Microbial impact on building materials: an overview

Microbial activity can have an important impact on the durability of building materials. It is important to understand this activity in order to select appropriate treatment strategies for the repair and restoration of buildings and monuments. This paper describes the microorganisms which can modify the properties of building materials such as concrete, mortars, composites, timber, gypsum, etc., as well as the mechanisms responsible for deterioration of these materials. The information provided by the members of TC 183-MIB via a questionnaire is discussed. Techniques currently utilised and areas requiring further study are identified. In addition to the references, a list of publications for further reading completes this article.

     

Advances in science and technology of modern energetic materials: an overview

Energetic materials such as explosives, propellants and pyrotechnics are widely used for both civilian and military explosives applications. The present review focuses briefly on the synthesis aspects and some of the physico-chemical properties of energetic materials of the class: (a) aminopyridine-N-oxides, (b) energetic azides, (c) high nitrogen content energetic materials, (d) imidazoles, (e) insensitive energetic materials, (f) oxidizers, (g) nitramines, (h) nitrate esters and (i) thermally stable explosives. A brief comment is also made on the emerging nitration concepts. This paper also reviews work done on primary explosives of current and futuristic interest based on energetic co-ordination compounds. Lead-free co-ordination compounds are the candidates of tomorrow's choice in view of their additional advantage of being eco-friendly. Another desirable attribute of lead free class of energetic compounds is the presence of almost equivalent quantity of fuel and oxidizer moieties. These compounds may find wide spectrum of futuristic applications in the area of energetic materials. The over all aim of the high energy materials research community is to develop the more powerful energetic materials/explosive formulations/propellant formulations in comparison to currently known benchmark materials/compositions. Therefore, an attempt is also made to highlight the important contributions made by the various researchers in the frontier areas energetic ballistic modifiers, energetic binders and energetic plasticizers.     

Deformation twinning in nanocrystalline materials

Nanocrystalline (nc) materials can be defined as solids with grain sizes in the range of 1-100 nm. Contrary to coarse-grained metals, which become more difficult to twin with decreasing grain size, nanocrystalline face-centered-cubic (fcc) metals become easier to twin with decreasing grain size, reaching a maximum twinning probability, and then become more difficult to twin when the grain size decreases further, i.e. exhibiting an inverse grain-size effect on twinning. Molecular dynamics simulations and experimental observations have revealed that the mechanisms of deformation twinning in nanocrystalline metals are different from those in their coarse-grained counterparts. Consequently, there are several types of deformation twins that are observed in nanocrystalline materials, but not in coarse-grained metals. It has also been reported that deformation twinning can be utilized to enhance the strength and ductility of nanocrystalline materials. This paper reviews all aspects of deformation twinning in nanocrystalline metals, including deformation twins observed by molecular dynamics simulations and experiments, twinning mechanisms, factors affecting the twinning, analytical models on the nucleation and growth of deformation twins, interactions between twins and dislocations, and the effects of twins on mechanical and other properties. It is the authors’ intention for this review paper to serve not only as a valuable reference for researchers in the field of nanocrystalline metals and alloys, but also as a textbook for the education of graduate students.     

Mechanical properties of nanocrystalline materials

The mechanical properties of nanocrystalline materials are reviewed, with emphasis on their constitutive response and on the fundamental physical mechanisms. In a brief introduction, the most important synthesis methods are presented. A number of aspects of mechanical behavior are discussed, including the deviation from the Hall-Petch slope and possible negative slope, the effect of porosity, the difference between tensile and compressive strength, the limited ductility, the tendency for shear localization, the fatigue and creep responses. The strain-rate sensitivity of FCC metals is increased due to the decrease in activation volume in the nanocrystalline regime; for BCC metals this trend is not observed, since the activation volume is already low in the conventional polycrystalline regime. In fatigue, it seems that the S-N curves show improvement due to the increase in strength, whereas the da/dN curve shows increased growth velocity (possibly due to the smoother fracture requiring less energy to propagate). The creep results are conflicting: while some results indicate a decreased creep resistance consistent with the small grain size, other experimental results show that the creep resistance is not negatively affected. Several mechanisms that quantitatively predict the strength of nanocrystalline metals in terms of basic defects (dislocations, stacking faults, etc.) are discussed: break-up of dislocation pile-ups, core-and-mantle, grain-boundary sliding, grain-boundary dislocation emission and annihilation, grain coalescence, and gradient approach. Although this classification is broad, it incorporates the major mechanisms proposed to this date. The increased tendency for twinning, a direct consequence of the increased separation between partial dislocations, is discussed. The fracture of nanocrystalline metals consists of a mixture of ductile dimples and shear regions; the dimple size, while much smaller than that of conventional polycrystalline metals, is several times larger than the grain size. The shear regions are a direct consequence of the increased tendency of the nanocrystalline metals to undergo shear localization.The major computational approaches to the modeling of the mechanical processes in nanocrystalline metals are reviewed with emphasis on molecular dynamics simulations, which are revealing the emission of partial dislocations at grain boundaries and their annihilation after crossing them.     

