High frequency properties of elastomeric silicones and engineered nanocomposites

摘要

Polydimethylsiloxane (PDMS) is an elastomeric silicone polymer utilised extensively in the realisation of micro-devices. Techniques compatible with microfabrication enable the realisation of thin, flexible, conformal, and biocompatible layers ranging from 5 um to a few mm. The inherent silicone chemistry of PDMS makes it favourable for applications as a high frequency dielectric. In this chapter, the dielectric properties of thin layers of PDMS will be characterised at radio frequencies (RF) and terahertz frequencies. The RF properties characterisation determined the average effective index of PDMS by transmission line measurements of coplanar waveguide structures. This was followed by finite element method (FEM) simulations of the coplanar waveguides, where a curve-fitting approach was used to extract the dielectric constant of PDMS from the experimentally measured average effective index and the FEM simulated dielectric constant results. The average effective index and the dielectric constant of PDMS were estimated to be 1.396 and 2.72, respectively. The suitability of PDMS as a RF substrate is demonstrated by the integration of a pneumatic valve, to switch RF transmission on or off without external electrical bias. The terahertz properties of PDMS were determined by terahertz time-domain spectroscopy (THz-TDS). The dielectric constant and loss tangent of PDMS were estimated to be 2.35 and 0.02-0.04, respectively, over the 0.2-2.5 THz band. The silicone substrate is used to fabricate a multi-layered flexible fishnet metamaterial operating at terahertz frequencies, with excellent agreement of simulation and experimental results demonstrated. Silicone nanocomposites that included dopants such as aluminium oxide and polytetrafluoroethylene (PTFE) of differing weight ratios are used to engineering dielectric properties. The ability to design dielectric properties, and the fundamental approach to engineer the process, were studied at both microwave and terahertz frequencies.