The Rayleigh wave propagation problem in the elastic halfspace - viscoelastic layer interface was analysed in the paper. The problem was formulated in the Fourier-Laplace space using the Biot viscoelastic solid model. The characteristic equation has taken the Rayleigh equation form with correction term describing viscoelastic layer properties influence on the wave velocity. The approach presented here seems to be useful for surface acoustic waves gas sensors modelling because many chemisensitive coatings applied to the sensors exhibit viscoelastic properties.
The Rayleigh wave propagation problem in the elastic substrate-viscoelastic layer interface was analysed in the paper. The fundamental solution of the problem has been found in the Fourier-Laplace space using Biot viscoelastic solid model with Maxwell relaxation time. The space-time form of the solution was obtained using Cagniard-deHoop method. The solution presented in the paper seems to be applicable to surface acoustic waves gas sensors with polymeric (viscoelastic) layer.
In general, there are three possible kinds of surface acoustic wave devices that may be applied for high frequency gas sensor construction: delay lines, one- and two-port resonators. Choice among them depends on individual preferences and sometimes it is the matter of chance. In the paper the key differences among the surface acoustic wave devices important from the gas sensors technology point of view are described.
The results of numerical simulations and experimental measurements of surface acoustic wave resonator centre frequency and amplitude dependence on external pressure changes in the range 0-1 atm are presented in the paper. According to the analytical model as well as numerical calculations the pressure influence on the resonator centre frequency is linear. Unfortunately, in the real measurements the linearity is difficult to observe. Additional effects that modify the linear dependence of the centre frequency on pressure are caused by water particles. It is well visible especially in the low pressure range. The phenomenon can be applied for research of water contents in dry gases as well as construction of acoustoelectronic vacuometers.
In the surface acoustic wave sensors response analysis the influence of different factors on acoustic wave propagation velocity and resonant frequency of the surface acoustic wave device plays an important role. Because the resonant frequency is commonly assumed to be a sensor response, knowing the magnitude of this influence and its mechanisms is very important. The factors which interfere the most with the surface acoustic wave sensor response include: temperature, pressure, and humidity. The influence of humidity is almost impossible to eliminate because of phenomena connected with water adsorption at the surface of the substrate as well as the electrodes of the device.
Mercury is the element commonly applied in industry. Because of poisoning properties the monitoring of the mercury vapour concentration in the environment is very important. Mercury forms amalgams in connection with gold that causes a change in the electrical resistance and mass of the gold specimens. This effect was applied to measure mercury vapour concentration in the environment using surface acoustic wave technology. Two kinds of surface acoustic wave mercury sensors have been described in the paper. First one utilizes a thin film of gold deposited between aluminium interdigital transducers and reflectors of two-port surface acoustic wave resonator, and the second one golden interdigital transducers and reflectors without any sensitive film between them.
Typical approach to the surface acoustic waves sensors response analysis is based on the use of self-oscillating circuits with surface acoustic wave device working inside positive feedback loop of an amplifier. Such kind of parametric measurement allows to track the center frequency of the sensor changes in particular. The method is widely used mainly due to their relative simplicity. Unfortunately, it has many disadvantages like frequency (phase) instability, sensitivity to unwanted factors, surface acoustic wave substrate mass-load limit etc. A new system to the analysis of surface acoustic wave gas sensors response as well as an exemplary measurement results are described in the paper. The presented system make the first step to the more complex conception realization.
Majority papers concerning surface acoustic wave sensors technology is devoted to the analysis of self-oscillating circuits with surface acoustic wave device and its basic frequency changes in particular. Such circuits are widely used mainly due to their relative simplicity. Unfortunately the price of the simplicity is such drawbacks like frequency instability, sensitivity to unwanted factors, surface acoustic wave surface load limit etc. A new approach to the analysis of surface acoustic wave gas sensors response is proposed in the paper. The approach significantly eliminates the disadvantages of commonly used so far methods of analysis.
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