Sorptionsinduzierte Verformung hierarchisch poröser Materialien

Translated title of the contribution: Sorption induced deformation of hierarchical porous materials

Roland Morak

Research output: ThesisDoctoral Thesis

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Abstract

Sorption induced deformations in porous materials, today, is not just a field of interest for scientific research but also important to industrial applications. So far only the porosity of the materials mattered for the use in catalytic or separation processes or in the field of lightweight construction. The usage of porous materials in sensors or actuators, however, requires the understanding of the mechanisms of the sorption induced deformation to be able to influence and control the process. The materials to be used for such applications have to have ordered pores at least on one hierarchical level. The silica material used in this thesis has hexagonally ordered mesopores which form tetragonally or star like linked struts connecting into a framework building a silica monolith. The free space between the struts forms the macropores and inside the mesopore walls micropores can be found. The goal of this thesis is to gain deeper understanding of the sorption induced deformation on the different hierarchical levels and how each level influences the whole process. Four different samples where used for this purpose. First sample (sample A) is a hierarchical porous silica monolith with organic residues still inside the micropores and at the mesopore walls. In the second sample (sample C) the organic residues have been eradicated to gain accessibility to the micropores. In the third sample (sample S) hardly any micropores are detectable and in the fourth sample (sample AA) the struts show a preferred orientation to the axial direction of the monolith and allow us to comment on the question, whether a preference in the orientation of the struts results in a preference in the orientation of the deformation. The structural characterisation of the samples was done by nitrogen and water sorption as well as with small angle X-ray scattering (SAXS). The results confirm the existence of organic residues in sample A, the accessibility of the micropores in sample C and the lack of micropores in sample S. A preference in the orientation of the struts was shown by SAXS measurements, however, further experiments could not confirm the hypothesized preference in the orientation of the deformation. The influence of the different pore hierarchies could only be detected by developing set ups for simultaneously in-situ dilatometry and small angle neutron scattering (SANS) experiments with a zero scattering length density adsorptive. In order to repeat those experiments with SAXS a whole sorption system set up had to be developed, constructed and implemented in a laboratory SAXS system. The results of the in-situ experiments show that the micropores have the largest impact on the sorption induced deformation of a hierarchical porous material. Organic residues inside those micropores can even enhance the effect and might give the opportunity to control the magnitude of the sorption induced deformation. The comparison of the in-situ SANS and dilatometry data features that the maximum strain of the fully filled sample is the same for both methods. This indicated that the radial deformation of the mesopores equals the macroscopic deformation of the whole monolith. In the area of capillary condensation, however, the deformations of both methods differ. Further findings show that it is possible to reconstruct the sorption isotherm from the SANS data to check the sorption isotherm measurements or to replace those. If the in-situ SAXS and in-situ SANS measurements are compared to each other, it can be shown that the maximum deformation for both experiments show different values. The reason for this can be found in a contrast variation of the SAXS measurements due to the sorption of the adsorbate, which leads to so called pseudo strains. These can influence the detected sorption induced deformation quite strongly. The comparison of the data allows an increase of knowledge due to the pseud
Translated title of the contributionSorption induced deformation of hierarchical porous materials
Original languageGerman
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Lichtenegger, Helga, Assessor B (external), External person
  • Paris, Oskar, Assessor A (internal)
Publication statusPublished - 2017

Bibliographical note

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Keywords

  • hierarchical porous Materials
  • sorption
  • sorption induce deformation
  • small angle X-ray scattering
  • small angle neutron scattering
  • dilatometry

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