Nanomaterials E-Book

Dieter Vollath

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 578

Veröffentlichungsjahr: 2013

Beschreibung

Successor of the highly acclaimed, first full-color introduction to nanomaterials - now including graphenes and carbon nanotubes

This full-colored introduction to nanomaterials and nanotechnology in particular addresses the needs of engineers who need to know the special phenomena and potentials, without getting bogged down in the scientific detail of the physics and chemistry involved.

Based on the author's own courses, this textbook shows how to produce nanomaterials and use them in engineering applications for novel products. Following an introduction, the text goes on to treat synthesis, characterization techniques, thermal, optical, magnetic and electronic properties, processing and, finally, emerging applications.

A sound overview of the "nano world" from an application-oriented perspective.

Reviews for the first edition:

"The reader [of this book] profits from the broad scientific teaching experience of the author.... This book is highly recommended for everyone who wants to step onto the new and fascinating field of nanomaterials." (International Journal of Materials Research, May 2009)

"The practical presentation and clarity in writing style makes this book a winner for anyone wanting to quickly learn about the fundamentals and practical side of nanomaterials." (IEEE Electrical Insulation Magazine, March/April 2009)

Leseprobe

Contents

Cover

Related Titles

Title Page

Copyright

Preface

Chapter 1: Nanomaterials: An Introduction

Chapter 2: Nanomaterials and Nanocomposites

2.1 Introduction

2.2 Elementary Consequences of Small Particle Size

References

Chapter 3: Surfaces in Nanomaterials

3.1 General Considerations

3.2 Surface Energy

3.3 Some Technical Consequences of Surface Energy

References

Chapter 4: Gas-Phase Synthesis of Nanoparticles

4.1 Fundamental Considerations

4.2 Inert Gas Condensation Process

4.3 Physical and Chemical Vapor Synthesis Processes

4.4 Laser Ablation Process

4.5 Radio- and Microwave Plasma Processes

4.6 Flame Aerosol Process

4.7 Synthesis of Coated Particles

References

Chapter 5: Nanotubes, Nanorods, and Nanoplates

5.1 General Considerations

5.2 Nanostructures Related to Compounds with Layered Structures

References

Chapter 6: Nanofluids

6.1 Definition

6.2 Nanofluids for Improved Heat Transfer

6.3 Ferrofluids

References

Chapter 7: Phase Transformations of Nanoparticles

7.1 Thermodynamics of Nanoparticles

7.2 Heat Capacity of Nanoparticles

7.3 Phase Transformations of Nanoparticles

7.4 Phase Transformation and Coagulation

7.5 Structures of Nanoparticles

7.6 A Closer Look at Nanoparticle Melting

7.7 Structural Fluctuations

References

Chapter 8: Magnetic Properties of Nanoparticles

8.1 Magnetic Materials

8.2 Superparamagnetic Materials

8.3 Susceptibility and Related Phenomena in Superparamagnets

8.4 Applications of Superparamagnetic Materials

8.5 Exchange-Coupled Magnetic Nanoparticles

References

Chapter 9: Optical Properties of Nanoparticles

9.1 General Remarks

9.2 Adjustment of the Index of Refraction

9.3 Optical Properties Related to Quantum Confinement

9.4 Quantum Dots and Other Lumophores

9.5 Metallic and Semiconducting Nanoparticles Isolated and in Transparent Matrices

9.6 Special Luminescent Nanocomposites

9.7 Electroluminescence

9.8 Photochromic and Electrochromic Materials

9.9 Materials for Combined Magnetic and Optic Applications

References

Chapter 10: Electrical Properties of Nanoparticles

10.1 Fundamentals of Electrical Conductivity in Nanotubes and Nanorods

10.2 Nanotubes

10.3 Photoconductivity of Nanorods

10.4 Electrical Conductivity of Nanocomposites

References

Chapter 11: Mechanical Properties of Nanoparticles

11.1 General Considerations

11.2 Bulk Metallic and Ceramic Materials

11.3 Filled Polymer Composites

References

Chapter 12: Characterization of Nanomaterials

12.1 General Remarks

12.2 Global Methods for Characterization

12.3 X-Ray and Electron Diffraction

12.4 Electron Microscopy

References

Index

Related Titles

Manasreh, O.

