3.5. Técnicas e instrumentos de análisis de datos
4.1.1. Descripción del Negocio
From the outset of this research program, it was the author's intention to develop highly sensitive gas sensors based on nanostructured materials. In this chapter, the author initially presented gas sensing mechanisms of MOS and polyaniline thin film based gas sensors while at the same time highlighting the difference between the nanostructured and bulk forms of materials for gas sensing. Then, a critical literature review was conducted for nanostructured metal oxides and polyaniline based sensors to justify the author's rationale for proposing them for gas sensing. The author believes that for nanostructured materials, structural morphology, orientation, crystallinity, film porosity and operating temperatures must be optimised in order to obtain the best possible gas sensing performance. Therefore, to achieve this, the films must be investigated using various characterisation techniques such as SEM, TEM, XRD etc. and undergo electrical testing to determine the optimal operating temperatures for maximum sensitivity.
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Chapter 3
Theoretical Background of SAW Sensors
3.1 Introduction
In Chapter 2, from a critical review of literature it was revealed that nanostructured materials have the potential to improve performances of traditional gas sensors. Thus, the knowledge gained from the previous chapters has formed the basis of the author’s novel approach to develop nanostructured material based SAW and conductometric gas sensors. The purpose of this chapter is to illustrate the author’s rationale for proposing novel layered nanostructured materials/ZnO/ surface acoustic wave (SAW) substrate structures to develop highly sensitive gas sensors.
In this PhD work, the author employed both SAW and conductometric transducing platforms to develop nanomaterial based gas sensors. As the theory and design aspects of SAW sensors are much more complex than that of conductometric sensors, this chapter will only focus on SAW technology.
In order to present author’s novel nanostructured material based SAW structures, it is necessary to first demonstrate a general understanding of acoustic wave transducers and the theoretical background of acoustic wave propagation and generation in piezoelectric media, and conductometric and mass sensitivity in SAW sensors. This will chapter will cover all of these aspects. Section 3.2 is devoted to the general overview of acoustic wave devices, piezoelectic substrate materials, and SAW devices with different acoustic modes. Section 3.3 covers the derivation of the electromechanical coupling co-efficient tensor from the constitutive and governing equations for piezoelectric materials. Section 3.4 describes the operational principles of the SAW gas sensors, in particular mass and conductometric sensitivity. Section 3.5 presents the novel nanostructured material based layered SAW structures developed by the author. Finally, section 3.6 provides a summary of this chapter.