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Silicon Nitride structures and properties

Crystal structure of Silicon Nitride
Si3N4 has 2 sort 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 belong to hexagonal system. The distinction in 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 six practically entirely symmetrical [SN4] tetrahedrons in the c-axis direction, while the α & alpha; stage is formed by two layers of concrete makeover and also different non-hexagonal ring layers. α & alpha; stage can dissolve oxygen in the series of crystal framework, and also the internal strain of α & alpha; stage is bigger than that of β & beta; stage, so the cost-free energy is higher than that of β & beta; phase. From the perspective of thermodynamics, the β & beta; stage is extra stable at higher temperature level. The & alpha; phase has low symmetry as well as is simple to develop. At the temperature level of regarding 1500 ℃, the α & alpha; phase undergoes a restoration change and is changed right into β & beta; phase. This makeover is irreparable, and also the presence of some process problems and also high quality is much more helpful to the makeover from α & alpha; phase to & beta; phase. & alpha;-Si3N4 is formed when the temperature level βis lower than 1350 ℃, and & beta;-Si3N4 can be straight prepared at a temperature

greater than 1500 ℃. Silicon Nitride residential or commercial properties The molecular formula of silicon nitride is Si3N4, of which Si represent 60.06% as well as dint N make up 39.94%. There is a solid covalent bond in between Si and N (of which the ion bond accounts for only 30%), so the Si3N4 has high firmness (Morse solidity 9), high melting point and also secure framework.
The Si-N in silicon nitride crystal is mainly covalent bond, as well as the bonding stamina 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 large, so this product is hard to create thermal stress, so it has good thermal shock resistance and good thermal shock resistance. It has sturdiness, high mechanical strength at heat and also small contortion at heat. (the high temperature deformation of silicon nitride ceramic with 2.5 g/cm3 density is 0.5% at 1200 × & times; 1000h ℃ and 23 × & times; 7kg/cm2 load). Not eroded by many metals, as a result of the formation of a silicon dioxide layer, good oxidation resistance and excellent electrical insulation.

Silicon nitride has no melting factor and also sublimates and also decays at 1900 ℃ under air pressure. The certain warmth is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and also & beta; stage is 10 ~ 16GPa as well as 24.5 ~ 32.65 Grade point average respectively. Due to the fact that it is a strong covalent bond compound, no liquid stage will be developed below its decomposition temperature (about 1900 ℃), so silicon nitride products can be sintered with the aid of oxide ingredients. The oxide materials that advertise sintering are primarily Y2O3, Al2O3, and so on, and the high addition quantity can reach 20%. The response concept is that the SiO2 oxide film formed on the surface area of silicon nitride bits responds with the added oxide to create fluid stage as well as penetrate at the grain boundary to make sure high diffusion capacity throughout product movement.

Chemical Stability of Silicon Nitride
Si3N4 is a thermodynamically secure compound. Silicon nitride ceramics can be used up to 1400 ℃ in oxidation environment as well as 1850 ℃ in neutral or minimizing atmosphere. As a matter of fact, the oxidation reaction of Si3N4 happens above 800C:
Si3N4 +3 O2=3SiO2+N2.

The weight of the example increased and a dense silica protective layer was progressively based on the surface, which protected against the additional oxidation of Si3N4. The weight gain was not apparent up until the temperature was above 1600 ℃. But in the damp environment, Si3N4 is extremely simple to oxidize, and also the surface area begins to oxidize at 200 ℃, which has to do with twice as rapid as that in completely dry air. The oxidation activation power of Si3N4 powder in water vapor is clearly less than that in oxygen as well as air. The factor is that water vapor can respond with Si3N4 with amorphous SiO2 films:
Si3N4 +6 H2O=3SiO2+NH3.

Silicon nitride is stable to most metal solutions, without rust and seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so forth. Nevertheless, for Cu solution, it is not worn down just in vacuum or inert ambience; Mg can react weakly with Si3N4; silicon option can damp Si3N4 as well as wear down a little; transition element service can highly damp Si3N4 and form silicide with Si to promptly break down silicon nitride and retreat N2 at the very 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 tool carbon steel, yet it is not immune to Ni-Cr alloy as well as stainless-steel.

Except for liquified NaOH and HF, silicon nitride has excellent chemical rust resistance. But the majority of the liquified antacid as well as salt can communicate with Si3N4 to disintegrate it.

Silicon Nitride made use of in Refractories.
Silicon nitride porcelains are called appealing high temperature structural products because of their excellent heat homes, such as heat toughness, excellent wear resistance and also corrosion resistance. Because of the solid covalent bond as well as reduced diffusion coefficient at high temperature, the manufacture of Si3N4 ceramics have to rely on heat, high stress and sintering representative, and the cost is too expensive, otherwise it is tough to generate excellent quality silicon nitride materials. These restrictions of manufacturing expenses and also tools are challenging to accept in the metallurgical sector, so the study in the area of refractories begins fairly late, and the research is not in-depth. Many theories come from ceramics, but there is very little advancement. In the past, silicon nitride normally existed in the kind of bonding stage in refractories. Through nitriding and firing of steel Si, aggregates such as corundum or silicon carbide were incorporated with great powder, so as to accomplish the function of incorporating refractory materials. Ceramic shed plate is silicon carbide accumulation and part of fine powder. Silicon nitride is formed by nitriding steel Si to form silicon nitride. Silicon carbide is combined to create silicon nitride bonded silicon carbide product, which is used in blast heater body and other parts, so that the performance of the material has been considerably boosted. Compared to the clay adhered silicon carbide lost plate, the heat efficiency of the product is very good, which fixes the problem of bulging failing caused by the oxidation of silicon carbide when the clay bound silicon carbide shed plate is made use of.

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