Hsin-Yuan Miao

Analysis of a laser post-process on a buckypaper field emitter for high and uniform electron emission

This study reports a laser irradiation process to enhance the field emission properties of buckypaper, which is a thin sheet of high-loading carbon nanotube networks. The scanning laser treated the selected regions of buckypaper to activate carbon nanotube (CNT) emitters. This post-process causes a decrease in turn-on field and increases the field enhancement factor (beta), luminance intensity, and uniformity of buckypaper emitters. The phosphorescence luminance intensity and uniformity of buckypaper emitters are measured and characterized. The low turn-on field of 0.56 V microm(-1), highest average luminance intensity of 235.9/255, and uniformity of 99.8% are achieved by adjusting the machining parameters of laser power, laser lens motion speed, laser resolution, laser beam size, and pattern orientation. Those parameters relate to the field emission properties of beta, turn-on electric field, luminance intensity, and uniformity. Using design of experiment (DOE) methodology, the optimal parameter settings for high and uniform electron emission of a buckypaper emitter are obtained within fewer experimental runs.

Fabrication of metal alloy-deposited flexible MWCNT buckypaper for thermoelectric applications

We report the fabrication of a flexible network of multiwall carbon nanotubes (MWCNTs) known as buckypaper (BP) for thermoelectric (TE) applications. A thermal evaporation method was used to deposit TE metal alloys onto the BP. The TE properties were improved primarily by increasing the Seebeck coefficient values (50 and 75 µV/K) and the electrical conductivity by approximately 10 000 S/m. High-temperature resistivity studies were performed to confirmthe semiconductivity of buckypaper. Variations in resistivity were observed to be the result of the metal alloys coated on the BP surface. We also demonstrated that a substantial increase in the Seebeck coefficient values can be obtained by connecting 3 and 5 layers of metal-deposited BP in series, thereby enhancing the TE efficiency of MWCNT-based BP for application in thermoelectric devices.

Application of buckypaper for preserving cholesteric liquid crystal cells within a certain temperature range

Application of buckypaper to cholesteric liquid crystals (CLCs) is demonstrated. The buckypaper functions as a thin film resistant heater and a near-perfect absorber in this study. A planar CLC cell with buckypaper pasted onto one of its surfaces is used to develop a voltage-induced optical attenuator. The intensity of the reflection band of the CLC attenuator can decrease (increase) by the application (removal) of a single-pulse voltage, and the wavelength of the reflection band remains constant as the reflection intensity decreases (increases). The decrease in the reflection intensity is attributable to the cholesteric→isotropic phase transition of the LCs via heating of the buckypaper, and absorption by the black buckypaper. The increase in the reflectance results from the isotropic→cholesteric phase transition of the LCs through cooling of the environment. During cooling, the application of a low DC voltage to the buckypaper can keep the cell temperature constant because thermal equilibrium between the heating of the buckypaper and the cooling of the environment is established. Using this method, the blue phase of a CLC cell can stably exist for more than an hour at room temperature, without the need for a temperature stage, polymer materials or particular LCs.

Electrically controllable all-optical switches using dye-doped liquid crystal cells with buckypapers

This work fabricates an electrically controllable all-optical switch using a dye-doped liquid crystal (DDLC) cell on which is pasted a buckypaper. The buckypaper serves as a thin film resistant heater; it can maintain the temperature of the DDLC cell constant by environmental cooling. The threshold power of the DDLC cell, which is the input optical power at which the output optical power begins to fall, is controlled by applied voltages, and the switch-off power of the cell, which is the minimum input optical power at which the output optical power falls to zero, is also controlled by the applied voltages. Therefore, such a cell can be developed into optical limiting devices with adjustable threshold and switch-off powers for multi-purpose use.

Preparation and Investigation of Co-Dispersed MWCNT Buckypaper for the Microwave Absorption

The paper deals with the preparation of cobalt nanoparticles dispersed multi walled carbon nanotube buckypaper by the dispersion and filtration method to study the electromagnetic (microwave) absorption properties. From the X-ray diffraction the crystallite size of the cobalt nanoparticles of 33.04 nm was calculated. FESEM image clearly indicates the presence of cobalt nanoparticles in the buckypaper. Microwave-absorbing properties were investigated in terms of measuring the transmission coefficient (S21) with MWCNT-buckypaper as the absorber using microstrip line in a frequency range of 2–20 GHz. Compared to the pure buckypaper the absorption peak of the Co-CNT composites move to the lower frequency by dispersing the Co nanoparticles into the MWCNT-BP.

