Showing papers in "Annual Review of Materials Science in 1988"

Shape Memory Alloys

Abstract: In some alloys, a given plastic strain recovers completely when the con­ cerned alloy is heated above a certain temperature. This phenomenon, shape memory effect (SME), was observed in Au-Cd (1) and In-Tl (2) alloys in the first half of 1950s. However, SME was not a focus of research until it was found in a Ti-Ni alloy (3) in 1963, when the phenomenon was first termed the shape memory effect. A similar phenomenon was found in a Cu-AI-Ni alloy as well (3a). At that time, however, SME was considered to be a peculiar phenomenon limited to the specific Ti-Ni alloy. In 1970, Otsuka & Shimizu (4, 4a) unambiguously demonstrated a one­ to-one correspondence between SME and the thermoelastic martensitic transformation in a Cu-AI-Ni alloy. Thus, they concluded that SME is characteristic of alloys exhibiting thermoelastic martensitic trans­ formations. They ascribed the origin to the crystallographic reversibility of the thermoelastic transformation and the presence of a recoverable deformation mode, i.e. twinning, in thermoelastic alloys. Since then, there

Aluminum-lithium alloys

Abstract: The importance attached to the development of aluminum-lithium based alloys may be deduced from the fact that over the past seven years four major international conferences (1-4) have been devoted to these materials: The first was held at Stone Mountain, Georgia, in 1980 (1), and the most recent was in Paris in 1987 (4). This rapid sequence of conferences reflects intense research and devel­ opment activity within the laboratories of the aerospace companies, the aluminum companies, many universities, research institutes, and govern­ ment research establishments. One reason for this activity was the need for more fuel efficient aircraft because of escalating fuel prices during the 1970s: the addition of lithium to aluminum simultaneously reduces the density and increases the elastic modulus of the resultant alloy. A second and continuing impetus for the development of these materials comes from the requirements of military aircraft.

Hydrogen in Crystalline Semiconductors

Abstract: The study of the influence of hydrogen on the properties and behavior of materials has been restricted to the case of metallic materials for a long time. In these materials, hydrogen, easily introduced from the surface (for example by corrosion), diffuses rapidly and interacts very strongly with all defects. This leads to a trapping phenomena and often has a deleterious effect on the material's properties (for example embrittlement). More recently, the decisive improvement of the electronic properties of amorphous silicon alloyed with hydrogen has promoted wide research on the behavior of hydrogen in amorphous semiconductors. The problem of hydrogen in crystalline semiconductors is of rapidly growing interest because it is a very rich source of fundamental investigation and poten­ tial applications. As in metals, hydrogen interacts strongly with most defects in semiconductors. Passivation of point and extended defects, neutralization of dopants, and trapping phenomena are examples of these interactions. More than 900 research articles on the subject of hydrogen in crystalline semiconductors have been published during the past six years. A recent review article (1) from Pearton et al provided a large comprehensive description of the most significant results achieved in this field. The aim of this article is to propose a didactic description of the present knowledge of the parameters and mechanisms that govern the behavior of hydrogen in common crystalline semiconductors. We present, in the first section, the physicochemical point of view of hydrogen incorporation and analysis techniques. In the second section we describe the solubility and diffusion mechanisms proposed for hydrogen in crystalline semiconductors. In the

Joining of Ceramics and Metals

Abstract: In recent years, there has been a considerable increase in the potential and actual uses of certain ceramics for structural applications in which strength is the basic requirement. Joining technology is critical for such applications of ceramics because of the physical and economic limitations of production of large and complex-shaped components with ceramics alone. A very important aspect of ceramic/metal joining is that it can increase the reliability of ceramics. Research on joining ceramics to metals for electrical and engineering applications has been performed for more than a decade, and a number of methods have been developed. Table I presents a brief comparison of those joining methods and gives a general trend but not a precise compari­ son. Mechanical joining, adhesives with organics, and cements have been widely used because of ease and inexpensiveness. Mechanical joining includes a variety of processes. Bolting and clamping are the simplest methods. The shrunk-in inserts were applied for part of the production of

