Graphite - an innovative material
The advantages of graphite are obvious, making it an attractive material for industry.
The process flow for the production of carbon graphites and special graphites corresponds to that of a ceramic manufacturing process. In the first process step, the raw materials used are crushed. In mixing units, the solid mixture components are then uniformly mixed with binders and homogenized. For subsequent forming, extrusion, die pressing and isostatic pressing are used.
In a first step, the pressed moldings are fired at approx. 1,000°C in the absence of air. In this process, the binder bridges are created between the solid particles. The hard carbon produced in this way is converted into the three-dimensionally ordered graphite in a further thermal process step at approx. 3,000°C. The graphite is then converted into the three-dimensionally ordered graphite.
Although they have the same chemical formula, the properties of synthetic carbon and graphite, expanded natural graphite and carbon fibers are extremely different.
Amorphous carbon is temperature resistant, electrically conductive, thermally conductive and wear resistant.
Shaped bodies of amorphous, coarse-grained carbon are used in thermal and electrolytic processes, for example in the extraction of aluminum, silicon, pig iron, ferrosilicon or phosphorus.
Anthracite and other aggregates are combined with coal tar pitch and shaped as a homogeneous mass and subjected to a thermal process, carbonization
Electrographite is extremely temperature-resistant, very good electrical conductor, mechanically resilient and energy-efficient. The special crystalline structure of synthetic graphite leads to unique properties.
Graphite electrodes can withstand temperatures of up to 3,000°C as current conductors in the arc furnace.
As current conductors, graphite cathodes withstand electrochemical and mechanical stress in the aluminum smelting electrolytic cell for 5 to 7 years temperatures of almost 1,000°C. They still remain highly energy efficient.
Coke and pitch are mixed, shaped and carbonized. Carbonization is followed by graphitization. In this high-temperature treatment at 2,500°C to 3,000°C, the typical graphite crystal lattice is formed.
Synthetically produced carbon and graphite materials with an average particle size of less than one millimeter are referred to as fine con- or special graphites.
The ability to selectively influence material properties through variations in raw materials and manufacturing technologies makes specialty graphites an indispensable key material for many applications.
The raw materials are prepared and mixed with carbonaceous binders in mixing units. Various processes are available for subsequent shaping: extrusion, vibratory compaction, and die or isostatic pressing. Subsequently, the pressed moldings are fired and grafitted under exclusion of air, mechanically processed and possibly finished.
Expanded natural graphite
The unique properties of natural graphite are tapped with expanded graphite for a wide range of applications. During the expansion process, the graphite retains its inherent very good electrical and thermal conductivity.
The starting material for expanded graphite is well-ordered, highly crystalline, flaky natural graphite. It is transferred to a graphite salt with an intercalation agent and expanded by a subsequent thermal shock treatment.
The graphite scales increase their volume by a factor of between 200 and 400. The Grafitexpandate consists of loose worms. These are then compacted without binders and fillers, for example, into sheets or films.
One of the central developments in carbon technology after 1945, in addition to the Castner process, was isostatic pressing, which is still used today in the field of fine-grain graphite. The users’ desire for ever larger and stronger graphite components could no longer be met with conventional manufacturing technologies.
Carbon fibers are a modern high-performance material. They are used in applications with high mechanical requirements combined with low weight, for example in aerospace, automotive, wind industry or modern sports equipment.
The material is very strong and stiff, light, electrically conductive and can be processed in many ways.
Carbon fibers are produced by oxidation and carbonization of the man-made fiber polyacrylonitrile (PAN). Then, depending on the application, the carbon fibers are further processed directly as fiber bundles, as woven fabrics, scrims or as semi-finished products impregnated with resin (prepregs)
Isostatically pressed fine-grained graphites are produced up to block sizes of 2400 x 500 x 400mm
Carbon fiber reinforced plastic
Carbon fiber reinforced plastic – CFRP – is made up of carbon fibers and matrix components.
Its properties are very strong and rigid, at the same time light, fatigue resistant, dimensionally stable and it has a high energy absorption.
This material is indispensable for demanding applications in high-tech areas, where high strength and stiffness combined with low weight are also important.
In addition to laminating and pressing textile semi-finished products or prepregs, there are various injection, pultrusion and winding processes for manufacturing components from CFRP. In addition, adapted extrusion and compression molding processes exist for thermoplastic resin systems.
Carbon fiber reinforced carbon / CFC
Hard and soft felt
Whenever the highest demands are made on insulation properties, soft felts made of carbon fibers are indispensable. In addition, soft felts are also used as battery felts in energy storage systems.
Typical properties of the material are their thermal conductivity, low heat capacity, resistance to high temperatures. In addition, they are highly pure.
The starting material for the production of carbon and graphite soft felts are felts made of needled cellulose fibers. These are converted into carbon soft felts by thermal treatment at 800 – 1000°C. If the felts are treated at even higher temperatures (>2000°C), the carbon fibers increasingly take on a graphite-like structure. One then speaks of “soft graphite felts” without a real graphite structure being present.
In contrast, rigid felts are a dimensionally stable insulation material with low thermal conductivity based on carbon fibers. They are suitable for temperature ranges greater than 800°C in inert gas and vacuum applications.
Typical properties of the material is their dimensional stability, low thermal conductivity and thermal shock resistance. They are also resistant to erosion.
Hard felt is produced by mixing and pressing fiber mixtures and binders such as phenolic resins. Subsequently, they are treated under high temperatures up to max. 2,800°C treated. The hard felt is then mechanically processed to the desired dimensions.
The main properties of carbon and graphite materials
The advantages of these materials are used in particular in the glass industry:
Other advantages include:
- The density of graphite ranges from 1.7g/cm³ to 1.9g/cm³, depending on the brand.
- Graphite is not fusible, but sublimates at about 3,900 K.
- In air, graphite is stable up to approx. 750 K.
- Graphite is extremely resistant to thermal shock. Thus, fast heating and cooling times are possible without any problems.
- The thermal conductivity of graphite is higher than that of many metals and decreases with increasing temperature.
- The coefficient of thermal expansion of graphite in the temperature range of 20 – 200°C is of the order of 3-5 x 10-6K-1. It varies from brand to brand and depends on the temperature.
- Unlike most materials, the tensile, compressive and flexural strength of graphite increases with increasing temperature up to 2,700 K and then decreases again.
- Graphite has about twice the strength at 2,700 K as at room temperature.
Text/Picture Source: SGL Group – www.sglgroup.com Stefan André