Contact mechanics and thermal conductance of carbon nanotube array interfaces

A model is developed in this work to predict the thermal contact resistance of carbon nanotube (CNT) array interfaces with CNT arrays synthesized directly on substrate surfaces. An analytical model for contact mechanics is first developed in conjunction with prior data from load–displacement experiments to predict the real contact area established in CNT array interfaces as a function of applied pressure. The contact mechanics model is utilized to develop a detailed thermal model that treats the multitude of individual CNT–substrate contacts as parallel resistors and considers the effects on phonon transport of the confined geometry that exist at such contacts. The influence of CNT array properties, e.g. diameter and density, are explicitly incorporated into the thermal model, which agrees well with experimental measurements of thermal resistances as a function of pressure for different types of interfaces. The model reveals that: (1) ballistic thermal resistance dominates at the CNT array interface; (2) the overall performance of CNT array interfaces is most strongly influenced by the thermal resistance at the contacts between free CNT ends and the opposing substrate surface (one-sided interface) or the opposing CNT array (two-sided interface); and (3) dense arrays with high mechanical compliance reduce the thermal contact resistance of CNT array interfaces by increasing the real contact area in the interface.     

碳纳米管纤维导热性能的实验研究

利用瞬态电热技术对碳纳米管纤维（CNTF）的导热性能进行测量，研究了不同加热电流下CNTF导热性能的变化规律。在测量过程中，发现热扩散系数存在“突变”现象，CNTF的高热扩散系数比低热扩散系数大约提高1.65-3.85倍，随着电流的增加，热扩散系数呈下降趋势。对电流加热过程中的声子散射机制进行了分析总结，并探究了CNTF中微观结构演变。研究表明，热扩散系数下降主要是由于高温加热过程中，倒逆声子散射增强和组成CNTF中的多壁碳纳米管结构变化造成。     

Modeling thermal resistance in carbon nanotube contacts

Carbon nanotube (CNT) contacts play a promising role in thermal management devices due to their high thermal conductivity. However, at the CNT–substrate contacts, interfacial thermal resistance (ITR) may significantly reduce the heat transfer ability of carbon nanotube interconnects. An in depth understanding of the thermal transport in CNT–substrate contacts is therefore essential, considering the fact that very few experimental results for these contacts are available. In this computational study, the heat transport in 3-D hollow CNT/SiO₂ and CNT/Si contacts at room temperature are modeled using the Boltzmann transport equation for phonons. An isotropic assumption for the dispersion relations of graphite has been used to calculate the material properties of CNT. The present simulation for the CNT/SiO₂ contact predict the ITR to be of the same order as that of the theory. However, the computed ITR is two orders of magnitude smaller than that of the experimental value. The discrepancy between the measured and predicted values of thermal contact resistances may be attributed to the imperfect contact and the presence of catalyst particles in between the CNT and SiO₂ substrate in the experiment, the assumption of isotropic phonon dispersion and the use of Debye model to calculate the material properties. For the CNT/Si contact, the value of ITR obtained using the phonon sine function dispersion model is an order of magnitude higher than that of the Debye Model. It is determined that the length of the CNT and substrate do not have a significant effect on the thermal contact resistance. The thermal contact resistances are found to decrease with increasing values of the CNT diameter and thickness and are relatively independent of substrate diameter.     

Measurement and model on thermal conductivities of carbon nanotube nanorefrigerants

The objective of this study is to test thermal conductivity characteristics of CNT nanorefrigerants and to build a model for predicting the thermal conductivities of CNT nanorefrigerants. The influences of CNT diameters and CNT aspect ratios on nanorefrigerant's thermal conductivity were reflected in the experiments, and R113 was used as the host refrigerant for the convenience of the experiments. The experimental results show that the thermal conductivities of CNT nanorefrigerants are much higher than those of CNT–water nanofluids or spherical-nanoparticle-R113 nanorefrigerants. Experiments also show that the smaller the diameter of CNT is or the larger the aspect ratio of CNT is, the larger the thermal conductivity enhancement of CNT nanorefrigerant is. The existent models for predicting thermal conductivity of CNT nanofluid, including Hamilton–Crosser model, Yu–Choi model and Xue model, were verified by the experimental data of CNT nanorefrigerants' thermal conductivities. The verification shows that Yu–Choi model has the mean deviation of 15.1% and it is more accurate than the other two models. A modified Yu–Choi model was presented by improving the empirical constant of Yu–Choi model, and the mean deviation of the modified Yu–Choi model from the experimental results is 5.5%.     

