Development of Dielectric Inks for Printed Capacitive Pressure Sensors Through Surfactant-Aided Exfoliation of hBN Nanosheets

With increase in development of specialized printable circuits and devices for wireless communication, dielectric materials with high dielectric constants and low losses at high frequencies are essential for additive manufacturing of flexible wearable devices. Hexagonal boron nitride (h-BN), an insulating analogue of graphene, is an excellent candidate due to its wide band gap and dielectric constant ranging from 2 to 4. This makes it ideal for applications such as gate dielectrics, capacitors, and passivation layers in a variety of sensor designs. However, there is a lack of research on the high-frequency dielectric properties of these materials. Additionally, stabilizing h-BN nanosheet dispersions for printing, while reducing the use of toxic solvents and excessive surfactants, is challenging. Current exfoliation techniques are often time-consuming and resource intensive. This study investigates a two-part, surfactant-assisted mechanochemical exfoliation method to obtain stable h-BN nanosheet dispersions quickly, using ball milling followed by probe sonication. Various concentrations (0 to 1 wt.%) of Triton X-100 were used to assist the exfoliation and stabilization. The yield of each mixture was quantified by thermogravimetric analysis (TGA), achieving a maximum yield of 18.4% with 1 wt.% surfactant. Colloidal stability was assessed using UV-VIS spectroscopy, with solutions remaining stable for up to 30 days. X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to evaluate the quality and size of the nanosheets. Dielectric properties were measured using a vector network analyser (VNA) at microwave frequencies (2 – 18 GHz), with real permittivity ranging from 2.1 to 3.7 and low dielectric loss tangents from 0.012 to 0.014, depending on the surfactant concentration. The two-part exfoliation technique required less than half the processing time than some of the conventional exfoliation techniques and produced highly stable aqueous dispersions. The resulting h-BN nanosheets exhibited tuneable real permittivity and low dielectric loss, making them promise for dielectric ink formulations. Eco-friendly and biocompatible water-based inks were then formulated with polyethylene oxide (PEO) as the polymer binder. Filler and binder concentrations were optimized to achieve inks with suitable rheological properties for direct ink writing, a printing technique that uses viscous fluids as inks. Capacitors with the formulated h-BN ink as the dielectric layer and silver interdigitated electrodes were printed. Electrical properties such as dielectric constant, impedance, and capacitance were measured over a range of frequencies (KHz to GHz). The flexible planar capacitors had capacitance values ranging from 3-5 pF and were tested as physical sensors for pressure sensing. These devices hold significant potential for a wide range of applications, particularly in the development of wearable sensors designed for comprehensive human health monitoring systems that provide real time data and insights.