High-current field emission of point-type carbon nanotube emitters on Ni-coated metal wires

The fabrication and field emission characteristics are reported for point-type carbon nanotube (CNT) emitters formed by transferring a CNT film onto a Ni-coated Cu wire with a diameter of 1.24 mm. A Ni layer plays a role in enhancing the adhesion of CNTs to the substrate and improving their field emission characteristics. On firing at 400 °C, CNTs appear to directly bonded to a Ni layer. With a Ni layer introduced, a turn-on electric field of CNT emitters decreases from 1.73 to 0.81 V/μm by firing. The CNT film on the Ni-coated wire produces a high emission current density of 667 mA/cm² at quite a low electric field of 2.87 V/μm. This CNT film shows no degradation of emission current over 40 h for a current density of 60 mA/cm² at electric field of 6.7 V/μm. X-ray imaging of a printed circuit board with fine features is demonstrated by using our point-type CNT emitters.     

Growth mechanism and field emission behavior of carbon nanotubes grown over 300 nm thick aluminium interlayer

The influence of thick aluminium (Al) ~ 300 nm interlayer on the growth and field emission (FE) properties of carbon nanotubes (CNTs) deposited on silicon coated with a 2 nm iron (Fe) catalyst was studied. The CNTs were grown over silicon substrate with and without Al-interlayer via CVD. It was observed that the presence of such high thickness of the interlayer on the substrate resulted in higher growth rate, narrower diameters and longer height of CNTs compared to CNTs grown on silicon (Si) substrate coated only with Fe. Al-interlayer hinders the diffusion of Fe into silicon, hence promotes the growth rate. Literature reports that a thick layer of Al causes Fe to diffuse into it, negatively affecting the growth. However, in our experiments, no evidence of depletion of Fe from the substrate was observed. Unique patterns of grown CNTs could be attributed to anisotropic Al-melting over the silicon substrate resulting in Al/Fe rich and deficient regions. The drastic improvement of current density from 0.41 mA/cm² to 20 mA/cm² at a field of 3.5 V/μm was found with Al-interlayer CNT grown samples. These mechanisms of improvements in field emission characteristics have been discussed in detail.     

Robust growth of herringbone carbon nanofibers on layered double hydroxide derived catalysts and their applications as anodes for Li-ion batteries

Herringbone carbon nanofibers (CNFs) were efficiently produced by chemical vapor deposition on Ni nanoparticles derived from layered double hydroxide (LDH) precursors. The as-obtained CNFs with a diameter ranging from 40 to 60 nm demonstrated herringbone morphologies when they grew on Ni/Al LDH derived catalysts both in the fixed-bed and fluidized-bed reactor. The Ni/Mg/Al, Ni/Cu/Al, as well as Ni/Mo/Mg/Al catalysts were also effective to grow herringbone CNFs. The diameter and specific surface area of the as-obtained CNFs highly depended on the catalyst composition and the growth temperature. When CNFs were grown at 550 °C on Ni/Al catalyst, the as-obtained products had an outer diameter of ca. 50 nm and a specific surface area of 242 m² g⁻¹, possessed a discharge capacity of 330 mAh g⁻¹ as the electrode in a two-electrode coin-type cell. With the increase of the surface area, the discharge capacity increased at a rate of 0.90 mAh cm⁻², while the initial coulombic efficiency decreased gradually on nanocarbon anodes. This is attributed to the fact that CNFs with higher surface area afford smaller sp² carbon layer that facilitated more Li ions to extract from the anodes.     

Growth of carbon nanotube forests between a bi-metallic catalyst layer and a SiO2 substrate to form a self-assembled carbon–metal heterostructure

A special nanostructure was formed by the growth of carbon nanotubes (CNTs) between a substrate and a thin bi-metallic catalyst layer using a thermal chemical vapor deposition process. The catalyst layer is composed of adjacently disposed Cr and Ni phases formed prior to CNT growth. The Cr/Ni layer serves as a bi-metallic catalyst layer, which is pushed away from the substrate as a thin and continuous nanomembrane with the growth of CNTs. The self-assembled CNT–catalyst heterostructure possesses a smooth surface (RMS = 2.9 nm) with a metallic shine. Directly interlinked to the Cr/Ni layer, dense and vertically aligned multi-walled CNTs are found. Compared to conventional CNT films, the structure has significant advantages for CNT integration. From technology point of view, the structure allows further processing without impact on the CNTs as well as transfer of pristine vertically aligned CNTs to arbitrary substrates. Moreover, the as-grown CNT films provide an interface ideal for further electrical, thermal and mechanical contacting of CNT films. We present structural investigations of this special CNT–metal heterostructure. Furthermore, we discuss possible interface mechanisms during catalyst layer formation and CNT growth.     

