Silicon Nitride structures and properties
Crystal framework of Silicon Nitride
Si3N4 has two kinds of crystal framework: α& alpha;- Si3N4 is granular crystal and also β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks composed of common vertices of [SN4] tetrahedron and come from hexagonal system. The distinction between them hinges on the order of [SiN4] tetrahedral layers. The β & beta; stage is created by the overlap of hexagonal ring layers made up of 6 almost totally in proportion [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; phase is developed by two layers of tangible transformation as well as various non-hexagonal ring layers. α & alpha; phase can dissolve oxygen in the variety of crystal structure, and the interior pressure of α & alpha; stage is bigger than that of β & beta; phase, so the complimentary energy is higher than that of β & beta; stage. From the point of view of thermodynamics, the β & beta; phase is more steady at higher temperature level. The & alpha; phase has low symmetry and also is very easy to form. At the temperature of about 1500 ℃, the α & alpha; stage undergoes a restoration improvement and also is transformed into β & beta; stage. This makeover is irreparable, and also the existence of some process conditions and top quality is extra beneficial to the improvement from α & alpha; stage to & beta; phase. & alpha;-Si3N4 is created when the temperature βis less than 1350 ℃, and also & beta;-Si3N4 can be directly prepared at a temperature
greater than 1500 ℃. Silicon Nitride homes The molecular formula of silicon nitride is Si3N4, of which Si make up 60.06% and also dint N accounts for 39.94%. There is a solid covalent bond between Si and also N (of which the ion bond accounts for just 30%), so the Si3N4 has high hardness (Morse solidity 9), high melting point and steady framework.
The Si-N in silicon nitride crystal is mostly covalent bond, as well as the bonding stamina is high, so it has a large flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is reduced, however the thermal conductivity is large, so this product is not easy to generate thermal tension, so it has excellent thermal shock resistance as well as great thermal shock resistance. It has toughness, high mechanical strength at high temperature as well as tiny deformation at heat. (the heat deformation of silicon nitride ceramic with 2.5 g/cm3 thickness is 0.5% at 1200 × & times; 1000h ℃ as well as 23 × & times; 7kg/cm2 load). Not eroded by numerous metals, as a result of the development of a silicon dioxide layer, good oxidation resistance as well as good electrical insulation.
Silicon nitride has no melting factor and sublimates as well as disintegrates at 1900 ℃ under air pressure. The details warm 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 Grade point average respectively. Because it is a strong covalent bond substance, no liquid stage will be created below its decomposition temperature level (concerning 1900 ℃), so silicon nitride products can be sintered with the help of oxide ingredients. The oxide materials that promote sintering are generally Y2O3, Al2O3, and so on, and also the high addition amount can get to 20%. The response principle is that the SiO2 oxide film formed on the surface of silicon nitride particles reacts with the included oxide to create liquid stage and permeate at the grain border to make certain high diffusion ability throughout material movement.
Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically steady compound. Silicon nitride ceramics can be consumed to 1400 ℃ in oxidation atmosphere and 1850 ℃ in neutral or reducing ambience. In fact, the oxidation response of Si3N4 occurs above 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the sample increased as well as a dense silica protective layer was progressively based on the surface area, which avoided the more oxidation of Si3N4. The weight gain was not evident till the temperature level was above 1600 ℃. Yet in the moist atmosphere, Si3N4 is really easy to oxidize, and the surface area starts to oxidize at 200 ℃, which is about two times as quick as that in dry air. The oxidation activation power of Si3N4 powder in water vapor is undoubtedly lower than that in oxygen and air. The reason is that water vapor can react with Si3N4 via amorphous SiO2 movies:
Si3N4 +6 WATER=3SiO2+NH3.
Silicon nitride is secure to most metal solutions, free from corrosion and infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and more. However, for Cu service, it is not eroded just in vacuum or inert ambience; Mg can react weakly with Si3N4; silicon solution can wet Si3N4 and deteriorate a little; shift element solution can strongly wet Si3N4 and also type silicide with Si to rapidly decompose silicon nitride and also getaway N2 at the exact same time. Si3N4 is secure to alloy options such as brass, difficult aluminum and nickel silver, as well as has great corrosion resistance to cast iron and also medium carbon steel, yet it is not resistant to Ni-Cr alloy as well as stainless steel.
Except for liquified NaOH as well as HF, silicon nitride has excellent chemical rust resistance. But the majority of the molten alkali and also salt can interact with Si3N4 to disintegrate it.
Silicon Nitride utilized in Refractories.
Silicon nitride ceramics are called encouraging high temperature architectural products due to their excellent high temperature buildings, such as heat strength, excellent wear resistance and also corrosion resistance. Due to the strong covalent bond and reduced diffusion coefficient at high temperature, the manufacture of Si3N4 ceramics have to count on heat, high stress as well as sintering agent, and also the expense is too high, otherwise it is challenging to produce excellent quality silicon nitride products. These restrictions of manufacturing expenses and equipment are hard to approve in the metallurgical sector, so the research in the field of refractories starts fairly late, and also the research is not thorough. Numerous theories originate from ceramics, however there is very little innovation. In the past, silicon nitride usually existed in the type of bonding phase in refractories. With nitriding as well as shooting of steel Si, accumulations such as corundum or silicon carbide were incorporated with great powder, so regarding attain the purpose of incorporating refractory products. Ceramic shed plate is silicon carbide accumulation as well as component of great powder. Silicon nitride is formed by nitriding steel Si to create silicon nitride. Silicon carbide is incorporated to form silicon nitride bound silicon carbide material, which is utilized in blast heater body and also various other parts, to ensure that the efficiency of the material has been greatly boosted. Compared to the clay adhered silicon carbide shed plate, the heat performance of the material is great, which addresses the problem of bulging failing triggered by the oxidation of silicon carbide when the clay bonded silicon carbide shed plate is utilized.
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