A normal solid becomes stiffer when squeezed and softer when heated. In contrast, silica glass behaves the opposite way: its elastic moduli decrease upon compression and increase upon heating. Silica glass is also known to densify under compression and radiations. These have been long-standing mysteries in materials science. Using molecular dynamics simulation, we uncovered the structural origins of the anomalous thermo-mechanical behaviors and mechanisms of permanent densification in silica glass. Accordingly, these anomalies can be attributed to localized structural transitions, analogous to those that occur in the crystalline counterparts. The irreversible densification in silica glass is achieved through structural transition involving bond breaking and re-formation under a combination of high pressure and temperature. We further revealed that the anomalous thermo-mechanical behaviors are inherently connected to the ability of the glass to undergo permanent densification. Our computer simulations demonstrate that by processing in ways that gradually eliminates anomalous thermo-mechanical behaviors, degree of the glass to undergo densification can be eventually eradicated. This provides the conceptual foundation for the bottom-up design of new glasses with tunable structure and properties.

Monodisperse ZnO quantum dots (QDs) with a particle size of about 5 nm have been synthesized. Isopropanol together with hexane were utilized to precipitate ZnO nanoparticles to form condensed phases, ranging from white flocculation, to gel-like fluid, and to transparent solid. The morphology and structure in the transparent ZnO solid was characterized by UV-vis, X-ray diffraction, small-angle X-ray scattering, transmission electron microscopy, and scanning electron microscopy. The mechanisms for the formation of transparent ZnO QDs close-packed structure were monitored via UV-vis spectra, and found likely to be a colloidal crystal. The colloidal crystal is transparent and absorbs UV light efficiently. Possible conditions for the formation of the ZnO QDs colloidal crystal are discussed.

For centuries, the manipulation of mechanical properties for the development of components has been extremely important. Its relevance is based on improving the service life in the components. The aim of some techniques that have been used is to introduce strain hardening (tensile) and a beneficial residual stress field. Nevertheless, the application of both methods is very common when the component is manufactured, but the lack of knowledge of the final physical state of the material could compromise the structural integrity of the final product. This work presents a numerical evaluation concerning the characterization of a stainless steel AISI 316L, having a homogeneous axial history and a residual stress field. The relevance of the work is focused in a new methodology that can be used to improve the mechanical resistance of the component and to arrest crack propagation. By altering the mechanical properties of the material, it could be possible to delay nucleation and interrupt the propagation of cracks. This study also shows that if the strain hardening behaviour and the introduction of the residual stress field is not done properly, it could result in a component susceptible to fail. In the same sense, bending tests are proposed to provide tensile and compressive stress profiles.

Exquisite control of surface functionality is essential to tailor the chemical and physical properties of metal nanocrystals to the requirements of specific applications. Hybridization of gold nanoparticles with other components such as polymers and metal oxides can effectively introduce appropriate functionalities on the surface without changing their own properties, and thereby become a basic architecture for various applications such as sensors and catalysts. In the present work, we report two hybrid nanostructures comprising gold nanocrystals. PDMAEMA (poly(dimethylaminoethylmethacrylate))–gold hybrid nanocrystals were synthesized via a polyol process, which produced carboxylate functionality on the gold surface. This hybrid structure was employed for a sensitive pH-sensor in solution. On the other hand, porous silica-gold hybrid nanoreactors were produced by selective etching of gold cores from gold@silica core-shell particles. The nanoreactor framework exhibited high and controllable activity on the reduction of aromatic nitroxides. These two examples of hybrid gold architectures would be able to apply for other metal and metal oxide systems to develop biosensors and energy production catalysts.

We demonstrate the applications of several novel techniques in solid-state nuclear magnetic resonance spectroscopy (SSNMR) to the structural studies of mesoporous organic-inorganic hybrid catalytic materials. Most of these latest capabilities of solid-state NMR were made possible by combining fast magic angle spinning (at ≥ 40 kHz) with new multiple RF pulse sequences. Remarkable gains in sensitivity have been achieved in heteronuclear correlation (HETCOR) spectroscopy through the detection of high-λ (1H) rather than low-λ (e.g., 13C, 15N) nuclei. This so-called indirect detection technique can yield through-space 2D 13C-1H HETCOR spectra of surface species under natural abundance within minutes, a result that earlier has been out of reach. The 15N-1H correlation spectra of species bound to a surface can now be acquired, also without isotope enrichment. The first indirectly detected through-bond 2D 13C-1H spectra of solid samples are shown, as well. In the case of 1D and 2D 29Si NMR, the possibility of generating multiple Carr-Purcell-Meiboom-Gill (CPMG) echoes during data acquisition offered time savings by a factor of ten to one hundred. Examples of the studied materials involve mesoporous silica and mixed oxide nanoparticles functionalized with various types of organic groups, where solid-state NMR provides the definitive characterization.

