Showing papers in "Materials Science & Engineering R-reports in 2011"

Nanostructured electrodes for lithium-ion and lithium-air batteries: the latest developments, challenges, and perspectives

Abstract: The urgency for clean and secure energy has stimulated a global resurgence in searching for advanced electrical energy storage systems. For now and the foreseeable future, batteries remain the most promising electrical energy storage systems for many applications, from portable electronics to emerging technologies such as electric vehicles and smart grids, by potentially offering significantly improved performance, energy efficiencies, reliability, and energy security while also permitting a drastic reduction in fuel consumption and emissions. The energy and power storage characteristics of batteries critically impact the commercial viability of these emerging technologies. For example, the realization of electric vehicles hinges on the availability of batteries with significantly improved energy and power density, durability, and reduced cost. Further, the design, performance, portability, and innovation of many portable electronics are limited severely by the size, power, and cycle life of the existing batteries. Creation of nanostructured electrode materials represents one of the most attractive strategies to dramatically enhance battery performance, including capacity, rate capability, cycling life, and safety. This review aims at providing the reader with an understanding of the critical scientific challenges facing the development of advanced batteries, various unique attributes of nanostructures or nano-architectures applicable to lithium-ion and lithium-air batteries, the latest developments in novel synthesis and fabrication procedures, the unique capabilities of some powerful, in situ characterization techniques vital to unraveling the mechanisms of charge and mass transport processes associated with battery performance, and the outlook for future-generation batteries that exploit nanoscale materials for significantly improved performance to meet the ever-increasing demands of emerging technologies.

Development of hafnium based high-k materials—A review

Abstract: The move to implement metal oxide based gate dielectrics in a metal-oxide-semiconductor field effect transistor is considered one of the most dramatic advances in materials science since the invention of silicon based transistors. Metal oxides are superior to SiO 2 in terms of their higher dielectric constants that enable the required continuous down-scaling of the electrical thickness of the dielectric layer while providing a physically thicker layer to suppress the quantum mechanical tunneling through the dielectric layer. Over the last decade, hafnium based materials have emerged as the designated dielectrics for future generation of nano-electronics with a gate length less than 45 nm, though there exists no consensus on the exact composition of these materials, as evolving device architectures dictate different considerations when optimizing a gate dielectric material. In addition, the implementation of a non-silicon based gate dielectric means a paradigm shift from diffusion based thermal processes to atomic layer deposition processes. In this report, we review how HfO 2 emerges from all likely candidates to become the new gold standard in the microelectronics industry, its different phases, reported electrical properties, and materials processing techniques. Then we use specific examples to discuss the evolution in designing hafnium based materials, from binary to complex oxides and to non-oxide forms as gate dielectric, metal gates and diffusion barriers. To address the impact of these hafnium based materials, their interfaces with silicon as well as a variety of semiconductors are discussed. Finally, the integration issues are highlighted, including carrier scattering, interface state passivation, phonon engineering, and nano-scale patterning, which are essential to realize future generations of devices using hafnium-based high- k materials.

Medium chain length polyhydroxyalkanoates, promising new biomedical materials for the future

Abstract: Medium chain length polyhydroxyalkanoates, mcl-PHAs (C6–C14 carbon atoms), are polyesters of hydroxyalkanoates produced mainly by fluorescent Pseudomonads under unbalanced growth conditions. These mcl-PHAs which can be produced using renewable resources are biocompatible, biodegradable and thermoprocessable. They have low crystallinity, low glass transition temperature, low tensile strength and high elongation to break, making them elastomeric polymers. Mcl-PHAs and their copolymers are suitable for a range of biomedical applications where flexible biomaterials are required, such as heart valves and other cardiovascular applications as well as matrices for controlled drug delivery. Mcl-PHAs are more structurally diverse than short chain length PHAs and hence can be more readily tailored for specific applications. Composites have also been fabricated using mcl-PHAs and their copolymers, such as poly (3-hydroxyoctanoate) [P(3HO)] combined with single walled carbon nanotubes and poly(3-hydroxbutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)] combined with hydroxyapatite. Because of these attractive properties of biodegradability, biocompatibility and tailorability, Mcl-PHAs and their composites are being increasingly used for biomedical applications. However, studies remain limited mainly to P(3HO) and the copolymer P(3HB-co-3HHx), which are the only mcl-PHAs available in large quantities. In this review we have consolidated current knowledge on the properties and biomedical applications of these elastomeric mcl-PHAs, their copolymers and their composites.

