Measurement of the Intrinsic Thermal Conductivity of a Multiwalled Carbon Nanotube and Its Contact Thermal Resistance with the Substrate

The intrinsic thermal conductivity of an individual carbon nanotube and its contact thermal resistance with the heat source/sink can be extracted simultaneously through multiple measurements with different lengths of the tube between the heat source and the heat sink. Experimental results on a 66‐nm‐diameter multiwalled carbon nanotube show that above 100 K, contact thermal resistance can contribute up to 50% of the total measured thermal resistance; therefore, the intrinsic thermal conductivity of the nanotube can be significantly higher than the effective thermal conductivity derived from a single measurement without eliminating the contact thermal resistance. At 300 K, the contact thermal resistance between the tube and the substrate for a unit area is 2.2 × 10^?8 m² K W^?1, which is on the lower end among several published data. Results also indicate that for nanotubes of relatively high thermal conductance, electron‐beam‐induced gold deposition at the tube–substrate contacts may not reduce the contact thermal resistance to a negligible level. These results provide insights into the long‐lasting issue of the contact thermal resistance in nanotube/nanowire thermal conductity measurements and have important implications for further understanding thermal transport through carbon nanotubes and using carbon nanotube arrays as thermal interface materials.     

Synthesis and thermal transport studies of nanofluids based on metal decorated photochemically oxidized multiwalled carbon nanotubes

Nanoparticle fluid suspensions were prepared using photochemically functionalized multiwalled carbon nanotubes in polar base fluids. Multiwalled carbon nanotubes prepared by catalytic chemical vapour deposition technique have been functionalized by irradiating with ultraviolet light of wavelength 254 nm. The photochemical oxidation of multiwalled carbon nanotubes under UV irradiation introduces oxygen containing functional groups onto the surface of the nanotubes, generating new defects on their structure. Silver nanoparticles have been deposited over multiwalled carbon nanotubes by chemical method. The enhancement in thermal conductivity of the prepared nanofluids using functionalized multiwalled carbon nanotubes and Ag nanoparticles deposited functionalized multiwalled carbon nanotubes with volume fraction, temperature and aspect ratio has been demonstrated. Silver deposited functionalized multiwalled carbon nanotubes based nanofluids in DI water with 0.02% volume fraction exhibit a thermal conductivity enhancement of 9.9% and 47% at room temperature and at 50 degrees C respectively.     

Optically transparent carbon nanowire thin film

Thin films of amorphous carbon nanowire (a-CNW) have been fabricated from crossed multi-walled carbon nanotube (MWCNT) thin film. The fabrication was done by means of ion beam irradiation on various substrates. It is found that the a-CNW thin films show electrical conduction behaviour, and electrical conductivity varies after annealing. In addition, the transmission spectra in the visible range reveal that the film has above 90% optical transmission. It can be ascribed to the fact that the decreased crystallinity of MWCNTs by ion beam irradiation has caused the incremental increase of optical transmission. We also report on a method for cutting or destroying a-CNWs using low-energy focused electron beam from a scanning electron microscope.     

Atypical self-activation of Ga dopant for Ge nanowire devices

In this Letter we report the atypical self-activation of gallium (Ga) implanted by focused ion beam (FIB) into germanium nanowires (Ge-NWs). By FIB implantation of 30 keV Ga(+) ions at room temperature, the Ge-NW conductivity increases up to 3 orders of magnitude with increasing ion fluence. Cu(3)Ge heterostructures were formed by diffusion to ensure well-defined contacts to the NW and enable two point I/V measurements. Additional four point measurements prove that the conductivity enhancement emerges from the modification of the wires themselves and not from contact property modifications. The Ga distribution in the implanted Ge-NWs was measured using atom probe tomography. For high ion fluences, and beginning amorphization of the NWs, the conductivity decreases exponentially. Temperature dependent conductivity measurements show strong evidence for an in situ doping of the Ge-NWs without any further annealing. Finally the feasibility of improving the device performance of top-gated Ge-NW MOSFETs by FIB implantation was shown.     

