In the modern world, where the development of new methodologies for the quantitative analysis of present day’s increasingly complex materials is absolutely necessary, the implementation of multiscale characterization approaches is essential to obtain their complete description and understand their properties at all scales of interest, between the macroscopic and the nanometre. For porous materials, the structural information obtained must be set up in the three directions of space, in order to extract quantitative information of porosity and to be able to optimize the characteristics of the porous network along with its connectivity to surface and transport properties. Alumina, a material which has numerous potential applications (such as its use as a support in heterogeneous catalysis in the field of refining, petrochemistry and environment) and whose specific properties of interest are diverse, exhibit multi-scale hierarchical porosity from nanometre to micrometre, inherited from the structure of boehmite nanocrystals aggregates.
The major objective of the project is to understand and determine the structural and textural relationships between the different hierarchical levels of boehmite, which are nanocrystals, aggregates, and agglomerates, through a multiscale and 3D characterization of the former hierarchical entities and the resulting porous medium. To do so, we have confronted three types of experimental techniques, namely high-resolution electron tomography, electron microscopy imaging on serial sections and finally X-ray microscopy on synchrotron. The implementation of this 3D multi-techniques approach required a specific preparation of the samples based on a laser cutting of the macroscopic blocks containing the boehmite grains impregnated in the resin, to obtain microscopic blocks of well-defined geometry allowing us to locate the representative zones of the sample and to facilitate the acquisition of the tomographic data. Once these microscopic blocks were analysed by X-ray microscopy, we cut them by ultramicrotomy for studies by other 3D imaging techniques, at smaller scales, with the objective of analysing an area previously imaged at a larger scale.
The implementation of a correlative tomographic approach on hierarchical porous materials and in such ranges of resolution and field of view has never been done before. We are in the process of exploiting all the acquired data, in order to be able to "nest" the 3D reconstructions recorded at different scales within each other, as well as the porosity information, with the final objective of obtaining a complete, multi-scale description of the internal structure of an alumina agglomerate.