Acoustics of activated carbon

Bechwati, F

Abstract

This thesis describes a study into how sound interacts with activated carbon, a material that
exhibits adsorbing and desorbing properties. Adsorption is where molecules from the
surrounding gas are attracted to the material microstructure and held in place by a weak
physical attraction force named after the scientist van der Waals\ desorption is the opposite
process. Activated carbons include a complex porous structure, with a large internal surface
area, and a considerable adsorption capacity caused by free electrons in the deformed
graphene layers. The process of adsorption and desorption is usually associated with energy
exchanges, caused by transfers of heat between the adsorbate molecules and the adsorbent
surface.
The study of acoustic interactions with granular activated carbons at normal conditions
makes the subject of this doctoral thesis. Two main physical phenomena were seen to
accompany sound propagation through the material: (i) an increase in volume compliance
which is assumed to be caused by a change in the density of the interacting gas, and (ii)
excess absorption at low frequencies thought to be due to the energy lost in the
adsorption/desorption hysteresis. For the former, measurements on the impedance of low
frequency Helmholtz resonators reveal significant shifts in resonance when activated
carbon is used as a porous liner in the backing volume. At constant aperture dimensions,
these shifts are attributed to a larger apparent volume of the resonator as compared to an
empty backing volume. This phenomenon is in direct contravention of the physical theory
associated with Helmholtz resonators as the resonant frequency of a device increases
slightly when a porous solid is placed in the backing volume. An upper frequency limit of
SOOHz is also determined where sorption effects in activated carbon are assumed to become
almost negligible in relation to sound propagation.
For the latter, the excess absorption at low frequency, a series of experiments to reveal the
physical cause of the phenomenon have been undertaken. Hysteresis was observed during
the sorption of humid air onto activated carbon at room temperature. At such conditions,
the different rates of adsorption and desorption lead to a disturbance in the system
equilibrium and cause a change in entropy. The return of the system to equilibrium is an
exothermic process hence involves energy losses between activated carbon and the
surrounding gas. This is suggested as a possible cause of the excess attenuation. However,the relaxation times are rather long for acoustic propagation, and further work is needed to
examine this.
An experimental apparatus to explore sound propagation through the material was devised.
Results showed a violation of the equation of state for the relationship between volume and
pressure: as the volume in a sealed chamber was reduced at constant temperature, the
measured pressure change was found to be lower for a sample of activated carbon than
when the chamber was empty; a phenomenon assumed due to the differences between
adsorption and desorption rates.
A new method for determining the porosity of a material exhibiting adsorption at acoustic
pressures has been devised and found to be 81 ±7% for the granular sample examined. BET
analysis and examination of electron microscope pictures allowed the pore size distribution
to be found. Although the activated carbon sample has many very small pores (0.7nm in
width), the BET isotherm showed that these will be saturated with water vapour in normal
conditions. Consequently, the pores that affect sound propagation are those between the
grains of the activated carbon, and the macropores (>50nm) on the surface of the grains.
A theoretical model is developed and outlined based on the Langmuir isotherm. This was
used to predict the sound propagation within the material and is compared to acoustic
impedance measured in a large low frequency impedance tube, which was constructed
especially for this project. The match between theory and measurement is rather poor,
thought to be due to the lack of modelling the hysteresis effects in the adsorption-
desorption cycle.
Two applications of the material are examined, within a Helmholtz resonator and the cups
of hearing defenders. In both cases, improved performance is seen. For instance, the use of
the material in hearing defenders showed that activated carbon could be used to improve
the attenuation at low frequencies in comparison to conventional foam liners.

Citation

Bechwati, F. Acoustics of activated carbon. (Thesis). Salford : University of Salford

• Thesis Type: Thesis
• Deposit Date: Oct 3, 2012
• Award Date: Jan 1, 2008