O. Ranasingha, M. Haghzadeh, C. Armiento, A. Akyurtlu
University of Massachusetts – Lowell,
Keywords: Barium strontium titanate, tunable dielectric inks, printed electronics, functional inks
Summary:Printed electronic devices aim to transform conventional electronic devices in the near future. The demand for printed electronics over conventional electronics is rising exponentially due to various reasons, such as the extremely low fabrication cost, low materials consumption and wastage, compatibility with flexible substrates, durability, and ease of customization. Functional inks are one of the key enablers in printed electronics. In order to improve the overall device performance and achieve the functionalities critical to target applications, it is necessary to have high quality and tailored printable functional inks. Tunable electronic devices play a crucial role in the electronics industry due to their low foot print, low cost and versatile performance. Barium Strontium Titanate (BST) is one of the mostly investigated materials due to its distinctive tunable properties. The dielectric constant of BST can be decreased by applying an external electric field. BST has a ferroelectric phase below the Curie temperature and a paraelectric phase above the Curie temperature. A continuous tunability of BST can be achieved in the paraelectric phase with low dielectric losses at microwave frequencies. BST has both a high dielectric constant and a low loss tangent and can also provide high speed tunability of its dielectric constant in the presence of an applied electric field. The tunability of BST allows the design of variable capacitors (varactors) for various types of electronic applications, such as tunable filters, frequency selective surfaces, and phased array and conformal antennas. Various deposition techniques, such as pulsed laser deposition, chemical vapor deposition, and sputtering are well established deposition techniques to fabricate BST based electronic devices. However, all these techniques require high processing temperatures that are not suitable for flexible substrates and fabrics. Moreover, it is difficult to vary the maximum dielectric constant in these deposition techniques. In this work, a series of BST nanocomposite inks were developed by varying the ink formula in order to achieve different dielectric constants. The processing temperature of these BST nanocomposite inks are approximately 150 0C, which is suitable for most flexible substrates. Also, these inks are compatible with a wide range of substrates as well as several printing technologies, such as Inkjet, Aerosol Jet, and micro dispensing. The inks in this study had a Barium molar fraction that was 0.67 to adjust the Curie temperature slightly below the room temperature, enabling the paraelectric phase of BST to be set at room temperature. This composition allows the fabrication of electronic devices that are tunable at room temperature. Ethylene glycol was used as the major solvent and polyvinyl alcohol and/or polyvinylpyrrolidone were used as the polymer matrix of these BST nanocomposite inks. The dielectric constant could be varied from 3.5 to 45 at 10 GHz by changing the compositions of these inks. The dielectric constant of the inks can be tuned up to 15% at 10 GHz for ink compositions with higher dielectric constant. These BST nanocomposite inks have been demonstrated to be suitable for printing of electronic devices that work up to microwave frequencies.