Biomedical Engineering Reference
In-Depth Information
1.
Discussion
The gas adsorption properties of materials have been studied for
many years: the gravimetric and volumetric techniques have been
in use for over a century [53, 54], and early adsorption studies date
back to the 18th and 19th centuries [55]. Furthermore, as a result
of the subsequent development of commercial instrumentation,
gas adsorption measurements at sub-ambient pressures are now
routinely used to characterize powders and porous solids [3, 6].
However, despite the significant technological advances made in the
intervening period, in the areas of computing, metrology, and vacuum
and pressure equipment technology, many of the issues that affect
the accuracy of gas adsorption measurements for porous materials,
particularly at elevated pressure, are of a more fundamental nature.
They include, for example, the definition of the location of the Gibbs
dividing surface [31-33], the potentially fractal nature of real surfaces
[35], the differences in the diameter of a molecule depending on
the chosen definition and its physical state, the differing behavior
of confined fluids compared to the bulk fluid phase [56] and the
practical difficulty of measuring either the adsorbed phase volume
or its density [23]. Such aspects will therefore continue to affect the
measurement of gas adsorption and are likely to be the subject of
further research.
Meanwhile, over the last decade or so, a wide range of values have
been reported in the literature for the hydrogen storage capacity of
carbon nanotubes and other carbon nanostructures [11, 15, 28-30,
36, 52, 57]. The uncertainty in the microstructural characterization
of the samples that were used in the studies made a significant
contribution to the inconsistencies in the reported results. However,
inaccurate hydrogen sorption measurements also contributed
significantly to the controversy. Although some of the erroneous
results were not necessarily affected by the fundamental issues
mentioned above, the episode served to highlight the challenging
nature of gas adsorption measurement. Careful consideration of
the experimental characterization equipment and measurement
methodology is therefore clearly important to ensure that future
research does not suffer unnecessarily from the repetition of past
mistakes. Although some discussion of the properties of a newly
discovered or developed material is inevitable during the scientific
process, it is nevertheless beneficial to enhance our understanding
