In-situ low-temperature synthesis of mullite whiskers from MoO3-doped industrial silica-alumina waste and growth kinetics study

Abstract

This study employed industrial silica-alumina waste residue as the principal raw material, achieving low-temperature synthesis of mullite whiskers through Al(OH)₃ supplementation for silica-alumina ratio adjustment, with MoO₃ as the sole sintering aid. An innovative conceptual design for exhaust gases recovery system was developed. The critical controlling factors and kinetic mechanisms governing whisker growth during low-temperature sintering were systematically investigated. Combined characterization techniques of X-ray diffraction analysis and scanning electron microscopy were employed to comprehensively analyze phase evolution patterns and microstructural characteristics. Experimental optimization determined the optimal temperature range for whisker growth and the optimum MoO₃ doping concentration. A physical model for whiskers agglomeration growth was established with corresponding formation mechanisms elucidated. Based on crystal growth theory and Arrhenius equation, growth kinetic parameters along different crystallographic orientations were quantitatively characterized, with apparent activation energies calculated. Results demonstrated that MoO₃ doping content significantly influenced phase purity and crystal morphology, with excessive doping (>7 wt%) inhibiting mullite crystal growth. Notably, MoO₃ catalysis enabled mullite nucleation temperature reduction to 700 °C, with stable crystallization temperature range maintained at 800–850 °C. The doped system induced markedly anisotropic growth characteristics in mullite whiskers, resulting in a maximum whisker length of 12 μm and an aspect ratio of 11. At 850 °C, longitudinal growth activation energies measured 306.5, 184.8, and 229.5 kJ/mol for 4 wt%, 7 wt%, and 10 wt% MoO₃ doping levels respectively, while transverse activation energies correspondingly reached 490.6, 658.7, and 671.6 kJ/mol. The activation energy differences confirmed the intrinsic crystallographic orientation-selective growth characteristics, highlighting the dual advantages of this process in low-temperature energy conservation and solid waste resource utilization.