WIRELESS AND BATTERYLESS SURFACE ACOUSTIC WAVE SENSORS FOR HIGH TEMPERATURE ENVIRONMENTS

International audienceSurface acoustic wave (SAW) devices are widely used as filter, resonator or delay line in electronic systems in a wide range of applications: mobile communication, TVs, radar, stable resonator for clock generation, etc. The resonance frequency and the delay line of SAW devices are depending on the properties of materials forming the device and could be very sensitive to the physical parameters of the environment. Since SAW devices are more and more used as sensor for a large variety of area: gas, pressure, force, temperature, strain, radiation, etc. The sensors based SAW present the advantage to be passive (batteryless) and/or wireless. These interesting properties combined with a small size, a low cost radio request system and a small antennas when operating at high frequency, offer new and exiting perspectives for wireless measurement processes and IDTAG applications. When the materials constituting the devices are properly selected, it becomes possible to use those sensors without embedded electronic in hostile environments (as high temperature, nuclear site, …) where no solutions are currently used. General principle of the SAW sensor in wired and wireless configurations will be developed and a review of recent works concerning the field of high temperature applications will be presented with specific attention given to the choice of materials constituting the SAW device

Characterization of the Elastic, Piezoelectric, and Dielectric Properties of Langatate At High Temperatures Up To 900 ◦C

The acoustic wave constants of the piezoelectric crystal langatate (LGT, La3Ga5.5Ta0.5O14) are characterized up to 900 ◦C for the first time in this work, targeting the development of high-temperature acoustic wave (AW) devices. There is a pressing need for sensors and frequency control systems that operate at high temperature, above 200 ◦C, and in harsh environments, with applications found in the industrial process control, automotive, aerospace, and power generation industries and in gas and petroleum exploration. Surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices, using piezoelectric crystals such as LGT, have the capability to provide the desired high-temperature sensors for temperature, pressure, strain, and gas species measurement. Langatate, a member of the langasite crystal family, retains crystal structure up to the melting point at 1470 ◦C and has higher piezoelectric coupling than quartz and langasite; however, a full set of LGT AW constants required for SAW and BAW device design had not previously been characterized above 150 ◦C. In this work, the LGT elastic, piezoelectric, and dielectric constants have been extracted up to 900 ◦C, providing a complete set of AW constants. The LGT elastic and piezoelectric constants were measured using resonant ultrasound spectroscopy (RUS) and determined by fitting predicted resonance modes with measured spectra of LGT samples heated in a custom-fabricated high-temperature furnace. The LGT dielectric permittivity and conductivity constants were extracted from parallel-plate capacitor measurements. Langatate SAW devices were fabricated and the measured properties up to 900 ◦C were found to be in very good agreement with the predictions, thus validating the extracted LGT high-temperature constants. The newly determined LGT constants were used to locate SAW orientations for high-temperature operation by calculating the SAW velocity and temperature coefficient of delay (TCD) up to 900 ◦C along multiple regions in space. Multiple SAW orientations were identified with potentially desirable properties such as turnover temperatures, TCD=0, at elevated temperatures up to 500 ◦C, and either low or high sensitivity to temperature. Differential high-temperature sensors utilizing a suite of SAW sensors on the same wafer were proposed and experimentally demonstrated. Additionally, BAW orientations were identified with turnover temperatures ranging from 100 ◦C to 550 ◦C

Microwave Acoustic SAW Resonators for Stable High-temperature Harsh-Environment Static and Dynamic Strain Sensing Applications

High-temperature, harsh-environment static and dynamic strain sensors are needed for industrial process monitoring and control, fault detection, structural health monitoring in power plant environments, steel and refractory material manufacturing, aerospace, and defense applications. Sensor operation in the aforementioned extreme environments require robust devices capable of sustaining the targeted high temperatures, while maintaining a stable sensor response. Current technologies face challenges regarding device or system size, complexity, operational temperature, or stability. Surface acoustic wave (SAW) sensor technology using high temperature capable piezoelectric substrates and thin film technology has favorable properties such as robustness; miniature size; capability of mass production; reduced installation costs; battery-free operation; maintenance-free; and offer the potential for wireless, multi-sensor interrogation. These characteristics are very attractive for static and dynamic strain sensors targeted to operate in high-temperature harsh-environment conditions. The investigation of harsh-environment static and dynamic SAW strain sensors requires addressing the issues of: (i) sensor platform endurance and stability; (ii) development of durable packaging and attachment techniques; (iii) temperature compensation techniques, to mitigate temperature cross-sensing; and (iv) methods of sensor interrogation and calibration at high temperatures. In this work, langasite-based SAW resonator (SAWR) sensors have been investigated. A stable sensor platform was verified for two types of thin-film electrode configurations, namely: co-deposited Pt/Al2O3 (up to 750oC) and multilayered PtNi|PtZr (up to 1000oC). High-temperature sensor attachment solutions for strain sensor applications were developed for temperatures up to 500oC. The developed SAWR sensors were tested and calibrated for both static and dynamic strain up to 400oC. A temperature compensation technique and a novel finite element analysis was used to perform high-temperature static strain calibration. A high-temperature dynamic strain test rig using a constant stress beam was designed, implemented and used to characterize the SAWR strain sensor performance in measuring dynamic strain. Using the in-phase and quadrature strain sensor signal analysis technique proposed and developed in this study, the existence of both amplitude and frequency modulations of the SAWR RF signal by the dynamic strain signal was confirmed, and the two types of modulations separated and quantified

