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Silicon Nitride structures and properties

Crystal framework of Silicon Nitride
Si3N4 has two kinds of crystal structure: 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 as well as come from hexagonal system. The distinction in between them lies in the order of [SiN4] tetrahedral layers. The β & beta; stage is developed by the overlap of hexagonal ring layers made up of six nearly entirely symmetrical [SN4] tetrahedrons in the c-axis direction, while the α & alpha; stage is created by 2 layers of substantial improvement as well as different non-hexagonal ring layers. α & alpha; phase can dissolve oxygen in the series of crystal structure, and the interior stress of α & alpha; stage is bigger than that of β & beta; stage, so the complimentary power is greater than that of β & beta; stage. From the perspective of thermodynamics, the β & beta; phase is extra steady at greater temperature level. The & alpha; stage has low symmetry and is easy to develop. At the temperature of about 1500 ℃, the α & alpha; stage goes through a restoration change and also is transformed into β & beta; phase. This improvement is irreparable, and the existence of some procedure conditions as well as high quality is a lot more helpful to the improvement from α & alpha; stage to & beta; phase. & alpha;-Si3N4 is formed when the temperature βis less than 1350 ℃, and also & beta;-Si3N4 can be directly prepared at a temperature

higher than 1500 ℃. Silicon Nitride homes The molecular formula of silicon nitride is Si3N4, of which Si make up 60.06% and also dint N make up 39.94%. There is a strong covalent bond between Si and also N (of which the ion bond make up only 30%), so the Si3N4 has high solidity (Morse hardness 9), high melting point and also steady structure.
The Si-N in silicon nitride crystal is mainly covalent bond, and also the bonding stamina is high, so it has a big flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is reduced, yet the thermal conductivity is huge, so this product is difficult to generate thermal stress, so it has good thermal shock resistance and excellent thermal shock resistance. It has sturdiness, high mechanical strength at high temperature and also tiny contortion at high temperature. (the heat deformation of silicon nitride ceramic with 2.5 g/cm3 density is 0.5% at 1200 × & times; 1000h ℃ and also 23 × & times; 7kg/cm2 load). Not eroded by several metals, because of the formation of a silicon dioxide layer, excellent oxidation resistance as well as great electrical insulation.

Silicon nitride has no melting factor and also sublimates and disintegrates at 1900 ℃ under atmospheric pressure. The specific warm is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and & beta; stage is 10 ~ 16GPa and 24.5 ~ 32.65 Grade point average respectively. Due to the fact that it is a strong covalent bond substance, no fluid phase will be formed listed below its decay temperature level (about 1900 ℃), so silicon nitride materials can be sintered with the aid of oxide additives. The oxide products that advertise sintering are mostly Y2O3, Al2O3, etc, and also the high addition amount can reach 20%. The response principle is that the SiO2 oxide film formed on the surface of silicon nitride particles responds with the included oxide to create liquid phase and also penetrate at the grain border to make certain high diffusion capacity during material migration.

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

The weight of the sample boosted and also a thick silica protective layer was gradually formed on the surface, which stopped the additional oxidation of Si3N4. The weight gain was not obvious till the temperature level was over 1600 ℃. However in the humid ambience, Si3N4 is really simple to oxidize, and also the surface area begins to oxidize at 200 ℃, which is about twice as fast as that in completely dry air. The oxidation activation energy of Si3N4 powder in water vapor is certainly less than that in oxygen and also air. The reason is that water vapor can respond with Si3N4 through amorphous SiO2 movies:
Si3N4 +6 WATER=3SiO2+NH3.

Silicon nitride is stable to most metal options, devoid of rust and also seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so on. Nonetheless, for Cu remedy, it is not worn down just in vacuum cleaner or inert atmosphere; Mg can react weakly with Si3N4; silicon remedy can wet Si3N4 and deteriorate a little; shift aspect option can highly damp Si3N4 and also form silicide with Si to quickly break down silicon nitride and also getaway N2 at the same time. Si3N4 is secure to alloy remedies such as brass, tough light weight aluminum and nickel silver, and also has good rust resistance to cast iron and medium carbon steel, yet it is not resistant to Ni-Cr alloy as well as stainless-steel.

Except for molten NaOH and HF, silicon nitride has great chemical deterioration resistance. Yet the majority of the liquified antacid as well as salt can interact with Si3N4 to disintegrate it.

Silicon Nitride used in Refractories.
Silicon nitride porcelains are called promising heat structural products as a result of their excellent high temperature residential or commercial properties, such as heat strength, good wear resistance and deterioration resistance. Because of the strong covalent bond and also reduced diffusion coefficient at heat, the manufacture of Si3N4 porcelains need to rely on heat, high pressure and sintering representative, and also the expense is too high, or else it is difficult to create excellent quality silicon nitride materials. These constraints of production expenses and also equipment are challenging to approve in the metallurgical industry, so the research in the field of refractories starts reasonably late, and the study is not in-depth. Many concepts originate from ceramics, but there is very little innovation. In the past, silicon nitride typically existed in the kind of bonding phase in refractories. Through nitriding and shooting of metal Si, accumulations such as diamond or silicon carbide were incorporated with fine powder, so regarding attain the function of combining refractory products. Ceramic shed plate is silicon carbide accumulation and also component of fine powder. Silicon nitride is formed by nitriding metal Si to develop silicon nitride. Silicon carbide is combined to create silicon nitride bound silicon carbide material, which is made use of in blast heater body and also various other parts, to make sure that the performance of the material has been considerably boosted. Compared with the clay bonded silicon carbide lost plate, the high temperature efficiency of the product is great, which resolves the issue of bulging failure triggered by the oxidation of silicon carbide when the clay bound silicon carbide shed plate is utilized.

Silicon nitride Rate
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Silicon nitride Vendor
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