bluuebunny/crossref_metadata_2025 huggingface dataset:
Texture development during hot extrusion of a gamma titanium aluminide alloy
Authors: Oehring M. , Appel F. , Brokmeier H.-G. , Lorenz U.
Published: April 2011

The evolution of preferred orientations during processing appears to be of significant importance for the use of γ titanium aluminide alloys, since the desired lamellar microstructures exhibit a strong anisotropy of mechanical properties. As texture evolution certainly is dependent on several factors, involving deformation properties, recrystallization kinetics and particularly the phase constitution, different processing temperatures were investigated. By comparing the results it is indicated that the determined textures can be understood by the deformation modes of the dominating phase at hot-working temperature and the subsequent phase transformations.
A Study of the Deformation Behavior of Lamellar γ-TiAl by Numeric Modeling
Authors: Schaden T. , Fischer F. D. , Clemens H. , Appel F. , Bartels A.
Published: April 2011

In this paper the deformation behavior of a fully lamellar microstructure, which is usually present in cast γ-TiAl based alloys, is studied by numerical modeling. After large compressive deformation at elevated temperatures the lamellar colonies are often bent or buckled depending on their orientation, which is representative for a deformation instability characterized by a large wave length. Such a deformation behavior is triggered by both, structural defects of the lamellae and their somewhat irregular arrangement. In addition, a shear band-type deformation mode occurs according to an instability mode exhibiting a short wave length. These two deformation modes interact in a rather subtle way, which leads to a very inhomogeneous deformation pattern.
Effect of Long-Term Aging and Creep Exposure on the Microstructure of TiAl-Based Alloy for Industrial Applications
Authors: Lapin Juraj , Nazmy Mohamed , Staubli Marc
Published: April 2011

The effect of long-term aging and creep exposure on the microstructure of a cast TiAl-based alloy with nominal chemical composition Ti-46Al-2W-0.5Si (at.%) was studied. The aging experiments were performed at temperatures between 973 and 1073 K for various times ranging from 10 to 14000 h in air. Constant load tensile creep tests were performed at applied stresses ranging from 150 to 400 MPa and at temperatures between 973 and 1123 K up to 25677 h. During aging and creep testing the α2(Ti3Al)-phase in the lamellar and feathery regions transforms to the γ(TiAl)-phase and fine needle-like B2 precipitates. Microstructural instabilities lead to a softening of the alloy. The effect of this softening on long-term creep resistance is negligible at temperatures of 973 and 1023 K.
Creep Behavior and Microstructural Stability of Ti-46Al-9Nb with Different Microstructures
Authors: Bystrzanowski S. , Bartels A. , Clemens H. , Gerling R. , Schimansky F.-P. , Dehm G.
Published: April 2011

In this paper the creep behavior and the microstructural stability of Ti-46Al-9Nb (in at.%) sheet material were investigated in the temperature range of 700°C to 815°C. The study involves three different types of microstructure, namely fully lamellar with narrow lamellar spacing, duplex and massively transformed. Short-term creep experiments conducted at 700°C and 225 MPa confirmed that the lamellar microstructure with narrow lamellar spacing exhibits a much higher creep resistance when compared to the massively transformed and duplex ones. During longterm creep tests up to 1500 hours stress exponents (in the range of 4.4 to 5.8) and apparent activation energies (of about 4 eV) have been estimated by means of load and temperature changes, respectively. Both, stress exponents and activation energies suggest that under the applied conditions diffusion-assisted climb of dislocations is the dominant creep mechanism. The thermal stability of the different microstructures under various creep conditions has been analyzed by electron microscopy and X-ray diffraction. Our investigations revealed considerable stress and temperature induced microstructural changes which are reflected in the dissolution of the α2 phase accompanied by precipitation of a Ti/Nb - rich phase situated at grain boundaries. This phase was identified as a ω-related phase with B82-type structure. It was shown, that in particular the duplex microstructure is prone to such microstructural instabilities.
TEM characterisation of fatigue tested lamellar Ti-44Al-8Nb-1B
Authors: Jiang H , Hu D , Jones I P
Published: April 2011

Detailed microstructural examination by TEM of fine-grained polycrystalline lamellar Ti-44Al-8Nb-1B after fatigue testing (R=0.1) at room temperature has been carried out. The operative slip systems were identified as 1/2<110> ordinary dislocations and 1/6<112> twinning. The results showed no strong relation between operative slip systems and macroscopic Schmid factor and it is believed the local stress conditions control the operation of the slip systems. Translamellar cracking was observed to be associated with fine transverse twins in the gamma lamellae.
Creep of TiAl alloys at 750°C under moderate stress
Authors: Couret Alain , Malaplate Joel
Published: April 2011

The creep properties at 750°C of TiAl alloys produced by cast and powder metallurgy processes are investigated by creep tests and subsequent TEM investigations. It is demonstrated that the same dislocation mechanism is controlling the primary and secondary creep stages in these two alloys. This mechanism is the mixed climb of ordinary dislocations. Explanations to the creep behaviour during primary and secondary stages are then proposed.
Microstructure-Property Relationships of Two Ti2AlNb-based Intermetallic Alloys: Ti-15Al-33Nb(at.%) and Ti-21Al-29Nb(at.%)
Authors: Cowen Christopher J. , Li Dingqiang , Boehlert Carl J.
Published: April 2011

