Maeum Han

Flexible transparent conductive heater using multiwalled carbon nanotube sheet

This paper reports highly flexible, transparent, conducting heaters based on multiwalled carbon nanotube (MWCNT) sheets. The MWCNT sheets were spun directly from a well-aligned MWCNT forest. The fabrication of the MWCNT sheet heater was quite simple and suitable for mass production, requiring only a one-step transferring process, in which the MWCNT sheet is drawn onto the target substrates. This study examined the parameters that affect the heat generation of the MWCNT sheet-based heater; input power, surface area, and thermal conductivity of the substrate. In particular, more effort was focused on how to increase the surface area and contact points between the individual MWCNTs; simple acid treatment and added metal nanoparticles increased the heat performance of the heater dramatically. Moreover, the heaters exhibited durability and flexibility against many bending cycles. Therefore, the MWCNT sheet-based heater can be used for versatile applications requiring transparency, conduction, and flexibility.

Fast-Response Room Temperature Hydrogen Gas Sensors Using Platinum-Coated Spin-Capable Carbon Nanotubes

We report the properties of a hydrogen (H2) gas sensor based on platinum (Pt)-coated carbon nanotubes (CNTs) in this paper. To fabricate the Pt-CNT composite sensor, a highly aligned CNT sheet was prepared on a glass substrate from a spin-capable CNT forest, followed by electrobeam (e-beam) deposition of Pt layers onto the CNT sheet. To investigate the effect of Pt on the response of the sensor, Pt layers of different thicknesses were deposited on the CNT sheets. A Pt thickness of 6 nm yielded the highest response for H2 detection, whereas Pt layers thinner or thicker than 6 nm led to a reduction of the surface area for gas adsorption and, consequently, decreased response. The Pt-CNT composite sensor detects H2 concentrations of 3-33% at room temperature and shows reproducible behavior with fast response and recovery times.

Effect of hydrogen pretreatment on the spin-capability of a multiwalled carbon nanotube forest

The authors report the effect of hydrogen (H2) pretreatment on the spin capability of vertically aligned multiwalled carbon nanotubes (MWCNTs) grown on an iron (Fe) catalyst by using a chemical vapor deposition system. Variations in H2 pretreatment time had significant effects on the distribution, morphology, diameter, and height of MWCNTs. A longer H2 pretreatment time to the Fe film increased the number of large Fe particles as well as crystallization of Fe3O4 phase. As a result, the areal density of carbon nanotubes (CNTs) was largely reduced, whereas the mean CNT diameter slightly increased. This worsened the alignment of the CNTs and reduced the spin-capability of MWCNTs.

Regrowth analysis of multiwalled carbon nanotube forests

The multiple regrowth of spin-capable multiwalled carbon nanotube (MWCNT) forests has been performed without additional catalyst films. Fe films of 6 nm thickness were deposited as catalysts, after which as-grown MWCNT forests were removed to reuse the initial catalyst. We observed that the initial catalysts could be reactivated by thermal annealing, and the annealing time affects the surface morphology of the catalyst. We determined the variable parameters for size control of the reactivated catalyst so that it can be reused for multiple growth cycles while still maintaining a sufficiently high particle density to support the growth of MWCNT forests.

Infrared sensing characteristics of multiwalled carbon nanotube sheet

In this paper, a multiwalled carbon nanotubes (MWCNT)-sheet-based infrared sensor is proposed along with an easy, simple, reliable, and low-cost fabrication process that does not require manipulation of individual or bunches of MWCNTs or complex, expensive techniques such as photolithography. The output signal (resistance) is produced by a temperature change induced by infrared illumination. The authors found that the responsivity of the sensor could be improved by acid treatment. In addition, the density of the MWCNTs played a critical role in the performance of the sensor.

