Silicate-CoNi-carbon triple shell sandwich structured composite hollow microspheres with low density boosted microwave absorption and high mechanical strength

Low density (ca. 0.69 g cm(-3)) sandwich structured silicate-CoNi-carbon triple shell composite hollow microspheres (CHMs) were prepared by a stepwise three-stage shell-assembly route. Hollow microspheres with robust silicate shells were fabricated firstly through spray drying-high temperature melting, which were subsequently employed as a support for the in situ formation and assembly of CoNi shells. Then the CoNi shells underwent a double-interface co-constricted crystallization during calcination-induced carbonization to form the final product. The density of the hollow microspheres increases gradually with the stepwise assembly of the CoNi and the carbon shells, while the broken ratio of the hollow microspheres at the same isostatic pressure varies in an opposite trend. Moreover, the enriched interfaces with distinguished conductivity, multiple electromagnetic dissipation mechanisms and suitable impedance matching favour the triple shell CHMs with excellent electromagnetic absorption properties. The minimum reflection loss value can reach -55.4 dB, and a broad effective frequency range of 6.8 GHz can be achieved. Moreover, when used as lightweight functional fillers, the as-obtained polymer-triple shell CHMs composite exhibits a low density of 0.92 g cm(-3) and high compression strength of 132 +/- 2.1 MPa.