Sculptured thin films — II. Experiments and applications

 A report on the preparation and potential applications of sculptured thin films (STFs) is presented. STFs are nano-engineered columnar thin films in which the columnar direction can made to change easily and often during growth. STFs of virtually any material can be prepared through directional vapor deposition onto any surface under low adatom-mobility conditions. Columnar shapes, such as zig-zag, C-, S-, helicoidal and superhelical, can be engineered in any sequence and with controlled density profiles. Although oblique deposition and the resulting anisotropic properties have been known for over a century, the general recognition that such nano-engineered morphologies can lead to unique and predictable optical, mechanical, electrical, magnetic, thermal, and biological properties has occured only recently. Received: 18 August 1998 / Reviewed and accepted: 28 September 1998     

Surface energy-mediated fibronectin adsorption and osteoblast responses on nanostructured diamond

Nanostructured diamond have potential applications in many biomedical related fields and demonstrated extraordinary capacity to influence cellular responses. Studying the surface property of nanodiamond and its influence to protein adsorption and subsequent cellular responses along with the mechanism behind such capacity becomes more important. Here the role of surface energy associated with nanostructured diamond in modulating fibronectin and osteoblast(OB, bone forming cells) responses was investigated. Nanocrystalline diamond(NCD) and submicron crystalline diamond(SMCD) films with controllable surface energy were prepared by microwave-enhanced plasma chemical vapor deposition(MPCVD) techniques. Fibronectin adsorption on the diamond films with varied surface energy values was measured via the enzyme-linked immunosorbent assay(ELISA) and the relationship between the surface energy and fibronectin adsorption was studied. The result indicated that fibronectin adsorption on nanostructured surfaces was closely related to both surface energy and material microstructures. The spreading and migration of OB aggregates(each containing 30–50 cells) on the NCD with varied surface energy values were also studied. The result indicates a correlation between the cell spreading and migration on nanodiamond and the surface energy of nanostructured surface.     

Growth and optical properties of ZnO low-dimensional nanostructures

Recent studies on the growth of ZnO nanostructures and their optical properties were reviewed. Using different methods, a variety of ZnO nanostructures, including quantum dots nanotowers, nanotubes, nanorods, nanowires, and nanosheets, displaying zero, one, and two dimensions, have been synthesized. The growth of ZnO low-dimensional nanostructures has been demonstrated. Their optical properties have been studied by means of room-temperature photoluminescence spectra, low-temperature photoluminescence spectra, temperature-dependent photoluminescence spectra, and pressure-dependent photoluminescence spectra. The optical properties can be adjusted by the surface features of ZnO low-dimensional nanostructures. The strong exciton emission has been observed in some nanostructures, showing promising potential in nanodevice applications.     

Effect of slurry composition on the chemical mechanical polishing of thin diamond films

Nanocrystalline diamond (NCD) thin films grown by chemical vapour deposition have an intrinsic surface roughness, which hinders the development and performance of the films’ various applications. Traditional methods of diamond polishing are not effective on NCD thin films. Films either shatter due to the combination of wafer bow and high mechanical pressures or produce uneven surfaces, which has led to the adaptation of the chemical mechanical polishing (CMP) technique for NCD films. This process is poorly understood and in need of optimisation. To compare the effect of slurry composition and pH upon polishing rates, a series of NCD thin films have been polished for three hours using a Logitech Ltd. Tribo CMP System in conjunction with a polyester/polyurethane polishing cloth and six different slurries. The reduction in surface roughness was measured hourly using an atomic force microscope. The final surface chemistry was examined using X-ray photoelectron spectroscopy and a scanning electron microscope. It was found that of all the various properties of the slurries, including pH and composition, the particle size was the determining factor for the polishing rate. The smaller particles polishing at a greater rate than the larger ones.     

