Silicon Nitride structures and properties

Crystal framework of Silicon Nitride
Si3N4 has 2 sort 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 usual vertices of [SN4] tetrahedron and come from hexagonal system. The difference in between them hinges on the order of [SiN4] tetrahedral layers. The β & beta; phase is formed by the overlap of hexagonal ring layers composed of six almost totally in proportion [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; stage is developed by two layers of substantial transformation as well as different non-hexagonal ring layers. α & alpha; stage can dissolve oxygen in the range of crystal structure, as well as the interior stress of α & alpha; phase is larger than that of β & beta; stage, so the free energy is more than that of β & beta; phase. From the point of view of thermodynamics, the β & beta; stage is extra steady at greater temperature level. The & alpha; stage has reduced proportion as well as is simple to create. At the temperature of concerning 1500 ℃, the α & alpha; phase undertakes a restoration change and is changed into β & beta; stage. This improvement is irreparable, and also the existence of some process problems and top quality is much more helpful to the transformation from α & alpha; stage to & beta; stage. & alpha;-Si3N4 is developed 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 make up 60.06% and dint N accounts for 39.94%. There is a solid covalent bond in between Si and also N (of which the ion bond accounts for just 30%), so the Si3N4 has high firmness (Morse solidity 9), high melting point and stable structure.
The Si-N in silicon nitride crystal is mainly covalent bond, and the bonding stamina is high, so it has a big flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal growth is reduced, but the thermal conductivity is big, so this material is challenging to create thermal stress, so it has excellent thermal shock resistance as well as good thermal shock resistance. It has durability, high mechanical strength at heat and also tiny deformation at heat. (the heat contortion of silicon nitride ceramic with 2.5 g/cm3 thickness is 0.5% at 1200 × & times; 1000h ℃ and also 23 × & times; 7kg/cm2 lots). Not deteriorated by lots of metals, because of the development of a silicon dioxide layer, excellent oxidation resistance as well as great electrical insulation.

Silicon nitride has no melting factor and sublimates as well as decays at 1900 ℃ under atmospheric pressure. The specific heat is 711.8 J/ kg ℃. The microhardness of α & alpha; phase as well as & beta; stage is 10 ~ 16GPa and also 24.5 ~ 32.65 GPa specifically. Due to the fact that it is a solid covalent bond substance, no liquid phase will certainly be developed below its decay temperature (concerning 1900 ℃), so silicon nitride materials can be sintered with the help of oxide additives. The oxide products that advertise sintering are primarily Y2O3, Al2O3, etc, as well as the high enhancement quantity can reach 20%. The response principle is that the SiO2 oxide movie based on the surface area of silicon nitride particles reacts with the added oxide to form liquid phase as well as permeate at the grain boundary to ensure high diffusion capacity during product movement.

Chemical Stability of Silicon Nitride
Si3N4 is a thermodynamically steady compound. Silicon nitride ceramics can be used up to 1400 ℃ in oxidation ambience and also 1850 ℃ in neutral or lowering ambience. Actually, the oxidation response of Si3N4 happens over 800C:
Si3N4 +3 O2=3SiO2+N2.

The weight of the sample increased and also a dense silica safety layer was slowly based on the surface, which stopped the more oxidation of Si3N4. The weight gain was not obvious up until the temperature was above 1600 ℃. But in the moist atmosphere, Si3N4 is really simple to oxidize, as well as the surface area begins to oxidize at 200 ℃, which has to do with two times as quick as that in dry air. The oxidation activation power of Si3N4 powder in water vapor is undoubtedly lower than that in oxygen as well as air. The factor is that water vapor can respond with Si3N4 with amorphous SiO2 movies:
Si3N4 +6 WATER=3SiO2+NH3.

Silicon nitride is steady to most metal remedies, without deterioration and also infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so forth. Nevertheless, for Cu remedy, it is not deteriorated only in vacuum cleaner or inert atmosphere; Mg can respond weakly with Si3N4; silicon service can wet Si3N4 and erode somewhat; transition component option can highly damp Si3N4 and form silicide with Si to quickly disintegrate silicon nitride and also escape N2 at the exact same time. Si3N4 is steady to alloy remedies such as brass, tough aluminum and nickel silver, as well as has excellent corrosion resistance to cast iron and also medium carbon steel, but it is not immune to Ni-Cr alloy and stainless steel.

With the exception of molten NaOH as well as HF, silicon nitride has excellent chemical corrosion resistance. However the majority of the liquified alkali and also salt can connect with Si3N4 to decay it.

Silicon Nitride made use of in Refractories.
Silicon nitride ceramics are called appealing high temperature architectural materials because of their excellent heat properties, such as high temperature strength, great wear resistance and also corrosion resistance. As a result of the solid covalent bond and low diffusion coefficient at high temperature, the manufacture of Si3N4 porcelains should rely on heat, high pressure as well as sintering representative, as well as the cost is expensive, or else it is tough to create high quality silicon nitride materials. These constraints of manufacturing prices and devices are difficult to approve in the metallurgical market, so the study in the area of refractories begins reasonably late, and also the research is not thorough. Many theories come from ceramics, however there is very little innovation. In the past, silicon nitride normally existed in the form of bonding stage in refractories. Through nitriding and firing of metal Si, accumulations such as corundum or silicon carbide were incorporated with fine powder, so regarding accomplish the purpose of combining refractory products. Ceramic shed plate is silicon carbide accumulation and also part of great powder. Silicon nitride is created by nitriding steel Si to create silicon nitride. Silicon carbide is integrated to develop silicon nitride bonded silicon carbide material, which is used in blast furnace body as well as other parts, so that the performance of the material has actually been considerably improved. Compared to the clay bonded silicon carbide shed plate, the high temperature performance of the material is very good, which resolves the issue of protruding failing brought on by the oxidation of silicon carbide when the clay bound silicon carbide dropped plate is used.

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