Preparation And Study On Novel Carbon Foam

Novel carbon material has been developed since 1980 and played an important role in the discipline of materials. Carbon material, a kind of special material, integrates with many advantage properties of metal, ceramic and polymer. With the development of new kind of carbon materials, the applications are expanding rapidly and widely. Carbon materials also exhibit unique properties in different forms such as powder (one dimension), fiber (two dimensions), film and block (three dimensions).The developments of science and technology require materials with excellent properties such as high temperature endurance, corrosion-proof, low density, oxidation resistant, excellent mechanical properties and heat insulation. Carbon foam is one of the materials meets all of these requirements. Recently, carbon foam has attracted a wide attention due to its excellent properties. Normal carbon foam was first produced in the late 1960s by Walter Ford from pyrolysis of thermosetting polymers including cell foam and reticulates vitreous carbon foam. The traditional carbon foam was produced from polymer materials such as phenolic resin and polyurethane or thermoplastic resin, which can't be graphitized and meet the requirement of structure materials however because of vitreous structure. With low thermal conductivity and high porosity, traditional carbon foam was usually used as thermal and electric insulator, energy absorbing materials, catalyst carrier, pore polar and gas filter. At Oak Ridge National Laboratory, scientists developed graphitic carbon foam in 1998. The patent was bought by the Department of America Defense and authorized to Poco Graphite Company to produce such foam at the trademark of Pocofoam. Based on its controllability of electric and thermal conductivity, low density, high temperature resistant, corrosion-proof, ready-process and radiation absorb, novel carbon foam are rapidly applied in the fields of defense, navigation and aviation.Mesophase pitch was first prepared by Australian researchers, Brooks and Tailor. Mesophase pitch is a kind of aromatics. Mesophase pitch (liquid crystal pitch) has become the precusor for many advanced carbon materials. Mesophase pitch consists of many kinds of disk-like aromatics with molecular weight ranging from 370 to 2000. Mesophase pitch is readily produced into advanced carbon materials in different shapes, for example high strength carbon flake and functional micro bead. Mesophase pitch has been viewed as excellent precursor for advanced carbon materials and attracted a wide attention.In this study, mesophase pitch is used as precursor of carbon foam and suitable for carbonization and graphitization in a more efficient and simple process.Coal tar based pitch is used as a raw material of mesophase pitch with a wide resource and low price. Mesophase pitch can be prepared after a series of reactions such as condensation, decomposition and filtration in coal tar pitch. In order to improve the mechanical properties of mesophase pitch, clay and carbon nano-tubes are added into raw pitch during the preparation of mesophase pitch. In such way, the content of isotropic and anisotropic fraction in mesophase pitch can be controlled. Carbon foam is formed in such a process: first the mold is loaded with mesophase pitch and placed in a proprietary reactor at low initial pressure and temperature. The temperature is then raised above decomposing temperature of mesophase pitch and maintained for hours. During the soaking process, mesophase pitch will be decomposed and release a series of light fractions, which act as blowing agent and expand into micro bubbles in the formation of carbon foam. The high viscosity and system pressure keep bubbles in the pitch and expanding into pitch foam. Pitch foam can be directly carbonized and graphitized, which contribute to excellent properties of graphitic foam such as high thermal conductivity, excellent mechanical and electric properties. Compared with traditional carbon foam, carbon foam obtained in such way has possessed controllable thermal conductivity and perfect structure by the control of foaming pressure and soaking time.In the first section of this paper, the properties of raw materials including synthesized naphthalene based mesophase pitch (PNJ), coal tar based mesophase pitch (PTZ) and isotropic pitch (PDD) are studied. The low viscosity and Newtonian fluid properties of PDD limit it to be matrix in the preparation of carbon foam. Both of PNJ and PTZ have high viscosity and softening point (200-370℃), exhibit thixotropic fluid at temperature above softening point and suitable for preparation of carbon foam. The content of aliphatic in mesophase pitch has important effect on the porosity and morphology of foam. When aromatic is higher than 90%, the properties of mesophase and foam will become poor, aromatics should therefore be controlled between 70% and 90%. The foaming temperature