Tailoring of Nanocomposite Dielectrics

From Fundamentals to Devices and Applications

Toshikatsu Tanaka, Alun S. Vaughan, Toshikatsu Tanaka, Alun S. Vaughan

• Publisher: Jenny Stanford Publishing
• Year: 2016
• ISBN:
  9781351782289
• Edition: 1
• Topic: Naturwissenschaften
• Subtopic: Physikalische & theoretische Chemie

This book illustrates interfacial properties, preparation, characterization, devices, and applications from the standpoint of nano-interfacial tailoring. Since the primary focus of the book is on the use of nanocomposite dielectrics in electrical applications, chapters are devoted to directly relevant topics, such as surface and bulk breakdown processes. However, the mechanisms that underpin such behavior are not unique. Therefore, the book also addresses related topics that range from the chemistry of polymer and nanocomposite degradation to the simulation of charge transport dynamics in disordered materials, thereby presenting a multi- and interdisciplinary approach to the area. It will serve as a practical handbook or graduate textbook and is supplemented by ample number of illustrations, case studies, practical examples, and historical perspectives.

Chapter 1

Introduction

Toshikatsu Tanaka

IPS Research Center, Waseda University,

Kitakyushu-shi, Fukuoka 808-0135, Japan

[email protected]

Nanotailoring is the theme of this book. In this way, you can formulate whatever materials you want. Such an era is coming near and polymer nanocomposites are an example of this. By adding a small amount of nanofiller into conventional polymers, you can actively design the very materials you might need for potential application. These materials have ever appeared as advanced dielectric and electrical insulating materials since the year 2000. They include inorganic fillers of nanometer scale and surprisingly are characterized by their interfaces that are internally formed between nanofillers and their surrounding polymermatrices. Such inner-interface dominant materials can exhibit much improved performance compared with neat polymers. Properly tailored interfaces will result in significant improvements in dielectric properties and, consequently, various applications of such materials are under R&D consideration. Indeed, some of these materials have been utilized in reality. This book is intended to give a comprehensive explanation of how we should tailor polymer materials, what characteristics such tailored materials exhibit, how anti-aging they are against ambient conditions and, finally, how they can be used in the electrical power and microelectronics sectors. Many leading international experts participated in contribution to this book. Each of the chapters has a special touch that is characteristic of each author’s talent and experience, but as a whole, the book is rather well balanced. Thereby, it has a strong appeal that will guide you to enjoy and understand such a nanodielectric world.

Tailoring of Nanocomposite Dielectrics: From Fundamentals to Devices and Applications
Edited by Toshikatsu Tanaka and Alun S. Vaughan
Copyright © 2017 Pan Stanford Publishing Pte. Ltd.
ISBN 978-981-4669-80-1 (Hardcover), 978-981-4669-81-8 (eBook)
www.panstanford.com

1.1 Invitation to the Nanodielectric World

Welcome to the nanodielectric world. Nanocomposites are composed of host and guest materials in general. They should be fabricated so that they may be endowed with the superb performance of the guest filler materials, while keeping the original performance of the host materials. In general, inorganic materials exhibit excellent optical, electrical, mechanical and thermal properties, while organic materials are superb in terms of light weight, flexibility, and processability. Such characteristics for both materials can be transferred, complementarily, to nanocomposites. Furthermore, novel performances that neither of them holds by nature will possibly appear in newly fabricated nanocomposites. There are three kinds of combination as for hosts and guests, i.e. inorganic-inorganic, inorganic-organic, and organic-organic composite systems. The materials that attract most attention to-date are nanocomposites that consist of organic polymers as host and inorganic substances as guest. This field was pioneered by the successful invention of polyamide/organic clay nanocomposites in the 1990s. Much attention was directed toward mechanical and optical properties in the beginning. Furthermore, recent investigations have, in addition, been directed to characteristics including gas barrier, lubrication, thermal endurance, heat radiation, electrical conductivity, and electrical insulation, with a view to producing drastic enhancements by controlling material structures in the nanometer scale through self-assembly. The investigation of various polymer nanocomposites has been intensively made concerning permittivity, loss tangent, electrical conductivity, space charge, thermally stimulated current (TSC), dielectric breakdown, treeing breakdown (short time breakdown and treeing V-t characteristics), partial discharge (PD) resistance, electroluminescence and many other relevant characteristics. It is now recognized that all the characteristics indicated above reveal the superiority of nanocomposites over conventional composites; the improvement of PD resistance, the prolongation of treeing lifetime, and the suppression of space charge formation are most prominent among these. New application innovation for power apparatus is certainly expected through the use of polymer nanocomposites.