Effect of interface on the thermal conductivity of carbon nanotube composites

This paper presents the effect of interface on the equivalent thermal conductivity of the carbon nanotube composites. The element free Galerkin method has been utilized as a numerical tool to evaluate the thermal conductivity of the composites. The numerical results have been obtained using continuum mechanics approach for a model composite problem, and it was found that the interface has a major effect on the thermal conductivity of the composites. The effect of interface on the effective conductivity of the composite is small for short nanotubes as compared to long nanotubes. Interface thickness also plays an important role on the effective thermal conductivity of the composite. Nanotube anisotropy has got a small effect on effective thermal conductivity of the composites. Transverse thermal conductivity of the composite has got nearly linear variation with nanotube length.     

Enhancement of thermal interface materials with carbon nanotube arrays

This paper describes an experimental study of thermal contact conductance enhancement enabled by carbon nanotube (CNT) arrays synthesized directly on silicon wafers using plasma-enhanced chemical vapor deposition. Testing based on the one-dimensional reference bar method occurred in a high-vacuum environment with radiation shielding, and temperature measurements were made with an infrared camera. Results from other thermal interface materials are presented, as well as combinations of these materials with CNT arrays. Dry CNT arrays produce a minimum thermal interface resistance of 19.8 mm² K/W, while the combination of a CNT array and a phase change material produces a minimum resistance of 5.2 mm² K/W.     

Enhancement of thermal conductivity with carbon nanotube for nanofluids

Thermal conductivity enhancements in ethylene glycol and synthetic engine oil in the presence of multi-walled carbon nanotubes (MWNTs) are investigated. CNT nanofluids are prepared using a two-step method. The volume concentration of CNT–ethylene glycol suspensions is below 1.0 vol.% and that of CNT–synthetic engine oil suspensions is below 2.0 vol.%. The thermal conductivities of the CNT suspensions are measured with a modified transient hot wire method. The results show that CNT–ethylene glycol suspensions have noticeably higher thermal conductivities than the ethylene glycol base fluid without CNT. The results for CNT–synthetic engine oil suspensions also exhibit the same trend. For CNT–ethylene glycol suspensions at a volume fraction of 0.01 (1 vol.%), thermal conductivity is enhanced by 12.4%. On the other hand, for CNT–synthetic engine oil suspension, thermal conductivity is enhanced by 30% at a volume fraction of 0.02 (2 vol.%). The rates of increase are, however, different for different base fluids. The CNT–synthetic engine oil suspension has a much higher enhanced thermal conductivity ratio than the CNT–ethylene glycol suspension.     

Influence of carbon nanotube suspension on the thermal performance of a miniature thermosyphon

An experimental study was carried out to understand the heat transfer performance of a miniature thermosyphon using water-based carbon nanotube (CNT) suspensions as the working fluid. The suspensions consisted of deionized water and multi-wall carbon nanotubes with an average diameter of 15 nm and a length range of 5–15 μm. Experiments were performed under three steady operation pressures of 7.4 kPa, 13.2 kPa and 20 kPa, respectively. Effects of the CNT mass concentration and the operation pressure on the average evaporation and condensation heat transfer coefficients, the critical heat flux and the total heat resistance of the thermosyphon were investigated and discussed. Experimental results show that CNT suspensions can apparently improve the thermal performance of the thermosyphon and there is an optimal CNT mass concentration (about 2.0%) to achieve the maximum heat transfer enhancement. The operation pressure has a significant influence on the enhancement of the evaporation heat transfer coefficient, and slight influences on the enhancement of the critical heat flux. The enhanced heat transfer effect is weak at low heat fluxes while it is increased gradually with increasing the heat flux. The present experiment confirms that the thermal performance of a miniature thermosyphon can be strengthened evidently by using CNT suspensions.     

Thin foil lithium-aluminium electrode. The effect of thermal treatment on its electrochemical behaviour in nonaqueous media

The effect of the microstructure of aluminium foil electrodes on the electrochemical behaviour of the Li-Al alloy at ambient temperature has been studied by X-ray diffraction, texture goniometric and scanning electron microscopic (SEM) techniques.It was found that thermal annealing of aluminium electrodes close to the theoretical recrystallization temperature of Al reduces the overvoltage of Li alloying and increases the cycling efficiency.It is suggested that the observed uniform, fine-grain microstructure of the β-Li-Al electrode on the annealed substrate reduces the effective current density during cycling.     

