Mauricio Pereira da Cunha

Recent advances in harsh environment acoustic wave sensors for contemporary applications

There is a significant need for wireless sensor systems capable of operation up to 1100°C and beyond, in abrasive or corrosive harsh environments, in particular for the energy, steel, aerospace, oil and gas exploration industries. These environments and applications preclude the use of batteries and normally require wireless and multiple sensor interrogation. The University of Maine and Environetix Technologies have successfully responded to these needs by researching and developing surface acoustic wave (SAW) sensors based on the langasite family of crystals and co-deposited Pt/Rh/ZrO2 thin-film electrode technology. This paper reports on the recent achievements, which include: long term operation in furnace and technology validation in jet-engine static and rotating parts up to 53,000 gs; stable and repetitive wired and wireless responses of temperature sensors; multiple wireless sensor interrogation; and associated packaging (tests run in the 200°C to 1000°C range).

Wireless sensing in hostile environments

Microwave acoustic devices have long been shown to provide sensitive platforms for physical and gas sensors. Piezoelectric substrates introduced over the past two decades, such as the langasite (LGS) family of crystals, gallium orthophosphate, and aluminum nitride have enabled the exploration of microwave acoustics sensing at temperatures above 500°C. However the ability of the substrate to withstand high temperatures is only one of the requirements for sensor operation in harsh environments. Other prerequisites are: the development of stable high-temperature device electrodes; appropriate packaging; and resilience of the entire system to thermal shock and thermal cycling. In this paper, the wireless operation of microwave acoustic wave sensors in harsh environments is reviewed, and recent achievements are highlighted. Particular focus is given to surface acoustic wave (SAW) sensor technology because it has the advantage of being wireless, battery-free, and allows interrogation of arrays of multiple devices. During the last decade, wireless hostile environment microwave acoustic sensors that can detect gases and monitor temperature and pressure have transitioned from laboratory proof-of-concept to commercial devices and systems. Recent accomplishments in thin film technology and packaging are extending the technology to even higher temperatures, beyond 1000°C. These technological advances are enabling wireless microwave acoustic sensor applications in hostile environments including power plants, turbine engines, oil/gas refineries, and high temperature industrial processing.

Hydrogen Fluoride Gas Detection Mechanism on Quartz Using SAW Sensors

Hydrogen fluoride (HF) is a hazardous compound used in a variety of industrial processes and is a decomposition product of many environmentally harmful fluorinated volatile organic compounds. Surface acoustic wave (SAW) resonators on quartz substrates are suited for HF sensing because the analyte reacts directly with the sensor substrate, producing H2O and the volatile compound, SiF4. This work shows evidence that during gas phase HF exposure to a generalized SAW (GSAW) resonator and a pure shear horizontal SAW (SH-SAW) resonator, the dominant sensing mechanism is the detection of a condensed liquid layer on the substrate surface, rather than simply material removal via SiF4 desorption. The GSAW and pure SH-SAW resonators, fabricated on ST-X and ST-90° quartz, respectively, have been simultaneously exposed to HF in a low-volume (1.3 cm3) test cell. An automated gas delivery system developed under this project varied HF concentrations from 1-18 ppm. Both resonators are sensitive to the formation of a condensed liquid layer, but the frequency shift of the pure SH-SAW resonator, due to this effect, is up to 4.6 times greater than that of the GSAW device for the HF concentrations investigated. The measured sensor frequency response to potential inteferents, such as R-134a (C2H2F4), isopropanol (C3H8O), propane (C3H8), acetone (C3H6O), and carbon monoxide (CO), is below the devices limit of detection, while its response to HF is as high as 7.5 times its limit of detection.

