A wide range of high-value applications, including power generation and chemical manufacturing, involve harsh chemical conditions and extreme temperatures. Options for in-situ monitoring of these processes are very limited, as traditional electronic sensing materials, packaging, and metallic interconnects rapidly degrade under such conditions. Even high-temperature stable electrochemical sensors require electrical feedthrough which may limit applicability. Silica optical fiber-based sensors provide a low cost and extremely rugged platform for applications up to 800 °C but tend to degrade over long time scales at higher temperatures. For higher temperature applications, single crystal optical fiber may be employed, limited only by the melting point of the material (e.g., approximately 2000 °C for sapphire). In this work, we discuss the implementation of evanescent field optical fiber sensors for distributed gas sensing of H2, focusing on results using a Ni/Gd-doped CeO2 nanocomposite sensing material for detection of low levels of H2 (0-4%) at 700 °C. This approach utilizes sensors prepared using a low-cost, wet chemical deposition process, in conjunction with a custom-built interrogator system leveraging optical time domain reflectometry (OTDR). Using a specially designed dual-gas flow reactor system, the sensor is tested by establishing a controlled equilibrium gradient of gas concentration. Initial results shown using silica fiber provide a pathway for utilization with high-temperature stable single crystal optical fiber for operation at higher temperatures and higher levels of H2 relevant for solid oxide fuel cell (SOFC) operating conditions.
High-temperature, chemically harsh processes underpin a wide range of applications ranging from power generation, infrastructure monitoring, chemical manufacturing, and many others. For such processes, in situ sensor data is a valuable tool for both optimization and safety, however, traditional sensor platforms can be limited in terms of stability at high temperatures or under highly corrosive, reducing, or oxidizing chemical conditions. Optical fiber-based sensing offers a unique tool for this type of harsh environment sensing application. Off-the-shelf silica fiber itself is highly stable up to ~800 °C, under a wide range of chemical conditions; while single crystal optical fiber expands this operational range even further, to temperatures well above 1000 °C. Work will be presented on the utilization of n-type semiconducting oxide thin films on single crystal sapphire fiber for the evanescent field-based sensing of reducing gas streams at temperatures up to 900 °C. The role of oxygen defects on the electrical and optical properties of the relevant films will be discussed, providing a theoretical background for the observed sensing response, time-dependence, and stability. Doped SrTiO3 systems (LaxSr1-xTiO3) will be discussed for hydrogen sensing at high temperatures. Strategies and challenges associated with pushing sensor and single crystal fiber performance above 1000 °C will also be discussed.
Many systems within the energy sector necessitate high-temperature or chemically harsh conditions (e.g., solid oxide fuel cells, power plant boiler systems, post-combustion facilities). Significant economic and technological value can be added through the integration of in-situ sensor technology; unfortunately, harsh environments pose a major challenge to traditional sensor materials. Optical fiber-based sensors provide a robust solution to this problem and offer capability for spatially distributed sensing. Silica fiber, with cladding removed and coated with bare or metallic nanoparticle incorporated sensing layers, exhibits stability up to 800-900°C under a wide range of chemical environments. As sensing layers, complex perovskites oxides - studied extensively as anode and cathode materials within the solid oxide fuel cell (SOFC) community – provide ideal tunability, stability, and defect-dependent optical properties for high-temperature gas-sensing applications. Modeled defect chemistry kinetics are presented in the context of experimental high-temperature (600-800°C) optical gas sensor data at visible and NIR wavelengths, both on planar substrates and on optical fiber. Doped SrTiO3 is highlighted as a model sensing material, due to strong Drude response under chemically reducing conditions, and due to its well-documented material / chemical properties. Equilibrium calculations are performed for ionic and electronic motion within thin films on fiber – using a ray-based approach for guided optical modes.
Real time gas sensing in high temperature energy conversion devices can enable optimal and efficient operation at both component and system levels, and the optical fiber based sensing platform shows significant advantages for harsh environment applications. In this research, (La0.8Sr0.2)0.95MnO3-δ (LSM), (La0.8Sr0.2)0.95CoO3-δ (LSC) and (La0.8Sr0.2)0.95Co0.2Fe0.8O3-δ (LSCF) films with thicknesses of several tens of nm are integrated with the optical fiber sensing platform as a functional sensor layer using a finely tuned-RF sputtering system designed for the fiber substrate deposition. Oxygen sensitivities, stabilities and overall feasibilities of these representative perovskite materials on the optical fiber platform are evaluated in the solid oxide fuel cell operational temperature regime at the oxygen concentration up to 19%, relevant for in-cell cathode stream gas composition sensing through optical transmission measurement which covers visible and near infrared wavelength ranges. Various sensitivity comparisons are carried out as a function of thickness, oxide composition, and deposition conditions. In general, the LSM sensor shows a stepwise absorption response to increasing levels of O2 in a N2 background, but also exhibits relatively slow kinetics including a continuous baseline drift. In contrast, LSCF based sensors exhibited enhanced transmittance responses in O2 containing gas and a more rapid recovery and response, presumably due to the enhanced oxygen ion diffusion kinetics as compared to LSM. The results presented here are promising for the broad application areas of high temperature O2 sensor research and a concomitantly wide range of energy related applications including combustion, solid oxide fuel cells, and others.
In-situ sensing in high temperature and chemically reactive environments – i.e., within solid oxide fuel cells or power plant boiler systems – is inherently challenging due to the rapid degradation of most traditional sensor materials within this regime. Although optical fiber based sensors provide clear advantages in this context, progress in this area of application has hinged on the development of (a) optical fiber materials and (b) thin film materials with strong optical response to gas environment, both of which must resist degradation under such conditions. Conducting metal oxide thin films have been examined in the literature as a candidate to solve the latter problem, due to a free-carrier governed optical response in the NIR (1-2 μm), that can be strongly dependent upon gas environment. In this work, we present the impact of incorporating gold nanoparticles in one such metal oxide, lanthanum-doped strontium titanite (LSTO), on the gas sensing response, both in the NIR and UV-VIS range. Via optical transmission measurements performed at high temperature (up to 800 C), the intertwined free-carrier response of the film and the localized surface plasmon response of the nanoparticles are examined in the presence of hydrogen of varying concentration. Measurements are presented for films coated both on planar substrates and on optical fibers.
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