Flexible liquid‐metal capacitors

Abstract

Researchers in the US report liquid-metal based tuneable capacitors with linear behaviour and large tuning ratios. Compact and flexible, they hold promise for reconfigurable and wearable RF circuits. The liquid metal research team conducting research at the University of Hawai'i at Manoa. Tuneable capacitors could allow next generation wireless communication systems to adapt to the needs of the user and environment by changing the frequency and bandwidth of RF circuits, optimising their performance in response to the circumstances. Work in tuneable capacitors can be broadly divided into mechanically and electrically controlled approaches. Mechanically controlled capacitors can provide high-power handling, a high Q-factor and often a linear response. However, they tend to be bulkier than electrically controlled capacitors and have a limited tuning range. On the other hand, Electrically-controlled tuneable capacitors tend to have nonlinear behaviour, also limiting their useful range of tuneability. The range of tuneability and the linearity of the change in capacitance are two critical parameters for RF tuneable capacitors. In this issue of Electronics Letters a team at the University of Hawai'i report tuneable capacitors which not only combine linear response with a wide tuning range but add mechanical flexibility, giving them potential in flexible electronics. These benefits derive from their use of liquid metal. “The significance of this work is the integration of rapid prototyping with the dynamic shape-changing characteristics of liquid metal to develop a tuneable element that provides better performance in several categories compared to existing tuneable capacitors,” explains Hawai'i team Member, Prof. Aaron Ohta. “Liquid metal is electrically and thermally conductive, and naturally deformable, allowing it to conform to the shape of its container. Liquid metal can also provide better contact with rough solid surfaces compared to a solid-solid contact. It is also stretchable, enabling its use in flexible electronics. The inherently dynamic behaviour of liquid metal makes it appropriate for reconfigurable electronics.” One of the capacitors they have developed has a tuning ratio of 42:1, and the team say there is potential to increase this. The tuneability of the capacitors is achieved by varying the overlap of parallel conductors, which is an established principle in tuneable capacitors using solid conductors. The key difference in the Hawaii team's approach is that the fluid nature of the liquid-metal conductor allows a wider range of overlap by actuating the liquid metal. Having a large overlap range with solid conductors can easily mean a bulky device; but using liquid metal also allows the team to create reconfigurable conductor shapes that keep the size of the device down. In their Letter, the team present four different geometries, including coils and spirals. The high tuning ratio and linear response of liquid-metal capacitors make them attractive propositions for RF circuits that require a high dynamic range and resolution, such as oscillators, filters, or antennas that need to operate over a wide range of frequency and bandwidth. “In addition, the rapid prototyping of the liquid-metal capacitors described in this Letter is very suitable for laboratory devices,” said Ohta. Left: tuneable liquid metal coil capacitor, formed by a pair of plastic tubes coiled together. One tube is completely filled with liquid metal and the other tube is incrementally filled with liquid metal, providing a tuneable capacitance. Right: The capacitor is attached to a PET substrate, demonstrating its flexibility. The team at the University of Hawai'i are now integrating their tuneable capacitors into various RF circuits including filters, resonators, oscillators and antennas. This includes optimising the frequency response, bandwidth, and loss in each case. They are also working to increase the tuning range of the capacitors and include electrical actuation of the liquid metal for tuning. This is an area in which their group already has significant expertise; having developed several electrical actuation methods to tune networks, phase shifters, switches and antennas. Developing actuation methods for liquid metals that are more easily integrated into common circuit technologies will be critical to seeing this kind of capacitor applied in real-world applications.