bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular Dynamics Simulations of Glassforming Network Fluids
Authors: Binder Kurt , Horbach Juergen , Hawlitzky Michael
Published: April 2011

Molecular Dynamics simulations of molten oxides, such as fluid silicon dioxide and germanium dioxide, based on simple classical pair potentials, are compared with corresponding Car-Parrinello “ab initio” Molecular Dynamics (CPMD) work and with experiment. It is shown that CPMD provides a significantly better account for properties on short length scales, but classical MD is still indispensable to deal with larger scales of length and time. The behavior of the mean square displacement of the particles as well as the incoherent intermediate scattering function is compatible with a mode coupling description, at least at very high temperatures, while the diffusion constants show a crossover to Arrhenius behavior near the mode coupling critical temperature of these systems. Finally, the results for the network forming liquids are compared to those from simulations of binary metallic alloys such as Al80Ni20, which form a structure similar to densely packed hard spheres.
Analysis of Indentation Creep
Authors: Stone Donald , Elmustafa A. A.
Published: April 2011

Increasingly, indentation creep experiments are being used to characterize rate-sensitive deformation in specimens that, due to small size or high hardness, are difficult to characterize by more conventional methods like uniaxial loading. In the present work we use finite element analysis to simulate indentation creep in a collection of materials whose properties vary across a wide range of hardness, strain rate sensitivities, and work hardening exponents. Our studies reveal that the commonly held assumption that the strain rate sensitivity of the hardness equals that of the flow stress is violated except for materials with low hardness/modulus ratios like soft metals. Another commonly held assumption is that the area of the indent increases with the square of depth during constant load creep. This latter assumption is used in an analysis where the experimenter estimates the increase in indent area (decrease in hardness) during creep based on the change in depth. This assumption is also strongly violated. Fortunately, both violations are easily explained by noting that the “constants” of proportionality relating 1) hardness to flow stress and 2) area to (depth)2 are actually functions of the hardness/modulus ratio. Based upon knowledge of these functions it is possible to accurately calculate 1) the strain rate sensitivity of the flow stress from a measurement of the strain rate sensitivity of the hardness and 2) the power law exponent relating area to depth during constant load creep.
A Thermo-Magneto-Mechanical Free Energy Model for NiMnGa Single Crystals
Authors: Morrison Phillip , Seelecke Stefan , Kohl Manfred , Krevet Berthold
Published: April 2011

The paper extends the authors' recent model for one-dimensional rate-dependent magneto-mechanical behavior of NiMnGa single crystals to account for temperature-dependent effects including austenite/martensite and ferro-/paramagnetic phase transitions. The magneto-mechanical model is based on the Helmholtz free energy landscape constructed for a meso-scale lattice element with strain and magnetization as order parameters. This two-dimensional energy landscape includes three paraboloidal wells representing the two easy-axis and one hard-axis martensite variants relevant for the structurally one-dimensional case. Phase transformations resulting from applied stresses and magnetic fields follow from a system of evolution laws based on the Gibbs free energy equations and the theory of thermally activated processes, which in the low-thermal-activation limit appropriately reproduce the athermal transformation behavior observed in these materials. The phase fractions subsequently determine the macroscopic strain and magnetization of a sample of NiMnGa by means of a standard averaging procedure. To account for the first-order phase transitions to austenite, additional temperature-dependent wells representing the stable states of austenitic NiMnGa are introduced into the Helmholtz energy landscape. The transition from ferromagnetic to paramagnetic states is modeled as a second order transformation based on the gradual degeneration of the ferromagnetic wells with increasing temperature.
Fast Characterization of Silicon Membrane Structures by Laser-Doppler Vibrometry
Authors: Gerbach Ronny , Ebert Matthias , Bagdahn Joerg
Published: April 2011

Micromechanical structures were investigated nondestructively via laser-Doppler-vibrometry to determine defect structures. Therefore, silicon membrane structures were characterized by their measured resonant frequencies and mode shapes. The influence of defects on the micromechanical structures is shown on the measured dynamic properties. Defect samples were indentified on the basis of the ratios of measured resonant frequencies and the quantified comparison of mode shapes without an identification of unknown parameters. The investigations showed that a fast determination of defect structures is possible by measured dynamic properties.
A Direct Method of Determining Complex Depth Profiles of Residual Stresses in Thin Films on a Nanoscale - Mechanics of Residually Stressed Systems
Authors: Massl Stefan , Keckes Jozef , Pippan Reinhard
Published: April 2011

The basic ideas behind the calculation procedure of the developed ion beam layer removal method (ILR method) to determine complex depth profiles of residual stresses are explained. In order to give the readers an understanding of the mechanics of films on substrates and the corresponding stresses in cantilevers fabricated from such systems, the equations necessary for the calculations are explained by means of a residually stressed model system.
Aligned Low Temperature Wafer Bonding for MEMS Manufacturing: Challenges and Promises
Authors: Dragoi Viorel , Matthias Thorsten , Mittendorfer Gerald , Lindner Paul
Published: April 2011