Current-induced domain wall motion in nanoscale ferromagnetic elements

Abstract: The manipulation of a magnetic domain wall (DW) by a spin polarized current in ferromagnetic nanowires has attracted tremendous interest during the last years due to fundamental questions in the fields of spin dependent transport phenomena and magnetization dynamics but also due to promising applications, such as DW based magnetic memory concepts and logic devices. We comprehensively review recent developments in the field of geometrically confined domain walls and in particular current induced DW dynamics. We focus on the influence of the magnetic and electronic transport properties of the materials on the spin transfer effect in DWs. After considering the different DW structures in ferromagnetic nanowires, the theory of magnetization dynamics induced by a spin polarized current is presented. We first discuss the different current induced torques and their origin in the light of recent theories based on a simple s-d exchange model and beyond. This leads to a modified Landau-Lifshitz-Gilbert equation of motion where the different spin transfer torques are included and we discuss their influence on the DW dynamics on the basis of simple 1D models and recent micromagnetic simulations studies. Experimental results illustrating the effects of spin transfer in different ferromagnetic materials and geometries constitute the body of the review. The case of soft in-plane magnetized nanowires is described first, as it is the most widely studied class of ferromagnetic materials in this field. By direct imaging we show how confined domain walls in nanowires can be displaced using currents in in-plane soft magnetic materials and that using short pulses, fast velocities can be attained. While a spin polarized current can trigger DW depinning or displacement, it can also lead to a modification of the DW structure, which is described in detail as it allows one to deduce information about the underlying spin torque terms. High perpendicular anisotropy materials characterized by narrow domain walls have also raised considerable interest. These materials with only a few nanometer wide DWs combined several key advantages over soft magnetic materials such as higher non-adiabatic effects leading to lower critical current densities and high domain wall velocities. We review recent experimental results obtained in this class of materials and discuss the important implications they entail for the nature of the spin torque effect acting on DWs. (C) 2011 Elsevier B.V. All rights reserved.

Strain effects in low-dimensional transition metal oxides

Abstract: Transition metal oxides offer a wide spectrum of properties which provide the foundation for a broad range of potential applications. Many of these properties originate from intrinsic coupling between lattice deformation and nanoscale electronic and magnetic ordering. Lattice strain thus has a profound influence on the electrical, optical, and magnetic properties of these materials. Recent advances in materials processing have led to the synthesis of low-dimensional single-crystal transition metal oxides, namely, epitaxial ultra-thin films and free-standing nano/microwires. Unlike bulk materials, these systems allow external tuning of uniform strain in these materials to tailor their properties and functionalities. This paper provides a comprehensive review of recent developments in studies of strain effects in transition metal oxide ultra-thin films and nano/microwires. In epitaxial thin films, biaxial strain is developed as a result of lattice mismatch between the film and the substrate. By choosing different substrates, a wide range of strain can be established at discrete values that allows for exploration of new phase space, enhancement of order parameters, creation of complicated domain textures, and stabilization of new phases. On the other hand, continuous tuning of uniaxial strain is possible in nano/microwires, where a variety of phase transitions and their dynamics could be probed at the single or few-domain scale. We focus on the work of strain-controlled electromechanical response in piezoelectric oxides and strain-induced metal–insulator transitions as well as domain physics in strongly correlated electron oxides. Related nanoscale device applications such as strain sensing and power generation will be highlighted as well.

Adhesion, friction, and compliance of bio-mimetic and bio-inspired structured interfaces

Abstract: The remarkable mechanical properties of natural contact surfaces have inspired a great deal of interest and research in recent years. The underlying driver of this interest is the surprising range of surface mechanical properties such as adhesion, friction, and compliance that can be attained primarily by design of near-surface architecture using generic materials properties. A considerable literature has developed spanning the range from biological studies of structure and properties, through models to understand these properties, to development of bio-mimetic and bio-inspired structures, along with theory to understand how structure leads to development of surface mechanical properties. The literature has matured sufficiently that common architectures and principles have emerged, for which we attempt here to present a unified view. The field remains vibrant so we hope that this review can at the same time help in its further progress. Our goal in this paper is to review the field from the point of view of scientists and engineers interested to learn about the architecture, properties, and mechanisms of contacting surfaces in nature and how these might be mimicked to create new materials with unique and interesting properties. We begin with a brief description of natural systems, their architectures and properties, and follow by a discussion of the main bio-mimetic and bio-inspired materials that have been developed recently. We then discuss surface mechanical properties – adhesion, friction, and compliance – how these are related to materials and architectural parameters, and how these properties are measured. Where possible, we provide quantitative models for the relationship between structure and properties. We conclude the paper with a discussion of outlook and future possibilities in this field.

