Biomedical Engineering Reference
In-Depth Information
0.347 nm, 0.354 nm, 0.364-0.380 nm and 0.376 nm, respectively,
according to values tabulated by Li
[34]. Firstly, it is worth
noting the exploitation of the different areas and volumes seen by
different molecules in the determination of the fractal dimension
of adsorbents. Such an approach yields fractal dimensions
because different molecules act as different size yardsticks [35].
Furthermore, adsorption in micropores can be viewed in terms of
the packing of different diameter spheres (depending on the steric
nature of the adsorbate) in narrow cylindrical or spherical pores,
or in different width slit pores, and so significantly different results
could clearly be obtained as a result of relatively small differences in
molecular size and shape due to packing geometry considerations.
Secondly, a further consideration for ultramicroporous materials
(with a pore dimension <0.7 nm) is the possibility that larger
molecules cannot enter the smallest pores. This is very dependent
on the species, but this molecular sieving effect should not be
ignored when analyzing potential error sources for high-pressure
gas adsorption. If helium can access a considerably greater volume
than the adsorptive of interest, the buoyancy effect (Section 1.5.10)
and dead volume (Section 1.5.11) corrections could be significantly
affected regardless of the occurrence of helium adsorption at the
calibration or sample volume determination temperature and
pressure.
et al.
1..
Gas Purity
The purity of the gas supply used for a sorption measurement
is a very important consideration because gas phase impurities
can preferentially adsorb onto the sample. Impurity adsorption
can block potential adsorption sites and block pores in the
case of microporous materials, as well as producing a potential
false measurement of adsorption. The adsorption of water as a
contaminant has been implicated as a major source of error in some
of the more controversial hydrogen storage capacity measurements
performed on carbon nanomaterials
5
[15]. The purity of hydrogen
should therefore ideally be >99.9999%. If the hydrogen supply
is not connected directly to the instrument, filtration should be
5
Yang [15] examined the work of Chen
[36] on lithium and potassium-doped
carbon nanotubes, in which the reported storage capacities were 20 wt.% and
14 wt.%, respectively, concluding that the high mass uptake was most likely due to
moisture contamination in the hydrogen supply.
et al.
