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The contents of this article will provide you with everything you will need to know about graphite blocks and their use.
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Graphite blocks are a crystalline form of carbon that are engineered to have special properties such density, electrical resistance, hardness, porosity, compressive strength, flexural strength, thermal expansion, and thermal conductivity. Although graphite occurs naturally, a major portion of graphite blocks for industrial use is synthesized and produced from petroleum coke or coal tar pitch. High purity graphite blocks, known as molded graphite, have a carbon content of 99.99% with special properties and characteristics for which graphite blocks are famous.
Natural graphite (NG) is a naturally occurring form of crystalline carbon that is found in metamorphic and igneous rocks. It is used as a thermal management material and mold lubricant as a heat dissipation material. There are several types of naturally occurring forms of carbon with diamonds and graphite being the most common.
Graphitization of carbon is a process that converts carbon into graphite. It involves heating amorphous carbon to rearrange the atomic structure to achieve the crystalline structure of a solid. During the process, carbon atoms are rearranged to fill atom openings for better atom layouts. The rearranging of atoms takes place in the presence of oxidizing gasses, which breaks the bonds of amorphous carbon.
In its natural form, graphite is grayish black and opaque with a black sheen and has metal and non-metal properties. It is a chemically inert and highly refractory material with high thermal and electrical conductivity. The unique properties of graphite are due to its crystalline structure with carbon atoms set hexagonally in a planar condensed ring system placed in layers that are stacked parallel to each other.
The two main types of graphite are natural and synthetic where natural graphite is composed of graphitic carbon and varies in crystallinity. Synthetic graphite is made from coke and coal pitch with a less crystalline structure than that of natural graphite and contains graphitic carbon produced by graphitization.
The first step in the manufacturing of graphite blocks is choosing the raw materials. The choice of raw graphite is dependent on the requirements of the application for which the material is being chosen and the desired properties of the product being produced with the main choice being between natural graphite and synthetic graphite. For the best results, the highest purity of graphite is chosen, which produces the highest quality products.
Natural graphite is available as amorphous graphite, flake graphite, and crystalline vein graphite. Each of the various types has properties and characteristics that make them ideal and appropriate for different applications. Amorphous graphite is formed by the metamorphosis of anthracite coal and a metamorphic agent, the result of which is a microcrystalline graphite. Flake graphite is formed by carbon being placed under high pressure and temperature. It is found in metamorphic rock. Crystalline vein graphite is thought to be a naturally occurring pyrolytic that has been melted naturally and flowed into the cracks and crevices of rocks.
Synthetic graphite, or artificial graphite, has two forms, which are primary and secondary graphite that are distinguished by the different methods used to produce them. Primary synthetic graphite is made from the high temperature treatment of coke while secondary synthetic graphite is a byproduct of graphite electrode and part manufacturing. Both types have a dark gray to black appearance and are graphitized at °C (°F). Synthetic graphite has high purity with excellent lubrication properties and electrical conductivity.
To ensure the quality of a graphite block, the raw graphite, whether it is natural or synthesized, undergoes a purification process. The methods for purification include chemical treatments, thermal processing, and mechanical methods. The types and amounts of impurities vary from batch to batch, which necessitates examining the raw graphite to determine the quantity of impurities.
The kinds of impurities found in graphite are potassium, sodium, aluminum, calcium, magnesium, and various silicate minerals. Most natural graphite has a low carbon content except for vein graphite. Flake graphite is purified to improve its carbon content. Prior to purification, the raw graphite is crushed to improve the efficiency of the purification process.
Thermal purification is completed at °C (°F) and produces high purity graphite. It is an expensive process that involves the use of specially designed furnaces and high energy consumption. The chemical method of purification involves the use of hydrofluoric acid, alkali acid, and chlorination roasting.
Chemical purification removes hydrophobic impurities using an acid wash that creates a chemical reaction between the acid and the impurities. The process purifies and acts as a pretreatment before the flotation process. The purity of graphite achieved using the chemical method is as high as 99.5% and is a low-cost method.
