Tesis doctoral de Joao Luis Ealo Cuello
Internally charged, cellular polymer electrets are a new class of materials which exhibit interesting electromechanical characteristics. Due to its piezoelectric behaviour, which resembles that of ferroelectric materials, the name ferroelectret has been coined for this kind of materials. Its recent appearance and remarkable properties open up new possibilities for the design and development of ultrasonic transducers. therefore, there is a need for quantifying the real potential of ferroelectrets in different fields, such as air-coupled ultrasonic applications, which comprises the main objective of this dissertation. ferroelectret technology is an excellent candidate for the current demand to develop airborne ultrasonic transducers, which satisfy, at the design stage, the trade-off among directivity, bandwidth, sensitivity and cost. This is possible thanks to its high d33 piezoelectric coefficient, its wide frequency range of operation (from 30 khz up to 2 mhz), its low acoustic impedance (approx. 0.03 mrayls) which results in a good adaptation to the air and its unprecedented mechanical flexibility. Furthermore, ferroelectrets are easy to use and have a low fabrication cost. up to our knowledge, the electromechanical film emfit is the only ferroelectreto material commercially available. For this reason, all the experimental results included in this work have been accomplished using this film. At present, different ferroelectret foils, which exhibit a superior performance compared to that of the emfit, have been developed by several researchers at a laboratory scale. However, the emfit electromechanical characteristics allowed us to demonstrate the potential and versatility of ferroelectrets in air-coupled applications. this thesis consists of five self-contained chapters. First, a complete state of the art of the ferroelectret technology is introduced. This includes the advantages and limitations of this material along with its potential in airborne ultrasound applications. Likewise, an outline of the well-known technologies used in air applications is shown in order to allow the reader to locate the ferroelectrets in the current scene. chapter two includes characterization results of emfit-based ultrasonic transducers. These are obtained using laser interferometry to measure the performance of single and double-layer prototypes fabricated on at substrates. Moreover, we show that in spite of the anisotropy of the ferroelectret emfit film and the inhomogeneity of its vibratory behavior, the average frequency response of multilayer transducers in air is fairly well predicted with lumped-element coupled models. in chapter three, a new concept of ultrasonic transducers is introduced. Thanks to the high mechanical flexibility of the emfi fillm along with its piston-like vibratory behavior, which remains unaltered when the film is glued on a curved substrate, it is possible to develop, what we have called, developable surface ultrasonic transducers. this result opens up the possibility of designing transducers with custom acoustic pattern, as long as the film is fixed on the substrate of a developable surface. Different transducer prototypes of special directivity patterns are modeled and empirically characterized, namely, omnidirectional and panoramic transducers and an acoustic vortex generator. subsequently, in the fourth chapter, a novel procedure that considerably simplifies the fabrication process of ferroelectret-based multielement array transducers is proposed and evaluated. Also the potential of ferroelectrets being used as active material for air-coupled ultrasonic transducer design is shown. Furthermore, we demonstrate the feasibility of using a ferroelectret-based phased array for the inspection of woven textiles. last chapter summarized the most relevant result of our research on ferroelectreto based transducers for airborne ultrasound applications. The main original contributions are carefully highlighted.
Datos académicos de la tesis doctoral «Transductores basados en ferroelectretos para aplicaciones ultrasónicas en aire«
- Título de la tesis: Transductores basados en ferroelectretos para aplicaciones ultrasónicas en aire
- Autor: Joao Luis Ealo Cuello
- Universidad: Politécnica de Madrid
- Fecha de lectura de la tesis: 14/12/2009
Dirección y tribunal
- Director de la tesis
- Fernando Seco Granja
- Tribunal
- Presidente del tribunal: Jesús Ureña ureña
- Fernando Javier álvarez franco (vocal)
- álvaro Hernández domínguez (vocal)
- tomas Gomez alvarez arenas (vocal)