Dislocation motion-induced strain in nanocrystalline materials: Overlooked considerations

The amount of plastic strain caused by the motion of a single dislocation across an individual nanosize grain is drastically higher than the amount recorded for larger grain sizes. As a result, in nanocrystalline materials, only a small number of dislocations would need to move within each individual grain in order to accommodate the plastic strain on the entire sample. This observation leads to a quantitative criterion for determining if observed dislocation activity is sufficient to accommodate realistic applied plastic strains. This new criterion is directly applicable to the interpretation of in situ TEM experiments and computational molecular dynamics simulations.     

Structure and properties of nanocrystalline materials

The present article reviews the current status of research and development on the structure and properties of nanocrystalline materials. Nanocrystalline materials are polycrystalline materials with grain sizes of up to about 100 nm. Because of the extremely small dimensions, a large fraction of the atoms in these materials is located at the grain boundaries, and this confers special attributes. Nanocrystalline materials can be prepared by inert gas-condensation, mechanical alloying, plasma deposition, spray conversion processing, and many other methods. These have been briefly reviewed. A clear picture of the structure of nanocrystalline materials is emerging only now. Whereas the earlier studies reasoned out that the structure of grain boundaries in nanocrystalline materials was quite different from that in coarse-grained materials, recent studies using spectroscopy, high-resolution electron microscopy, and computer simulation techniques showed unambiguously that the structure of the grain boundaries is the same in both nanocrystalline and coarse-grained materials. A critical analysis of this aspect and grain growth is presented. The properties of nanocrystalline materials are very often superior to those of conventional polycrystalline coarse-grained materials. Nanocrystalline materials exhibit increased strength/hardness, enhanced diffusivity, improved ductility/toughness, reduced density, reduced elastic modulus, higher electrical resistivity, increased specific heat, higher thermal expansion coefficient, lower thermal conductivity, and superior soft magnetic properties in comparison to conventional coarse-grained materials. Recent results on these properties, with special emphasis on mechanical properties, have been discussed. New concepts of nanocomposites and nanoglasses are also being investigated with special emphasis on ceramic composites to increase their strength and toughness. Even though no components made of nanocrystalline materials are in use in any application now, there appears to be a great potential for applications in the near future. The extensive investigations in recent years on structure-property correlations in nanocrystalline materials have begun to unravel the complexities of these materials, and paved the way for successful exploitation of the alloy design principles to synthesize better materials than hitherto available.     

Nanomechanics of Hall-Petch relationship in nanocrystalline materials

Classical Hall-Petch relation for large grained polycrystals is usually derived using the model of dislocation pile-up first investigated mathematically by Nabarro and coworkers. In this paper the mechanical properties of nanocrystalline materials are reviewed, with emphasis on the fundamental physical mechanisms involved in determining yield stress. Special attention is paid to the abnormal or ‘inverse’ Hall-Petch relationship, which manifests itself as the softening of nanocrystalline materials of very small (less than 12 nm) mean grain sizes. It is emphasized that modeling the strength of nanocrystalline materials needs consideration of both dislocation interactions and grain-boundary sliding (presumably due to Coble creep) acting simultaneously. Such a model appears to be successful in explaining experimental results provided a realistic grain size distribution is incorporated into the analysis. Masumura et al. [Masumura RA, Hazzledine PM, Pande CS. Acta Mater 1998;46:4527] were the first to show that the Hall-Petch plot for a wide range of materials and mean grain sizes could be divided into three distinct regimes and also the first to provide a detailed mathematical model of Hall-Petch relation of plastic deformation processes for any material including fine-grained nanocrystalline materials. Later developments of this and related models are briefly reviewed.Prof. Frank Nabarro was a physicist by training, a metallurgist by profession and a genius by nature, blessed with a unique ability to treat everyone as his equal. During his later years he was very much interested in the mechanical properties of nanocrystalline materials. This review on that topic is our contribution to the special issue of Progress in Materials Science honoring him.     