The increased complexity of current generations of MEMS devices imposes new requirements for wafer bonding. Among these can be mentioned low process temperature (<400°C), precise optical alignment of substrates, ability to bond a large variety of substrates and the possibility to bond with defined intermediate layers. An important aspect in aligned wafer bonding is that alignment accuracy needs to be correlated to the type of bond process. Especially in case of processes using bonding layers the post-bond alignment accuracy will be given by the behavior of the bonding layers. This paper aims to review the main criteria to be considered in defining aligned wafer bonding processes. Particularly bonding of substrates containing electronics (e.g. CMOS wafers) is currently of high technological interest.
RF MEMS Behavior, Surface Roughness and Asperity Contact
Authors: Rezvanian O. , Zikry M. A. , Brown C. , Krim J.
Published: April 2011

Modeling predictions and experimental measurements were obtained to characterize the electro-mechanical response of radio frequency (RF) microelectromechanical (MEM) switches due to variations in surface roughness and finite asperity deformations. A Weierstrass-Mandelbrot fractal representation was used to generate three-dimensional surface roughness profiles. Contact asperity deformations due to applied contact pressures, were then obtained by a creep constitutive formulation. The contact pressure is derived from the interrelated effects of roughness characteristics, material hardening and softening, temperature increases due to Joule heating, and contact forces. The numerical predictions were qualitatively consistent with the experimental measurements and observations of how contact resistance evolves as a function of deformation time history. This study provides a framework that is based on an integrated modeling and experimental measurements, which can be used in design of reliable RF MEMS devices with extended life cycles.
A Miniature Silicon Condenser Microphone Improved with a Flexure Hinge Diaphragm and a Large Back Volume
Authors: Kim Hyejin , Lee Sung Q , Lee Jaewoo , Lee Sangkyun , Park Kangho
Published: April 2011

We have developed a miniature silicon condenser microphone improved with a spring supported hinge diaphragm and a large back volume, which is designed in order to increase sensitivity of microphones. MEMS Technology has been successfully applied to miniature silicon capacitive microphones, and we fabricated the smallest condenser silicon microphone in the presented reports. We used the finite-element analysis (FEA) to evaluate mechanical and acoustic performances of the condenser microphone with a flexure hinge diaphragm. From the simulation results, we confirmed that the sensitivity of a flexure hinge diaphragm can be improved about 285 times higher than a flat diaphragm. The first and second modes occurred at 15,637Hz and 24,387Hz, respectively. The areas of the miniature condenser microphones with a hinge diaphragm are 1.5 mm × 1.5 mm. We measured the impedance characteristics and sensitivities of the fabricated condenser microphones. The sensitivities of microphones are around 12.87 μV/Pa (-60 dB ref. 12.5 mV/Pa) at 1 kHz under a low bias voltage of 1 V, and the frequency response is flat up to 13 kHz.
Comparison of 1D and 2D Theories of Thermoelastic Damping in Flexural Microresonators
Authors: Prabhakar Sairam , Vengallatore Srikar
Published: April 2011

Thermoelastic damping (TED) represents the lower limit of material damping in flexural mode micro- and nanoresonators. Current predictive models of TED calculate damping due to thermoelastic temperature gradients along the beam thickness only. In this work, we develop a two dimensional (2D) model by considering temperature gradients along the thickness and the length of the beam. The Green's function approach is shown to be a robust means of solving the coupled heat conduction equation in one and two dimensions. In the 1D model, curvature information is lost and, hence, the effects of structural boundary conditions and mode shapes on TED are not captured. In contrast, the 2D model retains curvature information in the expression for TED and can account for beam end conditions and higher order modes. The differences between the 1D and 2D models are systematically explored over a range of beam aspect ratios, frequencies, boundary conditions, and flexural mode shapes.
Bottom-up Fabrication of Individual SnO2 Nanowires-based Gas Sensors on Suspended Micromembranes
Authors: Romano-Rodriguez Albert , Hernandez-Ramirez Francisco , Prades Joan Daniel , Tarancon Albert , Casals Olga , Jimenez-Diaz Roman , Juli Miguel Angel , Morante Juan Ramon , Barth Sven , Mathur Sanjay , Helwig Andreas , Spannhake Jan , Mueller Gerhard
Published: April 2011

Bottom – up techniques were used to obtain gas sensors based on individual SnO2 nanowires placed over microhotplates with integrated heaters. These nanowires were electrically contacted to pre-patterned microelectrodes by means of Focused Ion Beam (FIB) nanofabrication methodologies. The performance of these sensors, which exhibit reproducible and stable responses, was evaluated as function of different gas atmospheres and dissipated power by the heater, demonstrating that this technological approach could be used to develop functional devices based on nanomaterials.
MEMS-Based MHz Silicon Ultrasonic Nozzles for Production of Monodisperse Drops
Authors: Song Y. L. , Cheng Chih H. , Wang Ning , Tsai Shirley C. , Chou Yuan F. , Lee Ching T. , Tsai Chen S.
Published: April 2011

This paper reports production of 4.5 μm-diameter monodisperse water drops using a micro electro-mechanical system (MEMS)-based 1 MHz 3-Fourier horn ultrasonic nozzle. The required electrical drive voltage for atomization was 6.5 V at 964±1 kHz that is in good agreements with the values obtained by impedance measurement and by the three-dimensional (3-D) simulation using a commercial finite element analysis program. Such small diameter drops with geometrical standard deviation (GSD) as small as 1.2 and 90% inhale-able fine particle fraction (<5.8 μm-diameter) were achieved in ultrasonic atomization for the first time. Therefore, the MEMS-based MHz ultrasonic nozzles should have potential application to targeted delivery of reproducible doses of medicine to the respiratory system.
Thermal Conductivity of Thermal Interface Materials
Authors: Fullem Travis Z. , Cotts Eric J.
Published: April 2011