Introduction to Nanomaterials and Devices

2012

Print ISBN 978-0-470-92707-6

Wolf, E.L., Medikonda, M.

Understanding the Nanotechnology Revolution

2012

Print ISBN 978-3-527-41109-2

Kumar, C.S.

Nanomaterials for the Life Sciences

10 Volume Set

2011

Print ISBN 978-3-527-32261-9

Schmid, G.

Nanoparticles

From Theory to Application

2 Edition

2010

Print ISBN 978-3-527-32589-4

Cademartiri, L., Ozin, G.A.

Concepts of Nanochemistry

2009

Print ISBN 978-3-527-32597-9

Wolf, E.L.

Quantum Nanoelectronics

An Introduction to Electronic Nanotechnology and Quantum Computing

2009

Print ISBN 978-3-527-40749-1

Gubin, S.P.

Magnetic Nanoparticles

2009

Print ISBN 978-3-527-40790-3

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No.: applied for

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

Bibliographic information published by the Deutsche Nationalbibliothek

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.

© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany

All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.

Print ISBN: 978-3-527-33379-0

ePDF ISBN: 978-3-527-67187-8

ePub ISBN: 978-3-527-67186-1

mobi ISBN: 978-3-527-67185-4

Cover Design Schulz Grafik-Design, Fußgönheim

Typesetting Thomson Digital, Noida, India

Preface

This book is an introduction to nanomaterials; one may consider it as an approach to a textbook. It is based on the course on Nanomaterials for Engineers that I give at the University of Technology in Graz, Austria, and on the courses that NanoConsulting organizes for participants from industry and academia. I want to provoke your curiosity. The reader should feel invited to learn more about nanomaterials, to use nanomaterials, and to want to go beyond the content of this book. However, even when it is thought of as an introduction, reading this book requires some basic knowledge of physics and chemistry. I have tried to describe the mechanisms determining the properties of nanoparticles in a simplified way. Therefore, specialists in the different fields may feel uncomfortable with the outcome, but I saw no other way to describe the mechanisms leading to the fascinating properties of nanoparticles for a broader audience.

I am fully aware of the fact that the selection of examples from the literature is, to some extent, biased against those who discovered these new phenomena. However, in most cases, where a new phenomenon was described for the first time, the effect is just shown in principle. Later papers only had a chance when they showed these phenomena very clearly. Therefore, from the viewpoint of a textbook, the later papers are preferred. I really apologize this selection of literature.

Many exciting phenomena and processes are connected with nanoparticles. However, the size of this book is limited and, therefore, I had to make a selection of the topics presented. Unavoidably, such a selection was inevitably influenced by personal experience and preferences. Again, I apologize if the reader does not find information on a field that is important for their company or institution.

I hope the reader will find this book inspiring, and will be motivated to go deeper into this fascinating field of science and technology.

This is now the second edition and I am so thankful for all the kind reviews of the previous edition in different journals. I have now reshuffled the chapters according to my current views. Some topics have been removed, because scientific or technological developments did not go as expected, and a few new and exciting topics have been added.

I want to thank my family, especially my wife Renate, for her steady support during the time when I wrote this book and her enduring understanding of my passion for science. Furthermore, I thank Dr. Waltraud Wüst from WILEY-VCH for her steady cooperation. Without her efforts, things would have been much more difficult for me.

Stutensee, June 2013

Dieter Vollath

1

Nanomaterials: An Introduction

Today, everybody is talking about nanomaterials, even advertisements for consumer products use the prefix “nano” as a keyword for special features, and, indeed, very many publications, books, and journals are devoted to this topic. Usually, such publications are directed towards specialists such as physicists and chemists, and the “classic” materials scientist encounters increasing problems in understanding the situation. Moreover, those people who are interested in the subject but who have no specific education in any of these fields have virtually no chance of understanding the development of this technology. It is the aim of this book to fill this gap. The book will focus on the special phenomena related to nanomaterials and attempt to provide explanations that avoid – as far as possible – any highly theoretical and quantum mechanical descriptions. The difficulties with nanomaterials arise from the fact that, in contrast to conventional materials, a profound knowledge of materials science is not sufficient. The cartoon shown in Figure 1.1 shows that nanomaterials lie at the intersection of materials science, physics, chemistry, and – for many of the most interesting applications – also of biology and medicine.

