| Electromechanical effects including piezoelectric, piezoresistive, capacitive and flexoelectric effects are ubiquitous in biological and material systems. Among them, the flexoelectric effect, the fundamental electromechanical coupling effect between electric polarization and mechanical strain/stress gradient, has drawn researchers' attention because of its uniqueness. In crystallography, flexoelectricity exhibits in all dielectric solids while piezoelectricity can only be found in non-centrosymmetric crystal system. Moreover, flexoelectric effect is size-dependent effect that becomes more significant in micro/nano scale. Thus, it is important to explore flexoelectric effect of materials and to explore the associated sensor applications. In this dissertation research, the characterizations of barium strontium titanate, bulk ceramic and thin films, were carried out. Furthermore, several types of flexoelectric sensors for vibration and sound pressure monitoring were designed, fabricated and tested.;Firstly, BST unimorph type micro-cantilevers with the thickness ranging from 25 mum to 100 mum were prepared for measurement of the transverse flexoelectric coefficient mu12. The measured mu12 of bulk BST was about 10 muC/m, suggesting that BST possesses excellent flexoelectric properties.;In order to exploit the flexoelectric effect in BST thin films, BST thin film with thickness of 130 nm was deposited on silicon wafers using the RF magnetron sputtering process. The flexoelectric coefficients of the thin films were then determined experimentally. It was revealed that the BST thin films possessed a transverse flexoelectric coefficient of 24.5 muC/m at Curie temperature (∼ 28 °C) and 17.44 muC/m at 41 of, respectively. The measured flexoelectric coefficients are comparable to that of bulk BST ceramics, which were reported to be 10 - 100 muC/m.;The relatively low flexoelectric coefficients of ferroelectrics inhibit the potential in developing flexoelectric sensing devices. Therefore, multilayered structure using BST ceramic was attempted to enhance effective flexoelectric coefficients. The performance of piezoelectric and flexoelectric cantilevers under the same dimensions and conditions was first compared analytically. Owing to the flexoelectric scaling effect, under the same force input, BST flexoelectric structure would generate higher charge output than that of its piezoelectric counterparts. Also, amplification of charge output using multilayered structure was experimentally verified. The prototyped structure consisted of three layers of 350 mum-thick BST plates with a parallel electric connection. The charge output of the multilayered structure was approximately 287 % of that obtained using a single-layer structure with the same total thickness and under the same end deflection input.;In order to demonstrate the sensing applications with the characterized flexoelectric BST ceramics, flexoelectric accelerometer was designed, fabricated and tested for vibration monitoring. The flexoelectric sensor is configured as a trapezoidal shape micro-unimorph with a BST layer bonded onto a steel substrate. A seismic mass was attached to the unimorph tip to amplify the transverse flexoelectric response of the BST layer. The theoretical model was developed and validated by vibration tests using the prototyped flexoelectric micro-unimorph. The prototyped accelerometer showed a stable sensitivity of 0.84 pC/g in the frequency range of 100 Hz - 1.6 kHz.;A flexoelectric bridge-structured microphone using BST ceramic was designed and fabricated. The prototyped flexoelectric microphone was tested in an anechoic box using a loud speaker as the pressure source. Charge sensitivity of the flexoelectric microphone was measured and calibrated using a reference microphone. The 1.5 mm x 768 mum x 50 mum flexoelectric microphone showed a sensitivity of 0.92 pC/Pa, and the resonance frequency of 98.67 kHz. The signal to noise ratio was measured to be 74 dB. The analytical and experimental results suggest that the flexoelectric microphone is featured with both high sensitivity and broad bandwidth.;The findings from this dissertation research strongly suggest that micro/nano flexoelectric sensing is promising for a broad range of applications. |
