Molecular dynamics simulations are used to evaluate the influence of Sb dopant atoms at the grain boundaries on plastic deformation of nanocrystalline Cu. Deformation is conducted under uniaxial tensile loading, and Sb atoms are incorporated as substitutional defects at the grain boundaries. The presence of randomly dispersed Sb atoms at the grain boundaries does not appreciably influence the mechanisms associated with dislocation nucleation in nanocrystalline Cu; grain boundary ledges and triple junctions still dominate as partial dislocation sources. However, the magnitude of the tensile stress associated with the partial dislocation nucleation event does increase with increasing Sb concentration and also with increasing grain size. The flow stress of nanocrystalline Cu increases with increasing Sb concentration up to 1.0 at.% Sb, with a maximum observed at a grain size of 15 nm for all Sb concentrations (0.0–2.0 at.% Sb).

We report the characteristics of epitaxial growth and properties of vanadium oxide (VO2) thin films on sapphire (0001) substrates. Pulsed laser deposition was used to grow (002) oriented VO2 films on sapphire (0001). Transmission electron microscopy studies showed that the orientation relationship between the substrate and the thin film is: (002)f2∥(0006)sub3 and [010]f2 ∥sub. It was also established that VO2 has three different orientations in the film plane which are rotated by 60° from each other. The epitaxial growth of vanadium oxide on sapphire (0001) has been explained in the framework of domain matching epitaxy (DME). Electrical resistivity measurements as a function of temperature showed a sharp transition with a hysteresis width ˜5 °C, and large resistance change (˜1.5 × 104) from the semiconductor phase to the metal phase. It is interesting to note that in spite of large angle twin boundaries in these VO2 films, the SMT characteristics are better than those observed for polycrystalline films. The higher width of thermal hysteresis for the VO2 film on c-sapphire compared to a bulk single VO2 crystal and a single-crystal VO2 film on r-sapphire can be attributed to the existence of these large-angle twin grain boundaries. These findings can provide insight into the phase transformation characteristics of VO2, which has important applications in switching and memory devices.

In situ residual gas analyzer techniques were used to identify process-property relationships that regulate microstructure evolution in chemical solution-deposited BaTiO3 films. In situ analysis of furnace exhaust gasses enabled quantitative exploration of thermolysis and crystallization reactions and an ability to identify processing parameters that influence the temperature ranges over which they occur. The atmospheric analysis was instrumental in identifying heat treatments that produced optimally consolidated precursor gels that crystallized into BaTiO3 layers with optimized structure and properties. Slow ramp rates resulted in higher porosity, larger grain size, and a dramatic drop in the capacitor yield. Fast ramp rates produced similar trends; however, the mechanisms were distinct. The effects of oxygen partial pressure were also explored. BaTiO3 grain size increased with increasing pO2, whereas there was no appreciable influence on density and capacitor yield. Optimal firing parameters, i.e., 20 °C/min ramp rate at a pO2 of 10−13 atm, were identified as those that produced an overlap in the temperature ranges of thermolysis and crystallization reactions and thus a precursor gel with a density and compliance that supports crystallization and densification while tolerating the associated volume contraction. This in situ approach to analyze downstream furnace gas is shown to be a generically applicable means to understand synthesis methods that are complicated by simultaneous mechanisms of precursor decomposition, extraction of volatile components, and crystallization.

Technological applications of high temperature superconductors (HTS) require high critical current density, Jc, under operation at high magnetic field strengths. This requires effective flux pinning by introducing artificial defects through creative processing. In this work, we evaluated the feasibility of mixed-phase LaMnO3:MgO (LMO:MgO) films as a potential cap buffer layer for the epitaxial growth and enhanced performance of YBa2Cu3O7-δ (YBCO) films. Such composite films were sputter deposited directly on IBAD-MgO templates (with no additional homo-epitaxial MgO layer) and revealed the formation of two phase-separated, but at the same time vertically aligned, self-assembled composite nanostructures that extend throughout the entire thickness of the film. The YBCO coatings deposited on these nanostructured cap layers showed correlated c-axis pinning and improved in-field Jc performance compared to those of YBCO films fabricated on standard LMO buffers. Microstructural characterization revealed additional extended disorder in the YBCO matrix. The present results demonstrate the feasibility of novel and potentially practical approaches in the pursuit of more efficient, economical, and high performance superconducting devices.

