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42747-AC10
The Behavior and Properties of New Piezoelectric Crystals at High Temperature (150 Degrees C to 900 Degrees C)

Mauricio Pereira da Cunha, University of Maine

The project entitled ‘The Behavior and Properties of New Piezoelectric Crystals at High Temperature (150ºC to 900ºC)' under development at the University of Maine achieved major breakthroughs during this past year towards the original postulated hypothesis of high temperature (150ºC to 900ºC) operation and material properties extraction of the langasite family of crystals (LGX). The investigation has successfully shown that langasite and langatate crystals, LGS and LGT, respectively, allow device operation for long term (about six months tested so far at 800ºC). The research progressed to the extraction of unknown relevant acoustic wave (AW) material properties at high temperatures to enable fundamental material investigation and also the design of devices for signal processing, frequency control, communications, and sensor applications.

Significant previous accomplishments reported by the PI and his team of graduate and undergraduate students included: (i) the consistent extraction of high temperature thermal expansion coefficients using both X-Ray diffractometry and dilatometry up to 1300ºC; (ii) verification of the crystal integrity up to 1300ºC; (iii) verification of the LGS crystal piezoelectric activity up to at least 1000ºC; (iv) extraction of low temperature range (<120ºC) elastic and dielectric material properties for starting point reference using several techniques (pulse echo; resonator techniques, in particular electromagnetic-EMAT transducer, thickness excitation, lateral field excitation techniques; and capacitance measurements) for consistency check and improved precision;  (v) crystal sample preparation for high temperature microwave acoustic properties material extraction using resonant ultrasound spectroscopy (RUS); and (vi) preliminary (RUS) measurements at the Oak Ridge National Laboratory (ORNL) at temperature up to 1100ºC.

During the presently reporting period, efforts have been dedicated to successfully implement the RUS measurement system at the University of Maine and to extract the high temperature elastic and piezoelectric constants using the high temperature RUS measurements. RUS measurements up to 1100ºC have been successfully performed at the Microwave Acoustic Materials Laboratory at the University of Maine. In addition, significant attention has been dedicated to the selection of modes from the measured resonance spectra and iterative minimization computation to extract microwave acoustic elastic and piezoelectric properties. A fairly high accuracy is required with respect to the identification and selection of modes; selection of starting frequencies and tracking in order for the non-linear iterative method to converge. For this purpose, the prior work of material property extraction at lower temperatures has been critical to enable the proper tracking of the elastic and piezoelectric constants.

In order to verify the original hypothesis that the LGX's unique combination of piezoelectric and anisotropic properties will yield orientations for bulk or surface acoustic waves with propagation characteristics which are markedly different than those found in today's common piezoelectric materials, efforts were dedicated to investigate the crystal operation at high temperature using surface acoustic wave (SAW) devices. High temperature layered and alloyed thin films electrodes (around 120 nm thick) were successfully researched and developed using platinum alloys combined with ceramic zirconia, which enabled unprecedented SAW device long-term tests (five months and a half) at 800 ºC, and medium-term tests (up to a few weeks) up to 1000ºC. Also very important, no permanent degradation in the crystal properties has been observed after exposing these devices to multiple cycles between room temperature and elevated temperatures.

Present on-going work relates to: (a) mode selection and high temperature elastic and piezoelectric microwave acoustic material properties extraction at temperatures between 150ºC to 1100ºC; (b) orientation, cut, grinding, and polishing of unique crystal orientations for the validation of the microwave acoustic material properties and temperature coefficients at low temperature (<150ºC) and high temperature (from 150ºC to 1100ºC) using SAW devices; (c) fabrication of SAW devices and test of these devices at the referred low and high temperature ranges for the validation of the extracted AW constants and temperature coefficients.

 Next steps in the work include: analysis of the RUS data; verification of the measured constants and temperature coefficients; further characterization of the LGT crystal, including conductivity measurements and long-term RUS LGT measurements, high temperature dielectric measurements, and AW device design and operation at high temperature.

This project and its related activities have generated so far seven papers, three workshop presentations, a master's thesis, and presently relate to the work of two PhD students, one of whom is directly supported by this project. The activities developed so far have profoundly influenced the research at UMaine, strongly affecting the PI's career and that of at least three of his students. In addition, the project has captured the attention of industry, Navy, and Air Force, with preliminary work under way to investigate the feasibility for high temperature sensor development using the proposed LGT crystal.

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