While detailed theories exist for thermal conduction due to electrons and phonons in crystalline solids, phonon scattering and transmission at solid/solid interfaces is not as well understood. Steady increases in the power density of microelectronic devices have resulted in an increasing need in the electronics industry for an understanding of thermal conduction in multilayered structures. The materials of interest in this study consist of a polymer matrix in which small (on the order of microns to tens of microns) highly conductive filler particles (such as Ag or alumina) are suspended. These materials are used to form a thermal interface material bondline (a fifty to several hundred micron bonding layer) between a power device and a heat spreader. Such a bondline contains many polymer/filler interfaces. Using a micro Fourier apparatus, the thermal conductivities of such thermal interface material (TIM) bondlines of various thicknesses, ranging from fifty microns to several hundred microns, have been measured. The microstructure of these bondlines has been investigated using optical microscopy and acoustic microscopy. Measured values of thermal conductivity are compared to values for bulk samples, and considered in terms of microstructural features such as filler particle depleted regions. The influence of polymer/filler particle interfaces in the TIM bondline on phonon transport through the bondline is also considered.
Enabling Technology Based on Leaky-Mode Resonance Effects in Periodic Films
Authors: Magnusson Robert , Shokooh-Saremi Mehrdad , Lee Kyu J. , Wawro Debra
Published: April 2011

The physical properties of resonant leaky modes in periodically patterned layered materials are summarized. Representative examples of photonic device applications including filters, polarizers, wideband reflectors, tunable elements, and biosensors are discussed.
Characterization and In Vitro Bioactivity Study of Ternary Glass-ceramic Coatings
Authors: Chatzistavrou Xanthippi , Kontonasaki Eleana , Kantiranis Nikolaos , Papadopoulou Lamprini , Koidis Petros , Hatzikraniotis Evripides , Paraskevopoulos Konstantinos
Published: April 2011

The aim of the present study was to test the in vitro bioactive behavior of two new ternary mixtures consisting of a glass-ceramic porcelain, a bioactive glass and alumina (PBA) as coatings on dental glass ceramic substrates. They consisted of bioactive glass- in the system SiO2-Na2O-CaO-P2O5−, powder from a leucite-fluorapatite glass ceramic and γ-Al2O3 in 50-42.5-7.5 wt % respectively, while coatings with the same two components in 50-50 wt % ratio, without Al2O3, served as control. The characterization of the coatings was performed by Scanning Microscopy (SEM-EDS), Fourier Transform Infrared Spectroscopy (FTIR) and X-Ray Diffraction (XRD). The in vitro bioactivity was tested in Simulated Body Fluid (SBF) and the surfaces of reacted specimens were examined by SEM and FTIR. All coatings presented rough surface, while FTIR spectra revealed the characteristic peaks of the crystallized phases in the bioactive glass network and dental glass ceramic, i.e. Sodium Calcium Silicate (Na4Ca4Si6O18), Leucite (KAlSi2O6) and Fluorapatite (Ca5(PO4)3F), and furthermore the very limited participation of Al3+ in the bioactive glass network. In the coatings the main phases detected by XRD were, also, Na4Ca4Si6O18 and Leucite, dispersed in an amorphous glassy matrix. Furthermore, a transformation of γ-Al2O3 to δ-Al2O3 was observed and a new phase of Nepheline (Na7.15Al7.2Si8.8O32) was detected in traces. Apatite was developed sporadically on the surfaces of the PBA coatings after 30 days in SBF. These results demonstrate a slight retardation of bioactive response of the coatings of the ternary mixtures compared to dual ones consisting of dental glass ceramic and bioactive glass. The incorporation of Al2O3 can enhance the mechanical properties without a substantial degradation of bioactivity. This finding renders these coatings as promising composite materials for potential dental applications.
Fabrication and Characterization of Size- and Shape-Controlled C60 Nano/microcrystals by Solvent-Induced Reprecipitation Process (SIRP)
Authors: Masuhara Akito , Tan Zhenquan , Kasai Hitoshi , Nakanishi Hachiro , Oikawa Hidetoshi
Published: April 2011

We have first developed useful Solvent-Induced Reprecipitation Process, and have first succeeded in fabricating C60 nano/microcrystals with various shape and size, using m-xylene and 2-propanol as good and poor solvents, respectively.

It was found that the resulting C60 nano/microcrystals had a hexagonal crystal structure, and that were a kind of crystal solvates in which the molar ratio of C60 to m-xylene was 3:2. C60 nano/microcrystals could be said to be important nano components of integrated devices in electronics.
Multi-color Photoresponse Based on Interband and Intersubband Transitions in InAs and InGaAs Quantum Dot Photodetectors
Authors: Passmore Brandon Scott , Wu Jiang , Decuir Eric A. , Manasreh Omar , Lytvyn Peter M. , Kunets Vasyl P. , Salamo Greg J.
Published: April 2011

