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Silicon Nitride structures and properties

Crystal structure of Silicon Nitride
Si3N4 has 2 type 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 typical vertices of [SN4] tetrahedron and also come from hexagonal system. The distinction in between them depends on the order of [SiN4] tetrahedral layers. The β & beta; stage is formed by the overlap of hexagonal ring layers made up of 6 practically completely in proportion [SN4] tetrahedrons in the c-axis direction, while the α & alpha; stage is created by two layers of concrete change and also different non-hexagonal ring layers. α & alpha; stage can liquify oxygen in the variety of crystal framework, and also the inner stress of α & alpha; phase is larger than that of β & beta; stage, so the complimentary energy is more than that of β & beta; phase. From the viewpoint of thermodynamics, the β & beta; phase is a lot more secure at greater temperature level. The & alpha; stage has low balance and also is easy to create. At the temperature level of about 1500 ℃, the α & alpha; phase goes through a reconstruction change as well as is transformed into β & beta; phase. This improvement is permanent, and also the existence of some process problems and also top quality is more advantageous to the change from α & alpha; phase 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 level

more than 1500 ℃. Silicon Nitride residential properties The molecular formula of silicon nitride is Si3N4, of which Si accounts for 60.06% and dint N accounts for 39.94%. There is a strong covalent bond between Si and also N (of which the ion bond accounts for only 30%), so the Si3N4 has high hardness (Morse hardness 9), high melting factor as well as secure framework.
The Si-N in silicon nitride crystal is mainly covalent bond, and also the bonding stamina is high, so it has a big elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal expansion is reduced, yet the thermal conductivity is large, so this product is not easy to generate thermal anxiety, so it has excellent thermal shock resistance as well as excellent thermal shock resistance. It has sturdiness, high mechanical stamina at heat as well as tiny deformation at high temperature. (the high temperature deformation 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 eroded by lots of steels, because of the formation of a silicon dioxide layer, excellent oxidation resistance and great electric insulation.

Silicon nitride has no melting point as well as sublimates and disintegrates at 1900 ℃ under air pressure. The particular warm is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and & beta; stage is 10 ~ 16GPa and also 24.5 ~ 32.65 Grade point average respectively. Due to the fact that it is a strong covalent bond compound, no liquid stage will be formed listed below its disintegration temperature (about 1900 ℃), so silicon nitride materials can be sintered with the assistance of oxide additives. The oxide products that advertise sintering are primarily Y2O3, Al2O3, and so on, and also the high addition amount can get to 20%. The response principle is that the SiO2 oxide film formed on the surface of silicon nitride bits responds with the included oxide to form liquid stage and penetrate at the grain boundary to guarantee high diffusion ability during material movement.

Chemical Stability of Silicon Nitride
Si3N4 is a thermodynamically stable compound. Silicon nitride porcelains can be used up to 1400 ℃ in oxidation ambience and also 1850 ℃ in neutral or reducing ambience. In fact, the oxidation response of Si3N4 takes place above 800C:
Si3N4 +3 O2=3SiO2+N2.

The weight of the sample boosted and also a thick silica protective layer was progressively formed on the surface, which prevented the more oxidation of Si3N4. The weight gain was not obvious till the temperature level was above 1600 ℃. Yet in the damp ambience, Si3N4 is very simple to oxidize, as well as the surface starts to oxidize at 200 ℃, which is about twice as rapid as that in completely dry air. The oxidation activation energy of Si3N4 powder in water vapor is undoubtedly lower than that in oxygen as well as air. The factor is that water vapor can react with Si3N4 with amorphous SiO2 movies:
Si3N4 +6 WATER=3SiO2+NH3.

Silicon nitride is secure to most metal solutions, devoid of rust and also seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so on. Nevertheless, for Cu remedy, it is not deteriorated only in vacuum cleaner or inert environment; Mg can respond weakly with Si3N4; silicon option can damp Si3N4 as well as wear down a little; change aspect remedy can strongly wet Si3N4 and kind silicide with Si to rapidly break down silicon nitride and retreat N2 at the exact same time. Si3N4 is stable to alloy solutions such as brass, tough light weight aluminum and nickel silver, as well as has excellent rust resistance to cast iron and also medium carbon steel, yet it is not resistant to Ni-Cr alloy and stainless steel.

With the exception of liquified NaOH as well as HF, silicon nitride has good chemical rust resistance. But most of the liquified antacid and salt can connect with Si3N4 to decompose it.

Silicon Nitride made use of in Refractories.
Silicon nitride ceramics are called encouraging high temperature architectural materials because of their exceptional heat residential properties, such as heat toughness, great wear resistance as well as corrosion resistance. Due to the solid covalent bond and reduced diffusion coefficient at high temperature, the manufacture of Si3N4 ceramics should count on heat, high pressure and also sintering representative, as well as the price is too high, otherwise it is difficult to create top quality silicon nitride materials. These restrictions of production prices as well as devices are hard to accept in the metallurgical sector, so the study in the field of refractories starts relatively late, and the research is not comprehensive. Many concepts originate from ceramics, but there is very little advancement. In the past, silicon nitride generally existed in the type of bonding stage in refractories. With nitriding as well as shooting of metal Si, aggregates such as corundum or silicon carbide were combined with fine powder, so as to achieve the function of incorporating refractory products. Ceramic shed plate is silicon carbide accumulation and component of great powder. Silicon nitride is formed by nitriding steel Si to create silicon nitride. Silicon carbide is combined to create silicon nitride adhered silicon carbide material, which is used in blast heating system body as well as other parts, so that the performance of the product has actually been substantially enhanced. Compared to the clay bound silicon carbide shed plate, the high temperature performance of the product is excellent, which resolves the trouble of protruding failing caused by the oxidation of silicon carbide when the clay adhered silicon carbide lost plate is made use of.

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