Figure 1.1 A basic understanding of physics and chemistry, and some knowledge of materials science, is necessary to understand the properties and behavior of nanomaterials. As many applications are connected with biology and medicine, some knowledge of these areas is also required.

However, this situation is less complicated than it first appears to the observer, as the number of additional facts introduced to materials science is not that large. Nonetheless, the user of nanomaterials must accept that their properties demand a deeper insight into their physics and chemistry. Whereas for conventional materials the interface to biotechnology and medicine is related directly to the application, the situation is different in nanotechnology, where biological molecules such as proteins or DNA are also used as building blocks for applications outside of biology and medicine.

So, the first question to be asked is: “What are nanomaterials?” There are two definitions. The first – and broadest – definition states that nanomaterials are materials where the sizes of the individual building blocks are less than 100 nm, at least in one dimension. This definition is well suited for many research proposals, where nanomaterials often have a high priority. The second definition is much more restrictive and states that nanomaterials have properties that depend inherently on the small grain size; as nanomaterials are usually quite expensive, such a restrictive definition makes more sense. The main difference between nanotechnology and conventional technologies is that the “bottom-up” approach (see below) is preferred in nanotechnology, whereas conventional technologies usually use the “top-down” approach. The difference between these two approaches can be explained simply by using an example of powder production, where chemical synthesis represents the bottom-up approach while crushing and milling of chunks represents the equivalent top-down process.

On examining these technologies more closely, the expression “top-down” means starting from large pieces of material and producing the intended structure by mechanical or chemical methods. This situation is shown schematically in Figure 1.2. As long as the structures are within a range of sizes that are accessible by either mechanical tools or photolithographic processes, then top-down processes have an unmatched flexibility in their application.

Figure 1.2 Conventional goods are produced via top-down processes, starting from bulk materials. The intended product is obtained by the application of mechanical and/or chemical processes.

The situation is different in “bottom-up” processes, in which atoms or molecules are used as the building blocks to produce nanoparticles, nanotubes, or nanorods, or thin films or layered structures. According to their dimensionality, these features are also referred to as zero-, one-, or two-dimensional nanostructures (see Figure 1.3). Figure 1.3 also demonstrates the building of particles, layers, nanotubes, or nanorods from atoms (ions) or molecules. Although such processes provide tremendous freedom among the resultant products, the number of possible structures to be obtained is comparatively small. In order to obtain ordered structures, bottom-up processes (as described above) must be supplemented by the self-organization of individual particles.

Figure 1.3 Nanotechnologies are usually connected to bottom-up processes and are characterized by the use of atoms or molecules as building blocks. Bottom-up processes result in particles, nanotubes, nanorods, thin films, or layered structures. These products are also characterized by their dimensionality, as is also indicated.

Often, top-down technologies are described as being “subtractive,” in contrast to the “additive” technologies that describe bottom-up processes. The crucial problem is no longer to produce these elements of nanotechnology; rather, it is their incorporation into technical parts. The size ranges of classical top-down technologies compared to bottom-up technologies are shown graphically in Figure 1.4. Clearly, there is a broad range of overlap where improved top-down technologies, such as electron beam or X-ray lithography, enter the size range typical of nanotechnologies. Currently, these improved top-down technologies are penetrating into increasing numbers of fields of application.

Figure 1.4 Estimated lateral limits of different structuring processes. Clearly, the size range of bottom-up and conventional top-down processes is limited. New, advanced top-down processes expand the size range of their conventional counterparts and enter the size range typical of bottom-up processes.