can be obtained from the TG analysis of mesophase pitch. And with the increase of light fraction in mesophase pitch, the porosity of foam will be increased.The morphology of carbon foam plays an important role in foam thermal conductivity and mechanical properties. Among the three kinds of foams, carbon foam obtained from PDD has inhomogenous pore diameters ranging from 100 to 500μm and small pore imbedded in the wall of big pore. On the contrast, foams obtained from PNJ and PTZ have homogenous pore. The pore of PNJ foam is of circle shape and that of PTZ foam is of ellipse shape. During carbonization and graphitzation, the foam is transformed from mesophase layer to graphite crystalline layer with the decrease of crystalline spacing and release of imperfections of mesophase pitch, which cause the appearing of cracking on the wall of foam pore. As for PNJ foam, cracks are long and paralleling to each other, more than those of PTZ foam. When foaming is conducted at 450℃, with decrease of temperature rate, the formed foam has homogenous pore and perfect pore morphology. During the preparation of foam, with the increase of foaming pressure, the pore diameter and porosity decrease apparently. In addition, the adequate increase of soaking time will increase porosity and decrease pore diameter.The factors affecting foam thermal conductivity include heat capacity, foaming temperature, heat-treat temperature and parameters of foam structure, heat-treat temperature is the most important. With the increase of foaming temperature, thermal conductivity decreases dramatically. But for the heat-treat temperature, it is on the contrast. When carbonization temperature is 1200℃, thermal conductivity of foam is only 2 W/m.K, thermal conductivity increases to 110 W/m.K when heat-treat temperature is 2600℃namely graphitization temperature. Thermal conductivity of carbon foam is decreased from 110 W/m.K to 50 W/m.K when porosity increased from 50% to 70%. The anisotropy of graphitic foam causes the anisotropy of thermal conductivity in the vertical and horizontal direction in the mold. The thermal conductivity in the vertical direction is higher than that in the horizontal direction of foam.The mechanical properties of carbon foam are also anisotropic. The compressive strength of vertical direction is 10MPa and 7MPa in horizontal direction. In addition, the imperfection in horizontal direction is more than those in vertical direction of foam. With the increase of porosity, mechanical properties of foam decreased. For example, compressive strength decreased from 12.98MPa to 9.98 MPa when porosity increased from 40% to 70%, but the compressive modulus is on the contrast. Due to the high treatment temperature and difference in coefficient of thermal expanding of multiphase in foam, the cracking appears on the wall of foam pore. Cracking in graphitized foam and PNJ foam are more than those in carbonized foam and PTZ foam. With the addition of clay, expanding of cracking can be avoided because clay becomes the energy-absorb center. As a result, mechanical properties of foam can be improved.In this study, mesophase pitch obtained by pyrolysising of pitch is used to prepare graphitic carbon foam. Compared with traditional carbon foam, the novel carbon foam has below advantages:First, compared with carbon foam prepared from rod-like naphthalene mesophase pitch, carbon foam prepared from coal tar based mesophase pitch, which is of disk-like structure, has perfect structure and fewer imperfects after heat treatment,Secondly, the process in this study is much simple than that of traditional carbon foam, in which blowing agent is used. Due to the high aromaticity of mesophase pitch, the time consuming and high cost step-oxygen stabilization of carbon foam-can be omitted.Thirdly, the isotropy of mesophase pitch in this study is controllable and so are the properties of the carbon foam prepared from this mesophase pitch such as the porosity, mechanical properties and thermal conductivity.The applications of carbon foam prepared in this study are expanded due to its excellent properties, which overcome the disadvantages of traditional carbon foam. The applications of traditional carbon foam are including insulation materials, filtration materials, catalyst carrier with however poor mechanical properties, thermal conductivity, thermal endurance and sound absorption. In this study, applications are extended to thermal conductive materials, thermal endurance materials.According to the high ratio of length and diameter and thermal conductivity, carbon nano-tube can be used as ideal reinforcement form carbon foam. But thermal conductivity is not enhanced apparently and as a result the poor compatibility and dispersion in carbon foam limit its application. In other way, layered clay can decrease the thermal conductivity of carbonized foam from 2 W/m.K to 0.25 W/m.K, which expanding the application of carbon foam as a kind of insulation material.