Graphite, Artificial (2003)

Abstract The graphite crystal, the fundamental building block for manufactured graphite, is one of the most anisotropic bodies known. Manufactured graphite is semimetallic in character and is strongly diamagnetic. Compared with other refractories, graphite has an unusually high thermal conductivity near room temperature; above room temperature, the conductivity decreases exponentially to approximately 1500°C. The volumetric thermal expansion (VTE) of manufactured graphite is anomalously low when compared to that of the graphite single crystal. Graphite reacts with oxygen to form CO 2 and CO, with metals to form carbides, with oxides to form metals and CO, and with many substances to form laminar compounds. Of these reactions, oxidation is the most important to the general use of graphite at high temperatures. The raw materials used in the production of manufactured carbon and graphite largely control the ultimate properties and practical applications of the final product. This dependence is related to the chemical and physical nature of the carbonization and graphitization processes. The carbonization process through the elimination of heteroatoms and substituent hydrogen converts the organic precursor into a carbon polymer. With continued heat treatment, this carbon is transformed to a more or less ordered three‐dimensional framework approaching the structure of graphite. Differences in the final material depend on the ease and extent of completion of these overall chemical and physical ordering processes. Filler materials include petroleum coke, coal‐tar pitch coke, natural graphite, carbon blacks and anthracite. Carbon articles are made by mixing a controlled size distribution of coke filler particles with a binder such as coal‐tar or petroleum pitch. In addition to the primary ingredients, the fillers and binders, minor amounts of other materials are added at various steps in the carbon and graphite manufacturing process. They can play an important role in determining the quality of the final product. Light extrusion oils and lubricants, are often added to the mix to improve the extrusion rates and structure of the extruded products. Chemical inhibitors are introduced to reduce the detrimental effects of sulfur in high sulfur cokes. Nearly all coke utilized in carbon manufacture is calcined. Calcined petroleum coke arrives at the graphite manufacturer's plant in particle sizes ranging typically from dust to 50–80 mm diameter. In the first step of artificial graphite production, the run‐of‐kiln coke is crushed, sized, and milled to prepare it for the subsequent processing steps. The size of the largest particle is generally set by application requirements. The manufacturing process begins with the mixing operation. The purpose of mixing is to blend the coke filler materials and distribute the pitch binder over the surfaces. The intergranular bond ultimately determines the properties and structural integrity of the graphite. Thus the more uniform the binder distribution throughout the filler components, the greater the likelihood for a structurally sound product. After the forming operation, the purpose of which is to compress the mix so that pitch‐coated filler particles and flour are in intimate contact, in the next stage, the baking operation, the product is fired to 800–1000°C. A variety of baking furnaces are in use. Graphitization is an electrical heat treatment of the product to ca 3000°C. The purpose of this step is to cause the carbon atoms in the petroleum coke filler and pitch coke binder to orient into the graphite lattice configuration, producing graphite with intermetallic properties useful in many applications. In the temperature range of 1500–2000°C, most petroleum cokes and coal‐tar pitch cokes undergo an irreversible volume increase known as puffing, associated with thermal removal of sulfur. Depending on particle size and on the product size, heating rates must be adjusted in the puffing range to avoid splitting the product. Fortunately, the use of puffing inhibitors has eased the problem. Graphite, with its exceptional strength and thermal stability at high temperatures, is a prime candidate material for many aerospace and nuclear applications. Its properties, through process modifications, are tailorable to meet an array of design criteria for survival under extremely harsh environmental operations. Aerospace and nuclear reactor applications of graphite demand high reliability and reproducibility of properties, physical integrity of product, and product uniformity. The manufacturing processes require significant additional quality assurance steps that result in high cost. Carbon and graphite exhibit excellent resistance to the corrosive actions of acids, alkalies, and organic and inorganic compounds, an attribute that has fostered the use of graphite in process equipment. Graphite is used extensively in the steel, food, petroleum, pharmaceutical, and metal finishing industries. The high thermal conductivity and thermal stability of graphite have made it a useful material in heat exchangers. Manufactured carbon and graphite exhibit varying degrees of porosity. Equipment fabricated from these materials must be operated at atmospheric pressure, or some degree of leakage must be tolerated. The resistance of graphite to thermal shock, its stability at high temperatures, and its resistance to corrosion permit its use as self‐supporting vessels to contain reactions at elevated temperatures (800–1700 °C), eg, self‐supporting reaction vessels for the direct chlorination of metal and alkaline‐earth oxides. For applications where fluids under pressure must be retained, imperviousness is attained by blocking the pores of the graphite or carbon material with themosetting resins. Impervious graphite shells and tubes are used in numerous applications for transferring thermal energy, eg, boiling, cooling, or condensing. Several grades of low density porous carbon and graphite are commercially available. Porous carbon and graphite are used in filtration of hydrogen fluoride streams, caustic solutions, and in aeration of waste sulfite liquors from pulp and paper manufacture. Carbon–graphite possesses lubricity, strength, dimensional stability, thermal stability, and ease of machining, a combination of properties that has led to its use in a wide variety of mechanical applications for supporting rotating or sliding loads in contact. Its principal applications are in bearings, seals, and vanes. With the exception of carbon use in the manufacture of aluminum, the largest use of carbon and graphite is as in electric‐arc furnaces. In general, graphite electrodes are restricted to open‐arc furnaces used in steel production. Because of their unique combination of physical and chemical properties, manufactured carbons and graphites are widely used in several forms in high temperature processing of metals, ceramics, glass, and fused quartz. Industrial carbons and graphites are available in a broad range of shapes and sizes. Various forms of carbon, semigraphite, and graphite materials have found wide application in the metals industry, particularly in connection with the production of iron, aluminum, and ferroalloys.