Silicon carbide (SiC), also known as emery. In 1891 American Acheson invented the industrial manufacturing method of silicon carbide. Silicon carbide is synthesized by using natural silica, carbon, wood chips, industrial salt as basic synthetic raw materials, and heating reaction in a resistance furnace. The wood chips are added to make the massive mixture porous at high temperature, which facilitates the removal of a large amount of gases and volatiles generated by the reaction, and avoids explosions, because the synthesis of IT silicon carbide will produce about 1.4t of carbon monoxide (CO). The role of industrial salt (NaCl) is to facilitate the removal of impurities such as alumina and iron oxide present in the feed.
Silicon carbide
(1) Synthesis and use of silicon carbide
The synthesis of silicon carbide is carried out in a special resistance furnace, which is actually just a graphite resistance heating element, which is formed by stacking graphite particles or carbon particles into columns. The heating element is placed in the middle. The above raw materials are uniformly mixed in the ratio of silica 52% to 54%, coke 35%, wood chips 11%, industrial salt 1.5% to 4%, and are closely packed around the graphite heating element. When energized and heated, the mixture undergoes a chemical reaction to produce silicon carbide. Its reaction formula is:
SiO2+3C→SiC+2CO↑
The starting temperature of the reaction is about 1400 ℃, the product is low-temperature type β-SiC, the base crystal is very fine, it can be stabilized to 2100 ℃, and then slowly converted to high-temperature type α-SiC. α-SiC can be stabilized to 2400°C without significant decomposition. When it exceeds 2600°C, it sublimates and decomposes, evaporating silicon vapor, and leaving graphite. Therefore, the final temperature of the reaction is generally selected to be 1900~2200℃. The product synthesized by the reaction is a massive crystalline polymer, which needs to be pulverized into particles or powders of different particle sizes, and at the same time remove impurities.
Sometimes to obtain high-purity silicon carbide, you can use the vapor deposition method, that is, using a mixed vapor of silicon tetrachloride and benzene and hydrogen, when passing through a hot graphite rod, a gas phase reaction occurs, and the generated silicon carbide is deposited on the graphite surface. Its reaction formula is:
6SiCl4+C6H6+12H2→6SiC+24HCl
Pure silicon carbide is colorless and transparent, but industrially produced silicon carbide has different colors such as yellow, black, dark green, and light green due to the presence of impurities such as free carbon, iron, and silicon. The common colors are light green and black. The relative molecular mass of silicon carbide is 40.09, of which silicon accounts for 70.04% and carbon accounts for 29.964. The true density is 3.21. Melting point (sublimation) 2600℃. The crystalline form has a low-temperature form of β-SiC with a cubic structure; high-temperature form of α-SiC has a hexagonal structure; and there are other series of deformed bodies due to the different arrangement of atoms in the silicon carbide crystal structure, there are about a hundred kinds , Commonly known as homogeneous heterocrystals. In addition, due to the different electron affinity in the crystalline structure, in addition to the main covalent bonds, there are still some ionic bonds. Silicon carbide is a hard material with a Mohs hardness of 9.2. At low temperatures, the chemical properties of silicon carbide are relatively stable, with excellent corrosion resistance, and will not be corroded in boiling hydrochloric acid, sulfuric acid and hydrofluoric acid. However, it can react with certain metals, salts and gases at high temperature. The reaction conditions are listed in Table 10-4-16. Silicon carbide is still stable in a reducing atmosphere until 2600℃, and oxidation will occur in a high-temperature oxidizing atmosphere:
SiC+2O2→SiO2+CO2
But its anti-oxidation ability between 800~1140℃ is not as good as 1300~1500℃, this is because the structure of the oxide film (SiO2) generated by oxidation at 800~1140℃ is relatively loose, which can not fully protect the substrate. The oxidation effect is significant above 1140°C, especially between 1300 and 1500°C. At this time, the oxide film formed covers the surface of the silicon carbide substrate, hindering the further contact of oxygen with silicon carbide, so the anti-oxidation ability Instead, strengthen. However, at higher temperatures, its oxidation protective layer is destroyed, causing the silicon carbide to undergo strong oxidation and decomposition and destruction.