Biological evaluation of ultrananocrystalline and nanocrystalline diamond coatings

Nanostructured biomaterials have been investigated for achieving desirable tissue-material interactions in medical implants. Ultrananocrystalline diamond (UNCD) and nanocrystalline diamond (NCD) coatings are the two most studied classes of synthetic diamond coatings; these materials are grown using chemical vapor deposition and are classified based on their nanostructure, grain size, and sp³ content. UNCD and NCD are mechanically robust, chemically inert, biocompatible, and wear resistant, making them ideal implant coatings. UNCD and NCD have been recently investigated for ophthalmic, cardiovascular, dental, and orthopaedic device applications. The aim of this study was (a) to evaluate the in vitro biocompatibility of UNCD and NCD coatings and (b) to determine if variations in surface topography and sp³ content affect cellular response. Diamond coatings with various nanoscale topographies (grain sizes 5–400?nm) were deposited on silicon substrates using microwave plasma chemical vapor deposition. Scanning electron microscopy and atomic force microscopy revealed uniform coatings with different scales of surface topography; Raman spectroscopy confirmed the presence of carbon bonding typical of diamond coatings. Cell viability, proliferation, and morphology responses of human bone marrow-derived mesenchymal stem cells (hBMSCs) to UNCD and NCD surfaces were evaluated. The hBMSCs on UNCD and NCD coatings exhibited similar cell viability, proliferation, and morphology as those on the control material, tissue culture polystyrene. No significant differences in cellular response were observed on UNCD and NCD coatings with different nanoscale topographies. Our data shows that both UNCD and NCD coatings demonstrate in vitro biocompatibility irrespective of surface topography.     

Polyurethanes for “Two-dimensional” applications

The unsurpassed versatility of polyurethanes is remarkably evident in the areas of two-dimensional applications. Polyurethanes are tailor-made for the wide range of rigid surface coatings, eg; for metal, concrete and wood; for flexible surface coatings; for textile substrates and leather; and for a wide variety of adhesives. Polyurethanes give unexcelled performance in almost all of these applications. In many cases this superiority is achieved by the fact that polyurethanes are generally reactive processing polymers which can be intrinsically cross-linked to the optimal degree. This key fact can be accounted for by the versatility of the NCO group, an “energy package” of unique quality. Two-component polyurethane coatings based on aliphatic polyisocyanates have experienced significant breakthroughs in automotive applications and for powder coatings. A prime target of coating textiles with polyurethanes is the achievement of the quality of natural leather. A highly water vapor permeable, but water repellent textile coating has recently become possible using a direct coating process. The application of a microporous polyurethane film on split leather by a reaction process leads to prime quality leather for shoe uppers. For many years, elastomeric polyurethanes have been the top-ranking adhesives for highly plasticized PVC articles. More recently, polyurethanes have been introduced into the market as structural adhesives, especially for the bonding of automotive body parts. A new principle of heterogeneous cross-linking of polyurethanes which has potential for one-package structural adhesives is presented. Aqueous polyurethane can also be tailor-made for the whole range of two-dimensional applications. They complement the high solids and the powder polyurethane coatings, as well as the solvent-free polyurethane adhesives in the fulfillment of ecological and industrial hygiene requirements.     

Hybrid Mesoporous Materials with Functionalized Monolayers

Mesoporous materials have great potential for environmental and industrial processes, but many applications require the materials to exhibit specific surface chemistry and binding sites. A new approach has been developed so that organized functional monolayers are covalently bound to mesoporous supports. The functionalized hybrid materials show exceptional selectivity and capacity for removing heavy metals from waste streams. Tailored hybrid materials have also shown potential to selectively bind anions and radionuclides. Rational design of the surface properties of mesoporous materials will lead to more sophisticated functional composites.     

