Phase transformation and morphology evolution of sepiolite fibers during thermal treatment

In this study, the phase transformation and micromorphology evolution of sepiolite fibers were studied along with the increase of temperature from 300 to 1300 °C, using raw sepiolite samples as raw materials. Through characterization by scanning electron microscope (SEM), high resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), thermo-gravimetric analysis (TGA/DTG) and differential scanning calorimeter (DSC), it was found that the determined transition temperature values for the dehydrations of the hygroscopic water and the zeolitic water were 120 °C and 340 °C, and the values for the dehydration of the bound water and the dehydroxylation of the hydroxyls were about 500 °C and 810 °C. Moreover, orthoenstatite (MgSiO3) was recrystallized from dehydroxylated phase at 852 °C and then transformed into protoenstatite at 970 °C, and enstatite crystals grew along with c-axis of the fiber with the increase of temperature. Amorphous phase as the other phase of sepiolite dehydroxylation recrystallized into cristobalite in the temperature range of 1130–1200 °C and the samples began to melt at the above temperature range. In terms of morphology, both sepiolite bundles and laths remained constant after heating up to 1000 °C and sintering phenomenon appeared when the temperature was up to 1100 °C. The bundles partly melted and adhered each other, which further formed a porous structure after calcination at 1200 °C. When the temperature was up to 1300 °C, the sample substantially melted except for a small amount of enstatite and cristobalite crystals. The present study reflects the thermal reaction of sepiolite, which could be applied in exploring the reinforcement mechanism of sepiolite reinforced ceramic body composites.