The following is an edited version of the David Turnbull Lectureship address, given by recipient Didier R. de Fontaine at the 1995 MRS Fall Meeting, De Fontaine received the lectureship “in recognition of fundamental contributions and insights in the fields of order/disorder phenomena in materials and computational techniques for phase diagrams.” De Fontaine is a professor in the Department of Materials Science and Mineral Engineering at the University of California—Berkeley and holds a joint appointment with the Lawrence Berkeley National Laboratory.

The materials used in magnetic recording heads have recently received a tremendous amount of attention. This has been the result of a fortunate set of circumstances. Ever-increasing demands for information storage, especially for graphics-intensive applications, have necessitated unprecedented increases in disk-drive areal densities. Combined with this are recent discoveries in the area of magnetoresistive materials, enabling the design and fabrication of much more sensitive recording heads. The end result is a flurry of activity that has come to dominate the field of magnetics. This article will explore choices for magnetoresistive read head materials, with an emphasis on the materials challenges.

The recording heads that are used in high-performance disk drives typically consist of separate magnetoresistive read and inductive write heads (see Figure 1) where previously a single inductive head performed both functions. Separation of the two heads allows each to be optimized for their individual function, an essential factor in enabling disk drives to contain gigabytes of storage. The write head is the simpler of the two, consisting of a U-shaped ferromagnet surrounding a set of coils. The ends of the ferromagnet are the magnetic poles defining the write gap. When current passes through the coils, a field bridges the gap, setting the orientation of the magnetization in the media. Information is stored by changing the polarity of the current in order to write a pattern of magnetic domains in the media. The materials used in write poles will be reviewed in the section, Write Head Materials.

One material exists in most types of flat panel displays that are used in high information content applications, from liquid crystal displays to electroluminescent displays. This material is glass, something we all use daily. As a substrate, glass plays a significant role in the manufacture and function of flat panel displays (FPDs). In the case of active-matrix liquid crystal displays (AMLCDs), the semiconductor-based thin-film transistors (TFTs) or diodes are fabricated on the active glass substrate. In addition, AMLCDs require color-filter structures consisting of dyed or pigmented resins built upon the passive plate of an AMLCD cell. For plasma and electroluminescent display panels, the glass provides the surface upon which are deposited phosphors, conductive lines, and dielectrics.

The excellent properties of glass make it the substrate of choice in FPDs. It is the only material that can meet the demanding requirements of high-information-content FPD manufacturing processes and operation. Glass' transparency is an obvious requirement. For all types of displays, the rigidity of glass also plays a important role because it adds overall mechanical integrity and strength.

One of the most important attributes of glass in terms of the display manufacturing process is its thermal stability. Glass substrates, particularly those made of “hard” or high temperature glasses, can be processed at elevated temperatures with minimal deformation. In addition, the dimensional precision required in the alignment of various display components is maintained throughout the specific manufacturing processes.

Just what is an intermetallic compound? An intermetallic compound is a true compound of two or more metals that has a distinctive structure in which the metallic constituents are in relatively fixed abundance ratios and are usually ordered on two or more sublattices, each with its own distinct population of atoms. Often substantial or complete disorder may be obtained as a result of a low ordering energy or the intervention of some external agency—for example, extreme cooling rates or radiation. Deviations from precise stoichiometry are common on one or both sides of the nominal atomic ratio, necessitating some misordering and/or the introduction of vacancies. Usually metal-metalloid compounds such as silicides, arsenides, or tellurides are also included in the category since the phenomenology of most such compounds is similar to that of metal-metal compounds. More than 25,000 distinct intermetallics are now known.

To many, intermetallic compounds (IMCs) are classed as “exotics” or “advanced materials.” Such categorization should apply only to the recent interest in their potential use as monolithic high-temperature structural materials. Actually IMCs have been used for thousands of years in an astonishing range of applications exploiting not only their mechanical properties but their chemical, electrical, magnetic, optical, and semiconducting properties as well. Most readers will be surprised to learn that they carry within their own bodies (dental fillings) or on their persons (jewelry or pocket lighters) IMCs for which selection and processing are uniquely appropriate to those applications. The economic impact of IMCs is as unappreciated as the diversity of their applications. Some are regularly produced in quantities of 1,000s of tons per year while others, although not in tonnage production, are key factors in devices with billion-dollar markets.