Low temperature growth mechanisms of vertically aligned carbon nanofibers and nanotubes by radio frequency-plasma enhanced chemical vapor deposition

Using radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD), carbon nanofibers (CNFs) and carbon nanotubes (CNTs) were synthesized at low temperature. Base growth vertical turbostratic CNFs were grown using a sputtered 8 nm Ni thin film catalyst on Si substrates at 140 °C. Tip growth vertical platelet nanofibers were grown using a Ni nanocatalyst in 8 nm Ni films on TiN/Si at 180 °C. Using a Ni catalyst on glass substrate at 180 °C a transformation of the structure from CNFs to CNTs was observed. By adding hydrogen, tip growth vertical multi-walled carbon nanotubes were produced at 180 °C using FeNi nanocatalyst in 8 nm FeNi films on glass substrates. Compared to the most widely used thermal CVD method, in which the synthesis temperature was 400–850 °C, RF-PECVD had a huge advantage in low temperature growth and control of other deposition parameters. Despite significant progress in CNT synthesis by PECVD, the low temperature growth mechanisms are not clearly understood. Here, low temperature growth mechanisms of CNFs and CNTs in RF-PECVD are discussed based on plasma physics and chemistry, catalyst, substrate characteristics, temperature, and type of gas.     

Over 99.6 wt%-pure,sub-millimeter-long carbon nanotubes realized by fluidized-bed with careful control of the catalyst and carbon feeds

To establish a method for sub-second conversion of acetylene to sub-millimeter-long carbon nanotubes (CNTs), we have proposed and developed an internal heat-exchange reactor for fluidized-bed chemical vapor deposition (FBCVD). This reactor enabled sufficient heating of the reaction gas and uniform heating of the bed of alumina beads at a space velocity as high as 3600 h⁻¹. The direct feeding of the catalyst vapors (aluminum isopropoxide for the alumina support layer and ferrocene for the iron particles) to the bed separately from the other gases, which were fed through the heat-exchange and preheating zone and the distributer, enabled the careful control of the catalyst particles deposited on the beads. By decreasing the acetylene feed concentration and preventing the deactivation of small Fe particles, we realized semi-continuous production of 99.6–99.8 wt%-pure, sub-millimeter-long, few-wall CNTs with an average diameter of 6.5 nm at a carbon yield of 42%. The FBCVD reactor with an internal heat-exchanger can be scaled-up for practical mass production with uniform and energy-saving heating.     

Effects of Fe addition on the structure and catalytic performance of mesoporous Mn/Al–SBA-15 catalysts for the reduction of NO with ammonia

Mesoporous Al–SBA-15 has been synthesized by a hydrothermal method and used as a support for Mn/Al–SBA-15, Fe/Al–SBA-15, and Mn–Fe/Al–SBA-15 catalysts. XRD, N₂ sorption, XPS, H₂-TPR and activity tests have been used to assess the properties of catalysts. The Mn–Fe/Al–SBA-15 catalyst exhibited a higher SCR activity than Mn/Al–SBA-15 or Fe/Al–SBA-15 due to a synergistic effect between Mn and Fe. After the addition of Fe, the binding energy of Mn 2p_3/2 on Mn–Fe/Al–SBA-15(573) decreased by about 0.4 eV and the Mn^4 +/Mn^3 + ratio decreased to 1.10. The appropriate Mn^4 +/Mn^3 + ratio may have a great effect on the reduction of NO over Mn–Fe/Al–SBA-15(573) catalyst.     

