Study On The Controllable Preparation And Properties Of Quasi-macroscopic Carbon Fibers

Currently,miniaturization of systems and parts have become a common pursuit for improving device performance as high as possible in limited assembly space of advanced equipment in many fields,such as electronic information,medical treatment,communication and so on.Complex metal micro-parts with highly specialization and custom design are needed more and more.MEMS technology are efficiently produced at very high volumes using large-scale semiconductor manufacturing techniques.However,these techniques are not viable for the manufacturing of specialized MEMS parts sized between micron and millimeter level.Hence it is urgent to solve the technical problems in complex metal micro-parts manufacturing field.Electron beam melting（EBM）technology can be used to fabricate complex metal parts,but cannot be used to fabricate complex metal micro-parts because it cannot achieve an electron convergence beam spot with small enough diameter and high enough energy density simultaneously.If the EBM electron gun is optimized into micro-beam electron gun,it is expected to develop the additive micro-manufacturing technology based on EBM technology.The core component of electron gun is filament,but the performance of existing filament material is difficult to be further improved,so in order to optimize the performance of electron gun,it is necessary to look for new filament materials.In this paper,carbon fibers（CF）with micrometer（10⁰～10¹μm）diameter and centimeter(10^-1～10¹cm)length is prepared by chemical vapor deposition（CVD）based on non-catalytic pyrolysis of methane.The growth mechanism,electrical properties,mechanical properties,thermal properties and electron emission characteristics of CF are analyzed and tested to prove that CF can be used as a new electron gun filament.The length of the grown carbon fiber has reached the macroscopic scale,but the diameter size is still microscopic,Therefore,the carbon fiber is defined as quasi-macroscopic carbon fiber（QMCF）in this paper.The main research work and conclusions are as follows:（1）Quasi-macroscopic carbon fiberby chemical vapor deposition（CVD）method based on non-catalytic pyrolysis of methane（nC-QMCF）.nC-QMCF of different lengths（5mm～10cm）and diameters（5μm～25μm）was controllable prepared by optimizing factors such as temperature,growing base,growing time and gas ratio.（2）The key factors for the growth of nC-QMCF were ascertained.The key lies in the use of physical space limiting effect,through the construction of activation points on the surface of the substrate,gaseous carbon is selectively enriched in the activation region and produces an effect similar to"root",which restricts the accumulation of carbon atoms in the quasi-0 dimensional region and induces the longitudinal growth of carbon atoms,thus starting the growth process of carbon fibers under non-catalytic conditions and getting rid of the randomness of carbon fiber growth.The success rate and yield of nC-QMCF are increased.The activation sites constructed by chemical etching method are more conducive to the preparation of nC-QMCF.（3）By observing and analyzing the morphology of nC-QMCF at the beginning,middle and end of growth,it is concluded that the growth process of nC-QMCF is as follows:gas phase carbon atoms-cluster carbon-carbon filament-carbon fiber.（4）The performance of nC-QMCF is tested.In mechanics,the elastic modulus of nC-QMCF can reach 1.15×10¹⁰N/m²;in terms of heat and electricity,it is found that nC-QMCF has a thermal-electrical coupling effect,when the temperature increases and the resistance decreases,the resistivity of nC-QMCF decreases with the increase of length-diameter ratio,the resistivity of the measured nC-QMCF reaches the minimum of 8×10^-6（?）·m,and nC-QMCF also has a strength-electrical coupling effect,when nC-QMCF with aspect ratio of 249 is compressed by 0.2%,the resistance decreases by0.1%,for every 0.2%stretch,the resistance increases by about 0.8%;in terms of electron emission characteristics,the opening field intensity reaches 0.285V/μm,the maximum emission current reaches 1.13mA,and the emission current density reaches1494A/cm².The research results of this paper are expected to create new micro-beam electron gun,which is of great significance in promoting the development of EBM technology to the direct manufacturing of metal micro-parts.