Two Ti2AlNb intermetallic orthorhombic (O) alloys, Ti-15Al-33Nb and Ti-21Al-29Nb(at.%), were subtransus processed into sheets, using pancake forging and hot-pack rolling, and evaluated in tension (25 and 650°C) and creep (650–710°C) and the properties and deformation behavior were related to microstructure. Some of the microstructural features evaluated were grain boundary character, grain size, phase volume fraction, and morphology. The alloy Al content was important to strength and elongation-to-failure (εf), where higher Al contents lead to greater tensile strengths and lower εf values and a corresponding brittle fracture response. However, the room temperature (RT) strengths of Ti-15Al-33Nb, which exhibited greater BCC phase volume fractions than Ti-21Al-29Nb and ductile failure (εf >2%), were always greater than 775 MPa. The creep stress exponents (n) and activation energies (Qapp) suggested that a transition in the dominant creep deformation mechanism exists and is dependent on stress and microstructure. Supertransus heat treatment, which increased the prior-BCC grain size and resulted in a lath-type O+BCC microstructure, resulted in reduced creep strains and strain rates. In fact, the supertransus heat-treated Ti-15Al-33Nb microstructures exhibited greater creep resistance than subtransus heat-treated Ti-21Al-29Nb microstructures. Combining the creep observations with the tensile response, the supertransus heat treated Ti-15Al-33Nb lath O+BCC microstructures exhibited the most attractive combination of tensile strength, εf values, and creep resistance.
Role of Biomechanics in Functional Tissue Engineering
Authors: An Kai-Nan
Published: April 2011

Functional tissue engineering establishes functional criteria for design and manufacture of the scaffold matrix for repair and replacement. It also provides useful and strategic information in mechanical stimulation of the cells seeded in the matrix before and after surgical placement to enhance the success of tissue engineering. Biomechanics plays an important role in accomplishing these requirements by assessing the in vivo environment and the material properties.
Atomistic And Continuum Studies Of Flaw Tolerant Nanostructuresin Biological Systems
Authors: Buehler Markus J. , Yao Haimin , Ji Baohua , Gao Huajian
Published: April 2011

Bone-like biological materials have achieved superior mechanical properties through hierarchical composite structures of mineral and protein. Geckos and many insects have evolved hierarchical surface structures to achieve superior adhesion capabilities. What is the underlying principle of achieving superior mechanical properties of materials? Using joint atomistic-continuum modeling, we show that the nanometer scale plays a key role in allowing these biological systems to achieve such properties, and suggest that the principle of flaw tolerance and design for robustness may have had an overarching influence on the evolution of the bulk nanostructure of bone-like materials and the surface nanostructure of gecko-like animal species. We illustrate that if the characteristic dimension of materials is below a critical length scale on the order of several nanometers, Griffith theory of fracture no longer holds. An important consequence of this finding is that materials with such nano-substructures become flaw-tolerant, as the stress concentration at crack tips disappears and failure always occurs at the theoretical strength of materials, regardless of defects. The atomistic simulations complement continuum analysis and reveal a smooth transition between Griffith modes of failure via crack propagation to uniform bond rupture at theoretical strength below a nanometer critical length. This modeling resolves a long-standing paradox of fracture theories, and these results have important consequences for understanding failure of small-scale materials. Additional investigations focus on shape optimization of adhesion systems. We illustrate that optimal adhesion can be achieved when the surface of contact elements assumes an optimal shape. The results suggest that optimal adhesion can be achieved either by length scale reduction, or by optimization of the contact shape. Whereas change in shape does not lead to robustness, reducing the dimension results in robust adhesion devices.
Contribution of Collagen, Mineral and Water Phases to the Nanomechanical Properties of Bone
Authors: Bembey Amanpreet K. , Koonjul Vanessa , Bushby Andrew J. , Ferguson Virginia L. , Boyde Alan
Published: April 2011

Cortical bone is an anisotropic material, and its mechanical properties are determined by its composition as well as its microstructure. Mechanical properties of bone are a consequence of the proportions of, and the interactions between, mineral, collagen and water. Mid-shaft palmar cortical tissue from the equine third metacarpal bone is relatively dense and uniform with low porosity. The mainly primary osteons are aligned to within a few degrees of the long axis of the bone. Beams of compact cortical bone were prepared to examine effects of dehydration and embedding and to study contribution of collagen and mineral to nano-scale material properties. Five beams were tested: untreated (hydrated); 100% ethanol (dehydrated); or embedded in poly-methylmethacrylate (PMMA) for one normal, one decalcified, and one deproteinated bone sample. Elastic modulus was obtained by nanoindentation using spherical indenters, with the loading direction transverse [1] and longitudinal to the bone axis. By selectively removing water, mineral and organic components from the composite, insights into the ultrastructure of the tissue can be gained from the corresponding changes in the experimentally determined elastic moduli.
Effects of Osteopontin Deficiency and Aging on Nanomechanics of Mouse Bone
Authors: Kavukcuoglu B. , West C. , Denhardt D.T. , Mann A. B.
Published: April 2011

