**Abstract**

The chapter presents the results of the numerical investigation of the SAW gas detector structures with selected polymer layers in steady-state conditions. The effect of SAW velocity changes vs. the surface electrical conductivity of the detector structures is predicted on the base of acoustoelectric elemental theory. The electrical surface conductivity of the rough polymer sensing layer placed above the piezoelectric waveguide depends on the profile of the diffused gas molecule concentration inside the whole detector structure. Numerical results in the steady state conditions for the gas molecules DMMP and polymer layer of (RR)-P3HT have been shown as well as for carbon oxide molecules with thin polyaniline and polypyrrole layer. The main aim of the investigations was to study a thin film's interaction with targeted gases in the SAW detector configuration based on diffusion equations for polymers. Numerical results for profile concentration in steady state conditions for gas molecules concentration, film thickness, roughness, and interaction temperature have been shown. The results of numerical analyzes allow for selecting better detector design conditions, including the morphology of the detector layer, its thickness, operating temperature, and layer type. The numerical results, based on the code written in Python, were shown.

**Keywords:** SAW (surface acoustic wave) gas detector, SAW, DMMP (dimethyl methylphosphonate) detection, polymer, (Regio-regular)-P3HT, (RR)-P3HT, polyaniline (PANI), polipirol (PPy), Nafion, numerical acoustoelectric analysis (NAA), Ingebrigtsen's formula, CO (carbon monoxide), python

### **1. Introduction**

Polymers are a very interesting material for testing the concentration of particularly dangerous gases, CO (the silent killer), and are also used in research on toxic warfare agents. The following publication presents two approaches that have evolved during research, both in terms of the laboratory method—optimization of the measurement system in terms of control, and thanks to technological progress and the use of atomic force microscopy (AFM). This lastly allows for the emergence of the

polymer structure and confirmation of the dependence of the shape of the detector surfaces, its morphology, and its influence on the SAW detector response. Historically, different thin film materials and the piezoelectric substrates (LiNbO3—lithium niobate Y cut-Z propagation) were utilized with the detector layers ranging from semiconducting to polymers—on which we focused in our research work—PANI (polyaniline), PPY (polypyrrole) with NAFION—to detect carbon oxide (CO), and RR-P3HT (regio-regular poly-3-hexylothiophen)—to detect DMMP (dimethylomethylo-phosphonate) [1]—a simulant of the poison of chemical warfare agent (CWA), like sarin. The development and optimization of the NNA direction (normalized numerical acoustoelectric analysis) over the analytical detector with an acoustic surface wave (SAW), which now, thanks to technological development, has been empirically confirmed.

Over the decade, the research team made efforts to select the appropriate detector layer, its thickness, porosity, and morphology for individual gases that are important for human safety, e.g. in CO detectors [2]. This publication was created from these research works. Particular importance, also from the point of view of safety, are polymer layers, those photoconductive, which are used to detect CWA (Chemical Warfare Agents), the use of which in systems can significantly prevent a tragedy. For the purposes of the experiment, an original analytical model of the gas detector was created. Initially, this model was based on Knudsen diffusion mechanisms and was inspired by the works of: Matsumaga N., Sakai G., Shimanoe K., Yamazoe N [3, 4]. The main aim of the investigation was to study thin film interaction with target gases in the SAW detector based on a simple reaction-diffusion eq. [5–11]. Diffusion equations provide theoretical bases for the analysis of physical phenomena like heat transport or mass transport in porous, roughness substrates [5].

In the initial version of the development of the analytical model, the porosity of the layer was taken into consideration. This model was a good illustration of the operation of semiconductor gas detectors. However, in the case of polymer layers and due to different physicochemical properties [12] of polymer layers and other differential equations, this model had to evolve toward a model that took into account the roughness of the layer.

Generally, these detectors have a very high sensitivity, much higher than the commercially available resistance gas detector. Detectors based on surface acoustic waves are widely used in many industries, especially in biochemical applications, allowing for monitoring of DNA mutation [13] and commercial applications, such as monitoring the quality of food, as well as monitoring the physical and chemical properties of solids, such as adsorption/desorption of the substance, humidity.

