Microstructural evolution and sintering properties of palygorskite nanofibers

In this study, the morphological evolution and sintering properties of the palygorskite nanofibers were studied along with the increase of temperature, using raw palygorskite as materials. The palygorskite powder was calcined at different temperatures in the range of 100 degrees C-1200 degrees C, and the microstructural evolution of the palygorskite nanofibers was investigated by thermogravimetric and differential thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscope (HRTEM). Furthermore, the palygorskite powder was shaped to bars by dry pressing and sintered from 700 degrees C to 1200 degrees C. The properties of the sintered palygorskite were characterized by bending strength, mercury intrusion porosimeter (MIP), and stepwise isothermal dilatometry (SID). The results showed that the morphology of palygorskite nanofibers maintained unchanged till 1000 degrees C. The palygorskite nanofibers molted to bind each other and formed a solid interwoven network structure at 1100 degrees C. Correspondingly, it was shown from the sharply decrease of the sintered palygorskite porosity from 45.46% at 1000 degrees C to 1.82% at 1100 degrees C that the dense sintering of palygorskite started at 1100 degrees C. With the sintering proceeding, some closed micropores fused each other to form bigger opening pores, resulting in a slight increase of porosity at 1200 degrees C. However, the pore size distribution got more uniform and the density of the sintered body increased. So the bending strength of the sintered body reached the maximum of 176.67 Mpa and finally the main crystalline phases of the sintered sample changed to quartz, enstatite, and kyanite. The sintering activation energy of the palygorskite was measured by means of SID with a value of 906.46 kJ center dot mol(-1).