Bottom-up growth of carbon nanotube multilayers: unprecedented growth

An unusual growth phenomenon, with no precedent in vapor-phase thin film growth, is described here, for the case of the growth of stacked multiple layers of vertically aligned carbon nanotubes(1-6) on solid substrates. As multiple layers of ordered nanotubes are sequentially deposited from the vapor onto the substrate, each layer nucleates and grows from the original substrate surface at the bottom of the existing multiple stacks of nanotubes. In contrast to conventional understanding of thin film deposition,(7) the mechanism here has similarities to porous oxide film formation on surfaces.(8) The stacked layers of aligned nanotubes act as fully permeable membranes for the downward diffusion of growth precursor vapors, allowing growth to occur at the buried solid interface. The preexisting multiple nanotube stacks lift up to accommodate the vertical growth of fresh layers, allowing the formation of nanotube towers extending in millimeter lengths. Our results provide evidence for a new growth phenomenon, characterized by selective, interface-driven, bottom-up growth of self-assembled nanowires at buried interfaces, covered with weakly adhering thick porous membranes.     

Investigation of initial growth of ZnO nanowires and their growth mechanism

ZnO nanowires were synthesized on Si substrates by a simple metal vapor deposition method without any catalysts. The initial growth and the growth mechanism of the ZnO nanowires were studied using scanning and transmission electron microscopy. We found that the ZnO nanowires grew on the Si substrate via a self-seeding vapor-solid mechanism. The growth process of the ZnO nanowires consisted of four steps: self-seeding, one-dimensional epitaxial growth of the nanowires on the seeds by a base-growth mode, further acceleration of nanowire growth with additional seeding, and active formation of the nanowires.     

Abnormal grain growth via the migration of planar growth interfaces

Nanocrystalline nickel electrodeposits undergo a sequence of morphologically distinct grain growth stages during annealing. The nanostructure initially undergoes a rapid sequence of abnormal grain growth, followed by a much slower normal grain growth stage. Out of this uniformly growing structure comes a second stage of abnormal grain growth which not only accelerates the overall growth rate, but the transformation also occurs by the migration of planar reaction fronts. These planar growth interfaces are composed of many individual grain boundary segments, migrating together at essentially the same velocity. Grain shape was studied from intergranular fracture surfaces; it was found that the abnormally growing grains were cuboidal in shape and were present either as individually growing grains or as cuboid clusters. Electron backscatter diffraction showed numerous twin-related cuboidal grain clusters having complex compound planes.     

The growth and growth mechanism of lithium tetraborate

The conditions required to grow high quality lithium tetraborate single crystals are described, along with the defects likely to be encountered during growth of the crystals. The relationship of the latter to the growth mechanism is discussed and a mechanism proposed which provides an explanation of the observed nature of the defects and of the manner in which the crystals grow and reject impurities.     

Synergetic nanowire growth

Interest in nanowires continues to grow because they hold the promise of monolithic integration of high-performance semiconductors with new functionality into existing silicon technology. Most nanowires are grown using vapour-liquid-solid growth, and despite many years of study this growth mechanism remains under lively debate. In particular, the role of the metal particle is unclear. For instance, contradictory results have been reported on the effect of particle size on nanowire growth rate. Additionally, nanowire growth from a patterned array of catalysts has shown that small wire-to-wire spacing leads to materials competition and a reduction in growth rates. Here, we report on a counterintuitive synergetic effect resulting in an increase of the growth rate for decreasing wire-to-wire distance. We show that the growth rate is proportional to the catalyst area fraction. The effect has its origin in the catalytic decomposition of precursors and is applicable to a variety of nanowire materials and growth techniques.     

Grain growth

Abstract

The processes and mechanisms controlling grain growth in crystalline solids are reviewed. The driving forces governing the overall process are considered, and a distinction is made between normal and anomalous grain growth. The influence of solutes and dispersions (soluble and insoluble, coherent and incoherent) is discussed. Recent experimental studies of grain growth in model and commercial materials are surveyed.