We report temperature-dependent electrical resistivity (or dc conductivity, σdc) down to 4 K for pristine and gamma-irradiated microwave plasma-assisted chemical vapor-deposited boron-doped diamond films with [B]/[C]gas = 4000 ppm to gain insights into the nature of conduction mechanism, distribution, and kinetics of point defects generated due to gamma irradiation prompted by the article [Gupta et al., J. Mater. Res.24, 1498 (2009)]. The pristine samples exhibit typical metallic conduction up to 50 K and with reduction in temperature to 25 K, the σdc decreases monotonically followed by saturation at 4 K, suggesting “disordered” metal or “localized” behavior. For irradiated films, continuous increasing resistivity with decreasing temperature demonstrates semiconducting behavior with thermal activation/hopping conduction phenomena. It is intriguing to propose that irradiation leads to substantial hydrogen redistribution leading to unexpected low-temperature resistivity behavior. Scanning tunneling microscopy/spectroscopy helped to illustrate local grain and grain boundary effects.

A pulsed direct current (dc) reactive ion beam sputtering system has been used to synthesize highly c-axis oriented aluminum nitride (AlN) thin films on (0002)-oriented 200-nm thin titanium layers deposited on a Si-(111) substrate. After a systematic study of the processing variables, high-quality polycrystalline films with preferred c-axis orientation have been grown successfully on the Ti (0002) layer using an Al target under a N2/(N2 + Ar) ratio of 70%, a 2 mTorr processing pressure, and keeping the temperature of the substrate holder at ambient temperature (no substrate heating). The crystalline quality of the AlN and the underlaying Ti thin films was characterized by high-resolution x-ray diffraction. Best ω- full width at half maximum values of the (0002) reflection for 1-μm thin AlN layers are 0.56°. Hence, the AlN layers show a high degree of orientation in the (0002) direction, which directly translates into a high Q value piezoelectric response. Atomic force microscopy measurements were used to study the surface morphology of the Ti layer in an attempt to understand its impact on the quality of the AlN films deposited on top of them. Transmission electron microscopy cross-section analysis has been carried out to investigate the AlN/Ti interface. Our observations reveal the presence of crack-free layers with a smooth surface and extremely low defect density. Even local epitaxy phenomena have been identified at the AlN/Ti interface. The processing conditions used to synthesize AlN layers on Ti at room temperature are efficient in reducing the dislocation density and in-plane residual strain. Such AlN/Ti bilayers can be applied to manufacture novel electroacoustic device structures (such as bulk acoustic wave filters) on silicon substrates in further investigations.

A solid state mechanochemical reaction (MCR) method for synthesizing AlN powder with aluminum and melamine powders as the reactants was proposed and put into practice. It was found that the solid state MCR between aluminum and melamine is an instantaneous and exothermic reaction. For a certain charge ratio, a critical ball milling time is needed for the MCR to occur. The higher the charge ratio, the faster the MCR. Cryogenic environments help to accelerate the MCR between Al and melamine. In addition to the direct one-step MCR synthesis approach mentioned above, AlN powder can also be synthesized by pre-ball-milling Al and melamine powders followed by heat treatment. Using this two-step approach, the heat treatment temperature is only about 638 °C, which is much lower than that used in other ways for synthesizing AlN powder. The lower heat treatment temperature can be attributed to the combined effect of both the adoption of melamine and the high reactivity of powders caused by ball milling. Comparatively, the present solid state MCR method for synthesizing AlN powder may be more cost-effective and hence more promising to be used to industrially produce both AlN powder and in situ AlNP reinforced aluminum matrix composites.