The interband and intersubband transitions in self-assembled InAs and In0.3Ga0.7As quantum dots grown by molecular beam epitaxy have been investigated for their use in visible, near-, and mid-infrared detection applications. Devices based on InAs quantum dots embedded in an InxGa1−xAs (0 to 0.3) graded well and In0.3Ga0.7As quantum dots were fabricated in order to measure the temperature dependent (77 – 300 K) photoresponse. The dark current was measured in the temperature range of 77 to 300 K for the devices. Room temperature photoresponse ranging between 0.6 to 1.3 μm was observed for the InAs and In0.3Ga0.7As quantum dot photodetectors. Furthermore, a dual band photoresponse in the visible, near-, and mid-infrared spectral regions for both devices was observed at 77 K. Using a self-consistent solution of Schrödinger-Poisson equations, the peak position energies of the interband and intersubband transitions in the two multi-color quantum dot infrared photodetector structures was calculated.
Prediction of Energies of <100> Tilt Boundaries in Al-Pb Alloy
Authors: Purohit Y. , Irving D. L. , Scattergood R. O. , Brenner D. W.
Published: April 2011

Energies for symmetric tilt grain boundaries in pure Al and in Al with substitutional Pb defects at coincident sites along the grain boundaries were calculated using a modified embedded atom method potential and density functional theory. The agreement between the analytic potential, the first principles calculations and experiment is reasonably good for the pure system. For the Al-Pb system both the analytic potential and first principles calculations predict that Pb segregation to the interface is energetically preferred compared to the dilute solution. The application of a disclination structural unit model to calculating grain boundary energies over the entire range of tilt angles is also explained.
High Yield Synthesis of single crystal FCC Silver Nanoparticles and their Size Control
Authors: Nakamura Hiroyuki , Shimizu Toshiyuki , Uehara Masato , Yamaguchi Yoshiko , Maeda Hideaki
Published: April 2011

Face centered cubic (FCC) single crystal silver nanoparticles(NPs) of nearly monodispersed particle size of 12nm were prepared by simple ripening multi-twined particles (MTP). The product yield was nearly 100% and growth mechanism was considered to be similar to Ostwald ripening but utilize chemical potential difference between MTP and FCC NPs. Controlling the ratio between defectless and defective particles before ripening by controlling kinetics of particle generation stage, it was possible to control the final defectless particle size from 7-18 nm maintaining almost 100% product yield.
Aluminum Nanoparticle Synthesis by Reduction of Halides with Na/K
Authors: Purdy Andrew P , Miller Joel B , Stroud Rhonda M , Pettigrew Katherine A
Published: April 2011

Solutions or mixtures of aluminum halides AlX3 (X = Cl, Br, I) or aluminum halide alkoxide (ROAlX2; R= i-Pr, X = Cl, Br) are prepared in toluene, or pentamethyldiethylenetriamine (PMDETA) and treated with a slight excess of 50:50 (wt.) sodium potassium alloy (Na/K). Upon agitation, the reaction takes place slowly. The reaction appears to be limited by the deposition of insoluble products on the surface of the Na/K since sonication in an ordinary ultrasonic cleaner is necessary to bring the reaction to completion in several hours. Aluminum nanoparticles (NPs) and Na and K halides are formed, and soluble Al compounds, are also formed in some cases. Lithium powder is used instead of Na/K to reduce Al(N(SiMe3)2)3 to Al NPs. In an effort to passivate the surface of the Al particles to oxidation or hydrolysis, various agents are added either during the reaction or afterward. The products are characterized by SEM, XRD, NMR, and TEM, and the stability to water and air is evaluated.
Doped Zirconia Luminescent Nanoparticles
Authors: Sample Jennifer , Zhang Dajie
Published: April 2011

Nanoparticles that fluoresce or absorb light in the visible and near-IR wavelengths are desirable for a variety of applications including biological tagging for medical imaging purposes and for solar control glazing in the automobile industry. Semiconductor quantum dots are commercially available and dyed polymer nanoparticles as well as organic dye/silica core shell nanoparticles have also been demonstrated. We report the synthesis, characterization, and optical properties of another luminescent nanoparticle: doped zirconia. The zirconia nanoparticles reported in this study are doped with up to 10% of the lanthanide dopants Er, Gd, Nd and Eu. These materials emit in the visible and near-IR wavelengths depending on the dopant and are refractory, making them useful for high temperature applications. These cations were found to stabilize the cubic phase over the monoclinic phase of zirconia, at approximately 10% dopant, as characterized by X-ray diffraction. We report the luminescence spectra of these nanoparticles at various wavelengths which reveal emissions from the matrix as well as from the dopants.
Local Elastic Constants For Epoxy-Nanotube Composites From Molecular Dynamics Simulation
Authors: Frankland S. J. V. , Gates T. S.
Published: April 2011

A method from molecular dynamics simulation is developed for determining local elastic constants of an epoxy/nanotube composite. The local values of C11, C33, K12, and K13 elastic constants are calculated for an epoxy/nanotube composite as a function of radial distance from the nanotube. While the results possess a significant amount of statistical uncertainty resulting from both the numerical analysis and the molecular fluctuations during the simulation, the following observations can be made. If the size of the region around the nanotube is increased from shells of 1Å to 6Å in thickness, then the scatter in the data reduces enough to observe trends. All the elastic constants determined are at a minimum 20Å from the center of the nanotube. The C11, C33, and K12 follow similar trends as a function of radial distance from the nanotube. The K13 decreases greater distances from the nanotube and becomes negative which may be a symptom of the statistical averaging.
Effects of Relative Humidity and Nanoparticle Incorporation on Nanostructures of PS-b-PEO Diblock Copolymer
Authors: Wu Juntao , Zhao Minhua , Nguyen Tinh , Gu Xiaohong
Published: April 2011

