Acoustics of activated carbon

Bechwati, F 2008, Acoustics of activated carbon , PhD thesis, Salford : University of Salford.

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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.

Item Type: Thesis (PhD)
Contributors: Cox, TJ (Supervisor) and Avis, MR (Supervisor)
Schools: Schools > School of Computing, Science and Engineering
Schools > School of the Built Environment
Depositing User: Institutional Repository
Date Deposited: 03 Oct 2012 13:34
Last Modified: 04 Aug 2022 11:27

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