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Silicon Nitride structures and properties

Crystal framework of Silicon Nitride
Si3N4 has two kinds of crystal structure: α& alpha;- Si3N4 is granular crystal and β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks composed of common vertices of [SN4] tetrahedron as well as belong to hexagonal system. The distinction between them hinges on the order of [SiN4] tetrahedral layers. The β & beta; stage is developed by the overlap of hexagonal ring layers composed of six virtually entirely in proportion [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; stage is developed by two layers of concrete transformation as well as different non-hexagonal ring layers. α & alpha; phase can liquify oxygen in the variety of crystal framework, and the inner stress of α & alpha; phase is bigger than that of β & beta; stage, so the complimentary energy is higher than that of β & beta; phase. From the point of view of thermodynamics, the β & beta; phase is a lot more secure at higher temperature level. The & alpha; stage has low symmetry as well as is easy to develop. At the temperature of concerning 1500 ℃, the α & alpha; stage undergoes a reconstruction improvement and is changed into β & beta; stage. This improvement is permanent, as well as the existence of some procedure problems and top quality is extra advantageous to the makeover 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

higher than 1500 ℃. Silicon Nitride residential or commercial properties The molecular formula of silicon nitride is Si3N4, of which Si accounts for 60.06% as well as dint N accounts for 39.94%. There is a strong covalent bond between Si and N (of which the ion bond accounts for only 30%), so the Si3N4 has high solidity (Morse solidity 9), high melting point as well as steady framework.
The Si-N in silicon nitride crystal is primarily covalent bond, as well as the bonding strength is high, so it has a huge flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal growth is reduced, yet the thermal conductivity is huge, so this material is challenging to create thermal stress and anxiety, so it has great thermal shock resistance and also great thermal shock resistance. It has toughness, high mechanical stamina at high temperature and also little contortion 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 lots). Not deteriorated by several metals, because of the formation of a silicon dioxide layer, excellent oxidation resistance and great electric insulation.

Silicon nitride has no melting factor and sublimates as well as decays at 1900 ℃ under air pressure. The certain heat is 711.8 J/ kg ℃. The microhardness of α & alpha; phase as well as & beta; phase is 10 ~ 16GPa as well as 24.5 ~ 32.65 Grade point average specifically. Because it is a solid covalent bond substance, no fluid stage will certainly be developed below its decomposition temperature (concerning 1900 ℃), so silicon nitride materials can be sintered with the assistance of oxide ingredients. The oxide products that promote sintering are generally Y2O3, Al2O3, etc, as well as the high addition amount can reach 20%. The reaction concept is that the SiO2 oxide movie based on the surface of silicon nitride particles responds with the added oxide to form fluid phase as well as permeate at the grain limit to make sure high diffusion capacity throughout product movement.

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

The weight of the sample enhanced and a dense silica safety layer was slowly formed on the surface, which protected against the additional oxidation of Si3N4. The weight gain was not evident up until the temperature level was over 1600 ℃. But in the moist ambience, Si3N4 is extremely easy to oxidize, as well as the surface area starts to oxidize at 200 ℃, which is about twice as fast as that in dry air. The oxidation activation power of Si3N4 powder in water vapor is clearly less than that in oxygen and air. The reason is that water vapor can respond with Si3N4 with amorphous SiO2 movies:
Si3N4 +6 WATER=3SiO2+NH3.

Silicon nitride is secure to most metal solutions, free from deterioration as well as infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so on. Nevertheless, for Cu service, it is not eroded just in vacuum cleaner or inert atmosphere; Mg can react weakly with Si3N4; silicon option can damp Si3N4 and erode slightly; shift element option can highly wet Si3N4 and also type silicide with Si to swiftly disintegrate 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 as well as nickel silver, as well as has good corrosion resistance to cast iron and also medium carbon steel, but it is not immune to Ni-Cr alloy as well as stainless-steel.

Except for liquified NaOH as well as HF, silicon nitride has excellent chemical corrosion resistance. Yet most of the molten alkali as well as salt can engage with Si3N4 to decompose it.

Silicon Nitride made use of in Refractories.
Silicon nitride ceramics are called encouraging high temperature structural products because of their excellent heat homes, such as high temperature stamina, excellent wear resistance and also corrosion resistance. Because of the strong covalent bond and also low diffusion coefficient at high temperature, the manufacture of Si3N4 ceramics have to rely on high temperature, high stress and sintering representative, and the cost is too expensive, otherwise it is hard to create premium quality silicon nitride products. These restrictions of manufacturing prices and also equipment are hard to approve in the metallurgical industry, so the research in the area of refractories starts relatively late, and the study is not comprehensive. Several theories come from ceramics, but there is not much technology. In the past, silicon nitride typically existed in the kind of bonding stage in refractories. With nitriding and shooting of metal Si, accumulations such as corundum or silicon carbide were integrated with fine powder, so as to accomplish the function of incorporating refractory materials. Ceramic shed plate is silicon carbide aggregate and also part of fine powder. Silicon nitride is formed by nitriding steel Si to create silicon nitride. Silicon carbide is integrated to develop silicon nitride adhered silicon carbide material, which is used in blast heater body as well as other parts, to make sure that the efficiency of the product has actually been greatly enhanced. Compared with the clay bonded silicon carbide shed plate, the high temperature performance of the material is excellent, which addresses the issue of bulging failure caused by the oxidation of silicon carbide when the clay bonded silicon carbide dropped plate is utilized.

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