1.2 Interfaces

Interface science and colloid science have dealt with nanometer size substances long before nanotechnology emerged. Gold sols and other nanomaterials with sizes down to 10 nm were studied in the 1920s, while Langmuir-Blodgett thin films consisting of one or more monolayers of organic material have attracted considered interest since the early 1950s. The first measurement of surface forces was made in the early 1950s; the characteristics of interfaces between mutually immiscible substances in colloids were also clarified. Colloid science reached a significant level of maturity during the second half of the 20th century. It is important to note that in colloid science the forces acting between particles and solvent are well understood for both dilute and condensed phases. Various forces act between colloidal particles and include excluded volume repulsion; electrostatic forces; van der Waals’ force; entropic effects; steric forces. Excluded volume repulsion, sometimes called Born repulsion, is caused by the impossibility of overlap between hard particles. Electrostatic forces surrounding a colloidal particle were analyzed in detail and clarified by Derjaguin, Landau, Verwey, and Overbeek, resulting in the DLVO theory. There is a deep minimum in the potential (attractive force) near the particle surface, a shallow minimum (attractive force) far from the particle surface, with a high maximum (repulsive force) in between. Entropic effects act to increase the entropy until thermal equilibrium is reached. Steric forces arise between polymer-covered surfaces, or in solutions containing non-adsorbing polymer, producing an additional steric repulsive force (which is predominantly conformational and thereby entropic in origin) or an attractive force due to depletion of counter ions. There are also other interfacial phenomena associated with the interaction between particles and their environment. Physical and chemical adsorption is one of them. The former is related to hydrogen, hydrophobic or van der Waals bonding. The latter is caused by chemical bonding between adsorbate and adsorbent. Surface tension appears on a soft surface, generated by the difference between the free energies of the surface and the bulk. Wetting is the contact of a solid surface by a liquid and includes adhesion, spreading and immersion wetting. Knowledge of interfacial phenomena in colloids has proved extremely useful in developing polymer nanocomposites.

Interfaces in dielectric nanocomposites are now discussed in terms of the following subjects:

1. Bound polymers
   • Silane coupling—bonding between organic and inorganic substances
   • Wettability and affinity between inorganic filler and polymer
   • Bound polymer involving covalent bonding
   • Evidence for bound polymers
2. Interface models
   • Wilkes’ model
   • Conceptual illustration of interfaces
   • Evidence for far-distance interaction
   • Charge tails in interface: Stern-Gouy-Chapman counter-charge layer and triboelectricity charge
   • Multicore model
   • Water shell model

We come to the stage at which we can tailor interfaces. Surprisingly we can control more than two characteristics at the same time, and thereby simultaneously improve both dielectric and thermal properties, for instance. Double core shell and core satellite structures are some of the examples that can be tailored by reversible addition-fragmentation chain-transfer (RAFT) and atom-transfer radical-polymerization (ATRP) techniques. Bimodal polymer brush structures can be formed by click chemistry such as “grafting from” and “grafting to” techniques. Then we can control dispersion and dielectric performances separately. In this way, we can seek to create the second generation of nanocomposites.

1.3 Improvement in Materials Performances

As polymer nanocomposites are expected to appear as advanced dielectric materials, significant investigations have been undertaken in connection with both fundamental characteristics and application specific characteristics. These include (i) electrical and dielectric properties, such as dielectric spectroscopy, dc conductivity at low and high electric field, space charge, dielectric breakdown, treeing resistance, partial discharge resistance, tracking resistance, and flame retardancy, and (ii) closely associated physical properties, such as thermal endurance, thermal conductivity, and glass transition temperatures. Overall, better properties have been obtained for polymer nanocomposites than for their base resins and conventional micro filler filled resins. However, the production of materials with excellent properties requires excellent material processing skills. This involves achieving not only a good dispersion with sound binding between the filler particles and the matrices, but also a freedom from inclusion of unwanted impurities. It is not possible to make a comprehensive description of all the data available for polymer nanocomposites. Basic properties have been obtained mainly through dielectric properties and dc conductivity to correlate with interfacial problems that are very important for nanocomposites. From the material application point of view, resistance against partial discharges, treeing lifetime and reduction of space charge formation are outstanding and extremely promising characteristics. Tracking resistance also seems to be positive. There are supportive data available for an increase of dielectric breakdown strength by nanofiller addition. Physical properties such as thermal endurance, thermal conductivity, and glass transition temperatures are used to explain why dielectric and electrical properties change when polymers are nanostructured. Additionally, one kind of polymer nanocomposite such as LDPE/MgO nanocomposite was assessed by cooperative individual groups to demonstrate its feasibility for application.