Langasite SAW pressure sensor for harsh environments

Wireless pressure sensing in harsh environments at temperatures in excess of 200°C has important applications in energy generation, aerospace, and industrial processing. Such environments restrict or prevent the use of silicon based technology and battery powered sensor devices. Microwave acoustics, in particular surface acoustic wave (SAW) technology, allows the design of wireless pressure sensors to operate battery-free in a harsh environment. Current SAW-based pressure sensors operate up to approximately 200°C and 150 psi and typically employ quartz substrates. The Langasite (LGS) family of crystals retains their piezoelectric properties up to their melting point of approximately 1470°C, thus opening up the possibility of designing and implementing very high temperature and harsh environment LGS SAW pressure sensors. A square shaped sealed cavity has been used in this work with sensors strategically placed at the edges of the cavity for improved differential sensitivity and temperature compensated response. The fabricated devices were tested up to 225 psia and 500°C.

Transverse waveguide mode suppression for Pt-electrode SAW resonators on Quartz and LGS

SAW resonators on ST-X quartz and langasite (LGS) [0°, 144°, 24°] are currently being used for hydrogen fluoride (HF) vapor sensing and high-temperature sensing, respectively. For these applications, the use of Pt-based electrodes allows the resonators to withstand the targeted harsh environments. This work reveals that for Pt-electrode resonators with conventional short-circuit gratings on the aforementioned quartz and LGS orientations, acoustic energy leaks from the grating region to the bus bars, thus degrading the resonator response. To resolve this problem, this paper proposes and implements open-circuit gratings for resonators fabricated with these substrate/metal combinations. The open-circuit gratings guide the acoustic energy within the grating region, resulting in greater quality factors and reduced losses in the resonator response. In addition, scalar potential theory is utilized in this work to identify transverse waveguide modes in the responses of open-circuit grating resonators on quartz and LGS. A transverse waveguide mode dispersion relation was derived to extend the scalar potential theory to account for asymmetry in the slowness curve around the propagation direction. This is the case for several commonly used LGS orientations, in particular LGS [0°, 144°, 24°]. Finally, this work addresses spurious transverse mode mitigation by scaling both the transducers grating aperture and electrode overlap width. Open circuit grating resonators with appropriately scaled transducer designs were fabricated and tested, resulting in a 71% increase in quality factor and a spurious mode rejection of over 26 dBc for Pt-electrode devices on ST-X quartz. This progress directly translates into better frequency resolution and increased dynamic range for HF vapor sensors and high-temperature SAW devices.

Electrically conductive Pt-Rh/ZrO2 and Pt-Rh/HfO2 nanocomposite electrodes for high temperature harsh environment sensors

Nanocomposite films comprised of either Pt-Rh/ZrO2 or Pt-Rh/HfO2 materials were co-deposited using multiple e-beam evaporation sources onto langasite (La3Ga5SiO14) substrates, both as blanket films and patterned interdigital transducer electrodes for surface acoustic wave (SAW) sensor devices. The films and devices were tested after different thermal treatments in a tube furnace up to 1200°C. X-ray diffraction and electron microscopy results indicate that Pt-Rh/HfO2 films are stabilized by the formation of monoclinic HfO2 precipitates after high temperature exposure, which act as pinning sites to retard grain growth and prevent agglomeration of the conductive cubic Pt-Rh phase. The Pt-Rh/ZrO2 films were found to be slightly less stable, and contain both tetragonal and monoclinic ZrO2 precipitates that also help prevent Pt-Rh agglomeration. Film conductivities were measured versus temperature for Pt-Rh/HfO2 films on a variety of substrates, and it was concluded that La and/or Ga diffusion from the langasite substrate into the nanocomposite films is detrimental to film stability. An Al2O3 diffusion barrier grown on langasite using atomic layer deposition (ALD) was found to be effective in minimizing interdiffusion between the nanocomposite film and the langasite crystal.

Conductivity and complex permittivity of langatate at high temperature up to 900°C

There are a large number of high-temperature sensing and frequency control applications that can be addressed using acoustic wave devices capable of operation at high-temperatures. For those applications, it is important to characterize the acoustic properties of the piezoelectric crystal used as substrate at elevated temperatures. Langatate (LGT) is one of the crystals which allow the fabrication of SAW devices at elevated temperatures. In a previous work, the authors measured and discussed the LGT elastic constants up to 900°C. This paper reports the langatate complex dielectric permittivity and conductivity from 25 to 900°C. The constants were extracted from impedance measurements of parallel-plate capacitors fabricated with Pt/Rh/ZrO2 electrodes on LGT wafers aligned along the X and Z crystalline axes. The real permittivities, έ11 and έ33, were found to change significantly in the range from 25 to 900°C with a 38% increase and a 49% decrease of their room temperature values, respectively. Thus, it is important to include the extracted high temperature permittivities when designing LGT acoustic wave devices and not simply to use extrapolated low temperature data. Both LGT conductivity and imaginary permittivity are necessary to quantify the electrical losses of sensors, signal-processing, and frequency-control devices operating with this substrate at high-temperatures.

