Silicon Nitride structures and properties
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Crystal structure of Silicon Nitride
Si3N4 has two kinds 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 common vertices of [SN4] tetrahedron as well as come from hexagonal system. The distinction in between them depends on the order of [SiN4] tetrahedral layers. The β & beta; phase is formed by the overlap of hexagonal ring layers made up of six nearly totally balanced [SN4] tetrahedrons in the c-axis direction, while the α & alpha; stage is created by two layers of substantial improvement and different non-hexagonal ring layers. α & alpha; phase can liquify oxygen in the range of crystal framework, and the interior strain of α & alpha; stage is larger than that of β & beta; phase, 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 secure at greater temperature level. The & alpha; phase has low balance and also is easy to create. At the temperature of concerning 1500 ℃, the α & alpha; stage goes through a repair transformation and also is transformed into β & beta; stage. This improvement is irreparable, and also the existence of some procedure problems as well as high quality is a lot more useful to the improvement from α & alpha; stage to & beta; phase. & alpha;-Si3N4 is developed when the temperature βis lower than 1350 ℃, as well as & beta;-Si3N4 can be directly prepared at a temperature level
higher than 1500 ℃. Silicon Nitride properties 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 in between Si as well as N (of which the ion bond represent only 30%), so the Si3N4 has high firmness (Morse firmness 9), high melting point and steady framework.
The Si-N in silicon nitride crystal is mostly covalent bond, and the bonding strength is high, so it has a large flexible modulus (4.7 × & times; 105kg/cm2). The coefficient of thermal expansion is reduced, however the thermal conductivity is large, so this material is challenging to generate thermal tension, so it has excellent thermal shock resistance and also good thermal shock resistance. It has toughness, high mechanical toughness at heat and also little contortion at high temperature. (the high temperature deformation of silicon nitride ceramic with 2.5 g/cm3 thickness is 0.5% at 1200 × & times; 1000h ℃ and 23 × & times; 7kg/cm2 tons). Not deteriorated by numerous steels, as a result of the formation of a silicon dioxide layer, great oxidation resistance as well as good electric insulation.
Silicon nitride has no melting factor and also sublimates as well as breaks down at 1900 ℃ under air pressure. The certain heat is 711.8 J/ kg ℃. The microhardness of α & alpha; phase as well as & beta; stage is 10 ~ 16GPa and 24.5 ~ 32.65 Grade point average respectively. Since it is a strong covalent bond substance, no liquid stage will be formed listed below its decay temperature level (regarding 1900 ℃), so silicon nitride materials can be sintered with the aid of oxide ingredients. The oxide products that promote sintering are generally Y2O3, Al2O3, and so on, as well as the high addition quantity can reach 20%. The response concept is that the SiO2 oxide film based on the surface of silicon nitride fragments responds with the included oxide to form fluid stage and also permeate at the grain border to make certain high diffusion capacity throughout product migration.
Chemical Security of Silicon Nitride
Si3N4 is a thermodynamically secure compound. Silicon nitride porcelains can be consumed to 1400 ℃ in oxidation ambience and also 1850 ℃ in neutral or reducing ambience. In fact, the oxidation reaction of Si3N4 takes place above 800C:
Si3N4 +3 O2=3SiO2+N2.
The weight of the example enhanced and a thick silica safety layer was progressively based on the surface, which avoided the additional oxidation of Si3N4. The weight gain was not evident till the temperature level was over 1600 ℃. Yet in the damp environment, Si3N4 is really simple to oxidize, as well as the surface begins to oxidize at 200 ℃, which has to do with twice as quick as that in dry air. The oxidation activation energy of Si3N4 powder in water vapor is obviously less 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 steady to most metal solutions, devoid of rust and also seepage, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and more. Nonetheless, for Cu remedy, it is not deteriorated just in vacuum cleaner or inert ambience; Mg can respond weakly with Si3N4; silicon solution can wet Si3N4 and wear down slightly; shift element option can strongly wet Si3N4 and also type silicide with Si to quickly break down silicon nitride as well as escape N2 at the very same time. Si3N4 is stable to alloy solutions such as brass, hard light weight aluminum and nickel silver, and also has excellent deterioration resistance to cast iron and also tool carbon steel, but it is not resistant to Ni-Cr alloy as well as stainless-steel.
With the exception of molten NaOH as well as HF, silicon nitride has excellent chemical corrosion resistance. Yet a lot of the liquified antacid and also salt can engage with Si3N4 to disintegrate it.
Silicon Nitride made use of in Refractories.
Silicon nitride ceramics are called encouraging high temperature architectural products as a result of their excellent heat residential or commercial properties, such as high temperature strength, good wear resistance as well as deterioration resistance. Because of the solid covalent bond and also reduced diffusion coefficient at high temperature, the manufacture of Si3N4 porcelains need to rely upon heat, high pressure and sintering agent, and the expense is too expensive, otherwise it is difficult to generate premium quality silicon nitride products. These constraints of manufacturing prices and devices are hard to approve in the metallurgical market, so the study in the field of refractories begins fairly late, and also the research is not extensive. Many theories come from porcelains, yet there is very little technology. In the past, silicon nitride normally existed in the form of bonding stage in refractories. With nitriding and also shooting of steel Si, accumulations such as diamond or silicon carbide were combined with fine powder, so as to accomplish the function of combining refractory products. Ceramic shed plate is silicon carbide accumulation and component of fine powder. Silicon nitride is developed by nitriding steel Si to develop silicon nitride. Silicon carbide is combined to develop silicon nitride adhered silicon carbide material, which is used in blast heater body and other parts, to ensure that the performance of the product has actually been substantially improved. Compared to the clay adhered silicon carbide dropped plate, the heat efficiency of the product is great, which addresses the issue of protruding failure caused by the oxidation of silicon carbide when the clay adhered silicon carbide dropped plate is utilized.
Silicon nitride Rate
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