Date of Award

8-2018

Document Type

Thesis - Open Access

Degree Name

Master of Science in Aerospace Engineering

Department

Graduate Studies

Committee Chair

Dr. Daewon Kim

First Committee Member

Dr. Marwan Al-Haik

Second Committee Member

Dr. Eduardo Rojas

Abstract

Surface acoustic wave (SAW) sensors have increasing demand in structural health monitoring due to its passive, variable bandwidth, reliable life-cycle, high accuracy, small size, and light nature. SAW sensors can not only provide the static states of the structural system, such as temperature and pressure but also enable continuous real-time monitoring of dynamic states including strains. SAW sensors are generally fabricated with a rigid piezoceramic substrate, incapable of adapting to highly curved surfaces or flexible objects. The ongoing demands of sensor adaptability with flexible substrates, which is also capable of wireless monitoring, is the basis of the research that can be applied to medical and aerospace fields.

The SAW fabrication involves developing a composite substrate using hot-pressing and depositing the interdigital transducers using additive manufacturing. The substrate of the SAW sensor is fabricated by integrating lead zirconate titanate (PZT) ceramic nanoparticles as a reinforcement into polyvinylidene fluoride (PVDF) polymer matrix in the 0-3 direction. This enables the substrate to attain enhanced piezoelectric properties along with improved mechanical strength. PVDF is dissolved in a strong polar solvent such as N, N- dimethyl sulfoxide to which 50 wt. % of PZT powder is added to impart optimum dielectric property maintaining flexibility. Stretching by hot-pressing this mixture above the melting point of PVDF enables the transformation of non-polar α to a polar β crystalline phase of PVDF. The amount of β crystalline phase of the substrate produced out of stretching the PVDF is observed under FTIR scanning.

Research has also been conducted by using a PVDF-TrFE, a copolymer of PVDF and brief comparison study on the flexibility of the composite developed using PZT/PVDF and PZT/PVDF-TrFE is performed.

Polarization is conducted by sputtering gold on both sides of the thin substrate and subjecting it to a high electric field in a silicone oil bath to prevent arcing. The dielectric properties of the sample are measured by which frequency and attenuation are calculated mathematically. Delay-line IDTs are attached to it using conventional photolithography technique. With the sensor being developed, radio frequency signal is passed through an interrogator to the antenna connected to the input transducer which transfers the signal in the form of Rayleigh waves. The frequency response of the SAW device changes in amplitude and phase when subjected to temperature, pressure, or strain changes. FEM model of the SAW sensor is conducted, and the resulting deformation simulation is performed. This thesis discusses the development process of a flexible piezocomposite SAW sensor.

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