Impact of high-temperature dielectric and piezoelectric behavior on LGT acoustic wave properties up to 900°C

The langatate (LGT) elastic, dielectric and piezoelectric constants with respective temperature coefficients up to 900°C are reported for the first time. The new set of constants was used to improve the predictions of high-temperature LGT surface acoustic wave (SAW) properties such as phase velocity (vp), temperature coefficient of delay (TCD), and electromechanical coupling (K2) along multiple orientation sweeps up to 900°C. These predictions were then compared to previous calculations, which ignored the temperature dependence of the dielectric and piezoelectric constants, and to measured data up to 900°C, obtained from SAW delay lines fabricated along 6 orientations in the LGT plane (90°, 23°, Ψ). The average discrepancy between predicted and measured vp and TCD responses between 25 and 900°C were reduced by a factor of 4 for vp and 13% for TCD when the temperature dependence of both dielectric and piezoelectric constants are considered. The extracted LGT piezoelectric constants and temperature coefficients show that e11 and e14 change by up to 62% and 77%, respectively, for the entire 25°C to 900°C range when compared to room temperature values. In addition, this paper uncovers the full set of high-temperature LGT elastic, piezoelectric, and dielectric constants and temperature coefficients applicable up to 900°C, including the respective estimated uncertainty.

Monitoring polymer properties using shear horizontal surface acoustic waves.

Real-time, nondestructive methods for monitoring polymer film properties are increasingly important in the development and fabrication of modern polymer-containing products. Online testing of industrial polymer films during preparation and conditioning is required to minimize material and energy consumption, improve the product quality, increase the production rate, and reduce the number of product rejects. It is well-known that shear horizontal surface acoustic wave (SH-SAW) propagation is sensitive to mass changes as well as to the mechanical properties of attached materials. In this work, the SH-SAW was used to monitor polymer property changes primarily dictated by variations in the viscoelasticity. The viscoelastic properties of a negative photoresist film were monitored throughout the ultraviolet (UV) light-induced polymer cross-linking process using SH-SAW delay line devices. Changes in the polymer film mass and viscoelasticity caused by UV exposure produced variations in the phase velocity and attenuation of the SH-SAW propagating in the structure. Based on measured polymer-coated delay line scattering transmission responses (S(21)) and the measured polymer layer thickness and density, the viscoelastic constants c(44) and eta(44) were extracted. The polymer thickness was found to decrease 0.6% during UV curing, while variations in the polymer density were determined to be insignificant. Changes of 6% in c(44) and 22% in eta(44) during the cross-linking process were observed, showing the sensitivity of the SH-SAW phase velocity and attenuation to changes in the polymer film viscoelasticity. These results indicate the potential for SH-SAW devices as online monitoring sensors for polymer film processing.

Langatate temperature-compensated BAW orientations identified using high-temperature constants

LGT BAW orientations are investigated up to 900°C for temperature-compensated orientations targeting harsh-environment applications. The study utilizes recently published LGT elastic, piezoelectric, and dielectric constants extracted by the authors using resonant ultrasound spectroscopy (RUS) up to 900°C based on the resonances of bulk crystal samples. Temperature-compensated LGT BAW orientations have been identified for operation at high temperatures. The orientations disclosed in this work have turnover temperatures between 100°C and 550°C for the one quasi-shear mode and up to 150°C for the other quasi-shear mode. Additionally, orientations were identified with pure transverse-shear modes that are selectively excitable and have temperature compensation up to 550°C. These orientations can be readily applied for both frequency control and sensor applications in harsh environments.