Ferro silicon nitride is a composite material of silicon nitride with iron and ferro silicon, usually from ferro silicon alloy FeSi75 directly nitriding at high temperature, of which the mass fraction of Si3N4 accounts for 75%~80%, the mass fraction of Fe accounts for 12%~17%, and the main phase is α-Si3N4 and β-Si3N4. In addition, there is some Fe3Si, a small amount of α-Fe and a very small amount of SiO2. As a new type of non-oxide refractory raw material, it has excellent sintering property and chemical stability, high refractoriness, low thermal expansion coefficient, excellent thermal shock resistance, high high temperature strength and thermal conductivity, and good erosion resistance and wear resistance.
The use of ferro silicon nitride
Ferro silicon nitride also has important uses in the gold industry. Because of its high hardness and high melting point, ferric silicon nitride is often used as a material for gold treatment molds. In the process of gold treatment, the metal liquid needs to be formed by the mold, while ordinary metal materials are easy to wear and deform under high temperature and pressure, and ferric silicon nitride can effectively extend the service life of the mold and improve the production efficiency of gold treatment because of its good wear resistance and high temperature resistance
As an important material, ferro silicon nitride has a wide range of applications in the electronics industry, energy industry and metallurgy industry. In the electronics industry, it can be used as a heat dissipation material to improve the reliability of electronic devices, and in the energy industry, it can be used as a high-temperature structural material to maintain the stable work of energy equipment; In the metallurgical industry, it can be used as a mold material to improve the production efficiency of gold treatment. With the continuous development of science and technology, it is believed that the application field of ferro silicon nitride will be further expanded.
Synthetic production methods are introduced
The main synthesis methods of ferro silicon nitride include direct nitriding, carbothermal reduction nitriding, vapor deposition, thermal decomposition, self-propagating high temperature combustion synthesis, flash combustion synthesis, microwave synthesis, ferro silicon melt nitriding and so on. The phase composition and structure of ferro silicon nitride are different with different synthesis methods and processes.
1, Flash combustion synthesis method
Flash combustion synthesis method, also known as vertical continuous combustion synthesis method, is a new process developed in the field of metal nitriding in recent years, with its synthesis of ferro silicon nitride does not contain free silicon, and is suitable for large-scale continuous production, production costs compared with other production methods are greatly reduced, only equivalent to other production methods of production costs 1/10~1/3. The process is to continuously add FeSi75 alloy raw materials with particle size of 74μm from the top of the flash furnace to 1400~1600℃ nitrogen (the volume fraction of 99.99%) furnace, FeSi75 alloy in high temperature nitrogen while falling flash combustion, the generated ferro silicon nitride by gravity into the product pool.
In addition to β-Si3N4 containing columnar crystals, α-Si3N4 and Fe3Si containing small round particles, the phase of ferric silicon nitride prepared by flash combustion synthesis method contains only a small amount of SiO2 and no Si2N2O. The length and diameter of a large number of columnar silicon nitride crystals are relatively high in the microstructure. The iron phase materials exist in the form of Fe3Si and α-Fe, and are distributed in the interior of the columnar Si3N4 crystals. The appearance structure is loose and the activity is strong. This process can continuously, large-scale and low-cost synthesis of ferro silicon nitride under low pressure (0.01~3MPa).
2, Self-propagating high temperature combustion synthesis method
Self-propagating (SHS), also known as combustion synthesis, is a new technology that utilizes the self-heating and self-conduction process of high chemical reaction heat between reactants to synthesize materials. In addition to β-Si3N4 and α-Si3N4, Si2N2O and FexSi are also contained in the phase of ferro silicon nitride prepared by self-spreading high temperature combustion synthesis, among which the content of incomplete nitriding FexSi is more. The microstructure is dense silicon nitride block, the surface of the block is covered with Si2N2O film, there are fine crystalline silicon nitride crystals in the gap of the block, and the iron-containing components are embedded in the dense block. The appearance structure is dense and the property is stable. This process has the advantages of low production cost and more suitable for industrial production, but its operation process is strict, the process is complex, the nitriding reaction is difficult to control, and there are high nitriding pressure, demanding equipment, difficult to continuous production and low output shortcomings.
3, Carbothermal reduction nitriding method
Carbothermic reduction nitriding is a kind of nitrogen reduction reaction with carbon as reducing agent. Chen Bo used quartz powder, iron ore powder and coke powder as raw materials to prepare ferro silicon nitride by carbothermic reduction nitride process. It was found that the best process conditions for preparing ferro silicon nitride by carbothermic reduction nitride are excess coke 50%(w) and held at 1450℃ for 3h. The main phases of ferro silicon nitride are β-Si3N4, β-SiC and Fe3Si. When the temperature is too high or the amount of carbon is greater than 50%(w), it is unfavorable to the preparation of ferro silicon nitride.
4, Direct nitriding method
Direct nitriding method is the direct use of ferro silicon powder to nitride reaction at high temperature, it is a traditional preparation method, with simple process, less equipment required, low reaction temperature, is a relatively simple and widely used in the nitriding reaction method of a test means.
5, Other synthesis methods
The preparation of ferro silicon nitride by the nitriding method of ferrosilicon melt is based on ferro silicon powder as raw material, liquefied at a certain temperature, then sprayed with high-pressure nitrogen, fully stirred the solution to boil, and the nitriding reaction is carried out quickly and efficiently, so as to obtain ferro silicon nitride. Han Junhua’s research found that in the nitriding process of ferrosilicon melt, the finer the particle size of ferro silicon, the greater the surface energy and the more intense the reaction, which will cause serious sintering phenomenon, resulting in porosity blockage and nitrogen diffusion difficulties. At the same time, the liquid phase at high temperature increases the densification rate of the sample, resulting in poor nitriding effect of ferro silicon powder. The coarser the particle size of ferro silicon powder, the nitriding reaction proceeds slowly, avoiding the sintering phenomenon, and the generated silicon nitride plays the role of crystal seed, which is more conducive to the nitriding reaction.
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