Anodic Aluminum Oxide (AAO) was grown both as free-standing membranes and as integrated layers on Si as templates for arrays of magnetoresistive nanowires. These structures will be useful for applications such as current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) sensors, magnetic random access memory (MRAM) and microwave oscillator arrays. As the AAO was formed, using a two-step anodization process, columnar nanopores self-assembled inside the oxide to form a close-packed array. The pore diameters could be varied from 10-60nm by changing the anodization conditions. As the diameter of the AAO nanopores decreased, the distance between the nanopores also decreased. The free-standing membranes had pores with lengths of 17um. The pores that were grown directly onto Si with an adhesion layer of Ti were 600nm in length. In addition to growing these latter pores directly onto Si, they were also grown onto Co/Cu/Co thin films that were evaporated onto the Si. Au nanocontacts were electroplated into these nanopores to study point-contact magnetoresistance and microwave response. For the magnetoresistive nanostructures, multilayered Co/Cu nanowires were fabricated via electrochemical deposition. The samples were measured with vibrating sample magnetometry (VSM) and also using ac and dc magnetotransport systems. The highest magnetoresistance was found in nanowires that had hysteresis loops that were identical as measured in plane and perpendicular to the plane. The highest measured MR (Delta R/R = 11%) of the multilayers was calculated as 33% by subtracting the resistance of the Cu leads on either side of the multilayers from the denominator. Shorter wires are currently under construction to avoid this effect. Spin transfer torque (STT) was also measured in the samples. For 10-60nm diameter nanowires, the change in resistance due to STT was around 6% which represents the full magnetoresistance of the larger wires, but only half that of the smaller nanowires. It is therefore concluded that the 10-nm Co layers do not align antiparallel to parallel as fully at the switching current density of JAP-P = 2.7 × 108 A/cm2 compared to the larger wires which switch at JAP-P = 3.2 × 107 A/cm2. With diameters in the 10-60 nm range and integration with Si, these nanostructures have great potential for future nanosensors, MRAM and microwave oscillator arrays.

The Cu-type and (Cu-Fe)-type film catalysts have been successfully prepared by the electroless plating on ZnO nanorods/stainless steel substrates. The microstructure features of the (Cu-Fe)-type films are high porosity and plate-type grains. The addition of iron into Cu-type film can improve the reducibility and the stability of the film catalysts. The reduction temperature of the (Cu-Fe)-type film catalysts decreases with increasing the addition of Fe. For Cu-5 at% Fe film, the reduction temperature is in the range of 195°C to 216°C as comparison in the range of 208°C to 233°C of the Cu-type film catalysts.

An informatics based approach to extract further refinements on the crystallographic information embedded in the Spatial Distribution Maps (SDMs) has been developed. The data mining based methods to generate and interpret spectra that de-convolute the SDMs are discussed. This work has resulted in a method to generate SDMs that can map three-dimensional crystallographic information as opposed to existing methods that map structural information on only one atomic plane at a time. The broader implications of this work on enhancing the interpretation and resolution of structural information in atom probe tomography studies is also discussed.

Effect of back reflectors on light trapping in μc-Si:H cells has been investigated with self-ordered Al substrates obtained by anodic oxidation. With increasing the period of the patterned substrates from 0 to 1.1 μm, 1-μm-thick μc-Si:H cells on the patterned substrates have shown a significant enhancement of spectral response in the near infrared region, giving an increment of the short circuit current density from 18 to 24 mA/cm2. This enhanced light trapping effect are attributed to the improved reflectivity of the rear side and effective light scattering at the front side, as well as light scattering at the rear side.

