Preparation And Properties Of Skin–core Structure Silicon Carbide Ceramic Fibers

Polycarbosilane（PCS）fibers were pulse treated by alternating air and vacuum atmosphere at thermal oxidation temperature（pulse thermal oxidation）.Skin-core structure silicon carbide（SiC）ceramic fibers were prepared by pyrolysis of the treated PCS fibers in inert atmosphere.The effect of pulse thermal oxidation temperature and alternating times on the oxygen element distribution,weight gain,chemical structure,Si—H reaction degree and gel fraction of the PCS fibers were investigated by SEM-EDS,oxidation weight gain,IR and gel fraction testing.Results showed that the oxygen content in the out surface and inner core in the PCS fibers increased gradually with increasing pulsed thermal oxidation temperature and times.SiC ceramic fibers which obtained by this method were a skin–core structure,and the oxygen content in the surface area was always higher than that in the core.The weight gain and Si—H reaction degree also showed a trend of increasing.Compared with the received PCS fibers,the heat treated PCS fibers formed Si-OH and C=O absorption peaks at 3680 cm^-1 and 1710 cm^-1,and the intensity increased with the temperature and times,while the Si-H absorption peak existed an opposite trend.The gel fraction appeared when the weight gain reached 7%,and increased with temperature and times.The oxygen element distribution,phase structure,tensile strength and microstructure of the sintered SiC fibers were investigated by SEM-EDS,XRD and single fiber strength tester.Results showed that the sintered fiber formedβ-SiC crystal structure,and had a similar oxygen distribution and skin-core structure as the treated PCS fibers.The tensile strength of the SiC fiber reached the maximum of 1.74±0.21 GPa when treated 50 times at185℃,and the micro structure was flawless.It’s a typical skin-core structure which showed an oxygen concentration gradient distribution.The high temperature properties of the skin-core structure SiC ceramic fibers were measured by SEM,IR,TG-DSC,XRD and tensile strength.It was found that the strength,micro structure and morphology retained stability after exposure at 1200℃for 1 h in air.In particular,the fibers retained a strength of 0.86±0.03 GPa and had a unbroken morphology after heat treated in N₂ at 1400℃,the strength retention rate is 49.43%.In addition,the XRD results showed that the growth ofβ-SiC andα-cristobalite crystals was an important reason for the reduction of tensile strength and increasing surface cracks.The SiC ceramic fibers formd SiO₂films in air and N₂,and the thickness in air was higher than that of N₂.The electrical resistivity and microwave absorbing properties of the skin-core structure SiC ceramic fibers were measured by high-insulation resistance tester and vector network analyzer.The resistivity of the ceramic fiber was8.60×10⁴Ω?cm,and the maximum reflection was-8.73 dB.It showed that the ceramic fibers had a certain microwave absorbing properties.Moreover,the strong acid and alkali corrosion resistance of the skin-core structure SiC ceramic fibers were examined by immersing in hydrofluoric（HF）acid and sodium hydroxide（NaOH）solutions.It showed that the fibers had excellent alkali corrosion resistance,and poor in HF acid.The Si-O structure in ceramic fibers was etched by HF acid and resulted in significantly increasing specific surface area.The maximum of the specific surface area was 9.76 m²/g.