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.     

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.     

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.     

Thermoelectric properties of carbon nanotube reinforced cement-based composites fabricated by compression shear

Carbon nanotubes (CNTs) with weight percent of 5.0%, 10.0% and 15.0% were added into the cement matrix to fabricate CNT reinforced cement-based composites (CNTs/CC) by mixing and dry compression shear methods. Seebeck coefficient, electrical conductivity and thermal conductivity of the as-received CNTs/CC were measured and analyzed in detail. The CNTs/CC exhibits the thermoelectric behavior of p-type semiconductor. CNTs were dispersed uniformly in cement matrix by compression shear stress, which promoted a relatively high electrical conductivity (0.818 S/cm) and Seebeck coefficient (57.98 μV/°C) of CNTs/CC. Combining with their lower thermal conductivity ranged from 0.734 to 0.947 W m^?1 K^?1, the CNTs/CC shows the highest thermoelectric figure of merit (ZT) has reached 9.33 × 10^?5, Which is benefit to the applications in large-scale energy harvesting in the buildings and pavements with low cost in the future cities.     

Theoretical investigation of mechanical and thermal properties of MPO4 (M = Al,Ga)

Minimum lattice thermal conductivities and mechanical properties of polymorphous MPO₄ (M = Al, Ga) are investigated by first principles calculations. The theoretical minimum thermal conductivities are found to be 1.02 W (m K)^?1 for α-AlPO₄, 1.20 W (m K)^?1 for β-AlPO₄, 0.87 W (m K)^?1 for α-GaPO₄ and 0.88 W (m K)^?1 for β-GaPO₄. The lower thermal conductivities in comparison to YSZ can be attributed to the lattice phonon scattering due to the framework of heterogeneous bonds. In addition, the low shear-to-bulk modulus ratio for both β-AlPO₄ (0.38) and β-GaPO₄ (0.30) is observed. Our results suggest their applications as light-weight thermal insulator and damage-tolerant/machinable ceramics.     

Assessment of thermal conductivity as a function of porosity in bread dough during proving

During the proving process, yeast fermentation in the aqueous phase of dough produces carbon dioxide which diffuses through the aqueous phase to the gas cell nuclei entrapped during dough mixing. CO₂ evaporates to generate within gas cell nuclei an excess pressure that provides the driving force for dough expansion. As a result, the void fraction of the dough increases and thermal conductivity decreases. These two properties are of considerable interest because of their technological significance in breadmaking (specific volume and processing times). The aim of the present study was therefore to investigate experimentally the thermal conductivity and porosity evolutions of bread dough during the proving process. Apparent density and thermal conductivity of fermenting dough were experimentally determined as a function of fermentation time using the line-heat source probe and image analysis methods, respectively. Thermal conductivity values were observed in the range of 0.36–0.12 W m^?1 K^?1 and values of void fraction air between 6 and 78% at proving conditions of 35 °C and 95% HR for 100 min of fermentation time. According to experimental data, relationship between thermal conductivity λ and porosity ? could be adequately described by a nonlinear equation: λ(?) = 0.421–0.134?–0.219?^0.5.     

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.     

Single-walled carbon nanotube/silicone rubber composites for compliant electrodes

Single-walled carbon nanotube (SWCNT)/silicone rubber composites that can be used in fabricating compliant electrodes are prepared by spraying a mixed solution of ionic-liquid-based SWCNT gel and silicone rubber onto an elastic substrate. Subsequently, the composites are exposed to nitric acid vapor. Scanning electron microscopy and atomic force microscopy images of the composites show that the SWCNTs are finely dispersed in the polymer matrix due to the addition of the ionic liquid. Doping of the SWCNTs by nitric acid can significantly lower the sheet resistance (R_s) of the composites; samples with 4 wt% of SWCNT content exhibit the lowest R_s value (50 Ω sq^?1). This sheet resistance corresponds to a conductivity value of 63 S cm^?1. In addition, the composites retain a high conductivity after several tensile strains are applied. Stretching the composite sample to 300% of the original length increased the R_s value to 320 Ω sq^?1 (19 S cm^?1). Even after 20th stretch/release/stretch cycle, the conductivity remains constant at a value of 18 S cm^?1. These results provide a scalable route for preparing highly stretchable and conductive SWCNT composites with relatively low SWCNT concentrations.     

