MRS Online Proceedings Library (OPL)

Nanocomposites and nanostructured polymers with unique opto-mechanical properties have been developed as smart coatings for use in a novel, high resolution, and non-contact strain-measuring application. Remote polarized Raman spectroscopy has been used to monitor optical strain sensitivity of deformed coatings (deformation micromechanics), and determine local strains on the micron scale directly from stress/strain induced Raman band shifts.

The research is aimed at providing a novel high-resolution non-contact technique for the determination of surface stresses and strains in a wide variety of engineering components used in both laboratory and in-the-field (external) applications.

Polyethylene terephthalate glycol (PETG) is a clear amorphous polymer, which is extensively used in flexible packaging. The dual packaging requirements of recyclability and long-term shelf life are often difficult to achieve. Meeting these needs become more urgent when considering food packaging for large volumes of soldiers positioned in different parts of the world. Our approach is to develop a high barrier PET packaging system via the Montmorillonite layered silicate (MLS) based nano technology. Prior research has indicated the significant impact of the polymer crystalline regions on the properties of the resultant nanocomposite. Therefore we must first investigate the amorphous PETG. We must also investigate the influence of increased matrix polarity on dispersion of the PETG by incorporating maleic anhydride (MA) onto the PETG backbone. The influence of the clay concentration and maleation are independently investigated. The glass transition of the as-processed and annealed samples are analyzed using Differential Scanning Calorimetry (DSC) while the thermal stability is determined using Thermogravimetric Analysis (TGA). Testing showed a slight depression in the glass transition temperature of PETG film when the MLS is introduced into the system. The nanocomposite films also demonstrated a lower thermal stability in relation to the neat PETG films. The barrier properties were determined on an in-house built calibration unit based on atomic mobility under high vacuum. X-ray diffraction and TEM were utilized to determine the dispersion of the MLS in PETG. The results indicate that the dispersion was concentration independent but maleation of the PETG led to a slight decrease in agglomeration. An increased ultimate tensile strength and modulus was observed in PETG nanocomposites. The barrier properties were improved by incorporating the MLS into the system. Maleation of the PETG resulted in significant yellowing of the nanocomposites.

High molecular weight polyaniline / multi-walled carbon nanotube composite films were fabricated using solution processing. Composite films with various weight percentages of multiwalled carbon nanotubes were fabricated. Physical properties of these composites were analyzed by thermogravimetric analysis, tensile testing, and scanning electron microscopy. These results indicate that the addition of multiwalled nanotubes to polyaniline significantly enhances the mechanical properties of the films. In addition, metal–semiconductor (composite) (MS) contact devices were fabricated, and it was observed that the current level in the films increased with increasing multiwalled nanotube content. Furthermore, it was observed that polyaniline containing one weight percent of carbon nanotubes appears to be the most promising composition for applications in organic electronic devices.

In nanostructured metallic multilayers, hardness and strength are greatly enhanced compared to their microstructured counterparts. As layer thickness is decreased, three different regions are frequently observed: the first region shows Hall-Petch behavior; the second region shows an even greater dependence on layer thickness; and the third region exhibits a plateau or softening of hardness and strength. The second and third regions are studied using our discrete dislocation simulation method. This method includes the effects of stress due to lattice mismatch, misfit dislocation substructure, and applied stress on multilayer strength. To do so, we study the propagation of existing threading and interfacial dislocations as the applied stress is increased to the macroyield point. Our results show that in region 2, dislocation propagation is confined to individual layers initially. This “confined layer slip” builds up interfacial content and redistributes stress so that ultimately, the structure can no longer confine slip. The associated macroyield stress in this region depends strongly on layer thickness. In region 3, layers are so thin that confined layer slip is not possible and the macroyield stress reaches a plateau that is independent of layer thickness.

The residual microstrains in cryomilled Ni powders, processed under various cryomilling conditions, are measured by XRD and analyzed using a single line approximation (SLA) method. The results show that the average residual microstrains are in the range of 2×10-3 ∼ 6×10-3, and the residual microstrain on the (200) plane is higher than those on the other planes by 33 %. The measured microstrain is proposed to evolve from the introduction of N as impurity atoms in Ni lattice may be attributed to the evolution of the residual microstrain. The N atoms tend to stay in the octahedral sites of Ni, and the diameter of N atom is larger than that of the octahedral site of Ni by 48 %. Accordingly, a lattice strain field is expected around interstitial N atoms that are located at octahedral sites. By comparing the crystal structure around the octahedral site of Ni with that of Ni3N structure, the lattice strains are estimated to be in the range of 5 ∼ 15 %. The results show that the (200) plane has the lattice strains that are two times higher than those in other planes, and this is argued to be the reason for the measured anisotropy of residual microstrain in Ni after cryomilling.