Effect of homogeneous Al(OH)3 covered MWCNT addition on the thermal conductivity of Al2O3/epoxy-terminated poly(dimethylsiloxane) composites

Aluminum hydroxide covered multiwalled carbon nanotubes (A-MWCNTs) were synthesized as a conducting additive to alumina-epoxy-terminated poly(dimethylsiloxane). The measured diffusivity and calculated conductivity exhibited dissimilar behavior between several Al₂O₃ concentrations as a function of A-MWCNT loading, which correlated with the interface density and interconnectivity of the structures. The fabricated heterostructured A-MWCNT did not have a significant effect on the thermal conductivity of the composite because of phonon scattering at the interface. A small amount of A-MWCNT was feasible for establishment of a heat conductive percolating network with the greatest enhancement of thermal conductivity and diffusivity at an A-MWCNT loading of 1.0 and 2.0 wt%. Continuously increasing thermal transport properties were observed with the 49.1 vol.% Al₂O₃ loading which derived from a lower interface density nanowire and polymer matrix with enhanced interconnectivity.     

Bulk thermal conductivity studies of Sn/SnO coated and filled multiwalled carbon nanotubes for thermal interface material

Multiwalled carbon nanotubes (MWCNTs) were successfully coated and filled with Sn using a simple method. Bulk thermal conductivity of Sn/SnO filled and coated multiwalled carbon nanotubes (MWCNTs) of ～1 mm thickness pellet by laser method reveals surge in hybrid carbon nanotubes in comparison to pristine nanotubes. The thermal diffusivity and thermal conductivity of hybrid nanotubes are increased to 4.41 mm²/sec, 5.39 Wm^?1K^?1 of as compared to 0.36 mm²/sec and 0.28 Wm^?1K^?1 of the pristine nanotubes. The enhancement in thermal conductivity is attributed to the presence of Sn coating on surface and inside the carbon nanotubes and the formation of compact structures by reducing the air gaps between nanotubes because of their joining during compression and sintering.     

pH sensitive multiwalled carbon nanotubes

The CNTs-based sensors have received considerable attention because of their outstanding properties, such as faster response, higher sensitivity, and lower operating temperature. And we expect that CNTs-based electrochemical sensors offer substantial improvements in the performance of pH sensing device. This letter reports experimental results that demonstrate the pH sensing capability of the multiwalled carbon nanotubes (MWCNTs) film by using the thermal chemical vapor deposition (thermal CVD). It was found that electronic properties of MWCNTs can be changed by the introduction of different pH value solutions. The absorption of the hydroxide in pH buffer solution changes conductivity of the MWCNTs. We observed in situ measurement of electrical conductivity by cycling solution range from acid to base.     

An experimental study on stability and some thermophysical properties of multiwalled carbon nanotubes with water–ethylene glycol mixtures

In this study, the stability and thermophysical properties of multiwalled carbon nanotubes (MWCNTs) with double-distilled water (W) and double-distilled water/ethylene glycol (W/EG) mixtures are investigated. Stability analyses are performed through visual observation, thermal conductivity measurements, spectrophotometry and zeta potential measurement methods. An increase in ethylene glycol ratio in water increases the stability of nanofluid, which helps the nanoparticles disperse uniformly in the base fluid for a longer duration. It is concluded from the results that MWCNT nanofluids with a W/EG system (50:50) has good stability, showing no agglomeration for 36 d as compared with other nanofluids. Thermophysical properties such as thermal conductivity, viscosity and density with temperature were also measured. Maximum thermal conductivity enhancement of 29% was observed for MWCNT-nanofluid with W/EG system (50:50) at 50°C. It is also observed that with the addition of MWCNT in W/EG mixtures, viscosity and density increase but the enhancement was comparatively low with reference to thermal conductivity. From these results, it was interpreted that both stability and thermal conductivity increase with increase in ethylene glycol ratio in water.     

单根沥青基炭纤维热导率的温度特性研究

采用"T"形法测量了温度100K～400K范围内单根沥青基炭纤维的热导率.结果表明,在300K以下,由于边界散射的影响,炭纤维热导率随着温度升高而增大,350K左右渐趋于饱和,对应热导率约为800W/(in·K),400K附近热导率又增大至920W/(in·K).在不改变接触点的前提下,通过测量同一根纤维小同长度对应的热导率,估计了炭纤维与热线节点处的接触热阻,并讨论了不同温度下辐射对热导率测量的影响,最后得到热导率的测量不确定度在±13%以内.     

