Showing papers in "Materials Science & Engineering R-reports in 2015"

Metallic implant biomaterials

Abstract: Human tissue is structured mainly of self-assembled polymers (proteins) and ceramics (bone minerals), with metals present as trace elements with molecular scale functions. However, metals and their alloys have played a predominant role as structural biomaterials in reconstructive surgery, especially orthopedics, with more recent uses in non-osseous tissues, such as blood vessels. With the successful routine use of a large variety of metal implants clinically, issues associated with long-term maintenance of implant integrity have also emerged. This review focuses on metallic implant biomaterials, identifying and discussing critical issues in their clinical applications, including the systemic toxicity of released metal ions due to corrosion, fatigue failure of structural components due to repeated loading, and wearing of joint replacements due to movement. This is followed by detailed reviews on specific metallic biomaterials made from stainless steels, alloys of cobalt, titanium and magnesium, as well as shape memory alloys of nickel–titanium, silver, tantalum and zirconium. For each, the properties that affect biocompatibility and mechanical integrity (especially corrosion fatigue) are discussed in detail. Finally, the most critical challenges for metallic implant biomaterials are summarized, with emphasis on the most promising approaches and strategies.

Stimulus-responsive hydrogels: Theory, modern advances, and applications

Abstract: Over the past century, hydrogels have emerged as effective materials for an immense variety of applications. The unique network structure of hydrogels enables very high levels of hydrophilicity and biocompatibility, while at the same time exhibiting the soft physical properties associated with living tissue, making them ideal biomaterials. Stimulus-responsive hydrogels have been especially impactful, allowing for unprecedented levels of control over material properties in response to external cues. This enhanced control has enabled groundbreaking advances in healthcare, allowing for more effective treatment of a vast array of diseases and improved approaches for tissue engineering and wound healing. In this extensive review, we identify and discuss the multitude of response modalities that have been developed, including temperature, pH, chemical, light, electro, and shear-sensitive hydrogels. We discuss the theoretical analysis of hydrogel properties and the mechanisms used to create these responses, highlighting both the pioneering and most recent work in all of these fields. Finally, we review the many current and proposed applications of these hydrogels in medicine and industry.

High-K materials and metal gates for CMOS applications

Abstract: The scaling of complementary metal oxide semiconductor (CMOS) transistors has led to the silicon dioxide layer used as a gate dielectric becoming so thin that the gate leakage current becomes too large. This led to the replacement of SiO2 by a physically thicker layer of a higher dielectric constant or ‘high-K’ oxide such as hafnium oxide. Intensive research was carried out to develop these oxides into high quality electronic materials. In addition, the incorporation of Ge in the CMOS transistor structure has been employed to enable higher carrier mobility and performance. This review covers both scientific and technological issues related to the high-K gate stack – the choice of oxides, their deposition, their structural and metallurgical behaviour, atomic diffusion, interface structure, their electronic structure, band offsets, electronic defects, charge trapping and conduction mechanisms, reliability, mobility degradation and oxygen scavenging to achieve the thinnest oxide thicknesses. The high K oxides were implemented in conjunction with a replacement of polycrystalline Si gate electrodes with metal gates. The strong metallurgical interactions between the gate electrodes and the HfO2 which resulted an unstable gate threshold voltage resulted in the use of the lower temperature ‘gate last’ process flow, in addition to the standard ‘gate first’ approach. Work function control by metal gate electrodes and by oxide dipole layers is discussed. The problems associated with high K oxides on Ge channels are also discussed.

A comprehensive review of Li4Ti5O12-based electrodes for lithium-ion batteries: The latest advancements and future perspectives

Abstract: Advanced electrical energy storage technology is a game changer for a clean, sustainable, and secure energy future because efficient utilization of newable energy hinges on cost-effect and efficient energy storage. Further, the viability of many emerging technologies depends on breakthroughs in energy storage technologies, including electric vehicles (EVs) or hybrid electric vehicles (HEVs) and smart grids. Lithium-ion batteries (LIBs), a great success in the portable electronics sector, are believed also the most promising power sources for emerging technologies such as EVs and smart grids. To date, however, the existing LIBs (with LiCoOx cathode and graphite anode) are still unable to meet the strict requirements for safety, cycling stability, and rate capability. The development of advanced anode materials, which can overcome the shortcomings of graphite anode (such as formation of dendritic lithium during charge and undesirable solid electrolyte interface), is of critical importance to enhancing the cycling stability and operational safety of LIBs. Lithium titanate (Li4Ti5O12) has recently attracted considerable attentions as a potential anode material of LIBs for high power applications due to several outstanding features, including a flat charge/discharge plateaus (around 1.55 V vs. Li/Li+) because of the two-phase lithium insertion/extraction mechanism and minimum chance for the formation of SEI and dendritic lithium, dramatically enhance the potential for high rate capability and safety. In addition, there is almost no volume change during the lithium insertion and extraction processes, ensuring a high cycling stability and long operational life. However, the electronic conductivity of Li4Ti5O12 is relatively low, resulting in large polarization lose, more so at higher cycling rates, and poor rate performance. Currently, considerable research efforts have been devoted to improving the performance of Li4Ti5O12 at fast charge/discharge rates, and some important progresses have been made. In this review, we first present a general overview of the structural features, thermodynamic properties, transport properties, and the electrochemical behavior of Li4Ti5O12 under typical battery operating conditions. We then provide a comprehensive review of the recent advancements made in characterization, modification, and applications of Li4Ti5O12 electrodes to LIBs, including nanostructuring, surface coating, morphological optimization, doping, and rational design of composite electrodes. Finally, we highlight the critical challenges facing us today and future perspectives for further development of Li4Ti5O12-based electrodes. It is hoped that this review may provide some useful guidelines for rational design of better electrodes for advanced LIBs.