Osteopontin (OPN), a phosphorylated glycoprotein, is among the most abundant non-collageneous bone matrix proteins produced by osteoblasts and osteoclasts. OPN has been implicated in bone formation, resorption and remodeling. However, previous studies have presented contradictory results regarding the effect of OPN on the mechanics and microstructure of bone. This study has used nanoindentation to identify local variations in elastic modulus and hardness of OPN deficient (OPN -/-) and wild-type control (OPN+/+) mouse bones. Specifically, the study has looked at changes in the mechanical properties of OPN-/- and OPN+/+ mouse bones with the mouse's age. Cortical sections of femurs from different age groups ranging from 3 weeks to 58 weeks were tested and compared. The results suggest that there are large, abrupt variations in mechanical properties across the femur's radial section for 3-week-old mouse bone. The hardness (H) drops significantly towards the inner and outer sections so the cortical bone has a mean H=3.66 GPa with a standard deviation of 2.44 GPa. In contrast, the hardness of the 58-week-old mouse bone had a standard deviation of 0.35 GPa and a mean H=1.45 GPa. The hardness across the radial axis of the 58-week-old bone was found to be quite uniform. The elastic modulus showed similar variations to the hardness with respect to age and position on the bone. We conclude that the mechanical properties of the mouse bones decrease substantially with maturity, and statistically the hardness and elastic modulus are more uniform in mature bones than young ones. Surprisingly we found a similar variation in both OPN-/- and OPN+/+ bones, with no statistically significant difference in the mechanical properties of the OPN -/- and OPN+/+ bones. The results for OPN-/- and OPN+/+ mouse bones are particularly important as control of OPN activity has been postulated as a potential treatment for bone pathologies that exhibit a change in the bone mineralization, such as osteoporosis, osteopetrosis and Paget's disease. Understanding the effects of OPN on bone mechanics is a vital step in the development of these new treatments.
Spatially Dependent Mechanical Properties Of Rat Whiskers For Tactile Sensing
Authors: Herzog E. K. , Bahr D. F. , Richards C. D. , Richards R. F. , Rector D. M.
Published: April 2011

A new generation of sensors based on biologically inspired whisking action will help determine the presence and location of solid objects and fluid vortices similar to mechanisms used by whisker bearing animals such as rats and seals. By using nanoindentation, we demonstrate that mechanical properties are essentially uniform by cross section, but vary longitudinally from the whisker base (a 3.9 GPa elastic modulus) to the tip (a 3.1 GPa elastic modulus). Several recent studies show propagation of high frequency information through whiskers that are tuned by their physical properties. In order to fully understand and model these properties, this study demonstrates a more complex whisker structure than previously assumed.
Nano-Mechanical Investigation of the Byssal Cuticle, a Protective Coating of a BioElastomer
Authors: Holten-Andersen Niels , Slack Nelle , Zok Frank , Herbert Waite J.
Published: April 2011

The mechanical properties of the mussel byssal thread have been investigated via nano-indentation, with the emphasis on the differences between the cuticle and the fibrous interior. The cuticle hardness was found to be 30–40% higher than that of the underlying fibrous interior. In contrast, the Young's moduli in the two regions were virtually identical to one another. Elemental analysis via energy dispersive spectroscopy indicated surprisingly high levels of Al and Br in the cuticle considering the low amounts found in seawater. A potential role of Al in byssal thread mechanics is discussed in light of the unique capability of the cuticle to accommodate strains of 70% by the underlying fibrils in the core without delamination.
Effects of Wetting and Desiccation on the Creep Properties of Spider Dragline Silk
Authors: Ritchie Joanne , Smith Christopher , Bell Fraser I. , McEwen Iain J. , Viney Christopher
Published: April 2011

We have previously devised a simple method for quantifying the creep resistance of silk fibres – i.e. the ability of silk to maintain dimensional stability while supporting a constant load. We demonstrated (C. Smith et al., J. Arachnol. 31:421–424, 2003) that creep of spider dragline is significant at stresses that are small compared to the conventional yield strength. In addition, the existence of a limiting creep stress was revealed: if samples are loaded smoothly and quickly to a constant stress lying above the limiting creep stress, they break within a few seconds of the stress being applied. The magnitude of the limiting creep stress is equal to approximately one fifth of the fracture stress recorded in conventional constant strain rate tests (in which the stress is applied more gradually).

Here we develop the method and broaden the study of creep to investigate the effects of immersion in water and in ethanol. Experiments are conducted by attaching a small weight (an appropriate number of office staples) to an approximately 20 cm length of dragline from Nephila clavipes spiders, and monitoring extension as a function of time while the samples are suspended vertically in the liquid. The silk is previously conditioned (immersed without imposing any geometrical constraint) in the liquid, to allow it to supercontract or relax if the liquid promotes such a response. Allowance for buoyancy is made when calculating the net force applied to samples by the weight of the staples. Water is found to have a plasticizing effect; it exacerbates the rate of creep in response to a given small load, and it decreases the limiting creep stress. Ethanol, a desiccant, delays the transition from creep dominated by changes in chain conformation to creep dominated by chain slip, and increases the limiting creep stress. These results draw attention to an important limitation that must be overcome if biomimetic silk is to be used in applications where dimensional stability is required while loads are being supported for long times, especially in a wet environment. The results also point to the molecular origins of the creep sensitivity, and thus to ways of making silk and silk analogues less susceptible to creep.
Micromechanics-Based Modeling of Biological Tissues Using the High-Fidelity Generalized Method of Cells
Authors: Pindera Marek-Jerzy
Published: April 2011