Effect of rapid thermal annealing treatment on the field-emission characteristics of nanocrystalline diamonds grown on various metal/silicon substrates

In this study, nanocrystalline diamond (NCD) films were deposited on various metal/silicon substrates using a microwave plasma chemical vapor deposition system. Metal layers used are chromium, titanium, aluminum and were used as the electron source for field emitters. These NCD/metal/silicon structures were subsequently annealed at 500 °C in a rapid thermal annealing (RTA) furnace. After RTA treatment, the surface of NCD films becomes flat and the grain boundaries can no longer be clearly seen. The intensity of graphitic peak is substantially decreased and the sp³ content of NCD films is increased. The chemical composition of NCD film remains unchanged after RTA treatment, but the sp³/sp² ratio in C 1s has been increased. It is found that the field-emission characteristics of diamond emitter not only can be effectively controlled by the metal used in the metal/NCD/Si structure, but also can be further enhanced by the improved microstructure of the NCD film obtained after RTA treatment.     

Sputtered tungsten-based ternary and quaternary layers for nanocrystalline diamond deposition

Many of today's demanding applications require thin-film coatings with high hardness, toughness, and thermal stability. In many cases, coating thickness in the range 2-20 microm and low surface roughness are required. Diamond films meet many of the stated requirements, but their crystalline nature leads to a high surface roughness. Nanocrystalline diamond offers a smoother surface, but significant surface modification of the substrate is necessary for successful nanocrystalline diamond deposition and adhesion. A hybrid hard and tough material may be required for either the desired applications, or as a basis for nanocrystalline diamond film growth. One possibility is a composite system based on carbides or nitrides. Many binary carbides and nitrides offer one or more mentioned properties. By combining these binary compounds in a ternary or quaternary nanocrystalline system, we can tailor the material for a desired combination of properties. Here, we describe the results on the structural and mechanical properties of the coating systems composed of tungsten-chromium-carbide and/or nitride. These WC-Cr-(N) coatings are deposited using magnetron sputtering. The growth of adherent nanocrystalline diamond films by microwave plasma chemical vapor deposition has been demonstrated on these coatings. The WC-Cr-(N) and WC-Cr-(N)-NCD coatings are characterized with atomic force microscopy and SEM, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and nanoindentation.     

多孔磁性复合微球的制备及其功能化研究进展

与常规磁性微球相比,多孔磁性复合微球具有比表面积大、密度小等特性,因此得到了人们的普遍关注。近几年来,多孔磁性复合微球的制备技术得到了很大发展。目前,基本实现了比表面积可控、孔径和表面化学性质可调。本文详细介绍了多孔磁性复合微球的制备方法,总结了成孔机理和影响孔结构的因素,并对磁性微球表面功能化的方法作了相关概述。     

Mechanisms,models and methods of vapor deposition

The condensation and assembly of atomic fluxes incident upon the surface of a thin film during its growth by vapor deposition is complex. Mediating the growth process by varying the flux, adjusting the film temperature, irradiating the growth surface with energetic (assisting) particles or making selective use of surfactants is essential to achieve the level of atomic scale perfection needed for high performance films. A multiscale modeling method for analyzing the growth of vapor deposited thin films and nanoparticles has begun to emerge and is reviewed. Ab-initio methods such as density functional theory are used to provide key insights about the basic mechanisms of atomic assembly. Recent work has explored the transition paths and kinetics of atomic hopping on defective surfaces and is investigating the role of surfactants to control surface atom mobility. New forms of interatomic potentials based upon the embedded atom method, Tersoff and bond order potentials can now be combined with molecular dynamics to investigate many aspects of vapor phase synthesis processes. For example, the energy distribution of atoms emitted from sputtering targets, the effects of hot atom impacts upon the mechanisms of surface diffusion, and the role of assisting ions in controlling surface roughness can all be investigated by this approach. They also enable the many activation barriers present during atomic assembly to be efficiently evaluated and used as inputs in multipath kinetic Monte Carlo models or continuum models of film growth. This hierarchy of modeling techniques now allows many of the atomic assembly mechanisms to be incorporated in film growth simulations of increasing fidelity. We identify new opportunities, to extend this modeling approach to the growth of increasingly complicated material systems. Using the growth of metal multilayers that exhibit giant magnetoresistance as a case study, we show that the approach can also lead to the identification of novel growth processes that utilize adatom energy control, very low energy ion assistance, or highly mobile, low solubility chemical species (surfactants) to control surface diffusion controlled film growth. Such approaches appear capable of enabling the creation of multilayered materials with exceptionally smooth, unmixed interfaces, with significantly superior magnetoresistance.     

