Silicon Nitride structures and properties
Crystal structure of Silicon Nitride
Si3N4 has two sort of crystal structure: α& alpha;- Si3N4 is granular crystal and β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks made up of usual vertices of [SN4] tetrahedron and also belong to hexagonal system. The distinction between them lies in the order of [SiN4] tetrahedral layers. The β & beta; phase is formed by the overlap of hexagonal ring layers composed of 6 virtually totally symmetrical [SN4] tetrahedrons in the c-axis direction, while the α & alpha; stage is created by 2 layers of tangible change and also different non-hexagonal ring layers. α & alpha; stage can dissolve oxygen in the variety of crystal framework, as well as the inner stress of α & alpha; phase is larger than that of β & beta; phase, so the totally free power is greater than that of β & beta; stage. From the viewpoint of thermodynamics, the β & beta; phase is more steady at higher temperature. The & alpha; stage has reduced balance and also is simple to form. At the temperature level of about 1500 ℃, the α & alpha; stage undertakes a reconstruction change and also is changed right into β & beta; stage. This change is irreversible, and also the presence of some procedure problems as well as quality is much more beneficial to the improvement from α & alpha; phase to & beta; phase. & alpha;-Si3N4 is formed when the temperature level βis lower than 1350 ℃, and also & 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 make up 60.06% and also dint N represent 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 firmness (Morse hardness 9), high melting factor and also stable framework.
The Si-N in silicon nitride crystal is generally covalent bond, and the bonding strength is high, so it has a large flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal expansion is reduced, however the thermal conductivity is big, so this material is hard to generate thermal stress and anxiety, so it has excellent thermal shock resistance as well as good thermal shock resistance. It has sturdiness, high mechanical toughness at heat and also little contortion at high temperature. (the high temperature contortion 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 steels, as a result of the development of a silicon dioxide layer, great oxidation resistance and also great electric insulation.
Silicon nitride has no melting point as well as sublimates as well as breaks down at 1900 ℃ under atmospheric pressure. The specific warmth is 711.8 J/ kg ℃. The microhardness of α & alpha; stage as well as & beta; phase is 10 ~ 16GPa and also 24.5 ~ 32.65 GPa respectively. Since it is a solid covalent bond compound, no fluid stage will certainly be developed listed below its decomposition temperature level (about 1900 ℃), so silicon nitride products can be sintered with the aid of oxide ingredients. The oxide products that promote sintering are mostly Y2O3, Al2O3, etc, as well as the high enhancement quantity can reach 20%. The reaction principle is that the SiO2 oxide film formed on the surface of silicon nitride fragments reacts with the added oxide to create fluid stage and permeate at the grain border to make sure high diffusion capability throughout product migration.
Chemical Stability of Silicon Nitride
Si3N4 is a thermodynamically secure substance. Silicon nitride porcelains can be consumed to 1400 ℃ in oxidation environment and also 1850 ℃ in neutral or minimizing environment. Actually, the oxidation reaction of Si3N4 occurs above 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the example enhanced and also a dense silica protective layer was progressively formed on the surface, which protected against the more oxidation of Si3N4. The weight gain was not evident till the temperature level was over 1600 ℃. However in the moist environment, Si3N4 is very easy to oxidize, and also the surface starts to oxidize at 200 ℃, which is about two times as rapid as that in completely dry air. The oxidation activation energy of Si3N4 powder in water vapor is obviously less than that in oxygen and also air. The reason is that water vapor can respond with Si3N4 with amorphous SiO2 movies:
Si3N4 +6 WATER=3SiO2+NH3.
Silicon nitride is stable to most metal services, free from deterioration and infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so forth. However, for Cu remedy, it is not worn down only in vacuum cleaner or inert atmosphere; Mg can respond weakly with Si3N4; silicon option can damp Si3N4 as well as deteriorate somewhat; shift element solution can highly damp Si3N4 and kind silicide with Si to quickly break down silicon nitride and also getaway N2 at the same time. Si3N4 is steady to alloy services such as brass, tough aluminum and nickel silver, and has good corrosion resistance to cast iron and also medium carbon steel, but it is not resistant to Ni-Cr alloy and stainless-steel.
Except for liquified NaOH as well as HF, silicon nitride has excellent chemical corrosion resistance. Yet the majority of the molten antacid as well as salt can interact with Si3N4 to decompose it.
Silicon Nitride made use of in Refractories.
Silicon nitride ceramics are called encouraging heat structural materials because of their exceptional high temperature residential or commercial properties, such as high temperature toughness, good wear resistance and also deterioration resistance. Because of the strong covalent bond and reduced diffusion coefficient at heat, the manufacture of Si3N4 ceramics should count on high temperature, high stress and also sintering representative, and also the price is too expensive, otherwise it is difficult to generate top quality silicon nitride materials. These restrictions of production prices and equipment are hard to approve in the metallurgical market, so the study in the area of refractories starts reasonably late, and the research is not comprehensive. Many concepts originate from porcelains, but there is very little technology. In the past, silicon nitride normally existed in the kind of bonding stage in refractories. Through nitriding and shooting of metal Si, accumulations such as diamond or silicon carbide were integrated with great powder, so regarding attain the purpose of combining refractory products. Ceramic shed plate is silicon carbide accumulation as well as part of fine powder. Silicon nitride is developed by nitriding steel Si to form silicon nitride. Silicon carbide is combined to form silicon nitride adhered silicon carbide material, which is utilized in blast furnace body and also various other components, to make sure that the efficiency of the material has actually been substantially improved. Compared with the clay bonded silicon carbide shed plate, the high temperature performance of the product is very good, which fixes the issue of bulging failing triggered by the oxidation of silicon carbide when the clay adhered silicon carbide lost plate is utilized.
Silicon nitride Cost
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