The fundamental problem of micromechanics is the prediction of overall response of a composite material given the properties or response of the individual constituents and their internal geometric arrangement. The recently developed High-Fidelity Generalized Method of Cells is a promising micromechanics model whose predictive capability has been demonstrated for infinitesimal deformations in the presence of inelastic constituent behavior. The extension of this micromechanics model to the finite-deformation regime and incorporation of the quasi-linear viscoelasticity theory for the constituent response extends the range of this model's applicability to the bio-engineering area. Herein, an application involving the response of mitral valve chordae tendineae is presented that demonstrates the model's capability to mimic experimentally-observed response of this class of biological tissues rooted in their characteristic microstructures.
Nanoscale Origins of Spider Dragline Mechanical Properties
Authors: Trancik Jessika E. , Czernuszka Jan T. , Bell Fraser I. , Viney Christopher
Published: April 2011

Several mechanical models, in which a material is treated as a composite of crystalline and amorphous and/or interphase material, were used to predict the tensile modulus of spider dragline along the fiber direction. The models included the Voigt average (which assumes that the fibers/crystals are continuous, and that the strain is the same in all components of the composite); a modified Halpin-Tsai model (which is suitable for predicting the longitudinal elastic modulus for short aligned fiber composites, and is thus more appropriate for silk); and the shear-lag or Cox model (which is a modification of the Voigt average that takes into account a discontinuous nature of stiff fibers/crystals and the resulting shear stress in the amorphous matrix). The latter two models yielded close approximations of an experimentally measured elastic modulus of Latrodectus hesperus (black widow spider) dragline under conditions of controlled temperature and humidity, given realistic inputs for the moduli of the individual components and the percent crystallinity. A literature model for the stress-strain behavior of silk [1] was also considered, in the context of our experimental results from transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies of L. hesperus dragline. TEM and XRD results indicated a bimodal size distribution of ordered regions; one population of crystals has a mean size of 2 nm, and another spans the size range 40–120 nm. The average elastic modulus measured from L. hesperus dragline is 23 GPa - close to the 25 GPa theoretical modulus for the case of large crystals in Termonia's model. The tensile strength of L. hesperus dragline is ca. 1.7 GPa, close to the case predicted for small crystals in Termonia's model. A combination of the small and large crystals could explain the forced elongation behavior of L. hesperus dragline.
Towards a Mechanical Model of Skin: Insights into Stratum Corneum Mechanical Properties from Hierarchical Models of Lipid Organisation
Authors: O'Malley Brendan , Moore David J. , Noro Massimo , Anwar Jamshed , Notman Becky , Dauskhardt Reinhold , Bedford Eilidh
Published: April 2011

The stratum corneum (SC), the outermost layer of the skin, provides the body with a physiologically essential barrier to unregulated water loss and the influx of exogenous substances. Furthermore, the 10–20 micron thick SC, composed of overlapping protein-rich corneocytes surrounded by a heterogeneous multilamellar lipid matrix, displays tremendous mechanical cohesion and thermal integrity. To understand the contribution of these components to SC mechanical properties requires building a complete mechanical model of the skin. In this study we focus on modelling the hierarchical microstructure of the lipid phase and its relation to mechanical properties using a combination of atomistic and mesoscale simulations. The modelling approaches are parameterised with experimental data from FT-IR spectroscopy, X-ray scattering and, in the case of the mesoscale simulations, with detailed density profiles derived from atomic models. The atomistic models are used to probe the role of specific lipid species in maintaining the thermal and structural stability of the SC extracellular lipid matrix and to investigate the role of hydrogen bonding networks in SC lipid cohesion. Mesoscale models are used to investigate domain formation and lipid bilayer organisation on length and time scales inaccessible with atomistic models. These coarse grained models display transitions between ordered hexagonal gel phases and fluid phases, reproducing the experimentally observed ordering of the hydrophilic and hydrophobic regions.
Solving materials design problems in biology and technology – a case study
Authors: Vincent Julian F. V.
Published: April 2011

In a series of studies we have reclassified the TRIZ categories of ‘contradiction’ and ‘inventive principle’ derived by Altshuller and his colleagues and show that the hierarchical relationships of the parts of a problem are important, such that at low levels of hierarchy, materials and structure predominate, but at high levels, it's information which is important.