Computer modeling of crystal defects ranges at present from generic empirical investigations to first-principle quantum-mechanical calculations (see, for example, References 1–4). Descriptions of atomic interactions in terms of pair potentials dominated such studies until the early 1980s, and many fundamental features of lattice defects and interfaces were revealed in these calculations. The generic results of these studies withstood the test of time, and calculations employing more sophisticated schemes usually confirmed their validity. An early example goes back to the late 1950s when Vineyard and co-workers pioneered the very first computer simulations in their studies of radiation damage. Empirical pair potentials were used in these investigations in which many fundamental, generic aspects of the effect of irradiation of crystalline materials by energetic particles were discovered.

Such simple treatments of atomic interactions may appear totally inadequate from the point of view of pure physics. However, it must be recognized that the purpose of the majority of atomistic studies of lattice defects has been to elucidate atomic structure and atomic-level properties in materials with given: (a) crystal structure, (b) elastic properties and possibly phonon spectra, (c) values of certain material parameters such as vacancy formation energies and stacking fault energies, and (d) in alloys, alloying and ordering energies, and possibly antiphase boundary energies. This is in contrast with ab initio studies, the objective of which is to determine all these properties from first principles. These goals of atomistic studies are, of course, the same for all semi-empirical approaches discussed in this collection of articles. In general, the validity of the structural features of lattice defects found in calculations using empirical schemes is best guaranteed if they can be related to fitted material properties and are not sensitively dependent on the deails of the fittings and functional forms employed.

Ferroelectric thin films have experienced tremendous development in recent years due to the advent of sophisticated film-synthesis techniques and an increase in the understanding of the related materials science and integration of films into various novel devices. Major advances in ferroelectric thin-film synthesis and characterization were addressed last month in the June MRS Bulletin. This volume is dedicated to selective device issues and applications.

Ferroelectric materials can be used to fabricate capacitors for integration into nonvolatile ferroelectric random-access memories (NVFRAMs) and as high di-electric layers for capacitors with high capacitance for manufacturing planar dynamic random-access memories (DRAMs). Ceramic conductors can be applied to ohmic, voltage-dependent, and thermally sensitive resistors; fast-ion conductors; and humidity and gas sensors. Piezoelectricity is being exploited in micromachines such as accelerometers, displacement transducers, and actuators such as those required for inkjet printers, for video-recording head positioning, and for micromachining. Pyroelectricity can be utilized in the fabrication of high-sensitivity room-temperature infrared detectors while electro-optic activity can be used in color-filter devices, displays, image-storage systems, and optical switches for integrated optical systems. These applications of electroceramic thin films are only part of a more extensive list.

What we see when we look at a painting—the image and our belief in it—is the result of choices in painting media, physical components, and methods made by the painter prior to and during the painting process. To a great extent, the number of choices, the way they are made, and the sequence in which they are used are influenced by the environment of the artist. Availability of materials, training, common practice, the demand of imagery, scale, and placement of the completed painting all influence these choices. We certainly cannot discount skill and invention as contributing factors in the making of the object for they guide the use of materials, and when the artist is truly inspired, allow us to be transported into the life of the painting.

The success of tissue engineering rests on the ability to direct specific cell types to multiply, migrate, and express normal physiologic behaviors in order to yield a cellular organization that performs the functions of the desired tissue. For example the engineering of living bone to repair skeletal defects has focused on growing osteoblasts—the cells responsible for bone formation—on degradable polymer matrices in vitro. The polymer matrix initially serves as the scaffold for bone-cell proliferation and maturation. Ideally the cells form a bonelike tissue that after implantation is fully integrated into the patient's own bone, thus repairing the bone injury or defect. Soon thereafter, its function complete, the polymer scaffold resorbs away. Readily apparent is the crucial role the scaffold material occupies in tissue engineering since it serves as the template for cell growth and tissue formation. It is the interaction between the cell and the material that dictates whether the cells will proliferate, mature, and express the desired tissue characteristics.

A critical issue facing the biomedical industry today is the availability of raw materials for medical-device manufacture. Furthermore it is now recognized that the materials base of the medical-device industry is outdated. Metals and various industrial plastics (e.g., polysiloxanes, polyurethanes, Dacron®, Teflon®, polyethylene) are the most commonly used biomaterials. These biostable, synthetic implant materials lack the biological sequences and patterns crucial to normal cell function and can trigger aberrant cell responses. Likewise few degradable polymers are available to the medical-device designer and tissue engineer, representing another limitation of the materials base of the medical-device industry (Table I).

