9 research outputs found
Synergetic Chemical Coupling Controls the Uniformity of Carbon Nanotube Microstructure Growth
Control of the uniformity of vertically aligned carbon nanotube structures (CNT “forests”), in terms of both geometry and nanoscale morphology (density, diameter, and alignment), is crucial for applications. Many studies report complex and sometimes unexplained spatial variations of the height of macroscopic CNT forests, as well as variations among micropillars grown from lithographically patterned catalyst arrays. We present a model for chemically coupled CNT growth, which describes the origins of synergetic growth effects among CNT micropillars in proximity. <i>Via</i> this model, we propose that growth of CNTs is locally enhanced by active species that are catalytically produced at the substrate-bound nanoparticles. The local concentration of these active species modulates the growth rate of CNTs, in a spatially dependent manner driven by diffusion and local generation/consumption at the catalyst sites. Through experiments and numerical simulations, we study how the uniformity of CNT micropillars can be influenced by their size and spacing within arrays and predict the widely observed abrupt transition between tangled and vertical CNT growth by assigning a threshold concentration of active species. This mathematical framework enables predictive modeling of spatially dependent CNT growth, as well as design of catalyst patterns to achieve engineered uniformity
Highly Consistent Atmospheric Pressure Synthesis of Carbon Nanotube Forests by Mitigation of Moisture Transients
Consistent synthesis of carbon nanotubes
(CNTs) using laboratory-scale
methods is essential to the development of commercial applications,
particularly with respect to the verification of recipes that achieve
control of CNT diameter, chirality, alignment, and density. Here,
we report that transients in the moisture level and carbon concentration
during the chemical vapor deposition (CVD) process for vertically
aligned CNT “forests” can contribute significantly to
run-to-run variation of height and density. Then, we show that highly
consistent CNT forest growth can be achieved by physically decoupling
the catalyst annealing and hydrocarbon exposure steps, to allow the
gas composition to stabilize between the steps. This decoupling is
achieved using a magnetically actuated transfer arm to move the substrate
rapidly into and out of the CVD reactor. Compared to a reference process
where the sample resides in the furnace throughout the process, the
decoupled method gives 21% greater CNT forest height, reduces the
run-to-run variance of height by 76%, and results in forests with
improved vertical alignment (Herman’s orientation parameter
of 0.68 compared to 0.50). Building on this foundation, we study the
influence of the moisture level during the CNT growth step and find
a 30% improvement in growth rate going from the baseline condition
(<15 ppm) to 40 ppm. Interestingly, however, the increased moisture
concentration does not improve the catalyst lifetime or the CNT forest
density, warranting further study of the role of moisture on CNT nucleation
versus growth
Measurement of the Dewetting, Nucleation, and Deactivation Kinetics of Carbon Nanotube Population Growth by Environmental Transmission Electron Microscopy
Understanding
the collective growth of carbon nanotube (CNT) populations
is key to tailoring their properties for many applications. During
the initial stages of CNT growth by chemical vapor deposition, catalyst
nanoparticle formation by thin-film dewetting and the subsequent CNT
nucleation processes dictate the CNT diameter distribution, areal
density, and alignment. Herein, we use <i>in situ</i> environmental
transmission electron microscopy (E-TEM) to observe the catalyst annealing,
growth, and deactivation stages for a population of CNTs grown from
a thin-film catalyst. Complementary <i>in situ</i> electron
diffraction and TEM imaging show that, during the annealing step in
hydrogen, reduction of the iron oxide catalyst is concomitant with
changes in the thin-film morphology; complete dewetting and the formation
of a population of nanoparticles is only achieved upon the introduction
of the carbon source, acetylene. The dewetting kinetics, i.e., the
appearance of distinct nanoparticles, exhibits a sigmoidal (autocatalytic)
curve with 95% of all nanoparticles appearing within 6 s. After nanoparticles
form, they either nucleate CNTs or remain inactive, with incubation
times measured to be as small as 3.5 s. Via E-TEM we also directly
observe the crowding and self-alignment of CNTs after dewetting and
nucleation. In addition, <i>in situ</i> electron energy
loss spectroscopy reveals that the collective rate of carbon accumulation
