Silicon Nitride structures and properties
Crystal structure of Silicon Nitride
Si3N4 has two type of crystal framework: α& alpha;- Si3N4 is granular crystal and β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks composed of typical vertices of [SN4] tetrahedron and also belong to 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 practically entirely balanced [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; stage is created by 2 layers of concrete change and also various non-hexagonal ring layers. α & alpha; phase can liquify oxygen in the range of crystal structure, as well as the inner stress of α & alpha; phase is bigger than that of β & beta; phase, so the cost-free energy is higher than that of β & beta; stage. From the perspective of thermodynamics, the β & beta; stage is much more steady at greater temperature level. The & alpha; phase has low symmetry and also is very easy to develop. At the temperature of concerning 1500 ℃, the α & alpha; phase goes through a reconstruction transformation and also is changed right into β & beta; phase. This improvement is irreparable, as well as the existence of some procedure problems as well as quality is much more beneficial to the transformation from α & alpha; phase to & beta; phase. & alpha;-Si3N4 is created when the temperature level βis lower 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 accounts for 60.06% and also dint N represent 39.94%. There is a strong covalent bond in between Si and N (of which the ion bond represent just 30%), so the Si3N4 has high firmness (Morse firmness 9), high melting factor and stable framework.
The Si-N in silicon nitride crystal is primarily covalent bond, and the bonding toughness is high, so it has a big flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is reduced, however the thermal conductivity is large, so this product is hard to produce thermal anxiety, so it has great thermal shock resistance and excellent thermal shock resistance. It has durability, high mechanical toughness at high temperature as well as small deformation at high temperature. (the heat deformation of silicon nitride ceramic with 2.5 g/cm3 thickness is 0.5% at 1200 × & times; 1000h ℃ and 23 × & times; 7kg/cm2 tons). Not worn down by lots of steels, as a result of the formation of a silicon dioxide layer, excellent oxidation resistance and great electrical insulation.
Silicon nitride has no melting factor as well as sublimates and decomposes at 1900 ℃ under atmospheric pressure. The certain warm is 711.8 J/ kg ℃. The microhardness of α & alpha; phase as well as & 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 compound, no liquid stage will certainly be created listed below its decomposition temperature level (about 1900 ℃), so silicon nitride materials can be sintered with the assistance of oxide additives. The oxide materials that promote sintering are mostly Y2O3, Al2O3, and so on, as well as the high addition amount can reach 20%. The reaction concept is that the SiO2 oxide film based on the surface area of silicon nitride bits responds with the added oxide to create fluid phase as well as penetrate at the grain border to guarantee high diffusion capacity throughout material movement.
Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically steady substance. Silicon nitride porcelains can be consumed to 1400 ℃ in oxidation atmosphere and 1850 ℃ in neutral or reducing ambience. In fact, the oxidation response of Si3N4 happens over 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the example boosted as well as a dense silica protective layer was slowly formed on the surface area, which prevented the more oxidation of Si3N4. The weight gain was not noticeable until the temperature was above 1600 ℃. Yet in the damp environment, Si3N4 is really simple to oxidize, and also the surface area begins to oxidize at 200 ℃, which is about two times as rapid as that in dry air. The oxidation activation power of Si3N4 powder in water vapor is obviously less than that in oxygen as well as air. The factor is that water vapor can react with Si3N4 with amorphous SiO2 films:
Si3N4 +6 WATER=3SiO2+NH3.
Silicon nitride is steady to most metal remedies, free from rust as well as seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so forth. Nonetheless, for Cu service, it is not worn down just in vacuum or inert atmosphere; Mg can react weakly with Si3N4; silicon option can damp Si3N4 as well as erode slightly; transition aspect service can strongly wet Si3N4 and also type silicide with Si to quickly disintegrate silicon nitride and also getaway N2 at the same time. Si3N4 is steady to alloy solutions such as brass, hard aluminum as well as nickel silver, and has good corrosion resistance to cast iron and tool carbon steel, but it is not immune to Ni-Cr alloy and stainless-steel.
With the exception of molten NaOH as well as HF, silicon nitride has excellent chemical rust resistance. But a lot of the molten antacid and also salt can engage with Si3N4 to disintegrate it.
Silicon Nitride used in Refractories.
Silicon nitride porcelains are called encouraging high temperature structural materials due to their superb heat homes, such as high temperature strength, excellent wear resistance as well as deterioration resistance. Because of the strong covalent bond as well as low diffusion coefficient at heat, the manufacture of Si3N4 porcelains should count on heat, high stress as well as sintering representative, and the expense is too expensive, otherwise it is tough to produce high quality silicon nitride materials. These limitations of production expenses and devices are difficult to approve in the metallurgical market, so the research study in the field of refractories begins reasonably late, and the research is not comprehensive. Many theories come from porcelains, yet there is not much technology. In the past, silicon nitride typically existed in the form of bonding phase in refractories. Through nitriding and shooting of steel Si, accumulations such as diamond or silicon carbide were incorporated with great powder, so as to achieve the objective of integrating refractory products. Ceramic shed plate is silicon carbide accumulation as well as component of fine powder. Silicon nitride is developed by nitriding metal Si to create silicon nitride. Silicon carbide is incorporated to develop silicon nitride bonded silicon carbide material, which is used in blast furnace body as well as other parts, so that the performance of the material has been considerably boosted. Compared to the clay adhered silicon carbide dropped plate, the heat performance of the product is very good, which fixes the problem of protruding failure caused by the oxidation of silicon carbide when the clay bonded silicon carbide shed plate is used.
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