Silicon Nitride structures and properties
Crystal framework of Silicon Nitride
Si3N4 has 2 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 composed of typical vertices of [SN4] tetrahedron and also belong to hexagonal system. The distinction in between them hinges on the order of [SiN4] tetrahedral layers. The β & beta; phase is created by the overlap of hexagonal ring layers composed of six almost totally symmetrical [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; stage is created by 2 layers of concrete improvement and various non-hexagonal ring layers. α & alpha; stage can liquify oxygen in the range of crystal structure, and also the internal stress of α & alpha; stage is larger than that of β & beta; stage, so the cost-free energy is higher than that of β & beta; stage. From the viewpoint of thermodynamics, the β & beta; phase is much more steady at higher temperature. The & alpha; phase has reduced proportion as well as is very easy to develop. At the temperature level of concerning 1500 ℃, the α & alpha; stage goes through a restoration improvement and also is changed into β & beta; stage. This makeover is irreversible, as well as the existence of some process conditions and quality is a lot more advantageous to the improvement from α & alpha; phase to & beta; stage. & alpha;-Si3N4 is developed when the temperature level βis lower than 1350 ℃, and & 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 dint N accounts for 39.94%. There is a strong covalent bond between Si and also N (of which the ion bond make up only 30%), so the Si3N4 has high solidity (Morse solidity 9), high melting factor and steady structure.
The Si-N in silicon nitride crystal is mostly 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 reduced, yet the thermal conductivity is huge, so this product is difficult to create thermal anxiety, so it has excellent thermal shock resistance as well as great thermal shock resistance. It has durability, high mechanical strength at heat and also tiny deformation at heat. (the high temperature contortion of silicon nitride ceramic with 2.5 g/cm3 thickness is 0.5% at 1200 × & times; 1000h ℃ and 23 × & times; 7kg/cm2 load). Not eroded by numerous metals, as a result of the development of a silicon dioxide layer, great oxidation resistance as well as good electric insulation.
Silicon nitride has no melting factor and also sublimates as well as disintegrates at 1900 ℃ under air pressure. The particular heat is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and also & beta; phase is 10 ~ 16GPa and 24.5 ~ 32.65 GPa respectively. Because it is a solid covalent bond compound, no liquid phase will be developed listed below its decay temperature (concerning 1900 ℃), so silicon nitride materials can be sintered with the help of oxide additives. The oxide materials that advertise sintering are generally Y2O3, Al2O3, etc, and the high addition amount can get to 20%. The response principle is that the SiO2 oxide movie formed on the surface of silicon nitride particles reacts with the included oxide to form fluid phase and permeate at the grain limit to guarantee high diffusion ability during product movement.
Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically stable compound. Silicon nitride porcelains can be consumed to 1400 ℃ in oxidation environment and also 1850 ℃ in neutral or decreasing atmosphere. In fact, the oxidation reaction of Si3N4 takes place over 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the example boosted and a thick silica protective layer was slowly formed on the surface, which stopped the further oxidation of Si3N4. The weight gain was not apparent up until the temperature level was above 1600 ℃. However in the damp atmosphere, Si3N4 is really easy to oxidize, as well as the surface area begins to oxidize at 200 ℃, which is about twice as fast as that in completely dry air. The oxidation activation energy of Si3N4 powder in water vapor is certainly less than that in oxygen and 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, without deterioration and also seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so forth. Nonetheless, for Cu solution, it is not worn down only in vacuum cleaner or inert environment; Mg can respond weakly with Si3N4; silicon remedy can wet Si3N4 and wear down a little; transition element service can strongly wet Si3N4 and also type silicide with Si to rapidly decompose silicon nitride and also retreat N2 at the exact same time. Si3N4 is steady to alloy solutions such as brass, tough light weight aluminum as well as nickel silver, as well as has good deterioration resistance to cast iron and medium carbon steel, but it is not immune to Ni-Cr alloy as well as stainless-steel.
Except for liquified NaOH and HF, silicon nitride has great chemical rust resistance. However the majority of the molten alkali as well as salt can interact with Si3N4 to decompose it.
Silicon Nitride made use of in Refractories.
Silicon nitride porcelains are called appealing heat structural products as a result of their outstanding heat properties, such as high temperature toughness, great wear resistance and deterioration resistance. Because of the solid covalent bond and also reduced diffusion coefficient at high temperature, the manufacture of Si3N4 ceramics have to rely on high temperature, high stress and also sintering representative, and the expense is too expensive, or else it is tough to generate excellent quality silicon nitride materials. These constraints of production prices and also tools are challenging to approve in the metallurgical sector, so the study in the area of refractories starts relatively late, and also the study is not in-depth. Several theories come from ceramics, but there is not much technology. In the past, silicon nitride normally existed in the kind of bonding stage in refractories. Through nitriding and shooting of steel Si, aggregates such as diamond or silicon carbide were integrated with fine powder, so as to accomplish the function of incorporating refractory products. Ceramic shed plate is silicon carbide aggregate as well as component of great powder. Silicon nitride is developed by nitriding metal Si to create silicon nitride. Silicon carbide is integrated to form silicon nitride bound silicon carbide material, which is utilized in blast heater body and also various other components, to ensure that the performance of the product has been greatly improved. Compared to the clay adhered silicon carbide dropped plate, the heat performance of the material is great, which addresses the problem of protruding failure caused by the oxidation of silicon carbide when the clay bonded silicon carbide lost plate is used.
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