Property and application
Graphite is carbon crystal andnonmetal material with the natures of silver gray, soft texture, metal luster,Mohs hardness 1~2, specific gravity 2.2~2.3 and volumetric weight 1.5~1.8 ingeneral.
Graphite features very highmelting point. It starts to melt at 3000°C in vacuum and evaporates at 3600°C.Strength of most materials decreases under high temperature but, on thecontrary, the strength of graphite will increase in double when it is heated to2000°C. However, graphite’s oxidation resistance property is poor andtemperature rise may quicken the Oxidation process of graphite.
Graphite is noted forexcellent thermal and electrical conductivity, which is 4 times that ofstainless steel, 2 times that of carbon steel and 100 times that of mostnonmetal materials. Of graphite, thermal conductivity is better than that ofmetals, including steel, iron and lead, but decreases when temperature rises.Unlike metals, when temperature is extremely high, graphite will be of thermalinsulation. So, the property of graphite in heat insulation under hightemperature is accountable.
Graphite features excellentlubrication and plasticity. Friction coefficient of graphite is less than 0.1.Graphite can be extended into poromeric and transparent thin flakes. Due tohigh strength, graphite is unlikely processed even with a diamond cutter.
Graphite has stable chemicalproperties and resists corrosions of acid, alkali and organic solvent.Currently, graphite has been widely used in modern industries.
1. Fireproof material: graphite and graphite products have theproperties of high temperature resistance and high strength and are used forproduction of graphite crucible, protection of iron ingot and inner lining inmetallic furnace.
2. Electric conducting material: in electrical industry,graphite is used for production of electrode, brush, carbon rod, carbon tube,positive pole of mercury rectifier, washer, telephone component and coating oftelevision tube.
3. Antifriction material: graphite is often used as lubricant inmachinery production. Common lubricating oil cannot be used under high rotationspeed, high temperature and high pressure. The equipment made of graphiteantifriction materials can be operated at high rotation speed under 200~2000°Cwithout any lubricating oil. Graphite is also used in piston ring, sealing ringand bearing of the equipment of corrosive medium delivery and does not requireany lubricant. Graphite emulsion can be used as lubricant in metal wire andtube drawing.
4. Graphite with stable chemical property: specially treatedgraphite features excellent corrosion resistance and thermal conductivity andlow penetration rate and, as a result, can be used in heat exchanger, reactiontank, condensator, combustion tower, absorbing tower, cooling and heatingsystems, as well as filter and pump in industries of petrochemical,hydrometallurgy, acid/alkali, synthetic fiber and paper making as asubstitution of metal materials.
5. Used in foundry, molding, pressing die and high tempmetallurgy: due to minor coefficient of thermal dilation and property ofthermal shock resistance, graphite is used in glass ware molding to ensureprecise dimensions of cast ferrous metal and excellent surface smoothness,resulting in significant reduction of metal consumption. In production of metalpowder, such as hard metal alloy, graphite is used in pressing die andsintering crucible. Highly purified graphite is used in single crystal silicongrowth crucible, regional refinery vessel, support clamp and induction heatingapparatus. In addition, graphite can also used in heat insulation, basesupport, high temperature furnace tube, rod, panel and grate in process ofvacuum metallurgy.
6. Used in A-energy and national defense industry: as a neutronmoderator, graphite is used in atomic reactor, ie. uranium-graphite reactor,the most popular one in the world at present. Graphite meets the requirementsof atomic reactor due to its properties of high melting point, stability and corrosionresistance when used as a moderator. When used in atomic reactor, impurity ingraphite must not be in excess of some dozens of PPM. Especially the content ofboron should be less than 0.5PPM. In national defense industry, graphite isused for production of nozzles of solid fuel for rockets, nose cone ofmissiles, and parts, insulation and ray-proof components of spacecrafts.
7. Graphite has the function to prevent scale from buildup inboilers. Experiment result shows that scale does not form on boiler surfaces ifa certain quantity of graphite is added into water, usually 4~5g per ton. Inaddition, graphite can prevent rusts on metal chimney, building roof, bridgeand pipeline.
8. Graphite can be used as pencil core, pigment and polishingagent. After special treatment, graphite is used in some industries asimpervious graphite, highly oriented graphite and graphite fabrics.