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Achieving the thermal conductivity required for efficient heat management in semiconductors and other devices requires the integration of thermally conductive ceramic fillers at concentrations of 60 vol% or higher. However, an increased filler content often negatively affects the mechanical properties of the composite matrix, limiting its practical applicability. To address this issue, in this paper, we present a new strategy to reduce the required ceramic filler content: the use of a thermally conductive ceramic composite filler with carbon nanotubes (CNTs) grown on aluminum nitride (AlN). We combined catalyst coating technology with vacuum filtration to ensure that the catalyst was uniformly applied to micrometer-sized AlN particles, followed by the efficient and uniform synthesis of CNTs using a water-assisted process in a vertical furnace. By carefully controlling the number of vacuum filtration cycles and the growth time of the CNTs, we achieved precise control over the number and length of the CNT layers, thereby adjusting the properties of the composite to the intended specifications. When AlN/CNT hybrid fillers are incorporated into silicone rubber, while maintaining the mechanical properties of rubber, the thermal diffusivity achieved at reduced filler levels exceeds that of composites using AlN-only or simultaneous AlN and CNTs formulations. This demonstrates the critical influence of CNTs on AlN surfaces. Our study represents a significant advancement in the design of thermally conductive materials, with potential implications for a wide range of applications.

We present a study quantitatively demonstrating that the method of synthesis (gas phase, fixed bed, non-fixed bed) represents a determining factor in the level of crystallinity in growing single wall carbon nanotubes (SWCNTs). Using far infrared spectroscopy, the "effective length" (associated with the level of crystallinity) was estimated for CNTs grown using various synthetic methods (lab-produced and supplemented by commercially purchased SWCNTs) as a metric for crystallinity (i.e., defect density). Analysis of the observed "effective lengths" showed that the SWCNTs fell into two general groups: long and short (high and low crystallinity) synthesized by gas-phase methods and all other supported catalyst methods, respectively. Importantly, the "long" group exhibited effective lengths in the range of 700-2200 nm, which was greater than double that of the typical values representing the "short" group (110-490 nm). These results highlight the significant difference in crystallinity. We interpret that the difference in the crystallinity stemmed from stress concentration at the nanotube-catalyst interface during the growth process, which originated from various sources of mismatch in growth rates (e.g., vertically aligned array) as well as impact stress from contact with other substrates during fluidization or rotation. These results are consistent with well-accepted belief, but now are demonstrated quantitatively.

The importance of hydrogen in carbon nanotube (CNT) synthesis has been known as it supports the critical processes necessary for CNT growth, such as catalyst reduction. However, within the scope of our mini microplasma CNT synthesis reactor, we found that hydrogen was critical for unexpected reasons. Without hydrogen, CNT growth was inhibited and characterized by amorphous carbon particles. Optical emission spectroscopy of the microplasma revealed that without hydrogen, the high-energy electrons induced the immediate decomposition of carbon feedstock simultaneously with the catalyst feedstock, thus suppressing the formation of catalyst nanoparticles and inducing catalyst deactivation. In contrast, the inclusion of hydrogen induced less-immediate decomposition of reactant gases, through the conversion of electron energy of the plasma to thermal energy, which provided the appropriate conditions for catalyst nanoparticle formation and subsequent CNT nucleation. A simple reaction pathway model was proposed to explain these observed results and underlying mechanisms., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

We report an approach to fabricate high conductivity graphite sheets based on a heat-and-current treatment of filtrated, exfoliated graphite flakes. This treatment combines heating (~ 900 °C) and in-plane electrical current flow (550 A·cm -2 ) to improve electrical conductivity through the reduction of crystalline defects. This process was shown to require only a 1-min treatment time, which resulted in a 2.1-fold increase in electrical conductivity (from 1088 ± 72 to 2275 ± 50 S·cm -1 ). Structural characterization by Raman spectroscopy and X-ray diffraction indicated that the improvement electrical conductivity originated from a 30-fold improvement in the crystallinity (Raman G/D ratio increase from 2.8 to 85.3) with no other observable structural transformations. Significantly, this treatment was found to act uniformly across a macroscopic (10 mm) sheet surface indicating it is on the development of applications, such as electrodes for energy generation and storage and electromagnetic shielding, as well as on the potential for the development of large-scale treatment technologies.

