Surface Acoustic Wave (SAW) Resonators for Nuclear Reactor Sensors

In-pile irradiation behavior of aluminum nitride (AlN) based Surface Acoustic Wave (SAW) resonators inside a nuclear reactor were analyzed. SAW devices were fabricated by depositing platinum interdigitated transducer (IDT) on the surface of piezoelectric AlN. Two forms of AlN were examined, AlN/sapphire bilayer and single-crystal AlN (sc-AlN). The resonant frequency of each device is mostly sensitive to the acoustic wave velocity, determined by the elastic constants of the device material, and is impacted by temperature and radiation fields present inside the nuclear reactor. A correlation is established between reactor operating conditions and characteristic signal, introducing sensor applications. Experiments were conducted at The Ohio State University Nuclear Reactor Laboratory (OSU-NRL) to study these devices under the combined influence of temperature up to 400^oC and reactor power up to 450 kW (5.4*10¹² n/cm²s). The resonant frequency of each device exhibits a linear relationship with temperature and neutron fluence, characterized by the temperature coefficient of frequency (TCF) and for nuclear applications we introduce the neutron flux coefficient of frequency (NFCF). Device are characterized by TCF of -69.5 ppm/K for AlN/sapphire and -25.2 ppm/K for sc-AlN. At 300^oC, response to neutron fluence is characterized by NFCF of -97.24 ppm/(n*10¹²/cm²s) for AlN/sapphire and -128.3 for sc-AlN. The response time of the frequency shift can be used to identify the mechanism responsible for the observed shift. Application of proposed kinetic models allows determination of defect absorption rate for 1-particle mechanism and vacancy-interstitial recombination rate for 2-particle mechanism.