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Silicon Nitride structures and properties

Crystal structure of Silicon Nitride
Si3N4 has two 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 made up of common 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 formed by the overlap of hexagonal ring layers made up of six virtually completely symmetrical [SN4] tetrahedrons in the c-axis direction, while the α & alpha; phase is created by 2 layers of tangible transformation and also different non-hexagonal ring layers. α & alpha; stage can liquify oxygen in the series of crystal framework, and also the internal stress of α & alpha; phase is bigger than that of β & beta; stage, so the totally free power is higher than that of β & beta; stage. From the perspective of thermodynamics, the β & beta; stage is more steady at higher temperature level. The & alpha; phase has low proportion and also is easy to form. At the temperature level of concerning 1500 ℃, the α & alpha; phase undertakes a repair improvement and is changed into β & beta; stage. This transformation is irreparable, and the existence of some procedure conditions and also high quality is much more advantageous to the makeover from α & alpha; stage to & beta; phase. & alpha;-Si3N4 is formed when the temperature βis lower than 1350 ℃, and also & beta;-Si3N4 can be straight prepared at a temperature

more than 1500 ℃. Silicon Nitride homes The molecular formula of silicon nitride is Si3N4, of which Si make up 60.06% and also dint N represent 39.94%. There is a strong covalent bond in between Si as well as N (of which the ion bond accounts for only 30%), so the Si3N4 has high hardness (Morse hardness 9), high melting factor and also stable framework.
The Si-N in silicon nitride crystal is mainly covalent bond, and also the bonding toughness is high, so it has a big elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal expansion is low, however the thermal conductivity is large, so this product is difficult to generate thermal stress, so it has good thermal shock resistance and also great thermal shock resistance. It has toughness, high mechanical stamina at heat as well as small deformation at heat. (the high temperature contortion 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 lots of steels, due to the formation of a silicon dioxide layer, great oxidation resistance and good electric insulation.

Silicon nitride has no melting factor as well as sublimates and also decays at 1900 ℃ under air pressure. The certain heat is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and & beta; stage is 10 ~ 16GPa as well as 24.5 ~ 32.65 GPa specifically. Since it is a solid covalent bond compound, no fluid phase will be formed listed below its decay temperature level (about 1900 ℃), so silicon nitride products can be sintered with the assistance of oxide additives. The oxide products that promote sintering are mainly Y2O3, Al2O3, and so on, as well as the high enhancement quantity can reach 20%. The reaction principle is that the SiO2 oxide film formed on the surface area of silicon nitride fragments reacts with the included oxide to create liquid stage and also penetrate at the grain limit to guarantee high diffusion ability throughout product movement.

Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically stable substance. Silicon nitride ceramics can be consumed to 1400 ℃ in oxidation environment and 1850 ℃ in neutral or lowering ambience. Actually, the oxidation reaction of Si3N4 occurs over 800C:
Si3N4 +3 O2=3SiO2+N2.

The weight of the sample raised and also a dense silica protective layer was gradually based on the surface, which protected against the additional oxidation of Si3N4. The weight gain was not apparent till the temperature was over 1600 ℃. However in the moist environment, Si3N4 is extremely easy to oxidize, and the surface starts to oxidize at 200 ℃, which is about two times as quick as that in completely dry air. The oxidation activation energy 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 films:
Si3N4 +6 H2O=3SiO2+NH3.

Silicon nitride is stable 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 service, it is not eroded only in vacuum cleaner or inert ambience; Mg can respond weakly with Si3N4; silicon option can wet Si3N4 as well as erode slightly; change component solution can highly wet Si3N4 and also kind silicide with Si to promptly break down silicon nitride and retreat N2 at the exact same time. Si3N4 is stable to alloy remedies such as brass, tough light weight aluminum as well as nickel silver, as well as has great rust resistance to cast iron as well as tool carbon steel, but it is not resistant to Ni-Cr alloy and also stainless-steel.

With the exception of molten NaOH and also HF, silicon nitride has good chemical deterioration resistance. Yet the majority of the liquified antacid and salt can connect with Si3N4 to disintegrate it.

Silicon Nitride made use of in Refractories.
Silicon nitride ceramics are called promising high temperature structural materials as a result of their outstanding heat buildings, such as heat strength, great wear resistance as well as rust resistance. Due to the solid covalent bond and also low diffusion coefficient at heat, the manufacture of Si3N4 ceramics need to rely upon high temperature, high pressure and also sintering representative, as well as the expense is expensive, or else it is challenging to generate top quality silicon nitride products. These restrictions of production prices and devices are tough to approve in the metallurgical market, so the research in the area of refractories begins reasonably late, as well as the research study is not thorough. Several concepts come from ceramics, but there is not much technology. In the past, silicon nitride normally existed in the form of bonding phase in refractories. Via nitriding as well as shooting of metal Si, aggregates such as corundum or silicon carbide were combined with great powder, so as to attain the objective of integrating refractory products. Ceramic shed plate is silicon carbide aggregate as well as part of fine powder. Silicon nitride is developed by nitriding steel Si to create silicon nitride. Silicon carbide is combined to form silicon nitride bound silicon carbide material, which is utilized in blast heating system body and various other components, to make sure that the performance of the material has been significantly boosted. Compared with the clay bonded silicon carbide dropped plate, the high temperature performance of the material is very good, which resolves the trouble of protruding failing caused by the oxidation of silicon carbide when the clay bonded silicon carbide shed plate is utilized.

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