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 usual vertices of [SN4] tetrahedron as well as belong to hexagonal system. The distinction between them lies in the order of [SiN4] tetrahedral layers. The β & beta; stage is developed by the overlap of hexagonal ring layers composed of six practically totally in proportion [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; phase is developed by 2 layers of tangible improvement as well as different non-hexagonal ring layers. α & alpha; stage can dissolve oxygen in the range of crystal framework, as well as the inner strain of α & alpha; stage is larger than that of β & beta; phase, so the free power is greater than that of β & beta; phase. From the point of view of thermodynamics, the β & beta; stage is more stable at higher temperature. The & alpha; phase has reduced balance and also is easy to develop. At the temperature of about 1500 ℃, the α & alpha; stage goes through a reconstruction transformation and also is transformed into β & beta; stage. This transformation is irreparable, and the presence of some procedure conditions as well as top quality is much more advantageous to the makeover from α & alpha; stage to & beta; stage. & alpha;-Si3N4 is created when the temperature βis less than 1350 ℃, as well as & beta;-Si3N4 can be directly prepared at a temperature level

greater 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 make up only 30%), so the Si3N4 has high hardness (Morse hardness 9), high melting factor and also stable framework.
The Si-N in silicon nitride crystal is mainly covalent bond, and also the bonding strength is high, so it has a big flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is reduced, yet the thermal conductivity is huge, so this material is difficult to produce thermal stress, so it has great thermal shock resistance and good thermal shock resistance. It has durability, high mechanical toughness at heat as well as small deformation at high temperature. (the heat contortion 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 deteriorated by lots of steels, as a result of the development of a silicon dioxide layer, excellent oxidation resistance and also good electrical insulation.

Silicon nitride has no melting point as well as sublimates and breaks down at 1900 ℃ under atmospheric pressure. The certain warmth is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and & 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 stage will be formed below its decomposition temperature (regarding 1900 ℃), so silicon nitride materials can be sintered with the help of oxide ingredients. The oxide products that advertise sintering are generally Y2O3, Al2O3, and so on, and the high enhancement amount can reach 20%. The response concept is that the SiO2 oxide movie formed on the surface of silicon nitride bits reacts with the added oxide to create fluid phase as well as penetrate at the grain limit to guarantee high diffusion ability during material migration.

Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically steady substance. Silicon nitride ceramics can be used up to 1400 ℃ in oxidation ambience and 1850 ℃ in neutral or reducing ambience. As a matter of fact, the oxidation reaction of Si3N4 takes place over 800C:
Si3N4 +3 O2=3SiO2+N2.

The weight of the example increased as well as a thick silica safety layer was slowly based on the surface area, which protected against the further oxidation of Si3N4. The weight gain was not apparent until the temperature was above 1600 ℃. Yet in the damp environment, Si3N4 is really simple to oxidize, and also the surface area starts to oxidize at 200 ℃, which is about two times as fast as that in completely dry air. The oxidation activation power of Si3N4 powder in water vapor is undoubtedly lower than that in oxygen as well as air. The reason is that water vapor can respond with Si3N4 with amorphous SiO2 movies:
Si3N4 +6 H2O=3SiO2+NH3.

Silicon nitride is stable to most metal options, free from deterioration and infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so on. Nonetheless, for Cu service, it is not worn down just in vacuum cleaner or inert environment; Mg can respond weakly with Si3N4; silicon option can damp Si3N4 as well as deteriorate a little; shift component remedy can strongly wet Si3N4 and form silicide with Si to rapidly disintegrate silicon nitride and escape N2 at the exact same time. Si3N4 is secure to alloy remedies such as brass, tough aluminum as well as nickel silver, as well as has excellent rust resistance to cast iron and medium carbon steel, but it is not immune to Ni-Cr alloy and stainless steel.

With the exception of molten NaOH and HF, silicon nitride has excellent chemical corrosion resistance. Yet the majority of the molten alkali and salt can engage with Si3N4 to disintegrate it.

Silicon Nitride made use of in Refractories.
Silicon nitride ceramics are called encouraging high temperature architectural materials due to their excellent heat residential properties, such as high temperature stamina, good wear resistance and rust resistance. Due to the solid covalent bond and low diffusion coefficient at high temperature, the manufacture of Si3N4 ceramics need to depend on heat, high pressure as well as sintering representative, and also the expense is expensive, otherwise it is tough to create premium quality silicon nitride products. These limitations of production expenses and also tools are hard to accept in the metallurgical industry, so the research in the area of refractories starts reasonably late, and the research is not in-depth. Lots of theories come from porcelains, but there is very little innovation. In the past, silicon nitride usually existed in the form of bonding phase in refractories. With nitriding and also firing of metal Si, accumulations such as diamond or silicon carbide were incorporated with great powder, so as to attain the function of combining refractory materials. Ceramic shed plate is silicon carbide aggregate and also part of great powder. Silicon nitride is created by nitriding metal Si to form silicon nitride. Silicon carbide is integrated to develop silicon nitride bonded silicon carbide product, which is utilized in blast furnace body and other components, so that the efficiency of the product has been greatly enhanced. Compared to the clay bound silicon carbide shed plate, the high temperature performance of the product is great, which solves the issue of bulging failure triggered by the oxidation of silicon carbide when the clay bound silicon carbide shed plate is used.

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