Nano- to Micro-Scale Volumetric Metrology
Dr. J.B.W. Webber.
Nano-scale to micro-scale volumetric metrology :
Lab-Tools performs contract analyses of Pore Size Distributions and Pore Volume using NMR Cryoporometry.
The main technique we have developed for nano-scale metrology uses NMR Cryoporometry, based on physical thermodynamics (Gibbs equations), and employs Gas Adsorption and neutron scattering for primary calibration.
NMR Cryoporometry (NMRC)
is a novel pore size distribution measurement technique
that we have developed at the University of Kent and at Lab-Tools Ltd.
It makes use of the fact that small crystals of a
liquid in the pores melt at a lower temperature than the bulk liquid.
This is the Gibbs-Thomson effect : The melting point depression is inversely proportional to the pore size.
A liquid is imbibed into the porous sample, the sample cooled until all
the liquid is frozen, and then warmed slowly while measuring the quantity
of the liquid that has melted. This data is converted to a pore-size distribution.
Lab-Tools performs contract analyses of Pore Size Distributions using NMR Cryoporometry :Lab-Tools has successfully measured pore sizes in a wide range of porous materials, including oil and/or water wet materials. NMR Cryoporometry is one of the few techniques that can be applied to materials that can not be dried out without losing their structure. The technique is non-destructive, and can be used to follow evolution of pore structure for evolving systems.
The Lab-Tools pore-size distribution measurement range extends from about 1nm to over 2µm. NMR Cryoporometry measures both the total pore volume and the pore volume distribution as a function of pore diameter.
NMR Cryoporometry measurement costs are very competitive.
Cost per sample depends on the pore-size range to be covered, and precision needed, but goes down markedly for multiple-sample batches. Two free pore size distribution measurements are normally available for you to evaluate the technique for your own samples.
Please contact Dr. Beau Webber to discuss pore size measurements on your samples.
Maximum sample size needed : enough sample to fill a tube 4mm dia. x 12mm : usually 30mg to 300mg, depending on density.
Figure 2: NMR Cryoporometric Pore Size Distributions
for porous carbonate and sandstone rocks.
Figure 3: NMR Cryoporometric Integral (cumulative) Pore Size Distributions
for porous carbonate and sandstone rocks.
Note: the divergence in the carbonate data at low pore diameter corresponds to less than a 1μl.g-1 base-line error, as can be seen on the pore integral graph.
With all these thermodynamic techniques, however there is increasingly a
supposition that as atomic dimensions are approached, the
calibration from the thermodynamics may change.
Thus we have mounted a separate metrology program using neutron and X-ray scattering. These have the advantage that they are inverse techniques, by which is meant that the smaller the structure being observed, the larger is the scattering angle.
Further, there is no existing reason to suppose that these scattering techniques are length-scale dependent. i.e. if one has a good calibration at one length-scale, it should also be good at all other length-scales.
To transform a measured scattering to a metric of the structures in the sample, we create extended models of pore systems, and calculate the scattering using numerical integration. These show very good agreement with measured scattering, figure 4.
Our existing measurements using sol-gel silicas have shown that while the thermodynamic techniques are in close agreement with the scattering measurements for dimensions above 10nm, below this dimension there appears to be an increasing divergence between the scales of the thermodynamic and scattering metrologies.
The equations describing these models have recently been published in Physics Reports :
Dr. Beau Webber was recently awarded further measurement time (as first user) on the excellent new wide-scattering range ISIS Target Station 2 instrument NIMROD, and 17 porous samples were measured on this most successful run, with very pleasing preliminary results. Simulation models are being run to match the results, and the models, results and conclusions will be presented in a paper.
Porous Media pages