Conducting polymers have attracted attention in many areas of materials chemistry due to their tunability and wide range of applications. We are interested in utilizing the thermo-switchable properties of precursor PPV polymers to develop capacitor dielectrics that will fail at specific temperatures due to the material irreversibly converting from an insulating to a conducting state. Here, we report the synthesis and characterization of a new halophenyl substituted PPV polymer. We show that the precursor polymer has good dielectric properties over a range of temperatures, but will fail at high temperatures due to the polymer backbone conjugating. By utilizing thermo-switchable dielectrics in capacitors, the unintentional discharge of electricity in the event of a fire or overheating could be averted, providing a fundamental safety mechanism for high-voltage electronics.

We report on a method based on cross-sectional scanning photoelectron microscopy and spectroscopy (XSPEM/S) for studying electronic structure of III-nitride surfaces and interfaces on a submicrometer scale. Cross-sectional III-nitride surfaces prepared by in situ cleavage were investigated to eliminate the polarization effects associated with the interface charges/dipoles normal to the cleaved surface. In contrast to the as-grown polar surfaces which show strong surface band bending, the cleaved nonpolar surfaces have been found to be under the flat-band conditions. Therefore, both doping and compositional junctions can be directly visualized at the cleaved nonpolar surfaces. Additionally, we show that the “intrinsic” valence band offsets at the cleaved III-nitride heterojunctions can be unambiguously determined.

Anodic aluminum oxide (AAO) template was prepared by using anodizing voltage step-decreasing method after two-step oxide method. Based on AAO template, Fe nanowires arrays were electrochemically deposited. Fe nanowires were coated by chitosan. Fe nanowires/chitosan was synthesized by glutaraldehyde as cross-linking reagent. By crosslinking α-human chorionic gonadotropin (α-HCG), biological probes with Fe nanowires/chitosan/antibody were prepared. An easy operating, easy taking and rapid reacting magnetic detecting system was developed after optimizing the geometry parameters of detect coil. Different concentration samples with 1, 2 and 5 g(Fe)/L were detected. The results show that the sensitivity of system is 0.2 g(Fe)/L and can be improve better.

Microcomposites composed of titanium dioxide nanoparticles embedded within cross-linked, thermally responsive microgels of poly(N-isopropylacrylamide) were prepared. These microcomposites showed rapid sedimentation, which is useful for gravity separation of the titania nanoparticles in applications such as environmental remediation. To investigate the degradation kinetics using these microcomposites in aqueous suspensions, methyl orange was employed as a model contaminant. The decline in the methyl orange concentration was monitored using UV-Vis spectroscopy. Degradation of methyl orange was also measured using only nanoparticles TiO2 (DegussaTM P25) for comparison with the microcomposites. Experiments were performed at different pH conditions that spanned acidic, neutral, and basic conditions to gain insight into the interplay of TiO2 surface charge, ionization of the polyelctrolyte chains in the microcomposites, and ionization of the methyl orange.

In the last decades, HRTEM approach has been quite fruitful to study structural characteristics of layered transition metal sulfide (LTMS) catalytic materials since providing direct local information about the structural organization of this quite important class of catalysts at the nanoscale level. However, up to now, HRTEM observations of some common localized structural organization like honeycomb-like structures have remained unexplained. In the present study, a structural model corresponding to stacked 2H-MoS2 slabs twisted along their basal direction is proposed to explain honeycomb like-structures observed by HRTEM. This model is based on a comparison between experimental and simulated images of 2H-MoS2 catalysts promoted with cobalt. The resulting Density of States (DOS) of the twisted structure was then calculated.

Immunoassays are currently the main analytical technique for quantification of a wide range of analytes of clinical, medical, biotechnological, and environmental significance with high sensitivity and specificity. Miniaturization of immunoassays is achieved using microfluidics coupled with integrated optical detection of the antibody-antigen molecular recognition reaction using thin-film amorphous silicon (a-Si:H) photodiodes. The detection system used consists of an a-Si:H photodiode aligned with a polydimethylsiloxane (PDMS) microchannel. An enzymatic reaction taking place in the microchannel yields a product which is a light-absorbent molecule and hence can be optically detected by the integrated photodiode. Specific antigen-antibody reaction was detected and distinguished from the non-specific reaction.