Physical properties of CVD boron-doped multiwalled carbon nanotubes

The effects of boron doping and electron correlation on the transport properties of CVD boron-doped multiwalled carbon nanotubes are reported. The boron-doped multiwalled carbon nanotubes were characterized by TEM as well as Raman spectroscopy using different laser excitations (viz. 488, 514.5 and 647 nm). The intensity of the D-band laser excitation line increased after the boron incorporation into the carbon nanotubes. The G-band width increased on increasing the boron concentration, indicating the decrease of graphitization with increasing boron concentration. Electrical conductivity of the undoped and boron-doped carbon nanotubes reveal a 3-dimensional variable-range-hopping conductivity over a wide range of temperature, viz. from room temperature down to 2 K. The electrical conductivity is not found to be changed significantly by the present levels of B-doping. Electron Paramagnetic Resonance (EPR) results for the highest B-doped samples showed similarities with previously reported EPR literature measurements, but the low concentration sample gives a very broad ESR resonance line.     

Nitrogen beam bombardment induce polarity of carbon nanotubes

The multiwalled carbon nanotubes (MWCNTs) were prepared on SiO₂ substrates using chemical vapor deposition (CVD). N ion beam bombardment to MWCNTs was performed at different beam currents of 5–15 mA in an ion-beam-assisted deposition (IBAD) system. Scanning electron microscope (SEM) and Transmission electron microscope (TEM) proved no significant crack and surface morphological change for MWCNTs after N ion beam bombardment. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrometry (FTIR), and Raman studies indicated that higher N ion beam current (15 mA) or N atomic concentration (8.6%) induced formation of polar N-containing functional groups of N–C and N–H bonds on the surfaces of MWCNTs. The content of N–C and N–H bonds increased with N ion beam current.     

Thermogravimetric analysis of the interaction of ferromagnetic metal atom and multiwalled carbon nanotubes

This paper describes the thermal oxidative behavior of atomized iron or atomized cobalt in the presence of multiwalled carbon nanotubes (MWCNT). The thermogravimetric analysis shows the atomized iron thermal oxidation starts at about 500 degrees C that is absent when the atomized iron is sintered with multiwalled carbon naonotubes. The thermal oxidation of iron in the sintered samples requires the collapse of the multiwalled carbon nanotubes. A similar behavior is observed with atomized cobalt when its oxidation requires the collapse of the nanotubes. This thermal oxidative shift is interpreted as due to the atomized iron or atomized cobalt atom experiencing extensive overlap and confinement effect with multiwalled carbon nanotubes causing a spin transfer. This confinement effect is suggested to produce a transformation of iron from the outermost electronic distribution of 3d64s2 to an effective configuration of 3d84s0 and for cobalt 3d74s2 to 3d94s0 producing spintronics effect.     

以预锂化多壁碳纳米管为负极的锂离子电容器性能

采用内部短路方式对多壁碳纳米管负极进行不同程度的预嵌锂处理,预嵌锂时间为5,30,60min,以预嵌锂多壁碳纳米管极片作为负极,活性炭极片作为正极,组装成锂离子电容器。利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)对多壁碳纳米管及电极极片进行表征分析,采用恒流充放电(GCD)和交流阻抗谱(EIS)研究预嵌锂多壁碳纳米管负极和未预嵌锂处理多壁碳纳米管负极锂离子电容器的性能。电化学测试结果表明,多壁碳纳米管负极预嵌锂大幅提高了电容器充放电性能,对比未嵌锂多壁碳纳米管电容器,在相同的电流密度下(100mA/g),能量密度提高400%。预嵌锂60min,电流密度100mA/g时,其比容量达到57F/g。在电流密度为100～3200mA/g范围内,其最高能量密度与功率密度分别达到90Wh/kg,4130W/kg。1000次充放电循环后,容量保持率维持在85%以上,表现出良好的超级电容器性能。     

Measurements of the Thermal Conductivity and Thermal Diffusivity of Polymer Melts with the Short-Hot-Wire Method

In this paper, the thermal conductivity and thermal diffusivity of four kinds of polymer melts were measured by using the transient short-hot-wire method. This method was developed from the hot-wire technique and is based on two-dimensional numerical solutions of unsteady heat conduction from a wire with the same length-to-diameter ratio and boundary conditions as those in the actual experiments. The present method is particularly suitable for measurements of molten polymers where natural convection effects can be ignored due to their high viscosities. The results have shown that the present method can be used to measure the thermal conductivity and thermal diffusivity of molten polymers within uncertainties of 3 and 6%, respectively. Further, the thermal conductivity and thermal diffusivity of solidified samples were also measured and discussed.     

