Microwave processing of glass matrix composites containing controlled isolated porosity

Porous glass matrix composites containing well-defined spherical porosity were fabricated employing microwave-assisted densification of powder compacts. The inverse thermal gradient typical of microwave heating was used to obtain a high concentration of spherical pores in the central region of the sample and a relatively dense outer shell. Pores in the central region were formed in the molten glass phase exploiting gas evolution and entrapment. The outer region, being at a lower temperature, was sintered by viscous flow. Minimal distortion of the part occurred. The diameter of the pores showed a wide size distribution, i.e. between ∼5 and ∼50 μm. In comparison to other methods described in the literature for the fabrication of hermetic porous materials, i.e. using hot-pressing, the present approach is advantageous due to high heating rates of microwave heating resulting in saving of time and energy. Moreover, parts of complex shape may be fabricated by this technique.