Silicon Nitride structures and properties
Crystal framework of Silicon Nitride
Si3N4 has 2 type of crystal framework: α& alpha;- Si3N4 is granular crystal and also β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks composed of typical vertices of [SN4] tetrahedron as well as come from hexagonal system. The difference between them lies in the order of [SiN4] tetrahedral layers. The β & beta; phase is formed by the overlap of hexagonal ring layers composed of 6 almost totally in proportion [SN4] tetrahedrons in the c-axis direction, while the α & alpha; stage is developed by two layers of tangible makeover as well as various non-hexagonal ring layers. α & alpha; phase can liquify oxygen in the series of crystal framework, and the interior stress of α & alpha; phase is larger than that of β & beta; phase, so the totally free power is greater than that of β & beta; stage. From the perspective of thermodynamics, the β & beta; phase is more stable at greater temperature. The & alpha; phase has reduced proportion and is easy to develop. At the temperature level of about 1500 ℃, the α & alpha; stage undertakes a repair transformation and also is changed right into β & beta; phase. This change is irreparable, as well as the existence of some process problems and quality is more helpful to the improvement from α & alpha; stage to & beta; stage. & alpha;-Si3N4 is developed when the temperature βis less than 1350 ℃, and also & beta;-Si3N4 can be straight prepared at a temperature level
more than 1500 ℃. Silicon Nitride buildings The molecular formula of silicon nitride is Si3N4, of which Si represent 60.06% and dint N represent 39.94%. There is a solid covalent bond in 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 also steady structure.
The Si-N in silicon nitride crystal is mainly covalent bond, and also the bonding strength is high, so it has a big flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is reduced, but the thermal conductivity is huge, so this product is hard to create thermal stress, so it has good thermal shock resistance and great thermal shock resistance. It has sturdiness, high mechanical strength at heat as well as small 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 ℃ as well as 23 × & times; 7kg/cm2 tons). Not worn down by lots of steels, due to the development of a silicon dioxide layer, good oxidation resistance and good electrical insulation.
Silicon nitride has no melting factor and also sublimates as well as decays at 1900 ℃ under air pressure. The certain heat is 711.8 J/ kg ℃. The microhardness of α & alpha; stage as well as & beta; phase is 10 ~ 16GPa and also 24.5 ~ 32.65 Grade point average respectively. Because it is a strong covalent bond compound, no liquid phase will be developed listed below its decay temperature level (about 1900 ℃), so silicon nitride materials can be sintered with the help of oxide additives. The oxide materials that advertise sintering are generally Y2O3, Al2O3, and so on, and also the high addition quantity can get to 20%. The response concept is that the SiO2 oxide movie formed on the surface area of silicon nitride fragments reacts with the included oxide to develop liquid stage and permeate at the grain boundary to make certain high diffusion capacity during material movement.
Chemical Stability of Silicon Nitride
Si3N4 is a thermodynamically secure substance. Silicon nitride ceramics can be consumed to 1400 ℃ in oxidation environment and also 1850 ℃ in neutral or decreasing environment. As a matter of fact, the oxidation response of Si3N4 happens over 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the example increased as well as a thick silica protective layer was progressively based on the surface, which stopped the further oxidation of Si3N4. The weight gain was not evident till the temperature level was above 1600 ℃. But in the damp environment, Si3N4 is really easy to oxidize, as well as the surface begins to oxidize at 200 ℃, which has to do with twice as quick as that in completely dry air. The oxidation activation power of Si3N4 powder in water vapor is obviously less than that in oxygen as well as air. The reason is that water vapor can respond with Si3N4 via amorphous SiO2 movies:
Si3N4 +6 H2O=3SiO2+NH3.
Silicon nitride is steady to most metal remedies, devoid of corrosion as well as seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so on. Nevertheless, for Cu option, it is not worn down just in vacuum cleaner or inert environment; Mg can react weakly with Si3N4; silicon service can wet Si3N4 and also deteriorate slightly; shift element service can highly wet Si3N4 as well as type silicide with Si to rapidly disintegrate silicon nitride and also getaway N2 at the exact same time. Si3N4 is stable to alloy remedies such as brass, hard light weight aluminum as well as nickel silver, and has excellent deterioration resistance to cast iron and also tool carbon steel, yet it is not immune 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 a lot of the molten antacid as well as salt can communicate with Si3N4 to decompose it.
Silicon Nitride utilized in Refractories.
Silicon nitride ceramics are called promising high temperature architectural products due to their exceptional heat properties, such as heat stamina, great wear resistance as well as deterioration resistance. As a result of the strong covalent bond and low diffusion coefficient at high temperature, the manufacture of Si3N4 porcelains have to rely upon heat, high pressure as well as sintering agent, and also the expense is expensive, otherwise it is hard to create top quality silicon nitride products. These constraints of manufacturing costs as well as devices are tough to accept in the metallurgical industry, so the study in the area of refractories starts relatively late, and also the study is not in-depth. Numerous theories originate from ceramics, however there is very little technology. In the past, silicon nitride typically existed in the kind of bonding stage in refractories. With nitriding and shooting of steel Si, aggregates such as corundum or silicon carbide were combined with fine powder, so as to accomplish the function of combining refractory products. Ceramic shed plate is silicon carbide accumulation and component of fine powder. Silicon nitride is developed by nitriding steel Si to create silicon nitride. Silicon carbide is integrated to create silicon nitride adhered silicon carbide product, which is made use of in blast heater body as well as various other parts, to ensure that the efficiency of the material has actually been significantly enhanced. Compared to the clay bonded silicon carbide dropped plate, the high temperature performance of the material is very good, which addresses the trouble of bulging failure brought on by the oxidation of silicon carbide when the clay adhered silicon carbide dropped plate is made use of.
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