Modeling of Sensing Layer of Surface Acoustic-Wave-Based Gas Sensors

Modeling of Sensing Layer of Surface Acoustic-Wave-Based Gas Sensors

Hediyeh Karimi (Universiti Teknologi Malaysia, Malaysia), Rasoul Rahmani (Swinburne University of Technology, Australia), Elnaz Akbari (Universiti Teknologi Malaysia, Malaysia), Ali Cheloee Darabi (Iran University Science and Technology, Iran), Meisam Rahmani (Universiti Teknologi Malaysia, Malaysia), Mohammad Taghi Ahmadi (Universiti Teknologi Malaysia, Malaysia & Urmia University, Iran) and Saba Anbari (Universiti Teknologi Malaysia, Malaysia)
DOI: 10.4018/978-1-5225-0736-9.ch009
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Industrial activities have polluted the atmosphere very rapidly in these days. There are a various varieties of air pollutants having a strong effect on human health as well as on climate and specially environment, such as nitrogen dioxide (NO2), oxides of carbon (COx) and hydrocarbons. The World Health Organization (WHO) estimates that each year about 4.6million people die directly from causes of air pollution that will be a serious threat to human health. Therefore, there is a growing demand towards highly sensitive, cheap, low consumption, user-friendly devices which could monitor the quality of air indoor and outdoor areas for protecting human health. It is proved to be an efficient and economically feasible alternative for measuring different gas concentrations.
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Acoustic Wave Devices

Acoustic wave devices are used as filters, delay-lines. There are also various applications for acoustic devices, such as medical (chemical sensors), automotive (torque and pressure sensors) and industrial and commercial applications (Drafts, 2001). Acoustic wave sensors specifications are defined as being small, inexpensive, very sensitive and easily designed for various required measurements.

Surface Acoustic Waves Devices

SAW devices have been developed as high sensitive gravimetric sensors due to their strong surface acoustic energy confinement (Schweyer, Andle, McAllister, French, & Vetelino, 1996). Surface acoustic wave devices have been used for about 40 years. In 1887, Lord Rayleigh who discovered surface acoustic propagation mode described its longitudinal and a vertical shear component which has the ability to contact with any media (Morgan, 1985).

Each SAW device is composed of two sets of interdigital transducers (IDTs) called input and output transducers which are patterned on a piezoelectric substrate. The IDTs are made of thin metal electrodes. In 1960s, White and Voltmer demonstrated that by depositing IDTs metal electrodes on piezoelectric substrates, surface acoustic waves could be generated (Strutt, 1885). Acoustic waves are generated when voltage changes through metal interdigital transducers on the input side. The generated acoustic wave travels through the substrate to reach the other side of output IDTs. The output IDT will detect the generated waves and convert the acoustic wave energy back into electrical energy. This region is called delay line where a voltage generates an output interdigital transducer by mechanical displacement (White & Voltmer, 1965).

SAW Sensors

Surface acoustic waves (SAW) have been used for chemical gas sensing purposes for about 20 years (Michigan). One of the distinctive advantages of SAW devices is that they are so sensitive to mass and conductivity changes. Mass loading mechanism is one of the commonly used ones. Change in the acoustic wave velocity is caused by a mass increase due to the molecules absorbed on the structure surface. Typically used mainly for liquid sensing, mass adsorption mechanism has been also employed for gas sensing, e.g. for volatile organic compounds.

For first time, the SAW technology was used as a sensor for pressure sensing in 1975(Nicolay et al., 2009; Reeder, Cullen, & Gilden, 1975). SAW temperature sensors have good resolution (about one-thousandth of a degree). A SAW device with a sensitive layer of polyaniline nanofibres for H2 sensing, which could be operated at room temperature, has been reported in 2005 (He et al., 2010). Recently, (Penza, Antolini, & Antisari, 2004) developed a highly sensitive layered SAW device for H2, NH3 and NO2 sensing using carbon nanotubes on ZnO/LiTaO3 obtaining high sensitivity at room temperature.

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