decays exponentially. We conclude that the kinetics of catalyst formation
and CNT nucleation must both be addressed in order to achieve uniform
and high CNT density, and their transient behavior may be a primary
cause of the well-known nonuniform density of CNT forests
Local Relative Density Modulates Failure and Strength in Vertically Aligned Carbon Nanotubes
Micromechanical experiments, image analysis, and theoretical modeling revealed that local failure events and compressive stresses of vertically aligned carbon nanotubes (VACNTs) were uniquely linked to relative density gradients. Edge detection analysis of systematically obtained scanning electron micrographs was used to quantify a microstructural figure-of-merit related to relative local density along VACNT heights. Sequential bottom-to-top buckling and hardening in stress–strain response were observed in samples with smaller relative density at the bottom. When density gradient was insubstantial or reversed, bottom regions always buckled last, and a flat stress plateau was obtained. These findings were consistent with predictions of a 2D material model based on a viscoplastic solid with plastic non-normality and a hardening–softening–hardening plastic flow relation. The hardening slope in compression generated by the model was directly related to the stiffness gradient along the sample height, and hence to the local relative density. These results demonstrate that a microstructural figure-of-merit, the effective relative density, can be used to quantify and predict the mechanical response
Measurement of the Dewetting, Nucleation, and Deactivation Kinetics of Carbon Nanotube Population Growth by Environmental Transmission Electron Microscopy
Understanding
the collective growth of carbon nanotube (CNT) populations
is key to tailoring their properties for many applications. During
the initial stages of CNT growth by chemical vapor deposition, catalyst
nanoparticle formation by thin-film dewetting and the subsequent CNT
nucleation processes dictate the CNT diameter distribution, areal
density, and alignment. Herein, we use <i>in situ</i> environmental
transmission electron microscopy (E-TEM) to observe the catalyst annealing,
growth, and deactivation stages for a population of CNTs grown from
a thin-film catalyst. Complementary <i>in situ</i> electron
diffraction and TEM imaging show that, during the annealing step in
hydrogen, reduction of the iron oxide catalyst is concomitant with
changes in the thin-film morphology; complete dewetting and the formation
of a population of nanoparticles is only achieved upon the introduction
of the carbon source, acetylene. The dewetting kinetics, i.e., the
appearance of distinct nanoparticles, exhibits a sigmoidal (autocatalytic)
curve with 95% of all nanoparticles appearing within 6 s. After nanoparticles
form, they either nucleate CNTs or remain inactive, with incubation
times measured to be as small as 3.5 s. Via E-TEM we also directly
observe the crowding and self-alignment of CNTs after dewetting and
nucleation. In addition, <i>in situ</i> electron energy
loss spectroscopy reveals that the collective rate of carbon accumulation
decays exponentially. We conclude that the kinetics of catalyst formation
and CNT nucleation must both be addressed in order to achieve uniform
and high CNT density, and their transient behavior may be a primary
cause of the well-known nonuniform density of CNT forests
Local Relative Density Modulates Failure and Strength in Vertically Aligned Carbon Nanotubes
Micromechanical experiments, image analysis, and theoretical modeling revealed that local failure events and compressive stresses of vertically aligned carbon nanotubes (VACNTs) were uniquely linked to relative density gradients. Edge detection analysis of systematically obtained scanning electron micrographs was used to quantify a microstructural figure-of-merit related to relative local density along VACNT heights. Sequential bottom-to-top buckling and hardening in stress–strain response were observed in samples with smaller relative density at the bottom. When density gradient was insubstantial or reversed, bottom regions always buckled last, and a flat stress plateau was obtained. These findings were consistent with predictions of a 2D material model based on a viscoplastic solid with plastic non-normality and a hardening–softening–hardening plastic flow relation. The hardening slope in compression generated by the model was directly related to the stiffness gradient along the sample height, and hence to the local relative density. These results demonstrate that a microstructural figure-of-merit, the effective relative density, can be used to quantify and predict the mechanical response