MST/1297     

Classification of growth models based on growth rates and its applications

In this paper, growth models are classified and characterised using two types of growth rates: from time t to t+1 and from time t to 2t. They are interesting in themselves but can also be used for a quick prediction of the type of growth model that is valid in a particular case. These ideas are applied on 20 data sets collected byWolfram, Chu andLu. We determine (using the above classification as well as via nonlinear regression techniques) that the power model (with exponent>1) is the best growth model for Sci-Tech online databases, but that Gompertz-S-shaped distribution is the best for social sciences and humanities online databases.One of the authors (I.K.R. Rao) is grateful to the Belgian National Science Foundation (NFWO) for financial support in the period that he was a visiting professor in LUC.     

Comparison of human and animal growth

New ad libitum feeding and growth equations found suitable to describe the growth of animals from the rat to the pig are used to conjecture a human growth equation. The equation is an exponential function involving eight parameters. The function was found to fit very well Baldwin's transverse data for human growth from birth to young adulthood. The values of the parameters are used to compare the growth of U.S. And Asian males and U.S. Males and females.     

Self-aligned growth of carbon nanosticks

Pulsed laser deposition of carbon on LiNbO(3) as substrate material leads for certain process parameters to the growth of self-aligned carbon stick-like nanoparticles (so called nanosticks). The carbon nanosticks and the growth conditions were investigated in detail by means of atomic force microscopy, scanning electron microscopy and conductivity measurements. A model for the growth mechanism is presented on the basis of the experimental investigations.     

A general mechanism of polycrystalline growth

Most research into microstructure formation during solidification has focused on single-crystal growth ranging from faceted crystals to symmetric dendrites. However, these growth forms can be perturbed by heterogeneities, yielding a rich variety of polycrystalline growth patterns. Phase-field simulations show that the presence of particulates (for example, dirt) or a small rotational-translational mobility ratio (characteristic of high supercooling) in crystallizing fluids give rise to similar growth patterns, implying a duality in the growth process in these structurally heterogeneous fluids. Similar crystallization patterns are also found in thin polymer films with particulate additives and pure films with high supercooling. This duality between the static and dynamic heterogeneity explains the ubiquity of polycrystalline growth patterns in polymeric and other complex fluids.     

Multiplicity of steady modes of nanowire growth

For nanowire growth by the vapor-liquid-solid process, we examine whether there is a unique steady-state growth morphology. Applying a continuum model for faceted nanowire evolution to a model crystal structure, we enumerate the possible growth morphologies and calculate their dynamical stability. We find that even for a single set of experimental conditions there can be multiple distinct modes of steady-state growth. The actual growth mode occurring in experiment thus depends on the initial conditions and growth history. Relevant experiments are discussed.     

On the non-steady-state growth of lamellar eutectics

The non-steady-state growth of the typical anomalous Al-Si eutectic is investigated experimentally by means of two methods of non-steady-state growth: abrupt change of growth rate and constant-acceleration (deceleration) growth. The uniqueness of steady-state growth of the eutectic is verified, and the details of the dependence on non-steady-state growth of the response features, such as silicon interflake spacing, to varying growth rate, are shown. A model for non-steady-state growth of a lamellar eutectic is proposed to describe the response dynamics of a eutectic: a new concept.     

Measuring the growth of science

A number of indicators of the growth of science are critically reviewed to asses their strengths and weaknesses. The focus is on the problems involved in measuring two aspects of scientific growth, growth in manpower and growth in knowledge. It is shown that the design of better indicators depends on careful consideration of the theoretical framework within which the indicators are intended to be used. Recent advances in the sociology of science suggest ways in which the validity of existing indicators may be assessed and improved.This paper is a revision of one presented to the International Symposium on Quantitative Methods in the History of Science, Berkely, California, August 25–27, 1976, under the title Measuring Science.     