For industrial applications, the most important question is the product's price in relation to its properties. In most cases, nanomaterials and products utilizing nanomaterials are significantly more expensive than conventional products. In the case of nanomaterials, the increase in price is sometimes more pronounced than the improvement in properties and therefore economically interesting applications of nanomaterials are often found only in areas where specific properties are demanded that are beyond the reach of conventional materials. Hence, as long as the use of nanomaterials with new properties provides the solution to a problem that cannot be solved with conventional materials, the price becomes much less important. Another point is that as the applications of nanomaterials using improved properties are in direct competition to well-established conventional technologies, they will encounter fierce price competition, and this may lead to major problems for a young and expensive technology to overcome. Indeed, it is often observed that marginal profit margins in the production or application of nanomaterials with improved properties may result in severe financial difficulties for newly founded companies. In general, the economically most successful application of nanomaterials requires only a small amount of material as compared to conventional technologies; hence, one is selling “knowledge” rather than “tons” (see Table 1.1). Finally, only those materials that exhibit new properties leading to novel applications, beyond the reach of conventional materials, promise interesting economic results.

Table 1.1 Relationship between the properties of a new product and prices, quantities, and expected profit (note that only those products with new properties promise potentially high profits)

3

Surfaces in Nanomaterials

3.1 General Considerations

In nanomaterials, the surface forms a sharp interface between a particle and its surrounding atmosphere or between a precipitated phase and the parent phase. These are free surfaces in the case of particulate materials or grain boundaries in bulk material. Nanomaterials have large surfaces, a fact that can be demonstrated by using spherical particles as examples. As mentioned Chapter 2, nanoparticles demonstrate a large ratio R ′ of surface area a to volume v. Assuming a mathematical surface, the surface area/volume ratio, R′ = a /v = 6/d, is inversely proportional to the particle diameter d. Realistically, however, the surface has a certain thickness, influencing partly the volume. Based on many physical properties, it is known that the region of a particle that is influenced by the surface has a thickness δ between 0.5 and 1.5 nm. Therefore, a modified, dimensionless ratio R * must be defined as:

(3.1)

This ratio, for an assumed surface thickness of 0.5 and 1.0 nm, is shown graphically in Figure 3.1. On examining Figure 3.1, it is clear that in the case of a 5-nm particle, 49% or 78%, respectively, of the volume belongs to the surface or, more precisely, to the surface-influenced volume.

Figure 3.1 Ratio of the surface layer over the total particle volume. The thickness of the surface layer was assumed to be 0.5 or 1.0 nm.

As surface is related to energy, the amount of surface energy per particle usurface is equal to γa, where γ is the specific surface energy and a is the surface area of one particle. In this context, one considers the geometric surface area of the particle, which is calculated from (physical values related to one particle are denoted by lower-case letters, while those related to molar quantities are denoted by upper-case letters.) For thermodynamic considerations, the surface energy per mole of material is the essential quantity. Hence, if N is the number of particles per mole, one obtains (where ρ is the density of the material, M is the molar weight, d is the particle diameter, and A is the surface area of 1 mole of particles). Finally, one obtains the surface energy of particles with diameter d per mole:

(3.2)

Equation (3.2) states that the surface energy per mole increases with 1/d and in some cases, especially those related to very small particles, this may have dramatic consequences.

The same considerations are valid for polycrystalline materials, where the volume related to the grain boundaries increases as the grain size decreases. In contrast to the well-ordered crystalline areas, the atoms or ions in the grain boundaries are, in a first approximation, arranged randomly. The famous picture of Gleiter [1] representing the arrangement of grains and grain boundaries is shown in Figure 3.2.

Figure 3.2 Grain boundaries in polycrystalline material with grains in the nanometer range. A large portion of the material is associated with the surface [1] (Reproduced by permission of Elsevier).

3.2 Surface Energy

The origin of surface energy is explained by a model that assumes that particles are produced by breaking a large solid piece of material into smaller parts. In order to achieve this, it is necessary to cut the bonds between the neighboring atoms. (In this simplified explanation, the term “atom” is used equally to describe atoms, ions, and molecules.) Between each two atoms in the lattice the energy of bonding u is active (see Figure 3.3).