Fast diffusion in semiconductors

Abstract: Elements diffusing in semiconductors are frequently classified as "slow" or "fast" diffusers (1). Slow diffusers have diffusion coefficients reasonably close to those of self-diffusion, whereas fast diffusers have diffusion coefficients that are many orders of magnitude larger than those of slow diffusers for a given temperature. In Figure 1 this classification is exem­ plified for the diffusion of various elements in crystalline silicon. The large difference in the diffusion rates of fast and slow diffusers derives mainly from their different diffusion mechanisms, which in turn are closely related to their incorporation in the semiconductor lattice. Slow diffusers, such as the common group III and group V dopants, are substitutionally dissolved and require intrinsic point defects (vacancies and/or self-interstitials) for their diffusion process, whereas fast diffusers, such as Cu, Li, H, or Fe, are predominantly interstitially dissolved and move by jumping from inter­ stitial site to interstitial site (Figure 2) without requiring the presence of intrinsic point defects. Since most interstitially dissolved elements are only weakly bound to the lattice, the jump process itself does not involve breaking bonds as in the case of substitutionally dissolved elements. There are also elements with diffusion coefficients between the extreme fast and slow diffusers, such as oxygen or gold in silicon. Oxygen is interstitially dissolved, but forms fairly strong bonds with the two neighboring silicon atoms. These bonds are broken during the jump from interstitial site to interstitial site (Figure 3), which slows down the diffusion process (24). Gold in silicon is an example of an element that is predominantly substitutionally dissolved but diffuses interstitially (5-6). The cor­ responding substitutional-interstitial diffusion mechanism (7, 8) also plays

Recent Advances in Solid-State NMR of Zeolites

Abstract: Zeolites have found widespread application as catalysts in a variety of important reactions, as sorbents and ion exchangers. While single crystals of natural minerals readily lend themselves to conventional methods of structural investigation, synthetic zeolites until very recently were studied by the less powerful powder x-ray diffractometry, which required the support of indirect methods. The development of high-resolution solid­ state NMR techniques, such as magic-angle spinning (MAS) gave zeolite chemistry a new and powerful structural tool. Zeolites are an attractive subject to an NMR spectroscopist since virtually every chemical element they contain can be profitably studied by NMR. The scope of this survey does not permit a comprehensive discussion of the more than a thousand papers published on the subject, most during the last five years, especially since an earlier review (I) and a recent book (2) are available. Therefore, firstly I consider only high-resolution studies; secondly, I provide the fundamentals of solid-state NMR of zeolites; thirdly, I place particular stress on recent work. My principal aim is to provide an up-to-date survey of the most important papers in the field published in the last three years. Since 1979, 29Si, 27Al, and 170 have been observed in the zeoli tic frame­ work using MAS NMR. In particular, 29Si studies have provided many new insights into the structure and chemistry of zeolites. Signals originating from crystallographically nonequivalent silicon atoms can now be resolved and related to structural parameters. MAS studies of protons in zeolites have led to considerable progress in understanding the nature of catalytic

Thermal Surface Modification by Electron Beam High-Speed Scanning

Abstract: Summary Highly productive techniques of thermal surface modification require the use of areal temperature fields that are restricted to a thin surface layer They can be generated by means of an electron beam scan with deflection speeds in a range of a few kilometers per second or scan frequencies of about 100 kHz A raster scan which allows an isothermal energy transfer to moving workpiece surfaces was developed for surface hardening Typical hardness depths are of the order of 05 – 12 mm At a beam power of 10 kW the areal throughout is around 10 cm2 s−1 This new technique is used successfully by some enterprises in the GDR Apart from the qualitative advantages, it is the financial advantages which determine its industrial application

Semimagnetic and Magnetic Semiconductor Superlattices

Abstract: The recent success in the growth of the II-VI class of semiconducting compounds by molecular beam epitaxy (MBE) has resulted in a sharp increase of interest in these materials and in their potential use. In addition, integration of several device functions onto a common substrate material (for example CdTe on GaAs or ZnSe on GaAs) is an area of focus for the optoelectronic community. One of the more important areas for appli­ cation of high quality II-VI films is infrared imaging where CdTe deposited by MBE onto GaAs substrates is proposed as the substrate for subsequent (Rg, Cd)Te and RgTe/CdTe superlattice deposition. Interest in the wide gap II-VI compounds is stimulated by the need for electronically address­ able flat panel display devices and for the development of wide gap (blue) LED and injection laser devices. For applications in the blue portion of the visible spectrum, Zn�e and ZnS have long been the favored candidates. Very high quality ZnSe has been grown by MBE. The optical character-

NEW PERSPECTIVES OF SILICON CARBIDE: An Overview, with Special Emphasis on Noise and Space-Charge- Limited Flow

Abstract: Silicon carbide single crystals were studied at the University of Florida as part of a large program on ultrastructure processing and environmental stability of advanced structural and electronic materials. We are well aware of the fact that we are one of the few research groups who have pursued interest in this material. Although initially there was much interest in this material, in particular since it is a high band gap, high temperature, inert and chemically stable material (see e.g. 1 ), research on this material slackened considerably from 1960-1 980. The reasons for this are discussed well by Campbell (2). First, as he indicates, in the late 1960s there was a decline in corporate and government research and development funding. At that time, SiC had not carved out its niche in the semiconductor device market, like Si and GaAs. Second, there was the disappearance of the small market in which SiC could have made an impact, such as in space missions to Venus or other hostile atmospheres. Third, the fabrication techniques for SiC were, and are, beset with difficulties, and the hope for cheap devices is, even now, nil. At the end of his article, Campbell expresses only gloom; the enthusiasm of those who were involved in the material development of SiC is over since no viable markets are in sight. Yet, hardly a few years after (or even simultaneously with) Campbell's article, new