The morphological structures of polystyrene-block-poly(ethylene oxide), PS-b-PEO, have been investigated under humidity controlled environments during film formation. Well-defined cylindrical microdomains oriented normal to the surface were induced by the high humidity during spin coating. Effects of the relative humidity (RH) on such ordered structures have also been studied. The results show that, the orientation and the ordering of the copolymer structures are very sensitive to the humidity of the environment during the film formation. Such ordered structure is stable at a variety of the RHs, and the phase contrast between the cylindrical PEO domains and the PS matrix domains can be significantly enhanced by the elevated RH. After the incorporation of the nanoparticles, however, the morphological structures of the PS-b-PEO are modified.
Computer Simulation of Nanoparticle Aggregate Fracture
Authors: Hawa Takumi , Henz Brian , Zachariah Michael
Published: April 2011

Nanoparticle aggregates have been found to possess unique mechanical properties. Aggregates of metal nanoparticles can be strained up to 100% before failure, and even typically brittle materials are observed to have a ductile failure mode. In this effort two materials; namely silver and silicon, were chosen to represent ductile and brittle materials, respectively. Aggregates with 2 to 6 particles were simulated using the molecular dynamics (MD) algorithm to determine the stress-strain behavior of the aggregate. Many interesting observations are made including the negligible affect of strain rate on ultimate tensile strength, and the direct relationship between Young's modulus and nanoparticle size.
Nanostructured indium tin oxide for application in optoelectronic devices
Authors: Fernando S. P. , Harris K. D. , Gerein N. J. , Brett M. J.
Published: April 2011

The electrical resistivities of nanostructured films of indium tin oxide were measured. The films were prepared using glancing angle deposition, a physical vapour deposition technique. This technique was used to prepare nanostructured indium tin oxide thin films of various morphologies. Films consisting of vertical posts were electrically characterized using a probing station. Compared to a normally evaporated planar film, the nanostructured films were found to exhibit resistivities approximately two orders of magnitude greater. Development of the measurement technique is ongoing, and several ideas for improvement are described.
New Polypropylene-boron Oxide Composite with a High Tensile Strength: A Possible Method for the Conversion of Microcomposite to Nanocomposite
Authors: Chopra Shwetha , Ge Changfeng , Santhanam K S.V.
Published: April 2011

Industrially viable blends having microstructures that can be converted into a nanostructured material by post chemical treatments, potentially should have a commercially viable solution to the development of nanostructured materials for practical applications. In this regard a polypropylene-boron oxide (PPBO) blend provides a model study here. In this paper we wish to report the synthesis of PPBO blends with different concentrations of boron oxide by melt grafting and through reactive extrusion. The tensile strengths of the ideal blend has been found to be 62% higher than polypropylene (PP). By chemically treating the blends for a period of 24 hours to 72 hours the tensile strength of the blends increased by 152%. The blends have been characterized by fourier transform infra red spectroscopy (FTIR) and scanning electron microscopy (SEM). An interesting characteristic of the PPBO blend has been the development of surface potential that changes upon visible light excitation. The increased tensile strength has been attributed to the conversion of micro structured blends to nanostructured blends. The improvement in thermal stability can be attributed to a good PP matrix-oxide interaction and also due to the thermal conductivity of the boron oxide. The good dispersion of the nanotubes in the polymer matrix allows the spreading of heat uniformly along the fiber.
Size Effects on the Plastic Deformation of Nanocomposites Prepared from SiCN and SiCNAlYO Nanopowders Synthesized by Laser Pyrolysis
Authors: Herlin-Boime Nathalie , Dez Romuald , Reynaud cecile , Besson Jean Louis , Vicens Jean
Published: April 2011

The aim of this work was to synthesise nanopowders by laser pyrolysis in order to elaborate Si3N4-SiC nanocomposites exhibiting a high ductility at high temperature. The synthesis of SiCN and SiCNYAlO nanopowders with a good thermal stability has been developed from a liquid mixture based on hexamethyldisilazane with a gaseous precursor (silane, SiH4) and ammonia. Si3N4-SiC nanocomposites exhibiting almost theoretical density have been elaborated by hot pressing under 35 MPa at 1600°C either from SiCNYAlO nanopowders or from SiCN nanopowders mixed with commercial sintering aids nanopowders (6 wt% yttria and 3 wt% alumina) in ethanol and de-agglomerated by milling with Si3N4 balls. The densification behaviour is better for nanopowders containing in-situ additives. In addition, using prealloyed powders enables to simplify the elaboration process since the mixing step of the nanopowders is eliminated. The nanocomposites elaborated from the mixture (SiCN + commercial sintering aids) exhibit a different microstructure, depending on the drying process (under vacuum or in an oven) of the nanopowders after the mixing step: When drying was performed in a confined atmosphere, a noticeable amount of Si2N2O was detected in addition to β-Si3N4 and the average grain size was 60 nm, whereas the crystalline phases were α- and β-Si3N4, when the slurry was dried under vacuum and the average grain size was 200 nm. The creep behaviour of the two grades was assessed under compressive loading in air and the deformation appears strongly influenced by this change of the microstructure: A strong ductility was observed for the smallest grain size: a deformation as high as 45 % was measured under a compressive load of 180 MPa at 1350°C under air.
Characterization of Thermo-Electric Interface Material with Carbon Nanotubes
Authors: Thakre Piyush R , Bisrat Yordanos , Lagoudas Dimitris C
Published: April 2011

