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Conductivity Changes in Tungsten Trioxide (WO3) When Exposed to Various Gases and Humidities The objective of our experiment is to observe and encapsulate the conductivity changes in Tungsten Trioxide (WO3) when it is exposed to various gases and to humidity. WO3 is a semi-conducting metal oxide (SMO), which is used in a number of gas sensors. Based on our experiment, we will identify any cross-sensitivity problems as well as any selectivity problems. We have two models for our experiment. The first model consists of a thin film of WO3 deposited onto a sapphire substrate, connecting electrodes above or embedded in the substrate, and placing all of this on top of a heating element. By varying the experimental parameters (frequency range, film thickness, temperature, and film structure), we will be able to capture the film?s conductivity response to various gases and to humidity. The method used to measure the conductivity response of the WO3 is called impedance spectroscopy (IS): Impedance Spectroscopy measures the impedance by applying a single-frequency voltage or current to the interface and measuring the phase shift and amplitude, or real and imaginary parts, of the resulting current at the frequency using either analog circuit or fast Final Report: 0401439 Page 5 of 18 Fourier transform analysis of response. From the data provided via impedance spectroscopy, we will fit the experimental response to that of an equivalent circuit. The second substrate we will be using is quartz, which is a piezoelectric material: Piezoelectricity is the ability of some materials to generate an electric charge in response to applied mechanical stress. If the materials is not short-circuited, the applied charge induces a voltage across the materials. A thin film of WO3 will be deposited onto the surface of a quartz substrate and two metal electrical connectors will be attached to the bottom of the quartz, which results in an electric field in the plane of the plate. This type of sensor is known as a lateral field excited acoustic wave sensor (an LFE sensor): Acoustic wave sensors are described by the mode of wave propagation through the substrate. As the acoustic wave propagates through the surface of the quartz, any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave. Changes in velocity can be monitored by measuring the frequency or phase characteristics of the sensor and can then be correlated to the corresponding physical quantity being measured. Similar to IS, the data will be fitted to that of an equivalent circuit. In closing, our experiment should capture whatever changes happen to the thin film WO3 while it?s being exposed to various gases and to humidity. Our goal is to gain knowledge of cross-sensitivity and selectivity issues with thin film WO3.