Date of Award

12-2013

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical and Computer Engineering

Advisor

Mauricio Pereira da Cunha

Second Committee Member

Nuri Emanetoglu

Third Committee Member

Donald Hummels

Abstract

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.

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