Silicon Nitride structures and properties
Crystal structure of Silicon Nitride
Si3N4 has 2 sort of crystal structure: α& alpha;- Si3N4 is granular crystal and also β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks composed of common vertices of [SN4] tetrahedron as well as come from hexagonal system. The difference in between them hinges on the order of [SiN4] tetrahedral layers. The β & beta; stage is developed by the overlap of hexagonal ring layers composed of six virtually completely symmetrical [SN4] tetrahedrons in the c-axis direction, while the α & alpha; phase is developed by two layers of substantial makeover and various non-hexagonal ring layers. α & alpha; stage can dissolve oxygen in the range of crystal structure, as well as the interior strain of α & alpha; phase is bigger than that of β & beta; phase, so the cost-free energy is greater than that of β & beta; phase. From the perspective of thermodynamics, the β & beta; stage is a lot more stable at higher temperature level. The & alpha; stage has low symmetry as well as is very easy to form. At the temperature level of about 1500 ℃, the α & alpha; phase undertakes a repair makeover and is transformed right into β & beta; stage. This improvement is permanent, and the existence of some process problems as well as quality is much more useful to the change from α & alpha; phase to & beta; stage. & alpha;-Si3N4 is created when the temperature βis less than 1350 ℃, and also & beta;-Si3N4 can be straight prepared at a temperature
more than 1500 ℃. Silicon Nitride residential or commercial properties The molecular formula of silicon nitride is Si3N4, of which Si make up 60.06% and dint N make up 39.94%. There is a solid covalent bond in between Si and also N (of which the ion bond accounts for only 30%), so the Si3N4 has high firmness (Morse solidity 9), high melting point and also secure framework.
The Si-N in silicon nitride crystal is primarily covalent bond, as well as the bonding stamina is high, so it has a huge elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal growth is low, yet the thermal conductivity is huge, so this material is challenging to produce thermal stress and anxiety, so it has good thermal shock resistance and also excellent thermal shock resistance. It has sturdiness, high mechanical strength at high temperature and small contortion at high temperature. (the heat deformation of silicon nitride ceramic with 2.5 g/cm3 thickness is 0.5% at 1200 × & times; 1000h ℃ and 23 × & times; 7kg/cm2 tons). Not deteriorated by many metals, because of the formation of a silicon dioxide layer, good oxidation resistance and also great electrical insulation.
Silicon nitride has no melting point and also sublimates as well as decays at 1900 ℃ under atmospheric pressure. The details warmth is 711.8 J/ kg ℃. The microhardness of α & alpha; phase as well as & beta; stage is 10 ~ 16GPa and also 24.5 ~ 32.65 GPa specifically. Since it is a solid covalent bond substance, no fluid stage will certainly be created below its decay temperature level (regarding 1900 ℃), so silicon nitride products can be sintered with the aid of oxide ingredients. The oxide products that promote sintering are mostly Y2O3, Al2O3, and so on, as well as the high addition amount can get to 20%. The reaction principle is that the SiO2 oxide film formed on the surface of silicon nitride fragments reacts with the added oxide to create liquid phase and also penetrate at the grain border to make certain high diffusion capacity throughout product movement.
Chemical Stability of Silicon Nitride
Si3N4 is a thermodynamically steady compound. Silicon nitride porcelains can be used up to 1400 ℃ in oxidation ambience and 1850 ℃ in neutral or decreasing environment. Actually, the oxidation reaction of Si3N4 happens above 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the example enhanced and also a dense silica safety layer was progressively based on the surface, which stopped the additional oxidation of Si3N4. The weight gain was not apparent up until the temperature was over 1600 ℃. Yet in the damp ambience, Si3N4 is extremely easy to oxidize, and the surface area begins to oxidize at 200 ℃, which is about twice as rapid as that in dry air. The oxidation activation power of Si3N4 powder in water vapor is clearly less than that in oxygen and air. The reason is that water vapor can respond with Si3N4 via amorphous SiO2 movies:
Si3N4 +6 WATER=3SiO2+NH3.
Silicon nitride is steady to most metal remedies, without rust and seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and more. However, for Cu remedy, it is not deteriorated just in vacuum or inert environment; Mg can react weakly with Si3N4; silicon service can wet Si3N4 and also deteriorate somewhat; transition component service can highly damp Si3N4 as well as kind silicide with Si to quickly disintegrate silicon nitride and retreat N2 at the very same time. Si3N4 is secure to alloy services such as brass, difficult aluminum and nickel silver, and also has good corrosion resistance to cast iron as well as medium carbon steel, but it is not immune to Ni-Cr alloy and stainless steel.
With the exception of molten NaOH and also HF, silicon nitride has great chemical corrosion resistance. Yet most of the liquified alkali and salt can connect with Si3N4 to decay it.
Silicon Nitride utilized in Refractories.
Silicon nitride porcelains are called appealing heat structural products as a result of their exceptional heat residential properties, such as high temperature stamina, good wear resistance and deterioration resistance. Because of the strong covalent bond as well as low diffusion coefficient at high temperature, the manufacture of Si3N4 porcelains have to rely on heat, high stress as well as sintering representative, and the expense is too expensive, otherwise it is hard to create premium quality silicon nitride materials. These constraints of production prices and devices are challenging to accept in the metallurgical sector, so the research in the area of refractories begins reasonably late, as well as the research is not comprehensive. Many concepts come from porcelains, yet there is not much technology. In the past, silicon nitride generally existed in the type of bonding phase in refractories. Through nitriding and firing of metal Si, accumulations such as diamond or silicon carbide were integrated with great powder, so as to achieve the purpose of incorporating refractory materials. Ceramic shed plate is silicon carbide aggregate and also component of fine powder. Silicon nitride is created by nitriding metal Si to create silicon nitride. Silicon carbide is incorporated to form silicon nitride bonded silicon carbide product, which is utilized in blast heating system body and other components, to make sure that the efficiency of the material has been considerably improved. Compared with the clay bonded silicon carbide dropped plate, the heat efficiency of the material is great, which resolves the trouble of protruding failing triggered by the oxidation of silicon carbide when the clay bound silicon carbide shed plate is used.
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