Optical Thin Films for Gas Sensing in Advanced Coal Fired Power Plants (2012)

Even for existing coal based plants, the opportunity for sensors and controls to improve efficiency is great. A wide range of gas species are of interest for relevant applications. Functional sensor layers for embedded sensing must be compatible with extreme conditions (temperature, pressure, corrosive). Au incorporated metal oxides have been looked at by a number of other authors previously for gas sensing, but have often focused on temperatures below 500{degree}C. Au nanoparticle incorporated metal oxide thin films have shown enhanced gas sensing response. In prior work, we have demonstrated that material systems such as Au nanoparticle incorporated TiO{sub 2} films exhibit a potentially useful optical response to changing gas atmospheres at temperatures up to ~800-850{degree}C. Current work is focused on sputter-deposited Au/TiO{sub 2} films. Au and Ti are multi-layered sputter deposited, followed by a 950{degree}C oxidation step. Increasing Au layer thickness yields larger particles. Interband electronic transitions significantly modify the optical constants of Au as compared to the damped free electron theory. A high temperature oxidation (20%O{sub 2}/N{sub 2}) treatment was performed at 700{degree}C followed by a reduction (4%H{sub 2}/N{sub 2}) treatment to illustrate the shift in both absorption and scattering with exposure to reducing gases. Shift of localized surface plasmon resonance (LSPR) absorption peak in changing gas atmospheres is well documented, but shift in the peak associated with diffuse scattering is a new observation. Increasing Au layer-thickness results in an increase in LSPR absorption and a shift to longer wavelengths. Diffuse scattering associated with the LSPR resonance of Au shows a similar trend with increasing Au thickness. To model the temperature dependence of LSPR, the modification to the plasmon frequency, the damping frequency, and the dielectric constant of the oxide matrix must be accounted for. Thermal expansion of Au causes a decrease in free-carrier concentration and plasmon frequency with increasing temperature. Increased scattering of electrons with increasing temperatures results in an increased damping frequency for free carriers. The thermo-optic coefficient of TiO{sub 2} is assumed to be constant or decreasing with increasing temperature. A Custom Designed System Can Be Used to Monitor Optical Transmission and Reflection of Films at Elevated Temperatures and Upon Exposure to Changing Ambient Gas Atmospheres. The calculated extinction peak broadens and reduces peak height with increasing temperature. Extinction spectra measured using the elevated temperature system deviate from measurements performed with an integrating sphere. Modifications to measured transmittance and reflectance spectra are observed with increasing temperatures. Optical constants of Au nanoparticles were measured for elevated temperature. The thermo-optic coefficient of TiO{sub 2} strongly affects the shift in LSPR absorption peak. Peak broadening is dictated by the increased damping frequency of Au. In all cases of the theoretical modeling, we are assuming a particle of Au embedded in a TiO{sub 2} matrix without any interparticle interaction. Localized surface plasmon resonance in noble metal nanoparticles is associated with the free electrons. Surface charges create an internal field that acts as a restoring force on displaced charge carriers resulting in an oscillation with an associated resonance. An estimate of the sensitivity as a function of wavelength for sensing done using a single wavelength transmission measurement can be derived by focusing on the partial derivative of Q{sub abs} with respect to the material parameter that we assume is changing during the experiment. We focus on the free carrier density. However, other material parameters may also change in some cases. Peak sensitivity for transmission or absorption based sensing occurs on either side of the LSPR absorption peak maximum. Theoretically predicted wavelength dependence is consistent with the literature. The wavelength of LSPR absorption peak sensitivity shifts with increasing temperature due to broadening and peak shifting. The maximum sensitivity decreases with increasing temperature. The sensitivity should also depend on the mechanism responsible for the solid-gas interaction.