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Silicon Nitride structures and properties

Crystal structure of Silicon Nitride
Si3N4 has 2 type 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 usual vertices of [SN4] tetrahedron as well as come from hexagonal system. The difference between them hinges on the order of [SiN4] tetrahedral layers. The β & beta; stage is created by the overlap of hexagonal ring layers composed of 6 nearly entirely balanced [SN4] tetrahedrons in the c-axis direction, while the α & alpha; stage is created by 2 layers of tangible makeover as well as different non-hexagonal ring layers. α & alpha; phase can liquify oxygen in the variety of crystal framework, and the internal pressure of α & alpha; phase is bigger than that of β & beta; stage, so the totally free energy is more than that of β & beta; stage. From the viewpoint of thermodynamics, the β & beta; stage is more secure at higher temperature level. The & alpha; stage has low symmetry as well as is easy to develop. At the temperature of about 1500 ℃, the α & alpha; stage undergoes a reconstruction change and is transformed into β & beta; phase. This transformation is irreparable, and also the presence of some procedure conditions and also high quality is a lot more useful to the change from α & alpha; stage to & beta; stage. & alpha;-Si3N4 is created when the temperature βis less than 1350 ℃, as well as & beta;-Si3N4 can be straight prepared at a temperature level

greater than 1500 ℃. Silicon Nitride buildings The molecular formula of silicon nitride is Si3N4, of which Si accounts for 60.06% and dint N make up 39.94%. There is a strong covalent bond in between Si and also N (of which the ion bond make up only 30%), so the Si3N4 has high hardness (Morse solidity 9), high melting point and also secure framework.
The Si-N in silicon nitride crystal is mostly covalent bond, and also the bonding stamina is high, so it has a large elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal expansion is reduced, yet the thermal conductivity is big, so this material is challenging to produce thermal tension, so it has great thermal shock resistance and great thermal shock resistance. It has strength, high mechanical stamina at heat and small contortion at high temperature. (the heat deformation of silicon nitride ceramic with 2.5 g/cm3 thickness is 0.5% at 1200 × & times; 1000h ℃ as well as 23 × & times; 7kg/cm2 load). Not eroded by lots of steels, because of the development of a silicon dioxide layer, great oxidation resistance as well as good electric insulation.

Silicon nitride has no melting factor as well as sublimates and also decays at 1900 ℃ under atmospheric pressure. The particular heat is 711.8 J/ kg ℃. The microhardness of α & alpha; phase and also & beta; stage is 10 ~ 16GPa as well as 24.5 ~ 32.65 GPa respectively. Because it is a strong covalent bond compound, no fluid phase will be developed below its decay temperature (concerning 1900 ℃), so silicon nitride materials can be sintered with the assistance of oxide additives. The oxide products that advertise sintering are mostly Y2O3, Al2O3, and so on, and also the high enhancement amount can get to 20%. The reaction concept is that the SiO2 oxide movie formed on the surface of silicon nitride bits reacts with the added oxide to create fluid stage and also penetrate at the grain limit to make certain high diffusion capacity throughout product migration.

Chemical Stability of Silicon Nitride
Si3N4 is a thermodynamically steady substance. Silicon nitride porcelains can be used up to 1400 ℃ in oxidation environment and also 1850 ℃ in neutral or lowering atmosphere. Actually, the oxidation reaction of Si3N4 takes place above 800C:
Si3N4 +3 O2=3SiO2+N2.

The weight of the sample enhanced and a dense silica protective layer was gradually formed on the surface, which avoided the more oxidation of Si3N4. The weight gain was not evident until the temperature level was over 1600 ℃. But in the damp atmosphere, Si3N4 is extremely simple to oxidize, and also the surface area starts to oxidize at 200 ℃, which has to do with twice as quick as that in completely dry air. The oxidation activation power of Si3N4 powder in water vapor is certainly lower than that in oxygen and 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 remedies, without corrosion and infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so forth. Nevertheless, for Cu option, it is not eroded only in vacuum or inert environment; Mg can respond weakly with Si3N4; silicon service can wet Si3N4 and also erode a little; shift aspect remedy can highly wet Si3N4 and also form silicide with Si to rapidly decompose silicon nitride as well as retreat N2 at the same time. Si3N4 is stable to alloy remedies such as brass, tough light weight aluminum and also nickel silver, and has good corrosion resistance to cast iron and also medium carbon steel, yet it is not immune to Ni-Cr alloy and stainless-steel.

Besides molten NaOH and also HF, silicon nitride has great chemical rust resistance. But a lot of the liquified alkali as well as salt can connect with Si3N4 to decay it.

Silicon Nitride made use of in Refractories.
Silicon nitride ceramics are called encouraging high temperature structural products as a result of their outstanding heat residential properties, such as heat toughness, excellent wear resistance and also corrosion resistance. As a result of the solid covalent bond and low diffusion coefficient at high temperature, the manufacture of Si3N4 porcelains have to depend on heat, high stress and sintering representative, and also the price is too expensive, otherwise it is hard to produce premium quality silicon nitride products. These limitations of manufacturing costs as well as devices are challenging to accept in the metallurgical industry, so the research in the field of refractories begins relatively late, and also the research is not in-depth. Numerous theories come from porcelains, yet there is very little technology. In the past, silicon nitride generally existed in the kind of bonding stage in refractories. With nitriding and shooting of metal Si, accumulations such as diamond or silicon carbide were incorporated with great powder, so as to accomplish the function of integrating refractory products. Ceramic shed plate is silicon carbide accumulation and also part of fine powder. Silicon nitride is formed by nitriding steel Si to form silicon nitride. Silicon carbide is integrated to form silicon nitride bonded silicon carbide product, which is used in blast heating system body as well as other parts, so that the efficiency of the product has actually been considerably improved. Compared to the clay bonded silicon carbide lost plate, the high temperature performance of the product is great, which solves the trouble of bulging failing brought on by the oxidation of silicon carbide when the clay adhered silicon carbide lost plate is utilized.

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