Silicon Carbides is the only chemical compound of carbon and silicon. It was originally produced by a high temperature electro-chemical reaction of sand and carbon. Silicon carbide is an excellent abrasive and has been produced and made into grinding wheels and other abrasive products for over one hundred years. Today the material has been developed into a high quality technical grade ceramic with very good mechanical properties. It is used in abrasives, refractories, ceramics, and numerous high-performance applications. The material can also be made an electrical conductor and has applications in resistance heating, flame igniters and electronic components. Structural and wear applications are constantly developing.
Silicon carbides is composed of tetrahedral (structure) of carbon and silicon atoms with strong bonds in the crystal lattice. This produces a very hard and strong material. Silicon carbide is not attacked by any acids or alkalis or molten salts up to 800°C. In air, silicon carbides forms a protective silicon oxide coating at 1200°C and is able to be used up to 1600°C. The high thermal conductivity coupled with low thermal expansion and high strength gives this material exceptional thermal shock resistant qualities. Silicon carbide ceramics with little or no grain boundary impurities maintain their strength to very high temperatures, approaching 1600°C with no strength loss. Chemical purity, resistance to chemical attack at temperature, and strength retention at high temperatures has made this material very popular as wafer tray supports and paddles in semiconductor furnaces. The electrical conduction of the material has lead to its use in resistance heating elements for electric furnaces, and as a key component in thermistors (temperature variable resistors) and in varistors (voltage variable resistors).
Production of Silicon Carbide
Silicon carbides powders are produced predominantly via the traditional Acheson method where a reaction mixture of green petroleum coke and sand is heated to 2500°C using two large graphite electrodes. Due to the high temperatures, the Acheson process yields the alpha form of SiC, i.e. hexagonal or Rhombohedral (α-SiC). The SiC product, usually in the form of a large chunk, is broken, sorted, crushed, milled, and classified into different sizes to yield the commercial grades of SiC powder. To produce ultrafine SiC powder, the finest grade of the Acheson product is further milled, typically for days, and then acid-treated to remove metallic impurities. Fine SiC powder can also be produced using a mixture of fine powders of silica and carbon reacted at lower temperatures for short periods of time followed by quenching to prevent grain growth. The product, however, is agglomerates of SiC and needs to be attrition milled to break up the agglomerates and reduce the particle size to submicron range.
Silicon carbide Powder
Silicon carbides fibers are produced via the pyrolysis of organosilicon polymers, such as polycarbosilane, and are commercially available. Briefly, the process consists of melt-spinning the polycarbosilane at approximately 300°C, unfusing with thermal oxidation at 110-200°C, and baking at 1000-1500°C under a flow of inert gas. Nicalon fibers are known for their excellent mechanical properties when used as reinforcement in ceramic matrix composites. (CMC). The drawback of Nicalon fibers has been their oxygen and free-carbon contents, which limit their high temperature applications. Recently, however, Hi-Nicalon SiC fibers have been introduced with much lower oxygen content. At present, much of the work in the SiC fiber reinforced CMC development is using Hi-Nicalon SiC fibers. Another method for producing SiC fibers is via the CVD method. In this process, SiC is deposited from the gas phase on a tungsten wire used as the substrate. These fibers are stronger and have higher thermal stability due to their higher stoichiometry and purity.
Silicon carbides whiskers, which are nearly single crystals, are produced (grown) using different methods, including the heating of coked rice hulls, reaction of silanes, reaction of silica and carbon, and the sublimation of SiC powder. In some cases a third element used as a catalyst, such as iron, is added to the reacting materials to facilitate the precipitation of the SiC crystals. In this arrangement, the mechanism for the SiC whisker growth is called the vapor liquid-solid (VLS) mechanism. SiC whiskers are in the order of microns in diameter and grow several hundred microns in length. Currently, commercially available SiC whiskers are produced using the rice-hull process with the whisker growth being largely of VLS mechanism due to the absence of a catalyst. Because of their excellent mechanical properties, SiC whiskers are very desirable as reinforcements of metal and ceramic matrix composites for structural applications where fracture toughness and strength are significantly improved.
