Preparation And Properties Of Silicon Carbide Fibers And Their Composities

Due to the unique physical and chemical properties of silicon carbide materials,it has the properties of wear resistance,corrosion resistance,oxidation resistance,high elastic modulus,high hardness and high temperature strength,etc.,and has received extensive attention in recent years.Especially the one-dimensional silicon carbide fiber,because its two dimensions structure are in the nano-micro level,with its high thermal conductivity,high chemical stability,oxidation resistance,and special optoelectronics and other excellent properties,in nanoelectronic devices,photonic devices,Pressure sensors,energy storage and conversion and other related fields have important scientific and industrial applications.In this paper,a high-efficiency process for preparing silicon carbide fibers is achieved through theoretical design.Starting from the morphology synthesis process of one-dimensional silicon carbide nanomaterials,one-dimensional silicon carbide nanomaterials with a series of special morphologies are obtained,with the growth mechanism of the material structure changes during the process,and further study on the structure-effect relationship between the internal structure and performance of one-dimensional silicon carbide fiber materials.On this basis,an extensive study on the preparation mechanism and application of one-dimensional silicon carbide fiber composite materials is provided,which provides a more comprehensive theoretical and experimental basis for the application of silicon carbide one-dimensional nanomaterials and composite materials in practical fields.The research of the paper includes the preparation of one-dimensional silicon carbide fiber by chemical vapor deposition method,the silicon carbide fiber composite aerogel obtained by freeze-drying method,the solvothermal synthesis of thermally enhanced SA/porous carbon ball/SiC fiber composite phase change material and its applications of pressure sensors and energy storage are mainly:(1)In order to achieve larger-scale production of one-dimensional silicon carbide fibers,this paper uses silicon powder and one-dimensional carbon materials as the silicon source and carbon source,and powdered iron oxide as the catalyst.First,the elemental silicon core-shell structure material is prepared through a design process to effectively control the volatilization of vapor-phase silicon.Through the VLS(gas-liquid-solid)mechanism as a guide in Ar atmosphere,by adjusting different synthesis temperature,holding time,cooling rate,etc.The growth rule of SiC fiber,and related characteristics of silicon carbide one-dimensional nanomaterials with different structures was systematically studied.And by adjusting the reaction conditions,the growth rule of the SiC fiber is obtained as follows:the fiber output is affected by the raw material ratio and the temperature of the heat treatment Large;bamboo-like fibers with high defect structure are formed in large quantities at a high temperature of 1600?;the morphology of the fiber is related to the cooling rate.When the cooling rate is too fast,the active SiO gas in the atmosphere quickly condenses and will produce bead-like SiC fiber material.At the same time,the synthesis mechanism of one-dimensional silicon carbide fiber is systematically analyzed,and the synthesis process is explained from the perspective of thermodynamics and kinetics.(2)Based on the VLS mechanism,three kinds of SiC fiber with straight rod shape,bamboo shape and bead-like shape were prepared by chemical vapor deposition method,and the structural,composition and morphology of these three fibers were analyzed.Its related thermal,electrical and optical properties are obtained.When conducting the thermal conductivity test,the results obtained were 25.7 W m-1 K-1(straight rod shape),22.3 W m-1 K-1(bamboo-like shape),18.5 W m-1 K-1(bead-like shape).The overall excellent performance is attributed to the effective network structure with heat conduction.When conducting the conductivity test,the conductivity result was k=0.157 S·cm-1(straight rod shape),0.123 S·cm-1(bamboo-like shape),0.09 S·cm-1(bead-like shape),and the corresponding to the resistivity is p=6.37 ?·cm,8.13 ?·cm,11.12 ?·cm.It was found that the straight rod-shaped SiC fiber with perfect crystal form obtained the best conductivity.In addition,the real value of the dielectric constant of the bamboo-like SiC fiber is between 4.4 and 4.8,the imaginary part is between 1.3 and 1.5,and the loss angle is between about 0.275 and 0.325.Compared with the straight rod-shaped SiC fiber RL-7.0 dB and the beaded SiC fiber is RL-5.0 dB,and of the bamboo-like SiC fiber of the reflection loss can reach RL-12 dB with the minimum value,which has a certain absorbing ability.Finally,a preliminary test of its optical performance revealed that the fiber is greatly affected by its morphology,such as a large number of defects,such as twins or stacking faults,as well as thin layers of amorphous silica affect its photoluminescence performance.It is shown that the bamboo-like silicon carbide fiber has the best photoluminescence performance.The relevant research and analysis in the paper widens the potential application of one-dimensional SiC fibers in various fields of science.