In today's world of increasing office automation and computer-aided personal-communications systems, display devices play a very important role as person-machine interfaces. Above all, high-definition, full-color flat-panel displays will be key devices in the near future when processing huge amounts of information—including pictorial images via computer networks and telecommunication systems that transcend the present limitations of time and place—will be possible.

Passive-matrix liquid-crystal displays (LCDs) represent the most widely used choice for portable display devices. Figure 1 illustrates the essential components and operating principle of a typical LCD. Each pixel is addressed by the top- and bottom-line electrodes of the cell based on information signals, producing a light image. By installing a color filter of red, green, or blue for each pixel, full-color images can be displayed. However, the essential problems of crosstalk among pixels and low response speed become serious with an increase in the number of pixels, resulting in a low contrast ratio and failure of the display to keep up with the signals.

A plasma, commonly referred to as the “fourth state of matter,” is an ensemble of randomly moving charged particles with a sufficient particle density to remain, on average, electrically neutral. While their scientific study dates from the 19th century, plasmas are ubiquitous, comprising more than 99% of the known material universe. The term “plasma” was first coined in the 1920s by Irving Langmuir at the General Electric Company after the vague resemblance of a filamented glow discharge to a biological plasma.

Plasmas are studied for many reasons. Physicists analyze the collective dynamics of ions and electron ensembles, utilizing principals of classical electromagnetics, and fluid and statistical mechanics, to better understand astrophysical, solar, and ionospheric phenomenon, and in applied problems such as thermonuclear fusion. Electrical engineers use plasmas to develop efficient lighting, and high-power electrical switchgear, and for magneto-hydrodynamic (MHD) power conversion. Aerospace engineers apply plasmas for attitude adjustment and electric propulsion of satellites. Chemists, chemical engineers, and materials scientists routinely use plasmas in reactive ion etching and sputter deposition. These methods are commonplace in microelec tronics since they allow synthesis of complex material structures with submicron feature sizes. A substantial portion of the multi-billion-dollar market for tooling used to manufacture semiconductors employs some form of plasma process. When compared with traditional wet-chemistry techniques, these dry processes result in minimal waste generation. Plasmas are also useful in bulk processing—for example as thermal sprays for melting materials.

While the quest for controlled thermonuclear fusion dominated much of plasma research in the 1960s and 1970s, in the last 20 years it has been the application of plasmas to materials processing that has provided new challenges for many plasma practitioners. It is not surprising that the guest editors and several of the authors for this issue of MRS Bulletin come from a fusion plasma-physics background.

One of the greatest challenges currently facing the materials community is the need to develop a new generation of materials to replace Ni-based superalloys in the hot sections of gas-turbine engines for aircraft-propulsion systems. The present alloys, which have a Ni-based solid-solution matrix surrounding Ni3Al-based precipitates, are currently used at temperatures exceeding 1100°C, which is over 80% of the absolute melting temperature. Since Ni3Al melts at 1395°C and Ni at 1453°C, it is clear that significantly higher operating temperatures, with the attendant improvements in efficiency and thrust-to-weight ratio, can only be attained by the development of an entirely new materials system. This problem is a primary reason for the current high level of interest in high-temperature intermetallic compounds.

The development of such a material system has important implications for national defense and for spin-offs to civilian technology, as well as for the economy and balance of payments. Obviously it would be a boon to any economy to have these new materials developed domestically, as was the case in the United States for the currently used single-crystal technology applied to Ni-based superalloys. As an example, the aerospace industry is one area where the United States is still the undisputed world leader, with net exports of $29 billion in 1989, twice that of any other U.S. industry.

Strain relaxation in lattice-mismatched, heteroepitaxial systems is one of the classic problems in materials physics, which has gained new urgency with the increased applications of strained layers in microelectronic systems. In general both the structure and the integrity of the thin films are strongly influenced by strain. For instance it has long been known that under strain, the growth changes from an initial layer-by-layer growth mode to one with three-dimensional islanding. In the seminal works of van der Merwe, and Matthews and Blakeslee, this change in growth mode is explained in terms of the introduction of strain-relieving misfit dislocations, which appear when the film has reached some critical thickness. Recently it has become clear that this change in growth mode can take place even without the introduction of misfit dislocations. Such dislocation-free coherent islanding, or “roughening,” has been observed experimentally both in Ge/Si and in InGaAs/GaAs systems. Furthermore recent experiments show that in Ge/Si(100) systems, the thin films display a curious asymmetry with respect to the sign of the strain: Films under compression roughen by forming coherent islands while those under tension remain relatively smooth. A possible mechanism behind this strain-induced type of roughening is the subject of this article.

