P. M. Davulis

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 acoustic wave sensors and systems for harsh environment applications

This paper reviews current progress in the area of wireless microwave acoustic sensor technology, and discusses advances in wireless interrogation systems that can operate in harsh environments. The use of wireless, battery-free, low maintenance surface acoustic wave (SAW) sensors has been successfully demonstrated in applications including high temperature turbine engines and inflatable aerospace structures. Wireless interrogation of multiple sensors up to 910°C has been established and sensor tests in gas turbine engine are reported. This paper elaborates on several aspects of the technology, including: high-temperature thin-film electrode and sensor development, temperature cycling, thermal-shock behavior, testing in turbine engine environments, sensor packaging and attachment, wireless operation, and adaptation to energy and industrial applications.

High-temperature langatate elastic constants and experimental validation up to 900°C

This paper reports on a set of langatate (LGT) elastic constants extracted from room temperature to 1100°C using resonant ultrasound spectroscopy techniques and an accompanying assessment of these constants at high temperature. The evaluation of the constants employed SAW device measurements from room temperature to 900°C along 6 different LGT wafer orientations. Langatate parallelepipeds and wafers were aligned, cut, ground, and polished, and acoustic wave devices were fabricated at the University of Maine facilities along specific orientations for elastic constant extraction and validation. SAW delay lines were fabricated on LGT wafers prepared at the University of Maine using 100-nm platinum rhodium- zirconia electrodes capable of withstanding temperatures up to 1000°C. The numerical predictions based on the resonant ultrasound spectroscopy high-temperature constants were compared with SAW phase velocity, fractional frequency variation, and temperature coefficients of delay extracted from SAW delay line frequency response measurements. In particular, the difference between measured and predicted fractional frequency variation is less than 2% over the 25°C to 900°C temperature range and within the calculated and measured discrepancies. Multiple temperature-compensated orientations at high temperature were predicted and verified in this paper: 4 of the measured orientations had turnover temperatures (temperature coefficient of delay = 0) between 200 and 420°C, and 2 had turnover temperatures below 100°C. In summary, this work reports on extracted high-temperature elastic constants for LGT up to 1100°C, confirmed the validity of those constants by high-temperature SAW device measurements up to 900°C, and predicted and identified temperature-compensated LGT orientations at high temperature.

Pulse echo and combined resonance techniques: a full set of LGT acoustic wave constants and temperature coefficients

This work reports on the determination of langatate elastic and piezoelectric constants and their associated temperature coefficients employing 2 independent methods, the pulse echo overlap (PEO) and a combined resonance technique (CRT) to measure bulk acoustic wave (BAW) phase velocities. Details on the measurement techniques are provided and discussed, including the analysis of the couplant material in the PEO technique used to couple signal to the sample, which showed to be an order of magnitude more relevant than the experimental errors involved in the data extraction. At room temperature, elastic and piezoelectric constants were extracted by the PEO and the CRT methods and showed results consistent to within a few percent for the elastic constants. Both raw acquired data and optimized constants, based on minimization routines applied to all the modes involved in the measurements, are provided and discussed. Comparison between the elastic constants and their temperature behavior with the literature reveals the recent efforts toward the consistent growth and characterization of LGT, in spite of significant variations (between 1 and 30%) among the constants extracted by different groups at room temperature. The density, dielectric permittivity constants, and respective temperature coefficients used in this work have also been independently determined based on samples from the same crystal boule. The temperature behavior of the BAW modes was extracted using the CRT technique, which has the advantage of not relying on temperature dependent acoustic couplants. Finally, the extracted temperature coefficients for the elastic and piezoelectric constants between room temperature and 120degC are reported and discussed in this work.

CDMA and FSCW surface acoustic wave temperature sensors for wireless operation at high temperatures

This paper reports on the successful operation of surface acoustic wave (SAW) devices at high temperature, including wireless measurements of SAW devices up to 750°C. Two types of SAW devices and corresponding systems were investigated: (1) 15-bit code-division multiple-access (CDMA) SAW tags, interrogated using a device-specific direct sequence spread spectrum (DSSS) binary phase shift key (BPSK) code; and (2) multi-track reflective delay lines, interrogated using a frequency-stepped continuous-wave (FSCW) radar signal. The SAW devices used in this work were fabricated on langasite (LGS, La3Ga5SiO14) using Pt/Rh/ZrO2 electrodes, which can withstand temperatures up to 1000°C. A FEM/BEM simulation package using both the Pt/Rh/ZrO2 thin film and LGS properties was used to simulate the interdigital transducers (IDTs) used in the fabricated structures. The two classes of SAW sensors were first directly wired and characterized at high temperature. The FSCW system was then used to test the wireless operation of LGS SAW devices up to 750°C.

