, physical and electrical properties. Silver uptake by pitch is dependent on chemical constitution of pitch. If the pitch is modified at higher temperature for long time, silver uptake decreases. Silver pick up decreases with several treatment conditions in blowing air. In metal impregnated PAN-ox fibers based composite, impregnation of silver gets enhanced considerably by using PAN-ox fiber and also intake of silver depends on the concentration of silver nitrate solutions. The silver intake is more from 0.5 N solution is 9-10% than from 0.2 N solution of silver nitrate is 4-5%. Surface oxygen complexes have been found to increase with increases in the air stabilization of the pitch is meq/loogm. On converting these stabilized pitches to coke at 600 C, the amount of these oxygen complex have been found to decrease to meq/loogm. A small increase in the group is noticed on activation in steam. But surface oxygen complexes on composite have been found to be about 10-15% lower than on the coke alone. This may be due to incorporation of metal ions. The electrical resistivity of cokes decreases on incorporation of silver. The electrical resistivity decreases and conductivity increases with intake of silver, which obviously depends on strength of silver solution. Electrical resistivity is found to decrease further with activation viii

9 Development of carbon products using industrial thermally cracked cokes and their characteristics have been compiled in chapter VII. Carbon products were made by using industrial thermally cracked cokes as filler and different binder such as coal-tar pitch, AR mesophase pitch, phenolic resin and Polyfurfuryl alcohol with proper ratio. Green products were made by using hot pressing at suitable temperature. These green products were heat treated at C. Finally the carbon products were characterized for density, porosity, microstructural analysis, electrical properties, mechanical properties. The various conclusions drawn from the present studies have been compiled in chapter VIII. Modification of coal-tar pitch through heat treatment, result in chemical crosslinking. The reaction is faster in air than in nitrogen. The properties of pitches can be controlled and materials with an excellent spinnability can be easily stabilized by modifying pitches either by nitrogen blowing or air blowing. On treatment with silver salt solution heat treated pitches under mild condition exhibit higher silver uptake than those treated at higher temperature for long times though latter exhibit higher softening point. By controlling the amount of metal ion (iron/silver) and method of modification, tailor made carbon products such as metal loaded pitch based carbon fibers, metal loaded coke and metal loaded PAN-OX fiber reinforced composite with desired electrical, magnetic and adsorption characteristics can be developed. IX