The adhesion of a polymer to itself, to a different polymer, or to an inorganic material can often be one of its most important properties. Adhesives are normally polymers, but then so are nonstick coatings. Clearly, polymer adhesion can vary from very strong to extremely weak. Adhesion is a sufficiently complex and broad area that there are a range of approaches to its understanding. My approach here is one that attempts to bridge between the mechanics of crack propagation and the atomic-scale processes that occur at a crack tip. I will not be concerned with the details of the interface chemistry and its modification. The interface chemical approach is very useful in many practical situations but is often very specific to particular systems. Adhesion (or perhaps its inverse) can be considered a failure property. We can only measure the adhesion between two items by breaking the joint between them. I will be mainly concerned with failure by crack propagation using a fracture-mechanics approach.

I will first consider the general question of the effects of bulk mechanical properties on adhesion and then describe some recent work on fracture mechanics of biomaterial interfaces as it relates to polymer glasses. I will then change the focus to the scale of the polymer chain to consider local processes involved in adhesion.

The continued exponential growth in areal density for longitudinal magnetic-recording devices places ever more stringent demands on disk performance. The design of materials and processes must provide the required advances in technology. The magnetic properties are controlled through the choice of underlayers, magnetic alloys, and the deposition processes that control crystallographic orientation and magnetic isolation between grains. The requirement of lower head-disk spacing places increasing stress on the tribological performance of the disks, controlled by a very thin overcoat and lubricant layer. This article reviews the various materials issues relevant to magnetic-disk technology.

The major obstacle for achieving high areal density in thin-film media is transition noise. This noise arises from the zig-zag transition boundaries that occur due to cooperative switching of the magnetic grains. Both exchange coupling between grains and magnetostatic interactions cause magnetic-cluster sizes larger than the grain size. The goal is to magnetically isolate the grains and keep the grain size small. As the dimensions of the bit cell shrink, smaller grain size is required to obtain enough grains per bit cell to maintain the required signal-to-noise ratio (SNR). In the 10–40-Gbits/in.2 areal-density range, the issue of thermal stability of small (<10 nm), isolated grains needs to be addressed.

In addition to good SNR, a narrow transition width is needed in order to pack the transitions closer together. The objective is to minimize interactions between transitions to reduce nonlinear amplitude loss and superlinear noise.

Ultrasonic waves have been used extensively for material characterization and for sensing in material process control. The waves produce small-amplitude mechanical vibrations and, depending on the mode being used, may induce both longitudinal and shear stresses in the solid. Information on the structural properties of a substance can be obtained by measuring both the velocity and the attenuation of the ultrasonic wave. The phase velocity of the wave depends on the elastic constants and density of the body while attenuation depends on microstructure and crystalline defects.

In an isotropic solid medium, which has only two independent elastic moduli, there exist two elastic waves: the longitudinal and the shear. Three kinds of bulk elastic waves may propagate in an anisotropic solid: a quasilongitudinal and two quasitransverse waves, differing in polarization and velocity. To determine the set of elastic constants, one must measure the phase velocity in several different directions relative to the crystallographic axes.

The attenuation of an ultrasonic wave is associated with absorption of elastic waves (inelastic effect) and the scattering of elastic waves by structural inhomogeneities. Scattering may be the governing attenuation mechanism in polycrystalline, composite, and ceramic materials. As a result of scattering, elastic energy is lost by the prime ultrasonic beam in the form of a stochastically scattered field, which is gradually absorbed in the material. The latter is associated with conversion of elastic into thermal energy as a result of various inelastic effects termed internal friction.

The strain-induced transition of a planar film to a three-dimensional island morphology is presently a significant issue in the growth of semiconductor thin films. Strain-induced roughening can be problematic in the fabrication of coherently strained device structures where it is important to understand the early stages of the transition to avoid or suppress three-dimensional (3D) growth. On the other hand, the strain-driven transition is beneficial for the self-assembly of quantum dots where it is necessary to control the size distribution and self-organizing behavior of the islands. In both cases, it is clearly important to identify and understand the kinetic pathways to island formation. From a more basic perspective, the strain-induced transition of epitaxial films allows us to study in detail the interplay between elastic stresses and surface energy in a carefully controlled experimental environment. One would therefore hope that the lessons learned from model semiconductor systems will be of relevance to understanding related phenomena in other areas of materials science and physical metallurgy.

