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.     

Rapid oxidative activation of carbon nanotube yarn and sheet by a radio frequency,atmospheric pressure,helium and oxygen plasma

Carbon nanotube yarn and sheet were activated using radio frequency, atmospheric pressure, helium and oxygen plasmas. The nanotubes were exposed to the plasma afterglow, which contained 8.0 × 10¹⁶ cm⁻³ ground state O atoms, 8.0 × 10¹⁶ cm⁻³ metastable O₂ (¹Δ_g), and 1.0 × 10¹⁶ cm⁻³ ozone. X-ray photoelectron spectroscopy and infrared spectroscopy revealed that 30 s of plasma treatment converted 25.2% of the carbon atoms on the CNT surface to oxidized species, producing 17.0% alcohols, 5.9% carbonyls, and 2.3% carboxylic acids. The electrical resistivity increased linearly with the extent of oxidation of the CNT from 4 to 9 × 10⁻⁶ Ω m. On the other hand, the tensile strength of the yarn was decreased by only 27% following plasma oxidation.     

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.     

Physical properties of carbon nanotube sheets drawn from nanotube arrays

We report mechanical, thermal, and electrical properties of novel sheet materials composed of multiwalled carbon nanotubes, drawn from a CNT array. At low loading there is some slippage of CNTs but at higher loading tensile strength σ₀ = 7.9 MPa and Young’s modulus E = 310 MPa. The room-temperature thermal conductivity of the CNT sheet was 2.5 ± 0.5 W m^?1 K^?1, giving a thermal conductivity to density ratio of κ/ρ = 65 W m^?1 K^?1 g^?1 cm³. The heat capacity shows 1D behavior for T > 40 K, and 2D or 3D behavior at lower temperatures. The room-temperature specific heat was 0.83 J g^?1 K^?1. The iV curves above 10 K have Ohmic behavior while the iV curve at T = 2 K is non-Ohmic, and a model to explain both ranges is presented. Negative magnetoresistance was found, increasing in magnitude with decreasing temperature (?15% at T = 2 K and B = 9 T). The tensile strength, Young’s modulus and electrical conductivity of the CNT sheet are low, in comparison with other CNT materials, likely due to defects. Thermal conductivity is dominantly phononic but interfacial resistance between MWCNTs prevents the thermal conductivity from being higher.     

Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns

The electrical resistivity of CNT yarns of diameters 10–34 μm, spun from multi-walled carbon nanotube arrays, have been determined from 2 to 300 K in magnetic fields up to 9 T. The magnetoresistance is large and negative at low temperatures. The thermal conductivity also has been determined, by parallel thermal conductance, from 5 to 300 K. The room-temperature thermal conductivity of the 10 μm yarn is (60 ± 20) W m^?1 K^?1, the highest measured result for a CNT yarn to date. The thermal and electrical conductivities both decrease with increasing yarn diameter, which is attributed to structural differences that vary with the yarn diameter.     

Effect of Nd-doping on structural,thermal and electrochemical properties of LaFe0.5Cr0.5O3 perovskites

The structural, thermal and electrochemical properties of the perovskite-type compound La_1−xNd_xFe_0.5Cr_0.5O₃ (x=0.10, 0.15, 0.20) are investigated by X-ray diffraction, thermal expansion, thermal diffusion, thermal conductivity and impedance spectroscopy measurements. Rietveld refinement shows that the compounds crystallize with orthorhombic symmetry in the space group Pbnm. The average thermal expansion coefficient decreases as the content of Nd increases. The average coefficient of thermal expansion in the temperature range of 30–850 °C is 10.12×10⁻⁶, 9.48×10⁻⁶ and 7.51×10⁻⁶ °C⁻¹ for samples with x=0.1, 0.15 and 0.2, respectively. Thermogravimetric analyses show small weight gain at high temperatures which correspond to filling up of oxygen vacancies as well as the valence change of the transition metals. The electrical conductivity measured by four-probe method shows that the conductivity increases with the content of Nd; the electrical conductivity at 520 °C is about 4.71×10⁻³, 6.59×10⁻³ and 9.62×10⁻³ S cm⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. The thermal diffusivity of the samples decreases monotonically as temperature increases. At 600 °C, the thermal diffusivity is 0.00425, 0.00455 and 0.00485 cm² s⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. Impedance measurements in symmetrical cell arrangement in air reveal that the polarization resistance decreases from 55 Ω cm⁻² to 22.5 Ω cm⁻² for increasing temperature from 800 °C to 900 °C, respectively.     

