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Silicon Nitride structures and properties

Crystal framework of Silicon Nitride
Si3N4 has 2 kinds of crystal structure: α& alpha;- Si3N4 is granular crystal and β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks composed of typical vertices of [SN4] tetrahedron and also belong to hexagonal system. The distinction in between them lies in the order of [SiN4] tetrahedral layers. The β & beta; stage is developed by the overlap of hexagonal ring layers composed of 6 practically entirely balanced [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; stage is formed by 2 layers of substantial improvement and different non-hexagonal ring layers. α & alpha; phase can dissolve oxygen in the variety of crystal structure, as well as the interior stress of α & alpha; stage is bigger than that of β & beta; stage, so the free energy is higher than that of β & beta; stage. From the point of view of thermodynamics, the β & beta; phase is more secure at higher temperature. The & alpha; phase has low proportion and also is simple to form. At the temperature of about 1500 ℃, the α & alpha; phase undertakes a reconstruction improvement and is transformed into β & beta; stage. This makeover is irreparable, and also the presence of some process problems as well as top quality is much more helpful to the change from α & alpha; stage to & beta; stage. & alpha;-Si3N4 is created 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 residential or commercial properties The molecular formula of silicon nitride is Si3N4, of which Si represent 60.06% and also dint N accounts for 39.94%. There is a solid covalent bond between Si and N (of which the ion bond accounts for only 30%), so the Si3N4 has high solidity (Morse hardness 9), high melting point and also stable structure.
The Si-N in silicon nitride crystal is primarily covalent bond, and also the bonding toughness is high, so it has a large elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal expansion is reduced, yet the thermal conductivity is huge, so this material is not easy to produce thermal stress and anxiety, so it has great thermal shock resistance and also excellent thermal shock resistance. It has toughness, high mechanical stamina at heat as well as little contortion at high temperature. (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 eroded by several steels, because of the development of a silicon dioxide layer, good oxidation resistance as well as good electric insulation.

Silicon nitride has no melting point and also sublimates and also disintegrates at 1900 ℃ under air pressure. The particular heat is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and & beta; stage is 10 ~ 16GPa and 24.5 ~ 32.65 Grade point average respectively. Since it is a solid covalent bond compound, no liquid stage will be developed below its decay temperature (about 1900 ℃), so silicon nitride materials can be sintered with the help of oxide additives. The oxide materials that advertise sintering are mainly Y2O3, Al2O3, etc, as well as the high enhancement quantity can get to 20%. The reaction principle is that the SiO2 oxide film based on the surface area of silicon nitride bits responds with the included oxide to form liquid stage and permeate at the grain boundary to make certain high diffusion capacity throughout product movement.

Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically steady substance. Silicon nitride porcelains can be consumed to 1400 ℃ in oxidation atmosphere 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 also a thick silica protective layer was slowly formed on the surface area, which protected against the additional oxidation of Si3N4. The weight gain was not obvious till the temperature was above 1600 ℃. However in the moist ambience, Si3N4 is extremely simple to oxidize, as well as the surface starts to oxidize at 200 ℃, which has to do with twice as quick as that in dry air. The oxidation activation power of Si3N4 powder in water vapor is undoubtedly less than that in oxygen and air. The reason is that water vapor can react with Si3N4 via amorphous SiO2 movies:
Si3N4 +6 WATER=3SiO2+NH3.

Silicon nitride is secure to most metal solutions, devoid of rust and also infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so forth. Nonetheless, for Cu solution, it is not eroded just in vacuum or inert ambience; Mg can respond weakly with Si3N4; silicon option can wet Si3N4 and also deteriorate a little; transition component solution can highly damp Si3N4 and type silicide with Si to swiftly decay silicon nitride and also getaway N2 at the same time. Si3N4 is stable to alloy solutions such as brass, hard light weight aluminum and also nickel silver, and also has great rust resistance to cast iron and medium carbon steel, but it is not immune to Ni-Cr alloy and stainless steel.

Except for molten NaOH and also HF, silicon nitride has excellent chemical deterioration resistance. However most of the liquified alkali as well as salt can interact with Si3N4 to disintegrate it.

Silicon Nitride used in Refractories.
Silicon nitride porcelains are called promising heat architectural products because of their outstanding heat buildings, such as high temperature toughness, excellent wear resistance as well as corrosion resistance. As a result of the strong covalent bond and low diffusion coefficient at heat, the manufacture of Si3N4 ceramics must depend on heat, high pressure and sintering agent, and the price is too expensive, or else it is tough to create high quality silicon nitride materials. These constraints of manufacturing expenses as well as equipment are hard to accept in the metallurgical sector, so the research in the area of refractories begins reasonably late, as well as the research is not thorough. Lots of concepts originate from ceramics, however there is very little innovation. In the past, silicon nitride usually existed in the kind of bonding phase in refractories. With nitriding as well as firing of steel Si, aggregates such as corundum or silicon carbide were combined with great powder, so as to attain the objective of incorporating refractory products. Ceramic shed plate is silicon carbide aggregate as well as component of fine powder. Silicon nitride is formed by nitriding metal Si to develop silicon nitride. Silicon carbide is integrated to develop silicon nitride bound silicon carbide material, which is made use of in blast heater body and also other components, to ensure that the efficiency of the material has actually been significantly boosted. Compared to the clay bound silicon carbide shed plate, the heat efficiency of the product is very good, which resolves the problem of protruding failing triggered by the oxidation of silicon carbide when the clay bonded silicon carbide dropped plate is utilized.

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