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Sample Sending Unit:Fengfeng New Oriental Science and Technology Development Co., Ltd.

With the development of science and technology, especially the development of energy and spacetechnology, materials are required with characteristics of high temperature resistance, corrosion resistance and abrasive resistance in order to be used in more demanding work environment. Because Pressureless SSiC ceramic material has oxidation resistance, good abrasive resistance,high temperature resistance, excellent thermal stability, high temperature strength, small coefficient of thermal expansion high thermal conductivity thermal shock resistance and chemical corrosion resistance, it has become an important part of advanced science. SiC ceramics are the new materials that have been developed in the last 20 years, in particular,pressureless SSiC ceramics has become as an advanced ceramics in last 8 or 9 years. As result of high strength and hardness, corrosion resistance and high temperature resistance of SiC ceramics,it develops quickly and is applied in the petrochemical, metallurgical machinery, aerospace,microelectronics, automotive, steel, power generation fields. It increasingly shows the advantages with which other special ceramic cannot be incomparable. Main characteristics and application Structure of SiC Silicon carbide is an artificial material. After the artificial synthesis of silicon carbide, it was confirmed that there occasionally exists silicon carbide in the meteorite and the crust. The molecular formula of silicon carbide is SiC, molecular weight of 40.07, with 70.045% of silicon and 29.955% of carbide. SiC`s density is 3.16-3.21g/m3 in theory. SiC is a covalent compound based on covalent bond. When carbon and silicon element are formedin SiC crystal, SiC crystal`s basic unit is tetrahedron. All SiC are formed by SiC tetrahedron, but the only difference is the parallel combination and anti-parallel combination, which lead to the formation of SiC with diamond. SiC has 75 variants, such as 3C-SiC.4H-SiC and 15R-SiC.Especially, α-SiC and β-SiC are two common variants. The crystal structure of -SiC is cubic crystal,Si and C are composed of a face centered cubic lattice; α-SiC has more than 100 kinds of polymorph, such as 4H, 15R and 6H, where 6H is the most common one in industrial application.There is a certain thermal stability relationship between SiC polymorphs. When the temperature is below 1600℃, SiC will be in the form of β-SiC. However, when the temperature is above 1600℃, β-SiC will transform slowly into various forms of α-SiC. 4H-SiC is easy to generate at 2000℃, 15R and 6H polymorph can be easy to be produced at above 2100℃. For 6H-SiC, even though the temperature is above 2200℃, it is still stable. The free energy difference between various forms of SiC polymorph is very small, therefore, the solid solution of microimpurity will result in changes in thermal stability relationship between polymorphs. Chemical properties of SiC The chemical stability of SiC is closely related to its oxidation characteristic. SiC itself is so easy to be oxidized, but after being oxidized, it will form a thin SiO2 film which will stop the oxidation. In the air, SiC starts to be oxidized at 800℃ in a very slow rate. As the temperature increases, the rate of oxidation is rapidly increasing. Its oxidation rate is 1.6 times faster than in the air, and the rate of oxidation will slow down over time. Vapor can react strongly with SiC under high temperature. When the temperature is above1100℃, silicon, carbon, or SiO2 will be generated depending on the circumstances. When the temperature is around 1000℃, SiC and sulfur compounds, like H2S, can generate brown vulcanized silicon. SiC ceramic has a unique structure that is closely related to its specific structure. SiC is a compound with strong covalent bond, where the iconicity of Si-C covalent bond in SiC is only about 12%. Thus, SiC has high strength, high elastic modulus and excellent abrasion resistance.Pure SiC will be eroded by alkali solution such as NaOH, and acid solution such as HCL, HNO3,H2SO4 and HF. Application SiC is used in mechanical seal with high PV value, where PV value is the product of the sealing fluid pressure V and the average sliding velocity of the sealing end P, and presents the limit value of the use of mechanical seal. 1.For high fluid pressure SiC is a material with high strength and rigidity. Since it has high modulus of elasticity, it will not be easy to become deformed under high pressure. Meanwhile, SiC has high compressionstrength. SiC has strong adhesiveness resistance and wear ability, therefore, it`s very useful as a sealing material in high pressure. 2.For mechanical seal under high temperature SiC has very high hot strength. At 1400℃, the flexural strength is the same as that in normaltemperature. 3.For high speed Under high velocity, the major problem for mechanical seal is heat and vibration from frictionpairs, as result of high linear velocity. Pressureless SSiC could totally meet the requirements of mechanical seal with high velocity. 4.For mechanical seal under corrosive condition Pressureless SSiC has single structure, no free silicon and free carbon. It also has strongcorrosion resistance, excellent chemical stability and can be used for all kinds of strongly corrosive acid and alkali medium. 5. For mechanical seal containing solid particles Many of the sealing fluids in industrial production contain solid particles which are easilydeposited on the surface of the seal. These particles not only cause serious erosion of the sealing surface from abrasion by particles, but also cause local heat on the sealing cover. SiC has high hardness, abrasion resistance and thermal shock stability. It also has outstanding emergency operation. Thus, adopting SiC self-contrast dense sealing pairs exists great potential for application in granular media. 6.We are working together with nuclear safety researcher from Hefei on the subject of delivery of liquid metal pipe within modern nuclear fusion applications. 7.We put forward that in order to increase the toughness of the SiC ceramic which is used forpump s, haft in production machinery and chemical industry, we add appropriate amount of carbon fiber in the original SiC. We have applied for a national invention patents which has already been accepted, and wait for the publicity.

