Biomedical Engineering Reference
In-Depth Information
a different meaning for different materials, but is particularly
pertinent to nanostructured carbons and hydrogen adsorption.
In general, with few exceptions, it is important to have a good
understanding of the chemical, physical and microstructural nature
of a material in order for accurate and reliable characterization of its
hydrogen sorption properties to be performed. For metal hydrides,
the sample purity can relate to the impurity levels in the bulk of the
host material, the presence of minority phases or inhomogeneity in
the stoichiometry of the host compound. In crystalline microporous
adsorbents it can involve the crystallography of the structure and
the presence of impurities in the pore network. An example of
the latter is the variation in the adsorption properties of MOF-5,
a metal-organic framework widely investigated for its hydrogen
storage properties. The variation was attributed by Hafizovic
et al.
[27] to framework interpenetration and the blocking of pores by
Zn(OH
) species. In the case of carbon nanomaterials, it can relate
to by-products of the synthesis or purification process, or the overall
concentration of the allotrope of interest in the sample. In the case
of the early work on the hydrogen adsorption properties of carbon
nanotubes [28], for example, the hydrogen sorption process was
later found to be affected by the presence of Ti alloy particles [29,
30], which had been deposited on the samples during purification.
2
3
In addition, the samples of Dillon
[28] consisted of a relatively
small proportion of nanotubes; the remainder was uncharacterized
soot, which cannot be assumed to be noninteracting with regard to
hydrogen [30]. Careful characterization of carbon nanomaterials
and their purity, combined with consideration of the effects of
any potential contaminant on their hydrogen sorption properties,
is therefore important to avoid errors in the determination or
assessment of their hydrogen storage capabilities. Less reactive, or
indeed inert, species may not be susceptible to the same specific
problems as hydrogen, but the sample purity clearly remains critical
to the success of the characterization process.
et al.
1..
Sample Density and Volume
In combination with a good understanding of the purity of a sample,
the sample density and volume are also required in one form or
3
The Ti-V-Al alloys in question are known to form hydrides, and so in this case the
impurities were reacting with the hydrogen during the characterization process.
