Alumina aerogels with unidirectional channels under different freezing temperatures during freeze casting—Part I: Control and analysis of pore channels

Unidirectional alumina aerogels (AAs) were prepared by simultaneously using boehmite hydrosols and alumina nano-powder as solid solutes at different freezing temperatures. In order to understand the porous structural characteristics under different freezing parameters, the real-time frozen parameters were recorded at the designated positions under different freezing temperatures. Fourier transform infrared (FT-IR) spectra, X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were used to investigate the functional groups, phase composites, microstructures and porous characteristics. The regularities and influence factors of unidirectional porous structures of AAs were detailedly explored under different frozen parameters. The results show that the AAs are characterized with unidirectional porous structures at the direction of ice-crystal growth. The cooling rate and ice front rate ( v inf ) increase and the structure wavelengths ( λ s) decrease with the decrease of the freezing temperature. The relationship between λ and v inf approximately satisfies a power law, which perhaps is related to the uniform dilute solution containing nanoparticle .