Al8B4C7 was used as a sintering additive for the densification of nano-SiC powder. The average grain size was approximately 70 nm after sintering SiC-12.5wt% Al8B4C7 at 1550 °C. The densification rate strongly depended on the sintering temperature and the applied pressure. The rearrangement of SiC particles occurred at the initial shrinkage, while viscous flow and liquid phase sintering became important at the middle and final stage of densification.

A novel colloidal co-casting process was developed to fabricate laser quality, multisegment composite ceramic laser gain materials. The approach was demonstrated for a three segment transparent composite rod 62 mm long by 3 mm diameter consisting of undoped yttrium aluminus garnet (YAG), 0.25% Er:YAG, and 0.5% Er:YAG. The Er concentration profile in the composite has steep, controllable gradients at the segment interfaces, while maintaining constant dopant concentrations within each segment. The composite rod has 84% transmittance at 1645 nm (the lasing wavelength) with a scatter loss of 0.4% cm−1. Laser operation of such a composite Er:YAG ceramic rod was demonstrated for the first time, with nearly equivalent lasing behavior to an Er:YAG single crystal rod.

Hafnium oxide films doubly doped with CeCl3 and TbCl3 and triply doped with CeCl3, TbCl3, and MnCl2 were deposited at 300 °C with the ultrasonic spray pyrolysis technique. The green and yellow emissions of Tb3+ ions and the yellow-red emission of Mn2+ ions can be generated upon ultraviolet (UV) excitation via a nonradiative energy transfer from Ce3+ to Tb3+ and Ce3+ to Mn2+. In the doubly doped film Ce3+ → Tb3+ energy transfer via an electric dipole–quadrupole interaction appears to be the most probable transfer mechanism; the efficiency of this transfer is about 81% upon excitation at 270 nm. In the HfO2 films activated with Ce3+, Tb3+, and Mn2+ the efficiency of energy transfer from Ce3+ to Tb3+ and Mn2+ ions is enhanced by increasing the Mn2+ concentration, up to about 76% for the film with the highest manganese content (1.6 at.%). In addition, it is demonstrated that these triply doped films can generate cold white light emission upon excitation at 270 nm (peak emission wave length of an AlGaN/GaN-based LEDs).

CdS nanocrystals embedded in sodium borosilicate glass were synthesized through sol-gel process. The CdS nanocrystals were usually 10 to 20 nm in size. The microstructure of CdS nanocrystals was determined to be of the hexagonal phase. The morphology and microstructure of the glass were examined using diverse techniques including scanning-probe microscopy (SPM), x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy dispersion x-ray spectra (EDAX), and high-resolution TEM (HRTEM). The linear optical absorption spectrum of the glass showed a blue shift as a result of quantum-size effect. Furthermore, the third-order optical nonlinearities of the glass were studied by Z-scan technique at a wavelength of 770 nm. The results showed that the third-order optical nonlinear refractive index γ, absorption coefficient β, and susceptibility χ(3) were determined to be −2.16 × 10−16 m2/W, 6.32 × 10−11 m/W, and 1.20 × 10−10 esu, respectively, which were greater than those reported previously for CdS nanocrystals embedded in different matrices.

We report the synthesis, characterization, and optical properties of high-temperature stable lanthanide-doped luminescent zirconia nanoparticles via a novel method using carbon black as template. Dopant concentrations were varied from 1 to 5% of Er3+ or Nd3+ and annealing temperatures were varied from 650 to 1100 °C. The effects of the dopant concentration on crystal structure and emission properties were evaluated using x-ray powder diffraction and fluorescence spectroscopy, respectively. The lanthanide cations were found to stabilize the tetragonal phase of zirconia over the monoclinic phase as dopant concentration was increased to 5%. Increasing the annealing temperature to 1100 °C had the opposite effect and was found to stabilize the monoclinic phase of zirconia. The luminescence intensity of the Nd-doped zirconia was enhanced by two orders of magnitude over the undoped or Er-doped zirconia. In all cases, the luminescence spectra revealed increasing intensity with increasing annealing temperature. Zirconia luminescence at near-infrared wavelengths is likely caused by oxygen vacancies. This work demonstrates that the spectral signatures of fluorescent zirconia nanoparticles can be modified with small lanthanide dopant concentration. These particles will have utility in fluorescent sensors and tags, as well as new in refractory materials.

