Silicon Nitride structures and properties
Crystal framework of Silicon Nitride
Si3N4 has two kinds 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 also belong to hexagonal system. The difference 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 six nearly completely in proportion [SN4] tetrahedrons in the c-axis direction, while the α & alpha; stage is formed by 2 layers of concrete improvement and also various non-hexagonal ring layers. α & alpha; stage can liquify oxygen in the variety of crystal framework, as well as the interior stress of α & alpha; phase is bigger than that of β & beta; phase, so the totally free energy is greater than that of β & beta; stage. From the viewpoint of thermodynamics, the β & beta; stage is more stable at higher temperature level. The & alpha; stage has low proportion and also is easy to develop. At the temperature of regarding 1500 ℃, the α & alpha; phase undergoes a repair change and also is transformed into β & beta; phase. This makeover is irreversible, and also the presence of some procedure conditions and also high quality is extra beneficial to the change from α & alpha; phase to & beta; stage. & alpha;-Si3N4 is created when the temperature βis less than 1350 ℃, and & beta;-Si3N4 can be straight prepared at a temperature level
higher 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 solid covalent bond between Si and also N (of which the ion bond represent just 30%), so the Si3N4 has high hardness (Morse solidity 9), high melting factor as well as secure framework.
The Si-N in silicon nitride crystal is generally covalent bond, as well as the bonding toughness is high, so it has a big elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal development is reduced, but the thermal conductivity is huge, so this product is challenging to generate thermal stress, so it has excellent thermal shock resistance and good thermal shock resistance. It has strength, high mechanical strength at heat as well as little contortion at heat. (the heat 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 lots of metals, due to the formation of a silicon dioxide layer, excellent oxidation resistance as well as great electric insulation.
Silicon nitride has no melting point and sublimates and disintegrates at 1900 ℃ under air pressure. The specific warm is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and also & beta; phase is 10 ~ 16GPa and 24.5 ~ 32.65 Grade point average specifically. Due to the fact that it is a solid covalent bond compound, no liquid phase will be formed listed below its decay temperature (about 1900 ℃), so silicon nitride materials can be sintered with the aid of oxide ingredients. The oxide products that promote sintering are primarily Y2O3, Al2O3, etc, and the high addition quantity can reach 20%. The reaction principle is that the SiO2 oxide movie based on the surface area of silicon nitride bits responds with the included oxide to develop liquid stage and permeate at the grain border to ensure high diffusion capacity during product movement.
Chemical Stability of Silicon Nitride
Si3N4 is a thermodynamically steady substance. Silicon nitride porcelains can be consumed to 1400 ℃ in oxidation ambience and 1850 ℃ in neutral or decreasing environment. As a matter of fact, the oxidation reaction of Si3N4 takes place above 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the sample raised as well as a thick silica protective layer was gradually based on the surface area, which prevented the more oxidation of Si3N4. The weight gain was not apparent until the temperature level was over 1600 ℃. However in the damp ambience, Si3N4 is really simple to oxidize, and also the surface area begins to oxidize at 200 ℃, which is about two times as quick as that in completely dry air. The oxidation activation power of Si3N4 powder in water vapor is undoubtedly lower than that in oxygen and air. The factor is that water vapor can react with Si3N4 via amorphous SiO2 films:
Si3N4 +6 H2O=3SiO2+NH3.
Silicon nitride is stable to most metal solutions, free from deterioration and also seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so on. Nevertheless, for Cu remedy, it is not worn down just in vacuum or inert environment; Mg can react weakly with Si3N4; silicon option can damp Si3N4 as well as erode somewhat; shift aspect remedy can highly wet Si3N4 and also form silicide with Si to rapidly decompose silicon nitride as well as retreat N2 at the exact same time. Si3N4 is secure to alloy services such as brass, difficult aluminum as well as nickel silver, and also has great corrosion resistance to cast iron and tool carbon steel, but it is not immune to Ni-Cr alloy as well as stainless-steel.
With the exception of liquified NaOH and also HF, silicon nitride has excellent chemical corrosion resistance. Yet most of the liquified alkali and also salt can engage with Si3N4 to disintegrate it.
Silicon Nitride made use of in Refractories.
Silicon nitride porcelains are called promising heat structural materials as a result of their superb heat residential or commercial properties, such as heat toughness, good wear resistance as well as rust resistance. Due to the solid covalent bond and also reduced diffusion coefficient at high temperature, the manufacture of Si3N4 ceramics must count on heat, high stress and sintering representative, as well as the price is too expensive, or else it is difficult to create excellent quality silicon nitride materials. These constraints of manufacturing costs and also devices are challenging to accept in the metallurgical industry, so the research in the field of refractories starts relatively late, and the research study is not thorough. Several concepts originate from ceramics, but there is not much advancement. In the past, silicon nitride normally existed in the type of bonding stage in refractories. Via nitriding and also shooting of metal Si, accumulations such as corundum or silicon carbide were incorporated with great powder, so as to accomplish the function of combining refractory materials. Ceramic shed plate is silicon carbide accumulation as well as part of fine powder. Silicon nitride is created by nitriding steel Si to form silicon nitride. Silicon carbide is combined to create silicon nitride bonded silicon carbide material, which is utilized in blast heating system body as well as other parts, to make sure that the performance of the product has actually been considerably boosted. Compared to the clay adhered silicon carbide dropped plate, the heat efficiency of the material is great, which fixes the issue of bulging failure caused by the oxidation of silicon carbide when the clay adhered silicon carbide shed plate is made use of.
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