Synthesis And Properties Of Carbon Microtubes

For a new form of carbon, carbon microtube with a micro-sized diameter and superthin wall have unique physical and chemical properties of carbon nanotubes and graphene. However, comparing with their, the reports of carbon microtubes are very little, and up to now, there is no report on the property research of carbon microtubes, which greatly limit its researches and applications owing to lack of feasible and reliable synthetic method. So, the important factor is controllable and large-scale synthesis of high quality carbon microtubes. In the present work, large-scale, high-purity and well graphite carbon microtubes were successful synthesized by a novel gas pressure enhanced chemical vapor deposition (CVD) method using urea as a main raw material. We propoesd a growth model and explained the growth mechanism, and explored the physical and chemical properties of the carbon microtubes. The main results are as follows:Carbon microtubes with diameter of about 1μm, wall-thickness of 5²0 nm and length of centenmeters were synthesized by gas pressure enhanced CVD method using urea and ethylene glycol as raw materials in the absence of metal catalysts. The results show that diameters of caron microtubes can be tuned by gas pressures. The diameter is about 100 nm under the gas pressure of 0.2 MPa and increases with the gas pressure enhancement. When the gas pressure is 0.5 MPa, the diameter of carbon microtubes is about 1μm. When the gas pressure increases to 1.5 MPa, the diamter of carbon microtubes is about 1.5μm. The results also indicate that nitrogen-doped carbon microtubes were obtained using urea as only raw material at low temperature and the nitrogen content is descreased dramatically with the synthetic temperature increasing. The nitrogen content is 7.89% in carbon microtubes obtained at l000℃and decreased to 3.2% when the temperature increases to 1250℃, and the nitrogen content is 0 with the temperature increasing to 1450℃. We can also fabricate carbon microtube membrane with uniform thickenss and trasparent property and carbon microtube buckypaper with thichness of 80μm and good flexibility by controlling the mass of raw materials.TG-MS analsis indicates that urea was decomposed into CO at about 250℃. The carbon micro-crystalline was generated by the CO disproportionation reaction at 850℃. The nucleation and growth theory studies indicate that the nuclea were firstly formed at defects of the crucible and gradually grow into crystalline particles, which constructed graphene by their diffusion and movement. We proposed a Vapor-Solid (VS) growth mechanism and created a growth model basing on the structure of carbon microtubes. Moreover, the effects of gas pressure on diameter and yield of the carbon microtubes were explained.The density of carbon microtube is calculated to be 0.16⁰.18 g/cm3. The electrical and mechanical measurements show that the average electrical conductivity and modulus are (1.37±0.23)×10⁷ S/m and 0.53±0.16TPa respectively. The thermal property measurements indicate that the carbon microtube buckypaper have a thermal conductivity of 5.516 W/(m·K) along the surface direction and 0.033 W/(m·K) along the thickness direction. The field-emission properties of the carbon microtube buckypaper were measured, the results imply that the turn-on field is 1.35 Vμm^-1 and produce a current density of 1.8 mAcm-2 for the anode-sample distances of 200μm. The current density arrive to the maximum of 8.8 mAcm-2 when the electric field is 1.75 Vμm^-1. There is almost no degradation at a current of 1 mAcm-2 hold for 20 h, indicating that the carbon microtube buckypapers have stable field-emission properties. The tensile and four-probe tests show that the carbon microtube buckypapers have specific tensile and electrical conductivity of 20²8 MPa g^-1cm3 and 2.7×104 S/m respectivily. The electrochemical measurements indicate that the carbon microtubes obtained at 1000℃have charge capacity of 2374 mAh/g, discharge capacity of 479 mAh/g, the first coulombic efficiency of 24%. They also exhibit a good capacity retention of 534 mAh/g after 40 cycles and a higher efficiency than 90%. The capacity of the carbon microtubes is decreased and the efficiency is increased with synthetic temperature increasing. As for capacitor, the symmetry of cycle curve of the carbon microtubes is decreased with the scan speed increasing. The specific capacity of the carbon microtubes is 23.1 F/g at a constant current of 1 mA. The specific capacities and charge/discharge time are decreased with current increasing.Carbon microtubes/epoxy composites with thickness of 2 mm and the carbon microtube contents of 1.0 wt% and 1.5 wt% exhibit the strongest microwave adsorption, and the minimum reflectivity is -23.1 dB and -24 dB in the 2^-18 GHz, the corresponding bandwidth is 6.5 GHz and 2.4 GHz respectively. Therefore, the microwave adsorbing property is the best when the carbon microtube content is 1.0 wt%. The adsorbing mechanism is mainly based on the loss of conductivity. In addition, the microwave is also attenuated by interference cancellation and rayleigh scattering. The mechanical properies of composite with carbon microtube content of 1.0 wt% are the best, which tensile strength and elongation at break is 41.55MPa and 5.89% respectively. Comparing with pure epoxy resin, the tensile strength and elongation at break of the composite are increased by 66.2 % and 113.4 %. The density of the composite is caculated to be 1.06g/cm3 and meets the light absorbing material requirements.