The flotation method is another process used to purify graphite and is based on the wettability difference between graphite and other minerals. During flotation, graphite floats up with bubbles, while hydrophilic useless minerals remain in the water leading to flotation separation. Although the flotation process is widely used, it is adjusted to conform with the type of graphite being processed and involves several steps that includes multiple grindings of the graphite.
The powder from the purification stage is mixed with synthetic resins, coal tar pitch, or petroleum pitch that serve as binders to produce a graphite paste. The binders help hold the particles together during the shaping and forming stages when the graphite powder is compressed into blocks. The amount of the binder differs between the various types of graphite due to the different binding points for each type.
There are several other factors that are an important part of the blending and mixing process, which include the requirements of the application for which the graphite is being prepared. Some of the influences include particle size, shaping method, and the specific needs of the graphite block user.
The shaping and forming of the graphite blocks are completed using isostatic pressing, extrusion, or die molding. The choice of method is generally the decision of the manufacturer and the needs of the customer.
Extrusion &#; Extrusion is a process where the graphite paste is forced through a die that has the desired shape of the graphite block. The paste is squeezed along the cylinder of the extruder toward a die that has the shape of the graphite block. During the process, the paste is compressed under pressure as the baffle moves along the cylinder. As the shaped paste exits the cylinder through the die, it is cut to the desired length, checked, and cooled. Extrusion is a continuous process that can produce any number of graphite blocks continuously.
Compression Molding &#; With compression molding, as with vibration molding, the mold has the shape and size of the graphite block. The mold is filled with the graphite paste and pressure is applied above and below the mold to compress and form the paste into the shape of the mold. It is a slow method for forming graphite blocks with one graphite block being formed during each cycle. Compression molding is used as an alternative to extrusion since it can produce any size of graphite block. The graphite blocks formed by compression molding have exceptional mechanical strength, friction resistance, density, hardness, and conductivity.
The compacted graphite blocks are heat treated in a furnace at temperatures that range between 900°C up to °C (°F up to °F). During the baking process, carbonization and thermal decomposition of the binder occurs where the binder turns into elemental carbon and volatile components. The purpose of the baking cycle is to turn the graphite blocks into solid carbon, a process that is carefully controlled and requires an extended period of time.
Graphite is a very porous material that is capable of absorbing water at a slow rate, an aspect of the material that is not ideal for certain applications. In order to overcome the porosity of graphite blocks, they are impregnated with other materials that alter the characteristics and properties of the blocks. The general term impregnation refers to several different processes used to inject materials into the graphite blocks. The choice of method is in accordance with how the graphite blocks will be used. The impregnation material has a lower viscosity than that of the binder material, which is necessary in order for the impregnated material to fill the gaps in the graphite blocks. In most cases, petroleum pitch is used. High density graphite grade blocks are impregnated and rebaked several times.
Graphitization crystallizes the carbon to create crystalline graphite due to the temperature used during graphitization with temperature ranging between °C and °C (°F and °F). During graphitization, crystallites grow and rearrange into a stacked parallel plane pattern, a factor that changes the properties of the graphite blocks. In addition, graphitization purifies the graphite blocks due to the high temperature of the process, which causes impurities, such as binder residue, gases, oxides, and sulfur, to vaporize.
The process of graphitization takes place in an Acheson furnace that has a central rectangular chamber surrounded by walls made of refractory materials. An Acheson furnace is designed to keep the heat generated by electrical resistance in the furnace. During graphitization, oxygen has to be eliminated, which is accomplished by covering the graphite blocks with oxygen scavenging material.
The wide use of graphite requires close adherence to the manufacturing and production steps of the process since each graphite block is designed to meet a specific industrial requirement. After the completion of the processing steps, each graphite block is carefully inspected to ensure that it meets the necessary qualifications of the industry for which it was produced. The blocks are labeled, documented, and given identifying data such that they can be traced for integration into an assembly.
The steps of the manufacturing process, from careful selection of raw materials to forming, baking, and molding, are followed with attention to details and close adherence to manufacturer quality standards. Each industry that uses graphite blocks has a set of standards, which necessitates that graphite block producers observe those standards such that the final products produced from the graphite blocks are of the highest quality.
