Silicon Nitride structures and properties
Silicon Nitride HTML3N4 contains two types of crystal structures: b–Si3N4 and . Each of these three-dimensional networks are composed of common verices of [SN4] Tetrahedron. They both belong to the hexagonal system. Their differences lie in the order and number of [SiN4] trihedral layers. The hexagonal ring layer of [SiN4] tetrahedrons arranged in a c-axis orientation, forming the b-phase. In contrast to the two layers of tangible transform and non-hexagonal rings layers that form the phase, the a-phase is composed of two layers of tangible conversion. A phase can dissolve oxygen from a variety of crystal structures. Also, the internal strain for a phase is stronger than that of the b-phase, which means that the free energy of a Phase is lower than that of the B phase. According to thermodynamics the stability of the b-phase is greater at higher temperatures. Because of its low symmetry, the aphase is very easy to form. At 1500°C, the aphase undergoes reconstruction and becomes the b phase. It is impossible to reverse this transformation, so it is important to have certain process conditions. When the temperature drops below 1350, a-Si3N4 will form. However, bSi3N4 can easily be made at higher temperatures than 1500.
Silicon Nitride properties
Si3N4 describes the molecular structure of silicon nitride. Si is responsible for 60.06% while dint N makes up for 39.94%. Si3N4 is strong because of the covalent bonds between N and Si (though the ion bond only accounts for 30%). This gives it high hardness (9 More hardness 9), high melting points, and stable structure.
Si-N crystals of silicon nitride are mainly composed of covalent bonds. Because the bonding strength and bonding strength are highs, they have a large elastic module (4.7x105kg/cm2). Although the coefficient of thermal extension is very low, it is high in thermal conductivity. It is therefore difficult to generate thermal stress. The material has excellent thermal shock resistance as well as good thermal shock resistant. The material has great toughness and high mechanical strength even at elevated temperatures. It also shows little deformation when heated up. At 1200 x 1000h the silicon nitride calcimic ceramic has a 2.5g/cm3 dense and is deformed at high temperatures of 0.5%. This also applies to 23 x 7.kg/cm2 (load). It is resistant to oxidation, and provides good insulation.
Silicon nitride does not melt and is sublimated and decomposed at 1800 under atmospheric pressure. The specific heat of silicon nitride is 7111.8J/kg. A phase's microhardness is 1016GPa while the phase in b is 24.532.65GPa. The strong covalent bonds compound means that no liquid phase is formed below the temperature at which it was decomposed (around 1900). Silicon nitride materials are therefore able to be sintered by using oxide additives. Al2O3, SiO2, and Y2O3 are some of the oxide materials that encourage sintering. A high level of additive can even reach 20%. It is a reaction process where the SiO2 oxide layer on the silicon nitride particle's surface reacts with the additional oxide, forming a liquid phase. The grain boundary permeates the liquid phase to allow for good diffusion.
Chemical stability of Silicon Nitride
Si3N4 can be used as a thermodynamically stable material. Silicon nitride ceramics may be used as high as 1400 degrees in an oxygen atmosphere, and up to 1850 in a neutral or reducing environment. Si3N4's oxidation reaction occurs at temperatures above 800C.
A dense layer of silica protection was slowly formed over the sample, which stopped further oxidation. The temperature reached above 1600, and weight gains were not evident. In humid environments, Si3N4 is much more difficult to oxidize. Surface oxidization begins at 200 and is twice as rapid as in dry atmosphere. Si3N4 in water vapour has an oxidation activation energetic that is lower than the one in oxygen or air. This is because Si3N4 can be reacted with water vapor through SiO2 films.
Silicon nitride does not react to corrosion. Cu solution cannot be eroded by vacuum, inert atmosphere, or Mg. Silicon solution can weakly react with Si3N4 while silicon solution can wet Si3N4 causing it to erode slightly. The transition element solution can strongly wet Si3N4 forming silicide with Si, which can quickly dissolve silicon nitride, and also escape N2. While Si3N4 can withstand alloy solutions like brass, aluminum, and nickel silver it does not corrode well to stainless steel or Ni-Cr.
Other than molten NaOH, HF and Si3N4, silicon nitride exhibits good resistance to chemical corrosion. Si3N4 is able to interact with most alkali, salt, and molten acids that can decompose the silicon nitride.
Silicon Nitride for Refractories.
High temperature ceramics made of silicon nitride are known for their promise as promising materials. They have excellent properties at high temperatures, including high heat strength, wear resistance, and resistance to corrosion. The strong covalent bond at high temperatures and low diffusion coefficient make Si3N4 ceramics difficult to manufacture. The limitations of equipment and production costs are not easily accepted by the metallurgical sector. This means that research into refractories is often late in its development and does not go deep. While there are many theories based on ceramics, not much new research is available. In the past, silicon Nitride was found as a bonding component in refractories. Fine powder was combined with corundum and silicon carbide aggregates by nitriding or firing metal Si. This allowed them to combine refractory materials. Part of the fine powder and silicon carbide aggregate ceramic shed plate are made from fine powder. The nitriding of Si metal to create silicon nitride forms silicon nitride. Combining silicon carbide with silicon nitride is what results in silicon nitride-bonded silicon carbide material. This material can be used for blast furnace bodies and other applications. The material's performance has been significantly improved. It is much more stable than clay-bonded silicon caride shed plate. The high temperature performance solves bulging problems caused by the oxidation.
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