Introduction
Carbon electrodes are applied for the production of silicon metal as they have peculiar characteristics. Carbon electrodes are used as electric current conductors in electric furnaces where silicon metal, elemental phosphorus or - to a small extent - other specialties are produced. The production of silicon metal and phosphorus is quite a specialised process and therefore only a small number of companies is engaged in this business world-wide. Most of the silicon metal produced is used in two main industrial fields: in metallurgy as an alloying element for aluminium and in the chemical industry for the production of a large family of plastics called silicones. Silicon metal is also employed, after a long processing chain, in the electronic field.
Production of silicon metal
Carbon electrodes play a main role in the production processes in submerged arc furnaces. Carbon electrodes are most largely used for the production of silicon metal. The process consists in the continuous reduction of quartz (SiO2) into silicon by a reducing mixture according to the simplified relation SiO2, + 2C -> Si + 2CO. As components of the mixture, carbon in the form of mineral carbon, petroleum coke, charcoal, wood-chips can be used. The choice and proportions of these materials vary depending on the local availability and cost. The electric current runs through the electrode between the contact plates and the tip of the electrode causing the ignition of the electric arc with its extremely high temperatures (> 2000°C) necessary for the reduction of quartz into silicon. The silicon is then tapped from the bottom of the furnace. Every day, 15 to 50 tons of silicon are produced according to the power applied to the furnace. During the process the tips of the columns submerged in the reaction area are gradually consumed. Each of the three electrode columns is built with 5-6 electrodes.
Drawing of a silicon furnace
Carbon electrodes for silicon metal production
Amorphous carbon electrodes are produced with electrographite and/or anthracite, bound with coal tar pitch. The production cycle includes: raw materials, crushing and selection of granulometry, mixing with pitch as binder, forming, baking and final machining. Click here for the production scheme.
Carbon electrodes are produced with several types of joints: conical (male-female) or pin type joint. The choice between the two types depends on the characteristics and operative conditions of the electric furnace.
Production scheme for electrodes
Peculiar characteristics of carbon electrodes for silicon metal production
• carry large amounts of electrical current
• withstand very high temperatures
• keeping good mechanical properties under above-described circumstances
Other industrial applications
Fibres
Felt
Mechanical
applications
Electrical
applications
Foils
Specialty graphite
Nuclear
Semi conductors
Aerospace
Fuel cell
What is a fuel cell?
A fuel cell is a power generator that produces electricity through the chemical reaction of hydrogen and oxygen, without combustion, giving off heat and water as the primary by-products.
A fuel cell consits of two electrodes, an anode and a cathode, separated by an electrolyte. Power is produced electrochemically when ions are formed at one end of the electrodes with the aid of a catalyst, and are then passed through the electrolyte. The current produced can be used for electricity.
Components
The bipolar plate that is made of graphite is one of the most important components for fuel cell. Until now no other material is able to meet the extremely high requirements with respect to chemical stability, conductivity and thermal stability. Graphite will surely plan an important role in this technology of the future.
Fuel cell components
These new materials are non-woven papers and felts specifically designed to transport gases into, and water out of fuel cell stacks. They have an open pore structure, good mechanical strength and high electrical conductivity. They are most cost effective for use in moderate temperature Polymer Electrolyte Fuel Cells and in Direct Methanol Fuel Cells.
Carbon & graph. Fibre
Industrial applcation : Fibres and Fabrics
Carbon fibres
Definition
Since technical grade carbon fibres were developed in the mid 1960s, they have been gradually introduced in technical products. The application is connected with material questions such as matrix materials, fibre/matrix adhesion promoters and long term behaviour, component production techniques or textile semi-finished materials. Precursors for carbon fibres can be rayon, polyacrylnitril or pitch. Depending on applications carbon fibres can even be graphitised.
Production of PAN Fibres
Production Process of Carbon Fibres
Carbon fibres applications
Carbon fibres are applied in flat and tubular components, for supports, rods, and as reinforcements as well as in special components.