Some composite materials of poly (lactic acid) (PLA) with organophilic clays and an organic crystal accelerator (OCA) were prepared by melt compounding method to improve the rate of crystallization and mechanical properties, especially heat distortion temperature (HDT). The PLA/clay nanocomposite with 2.9wt% of finely dispersed organophilic clay and 1wt% of the OCA showed an exothermic peak corresponding to its crystallization at 1.9 minutes in the isothermal measurement of DSC at 100°C from its molten state, although PLA itself did not show a clear peak in the same procedure. Crystallized specimens of the nanocomposite were well obtained by injection molding when the hold time was set at more than 90 seconds. The HDT of the specimens that were held for more than 120 seconds in the mold exceeded 120°C. It is about 65°C higher than amorphous PLA. The crystallized specimen with 1wt% of the clay showed higher tensile modulus and Izod impact strength than the amorphous one.

A range of multi-wall carbon nanotubes and carbon nanofibres were mixed with a polyamide-12 matrix using a twin-screw microextruder, and the resulting blends used to produce a series of reinforced polymer fibres. The aim was to compare the dispersion and mechanical properties achieved for nanofillers produced by different techniques. A high quality of dispersion was achieved for all the catalytically-grown materials and the greatest improvements in stiffness were observed using aligned, substrate-grown, carbon nanotubes. The use of entangled multi-wall carbon nanotubes led to the most pronounced increase in yield stress. The degrees of polymer and nanofiller alignment and the morphology of the polymer matrix were assessed using X-ray diffraction and calorimetry.

Abstract Superhard nanocomposites, nc-MnN/a-XxNy (M = Ti, W, V, Zr, (Al1-xTix)N; X = Si, B) with hardness of 40–100 GPa are prepared by plasma CVD or PVD under a sufficiently high nitrogen activity and deposition temperature that allow the formation of a stable nanostructure by self-organization upon strong thermodynamically driven, spinodal phase segregation. These nanocomposites display an extraordinary combination of a high hardness, high elastic recovery, high resistance against brittle fracture and tensile strength of 5 to 40 GPa approaching the ideal strength of flaw-free materials. These properties can be understood in terms of conventional fracture physics scaled appropriately down to crystallite sizes of few nm. The interfacial monolayer of Si3N4 or BN with strong bonding to the nanocrystallites and high structural flexibility avoids grain boundary sliding. With increasing thickness of this interface the hardness decreases, possibly due to an increase of this “liquid-like” component in which plastic transformation can be triggered.

The crystallization behavior of polymers in nanocomposites with inorganic fillers (montmorillonite layered silicate, MMT) is reviewed. Various different polymers are comparatively discussed [poly(vinyl alcohol) (PVA), polypropylene (PP), syndiotactic-polystyrene (sPS), and poly(ethylene oxide) (PEO)] representing three types of filler/matrix interactions: strong specific interactions (PVA/MMT), weak/negligible interactions (sPS and PP/o-MMT), and “unfavorable” (PEO/MMT). In the case of PVA/MMT, crystallization of PVA is strongly promoted by MMT, also stabilizing a new crystal form not found in bulk PVA. For sPS and PP/o-MMT, crystallization is only moderately affected, exhibiting traces of simple heterogeneous nucleation and mostly bulk-like crystal structures, with very small traces on non-bulk crystals. For PEO, crystallization is impeded near the MMT surfaces, due to coordination of the surface cations to the PEO. In all cases smaller spherulite sizes develop when filler is added, independent of the size of the bulk polymer spherulites, whereas the crystallization temperature changes reflect the strength of the polymer/surface interactions.