Structure and thermal stability of nanocrystalline materials

Nanocrystalline materials, which are expected to play a key role in the next generation of human civilization, are assembled with nanometre-sized “building blocks” consisting of the crystalline and large volume fractions of intercrystalline components. In order to predict the unique properties of nanocrystalline materials, which are a combination of the properties of the crystalline and intercrystalline regions, it is essential to understand precisely how the structures of crystalline and intercrystalline regions vary with decrease in crystallite size. In addition, study of the thermal stability of nanocrystalline materials against significant grain growth is both scientific and technological interest. A sharp increase in grain size (to micron levels) during consolidation of nanocrystalline powders to obtain fully dense materials may consequently result in the loss of some unique properties of nanocrystalline materials. Therefore, extensive interest has been generated in exploring the size effects on the structure of crystalline and intercrystalline region of nanocrystalline materials, and the thermal stability of nanocrystalline materials against significant grain growth. The present article is aimed at understanding the structure and stability of nanocrystalline materials.     

Grain size dependence of strength of nanocrystalline materials as exemplified by copper: an elastic-viscoplastic modelling approach

The purpose of this work is to model the mechanical behavior of nanocrystalline materials. Based on previous rigid viscoplastic models proposed by Kim et al. (Acta Mater, 48: 493, 2000) and Kim and Estrin (Acta Mater, 53: 765, 2005), the nanocrystalline material is described as a two phase composite material. Using the Taylor–Lin homogenisation scheme in order to account for elasticity, the yield stress of nanocrystalline materials can be evaluated. The transition from a Hall–Petch relation to an inverse Hall–Petch relation is defined and is related to a change in plastic deformation mode in the crystallite phase from a dislocation glide driven mechanism to a diffusion-controlled process.     

Grain growth in porous two-dimensional nanocrystalline materials

Grain growth in two-dimensional polycrystals with mobile pores at the grain boundary triple junctions is considered. The kinetics of grain and pore growth are determined under the assumption that pore sintering and pore mobility are controlled by grain boundary and surface diffusion, respectively. It is shown that a polycrystal can achieve full density in the course of grain growth only when the initial pore size is below a certain critical value which depends on kinetic parameters, interfacial energies, and initial grain size. Larger pores grow without limits with the growing grains, and the corresponding grain growth exponent depends on kinetic parameters and lies between 2 and 4. It is shown that for a polycrystal with subcritical pores the average grain size increases linearly with time during the initial stages of growth, in agreement with recent experimental data on grain growth in thin Cu films and in bulk nanocrystalline Fe.     

Some aspects of synthesis of nanocrystalline materials

Abstract

This paper reviews research work at the Institute for Materials and Advanced Processes, University of Idaho, on the synthesis of nanocrystalline materials and their consolidation. Nanocrystalline materials have been synthesised by a number of ‘far from equilibrium’ processes including mechanical alloying (MA), mechanochemical processing (MCP), supercritical fluid processing (SCFP), and severe plastic deformation (SPD). Examples of the materials include the TiAl based intermetallic compounds and composites produced by MA and SPD, Ti base alloys and metal carbides synthesised by MCP, thin film Cu produced by SCFP, and Al–Fe alloys produced by SPD. Details of the processes used and the enhancement of properties owing to the nanoscale structures in consolidated material will be presented. The potential of these processes to substitute for conventional methods of production will also be discussed.     

天然有机纤维吸油材料的结构特点及其功能化改性技术的研究进展

天然有机纤维作为一种可再生生物质资源,具有可生物降解性,显示出一定的吸油能力,近年来作为吸油材料的研究备受关注。首先概述了有机吸油材料的研究发展动态和吸油机理,然后详述了天然有机纤维的结构特点,并归纳和评价了其功能化改性技术,指出以天然有机纤维为基质,通过疏水改性和构筑表面微观粗糙度获得超疏水亲油表面,同时构建适宜的多孔性结构并引入光降解等模式,可望获得兼具突出吸油性能和可控降解能力的生态环保吸油材料。最后,对蛋白纤维(尤其是胶原纤维)在吸油领域的应用潜力及存在问题进行了总结和展望。     

Rail defects: an overview

For about 150 years, the steel rail has been at the very heart of the world's railway systems. The rail works in a harsh environment and, as part of the track structure, it has little redundancy; thus, its failure may lead to catastrophic derailment of vehicles, the consequences of which can include death, injury, costs and loss of public confidence. These can have devastating and long‐lasting effects on the industry. Despite the advances being made in railway permanent way engineering, inspection and rail‐making technology, continually increasing service demands have resulted in rail failure continuing to be a substantial economic burden and a threat to the safe operation of virtually every railway in the world. This paper presents an overview of rail defects and their consequences from the earliest days of railways to the present day.     