Textile components – components for radiography equipment – components produced by filament winding – components for measuring technology and optical equipment – components for automation – components for general machine construction
Fibre composites
Fibre composites can be used in machine, equipment and apparatus construction, medical technology and vehicle building.
Carbon fabric
Definition
When the fibres are processed into knitted or woven artefacts, they make up a fabricyl.
Applications of carbon fabric
This fabric can be used for interior furnishings, protective clothing or as industrial applications for packing and gaskets.
Carbon fabric: characteristics
High tensile strength
High Young's modulus
Carbon content > 95%
Suitability for reinforcement
Electrical conductivity
Fibre sizing adapted to end matrix
or respective application
Carbon fibre reinforced plastics
Definiton:
Fibre-reinforced plastics are composite materials with tailor-made properties.
Reinforcement fibres are embedded in a matrix of resin Carbon reinforced plastics
Properties
The properties of the fibre-reinforced plastic articles are governed mainly by the properties of the fibre, in particular the carbon fibre, and the form of textile into which the fibre is processed. Pre-impregnated materials (prepregs) offer a precise and economical way of combining reinforcements with a resin matrix. Prepregs consist of high-quality textile fabrics impregnated with curable resins. The fibre type is the main factor governing the strength. Young's modulus and other important properties of fibre composite products. High strength, rigidity and pronounced anisotropy are achieved by a unidirectional (UD) arrangement of the fibres or the prepregs themselves. As the fibres are arranged in dense bundles, the unidirectional prepregs contain at least 60 percent fibres by volume. In principle, prepregs made from woven fabrics are employed for components that have to be isotropic in one plane (orthotropic). This can be achieved with plain-weave fabrics, in which warp and weft are arranged at angles of +45°/-45° and 0°/90° to the main axis of the laminate. In general, the fibre content of such elements will be about 50 percent by volume. Not only does the resin influence the essential properties of the resulting products, but it also determines their processibility, manufacturing time and cost.
Prepreg composite properties
Properties at a glance -
superior mechanical, dynamic and chemical properties in comparison with conventional
- isotropic materials (metals, plastics, ceramics)
- low density and mass
- high rigidity and strength
- high fatigue strength
- very good corrosion resistance
- suitable for integral construction techniques
Carbon fibre reinforced plastics: applications
Applications
The materials currently used are glass, aramid, carbon and graphite fibres in combination with epoxy and phenolic resins. Elements manufactured from fibre composites can be designed in such a way that they exactly meet the requirements imposed on them.
Advantages of fibre composite products
Resistance to high temperatures and weathering, low flammability, low smoke density, low toxicity of decomposition products . Temperature resistance of course depends on choice of resin.
High chemical stability
Large variety of possible component shapes and sizes
High durability due to long prepreg storage life.
Prepregs comprise the range of reinforcements and resin matrix combinations. They are manufactured on a state-of-the-art fusible resin plant. Fusible resins have fewer volatile constituents and increase the composite materials' mechanical strength. The prepreg manufacturing plant is accredited to DIN AND ISO 9001 quality assurance standards
Carbon fibre reinforced plastics: applications
Typical prepreg composite properties
Table: Component figuration of fibres
Prepreg composites: density, tensile strength and specific strength compared to other materials
Graphite foil
Industrial application: Foils
Definition
When the fibres are processed into knitted or woven fabrics, they make up a foil.
Applications of carbon foil
This foil can be used for interior furnishings, protective clothing or as industrial applications for packing and gaskets.
Carbon foil: characteristics
High tensile strength
High Young's modulus
Carbon content > 95%
Suitability for reinforcement
Electrical conductivity
Fibre sizing adapted to end matrix
or respective application.
Carbon & graph. Felt
Carbon felt as well as graphite felt are produced and have unique textile, chemical and thermal properties.
Carbon felt
This type of felt is manufactured by carbonisation of natural and synthetic fibres. Thermal properties Chemical properties Applications
Carbon felt – thermal properties
• Thermal properties:
• Low thermal conductivity
• Low specific heat
• Permits rapid heating and cooling of furnace.
