Beamline BL16B1 at Shanghai Synchrotron Radiation Facility(SSRF) is dedicated to studying the microstructure and dynamic processes of polymers, nanomaterials, mesoporous materials, colloids, liquid crystals,metal materials, etc. At present, SAXS, wide angle X-ray scattering(WAXS), simultaneous SAXS/WAXS,grazing incident SAXS, and anomalous SAXS techniques are available for end user to conduct diverse experiments at this beamline. The sample-to-detector distance is adjustable from 0.2 m to 5 m. The practicable q-range is 0.03–3.6 nm-1at incident X-ray of 10 ke V for conventional SAXS whilst a continuous q-region of0.06–33 nm-1can be achieved in simultaneous SAXS/WAXS mode. Time-resolved SAXS measurements in sub-second level was achieved by the beamline upgrating in 2013. This paper gives detailed descriptions about the status, performance and applications of the SAXS beamline.
Elongated microvoids, internal fibrillar structure, and edge scattering from both surface refraction cause an equatorial streak in small angle X-ray scattering. A model for analyzing the edge scattering of fibers is proposed. Simulation results indicate that the intensity of edge scattering from surface refraction of a cylindrical fiber is strong and makes an important contribution to the equatorial streak. Two factors influence edge scattering intensity. One is the sample-to-detector distance （D）; edge scattering intensity increases with increasing D. The equatorial streak becomes weak when D is shortened. The other factor is the refraction index. Edge scattering intensity increases as the real component of the refraction index decreases. In experiment, weak or even no equatorial streaks were found for samples measured in a roughly index-matching fluid. Edge scattering can be eliminated or weakened, and it can be calculated by comparing the intensities of a cylindrical fiber when it is measured in air and in index-matching fluid. The simulation data are basically in agreement with the experimental data.
Structure of PAN fibers during pre-oxidation and carbonization was studied using two dimensional small angle X-ray scattering/wide angle X-ray diffraction （2D SAXS/WAXD）. The SAXS results show that during pre-oxidation between 180 ℃ and 275 ℃, the volume content of microvoids increases with the temperature increasing, which may be one of reasons for the decrease of tensile strength ofpre-oxidized fibers. 253 ℃ was the critical transition temperature, the length, diameter, aspect ratio and orientation distribution of microvoids increased with temperature before this temperature and decreased after this temperature. After the high temperature carbonization, lots of spindly microvoids formed. WAXD patterns demonstrate that the crystallite size of PAN fibers first increased before 230 ℃ and then decreased with the increase of temperature during the pre-oxidation. The diffraction peak of PAN fibers at 2θ ≈ 17°almost disappeared at the end of pre- oxidation while the diffraction peak of aromatic structure at 2θ ≈ 25° appeared at 253 ℃. During carbonization, the peak intensity at 2θ ≈ 25° increased apparently due to the formation of graphite structure. The results obtained give a deep understanding of the microstrncture development in the PAN fibers during pre-oxidation and carbonization, which is important for the preparation of high performance carbon fibers.