A statistical model is developed to study the loading-rate dependent mechanical response of biopolymer arrays. Using the kinetic Monte Carlo method, bundles of fibers are studied under load-controlled and displacement-controlled conditions.

Static leach tests were conducted for simulated low-level radioactive waste (LLW) glass in deionized water at 90 °C for up to one year to investigate the dissolution mechanism of LLW glass. Widely studied leaching behavior of high-level radioactive waste (HLW) glass is referred in discussing the dissolution mechanism. LLW glass is characterized by higher sodium (Na) and aluminum (Al) contents than HLW glass, about twice as high as R7T7, with its SiO2 content close to HLW glass. Powdered simulated LLW glass of three different chemical compositions was tested with the glass-surface-to-water-volume ratio of 2,000 m−1. The release rates of boron (B), widely used as an indicator of dissolution for HLW glass, decreased with time during leaching, as commonly observed in similar tests for HLW glass. The pH of the leachate was stable around 11.3 - 11.6, which is higher than those in similar tests for HLW glass by one pH unit or more. The concentrations of Al in the leachates were higher compared to data for HLW glass by two orders of magnitude. The high concentration seems to be caused by higher pH. In the leachate condition of the present tests, a zeolitic mineral (analcime) is thermodynamically more stable than amorphous silica (SiO2(am)) which is known to control the concentration of dissolved silica (Si) with respect to HLW glass. The present results imply that dissolution of the LLW glass is accompanied with formation of analcime under virtually closed systems such as geological repository where the groundwater flow rate is quite low.

Tin oxide (SnO2) is a durable, inexpensive transparent conducting oxide (TCO) material used for thin-film photovoltaic devices. However, the optical properties of conducting SnO2:F are generally not as good as in other conducting TCO materials such as ITO and ZnO:Al. Our previous analyses indicate that for thin-film solar cells, improving the optical properties of SnO2-coated glass could enhance photon collection and gain up to 10% additional photocurrent. Previously, we showed that some commercial SnO2 samples could have much higher optical absorption than others [2]. In this work, we continue our study on causes that could contribute to the high optical absorption of SnO2 films. The SnO2:F samples are fabricated by low-pressure metal-organic chemical vapor deposition or atmospheric-pressure chemical vapor deposition with tin precursors that includes different amounts of chlorine. Optical, electrical, and compositional analyses were performed. In addition to the free-carrier-introduced optical absorption, the non-active dopant also impacts the optical absorption. Among the SnO2 films fabricated with different precursors, the optical properties show a relationship based on the level of chlorine in the precursors and films. With a low-optical-absorption SnO2 layer, the solar cell could have better photon collection and a higher short-circuit current density.

Electronic and magnetic properties of small Co-O atomic clusters (“quantum wires”) have been studied in the framework of the Hartree-Fock (HF) method. The obtained results indicate that non-stoichiometric Co - O molecules with more than two O atoms possess at least one remarkably stretchable O-O bond that may facilitate significant re-construction of such molecules to larger structures. This re-construction may result in energetically favorable spin re-alignment in “antiferromagnetic” HF singlet Co-O molecules converting the singlets to larger “ferromagnetic” HF triplets and pentets. Such a spin re-alignment is energetically favorable, and may happen at the antiferromagnet-ferromagnet interface in “critical” core (Co) – shell (CoO) exchange-biased nanoclusters, providing for minimization of the surface energy, and leading to a loss of exchange bias. The obtained results are in agreement with available experimental and computational data.

Solidification of a simple casting made of ductile iron is mathematically modeled in this work. The model is able to numerically simulate the cooling rate and solidification of the whole casting system composed by a cubic piece, a blind riser connected through a rectangular neck, and immersed in a green sand mold. The center of the neck acts as a valve that allows the flow of liquid metal between the casting and the riser based on the feeding technique known as Pressure Control Risering (PCR). The developed model couples the energy conservation equation and the solidification kinetics of ductile iron, through the statement of proper nucleation and growth laws. This model is satisfactorily validated by comparing the thermal histories predictions with experimental cooling curves obtained in the foundry laboratory for the same casting. According to a process analysis developed in this work, the pouring temperature is the variable that affects the most the solidification and the feeding behavior, since it increases significantly the solidification times in all regions of the casting system.