Porous Media Characterisation
Structural studies of sol-gel and related materials by SAS and diffraction
Dore and Webber
1) Pore size distribution functions
Sol-gel silicas have a complex structure on a mesoscopic scale and various experimental techniques may be used to determine the characteristics. One of the most direct methods is to use small-angle scattering of either x-rays or thermal neutrons. The scattering 'contrast' between different regions gives an intensity profile that is representative of the distribution of mass [or voids] within the material and since it relies on the interference of coherently scattered radiation there are no approximations in the formalism.
The SAXS or SANS pattern is readily measured using either reactor neutron or synchrotron x-ray sources; the count rates are large [for a 1 mm thick sample] and the SAS profile is usually measured in a period of a few minutes. The interpretation of the results is more complicated and requires modelling techniques in which polydispersity and fractal features may need to be incorporated into the fitting routine.
2) Experimental facilities
The UKC group uses various international facilities for studies of sol-gel silicas and other mesoporous materials. Recent neutron work has been conducted on the D22 instrument at the Institut Laue Langevin [ILL] and PAXE at the Orphee reactor, CEA, Saclay for SANS. X-ray work has been developing on the ID01 instrument at the European Synchrotron Radiation Facility [ESRF] for SAXS; all these facilities are in France and are supported by European or national funding resources. Beam-time is allocated on a competitive basis with applications submitted for two periods of six months. Frequently, the work is conducted as a collaboration with the instrument scientists and/or with groups from other laboratories.
3) Experimental techniques
i) Spatial range
The principles of small angle scattering are given in the literature and the details of the available instruments can be accessed through the websites of the central facilities. Essentially, the scattered intensity I(Q) is measured as a function of the scattering vector, Q [or k] using a multidetector to record the profile for an incident monochromatic beam; most SAS instruments cover a range of 10-3 to 0.1 Å-1, corresponding to a spatial range of 50 Å to 1µ. The intensity may vary over many decades in magnitude. Special techniques of ultra-low small-angle scattering [USAXS] may be used to study the profile at lower Q-values corresponding to larger distributions in real space.
ii) Confined liquids and contrast-matching
In many cases it is useful to study materials confined within the pores. If the pores are fully filled the contrast is modified and the intensity of the SAS signal is changed. In the case of SANS, scattering length density,, can be varied by selective H/D isotopic substitution to vary the contrast in a controlled manner. In some situations it is possible to contrast match the pore matrix with the liquid so that additional information on the filling can be studied. Another development is to study the time and spatial evolution of the I(Q,x,t) pattern through vapour phase condensation although few experiments have yet exploited this process.
iii) Other micro/mesoporous materials
The ordered microporous silicas, such as MCM41 and MCM48 are fabricated with a liquid crystal template and have channels within the range 25-35 Å. These powdered materials give a pattern which has Bragg peaks representing the ordered array, convoluted with a cylinder form-factor; the SAS pattern enables the geometrical features to be evaluated.
On a larger scale, the anodised mesoporous alumina membranes have a partially ordered array of cylindrical channels with sizes varying from 10 to 50 nm. In this case the scattering pattern is strongly dependent on the orientation of the membrane axis and this feature is currently under detailed study using both SAXS and SANS techniques to extract details of the pore characteristics.
4) Complementarity of SANS and SAXS
Comparison with other techniques :
The main advantages of SAXS is that the beam size can be very small [< 1mm] and consequently very small quantities of sample material can be studied. Furthermore, the contribution from incoherent scattering is negligible so that the pattern can be readily measured at the higher Q-values. The advantage of SANS is that the contrast can be varied and contrast-match conditions can be obtained which allows a more detailed study of composite systems. Larger quantities of material are needed as the beam size is typically 10mm square but this does offer the possibility of studying spatial variations within the sample volume.
Analysis of the intensity profile allows the pore size distribution function to be extracted from the data using appropriate models for the description. Good agreement is obtained with the results from NMR cryoporimetry for sol-gel silicas in the region >50 Å but there are deviations at lower vales which are currently under investigation; comparisons with gas phase data are also being made. It should be emphasised that the different methods yield pore distribution results that are effectively based on different assumptions and there is no guarantee that the results will be the same. The SAS techniques are sensitive to pore shapes and may indicate that the assumptions of spheres or slits made in the modelling of the pore structure are incorrect! It is anticipated that a comprehensive analysis of the data from the varying techniques can give a much more precise description of the complexities of the pore structure and this approach will require a more detailed consideration of systematic errors in the analysis procedures.
5) Further information
The group have been involved with SAS techniques for some time and several publications have been produced. An invited review of synchrotron-based SAXS has been written and current work on recent ESRF developments is in preparation. The SANS studies have been partly written up in individual papers but recent studies have not yet been published. Much of the work is contained in thesis presentation and the most recent work on the sol-gel system is primarily contained in the
of Beau Webber which incorporates both SANS and NMR cryoporimetry.
Related publications are in preparation.
Porous Media Characterisation
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