(3)Based on the excellent performance of one-dimensional silicon carbide fiber,the application in the composite material of resistance pressure sensor was explored.Making full use of the high modulus characteristics of one-dimensional silicon carbide fibers and organic fiber materials,a novel process is used to prepare high porosity and high specific surface silicon carbide aerogel composite materials,and finally the piezoresistive effect is achieved by loading organic conductive materials.The composite material provides a new solution in the direction of sensor miniaturization and light weight.In the test process,polyacrylonitrile(PAN)and silicon carbide fiber were used as precursors,and the silicon carbide fiber aerogel composite material was prepared through electrostatic spinning process and freeze-drying process.The composite aerogel material synthesized by this method has a certain degree of elastic properties.The porosity can be calculated to reach 99.24%.During the thermal modification,the sample shrinks moderately,and the porous structure remains unchanged.Under the condition of less than 300?,the gel material of PAN/SiC has almost no thermogravimetric loss,and has good pyrolysis resistance.This property can make the material have certain commercial value.By in-situ composite PANI material on the fiber surface in aerogel,a three-dimensional porous conductive composite material was prepared.After the polymerization of PANI,the mass increased by 400%,and the volume density remained low at 0.211 g/cm3.At the same time,the porosity of the prepared PAN/SiC/PANI composite is 76.44%,and its electrical conductivity is 0.013 S·m-1.Because the material has a special porous structure and conductive properties,it can be used as a good pressure sensing device.Under pressure test conditions,the resistance value is negatively correlated with the increase of pressure,based on the transient effect of the composite material and the good performance of micro-pressure in response time during the pressure-sensitive tests.(4)The phase change material(PCM)can store and release a large amount of latent heat during the phase change,and then can efficiently store energy and release latent heat,which is an important basic course in the process of storing latent heat energy.However,in the process of energy storage and energy release of phase change materials,the higher thermal impedance will affect the heat transfer efficiency.Compared with organic phase change materials,one-dimensional silicon carbide fibers have better thermal conductivity and have a better effect on improving the thermal conductivity of the system.The phase change energy storage carrier is in situ synthesized on the surface of the silicon carbide fiber,which further reduces the interface thermal resistance of the material and makes full use of the high specific surface characteristics of the nanomaterials to increase the effective load of the phase change carrier and achieve better heat transfer efficiency.In this paper,based on the high thermal conductivity of one-dimensional silicon carbide fiber,as a heat transfer medium,the porous carbon nanospheres is coated to obtain porous carbon sphere/SiC fiber composite materials by hydrothermal method,and using it as a carrier,The phase change material octadecyl alcohol is supported on the carrier by the adsorption method by physical process,and finally the octadecyl alcohol/porous carbon ball/SiC fiber composite phase change material is prepared.In the carrier material,the porous carbon with strong capillary force can absorb the stearyl alcohol into the nanopores of the porous carbon sphere,and the presence of the pores is conducive to the free crystallization of the core molecular chains material to obtain high energy storage efficiency,at the same time,the chemical bond and hydrogen bond between the interface of the core material octadecyl alcohol and the porous carbon sphere and the capillary force can play an effective shape setting function for the molten octadecyl alcohol in the pore channel.Silicon carbide fiber improves the thermal conductivity of the entire composite material,with an increase of 71%and 62%and the stearyl alcohol/porous carbon ball/SiC fiber composite phase change material still maintains excellent heat storage performance after 300 cycles.