Figure 3.3 Creation of new surfaces (e.g., by breaking a larger portion into smaller pieces) requires energy u for each bond to be broken.

In order to separate one bond, energy u (symbolized as arrows in Figure 3.3) is required; therefore, to break a large piece of material into smaller pieces, energy nu is required, where n is the number of broken bonds at the surface. After breaking, two new surfaces emerge; consequently, for each broken bond of the new surface, energy u /2 is required. It follows, therefore, that the total energy required to remove one particle from a larger piece of material is , where ns is the number of atoms at the surface of the particle. The number of broken bonds per unit area is used to estimate the contribution γ0 of the broken bonds to the surface energy:

(3.3a)

Within the interior of a particle, an atom or ion is held in a mechanical equilibrium by binding forces, which fix the ions in their lattice positions. These forces are indicated by arrows in Figure 3.4, from which it is clear that those atoms at the surface have lost their bonds to the outside.

Figure 3.4 Forces acting between atoms or ions at lattice positions. Note that atoms at the surface are attracted into the interior of the particle, as they have a reduced number of neighbors. This does not lead to a pressure comparable with a hydrostatic pressure; rather, it leads to stress in the surface (the surface stress).

Due to the reduced number of neighbors, at each surface of the atom, a force f acts perpendicular to the surface. At a plane surface (to be mathematically exact: the surface of a plane infinite half space), this does not cause any hydrostatic pressure in the material, but rather leads to stress in the surface plane; surface stress σ = f /a, where a is the area occupied by one atom of the surface. Consequently, a surface stress that deforms the surface will result in surface stretching, and this allows the surfaces of particles to be modeled as an elastic material skin. According to Gurtin et al. [2,3] and Fischer et al. [4] (this paper provides a broad overview on the problems connected with surface energy), this provides an additional contribution to the surface-free energy as a function of the surface stretching s (much like the stretching of a rubber skin) and the surface stress . Consequently, the surface energy is described by the relationship:

(3.3b)

where γs is the contribution of the surface stress to the surface energy. The surface stress and , the corresponding stretch, are assumed to be constant in any direction of the particle's tangent plane. It follows that:

(3.3c)

In the case of liquids, the second term of Eq. (3.3c) vanishes as . This often raises confusion between and , especially as both have the same dimension. In order to estimate thermal effects, as for example during the coagulation of two particles, the sum value from Eq. (3.3b) must be used. For a spherical particle of limited size and with a radius of curvature r at the surface, the situation is different. Due to the curvature, and in connection with the surface stress, a hydrostatic pressure within the particle, and which is comparable to that stemming from a gas or a liquid at the outside, comes into action. To calculate the hydrostatic pressure caused by surface stress, σ must be applied, the pressure being given by p = 4(σ /d).

Even when the situation at the surface can be described by quite plausible physical and exact mathematical models, the experimental situation is poor. To date, no data have been reported for the surface energy discriminating between γ, γs, and σ, and therefore it is necessary to use published values of the surface energy γ for all applications. Based on the considerations above, it is clear that the determination of surface energy by measuring interface stress is insufficient as these methods deliver only γs, whereas calorimetric measurements (e.g., connected to grain growth) result in a value for . Lastly, only these values are useful for thermodynamic considerations.

A more general situation is depicted in Figure 3.5, where the angle between two planes at the surface is assumed to differ from 90°. It may now also be considered how this configuration influences the surface energy.

Figure 3.5 Angle θ between an arbitrary crystallographic plane and the reference plane must be taken into account when modeling the surface energy.

Figure 3.5 illustrates an additional fact, namely that the energy related to the surface depends on the crystallographic orientation, while the number of broken bonds per surface unit depends on the orientation. In a cubic system, the surface energy related to different crystallographic planes can easily be calculated. If the angle between a reference plane and a second plane is termed θ (see Figure 3.5), then the surface energy of this second plane is given by:

(3.4)

The dependency of the surface energy as a function of the angle θ is displayed in Figure 3.6.

Figure 3.6 Surface energy as a function of the angle θ from a reference plane. As a function of the crystallographic orientation θ, the number of broken bonds per surface unit is different. In a cubic system, the anisotropic surface energy of the different crystallographic planes may be calculated using Eq. (3.4).

