Biomedical Engineering Reference
In-Depth Information
pressure measurements, which are often required for practical
applications, but are considerably more susceptible to error than
those performed at sub-ambient pressure.
1.1
Introduction
The quantitative determination of the gas sorption properties of
materials is typically performed using the gravimetric and volumetric
techniques. In addition, temperature-programmed methods have
also been used recently to determine the hydrogen desorption
properties of nanostructured and nanoporous carbons. The principal
aim of a gas sorption measurement is often to determine a sorption
isotherm, which is a plot of the quantity of gas adsorbed versus
pressure at the measurement temperature. Gravimetric methods
determine the amount of gas adsorbed or desorbed by measuring
the weight change of a sample in response to its exposure to a step
change in the gas pressure. Once equilibrium has been achieved,
the weight reading is recorded and then the pressure changed.
This process is repeated until a full isotherm has been determined.
Volumetric techniques, on the other hand, determine the amount
of gas adsorbed or desorbed using the real gas law,
PV
=
nZRT
(Eq. 1.1). The most common implementation of the technique
measures the pressure change in a system of a fixed, known volume.
An aliquot of gas is dosed from a calibrated volume into the sample
cell. A decrease in pressure beyond that expected from the ratio of
the dosing volume to the total system volume is then assumed to
be due to adsorption. Further doses are delivered to the sample
cell in order to construct a complete isotherm. In temperature-
programmed hydrogen desorption techniques, a material is dosed
with hydrogen and the temperature decreased below that required
for desorption to occur. A temperature ramp is then applied and the
desorbed quantity of hydrogen is monitored in one of a number of
ways. Temperature-programmed desorption (TPD) from carbon
nanomaterials typically requires a mass spectrometer. The total
desorbed quantity can be determined from the integral of the mass
spectrometer signal, providing it can be adequately calibrated.
Otherwise, the temperature of the desorption peaks and the form of
the spectrum provides information regarding the adsorbed state of
the hydrogen.
