Materials Science Reports最新文献

Renewable natural materials have been exploited for several millennia. Within the past two decades, it has become apparent that materials science can benefit from a detailed knowledge of the synthetic pathways and molecular self-assembly mechanisms by which natural materials are produced. This review describes the most significant classes of macromolecule used in the synthesis of biological materials. It explains how the techniques of genetic engineering can be employed to modify the structure or quantitative yield of these materials. The role of the liquid crystalline state in materials self-assembly, and the effects of hierachical molecular order on the properties of natural materials, are emphasized. The wide range of contexts in which biological principles have impacted materials science is illustrated with several specific examples.

The application of Mössbauer effect spectroscopy to the study of both icosahedral and decagonal quasicrystalline solids is reviewed. The application of different fitting models to both paramagnetic and weakly ferromagnetic quasicrystalline alloys is discussed, as well as the interpretation of information obtained from these data. Measurements from the literature are reviewed and are considered in the context of structural, electronic and magnetic models of quasicrystalline ordering in solids. New experimental results for stable quasicrystalline alloys with both icosahedral and decagonal symmetry are presented, as are extensions of the analyses previously presented in the literature for a number of systems, and these results are discussed in the context of the fundamental physical properties of these materials.

Experimental studies on the electronic and optical properties of conductingpolymer thin films are reviewed. Conducting polymers have been processed into various forms of undoped and doped thin films. The electrical conduction properties have been characterized by conventional transport measurements as well as by the fabrication and evaluation of polymeric semiconductor devices. Important electrical and structural material parameters are extracted by fitting the results to conductivity models. Optical and electro-optical measurements give valuable information on the structural order, the electronic structure, and the unusual nature of charge carriers in these novel materials. The polymeric devices are mainly utilized in material characterization, but possible implications for applications are also discussed.

An effective heat of formation (EHF) model for predicting compound phase formation sequence is described. The EHF model defines an effective heat of formation ΔH′, which is concentration-dependent. By choosing the effective concentration of the interacting species at the growth interface to be that of the liquidus minimum, the model correctly predicts first phase formation during formation of silicides, germanides, aluminides and other metal-metal binary systems. The ability to predict phase formation sequence and phase decomposition is illustrated for some silicide and aluminide systems. The EHF model is also used to describe phase formation in lateral and bulk diffusion couples and for the interpretation of the effects of impurities and diffusion barriers on phase formation. It is also shown how the EHF model can be applied to laser and ion-beam induced phase formation and to the formation of amorphous and metastable phases. A compilation of recommended heats of formation for many silicide, germanide, aluminide and other metal-metal binary systems is given. In cases where experimental heats of formation were not available or if they were suspected of being unreliable, use was made of calculated values according to Miedema's model.

This review has two primary aims. First, it describes how the composition of materials may be quantitatively mapped with near-atomic resolution and sensitivity. Second,it outlines how this capability may be used to investigate important solid-state phenomena at the microscopic level. Specifically, the following topics are considered: interfacial roughness in semiconductor heterostructures and its effect on the optical properties of quantum wells; stability of modern interfaces against interdiffusion during typical annealing treatments; diffusion of intrinsic point defects in semiconductors; and the microscopics of ion implantation.

The embedded-atom method is a semi-empirical method for performing calculations of defects in metals. The EAM incorporates a picture of metallic bonding, for which there is some fundamental basis. The limitations of the EAM are fairly well characterized: it works best for purely metallic systems with no directional bonding; it does not treat covalency or significant charge transfer; and it does not handle Fermi-surface effects. The main physical property incorporated in the EAM is the moderation of bond strength by other bonds (coordination-dependent bond strength). Within these constraints, the EAM provides a very useful and robust means of calculating approximate structure and energetics, from which many interesting properties of metals can be obtained.

We believe that atomistic calculations will continue to play an important role in the development of materials theory. Where the EAM can be useful, there is a tremendous number of interesting projects that have yet to be carried out. The understanding of mechanical properties on an atomistic level has only just begun. For materials where the EAM is not expected to work well, there are recent developments which may allow calculations similar to those presented here. We have mentioned already the problem of treating directional bonding in semiconductors and elements from the transition series. One approach which promises to be useful for treating directional bonding is reviewed by Carlsson [70]; the interested reader is encouraged to start there.

Electrical transport and optical properties of transition-metal silicides are reviewed. They are integrated with thermal properties of single-crystal silicides. Most of these compounds behave as metals while some of them behave as semiconductors. The former show an increasing electrical resistivity ρ with increasing temperature. Several of them show a non-classical deviation of ρ(T) from linearity in the high-temperature limit. This deviation, related to intrinsic properties of the compound, can be affected both in sign and in amount by the presence of foreign atoms (impurities) and structural defects. Moreover, defects dominate the electrical transport at low temperatures both in metallic and semiconducting compounds. Therefore, the interpretation of the electrical properties measured as a function of temperature may give a non-realistic description of silicide intrinsic properties. Since also other physical properties, like thermal and optical ones, can be strongly affected by impurities and defects, results about single-crystal silicides will be first illustrated. Single-crystal preparation and structural characterization are described in detail, with emphasis on crystalline quality in terms of residual resistivity ratio. The electrical quantities, resistivity and magnetoresistance, are measured as a function of temperature and along the main crystallographic directions. The effect of impurities and defects on the transport properties is then evaluated by examining the electrical transport of polycrystalline thin-film silicides. The different contributions to the total resistivity are measured by changing: (i) film stoichiometry, (ii) impurity concentration, (iii) texture growth and (iv) film thickness. Hall-coefficient measurements are briefly discussed with the main purpose to evidence that great caution is necessary when deducing mobility and charge-carrier density values from these data. The theoretical models currently used to interpret the low- and high-temperature resistivity behavior of the metallic silicides are presented and used to fit the experimental resistivity curves. The results of these studies reveal that in several cases there are well-defined temperature ranges in which a specific electron—phonon scattering mechanism dominates. This allows a more detailed study of the microscopic processes. The optical functions from the far-infrared to the vacuum ultraviolet, derived from Kramers—Krönig analysis of reflectance spectra or directly measured by spectroscopic ellipsometry, are presented and discussed for some significant metallic disilicides, both single crystals and polycrystalline films. Different physical phenomena are distinguished in the spectra: intraband transitions at the lowest photon energies, interband transitions at higher energies, and collective oscillations. In particular, the free-carrier response derived from this analysis is compared with the transport results. The interpretation of