Thermoelectric power factor: Enhancement mechanisms and strategies for higher performance thermoelectric materials

Abstract: Thermoelectric research has witnessed groundbreaking progress over the past 15–20 years. The thermoelectric figure of merit, ZT, a measure of the competition between electronic transport (i.e. power factor) and thermal transport (i.e. total thermal conductivity), has long surpassed once a longtime barrier of ∼1 and thermoelectric scientists are targeting ZT > 2 as the new goal. A majority of this recent improvement in ZT has been achieved through the reduction of lattice part of thermal conductivity (κl) using nanostructuring techniques. The rapid progress in this direction focused the efforts on the development of experimental methods and understanding phonon transport to decrease lattice thermal conductivity. This fact left the development of ideas to improve electronic transport and thermoelectric power factor rather overlooked. With thermal conductivity of the potential thermoelectrics approaching the minimum theoretical limit, on the journey to higher ZT values, a paradigm shift is necessary toward the enhancement of the thermoelectric power factor. This article discusses the ideas and strategies proposed and developed in order to improve the thermoelectric power factor and thus hopefully move us closer to the target of a ZT > 2!

Spherical nanoindentation stress–strain curves

Abstract: Although indentation experiments have long been used to measure the hardness and Young's modulus, the utility of this technique in analyzing the complete elastic–plastic response of materials under contact loading has only been realized in the past few years – mostly due to recent advances in testing equipment and analysis protocols. This paper provides a timely review of the recent progress made in this respect in extracting meaningful indentation stress–strain curves from the raw datasets measured in instrumented spherical nanoindentation experiments. These indentation stress–strain curves have produced highly reliable estimates of the indentation modulus and the indentation yield strength in the sample, as well as certain aspects of their post-yield behavior, and have been critically validated through numerical simulations using finite element models as well as direct in situ scanning electron microscopy (SEM) measurements on micro-pillars. Much of this recent progress was made possible through the introduction of a new measure of indentation strain and the development of new protocols to locate the effective zero-point of initial contact between the indenter and the sample in the measured datasets. This has led to an important key advance in this field where it is now possible to reliably identify and analyze the initial loading segment in the indentation experiments. Major advances have also been made in correlating the local mechanical response measured in nanoindentation with the local measurements of structure at the indentation site using complementary techniques. For example, it has been shown that the combined use of Orientation Imaging Microscopy (OIM, using Electron BackScattered Diffraction (EBSD)) and nanoindentation on polycrystalline metallic samples can yield important information on the orientation dependence of indentation yield stress, which can in turn be used to estimate percentage increase in the local slip resistance in deformed samples. The same methods have been used successfully to probe the intrinsic role of grain boundaries in the overall mechanical deformation of the sample. More recently, these protocols have been extended to characterize local mechanical property changes in the damaged layers in ion-irradiated metals. Similarly, the combined use of Raman spectroscopy and nanoindentation on samples of mouse bone has revealed tissue-level correlations between the mineral content at the indentation site and the associated local mechanical properties. The new protocols have also provided several new insights into the buckling response in dense carbon nanotube (CNT) brushes. These and other recent successful applications of nanoindentation are expected to provide the critically needed information for the maturation of physics-based multiscale models for the mechanical behavior of most advanced materials. In this paper, we review these latest developments and identify the future challenges that lie ahead.

