Jong Hyuk Lee

Highly flexible, stretchable, wearable, patternable and transparent heaters on complex 3D surfaces formed from supersonically sprayed silver nanowires

Here we demonstrate a scalable production process for a highly transparent, flexible, patternable, and wearable heater using a single-step supersonic kinetic spraying technique that deposits silver nanowires (AgNWs) on rollable substrates, facilitating a roll-to-roll process. AgNWs were suspended in an aqueous solution and supersonically sprayed onto a rolling substrate to produce a flexible heater film without use of any binders or additional post-process treatments. Because of the high-speed impact, the intersections of the nanowires were fused, thus creating a junction-free network of nanowires, which significantly reduced the contact and thus the sheet resistance. Cyclic temperature testing confirmed the thermal stability of the AgNW heater. A heater bent to a radius of less than 2 mm was tested for 600000 cycles; the heater exhibited little change in the sheet resistance. Moreover, it does not experience significant thermal expansion, which would manifest itself in buckling, and thus such heaters do not buckle during operation. AgNWs were sprayed onto a complex surface of a replica of Venus de Milo and Jejus Dol Hareubang statues, demonstrating the deposition capability onto a 3D surface. Defogging and defrosting tests showed potential applications of this heater in smart mirrors or windows. The highest heating temperature of 160 °C was achieved in a transparent fibrous film having 95% transparency and 15 Ω sq−1 sheet resistance at a supplied voltage of 8 V. Because the film fabrication method is rapid and scalable with the installation of multiple nozzles, the method is commercially viable.

Scalable Binder-Free Supersonic Cold Spraying of Nanotextured Cupric Oxide (CuO) Films as Efficient Photocathodes

We demonstrate production of nanotextured p-type cupric oxide (CuO) films via a low-cost scalable supersonic cold spray method in open air conditions. Simply sweeping the spray nozzle across a substrate produced a large-scale CuO film. When used as hydrogen evolution photocathodes, these films produced photocurrent densities (PCD) of up to 3.1 mA/cm(2) under AM1.5 illumination, without the use of a cocatalyst or any additional heterojunction layers. Cu2O particles were supersonically sprayed onto an indium tin oxide (ITO) coated soda lime glass (SLG) substrate, without any solvent or binder. Annealing in air converted the Cu2O films to CuO, with a corresponding decrease in the bandgap and increase in the fraction of the solar spectrum absorbed. Annealing at 600 °C maximized the PCD. Increasing the supersonic gas velocity from ∼450 to ∼700 m/s produced denser films with greater surface roughness, in turn producing higher PCD. The nanoscale texture of the films, which resembles the skin of a dinosaur, enhanced their performance, leading to one of the highest PCD values in the literature. We characterized the films by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy to elucidate the origins of their outstanding performance. This supersonic cold spraying deposition has the potential to be used on a commercial scale for low cost mass production.

Robust Mechanical Properties of Electrically Insulative Alumina Films by Supersonic Aerosol Deposition

Electrically insulating alumina films were fabricated on steel substrates using supersonic aerosol deposition and their hardness and scratchability were measured. Alumina particles (0.4-μm diameter) were supersonically sprayed inside a low-pressure chamber using between 1 and 20 nozzle passes. These alumina particles were annealed between 300 and 800 K to determine the temperature’s effect on film crystal size (37-41 nm). Smoother surface morphology and increased electrical resistance of the thin films were observed as their thicknesses grew by increasing the number of passes. Resistances of up to 10,000 MΩ demonstrate robust electrical insulation. Significant hardness was measured (1232 hv or 13.33 GPa), but the alumina films could be peeled off with normal loads of 36 and 47 N for films deposited on stainless steel and SKD11 substrates, respectively. High insulation and hardness confirm that these alumina films would make excellent electrical insulators.