Impacts of high-electroactive-surface-area buckypaper counter electrodes on the photovoltaic performance of dye-sensitized solar cells

In this paper we study the photovoltaic performance of dye-sensitized solar cells (DSCs) with high-electroactive-surface-area buckypaper (BP) counter electrodes (CEs). BP is made of entangled multi-walled carbon nanotubes and has a high electroactive surface area, which is effectively 11.5 times higher than that of Pt. The high surface area of BP causes a high reduction rate at the CEs and suppresses the charge recombination at the anodes. Accordingly, the open-circuit voltage, the filling factor, and the power conversion efficiency of DSCs with BP CEs are enhanced, and these photovoltaic parameters exhibit a better trend with the increase of solar power compared with conventional DSCs with Pt CEs.

Thermoelectric Properties of Transition Metal Deposited MWCNT Buckypaper

In this work, we report the preparation of transition metal deposited flexible multiwalled carbon nanotube buckypaper for thermoelectric applications. MWCNT buckypaper prepared by dispersion and filtration method was then deposited with the transition metals such as silver (Ag) and copper (Cu) by the electrodeposition method. We measured the voltage yield of Ag and Cu-doped buckypaper by making the temperature gradient along the sample. We established the temperature dependent Seebeck coefficient for Ag and Cu-doped buckypaper and found significant increase in the S(T). It is also revealed that remarkable rise in the value of S(T) and output voltage by connecting 3-sheets of BP in series. Here we determined the enhancement of Seebeck coefficient by increasing the number of BP sheets, thereby improving the thermoelectric efficiency. Furthermore, these paper-like CNT films show good flexibility, which makes them possible to be widely applied in various flexible energy conversion devices.

Effect of swelling of a photoresist on electromagnetic resonance of terahertz metamaterials.

This work uses isopropyl alcohol (IPA) to develop a photoresist. IPA dissolves the photoresist that is not exposed to UV light, and swells the photoresist that is exposed to UV light. The swelling of the photoresist distorts the split-ring resonators (SRRs). The distorted SRRs have a larger loop length, smaller line width, and smaller split gap than undistorted SRRs. The change in the dimensions of the SRRs is caused by the extension of the SRR arms in their longitudinal directions. The resonance frequency of the distorted SRRs is smaller than that of the undistorted SRRs, and the resonance frequency decreases with the development time. The resonance frequency of the distorted SRRs depends on not only their dimensions, but also the bending of their arms. The distorted SRRs in this work have a frequency tuning range with a maximum width of 0.13 THz. The method that is proposed herein uses IPA to fabricate passively tunable terahertz metamaterials, which exhibit the advantages of high reliability, low cost, and ease of fabrication.

Plasma functionalization and effect of annealing on electrical properties of MWCNT buckypaper

We report the fabrication of flexible high conductive multi walled carbon nanotube buckypaper (BP) by the dispersion and filtration method. The work demonstrates the changes observed in the electrical resistivity by both plasma post-treatment and by high temperature thermal annealing. Here we also illustrate the resistivity changes occurred due to different gas atmosphere as Ar, N₂, O₂ and studied the difference in resistivity in vacuum and air. Our R_H data yields a carrier concentration of about ~10²⁸-10³⁰ m^-3 for plasma treated and annealed BP samples.

A study on the properties of photoconductivity and photo voltage of buckpaper

Based on the remarkable properties and the extraordinary performance in theory, carbon nanotube (CNT) has been attracted numerous of attention and research interest since it has been found from 1991. However, until now there are no mature applications in our daily life, because of its singularity properties (even in one single tube) and nano-scale size (hard to handle). In this study, the unique dispersion and filtration process were introduced to form a bulk material, so called buckypaper (BP), from few grams of CNTs. That causes the superior nature of CNTs could expand and homogenize from a nano scale to macro scale. And, make a great breakthrough for real application of CNTs.