Electrophoretic Deposition of Barium Strontium Titanate Functionally Graded Thick Film Composites for Electronically Sca
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Electrophoretic Deposition of Barium Strontium Titanate Functionally Graded Thick Film Composites for Electronically Scanning Antennas Bonnie Gersten and Jennifer Synowczynski Weapons and Materials Directorate, U.S. Army Research Laboratory Aberdeen Proving Grounds, MD 21005-5069, U. S. A. ABSTRACT The composition 40 wt% Ba0.55Sr0.45TiO3 (BST) – 60 wt% MgO bulk material was recently found to be a promising candidate as a low loss, tunable material for electronically scanning antennas (ESA). However, due to the high permittivity of the BST-MgO composite impedance mismatch within the antenna could be a source of loss. Electrophoretic deposition (EPD) could be used as a processing tool to develop functionally graded structures tailored to reduce the impedance mismatch without a reduction in the tunability. In addition, EPD could be used in thin or thick films to decrease the required biasing fields. In this study, EPD was used as a method for deposit thick films of BST functionally graded composites. The concentration of the suspension, solvent type, mixed solvent concentrations, bias field, time, particle size of powders and distance between electrodes were studied to control the deposition yield and thickness. SEM or calipers were used to measure deposit thickness. The relative weight ratio of BST to MgO in the suspension was compared to the final structure as measured by EDS. INTRODUCTION Barium strontium titanate is a ferroelectric material that has a wide variety of applications (i.e., capacitors, non-volatile memories, actuators, transducers, tunable filters) and phase shifters in electronically scanning antennas. In an electronically steering antenna (ESA) the relative phases of the feed signals are electronically varied to reinforce the effective radiation pattern in a desired direction and suppress it in undesired directions thereby steering the radiation. The ferroelectric material can be used for this antenna since it has a permittivity (ε) that decreases under a bias field. For example, Ba0.6Sr 0.4TiO3 has a percent change in permittivity with bias ((ε-εb)/εb) equal to 56% when 2 V/µm is applied at 250 kHz [1]. This percent change is defined as the tunability. Both a high tunability and low-loss are desired for the ESA. However, barium strontium titanate is a lossy material where the losses increase with frequency of operation (e.g., Ba0.6Sr 0.4TiO3 tanδ=0.0908 at 10 GHz [1]). Therefore, recently the material was composited with a low-loss dielectric MgO [1,2] thus resulting in a low-loss, tunable composite ferroelectric material (e.g., 60 wt% MgO-40 wt% Ba 0.55Sr 0.45TiO3 tanδ=0.008 at 10 GHz, tenability equal to 6.5% at 250 kHz and 2V/µm [1,2]). As shown, the tunability decreases by this dilution effect (e.g., an order of magnitude at 250 kHz [1,2]). One way to increase the tunability is by increasing the connectivity between barium strontium titanate grains within the magnesium oxide matrix [3]. One question in this study is how the tunability is affected by a continuously graded structure of connectivit
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