Atomistic theory for predicting the binary metallic glass formation

Abstract: In the present review article, firstly, the experimental observations of the binary metallic glass formation by various glass-producing techniques are briefly summarized. Secondly, a detailed discussion is presented concerning the concepts of the glass-forming ability and glass-forming range (GFA/GFR) of a binary metal system. Meanwhile, some of the proposed empirical criteria or rules for predicting the binary metallic glass formation are discussed and compared with the experimental observations. Thirdly, it is proposed to take the interatomic potential of a binary metal system as the starting base to develop an atomistic theory capable of predicting the binary metallic glass formation. Accordingly, eight binary metal systems are selected as representatives to cover the various structural combinations as well as various thermodynamic characteristics. The n-body potentials of eight representative systems are then constructed with the aid of ab initio calculations. Applying the constructed and proven realistic n-body potentials, a series of molecular dynamics simulations are carried out. In the simulations, solid solution models are employed to compare the relative stability of the solid solution versus its competing disordered counterpart (i.e. the amorphous or metallic glass phase) as a function of the solute concentration. Finally, based on the realistic interatomic potentials, molecular dynamics simulations not only reveal the physical origin of the binary metallic glass formation, but also quantitatively determine, for each system, an alloy composition range, within which the disordered state is energetically favored, thus leading to establish the atomistic theory capable of predicting the GFR, i.e. the quantitative GFA, of the binary metal systems.

Electromigration in submicron interconnect features of integrated circuits

Abstract: Electromigration (EM) is a complex multiphysics problem including electrical, thermal, and mechanical aspects. Since the first work on EM was published in 1907, extensive studies on EM have been conducted theoretically, experimentally, and by means of computer simulation. Today EM is the most significant threat for interconnect reliability in high performance integrated circuits. Over years, physicists, material scientists, and engineers have dealt with the EM problem developing different strategies to reduce EM risk and methods for prediction of EM life time. During the same time a significant amount of work has been carried out on fundamentally understanding of EM physics, of the influence of material and geometrical properties on EM, and of the interconnect operating conditions on EM. In parallel to the theoretical studies, a large amount of work has been performed in experimental studies, mostly motivated by urgent and specific problem settings which engineers encounter during their daily work. On the basis of accelerated electromigration tests, various time-to-failure estimation methods with Blacks equation and statistics have been developed. The big question is, however, the usefulness of this work, since most contributions about electromigration and the accompanying stress effects are based on a very simplified picture of electromigration. The intention of this review paper is to present the most important aspects of theoretical and experimental EM investigations together with a brief history of the development of the main concepts and methods. We present an overview of EM models from their origins in classical materials science methods up to the most recent developments for submicron interconnect features, as well as the application of ab initio and first principle methods. The main findings of experimental studies, important for any model development and application, will also be presented.

Biologically inspired hairy structures for superhydrophobicity

Abstract: Superhydrophobic surfaces have received tremendous attention in the last decade, owing to the number of emerging applications in conservation of environment. These surface properties are based on physio-chemical principles and can be transferred into technical “biomimetic” materials, as successfully done for the Lotus leaves. This article provides a review of the most recent development in superhydrophobic surfaces. Examples of superhydrophobic surfaces from nature are presented. It focuses on the hairy exterior of many different plant and animal species which renders them water repellent for protecting and maintaining crucial life functions. The classical Wenzel and Cassie–Baxter models along with manufacturing and understanding of the wettability of flexible hairy structures are reviewed.

