Nguyen Tuan Hong

Comparison of field-electron emission from different carbon nanotube array structures

The effect of macroscopic cathode structures, which consisted of an array of the identical vertically aligned carbon nanotube (VACNT) columns in a particular arrangement, on the field emission was investigated. The electric field simulation based on the FEMLAB code showed characteristics of edge-induced U-shaped field distribution on CNT column. It was found that the degree of the field screening was dependent on VACNT-column array patterns, and that the overall field distribution depended on a number of VACNT columns at the periphery for the finite array size. Three different types of VACNT-column array cathodes were fabricated and tested for the field-electron emission: square, hexagonal, and triangular pattern arrays. All these VACNT-column array cathodes showed excellent electron-emission characteristics and a general trend consistent with field distribution simulations.

Enhanced thermal conductivity of nanofluid-based ethylene glycol containing Cu nanoparticles decorated on a Gr–MWCNT hybrid material

In this study, nanofluid based ethylene glycol (EG) containing Cu nanoparticles decorated on a Gr–MWCNT hybrid material (Gr–MWCNT/Cu) was synthesized successfully for the first time via a chemical reduction method. The SEM, HRTEM, FTIR and XRD studies revealed that Cu nanoparticles with an average diameter of 18 nm were well decorated on the surface of both MWCNTs and graphene sheets. The nanofluids containing Gr–MWCNT/Cu material showed good stability and a maximum thermal conductivity enhancement of 41% at 60 °C for the nanofluid containing 0.035 vol% material compared to EG alone. The enhancement is due to the combination of the high thermal conductivity of graphene, CNT and Cu nanoparticles as well as the higher surface area of the Gr–MWCNT/Cu hybrid structure. Experimental results of thermal conductivity were evaluated using different theoretical models, amongst which the Hamilton–Crosser model was found suitable for predicting the thermal conductivity of the nanofluid.

Field electron emission from free-standing flexible PDMS-supported carbon-nanotube-array films

Fabrication of free-standing carbon-nanotube- (CNT) array films supported by PDMS (poly-dimethylsiloxane) matrix, which opens up the possibility to transform deliberate CNT-array architecture into flexible electronic and photonic components, is reported. Low viscosity and good wetting characteristics of uncured PDMS prepolymer solution are important for thorough infiltration, and robustness and elasticity of cured PDMS are important for realization of deformable free-standing films detachable from substrates. Field-emission characterization of a series of CNT column arrays in diode configuration shows that fabrication of a flexible electron emitter is feasible, but the excellent emission characteristics of as-grown CNT column arrays is compromised after PDMS infiltration. In particular, emission current level and stability of a free-standing PDMS-supported CNT array is inferior to those of as-grown or PDMS-infiltrated CNT arrays on silicon substrates.

Field-electron emission from flexible carbon nanotube array cathodes

The authors report two approaches to fabricate flexible cold cathodes having vertically aligned carbon nanotubes (VACNTs) as active electron-emitting material. In the first approach, VACNT arrays were removed from substrates, transferred to flexible metal foils or plastic films, and secured by conductive epoxy. In the other approach, polydimethylsiloxane (PDMS) was used as a supporting matrix to fabricate more sturdy freestanding flexible cathodes. Controlled infiltration of PDMS to keep the top surfaces of VACNT columns from being buried underneath PDMS was the key of this approach. Both of these methods allowed fabrication of flexible cold cathode, and preliminary test results of fabricated flexible cold cathodes showed good field-emission characteristics.

Flexible carbon nanotube-array cathodes: Fabrication and bending effect on field-electron emission

The authors report the fabrication of freestanding field-electron emitters based on arrays of vertically aligned carbon nanotubes (VACNTs) and flexible polydimethylsiloxane (PDMS). Transplant of VACNT arrays from silicon substrates to flexible PDMS platforms through a press-and-curing process resulted in PDMS-supported VACNT-array electron emitters. Test of field-electron emission from the PDMS-supported VACNT columns in a diode configuration showed good field-emission results regardless of cathode geometry, either planar or convex shape cathodes. Furthermore, the repeated bending of the PDMS-supported VACNT-column cathodes up to a few hundred times showed no noticeable degradation in field emission. Numerical simulations of electric field distribution at various bending angles and anode-cathode distance show that the general trend in emission-current variations is consistent with the difference in maximum electric field strength at the cathode surface.

