Recrystallized silicon carbide has become one of the most versatile and widely used refractory ceramics. It offers the unique combination of excellent hardness and chemical resistance as well as a broad range of applications in a wide cross-section of industrial markets.Its properties are particularly good for a number of high-temperature structural materials and kiln furniture as well as thermoelectric energy conversion, etc.
Hardness
Silicon carbide (SiC) is a highly hard, brittle and fracture-resistant material. It is a versatile component used in a wide range of industries.SiC is formed from a chemical reaction between carbon and silicon. The most common production method is the Acheson process, where finely ground coke is surrounded by a carbon conductor within a brick electrical resistance-type furnace. Electric current is passed through the conductor, bringing about a chemical reaction between the carbon in the coke and silicon in the sand to produce SiC and carbon monoxide gas.A-SiC has a hexagonal crystal structure and is the most common polymorph form. It is commonly produced by sintering alpha silicon carbide (a-SiC) at temperatures above 1700 deg C, but can also be made by sintering beta silicon carbide (b-SiC).In addition to the a-SiC polymorphs, there are a number of other silicon carbide materials with various chemical structures. One of these is called b-SiC, which is characterized by a zinc blende crystal structure that is similar to diamond. Another is p-Al8FeMg3Si6, which has a crystalline structure with a cubic phase and a brittleness that is similar to tungsten.Another silicon carbide material with a high density is liquid-phase sintered silicon carbide (LPSIC). This material is manufactured from silicon carbide and an oxide or oxynitride ceramic powder. The oxide components are responsible for the material’s high density which is approx. 3.24 g/cm3 and can be found in a variety of applications including thermocouple protection tubes and burner nozzles.Liquid-phase LPSIC is manufactured from a mixture of SiC powder and oxide powders, often based on aluminium oxide. It is characterised by a fine-grained matrix with grain sizes 2 um, almost free from pores and very high strength and fracture toughness.Porous RSiC ceramics have been prepared by a new approach combination of carbothermal reduction and recrystallization sintering. Firstly, micro-sized SiC particles were used as the skeleton, and SiC spherical nanocrystals were in-situ synthesized by a vapor-solid reaction between carbon nanoparticles and silicon monoxide vapor. After recrystallization, high purity a-SiC porous ceramics with tailored necking area were obtained. X-ray diffraction (XRD) patterns showed that the shape and diameter of the pristine carbon nanoparticles were related to those of the SiC nanocrystals, which can be observed as a shape memory effect. This property can be exploited to enhance flexural strength of the porous RSiC ceramics by optimizing the d/d0 value, which is the ratio of the neck diameter to the coarse micron-sized grains diameter.
Corrosion Resistance
Recrystallized silicon carbide is a ceramic material that can be used in a wide range of applications. It has excellent thermal conductivity, resistance to corrosion, and low permeability. It also provides a good electrical insulation.It is a very popular material for producing nozzles because it can provide long life and minimal wear, which means reduced downtime. It is about one fifth the weight of tungsten carbide and provides superior corrosion and abrasion resistance. It is often produced as a semi-finished component and therefore a more cost effective choice than WC.Another use for the material is in forming abrasives, as it has high hardness and can be made to lap and grind hard materials. It can also be mixed with a vehicle to form a paste or stick and shaped to form abrasive sheets, disks and belts.Sintered silicon carbide is the most common ceramic alternative to tungsten carbide for blast nozzles because it offers superior corrosion and abrasion resistance. The low wear rate also means that it maintains the internal geometry of the nozzle, so it has a longer service life than tungsten carbide.It is also very useful in refractories because it can provide high temperature strength and oxidation resistance. It is a good choice for making boiler furnace walls, checker bricks, muffles, kiln furniture, furnace skid rails, try for zinc purification plants and more.Besides, it has high abrasion resistance and can be used to form wear resistant products such as brake linings and electrical contacts. It can also be used in a range of industrial applications including mining, chemical processing and pulp and paper production.Nitride bonded silicon carbide is another type of ceramic that can be used in a variety of applications. It is manufactured by nitriding silicon carbide grains and metallic silicon powder in a nitrogen atmosphere at approximately 1,400 degC. This results in a chemical reaction between the carbon and silicon atoms, which creates a bond between the grains of silicon carbide and the nitrided silicon.The nitride bonds between the SiC grains form a thin layer of oxidation that acts as a barrier to corrosion. This type of refractory is generally only used above a temperature of approximately 1,380 degC, as the melting point of metallic silicon below this temperature prevents its use.