Thermal performance of vertically-aligned multi-walled carbon nanotube array grown on platinum film

Vertically-aligned carbon nanotube array is expected to inherit high thermal conductivity and mechanical compliance of individual carbon nanotube and serve as thermal interface material. In this paper, vertically-aligned multi-walled carbon nanotube arrays have been directly grown on Pt film and the thermal performance has been studied by using laser flash technique. The determined thermal diffusivity decreases from 0.187 to 0.135 cm² s⁻¹ and the thermal conductivity increases from 1.8 to 3.1 W m⁻¹ K⁻¹ as temperature increases from 243.2 to 453.2 K. The fracture surface of the array peeled off the Pt film was characterized by scanning electron microscopy. It has been illustrated that the tearing surface is not smooth but fluffy with torn carbon nanotubes, indicating strong interfacial bonding and consequent small interface resistance between carbon nanotube array and Pt film. According to Raman spectra and transmission electron microscopy image, the possible mechanisms responsible for the thermal transport degradation are low packing density, twist, and the presence of impurities, amorphous carbon, defects and flaws. The influence of intertube van der Waals interactions has been studied by comparing the phonon dispersion relations and is expected to be not significant.     

The hierarchical structure and properties of multifunctional carbon nanotube fibre composites

The axial mechanical, electrical and thermal properties of carbon nanotubes (CNTs) can be exploited macroscopically by assembling them parallel to each other into a fibre during their synthesis by chemical vapour deposition. Multifunctional composites with high volume fraction of CNT fibres are then made by direct polymer infiltration of an array of aligned fibres. The fibres have a very high surface area, causing the polymer to infiltrate them and resulting in a hierarchical composite structure. The electrical and thermal conductivities of CNT/epoxy composites are shown to be superior to those of equivalent specimens with T300 carbon fibre (CF) which is widely used in industry. From measurements of longitudinal coefficient of thermal expansion (CTE) of the composites we show that the CTE of CNT fibres is approximately ?1.6 × 10^?6 K^?1, similar to in-plane graphite. The combination of electrical, thermal and mechanical properties of CNT fibre composites demonstrates their potential for multifunctionality.     

Thermal analysis of submicron nanocrystalline diamond films

The thermal properties of sub-μm nanocrystalline diamond films in the range of 0.37–1.1 μm grown by hot filament CVD, initiated by bias enhanced nucleation on a nm-thin Si-nucleation layer on various substrates, have been characterized by scanning thermal microscopy. After coalescence, the films have been outgrown with a columnar grain structure. The results indicate that even in the sub-μm range, the average thermal conductivity of these NCD films approaches 400 W m^− 1 K^− 1. By patterning the films into membranes and step-like mesas, the lateral component and the vertical component of the thermal conductivity, k_lateral and k_vertical, have been isolated showing an anisotropy between vertical conduction along the columns, with k_vertical ≈ 1000 W m^− 1 K^− 1, and a weaker lateral conduction across the columns, with k_lateral ≈ 300 W m^− 1 K^− 1.     

Enhanced thermoelectric properties and electronic structures of p-type BiCu1−xAgxSeO ceramics

The thermoelectric properties and electronic structures were investigated on p-type BiCu_1-xAg_xSeO (x=0, 0.02, 0.05, 0.08) ceramics prepared using a two-step solid state reaction followed by inductively hot pressing. All the samples consist of single BiCuSeO phase with lamella structure and no preferential orientation exists in the crystallites. Upon replacing Cu⁺ by Ag⁺, maximum values of electrical conductivity of 36.6 S cm⁻¹ and Seebeck coefficient of 350 μV K⁻¹ are obtained in BiCu_0.98Ag_0.02SeO and BiCu_0.92Ag_0.08SeO, respectively. Nevertheless, a maximum power factor of 3.67 μW cm⁻¹K⁻² is achieved for BiCu_0.95Ag_0.05SeO at 750 K owing to the moderate electrical conductivity and Seebeck coefficient. Simultaneously, this oxyselenide exhibits a thermal conductivity as low as 0.38 W m⁻¹ K⁻¹ and a high ZT value of 0.72 at 750 K, which is nearly 1.85 times as large as that of the pristine BiCuSeO. The enhancement of thermoelectric performance is mainly attributed to the increased density of states near the Fermi level as indicated by the calculated results.     