Influence of defects induced by chemical treatment on the electrical and thermal conductivity of nanofluids containing carboxyl-functionalized multi-walled carbon nanotubes

In this paper, we present the results on the influence of chemical treatment time on the structure of carboxyl-functionalized MWCNTs (MWCNT–COOH) and their nanofluids. The morphological and structural studies investigated by FTIR, HRTEM and Raman scattering demonstrated that the structural defects of MWCNT–COOH increase with increasing chemical treatment time. Nanofluids containing MWCNT–COOH treated for a longer time showed better stability due to the increasing of COOH functional groups attached to the surface of MWCNTs. The electrical conductivity of the nanofluids increases with increasing CNT concentration and decreases with increasing chemical treatment time. The thermal conductivity of the nanofluids enhanced when increasing CNT concentration and reached the highest value for MWCNT–COOH with 5 h chemical treatment. By using the effective medium theory (EMT) and experimental data fitting, the thermal boundary resistance (TBR) and the thermal boundary conductance (TBC) of MWCNT–COOH/water were found to be 90 × 10−8 m2 K W−1 and 1.1 MW m−2 K−1, respectively. The interfacial layer thermal conductivity (Ki) between CNTs and base fluid was estimated by using Mursheds model. The highest Ki was obtained to be 2.6 W m−1 K−1 for a nanofluid with 5 h chemical treatment. The results implied that the thermal conductivity of CNT based nanofluids could be improved by increasing the Ki via optimizing of the chemical treatment conditions.

Effect of Surface Morphology and Dispersion Media on the Properties of PEDOT:PSS/n-Si Hybrid Solar Cell Containing Functionalized Graphene

We present the results on the effect of surface morphology and dispersion media on the properties of PEDOT:PSS/n-Si hybrid solar cell containing functionalized graphene (Gr). The hybrid solar cells based on SiNWs showed higher power conversion efficiency (PCE) compared to the planar based cells due to suppressing the carrier recombination and improving carrier transport efficiency. The PCE of hybrid solar cells could be improved by adding Gr into PEDOT:PSS. Different solvents including deionized (DI) water, ethylene glycol (EG), and isopropyl alcohol (IPA) were used as media for Gr dispersion. The best performance was obtained for the cell containing Gr dispersed in EG with a measured PCE of 7.33% and nearly 13% and 16% enhancement in comparison with the cells using Gr dispersed in IPA and DI water, respectively. The increase in PCE is attributed to improving the carrier-mobility, electrical conductivity, PEDOT crystallinity, and ordering.

Carbon nanotubes based lubricating oils for engines

Carbon nanotubes (CNTs) are well-known nanomaterials with many excellent properties such as high hardness, high strength, and excellent thermal conductivity. Owing to their very high thermal conductivity (2000 W/m.K compared to thermal conductivity of Ag 419 W/m.K), CNTs become ones of the most suitable nano additives for fabricating the lubricating oils in order to increase the thermal conductivity of lubricating oils, to enhance the efficiency of heat dissipation for the engine, and to improve the performance efficiency of engine. In this work, we present the obtained results on application of the CNTs in lubricating oils for some engines. The results showed that with the addition of CNTs, the thermal conductivity of lubricating oils increase about 15%, this helps improve the efficiency of heat dissipation for the engine. Experimental results show that when using the lubricating oils containing carbon nanotubes, the temperature of engine dropped about 10°C, fuel saving was upto 15% and longevity of lubricating oil increased upto 20,000 km.

Schottky emitters with carbon nanotubes as electron source

In this paper we present a simple approach for making a Schottky emitter with carbon nanotubes (CNT). A tungsten wire of 0.3 mm diameter was used as a heating filament. An iron rod of 0.3 mm diameter having a sharp tip was weld-bonded to the filament. By heating the filament to around 750/spl deg/C in a mixture of (C/sub 2/H/sub 2/+Ar), multi-wall CNT was grown at the Fe tip. Electron field emission and thermal-field emission properties of the fabricated Schottky emitter were measured at different temperatures. At room temperature, the field emission current well obeyed the Fowler-Nordheim (F-N) mechanism with threshold voltage of around 0.2 V//spl mu/m. At the temperature of higher than 780/spl deg/C, the thermal-field emission dominated and both field emission and thermal-field emission were very stable with very low fluctuation. Enhancement factor of 1.3 /spl times/ 10/sup 7/ m/sup -1/ and work function of 4.95 eV were evaluated for the grown CNT. The mentioned CNT-Schottky emitters for advanced electron beam instruments are expected.

Thermal Conductivity of Ethylene Glycol Based Copper Nanoparticle Decorated Graphene Nanofluids

In this paper, thermal conductivity of ethylene glycol based copper nanoparticle decorated graphene (Cu/Gr) nanofluids are successfully synthesized by a chemical reduction technique using ascorbic acid antioxidant agent. Nano-sized copper nanoparticles are decorated on functionalized graphene, and then dispersed uniformly in ethylene glycol (EG) to make the nanofluids. Morphology, phase composition and thermal conductivity of nanofluids are investigated in detail. Thermal conductivity of the nanofluid containing Cu/Gr with mass ratio of 5:1 shows an enhancement about 10% and 29% at 30 ̊C and 60 ̊C comparing with the EG fluid only. The results show the high potential application of Cu/Gr nanofluids in heat transfer fields.