An approach has been presented in the current work to fabricate and characterize nanocomposite systems for optimizing electrical and thermal properties without sacrificing mechanical properties. An epoxy matrix based nanocomposite system has been processed with different volume fractions of carbon nanotubes. The purpose was to tailor macroscale properties to meet competing performance requirements in microelectronics industy. The nanofiller consisted of comparatively low cost XD grade carbon nanotubes (XD-CNTs) that are optimized for electrical properties. This system was compared with another system consisting of single wall carbon nanotubes (SW-CNTs) as nano-reinforcements in epoxy matrix. The electrical percolation threshold (about seven orders of magnitude increase in electrical conductivity) measured by dielectric spectroscopy was found to be at lower loading weight fraction of SWCNTs (0.015 weight %) as compared to XD-CNTs (0.0225 weight %). However, the electrical conductivity after percolation was higher for XD-CNTs reinforced epoxy with respect to SW-CNTs filled nanocomposites. The governing mechanisms for this phenomenon were investigated using transmission optical microscope. The enhancement in thermal conductivity, measured using differential scanning calorimetry, was found to be moderate at lower weight loadings corresponding to electrical percolation. However, a 90% improvement in thermal conductivity was observed for 0.3 weight percent of XD-CNTs. Dynamic mechanical analysis was performed to measure the storage and loss modulus along with the glass transition temperature. No significant change in modulus values and glass transition temperature was measured for nanocomposites varied filler contents with respect to neat matrix.
Percolation of Diffusionally Evolved Two-Phase Systems
Authors: Brunini Victor Eric , Schuh Christopher A. , Carter W. Craig
Published: April 2011

Although the phase fractions in dual phase systems are often compared with the percolation threshold for a randomly-assembled composite, most two-phase systems are non-random by virtue of correlations introduced during processing or as a consequence of microstructural evolution. This study examines the two dimensional percolation threshold in systems with soft impingement, i.e., when the phase distribution is affected by diffusional interactions between growing second phase particles. Phase field modeling is used to simulate the nucleation and growth process, with many simulations conducted at various system sizes and equilibrium phase fractions to obtain percolation probabilities. The value of the percolation threshold in the thermodynamic limit is estimated based on the finite size scaling behavior of the system. The value of the critical exponent Î½ (the size scaling exponent) is also estimated.
Growth of Branched Carbon Nanostructures in Nanopatterned Surfaces Created by Focused Ion Beam
Authors: Solá Francisco , Resto Oscar , Biaggi-Labiosa Azlin , Fonseca Luis F
Published: April 2011

A method to grow arrays of multibranched carbon nanostructures is explained. We use the electron-beam-induced deposition method using a transmission electron microscope with ∼10−6 Torr vacuum where hydrocarbons are present in the chamber. Saw-tooth nano-patterns were made with a focused ion beam in porous silicon substrates. Due to the dielectric properties of the films the patterns provide the corresponding sites in which high local electric fields appear during irradiation thus allowing the hydrocarbons to become preferentially attracted to those active sites. We found that the adequate ion dose to create well defined saw-tooth nano-patterns was between 8 and 10 nC/μ2 at 30 kv. Electron energy-loss spectroscopy on the branched carbon nanostructures show a high concentration of sp2 sites suggesting that they are made of graphite-like amorphous carbon.
Fabrication of Periodic Arrays of Nano-sized Si and Ni dots on SiO2 Using Linearly Polarized Nd:YAG Pulsed Laser
Authors: Nishioka Kensuke , Horita Susumu
Published: April 2011

Periodic arrays of nano-sized Si and Ni dots were fabricated by only irradiating a linearly polarized Nd:YAG pulsed laser beam to Si and Ni thin films deposited on silicon dioxide (SiO2) film. The interference between an incident beam and a scattered surface wave leads to the spatial periodicity of beam energy density distribution on the surface of the irradiated samples. A thin film was melted using a laser beam, and the molten film was split and condensed owing to its surface tensile according to the periodic energy density distribution. Then, the fine lines (line and space structure) were formed periodically. After the formation of fine lines, the sample was rotated by 90°, and the laser beam was irradiated. The periodic energy density distribution was generated on the fine lines, and the lines split and condensed according to the periodic energy density distribution. Eventually, the periodically aligned nano-sized dots were fabricated on the SiO2 film.
Nano-sized Taper Structure Formed by Wet Process Using Catalysis of Gold Nanoparticle
Authors: Nishioka Kensuke , Horita Susumu
Published: April 2011

This paper describes a new technique for fabrication of nano-sized taper structure formed by simple wet chemical etching using catalysis of gold (Au) nanoparticle. Single nano-sized Au particle dispersion solution was coated onto Si(111) substrate with polished surface. Then, the samples were soaked in an aqueous etching solution of hydrofluoric acid (HF) and hydrogen peroxide (H2O2). The surface of 15-min-etched Si substrate appeared black. The reflectivity of the Si substrate was reduced to below 5% throughout the entire spectrum from 300 to 800 nm owing to nano-sized taper structure called subwavelength structure (SWS). The fractional area occupied by Si as a function of the depth across the textured layer showed a smooth increase of density up to a depth. The observed optical effects are explained by the formation of a nanoscale taper structure, representing an effective medium with a smooth transition of the refractive index from that of air to that of Si.
Optimization and Constraints in Sonolithography
Authors: Campbell Paul , Prentice Paul
Published: April 2011

