*2.4. Characterization*

X-ray diffraction (XRD) studies were carried out with a PANalytical X'Pert-PRO diffractometer (Eindhoven, The Netherlands) at 2θ = 10–90◦ with Cu Kα radiation (λ = 0.15406 nm at 40 kV and 40 mA). The chemical bonding states were obtained by X-ray Photoelectron Spectroscopy (XPS, ESCALAB 250Xi) (Thermo Fisher Scientific, Waltham , Massachusetts, USA). The element analysis of silicon and boron was conducted using ICP-OES in a ThermoFisher iCAP6300 spectrometer (Waltham, MA, USA), and the nitrogen, carbon and oxygen contents were measured by a vario EL cube analyzer (elementar, Germany). Fiber morphologies were revealed by scanning electron microscopy (SEM) using a JSM-7001F system (JEOL, Tokyo, Japan). Element distribution along the fiber diameter was measured by an EPMA-1720 (Shimadzu, Japan). Single filament tensile properties were determined using an Instron5944 tensile tester (Norwood, MA, USA) with a gauge length of 25 mm, a load cell of 10 N, and a crosshead speed of 5 mm/min. The mean tensile strength of fibers was calculated based on 25 tested fibers using the Weibull statistic, and the Young's modulus of the fibers was evaluated from the strain-stress curves. The dielectric properties of ceramic fibers determined at 10 GHz were measured by the short-circuited wave guide method using an Agilent HP8722ES vector network analyzer (Santa Clara, CA, USA) at ambient temperature based on the Chinese National Standard GB/T 5597-1999.