Here, we demonstrate an approach of increasing the density of ultralong carbon nanotube (CNT) growth by combining a fast-heating method developed by Huang et al. ( J. Am. Chem. Soc. , 2003, 125 , 5636-5637) with catalyst support engineering. Specifically, using graphene oxide as a catalyst support for iron oxide (Fe 3 O 4 ) catalyst nanoparticles, we achieved high density growth of CNTs grown by the "kite-mechanism". Our analysis revealed that the fast-heating method reduced undesired aggregation of the catalyst nanoparticles, which has been reported to be a primary limitation mechanism, by shortening the time between substrate heating and CNT growth. In addition, the use of the graphene oxide support led to controllable and uniform dispersion of catalyst nanoparticles in relatively high density which provided increased process control by extending the time before the onset of catalyst aggregation. Together, these approaches suppressed the aggregation of the catalyst nanoparticles, which facilitated the "tip-growth" mode instead of the "root-growth" mode, and led to the high density growth of ultralong CNTs. Our results also indicate additional limitations and complexities on the high density CNT growth by the kite-growth approach, which limit high density synthesis., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We report an exceptionally high-efficiency synthesis of long single-wall carbon nanotube (SWCNT) forests using porous substrates (metal meshes) in place of nonporous flat substrates. This study examined the dependence of the growth efficiency on various mesh structures, including wire diameter, aperture size, and total surface area. We demonstrated that the synthesis of SWCNT forests is highly dependent on the initial porosity as well as maintaining the open pores throughout the duration of the growth. Our results show that carbon nanotubes (CNTs) can be grown on all surfaces of the mesh in high efficiency with the optimum growth efficiency observed for a mesh porosity of ∼30%. Based on these results, we demonstrated the high efficiency synthesis of SWCNT forests (height: >3.47 mm, average growth rate: 301 μm min -1 , and yield: 12.7 mg cm -2 in 10 min growth time). Furthermore, we showed that the initial growth rates exceeded 1 millimeter per minute (1000 μm min -1 ). Our results further indicate that metal meshes represent a viable alternative to nonporous flat substrates for the efficient synthesis of tall and high yielding SWCNTs., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We propose a porosimetry-based method to characterize pores formed by carbon nanotubes (CNTs) in the CNT agglomerates for designing neat CNT-based materials and composites. CNT agglomerates contain pores between individual CNTs and/or CNT bundles (micropore < 2 nm, mesopores 2-50 nm, and macropores > 50 nm). We investigated these pores structured by CNTs with different diameters and number of walls, clarifying the broader size distribution and the larger volume with increased diameters and number of walls. Further, we demonstrated that CNT agglomerate structures with different bulk density were distinguished depending on the pore sizes. Our method also revealed that CNT dispersibility in solvent correlated with the pore sizes of CNT agglomerates. By making use of these knowledge on tailorable pores for CNT agglomerates, we successfully found the correlation between electrical conductivity for CNT rubber composites and pore sizes of CNT agglomerates. Therefore, our method can distinguish diverse CNT agglomerate structures and guide pore sizes of CNT agglomerates to give high electrical conductivity of CNT rubber composites.

One aspect of carbon nanotube (CNT) synthesis that remains an obstacle to realize industrial mass production is the growth efficiency. Many approaches have been reported to improve the efficiency, either by lengthening the catalyst lifetime or by increasing the growth rate. We investigated the applicability of dwell time and carbon flux control to optimize yield, growth rate, and catalyst lifetime of water-assisted chemical vapor deposition of single-walled carbon nanotube (SWCNT) forests using acetylene as a carbon feedstock. Our results show that although acetylene is a precursor to CNT synthesis and possesses a high reactivity, the SWCNT forest growth efficiency is highly sensitive to dwell time and carbon flux similar to ethylene. Through a systematic study spanning a wide range of dwell time and carbon flux levels, the relationship of the height, growth rate, and catalyst lifetime is found. Further, for the optimum conditions for 10 min growth, SWCNT forests with ~2500 μm height, ~350 μm/min initial growth rates and extended lifetimes could be achieved by increasing the dwell time to ~5 s, demonstrating the generality of dwell time control to highly reactive gases.

