A hybrid sol-gel method was employed to develop a uniform and highly dispersed alumina nanopowder in the presence of hyperbranched dendritic poly(ethylene)imine (PEI) acting as template material and complexation agent for aluminium ions. For this purpose, the hydrolysis and polycondensation reactions followed the complexation reaction between the Al(NO3)3precursor and PEI, whereas ammonium polymethacrylate was added to improve the powder dispersion. The as-formed nanopowder was characterized before and after calcination studies carried out in the temperature range 100-1200 °C. For this purpose Scanning Electron Microscopy (SEM), Field Emission SEM, Transmission Electron Microscopy (TEM), X Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric and Differential Thermal Analysis (TG-DTA), N2porosimetry and ζ-potential measurements at different pH were carried out. The analysis confirmed the successful formation of a boehmite-PEI hybrid material of uniform tiny spheroid crystals (~ 1-2 nm) and small agglomerates. The boehmite phase is kept up to 300 °C, whereas after calcination at 600 °C a stabilized γ-alumina powder of high surface area and crystal sizes around 2-5 nm results. This phase is quite stable being kept even after calcination at 1000 °C. The transformation to the stable α-alumina phase is completed at 1100 °C leading to an easily dispersed nanopowder with crystal sizes ranging between 5-25 nm.
Synthesis of a hierarchical carbon fiber@cobalt ferrite@manganese dioxide composite and its application as a microwave absorber