Thermal conductivity of MWCNT/epoxy composites: The effects of length,alignment and functionalization

Carbon nanotubes (CNTs) show great promise to improve composite electrical and thermal conductivity due to their exceptional high intrinsic conductance performance. In this research, long multi-walled carbon nanotubes (long-MWCNTs) and its thin sheet of entangled nanotubes were used to make composites to achieve higher electrical and thermal conductivity. Compared to short-MWCNT sheet/epoxy composites, at room temperature, long-MWCNT samples showed improved thermal conductivity up to 55 W/mK. The temperature dependence of thermal conductivity was in agreement with κ ∝ Tⁿ (n = 1.9–2.3) below 150 K and saturated around room temperature due to Umklapp scattering. Samples with the improved CNT degree of alignment by mechanically stretching can enhance the room temperature thermal conductivity to over 100 W/mK. However, functionalization of CNTs to improve the interfacial bonding resulted in damaging the CNT walls and decreasing the electrical and thermal conductivity of the composites.     

Synthesis and mechanical properties of carbon nanotube/diamond-like carbon composite films

Diamond-like carbon (DLC) coatings were successfully deposited on carbon nanotube (CNT) films with CNT densities of 1 × 10⁹/cm², 3 × 10⁹/cm², and 7 × 10⁹/cm² by a radio frequency plasma-enhanced chemical vapor deposition (CVD). The new composite films consisting of CNT/DLC were synthesized to improve the mechanical properties of DLC coatings especially for toughness. To compare those of the CNT/DLC composite films, the deposition of a DLC coating on a silicon oxide substrate was also carried out. A dynamic ultra micro hardness tester and a ball-on-disk type friction tester were used to investigate the mechanical properties of the CNT/DLC composite films. A scanning electron microscopic (SEM) image of the indentation region of the CNT/DLC composite film showed a triangle shape of the indenter, however, chippings of the DLC coating were observed in the indentation region. This result suggests the improvement of the toughness of the CNT/DLC composite films. The elastic modulus and dynamic hardness of the CNT/DLC composite films decreased linearly with the increase of their CNT density. Friction coefficients of all the CNT/DLC composite films were close to that of the DLC coating.     

Reducing thermal contact resistance using a bilayer aligned CNT thermal interface material

A thermal interface material (TIM) was fabricated by synthesizing aligned carbon nanotubes (CNT) on both sides of a thin copper foil. The Hot Disk^® method was applied to measure the thermal conductivity of these CNT-TIMs. Results showed that a thicker copper foil substrate or CNT layer led to a lower overall thermal resistance. The laser flash method was used to study the performance of the bilayer aligned CNT-TIMs using two copper plates as heat source and sink. An enhancement in thermal conductivity of more than 290% could be obtained under an applied contact pressure of 0.01 MPa, as compared with two copper plates in direct contact. By filling in the space between the CNTs in the CNT layer with a conventional thermal conductive elastomer, Sylgard 160, the thermal resistance of the TIM was reduced to 8.78 mm² K/W, a value that is better than similar devices in the literature.     

Aligned diamond nano-wires: Fabrication and characterisation for advanced applications in bio- and electrochemistry

Nano-wires have become promising tools in a vast field of applications. Due to the many unique properties of diamond, the use of diamond nano-wires in biosensors attracts increasing attention. In this paper we introduce the realisation of wires from diamond using self-aligned nickel nano-particles as etching mask in an oxygen ICP dry etching step. With this process it is possible to create wires of high aspect ratios of 50, with diameters as small as 20 nm, and typical lengths of up to 1 μm on a large area in a dense pattern of about 10¹¹ cm^? 2. The Ni nano-particles are formed by thermal annealing at 700 °C for 5 min of a thin (1 nm) Ni film that is deposited onto the diamond surface. The surface enhancement factor due to wires is dependent on the geometrical details of wires and was measured to be 10 to 80. The electrochemical properties of wires have been characterized by cyclic voltammetry using Fe(CN)₆^? 3/? 4 which shows that such topographies act as filter for redox molecules.     

Low cost preparation of high thermal conductivity carbon blocks with ultra-high anisotropy from a commercial graphite paper

A carbon block with ultra-high anisotropy was produced from a commercial graphite paper as the thermal reinforcement and a thermosetting phenolic resin as the binder. Hot-pressing at a maximum temperature of 200 °C was used to densify and integrate the graphite paper stacks. It has been found that the graphite paper blocks have high thermal conductivities in the paper direction and low ones perpendicular. An anisotropy of 98.8% and a thermal conductivity of 197.8 W m^?1 K^?1 in the paper direction were achieved when the density was 1.1 g cm^?3. The thermal conductivity increased to 284.8 W m^?1 K^?1 with a decrease of anisotropy to 98.3% with a density of 1.56 g cm^?3.     

