Preparation of closed-cell Mg foams using SiO2-coated CaCO3 as blowing agent in atmosphere

In melt foaming method, the thermal stability and foaming speed of blowing agent significantly affect the pore structure, pore size and porosity of metal foams. To retard the foaming speed and increase thermal stability, Na2O·nSiO2 and dilute hydrochloric acid were used to coat SiO2 passive layer on the surface of CaCO3 which is the blowing agent of Mg foams. Thermal stability and microstructure of the SiO2 passive layer were studied by thermo gravimetric analyzer-differential thermal analysis (TGA-DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the thermal stability of coated CaCO3 is improved. The SiO2 layer is amorphous with reticular structure. Contrasting experiments reveal that the coated CaCO3 can foam placidly. Moreover, large size and homogeneous Mg foams were prepared without protective gas with the ignition- proof by alloying. And the porosity of the Mg foams is 60%-70%.