• High thermal stability
• In oxidizing atmosphere up to 350°C, in protective atmosphere or vacuum up to about
• 3000°C.
• Favourable resistivity
• Coupling in an inductive field occurs only above 12kHz.
• Easy of handling
• Felt can be cut with scissors or knife and adapts to small bending radii.
• Favourable surface properties
• Next Typical properties
Chemical properties of carbon felt:
Ø For hot and/or corrosive gases and liquids
Ø For molten metals (no wetting)
Ø Catalyst support: simplified recovery of the catalyst by heat treatment
Ø Porous electrodes: for accumulators and fuel cells
Typical properties
Applications of carbon felt:
Ø For resistance- or induction heated vacuum furnaces and inert gas furnaces, such as degassing furnaces, brazing furnaces, soft and bright annealing furnaces, sintering furnaces for hard metals, carburising furnaces, laboratory graphitising furnaces.
Ø For inductively heated melting and heat treatment furnaces, in which a proportion of the ceramic insulating material is replaced by felt in order to increase the electrical efficiency and prevent the liquid metal from coming into contact with the induction coil in the event of crucible fracture (no wetting).
Ø For nuclear reactor technology (small cross-sectional area for neutron absorption).
Ø Felt is applied as thermal insulation, as filters, and as catalyst support. Ø It can also be applied for porous electrodes.
Customized Products:
Insulationcylinder from felt with CFC support Graphite foil can be applied in order to avoid diffusion Flexible quantities according to application Customized self supporting insulation cylinder consisting of solf felt, cfc and graphite foil
Graphite felt:
Graphite felt is manufactured by graphitisation of carbon felt. Graphite felt is applied as insulating material, for filters, heat shields or insulating claddings as well.
Properties
The characteristic properties of graphite felt are:
Ø Low thermal conductivity: Graphite foil-faced felts ensure heat reflection (to the furnace interior) and the foil acts as a diffusion barrier
Ø Low specific heat: Graphite felt permits rapid heating and cooling of the furnace.
Ø High thermal stability: The felt is stable in oxidizing atmospheres up to 400°C, in vacuum up to about 2500°C and under inert atmospheres up to 3000°C.
Ø Shape retentivity: The felt does not undergo compression under normal operating conditions. The bulk density thus remains unchanged throughout the entire insulating layer. The properties are constant and no voids, channels or hot sports occur.
Ø Low absorption of gases or vapours: Graphite felts hardly absorb any moisture in difference to carbon felt. Brief pumping times in vacuum furnaces are possible.
Ø Erosion resistance Due to the carbon binder, the rigid felt fibres are securely anchored and not even torn off at high gas velocities. Low ash content Due to the temperature treatment during graphitisation ash content is much lower than for carbon felt
Specialty graphite
Industrial applications Specialty graphite
Specialty Graphite
Definition and special characteristics
Examples of application
Definition
Specialty graphites are man made artificial carbon and graphite grades with an average grain size typically less than 1mm (0.04in). Apart from being very pure, thermally and chemically stable, resistant to thermal shocks, thermally conductive (up to 3000°C) and corrosion resistant, specialty graphite has a programmable electrical resistance. The application of specialty graphite is a.o. electrodes, heating elements and susceptors. Very pure specialty graphite is called nuclear graphite as it can be applied in the nuclear industry as a moderator for thermal neutrons in water- and gas-cooled nuclear fission reactors. This is thus high-performance industrial material, applied e.g. for the production of computer chips.