Kinetics of epitaxial growth and roughening

We review some recent results on epitaxial growth and surface roughening. Particular emphasis is placed on the concept of the critical island size in submonolayer growth and on the existence of scaling in both the submonolayer and multilayer growth regimes. The use of scaling ideas as well as Monte Carlo simulations and continuum equations is shown to be effective in understanding experimental results for submonolayer growth and surface roughening.     

A study of multi-segment fatigue crack growth data analysis procedure for probabilistic crack growth prediction

Commonly used fatigue crack growth prediction models estimate life in a deterministic manner. Realizing that several variables influence fatigue crack growth, it is pertinent to assess crack growth using probabilistic models. In the present work, a probabilistic model is studied using continuous and segmented crack growth rate data models. It is observed that the prediction of life using Paris constants from continuous data model is accurate only in a finite region of the crack growth. The model based on segmented data provides more accurate life predictions with lesser variance with experimental data than the continuous data model.     

The calculation of growth and nucleation kinetics from MSMPR crystallizer data including growth rate dispersion

One of the most important applications of the population balance approach to crystallizer modeling is the recovery of crystal nucleation and growth rate data from steady state crystal size distributions as described by Randolph and Larson in 1971. Recent crystallization studies have revealed that growth rate dispersion, wherein crystals of the same size do not have the same growth rate, causes nonidealities that limit the usefulness of the Randolph and Larson approach. The present work presents and shows application of a procedure that allows calculation of unambiguous nucleation and growth rate kinetics in the presence of growth rate dispersion. It requires the appropriate choice of growth rate distribution for a fit to the experimental data. The method is applied to analysis of pilot scale crystallization experiments with the sucrose-water system and indicates that unimodal distributions may be incapable of describing such data.     

Mixed-mode crack growth predictions

The problem of slow stable growth of an inclined crack in a plate subjected to uniaxial tension is studied by the strain energy density criterion. The stable crack growth process is simulated by predicting a series of crack growth steps corresponding to a piecewise loading increase when material elements along the direction of crack extension absorb a critical amount of elastic strain energy density. Crack instability takes place when the last ligament of crack extension takes a critical value which is a material constant. The critical stress at the onset of crack initiation and unstable crack extension is determined for various crack inclination angles. Three different loading step increments corresponding to three different loading rates are considered and their effect on stable crack growth is analysed. Furthermore, the influence of loading history on the crack growth process for three different loading types is studied. The complete crack growth patterns for all types of load are determined and analysed. It is obtained that the amount of slow crack growth can be increased by lowering the rate of loading. The effect of the loading history on the failure load and the crack paths is established.     

Crystal growth of KCl in a reactive atmosphere II: The crystal growth technique

The crystal growth technique employed in the growth of large single crystals of KCl while the material is undergoing reactive-atmosphere processing is described in detail.     

Elementary processes in nanowire growth

We propose that many of the complex morpho-logical phenomena observed during nanowire growth arise from the interplay of just three elementary processes: facet growth, droplet statics, and the introduction of new facets. We incorporate these processes into an explicit model for the vapor-liquid-solid growth of fully faceted nanowires. In numerical simulations with this model, different conditions can lead to either growth of a free-standing wire or lateral growth where the catalyst droplet crawls along the surface. An external perturbation can cause the wire to kink into a different direction. Different growth conditions can also change the shape of the growth tip. All of these phenomena have been observed, and the model behavior is consistent with the experimental observations.     

Filament-assisted growth of diamond films

Filament-assisted pyrolytic growth of diamond films on (100) Si wafers was investigated in an attempt to grow quality layers for semiconductor applications. The work was carried out in hydrogen ambient under a reduced pressure condition of about 100 torr (133, 322×10² Pa). Using isopropanol and methanol as carbon source chemicals, the growth process and film properties were characterized as functions of reactant concentration, filament and substrate temperature, reaction pressure and the total gas flow rate. Diamond films of good quality were grown under condition of low source concentration and small flow rate. However, the growth rates were generally slow. The films were polycrystalline. The filament and substrate temperatures were fairly critical to the nucleation and growth processes. The substrate surface finishing from diamond paste polishing predominated the nucleation site and grain size of the deposits.