Modeling and preliminary characterization of passive, wireless temperature sensors for harsh environment applications based on periodic structures

Wireless temperature sensing has attained significant attention in recent years due to the increasing need to develop reliable and affordable sensing solutions for energy conversion systems and other harsh environment applications. The development of next generation sensors for energy production processing parameters, such as temperature and pressure, can result in better performance of the system. Particularly, continuous temperature monitoring in energy conversion systems can result in enhancements such as better system integrity, less pollution and higher thermal efficiencies. However, the conditions experienced in these system components hinder the performance of current solutions due to the presence of semi-conductor materials and welded joints. Additionally, the use of wired systems can result in complex wiring networks, increasing the cost of installation, maintenance and sensor replacement. Therefore, next generation sensing solutions must be developed to overcome current challenges in systems where adverse conditions are present. This research project proposes two novel passive, wireless temperature sensor designs based on concepts of guided mode resonance filters (GMRF) and metamaterials. For the GMRF, a tri-layer structure using a metallic encasing and a circular aperture grating layer was developed to have a resonance frequency of 10 GHz. While for the metamaterial-based sensor a continuation of previous work was presented by utilizing a dielectric substrate and an array of commercially available metallic washers divided in two layers. For both designs, High Frequency Structure Simulator (HFSS) from ANSYS® was employed to assess the feasibility of the sensor as well as to optimize the geometry and guide the fabrication process. A systematic approach consisting of evaluating the unit cell, then assessing the number of periods needed, and finally characterizing the response of the final sensor was followed for each case. After the modeling process was completed, the optimal configuration for the GMRF sensor was found to be the with an alumina slab with a thickness of 1.524 mm, two titanium screens with a thickness of 0.508, the use of metallic side reflectors and a side length of 49.525 mm. For the metamaterial, the process aforementioned resulted in a sensor design composed of a BTO/BN ceramic substrate and copper washers with 3.5 mm in OD and 1.6 mm in ID; the sensor side length was of 101.7 mm and design thickness was chosen to be 3.175 mm. The performed simulations resulted in several peaks in a 6 – 18 GHz frequency range for both the reflection and transmission spectra. The limitation of the periodicity had a detrimental effect on the response of the sensor; however, a final sensor design was achieved with visible response in both the reflection and transmission regions. Fabrication was carried over using water-jet cutting and traditional machining methods for the GMRF sensor, while a traditional powder compression method was employed for the metamaterial sensor. For the former, titanium screens were used, while aluminum and steel plates were employed on the second one. Commercially available alumina ceramic was employed for both fabrication methods. As for the metamaterial sensor, the fabrication was done by utilizing a mixture of 70% boron nitride/30% barium titanate with an added 7.5% wt. PVA for structural rigidity. Final dimensions of 50.8 mm in side length and a thickness of 3.175 mm were achieved. Samples fabricated showed good structural integrity and manageability. Preliminary free space measurements were performed using a Programmable Network Analyzer (PNA) and a set of X-band horn antennas and Gaussian beam antennas to characterize the response of both the GMRF and the metamaterial sensors, respectively. No visible peak was observed for the GMRF sensor in the frequency region. The lack of response might be attributed to fabrication errors. For the metamaterial sensor, a strong response at 14.47 GHz mark with an intensity of -33.05 dB was observed. The response found could be employed for temperature measurements. Finally, suggestions for future work are given to overcome the challenges present in current sensor designs and fabrication processes