Emerging Needs for Pervasive Passive Wireless Sensor Networks on Aerospace Vehicles

NASA is investigating passive wireless sensor technology to reduce instrumentation mass and volume in ground testing, air flight, and space exploration applications. Vehicle health monitoring systems (VHMS) are desired on all aerospace programs to ensure the safety of the crew and the vehicles. Pervasive passive wireless sensor networks facilitate VHMS on aerospace vehicles. Future wireless sensor networks on board aerospace vehicles will be heterogeneous and will require active and passive network systems. Since much has been published on active wireless sensor networks, this work will focus on the need for passive wireless sensor networks on aerospace vehicles. Several passive wireless technologies such as microelectromechanical systems MEMS, SAW, backscatter, and chipless RFID techniques, have all shown potential to meet the pervasive sensing needs for aerospace VHMS applications. A SAW VHMS application will be presented. In addition, application areas including ground testing, hypersonic aircraft and spacecraft will be explored along with some of the harsh environments found in aerospace applications

SAW Sensor for Fastener Failure Detection

The proof of concept for using surface acoustic wave (SAW) strain sensors in the detection of aircraft fastener failures is demonstrated. SAW sensors were investigated because they have the potential for the development of passive wireless systems. The SAW devices employed four orthogonal frequency coding (OFC) spread spectrum reflectors in two banks on a high temperature piezoelectric substrate. Three SAW devices were attached to a cantilever panel with removable side stiffeners. Damage in the form of fastener failure was simulated by removal of bolts from the side stiffeners. During testing, three different force conditions were used to simulate static aircraft structural response under loads. The design of the sensor, the panel arrangement and the panel testing results are reported. The results show that the sensors successfully detected single fastener failure at distances up to 54.6 cm from the failure site under loaded conditions

Characteristics of CVD Grown Diamond Films on Langasite Substrates

Surface acoustic wave (SAW) devices consist of a piezoelectric substrate with interdigitated (IDT) electrodes. These devices can be used to fabricate wireless and passive sensors that can be mounted in remote and/or inaccessible places. If encapsulated with CVD diamond, the SAW devices can be made to operate under extremely hostile conditions. The piezoelectric layer (AlN, ZnO etc.) deposited on the diamond or an inverse system can increase the frequency of the SAW device. Most piezoelectric materials (such as quartz) show phase transition temperatures below diamond deposition temperature (650º-1100ºC), preventing their use as a substrate for diamond growth. Langasite La3Ga5SiO14 (LGS) is recently fabricated piezoelectric material that can withstand high temperatures without being deteriorated. LGS does not have phase transitions up to its melting point of 1470°C.Here we report the deposition of diamond films by microwave plasma CVD in methane-hydrogen gas mixtures on polished and rough surfaces of the LGS substrates seeded with nanodiamonds. No buffer layer between the substrate and the coating had been used. The effect of substrate pretreatment (PT) was also investigated on the growth behaviour of diamond films on LGS. The resulting films are characterised by Raman spectroscopy, X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS). The effect of substrate roughness on the growth behaviour was found to favour bigger grain sizes on the unpolished substrates. Whereas, the effect of substrate pretreatment (PT) was found to produce unique microstructural features with better polycrystalline diamond (PCD) quality than on the substrates without PT. Raman signals confirm the deposition of PCD in all the cases but the X-ray results interestingly show new phase formation of hcp and rhombohedral diamond lattice structures under CVD growth environment

Temperature and Strain Coefficient of Velocity for Langasite SAW Devices

Surface Acoustic Wave sensors on Langasite substrates are being investigated for aerospace applications. Characterization of the Langasite material properties must be performed before sensors can be installed in research vehicles. The coefficients of velocity for both strain and temperature have been determined. These values have also been used to perform temperature compensation of the strain measurements

Wireless Sensor Applications in Extreme Aeronautical Environments

NASA aeronautical programs require rigorous ground and flight testing. Many of the testing environments can be extremely harsh. These environments include cryogenic temperatures and high temperatures (greater than 1500 C). Temperature, pressure, vibration, ionizing radiation, and chemical exposure may all be part of the harsh environment found in testing. This paper presents a survey of research opportunities for universities and industry to develop new wireless sensors that address anticipated structural health monitoring (SHM) and testing needs for aeronautical vehicles. Potential applications of passive wireless sensors for ground testing and high altitude aircraft operations are presented. Some of the challenges and issues of the technology are also presented