A dual beam gun for neutral or ionic primary beams in static SIMS

A dual primary beam gun for secondary ion mass spectroscopy (SIMS) is presented. The beam source produces, without any mechanical change, either an ion beam or a neutral beam generated by resonant charge transfer. The energy of the ion beam ranges from less than 1 keV up to 3.0 keV at beam currents from 10^?11 A to 1×10^?6A. The diameter of the ion beam can be focused additionally from 3.0 mm down to 0.4 mm FWHM. The particle density and the homogeneity of the neutral beam is of the same order as that of the ion beam. SIMS analysis of solid surfaces is possible without any restrictions arising from the conductivity of the specimen.     

Thermal conductivity of lead in the range −180 to 500°C

The thermal conductivity of lead (99.99%) has been measured in the range –180 to 500°C using four measuring devices (steady-state method). The thermal conductivity of both solid and liquid lead can be represented as a linear function of the temperature. The uncertainty of the measured values is estimated at 2.5% (solid) and 3% (liquid). Between the melting point and 500°C, the thermal conductivity increases by 14%. The ratio of the thermal conductivity of solid to liquid lead at the melting point corresponds to the ratio of the electric conductivities. The Lorenz function for liquid lead is approximately 1% above the ideal value at the melting point and some 3% lower than the ideal value at 500°C.     

Advanced elastomer nano-composites based on CNT-hybrid filler systems

Different techniques to disperse multiwalled carbon nanotubes (CNT) in elastomers using an internal mixer are applied and physical properties of the composites are evaluated: stress–strain behavior, dynamic-mechanical, thermal diffusivity, dielectric and fracture mechanical properties. The electrical percolation threshold is found to decrease by using ethanol as dispersion agent, compared to “dry” mixing, correlating with improved optical dispersion. The effect of nanoscopic gaps between adjacent CNTs on the electrical and thermal conductivity of the composites and the missing percolation behavior of the thermal conductivity are discussed. We have found some technically promising synergetic effects of the hybrid filler systems. For all systems one observes significantly steeper stress–strain curves by addition of 1.6 vol.% CNT to the systems with conventional fillers. In natural rubber the fatigue crack propagation resistance, tensile strength and electrical conductivity is found to be improved also for dry mixed CNT-silica hybrid systems.     

Measurements of Thermal Conductivity and Electrical Conductivity of a Single Carbon Fiber

In this paper, the thermal conductivity of a single carbon fiber under different manufacturing conditions is measured using the steady-state short-hot-wire method. This method is based on the heat transfer phenomena of a pin fin attached to a short hot wire. The short hot wire is supplied with a constant direct current to generate a uniform heat flux, and both its ends are connected to lead wires and maintained at the initial temperature. The test fiber is attached as a pin fin to the center position of the hot wire at one end and the other end is connected to a heat sink. One-dimensional steady-state heat conduction along the hot wire and test fiber is assumed, and the basic equations are analytically solved. From the solutions, the relations among the average temperature rise of the hot wire, the heat generation rate, the temperature at the attached end of the fiber, and the heat flux from the hot wire to the fiber are accurately obtained. Based on the relations, the thermal conductivity of the single carbon fiber can be easily estimated when the average temperature rise and the heat generation rate of the hot wire are measured for the same system. Further, the electrical conductivity of the single carbon fiber is measured under the same conditions as for the thermal conductivity using a four-point contact method. The relation between the thermal conductivity and electrical conductivity is further discussed, based on the crystal microstructure.     

Study of the growth of carbon on targets during ion bombardment

The phenomenon of carbon growth on targets irradiated with MeV ions was studied as a function of various parameters: beam intensity, target temperature, nature of the target, nature of the incident ion. It is proposed that the observed phenomenon of carbon growth is due to the dissociation of hydrocarbons of the residual atmosphere, by thermal cracking and by the secondary electrons emitted by the targets. The dissociation is followed by polymerization at the beam spot and partial carbonization.     

The effect of magnetic quadrupole lens hysteresis on the focusing of an ion beam with variable energy in a nuclear scanning microprobe

The influence of the hysteresis of magnetic quadrupole lenses on the transverse size of a beam focused on the target in a probe-forming system with a separated Russian quadruplet configuration has been experimentally studied. The measurements were performed in a nuclear scanning microprobe using a proton beam with the ion energy varied within 0.8–1.6 MeV. The beam size was determined by processing the secondary electron emission profiles measured during scanning of a calibrated micrometric square mesh. A new approach is proposed that allows a beam with variable energy in the nuclear microprobe to be focused so as to keep the probe size within preset limits.