Silicon Nitride structures and properties
Crystal structure of Silicon Nitride
Si3N4 has 2 type of crystal framework: 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 usual vertices of [SN4] tetrahedron as well as belong to hexagonal system. The distinction between them hinges on the order of [SiN4] tetrahedral layers. The β & beta; phase is created by the overlap of hexagonal ring layers composed of 6 almost entirely balanced [SN4] tetrahedrons in the c-axis instructions, while the α & alpha; stage is created by 2 layers of substantial improvement and various non-hexagonal ring layers. α & alpha; phase can liquify oxygen in the variety of crystal structure, and the interior strain of α & alpha; phase is larger than that of β & beta; stage, so the cost-free power is greater than that of β & beta; stage. From the point of view of thermodynamics, the β & beta; phase is extra stable at greater temperature level. The & alpha; phase has low proportion as well as is easy to create. At the temperature of regarding 1500 ℃, the α & alpha; stage undertakes a repair change and is changed into β & beta; stage. This transformation is irreversible, as well as the presence of some process problems and top quality is much more useful 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 properties The molecular formula of silicon nitride is Si3N4, of which Si accounts for 60.06% and dint N make up 39.94%. There is a strong covalent bond in between Si as well as N (of which the ion bond make up just 30%), so the Si3N4 has high firmness (Morse firmness 9), high melting point as well as stable structure.
The Si-N in silicon nitride crystal is mostly covalent bond, as well as the bonding stamina is high, so it has a large elastic modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal expansion is reduced, however the thermal conductivity is big, so this material is difficult to generate thermal stress and anxiety, so it has great thermal shock resistance and excellent thermal shock resistance. It has toughness, high mechanical toughness at high temperature and small deformation at heat. (the heat deformation of silicon nitride ceramic with 2.5 g/cm3 density is 0.5% at 1200 × & times; 1000h ℃ and also 23 × & times; 7kg/cm2 tons). Not worn down by numerous steels, because of the development of a silicon dioxide layer, good oxidation resistance as well as great electric insulation.
Silicon nitride has no melting factor and sublimates as well as decays at 1900 ℃ under air pressure. The details heat is 711.8 J/ kg ℃. The microhardness of α & alpha; stage and & beta; phase is 10 ~ 16GPa as well as 24.5 ~ 32.65 GPa specifically. Since it is a strong covalent bond substance, no fluid phase will be created listed below its decay temperature (concerning 1900 ℃), so silicon nitride products can be sintered with the aid of oxide additives. The oxide products that promote sintering are mostly Y2O3, Al2O3, and so on, and the high enhancement quantity can reach 20%. The reaction principle is that the SiO2 oxide movie based on the surface of silicon nitride particles responds with the included oxide to develop liquid phase and also permeate at the grain limit to guarantee high diffusion capacity throughout material migration.
Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically steady compound. Silicon nitride porcelains can be consumed to 1400 ℃ in oxidation ambience and also 1850 ℃ in neutral or reducing environment. Actually, the oxidation response of Si3N4 occurs above 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the sample enhanced and a thick silica protective layer was progressively based on the surface, which prevented the further oxidation of Si3N4. The weight gain was not noticeable until the temperature was over 1600 ℃. Yet in the damp environment, Si3N4 is very easy to oxidize, and also the surface area begins to oxidize at 200 ℃, which has to do with twice as rapid as that in dry air. The oxidation activation power of Si3N4 powder in water vapor is certainly less than that in oxygen as well as air. The factor is that water vapor can respond with Si3N4 through amorphous SiO2 movies:
Si3N4 +6 WATER=3SiO2+NH3.
Silicon nitride is stable to most metal remedies, without rust as well as seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so forth. Nevertheless, for Cu option, it is not deteriorated just in vacuum or inert ambience; Mg can respond weakly with Si3N4; silicon remedy can damp Si3N4 as well as erode a little; transition aspect remedy can strongly wet Si3N4 and kind silicide with Si to promptly decay silicon nitride and also getaway N2 at the very same time. Si3N4 is secure to alloy solutions such as brass, hard aluminum as well as nickel silver, and has great corrosion resistance to cast iron and tool carbon steel, but it is not resistant to Ni-Cr alloy and also stainless-steel.
With the exception of molten NaOH as well as HF, silicon nitride has great chemical deterioration resistance. However the majority of the molten antacid and also salt can connect with Si3N4 to decay it.
Silicon Nitride used in Refractories.
Silicon nitride ceramics are called appealing high temperature structural materials as a result of their exceptional high temperature buildings, such as heat stamina, great wear resistance and deterioration resistance. As a result of the strong covalent bond as well as reduced diffusion coefficient at heat, the manufacture of Si3N4 ceramics must count on high temperature, high pressure and also sintering agent, and the price is too expensive, otherwise it is difficult to produce top quality silicon nitride products. These limitations of production expenses as well as equipment are hard to accept in the metallurgical market, so the study in the area of refractories begins fairly late, and the study is not in-depth. Several theories originate from porcelains, but there is not much innovation. In the past, silicon nitride generally existed in the form of bonding phase in refractories. With nitriding as well as firing of metal Si, accumulations such as diamond or silicon carbide were integrated with great powder, so regarding accomplish the purpose of integrating refractory materials. Ceramic shed plate is silicon carbide aggregate and part of great powder. Silicon nitride is formed by nitriding metal Si to create silicon nitride. Silicon carbide is integrated to develop silicon nitride bound silicon carbide product, which is used in blast heater body and other parts, to ensure that the performance of the product has actually been greatly enhanced. Compared with the clay bound silicon carbide lost plate, the high temperature efficiency of the product is very good, which resolves the problem of bulging failure caused by the oxidation of silicon carbide when the clay bound silicon carbide lost plate is used.
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