Influence of a Ni interlayer on the optical and electrical properties of trilayer GZO/Ni/GZO films

Trilayer GZO/Ni/GZO films were deposited onto polycarbonate (PC) substrates with RF and DC magnetron sputtering, and then the influence of a Ni interlayer on the optical and electrical properties of the films was investigated. A 2-nm-thick Ni interlayer decreased the resistivity to 6.4×10⁻⁴ Ω cm and influenced the optical transmittance.Although optical transmittance deteriorated with Ni insertion, the films showed a relatively high optical transmittance of 74.5% in the visible wavelength region. The figure of merit (FOM) of a GZO single layer film was 1.2×10⁻⁴ Ω⁻¹, while that of the GZO/Ni/GZO films reached a maximum of 8.2×10⁻⁴ Ω⁻¹.Since a higher FOM results in higher quality transparent-conductive oxide (TCO) films, it is concluded that GZO films with a 2 nm Ni interlayer have better optoelectrical performance than single-layer GZO films.     

Co-production of hydrogen and multi-wall carbon nanotubes from ethanol decomposition over Fe/Al2O3 catalysts

The co-production of hydrogen and carbon nanotubes (CNTs) from the decomposition of ethanol over Fe/Al₂O₃ at different temperatures and feeding rates of ethanol was investigated systematically. The results indicated that Fe/Al₂O₃ was a quite active catalyst for the co-production of hydrogen and CNTs and that its activity and stability depended strongly on the Fe loading. Among all catalysts tested, 10 mol% Fe/Al₂O₃ was the most effective catalyst based on the ratio of hydrogen production, the total H₂ yield, and the quality of the CNTs formed. The efficiency of hydrogen production from ethanol decomposition over 10 mol% Fe/Al₂O₃ reached a maximum of ∼80% at 800 °C and the yield of CNTs with well-oriented growth and uniform diameter was 141%. In addition, the reaction of hydrogen and CNTs co-produced from ethanol decomposition was proposed.     

Transparent and high infrared reflection film having sandwich structure of SiO2/Al:ZnO/SiO2

New transparent and high infrared reflection films having the sandwich structure of SiO₂/Al:ZnO(AZO)/SiO₂ were deposited on the soda-lime silicate glass at room temperature by radio frequency (R.F.) magnetron sputtering. The optical and electrical properties of SiO₂ (110 nm)/AZO (860 nm)/SiO₂ (110 nm) sandwich films were compared with those of single layer AZO (860 nm) films and double layer SiO₂ (110 nm)/AZO (860 nm) films. The results show that these sandwich films exhibit high transmittance of over 85% in the visible light range (380–760 nm), and low reflection rate of below 4.5% in the wavelength range of 350–525 nm, which is not shown in the conventional single layer AZO (860 nm) films and double layer SiO₂ (110 nm)/AZO (860 nm) films. Further these sandwich films display a low sheet resistance of 20 Ω/sq by sheet resistance formula and high infrared reflection rate of above 80% in the wavelength range of 15–25 μm. In addition, the infrared reflection property of these sandwich films is determined mainly by the AZO film. The outer SiO₂ film can diminish the interference coloring and increase transparency; the inner SiO₂ film improves the adhesion of the coating to the glass substrate and prevents Ca²⁺ and Na⁺ in the glass substrate from entering the AZO film.     

Improved thermal interface property of carbon nanotube–Cu composite based on supercritical fluid deposition

In this paper, we present a new synthesis method of carbon nanotubes (CNTs)-copper (Cu) composite on a silicon substrate using combination of supercritical fluid deposition (SCFD) and electrochemical plating (ECP) process. Deposition of a Cu layer onto CNTs is carried out under supercritical condition, and the CNTs–Cu composite with high-density Cu is synthesized by additional ECP process. The Cu layer deposited by SCFD functions as a seed layer for ECP, and spaces between neighboring CNTs are filled by Cu. The measured density of the CNTs–Cu composite is 8.2 ± 0.3 g/cm³, and the volume percentage of voids is 3–6%. The evaluated thermal resistance including the thermal interface resistance and bulk resistance of the composite is as low as 28.4 mm² K W⁻¹ at a contact pressure of 0.2 MPa. A CNT brush formed on the composite surface can reduce the thermal resistance to be 68.4 mm² K W⁻¹ at a contact pressure of 0.25 MPa. The CNTs–Cu composite shows the ability applicable to many microelectronics applications as a thermal interface material.     

