Preparation And Characterization Of Boron Nitride Fibers Via The Polymer-Derived Ceramics Method

With increasing of the ultra-high speed missle radomes'demand for materials with high heat resistance, load bearing and wave transparence, the existing wave transparent fibers reinforced ceramic matrix composites have been a major bottleneck regarding the development of technology for radome. Nitride ceramic fibers, possessing high temperature resistance, good ablation performance and excellent stability, are one of novel intensifiers for wave transparent ceramic matrix composites. Hence, the major objective of the present work is dealt with the basic research on the preparation and characterization of boron nitride (BN) fibers via the polymer-derived ceramics (PDCs) method.According to the requirement of polymeric precursor's structure for BN fibers, three soluble and fusible polyborazines were synthesized. These low cost synthetic routes were under mild conditions without extremely low temperature, facilitating the manipulation. These preceramic polymers were characterized by NMR,FTIR,XPS,TGA and elemental analysis.PTMB synthesized from boron trichloride and methylamine (MA) was composed of B3N3 hexagons, C-H, B-N, N-H and C-N bonds. PTMB was soluble in toluene and oxylene. The typical composition of PTMB was (wt%):B(24.5), C(32.8), N(29.9) and H(10.2) with a chemical formula of BC1.20N0.94H4.5。The ceramic yield of PTMB was improved by treating under NH3. The carbon in PTMB was removed by pyrolysis in NH3. The major weight loss occurred below 600℃and the transition from organics to mineral basically happened at 900℃. The molecular structure and low ceramic yield of PTMB were not suitable for the preparation of BN fibers, while hopefully, for other forms of BN. Turbostratic BN (t-BN) was obtained by pyrolysis PTMB in NH3 and then in Ar. The t-BN showed a weight loss of less than 3.0wt% at 900℃in air, exhibiting its good oxidation resistance.PTPiAB synthesized from 2,4,6-trichloroborazine (TCB) and iso-propylamine showed a higher ceramic yield in Ar than that in NH3. PTPiAB was composed of B3N3 hexagons, C-H, B-N, N-H and C-N bonds. PTPiAB was soluble in toluene and oxylene. The typical composition of PTPiAB was (wt%): B(12.4), N(35.8), C(37.2), H(9.3) with a chemical formula of BC2.7N2.2H8.1。The weight loss in NH3 mainly occurred under 800℃and the organic groups in PTPiAB almost disappeared at 800℃. The removal of carbon in NH3 was evident and 94.0 weight percent of carbon was removed at 600℃. The range for the crystallization was mainly below 1400℃. The sample acquired by pyrolysis of PTPiAB in NH3 at 1000 ℃and then in Ar at 1800℃showed the (002) interlayer spacing of 0.334nm, with a density of 2.03g·cm-3. At 900℃, the weight loss of the sample obtained at 1800℃was less than 0.3wt% in air and the oxidation resistance was improved with the increase of crystallization.PPAB was synthesized from n-propylamine (n-PA), MA and TCB. Through controlling the ratio of raw materials, temperature and holding time, the soluble and fusible PPAB can be synthesized. The softening point of PPAB was heightened by the increase of temperature and holding time. The appropriate condition for the synthesis of PPAB was: n-PA?MA?TCB=2?1?1 (molar ratio), 150~170℃(polymerization temperature), 5~7h (holding time). The softening point of the as-synthesized PPAB was 90~100℃. PPAB was composed of N-H,C-H,B-N and C-N bonds, accompanied by the B3N3 hexagons linked with -B-(NCH3)-B- groups. The typical composition of PPAB was (wt%): B (22.28), C(23.24), N(44.75) and H(7.78) with a chemical formula of BC0.94N1.55H3.75.The number average molecular weight of typical PPAB was 1002 (relative to PS standards) and the weight number average molecular weight was 1359 with a polydispersity index of 1.50. The PPAB was soluble in toluene and oxylene with a ceramic yield of 50wt% at 1000℃in Ar.The melt spinning test for three polymeric precursors showed that PPAB possessed best spinnability, adapting to the preparation of BN fibers. Besides, PTMB and PTPiAB could be used to fabricate porous BN and BN coatings.The research on the PPAB's stability in air showed: PPAB hydrolyzed in air easily and the hydrolyzation occurred badly with the increase of humidity, temperature and holding time. PPAB was insensitive to O2 in air and the hydrolyzation of PPAB in air was mainly due to H2O. Oxygen came into PPAB during the hydrolyzation process to engender B-O bonds, helping to the formation of insoluble and infusible structure of three-dimensional reticulation. The hydrolyzed PPAB could not be melt processed and the pyrolyzed sample in Ar was mainly BN and B2O3.The composition, structure and properties of BN derived from PTPiAB and PPAB were compared and the results showed that the two BN samples without carbon impurity both possessed nearly stoichiometric composition. BN derived from PTPiAB had a higher crystallinity and exhibited better oxidation resistance. This is due to that the structure of symmetric molecular derived polymeric precursor is close to that of hexagonal BN.The influence of softening point, spinning temperature and spinning pressure in the melt spinning process was studied. The optimum spinning condition was found to be the softening point of 93~112℃, the spinning temperature of 20~70℃higher than softening point and the spinning pressure of 0.4~0.6MPa. The corresponding PPAB fibers with a length of longer than 200m could be obtained.The curing process of PPAB fibers in NH3 was accompanied by the increase of gel content, weight loss and decrease of carbon content. The drop of carbon content in cured fibers was mainly due to removal of alkyl-units during curing, whereas independent of C-N bonds. The optimum curing condition was found to be the heating rate of 0.5℃/min, the temperature of 70~80℃and the holding time of 80min. The gel content of corresponding cured fibers was more than 90wt%.Pyrolysis of PPAB in N2 resulted in BN with carbon contaminant, whereas in NH3 led to the removal of carbon and obtainment of BN with nearly stoichiometric composition. The ceramic yield of PPAB in N2 was higher than that in NH3. The products acquired in NH3 possessed better crystallization, oxidation resistance and dielectric property than those in N2.The influence of temperature, NH3 concentration, holding time and heating rate during inorganic transition of cured PPAB fibers was researched. The result showed the inorganic transition of cured fibers was almost completed at 800℃. The removal of carbon could be accomplished in the mixed atmosphere of NH3/N2=1:1(vol). The optimum inorganic condition was found to be the NH3 concentration of 50vol%, the heating rate of 4℃/min, the temperature of 800℃and the holding time of 2h. The carbon content in the corresponding fibers was less than 0.5wt%.The high temperature treatment of inorganic fibers favored BN fibers'better properties. The influence of temperature in the crystallinity, density, tensile strength and oxidation resistance was studied. The results showed the density of BN fibers increased mainly from 600℃to 1200℃and that of BN fibers prepared by pyrolysis of cured PPAB fibers in NH3 at 800℃and then in Ar at 1800℃was 1.92g·cm-3. For the BN fibers obtained at 1800℃, the (002) interlayer spacing and the crystalline size were 0.337nm and 6.50nm, respectively. The BN fibers with a tensile strength of 850MPa showed a good oxidation resistance. Besides, the BN fibers possessed a dielectric constant of 3 and a dielectric loss of 10-3 at 10GHz.