Cold electronics: an overview

Low-temperature operation is being applied and contemplated for electronic systems ranging from single-transistor circuits for basic research to VLSI integrated circuits for ultra-fast computers. It is seen as both a means of extracting better performance from present technology and as an important ingredient of the next generation of devices and circuits. This overview is concerned with electronics based on semiconductors; for low temperatures the primary material is Si, although GaAs also has considerable potential, and the primary device is the field-effect transistor in various forms. Reduced temperature operation offers improvements in performance through improvement of materials-related properties such as electronic carrier mobility, thermal conductivity, and electrical conductivity. Substantial improvements in reliability are also expected since degradation mechanisms are thermally activated; however, this could be negated unless problems of thermal expansion mismatch and cycling are overcome. Refrigeration continues to be a central concern; mechanical cycles are still the mainstay and progress is being made in systems applicable to electronics, although further development is needed.     

Structural effect on intrinsic stress in nanocrystalline Si:H films

Intrinsic stress in nanocrystalline Si:H films which prepared by the plasma enhanced chemical vapor deposition (PECVD) technique, was illustrated as a compressive stress by means of Raman scattering and radius of curvature measurement. The Raman signals can be well fitted by a model of strain-calibrated phonon confinement, where the sole effect of phonon confinement and Fano interference on Raman scattering was excluded, respectively. The ion bombardment effect on the origination of intrinsic stress in the PECVD films was discussed. The formation of nc-Si:H was explained by etching model in present experimental parameters’ range. The results infer that the intrinsic compressive stress shows intensive correlation to amorphous Si:H, grain boundaries and hydrogen incorporation in the as-deposited materials.     

Structural effect on intrinsic stress in nanocrystalline Si:H films

Intrinsic stress in nanocrystalline Si:H films which prepared by the plasma enhanced chemical vapor deposition (PECVD) technique, was illustrated as a compressive stress by means of Raman scattering and radius of curvature measurement. The Raman signals can be well fitted by a model of strain-calibrated phonon confinement, where the sole effect of phonon confinement and Fano interference on Raman scattering was excluded, respectively. The ion bombardment effect on the origination of intrinsic stress in the PECVD films was discussed. The formation of nc-Si:H was explained by etching model in present experimental parameters’ range. The results infer that the intrinsic compressive stress shows intensive correlation to amorphous Si:H, grain boundaries and hydrogen incorporation in the as-deposited materials.     

CdBr2 nanocrystalline layers as nonlinear optical materials

A new type of nanomaterial for optoelectronic is proposed. As such a material cleaved nanolayers of layered CdBr₂ single crystals is chosen. A principal possibility to prepare thin hexagonal single crystalline layers of CdBr₂ nanocrystals (possessing 6H polytype) with thickness up to several nanometers was shown. The studies of the optical absorption clearly show an occurrence of the blue spectral shift for the absorption edge up to 30 nm. During illumination by the 5 ns nitrogen laser at the wavelength at about 337 nm below the energy band gap it was established a substantial increase of the photoinduced absorption coefficient up to 25 cm^?1 at pump power density equal to about 1 GW cm^?2. The maximal photoinduced effect was observed for the thinnest film possessing thickness about 4 nm. The linear electrooptical effect was maximal at ambient temperature.     

Alloying behaviour in nanocrystalline materials during mechanical alloying

The alloying behaviour in a number of systems such as Cu-Ni, Cu-Zn, Cu-Al, Ni-Al, Nb-Al has been studied to understand the mechanism as well as the kinetics of alloying during mechanical alloying (MA). The results show that nanocrystallization is a prerequisite for alloying in all the systems during MA. The mechanism of alloying appears to be a strong function of the enthalpy of formation of the phase and the energy of ordering in case of intermetallic compounds. Solid solutions (Cu-Ni), intermetallic compounds with low ordering energies (such as Ni₃Al which forms in a disordered state during MA) and compounds with low enthalpy of formation (Cu-Zn, Al₃Nb) form by continuous diffusive mixing. Compounds with high enthalpy of formation and high ordering energies form by a new mechanism christened as discontinuous additive mixing. When the intermetallic gets disordered, its formation mechanism changes from discontinuous additive mixing to continuous diffusive one. A rigorous mathematical model, based on iso-concentration contour migration method, has been developed to predict the kinetics of diffusive intermixing in binary systems during MA. Based on the results of Cu-Ni, Cu-Zn and Cu-Al systems, an effective temperature (T _eff) has been proposed that can simulate the observed alloying kinetics. TheT _eff for the systems studied is found to lie between 0·42–0·52T ₁.