Dynamics and Limiting Mechanisms of Self-Aligned Carbon Nanotube Growth.

Carbon nanotubes (CNTs) are long, cylindrical molecules, which boast exceptional tensile strength and large thermal and electrical conductivities. Vertically aligned CNT “forests” have promising potential uses, including dry adhesives, electrical interconnects, light emitters, thermal interface materials, gas and liquid filters, composite reinforcements, and photonic crystals. Manufacturing indefinitely long CNTs may realize dreams of CNT-based cables and wires having stiffness, strength, and transport properties exceeding today’s best metal alloys and advanced fibers. However, the functional properties of CNT forests have so far fallen short of those of individual CNTs due to low packing fraction, polydisperse diameters, and relatively short lengths. Toward the eventual goal of bridging this structure-property relationship, my dissertation presents a novel set of in situ and ex situ characterization tools for CNT forest growth by chemical vapor deposition (CVD), as well as the use of these tools to investigate the limiting mechanisms thereof. In situ X-ray scattering reveals the dynamics of catalyst thin film dewetting into nanoparticle growth sites, the initial self-organization of the CNT forest, and the abrupt self-termination of growth. Quantification of catalyst and CNT sizes show that they are inevitably polydisperse, regardless of synthesis conditions. To overcome this, a novel method is introduced for templated dewetting of the catalyst film toward the formation of ordered, monodisperse particles using nanoporous anodic alumina. Further, a map of thermal conditions is explored by independently tuning the temperatures of the catalyst and gaseous precursors, thereby establishing a set of rules for engineering crucial characteristics of forest growth, including CNT diameter, structural quality, vertical alignment, as well as rate and lifetime of the reaction. Finally, aligned CNT ensembles are used as templates to direct the self-assembly of fullerene C60, creating hybrid films with high photoconductive gain, thereby demonstrating an immediate application of this exciting material. These studies represent many new insights into the so-called “birth, life, and death” of CNT growth, and they have important implications for future work in synthesis of advanced carbon materials, including CNTs, fullerenes, and graphene. Meanwhile, these results have immediate applicability to efficient CNT manufacturing, improved characterization, and new hybrid materials for energy conversion.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91427/1/emeshot_1.pd

Shock formation and rate effects in impacted carbon nanotube foams

We investigate rate-effects in the dynamic response of vertically aligned carbon nanotube (VACNT) foams excited by impacts at controlled velocities. They exhibit a complex rate-dependent loading–unloading response at low impact velocities and they support shock formation beyond a critical velocity. The measured critical velocities are ∼10 times lower than in other foams of similar densities—a desirable characteristic in impact protective applications. In-situ high-speed microscopy reveals strain localization and progressive buckling at low velocities and a crush-front propagation during shock compression. We correlate these responses to quantitative measurements of the density gradient and fiber morphology, obtained with spatially resolved X-ray scattering and mass attenuation

Interactive hypermedia : a comparative study of the effects of real-time motion videodisc versus still frame and of cognitive style on cetacea animals knowledge test for second grade students

The purpose of this study was to determine the effects of real-time motion vs still frame presentation mode and cognitive style (field dependent versus field independent) on a interactive hypermedia knowledge task. The field dependent and field independent cognitive style dimensions of 121 second grade students were determined by the administration of the Children's Embedded Figure Test. Forty field dependent individuals and 40 field independent individuals were selected, randomly assigned to treatment groups, and administered the Cetacea Animals Knowledge Test pre-test. Two groups each of 20 field dependent individuals and 20 field independent individuals received the hypermedia still frame presentation; two groups each of 20 field dependent individuals and 20 field independent individuals received the hypermedia real-time motion presentation. All groups were administered the Cetacea Animals knowledge test post-test. The results of a 2x2 analysis of covariance indicated a significant effect of cognitive style on the post-test scores; field independent students scored higher than field dependent students. There were no differences between hypermedia still frame and realtime motion treatment sub-groups, and no interaction effects between cognitive style field independent and field dependent dimension and hypermedia still frame and real-time motion presentation treatment

