Development Of PVDF Thick Film Interdigitated Capacitors For Pressure Measurement On Flexible Melinex Substrates
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Development Of PVDF Thick Film Interdigitated Capacitors For Pressure Measurement On Flexible Melinex Substrates Arous Arshak1, Khalil Arshak2, Deirdre Morris1, Olga Korostynska2, Essa Jafer2 1 Department of Physics, University of Limerick, Plassey Technological Park, Castletroy, Limerick, Ireland 2 Electronic and Computer Engineering Department, University of Limerick, Plassey Technological Park, Castletroy, Limerick Ireland ABSTRACT
In this work, a PVDF thick film paste was deposited onto interdigitated electrodes to form a capacitor. Two different substrates, alumina and Melinex® were used. Capacitors, fabricated on alumina substrates were tested as strain gauges, and showed a high sensitivity with low hysteresis. Capacitors on Melinex® substrates were tested as pressure sensors by adhering them to planar and cylindrical surfaces and subjecting them to pressures up to 300 kPa. Their sensitivity and hysteresis during cycling were examined and compared. It was found that sensors on cylindrical surfaces showed a higher sensitivity, however the hysteresis was also increased. It is thought that this is due to instabilities in the polymer film, accentuated by stretching of the substrate. INTRODCTION
Thick film sensors are commonly used in automotive industry, aerospace and robotics for the measurement of strain and pressure [1]. This popularity is mainly due to their rugged nature, cost effectiveness and reproducibility. A further advantage is the flexibility of thick film technology. It allows the use of a wide range of materials and structures, which can be deposited and fused together so that sensors with the required physical properties can be achieved [2]. Recently, such devices have been identified as suitable for integration into remote monitoring systems [3,4]. These systems could be used to observe environmental or biological events. For example, continuous monitoring of inter-cranial pressure in patients suffering from trauma, caused by a serious accident [5]. Sensors on flexible substrates could also find applications in long term monitoring of a particular region of the body, as they could be easily adhered to a wide range of surfaces. For example, strain gauges are often placed on limb implants to monitor their performance [6]. These devices should be small in size, reliable and cost-effective since they may only be allowed for one-time use. It has been documented, that to obtain a high sensitivity and reduce power consumption, it is likely that a capacitive structures would be used [4]. As a result, this work has focused on the development of a thick film capacitor, using Poly vinylidene fluoride (PVDF) as the dielectric.
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PVDF is a low density, semi-crystalline polymer, consisting of a long chain of repeating CF2-CH2 molecules. The crystalline regions consist of five polymorphs known as α, β, γ, δ, and ε. However, the α- and β-phases are most common. The β-phase is piezoelectric and is commonly used for the fabrication of hydrophones. PVDF with a dominant α-phase has been chosen for th
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