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Silicon Nitride structures and properties

Crystal structure of Silicon Nitride
Si3N4 has two kinds of crystal framework: α& alpha;- Si3N4 is granular crystal and also β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks made up of common vertices of [SN4] tetrahedron and also belong to hexagonal system. The difference in between them hinges 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 symmetrical [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; phase is created by two layers of concrete change and different non-hexagonal ring layers. α & alpha; stage can liquify oxygen in the series of crystal structure, and the interior stress of α & alpha; phase is larger than that of β & beta; phase, so the free energy is higher than that of β & beta; phase. From the point of view of thermodynamics, the β & beta; phase is more secure at greater temperature level. The & alpha; phase has low balance as well as is easy to form. At the temperature of regarding 1500 ℃, the α & alpha; phase goes through a restoration makeover and also is transformed into β & beta; phase. This change is irreparable, as well as the existence of some procedure problems and also quality is a lot more beneficial to the transformation from α & alpha; stage to & beta; stage. & alpha;-Si3N4 is formed when the temperature βis lower than 1350 ℃, as well as & beta;-Si3N4 can be straight prepared at a temperature level

more than 1500 ℃. Silicon Nitride residential or commercial properties The molecular formula of silicon nitride is Si3N4, of which Si represent 60.06% and also 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 solidity (Morse hardness 9), high melting factor as well as stable structure.
The Si-N in silicon nitride crystal is generally covalent bond, as well as the bonding toughness is high, so it has a big elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal expansion is low, but the thermal conductivity is large, so this material is not easy to produce thermal anxiety, so it has great thermal shock resistance as well as great thermal shock resistance. It has sturdiness, high mechanical toughness at heat as well as tiny deformation at high temperature. (the heat 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 deteriorated by numerous steels, due to the development of a silicon dioxide layer, good oxidation resistance and also good electric insulation.

Silicon nitride has no melting point and sublimates as well as decays at 1900 ℃ under air pressure. The specific warmth is 711.8 J/ kg ℃. The microhardness of α & alpha; stage as well as & beta; phase is 10 ~ 16GPa and 24.5 ~ 32.65 GPa respectively. Because it is a solid covalent bond compound, no liquid stage will certainly be formed listed below its disintegration temperature level (regarding 1900 ℃), so silicon nitride products can be sintered with the help of oxide additives. The oxide products that promote sintering are generally Y2O3, Al2O3, etc, as well as the high enhancement quantity can reach 20%. The reaction principle is that the SiO2 oxide film based on the surface of silicon nitride particles responds with the included oxide to develop liquid stage and permeate at the grain limit to make certain high diffusion capability throughout material movement.

Chemical Stability of Silicon Nitride
Si3N4 is a thermodynamically secure compound. Silicon nitride porcelains can be consumed to 1400 ℃ in oxidation environment as well as 1850 ℃ in neutral or decreasing environment. In fact, the oxidation reaction of Si3N4 takes place above 800C:
Si3N4 +3 O2=3SiO2+N2.

The weight of the example raised and a thick silica protective layer was progressively formed on the surface, which stopped the additional oxidation of Si3N4. The weight gain was not obvious until the temperature level was above 1600 ℃. However in the moist atmosphere, Si3N4 is extremely easy to oxidize, as well as the surface starts to oxidize at 200 ℃, which has to do with two times as fast as that in dry air. The oxidation activation power of Si3N4 powder in water vapor is obviously less than that in oxygen and also air. The factor is that water vapor can react with Si3N4 through amorphous SiO2 films:
Si3N4 +6 H2O=3SiO2+NH3.

Silicon nitride is secure to most metal remedies, without rust as well as infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so forth. Nevertheless, for Cu solution, it is not worn down just in vacuum cleaner or inert ambience; Mg can react weakly with Si3N4; silicon solution can damp Si3N4 as well as erode a little; transition component remedy can highly wet Si3N4 and form silicide with Si to quickly break down silicon nitride as well as retreat N2 at the exact same time. Si3N4 is stable to alloy options such as brass, tough light weight aluminum and nickel silver, and has good corrosion resistance to cast iron and medium carbon steel, however it is not immune to Ni-Cr alloy and also stainless steel.

With the exception of liquified NaOH and also HF, silicon nitride has good chemical corrosion resistance. Yet a lot of the liquified alkali as well as salt can interact with Si3N4 to disintegrate it.

Silicon Nitride made use of in Refractories.
Silicon nitride porcelains are called encouraging high temperature structural products as a result of their excellent high temperature buildings, such as heat strength, good wear resistance and also deterioration resistance. As a result of the strong covalent bond as well as reduced diffusion coefficient at heat, the manufacture of Si3N4 porcelains should rely on heat, high pressure as well as sintering representative, as well as the expense is too expensive, otherwise it is tough to create premium quality silicon nitride materials. These restrictions of manufacturing costs and equipment are challenging to approve in the metallurgical market, so the research in the area of refractories begins fairly late, and also the research is not comprehensive. Many concepts originate from porcelains, yet there is not much technology. In the past, silicon nitride typically existed in the kind of bonding stage in refractories. Via nitriding and also firing of metal Si, accumulations such as corundum or silicon carbide were combined with great powder, so regarding achieve the function of incorporating refractory materials. Ceramic shed plate is silicon carbide aggregate as well as part of fine powder. Silicon nitride is formed by nitriding steel Si to develop silicon nitride. Silicon carbide is integrated to develop silicon nitride bound silicon carbide product, which is made use of in blast heating system body as well as other components, to make sure that the performance of the material has been significantly enhanced. Compared to the clay bonded silicon carbide shed plate, the high temperature performance of the material is excellent, which solves the problem of bulging failing brought on by the oxidation of silicon carbide when the clay bound silicon carbide shed plate is utilized.

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