GaPO4 Stiffness and Piezoelectric Constants Measurements using the Combined Thickness Excitation and Lateral Field Technique

Gallium orthophosphate (GaPO4) is now a commercially available piezoelectric crystal with great potential as high temperature gas, temperature and pressure sensors for a number of challenging harsh environment applications in the automotive, aerospace, gas and oil well industries. The higher piezoelectric coefficients of GaPO4 with respect to quartz (about 1.2 times higher for e11 2.5 times higher for e14) and the existence of temperature compensated orientations for bulk acoustic waves (BAW) and surface acoustic waves (SAW) are positive characteristics for communications and frequency control applications. In addition, SAW phase velocities along temperature compensated orientations circa 30% lower than that found on ST-X quartz have been predicted, which favors more compact devices. Naturally, the precise determination of any crystal orientation for applications in frequency control, communication, or sensing relies on the measurement of the acoustic properties of the material. In particular, special attention must be devoted to the room temperature measurements performed, otherwise the temperature coefficients, albeit precise, will not result in the correct identification of the temperature compensated orientations. In this work, the elastic and piezoelectric constants of GaPO4 are measured using the combined thickness excitation and lateral field excitation methods, through which multiple harmonics are measured and verified for consistency. Piezocryst Advanced Sensorics GmbH, Graz, Austria, provided the five different plate orientations used in the measurements, namely X, Y, plusmn45deg Y-rotated, and Z cuts. Based on relative precision better than 10-3 for the thickness measurements and 10-4 for the frequency measurements, the resulting phase velocities obtained also have uncertainties in the 10-3 range. The comparison between GaPO4 BAW phase velocities previously published in the literature and the results obtained from this work indicate discrepancies ranging from 0.1 to 3.2%. For instance, along the Y-cut maximum discrepancies around 30 m/s are observed for the quasi longitudinal and quasi fast shear waves, and discrepancies around 55 m/s for the pure shear wave. Consistent e11 values have been extracted from X and Y cuts, which are within 10% of those reported in the literature. The complete set of stiffness and piezoelectric constants measured, the errors involved, and the comparison with the literature are given and discussed in the paper

A full set of langatate high-temperature acoustic wave constants: elastic, piezoelectric, dielectric constants up to 900°c

A full set of langatate (LGT) elastic, dielectric, and piezoelectric constants with their respective temperature coefficients up to 900°C is presented, and the relevance of the dielectric and piezoelectric constants and temperature coefficients are discussed with respect to predicted and measured high-temperature SAW propagation properties. The set of constants allows for high-temperature acoustic wave (AW) propagation studies and device design. The dielectric constants and polarization and conductive losses were extracted by impedance spectroscopy of parallel-plate capacitors. The measured dielectric constants at high temperatures were combined with previously measured LGT expansion coefficients and used to determine the elastic and piezoelectric constants using resonant ultrasound spectroscopy (RUS) measurements at temperatures up to 900°C. 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 with room-temperature values. The LGT high-temperature constants and temperature coefficients were verified by comparing measured and predicted phase velocities (vp) and temperature coefficients of delay (TCD) of SAW delay lines fabricated along 6 orientations in the LGT plane (90°, 23°, Ψ) up to 900°C. For the 6 tested orientations, the predicted SAW vp agree within 0.2% of the measured vp on average and the calculated TCD is within 9.6 ppm/°C of the measured value on average over the temperature range of 25°C to 900°C. By including the temperature dependence of both dielectric and piezoelectric constants, the average discrepancies between predicted and measured SAW properties were reduced, on average: 77% for vp, 13% for TCD, and 63% for the turn-over temperatures analyzed.

High temperature elastic constants of langatate from RUS measurements up to 1100°C

This paper reports on the langatate (LGT) elastic constants and their temperature coefficients measured from room temperature (25degC) to 1100degC using resonant ultrasound spectroscopy (RUS). The constants were extracted by iteratively fitting the resonant peaks with those calculated by Lagrangian mechanics at each temperature where the RUS measurements were taken. In addition, the RUS technique was used to extract the elastic and piezoelectric constants in the 25degC to 120degC temperature range. The extraction of LGT elastic constants up to 1100degC presented in this paper represents a critical step towards the design and fabrication of LGT acoustic wave devices for high temperature and harsh environment applications.

P1F-4 Revisiting LGT Dielectric Constants and Temperature Coefficients Up to 120°C

Langatate (LGT) has been grown and characterized more intensively in the past decade and the reported acoustic wave properties of this relatively recent crystal have shown significant variations among different groups. Yet to be determined is how much of this dissimilarity is attributable to variations in the growth process or to different measurement techniques. For the dielectric permittivity, in particular, previously published values of epsivS 11/epsiv0 differ from each other by as much as 33% while those of epsivS 33/epsiv0 differ by up to 25% at room temperature. In this work, the dielectric constants of LGT are determined by measurements made from room temperature (25degC) up to 120degC. The permittivity was extracted from capacitance measurements using a precision LCR meter and computer controlled oven. LGT plates oriented along the X, Y, and Z crystalline axes were cut, ground, and polished to an optical finish at the University of Maines Microwave Acoustic Lab facilities. The capacitor electrodes were deposited using an aerosol spray method chosen for ease of fabrication and to allow for multiple uses of each of the LGT sample. The measured relative dielectric constants reported in this work are: epsivS 11/epsiv0 is 17.69 +/- 0.30 and epsivS 33/epsiv0 is 70.73 +/- 1.24, which are 11.5% and 7.3% lower then an average of previously published values. The paper discusses the data provided and the associated uncertainties.

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.