Sensitivity and Frequency-Response Improvement of a Thermal Convection–Based Accelerometer

This paper presents a thermal convection–based sensor fabricated using simple microelectromechanical systems (MEMS)-based processes. This sensor can be applied to both acceleration and inclination measurements without modifying the structure. Because the operating mechanism of the accelerometer is the thermal convection of a gas medium, a simple model is proposed and developed in which the performance of the thermal convection–based accelerometer is closely associated with the Grashof number, Gr and the Prandtl number, Pr. This paper discusses the experiments that were performed by varying several parameters such as the heating power, cavity size, gas media, and air pressure. The experimental results demonstrate that an increase in the heating power, pressure, and cavity size leads to an increase in the accelerometer sensitivity. However, an increase in the pressure and/or cavity size results in a decrease in the frequency bandwidth. This paper also discusses the fact that a working-gas medium with a large thermal diffusivity and small kinematic viscosity can widen the frequency bandwidth and increase the sensitivity, respectively.

Sensitivity improvement of a thermal convection-based tilt sensor using carbon nanotube

This paper presents a thermal convection-based sensor, which is fabricated using carbon nanotube (CNT) yarn. The key element in this device is the non-symmetrically distributed, heated air medium around the heater, particularly when it experiences acceleration and/or changes in inclination. Therefore, it can withstand much higher accelerations/inclination than conventional sensors that use a proof mass. However, a major challenge for the design of this type of sensor is the high heating power (in the order of tens of milliwatts) required to facilitate thermal convection in a sealed chamber. In order to reduce the high heating power, CNTs are investigated as materials for both the heater and the temperature sensors. Moreover, this paper discusses experiments that were performed by varying several parameters, such as the heating power, distance between the heater and temperature sensors, the gas medium used, and air pressure.

Spin-capable carbon nanotube sheet as a substitute for TCO in transparent electronics and displays

ABSTRACT Spin-capable carbon nanotubes (CNTs) are proposed as a promising material for transparent conductive films (TCFs) to replace indium tin oxide (ITO) in optoelectronic and flexible applications. CNT-TCFs were prepared by a dry-spun method, a straightforward transfer process for fabricating CNT-based films. The effects of acid treatment on the electrical and optical properties of CNT-TCF were evaluated. After acid treatment, the CNT-TCF possesses a much higher electrical conductivity and slightly improved transparency compared to films that have not undergone acid treatment. The electrical properties of the CNT films were dramatically affected by the type of acid treatment. The CNT-TCF treated with a fuming acid treatment exhibited better performance than the CNT-TCF treated with immersion acid treatment only, with a low sheet resistance (210 Ω/sq) and high transmittance (90%) comparable to those of ITO films.

Multi-axis Response of a Thermal Convection-based Accelerometer

A thermal convection-based accelerometer was fabricated, and its characteristics were analyzed in this study. To understand the thermal convection of the accelerometer, the Grashof and Prandtl number equations were analyzed. This study conducted experiments to improve not only the sensitivity, but also the frequency band. An accelerometer with a more voluminous cavity showed better sensitivity. In addition, when the accelerometer used a gas medium with a large density and small viscosity, its sensitivity also improved. On the other hand, the accelerometer with a narrow volume cavity that used a gas medium with a small density and large thermal diffusivity displayed a larger frequency band. In particular, this paper focused on a Z-axis response to extend the performance of the accelerometer.

Gas-sensing properties of multi-walled carbon-nanotube sheet decorated with cobalt oxides

A hybrid, nanocomposite carbon nanotube (CNT) sheet decorated with cobalt oxide (Co₃O₄) serving as gas sensor was fabricated for detecting hydrogen (H₂). In order to improve performance of the sensor, Co₃O₄ nanoparticles were decorated on the surface area of the CNT sheets by electrodeposition, which is an easy and simple method for fabricating hybrid composites. Moreover, the effect of surface area on sensitivity of CNT/Co₃O₄ composite was further investigated, and we found the mean sizes of the Co₃O₄ nanoparticles significantly affected the sensitivity of the composite.