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Silicon Nitride structures and properties

Crystal framework of Silicon Nitride
Si3N4 has two sort of crystal framework: α& alpha;- Si3N4 is granular crystal as well as β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks made up of typical vertices of [SN4] tetrahedron and also belong to hexagonal system. The distinction between them depends on the order of [SiN4] tetrahedral layers. The β & beta; phase is developed by the overlap of hexagonal ring layers composed of 6 almost completely symmetrical [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; phase is developed by 2 layers of substantial makeover and different non-hexagonal ring layers. α & alpha; stage can dissolve oxygen in the range of crystal structure, as well as the inner stress of α & alpha; stage is bigger than that of β & beta; stage, so the complimentary power is higher than that of β & beta; stage. From the point of view of thermodynamics, the β & beta; phase is extra stable at higher temperature. The & alpha; phase has low symmetry and is simple to develop. At the temperature of about 1500 ℃, the α & alpha; stage undertakes a reconstruction change and also is transformed right into β & beta; stage. This improvement is permanent, and also the presence of some process conditions and also top quality is much more valuable to the transformation from α & alpha; phase to & beta; phase. & alpha;-Si3N4 is created when the temperature level βis less than 1350 ℃, as well as & beta;-Si3N4 can be straight prepared at a temperature level

greater than 1500 ℃. Silicon Nitride properties The molecular formula of silicon nitride is Si3N4, of which Si represent 60.06% as well as dint N accounts for 39.94%. There is a solid covalent bond in between Si and N (of which the ion bond represent just 30%), so the Si3N4 has high hardness (Morse solidity 9), high melting point as well as steady structure.
The Si-N in silicon nitride crystal is mostly covalent bond, and the bonding stamina is high, so it has a large elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is low, however the thermal conductivity is huge, so this material is hard to create thermal anxiety, so it has great thermal shock resistance as well as great thermal shock resistance. It has strength, high mechanical stamina at heat as well as small contortion at high temperature. (the heat contortion of silicon nitride ceramic with 2.5 g/cm3 density is 0.5% at 1200 × & times; 1000h ℃ as well as 23 × & times; 7kg/cm2 tons). Not deteriorated by numerous steels, because of the development of a silicon dioxide layer, excellent oxidation resistance as well as excellent electrical insulation.

Silicon nitride has no melting factor and also sublimates and breaks down at 1900 ℃ under air pressure. The certain warm is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and also & beta; phase is 10 ~ 16GPa and 24.5 ~ 32.65 Grade point average specifically. Since it is a solid covalent bond compound, no liquid phase will be formed listed below its decay temperature (regarding 1900 ℃), so silicon nitride materials can be sintered with the aid of oxide additives. The oxide materials that promote sintering are mostly Y2O3, Al2O3, and so on, and also the high addition amount can reach 20%. The response concept is that the SiO2 oxide movie based on the surface area of silicon nitride bits reacts with the included oxide to create liquid stage as well as permeate at the grain border to ensure high diffusion capacity during material movement.

Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically stable compound. Silicon nitride porcelains can be consumed to 1400 ℃ in oxidation atmosphere as well as 1850 ℃ in neutral or lowering environment. Actually, the oxidation reaction of Si3N4 happens above 800C:
Si3N4 +3 O2=3SiO2+N2.

The weight of the example enhanced and also a thick silica safety layer was slowly based on the surface area, which avoided the additional oxidation of Si3N4. The weight gain was not noticeable until the temperature was over 1600 ℃. But in the damp environment, Si3N4 is really simple to oxidize, and also the surface area begins to oxidize at 200 ℃, which has to do with twice as quick as that in dry air. The oxidation activation energy of Si3N4 powder in water vapor is clearly lower than that in oxygen and also air. The factor is that water vapor can react with Si3N4 with amorphous SiO2 movies:
Si3N4 +6 WATER=3SiO2+NH3.

Silicon nitride is steady to most metal solutions, devoid of corrosion as well as infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and more. Nonetheless, for Cu service, it is not worn down just in vacuum cleaner or inert environment; Mg can react weakly with Si3N4; silicon solution can damp Si3N4 and wear down somewhat; transition component option can highly wet Si3N4 and also type silicide with Si to rapidly break down silicon nitride and escape N2 at the same time. Si3N4 is secure to alloy remedies such as brass, difficult aluminum and nickel silver, and has good deterioration resistance to cast iron and medium carbon steel, however it is not resistant to Ni-Cr alloy and also stainless steel.

Except for molten NaOH as well as HF, silicon nitride has good chemical corrosion resistance. Yet the majority of the liquified alkali and also salt can interact with Si3N4 to decay it.

Silicon Nitride utilized in Refractories.
Silicon nitride porcelains are called encouraging high temperature structural materials because of their outstanding heat residential or commercial properties, such as high temperature strength, great wear resistance as well as rust resistance. As a result of the strong covalent bond and also low diffusion coefficient at high temperature, the manufacture of Si3N4 ceramics should count on high temperature, high pressure and sintering agent, as well as the price is too expensive, or else it is tough to produce premium quality silicon nitride products. These limitations of manufacturing costs and also devices are challenging to accept in the metallurgical industry, so the study in the area of refractories begins relatively late, and the research study is not comprehensive. Several theories come from porcelains, yet there is not much innovation. In the past, silicon nitride typically existed in the form of bonding stage in refractories. With nitriding and shooting of steel Si, accumulations such as diamond or silicon carbide were incorporated with great powder, so as to accomplish the purpose of incorporating refractory products. Ceramic shed plate is silicon carbide aggregate and part of fine powder. Silicon nitride is formed by nitriding metal Si to form silicon nitride. Silicon carbide is integrated to develop silicon nitride bonded silicon carbide material, which is made use of in blast furnace body and also various other parts, to make sure that the performance of the material has actually been substantially boosted. Compared to the clay bonded silicon carbide dropped plate, the heat efficiency of the product is great, which resolves the issue of protruding failure caused by the oxidation of silicon carbide when the clay bound silicon carbide lost plate is made use of.

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