Piezoelectric-based gas sensors

The history of electrochemical sensors began in the thirties of the twentieth century, when the pH-sensitive glass electrode was deployed, but no noteworthy development was carried out till the middle of that century. In 1956, Clark invented his oxygen-sensor based on a Ft electrode in 1959, the first piezoelectric mass-deposition sensor (a quartz crystal microbal-ance) was produced. In the sixties, the first biosensors (Clark and Lyons, 1962) and the first metal oxide semiconductor-based gas sensors (Taguchi, 1962) started to appear. 

Sensor performance for different applications is defined by various features of the ceramic. For example, the electrical output of most pressure sensors is dependent on the bulk piezoelectric properties of a PZT ceramic. Oxygen gas sensor performance is defined by the conductivity behavior of Zr02 ceramics, which is in turn dependent on the oxygen vacancy content of the material. The performance of still other sensors, for example, ceramic thermistors, is dependent on the grain boundary characteristics of doped BaTi03 ceramics. For humidity sensors based on NiO/ZnO, the p—n junction characteristics of the interface define sensor performance. 

It was found that in several polymers, such as stretched and poled poly(vinylidene fluoride) (PVDF) and its copolymer, poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), a strong polarization effect is observed under influence on mechanical stress and temperature. This means that piezoelectric and pyroelectric gas sensors can also be designed based on polymers (see Chap. 13 [Vol. 1]). 

Miura N (1991) New-type calorimetric gas sensor using temperature characteristics of piezoelectric quartz crystal fitted with noble metal catalyst film. Sens Actuators B 5 211-217 Monz6n-Hemdndez D, Luna-Moreno D, Martfnez-Escobar D (2009) Fast response fiber optic hydrogen sensor based on palladium and gold nano-layers. Sens Actuators B 136 562-566 Mueller WM, Blackledge IP, Libowitz GG (1968) Metal hydrides. Academic, New York, NY Noh J-S, Lee JM, Lee W (2011) Low-dimensional palladium nanostmctures for fast and rehable hydrogen gas detection. Sensors 11 825-851... 

Piezoelectric-based or acoustic wave (AW) sensors such as surface acoustic wave (SAW), quartz crystal microbalance (QCM) or bulk acoustic wave (BAW), and cantilever-based devices create a specific class of gas sensors widely used in various applications (Ippolito et al. 2009 Korotcenkov 2011) (see Fig. 13.1). Virtually all acoustic wave-based devices use a piezoelectric material to generate the acoustic wave which propagates along the surface in SAW devices or throughout the bulk of the structure in BAW devices. Piezoelectricity involves the ability of certain crystals to couple mechanical strain to electrical polarization and will only occur in crystals that lack a center of inversion symmetry (Ballantine et al. 1996). 

The nature of acoustic waves generated in piezoelectric materials is determined by the piezoelectric material orientation as well as the metal electrodes configuration employed to generate the electric field that induces acoustic waves by converse piezoelectric effect. As gas sensors, the resonators are coated with layers which selectively absorb or adsorb analytes of interest and thereby induce a mass change that is then detected via a shift in the resonant frequency of the device (Kurosawa et al. 1990). The detection limits and the relative (5) mass sensitivities for different types of acoustic sensors are presented in Table 13.1. The comparison of various types of AW sensors is also presented in Table 13.2. Several books and reviews (Ballantine et al. 1997 Ippolito et al. 2009) provide a more detailed analysis of AW-based sensors operation. 

Different gas sensors are presently used for eonstruetion of eleetronie noses. They are based on different sensitive materials (i.e., metal oxides, eonducting polymers etc.) and detection principles (resistive, piezoelectric, optical, electrochemical sensors, etc.) (Patel 2014). 

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