Silicon Nitride structures and properties
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Crystal structure of Silicon Nitride
Si3N4 has two kinds of crystal framework: α& alpha;- Si3N4 is granular crystal and 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 distinction in between them depends on the order of [SiN4] tetrahedral layers. The β & beta; stage is developed by the overlap of hexagonal ring layers composed of 6 nearly entirely in proportion [SN4] tetrahedrons in the c-axis direction, while the α & alpha; phase is developed by 2 layers of tangible change and different non-hexagonal ring layers. α & alpha; phase can liquify oxygen in the variety of crystal framework, as well as the internal pressure of α & alpha; phase is larger than that of β & beta; phase, so the cost-free energy is higher than that of β & beta; phase. From the viewpoint of thermodynamics, the β & beta; stage is extra steady at higher temperature level. The & alpha; stage has reduced proportion as well as is very easy to create. At the temperature level of concerning 1500 ℃, the α & alpha; stage undertakes a reconstruction makeover and is transformed into β & beta; phase. This improvement is permanent, and also the existence of some procedure conditions as well as quality is extra advantageous to the transformation from α & alpha; stage to & beta; stage. & alpha;-Si3N4 is created when the temperature level βis less than 1350 ℃, and also & beta;-Si3N4 can be directly prepared at a temperature
higher than 1500 ℃. Silicon Nitride properties The molecular formula of silicon nitride is Si3N4, of which Si accounts for 60.06% as well as dint N make up 39.94%. There is a solid covalent bond between Si and N (of which the ion bond accounts for only 30%), so the Si3N4 has high firmness (Morse firmness 9), high melting point and secure structure.
The Si-N in silicon nitride crystal is generally covalent bond, and the bonding stamina is high, so it has a huge flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal expansion is low, but the thermal conductivity is large, so this product is challenging to generate thermal anxiety, so it has great thermal shock resistance and great thermal shock resistance. It has strength, high mechanical strength at high temperature and also tiny contortion at high temperature. (the high temperature deformation of silicon nitride ceramic with 2.5 g/cm3 thickness is 0.5% at 1200 × & times; 1000h ℃ and 23 × & times; 7kg/cm2 lots). Not eroded by many steels, due to the formation of a silicon dioxide layer, excellent oxidation resistance as well as excellent electric insulation.
Silicon nitride has no melting point and sublimates as well as decomposes at 1900 ℃ under air pressure. The details warmth is 711.8 J/ kg ℃. The microhardness of α & alpha; phase as well as & beta; stage is 10 ~ 16GPa as well as 24.5 ~ 32.65 GPa specifically. Since it is a strong covalent bond substance, no fluid phase will be created below its decay temperature (about 1900 ℃), so silicon nitride products can be sintered with the assistance of oxide additives. The oxide products that advertise sintering are mainly Y2O3, Al2O3, and so on, and the high addition quantity can reach 20%. The response concept is that the SiO2 oxide film based on the surface of silicon nitride bits reacts with the included oxide to develop liquid stage and also penetrate at the grain boundary to guarantee high diffusion ability during product migration.
Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically stable compound. Silicon nitride porcelains can be consumed to 1400 ℃ in oxidation atmosphere and also 1850 ℃ in neutral or lowering atmosphere. In fact, the oxidation response of Si3N4 happens over 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the sample boosted and also a thick silica protective layer was gradually formed on the surface area, which avoided the further oxidation of Si3N4. The weight gain was not obvious until the temperature was over 1600 ℃. Yet in the humid atmosphere, Si3N4 is extremely easy to oxidize, and the surface area starts to oxidize at 200 ℃, which is about twice as quick as that in completely dry air. The oxidation activation energy of Si3N4 powder in water vapor is obviously lower than that in oxygen and air. The factor is that water vapor can react with Si3N4 through amorphous SiO2 movies:
Si3N4 +6 WATER=3SiO2+NH3.
Silicon nitride is steady to most metal options, without corrosion and infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so on. Nonetheless, for Cu solution, it is not deteriorated just in vacuum or inert environment; Mg can respond weakly with Si3N4; silicon option can wet Si3N4 and deteriorate a little; change aspect option can strongly damp Si3N4 and also type silicide with Si to swiftly decay silicon nitride as well as getaway N2 at the exact same time. Si3N4 is secure to alloy services such as brass, tough aluminum as well as nickel silver, and also has great corrosion resistance to cast iron as well as tool carbon steel, yet it is not resistant to Ni-Cr alloy as well as stainless steel.
With the exception of liquified NaOH and also HF, silicon nitride has good chemical corrosion resistance. However most of the liquified antacid and also salt can communicate with Si3N4 to decompose it.
Silicon Nitride utilized in Refractories.
Silicon nitride ceramics are called appealing heat structural materials due to their excellent high temperature homes, such as heat strength, good wear resistance and deterioration resistance. Because of the strong covalent bond and also low diffusion coefficient at heat, the manufacture of Si3N4 porcelains should count on heat, high pressure and also sintering agent, as well as the cost is too high, or else it is hard to produce high quality silicon nitride materials. These constraints of production costs and equipment are tough to approve in the metallurgical market, so the research study in the field of refractories starts fairly late, as well as the study is not in-depth. Several theories come from porcelains, but there is not much innovation. In the past, silicon nitride generally existed in the kind of bonding stage in refractories. Through nitriding and also shooting of metal Si, accumulations such as diamond or silicon carbide were integrated with fine powder, so as to accomplish the purpose of incorporating refractory materials. Ceramic shed plate is silicon carbide aggregate and part of great powder. Silicon nitride is developed by nitriding metal Si to create silicon nitride. Silicon carbide is combined to develop silicon nitride bound silicon carbide material, which is made use of in blast furnace body and various other components, to ensure that the performance of the product has actually been considerably boosted. Compared with the clay bonded silicon carbide lost plate, the high temperature efficiency of the material is great, which resolves the trouble of bulging failing brought on by the oxidation of silicon carbide when the clay adhered silicon carbide lost plate is used.
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