The violent interaction between pressure driven cavitation nuclei and nearby rigid substrates is usually a troublesome occurrence, giving rise to damage, and often system failure in hydraulic systems. However, the extreme nature of the phenomenon can also be exploited in situations where deliberate wear or erosion of a material is desireable, such as with the application of shock wave lithotripsy to fragment kidney stones in a medical context. The purpose of the present study was to examine whether a system can be designed so as to afford a level of control over cavitation processes, so that other useful applications might arise. Specifically, we looked at controlling single cavitation nuclei, constituted by encapsulated microbubbles, in proximity to a nearby rigid substrate and activated by ultrasound. This was achieved using a novel optical trapping arrangement, which facilitated establishment of an arbitrary, stable, initial spatial configuration for a bubble system. Critically, exercising optical control in such a way meant that a microbubble could be isolated from a resident population during insonation thus ensuring that ‘cross-talk’ with the rest of the bubble population was minimised. We observed, using high speed microphotography at circa one million frames per second that fine microjets are issued from cavitation microbubbles, and these impact the nearby surface, creating indentations of controllable size. Specific applications may arise that exploit this action, which, in the general case we refer to as ‘sonolithography’. However, scaling up the process to activate multiple bubbles at once, may lead to complications arising via the action of secondary radiation forces. We discuss the salient aspects of our preliminary findings on this subject herein.
PEG-POSS Hybrid Polyurethanes: Mechanically Robust Nanostructured Hydrogels
Authors: Wu Jian , Ge Qing , Mather Patrick T
Published: April 2011

A series of unique hybrid thermoplastic polyurethanes (TPUs) was synthesized using PEG as soft segment and incorporating an isobutyl-functionalized POSS diol (TMP POSS diol) in the hard segment. The molecular weight of PEG was systematically varied to include 10, 20, and 35 kDa, while the mole ratio of POSS diol (as chain extender) to PEG was in range from 3:1 to 8:1 with samples featuring a PEG molecular weight of 10 kDa. The diisocyanate employed for TPU polymerization was 4,4'-methylenebis(phenyl-isocyanate) (MDI). We found that the hydrophobic hard segments (POSS) can form crystalline structures driven by micro-phase separation, this being due to significant thermodynamic incompatibility between POSS and ethylene oxide units. The POSS nano-crystals thus formed serve as physical crosslinking sites within an inorganic-organic hybrid network. This affords a new hybrid organic-inorganic hydrogel in the water-swollen state. The equilibrium swelling ratio increased monotonically with PEG loading and ranged from ∼70% to ∼600%. The shear modulus, G, of the hybrid hydrogels was observed to span 0.3 < G < 4.0 MPa – C values commonly found for elastomers, not hydrogels. Indeed, the hydrogel stiffness can be finely tuned through the POSS:PEG molar ratio, as this predictably controls swelling in water.
Refractive Index Change in Nanoscale Thermosensitive Hydrogel for Optoelectronic and Biophotonic Applications
Authors: Garner Brett W , Hu Zhibing , McDaniel Floyd D. , Neogi Arup
Published: April 2011

The familiar property responses of isopropylacrylamide hydrogel, cause the hydrogel to undergo a discontinuous volumetric phase across a critical level of stimuli. Poly(N-isopropylacrylamide) hydrogel is a recognized response to global temperature stimuli across the low critical solution temperature (LCST). The basic PNIPAM hydrogel undergoes its phase change at a LCST of 34°C, where water is expelled from the interior of the gel microsphere, as the temperature increases past LCST. The responsiveness of PNIPAM hydrogel offers the potential for controlling optical properties of a medium such as refractive index and scattering index. In the present work we present the refractive index of microsphere/microbeaded hydrogel structures necessary for optical application. The particle size of the hydrogels nanostructures at room temperature is observed to be 300-400 nm as estimated by dynamic light scattering and agree well with scanning electron microscopy measurements. The temperature dependent refractive index change of hydrogel microspheres have been measured using variable angle spectroscopic ellipsometry and shows a 10% change as the temperature changes from 33°C to 34°C.
Synthesis and Self-assembly of DMPC-conjugated Gold Nanoparticles
Authors: Chatterjee Subhasish , Krikorian Markrete , Gafney Harry D. , Gersten Bonnie
Published: April 2011

Bio-conjugated nanomaterials play a promising role in the development of novelsupramolecular structures, molecular machines, and biosensing devices. In this study, lipid-capped gold nanoparticles were synthesized and allowed to form a self-assembled monolayer structure. The nanoparticles were prepared by a phase transfer method, which involved the reduction of potassium tetrachloroaurate(III) by sodium citrate in an aqueous solution and the simultaneous transfer of the reduced species to an organic medium containing DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine). The gold nanoparticles were characterized using Uv-vis spectroscopy and dynamic light scattering (DLS) particle-size analysis. In addition, the resulting nanoparticles were examined using transmission electron microscopy (TEM). The Langmuir-Blodgett (LB) technique was used to assemble the DMPC-capped nanoparticles onto a water subphase at room temperature. The measurement of the compression isotherm confirmed the assemblage of lipid capped gold nanoparticles. This method of synthesis of ordered structures utilizing molecular interactions of lipids will be useful in developing novel metamaterials and nanocircuits.
The Mechanism of Chitosan Enhanced Lung Surfactant Adsorption at the Air-Liquid Interface in the Presence of Serum Proteins
Authors: Stenger Patrick C , Palazoglu Omer M , Zasadzinski Joseph A
Published: April 2011

