Silicon Nitride structures and properties
Crystal framework of Silicon Nitride
Si3N4 has 2 sort of crystal structure: α& alpha;- Si3N4 is granular crystal as well as β& beta; -Si3N4 is needle crystal. Both of them are three-dimensional networks made up of typical vertices of [SN4] tetrahedron as well as belong to hexagonal system. The distinction between them lies in the order of [SiN4] tetrahedral layers. The β & beta; stage is created by the overlap of hexagonal ring layers composed of 6 nearly entirely symmetrical [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; phase is created by two layers of substantial transformation and different non-hexagonal ring layers. α & alpha; phase can liquify oxygen in the series of crystal framework, as well as the interior pressure of α & alpha; stage is bigger than that of β & beta; stage, so the totally free energy is greater than that of β & beta; phase. From the viewpoint of thermodynamics, the β & beta; stage is more secure at greater temperature level. The & alpha; stage has reduced balance and also is very easy to develop. At the temperature level of regarding 1500 ℃, the α & alpha; phase undertakes a repair change and is transformed right into β & beta; stage. This transformation is irreparable, and the presence of some procedure problems and high quality is more useful to the makeover from α & alpha; phase to & beta; phase. & alpha;-Si3N4 is created when the temperature βis less than 1350 ℃, as well as & beta;-Si3N4 can be straight prepared at a temperature level
greater than 1500 ℃. Silicon Nitride residential or commercial properties The molecular formula of silicon nitride is Si3N4, of which Si accounts for 60.06% as well as dint N make up 39.94%. There is a strong covalent bond in between Si and also N (of which the ion bond make up only 30%), so the Si3N4 has high firmness (Morse solidity 9), high melting factor as well as secure structure.
The Si-N in silicon nitride crystal is mainly covalent bond, and also the bonding strength is high, so it has a big elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal growth is low, however the thermal conductivity is huge, so this product is hard to generate thermal anxiety, so it has great thermal shock resistance as well as excellent thermal shock resistance. It has strength, high mechanical toughness at heat as well as tiny 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 lots). Not worn down by lots of metals, because of the formation of a silicon dioxide layer, good oxidation resistance and good electrical insulation.
Silicon nitride has no melting factor as well as sublimates as well as decomposes at 1900 ℃ under atmospheric pressure. The specific warm is 711.8 J/ kg ℃. The microhardness of α & alpha; stage as well as & beta; stage is 10 ~ 16GPa as well as 24.5 ~ 32.65 Grade point average respectively. Since it is a strong covalent bond compound, no fluid phase will be formed listed below its disintegration temperature level (concerning 1900 ℃), so silicon nitride products can be sintered with the assistance of oxide additives. The oxide products that promote sintering are mostly Y2O3, Al2O3, and so on, as well as the high addition amount can get to 20%. The response principle is that the SiO2 oxide film formed on the surface area of silicon nitride fragments responds with the included oxide to develop fluid stage and also permeate at the grain border to make sure high diffusion capacity throughout material movement.
Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically steady substance. Silicon nitride porcelains can be consumed to 1400 ℃ in oxidation atmosphere and 1850 ℃ in neutral or minimizing environment. As a matter of fact, the oxidation response of Si3N4 happens over 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the sample increased and also a thick silica safety layer was slowly based on the surface, which protected against the additional oxidation of Si3N4. The weight gain was not apparent until the temperature was above 1600 ℃. Yet in the moist ambience, Si3N4 is really easy to oxidize, as well as the surface area begins to oxidize at 200 ℃, which has to do with twice as rapid as that in completely dry air. The oxidation activation energy of Si3N4 powder in water vapor is obviously 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 secure to most metal services, without rust and seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so on. Nevertheless, for Cu service, it is not worn down only in vacuum cleaner or inert ambience; Mg can react weakly with Si3N4; silicon option can wet Si3N4 and also deteriorate slightly; shift element remedy can strongly wet Si3N4 and kind silicide with Si to rapidly disintegrate silicon nitride as well as getaway N2 at the same time. Si3N4 is secure to alloy options such as brass, hard light weight aluminum and nickel silver, and also has excellent rust resistance to cast iron as well as tool carbon steel, however it is not immune to Ni-Cr alloy and also stainless-steel.
Besides molten NaOH and HF, silicon nitride has great chemical rust resistance. However the majority of the molten antacid and also salt can engage with Si3N4 to decompose it.
Silicon Nitride used in Refractories.
Silicon nitride ceramics are called appealing high temperature architectural products as a result of their superb high temperature homes, such as high temperature strength, good wear resistance and also deterioration resistance. Due to the strong covalent bond as well as reduced diffusion coefficient at high temperature, the manufacture of Si3N4 porcelains must depend on heat, high stress as well as sintering agent, and the expense is expensive, otherwise it is tough to generate top quality silicon nitride materials. These constraints of manufacturing costs and also equipment are difficult to accept in the metallurgical sector, so the research in the field of refractories begins relatively late, as well as the study is not comprehensive. Several concepts originate from porcelains, but there is very little technology. In the past, silicon nitride normally existed in the type of bonding stage 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 purpose of incorporating refractory products. Ceramic shed plate is silicon carbide aggregate and also component of great powder. Silicon nitride is formed by nitriding steel Si to form silicon nitride. Silicon carbide is incorporated to develop silicon nitride bound silicon carbide material, which is utilized in blast heating system body and various other components, to ensure that the performance of the product has been greatly improved. Compared to the clay adhered silicon carbide dropped plate, the high temperature efficiency of the material is very good, which resolves the issue of protruding failure triggered by the oxidation of silicon carbide when the clay adhered silicon carbide shed plate is used.
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