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Silicon Nitride structures and properties

Crystal structure of Silicon Nitride
Si3N4 has two kinds of crystal framework: α& alpha;- Si3N4 is granular crystal as well as β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks composed of common 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 formed by the overlap of hexagonal ring layers made up of 6 nearly totally balanced [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; phase is developed by two layers of tangible makeover as well as different non-hexagonal ring layers. α & alpha; phase can liquify oxygen in the variety of crystal framework, and also the interior strain of α & alpha; stage is bigger than that of β & beta; phase, so the free energy is greater than that of β & beta; phase. From the perspective of thermodynamics, the β & beta; stage is extra secure at greater temperature level. The & alpha; stage has reduced balance and is very easy to develop. At the temperature level of about 1500 ℃, the α & alpha; phase goes through a restoration improvement as well as is changed right into β & beta; phase. This makeover is irreparable, and the presence of some procedure problems and high quality is extra valuable to the improvement from α & alpha; stage to & beta; phase. & alpha;-Si3N4 is formed when the temperature βis less than 1350 ℃, and also & beta;-Si3N4 can be straight prepared at a temperature level

higher than 1500 ℃. Silicon Nitride buildings 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 solid covalent bond in 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 structure.
The Si-N in silicon nitride crystal is generally covalent bond, as well as the bonding strength is high, so it has a huge elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is low, yet the thermal conductivity is large, so this product is not easy to create thermal stress, so it has excellent thermal shock resistance as well as good thermal shock resistance. It has strength, high mechanical strength at high temperature as well as tiny 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 also 23 × & times; 7kg/cm2 load). Not worn down by several metals, due to the formation of a silicon dioxide layer, good oxidation resistance and also good electrical insulation.

Silicon nitride has no melting point and also sublimates as well as decays at 1900 ℃ under air pressure. The specific heat is 711.8 J/ kg ℃. The microhardness of α & alpha; phase and & beta; stage is 10 ~ 16GPa and 24.5 ~ 32.65 GPa respectively. Since it is a solid covalent bond compound, no fluid stage will be created below its disintegration temperature level (concerning 1900 ℃), so silicon nitride materials can be sintered with the aid of oxide ingredients. The oxide products that advertise sintering are generally Y2O3, Al2O3, and so on, as well as the high addition amount can get to 20%. The response concept is that the SiO2 oxide movie formed on the surface area of silicon nitride fragments responds with the included oxide to create liquid stage as well as permeate at the grain boundary to ensure high diffusion ability throughout product movement.

Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically steady compound. Silicon nitride ceramics can be consumed to 1400 ℃ in oxidation atmosphere and also 1850 ℃ in neutral or reducing environment. Actually, the oxidation reaction of Si3N4 takes place over 800C:
Si3N4 +3 O2=3SiO2+N2.

The weight of the example increased and also a dense silica safety layer was slowly formed on the surface area, which protected against the additional oxidation of Si3N4. The weight gain was not obvious till the temperature was over 1600 ℃. Yet in the humid ambience, Si3N4 is extremely easy to oxidize, as well as the surface area starts to oxidize at 200 ℃, which is about two times as fast as that in completely dry air. The oxidation activation energy of Si3N4 powder in water vapor is clearly lower than that in oxygen and air. The factor is that water vapor can respond with Si3N4 via amorphous SiO2 movies:
Si3N4 +6 H2O=3SiO2+NH3.

Silicon nitride is stable to most metal services, devoid of corrosion as well as seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so on. However, for Cu service, it is not worn down just in vacuum or inert environment; Mg can react weakly with Si3N4; silicon service can damp Si3N4 and also erode slightly; change aspect remedy can highly wet Si3N4 as well as type silicide with Si to quickly disintegrate silicon nitride as well as retreat N2 at the exact same time. Si3N4 is stable to alloy solutions such as brass, hard aluminum and also nickel silver, and also has excellent corrosion resistance to cast iron as well as medium carbon steel, however it is not resistant to Ni-Cr alloy and also stainless-steel.

Besides molten NaOH as well as HF, silicon nitride has great chemical rust resistance. But a lot of the molten alkali as well as salt can connect with Si3N4 to decompose it.

Silicon Nitride made use of in Refractories.
Silicon nitride ceramics are called promising heat architectural materials due to their outstanding heat properties, such as heat toughness, good wear resistance as well as deterioration resistance. Due to the strong covalent bond and reduced diffusion coefficient at heat, the manufacture of Si3N4 ceramics must count on high temperature, high pressure and sintering agent, and also the expense is too expensive, otherwise it is hard to produce premium quality silicon nitride products. These limitations of production expenses as well as equipment are challenging to approve in the metallurgical industry, so the study in the area of refractories starts reasonably late, as well as the research is not thorough. Numerous concepts originate from porcelains, but there is not much technology. In the past, silicon nitride usually existed in the kind of bonding stage in refractories. Via nitriding and shooting of metal Si, accumulations such as diamond or silicon carbide were integrated with fine powder, so as to accomplish the purpose of incorporating refractory products. Ceramic shed plate is silicon carbide aggregate and component of great powder. Silicon nitride is formed by nitriding metal Si to develop silicon nitride. Silicon carbide is integrated to develop silicon nitride bonded silicon carbide material, which is made use of in blast heater body and other components, so that the performance of the material has been significantly improved. Compared to the clay adhered silicon carbide dropped plate, the high temperature efficiency of the product is very good, which solves the trouble of bulging failure caused by the oxidation of silicon carbide when the clay bonded silicon carbide lost plate is utilized.

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