Silicon Nitride structures and properties

Crystal structure of Silicon Nitride
Si3N4 has 2 type of crystal structure: α& alpha;- Si3N4 is granular crystal and also 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 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 6 practically totally symmetrical [SN4] tetrahedrons in the c-axis direction, while the α & alpha; phase is formed by 2 layers of tangible change as well as different non-hexagonal ring layers. α & alpha; stage can liquify oxygen in the series of crystal framework, as well as the inner strain of α & alpha; stage is bigger than that of β & beta; phase, so the cost-free energy is greater than that of β & beta; phase. From the viewpoint of thermodynamics, the β & beta; phase is extra steady at higher temperature. The & alpha; phase has low symmetry and is simple to develop. At the temperature of about 1500 ℃, the α & alpha; stage undertakes a restoration transformation and is changed right into β & beta; phase. This makeover is irreparable, as well as the existence of some procedure problems and top quality is extra useful to the improvement from α & alpha; phase to & beta; stage. & alpha;-Si3N4 is developed when the temperature βis less than 1350 ℃, as well as & beta;-Si3N4 can be straight prepared at a temperature level

higher than 1500 ℃. Silicon Nitride properties The molecular formula of silicon nitride is Si3N4, of which Si represent 60.06% and also dint N represent 39.94%. There is a strong covalent bond between Si as well as N (of which the ion bond make up only 30%), so the Si3N4 has high hardness (Morse firmness 9), high melting point and secure structure.
The Si-N in silicon nitride crystal is mainly covalent bond, and the bonding strength is high, so it has a large elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is low, however the thermal conductivity is large, so this product is not easy to produce thermal stress and anxiety, so it has excellent thermal shock resistance as well as excellent thermal shock resistance. It has durability, high mechanical toughness at heat and small deformation at heat. (the heat 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 steels, as a result of the development of a silicon dioxide layer, good oxidation resistance and also good electric insulation.

Silicon nitride has no melting factor and sublimates and decomposes at 1900 ℃ under atmospheric pressure. The details warm is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and & beta; phase is 10 ~ 16GPa as well as 24.5 ~ 32.65 GPa specifically. Because it is a strong covalent bond substance, no fluid stage will be formed below its decomposition temperature (regarding 1900 ℃), so silicon nitride products can be sintered with the help of oxide additives. The oxide materials that advertise sintering are mostly Y2O3, Al2O3, etc, and the high addition amount can reach 20%. The response concept is that the SiO2 oxide film based on the surface area of silicon nitride particles reacts with the included oxide to create fluid phase as well as penetrate at the grain boundary to guarantee high diffusion ability during material movement.

Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically secure compound. Silicon nitride porcelains can be used up to 1400 ℃ in oxidation atmosphere as well as 1850 ℃ in neutral or decreasing atmosphere. In fact, the oxidation response of Si3N4 happens over 800C:
Si3N4 +3 O2=3SiO2+N2.

The weight of the sample boosted and a dense silica protective layer was slowly based on the surface, which stopped the further oxidation of Si3N4. The weight gain was not evident up until the temperature level was above 1600 ℃. Yet in the moist environment, Si3N4 is extremely simple to oxidize, as well as the surface begins to oxidize at 200 ℃, which is about two times as rapid as that in dry air. The oxidation activation power of Si3N4 powder in water vapor is obviously less than that in oxygen as well as air. The factor is that water vapor can react with Si3N4 with amorphous SiO2 movies:
Si3N4 +6 WATER=3SiO2+NH3.

Silicon nitride is steady to most metal services, free from deterioration and also infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so on. However, for Cu solution, it is not worn down only in vacuum cleaner or inert ambience; Mg can respond weakly with Si3N4; silicon option can damp Si3N4 and also erode slightly; transition component solution can strongly damp Si3N4 and kind silicide with Si to promptly decay silicon nitride and also escape N2 at the same time. Si3N4 is secure to alloy options such as brass, difficult light weight aluminum as well as nickel silver, as well as has excellent deterioration resistance to cast iron and also medium carbon steel, yet it is not immune to Ni-Cr alloy as well as stainless-steel.

Besides liquified NaOH as well as HF, silicon nitride has good chemical rust resistance. However a lot of the molten antacid as well as salt can connect with Si3N4 to decay it.

Silicon Nitride utilized in Refractories.
Silicon nitride porcelains are called promising high temperature structural materials as a result of their excellent high temperature residential or commercial properties, such as heat toughness, great wear resistance and deterioration resistance. As a result of the solid covalent bond and also low diffusion coefficient at high temperature, the manufacture of Si3N4 ceramics have to depend on heat, high pressure and also sintering agent, and the cost is too expensive, or else it is tough to produce high quality silicon nitride products. These limitations of production expenses and tools are difficult to accept in the metallurgical industry, so the research in the area of refractories begins reasonably late, and the study is not comprehensive. Many concepts originate from porcelains, but there is very little development. In the past, silicon nitride typically existed in the form of bonding phase in refractories. With nitriding and shooting of steel Si, aggregates such as diamond or silicon carbide were integrated with fine powder, so regarding attain the function of incorporating refractory products. Ceramic shed plate is silicon carbide aggregate and also part of fine powder. Silicon nitride is created by nitriding metal Si to form silicon nitride. Silicon carbide is incorporated to form silicon nitride bonded silicon carbide material, which is used in blast heating system body and other components, to ensure that the efficiency of the product has actually been significantly boosted. Compared to the clay bonded silicon carbide dropped plate, the high temperature performance of the material is very good, which resolves the issue of protruding failing triggered by the oxidation of silicon carbide when the clay adhered silicon carbide lost plate is utilized.

Silicon nitride Cost
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