We have investigated the scalability of our post-synthesis graphitization process for single-walled carbon nanotubes (SWCNTs), which applies heat and current to SWCNTs to improve the thermal and electrical conductivities. This investigation was performed by examining the relationship between the processing conditions and the amount of treated SWCNTs. Characterization of all cases of treated SWCNTs showed the same level of improvement of ~3 times to both the thermal and electrical conductivities and that the SWCNTs remained SWCNTs, i.e., no change in diameter or wall number. These results provided evidence that the ability to improve the crystallinity of the SWCNTs was independent of the treatment amount. Further, our results showed that an increase in SWCNT amount required increased applied current density or increased in applied temperature to achieve optimum property improvement. Finally, we found a trade-off between the current density and temperature indicating that either a high current or high temperature was required to achieve the optimum process conditions. These results demonstrated that our heat and current SWCNT treatment was fundamentally scalable and applied towards larger scale (i.e., gram-level or more) amounts of SWCNT.

We report a general approach to fabricate elastomeric composites possessing high electrical conductivity for applications ranging from wireless charging interfaces to stretchable electronics. By using arbitrary nine kinds of rubbers as matrices, we experimentally demonstrate that the matching the solubility parameter of CNTs and the rubber matrix is important to achieve higher electrical conductivity in CNT/rubber composite, resulting in continuous conductive pathways leading to electrical conductivities as high as 15 S/cm with 10 vol% CNT in fluorinated rubber. Further, using thermodynamic considerations, we demonstrate an approach to mix CNTs to arbitrary rubber matrices regardless of solubility parameter of matrices by adding small amounts of fluorinated rubber as a polymeric-compatibilizer of CNTs. We thereby achieved electrical conductivities ranging from 1.2 to 13.8 S/cm (10 vol% CNTs) using nine varieties of rubber matrices differing in chemical structures and physical properties. Finally, we investigated the components of solubility parameter of CNT by using Hansen solubility parameters, these findings may useful for controlling solubility parameter of CNTs.

We propose an approach to disperse long single-wall carbon nanotubes (SWCNTs) in a manner that is most suitable for the fabrication of high-performance composites. We compare three general classes of dispersion mechanisms, which encompass 11 different dispersion methods, and we have dispersed long SWCNTs, short multi-wall carbon nanotubes, and short SWCNTs in order to understand the most appropriate dispersion methods for the different types of CNTs. From this study, we have found that the turbulent flow methods, as represented by the Nanomizer and high-pressure jet mill methods, produced unique and superior dispersibility of long SWCNTs, which was advantageous for the fabrication of highly conductive composites. The results were interpreted to imply that the biaxial shearing force caused an exfoliation effect to disperse the long SWCNTs homogeneously while suppressing damage. A conceptual model was developed to explain this dispersion mechanism, which is important for future work on advanced CNT composites.

We present a method to both precisely and continuously control the average diameter of single-walled carbon nanotubes in a forest ranging from 1.3 to 3.0 nm with ~1 Å resolution. The diameter control of the forest was achieved through tuning of the catalyst state (size, density, and composition) using arc plasma deposition of nanoparticles. This 1.7 nm control range and 1 Å precision exceed the highest reports to date.

The electrical conductivity and mechanical strength of carbon nanotube (CNT) buckypaper comprised of millimeter-scale long single-walled CNT (SWCNT) was markedly improved by the use of longer SWCNTs. A series of buckypapers, fabricated from SWCNT forests of varying heights (350, 700, 1,500 μm), showed that both the electrical conductivity (19 to 45 S/cm) and tensile strength (27 to 52 MPa) doubled. These improvements were due to improved transfer of electron and load through a reduced number of junctions for longer SWCNTs. Interestingly, no effects of forest height on the thermal diffusivity of SWCNT buckypapers were observed. Further, these findings provide evidence that the actual SWCNT length in forests is similar to the height.