Table 1.1 Bird's-eye view of the characteristics of polymer nanocomposites

1.4 Expectation to Computer Simulation

Dielectric Breakdown of Solids and Theory of Dielectrics were published by Stanley Whitehead in 1953 and by Herbert Frohlich in 1958, respectively. To our surprise, such theories include quantum mechanics. Evidently, these theoretical treatments were completed in the middle of the 20th century, and for more than half a century since then, engineering problems have been solved on the basis of such approaches, in connection with practical applications, materials aging, etc. In the 1940s during the Second World War, computer technology began to appear, especially to calculate ballistic trajectories and, since then, has rapidly evolved with the development of semiconductor devices and memory technologies in microelectronics. Moore’s law (1965) tells that the number of components per unit area in integrated circuits would double every year and, now, we live in an era in which we cannot survive without computers. In the dielectrics world, on the other hand, computer simulation has only recently emerged, but its role is crucial now.

Although dielectric phenomena have been simulated by a variety of methods, thus far, insufficient reviews and discussions have been made to provide comprehensive understanding. It is now understood from a preliminary survey that simulation works in dielectrics can be represented by the following methods:

1. Ab initio with density-functional theory
2. Molecular dynamics (coarse-grained MD)
3. Monte Carlo simulation
4. Cellar automata with finite element analysis
5. Fractal analysis
6. Stochastic analysis
7. Direct solution from governing formulae

Computer simulation of the mechanical properties of composites is popular. It includes molecular dynamics simulation, Monte Carlo simulation and ab initio computation. It would also be useful for clarifying dynamic behavior such as the formation of interfaces and dispersion of nanoparticles in polymer matrices. Some phenomena and characteristics have been qualitatively explained, but the technique is still maturing in the context of nanocomposites. Molecular dynamics simulation of thermosetting resins incorporating nanofillers has demonstrated several features. First, the distribution of created bonds is changed by the asymmetrical interaction between monomer and surface, and the affinity of the hardener brings about an increase in the number of near-surface bonds. Second, the segmental motion of reacted resins is not affected by the surface affinity in regions far from the surface, but it changes drastically near the surface. It was suggested that the state of interfaces is correlated with the multicore interfacial model. In an ab initio computational study, the theory of local dielectric permittivity was applied in order to understand variations in the static and optical dielectric constant across idealized Si-SiO₂, Si-HfO₂, and SiO₂-polymer interfaces over interatomic distances. These variations have been correlated with the chemistry of the interfaces, e.g., dangling bonds and multiple oxidation states. The layer-decomposed density of states approach has been used to compute the electronic structure variation across Si-HfO₂ and SiO₂-polymer interfaces and to elucidate band bending, band offsets and the creation of trap states at interfaces. Filler dispersion in a polyamide-imide/silica nanocomposite system has been studied by molecular dynamics simulation; adhesive fracture energy at the interfaces between silica and solvents was calculated and Brownian motion of the silica particles was simulated to clarify their dispersion. It was found that the colloidal state of silica is maintained by covering the silica surface with a new low-hygroscopicity solvent and the chemical structure of the polymer influences the dispersion of the silica. It was claimed that the method is useful when attempting to determine the optimum conditions for good filler dispersion in nanocomposite systems.

These techniques are essentially beneficial to the R&D of nanocomposites. This approach is linked with the exploration of nanocomposites with new possible structures that it is rather difficult to obtain experimentally.

1.5 Applications

Some of the possible applications of nanocomposites lie in motor windings (synthetic enamel), generator windings (epoxy), switchgear spacers (epoxy), capacitors (polypropylene), ac and dc cable insulation (polyethylene and cross-linked polyethylene (XLPE)), silicone elastomers for outdoor use, microelectronics PWB (printed wiring board) and packaging (epoxy). Plausible applications of nanocomposites are under R&D in the electric power field. They seem to offer advantages of higher efficiency, longer service life and reduced size of equipment. At present, energy consumption is increasing around the world and global warming is becoming more serious every year. Energy saving and environmental friendliness are international concerns. Nanocomposites have the potential to help solve these problems. When the use of nanotechnology and nanocomposites is widespread, one might reasonably be concerned about potential nanohazards in work places and in homes. Important as it is, that topic is beyond the scope of this book.