Improvement on characteristics of porous alumina from platelets using a TEOS treatment

A porous alumina body was synthesized from anisotropic alumina particles, namely platelets. When green compacts, which had been uniaxially pressed at 1 MPa, were heated at 1200 and 1500 °C for 1 h, the average porosity of the resulting alumina bodies was 75.5 and 71.0%, respectively. The thermal conductivity of the porous alumina fabricated at 1400 °C for 1 h with 72.3% in porosity was 0.8 W m^?1 K^?1. In an attempt to increase the compressive strength of the porous alumina bodies, TEOS (tetraethyl orthosilicate) solution treatment was carried out, followed by reheating to 1400 °C for 1 h. The compressive strength of the porous alumina body increased from 3.8 MPa (without TEOS solution treatment) to 10.2 MPa (with three rounds of TEOS treatment), with the porosity decreasing to 65.5% and the thermal conductivity increasing to1.2 W m^?1 K^?1.     

Enhanced thermoelectric properties of CNT dispersed and Na-doped Bi2Ba2Co2Oy composites

The thermoelectric properties of Bi₂Ba₂Co₂O_y and Bi_1.975Na_0.025Ba₂Co₂O_y+x wt% carbon nanotubes (CNT; x=0.00, 0.05, 0.10, 0.15, 0.5, and 1.0) ceramic samples synthesised by the solid-state reaction method were investigated from 300K to 950K. Na doping with a small amount played an important role in reducing resistivity and slightly reduced the Seebeck coefficients and the thermal conductivity. The CNT dispersant increased resistivity, but the thermal conductivity was reduced remarkably. In particular, the Bi_1.975Na_0.025Ba₂Co₂O_y+1.0wt% CNT sample exhibited an ultralow thermal conductivity of 0.39 W K⁻¹ m⁻¹ at 923K. This was attributed to the point defects caused by Na doping and the interface scattering caused by the CNT dispersant. The combination of Na doping and CNT dispersion had better effects on thermoelectric properties. The Bi_1.975Na_0.025Ba₂Co₂O_y+0.5wt% CNT sample exhibited a better dimensionless figure of merit (ZT) value of 0.2 at 923K, which was improved by 78.2%, compared with the undoped Bi₂Ba₂Co₂O_y sample.     

Thermal fatigue and hypothermal atomic oxygen exposure behavior of carbon nanotube wire

In low earth orbit (LEO), components of space systems are exposed to damaging hypothermal atomic oxygen and thermal fatigue. Carbon nanotube (CNT) wires are candidate materials for different applications in space systems. Thirty-yarn CNT wire’s behavior was evaluated when exposed to hypothermal atomic oxygen and thermal fatigue. CNT wire specimens were exposed to a nominal fluence of hypothermal atomic oxygen of 2 × 10²⁰ atoms/cm². The erosion rate due to hypothermal collision between atomic oxygen and CNT wires was calculated to be 2.64 × 10⁻²⁵ cm³/atom, which is comparable to highly ordered pyrolytic graphite. The tensile strength of CNT wire was not affected by this exposure, and a minor reduction of electrical conductivity (2.5%) was found. Scanning electron microscopy (SEM) and Energy Dispersive X-ray spectroscopy analysis showed erosion of surface layer with depleted carbon and increased oxygen. Thermal fatigue excursion of 5000 cycles from 70 to −50 °C at a rate of 55 °C/min showed no loss in tensile strength; however a large decrease in conductivity (18%) was seen. SEM analysis showed that the thermal fatigue created buckling of yarn and fracture of individual CNTs bundles. These reduced the effective area and electrical conductivity of CNT wire.     

Graphene nanoplatelet paper as a light-weight composite with excellent electrical and thermal conductivity and good gas barrier properties

Paper forms (i.e. thin free-standing films) of carbon-based materials have received increasing attention. Here we present a novel approach to fabricating a binder free, self-standing flexible paper consisting of exfoliated graphite nanoplatelets (GNPs). It is found that the electrical conductivity of the GNP paper can be as high as 2200 S cm^?1 and the thermal conductivity reaches 313 W m^?1 K^?1. Both thermoset and thermoplastic matrices were used to impregnate the porous GNP paper and an extremely high tensile modulus was attained. Even with 30 vol.% polymer, the GNP paper composite can still exhibit ～700 S cm^?1 electrical conductivity thanks to the highly continuous GNP network formed in the paper making process. The impregnated GNP paper was also investigated as a component in carbon fiber composite. It is found that when inserted into a layered laminate composite construction, gas permeability can be severely reduced and electrical and thermal conductivity can be greatly enhanced.     