By identifying the functional conflicts in its design, the cuticle of arthropods can be shown to cope with IR and UV irradiation in the same manner as our technology – by controlling spectral properties. However the skeletal properties of cuticle are integrated with demands for sensing, movement, etc, by controlling the local properties of the material rather than by changing global parameters (which would be the technical solution). The biomimetic similarity of cuticle with technology is only about 20%, suggesting that we can learn from the design of arthropod cuticle.
Effects of Aging on the Toughness of Human Cortical Bone: A Study from Nano to Macro Size-Scales
Authors: Nalla Ravi K. , Kruzic Jamie J. , Kinney John H. , Balooch Mehdi , Ager Joel W. , Martin Michael C. , Tomsia Antoni P. , Ritchie R. O.
Published: April 2011

Age-related deterioration of both the fracture properties and the architecture of bone, coupled with increased life expectancy, are factors leading to the increasing incidence of bone fracture in the elderly. In order to facilitate the development of treatments which counter this increased fracture risk, a thorough understanding of how fracture properties degrade with age is required. The present study describes ex vivo fracture experiments to quantitatively assess the effects of aging on the fracture toughness of human cortical bone in the longitudinal direction. Because cortical bone exhibits rising crack-growth resistance with crack extension, we depart from most previous studies by evaluating the toughness in terms of resistance-curve (R-curve) behavior, measured for bone taken from donors 34 to 99 years old. Using this approach, both the crack-initiation and crack-growth toughness are determined and are found to deteriorate with age; the initiation toughness decreases ∼40% from 40 to 100 years, while the growth toughness is effectively eliminated over the same age range. Evidence from x-ray synchrotron tomography is provided to support the hypothesis that the reduction in crack-growth toughness is associated primarily with a degradation in the degree of extrinsic toughening, in particular involving crack bridging at the microstructural level in the wake of the crack. Atomic force microscope-based nanoidentation of individual collagen fibers revealed changes at the collagen fibrillar level and deep-ultraviolet Raman spectroscopy showed that the cross-linking at the nanostructural level also changes with age. These results should provide for a better mechanistic understanding of the increased propensity for bone fracture with age.
Poro-Micromechanics of Bone: Impact Loading and Wave Propagation
Authors: Hellmich Christian , Ulm Franz-Josef
Published: April 2011

Based on ‘universal’ mechanical building blocks inherent to all different bones, this paper shows the integration of two classically separated fields of bone biomechanics, namely bone micromechanics[1] on the one hand, and bone poromechanics[2] on the other. Indeed, despite the complex hierarchical organization of bone, it was recently possible to identify such elementary components at the micro and nanolevel of the material for the explanation of the diversity of macroscopic (poro-)elastic properties of different bones. The mechanical properties (i.e. elasticity) of these elementary components are (within limits of experimental scatter) the same for all bones; they are ‘universal’, i.e. independent of tissue type, species, and anatomical location. The mechanical interaction between these elementary components (mechanical morphology) and the dosages of these components in different tissues determine the macroscopic material properties.

Continuum micromechanics has turned out as well-suited theoretical tool to derive tissue-dependent elastic properties at different length scales from upscaling the elasticity of ‘universal’ building blocks from the nanometer scale. Once respective micromechanics models are developed and validated, poro-micromechanics allows for the quantification of poroelastic properties such as the Biot and Skempton coefficients, as functions of the volume fractions of mineral, collagen, and the micropore space (Haversian and Volkmann canals, and the inter-trabecular space). Skempton coefficients, related to undrained conditions, allow for quantification of the marrow pressure rise due to impact loading, as can be shown by model predictions of non-destructive impact experiments. Moreover, the aforementioned poroelastic properties enter the governing equations for wave propagation in anisotropic porous media. They allow for the prediction of undrained, fast and slow waves, as is verified by comparison of model results with experimental findings.
Controlling Fracture Behavior of Polymeric Hydrogels
Authors: Kong Hyun Joon , Mooney David J.
Published: April 2011

Refined control over the mechanical properties of hydrogels formed from cross-linking of polymers is increasingly regarded as critical for their successful application. In general, increasing the cross-linking density (ρ) of polymer gels raises the mechanical rigidity, but makes the gels more brittle. We proposed that controlling properties of the cross-linking junction and the cross-linking type would mediate the fracture response of the gels, and allow one to decouple the dependency of the mechanical stiffness and toughness from ρ of the gel. This possibility was investigated with alginate hydrogels, because alginate can be gelled via ionic or covalent cross-linking. Increasing ρ of the gels formed using covalent cross-linking with adipic acid dihydrazide or poly (acrylamide-co-hydrazide) raised the elastic modulus (E), but led to a reduction in the toughness of the gels. In contrast, increasing the number of calcium cross-links slowed the crack opening of the gels, and subsequently raised both E and work to fracture (W). From the results of this study, we could demonstrate a novel approach to regulate different mechanical properties of gels in an independent manner. This study provides a valuable guideline to the design of a broad array of polymer hydrogels.
Synthesis of Hydrophilic Two-Photon Absorptive Fullerene-diphenylaminoflourene Dyads for Molecular Self-assembly in Water
Authors: Verma Sarika , Hauck Tanya , Padmawar Prashant A. , Canteenwala Taizoon , Chiang Long Y. , Pritzker Kenneth P.H.
Published: April 2011