Pressure-area isotherms and fluorescence microscopy were used to investigate the impact of chitosan on the competitive adsorption between lung surfactant (LS) and serum proteins at the air-liquid interface. Isotherms demonstrate an optimum chitosan concentration to mediate LS adsorption; higher concentrations actually reduce the amount of LS which can adsorb. Fluorescence microscopy images show the transition from a serum protein to LS-covered interface for the optimum chitosan concentration; this transition goes through a sharply phase separated coexistence region. The results suggest that the cationic chitosan molecules mediate adsorption of the negatively charged LS aggregates by reducing the electrostatic barrier imposed by negatively charged interfacial serum proteins.
Generation of Bioactive Nanostructured Interfaces to Mimic Cellular Microenvironments
Authors: Wolfram Tobias , Belz Ferdinand , Schoen Tobias , Aydin Daniel , Spatz Joachim P
Published: April 2011

There is currently great interest in material science to mimic cellular microenvironments with controlled bioactive features and biomolecule presentation at the nanometer length scale. Target cells exposed to those 2- or 3D-substrates are stimulated to manifest a variety of cellular functions like cell adhesion, migration and differentiation.

Based on micellar nanolithography, we developed an experimental setup to present single biomolecules on a 2D-nanoarray over large areas on cell-adhesive or cell-repellent substrates. The spacing between biomolecules can be adjusted to 30-250 nm. To mimic a more complex cellular micro-environment with cell-cell contacts as well as cell-extracellular matrix (ECM) contacts, we used biomolecules from different cell adhesion molecule families, like cadherins, immunglobulin-superfamily, integrins and laminins. Via a Ni2+- nitrilo triacetic acid (NTA) system, the amount and the orientation of proteins is chemically controlled such that it resembles the native in vivo settings in the cellular microenvironment. By using poly-l-lysine-grafted-polyethylene glycol (PLL-g-PEG) molecules, we generated 2D-protein-nanoarrays with a cell repellent background matrix. After modification of this matrix we incorporated small bioactive peptides to create a microenvironment which allows cell adhesion. These approaches resulted in substrates with specific presentation of biomolecules to mimic cell-cell contacts and cell-ECM interactions simultaneously.

With these substrates we investigated primary fibroblast and neuronal cell adhesion mediated by different biomolecules and with different nanometer spacing. We quantified neurite outgrowth of dorsal root ganglion (DRG) neurons as a differentiation paradigm on nanostructured substrates. Gold nanoparticles were functionalized with peptides, containing the arginine-glycine-aspartic acid (RGD) motif, an integrin activation sequence. Cell adhesion as well as differentiation showed a dependency on the nanometer length scale and on the presented biomolecules.
Patterning Cylindrical Fibers with Long-Range Electrostatic Forces
Authors: Kohlstedt Kevin L. , Vernizzi Graziano , de la Cruz Monica Olvera
Published: April 2011

We report our findings with theoretical arguments on chiral symmetry breaking on the surface of charged cylinders. We use a model for periodic patterns of charges constrained over a cylindrical surface. In particular we focus on patterns of oriented lamellar patterns, such as, chiral helices, achiral rings or vertical lamellae, with the constraint of global electroneutrality. We study the dependence of the patterns' size and pitch angle on the radius of the cylinder and salt concentration. We obtain a phase diagram by using numerical and analytic techniques. For pure Coulomb interactions, we find a ring phase for small radii and a chiral helical phase for larger radii. We extend the findings to discrete triangular lattices wrapped over a cylindrical geometry. We find no symmetry breaking chiral helical phase in the discrete wrapping when using just an electrostatic potential and the minimum energy configuration is an achiral lattice matching the six-fold symmetry of triangular lattice. Conversely, with the addition of an elastic potential between the charges on the surface of the cylinder we find a stable chiral configuration. We discuss possible consequences and generalizations of our model.
Micromechanical Investigations on Films made of Recombinant Spider Silk Proteins and Silk Fibroin
Authors: Junghans Frauke , Conrad Udo , Heilmann Andreas , Spohn Uwe
Published: April 2011

Films made of a recombinant spider silk protein and silk fibroin were prepared by spincoating and casting. Therefore the solubility of these substances was investigated in hexafluoroisopropa-nol, ionic liquids and concentrated salt solutions. The roughness and the thickness of the protein films were determined by the Atomic Force Microscopy (AFM) and by mechanical profilometry. The micromechanical behaviour was investigated by acoustic impedance analysis using a quartz crystal microbalance (QCMB) as well as by microindentation. Films with thickness less than 350 nm revealed an almost ideal elastic behaviour in the range of 5 to 75 MHz. At a higher film thickness the half-band-half width increases considerably and the films show a viscoelastic be-haviour with a considerable dissipation. The relative humidity significantly influences the me-chanical behaviour of protein films. Hence the micro-hardness and the ability of water adsorption were determined in dependence on the relative humidity.