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Silicon Nitride structures and properties

Crystal structure of Silicon Nitride
Si3N4 has two kinds of crystal structure: 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 common vertices of [SN4] tetrahedron and come from hexagonal system. The difference between them depends on the order of [SiN4] tetrahedral layers. The β & beta; phase is formed by the overlap of hexagonal ring layers made up of six almost entirely in proportion [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; phase is created by two layers of concrete improvement and various non-hexagonal ring layers. α & alpha; phase can dissolve oxygen in the series of crystal framework, and also the internal stress of α & alpha; phase is bigger than that of β & beta; phase, so the free power is greater than that of β & beta; phase. From the viewpoint of thermodynamics, the β & beta; stage is much more secure at greater temperature. The & alpha; phase has low symmetry and is very easy to create. At the temperature of regarding 1500 ℃, the α & alpha; phase goes through a reconstruction improvement as well as is transformed into β & beta; stage. This change is irreversible, as well as the presence of some process conditions as well as top quality is extra useful to the improvement from α & alpha; stage to & beta; stage. & alpha;-Si3N4 is formed when the temperature βis lower than 1350 ℃, and & beta;-Si3N4 can be directly prepared at a temperature

higher than 1500 ℃. Silicon Nitride buildings The molecular formula of silicon nitride is Si3N4, of which Si make up 60.06% and dint N accounts for 39.94%. There is a solid covalent bond in between Si and N (of which the ion bond accounts for just 30%), so the Si3N4 has high solidity (Morse hardness 9), high melting factor as well as secure framework.
The Si-N in silicon nitride crystal is primarily covalent bond, as well as the bonding stamina is high, so it has a large flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is low, yet the thermal conductivity is huge, so this material is not easy to generate thermal anxiety, so it has great thermal shock resistance and good thermal shock resistance. It has strength, high mechanical toughness at high temperature as well as small deformation at heat. (the high temperature contortion of silicon nitride ceramic with 2.5 g/cm3 thickness is 0.5% at 1200 × & times; 1000h ℃ as well as 23 × & times; 7kg/cm2 lots). Not eroded by many metals, because of the development of a silicon dioxide layer, good oxidation resistance as well as excellent electrical insulation.

Silicon nitride has no melting factor and also sublimates and also decays at 1900 ℃ under air pressure. The specific heat is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and & beta; phase is 10 ~ 16GPa and also 24.5 ~ 32.65 GPa specifically. Because it is a solid covalent bond compound, no fluid phase will certainly be created below its decomposition temperature level (concerning 1900 ℃), so silicon nitride products can be sintered with the help of oxide ingredients. The oxide materials that advertise sintering are primarily Y2O3, Al2O3, and so on, and also the high enhancement quantity can get to 20%. The reaction concept is that the SiO2 oxide movie formed on the surface area of silicon nitride particles responds with the included oxide to create fluid phase and also penetrate at the grain limit to make sure high diffusion ability throughout product migration.

Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically steady substance. Silicon nitride porcelains can be consumed to 1400 ℃ in oxidation environment and also 1850 ℃ in neutral or lowering ambience. As a matter of fact, the oxidation response of Si3N4 happens over 800C:
Si3N4 +3 O2=3SiO2+N2.

The weight of the example enhanced and a thick silica protective layer was slowly formed on the surface, which stopped the more oxidation of Si3N4. The weight gain was not apparent until the temperature level was above 1600 ℃. But in the damp ambience, Si3N4 is very easy to oxidize, and also the surface area starts to oxidize at 200 ℃, which has to do with twice as rapid as that in completely dry air. The oxidation activation energy of Si3N4 powder in water vapor is clearly lower than that in oxygen and air. The reason is that water vapor can react with Si3N4 via amorphous SiO2 films:
Si3N4 +6 WATER=3SiO2+NH3.

Silicon nitride is secure to most metal solutions, without rust as well as infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so on. Nevertheless, for Cu option, it is not deteriorated only in vacuum or inert environment; Mg can respond weakly with Si3N4; silicon solution can wet Si3N4 and also deteriorate slightly; change element remedy can strongly damp Si3N4 and type silicide with Si to rapidly decay silicon nitride as well as escape N2 at the exact same time. Si3N4 is steady to alloy solutions such as brass, difficult aluminum and nickel silver, as well as has great corrosion resistance to cast iron and also medium carbon steel, yet it is not resistant to Ni-Cr alloy and stainless-steel.

Except for liquified NaOH and HF, silicon nitride has great chemical corrosion resistance. But most of the liquified alkali and salt can engage with Si3N4 to disintegrate it.

Silicon Nitride made use of in Refractories.
Silicon nitride porcelains are called appealing high temperature architectural products as a result of their excellent high temperature buildings, such as heat strength, excellent wear resistance and also rust resistance. Because of the strong covalent bond and also reduced diffusion coefficient at heat, the manufacture of Si3N4 porcelains must rely on heat, high pressure as well as sintering agent, and also the price is too expensive, otherwise it is hard to generate top quality silicon nitride materials. These restrictions of manufacturing expenses and tools are challenging to approve in the metallurgical market, so the research in the field of refractories begins reasonably late, as well as the research is not extensive. Many theories come from ceramics, but there is not much technology. In the past, silicon nitride normally existed in the form of bonding stage in refractories. With nitriding and also shooting of metal Si, accumulations such as diamond or silicon carbide were integrated with great powder, so regarding attain the objective of combining refractory products. Ceramic shed plate is silicon carbide accumulation as well as component of fine powder. Silicon nitride is created by nitriding steel Si to create silicon nitride. Silicon carbide is incorporated to develop silicon nitride bonded silicon carbide material, which is used in blast furnace body and also other parts, to ensure that the efficiency of the product has actually been considerably enhanced. Compared with the clay bonded silicon carbide shed plate, the high temperature performance of the product is excellent, which resolves the problem of bulging failing caused by the oxidation of silicon carbide when the clay bonded silicon carbide shed plate is utilized.

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