Precision X-Ray Techniques for Semiconductors

Abstract: Single crystal semiconductors are required in many modern electronic and optoelectronic devices, and in most instances a high degree of perfection is required for reliable performance. High yields of reliable devices can be more reasonably assured if substrate-threading dislocation densities are low, interfaces are sharp and free of misfit dislocations, stresses and strains are small and thus warpage and cracking are prevented, and epitaxial structures are uniform over large areas. Double crystal and multiple crystal x-ray diffraction are powerful techniques that are very well suited to the determination of crystalline perfection because they can precisely monitor the intrinsic x-ray reflectivity of a semiconductor wafer.

Field Ion Studies of Structure of GP Zones

Abstract: During precipitation from a supersaturated solid solution of an alloy, precipitation of metastable phases prior to the equilibrium phase is often observed. In some alloys, especially aluminum alloys, solute clusters on the matrix lattice form in the initial stage of precipitation or aging. Such clusters of solute atoms are called GP zones after the names of their discoverers, Guinier (1) and Preston (2). GP zones were found first in AI­ Cu alloy and thcn in most of thc age hardenable Al alloys and various other alloy systems such as Fe-Cu, Cu-Fe, Fe-Au, Fe-Mo, Cu-Ti, and Cu­ Be. One common feature of GP zones is that they are coherent with the matrix and very effectively strengthen the alloy. In x-ray diffraction patterns, the structural information on GP zones and the matrix is super­ imposed. Determination of the structure of GP zones by weak diffuse x­ ray or electron scattering is not easy. Usually the size of GP zones is too small to be analyzed by conventional techniques, such as electron probe microanalysis (EPMA) or the analytical electron microscope (AEM). Thus it can be said that exact nature of GP zones is not well under­ stood, even in the case of the most extensively studied alloy, AI-Cu. In the case of commercial alloys, our knowledge is even less because they contain more than one alloying element. Importance of GP zones in the most popular high strength Al alloys, such as AI-Cu-Mg and AI-Zn-Mg, is

Laser Spectroscopy of Ultrafast Solid-State Phenomena

Abstract: Ultrafast phenomena is a term commonly being used to designate processes that are too fast to be time-resolved by ordinary electronic measuring techniques. Very roughly speaking, the present limit of electronic time resolution is about 10-10 s. On the other hand, it is now possible to generate laser pulses with durations corresponding to only a few optical cycles. The shortest laser pulses available today are just a few femtoseconds ( l0-15 s) in duration. The remarkable progress in the generation of ultrashort laser pulses has coincided with the development of new experimental methods of time-resolved spectroscopy. Today optical measurements can be per­ formed with a time resolution limited only by the duration of the available laser pulses. Time resolution of 10-14 s is of great importance in solid-state physics. There is a large variety of interesting ultrafast processes, including phenomena such as cooling of a gas of photoexcited excitons (10-10 s), damping of lattice vibrations (10-12 s), and scattering of electrons and holes out of excited band states, which may be as fast as 10-14 s. The ability to measure these ultrafast solid-state phenomena directly as a function of time often provides a chance to obtain new information that may be hard to determine with other experimental techniques. In this article applications of ultrafast laser spectroscopy to the study of solids are considered. The first section gives an overview of fast exciton phenomena. Dynamics of photoexcited carriers and of lattice vibrations are discussed in the second and third sections. The final part of the article

Dislocations and Disclinations Past and Present: Some Personal Views and Reminiscences

Abstract: Dislocations and disclinations, the singularity lines in long-range trans­ lational or rotational order in materials, date back as concepts to the early years of this century. Their golden age was probably early after the Second World War, when many concepts wcreJorged by a small group of physi­ cists, chemists, and metallurgists. Dislocations and disclinations have recently regained impetus with the development of new observational techniques, such as high resolution electron microscopy, with a better understanding of their applications outside metallurgy, in fields such as semiconductors, surfaces, geophysics, liquid crystals, or biological materials, and with the realization that ideas and models developed for dislocations or disclinations have their counterparts for singularity points, lines, and walls in the order parameter of many materials systems. These include Bloch walls, Neel lines, and Bloch points in ferromagnets; vortex lines in Benard instabilities of normal fluids, in superfluids, and super­ conductors, etc. Furthermore it is in this context that the concept of a hierarchy of successive orders was first imagined: the regular spacing of epitaxial dislocations builds a new lattice that can be incommensurate with the underlying crystal period; disorder in such a spacing can be analyzed in terms of dislocations in the lattice of epitaxial dislocations. Such con-