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.     

A new estimation method for the intrinsic thermal conductivity of nonmetallic compounds: A case study for MgSiN2, AlN and β-Si3N4 ceramics

A new method for estimating the maximum achievable thermal conductivity of non-electrically conducting materials is presented. The method is based on temperature dependent thermal diffusivity data using a linear extrapolation method enabling discrimination between phonon-phonon and phonon-defect scattering. The thermal conductivities estimated in this way for MgSiN₂, AlN and β-Si₃N₄ ceramics at 300 K equal 28, 200 and 105 W m⁻¹ K⁻¹, respectively in favourable agreement with the highest experimental values of 23, 266 and 106–122 W m⁻¹ K⁻¹. This suggests the general applicability of the proposed estimation method for non-metallic compounds. It is expected that when optic phonons contribute to the heat conduction (as is the case for AlN) the intrinsic thermal conductivity at lower temperatures (e.g. 300 K) is underestimated. However, the reliability and accuracy of the presented ‘easy to use’ estimation method seems to be much better than several other estimation methods. Furthermore the needed input for this method can provide information about which processing parameters should be optimised to obtain the highest thermal conductivity.     

Effects of reduction process and carbon nanotube content on the supercapacitive performance of flexible graphene oxide papers

The effects of the reduction process and carbon nanotube (CNT) content on the supercapacitive behavior of electrodes made from flexible, binder-free thick graphene oxide (GO) papers are studied. It is found that the supercapacitive performance depends on several factors, including the presence of oxygenated functional groups after reduction, the interlayer spacing of the GO papers and their wettability with electrolyte. A moderate reduction of GO papers using hydrazine or annealing at a low temperature of 220 °C in air is proven to be more beneficial to achieve a high capacitance than the heavy reduction using a hydrazine vapor or a high temperature thermal treatment. The addition of a small amount of CNT, typically 12.5 wt.%, to form thick GO/CNT sandwich papers gives rise to an excellent specific capacitance of 151 F g^?1 at a current density of 0.5 A g^?1, as well as a retention ratio of 86% of the initial value after 6000 charge/discharge cycles at 5 A g^?1. These improvements arise from the synergistic effects of the increased electronic conductivity and effective surface area associated with large electrochemical active sites due to the presence of intercalated CNT.     

Thermal transport phenomena and limitations in heterogeneous polymer composites containing carbon nanotubes and inorganic nanoparticles

An Off-Lattice Monte Carlo model was developed to investigate effective thermal conductivities (K_eff) and thermal transport limitations of polymer composites containing carbon nanotubes (CNTs) and inorganic nanoparticles. The simulation results agree with experimental data for poly(ether ether ketone) (PEEK) with inclusions of CNTs and tungsten disulfide (WS₂) nanoparticles. The developed model can predict the thermal conductivities of multiphase composite systems more accurately than previous models by taking into account interfacial thermal resistance (R_bd) between the nanofillers and the polymer matrix, and the nanofiller orientation and morphology. The effects of (i) R_bd of CNT–PEEK and WS₂–PEEK (0.0232–115.8 × 10⁻⁸ m²K/W), (ii) CNT concentration (0.1–0.5 wt%), (iii) CNT morphology (aspect ratio of 50–450, and diameter of 2–8 nm), and (iv) CNT orientation (parallel, random and perpendicular to the heat flux) on K_eff of a multi-phase composite are quantified. The simulation results show that K_eff of multiphase composites increases when the CNT concentration increases, and when the R_bd of CNT–PEEK and WS₂–PEEK interfaces decrease. The thermal conductivity of composites with CNTs parallel to the heat flux can be enhanced ∼2.7 times relative to that of composites with randomly-dispersed CNTs. CNTs with larger aspect ratio and smaller diameter can significantly improve the thermal conductivity of a multiphase polymer composite.