Energetic cluster ion beams: Modification of surfaces and shallow layers

Abstract: Atomic and molecular clusters can be considered to be a distinct form of matter, a “bridge” between atoms on the one hand and solids on the other. Interest in clusters comes from various fields. They can be used as models for investigation of fundamental physical aspects of the transition from the atomic scale to bulk material as well as controllable and versatile tools for modification and processing of surfaces and shallow layers on the nanometer scale. One of the important parameters in the application of cluster beams is the impact (or kinetic) energy. Current paper presents a state-of-the-art review in the field of cluster–surface interaction. The main emphasis is put on cluster collisions leading either to surface modification or implantation of cluster constituents. Both experimental results and data of theoretical modeling are considered. In particular, fundamental physical aspects and possible practical applications of pinning regime (slight cluster embedding into the surface) are under the discussion. Mechanisms of crater and hillock formation on the individual cluster impacts as well as of surface erosion on macroscopic scale (smoothing or dry etching) under the high fluence cluster bombardment are analysed. Specific phenomena of cluster stopping in matter and formation of radiation damage under keV-to-MeV energy implantation are critically analysed and an approach towards finding a universal scaling law for the cluster implantation is suggested. A number of advantages peculiar to the cluster beam technique are discussed in terms of designing and engineering the physical and chemical properties of materials for practical applications.

Threshold voltage shifting for memory and tuning in printed transistor circuits

Abstract: Multiple mechanisms for controllably shifting the threshold voltage of printed and organic transistors have been identified during the last few years, including some just in the past year, that are analogous in some ways to silicon floating gate memory elements. In addition, printed electronic memory is emerging as a serious product technology for identification and banking cards and for responsive systems through the efforts of startup companies. Other circuit applications are also being identified. Memory and tuning are not as prominently discussed in the literature as simpler and more accessible topics such as display driving, charge carrier mobility, voltage reduction, and high-frequency response. This report summarizes the numerous approaches being considered for the definition and control of transistor threshold voltage in alternative electronic technologies, including the theoretical basis for the effects utilized. Higher and more reliable performance parameters and entirely new functionality are among the advantages to be highlighted.

Exploring nanoscale magnetism in advanced materials with polarized X-rays

Abstract: Nanoscale magnetism is of paramount scientific interest and high technological relevance. To control magnetization on a nanoscale, both external magnetic fields and spin polarized currents, which generate a spin torque onto the local spin configuration, are being used. Novel ideas of manipulating the spins by electric fields or photons are emerging and benefit from advances in nano-preparation techniques of complex magnetic materials, such as multiferroics, ferromagnetic semiconductors, nanostructures, etc. Advanced analytical tools are needed for their characterization. Polarized soft X-rays using X-ray dichroism effects are used in a variety of spectroscopic and microscopic techniques capable of quantifying in an element, valence and site-sensitive way basic properties of ferro(i)- and antiferromagnetic systems, such as spin and orbital moments, nanoscale spin configurations and spin dynamics with sub-ns time resolution. Future X-ray sources, such as free electron lasers will provide an enormous increase in peak brilliance and open the fs time window to studies of magnetic materials. Thus fundamental magnetic time scales with nanometer spatial resolution can be addressed. This review provides an overview and future opportunities of analytical tools using polarized X-rays by selected examples of current research with advanced magnetic materials.

Aging-induced anisotropy of mechanical properties in steel products: implications for the measurement of engineering properties | nist

Abstract: Abstract Strain aging in low-carbon steels is a well-known strengthening phenomenon, the typical results of which are an increase in yield stress and/or an increase in the extent of discontinuous yielding. Aging effects are generally characterized through the use of results from mechanical tests in which the strain path prior to aging (prestrain) and the strain path after aging are in the same direction. However, these tests do not completely characterize the properties of aged materials, since the effects of aging are reduced when materials are tested in directions different than the direction of prestrain. The result is anisotropy of properties which can affect the performance of industrial products. In this paper, the effect is demonstrated in two examples of industrial products made from low carbon steels, the aging of which during processing results in performance changes that are not predicted through standard tensile testing of as-fabricated products. The first example compares the effect of aging on yield strength and dent resistance of stamped hood panels on an electro-galvanized, Al killed, bake hardenable sheet steel for auto body panel applications. The second example shows the effect of aging on the anisotropy of tensile data from two American Petroleum Institute (API) grade X100 pipe steels in the as-received condition. The data show that the performance gains realized from strain-aging in the tensile tests on base material are not apparent in the tensile data from the stamped panels after aging, but the dent resistance clearly demonstrated the beneficial effect of aging. The high degree of anisotropy in the yield strength and yielding behaviors between the circumferential and longitudinal tensile data in the two pipe steels demonstrates the effect of strain path on a materials response to aging, which may occur during downstream processing or in field service. The manifestation of material properties that are dependent upon the relationship between the pre-aging strain direction and the post-aging strain direction underscores the importance of correct evaluation of mechanical performance in the design of structural components in materials which undergo aging.