In the case of more anisotropic lattices, the relationships are more complicated as directional bonds are also present. In order to minimize the surface energy, these directed bonds raise crystallization in rods or platelets, while surface-active substances can also influence the surface energy. From a technical aspect, this is used in the production of one- or two-dimensional particles such as needles or plates.

In the case of oxides, it is advantageous to examine the surface is greater detail. Depending on the nature of the terminating ion, which, in most cases, is oxygen, termination by hydrogen or a hydroxid ion is also possible; the surface energy of the different crystallographic planes is also changing. Excellent reviews of this subject have been produced by Barnard et al. [5,6]. As the termination changes the surface energy of dissimilar crystallographic planes in different ways, facetted particles appear with crystallographic planes, leading to a minimum surface energy. However, in experimental procedures, small particles are usually spherical (or close to being spherical) due to the vapor pressure, increasing with curvature 1/r (where r is the radius of the edge; see Section 3.3). Therefore, sharp edges or tips, which energetically are unfavorable, are removed by evaporation and condensation processes. However, particles of materials with an extremely low vapor pressure may be facetted, even when produced by high-temperature processes. An example of facetted particles, ceria (CeO2), is shown in Figure 3.7.

Figure 3.7 Facetted ceria (CeO2) nanoparticles. (Reproduced by permission of Nanophase Technologies Inc, Romeoville, IL, USA; www.nanophase.com). Particles of materials with an extremely low vapor pressure may be facetted, even when produced by high-temperature processes.

An example of how surface energy has a major influence on the behavior of particles, in relation to particle synthesis, may be of benefit here, whereby the question might be asked as to what is the consequence of the coagulation of two particles. For reasons of simplicity, it is assumed that both coagulating particles are spherical and equal in size, and that the new particle is also assumed to be spherical. The difference in surface energy per particle between the surface of two particles with the diameter d and that of the coagulated particle with the diameter is:

(3.5)

The reduction in surface energy leads, due to dissipation, to an increase in temperature ΔT:

(3.6)

If zirconia particles are assumed to have density ρ = 5.6 g cm−3, surface energy γ = 1 J m−2 (in the literature there are indications that this value may be significantly larger; however, to avoid exaggerations, this extremely conservative value was selected), and heat capacity Cp = 56.2 J mol−1 K−1, this will cause an increase in temperature during the adiabatic coagulation process (see Figure 3.8). (For reasons of simplicity, the materials' data are those of conventional materials; the surface energy value is roughly approximated.)

Figure 3.8 Temperature flash after adiabatic coagulation of two ZrO2 nanoparticles of equal size.

Based on data in Figure 3.8, it can be seen that via the exchange of surface energy a remarkable temperature flash occurs during the coagulation of two equal-sized particles. It is this temperature flash that makes coagulation possible, as the rise in temperature causes the mobility of the atoms to be increased. The strong decrease in temperature flash with increasing particle size explains the occurrence of odd-shaped particles in the size range above 3 or 4 nm. This situation is not purely theoretical; rather, the coagulation of nanoparticles is a phenomenon that makes the production of small particles difficult as they tend to agglomerate when they come into contact with each other. The process of agglomeration may also be observed in the electron microscope; a series of excellent electron micrographs showing coagulation between two gold particles are shown in Figure 3.9. The sequence starts with two particles, with one oriented such that lattice fringes are visible. The particles are moving, as indicated by the change in the lattice fringes. When the particles touch each other, they rotate until their orientation is equal, at which moment the coagulation begins as the larger particles engulf their smaller counterparts. For such a process to occur, significant thermal mobility of the atoms is essential, while the required energy is provided via a reduction of the surface.

Figure 3.9 Series of electron micrographs depicting the coagulation of two gold particles. The orientation of the lattice fringes changes from frame to frame, indicating movement of the particles. During the process of coagulation, a grain boundary is not formed; rather, the orientation of the two particles is aligned (Jorge A. Ascencio, UNAM Mexico; private communication).