Research On The Fabrication Of Ultrafine Carbon Fibers And Its Thermal Properties

According to the high mechanic strength, low density, high temperature resistance and outstanding obscur properties to Infrared radiation of ultrafine carbon fiber (UHCf), UHCf reinforced aerogels are expected to possess high mechanic strength and excellent thermal insulating performance at high temperature, which is quite important for the development of space technology and military weapon technology.Polyacrylnitrile (PAN) was used as the raw material and dissolved in N,N-dimethylformamide(DMF) or dimethylsulfoxide(DMSO) to form a spinning solution. Ultrafine PAN fibers were fabricated via electrospinning and then stabilized and carbonized to form UHCfs, whose diameter was at the range of 220nm¹Î¼m and the ratio of fiber length to diameter was more than 5000. The as-received UHCfs were used as reinforcement to prepare SiO₂ aerogels composites. FT-IR, XRD, Raman and TGAA-DSC were used to study the influence of PAN concentration, supplied voltage, spinning distance, needle pin diameter, feedrate and ambient temperature on the morphology and fiber diameter, and the effect of temperature and heat rate on the morphology, microstructure and mechanic strength have also been researched. And finally a proper way was found to prepare UHCf. The thermal properties of UHCf mats and UHCf reinforced SiO₂ aerogels composites were studied primarily.The results show that the PAN concentration is the greatest factor to influence the PAN fiber diameter, then is the supplied voltage, spinning distance and the diameter of needle pin, while the effects of feedrate and ambient temperature on the fiber diameter are the lowest. The electrospinning process on the ultrafine PAN fiber is the PAN concentration around 13¹7wt%, supplied voltage is 15²5kV, spinning distance is 15²0cm, feedrate is 10²0μL/min, ambient temperature is 30⁴0℃and the diameter of needle pin is 0.8⁰.9mm.Before stabilizing, PAN fibers were pretreated by lay in the thermostatic oven at 50oC or dealt with ultrasonic in water. The results show that the pretreatment manner and inflict tension on the mats have small influence on the stabilization degree of PAN fiber while great influence on the mechanic strength of cured PAN fiber. The mechanic strength of PAN fiber pretreated at 50℃is 30% higher that of original PAN fiber. The mechanic strength of cured PAN fiber increases with the increasing tension. The highest strength of cured PAN fiber is 2.0cN/tex when the tension is 150N. The cyclization index is 91.8% at the temperature of 280℃. At the temperature of which the stabilization of PAN fiber was almost finished. The cured fiber possesses higher strength of 2.42cN/tex when the heating rate is 5℃/min. A much proper way to the stabilization of PAN fiber is that PAN fiber was pretreated by lay in the thermostatic oven at 50oC for 10h, stabilization temperature is 260²80℃, heating rate is 5℃/min, the tension is 100¹50N, and holding for 1h at the peak stabilization temperature. The degree of carbonization enhances with the sintering temperature increases. The degree of carbonization is the highest at the temperature of 1200℃, and at the same time C-N bond or C=N bond in the fiber are almost disappear, the RI value is 0.87 and the size of crystallite is 5.06nm. When the sintering temperature is higher than 1000℃, graphite crystallites are found in the UHCf.The specific extinction coefficient (SEC) of UHCf mats are all above 700m²/kg, which is much higher than conventional carbon fiber mats, basalt fiber, ultrafine high silicon and oxygen content fiber and industrial aluminum silicate fiber whose SEC are all lower than 100m2/kg. It suggests that UHCf mats have outstanding ability to obscur Infrared radiation. It is independent on the fiber diameter, the thermal conductivity of UHCf mats enhances when the test temperature increase from 25℃to 300℃. The thermal conductivities of as-received UHCf mats with fiber diameter is at the range of 220nm¹Î¼m is 0.012⁰.024W/m·K at 25℃and 0.054⁰.153W/m·K at 300℃, which is much lower than the thermal conductivities of conventional carbon fiber mats at 25℃(0.073W/m·K) and at 300℃( 0.520W/m·K), respectively.The thermal conductivity of UHCf reinforced SiO₂ (UHCf/SiO₂) aerogels increases with temperature increases at the range of 100³00℃. The thermal conductivity of UHCf with diameter is 1μm reinforced SiO₂ (UHCf-1000/SiO₂) aerogels is lower than that of carbon fiber with diameter is 373nm. UHCf-1000/SiO₂ aerogels possess the lowest thermal conductivity which is 0.012W/m·K at 100℃when the apparent density of fiber is 0.125g/cm3. Compared to ultrafine mineral fiber reinforced SiO₂ aerogels, UHCf-1000/SiO₂ aerogels possess higher thermal conductivity when the temperature is lower than 200℃and lower thermal conductivity when the temperature is higher than 200℃. That suggests ultrafine UHCf/SiO₂ aerogels express a more excellent thermal insulating performance at higher temperature.