Silicon Nitride structures and properties

Crystal structure of Silicon Nitride
Si3N4 has two sort 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 typical vertices of [SN4] tetrahedron and also come from hexagonal system. The distinction between them hinges on the order of [SiN4] tetrahedral layers. The β & beta; stage is formed by the overlap of hexagonal ring layers made up of six virtually entirely symmetrical [SN4] tetrahedrons in the c-axis direction, while the α & alpha; stage is created by two layers of tangible transformation and also different non-hexagonal ring layers. α & alpha; stage can liquify oxygen in the range of crystal structure, and the interior pressure of α & alpha; phase is bigger than that of β & beta; stage, so the totally free energy is more than that of β & beta; phase. From the perspective of thermodynamics, the β & beta; stage is extra stable at greater temperature level. The & alpha; phase has reduced symmetry and also is very easy to develop. At the temperature level of concerning 1500 ℃, the α & alpha; stage undertakes a repair change and is changed into β & beta; phase. This improvement is permanent, and the presence of some procedure problems and top quality is a lot more helpful to the transformation from α & alpha; phase to & beta; phase. & alpha;-Si3N4 is developed when the temperature level βis lower than 1350 ℃, as well as & beta;-Si3N4 can be straight prepared at a temperature level

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 accounts for 39.94%. There is a strong covalent bond in between Si and N (of which the ion bond accounts for only 30%), so the Si3N4 has high firmness (Morse firmness 9), high melting factor as well as stable framework.
The Si-N in silicon nitride crystal is mostly covalent bond, and also the bonding strength is high, so it has a large elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is reduced, but the thermal conductivity is big, so this material is hard to produce thermal tension, so it has good thermal shock resistance and excellent thermal shock resistance. It has sturdiness, high mechanical strength at high temperature and also little deformation at heat. (the high temperature deformation of silicon nitride ceramic with 2.5 g/cm3 density is 0.5% at 1200 × & times; 1000h ℃ and 23 × & times; 7kg/cm2 load). Not worn down by many metals, because of the formation of a silicon dioxide layer, excellent oxidation resistance and good electrical insulation.

Silicon nitride has no melting point and also sublimates as well as decays at 1900 ℃ under atmospheric pressure. The certain warm is 711.8 J/ kg ℃. The microhardness of α & alpha; phase and also & beta; phase is 10 ~ 16GPa and also 24.5 ~ 32.65 Grade point average specifically. Due to the fact that it is a solid covalent bond substance, no liquid stage will certainly be created listed below its decomposition temperature level (concerning 1900 ℃), so silicon nitride materials can be sintered with the help of oxide ingredients. The oxide products that advertise sintering are generally Y2O3, Al2O3, etc, and also the high addition amount can reach 20%. The reaction concept is that the SiO2 oxide movie based on the surface of silicon nitride particles responds with the added oxide to create liquid stage and permeate at the grain border to make sure high diffusion ability throughout product movement.

Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically stable compound. Silicon nitride porcelains can be used up to 1400 ℃ in oxidation ambience and 1850 ℃ in neutral or minimizing ambience. In fact, the oxidation reaction of Si3N4 takes place over 800C:
Si3N4 +3 O2=3SiO2+N2.

The weight of the sample increased as well as a thick silica protective layer was gradually based on the surface, which prevented the further oxidation of Si3N4. The weight gain was not noticeable up until the temperature was over 1600 ℃. Yet in the humid environment, Si3N4 is extremely simple to oxidize, as well as the surface area starts to oxidize at 200 ℃, which is about twice as fast as that in dry air. The oxidation activation energy of Si3N4 powder in water vapor is certainly lower than that in oxygen and also air. The reason is that water vapor can respond with Si3N4 through amorphous SiO2 movies:
Si3N4 +6 H2O=3SiO2+NH3.

Silicon nitride is secure to most metal remedies, devoid of rust as well as seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and more. Nevertheless, for Cu option, it is not worn down just in vacuum or inert atmosphere; Mg can react weakly with Si3N4; silicon option can wet Si3N4 and also wear down a little; change component remedy can highly damp Si3N4 as well as form silicide with Si to swiftly break down silicon nitride as well as retreat N2 at the exact same time. Si3N4 is stable to alloy services such as brass, tough aluminum and also nickel silver, as well as has excellent rust resistance to cast iron and tool carbon steel, however it is not resistant to Ni-Cr alloy as well as stainless steel.

Besides liquified NaOH and HF, silicon nitride has great chemical corrosion resistance. However a lot of the liquified antacid and also salt can connect with Si3N4 to disintegrate it.

Silicon Nitride used in Refractories.
Silicon nitride porcelains are called encouraging high temperature structural products due to their excellent heat residential properties, such as heat stamina, good wear resistance as well as deterioration resistance. As a result of the strong covalent bond and also low diffusion coefficient at heat, the manufacture of Si3N4 ceramics must rely upon heat, high stress and sintering agent, as well as the cost is expensive, otherwise it is difficult to create excellent quality silicon nitride materials. These restrictions of production prices as well as devices are difficult to accept in the metallurgical market, so the research study in the field of refractories starts relatively late, and the research is not in-depth. Numerous concepts come from ceramics, yet there is very little advancement. In the past, silicon nitride normally existed in the type of bonding phase in refractories. Through nitriding and firing of metal Si, accumulations such as diamond or silicon carbide were incorporated with fine powder, so as to accomplish the function of combining refractory materials. Ceramic shed plate is silicon carbide accumulation as well as part of great powder. Silicon nitride is developed by nitriding metal Si to create silicon nitride. Silicon carbide is combined to create silicon nitride bound silicon carbide material, which is utilized in blast furnace body as well as other components, so that the efficiency of the product has been substantially boosted. Compared to the clay bound silicon carbide shed plate, the heat efficiency of the product is great, which solves the trouble of protruding failure caused by the oxidation of silicon carbide when the clay bound silicon carbide dropped plate is used.

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