Showing papers in "Annual Review of Materials Science in 1977"

High Rate Thick Film Growth

Abstract: Thick films will be defined here as those sufficiently thick to permit evolutionary selection processes during growth to influence their structures. High rates are defined as those sufficient to deposit thick films in a reasonable time. To avoid superficiality, this review is restricted to evaporation and sputtering, i.e. to physical vapor deposition (PVD). PVD is finding increased use for applications ranging from micro­ electronics to corrosion-barrier and wear-resistant coatings, and to the synthesis of free-standing shapes with unique mechanical properties. The emphasis here is on metallic deposits and on the physics of their growth and structure. Particular attention is given to sputtering, because recent developments ih sputtering tech­ nology make thick film deposition feasible, and because the subject has not been reviewed. Several reviews have concentrated on thick film deposition by evaporation (1, 2). Structure zone models (3-5) [particularly the model proposed by Movchan & Demchishin (3), which predicts three structural forms or zones as a function of T/Tm. where T is the substrate temperature and Tm is the coating-material melting point] have come into increased use in interpreting coating microstructures. There­ fore this review is organized from thc viewpoint of the zone models. After a brief survey of certain pertinent features of evaporation and sputtering, subsequent sections discuss each of the structural zones, metallurgical phase formation, and the mechanical properties of coatings. In this review the structure zones are defined in terms of dominant physical processes rather than structural forms. This generalization permits a broader correlation with experimental observations.

Deep Level Impurities in Semiconductors

Abstract: Solids are usually divided into metals and nonmetals. In a simplified scheme we may say that nonmetals consist of insulators and semiconductors. Both can be described by energy bands separated by a forbidden energy gap Eg• Whether a nonmetal is considered to be an insulator or a semiconductor often depends on the temperature at which its properties are to be investigated or used. In this chapter we use "semi­ conductor" to mean a nonmetal with a band gap from at least a few tenths of an eV up to a maximum of 10 eV. The considerable interest in semiconductors since the 1930s has been stimulated by their technical significance. Unlikl� metals, the physical properties of semi­ conductors can be considerably modified by introducing small amounts of foreign atoms. In this manner the resistivity of a silicon crystal can be changed by about seven orders of magnitude when one-millionth of the atoms in the crystal are replaced by suitable foreign atoms. Note that impurity substitution of this magni­ tude does not affect the band gap or other basic characteristics of the host material. Moreover, depending on the kind of foreign atom, the electrical current through the crystal is carried by electrons or holes. Hence multilayer structures with different types of conductivity can be constructed by doping a semiconductor crystal in­ homogeneously, resulting in devices such as diodes, transistors, lasers, etc. Experimentally it is found that replacing an atom of the host lattice by a foreign atom results in lattice defects with physical properties depending considerably on the particular atom introduced. Good understanding of such impurity centers has been achieved at least for Si and Ge when the foreign atom belongs to the groups of the periodic table closest to that of the semiconductor. They introduce localized donor and acceptor levels in the otherwise forbidden energy gap and are often described by the effective mass theory of Kahn & Luttinger (KL-EMT) (1-3), giving a hydrogen-like spectrum of levels with binding energies En' which can be written as

Hot Corrosion of High-Temperature Alloys

Abstract: : Hot corrosion is a form of accelerated oxidation of alloys which can occur in combustion gases in the presence of impurities. The characteristics of the reaction are defined by describing practical examples, and then the results of a variety of laboratory experiments are considered in relation to the practical process. The temperature dependence and kinetics of the corrosion are briefly presented, and the effect of alloying elements is discussed; high chromium contents are required for good corrosion resistance, but there is no general agreement on the effect of other alloy elements. A number of mechanisms for hot corrosion are described and discussed in relation to the practical results.

Defect Chemistry in Crystalline Solids

Abstract: Many of the physical properties of crystalline solids depend on the presence of native or foreign point def ects. In pure compound crystals the native defects are atoms missing from lattice sites where, according to the crystal structure, atoms should be (vacancies); atoms present at sites where atoms should not be (inter­ stitials); and atoms occupying sites normally occupied by other atoms (misplaced atoms). In addition, there may be defects in the electronic structure: quasi-free electrons in the conduction band or electrons missi ng from the valence band (holes). In impure or doped crystals there are also defects involving the foreign atoms. These may occupy normal lattice sites (substitut ional foreign atoms) or interstitial sites (interstitial foreign atoms). In elemental crystals simil ar point defects occur, only misplaced atoms are missing. Defects can be represented by symbols, atoms being represented by their normal chemical symbol, vacancies by V. Subscripts indicate the lattice site, characterized by the atom normally occupying that site. Thus VA is a vacancy at an A site. Inter­ stitial sites are indicated by a subscript i, Ai being A at an interstitial site. Electrons are represented bye, holes by h.