Losses in ferroelectric materials

Abstract: Ferroelectric materials are the best dielectric and piezoelectric materials known today. Since the discovery of barium titanate in the 1940s, lead zirconate titanate ceramics in the 1950s and relaxor-PT single crystals (such as lead magnesium niobate-lead titanate and lead zinc niobate-lead titanate) in the 1980s and 1990s, perovskite ferroelectric materials have been the dominating piezoelectric materials for electromechanical devices, and are widely used in sensors, actuators and ultrasonic transducers. Energy losses (or energy dissipation) in ferroelectrics are one of the most critical issues for high power devices, such as therapeutic ultrasonic transducers, large displacement actuators, SONAR projectors, and high frequency medical imaging transducers. The losses of ferroelectric materials have three distinct types, i.e., elastic, piezoelectric and dielectric losses. People have been investigating the mechanisms of these losses and are trying hard to control and minimize them so as to reduce performance degradation in electromechanical devices. There are impressive progresses made in the past several decades on this topic, but some confusions still exist. Therefore, a systematic review to define related concepts and clear up confusions is urgently in need. With this objective in mind, we provide here a comprehensive review on the energy losses in ferroelectrics, including related mechanisms, characterization techniques and collections of published data on many ferroelectric materials to provide a useful resource for interested scientists and engineers to design electromechanical devices and to gain a global perspective on the complex physical phenomena involved. More importantly, based on the analysis of available information, we proposed a general theoretical model to describe the inherent relationships among elastic, dielectric, piezoelectric and mechanical losses. For multi-domain ferroelectric single crystals and ceramics, intrinsic and extrinsic energy loss mechanisms are discussed in terms of compositions, crystal structures, temperature, domain configurations, domain sizes and grain boundaries. The intrinsic and extrinsic contributions to the total energy dissipation are quantified. In domain engineered ferroelectric single crystals and ceramics, polarization rotations, domain wall motions and mechanical wave scatterings at grain boundaries are believed to control the mechanical quality factors of piezoelectric resonators. We show that a thorough understanding on the kinetic processes is critical in analyzing energy loss behavior and other time-dependent properties in ferroelectric materials. At the end of the review, existing challenges in the study and control of losses in ferroelectric materials are analyzed, and future perspective in resolving these issues is discussed.

Deep traps in GaN-based structures as affecting the performance of GaN devices

Abstract: New developments in theoretical studies of defects and impurities in III-Nitrides as pertinent to compensation and recombination in these materials are discussed. New results on experimental studies on defect states of Si, O, Mg, C, Fe in GaN, InGaN, and AlGaN are surveyed. Deep electron and hole traps data reported for GaN and AlGaN are critically assessed. The role of deep defects in trapping in AlGaN/GaN, InAlN/GaN structures and transistors and in degradation of transistor parameters during electrical stress tests and after irradiation is discussed. The recent data on deep traps influence on luminescent efficiency and degradation of characteristics of III-Nitride light emitting devices and laser diodes are reviewed.

Overview of the current issues in austenite to ferrite transformation and the role of migrating interfaces therein for low alloyed steels

Abstract: Solid state phase transformations in metals, and more precisely the science of transformation interfaces, is a key point to understand the formation of nano/microstructure, and thus, as a result, many physical properties such as mechanical properties, conductivity, thermoelectric and magnetic properties of materials. Steels are by far the most widely used metallic alloys, and a deep understanding of their microstructure is essential to tailor their service properties. The transformation of high temperature parent austenite to ferrite is one of the main issues controlling the final microstructures, and for more than a century, this has driven metallurgists to investigate in detail this solid state transformation, and, particularly, the details of austenite to ferrite interface migration. In this paper, we review the evolution of the different concepts and experiments developed in the last century to investigate this transformation mechanism. After a brief introduction, most of the physical models developed, which reduce the α/γ interface into a mathematical body with its own properties, are reviewed and discussed with regard to experimental data. The increased availability of highly sophisticated experimental and modelling tools in recent decades has considerably clarified the perceptions of transformation interfaces. These recent advances are presented, and their contribution to the field of migrating austenite–ferrite interfaces are highlighted in a third section. In the fourth section, the latest developments in experimental methods, which now allow the quasi atomistic direct characterization of the interface chemistry, are presented. The observed conditions at the interfaces can be compared with model predictions, which is believed to be a critical step for the refinement of the theoretical concepts guiding the understanding of the interface migration. Finally, in the concluding section, the present situation of the field is summarized, and some perspectives regarding the expected future developments are sketched.

Porous and high surface area silicon oxycarbide-based materials—A review

Abstract: Silicon oxycarbide (SiOC)-based materials are a class of polymer-derived ceramics that enables the formation of a homogeneous structure at the molecular level starting from polymer precursors. In this system, oxygen and carbon atoms share bonds with silicon atoms in the amorphous network structure while elemental carbon, and possibly nanosized SiO 2 and SiC nanodomains may co-exist. Because of the flexibility of molecular level composition and microstructure designs, the systems can be made porous with high specific surface areas by changing the precursor compositions and the ceramization conditions. In this review, two strategies of creating porous SiOCs are discussed: conventional approach of using fugitive fillers, as well as pore formation and selective removal of certain SiOC matrix compositions (such as carbon, SiO 2 , or SiC) at the molecular level. For the former, it includes ceramic replication of an organic template, direct foaming, and sacrificial pore formers. For the latter, it includes molecular level pore formation, molecular level species removal, and SiOC porous network creation through molecular templates. Direct pore formation can be achieved by changing processing conditions, using different precursor architectures, and using different hydrosilylation agents. For SiOC porous network creation through molecular level species removal, it includes molecular level free carbon removal, molecular level SiO 2 nanocluster removal, and molecular level carbon removal from SiC (and possibly BC x for SiOBC). To understand single nanometer (