As one of the modern engineering ceramics, SiC ceramic is a new artificial kind of inorganicnonmetallic materials. Its hardness is next only to diamond. It has variety of characteristics, such as good chemical corrosion resistance, abrasion resistance, high temperature resistance, high mechanical strength, excellent self-lubricating, anti-creep under high temperature, low friction coefficient, high thermal conductivity, and low thermal expansion coefficient. It can be widely used in petroleum, metallurgy, chemical, machinery, aviation, and aerospace fields. It usually be used as material of friction pairs. A lot of experiments show that SiC is the most excellent material for friction pair, especially Pressureless SSiC and hot sintering SiC. Currently, the main performance indicators of Pressureless SSiC produced domestically has beenclose to the level of similar foreign products. Although the hot pressing sintering SiC has the best performance, the cost of manufacture is very high. Thus, it has not yet been applied totally. Due to the excellent characteristics of SiC, it has been applied in the shielded electric pump and magnetic pump fields. From 2001, Dalian Sifang electric pump Co. started to do research and development of SiC friction pairs. Bearing, bearing sleeve and thrust plate are made of Pressureless SSiC. From 2002, it has been applied in products and the company achieved good results. At the beginning of 2003, based on the original results, the improvements were carried out: 1.Improve the torsion structure of bearing sets, solve the stress concentration caused by theopening of the bearing sets and increase the reliability. 2.Add an elastic automatic compensation sleeve, have the ability to automatically compensate the material expansion difference caused by temperature rise, and make pump operate normallywithin the temperature range of 0℃-250℃. 3.Unique thrust collar structure makes it more reliable. As a shielded electric pump bearing, the problems of SiC, such as thermal expansion coefficient is small, material is more brittle, and it` s easy to be broken, need to be solved. The thermal expansion coefficient of SiC is 1/4 of stainless steel, when the medium temperature is higher, and its stainless steel parts to produce a gap or interference and lead to bearing damage;its fracture toughness is 1/50 of stainless steel, the material is brittle, and it is easy to be broken subjected to impact force, especially the edges and corners. The elastic coefficient of SIC is the 1/50 of stainless steel, the mechanical property is more fragile,and easily damaged. The thermal expansion coefficient is the 1/4 of the stainless steel, which should be paid attention to when combined with stainless steel. Problems of structure If a rotor shaft (made of stainless steel) with diameter of 50mm is applied in the pump of liquid temperature of 100℃, it will have the following problems: the gap between shaft sleeve and rotor shaft will reach 0.052mm,that is, the gap will be decreased by 0.052mm compared at room temperature (20℃). To obtain the gap at room temperature, the gap between shaft sleeve and rotor shaft needs to be largen. Thus, shaft sleeve is easy to be damaged when it operates at room temperature, and more prone to be broken if it is used in high temperature. Through continuous improvements, the stress concentration caused by the opening of the bearingsets has been solved, the reliability has been increased, an elastic automatic compensation sleeve has been added and the material expansion difference caused by temperature rise has beenautomatically compensated. All of these make SiC bearing be successfully applied in the shieldedelectric pump and make pump operate normally in the temperature range of 0℃-250℃. SiC bearing has been completely applied in shielded electric pump, such as basic type PB, reverse cycle type PBN, water cooled high temperature type PBG, slurry type PBLK, self-priming type PBZ ,insulation type PBB, high power type PBGL and little capacity type PBX. Its use power is within the range of 0.75-75Kw, and the temperature is within the range of 0℃-250℃. The main application conditions include: a. its useful life should be longer; b. medium should have strong chemical corrosion; c. medium should have low density and be easy to be vaporized; d. high temperature; e. there are traces of fine and soft particles in medium; f. there is not mixed with power from friction pairs wear, such as pure water industry and working conditions with high purity requirements. Characteristics of SiC bearing: 1.Bearing is made of Pressureless SiC crystal, name: SSiC, purity: ≥99.5%. 2.With unique structure, it can automatically compensate the differences of material expansioncaused by increase in temperature and make pump normally operate in the temperature range of 0℃-250℃. 3.Its useful life is 10 times more than the ordinary graphite bearing.