Structural Aspects of One-Dimensional Conductors

Abstract: This review emphasizes recent advances in the investigation of the structural aspects of 1-D conductors. The discussion is limited to the early transition metal complexes, the organic charge-transfer complexes, and the partially oxidized transition metal d/sup 8/ complexes. Since our goal has not been to provide a totally comprehensive survey, many interesting materials which have electronic properties that are 3-D but have some 1-D structural character, such as (SN)/sub x/ (2) and Hg/sub 2/./sub 86/AsF/sub 6/ (3), are not included within the scope of this review. In order to define certain terms and concepts, a brief introduction to the basic theories of 1-D conductivity is given in the following section.

Reversible Temper Embrittlement

Abstract: Recent work with emphasis on the direct measurement of segregation of impurities and alloying elements as they relate to temper embrittlement of low-alloy steels is reported. Low-alloy steels tempered between 320 to 525/sup 0/C are embrittled by the segregation of impurity and alloy elements to the grain boundaries. Segregation occurs during tempering. Segregation in austenite is not important. Alloy elements promote impurity segregation. Embrittlement occurs by the reduction of cohesion at the grain boundaries. Antimony and tin are most effective embrittlers. Carbon and boron enhance ductility. (FS)

Acoustic Emission in Brittle Materials

Abstract: Acoustic emission may be defined as the generation of stress waves due to a rapid change of displacement in a material. An instantaneous displacement can be produced by a change in an internal or external boundary of the medium (crack motion, projectile impact, interface slippage), by a localized change in the volume or elastic constants of the medium (inhomogeneity motions such as twinning, martensitic transformations, phase changes), or by plastic deformation (dislocation motion). A large number of different stress-wave sources can exist in any given system. In principle, these sources could be distinguished by a detailed examination of their complete stress-wave patterns. However, since acoustic emission is usually detected by a single transducer located at the boundary of the solid, both the

Fundamental Optical Phenomena in Infrared Window Materials

Abstract: A wide variety of high-energy lasers and associated applications (1) and optical systems with very long path lengths [e.g. fiber optics (2)J have emerged in recent years. These developments have led to corresponding requirements for highly transparent window materials (3). Ideally, a window material would transmit light without any effect on either the light beam or the window. Obviously, no such ideal material exists or is really required; moreover, specific types of beam modi­ fications are often not only tolerable but desirable. Thus, for example, we may require a material which refracts light, modulates light intensity, modifies polariza­ tion, or the like, yet acts as a window in all other respects. Our principal emphasis here is on properties of materials for totally passive windows, although those appropriate for active components are considered briefly. We are concerned with the fundamental optical phenomena of absorption, refraction, and photo­ elasticity in window materials. The latter phenomena underly or contribute to more complex optical effects such as thermally induced lensing (4), laser-induced interference (5), laser-induced breakdown (6), ilnd self-focusing (7), for example. Typical observations of laser-induced lensing and interference in infrared materials are indicated in Figure 1. The primary emphasis here is on bulk infrared materials, especially those for 10 11m (C02) and 3-5 11m (chemical) laser applications. For this case, one is then concerned with the highly transparent frequency regime in insulating or semiconducting solids, which lies well above the principal lattice absorption bands, but well below the onset of the electronic absorption edge. Clearly, in order to achieve high transparency over an extended frequency region, materials with low lattice resonances Wo and high-energy gap frequencies Wg are desirable. It is useful to correlate the latter properties with molecular weight and with families of compounds of the same structure, as pointed out by Hilton (8). The graphs in Figure 2 vividly indicate the decrease in wyand increase in Wo as

Structural transformations during aging of metal alloys

Abstract: Decomposition of supersaturated solid solutions (aging) is a complex process of establishing a two-phase (multiphase) equilibrium. The process is accompanied by changes in the dist ribution of the alloy 's components and of the structure of the preci pitating phases. The complex character of the aging process is connected with the major role of elastic energy induced by precipitation of the coherent or semi­ coherent second phase. In some cases elastic energy plays the decisive role in the formation of the precipitate's structure. It is impossible to consider in one short review all the details of a proce ss as complex as the decomposition of supersaturated solid soluti ons. Consequently, we focus on the results in the field that have developed most rapidly during recent years. We examine first investigations of the spinodal decomposition mechanism.