Synthetic conditions for the preparation of amphiphilic two-photon absorptive poly(ethylene glycolated) diphenylaminofluorene[60]fullerene conjugates C60(>DPAF-PEG6) were investigated. UV-vis spectra of C60(>DPAF-PEG6) collected at the concentration of 1.0 × 10–5 M in water showed absorption bands with a slight red shift, broadened, and in a less intensity as compared with those of the bands obtained in CHCl3. These features are important characteristics of C60(>DPAF-PEG6) molecular aggregation in water. Transmission electron micrographs taken at the concentration of 1.0 × 10–5 M in H2O clearly displayed images of spherical aggregates in a diameter of 200–400 nm. That substantiated high hydrophobic intermolecular interactions between these amphiphilic molecules for assemblies in water.
Fabrication of Polydiacetylene Nanocrystals Deposited with Silver Nanoparticles for a Nonlinear Optical Material
Authors: Onodera Tsunenobu , Oikawa Hidetoshi , Kasai Hitoshi , Nakanishi Hachiro , Sekiguchi Takashi
Published: April 2011

Noble metal-coated polydiacetylene (PDA) nanocrystals are expected to enlarge effective third-order nonlinear optical susceptibility, owing to the enhancement of optical electric field induced by localized surface plasmon. The surface of PDA nanocrystals was decorated with silver nanoparticles, and the linear optical properties were investigated. The well-defined PDA nanocrystals were first prepared by the conventional reprecipitation method as an aqueous dispersion. The following two-steps of reduction methods, namely, seed deposition and seed growth, enabled higher silver coverage on the surface of PDA nanocrystals. Silver fine seeds were successfully deposited on the PDA surface by particularly employing an anionic surfactant as a binder. The extinction spectra of the PDA nanocrystals covered with silver exhibited the red-shift and broadening of surface plasmonic peak for silver nanoparticles, which would indicate the interaction among silver nanoparticles on the same PDA nanocrystal. In addition, the red-shift of excitonic peak for PDA nanocrystals was observed, resulting from the changes of dielectric properties around PDA nanocrystal. Both spectral changes would have an influence on nonlinear optical properties of PDA nanocrystal.
Size-effect on Fluorescence Spectrum of Perylene Nanocrystal Studied by Single-particle Microspectroscopy Coupled with Atomic Force Microscope Observation
Authors: Matsune Hideki , Asahi Tsuyoshi , Masuhara Hiroshi , Kasai Hitoshi , Nakanishi Hachiro
Published: April 2011

This paper presents single particle fluorescence spectroscopy of perylene nanocrystals coupled with AFM observation of their topographic shapes. The fluorescence spectra of individual nanocrystals confirmed clearly and precisely a blue-shift in the excimer emission maximum on the reduction of their volume. The result can be explained in terms of “lattice softening”, which makes intermolecular interaction weaker and modified the energy level of the excimer state in nanocrystal.
A Novel Light-Emitting Mixed-Ligand Iridium(III) Complex With a Polymeric Terpyridine-PEG Macroligand: Synthesis And Characterization
Authors: Holder Elisabeth , Marin Veronica , Meier Michael A. R. , Kozodaev Dmitry , Schubert Ulrich S.
Published: April 2011

The synthesis of mixed ligand orthometalated iridium(III) complexes as well as the characterization of their electro-optical properties is briefly illustrated. The cyclic voltammetry results exemplify an enhanced stability of the complex with a poly(ethylene glycol (PEG) side chain compared to the corresponding organometallic model material. Less degradation of the electrically active centers was found to occur on the electrode surface. Furthermore the morphology of thin films of the described polymeric material was investigated with respect to potential applications in thin film devices.
Optical Properties of Polystyrene Opals Infiltrated with Cyanine Dyes in the form of J-Aggregates
Authors: Marabelli F. , Comoretto D. , Bajoni D. , Galli M. , Fornasari L.
Published: April 2011

We report preliminary data on the infiltration of polystyrene opals with cyanine dye water solutions, which are known to form J-aggregates. Different approaches are used to infiltrate cyanines inside opals. The degree of infiltration is evaluated by scanning and transmission electron microscopy as well as micro reflectance spectroscopy. Data show that in spite of the low wettability of polystyrene with water solutions, some infiltration can be achieved. Even though this degree of infiltration is still not enough to observe strong modification on the photonic band structures, further strategies to improve it are in progress.
Complex, 3D Photonic Crystals Fabricated by Atomic Layer Deposition
Authors: King J. S. , Gaillot D. , Yamashita T. , Neff C. , Graugnard E. , Summers C. J.
Published: April 2011

Recently we have demonstrated the potential of Atomic Layer Deposition (ALD) for the fabrication of advanced luminescent photonic crystal (PC) structures based on the inverse opal architecture.[1–3] PC's offer efficiency enhancement, decreased threshold, and other enhancements that improve phosphor performance. 3D PC structures are being extensively modeled, revealing that changes in the structures such as shifting the distribution of dielectric material can significantly improve photonic band gap (PBG) properties. For example, in the inverted “shell” structure, the width of the PBG can be increased from 4.25% to 8.6%.[4] Similarly, the PBG width can also be increased to 9.6% by formation of a non-close-packed structure.[5] Using the FDTD method, we have found that the PBG in a TiO2 non-closed packed structure can be as high as 5%. The performance of these structures depends critically on precisely and accurately placed high dielectric material. Using ALD, we have demonstrated infiltration of TiO2 films with extremely smooth surfaces (0.2–0.4 nm RMS roughness) while maintaining a high level of control over the infiltration coating thickness, enabling formation of composite infiltrated and inverse opals with nano-scale precision.

