Silicon Nitride structures and properties
Crystal structure of Silicon Nitride
Si3N4 has 2 kinds 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 usual vertices of [SN4] tetrahedron and come from hexagonal system. The distinction in between them depends on the order of [SiN4] tetrahedral layers. The β & beta; phase is developed by the overlap of hexagonal ring layers made up of 6 nearly totally symmetrical [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; stage is formed by 2 layers of concrete transformation and also various non-hexagonal ring layers. α & alpha; stage can dissolve oxygen in the variety of crystal structure, and the inner strain of α & alpha; phase is larger than that of β & beta; stage, so the free energy is greater than that of β & beta; stage. From the point of view of thermodynamics, the β & beta; stage is much more secure at greater temperature. The & alpha; phase has low balance and is simple to form. At the temperature level of about 1500 ℃, the α & alpha; phase undertakes a repair makeover and also is transformed right into β & beta; stage. This makeover is permanent, as well as the existence of some procedure problems and high quality is more beneficial to the makeover from α & alpha; stage to & beta; stage. & alpha;-Si3N4 is developed when the temperature level βis less than 1350 ℃, and also & beta;-Si3N4 can be straight prepared at a temperature level
greater than 1500 ℃. Silicon Nitride buildings The molecular formula of silicon nitride is Si3N4, of which Si make up 60.06% as well as dint N make up 39.94%. There is a solid covalent bond in between Si as well as N (of which the ion bond represent only 30%), so the Si3N4 has high hardness (Morse solidity 9), high melting point as well as steady framework.
The Si-N in silicon nitride crystal is mostly covalent bond, and the bonding stamina is high, so it has a big flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal expansion is low, but the thermal conductivity is huge, so this product is difficult to create thermal stress and anxiety, so it has excellent thermal shock resistance as well as good thermal shock resistance. It has sturdiness, high mechanical stamina at high temperature and tiny deformation at high temperature. (the high temperature deformation of silicon nitride ceramic with 2.5 g/cm3 density is 0.5% at 1200 × & times; 1000h ℃ as well as 23 × & times; 7kg/cm2 tons). Not eroded by several metals, due to the formation of a silicon dioxide layer, great oxidation resistance and excellent electrical insulation.
Silicon nitride has no melting point and sublimates and also breaks down at 1900 ℃ under air pressure. The particular warm is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and & beta; stage is 10 ~ 16GPa and 24.5 ~ 32.65 GPa respectively. Since it is a solid covalent bond compound, no liquid phase will be created listed below its decay temperature (about 1900 ℃), so silicon nitride materials can be sintered with the aid of oxide ingredients. The oxide materials that advertise sintering are mainly Y2O3, Al2O3, and so on, and also the high addition quantity can get to 20%. The response concept is that the SiO2 oxide movie based on the surface of silicon nitride particles reacts with the added oxide to form liquid stage and also permeate at the grain boundary to make sure high diffusion capability during material movement.
Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically steady substance. Silicon nitride porcelains can be used up to 1400 ℃ in oxidation atmosphere and also 1850 ℃ in neutral or minimizing atmosphere. Actually, the oxidation response of Si3N4 happens above 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the sample raised and also a dense silica protective layer was gradually formed on the surface area, which avoided the further oxidation of Si3N4. The weight gain was not obvious up until the temperature level was above 1600 ℃. Yet in the damp environment, Si3N4 is really easy to oxidize, and the surface begins to oxidize at 200 ℃, which has to do with twice as quick as that in dry air. The oxidation activation energy of Si3N4 powder in water vapor is obviously lower than that in oxygen as well as air. The factor is that water vapor can react with Si3N4 through amorphous SiO2 films:
Si3N4 +6 WATER=3SiO2+NH3.
Silicon nitride is steady to most metal services, devoid of deterioration as well as infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so on. Nonetheless, for Cu service, it is not deteriorated just in vacuum or inert environment; Mg can react weakly with Si3N4; silicon service can wet Si3N4 as well as deteriorate a little; transition aspect solution can highly damp Si3N4 and kind silicide with Si to promptly decay silicon nitride as well as escape N2 at the exact same time. Si3N4 is secure to alloy remedies such as brass, difficult light weight aluminum and also nickel silver, and has excellent rust resistance to cast iron as well as medium carbon steel, however it is not resistant to Ni-Cr alloy as well as stainless steel.
Except for molten NaOH as well as HF, silicon nitride has good chemical rust resistance. However most of the molten antacid as well as salt can communicate with Si3N4 to decay it.
Silicon Nitride utilized in Refractories.
Silicon nitride ceramics are called appealing heat architectural products due to their superb high temperature homes, such as heat strength, great wear resistance and also rust resistance. Because of the solid covalent bond and low diffusion coefficient at high temperature, the manufacture of Si3N4 ceramics have to depend on heat, high pressure and also sintering agent, as well as the expense is expensive, otherwise it is difficult to produce high quality silicon nitride materials. These constraints of production expenses and tools are hard to approve in the metallurgical industry, so the research in the area of refractories starts fairly late, and also the study is not in-depth. Numerous theories come from porcelains, but there is very little innovation. In the past, silicon nitride normally existed in the form of bonding phase in refractories. Via nitriding and firing of metal Si, aggregates such as corundum or silicon carbide were incorporated with great powder, so as to attain the objective of combining refractory materials. Ceramic shed plate is silicon carbide aggregate and part of great powder. Silicon nitride is formed by nitriding steel Si to develop silicon nitride. Silicon carbide is integrated to form silicon nitride bound silicon carbide product, which is utilized in blast heating system body and also various other components, so that the performance of the material has been considerably enhanced. Compared with the clay adhered silicon carbide lost plate, the high temperature performance of the product is great, which addresses the issue of protruding failure caused by the oxidation of silicon carbide when the clay bonded silicon carbide lost plate is used.
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