Additive-free macroporous glasses by thermal amorphization of self-foaming copper silicate

To foam a silica glass at high temperature, the melt needs to generate or trap a gas, with viscosity balance that allows bubble growth but prevent gas escape. These foaming glasses are generally prepared by the addition of gas-generating agents. Unlike this approach, this work reveals a crystalline copper silicate (CuSH–1Na) that has an inherent ability to foam upon heating without any gas-generating additives, swelling up to 271 ± 7% of its initial volume. Upon rising temperature, the crystalline particles  amorphize, melt, and merge into a porous solid with progressively decreasing bulk density (from 0.49 to 0.26 g/cm³) and expanding pore sizes (25–3692 µm). By controlling the melting time and temperature, the interconnectivity of the pores can be tuned from closed to opened cell structures. In situ thermal observation and backscattered scanning electron microscopy reveal that pore formation is initiated by coupled liquid–liquid and liquid–gas phase separation mechanisms. A critical comparison with other porous silicates subjected to melting suggests that the presence of structural water may serve as an indirect predictor of foaming behavior in crystalline porous metal silicates.