Effect of grain growth on the thermal conductivity of liquid-phase sintered silicon carbide ceramics

The effect of grain growth on the thermal conductivity of SiC ceramics sintered with 3 vol% equimolar Gd₂O₃-Y₂O₃ was investigated. During prolonged sintering at 2000 °C in an argon or nitrogen atmosphere, the β → α phase transformation, grain growth, and reduction in lattice oxygen content occurs in the ceramics. The effects of these parameters on the thermal conductivity of liquid-phase sintered SiC ceramics were investigated. The results suggest that (1) grain growth achieved by prolonged sintering at 2000 °C accompanies the decrease of lattice oxygen content and the occurrence of the β → α phase transformation; (2) the reduction of lattice oxygen content plays the most important role in enhancing the thermal conductivity; and (3) the thermal conductivity of the SiC ceramic was insensitive to the occurrence of the β → α phase transformation. The highest thermal conductivity obtained was 225 W(m K)⁻¹ after 12 h sintering at 2000 °C under an applied pressure of 40 MPa in argon.     

Oxygen permeation properties of BSCF5582 tubular membrane fabricated by the slip casting method

BSCF5582 tubular oxygen separation membranes were prepared using the most cost effective slip casting techniques. The optimum slurry composition was identified and a dense, and crack free 60 mm long BSCF5582 tubular membrane being successfully prepared after the programmed sintering process. The effects of the feed flow rate and the sweeping flow rate on the oxygen permeation flux of the tubular BSCF5582 membrane were investigated. The oxygen permeation flux increased with an increase of the oxygen chemical potential gradient to a maximum of 42.5 cm³/min in an O₂/N₂ condition at 1223 K for the 1.5 mm thick, 60 mm long BSCF tube, a value which corresponds to 1.42 cm³/min cm². The ionic conductivity of the oxygen was successfully calculated in the dominant electron conducting regime. The ionic conductivity was found to increase with an increase of the temperature to 900 °C, indicating that it is a thermally activated process with an activation energy of 0.70 ± 0.1 eV in an air environment.     

Surface properties of vertically aligned carbon nanotube arrays

This study focuses on the structural changes of vertically aligned carbon nanotube (CNT) arrays while measuring their adhesive properties and wetting behaviour. CNT forests grown by chemical vapor deposition with a height of ~ 100 µm, an outer CNT diameter of ~ 10 nm and a density of the order of ~ 10¹⁰ CNTs/cm² show an average adhesion of 4 N/cm² when pressed against a glass surface. The applied forces lead to the collapse of the regular CNT arrays which limits their reusability as functional dry adhesives. Goniometric water contact angle (CA) measurements on CNT forests show a systematic decrease from an initial value of ~ 126° to a final CA similar to highly orientated graphite. Environmental scanning electron microscopy shows that this loss of hydrophobicity is due to an evaporation induced compaction of CNTs together with the loss of their vertical alignment. We observe the formation of cellular patterns for controlled drying.     

Electrical characterization of multidoped ceria ceramics

Ceria ceramics was obtained from multi-doped nanosized ceria powders prepared by both modified glycine nitrate procedure (MGNP) and self-propagating reaction at room temperature (SPRT). Rare earth elements such as Nd, Sm, Gd, Dy, Y, Yb were used as dopants. The overall mole fraction of dopants was 0.2. One-hour long sintering of powder compacts was performed at 1500 °C in oxygen atmosphere. Phase composition, microstructure and ionic conductivity of sintered samples were analysed. Single-phase ceria was detected in all samples. In general, the increase in the number of dopants improved the ionic conductivity. The samples doped simultaneously with five dopants had the highest ionic conductivity, as evidenced by the impedance measurements. At 450 °C, the conductivity of sample obtained by MGNP was 3.94×10^?3 Ω^?1 cm^?1 whereas the conductivity of sample obtained by SPRT was 2.61×10^?3 Ω^?1 cm^?1. The conductivity activation energy for MGNP and SPRT samples was measured to be 0.348 and 0.385 eV, respectively. Finally, the conductivity decreased as the number of dopants increased to six.     

Large-area reduced graphene oxide thin film with excellent thermal conductivity and electromagnetic interference shielding effectiveness

In this work, we fabricated reduced large-area graphene oxide (rLGO) with maximum surface area of 1592 μm² through a cost-effective chemical reduction process at low temperature. The product revealed large electrical conductivity of 243 ± 12 S cm⁻¹ and thermal conductivity of 1390 ± 65 W m⁻¹ K⁻¹, values much superior to those of a conventional reduced small-area graphene oxide (with electrical conductivity of 152 ± 7.5 S cm⁻¹ and thermal conductivity of 900 ± 45 W m⁻¹ K⁻¹). The rLGO thin film also exhibited not only excellent stiffness and flexibility with Young’s modulus of 6.3 GPa and tensile strength of 77.7 MPa, but also an efficient electromagnetic interference (EMI) shielding effectiveness of ∼20 dB at 1 GHz. The excellent performance of rLGO is attributed to the fact that the larger area LGO sheets include much fewer defects that are mostly caused by the damage of graphene sp² structure around edge boundaries, resulting in large electrical conductivity. The manufacturing process of rLGO is an economical and facile approach for the large scale production of highly thermally conducting graphene thin films with efficient EMI shielding properties, greatly desirable for future portable electronic devices.     