The chapter summarizes the acoustoelectric theory, i.e. Ingebrigtsen's formula, dynamics gas diffusion concentration profiles in steady-state, and predicts the influence of a thin polymer detector layer with new polymer gas diffusion model on the SAW wave velocity in a piezoelectric acoustic waveguide [14].

Delay lines, with acoustic surface wave (SAW), enable the detection of very small concentrations of chemical compounds in gas mixtures [15]. The miniaturization of these transducers resulted in a significant increase in the frequency of SAW detector. However, miniaturization of the detector requires the construction of more complicated or technically advanced electronic devices (**Figure 1**), while the test and measurement system for gas detection [6, 17–20] has been designed and developed independently.

The detection materials used here, such as DMMP, are not only valuable due to the fact that they can be used as sarin simulants for the calibration of organophosphorus

*Numerical Analysis of the Steady State in SAW Sensor Structures with Selected Polymers… DOI: http://dx.doi.org/10.5772/intechopen.109367*

#### **Figure 1.**

*Measurement appliance—patent no PAT.230526—system for detecting chemical compounds in gaseous atmospheres, with a detector using surface acoustic waves (SAW) and digital switches SW1 and SW2 ([16], μP microprocessor, OVG-4—the owlstone calibration gas generator, G—generator design and implementation by authors: J. Wrotniak and M. Magnuski).*

detectors, but also due to the obvious fact that DMMP can be used in the production of chemical weapons (sarin and soman) and is applicable to the construction of very sensitive detectors of this unnoticeable lethal weapon—chemical weapon, which in war conditions (in violation of international rules of war) has a special military and strategic importance.

Chemical warfare agents (CWA), especially nerve agents (e.g. sarin and soman) are highly lethal compounds. Sarin is one of the best-known chemical weapons of mass destruction. This odorless and colorless compound causes neuromuscular paralysis and death by suffocation within 1–10 min. Disabling and lethal exposures to sarin occur above 15 ppb and 64 ppb, respectively for 10 min of exposure [21].

In gas detectors based on SAW, the mechanism of detecting the concentration of the gas or vapors of a chemical compound depends on the interaction of its molecules with a properly selected detector layer sensitive to its presence [22]. The processes of interaction between gas molecules with the layer are kinetic phenomena, mainly sorption (in volume) and adsorption (on the surface), resulting from the entrapment of the molecule in the layer or on its surface. Sorption of gas or vapor molecules through the detector layer causes a change in its mass and electrical conductivity (change in conductivity affects the change of SAW propagation velocity) which in the measurement system leads to a change in the generator's operating frequency [23]. The channel with the detector layer generates as a result oscillations with a different frequency (usually lower) and is shifted in phase relative to the signal generated in the reference path (**Figure 2a**). The work focused on the electrical effect is a new contribution to the SAW gas detector technology [7].

The results of the research on the application of RR-P3HT (poly 3-hexylthiophene regioregular type) produced by means of air spraying on a quartz module with SAW 205 MHz to detect traces of DMMP (Dimethyl methylphosphonate) molecules were presented [1, 15]. DMMP is a non-toxic substance with a similar chemical structure to sarin (Combat Poisoning Agent) and it allows for safe experiments.

Due to the photoconductive properties of the P3HT polymer [24], the layer was additionally activated by LED light (**Figure 2b**). The sensitivity of the layer in the system with SAW [25] to the presence of DMMP in this manner was increased (different wavelengths of LED illumination). Oscillations (in reference and measurement line) were excited. Generator 205 MHz with switched channels was used.

**Figure 2.**

*Measuring appliance: (a) the idea of the historical measuring system [2, 14], and (b) the newest measurement system with LED lighting with switched channels [1, 8, 16].*

The essence of the conducted research is the search for new materials and ways to activate them in order to detect trace concentrations of DMMP in the air without the need of applying high temperatures (above 100°C) [1].
