Silicon Nitride structures and properties
Crystal framework of Silicon Nitride
Si3N4 has 2 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 made up of typical vertices of [SN4] tetrahedron and also come from hexagonal system. The difference in between them lies in the order of [SiN4] tetrahedral layers. The β & beta; stage is created by the overlap of hexagonal ring layers composed of six practically totally in proportion [SN4] tetrahedrons in the c-axis direction, while the α & alpha; stage is created by two layers of substantial transformation and also different non-hexagonal ring layers. α & alpha; phase can liquify oxygen in the variety of crystal structure, and the internal pressure of α & alpha; phase is larger than that of β & beta; stage, so the cost-free energy is higher than that of β & beta; stage. From the point of view of thermodynamics, the β & beta; phase is a lot more steady at greater temperature level. The & alpha; stage has reduced balance and is very easy to create. At the temperature of regarding 1500 ℃, the α & alpha; stage goes through a repair change and is changed into β & beta; phase. This makeover is irreversible, as well as the existence of some procedure conditions as well as quality is extra valuable to the change from α & alpha; phase to & beta; stage. & alpha;-Si3N4 is formed when the temperature level βis lower than 1350 ℃, as well as & beta;-Si3N4 can be directly prepared at a temperature
more than 1500 ℃. Silicon Nitride residential properties 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 in between Si and also N (of which the ion bond represent only 30%), so the Si3N4 has high firmness (Morse firmness 9), high melting factor as well as stable framework.
The Si-N in silicon nitride crystal is generally covalent bond, and the bonding strength is high, so it has a big elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is low, however the thermal conductivity is huge, so this product is difficult to produce thermal tension, so it has excellent thermal shock resistance as well as excellent thermal shock resistance. It has strength, high mechanical strength at heat as well as small contortion at heat. (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 eroded by several metals, due to the formation of a silicon dioxide layer, great oxidation resistance as well as good electric insulation.
Silicon nitride has no melting point and sublimates and also decomposes at 1900 ℃ under air pressure. The details warm is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and & beta; phase is 10 ~ 16GPa and 24.5 ~ 32.65 Grade point average respectively. Since it is a strong covalent bond compound, no fluid phase will be created listed below its disintegration temperature (regarding 1900 ℃), so silicon nitride materials can be sintered with the help of oxide additives. The oxide materials that advertise sintering are primarily Y2O3, Al2O3, etc, and also the high addition quantity can reach 20%. The response principle is that the SiO2 oxide film based on the surface area of silicon nitride bits reacts with the added oxide to create fluid stage and permeate at the grain boundary to guarantee high diffusion ability during product migration.
Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically steady compound. Silicon nitride porcelains can be used up to 1400 ℃ in oxidation ambience as well as 1850 ℃ in neutral or reducing environment. As a matter of fact, the oxidation reaction of Si3N4 takes place above 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the sample raised and a thick silica safety layer was gradually formed on the surface area, which prevented the additional oxidation of Si3N4. The weight gain was not evident until the temperature level was over 1600 ℃. Yet in the moist ambience, Si3N4 is really easy to oxidize, and the surface starts to oxidize at 200 ℃, which is about twice as quick as that in completely dry air. The oxidation activation power of Si3N4 powder in water vapor is clearly less than that in oxygen and also 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 services, devoid of rust and infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so on. However, for Cu service, it is not eroded only in vacuum cleaner or inert atmosphere; Mg can respond weakly with Si3N4; silicon service can wet Si3N4 and also wear down a little; shift aspect solution can strongly damp Si3N4 and form silicide with Si to promptly disintegrate silicon nitride and escape N2 at the exact same time. Si3N4 is steady to alloy services such as brass, hard aluminum as well as nickel silver, and has excellent deterioration 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 and HF, silicon nitride has great chemical corrosion resistance. But most of the molten antacid and also salt can interact with Si3N4 to decay it.
Silicon Nitride utilized in Refractories.
Silicon nitride ceramics are called promising high temperature structural materials because of their excellent high temperature residential properties, such as high temperature stamina, good wear resistance and also rust resistance. Because of the strong covalent bond and also low diffusion coefficient at high temperature, the manufacture of Si3N4 porcelains need to count on high temperature, high pressure and also sintering representative, and the expense is too expensive, or else it is difficult to produce high quality silicon nitride products. These constraints of manufacturing expenses as well as equipment are difficult to approve in the metallurgical industry, so the study in the area of refractories begins reasonably late, and also the study is not in-depth. Many concepts originate from porcelains, however there is not much innovation. In the past, silicon nitride normally existed in the form of bonding stage in refractories. Via nitriding and also shooting of steel Si, aggregates such as corundum or silicon carbide were combined with great powder, so as to attain the objective of combining refractory products. Ceramic shed plate is silicon carbide aggregate and also component of fine powder. Silicon nitride is developed by nitriding steel Si to develop silicon nitride. Silicon carbide is integrated to create silicon nitride adhered silicon carbide product, which is utilized in blast heating system body and also various other components, so that the performance of the material has actually been substantially enhanced. Compared to the clay bonded silicon carbide dropped plate, the high temperature performance of the material is great, which fixes the problem of bulging failure caused by the oxidation of silicon carbide when the clay bonded silicon carbide dropped plate is utilized.
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