Since silicon carbide has excellent physical and chemical properties, it is widely used as an important industrial raw material. Its main uses have three aspects: for the manufacture of abrasives; for the production of resistance heating elements-silicon carbon rods, silicon carbon tubes, etc.; for the manufacture of refractory products. As a special refractory material, it is used as a refractory product in the smelting of iron and steel smelting furnaces, such as blast furnaces, iron furnaces, etc., where it is severely stamped, corroded, and worn. Pipes, filters, crucibles, etc.; used as rocket engine tail nozzles and high-temperature gas turbine blades in space technology; in the silicate industry, they are widely used as sheds, muffle furnace linings and cassettes for various kilns In the chemical industry, it is used as oil and gas generation, oil gasifier, desulfurization furnace lining, etc.
(2) Product manufacturing process
Products made solely from α-SiC are difficult to grind into micron-level fine powder due to their high hardness, and the particles are in the form of plates or needles. The green body pressed with it, even when heated to its decomposition temperature There is no obvious shrinkage nearby, it is difficult to sinter, the density of the product is low, and the oxidation resistance is also poor. Therefore, in the industrial production of products, a small amount of spherical β-SiC fine powder is added to α-SiC and additives are used to obtain dense products. As an additive to the product binder, it can be divided into various types such as oxide, nitride, graphite, etc., such as clay, alumina, zircon, mullite, lime, glass, silicon nitride, silicon oxynitride , Graphite, etc. The molding binder solution can be one or more of carboxymethyl cellulose, polyvinyl alcohol, lignin, starch, alumina sol, silica sol and the like.
Depending on the type and amount of additives, the firing temperature of the green body is also different, and its temperature range is 1400~2300℃. For example, α-SiC 70% with a particle size greater than 44 μm, 20% β-SiC with a particle size less than 10 μm, 10% clay, plus 8% lignin aqueous solution with 4.5%, after uniform mixing, molding with a pressure of 50 MPa, 1400°C for 4 hours in air After firing, the bulk density of the product is 2.53g/cm3, the apparent porosity is 12.3%, and the flexural strength is 30~33MPa. The sintering properties of products with several different additives are listed in Table 2.
In general, silicon carbide refractories have many excellent properties, such as high strength, high thermal shock resistance, excellent wear resistance, high thermal conductivity, and chemical resistance in a relatively wide temperature range Corrosion, etc. However, it should also be noted that its weakness is its poor anti-oxidation ability, which results in volume expansion and deformation at high temperatures, which reduces its service life.
In order to improve the oxidation resistance of silicon carbide refractories, a lot of selection work has been done on the bonding agent. Clay (including oxide) was initially used for bonding, but it did not play a protective role, and the silicon carbide particles were still oxidized and eroded. At the end of the 1950s, silicon nitride (Si3N4) was chosen to be combined as an improved product of silicon carbide refractory. It does have good oxidation resistance (see Figure 1), and there is no significant expansion phenomenon. But the price is more expensive; coupled with the possibility of sudden destruction during repeated heating and cooling; and the network structure of silicon nitride itself is permeable and cannot fundamentally protect silicon carbide from oxidation. In the early 1960s, silicon carbide refractory materials combined with silicon oxynitride (Si2ON2) appeared, which has better oxidation resistance than silicon nitride, because silicon oxynitride adheres to silicon oxide on the surface of silicon carbide Thin film, and react with it to form a continuous protective film that is firmly combined with silicon carbide. At the same time, the price of this material is appropriate, which is equivalent to the silicon carbide material combined with oxide.
In order to obtain the dense ceramic products of pure silicon carbide in order to make the most of the characteristics of silicon carbide itself, the manufacturing process of self-bonding reaction sintering method and hot pressing method has been developed.
