Screen Printed Frequency-Selective Surfaces on Rigid, Flexible and Elastic Substrates
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0964-R01-09
Screen Printed Frequency-Selective Surfaces on Rigid, Flexible and Elastic Substrates Thomas Kistenmacher, Shaun Francomacaro, Ben Brawley, Ra'id Awadallah, Paul Vichot, Michael Fitch, Jane Spicer, and Dennis Wickenden Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD, 20723
ABSTRACT A series of frequency-selective surface (FSS) arrays based on nested split-ring triangle resonators have been fabricated using screen printing. A silver-filled polymer thick film (PTF) paste was selected as the active medium for the FSS arrays as it has good compatibility with the various substrates employed and is in itself naturally flexible. Substrates included FR4 boards and polyimide (PI), polyethylene terephthalate (PET) and silicone sheeting. Compared to arrays fabricated from Cu-clad FR4 board, the screen-printed arrays are resonance shifted owing to the magnitude of the dielectric constant and thickness of the various substrates. In addition, the quality factors of the screen-printed arrays are reduced compared to those fabricated from the more conductive Cu resonators. Despite these limitations, screen-printed arrays have considerable potential as components for low-cost flexible and conformal microwave devices. INTRODUCTION Over the past several years, there has been a growing interest in frequency-selective surfaces with applications ranging from RFID tags to multimodal array antennas. [1-5] Split-ring resonators (SRRs) are becoming important elements of such surfaces, and the properties of a number of two-dimensional arrays of SRRs on rigid substrates (commonly FR4 board) have been reported. [6-11].
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Figure 1. Examples of a nested split-ring triangle resonators: (a) idealized CAD drawing; (b) Cu on FR4; (c) PTF on PI; and, (d) PTF on silicone Less attention has been devoted to the fabrication of SRR arrays on flexible and elastic substrates [see reference 7 and Figure 1, for example]. Herein, transmission spectra are reported for two SRR arrays fabricated by screen printing Ag-filled PTF paste on rigid, flexible and elastic substrates. In a simpler case, a primitive 2D array was printed with one nested, split-ring triangular resonator (see Figure 1) per unit cell; in a more complex case, a primitive 2D array
was printed with four nested, split-ring triangular resonators [with geometrical scale factors of 1.1 (A), 1.0 (B), 0.9 (C), and 0.8 (D)] per unit cell. EXPERIMENT A silver-filled polymer thick film (PTF) paste (DuPont 5025) was selected as the active medium for the SRR arrays as it has good compatibility with the various substrates employed and is in itself naturally flexible. The PTF paste consists of a mixture of polymeric materials embedded with silver flakes. Unlike both refractory and cermet thick film pastes, Ag-filled PTF pastes are not fired at high temperatures (850 oC and higher), but rather dried/cured at temperatures typically below 200 oC. The DuPont 5025 paste has 61% silver flake by weight, translating to a sheet resistance of 1
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