Charge transfer and storage in nanostructures

Abstract: Efficient storage and conversion of electrical charge in materials, to a voltage and current, provides the basis for batteries and capacitors. Given the widespread usage of portable electronics there is a continual need to further enhance the energy and power density of such devices, which could be accomplished through the use of nanostructured materials. The large surface area to volume ratio and the possibilities of new materials physics and chemistry provide the rationale for their use and is discussed. The former aspect considers the relevance to the area-dependent capacitance as well as the parasitic elements that reduce the charge and energy delivery from the theoretical maximum values. Specific instances of electrode materials, as well as the electrode–electrolyte interface and electrolyte properties, with respect to their capability and prospects are examined. Alternate internal and external surface dependent Faradaic reactions and concomitant pseudocapacitance based mechanisms, seem to have the ability to bridge the large energy densities of batteries to the power density of the capacitors perhaps helping in realizing a truly useful hybrid device. While much of the report relates to presently used devices such as Li-ion batteries and activated carbon based electrochemical capacitors, the relevant principles are shown to be valid for other types of charge conversion agents such as photoelectrochemical and dye-sensitized solar cells. The review also considers perspectives on alternate materials and architectures.

Lanthanum chromite based perovskites for oxygen transport membrane

Abstract: Judicious selection of mixed ionic–electronic conducting (MIEC) perovskite oxide as oxygen transport membrane (OTM) offers the potential to enhance overall process economics and systems performance for a wide variety of industrial applications ranging from clean and efficient energy conversion (oxy-combustion) to selective gas separation (high purity oxygen production) and value added chemicals (syngas and liquid fuel) production with near-zero greenhouse gas emissions. Doped lanthanum chromite perovskites have been considered as promising material of choice for oxygen transport membrane (OTM) due to their superior thermo-chemical stability in aggressive environment (800–1000 °C, 0.21–10−20 P O 2 ) than the other mixed ionic–electronic conducting (MIEC) perovskites such as ferrites and cobaltite's. Thermo-physical properties of the lanthanum chromite, required for optimum oxygen transport can be tuned by modifying the crystal structure, chemical bonding, and ionic and electronic transport properties through selection of dopant's type and level. A perspective on the development of lanthanum chromite-based oxygen transport membranes is presented with an insight based on the pertinent literature and data analysis. The role of various A- and B-site dopants on the crystal structure, densification, thermal expansion, electrical transport, oxygen permeation, mechanical properties, and thermochemical stability of lanthanum chromite is discussed to enlighten ‘composition–structure–property’ correlations. It has been found that: the preferred dopants are strontium at A-site and manganese, nickel, iron, and titanium at B-site to obtain the desired thermo–chemo–electro–mechano properties. Challenges for long term performance and structural stability of doped lanthanum chromite as an oxygen transport membrane are outlined for the applications under ‘real system’ exposure conditions.

Friction, wear and mechanical behavior of nano-objects on the nanoscale

Abstract: Nano-objects are used in various applications where they come into sliding contact with each other and the surfaces where they are used. This can lead to nano-object deformation. Some examples of these applications include drug delivery for cancer treatment, oil detection, contaminant removal, catalysis, and tribology on the macro- to nanoscale. Fundamental understandings of friction and wear of nano-objects, mechanical properties, and deformation mechanisms have been gained through studies. These examined friction mechanisms in nano-object(s) contact sliding on dry and submerged-in-liquid surfaces, sliding on a single nano-object, and mechanical behavior. Nano-object friction studies used an atomic force microscope. Single nano-object contact studies (lateral-push) provide understanding of friction mechanisms, showing friction is influenced by real area of contact, roughness, and work of adhesion. Friction is lower in liquid environments versus dry environments. Contact studies of multiple nano-objects investigate whether several nano-objects reduce friction and wear between sliding surfaces as a result of lower contact area. Further studies on single nano-object friction reveal dependence on topography, scale, and material. Mechanical behavior studies investigate deformation during indentation and compression. Indentation studies investigate scale effects on hardness and Young's modulus. Compression studies investigate reverse plasticity and deformation resistance. This comprehensive study review assists understanding of fundamental interfacial interactions and deformation mechanisms. Studies reported use gold nano-objects, molybdenum disulfide and tungsten disulfide multi-walled nanotubes, and carbon nanohorns, which are of general interest.