Statistical Analysis of Variation in Laboratory Growth of Carbon Nanotube Forests and Recommendations for Improved Consistency
While many promising applications have been demonstrated for vertically aligned carbon nanotube (CNT) forests, lack of consistency in results (<i>e</i>.<i>g</i>., CNT quality, height, and density) continues to hinder knowledge transfer and commercialization. For example, it is well known that CNT growth can be influenced by small concentrations of water vapor, carbon deposits on the reactor wall, and experiment-to-experiment variations in pressure within the reaction chamber. However, even when these parameters are controlled during synthesis, we found that variations in ambient lab conditions can overwhelm attempts to perform parametric optimization studies. We established a standard growth procedure, including the chemical vapor deposition (CVD) recipe, while we varied other variables related to the furnace configuration and experimental procedure. Statistical analysis of 280 samples showed that ambient humidity, barometric pressure, and sample position in the CVD furnace contribute significantly to experiment-to-experiment variation. We investigated how these factors lead to CNT growth variation and recommend practices to improve process repeatability. Initial results using this approach reduced run-to-run variation in CNT forest height and density by more than 50%
High-Speed <i>in Situ</i> X-ray Scattering of Carbon Nanotube Film Nucleation and Self-Organization
The production of high-performance carbon nanotube (CNT) materials demands understanding of the growth behavior of individual CNTs as well as collective effects among CNTs. We demonstrate the first use of grazing incidence small-angle X-ray scattering to monitor in real time the synthesis of CNT films by chemical vapor deposition. We use a custom-built cold-wall reactor along with a high-speed pixel array detector resulting in a time resolution of 10 msec. Quantitative models applied to time-resolved X-ray scattering patterns reveal that the Fe catalyst film first rapidly dewets into well-defined hemispherical particles during heating in a reducing atmosphere, and then the particles coarsen slowly upon continued annealing. After introduction of the carbon source, the initial CNT diameter distribution closely matches that of the catalyst particles. However, significant changes in CNT diameter can occur quickly during the subsequent CNT self-organization process. Correlation of time-resolved orientation data to X-ray scattering intensity and height kinetics suggests that the rate of self-organization is driven by both the CNT growth rate and density, and vertical CNT growth begins abruptly when CNT alignment reaches a critical threshold. The dynamics of CNT size evolution and self-organization vary according to the catalyst annealing conditions and substrate temperature. Knowledge of these intrinsically rapid processes is vital to improve control of CNT structure and to enable efficient manufacturing of high-density arrays of long, straight CNTs
High-Speed <i>in Situ</i> X-ray Scattering of Carbon Nanotube Film Nucleation and Self-Organization
The production of high-performance carbon nanotube (CNT) materials demands understanding of the growth behavior of individual CNTs as well as collective effects among CNTs. We demonstrate the first use of grazing incidence small-angle X-ray scattering to monitor in real time the synthesis of CNT films by chemical vapor deposition. We use a custom-built cold-wall reactor along with a high-speed pixel array detector resulting in a time resolution of 10 msec. Quantitative models applied to time-resolved X-ray scattering patterns reveal that the Fe catalyst film first rapidly dewets into well-defined hemispherical particles during heating in a reducing atmosphere, and then the particles coarsen slowly upon continued annealing. After introduction of the carbon source, the initial CNT diameter distribution closely matches that of the catalyst particles. However, significant changes in CNT diameter can occur quickly during the subsequent CNT self-organization process. Correlation of time-resolved orientation data to X-ray scattering intensity and height kinetics suggests that the rate of self-organization is driven by both the CNT growth rate and density, and vertical CNT growth begins abruptly when CNT alignment reaches a critical threshold. The dynamics of CNT size evolution and self-organization vary according to the catalyst annealing conditions and substrate temperature. Knowledge of these intrinsically rapid processes is vital to improve control of CNT structure and to enable efficient manufacturing of high-density arrays of long, straight CNTs