The Carbon Age
Carbon ProductsIn the carbon industry, we normally make a distinction between carbon products based on final processing temperature along with forming techniques.
Carbon products that are processed to a temperature of approximately 1800°F are called carbon or baked carbon, and carbon products that are processed to approximately 4500°F are referred to as graphite, synthetic graphite or even electro-graphite. The sidebar is a schematic of graphite processing.
In addition to the baked carbon and graphite products, the carbon family also includes many other products that are reduced to a pure carbon form. Some examples of these products are flexible graphite felt, rigid graphite insulation, porous graphite, carbon or graphite cord or yarns, expanded graphite foil, and carbon-fiber-reinforced carbon, sometimes referred to as carbon-carbon (C/C), CFC, CFRC or just carbon fiber. And just as mankind progressed from the Copper Age to the Bronze Age to the Iron Age, we see a transition in the carbon industry from the original carbon and graphite products to the higher-performing CFC materials.
Extruded graphite is probably the most common form of synthetic graphite made in the world today. A mixture of metallurgical or petroleum coke along with natural or synthetic graphite powders is held together with some form of high-carbon-content resin or pitch. This mixture is extruded through a die using hydraulic pressure to form a green artifact of, typically, a round or rectangular shape. Subsequent baking and graphitization heat treating will transform this green artifact into a final carbon form of synthetic crystalline graphite, which is ready for transformation into hundreds of possible uses. Extruded graphite has the advantage of high-volume production, a wide range of cross-section sizes and long lengths.
Die-molded graphite products were developed to provide high-volume, pressed-to-size parts or blanks for further processing and to take advantage of high-speed pressing equipment. Combining finer-grain mixtures than extruded materials, die-mold materials form an important class of carbon and graphite products.
Isostatic graphite is the result of limitations in the extruded graphite and die-mold forming processes. Progressions in the development of finer-grained carbon materials met technical limitations in the extrusion process of forming the green artifact. Members of the carbon industry devised an alternative form of pressing to hold together the particles in the green form. This entails placing the mix – once again a combination of components similar to extruded graphite except normally in a much finer-grain form – in a rubber bag and putting it in a high-pressure, liquid-filled chamber, causing the compaction of the mix in the rubber bag. Because the pressing is done from all directions, the final graphite product has not only a finer grain structure, but it also has highly isotropic properties. This is unlike extruded graphite and die-molded graphite, which are highly anisotropic in property values such as electrical resistivity and flexural strength.
Vibration-molded materials are an additional way of forming that can produce a finer-grained material in large cross-sectional shapes. Not as common as extruded or isostatic graphite, it is nonetheless important when engineers and scientists consider these materials for industrial processes. Vibration molding involves the use of large containers of mix that are shaken or vibrated to facilitate the compaction of the carbon mix into a green artifact. Hydraulic compression is sometimes used in conjunction with vibration molding in the compaction process.
Porous graphite is the name of a material usually made through a process of pressureless sintering. Special care in powder processing provides for a material that – under the right conditions – will bond together under heat, minus pressure, to form a high-porosity material (50% range). This material is utilized for filter media, crystal growth and metal-infiltration applications.
High-Temperature ApplicationsHigh-temperature processes require specialized materials to provide the protection needed to safely process the designated load and also guarantee a successful end product. Insulations made of rayon, PAN or pitch-based felt can be an option for furnace manufacturers and end users. Soft and flexible, it can be molded, wrapped and layered in a large number of combinations. Because of the density and fiber orientation, it will not couple with induction coils. Therefore, it can be utilized in high-temperature induction furnaces.
With the reduced use of asbestos, expanded-graphite products have been utilized to fill the gap. Soft and pliable, yet gas tight and chemically resistant, expanded-graphite foils are an indispensable addition to many industrial processes such as sealing, gasketing and gas barriers.
Carbon-fiber materials – sometimes referred to as CFC, carbon/carbon (C/C) or CFRC – are a class of material based on a precurer material made from polyacrylonitrile, or PAN, a derivative of petroleum. First processed into single strands and then grouped into bundles known as a tow, it forms the basis for the wide range of carbon-fiber products currently in use today and being designed for the future. Two major application areas exist for carbon-fiber products – low temperature and high temperature. Low-temperature applications are the items most people are familiar with (i.e. golf-club handles, fishing rods, the A380 airbus or blades for windmills). High-temperature applications are more industrial-based and focus on the high-heat furnaces where other materials are at the peak of their performance limits. CFCs can provide increased performance and energy savings as a function of their unique properties. Figure 1 shows a comparison of CFC sintering trays versus extruded-graphite trays that demonstrates the relative difference in mass of trays needed to process the same amount of end product.
Carbon fiber encompasses a large area of products all the way from the single strand of oxidized pan fiber to groupings of fibers known as tow that are sold on spools for the production of an array of products. CFC tow can be used to weave cloth and then be impregnated with resin. These are referred to as prepregs. Prepregs can be used as layer material. Building upon a tool the shape the designer wants to create, the prepreg becomes a finished product based on carbon fiber, i.e. the hood of a racing car. Prepreg material, which once again is carbon-fiber cloth impregnated with resin, can be placed layer upon layer to create a material of varying thickness known as CFC. Pressed under heat to cure the resin and then processed through consecutively higher-temperature processes, a product is created that is extremely strong, very lightweight, relatively easily machined and capable of withstanding very high temperatures. These materials find utilization in all the high-temperature areas in which the more traditional extruded and isostatic graphite are currently used. As industry moves to a higher level of performance, these materials find increased applications. Figure 2 gives the production scheme for carbon-fiber-reinforced carbon (C/C).
ConclusionWhat do all these options mean for today’s engineers, designers and scientists? It means that a material with over 100 years of utilization and application is as viable today as ever. Extruded and isostatic graphites form the basis of numerous industrial processes critical to current needs of industry and society. As applications are reviewed for efficiency and longevity, new usages are emerging for these materials and the other carbon products such as soft and rigid carbon or graphite insulation, expanded graphite foils and CFCs.
These materials find application in numerous areas such as semiconductor, metallurgy, high-temperature processes, tool manufacturing, chemicals, glass, ceramics, energy, nuclear, medical and laboratory needs. Figure 3 shows an example of the many ways these materials are used in a single application such as a vacuum furnace. The internals and door and wall heaters are extruded graphite, the insulation is rigid graphite insulation and CFC is used as securing bolts for the insulation and heaters. Figure 4 is another multiple-use example. The heaters in this furnace are made of expanded foil, the heater mounts are extruded graphite and the insulation is rigid graphite.
For more information: Joseph Labant is product manager, high-temperature applications for SGL Group, The Carbon Company - Graphite Specialties, 900 Theresia St., St. Marys, PA 15857; tel: 814-781-2729; fax: 814-781-2697; e-mail: email@example.com; web: www.sglcarbon.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: flexible-graphite felt, carbon-fiber-reinforced carbon, extruded graphite, isostatic graphite, vibration-molded materials