FROM MICRON-SIZED PARTICLES TO NANOPARTICLES AND NANOBELTS: STRUCTURAL NON-UNIFORMITY IN THE SYNTHESIS OF BORON CARBIDE BY CARBOTHERMAL REDUCTION REACTION

摘要

Boron carbide (B_4C) powders were synthesized by the carbothermal reduction (CTR) reaction of molecular-scale mixed boron trioxide (B_2O_3) and carbon. Low cost, water soluble boric acid and sucrose were used as the boron trioxide and carbon precursors, respectively, which were mixed in water and yielded uniform precursor mixture after 800 °C pyrolysis. CTR of pyrolyzed materials was carried out in an inert atmosphere for various holding times at temperatures ranging from 1250 to 1450 °C. The synthesized materials were characterized using X-ray diffraction (XRD) for phase purity and scanning electron microscopy (SEM) for microstructure. Contrary to expectation, significant non-uniformity in product micro-morphology was observed despite uniformly mixed starting materials: At lower temperature (e.g., 1250 °C), large micron sized polyhedral B_4C particles surrounded with B_4C nanowires and nanobelts were obtained on the surface of pyrolyzed material; while at higher temperature of 1450 °C, a vast distribution of micron to fine nano-sized particles to micron-sized platelets were obtained. The observation of such non-uniformity is briefly discussed based on the complex nature of the CTR reaction involving different liquid-solid and vapor-solid reaction pathways. The direction for future research to fully understand such phenomena and achieve better control of product microstructure is pointed out.