Modeling, Fabrication, And Characterization Of Rf-Based Passive Wireless Sensors Composed Of Refractory Semiconducting Ceramics For High Temperature Applications

Real-time health monitoring of high temperature systems (\u3e500oC) in harsh environments is necessary to prevent catastrophic events caused by structural failures, varying pressure, and chemical reactions. Conventional solid-state temperature sensors such as resistance temperature detectors (RTDs) and thermocouples are restricted by their operating environments, sensor dimensions and often require external power sources for their operation. The current work presents the research and development of RF-based passive wireless sensing technology targeting high temperatures and harsh environmental conditions. Passive wireless devices are generally classified as near-field and far-field devices based on the interrogation distance. Near-field sensors are placed at 1 - 4 cm from the antenna whereas, the far-field sensor has a range exceeding several meters (\u3e50 cm). Both near-field and far-field sensor architectures were modeled in this work for application within high temperature environments. Computational modeling of multiple sensor architectures was completed using a high-frequency structure simulator (HFSS) to understand the effect of materials properties, especially electrical conductivity, on the wireless response of the sensor. The sensor designs were modeled using the properties of a select grouping of high-temperature stable ceramics and metals, not traditionally used in these passive sensor designs. Because signal strength, sensitivity, and repeatability of the sensor were determined by the electrical properties of the materials used in the fabrication. For the near-field sensors, an LC-resonator with an interdigitated capacitor (IDC) coupled with a planar inductor was initially chosen for wireless sensing up to 1200oC. Later, the IDC was replaced with a parallel plate capacitor architecture to minimize sensor geometry and compare the effect of the architecture on the wireless response. An advanced real-time signal processing technique was introduced to extrapolate the temperature information from the complex wireless signal. The far-field sensors were developed using metamaterial-based split-ring resonator (SRR) architecture. SRR architectures are simple to fabricate and can be embedded within the dielectric ceramic matrix, making it a great candidate for high temperature application. An analytical model was employed to study the effect of refractory metals, intermetallics, and polymer derived ceramics on the wireless response. The research includes the fabrication and implementation of various conductive ceramics at high temperatures, such as lanthanum nickelate, lanthanum chromate, and indium tin oxide. Also, non-oxide materials were evaluated based on the working environment of the sensor, where the fabrication was performed using a novel polymer-derived ceramic (PDC) synthesis by stereolithography technique. The material properties of the semiconducting ceramics were fabricated and evaluated by a suite of characterization tools, namely scanning electron microscope (SEM), energy dispersive x-ray analysis (EDS), and X-ray diffractometer (XRD). The optimal material sets were implemented into the previously described sensor designs, and their signal and sensitivity were evaluated using the signal processing methods developed. The work resulted in the first demonstrations of truly passive wireless temperature sensors completed fabricated by stable, high-temperature conductive ceramic materials ever reported in the literature