Properties
- homogeneous fine-grain structure
- high density
- suitable resistivity
- very high purity
The properties of specialty graphite involve a special industrial application of the product. Thermal application: heating elements for pulling optical fibres, Stable material: blocks for casting dies for railroad wheels, graphite electrodes for electrical discharge machining, vacuum sintering furnaces systems equipped with heating systems and structural elements made of specialty graphite. Corrosion/chemical resistant: bearings and sealing elements for pumps, electrolytic cells, carbon brushes for asynchronous slip ring rotor machines, sliding strips. Electrical conductivity: multipart sealing rings for turbines, lings of nuclear fusion reactors. Nuclear application: pyrolytically coated graphite tubes made of super-pure specialty graphite for atomic absorption spectroscopy
Ø Medical and laboratory technology: atomic absorption spectroscopy for detecting trace elements
Examples of Applications
Specialty graphites are used for
Computer (chips)
Semiconductor technology
Non-ferrous metals
Industrial furnace technology
Glass industry
Ceramics industry
Mechanical components
Heat treatment industry
Medical and laboratory technology
Semi-Conductors
Industrial applications: Semi-conductors
Technology
The key properties of graphite to be used in the semi-conductor technology are the unusual combination of its properties and the possibility to match certain material properties with a given specification through varying raw materials and production methods. There are several major applications for graphite in the semiconductor sector which are requiring purification to at least 5 ppm ash.
- Single Crystal Growth
- Silicon Carbide Single Crystal Growth
- Silicon Epitaxial Deposition Glass
- To-Metal Sealing
- Coatable Graphites Fiber
- Optics
Single Crystal Growth - Silicon Carbide:
Purified graphite with its enviable thermal properties and inertness provides a suitable constructional material in which single crystals of silicon carbide can be grown. The isomolded grades are ideal for this application.
Single Crystal Growth
- Silicon: (1)
Silicon is the most widely used material for the manufacture of integrated circuits and other semiconductor devices. Silicon, the second most abundant element, comprising 28 percent of the earth's crust, is a good material for circuits because its electrical properties can be precisely altered by adding controlled amounts of impurities, called dopants. One of the most essential operations in the manufacture of an integrated circuit is the conversion of this silicon into single crystal form. The most common method of converting the polycrystalline silicon to a single crystal ingot is through growing the ingot from molten silicon by the Czochralski crystal-growing process. This process involves wetting a properly-oriented seed in a melt and withdrawing it vertically to grow a crystal which might be up to 12" (300 mm) in diameter.
Graphite component
(2) Graphite is extensively used in this application for a number of reasons. First, graphite has electrical properties that are necessary for good heaters. Second, it has the required thermal properties. Material is needed that can withstand temperature cycling from room temperature to above 1400°C, maintaining strength at these high temperatures. Graphite has excellent thermal conductivity, enabling it to maintain thermal equilibrium in the growing process. Third, the combination of cost, availability, machinability, and life of the graphite parts has not been improved upon by other materials. Purified fine-grain graphites are used as resistance heaters, which melt the silicon and maintain it at a temperature of approximately 1400°C. Purified isomolded superfine graphites are used as holders or crucibles to hold a quartz crucible liner containing the molten silicon. These same grades are used as a graphite pedestal which support the crucible, rotate it, and lift it as necessary. The graphite is subjected to the same thermal conditions as the crucible and also acts as a path for removal of excess heat of fusion from the puller. A graphite heat shield, surrounds the heater. This shield prevents the loss of heat outward from the crucible area. Lastly, graphite chucks are used in the polysilicon reactors. These hold filaments upon which the polysilicon is grown. The choice of grade depends upon the size of the hot zone, and the graphite performance required.
Epitaxial Deposition:
Another application for graphite is epitaxial deposition, a process utilized throughout this industry for doping silicon or III - V compound substrates. Epitaxy, in silicon semiconductor technology, refers to the oriented overgrowth of one crystalline material on to another. This process is known as "vapor phase epitaxy" and uses silicon carbide coated graphite susceptors to hold the silicon wafers. Controlled doping of "P" or "N" type impurities is also possible in silicon epitaxial technology. Graphite is used in epitaxial deposition because of its high purity, electrical properties, thermal conductivity, low gas evolution, and coating characteristics. Graphites are generally superfine grain and possess CTE characteristics compatible with the coatings.
Glass-To-Metal Sealing:
In glass-to-metal sealing, a "chip" mounted to a metal lead frame must be protected, quite often by glass encapsulation. The device is mounted in a graphite fixture, heated to a desired temperature and the molten glass applied. Graphites are generally used due to their low gas evolution, high thermal conductivity, ability to be machined to very close tolerances, excellent resistance to abrasion and wear, and superior oxidation resistance.