Anomalous impact and strain responses in helical carbon nanotube foams

We describe the quasistatic and dynamic response of helical carbon nanotube (HCNT) foams in compression. Similarly to other CNT foams, HCNT foams exhibit preconditioning effects in response to cyclic loading; however, their fundamental deformation mechanisms are unique. In quasistatic compression, HCNT foams exhibit strain localization and collective structural buckling, nucleating at different weak sections throughout their thickness. In dynamic compression, they undergo progressive crushing, governed by the intrinsic density gradient along the thickness of the sample. HCNT micro-bundles often undergo brittle fracture that originates from nanoscale defects. Regardless of this microstructural damage, bulk HCNT foams exhibit super-compressibility and recover more than 90% of large compressive strains (up to 80%). When subjected to striker impacts, HCNT foams mitigate impact stresses more effectively compared to other CNT foams comprised of non-helical CNTs ([similar]50% improvement). The unique mechanical properties we revealed demonstrate that the HCNT foams are ideally suited for applications in packaging, impact protection, and vibration mitigation

Spring-block approach for nanobristle patterns

A two dimensional spring-block type model is used to model capillarity driven self-organization of nanobristles. The model reveals the role of capillarity and van der Waals forces in the pattern formation mechanism. By taking into account the relevant interactions several type of experimentally observed patterns are qualitatively well reproduced. The model offers the possibility to generate on computer novel nanobristle based structures, offering hints for designing further experiments.Comment: 6 pages, 6 figure

Decoupled Control of Carbon Nanotube Forest Density and Diameter by Continuous‐Feed Convective Assembly of Catalyst Particles

The widespread potential application of vertically aligned carbon nanotube (CNT) forests have stimulated recent work on large‐area chemical vapor deposition growth methods, but improved control of the catalyst particles is needed to overcome limitations to the monodispersity and packing density of the CNTs. In particular, traditional thin‐film deposition methods are not ideal due to their vacuum requirements, and due to limitations in particle uniformity and density imposed by the thin‐film dewetting process. Here, a continuous‐feed convective self‐assembly process for manufacturing uniform mono‐ and multi‐layers of catalyst particles for CNT growth is presented. Particles are deposited from a solution of commercially available iron oxide nanoparticles, by pinning the meniscus between a blade edge and the substrate. The substrate is translated at constant velocity under the blade so the meniscus and contact angle remain fixed as the particles are deposited on the substrate. Based on design of the particle solution and tuning of the assembly parameters, a priori control of CNT diameter and packing density is demonstrated. Quantitative relationships are established between the catalyst size and density, and the CNT morphology and density. The roll‐to‐roll compatibility of this method, along with initial results achieved on copper foils, suggest promise for scale‐up of CNT forest manufacturing at commercially relevant throughput. Continuous‐feed evaporative self‐assembly is used to create nanoparticle arrays for carbon nanotube (CNT) film growth. This versatile method enables specification of the CNT film morphology, and wide‐range tuning of the diameter and density of CNT forests. The present results exceed the performance limits of thin‐film catalyst dewetting, and the process is compatible with roll‐to‐roll manufacturing on flexible substrates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/99619/1/2564_ftp.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/99619/2/smll_201202878_sm_suppl.pd

Influence of Ni Catalyst Layer and TiN Diffusion Barrier on Carbon Nanotube Growth Rate

Dense, vertically aligned multiwall carbon nanotubes were synthesized on TiN electrode layers for infrared sensing applications. Microwave plasma-enhanced chemical vapor deposition and Ni catalyst were used for the nanotubes synthesis. The resultant nanotubes were characterized by SEM, AFM, and TEM. Since the length of the nanotubes influences sensor characteristics, we study in details the effects of changing Ni and TiN thickness on the physical properties of the nanotubes. In this paper, we report the observation of a threshold Ni thickness of about 4 nm, when the average CNT growth rate switches from an increasing to a decreasing function of increasing Ni thickness, for a process temperature of 700°C. This behavior is likely related to a transition in the growth mode from a predominantly “base growth” to that of a “tip growth.” For Ni layer greater than 9 nm the growth rate, as well as the CNT diameter, variations become insignificant. We have also observed that a TiN barrier layer appears to favor the growth of thinner CNTs compared to a SiO2 layer