Nanoscale polycrystalline metals typically exhibit increasing hardness with decreasing grain size down to a critical value on the order of 5 to 30 nm. Below this, a plateau or decrease is often observed. Similar observations are made for nanoscale multilayer thin films. There, TEM observations and modeling suggest that the hardness peak may be associated with the inability of interfaces to contain dislocations within individual nanoscale layers. This manuscript pursues the same concept for nanoscale polycrystalline metals via an analytic study of dislocation nucleation and motion within a regular 2D hexagonal array of grains. The model predicts a hardness peak and loss of dislocation confinement in the 5 to 30 nm grain size regime, but only if the nature of dislocation interaction with grain boundaries changes in the nanoscale regime.

The problems of the creation of composite polymer-based materials with the use of amorphous nanosized molybdenum and molybdenum dioxide fillers with the particle size of less than 5 nm are considered. The copolymer of formaldehyde and dioxolane (CFD) and the polyphenylene sulphide (PPS) are chosen as polymeric matrixes. A comparative analysis of the tribological properties of composite materials with amorphous nanosized fillers and crystalline microsized analogues is made. It is determined that the use of isotropic nanosized amorphous fillers leads to improved wear-resistance up to three times for CFD-based composites and up to an order for PPS-based ones.

The early stages of stress development during epitaxial growth of metal layers with a large misfit in lattice parameters still need in-depth understanding. In this particular study we have focused on Ag-Cu system, which is immiscible and exhibit a large 14% misfit in lattice parameters. Ag/Cu multilayers have been grown by ultrahigh-vacuum evaporation on Si (111) maintained at -20°C, 35°C or 110°C. The thickness of the individual layers is about 100 Å. All the films present the same (111) orientation with a well defined in-plane orientation: <110> Cu or Ag // <110> Si. The stress was monitored during growth with a home-made laser curvature measurement device. The stress vs thickness behaviour is highly asymmetric when comparing Ag/Cu and Cu/Ag. Indeed Ag grown on Cu does not develop any measurable stress at any thickness or temperature, whereas Cu grows on Ag under tensile temperature and thickness-dependent stress. The temperature dependence of this stress relaxation cannot be interpreted with a standard relaxation model including dislocation motion. A possible way to understand the stress temperature dependence is to consider the evolution of microstructure during growth.

Quantized high-frequency (∼1016 Hz) correlated longitudinal electron excitations (plasmons) generated in the energy-loss range 0–50 eV by fast electrons passing through any solid enable one to probe various states of matter. Their energy, Ep, is directly related to the density of valence electrons, thus allowing determination of solid-state properties that are governed by ground-state densities. Universal features and scaling in relations between Ep and the cohesive energy per atomic volume, bonding electron density and elastic constants have been established. The resulting correlations follow the universal binding energy relationship, thus providing new insights into the fundamental nature of structure-property relationships. They allow direct in situ determination of local material properties in an analytical electron microscope, as illustrated by examples utilizing Al- and Ti-based structural alloys.

Single wall (SWNT) and multi-wall carbon nanotubes (MWNT) were electrostatically assembled into nanofibers through an electrospinning process in order to increase the strength and toughness of polyacrylonitrile (PAN)-derived carbon fibers. It was found that the effectiveness of carbon nanotubes (CNT) in reinforcing the PAN precursor is highly dependent on the dispersion and the alignment of the CNT. Alignment was achieved during electrospinning by the flow of polymer, electrostatic charge and diameter confinement. Up to 10 wt. % SWNT co-electrospun with PAN was successfully produced with fiber diameters in the range of 40 nm to 400 nm. With the addition of 1 wt. % SWNT, a two-fold increase in strength and modulus was obtained in the as-spun nanofibers mat. These encouraging results show a promising pathway to produce the next generation of high performance carbon fibers that will help bridge dimensional and properties gap between nanoscopic and macroscopic structures.

We report the results of a study of adsorption of small molecules on the surface of buckminsterfullerene, C60. The pressure dependence of the Raman spectrum was investigated over the range 0–10 GPa in methanol-water mixtures that were used as the pressure transmitting fluid (PTF) in a diamond anvil cell. It is found that the spectral shift and its pressure derivative are sensitive to both the applied pressure and to the composition of the PTF. These observations are consistent with an explanation that involves preferential adsorption onto the surface of the C60. In particular, the notion of C60 collapse needs not be invoked to explain the observations.

