Hyun Pyo Hong

Microfabrication and characterization of spray-coated single-wall carbon nanotube film strain gauges

We present the design, fabrication, and characterization results of single-wall carbon nanotube (SWCNT) film strain gauges for potential applications as highly sensitive strain, weight, or pressure sensors on the macro-scale. A batch microfabrication process was developed for practical device construction and packaging using spray-coated SWCNTs and a conventional semiconductor process. The prototype was characterized using a commercial metal foil gauge with tensile and compressive testing on a binocular load cell. Our test results demonstrated that the proposed SWCNT film gauges have a linear relationship between resistance changes and externally applied strain. The gauge factor ranged from 7.0 to 16.4 for four different micro-grid configurations, indicating that the maximum strain sensitivity of the prototype was approximately eight times greater than that of commercial gauges.

Percolated pore networks of oxygen plasma-activated multi-walled carbon nanotubes for fast response, high sensitivity capacitive humidity sensors

We report on the preparation of capacitive-type relative humidity sensors incorporating plasma-activated multi-wall carbon nanotube (p-MWCNT) electrodes and on their performance compared with existing commercial technology. Highly open porous conductive electrodes, which are almost impossible to obtain with conventional metal electrodes, are fabricated by spray-depositing MWCNT networks on a polyimide layer. Oxygen plasma activation of the MWCNTs is also explored to improve the water adsorption of the MWCNT films, by introducing oxygen-containing functional groups on the CNT surface. Polyimide humidity sensors with optimized p-MWCNT network electrodes exhibit exceptionally fast response times (1.5 for adsorption and 2 s for desorption) and high sensitivity (0.75 pF/% RH). These results may be partially due to their percolated pore structure being more accessible for water molecules, expending the diffusion of moisture to the polyimide sensing film, and partially due to the oxygenated surface of p-MWCNT films, allocating more locations for adsorption or attraction of water molecules to contribute to the sensitivity.

A highly fast capacitive-type humidity sensor using percolating carbon nanotube films as a porous electrode material

Open porous multi-walled carbon nanotube (MWCNT) film networks were used as an upper electrode of polyimide (PI)-based capacitive humidity sensor for highly fast response time. MWCNT films were deposited by spray-dry method and patterned by O2 plasma. PI capacitive humidity sensors with MWCNT film electrodes showed a very short response time of less than 3 s and a good linearity of 0.9995 compared to the humidity sensors with porous Cr electrodes. The large improvement in response time was explained on the base of a percolated pore network that is more accessible for water molecules.

Fabrication, characterization and application of a microelectromechanical system (MEMS) thermopile for non-dispersive infrared gas sensors

We report on the design, fabrication, and characterization of a non-dispersive infrared (NDIR) gas sensor using an integrated thermopile on a micromachined silicon nitride membrane. The NDIR sensor consists of an optical cavity with new specular reflectors around the light bulb. The multi-layer absorber showed an absorptance of over 90% at 3.3–4.9 µm. The thermopile with this absorber has an output voltage of 144.83 mV at a 5 mW incident power and a sensitivity of 30 V W−1. The sensitivity of the thermopile packaged with a Fresnel lens was 51 V W−1, approximately 1.7 times higher than that of a thermopile with only an absorber. This is due to the decrease in thermal mass and heat loss from a hot junction, and due to the increase in absorptance. Using this newly fabricated thermopile, we developed a small and sensitive NDIR gas detector module for accurate air quality monitoring systems for energy-saving buildings and automotive applications. Our novel sample cavity design is configured to uniformly emit collimated light into the entrance aperture of the cavity to enhance the sensitivity of the NDIR gas detector.

Comparison of Gas Sensors Based on Oxygen Plasma-Treated Carbon Nanotube Network Films with Different Semiconducting Contents

We report on the effect of oxygen plasma treatment on the performance of single-wall carbon nanotube (SWCNT) NH3 gas sensors with different semiconducting contents (66% and 90% semiconducting SWCNTs). The performance of chemical sensors based on SWCNT networks depends on the concentration of semiconducting SWCNTs (s-SWCNTs), whose conductance can be significantly modulated by the absorbed molecules and the surface functionalization. After oxygen plasma treatment, the 66% s-SWCNT sample showed an increase in sensitivity from 0.0275%/ppm to 0.1525%/ppm (5.5 times), while the 90% s-SWCNT device demonstrated an increase in sensitivity from 0.1184%/ppm to 1.5707%/ppm (13 times). These results correspond to improvements in sensitivity of 57 times and 10 times compared with pristine and plasma-treated 66% s-SWCNT samples, respectively. In addition, the plasma-treated sensors exhibited much faster response and recovery times than the pristine one. The large improvement in performance was explained by the presence of oxygen-containing functional groups and the sp2–sp3 structure change of SWCNTs, which changes the binding energy while increasing the uptake of polar molecules such as NH3.