Biomedical Engineering Reference
In-Depth Information
different materials, are the bulk and the tap density. The
bulk density
is calculated from the volume occupied by the solid, including all
internal pore space and the volume of the voids between particles.
The
is similar except it is obtained after the container
holding the material is tapped in a specified manner to allow more
efficient packing of the bed. This should result in the tap density
being greater than the bulk density.
With regard to the determination of the skeletal density using
helium, the origins of the errors for microporous materials are two-
fold. Firstly, for micropores in which the potential fields of the pore
walls overlap, helium is likely to adsorb to a certain extent, although
this is dependent on both temperature and pressure. Secondly, probe
molecules of different sizes should 'see' different volumes depending
on their molecular diameters. It is generally accepted that helium
adsorbs in micropores at liquid nitrogen temperature (77 K) [31],
for example, and so helium pycnometry should not be carried out
at such a low temperature. Helium adsorption is also considered
to be significant at ambient temperature [32] and, as a result,
Malbrunot
tap density
[33] suggested that helium density determination
should be performed at elevated temperatures. The approach
advocated by Gumma and Talu [32] is to perform a correction to
account for the assumed adsorption of helium by measuring the
weight change of the sample as a function of helium pressure at a
number of different temperatures. These data are then used to plot
β
et al.
is a parameter that is dependent on
the isosteric heat of adsorption, a second parameter related to the
entropy of adsorption, and the volume of the solid. By fitting the
experimentally determined values of
versus temperature, where
β
, the volume of the solid and
hence the location of the Gibbs dividing surface can be determined.
Their method was demonstrated using helium adsorption data
measured for a silicalite sample at 12 temperatures in the range
93-515 K, up to approximately 3.5 MPa. They also suggest that the
conditions under which helium experiments are performed, for the
purpose of sample volume determination, should be standardized.
The second point mentioned above, relating to the different
volume seen by probe molecules of different size, is probably
secondary to the problem of helium adsorption, but it is dependent on
the diameters of the species being considered. The kinetic diameter
of helium is 0.255 nm, while those of hydrogen, carbon dioxide,
oxygen, argon, nitrogen and methane are 0.283-0.289 nm, 0.330 nm,
β