Optical absorption and scattering behaviors of lanthanum hexaboride (LaB6) nanoparticulate dispersion coatings with various particle sizes have been investigated in the ultraviolet to near-infrared (NIR) wavelengths for application to solar control filters. Large characteristic near-infrared absorption is introduced as the decreased particle size falls into nanoscale, and its origin is discussed in terms of a localized surface plasmon resonance (LSPR) of conduction electrons. Optical constants of LaB6 have been measured and Mie scattering theory analysis was conducted. The theory was found to generally replicate the optical characteristics, and variations of absorbed and scattered wave fractions as well as the LSPR wavelength have been discussed with varying particle size. The absorption peaks are found as shaped narrower and located at shorter wavelength in theory than in experiment, which is suggested as ascribed mainly to the nonspherical distorted shape of LaB6 nanoparticles.

We have developed dye-sensitized nanocrystalline CdS and ZnS solar cells (DSSCs) based on crystal violet, methylene blue, and fluorescein photosensitizers. Nanocrystalline CdS and ZnS were synthesized by a green synthesis method using starch as the capping agent. Characterization of nanocrystalline CdS and ZnS was carried out by optical absorption and x-ray diffraction. The results indicate that CdS and ZnS prepared by this method may be used as photoelectodes in photo-electro-chemical energy conversion systems. DSSCs have been built and their photocurrent, open-circuit voltage, fill factor, and efficiency have been measured under direct sunlight illumination (1000 Wcm−2). The efficiency of the cells made from dye-CdS was much higher than that of the cells made from dye-ZnS. This can be attributed to the particle size effect. Among the prepared dye-sensitized solar cells, a DSSC based on fluorescein dye as the photosensitizer produced the highest overall light solar energy to electricity conversion efficiency.

Nanoindentation and the viscous-elastic–plastic (VEP) model developed by Oyen and Cook for lightly filled thermoplastic polymer composites were used to characterize the elastic modulus, hardness, and viscoelastic response of a new high elastic modulus dental resin composite. The VEP model was used because loading rate studies indicated a viscous component in the loading/unloading response of our highly filled, thermosetting acrylic resin composites. Increasing the volume fraction of our high modulus filler increased the elastic modulus and hardness and decreased the viscous response in our composites. Coupling the filler and resin matrix with a commercial coupling agent like Metaltite or MPTMS (3-methacryloxypropyltrimethoxysilane) that ionically bonds to the filler and covalently bonds to the matrix decreases the viscous response and increases the hardness of the composite. The coupling agents did not affect the elastic modulus. The ability of the VEP model to predict load–displacement trajectories and the correlation of the elastic modulus and hardness values determined from the VEP model with those from the direct continuous stiffness measurement mode nanoindentation measurements indicate that the VEP model can be extended to highly filled, thermosetting systems. This is valuable since the potential to predict elastic, plastic, and viscous contributions to behavior should be valuable in the design and understanding of future highly filled resin composite systems.

Mechanical and thermodynamical properties of bulk polyethylene have been scrutinized using coarse-grained (CG) molecular dynamics simulations. Entangled but cross-link-free polymer clusters are generated by the semicrystalline lattice method for a wide range chain length of alkane modeled by CG beads, and tested under compressive and tensile stress with various temperature and strain rates. It has been found that the specific volume and volumetric thermal expansion coefficient decrease with the increase of chain length, where the specific volume is a linear function of the bond number to all bead number ratios, while the thermal expansion coefficient is a linear rational function of the ratio. Glass-transition temperature, however, does not seem to be sensitive to chain length. Yield stress under tension and compression increases with the increase of the bond number to all bead number ratio and strain rate as well as with decreasing temperature. The correlation found between chain length and these physical parameters suggests that the ratio dominates the mechanical properties of the present CG-modeled linear polymer material.