Coatable Graphites:
Coating of graphite with silicon carbide extends the applications of graphite into high temperature oxidation regimes and eliminates interaction with potentially reactive materials such as silicon.
Graphite coated with SiC
Fiber-Optics
High grade quartz is drawn at temperatures around 1700°C to form a cable with fiber-optic qualities. In one industrial process the furnace container and heater are fabricated from high purity isomolded superfine graphite. Graphite with its combination of thermal characteristics, chemical inertness, high purity, superfine grain texture, and structural integrity at elevated temperatures is eminently suitable.
picture of graphite components for fiber optics
Aerospace
Industrial Applications Aerospace [image] Aerospace
Rockets and Missiles
The vast field of rockets and missiles takes advantage of many of the properties of graphite. Applications include tactical rockets, strategic rockets and missiles, and large, advance-launch systems. Rapid temperature rise and unusually high operating temperatures are encountered and unusual cone, nozzle and vane shapes are needed. Graphite is one of the few materials that can reasonably meet the demands encountered under these conditions. Of particular importance in this type of application are the excellent thermal properties of graphite – high thermal shock resistance, high thermal stress resistance, and a strength increase with temperature increase. In addition, its excellent machinability makes it possible to maintain the required close tolerances. For most short-time applications, firing times are normally less than one minute, graphite is very cost effective in the hot, reactive atmosphere.
Nuclear
Industrial Applications
Nuclear Industry
Nuclear industry
Nuclear graphite
Nuclear Technology
Requirements for specialty graphite
Examples of Application
Nuclear graphite
The element carbon has the ability of slowing down fast neutrons without capturing them. Carbon is therefore a valuable material for the construction and operation of power reactors of various types ranging from the CO2-cooled reactor with a cooling gas temperature of 400°C to the helium-cooled high temperature reactor with operating temperatures beyond 800°C. Only purified graphite can meet this special requirement combining strength and good neutron irradiation behaviour.
Nuclear graphite
This means that its mechanical or physical properties, e.g. strength or lattice constants, are not allowed to be strongly influenced or affected by irradiation. This objective can be achieved by using isotropic graphite. For the manufacture of such isotropic graphite, either an isotropic coke is used as the starting material, or one has to exploit the opportunities of process engineering. The fields of application in the reactor industry are widely varied and comprise the use of graphite as moderators and in fuel matrices, as reflectors in heavy-water reactors and as neutron shields in sodium-cooled fast-breeding reactors. back
Nuclear technology
Graphite finds widespread use in many areas of nuclear technology based on its excellent moderator and reflector qualities, which are combined almost uniquely with strength and high temperature stability. The function of a moderator is to slow fast neutrons to thermal velocities at which fission in Uranium-235 and Uranium-233 are most efficient. The reflector serves to reflect neutrons, which otherwise would escape, back into the active core region.
Nuclear grade graphite was developed for fission reactors.
Nuclear Industry
Requirements for specialty graphite (depending on the nuclear application)
- fine-grain to ultra
-fine-grain graphites
- high to ultra
-high chemical purity
- high mechanical strength
- constant properties in accordance with specification
- good moderating ration
- good surface structure
- small neutron absorption cross-section
Nuclear industry
Examples of applications
- tubes for the installation of fuel elements for nuclear power plants
- graphite sheets
- graphite lining for the torus
- graphite protection for antennas in nuclear fusion reactors
- specialty graphite components for the core structure
- moderation spheres made from specialty graphites
- graphite powder for the production of spherical fuel elements
Brakes (part of Aerospace)
Brakes
Composite materials composed of a carbon matrix reinforced by long carbon fibers can withstand high temperatures, and are very resistant to wear. Brakes made from such compositions are more reliable, reduce vibration and cause less pollution than traditional braking systems fitted on planes and road vehicles. The novel carbon braking system is produced for use on aircraft such as airbus. Similar braking systems are employed on racing cars, road vehicles and passenger trains.