Washington Mills offers an intricate process for manufacturing silicon carbide. CARBOREX (r) SiC can be found across industries like abrasives, metalcasting, metallurgy and refractories – offering multiple chemistries and sizes suitable for many different applications.
Edward Acheson first created his Acheson process in 1893 and it remains one of the primary methods for producing SiC. It combines petroleum coke with high purity crystalline silica in an open atmosphere 30-kA electrical resistance furnace to form SiC; emitting toxic SOx, NOx and heavy metals into the atmosphere during processing.
Raw Materials
Silicon carbide manufacturing processes involve mixing high-grade quartz sand with petroleum coke as raw materials. After being ground and acid washed, these materials are magnetically or physically screened to yield products of various particle sizes. Silicon carbides are widely used as raw materials in functional ceramics, advanced refractory materials and abrasives production processes, as well as being utilized as metallurgical raw materials. Most commonly, black silicon carbide (a-SiC) and green silicon carbide (b-SiC) varieties of silicon carbide are employed. Black silicon carbide boasts greater toughness than green silicon carbide and is typically employed for grinding of low-tensile strength materials such as glass, ceramics, stone and refractory materials. Additionally, this grade is used for honing cylinder liners and precision grinding of high-speed steel cutting tools.
A-SiC is an extremely pure material with great hardness and thermal conductivity properties. With its wide band gap, A-SiC makes an excellent material choice for semiconductor applications; in addition, its thermal conductivity allows it to increase electric power utilization rates and energy efficiency for new energy vehicles and high-tech equipment.
Silicon carbide synthesis is a complex process, with its quality being determined by both its chemical composition and crystal structure. There have been various techniques developed for producing SiC; one of the most popular being Acheson process. This continuous sealed method takes place inside an electric resistance furnace called an Acheson graphite furnace; however it involves placing silicon and carbon atoms together in an enclosed high temperature cavity which requires skillful manipulation by skilled artisans.
Acheson Process
The Acheson Process is a traditional method for producing silicon carbide. It involves high-temperature reactions between silica and carbon that produce a fine powder comprised of quartz sand (silicon dioxide) and petroleum coke that is then compressed into an ingot which can later be cut and ground to meet specific industrial applications.
An Acheson furnace features a graphite core which serves both as the heat source and nucleation site for SiC crystal growth. Surrounding this core lies a reaction mixture composed of silica in the form of sand or quartz combined with carbon from previous production runs (typically petroleum coke from previous runs), often mixed with sawdust for control purposes and additives to ensure purity and favorable reaction conditions.
Initial experiments conducted by Acheson proved fruitless, yet he persisted with different ratios and temperatures until finally patenting his work in 1891 and giving the material its trademark name of Carborundum which comes from Latin for fused alumina minerals such as corundum.
Manufacturers have attempted to increase yield of coarse crystalline silicon carbide product by increasing furnace size, power of electrical resistor and changing other process variables; however, these modifications have failed to significantly enhance efficiency of production process. Furthermore, any unreacted fire sand that remains after each production run must be recycled using significant energy consumption as well as manpower resources to clear away and prepare for subsequent cycles.
Cold Isostatic Pressing
Cold Isostatic Pressing is a method for powder compaction used to compact ceramics, graphite and metals. This technique involves encasing materials within an elastomer mold filled with liquid medium; pressure is then evenly applied across all sides of the mold for compression without any distortion, producing isotropic high-density bodies without lubricants being necessary.
Once complete, the resulting saggar body can then be sintered at high temperatures to achieve the desired physical properties. Accurate control of sintering temperatures is key in producing an acceptable densification and microstructure; additionally, avoiding “elephant feet”, areas with larger diameter than surrounding parts which must be eliminated, must also be considered when producing satisfactory sintered samples.
CIP processing is typically used to manufacture ceramics; however, its use for metal processing remains less well-known. With improvements to powder metallurgy and sintering capabilities making CIP more feasible for some applications – for instance tungsten carbide can now be produced using CIP, with its properties greatly enhanced due to this method. It is also useful in producing complex-shaped parts which cannot be made through traditional green machining methods; however these parts must undergo extensive post-processing inspection in order to meet quality standards.
Heat Treatment
Silicon Carbide (SiC) is a wide band semiconductor material, known for its superior voltage and current switching characteristics as well as radiation resistivity and thermal conductivity. Fabricated through various methods including confinement controlled sublimation (CCS), vapor deposition method and an ion beam etching process, SiC can also be doped n-type with nitrogen or phosphorus dopants and doped p-type with beryllium, boron or aluminium for metallic conductivity purposes – plus it comes in various sizes and shapes to meet specific application needs.
SiC’s alpha form (a-SiC) is the most frequently occurring polymorph, featuring an hexagonal crystal structure similar to Wurtzite. Meanwhile, beta modification (b-SiC) requires more effort to form; it has zinc blende crystal structure which makes its formation harder. A-SiC polymorphs tend to form more readily at higher temperatures; however b-SiC polymorphs offer superior thermal shock resistance making them preferable in certain applications.
In order to prevent cracking during processing, silicon carbide ingots must first be heat treated prior to being cut into wafers. This invention provides an improved method for heat treating silicon carbide ingots by lowering both crucible temperature and atmospheric pressure during crystal growth, thus reducing stress in the resultant silicon carbide ingot and increasing crystal quality; constituent stress states in such ingots can then be determined using high-energy X-ray diffraction and Raman microspectroscopy.
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