Here we report progress in fabrication of multi-layered and non-close packed PCs using ALD. Two and three-layer inverse opals were formed by the deposition of thin layers of ZnS:Mn and TiO2 in stacked configuration, each exhibiting luminescence when excited by UV light. Evidence for modification of the emission characteristics by high order PBGs (gaps other than between the 2nd and 3rd bands) has been observed. In addition, non-close packed inverse opals have been formed by infiltrating heavily sintered silica opals with TiO2, etching the spheres with hydrofluoric acid, and backfilling the resulting inverse opal. Resulting structures were characterized using specular reflectance and transmission, photoluminescence, and SEM. This work demonstrates the enormous potential that ALD offers for the realization of high performance photonic crystal structures.
Two-dimensional Magneto-photonic Crystal Circulators
Authors: Wang Zheng , Fan Shanhui
Published: April 2011

Previous research has demonstrated enhanced Faraday rotation in one-dimensional magnetic photonic crystals, where nonreciprocity in magneto-optical cavities is resonantly enhanced to provide optical isolation in optical paths on the scale of a few microns. In this paper, we study the nonreciprocity of two-dimensional magnetic photonic crystal resonators to allow further miniaturization and monolithic in-plane integration with current integrated optical devices. The nonreciprocal magnetic resonators are constructed by alternating the magnetization directions of the ferromagnetic domains in cavities side-coupled to photonic crystal waveguides. We show analytically that the gyrotropic splitting and the strength of the magnetic hybridization of the cavity modes are determined by the overlap integral between the domain magnetization vector and the modal cross product. With a large overlap obtained from optimizing the domain structures, we circularly hybridize two nearly degenerate modes to form a pair of counter-rotating whispering-gallery like modes, oscillating at different frequencies. As a physical realization, we synthesize two singly-degenerate circularly-hybridized modes in a two-dimensional crystal formed of a triangular air hole lattice in bismuth iron garnet with a TE bandgap. We tune the magnetic splitting and the decay constants of the rotating modes to demonstrate numerically a three-port optical circulator with a 30dB extinction bandwidth of 35GHz at 1550nm. An alternative implementation of a four-port circulator is achieved by side-coupling a point defect to two parallel waveguides. Our numerical experiments are performed with finite-difference time-domain simulations and agree well with the analytical coupled-mode theory predictions.
Highly Ordered Pseudo-Discotic Chromophore Systems for Electro-Optic Materials and Devices
Authors: Bhatambrekar Nishant , Hammond Scott , Sinness Jessica , Clot Olivier , Rommel Harry , Chen Antao , Robinson Bruce , Jen Alex K-Y. , Dalton Larry
Published: April 2011

In order to achieve the near-ferroelectric order desired in organic electro-optic (EO) chromophore systems, a pseudo-discotic chromophore is under investigation. Calculations suggest head-to-tail inter-chromophore dipole-dipole interactions should drive chromophores with an appropriate aspect ratio into ferroelectric columns similar to those seen in discotic liquid crystals (DLCs). Therefore, the liquid crystalline properties of these chromophores are being examined by differential scanning calorimetery (DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD). Furthermore, the effect of this discotic behavior on the order and EO properties of the system are being examined both dynamically by second harmonic generation (SHG) and statically by attenuated total reflection (ATR). Additionally, these chromophores are being incorporated into waveguide-based photonic devices.
Large Area Microcontact Printing Presses for Plastic Electronics
Authors: Lee Hee Hyun , Menard Etienne , Tassi Nancy G. , Rogers John A. , Blanchet Graciela B.
Published: April 2011

Low cost fabrication is key to the successful introduction of organic electronics and roll to roll manufacturing processes. We propose here that extending flexography into the micron size resolution regime may provide an economical commercialization path for plastic devices. Flexography is a high-speed technique commonly used for printing onto very large area flexible substrates.[1] Although low resolution and poor registration are characteristics of today's flexographic process, it has many similarities with soft lithographic techniques. This work shows that large, (12”×12”) high-resolution printing plates appropriate for use on small tag and label flexographic presses can be prepared using simple and inexpensive flexographic compatible processes. We illustrate the use of these plates for three representative soft lithographic processes: microcontact printing, replica molding, and phase shift lithography.
Highly Luminescent Composite Films from Core-Shell Oxide Nanocrystals
Authors: Buissette Valérie , Moreau Mélanie , Gacoin Thierry , Mercier Thierry Le , Boilot Jean-Pierre
Published: April 2011

Luminescent nanocrystals can find interesting applications for the elaboration of light emitting transparent materials. The work described here is based on the use of lanthanide doped vanadate (YVO4:Eu) and phosphate (La, Ce, Tb)PO4-0, 7H2O nanoparticles grown through aqueous colloidal synthesis, with average sizes below 10 nm. The well-dispersed colloids are transparent and respectively exhibit red and green luminescence under U.V. excitation with high luminescence yields (20 – 50 %). Improvement of luminescence properties of the nanocrystals is achieved through the elaboration of core/shell nanostructures, obtained after the growth at the surface of an amorphous silica shell or a crystalline lanthanum phosphate shell. Surface derivatization is further achieved through the controlled growth of an organically modified silica coating using a functionalized silane precursor. Concentrated sols are obtained, which are highly luminescent and well-dispersed. They can be spin-coated on various substrates, leading to perfectly transparent and highly luminescent thin films. Multi-layers films and heating treatments are performed, leading to optimized materials.
Microscopic Theory of Surface-Enhanced Raman Scattering in Noble-Metal Nanoparticles Vitaliy
Authors: Pustovit N. , Shahbazyan Tigran V.
Published: April 2011

