REPOSITORY > RESULTS

Abstract

Highly porous ceramic γ‒Al2O3 monoliths are important in many applications, e.g., catalysis, thermal insulation and adsorption, yet they still have a lot of unrecognized potential. There is a lot of incentive to employ simple and green fabrication processes for the preparation of such porous ceramic materials; however, many challenges, especially the loss of mechanical strength and rigidity due to the increase of porosity, remain unresolved.
The presented doctoral thesis was set to establish an unconventional but genuine fabrication approach of highly hierarchically macro-mesoporous alumina (HMMA) monoliths prepared from hierarchically assembled, mesoporous AlN-powder-hydrolysis-derived γ‒Al2O3 (MA) powder of high surface area (~180 m2/g) and mesoporosity (0.47 mL/g).
In the first part of the doctoral thesis, the objective was to prepare stable aqueous suspensions containing MA powder and to understand the suspensions’ electrokinetic, rheological and sedimentation properties. The evaluation of rheological and sedimentation behaviour of aqueous suspensions containing MA particles dispersed with sodium polyacrylate (NaPAA) showed their proneness to undesired sedimentation and segregation and were as such not suitable for further green body consolidation. Thus, two mechanisms of stabilization were tested in order to delay or completely prevent sedimentation and segregation. The addition of divalent cations (Mg2+, Ca2+) or cellulose nanofibers (CNF) triggered the formation of interparticle association networks in the suspensions via bridging flocculation. While the former only partially prevented sedimentation, in the latter case, long-term stability lasting more than 12 weeks was achieved.
In the second part of the thesis the focus was shifted to the consolidation of as-prepared suspensions into structurally stable and highly-porous MA ceramic monoliths (or foams) with hierarchically distributed pores, high specific surface area, high permeability and low thermal conductivity. For this purpose, freeze casting (FC) was employed as a simple but powerful technique commonly used for fabrication of highly porous, columnar monolithic materials. By unidirectional freezing of aqueous suspension containing NaPAA-dispersed MA powder and CNF hierarchically macro-mesoporous alumina monoliths were successfully prepared. As-prepared, freeze-cast monoliths possessed relatively high surface areas (91‒134 m2/g) and high hierarchical porosity (93.1‒99.2 %). Owing to the columnar porosity HMMA monoliths also exhibited high permeability (k1 =2.39‒4.31×10-12 m2 and k2 =2.23‒9.15×10-7 m) and low, anisotropic thermal conductivity ranging from 0.039 W/m∙K to 0.071 W/m∙K that depended on the pore orientation. Despite their high porosity, monoliths still displayed remarkable Young’s modulus and high compressive strengths (up to 52.0 kPa).
The bridging flocculation of suspensions with divalent cations did not provide sufficient stability after freeze casting. On the other hand, the CNF turned out to be a superior nanofiller not only in terms of making MA containing suspensions stable but also gave rise to remarkable compressive strengths and rigidness rarely seen in green bodies of such high porosity.
The results of the thesis also showed for the first time how freeze-casting of stable aqueous suspensions containing MA particles can be appropriate for fabrication of hierarchical and highly porous HMMA monoliths that are pivotal in advancing many prominent applications, such as Li-ion batteries separators and inductively heated catalyst reactors.