Besides biological and chemical cues, cellular behavior has been found to be affected by mechanical cues such as traction forces, surface topology, and in particular the mechanical properties of the substrate. The present study focuses on completely characterizing the bulk linear mechanical properties of such soft substrates, a good example of which are hydrogels. The complete characterization involves the measurement of Young's modulus, shear modulus, and Poisson's ratio of these hydrogels, which is achieved by manipulating nonspherical magnetic microneedles embedded inside them. Translating and rotating these microneedles under the influence of a known force or torque, respectively, allows us to determine the local mechanical properties of the hydrogels. Two specific hydrogels, namely bis-cross-linked polyacrylamide gels and DNA cross-linked polyacrylamide gels were used, and their properties were measured as a function of gel concentration. The bis-cross-linked gels were found to have a Poisson's ratio that varied between 0.38 and 0.49, while for the DNA-cross-linked gels, Poisson's ratio varied between 0.36 and 0.49. The local shear moduli, measured on the 10 μm scale, of these gels were in good agreement with the global shear modulus obtained from a rheology study. Also the local Young's modulus of the hydrogels was compared with the global modulus obtained using bead experiments, and it was observed that the inhomogeneities in the hydrogel increases with increasing cross-linker concentration. This study helps us fully characterize the properties of the substrate, which helps us to better understand the behavior of cells on these substrates.

The relationship between atomic force microscopy probe-sample adhesion force and relative humidity (RH) at five different levels of surface free energy (γs) of an organic self-assembled monolayer (SAM) has been investigated. Different γs levels were achieved by exposing a patterned SiO2/CH3-terminated octyldimethylchlorosilane SAM sample to an ultraviolet (UV)/ozone atmosphere. A model consisting of the Laplace-Kelvin theory for capillary condensation for nanosized probe and probe-sample molecular interaction was derived to describe the adhesion force as a function of RH from 25 to 90% for different SAM γs values. The equations were solved analytically by using an equivalent curvature of the probe tip shape. Experimental results show that the adhesion force increases slightly with RH for nonpolar SAM. However, for polar SAM surfaces, it increases at first, reaches a maximum, and then decreases. Both the rate of increase and the maximum of the adhesion force with humidity are γs-dependent, which is in good agreement with theoretical prediction. The large rise in the adhesion force in this RH range is due to the capillary force.

In this article, the shear-banding behavior in bulk metallic-glasses (BMGs) is studied using a focused ion beam (FIB)-based nanoindentation method, which involves cylindrical nanoindentation of a FIB-milled BMG microlamella and is capable of revealing the subsurface shear-band patterns down to the submicron scale. The results of the current study on a Zr-based BMG clearly show that short shear bands, with the lengths of a few hundred nanometers, could be severely kinked before growing into a longer one, which implies that structural heterogeneity plays an important role in the microplasticity of BMGs. Furthermore, through the three-dimensional finite-element simulation combined with the theoretical calculation based on the Mohr–Coulomb law, it is found that the yield strengths exhibit a large scatter as a consequence of the structural heterogeneity when microplasticity occurs in the Zr-based BMG, which is consistent with our recent findings obtained from the microcompression experiments.

The phase equilibria at 200 °C, 250 °C, 300 °C, and 400 °C and the phase transformation of the Sn-Au-Bi system were investigated by using the electron probe micro-analyzer (EPMA) and differential scanning calorimeter (DSC), respectively. It is found that there is a new ternary intermetallic compound with a possible AuSn structure (called the ϕ phase in the present work), which has a limited solubility of Au in the Au-rich portion, and the ϕ phase decomposes peritectically at about 313 °C. Based on the experimental data reported in the previous papers and new experimental data determined by the present work, thermodynamic assessments of the Sn-Au-Bi system were carried out by the calculation of phase diagrams (CALPHAD) method. The thermodynamic parameters for describing the Gibbs free energy of each phase were optimized, and reasonable agreement between the calculated results and experimental data was obtained in the Sn-Au-Bi ternary system.