There are certain factors that differentiate synthetic graphite blocks from natural graphite blocks. Both forms have carbon as their base material but have distinctly different characteristics, processing methods, and applications. Natural graphite blocks are less expensive, have high capacity, and involve less energy consumption. Synthetic graphite blocks have higher density with thermal conductivity of 700 W up to W.
Most of the steps that are used to produce graphite blocks from natural graphite are used for the production of synthetic graphite with the key difference between the processes being the raw material from which synthetic graphite is synthesized. The production of synthetic graphite blocks starts with green petroleum coke extracted from the refining or catalytic cracking of heavy oils.
The methods used to produce synthetic graphite are similar to the methods used to produce ceramic materials. Coke and graphite are ground and mixed with carbon-based pitches to form a homogeneous mass. Once the mass is formed, the steps of the manufacturing of synthetic graphite blocks follow the same steps for the production of any other forms of graphite blocks.
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The process for the manufacture of synthetic graphite was introduced in by Charles Street. To produce synthetic graphite, Street graphitized amorphous carbon. The preferred material that is used to produce synthetic graphite is petroleum coke, which has heavy fractions that are a waste product of crude oil production. A less expensive type of raw material for the manufacture of synthetic graphite is pitch coke that comes from coal tar.
The base material for all graphite is carbon, which is found in diamonds and graphite. The difference between the two forms is the shape of their atoms with diamonds having atoms with a tetrahedral shape while graphite atoms having a hexagonal crystalline structure that forms a plane.
To produce synthetic graphite, amorphous carbon is heated for an extended period of time, a process known as graphitization rearranges the atomic structure of the graphite atoms. In essence, carbon atoms are changed by having the openings in its atoms filled and the layout of the atoms adjusted. Steady temperature increases change the crystal structure of carbon and brings it closer to forming graphite with its mechanical properties and layered structure. The resulting characteristics from graphitization include an increase in graphite&#;s lubrication properties, oxidation resistance, and thermal properties.
Graphitization Process Temperature Changes Caused by Temperature Increases Room Temperature - °C There are no changes in the carbon atoms, but minimal structural changes begin. °C - °C With the increase in temperature, crystal structures grow, which indicates movement and the rearrangement of atoms. As the atoms rearrange and change, the spacing between them changes and they shrink. °C - °C As the crystal growth increases, spacing continues to decrease and open, spaces diminish and are filled.Electrographite synthetic graphite is produced from pure carbon taken from coal tar pitch and calcined petroleum coke that is heated in an electric furnace. Another form of synthetic graphite is made from heated calcined petroleum pitch. Regardless of the raw materials used to produce synthetic graphite, each form does not have the same crystalline structure as natural graphite but is of exceptionally high purity.
The key to synthetic graphite is the graphitic carbon used in its formation, which comes from graphitization. Included in the characteristics of synthetic graphite is its high electrical resistance and porosity as well as its very low density. The porosity of synthetic graphite makes it inappropriate for refractory applications.
Characteristics of Synthetic Graphite Property Effect of Graphitization on the Properties of Graphite Reason for the Change Lubricity Increase Van der Waals forces in graphite&#;s atomic structure are broken to allow the layers of graphene to slide off and deposit onto a counter surface to provide lubrication for applications. Oxidation Resistance Increase As graphitization progresses, a crystal structure forms and there are less non-bonded atoms leaving fewer places for oxidation, giving graphitized graphite oxidation resistance. Thermal Conductivity Increase The structure of graphite allows for heat to flow through the material to avoid heat buildup. Coefficient of Friction Decrease The layered structure of the graphite allows graphene to rub off when placed against a counter surface because the Van der Waals forces connecting graphene layers are easily broken. Hardness Decrease The Van der Waals forces in graphite can be easily broken compared to its intertwined amorphous layout, which makes graphite a much softer material. Strength Decrease Graphite&#;s layered structure results in it being softer and having less strength compared to the harder and stronger carbon graphite that has amorphous carbon in it.Graphite blocks come in several different types with each type designed for a specific manufacturing and industrial application. When examining the types of graphite blocks, the first classification is by the grain structure of the blocks, which can be fine, medium, or coarse. Other methods used to define and categorize graphite blocks are by their purity, crystalline structure, and their characteristics and properties.