Properties of silicon carbide
In general, Silicon carbides has excellent oxidation resistance up to 1650°C. Oxidation resistance, however, depends largely on the amount of open porosity and particle size, which determine the surface area exposed to oxygen. The higher is the surface area the higher is the oxidation rate. Kinetically, SiC is stable in air up to ~1000°C.
Density and Porosity
Density, ρ, of a material is a measure of the mass, m, per unit volume, V, and is reported in units such as g/cm3, lb/in3, etc. Factors affecting the density include the size and atomic weight of the elements comprising the material, the tightness of packing of the atoms in the crystal structure, and the amount of porosity in the microstructure.
Porosity, which is occasionally reported along with density, is another important physical property used to indicate the amount of free space, i.e. not occupied by solid material. Porosity in general, open or closed, is very detrimental to the strength of the material, which is inversely exponentially proportional to the total porosity. Open porosity reduces the oxidation resistance of the non-oxide materials by allowing oxygen gas diffusion. In addition, a material with open porosity presents out gassing problems under high vacuum conditions. Therefore, it is very important to accurately measure total porosity and determine what percentage is open porosity.
The flexural strength is defined as a measure of the ultimate strength of a specified beam in bending. The beam is subjected to a load at a steady rate until rupture takes place. If the material is ductile, like most metals and alloys, the material bends prior to failure. On the other hand, if the material is brittle, such as ceramics and graphite, there would be a very slight bending followed by a catastrophic failure. There are two standard tests to determine the flexural strength of materials: the four-point test and the three-point test. In the four-point test, the specimen is symmetrically loaded at two locations that are situated one quarter of the overall span between two support spans. In the three-point test, the load is applied at the middle of the specimen between two support bearings.
Due to its high thermal conductivity, silicon carbide is a very attractive material for high temperature applications. From the device design point of view, the thermal conductivity of SiC exceeds that of Cu, BeO, Al2O3, and AlN. The thermal conductivity of SiC single crystal has been reported as high as 500 W/m⋅K. However, most commercial silicon carbides grades have thermal conductivity in the range 50-120 W/m⋅K. The high thermal conductivity of other commercial SiC products, such as POCO’s SUPERSiC, is attributed to the absence of thermal-conduction-inhibiting impurities on the crystal grain boundaries. Basically, SUPERSiC is a continuous phase of SiC with no obvious grain boundaries.
Applications of Silicon carbide
There are many uses of Silicon Carbide in different industries. Its physical hardness makes it ideal to be used in abrasive machining processes like grinding, honing, sand blasting and water jet cutting.
The ability of Silicon Carbide to withstand very high temperatures without breaking or distorting is used in the manufacture of ceramic brake discs for sports cars. It is also used in bulletproof vests as an armor material and as a seal ring material for pump shaft sealing where it frequently runs at high speed in contact with a similar silicon carbide seal. One of the major advantages in these applications being the high thermal conductivity of Silicon Carbide which is able to dissipate the frictional heat generated at a rubbing interface.
The high surface hardness of the material lead to it being used in many engineering applications, in which high degree of sliding, erosive and corrosive wear resistance is required. Typically this can be in components used in pumps or for example as valves in oilfield applications where conventional metal components would display excessive wear rates that would lead to rapid failures.
The unique electrical properties of the compound as a semiconductor make it ideal for manufacturing ultra fast and high voltage light emitting diodes, MOSFETs and thyristors for high power switching.
The material’s low thermal expansion coefficient, hardness, rigidity and thermal conductivity make it an ideal mirror material for astronomical telescopes. Silicon Carbide fibers, known as filaments are used to measure gas temperatures in an optical technique called thin filament pyrometry.
It is also used in heating elements where extremely high temperatures need to be accommodated. It is even used in nuclear power to provide structural supports in high temperature gas cooled reactors.
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