Silicon Nitride structures and properties
Crystal structure of Silicon Nitride
Si3N4 has 2 sort of crystal structure: α& alpha;- Si3N4 is granular crystal and also β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks composed of usual vertices of [SN4] tetrahedron and also come from hexagonal system. The difference 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 6 almost entirely in proportion [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; stage is developed by 2 layers of concrete improvement and different non-hexagonal ring layers. α & alpha; stage can dissolve oxygen in the range of crystal framework, as well as the inner strain of α & alpha; phase is bigger than that of β & beta; phase, so the totally free power is greater than that of β & beta; stage. From the point of view of thermodynamics, the β & beta; stage is much more secure at greater temperature. The & alpha; stage has reduced proportion and is simple to create. At the temperature level of concerning 1500 ℃, the α & alpha; stage undergoes a reconstruction makeover and is transformed into β & beta; stage. This change is irreversible, as well as the existence of some procedure conditions and high quality is much more beneficial to the improvement from α & alpha; phase to & beta; phase. & alpha;-Si3N4 is created when the temperature βis lower than 1350 ℃, and also & beta;-Si3N4 can be straight prepared at a temperature
greater 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 in between Si and N (of which the ion bond make up just 30%), so the Si3N4 has high firmness (Morse solidity 9), high melting point as well as stable structure.
The Si-N in silicon nitride crystal is primarily covalent bond, as well as the bonding toughness is high, so it has a large flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal growth is reduced, however the thermal conductivity is large, so this material is difficult to generate thermal tension, so it has good thermal shock resistance and also excellent thermal shock resistance. It has durability, high mechanical stamina at high temperature as well as little deformation at high temperature. (the high temperature deformation of silicon nitride ceramic with 2.5 g/cm3 thickness is 0.5% at 1200 × & times; 1000h ℃ and also 23 × & times; 7kg/cm2 load). Not eroded by numerous steels, because of the formation of a silicon dioxide layer, great oxidation resistance and good electrical insulation.
Silicon nitride has no melting factor and also sublimates and also decays at 1900 ℃ under air pressure. The specific warmth 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 specifically. Due to the fact that it is a solid covalent bond substance, 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 materials that promote sintering are primarily Y2O3, Al2O3, etc, and the high addition amount can get to 20%. The response principle is that the SiO2 oxide movie based on the surface of silicon nitride bits reacts with the included oxide to create liquid stage and permeate at the grain boundary to ensure high diffusion capability throughout product movement.
Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically stable compound. Silicon nitride ceramics can be consumed to 1400 ℃ in oxidation ambience and also 1850 ℃ in neutral or reducing atmosphere. In fact, the oxidation reaction of Si3N4 happens above 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the example boosted and a dense silica safety layer was slowly formed on the surface, which stopped the more oxidation of Si3N4. The weight gain was not apparent up until the temperature was above 1600 ℃. However in the damp ambience, Si3N4 is extremely simple to oxidize, and also the surface area begins to oxidize at 200 ℃, which is about two times as quick as that in completely dry air. The oxidation activation energy of Si3N4 powder in water vapor is obviously less than that in oxygen and air. The reason is that water vapor can respond with Si3N4 through amorphous SiO2 movies:
Si3N4 +6 WATER=3SiO2+NH3.
Silicon nitride is steady to most metal solutions, without rust as well as seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so forth. However, for Cu remedy, it is not deteriorated just in vacuum cleaner or inert atmosphere; Mg can react weakly with Si3N4; silicon remedy can wet Si3N4 and also deteriorate a little; shift component service can highly wet Si3N4 as well as type silicide with Si to rapidly break down silicon nitride and also retreat N2 at the very same time. Si3N4 is secure to alloy services such as brass, difficult light weight aluminum and nickel silver, and has excellent deterioration resistance to cast iron and also tool carbon steel, but it is not immune to Ni-Cr alloy and also stainless steel.
With the exception of molten NaOH as well as HF, silicon nitride has great chemical deterioration resistance. However most of the molten antacid and also salt can connect with Si3N4 to decay it.
Silicon Nitride used in Refractories.
Silicon nitride porcelains are called encouraging heat architectural materials due to their excellent high temperature homes, such as high temperature toughness, great wear resistance and also deterioration resistance. Due to the strong covalent bond and reduced diffusion coefficient at heat, the manufacture of Si3N4 porcelains need to depend on heat, high stress and sintering representative, as well as the price is too expensive, otherwise it is hard to produce premium quality silicon nitride products. These constraints of production prices as well as devices are hard to accept in the metallurgical market, so the research study in the area of refractories begins fairly late, and the research is not thorough. Several concepts originate from ceramics, but there is not much innovation. In the past, silicon nitride typically existed in the form of bonding stage in refractories. Via nitriding and also firing of metal Si, aggregates such as corundum or silicon carbide were integrated with great powder, so as to accomplish the objective of combining refractory products. Ceramic shed plate is silicon carbide aggregate as well as part of great powder. Silicon nitride is formed by nitriding steel Si to form silicon nitride. Silicon carbide is incorporated to create silicon nitride bound silicon carbide product, which is used in blast furnace body and also various other parts, to make sure that the performance of the product has actually been considerably improved. Compared with the clay adhered silicon carbide shed plate, the high temperature efficiency of the product is great, which fixes the issue of bulging failing triggered by the oxidation of silicon carbide when the clay bonded silicon carbide dropped plate is used.
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