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Silicon Nitride structures and properties

Crystal framework of Silicon Nitride
Si3N4 has two type 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 made up of usual vertices of [SN4] tetrahedron and come from hexagonal system. The difference in between them hinges on the order of [SiN4] tetrahedral layers. The β & beta; phase is formed by the overlap of hexagonal ring layers made up of 6 almost totally balanced [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; phase is created by two layers of concrete change and different non-hexagonal ring layers. α & alpha; stage can liquify oxygen in the range of crystal framework, as well as the internal strain of α & alpha; phase is larger than that of β & beta; stage, so the totally free energy is more than that of β & beta; phase. From the viewpoint of thermodynamics, the β & beta; phase is a lot more stable at greater temperature level. The & alpha; phase has low symmetry as well as is simple to form. At the temperature of about 1500 ℃, the α & alpha; stage undertakes a restoration transformation as well as is changed right into β & beta; stage. This improvement is irreversible, and also the presence of some procedure conditions and also quality is much more valuable to the makeover from α & alpha; phase to & beta; stage. & alpha;-Si3N4 is created when the temperature βis less than 1350 ℃, and also & beta;-Si3N4 can be straight prepared at a temperature

greater than 1500 ℃. Silicon Nitride properties The molecular formula of silicon nitride is Si3N4, of which Si make up 60.06% as well as dint N represent 39.94%. There is a strong covalent bond between Si and N (of which the ion bond accounts for just 30%), so the Si3N4 has high hardness (Morse solidity 9), high melting factor and also secure structure.
The Si-N in silicon nitride crystal is mainly covalent bond, as well as the bonding strength is high, so it has a big elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal growth is low, however the thermal conductivity is large, so this material is difficult to create thermal tension, so it has great thermal shock resistance as well as excellent thermal shock resistance. It has strength, high mechanical strength at high temperature as well as small contortion at high temperature. (the heat contortion of silicon nitride ceramic with 2.5 g/cm3 density is 0.5% at 1200 × & times; 1000h ℃ and also 23 × & times; 7kg/cm2 load). Not worn down by many metals, as a result of the formation of a silicon dioxide layer, great oxidation resistance and also good electric insulation.

Silicon nitride has no melting factor as well as sublimates as well as decomposes 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 and also 24.5 ~ 32.65 Grade point average respectively. Due to the fact that it is a strong covalent bond compound, no fluid phase will be formed listed below its disintegration temperature (about 1900 ℃), so silicon nitride materials can be sintered with the aid of oxide ingredients. The oxide materials that promote sintering are mainly Y2O3, Al2O3, etc, and also the high addition amount can get to 20%. The reaction concept is that the SiO2 oxide movie formed on the surface area of silicon nitride fragments reacts with the included oxide to develop fluid stage and permeate at the grain boundary to ensure high diffusion capacity throughout material movement.

Chemical Stability of Silicon Nitride
Si3N4 is a thermodynamically secure compound. Silicon nitride porcelains can be consumed to 1400 ℃ in oxidation ambience as well as 1850 ℃ in neutral or lowering environment. Actually, the oxidation reaction of Si3N4 takes place above 800C:
Si3N4 +3 O2=3SiO2+N2.

The weight of the example raised and a thick silica protective layer was slowly based on the surface, which protected against the further oxidation of Si3N4. The weight gain was not noticeable until the temperature level was over 1600 ℃. But in the damp ambience, Si3N4 is really easy to oxidize, and the surface begins to oxidize at 200 ℃, which has to do with two times as quick as that in dry air. The oxidation activation power of Si3N4 powder in water vapor is certainly 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 steady to most metal services, without deterioration and infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and more. Nonetheless, for Cu solution, it is not deteriorated only in vacuum cleaner or inert atmosphere; Mg can respond weakly with Si3N4; silicon service can wet Si3N4 as well as erode somewhat; shift component remedy can highly damp Si3N4 and form silicide with Si to quickly decay silicon nitride as well as escape N2 at the same time. Si3N4 is steady to alloy services such as brass, tough light weight aluminum as well as nickel silver, as well as has great corrosion resistance to cast iron and tool carbon steel, but it is not resistant to Ni-Cr alloy and stainless-steel.

Except for molten NaOH as well as HF, silicon nitride has great chemical corrosion resistance. But the majority of the molten alkali and salt can interact with Si3N4 to decompose it.

Silicon Nitride utilized in Refractories.
Silicon nitride ceramics are called encouraging high temperature architectural materials because of their exceptional high temperature residential properties, such as heat stamina, excellent wear resistance and deterioration resistance. Because of the solid covalent bond and reduced diffusion coefficient at high temperature, the manufacture of Si3N4 ceramics have to count on high temperature, high pressure as well as sintering representative, as well as the expense is expensive, otherwise it is difficult to create premium quality silicon nitride products. These limitations of manufacturing costs and also tools are hard to accept in the metallurgical industry, so the study in the area of refractories starts fairly late, and the study is not thorough. Numerous concepts come from ceramics, yet there is very little advancement. In the past, silicon nitride typically existed in the type of bonding stage in refractories. With nitriding and also shooting of steel Si, accumulations such as corundum or silicon carbide were combined with great powder, so as to attain the purpose of incorporating refractory products. Ceramic shed plate is silicon carbide accumulation as well as part of fine powder. Silicon nitride is developed by nitriding metal Si to develop silicon nitride. Silicon carbide is combined to form silicon nitride adhered silicon carbide product, which is made use of in blast heating system body as well as other parts, so that the performance of the material has been significantly enhanced. Compared with the clay bonded silicon carbide shed plate, the heat efficiency of the product is excellent, which fixes the issue of bulging failure brought on by the oxidation of silicon carbide when the clay bonded silicon carbide lost plate is utilized.

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