Pyrolytic graphite blocks are made by the decomposition of hydrocarbon gas, normally methane, in a vacuum furnace to create exceptionally pure graphite. Processing is very slow, takes a long time, and is very expensive. Graphite methane or hydrocarbon gas is heated under low pressure at °C (°F), a process that generates layers of graphite with an easy to machine surface non-porous.
One of the interesting properties of pyrolytic graphite is its diamagnetism, which is the ability to repel or be repelled by a magnetic field. The uses of pyrolytic graphite include heating and cooling conductors in the rocket industry, a neutron modulator for nuclear reactors, and in high power vacuum lamps. The list of other products produced from PG block includes sputtering targets, ion beam grids, ion implant hardware, liquid phase epitaxy hardware, crucibles for ultra-high vacuum, thermal insulators, rocket nozzles, and heater elements.
Amorphous graphite blocks are made of microcrystalline graphite and are known as aphanitic or cryptocrystalline graphite. They are a dense aggregate graphite made up of tiny natural graphite crystals with a gray black or steel gray color that gives the graphite a shiny metallic appearance.
Unlike its metallic appearance, amorphous graphite blocks are soft to the touch with a texture that feels smooth and easily colors your hands. Aside from its appearance, amorphous graphite blocks include chemical stability, thermal and electrical conductivity, high temperature resistance, and resistance to acid and alkali, corrosion, and oxidation. The positive properties of amorphous graphite blocks make amorphous blocks ideal for casting, coatings, batteries, and carbon because of its small crystals, plasticity, and good adhesion.
Flake graphite blocks are formed from natural graphite taken from metamorphic rock. It has a layered structure with carbon atoms formed in a hexagonal lattice with each layer having carbon atoms with a sp2 configuration. The layers are weakly bonded by Van der Waals force, which gives the flake graphite blocks their flaky nature.
With its shiny appearance and surface, flake graphite blocks are an excellent reflector of light, which allows it to align with any surface in order to provide lubrication that can withstand high temperatures for extended periods of time. When chemicals are added to the layers of flake graphite blocks, the Van der Waals bonds weaken and the volume of the blocks can expand to 300 times their volume, a factor that has caused flake graphite blocks to be known as expandable graphite.
Crystalline vein graphite is a natural form of pyrolytic carbon that can be flake like with fine particles or have medium particles. Unlike other forms of graphite blocks, crystalline vein graphite blocks are the most crystalline of the various graphites with purities ranging from 80% up to 90% carbon and come in powder form of 3 µ and lumps of 8 cm up to 10 cm.
Known as crystalline vein graphite, plumbago, Sri Lankan, and Ceylon graphite, it is one of the most difficult types of graphite to describe, which has led to a wide range of theories regarding its origins. It is a vein graphite that is unlike amorphous graphite or other minerals and is found in veins and fissures in rocks. Crystalline vein graphite is formed by the deposition of graphitic carbon that has been melted by naturally occurring high temperatures. The deposits of crystalline vein graphite are exceptionally pure and are typically above 90% with most being at 99.5%.
The main use of crystalline vein graphite blocks is in electrical applications with current carrying electrical motors using brushes made from crystalline vein graphite. It is also used for brake and clutch applications where it lines brake shoes as a substitute for asbestos.
Synthetic graphite blocks have become extremely popular due to their ready availability and their higher purity. The form of synthetic graphite blocks determines the industry for which it will be used. Common uses for synthetic graphite blocks, referred to as isotropic graphite, are energy storage in the solar industry. Synthetic graphite blocks are made from petroleum coke with the final graphite having a slightly different structure than that of other forms of graphite blocks.
The wide use of synthetic graphite blocks is in steel furnaces and aluminum smelters where synthetic graphite&#;s high energy density, low cost, and scalability provide a cost advantage. The blocks of synthetic graphite are heated to drive turbines where the infrared radiation from the synthetic graphite blocks is converted into electricity. Although the cost of synthetic graphite is higher than natural graphite, in the case of graphite blocks for producing energy, the blocks are produced in high volume, which significantly lowers their cost.