We study the role of a strong electron confinement on the surface-enhanced Raman scattering from molecules adsorbed on small noble-metal nanoparticles. We describe a novel enhancement mechanism which originates from the different effect that confining potential has on s-band and d-band electrons. We demonstrate that the interplay between finite-size and screening efects in the nanoparticle surface layer leads to an enhancement of the surface plasmon local field acting on a molecule located in a close proximity to the metal surface. Our calculations, based on time-dependent local density approximation, show that the additional enhancement of the Raman signal is especially strong for small nanometer-sized nanoparticles.
Layered Double Hydroxides as a Matrix for Luminescent Rare Earth Complexes
Authors: Zhuravleva Natalia G. , Eliseev Andrei A. , Lukashin Alexey V. , Kynast Ulrich , Tretyakov Yuri D.
Published: April 2011

In the present work new luminescent materials with high quantum efficiencies based on layered double hydroxides (LDH) were obtained. Intercalation of complexes into the interlayer space of LDH doesn't affect their luminescent properties, forming non-volatile solid state material with good optical properties. The Coulomb interactions between LDH layers and complex can result in a change of complex structure in comparison with free complexes and in decreasing the number of the organic ligands per Ln atom. The energy transfer in the system was also studied.
Evolution of Porosity and Morphology in Alkylene-Bridged Polysilsesquioxane Xerogels as a Function of Gel Aging Time
Authors: Loy Douglas A. , Small James H. , DeFriend Kimberly A. , Wilson Kennard V. , Minke McKenzie , Baugher Brigitta M. , Baugher Colleen R. , Schneider Duane A. , Shea Kenneth J.
Published: April 2011

Aging of silica gels before drying is known to result in significant changes in xerogel morphology, porosity and properties. In this study, the influence of aging gels on the porosity and morphology of alkylene-bridged polysilsesquioxane xerogels was examined. Gels of hexylene-, heptylene, octylene, nonylene, and decylene-bridged polysilsesquioxanes were prepared by the sol-gel polymerization of the respective bis(trimethoxysilyl)alkane monomers under acidic or basic conditions in methanol and in tetrahydrofuran. The gels were aged 3, 7, 14, 28, 35, 42, 49, and 56 days before drying to afford xerogels. The xerogels were characterized by nitrogen sorption porosimetry. Xerogels prepared in THF were non-porous. Those prepared and aged under basic conditions in methanol or tetrahydrofuran exhibited coarsening of porosity with aging time. With the exception of the hexylene-bridged gels, those prepared and aged in acidic methanol showed little change with aging. The surface area of the hexylene-bridged xerogels nearly tripled with aging times of up to several weeks, then decreased, for the gels aged for more than two weeks, to around 100 meters squared per gram.
Hybrid Materials Approach In The Design Of Electrodes And Electrolytes For Energy Storage And Conversion
Authors: Cuentas-Gallegos Karina , Lira-Cantú Mónica , Casañ-Pastor Nieves , Asensio Juan A. , Gómez-Romero Pedro
Published: April 2011

The integration of electro-ionically active inorganic species in polymer matrices allows for the design of either electrode or electrolyte materials depending on the conducting or insulating properties of the polymer used. Conducting polymers can be used as the basis for a variety of hybrid electrode systems, whereas other polymers such as polybenzimidazoles have been used as electrolyte membranes by themselves or in combination with inorganic solid acids. We will discuss the general approach of hybrid design with this in mind and specifically we will describe our recent results on the use of polyoxometalate-containing hybrids in energy storage and conversion devices. In this respect we have worked in our laboratory on electrochemical supercapacitors and fuel cells but emphasis should be made on the broader potential fields of application of this type of materials.
Ligand Control of Semiconductor Nanocrystals for Efficient Carrier Injection
Authors: Murase Tomohide , Asami Harumi , Kamiya Itaru
Published: April 2011

Inorganic/organic hybrid material of CdSe/ZnS core/shell nanocrystals (NCs) covered with organic ligands was prepared with ligand exchange reaction in order to facilitate carrier injection into NCs through the organic ligands. The organic ligands used in this study, 2-naphtalenethiol and bathophenanthroline, were selected as model compounds according to adjustment of highest occupied molecular orbital and lowest unoccupied molecular orbital of the ligand to valence band maximum and conduction band minimum of the CdSe NCs, respectively. The prepared hybrid NCs easily disperse in polar organic solvents such as CHCl3 and show photoluminescence only from the CdSe/ZnS core/shell NCs. Current-voltage characteristics of the ligand exchanged NCs thin films sandwiched between electrodes were investigated. It was shown that conductivity of the film was improved by the introduction of carrier injecting ligands in comparison to the ligand unexchanged trioctylphosphine oxide-capped CdSe/ZnS NC thin film.