printing precursor inks by ink jetting [4-7] or screen-printing [8-9]. ... applications such as flexible circuits, interconnections in devices and radio-frequency.
Conformal Passive Sensors for Wireless Structural Health Monitoring Sharavanan Balasubramaniam1, Jung-Rae Park1, Tarisha Mistry2, Niwat Angkawisittpan2, Alkim Akyurtlu2, Tenneti Rao2 and Ramaswamy Nagarajan1 1
Department of Plastics Engineering, University of Massachusetts, Lowell, MA 01854.
2
Department of Electrical & Computer Engineering, University of Massachusetts, Lowell,
MA 01854. ABSTRACT The application of conductive inks for the low-cost fabrication of wireless sensors is described. Binder-free silver inks have been formulated, which can form pure metallic patterns, leading to devices with high quality factors. The conductive formulations have been screen-printed on flexible polyester films and thermally converted to silver traces at low temperatures to yield resonant inductor-interdigital capacitor elements. The radiofrequency (RF) response of the printed inductor-capacitor (LC) resonators is measured using a vector network analyzer. Structural damage such as a crack in the substrate disrupts the resonant characteristics of the sensing elements, resulting in the loss of the characteristic response of the LC circuit. The application of these screen-printable sensors for the detection of cracks is demonstrated. INTRODUCTION Low-cost fabrication of sensors for wireless detection of structural defects like cracks is currently a challenge. Sensors for wireless structural health monitoring are fabricated using complex or expensive methods like photolithography [1] and laser micromachining techniques [2]. The commercial success of these sensors depends on both performance and ease of manufacture. We have developed low-temperature curing conductive silver inks for screen-printing resonant LC circuits on flexible plastic substrates. Silver inks based on flaky powder and nanoparticles [3] are very popular in the area of printed electronics due to their high electrical conductivity and environmental stability. The metallo-organic decomposition (MOD) technology is well known for printing precursor inks by ink jetting [4-7] or screen-printing [8-9]. In this method, the weak bonds holding the metal atom and the organic moieties can be thermally broken, paving the way for low-temperature conversion to metal. Thus, it has been used as a simple and convenient approach to the fabrication of metallic patterns for a variety of applications such as flexible circuits, interconnections in devices and radio-frequency identification (RFID) readers. Screen-printing has been used for the deposition of the formulated silver inks because of its compelling economic advantages and the ability to transfer a pattern of materials on any substrate at a high rate. Also, the thickness of the materials deposited by screen printing after sintering or firing is of the order of 10 microns. Thus the method lends
itself very well to highly conductive circuit elements. The conformal sensor reported here employs inductor-interdigital capacitor elements printed on a flexible polyester film using conductive silver inks. The LC resonators are activated by a radio-frequency signal and can be interrogated wirelessly using a single loop antenna. EXPERIMENTAL Silver acetylacetonate was used as the organic silver precursor for the conductive inks. The binder-free silver inks were prepared from silver flakes (
