Silicon Nitride structures and properties
Crystal structure of Silicon Nitride
Si3N4 has 2 type of crystal structure: α& alpha;- Si3N4 is granular crystal and β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks made up of usual vertices of [SN4] tetrahedron and also belong to hexagonal system. The difference between them hinges on the order of [SiN4] tetrahedral layers. The β & beta; phase is developed by the overlap of hexagonal ring layers made up of six practically totally balanced [SN4] tetrahedrons in the c-axis direction, while the α & alpha; stage is formed by 2 layers of concrete makeover and also various non-hexagonal ring layers. α & alpha; phase can dissolve oxygen in the range of crystal framework, as well as the interior pressure of α & alpha; phase is bigger than that of β & beta; phase, so the complimentary power is greater than that of β & beta; phase. From the perspective of thermodynamics, the β & beta; stage is extra steady at greater temperature level. The & alpha; phase has low symmetry as well as is very easy to form. At the temperature of regarding 1500 ℃, the α & alpha; phase goes through a restoration makeover and also is changed right into β & beta; phase. This transformation is permanent, and also the existence of some process conditions as well as top quality is more valuable to the makeover from α & alpha; phase to & beta; stage. & alpha;-Si3N4 is created when the temperature level βis lower than 1350 ℃, and also & beta;-Si3N4 can be straight prepared at a temperature
higher than 1500 ℃. Silicon Nitride residential or commercial properties The molecular formula of silicon nitride is Si3N4, of which Si accounts for 60.06% and also dint N make up 39.94%. There is a strong covalent bond in between Si and also N (of which the ion bond accounts for just 30%), so the Si3N4 has high hardness (Morse firmness 9), high melting point as well as stable framework.
The Si-N in silicon nitride crystal is mostly covalent bond, and the bonding toughness is high, so it has a big flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal expansion is reduced, yet the thermal conductivity is huge, so this material is challenging to create thermal anxiety, so it has excellent thermal shock resistance as well as excellent thermal shock resistance. It has durability, high mechanical stamina at heat as well as little deformation at high temperature. (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 load). Not eroded by several steels, due to the development of a silicon dioxide layer, great oxidation resistance and great electrical insulation.
Silicon nitride has no melting factor and also sublimates as well as breaks down at 1900 ℃ under air pressure. The particular heat is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and also & beta; stage is 10 ~ 16GPa and also 24.5 ~ 32.65 Grade point average respectively. Due to the fact that it is a solid covalent bond compound, no liquid stage will be created listed below its disintegration temperature (concerning 1900 ℃), so silicon nitride products can be sintered with the aid of oxide additives. The oxide materials that promote sintering are generally Y2O3, Al2O3, and so on, and also the high enhancement quantity can reach 20%. The reaction principle is that the SiO2 oxide film formed on the surface of silicon nitride particles reacts with the included oxide to create liquid stage as well as permeate at the grain limit to make sure high diffusion capacity throughout product migration.
Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically stable substance. Silicon nitride porcelains can be used up to 1400 ℃ in oxidation atmosphere and also 1850 ℃ in neutral or decreasing atmosphere. In fact, the oxidation reaction of Si3N4 takes place above 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the example increased and also a thick silica safety layer was progressively based on the surface, which stopped the additional oxidation of Si3N4. The weight gain was not obvious till the temperature was above 1600 ℃. Yet in the moist ambience, Si3N4 is really simple to oxidize, and also 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 lower than that in oxygen and air. The reason is that water vapor can react with Si3N4 through amorphous SiO2 films:
Si3N4 +6 WATER=3SiO2+NH3.
Silicon nitride is secure to most metal options, without deterioration as well as seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so forth. Nonetheless, for Cu option, it is not deteriorated only in vacuum or inert atmosphere; Mg can react weakly with Si3N4; silicon remedy can wet Si3N4 and deteriorate a little; change aspect solution can highly wet Si3N4 and type silicide with Si to quickly decay silicon nitride as well as retreat N2 at the same time. Si3N4 is stable to alloy remedies such as brass, hard aluminum and also nickel silver, as well as has great deterioration resistance to cast iron as well as tool carbon steel, however it is not immune to Ni-Cr alloy as well as stainless steel.
Besides liquified NaOH and HF, silicon nitride has good chemical corrosion resistance. Yet most of the liquified antacid and salt can interact with Si3N4 to decompose it.
Silicon Nitride used in Refractories.
Silicon nitride porcelains are called encouraging heat architectural products as a result of their outstanding high temperature homes, such as heat toughness, great wear resistance and rust resistance. Because of the solid covalent bond and also reduced diffusion coefficient at high temperature, the manufacture of Si3N4 porcelains need to rely on high temperature, high pressure as well as sintering representative, as well as the cost is expensive, otherwise it is hard to create premium quality silicon nitride products. These restrictions of production expenses and devices are hard to approve in the metallurgical sector, so the study in the area of refractories starts reasonably late, and also the research study is not thorough. Many theories originate from ceramics, but there is not much advancement. In the past, silicon nitride generally existed in the type of bonding stage in refractories. Via nitriding and also firing of steel Si, accumulations such as corundum or silicon carbide were incorporated with fine powder, so regarding achieve the objective of integrating refractory materials. Ceramic shed plate is silicon carbide aggregate and part of great powder. Silicon nitride is created by nitriding metal Si to form silicon nitride. Silicon carbide is integrated to form silicon nitride bonded silicon carbide product, which is utilized in blast heating system body as well as other parts, so that the efficiency of the material has been greatly improved. Compared with the clay adhered silicon carbide dropped plate, the high temperature efficiency of the material is very good, which addresses the problem of bulging failing brought on by the oxidation of silicon carbide when the clay bonded silicon carbide shed plate is utilized.
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