Self-bonding silicon carbide is to mix α-SiC with carbon powder, shape it by various molding methods, and then heat the green body in silicon vapor, so that the carbon powder in the green body becomes siliconized into β-SiC. The particles of α-SiC are tightly combined into a dense product. Therefore, self-bonding silicon carbide is actually a kind of α-SiC bonded by β-SiC. This manufacturing process is also known as reaction sintering. Examples of specific processes are as follows.
Mix α-SiC powder with various particle size ratios and colloidal graphite in a porcelain ball mill cylinder for 20 hours, then add carboxymethyl cellulose aqueous solution or polyvinyl alcohol alcohol solution as a binder, use 50~ in steel mold 70MPa pressure forming. The amount of graphite added has a great influence on the density of the green body. In order to make the final density of the siliconized silicon carbide product close to the theoretical value, it is required to achieve the expected green body density during compression molding. According to the green body density Value, in turn, the following formula can be used to calculate the amount of graphite required.
The silicification can be carried out in a carbon tube furnace of ordinary atmospheric pressure, and the silicidation temperature must be greater than 2000°C. If carried out in a vacuum furnace of 66.65MPa, the silicidation temperature can be reduced to 1500~1600℃. The particle size of silicon powder used to generate silicon vapor is 0.991~4.699mm. When siliconizing under atmospheric pressure, silicon powder can be packed in a graphite crucible. When siliconizing under vacuum, it should be installed in a boron nitride (BN) crucible, because silicon will infiltrate into graphite and form silicon carbide to break the graphite crucible, but boron nitride and silicon are not wet. The time required for silicidation varies depending on the temperature of silicidation and the volatilization of silicon at that temperature. After the silicidation is completed, there should normally be no silicon remaining in the crucible and all of it should evaporate. Silicon attached to the surface of the product due to evaporation can be removed by hot sodium hydroxide treatment. The strength of self-bonding silicon carbide products is 7 to 10 times that of general silicon carbide products, and the oxidation resistance is improved.
In addition to manufacturing silicon carbide products by sintering, since the hot press sintering technology was invented, silicon carbide products can also be manufactured by hot pressing, and more excellent sintering performance can be obtained. The hot pressing process combines the forming and firing of the billet into a process, that is, the billet is formed and sintered at a high temperature while under pressure at the same time. This method has been used in powder metallurgy for decades in the metallurgical industry, and has been gradually promoted and applied in the industrial production of special refractory materials. The use of hot press forming sintering can shorten the manufacturing time, lower the sintering temperature, improve the microstructure of the product, increase the density of the product, and improve the performance of the material. Selecting appropriate hot pressing process conditions such as temperature, pressure and billet size can achieve excellent hot pressing effect. The hot pressing process is particularly useful for the manufacture of refractory compounds. Because the mold for hot pressing has to withstand a high temperature of more than 1000°C and also bears a pressure of several kN at high temperature, high-strength graphite is generally used as a mold for manufacturing refractory compound products.
The heating of the mold can be radiant heating, high frequency induction heating or the resistance heating of the mold itself. The pressurization of the blanks can be done with hydraulic presses or ordinary jacks. The biggest disadvantage of the hot pressing method is that the shape of the product is limited, and the manufacturing efficiency is low, so this method is not as widely used as the reaction sintering method. But the performance of hot-pressed products is much better. For example, at a temperature of 1350°C, hot pressing with a pressure of 70 to 90 MPa, if the raw material is high-temperature α-SiC, the density does not exceed 96% of the theoretical value; if using low-temperature β-SiC, the heat The compressive density can reach 3.20g/cm3, which is close to the theoretical value, and transforms into high-temperature α-SiC during sintering. The strength of this hot-pressed sintered body is 380 MPa at normal temperature and 500 MPa at 1370°C. The thermal shock resistance is also quite good, and the oxidation resistance in the cross-temperature air is also very good.