Biomaterials for ocular reconstruction

Synthetic materials have played a significant role in ophthalmic applications to improve vision for many years. This has been in four main areas in ophthalmology: ocular surface reconstruction, lens replacement, vitreous replacement and structural support and cell transplantation in the retina. Corneal replacement therapies have been developed using both synthetic acrylic-based materials and more recently naturally derived materials such as amniotic membrane. Intraocular lenses as a replacement for the natural lens post cataract surgery has been used for many years. Newer developments include the opportunity to use gels so that the lenses can accommodate but these need improving in terms of the cross-linking chemistry. Silicone oils have been used as long-term tamponade agents as vitreous replacements but recent developments in their properties has enhanced the clinical outcomes and further research into their use as drug delivery vehicles will be a major advancement. Regenerative medicine therapies to repopulate the retina to repair and replace specific cell layers require the optimisation of synthetic scaffolds and this is a major area for development. Recent developments in biomaterials have emphasised the importance of the physical, chemical and mechanical properties specific to a particular ophthalmic application. Materials science has a critical role in developing strategies to overcome vision loss.     

Detecting barely visible impact damage detection on aircraft composites structures

Composites have many advantages as aircraft structural materials and for this reason their use is becoming increasingly widespread. Fragility of composite material to impact loading limits their application in aircraft structures. In particular, low velocity impacts can cause a significant amount of delamination, even though the only external indication of damage may be a very small surface indentation. This type of damage is often referred to as barely visible impact damage (BVID), and it can cause significant degradation of structural properties. If the damaged laminate is subjected to high compressive loading, buckling failure may occur. Therefore, there is the need to develop improved and more efficient means of detecting such damage. In this work a new NDT approach is presented, based on the monitoring of the nonlinear elastic material behaviour of damaged material. Two methods were investigated: a single-mode nonlinear resonance ultrasound (NRUS) and a nonlinear wave modulation spectroscopy (NWMS). The developed methods were tested on different composite plates with unknown mechanical properties and damage size and magnitude.The presence of the nonlinearities introduced by the damage was clearly identified using both techniques. The results showed that the proposed methodology appear to be highly sensitive to the presence of damage with very promising future applications.     

Factors affecting bonding of metals

Abstract

Adhesives have several advantages over mechanical fastening, and the use of bonding is steadily increasing in many applications. Bonding of primary structures in aircraft has been used for many years and much more research has been directed towards aluminium than any other material. Joint performance depends on the alloy, pretreatment, primer, and adhesive. Many studies of these variables have been made, often making use of modern surface analytical techniques. Informative studies have also been carried out on titanium and steel. Hot, humid conditions, especially combined with stress, can be especially damaging to metal joints. A full understanding of the factors that provide the best resistance to this damaging effect has not yet been achieved. However, stable oxide layers with the correct topography appear critical and selection of the correct pretreatment is therefore of paramount importance.

MST/475     

Analysis of early-stage frost formation in natural convection over a horizontal cylinder

Frost growth process on a cold surface consists of two stages: The early-stage or one-dimensional growth of ice columns and multidimensional growth in the form of a porous structure. The transition time which marking these two stages is important for any numerical modeling of frost formation. This paper proposes a mathematical model to predict the transition time and frost properties in natural convection of frost formation over a cooled horizontal cylinder in the first stage of growth period. Comparison is performed among the results of this model and experimental observations reported in the literatures. It is observed that the presented model can be used more efficiently to determine transition time and frost properties in the early-stage of frost formation. Based on the obtained results a new correlation is developed for the duration time of early-stage of frost formation process (transition time) in natural convection.     

