Biomedical Engineering Reference
In-Depth Information
The chapter is organized as follows. In the next section, we
will describe each of the gas sorption techniques in more detail.
We will then discuss some of the critical aspects of gas sorption
measurement methodology, before briefly covering the definition of
the excess and absolute adsorbed quantities, which are important in
the characterization of the gas adsorption properties of materials.
In the fifth section, various sources of experimental error in
the characterization of the gas sorption properties of carbon
nanomaterials are discussed in detail. We will focus on the accuracy of
hydrogen adsorption measurements due to the erroneous capacities
that have been reported for some nanostructured carbons, although
many of the error sources also apply to gas adsorption measurement
in general. In the sixth section, we briefly discuss our current level
of understanding of the accuracy of gas sorption characterization
methods and recent research into the topic, before concluding the
chapter with a summary.
1.
Gas Sorption Measurement Techniques
In this section, we cover the three general types of gas phase
measurement that can be used to quantitatively determine the
gas sorption properties of carbon nanomaterials. We discuss
the important aspects of the measurement methods, in each case, in
the next section.
1..1
Gravimetric Techniques
The gravimetric technique determines the amount of gas adsorbed or
desorbed by measuring the weight change of the sample. Gravimetric
gas sorption apparatus typically consists of a microbalance housed
in an ultra-high vacuum (UHV) compatible and pressure-rated
chamber. A gas delivery system is coupled to the chamber to provide
full control of the gas pressure within the system, and a vacuum
pump allows the evacuation of the apparatus and the degassing
of the sample prior to measurement. A method of controlling the
sample temperature is also required. This can include a furnace,
cryostat, cryofurnace or a fluid bath.
Once a sample has been degassed, the measurement begins
at vacuum with the dry sample weight being recorded. The first
isotherm point is determined by applying a pressure to the