We report the virtually infinite possible carbon feedstocks which support the highly efficient growth of single-wall carbon nanotubes (SWCNTs) using on the water-assisted chemical vapor deposition method. Our results demonstrate that diverse varieties of carbon feedstocks, in the form of hydrocarbons, spanning saturated rings (e.g. trans-deca-hydronaphthalene), saturated chains (e.g. propane), unsaturated rings (e.g. dicyclopentadiene), and unsaturated chains (e.g. ethylene) could be used as a carbon feedstocks with SWCNT forests with heights exceeding 100 ums. Further, we found that all the resultant SWCNTs possessed similar average diameter indicating that the diameter was mainly determined by the catalyst rather than the carbon feedstock within this synthetic system. A demonstration of the generality was the synthesis of a carbon nanotube forest from a highly unorthodox combination of gases where trans-decahydronaphthalene acted as the carbon feedstock and benzaldehyde acted as the growth enhancer.

Understanding the fundamental mechanisms and limiting processes of the growth of single-walled carbon nanotube (SWCNT) would serve as a guide to achieve further control on structural parameters of SWCNT. In this paper, we have studied the growth kinetics of a series of SWCNT forests continuously spanning a wide range of diameters (1.9-3.2 nm), and have revealed an additional fundamental growth limiting process where the mass of the individual SWCNT is determined by the individual catalyst volume. Calculation of the conversion rate of carbon atoms into CNTs per Fe atom is 2 × 10² atoms per second. This rate limiting process provides an important understanding where the larger diameter SWCNT would grow faster, and thus be more suited for mass production.

Increased portability, versatility and ubiquity of electronics devices are a result of their progressive miniaturization, requiring current flow through narrow channels. Present-day devices operate close to the maximum current-carrying-capacity (that is, ampacity) of conductors (such as copper and gold), leading to decreased lifetime and performance, creating demand for new conductors with higher ampacity. Ampacity represents the maximum current-carrying capacity of the object that depends both on the structure and material. Here we report a carbon nanotube-copper composite exhibiting similar conductivity (2.3-4.7 × 10(5) S cm(-1)) as copper (5.8 × 10(5) S cm(-1)), but with a 100-times higher ampacity (6 × 10(8) A cm(-2)). Vacuum experiments demonstrate that carbon nanotubes suppress the primary failure pathways in copper as observed by the increased copper diffusion activation energy (~2.0 eV) in carbon nanotube-copper composite, explaining its higher ampacity. This is the only material with both high conductivity and high ampacity, making it uniquely suited for applications in microscale electronics and inverters.

Viscoelasticity describes the ability of a material to possess both elasticity and viscosity. Viscoelastic materials, such as rubbers, possess a limited operational temperature range (for example, for silicone rubber it is -55° to 300°C), above which the material breaks down and below which the material undergoes a glass transition and hardens. We created a viscoelastic material composed from a random network of long interconnected carbon nanotubes that exhibited an operational temperature range from -196° to 1000°C. The storage and loss moduli, frequency stability, reversible deformation level, and fatigue resistance were invariant from -140° to 600°C. We interpret that the thermal stability stems from energy dissipation through the zipping and unzipping of carbon nanotubes at contacts.

Among all known materials, we found that a forest of vertically aligned single-walled carbon nanotubes behaves most similarly to a black body, a theoretical material that absorbs all incident light. A requirement for an object to behave as a black body is to perfectly absorb light of all wavelengths. This important feature has not been observed for real materials because materials intrinsically have specific absorption bands because of their structure and composition. We found a material that can absorb light almost perfectly across a very wide spectral range (0.2-200 mum). We attribute this black body behavior to stem from the sparseness and imperfect alignment of the vertical single-walled carbon nanotubes.

We propose a statistical and macroscopic analysis to estimate the catalyst activity of water-assisted growth (super-growth) of single-walled nanotubes (SWNT) and to characterize SWNT forests. The catalyst activity was estimated to be 84% (+/-6%), the highest ever reported. The SWNT forest was found to be a very sparse material where SWNTs represent only 3.6% of the total volume. This structural sparseness is believed to play a critical role in achieving highly efficient growth.

We demonstrate the efficient chemical vapor deposition synthesis of single-walled carbon nanotubes where the activity and lifetime of the catalysts are enhanced by water. Water-stimulated enhanced catalytic activity results in massive growth of superdense and vertically aligned nanotube forests with heights up to 2.5 millimeters that can be easily separated from the catalysts, providing nanotube material with carbon purity above 99.98%. Moreover, patterned, highly organized intrinsic nanotube structures were successfully fabricated. The water-assisted synthesis method addresses many critical problems that currently plague carbon nanotube synthesis.