Carbon nanotube modified air-cathodes for electricity production in microbial fuel cells

The use of air-cathodes in microbial fuel cells (MFCs) has been considered sustainable for large scale applications, but the performance of most current designs is limited by the low efficiency of the three-phase oxygen reduction on the cathode surface. In this study we developed carbon nanotube (CNT) modified air-cathodes to create a 3-D electrode network for increasing surface area, supporting more efficient catalytic reaction, and reducing the kinetic resistance. Compared with traditional carbon cloth cathodes, all nanotube modified cathodes showed higher performance in electrochemical response and power generation in MFCs. Reactors using carbon nanotube mat cathodes showed the maximum power density of 329 mW m⁻²; more than twice that of the peak power obtained with carbon cloth cathodes (151 mW m⁻²). The addition of Pt catalysts significantly increased the current densities of all cathodes, with the maximum power density obtained using the Pt/carbon nanotube mat cathode at 1118 mW m⁻². The stable maximum power density obtained from other nanotube coated cathodes varied from 174 mW m⁻² to 522 mW m⁻². Scanning electron micrographs showed the presence of conductive carbon nanotube networks on the CNT modified cathodes that provide more efficient oxygen reduction.     

Enhanced thermal conductivity of ethylene glycol with single-walled carbon nanotube inclusions

In the present work, we report measurements of the effective thermal conductivity of dispersions of single-walled carbon nanotube (SWNT) suspensions in ethylene glycol. The SWNTs were synthesized using the alcohol catalytic chemical vapour deposition method. Resonant Raman spectroscopy was employed to estimate the diameter distribution of the SWNTs based on the frequencies of the radial breathing mode peaks. The nanofluid was prepared by dispersing the nanotubes using a bile salt as the surfactant. Nanotube loading of up to 0.2 vol% was used. Thermal conductivity measurements were performed by the transient hot-wire technique. Good agreement, within an uncertainty of 2%, was found for published thermal conductivities of the pure fluids. The enhancement of thermal conductivity was found to increase with respect to nanotube loading. The maximum enhancement in thermal conductivity was found to be 14.8% at 0.2 vol% loading. The experimental results were compared with literature results in similar dispersion medium. Experimental results were compared with the Hamilton–Crosser model, the Lu–Lin model, Nan’s effective medium theory and the Hashin–Shtrikman model. Effective medium theory seems to predict the thermal conductivity enhancement reasonably well compared to rest of the models. Networking of nanotubes to form a tri-dimensional structure was considered to be the reason for the thermal conductivity enhancement.     

Carbon nanotube/polyaniline composite as anode material for microbial fuel cells

A carbon nanotube (CNT)/polyaniline (PANI) composite is evaluated as an anode material for high-power microbial fuel cells (MFCs). Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) are employed to characterize the chemical composition and morphology of plain PANI and the CNT/PANI composite. The electrocatalytic behaviour of the composite anode is investigated by means of electrochemical impedance spectroscopy (EIS) and discharge experiments. The current generation profile and constant current discharge curves of anodes made from plain PANI, 1 wt.% and 20 wt.% CNT in CNT–PANI composites reveal that the performance of the composite anodes is superior. The 20 wt.% CNT composite anode has the highest electrochemical activity and its maximum power density is 42 mW m⁻² with Escherichia coli as the microbial catalyst. In comparison with the reported performance of different anodes used in E. coli-based MFCs, the CNT/PANI composite anode is excellent and is promising for MFC applications.     

Single-step anodization for synthesis of hierarchical TiO2 nanotube arrays on foil and wire substrate for enhanced photoelectrochemical water splitting

A hierarchical nanoporous-TiO₂ nanotube composite structure consisting of a nanoporous top layer with smooth underlying nanotubes was obtained by a single-step anodization technique. The growth of such composite structure has also been applied to Ti wire substrate for the first time. The photoelectrochemical performance was examined under AM 1.5 simulated solar irradiation in 1 M KOH electrolyte. In general, the hierarchical architecture demonstrated improved photoelectrochemical activity over plain TiO₂ nanotubes. Furthermore, the composite structure on a wire substrate was also found to enhance photoelectrochemical activity over a foil substrate. Under optimized conditions, over a 40% increase in hydrogen generation for hierarchical nanotubes over plain nanotubes is observed and over 25% increase in hydrogen generation using a wire substrate over a foil substrate is observed.     

