Molten salt synthesis of mullite whiskers entirely derived from fly ash for electronic packaging toughening ceramic applications

Preparation of mullite from high-alumina coal fly ash (CFA) has become one of the most important ways for high-value utilization. However, low Al2O3/SiO2 ratio, poor reaction activity and various impurities in fly ash led to poor quality of mullite products. Conventional sintering method requires addition of aluminum sources and sintering additives, and high synthesis temperature. This paper proposes a new technical route for preparation of mullite whiskers entirely from fly ash by molten salt method based on additive activation-acid leaching process. The growth process and synthesis mechanism of mullite whiskers in Na2SO4 molten salt from fly ash were systematically investigated, and the application of fly ash-derived mullite whiskers in electronic packaging toughening ceramic was also explored. The results show that leaching liquor containing Al3+, Na+, and SO42−, and amorphous silica slag can be separated and obtained after acid leaching of activated fly ash at 80 °C, and the fly ash-derived mullite whiskers with a length of about 1 μm can be synthesized from the mixture of crystalline products and silica slag by the proposed method under the optimal synthesis conditions. Using amorphous silica slag with higher dissolution activity as the silicon source, the nucleation of mullite whiskers with homogeneous nucleation process is promoted and highly monodispersed mullite whiskers derived from fly ash form. The fly ash-derived mullite whiskers can improve the mechanical strength, reduce the dielectric constant and dielectric loss of the composite ceramic substrates, and the dielectric constant and dielectric loss of the composite ceramic are the lowest 5.16 and 0.006, respectively, when the mullite addition content is 20 wt.%, indicating that they could be used as electronic packaging materials. This paper provides a novel pathway for synthesis of mullite whiskers and high-value utilization of fly ash.