Surface Tailoring of Nanoparticles via Mixed‐Charge Monolayers and Their Biomedical Applications

The recent convergence of nanomaterials and medicine has provided an expanding horizon for people to achieve encouraging advances in many biomedical applications such as cancer diagnosis and therapy. However, to realize desirable functions in the rather complex biological systems, a suitable surface coating is greatly in need for nanoparticles (NPs), regardless of the species. In this review, a recently developed surface modification strategy is highlighted—mixed‐charge monolayers—with an emphasis on the nanointerfaces of inorganic NPs. Two typical mixed‐charge gold NPs (AuNPs) prepared from surface modifications with different combinations of oppositely charged alkanethiols are shown as detailed examples to discuss how the mixed‐charge monolayer can help NPs meet the criteria for in vitro and in vivo biomedical applications, including those critical issues like colloidal stability, nonfouling properties, and smart responses (pH‐sensitivity) for tumor targeting.     

高分子基磁性纳米复合物的研究进展

磁性纳米颗粒由于具有不同于大块样品的物理和化学性质,因而在日常生活中具有广泛的应用并且日趋复杂.因此控制磁性纳米材料的稳定性及表面的功能化就变得尤为重要.聚合物作为覆盖层可以很好地控制磁性纳米颗粒的成核、生长和团聚,此外,两者复合后还可改善聚合物的性质.评述了近些年来聚合物磁性纳米复合物的研究进展,并提出了纳米复合物需要解决的问题.     

Electromagnetism as a means for understanding materials mechanics phenomena in magnetic materials

Electromagnetic properties are an interesting means for monitoring a variety of materials' mechanical properties in ferro‐ and paramagnetic materials non‐destructively. Those properties include uni‐ und multi‐axial stress states as well as plasticity and fatigue damage and can be measured at macro‐ as well as at microscopic scales, depending on what measurement equipment will be used. The article describes the general electromagnetic phenomena to be considered as well as the equipment to be used before presenting a variety of different experimental results from which the materials mechanical properties mentioned above can be directly derived being an ideal means for monitoring the health of any magnetic metallic structure.     

Laser Hardening of Austempered Ductile Cast Iron (ADI)*

Austempered ductile cast iron (ADI) has emerged as a major engineering material in recent years. In addition to high strength and relatively light weight (compared to steel), it has high ductility, good wear resistance and good damping capacity. It has many potential applications such as automotive components (e.g. crank shafts and gear boxes) as well as aircraft components (landing gears).

In many structural applications, (e.g. aircraft landing gear) it is often required that the material be hardened at the surface while the interior of the material must remain soft or ductile. The higher hardness at the surface layer imparts excellent wear resistance while the soft inner core provides higher toughness and fracture resistance. The conventional methods of surface hardening such as carburizing and nitriding or shot peening have several limitations, e.g. retained austenite, massive carbide formations and insufficient case depth. In recent years, there has been significant interest in use of laser in surface treating of materials. Surface hardening by means of laser is a very useful technique because of self quenching and minimum of distortion. Laser hardening can also improve significantly the surface properties such as wear and fatigue resistance.     

Rediscovering the Crystal Chemistry of Borides

For decades, borides have been primarily studied as crystallographic oddities. With such a wide variety of structures (a quick survey of the Inorganic Crystal Structure Database counts 1253 entries for binary boron compounds!), it is surprising that the applications of borides have been quite limited despite a great deal of fundamental research. If anything, the rich crystal chemistry found in borides could well provide the right tool for almost any application. The interplay between metals and the boron results in even more varied material's properties, many of which can be tuned via chemistry. Thus, the aim of this review is to reintroduce to the scientific community the developments in boride crystal chemistry over the past 60 years. We tie structures to material properties, and furthermore, elaborate on convenient synthetic routes toward preparing borides.