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42747-AC10
The Behavior and Properties of New Piezoelectric Crystals at High Temperature (150 Degrees C to 900 Degrees C)
This project has made significant progresses during this past year towards the initial postulated hypothesis of high temperature (150șC to 900șC) operation of the langasite family of crystals (LGX) and also towards the extraction of previously inexistent relevant material properties to enable the usage of these crystals for science and for signal processing, frequency control, communications, and sensor applications.
During the previously reported period, the PI and his team of graduate and undergraduate students looked into the extraction of thermal expansion coefficients using both X-Ray diffractometry and dilatometry up to 1300șC, and successfully obtained consistent and repetitive measurements out of about two dozen data sets based on both measurement techniques. These results, reported in the literature, verified the crystal integrity during the harsh conditions, thus encouraging the continuation of the work towards the initially suggested hypothesis of high temperature operation for these crystals. Also as part of the previous reported period, the UMaine team looked into several techniques to perform the required elastic and dielectric material characterization first in the low temperature range (<120șC) and then at elevated temperature (150șC to 900șC). These techniques have been employed during the presently reported period and are described next.
The measurement of the dielectric and elastic constants in the temperature range below 120șC is important to be used as a starting point for the high temperature measurements and also to establish an independent set of data to be compared with the high temperature technique at the first measured temperatures. Among the techniques used below 120șC are: pulse echo (PE) and resonator techniques (RT), in particular electromagnetic transducer (EMAT), thickness excitation (TE), and lateral field excitation (LFE) techniques. The data obtained from these measurements have been published. The upper 120șC temperature limit for these techniques was imposed by the degradation of the measurement system, namely the magnetic material used in the EMAT, or the electronic fixtures, transducers and cables used in the PE, TE, and LFE techniques.
Above 120șC and up to 1100șC the Resonant Ultrasound Spectroscopy (RUS) method has been selected. This technique measures the mechanical resonances of a sample across a frequency spectrum and compares the measurements with the resonance frequencies calculated by a computer model. The acoustic properties of the material being probed are then extracted from an iterative best-fit process. Advantages of the RUS technique are: (i) possibility of extraction of all the elastic and piezoelectric constants simultaneously from a single measurement of a single sample; (ii.) the bare sample (no electrodes) is contacted with buffer rods that can withstand very high temperatures, thus avoiding mounting fixtures which limit the operation range to less than 150 șC; (iii) the shape of the resonant peaks allows for the extraction of acoustic loss properties of the crystal. The challenging issue with the RUS technique is the significant post-measurement computation time to extract the acoustic properties from the measured resonance spectra, and the fairly high accuracy required from the acoustic properties starting point in order to the non-linear iterative fitting to converge.
Six precisely cut, grinded, and polished LGT crystal parallelepiped samples, with dimension of 3.3 mm wide by 19.2 mm long by 12.9 mm high, were prepared at the University of Maine to meet the required RUS system specification. High temperature (up to 1100șC) RUS measurements were performed by one PhD student and the PI at the Oak Ridge National Laboratories and served the following purposes: (i) to verify the feasibility of the technique to achieve the required high temperature material characterization; (ii) to get better acquainted with RUS techniques; and (iii) to learn how to implement an RUS system at UMaine for further characterization. The measured resonant peaks of the crystalline bars were well defined up to 1100șC, thus providing very encouraging results towards the verification of the original postulated hypothesis on the feasibility of high temperature operation of the LGX crystals.
The next phases of the work include: analysis of the RUS data obtained for the extraction of high temperature elastic and piezoelectric coefficients; assembly of the UMaine RUS system; verification of the measured constants at low temperature; and further characterization of the LGT crystal, including conductivity measurements and long term RUS LGT measurements.
This effort and its related activities have generated so far five papers, two 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 two of his students. In addition, the project has captured the Air Force's attention, with preliminary work under way to investigate the feasibility for high temperature sensor development using the proposed LGT crystal.