Ihe strain-induced two-dimensional (2D)-to-3D transition in the Si-Ge system is manifested by a rich variety of observed surface morphologies. In the case of pure Ge on Si(OO1), the 4% misfit strain induces the formation of so-called hut clusters with curious elongated shapes. Such islands form almost immediately after the deposition of a wetting layer. In the case of lower misfit alloys, a more gentle ripple morphology can result that develops far from the interface. A general trend in all of the experiments is the decreasing size of typical morphological features with increasing misfit stress. In this article, largely guided by our experimental results, we adopt a nucleation and growth description of the 2D-to-3D transition. This approach appears particularly well-suited to explaining the wide spectrum of morphological development present in the Si-Ge system.

This article discusses simple but realistic models of the bonding in metallic systems. As is discussed in the introduction, such methods are valuable both to study complex systems that are intractable with more rigorous methods and to study generic properties that do not depend on the fine details of the energetics. In addition, simple models are useful for gaining a physical understanding of a system.

As discussed in the article by Vitek, a great deal of progress in the understanding of defect structures has been gained by the use of constant-volume pair potentials, that is, pair-potential models that include a volume-dependent term. In this picture, the energy is assumed to have two parts, a large density-dependent but structure-independent part and a structure-dependent part that is represented by pair interactions. This view follows from a physical picture where the metal is viewed as a uniform electron gas and the interactions between the atoms are obtained by performing perturbation theory on this reference system. This approach has a serious limitation, though, in that it is restricted to situations where the system is essentially uniform such as when the bulk or defects do not introduce significant changes in the local density. This is true for two reasons. First, there is not a clear prescription for how the structure-independent part of the energy should be treated in an inhomogeneous region. This is a serious problem since a large part of the binding energy is included in this term. Second, the pair-potential term in this picture depends on the overall density. In the vicinity of a inhomogeneity such as a surface, there is no prescription for how the pair-potential term should vary.

Acoustic microscopy is useful for characterizing with high spatial resolution the elastic structure and properties of an object. A range of techniques is now available for doing this, which enables the user to select the method and instrument that is most appropriate for a particular requirement. For imaging the interior of structures such as electronic-component packaging, an acoustic microscope operating at a relatively modest frequency can provide advanced nondestructive testing. For characterizing surface coatings and layers that may be only a fraction of a micrometer thick, higher frequency quantitative techniques are needed. For a given application, three questions should be asked at the outset: (1) What depth of material do I wish to include in my inspection? (2) Do I wish to image structures and/or defects, or do I wish to characterize elastic properties? (3) What is the minimum size of a defect or inhomogeneity that I wish to resolve or characterize (at a given depth) during my inspection? Selection of the appropriate technique will depend on the answers.

Tissue engineering offers the possibility to create completely natural tissue and replace failing or malfunctioning organs. In many cases, biocompatible, biodegradable polymers are utilized to either induce surrounding tissue and cell ingrowth or to serve as a temporary scaffold for transplanted cells to attach, grow, and maintain differentiated functions. Various processing techniques have therefore been developed to fabricate polymers with specific properties to meet the needs of a particular organ.

Polymer scaffolds must possess unique physical and chemical properties for specific applications and must satisfy some basic requirements for tissue engineering. These scaffolds may be implanted without cells, and the regeneration depends on ingrowth of surrounding tissue to such materials—a process known as tissue induction. Alternatively cells may be seeded into a porous polymer. The cell-polymer construct is then transplanted. In either case, one essential criterion for the scaffold is biocompatibility—that is, the polymer scaffolds and their degradation products should not invoke an adverse immune response or toxicity.

Because of the problems associated with long-term implants, such as infection, fibrous tissue formation, and the possible need for retrieval, the role of polymer scaffolds should only be a temporary one. The degradation rate is optimized to allow transplanted cells to proliferate and secrete their own extracellular matrix (ECM) while polymer scaffolds can vanish when necessary to leave enough space for new tissue growth. Biodegradability of a polymer is determined by its composition, molecular weight (MW), MW distribution, degree of crystallinity, and environmental conditions such as temperature and pH. Mechanical loading of the scaffold may affect its degradation.