Dae-Sung Kwon

A flexible hybrid strain energy harvester using piezoelectric and electrostatic conversion

A new design of flexible energy harvester to utilize piezoelectric and electrostatic energy conversion mechanisms simultaneously from a single mechanical energy source is proposed. This non-resonant type harvester enables low-frequency mechanical inputs to be converted to electricity, and the polymeric structures make the harvester mechanically flexible, allowing it to be applied to non-planar surfaces. The fabricated harvester generated peak- and average power densities of 159 and 1.79 μW cm−2 respectively by piezoelectric conversion, and 52.9 μW cm−2 and 1.59 nW cm−2 respectively by electrostatic conversion from an input force of 1.2 N at 3 Hz. Considering its flexibility and ability to harvest mechanical inputs at frequencies below 3 Hz, low-frequency human movements could be a potential energy source for the proposed hybrid harvester to exploit.

Using confined self-adjusting carbon nanotube arrays as high-sensitivity displacement sensing element.

Displacement sensing is a fundamental process in mechanical sensors such as force sensors, pressure sensors, accelerometers, and gyroscopes. Advanced techniques utilizing nanomaterials have attracted considerable attention in the drive to enhance the process. In this paper, we propose a novel and highly sensitive device for detecting small displacements. The device utilizes the changes in contact resistance between two sets of vertically aligned carbon nanotube (CNT) arrays, the growth of which was confined to enable their facile and reliable integration with fully fabricated microstructures. Using the displacement transduction of the proposed device, we successfully demonstrated a 3-axis wide bandwidth accelerometer, which was experimentally confirmed to be highly sensitive compared to conventional piezoresistive sensors. Through a test involving 1.2 million cycles of displacement transductions, the contact resistance of the CNT arrays was proved to be excellently stable, which was a consequence of the high electrical stability and mechanical durability of the CNTs.

Shock protection based on confined self-adjusting carbon nanotube arrays

We demonstrate a novel shock protector based on confined self-adjusting carbon nanotube (CNT) arrays. The CNTs with self-adjusted length are selectively synthesized on fully fabricated single crystal silicon microstructures to generate coulomb damping. The frictional contact between CNT arrays dissipates energy during impact and thus reduces the impact force applied on microstructures. The outstanding mechanical flexibility and resilience of CNTs make them suitable as a contact material that effectively absorbs energy through frictional contact preventing mechanical failure of microstructures. Experimental shock tests verify that CNT-based shock protector provides substantial survival rate of movable proof-mass compared with hard stop or compliant spring stop.

Widely Tunable Variable Capacitor With Switching and Latching Mechanisms

A widely tunable variable capacitor using mechanical switching and a reversible latching mechanism was developed. This variable capacitor can increase the total capacitance by utilizing three discrete switches to sequentially connect four sets of fixed capacitors arranged in parallel. Continuous fine tuning is achieved by closing gaps with these interdigitated capacitors, and connected states are maintained through the use of a mechanical latching mechanism. By combining switching and gap-closing modes, a maximum tuning ratio of 9.42 was obtained.

Variable capacitor with switching mechanism for wide tuning range

We developed a variable capacitor with mechanical switching mechanism and reversible mechanical latching component to enhance tuning ratio. The switching mechanism could connect four sets of capacitors arranged in parallel sequentially by controlling the displacement of a microactuator for abrupt and coarse tuning of total capacitance. Continuous and fine tuning was also achieved by gap-closing mode of interdigitated capacitors. The resultant maximum tuning ratio was 5.71 by combining coarse and fine tuning.