The elastic and plastic deformation of micron size anisotropic polycrystals of Ni3Al and Cu3Au intermetallic alloys have been studied under non-hydrostatic conditions by energy-dispersive X-ray diffraction (EDX) in a diamond-anvil cell. Compression was achieved by confining the samples in a viscous fluid or directly between the diamond anvils. Deviatoric forces are introduced in the samples as a result of the increasing viscosity with pressure and the eventual glassification of the pressurizing medium or by the contact forces of the diamond anvils. Line shifts and line profiles were used to analyze elastic and plastic strains. Plastic deformation is due to the onset of non-hydrostatic stresses and the introduction of stacking faults and dislocations. A volume incompressibility due to plastic deformation and the saturation of the stacking fault probability is followed by an elastic compression of a fully plastically deformed state. The compression of this state is isotropic and independent of the presence and type of the pressurizing medium. From the measured strains at different crystallographic orientations, the uniaxial stress and the stacking fault probability as a function of the confining pressure are derived and their role in the equation of state is examined. Using finite elasticity, the equation of state is derived in the presence of uniaxial stresses causing stacking faults, defects and dislocations.

Ceramic coatings can provide an ideal protection for MEMS (MicroElectroMechanical Systems) structures while imposing a great challenge in processing a prime, reliant coating due to their inherent brittleness and defects formed during processing. In an attempt to compensate for the weakness of the ceramic coating, we have developed a low-temperature solution precursor process to create strain-tolerant, protective bilayer coating consisting of an integrated ceramic-organic hybrid material. The top ceramic coating offers an inert, protective layer whereas the underlying nanometer scale self-assembled organic coating provides compliance for the overlying hard coating. Together, these bilayers minimize mechanical and thermal stresses. In addition, organic self-assembled monolayers(SAM) act as a ‘template’ by forming a proper surface functionality for the subsequent growth of hard ceramic coatings. Molecular level understanding of the microstructure and micromechanics involved in the synthesis and processing of the coating is systematically studied by a variety of characterization techniques such as XRD, AFM, SEM/EDS and nanoindentation. This work is also complemented by numerical simulation to provide a clearer understanding of the stress development in the ceramic coating and its interfacial properties.

By using elastic continuum model for a spherical body along with the appropriate boundary conditions at the surface of the body, vibrational frequencies of MgO nanoparticles are calculated in the present paper. It is found that the vibrational frequencies corresponding to these eigen values depend strongly on the size of the nano particles and the peaks in the Raman spectra shift to the higher frequencies side as size of the nano particles decreases. We have also investigated the effect of a polymer matrix on the frequencies and Raman shift of MgO nanoparticles. The damping constant is found to vary linearly with the size of nano particles which shows that the mechanical damping is the prominent mechanism in the damping of the acoustic phonon modes.

The creep resistance of the directionally solidified ceramic eutectic of Al2O3/c-ZrO2(Y2O3) was studied in the temperature range of 1200–1520°C both experimentally and by mechanistic dislocation models. The creep of the eutectic in its growth direction exhibits an initial transient that is attributed to stress relaxation in the c-ZrO2(Y2O3) phase, but otherwise in steady state shows many of the same characteristics of creep in sapphire single crystals with c-axis orientation. The creep strain rate of the eutectic has stress exponents in the range of 4.5–5.0 and a temperature dependence suggesting a rate mechanism governed by oxygen ion diffusion in the Al2O3. A detailed dislocation model of the creep rate indicates that much of the nano-scale encapsulated c-ZrO2(Y2O3) is too small to deform by creep so that the major contribution to the recorded creep strain is derived from the diffusion-controlled climb of pyramidal edge dislocations in the Al2O3 phase. The evidence suggests that the climbing dislocations in Al2O3 must repeatly circumvent the c-ZrO2(Y2O3) domains acting as dispersoids resulting in the stress exponents larger than 3. The creep model is in very good agreement with the experiments.

We report the results of first-principles calculations showing that boron can form a wide variety of metastable planar and tubular forms with unusual electronic and mechanical properties. The preferred planar structure is a buckled triangular lattice that breaks the threefold ground state degeneracy of the flat triangular plane. When the plane is rolled into a tube, the ground state degeneracy leads to a strong chirality dependence of the binding energy and elastic response, an unusual property that is not found in carbon nanotubes. The achiral (n, 0) tubes derive their structure from the flat triangular plane. The achiral (n, n) boron nanotubes arise from the buckled plane, and have large cohesive energies and novel structures as a result.