Alexander A. Chernov

Mechanism and Kinetics of Growth Termination in Controlled Chemical Vapor Deposition Growth of Multiwall Carbon Nanotube Arrays

We have investigated growth kinetics of multiwall carbon nanotube (MWCNT) arrays produced by catalytic thermal decomposition of ethylene gas in hydrogen, water, and argon mixture. The MWCNT growth rate exhibits a nonmonotonic dependence on total pressure and reaches a maximum at approximately 750 Torr of total pressure. Water concentrations in excess of 3000 ppm lead to the decrease in the observed growth rate. Optimal pressure and water concentration combination results in a reliable growth of well-aligned MWCNT arrays at a maximum growth rate of approximately 30 microm/min. These MWCNT arrays can reach heights of up to 1 mm with typical standard deviations for the array height of less than 8% over a large number of process runs spread over the time of 8 months. Nanotube growth rate in this optimal growth region remains essentially constant until growth reaches an abrupt and irreversible termination. We present a quantitative model that shows how accumulation of the amorphous carbon patches at the catalyst particle surface and the carbon diffusion to the growing nanotube perimeter causes this abrupt growth cessation. The influence of the partial pressures of ethylene and hydrogen on the ethylene decomposition driving force explains the nonlinear behavior of the growth rate as a function of total process pressure.

Deuterium-Tritium Fuel Layer Formation for the National Ignition Facility

Abstract Inertial confinement fusion requires very smooth and uniform solid deuterium-tritium (D-T) fuel layers. The National Ignition Facility (NIF) point design calls for a 65- to 75-μm-thick D-T fuel layer inside of a 2-mm-diam spherical ablator shell to be 1.5 K below the D-T melting temperature (Tm) of 19.79 K. We find that the layer quality depends on the initial crystal seeding, with the best layers grown from a single seed. The low modes of the layer are controlled by thermal shimming of the hohlraum and meet the NIF requirement with beryllium shells and nearly meet the requirement with plastic shells. The remaining roughness is localized in grain-boundary grooves and is minimal for a single crystal layer. Once formed, the layers need to be cooled to Tm - 1.5 K. We have studied dependence of the roughness on the cooling rate and found that cooling at rates of 0.03 to 0.5 K/s is able to preserve the layer structure for a few seconds after reaching the desired temperature. The entire fuel layer remains in contact with the shell during this rapid cooling. Thus, rapid cooling of the layers is able to satisfy the NIF ignition requirements.

Hidden role of trace gas impurities in chemical vapor deposition growth of vertically-aligned carbon nanotube arrays

Carbon nanotubes (CNTs) grow in a seemingly simple catalytic chemical vapor deposition (CVD) process, yet the detailed mechanism of the process has continued to puzzle researchers. We have examined the role of trace amounts of gas impurities on the kinetics of atmospheric pressure CVD growth of CNTs. Our studies, which used an in situ height monitoring system, revealed that even the nominally ultrapure gases contain enough trace amounts of oxygen-containing species to affect the growth drastically. We were able to obtain the “clean” kinetics of the CNT array growth by passing the feed gases through the high performance gas purifiers. Our data show a remarkable decrease in the catalytic lifetime after the removal of the trace oxygen containing impurities. We suggest that the gas purification is an essential step in obtaining reliable nanotube growth data.

Single crystal growth and formation of defects in deuterium-tritium layers for inertial confinement nuclear fusion

We identify vapor-etched grain boundary grooves on the solid-vapor interface as the main source of surface roughness in the deuterium-tritium (D–T) fuel layers, which are solidified and then cooled. Current inertial confinement fusion target designs impose stringent limits to the cross-sectional area and total volume of these grooves. Formation of these grain boundaries occurs over time scales of hours as the dislocation network anneals and is inevitable in a plastically deformed material. Therefore, either cooling on a much shorter time scale or a technique that requires no cooling after solidification should be used to minimize the roughness.

Tuning the rheological properties of sols for low-density aerogel coating applications

Coating of cylindrical and spherical surfaces with thin and homogeneous low-density aerogel films requires precise control over viscosity and gel time. If the viscosity is too low, shear forces can damage the growing gel network and prevent the formation of uniform coatings. Using the example of dicyclopentadiene-based polymer gels, we demonstrate that the gelation behaviour can be manipulated by reducing the amount of cross-linking through co-polymerization with a monomer that can only form linear chains. Even small additions of a linear co-polymer (1–10 wt. %) increase the viscosity at the sol–gel transition by several orders of magnitude, and drastically improve the uniformity of gel films formed under the influence of shear. These results are discussed in the context of the classical gel theory.

Solvent-mediated repair and patterning of surfaces by AFM

A tip-based approach to shaping surfaces of soluble materials with nanometer-scale control is reported. The proposed method can be used, for example, to eliminate defects and inhomogeneities in surface shape, repair mechanical or laser induced damage to surfaces, or perform 3D lithography on the length scale of an AFM tip. The phenomenon that enables smoothing and repair of surfaces is based on the transport of material from regions of high to low curvature within the solution meniscus formed in a solvent-containing atmosphere between the surface in question and an AFM tip scanned over the surface. Using in situ AFM measurements of the kinetics of surface remodeling on KDP (KH(2)PO(4)) crystals in humid air, we show that redistribution of solute material during relaxation of grooves and mounds is driven by a reduction in surface free energy as described by the Gibbs-Thomson law. We find that the perturbation from a flat interface evolves according to the diffusion equation, where the effective diffusivity is determined by the product of the surface stiffness and the step kinetic coefficient. We also show that, surprisingly, if the tip is instead scanned over or kept stationary above an atomically flat area of the surface, a convex structure is formed, with a diameter that is controlled by the dimensions of the meniscus, indicating that the presence of the tip and meniscus reduces the substrate chemical potential beneath that of the free surface. This allows one to create nanometer-scale 3D structures of arbitrary shape without the removal of substrate material or the use of extrinsic masks or chemical compounds. Potential applications of these tip-based phenomena are discussed.

Coating functional sol-gel films inside horizontally-rotating cylinders by rimming flow/state

The fabrication of uniform sol–gel coatings with embedded functional nanomaterials inside cylinders requires detailed understanding of the gelation behavior. For sol–gel systems the viscosity is a function of gelation time that affects sol–gel coatings on the inside of a slowly, horizontally rotating cylinder. Therefore the angular velocity has to be adjusted to this time dependence. The higher the viscosity the more liquid is dragged along with the moving cylinder wall while the balance of gravity and drag limits the layer thickness. In addition, inertial forces and surface tension can create instabilities within the coated layer. Here, we show that it is important to suppress these instabilities by transitioning the viscous sol directly to a velocity that allows for the formation of an almost uniform layer. In this regime, which is the so-called rimming state, the recirculation of the gel precursor solution is strongly reduced which allows to fabricate coatings with shear sensitive sol–gel chemistries. Here, we tested this approach with 4 different aerogel systems, with low-density CH-based-, TiO2-, SiO2- and Fe2O3-aerogels, that represent a wide variety of different sol–gel behaviors. We show that the required rotational velocities for these aerogel systems can be predicted with a simple analytical approximation, and we performed computational fluid dynamics simulations to predict local shear and thickness uniformity.