Background

Polymer electrolytes are the newest area of solid ionics to receive wide attention for applications in electrochemical devices such as batteries and electrochromic windows. Unlike inorganic glass or ceramic electrolytes, the polymer electrolytes are compliant and this property makes it possible to construct solid state batteries in which the polymer conforms to the volume changes of the electrodes that typically occur during discharge and charging cycles. The potential utility of polymer electrolytes has stimulated the synthesis of new polymer electrolytes, physical studies of their structure and charge transport, and theoretical modelling of the charge transport processes. The rapid progress in this field has led to many reviews (Armand, 1986; MacCallum and Vincent, 1987, 1989; Ratner and Shriver, 1988; Vincent, 1989; Tonge and Shriver, 1989; Cowie and Cree, 1989; Bruce and Vincent, 1993; Linford, 1987, 1990).

The structures and charge transport mechanisms for polymer electrolytes differ greatly from those of inorganic solid electrolytes, therefore the purpose of this chapter is to describe the general nature of polymer electrolytes. We shall see that most of the research on new polymer electrolytes has been guided by the principle that ion transport is strongly dependent on local motion of the polymer (segmental motion) in the vicinity of the ion.

Significant advances in our understanding of solid state electrochemistry have taken place over the last 30 years. The subject has grown rapidly and is, as illustrated in Chapter 1, much closer to the more established electrochemistry in liquid electrolytes than at any time since Faraday's pioneering work in both fields some 150 years ago. Although several books have appeared dealing with ionically conducting materials from a structural point of view, very few texts are available which describe the physical electrochemistry of solids. The present textbook aims to present the fundamentals of solid state electrochemistry with a strong emphasis on the physical aspects. It is directed primarily towards postgraduate students and other scientists and engineers entering the field for the first time, as well as those active in the areas of batteries, fuel cells, sensors and electrochromic devices, topics for which solid state electrochemistry makes a major contribution. Although the reader requires little prior knowledge of solid state electrochemistry, the subject is treated at a relatively advanced level and therefore significant sections of the book should be of interest to all electrochemists, as well as those already active in the electrochemistry of solids. The structural and material aspects of the subject are not ignored, indeed some knowledge of the most important solid electrolytes and intercalation electrodes is an essential foundation on which to build an understanding of the physical properties.

Excellent reviews of various aspects of ferrites (Smit & Wijn, 1961; Smit, 1971; Wohlfarth, 1980, 1982; Goldman, 1990) and magnetism (Cullity, 1972; Jiles, 1991) have been published; however, the need for a monograph covering recent developments on ferrites, such as new preparation technologies, the now-established correlations between polycrystalline structure and magnetic properties, and the most recent applications in magnetic recording, at an introductory level, was felt. The aim of this book is to satisfy that need.

The book corresponds to a final-year undergraduate or graduate level in solid-state chemistry, solid-state physics, or materials science, but is expected to be useful also for the researcher specialising in other areas who is interested in a basic overview of magnetic ceramics. It is hoped, additionally, that it will be helpful for the development engineer concerned with the science of ferrites which underpins their applications.

The book was conceived to cover all the important aspects of ferrites. The different crystalline structures are described in Chapter 2, with brief discussions on cation site occupancy and the basic features of the polycrystalline state. Chapter 3 is devoted to preparation methods for powders, ceramics, thin films and single crystals, including classical solid-state sintering as well as new techniques involving the manipulation of small particles. Magnetic properties are discussed in Chapter 4; the approach is developed from the microscopic scale (magnetic moments localised on atoms) to the macroscopic level (domains, granular and shape effects). Electrical and magnetooptic properties are also briefly discussed.

We are surrounded by magnetic materials playing a crucial role in many devices of every-day life: ac and dc motors which perform many operations (a top-of-the-range car has more than 20 dc motors); power distribution systems, based on power transformers, which deliver energy for home and industrial use; video and audio applications (tapes, writing/reading heads) which provide information and entertainment on a massive scale; telephone and telecommunication systems (microwave devices) which link continents at nearly the speed of light; data storage systems (discs, disc drives) which pervade virtually every human activity.

The history of magnetic materials is as old as that of man; the strange properties of magnetite (a magnetic ceramic!) have been linked to the military successes of an ancient Chinese Emperor (Huang-Ti, 2600 BC). The word magnet is derived from a Greek word used to indicate magnetite deposits in the district of Magnesia. The first study of magnetism is the book De Magnete, by W. Gilbert, published in 1600. The next significant development in magnetism occurred in 1825, when H. C. Oersted reported the crucial fact that magnetic fields can be produced by means of electrical currents. This discovery opened the way to the first applications of magnetism. Discoveries, models and theories have developed since at an increasingly accelerated pace. An amazing fact is that all the impressive variety of magnetic materials and their properties originate mainly from the three elements that are ferromagnetic at room temperature: iron, cobalt and nickel.

In order to complete this review, a brief overview of magnetic materials other than oxides is presented in this chapter. Soft and hard metallic alloys are discussed first; instead of a detailed account of the numerous alloy systems, this overview focuses on the mechanisms for obtaining specific microstructures, which, in turn, lead to a precise control of anisotropy in soft materials, and to coercivity in hard materials. These discussions include examples of ‘classic’ alloys, as well as the recently developed soft amorphous alloys and the impressive supermagnets with extremely high coercive fields.

Due to their impact on information storage technology, a brief account of materials for magnetic recording media and heads is presented. Materials for magnetooptical recording are also discussed.

The next section deals with magnetic materials and applications that are only now emerging, but which arouse a strong and exciting interest, i.e. molecular magnets, magnetic imaging techniques and non-destructive evaluation by magnetic methods.

The last section is a short review of superconductors; their amazing magnetic properties, as well as two of their present applications are discussed finally. In each section, an effort has been made to indicate new directions in magnetic materials research.

Soft magnetic materials

The basic magnetic properties expected in soft magnets are high initial and maximum permeabilities, μi and μmax, and low coercivity, Hc (lower than ∼ 10 A/m). In many applications, soft magnets are subjected to ac fields; their frequency response and, particularly, their ac losses are therefore important.

Information on orientational relaxation may be obtained by a wide range of techniques. Dielectric relaxation and magnetic resonance, neutron and light scattering, infrared spectroscopy and fluorescence depolarization are widely used. These different experimental probes of the phenomenon characterize it in different ways. The advantage of spectroscopic investigations is that they give information on relaxation times as well as on the corresponding correlation functions and their spectra. In particular, by combining the information in an absorption spectrum with that obtained from Raman scattering, one can determine the two lowest correlation functions of a molecule's axis position. A complete description of orientational relaxation is given by the infinite set of these functions.

The orientation of linear rotators in space is defined by a single vector directed along a molecular axis. The orientation of this vector and the angular momentum may be specified within the limits set by the uncertainty relation. In a rarefied gas angular momentum is well conserved at least during the free path. In a dense liquid it is a molecule's orientation that is kept fixed to a first approximation. Since collisions in dense gas and liquid change the direction and rate of rotation too often, the rotation turns into a process of small random walks of the molecular axis. Consequently, reorientation of molecules in a liquid may be considered as diffusion of the symmetry axis in angular space, as was first done by Debye.