Silicon Nitride structures and properties
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Crystal framework of Silicon Nitride
Si3N4 has two sort of crystal structure: α& alpha;- Si3N4 is granular crystal as well as β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks composed of usual vertices of [SN4] tetrahedron and belong to hexagonal system. The distinction between them depends on the order of [SiN4] tetrahedral layers. The β & beta; phase is created by the overlap of hexagonal ring layers made up of six nearly completely symmetrical [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; phase is created by 2 layers of substantial change and various non-hexagonal ring layers. α & alpha; phase can liquify oxygen in the variety of crystal framework, and also the internal stress of α & alpha; stage is bigger than that of β & beta; stage, so the totally free energy is more than that of β & beta; phase. From the perspective of thermodynamics, the β & beta; stage is much more stable at higher temperature level. The & alpha; stage has low symmetry and is easy to create. At the temperature of regarding 1500 ℃, the α & alpha; phase undergoes a repair transformation as well as is transformed right into β & beta; stage. This transformation is permanent, and the presence of some process problems as well as high quality is more useful to the improvement from α & alpha; phase to & beta; stage. & alpha;-Si3N4 is created when the temperature level βis less than 1350 ℃, and & beta;-Si3N4 can be directly prepared at a temperature level
higher than 1500 ℃. Silicon Nitride properties 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 as well as N (of which the ion bond make up just 30%), so the Si3N4 has high hardness (Morse solidity 9), high melting factor as well as secure structure.
The Si-N in silicon nitride crystal is mainly covalent bond, and also the bonding stamina is high, so it has a huge flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is reduced, however the thermal conductivity is large, so this product is difficult to create thermal stress, so it has great thermal shock resistance and also good thermal shock resistance. It has durability, high mechanical toughness 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 deteriorated by numerous steels, as a result of the development of a silicon dioxide layer, good oxidation resistance as well as great electric insulation.
Silicon nitride has no melting factor and also sublimates and decays at 1900 ℃ under air pressure. The details warm is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and & beta; phase is 10 ~ 16GPa and 24.5 ~ 32.65 Grade point average specifically. Since it is a strong covalent bond compound, no liquid stage will certainly be formed below its decomposition temperature (concerning 1900 ℃), so silicon nitride materials can be sintered with the assistance of oxide ingredients. The oxide materials that promote sintering are mostly Y2O3, Al2O3, etc, as well as the high addition amount can get to 20%. The response principle is that the SiO2 oxide film based on the surface area of silicon nitride fragments reacts with the added oxide to create fluid phase and penetrate at the grain boundary to make certain high diffusion capacity during material movement.
Chemical Stability of Silicon Nitride
Si3N4 is a thermodynamically steady compound. Silicon nitride ceramics can be consumed to 1400 ℃ in oxidation environment and 1850 ℃ in neutral or lowering atmosphere. Actually, the oxidation reaction of Si3N4 happens above 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the sample increased and also a dense silica safety layer was slowly based on the surface, which protected against the further oxidation of Si3N4. The weight gain was not obvious until the temperature level was above 1600 ℃. But in the damp ambience, Si3N4 is really easy to oxidize, and also the surface area starts to oxidize at 200 ℃, which has to do with twice as rapid as that in completely dry air. The oxidation activation energy of Si3N4 powder in water vapor is certainly lower than that in oxygen and also air. The factor is that water vapor can respond with Si3N4 through amorphous SiO2 movies:
Si3N4 +6 H2O=3SiO2+NH3.
Silicon nitride is steady to most metal solutions, free from rust and infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so forth. However, for Cu service, it is not deteriorated just in vacuum or inert ambience; Mg can react weakly with Si3N4; silicon option can wet Si3N4 and wear down slightly; change aspect service can highly damp Si3N4 and kind silicide with Si to promptly decay silicon nitride as well as retreat N2 at the very same time. Si3N4 is secure to alloy options such as brass, hard light weight aluminum and also nickel silver, as well as has great deterioration resistance to cast iron as well as tool carbon steel, however it is not immune to Ni-Cr alloy as well as stainless steel.
Besides molten NaOH and HF, silicon nitride has good chemical corrosion resistance. But the majority of the molten antacid and salt can connect with Si3N4 to decay it.
Silicon Nitride made use of in Refractories.
Silicon nitride porcelains are called encouraging high temperature structural materials because of their excellent high temperature buildings, such as high temperature strength, great wear resistance as well as corrosion resistance. Due to the solid covalent bond as well as reduced diffusion coefficient at high temperature, the manufacture of Si3N4 porcelains must rely on heat, high stress and also sintering agent, and the price is expensive, otherwise it is hard to produce high quality silicon nitride products. These constraints of manufacturing expenses and also devices are challenging to approve in the metallurgical sector, so the research in the area of refractories begins reasonably late, as well as the research study is not in-depth. Lots of theories come from porcelains, yet there is very little development. In the past, silicon nitride usually existed in the form of bonding phase in refractories. With nitriding as well as shooting of steel Si, accumulations such as corundum or silicon carbide were integrated with great powder, so as to accomplish the objective of incorporating refractory materials. Ceramic shed plate is silicon carbide accumulation and component of great powder. Silicon nitride is created by nitriding steel Si to develop silicon nitride. Silicon carbide is integrated to develop silicon nitride adhered silicon carbide product, which is utilized in blast heating system body and other parts, so that the performance of the material has actually been greatly boosted. Compared with the clay bonded silicon carbide shed plate, the heat efficiency of the material is excellent, which resolves the issue of bulging failing triggered by the oxidation of silicon carbide when the clay adhered silicon carbide shed plate is used.
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