Preparation of a highly dispersed Ni2P/Al2O3 catalyst using Ni–Al–CO32 − layered double hydroxide as a nickel precursor

We now report a novel method for the synthesis of a Ni₂P/Al₂O₃-LW catalyst using Ni–Al–CO₃^2 − layered double hydroxide (Ni–Al–CO₃^2 −-LDH) as a nickel precursor and ammonium dihydrogen phosphate as a phosphorous precursor under microwave–hydrothermal (MWH) treatment for 20 min at 363 K. The catalysts were characterized by XRD, TPR, BET, CO uptake and XPS. MWH treatment can promote the formation of smaller and highly dispersed Ni₂P particles and a higher surface area of the catalyst. The Ni₂P/Al₂O₃-LW shows hydrodesulfurization activity of 99.3%, which was much higher than that found for the Ni₂P/Al₂O₃ catalyst obtained via an impregnation method.     

Diffusion coefficient of carbon in Fe–Ni alloy during synthesis of diamond under high temperature and high pressure

Synthetic diamond particles were prepared under high temperature and high pressure using arrayed seeds. A dense Fe–Ni alloy shell covered each diamond seed during synthesis; the growth of diamond particles was controlled by the diffusion of carbon through the metallic shell. The diffusion coefficient of carbon through Fe–Ni melt at 1600 K and 5.5 GPa is about 5×10^?6 cm²/s, with an activation energy for diffusion of 336 kJ/mol.     

Fabrication and electromagnetic interference shielding effectiveness of carbon nanotube reinforced carbon fiber/pyrolytic carbon composites

Carbon nanotube reinforced carbon fiber/pyrolytic carbon composites were fabricated by precursor infiltration and pyrolysis method and their electromagnetic interference shielding effectiveness (EMI SE) was investigated over the frequency range of 8.2–12.4 GHz (X-band). Carbon nanotubes (CNTs) were in situ formed through catalyzing hydrocarbon gases evaporating out of phenolic resin with nano-scaled Ni particles. The content of CNTs increased with the increase of Ni loadings (0.00, 0.50, 0.75 and 1.25 wt.%) in phenolic resin. Thermal gravimetrical analysis results showed that the carbon yield of phenolic resin increased with the addition of Ni catalyst. With the formation of CNTs, the EMI SE increased from 28.3 to 75.2 dB in X-band. The composite containing 5.0 wt.% CNTs showed an SE higher than 70 dB in the whole X-band.     

Carbon nanotubes for interconnects in future integrated circuits: The challenge of the density

A CVD process with a high density of CNTs has been developed on doped silicon material thanks to plasma pre-treatment of the catalyst. With this process small diameter double and triple wall CNTs with an average diameter of 3.8 nm have been grown. The density of the best materials on blanket substrate is larger than 10¹² cm^? 2. These materials have been successfully integrated in via holes with a diameter ranging between 1 µm and 0.3 µm with an equivalent density. In 140 nm hole diameter large 70 nm bundle formations have been observed. In these bundles a density of CNT walls close to 10¹³ cm^? 2 has been estimated.     

A kinetic study on hydrochloric acid leaching of nickel from Ni–Al2O3 spent catalyst

Hydrochloric acid leaching of nickel from spent Ni–Al₂O₃ catalyst (12.7% Ni, 39.2% Al and 0.68% Fe) has been investigated at a range of conditions by varying particle size (50–180 μm), acid concentration (0.025–2 M), pulp density (0.2–0.4%, w/v) and temperature (293–353 K). Nickel was selectively leached from the catalyst, irrespective of the different conditions. Under the most suitable conditions (1 M HCl, 323 K, stirring at 500 rpm, 50–71 μm particle size), the extent of leaching of Ni and Al after 2 h was 99.9% and 1%, respectively. The XRD pattern of the spent catalyst corresponded to crystalline α-Al₂O₃ along with elemental Ni. The peak due to elemental Ni was absent in the residue sample produced at the optimum leaching conditions, confirming the complete dissolution of Ni from the spent catalyst. The leaching results were well fitted with the shrinking core model with apparent activation energy of 17 kJ/mol in the temperature range of 293–353 K indicating a diffusion controlled reaction.     

