Silicon Nitride structures and properties
Crystal framework of Silicon Nitride
Si3N4 has 2 sort of crystal framework: α& alpha;- Si3N4 is granular crystal and β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks made up of common vertices of [SN4] tetrahedron and also belong to hexagonal system. The distinction between them hinges on the order of [SiN4] tetrahedral layers. The β & beta; phase is created by the overlap of hexagonal ring layers composed of 6 virtually totally symmetrical [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; stage is formed by two layers of substantial makeover and different non-hexagonal ring layers. α & alpha; stage can dissolve oxygen in the series of crystal structure, as well as the interior stress of α & alpha; phase is larger than that of β & beta; phase, so the cost-free energy is higher than that of β & beta; stage. From the viewpoint of thermodynamics, the β & beta; phase is a lot more secure at greater temperature. The & alpha; stage has reduced proportion and is very easy to develop. At the temperature level of about 1500 ℃, the α & alpha; phase undergoes a restoration makeover and also is changed right into β & beta; stage. This makeover is irreversible, and also the presence of some process conditions and quality is more beneficial to the transformation from α & alpha; stage to & beta; stage. & alpha;-Si3N4 is developed when the temperature level βis less than 1350 ℃, and also & beta;-Si3N4 can be straight prepared at a temperature
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 strong covalent bond between Si as well as N (of which the ion bond make up just 30%), so the Si3N4 has high solidity (Morse firmness 9), high melting point and also steady framework.
The Si-N in silicon nitride crystal is primarily covalent bond, and the bonding strength is high, so it has a huge elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is reduced, but the thermal conductivity is huge, so this material is difficult to create thermal anxiety, so it has great thermal shock resistance and also great thermal shock resistance. It has strength, high mechanical strength at heat and also small contortion at high temperature. (the high temperature contortion of silicon nitride ceramic with 2.5 g/cm3 thickness is 0.5% at 1200 × & times; 1000h ℃ as well as 23 × & times; 7kg/cm2 tons). Not worn down by lots of steels, as a result of the formation of a silicon dioxide layer, great oxidation resistance and great electric insulation.
Silicon nitride has no melting factor and also sublimates and also breaks down at 1900 ℃ under atmospheric pressure. The certain heat is 711.8 J/ kg ℃. The microhardness of α & alpha; phase as well as & beta; stage is 10 ~ 16GPa as well as 24.5 ~ 32.65 GPa respectively. Since it is a strong covalent bond substance, no liquid stage will be created listed below its disintegration temperature level (concerning 1900 ℃), so silicon nitride materials can be sintered with the help of oxide ingredients. The oxide materials that promote sintering are mostly Y2O3, Al2O3, etc, and also the high addition quantity can get to 20%. The response principle is that the SiO2 oxide film based on the surface area of silicon nitride bits responds with the added oxide to create liquid stage as well as penetrate at the grain border to make sure high diffusion capacity throughout material migration.
Chemical Stability of Silicon Nitride
Si3N4 is a thermodynamically stable compound. Silicon nitride porcelains can be consumed to 1400 ℃ in oxidation atmosphere and also 1850 ℃ in neutral or lowering atmosphere. As a matter of fact, the oxidation reaction of Si3N4 takes place over 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the example raised as well as a thick silica protective layer was progressively formed on the surface, which protected against the further oxidation of Si3N4. The weight gain was not obvious until the temperature level was over 1600 ℃. However in the damp atmosphere, Si3N4 is extremely simple to oxidize, as well as the surface area starts to oxidize at 200 ℃, which is about twice as rapid as that in dry air. The oxidation activation energy of Si3N4 powder in water vapor is clearly lower than that in oxygen and air. The reason is that water vapor can respond with Si3N4 with amorphous SiO2 movies:
Si3N4 +6 H2O=3SiO2+NH3.
Silicon nitride is steady to most metal solutions, free from deterioration and also infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so forth. Nevertheless, for Cu remedy, it is not worn down just in vacuum or inert ambience; Mg can respond weakly with Si3N4; silicon service can damp Si3N4 and wear down a little; change aspect service can highly damp Si3N4 and also type silicide with Si to swiftly break down silicon nitride as well as escape N2 at the very same time. Si3N4 is steady to alloy solutions such as brass, tough aluminum as well as nickel silver, as well as has good deterioration resistance to cast iron and also tool carbon steel, but it is not immune to Ni-Cr alloy as well as stainless steel.
Besides liquified NaOH as well as HF, silicon nitride has great chemical deterioration resistance. But most of the liquified alkali and also salt can engage with Si3N4 to decay it.
Silicon Nitride utilized in Refractories.
Silicon nitride porcelains are called appealing high temperature architectural materials because of their superb high temperature residential or commercial properties, such as high temperature strength, great wear resistance and corrosion resistance. Because of the strong covalent bond and low diffusion coefficient at heat, the manufacture of Si3N4 ceramics must rely upon heat, high pressure and sintering representative, and the cost is expensive, otherwise it is difficult to create premium quality silicon nitride materials. These restrictions of production prices and also tools are hard to approve in the metallurgical industry, so the research study in the area of refractories starts relatively late, as well as the research is not extensive. Numerous theories originate from porcelains, however there is very little advancement. In the past, silicon nitride generally existed in the type of bonding stage in refractories. Via nitriding and also shooting of metal Si, accumulations such as diamond or silicon carbide were incorporated with fine powder, so regarding accomplish the purpose of integrating refractory materials. Ceramic shed plate is silicon carbide accumulation and part of great powder. Silicon nitride is developed by nitriding metal Si to form silicon nitride. Silicon carbide is combined to develop silicon nitride adhered silicon carbide product, which is used in blast heating system body as well as other parts, to ensure that the performance of the material has actually been greatly improved. Compared with the clay adhered silicon carbide shed plate, the high temperature performance of the material is great, which solves the problem of protruding failure caused by the oxidation of silicon carbide when the clay bound silicon carbide lost plate is used.
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