Engineering Hierarchical Nanostructures by Elastocapillary Self‐Assembly

Surfaces coated with nanoscale filaments such as silicon nanowires and carbon nanotubes are potentially compelling for high‐performance battery and capacitor electrodes, photovoltaics, electrical interconnects, substrates for engineered cell growth, dry adhesives, and other smart materials. However, many of these applications require a wet environment or involve wet processing during their synthesis. The capillary forces introduced by these wet environments can lead to undesirable aggregation of nanoscale filaments, but control of capillary forces can enable manipulation of the filaments into discrete aggregates and novel hierarchical structures. Recent studies suggest that the elastocapillary self‐assembly of nanofilaments can be a versatile and scalable means to build complex and robust surface architectures. To enable a wider understanding and use of elastocapillary self‐assembly as a fabrication technology, we give an overview of the underlying fundamentals and classify typical implementations and surface designs for nanowires, nanotubes, and nanopillars made from a wide variety of materials. Finally, we discuss exemplary applications and future opportunities to realize new engineered surfaces by the elastocapillary self‐assembly of nanofilaments. New insights in capillary interactions between nanofilaments have led to versatile and scalable methods to build complex structures that cannot be achieved by any other processing technique. Understanding the control of this process is conducive to the development of high‐performance battery and capacitor electrodes as well as photovoltaics, electrical interconnects, and other smart materials.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96738/1/2412_ftp.pd

Synthese von hierarchischen Nanostrukturen durch elastokapillare Selbstorganisation

Oberflächen, die mit nanoskaligen Filamenten wie Siliciumnanodrähten und Kohlenstoffnanoröhren beschichtet sind, sind potenziell hochinteressant für Hochleistungsbatterien und Kondensatorelektroden, Photovoltaik, elektrische Schaltungen, Substrate für Zellzüchtungen, Trockenklebstoffe und andere intelligente Materialien. Viele diese Anwendungen erfordern eine nasse Umgebung oder umfassen Verarbeitungsschritte in Lösung. Die durch diese nassen Umgebungen erzeugten Kapillarkräfte können einerseits zu einer unerwünschten Aggregation der nanoskaligen Filamente führen, andererseits kann über die Steuerung der Kapillarkräfte jedoch ein Manipulieren dieser Filamente zur Bildung diskreter Aggregate und neuer hierarchischer Strukturen möglich werden. Viele neuere Arbeiten deuten darauf hin, dass die elastokapillare Selbstorganisation von Nanofilamenten eine vielseitige und skalierbare Methode zum Aufbau komplexer und robuster Oberflächenarchitekturen sein kann. Mit diesem Aufsatz möchten wir zum Verständnis der elastokapillaren Selbstorganisation als Herstellungsmethode beitragen und deren Anwendungspotenzial vorstellen. Wir erläutern die Grundlagen und klassifizieren typische Anwendungen und Oberflächendesigns für Nanodrähte, Nanoröhren und Nanosäulen aus verschiedensten Materialien. Schließlich diskutieren wir beispielhafte Anwendungen und zukünftige Möglichkeiten, um neue Oberflächen über die elastokapillare Selbstorganisation von Nanofilamenten zu realisieren. Neue Einblicke in Kapillarwechselwirkungen zwischen Nanofilamenten haben zu vielseitigen und skalierbaren Methoden zum Aufbau komplexer Strukturen geführt, die mit anderen Techniken nicht zugänglich sind. Das Verständnis dieser Prozesse ist wichtig für die Entwicklung von Hochleistungsbatterien und Kondensatorelektroden sowie für die Photovoltaik, elektrische Schaltungen und andere intelligente Materialien.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96679/1/2470_ftp.pd