Carbon nanotube supported Pt-Ni catalysts for preferential oxidation of CO in hydrogen-rich gases

A series of carbon nano-tubes supported platinum-nickel catalysts were prepared and used for CO preferential oxidation in H₂-rich streams. The catalysts were characterized by using N₂-adsorption, XRD, HRTEM, H₂-TPD and H₂-TPR techniques. Effects of platinum and nickel loading amount, CO₂ and H₂O in the feed stream on the activity and selectivity over the catalysts were investigated. The results of catalytic performance tests show that the carbon nano-tubes supported Pt-Ni catalysts are very active and highly selective at low temperature for CO preferential oxidation in 1 vol. % CO, 1 vol. %O₂, 50 vol. % H₂ and N₂ gases. Adding 12.5 vol. % of CO₂ into the feed gases has slight negative influence on CO conversion. Adding 15 vol. % of H₂O leads to a little decrease of CO conversion at the temperature range of 100-120 °C, which is proposed to be caused by capillary wetting of water in the micro-pores of carbon nano-tubes. As the reaction temperature is higher, adding water can improve CO conversion. The characterization results indicate that platinum species are in nano-particles uniformly dispersed on the carbon nano-tubes surface. There are two kinds of nickel species, one is interacted with platinum and likely to form Pt-Ni alloy in reduction process, the other is much highly dispersed on carbon nano-tubes and strongly interacted with the supports. The high activity of the catalysts is attributed to the interaction between Pt and Ni with the formation of Pt-Ni alloy.     

The role of different parameters on the stability and thermal conductivity of carbon nanotube/water nanofluids

It is obvious that the applicability and efficiency of nanofluids (suspensions contained nanoparticles) are related to their high heat transfer coefficients, especially thermal conductivity. Many parameters affect this property including size, shape and source of nanoparticles, surfactants, power of ultrasonic, time of ultrasonication, elapsed time after ultrasonication, pH, temperature, particle concentration and surfactant concentration. Some of these parameters may have interaction effects. An accepted way for obtaining the optimized condition is based on the design of experiments and statistical analysis. In this paper we investigate the stability and thermal conductivity of carbon nanotube (CNT)/water nanofluids and propose the optimum condition for the production and application of nanofluids. It has been shown that the significant factors on the thermal conductivity and stability are not precisely similar to one another.     

Cross-plane thermal conductivity of superlattices with rough interfaces using equilibrium and non-equilibrium molecular dynamics

This paper describes a heat transfer study of binary Lennard Jones superlattices, focusing on the influence of interface topology on cross-plane thermal conductivity, by using both non-equilibrium and equilibrium molecular dynamics methods. Both methods reveal the same trends of thermal conductivity. In particular, interfacial roughness is shown to slightly increase cross-plane thermal conductivity in comparison to smooth interfaces. Our results highlight paths for optimizing superlattices for thermoelectric conversion applications and for thermal management solutions in micro- and nano-systems.     

The experimental exploration of carbon nanofiber and carbon nanotube additives on thermal behavior of phase change materials

In this study, thermal properties of carbon nanofiber (CNF) and carbon nanotube (CNT) filled phase change materials (soy wax and paraffin wax) were studied experimentally, aiming to improve their thermal conductivities. The composite phase change materials (PCMs) were prepared by the stirring of CNF or CNT in liquid wax at 60 °C, with CNF and CNT doping levels of 1, 2, 5, and 10 wt%. The experimental results show that the thermal conductivity of composite PCMs increases as CNF or CNT loading contents. Both CNF and CNT can improve the thermal conductivity of the composite, while CNF is shown to be more effective than CNT as the thermal conductive filler because of its better dispersion in the matrix.     

The temperature dependence of effective thermal conductivity of the samples of glass wool reinforced with aluminium foil

In this study, the temperature dependence of effective thermal conductivity (ETC) for samples of binary, ternary, and quadruple glass wools reinforced with aluminium foil was examined. The experiments were realized by the guarded hot plate in temperature differences of 5, 10 and 15 °C and the temperatures of 25 and 40 °C. The results revealed that in the case of reinforcing the aluminium foil, ETC increased with increasing the temperature or changing of temperature difference (5, and 15 °C). Also, an increase of additional layers decrease its' influence on low temperature. Consequently, reflectivity materials may increase or reduce ETC.     

Carbon footprint of thermal insulation materials in building envelopes

With increasing world population and economic development, the strain on resources is increasing. Energy consumption during construction and use of building is enormous. In this study, a quantitative comparison of various thermal insulation materials used in construction, from most commonly used to new, highly efficient insulation materials, was performed. It was demonstrated that the evaluation and consideration of environmental impact per unit weight of thermal insulation materials are inappropriate and can lead to misleading decisions, since it is imperative that the analysis considers the difference in the density of each thermal insulation material, as well as differences in their thermal conductivity. Furthermore, the environmental neutrality, i.e. the time needed to offset the carbon footprint of the manufacturing and the installation of thermal insulation materials in the building envelope with the difference between the carbon footprint of the heat losses in the heating season through a currently averagely insulated external envelope and a well-insulated external envelope, is achieved in very short-time periods. For the thermal insulation materials with the lowest environmental impact, it is reached in less than one heating season and soon after tenth heating seasons for the insulation with the highest environmental impact.