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 typical vertices of [SN4] tetrahedron and belong to hexagonal system. The difference between them depends on the order of [SiN4] tetrahedral layers. The β & beta; phase is created by the overlap of hexagonal ring layers made up of six practically entirely symmetrical [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; phase is developed by two layers of substantial improvement and different non-hexagonal ring layers. α & alpha; phase can liquify oxygen in the series of crystal structure, and the interior pressure of α & alpha; stage is larger than that of β & beta; phase, so the complimentary energy is higher than that of β & beta; phase. From the perspective of thermodynamics, the β & beta; stage is more stable at greater temperature level. The & alpha; stage has low balance as well as is very easy to develop. At the temperature of concerning 1500 ℃, the α & alpha; phase goes through a restoration change as well as is changed right into β & beta; stage. This makeover is permanent, and the presence of some process conditions and top quality is more valuable to the improvement from α & alpha; phase to & beta; stage. & alpha;-Si3N4 is created when the temperature βis lower than 1350 ℃, and also & beta;-Si3N4 can be directly prepared at a temperature level
higher than 1500 ℃. Silicon Nitride residential or commercial properties The molecular formula of silicon nitride is Si3N4, of which Si represent 60.06% as well as dint N accounts for 39.94%. There is a solid covalent bond between Si and also N (of which the ion bond represent only 30%), so the Si3N4 has high firmness (Morse hardness 9), high melting point as well as steady structure.
The Si-N in silicon nitride crystal is generally covalent bond, as well as the bonding toughness is high, so it has a large elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is reduced, however the thermal conductivity is big, so this material is challenging to create thermal stress and anxiety, so it has good thermal shock resistance as well as good thermal shock resistance. It has toughness, high mechanical strength at high temperature and tiny contortion 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 worn down by lots of metals, as a result of the development of a silicon dioxide layer, good oxidation resistance and also great electric insulation.
Silicon nitride has no melting factor and sublimates as well as breaks down at 1900 ℃ under air pressure. The particular warm is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and also & beta; stage is 10 ~ 16GPa as well as 24.5 ~ 32.65 Grade point average specifically. Since it is a strong covalent bond compound, no liquid stage will certainly be developed listed below its disintegration temperature level (regarding 1900 ℃), so silicon nitride products can be sintered with the aid of oxide ingredients. The oxide materials that advertise sintering are mostly Y2O3, Al2O3, and so on, as well as the high enhancement amount can reach 20%. The reaction principle is that the SiO2 oxide movie based on the surface of silicon nitride bits reacts with the included oxide to form fluid stage and also permeate at the grain boundary to make certain high diffusion ability during material movement.
Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically secure compound. Silicon nitride ceramics can be consumed to 1400 ℃ in oxidation environment as well as 1850 ℃ in neutral or minimizing atmosphere. In fact, the oxidation response of Si3N4 takes place above 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the sample enhanced and a thick silica protective layer was gradually formed on the surface area, which stopped the additional oxidation of Si3N4. The weight gain was not obvious up until the temperature level was over 1600 ℃. However in the humid atmosphere, Si3N4 is very easy to oxidize, as well as the surface area begins to oxidize at 200 ℃, which is about two times as quick as that in dry air. The oxidation activation energy of Si3N4 powder in water vapor is certainly less than that in oxygen and also air. The factor is that water vapor can respond with Si3N4 through amorphous SiO2 films:
Si3N4 +6 H2O=3SiO2+NH3.
Silicon nitride is secure to most metal remedies, without rust and also infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so forth. However, for Cu remedy, it is not eroded only in vacuum or inert environment; Mg can react weakly with Si3N4; silicon service can wet Si3N4 as well as erode slightly; change element option can strongly damp Si3N4 as well as kind silicide with Si to promptly decompose silicon nitride and also escape N2 at the exact same time. Si3N4 is steady to alloy solutions such as brass, difficult aluminum and nickel silver, and has excellent corrosion resistance to cast iron and also medium carbon steel, yet it is not immune to Ni-Cr alloy and also stainless steel.
Besides molten NaOH as well as HF, silicon nitride has excellent chemical corrosion resistance. Yet most of the liquified antacid and also salt can communicate with Si3N4 to decompose it.
Silicon Nitride utilized in Refractories.
Silicon nitride porcelains are called encouraging high temperature structural materials as a result of their excellent heat properties, such as high temperature toughness, good wear resistance as well as deterioration resistance. As a result of the solid covalent bond and also reduced diffusion coefficient at heat, the manufacture of Si3N4 porcelains must count on high temperature, high pressure and also sintering representative, and the expense is too expensive, or else it is difficult to generate top quality silicon nitride products. These limitations of production expenses as well as devices are tough to accept in the metallurgical market, so the study in the area of refractories begins fairly late, and the study is not thorough. Lots of concepts originate from porcelains, but there is very little advancement. In the past, silicon nitride normally existed in the form of bonding stage in refractories. Via nitriding and also firing of steel Si, aggregates such as diamond or silicon carbide were combined with great powder, so regarding achieve the function of incorporating refractory products. Ceramic shed plate is silicon carbide accumulation and also part of fine powder. Silicon nitride is formed by nitriding steel Si to create silicon nitride. Silicon carbide is incorporated to develop silicon nitride bound silicon carbide material, which is used in blast heater body as well as other components, to ensure that the performance of the material has been greatly enhanced. Compared to the clay bonded silicon carbide shed plate, the high temperature efficiency of the product is excellent, which solves the problem of bulging failing caused by the oxidation of silicon carbide when the clay bound silicon carbide shed plate is used.
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