Thermal Conductivity
Recrystallized silicon carbide is a very hard and durable material, which is used in a variety of industrial applications due to its high strength and excellent thermal shock resistance. It has been found to be a good choice for bearing components, such as thrust and journal bearings that operate at very high speeds.This type of ceramic is also a good choice for state-of-the-art magnetically driven pumps, as it has high thermal conductivity and is corrosion resistant in chemical environments. It is also used as a substrate material for ultra-pure semiconductor devices.There are many different ways of preparing SiC ceramics and these methods all result in a wide range of properties. This is because each method produces a different microstructure and each material has its own unique properties.For instance, a boron carbide based ceramic has a higher thermal conductivity than a silicon carbide based material. However, this material is prone to cracking at low temperatures and is not as durable at high temperatures.On the other hand, a ceramic made from a boron carbide based material is also more difficult to sinter at a lower temperature, as it has more porosity and fewer grain boundaries than a pure silica based ceramic. The sintering process for these types of materials can be either liquid phase or hot pressing and isostatic sintering, depending on the application and desired property.In order to achieve a higher density and better thermal conductivity, sintering aids such as Al2O3 and Y2O3 are added to the sintering mixture. These sintering aids help promote the densification of the green body. They also increase the sintering temperature, as well as promoting a higher rate of sintering.Another type of sintering aid used to promote the densification of SiC is boron carbide powder. This powder is mixed with a silica powder in a molar ratio of 3:1 to 5:1. It is then heated in a closed electrothermic smelter furnace until it reaches the desired sintering temperature.A new technique to prepare SiC from spent pot liners has been developed by Denis A Brosnan [415]. In this process, the raw materials are first decontaminated by reacting with strong oxidizing acids such as chromic or perchloric acid, and then the spent pot liner is crushed and powdered before it is subjected to a high temperature reaction with silica powder in an argon plasma furnace. This prevents the formation of toxic gases such as fluoride, cyanide, and alkali in the final product and the furnace lining.
Electrical Insulation
Recrystallized silicon carbide has good electrical insulation properties and is used in electrical heating elements such as resistors, thermistors, and the like. It is also an ideal material for thermocouple protection tubes due to its high thermal conductivity and low temperature coefficient of electrical resistance.The resistors are made by forming a mixed composition of silicon carbide grains and a temporary binder, and then subjecting the formed resistors to externally applied heat at a temperature of at least 22000 C. in a chamber of a kiln, furnace, or the like for not more than thirty minutes while protecting the resistors from oxidation by means of an inert gas.This process requires the use of a large volume of an inert gas at a constant rate of flow. The inert gas removes impurities such as SiO2 which exist on the surface of the silicon carbide particles, thereby improving their electrical properties. The inert gas also allows the SiC particles to grow large, so that they can come into contact with one another and form a dense, high-temperature-resistant ceramic.However, this method is not practical because it takes a very long time to prepare the inert gas and because it also produces corrosive fumes. The fumes may damage the furnace lining and decrease its life significantly.In addition, this process does not allow the continuous production of resistors with satisfactory electrical properties. It only permits the manufacture of resistors of limited length and width for a single heating application, and it is not possible to make any more than a certain number of these resistors in any given operation.Therefore, a process is needed which would allow the continuous production of a vast amount of silicon carbide resistors with satisfactory electrical properties. It is a difficult task because the unburned mixture of silicon carbide grains from which the resistors are made is practically an electrical insulator and requires a very high voltage to force a current through it.We have recently developed a novel technique for producing silicon carbide resistors having very acceptable electrical properties. This method is based on the fact that when the resistors are fabricated by the conventional method of forming them from a mixture of silicon carbide grains and a temporary binding agent, and then subjected to heat from a highly heated chamber furnace, the grains recrystallize almost instantaneously and the electrical properties of the resistors are improved greatly.