Electrothermal halogenation of carbon nanotube films

We report an innovative approach to halogenation of carbon nanotube (CNT) films. Application of bias voltage across horizontally aligned CNT films resulted in the electrothermal effect, which enabled us to chemically modify CNT films at the entirely controlled range of temperatures up to 300 °C. Such heated CNT membranes were exposed to gaseous halogens (Cl₂, Br₂, I₂) whilst their electrical properties were monitored. The experiments were carried out at room temperature and 100–300 °C (in 50 °C steps) until no further change in electrical resistance could be seen. The procedure lasted less than one minute, during which we were able to successfully introduce up to 6.7%, 6.0% and 1.5% at. of Cl, Br and I into the CNT framework. In parallel, we observed a permanent increase in conductivity of the films and mild purification of the starting material due to the removal of various carbon–oxygen functional groups.     

Evaluation of (Ba0.5Sr0.5)0.85Gd0.15Co0.8Fe0.2O3−δ cathode for intermediate temperature solid oxide fuel cell

A perovskite-type (Ba_0.5Sr_0.5)_0.85Gd_0.15Co_0.8Fe_0.2O_3?δ (BSGCF) oxide has been investigated as the cathode of intermediate temperature solid oxide fuel cells (IT-SOFCs). Coulometric titration, thermogravimetry analysis, thermal expansion and four-probe DC resistance measurements indicate that the introduction of Gd³⁺ ions into the A-site of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) leads to the increase in both oxygen nonstoichiometry at room temperature and electrical conductivity. For example, the conductivity of BSGCF is 148 S cm^?1 at 507 °C, over 4 times as large as that of BSCF. Furthermore, the electrochemical activity toward the oxygen reduction reaction is also enhanced by the Gd doping. Impedance spectra conducted on symmetrical half cells show that the interfacial polarization resistance of the BSGCF cathode is 0.171 Ω cm² at 600 °C, smaller than 0.297 Ω cm² of the BSCF cathode. A Ni/Sm_0.2Ce_0.8O_1.9 anode-supported single cell based on the BSGCF cathode exhibits a peak power density of 551 mW cm^?2 at 600 °C.     

Preparation by tape casting and hot pressing of copper carbon composites films

During this last decade, the use of metal matrix composites (MMCs) such as AlSiC or CuW for heat dissipation in microelectronic devices has been leading to the improvement of the reliability of electronic power modules. Today, the continuous increasing complexity, miniaturization and density of components in modern devices requires new heat dissipating films with high thermal conductivity, low coefficient of thermal expansion (CTE), and good machinability. This article presents the original use of copper carbon composites, made by tape casting and hot pressing, as heat dissipation materials. The tape casting process and the sintering have been adapted and optimised to obtain near fully dense, flat and homogeneous Cu/C composites.A good electrical contact between carbon fibres and copper matrix and a low porosity at matrix/copper interfaces allow obtaining a low electrical resistivity of 3.8 μΩ cm⁻¹ for 35 vol.% carbon fibre (electrical resistivity of copper = 1.7 μΩ cm⁻¹). The CTE and the thermal conductivity are strongly anisotropic due to the preferential orientation of carbon fibres in the plan of laminated sheets. Values in the parallel plan are, respectively, 9 × 10⁻⁶ °C⁻¹ and 160–210 W m⁻¹ K⁻¹ for 40 vol.% fibres. These CTE and thermal conductivity values are in agreement with the thermo-elastic Kerner's model and with the Hashin and Shtrikman model, respectively.     

Study of processing variables on the electrical resistivity of conductive adhesives

In this paper, the authors explored the effects of processing variables, including carbon nanotube (CNT) concentration, assembly pressure, and processing temperature, on electrical conductivity of CNT-included electrically conductive adhesives (ECAs). The main effects of these variables were analyzed under specific range for each variable. Response surface methodology was used to investigate the cross-effects of these variables on ECA conductivity. By fitting the experimental data to the response function, minimum bulk resistivity of 1.5×10^?4 Ω cm was obtained at the optimum settings of processing variables (CNT concentration 2%, processing temperature 199 °C, pressure 6000 psi).