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Schlagworte
X-ray refraction reveals the inner surface concentrations of porous materials within the range of (µm- to nm-dimensions. This technique is compared to mercury pressure porosimetry which is a standard method for pore size determination in porous solids. The mean pore size of several ceramics are investigated over a range of porosity between 30% to 60%. Complementary to high pressure mercury intrusion the refraction technique detects the spatially resolved inner surfaces of both, open and closed pores. Furthermore, X-ray refraction scanning topography visualizes integral interface properties up to 50 m spatial resolution by two-dimensional topographic images. Characteristic microstructure parameters, i. e. inner surface density, porosity and mean pore sizes of some Al2O3 and SiC ceramics are presented. The specific surface data of Al2O3 ceramics obtained from X-ray refraction agree well with the results given by mercury porosimetry. In contrast, the related mean pore sizes differs systematically by about 30%.
Porous and finely divided solids are distinguished by a large surface to volume ratio. As a result, they have specific properties, such as enhanced reactivity, a high adsorption capacity, and a lowered sinter temperature.
This contribution gives a survey over selected aspects of porous solids, with tlfe main emphasis on methods of characterization and their applications. We treat porous solids with a regular pore network (zeolites) and irregular pore structure (sintered compacts,rocks). After a short review of the possibilities orf characterization, we wilI describe the methods of gas adsorption and Hg intrusion in detail. Furthermore, we will give a critical assessment of the measurement of typical characterization parameters, such as the spedfic surface area, the pore volume and the pore volume distribution. The fractal character of large-surface and porous solids and the related uncertainty of the outcomes of measurements result in the necessity of standardization of experimental procedures and in the need for reference materials.
The section on applications treats examples of porous solids from different areas: densification of ceramic materials, porous ceramic carriers for flue gas desulfurization, technical catalysts (zeolite/binder), zeolites for the separation of substances, sedimentary and crystalline rocks.
The main problem in the certification of pressure-volume curves for the high-pressure range of the mercury intrusion is the fact that the uncertainty of the pressure (x-axis) should not be neglected. Additionally, the use of pressure-volume curves from different laboratories using different apparatus leads to incomparability of these curves and requires a correction. This is known in the statistical literature as warping. The use of point wise intervals causes
difficulties in the interpretation of the results. Such problems have been eliminated in the certification of pressure-volume curves of the new standards ERM®-FD123 and BAM-P124 to BAM-P126. These standards are reference materials for the comparatively low-pressure range of the mercury intrusion method.
The materials BAM-P124 and BAM-P126 are made up as flat membranes. The production of these membranes (a-alumina ceramics) was carried out in the HITK, Hermsdorf, Germany. The long ceramic tape was divided into 50 rectangular tapes. These tapes were sintered. Each of the tapes has been cut in 30 dies, numbered by a laser beam.
Samples were assigned systematically covering all 50 tapes to allow for an optimal modelling of material inhomogeneities.
The pore size analysis of solids is widely applied in chemical industries, materials engineering, ceramic production, environmental engineering, catalysis, chromatography, nanotechnology, and many other fields. In spite of several new methods used for determining the pore size distribution of meso- and macropores [see IUPAC Recommendations of 1994], mercury porosimetry has remained one of the most popular methods employed for the characterisation of porous materials. In this paper, a new way is described for the estimation of certified pressure-volume curves from experimental curves measured by different laboratories in connection with the certification of new reference materials for a comparatively low-pressure range of mercury intrusion (<2 MPa). Simultaneous confidence and prediction bands for the certified pressure-volume curves are constructed by bootstrapping.
Film formation and capillary condensation of nitrogen at 78 K on the mesoporous controlled pore glass CPG-10-75 have been studied at certain relative pressures by in situ small-angle neutron scattering. On desorption ramified clusters of vapor filled voids have been observed, but not on adsorption. The kinetics of adsorption and desorption have been followed. The experimental results are discussed with respect to recent theoretical studies of fluids in complex pore systems.
An evolutionary optimization technique is used to reconstruct digitized material models of 3003 nm3 size for mesoporous two-phase systems. The models are adapted to the two-point probability (TPP) and to a volume-based pore-size distribution (PSD) which were derived from SANS and adsorption experiments and which carry statistical information about morphology and topology of the pore system. To avoid extreme update-costs, the bulk of mutations are assessed by means of a suitable approximation of the PSD; it is demonstrated that a sporadic insertion of the PSD suffices to drive the algorithm towards satisfactory models in acceptable time. Our approach is knowledge-based in the sense that (i) the mutations are restricted to expedient exchanges of phase-voxels by a heuristic rule, and (ii) the sporadic calculation of the PSD from the current state of the model, in essence, provides an efficient self-control for the evolutionary process. We applied the method to reconstruct periodic models of the xerogel Gelsil 200. Such reconstructs of real mesoporous solids could be utilized, for instance, to verify theories of adsorption and capillary condensation.
Previous in situ small-angle neutron scattering studies of nitrogen adsorption and desorption at 78 K on the mesoporous silica xerogels Gelsil 50 and Gelsil 75 revealed the formation of ramified clusters of vapor-filled pores on desorption, which is characteristic for a percolation process. In the present work, we check whether the adsorption/desorption isotherm data for a monolithic sample of Gelsil 50 can be analyzed in terms of a bond-percolation model. Three powder samples were studied too. Percolation probability data are presented and the effects of heterogeneous nucleation, finite size, and surface clusters on drainage from Gelsil 50 are addressed. The mean coordination number was derived. The results of the analysis are discussed with respect to recent theoretical work for interactions of fluids with complex pore systems. The monolithic sample and a powder sample were characterized by small-angle neutron scattering data.