One-step synthesis of a graphene-carbon nanotube hybrid decorated by magnetic nanoparticles

Graphene-carbon nanotube (G-CNT) hybrids were synthesized by a one-step chemical vapor deposition process using a mixed catalyst of MgO and Fe/MgO. MgO layers acted as templates for the growth of graphene, and Fe particles on the MgO layers catalyzed the growth of single or double-walled CNTs. The G-CNT hybrids had porous structures with hierarchical pore distributions due to the composition of graphene with CNT network. Superparamagnetism with a saturation magnetization of 2.7 emu/g was found in the G-CNT hybrids due to the existence of Fe₃C nanoparticles of size ～3 nm. The graphene to CNT ratio was conveniently changed by varying the MgO to Fe/MgO ratio, as characterized by Raman analysis and specific surface area measurements. Furthermore, a simplified synthesis of G-CNT hybrids was demonstrated by using MgO supported Fe or Ni catalysts with a low metal concentration.     

Overcoming the quality–quantity tradeoff in dispersion and printing of carbon nanotubes by a repetitive dispersion–extraction process

Dispersion-printing processes are essential for the fabrication of various devices using carbon nanotubes (CNTs). Insufficient dispersion results in CNT aggregates, while excessive dispersion results in the shortening of individual CNTs. To overcome this tradeoff, we propose here a repetitive dispersion–extraction process for CNTs. Long-duration ultrasonication (for 100 min) produced an aqueous dispersion of CNTs with sodium dodecylbenzene sulfonate with a high yield of 64%, but with short CNT lengths (a few μm), and poor conductivity in the printed films (∼450 S cm⁻¹). Short-duration ultrasonication (for 3 min) yielded a CNT dispersion with a very small yield of 2.4%, but with long CNTs (up to 20–40 μm), and improved conductivity in the printed films (2200 S cm⁻¹). The remaining sediment was used for the next cycle after the addition of the surfactant solution. 90% of the CNT aggregates were converted into conductive CNT films within 13 cycles (i.e., within 39 min), demonstrating the improved conductivity and reduced energy/time requirements for ultrasonication. CNT lines with conductivities of 1400–2300 S cm⁻¹ without doping and sub-100 μm width, and uniform CNT films with 80% optical transmittance and 50 Ω/sq sheet resistance with nitric acid doping were obtained on polyethylene terephthalate films.     

Parametric study of intrinsic thermal transport in vertically aligned multi-walled carbon nanotubes using a laser flash technique

The thermal diffusivity of vertically aligned carbon nanotube (VACNT, multi-walled) films synthesized by thermal chemical vapor deposition was measured by a laser flash technique, and shown to be ～30 mm² s^?1 along the tube-alignment direction. The calculated thermal conductivities of the VACNT films and the individual CNTs were ～27 and ～540 W m^?1 K^?1, respectively. The technique was verified to be reliable although a proper sampling procedure is critical. A systematic parametric study of the effects of defects, buckling, tip-to-tip contacts, packing density, and tube–tube interaction on the thermal diffusivity was carried out. Defects and buckling decreased the thermal diffusivity dramatically. An increased packing density was beneficial in increasing the collective thermal conductivity of the VACNT film; however, the increased tube–tube interaction in dense VACNT films decreased the effective thermal conductivity of the individual CNTs in the films. The tip-to-tip contact resistance was shown to be ～1 × 10^?7 m² K W^?1. The study will shed light on the potential application of VACNTs as thermal interface materials in microelectronic packaging.     

Effect of substrate heating and microwave attenuation on the catalyst free growth and field emission of carbon nanotubes

Carbon nanotubes (CNTs) have been directly grown on Inconel 600 substrates by microwave plasma enhanced chemical vapor deposition without using any external catalyst. Grown CNTs were characterized by field emission scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy and field emission measurements. Characterization results show that field emission current density increases from 200 μA/cm² at ∼5.5 V/μm to 14.5 mA/cm² at ∼1.6 V/μA when substrate is heat-treated and incident microwave is attenuated before reaching it. Detailed characterization reveals that heat-treatment results in migration of Cr and Fe oxides towards the top surface which completely changes substrate morphology also. Microwave attenuation reduces reflection of microwaves from the substrate and increases residence time of the precursor over the substrate promoting high density growth of CNTs. The combination of these two process parameters resulted in growth of long, dense CNTs with bamboo-like defects that contributes to enhanced current density at lower applied field.