One of the ways to separate the many types of graphite blocks is by their grain size, which helps in deciding what type of graphite block is appropriate for a specific application. As a part of the selection process, grain size determines the appropriateness of a particular graphite block for an application.
Fine grain graphite has high density that produces precision machined details with exceptional finishes, a factor that reduces wear. For a graphite block to be designated as fine grain, its material has particles that range in size from 0. in up to 0.005 in (0. mm up to 0.127 mm) with the grains having been milled to achieve the particle size and pressed into the graphite block shape. Approximately 5% to 15% of the volume of fine grain graphite is made up of openings between the particles that are hard to see due to their very small size. Since fine grain graphite is very dense, it is very commonly produced in small cross sectional blocks.
There are an endless number of uses for fine grain graphite due to its formability and high density. Crucibles, continuous casting dies, rocket nozzles, electrical brushes, heating elements, seals and jigs are a few of the components that are produced using fine grain graphite blocks.
Medium grain graphite blocks are used for roughing and finishing applications and have a grain size of 0.020 in up to 0.062 in (0.508 mm up to 1. mm) with 12% to 20% of the volume of the blocks being porous and are visible with the naked eye due the particles being the base material. The production of medium grain blocks is less expensive and does not involve the use of isostatic molding but is completed using extrusion or compression molding. Medium grain graphite is used to produce furnaces, trays, extrusion guides, heating elements, crucibles, and self-lubricating bears, which is one of its major uses.
Coarse grain graphite is an economical solution for processes that require large amounts of raw materials. The grain size for coarse grain graphite ranges between 0.040 in and 0.25 in (1.016 mm and 6.35 mm) with porosity ranging between 12% and 20%. Coarse grain graphite is prized for the manufacture of crucibles, large ingot molds, and pouring troughs due to its ability to handle thermal shock and rapid changes in temperature from molten metals. The very large particles of coarse grain graphite are easily visible to the naked eye. The strength and stability of coarse grain graphite makes it ideal for forming large parts.
The main use of graphite blocks is in furnaces due to their thermal shock resistance and low thermal expansion. These particular characteristics of graphite blocks have made graphite blocks valuable for a wide range of industrial applications. Chemically stable, machinable, and lightweight, graphite blocks have become an essential part of manufacturing.
Graphite blocks producers offer graphite blocks in a wide range of sizes to fit the needs of any size company. In many cases, graphite blocks are custom ordered to meet specific needs and requirements. It is this flexibility that has made graphite blocks so important and an easy way to put graphite to use.
In powder metallurgy, graphite blocks are used in sintering where raw materials are placed on a graphite block and melted. The high temperature and oxidation resistance of graphite blocks meets the demanding requirements of the powder metallurgy industry. The blocks can be used repeatedly, which saves users the cost of production.
Metals can be heated in a graphite crucible up to °F (°C) to convert metals into liquid form for graphite mold casting, a method that can be used to cast all forms of industrial products. Graphite molds are similar to metal molds and have good thermal conductivity and thermochemical stability. Castings using tin bronze and aluminum iron bronze use graphite molds to eliminate casting defects like shrinkage, porosity, and pinholes with the added benefit of better mechanical properties.
Graphite blocks used for the manufacture of electrodes have high electrical conductivity and refractory properties such as thermal shock resistance and low thermal expansion. They are the only products that are able to endure the necessary electrical conductivity for electric arc furnaces and endure the high levels of heat.
Synthetic graphite blocks are most commonly used for moderators or reflectors for nuclear reactors. For uranium fission to occur properly, the neutrons created have to be slowed down by a neutron moderator, an element with low atomic weight. Initially, heavy water was used but was replaced by graphite with very high purity. Graphite blocks for nuclear fusion have to be of the highest purity and be free of boron, which absorbs neutrons.
High-purity graphite block fixed carbon content greater than or equal to 99.9%, mainly for flexible graphite sealing materials, nuclear graphite, instead of platinum crucible for chemical reagents melting and lubricant base, high purity graphite block for silicon carbide furnace, graphite Furnace and other metallurgical furnaces, furnace resistance furnace lining and conductive materials, as well as impermeable graphite heat exchanger.