Research On Key Technology Of Nanomaterials Synthesis By Atmospheric Pressure Microwave Plasma

Microwave plasma chemical vapor deposition(MPCVD),as a new type of nanomaterials synthesis technology,is widely used to synthesize diamond,graphene,carbon nanotubes,ceramics,and so on.Compared with the thermal chemical vapor deposition technology,the high-energy electrons in microwave plasma can effectively promote the chemical bond breaking and recombination of the reaction gas to generate highly active chemical groups.The plasma can decompose the gas compounds more effectively than thermal to realize the rapid growth of nanomaterials.The production of atmospheric pressure microwave plasma torches is complex because it relates to electromagnetics,hydrodynamics,chemistry,and molecular dynamics.For now,low-pressure microwave plasma technology is mainly used in the synthesis of nanomaterials by microwave plasma.There is no microwave-plasma nanomaterial synthesis system working at atmospheric pressure or above.Low-pressure microwave plasma generally works in a low-pressure environment below 10 KPA.Because of its low ion energy,plasma density,and gas temperature,the efficiency of cracking gas molecules is much lower than that of atmospheric pressure microwave plasma.Due to the influence of plasma volume and uniformity,it is challenging to achieve large-area uniform growth of nanomaterials,which limits the application of microwave plasma nanomaterials technology.For now,the most commonly used microwave plasma for nanomaterials synthesis is MPCVD,and its plasma is generally low-pressure microwave plasma.The plasma generation area maintains in a low-pressure environment below 10 k Pa,and the plasma suspends in the reaction chamber.The direct synthesis of nanomaterials by atmospheric pressure microwave plasma is similar to the combustion method,which uses an atmospheric pressure high-power microwave plasma torch with high temperature and electron density to decompose the precursor and catalyst.It then reacts in the exhaust gas of the plasma to synthesize nanomaterials.Compared with low-pressure plasma,atmospheric pressure microwave plasma has higher ion energy,higher plasma density,and higher gas temperature.The gas decomposes of an atmospheric pressure microwave plasma are stronger and can directly split gas compounds into atoms,with research and application value in nanomaterial synthesis.Therefore,the main research content of this dissertation is to design an atmospheric-pressure microwave-plasma nanomaterial synthesis system for the growth of high-quality nanomaterials.In this dissertation,an atmospheric pressure microwave-plasma nanomaterial synthesis system suitable for high-quality nanomaterial growth was designed by combining atmospheric pressure microwave plasma torch and traditional hightemperature tubular furnace.Aiming at the technical problems of large volume,difficult control,and high integration of atmospheric microwave plasma torch generator,a miniaturized atmospheric microwave plasma torch generator system suitable for the synthesis of nanomaterials was designed by studying the structure of atmospheric microwave plasma torch generator,the influence of microwave energy and gas flow on the plasma torch,and the efficiency of microwave plasma torch was improved.Through the simulation and experimental study of plasma under different air inlet conditions,the double-layer gas film plasma torch stabilization technology is proposed,which solves the critical problems of poor stability and carrying capacity of the atmospheric microwave plasma torch,and provides technical support for the synthesis of atmospheric microwave plasma torch nanomaterials.The pyrolysis ability of atmospheric pressure microwave plasma torch to gaseous precursor was studied by spectral diagnosis and gas chromatography analysis.Which proved that the pyrolysis rate of atmospheric pressure microwave plasma torch with 200 W power to gaseous precursor was more than 99%,and the pyrolysis products were atoms and diatomic molecules,which could be directly used for the growth of nanomaterials.Compared with the thermochemical vapor deposition,the precursor utilization efficiency of atmospheric pressure microwave-plasma nanomaterial synthesis system has been dramatically improved.Finally,MWCNTs were synthesized on stainless steel and commercial aluminum foil substrates by atmospheric pressure microwave plasma chemical vapor deposition system,which fully proves the practicability of this technology in the synthesis of nanomaterials.The main innovations of this dissertation are as follows:(1)The synthesis technology of microwave plasma nanomaterials at atmospheric pressure is realized.Because of the technical problems of large volume,difficult control,and integration of atmospheric microwave plasma torch generator,a coaxial atmospheric microwave plasma generator suitable for nanomaterials synthesis is designed.The synthesis system of microwave plasma nanomaterials at atmospheric pressure is realized by combining with a traditional high-temperature tube furnace.The high molecular cracking capacity of atmospheric microwave plasma torch can realize the high efficient cracking of precursor in material synthesis.The high active chemical groups produced are reformed as the target nanomaterials in the high-temperature tubular furnace,which significantly improves the precursor's utilization rate and growth rate.(2)A new synthesis method of carbon nanomaterials based on atmospheric-pressure microwave-plasma torch technology is established.Carbon nanotubes were synthesized on stainless steel and commercial aluminum foil substrates using the atmosphericpressure microwave-plasma nanomaterial synthesis system and ethanol vapor as a carbon source.By comparing the cracking effect of the atmospheric microwave plasma torch and high temperature on ethanol steam,combined with the spectral measurement of the atmospheric microwave plasma torch,it is proved that the 200 W atmospheric pressure microwave plasma torch can completely break ethanol steam into active chemical groups such as C2 and CH.These functional groups can be synthesized under the appropriate substrate and temperature conditions.(3)The study of the mechanism of the two-layer gas film and plasma torch based on hydrodynamics,the bottleneck of atmospheric microwave plasma torch technology is broken through,and the stable technology of atmospheric pressure microwave plasma torch is realized.Because of the problem that the atmospheric pressure microwave plasma torch is very susceptible to the influence of the stability of the airflow during the operation,a method of shielding gas is proposed by the fluid dynamics simulation and the verification experiment of the miniaturized coaxial microwave plasma torch.The problem is that plasma can not maintain torch shape when the plasma generator is connected to the tube furnace.The stability of the plasma torch is effectively improved,which provides strong technical support for the combination of the atmospheric microwave plasma torch and high-temperature tube furnace.