In this work we report a specialized reactive force field (ReaxFF) developed for the study of alumina/epoxy interfaces. Force field parameters were obtained by fitting the reactions of small clusters and separate components of epoxies on alumina surfaces in the alpha phase. We also introduce a procedure to obtain crosslinked epoxies based on a proximity criterion to drive reactions and induce crosslinking. Properties of the resulting polymer, like the coefficient of thermal expansion, are found to be of the same order of magnitude as in experiments. Molecular dynamics was used to calculate the adhesion between these polymers and different alumina surfaces: Al2O3-deficient, Al-terminated, O-terminated, 12% and 75% hydroxylated. Typical values for strong adhesion are about 0.70 J/m2 which compare well with previously reported works. The role of defects is also studied.

In order to understand the vacancy behavior during incubation period before steady state void swelling, positron annihilation lifetime measurements was performed after isochronal annealing of austenitic stainless steel (Ti added modified SUS316SS) and ferritic stainless steel (F82H) irradiated by neutrons and electrons to a dose of 0.2 dpa. By electron and neutron irradiations below 363 K, vacancies and nano-voids containing of few vacancies were formed in both alloys. By increasing annealing temperatures, the lifetime decreased without forming nano-voids. The change of lifetime during the annealing indicated the formation and growth of staking fault tetrahedra (Ti added modified SUS316SS) and the annihilation of vacancies at precipitates (F82H).

A sharp-interface model to study radiation-induced segregation in binary alloy has been developed. This model is based on a set of reaction-diffusion equations for the point defect and atomic species concentrations, with a stochastic, spatially-resolved, discrete defect generation terms representing the cascade damage. An important feature of this model, which is significantly different from the way radiation-induced segregation has been studied in the past, is that the role of the boundaries as defect sinks has been ensured by defining defect-boundary interactions via a set of reaction boundary conditions. Defining defect-boundary interactions in this way makes it possible to capture the process of segregation as a consequence of boundary motion. The model is tested in 2D for Cu-Au solid solution with the material surface being free to move. The Gear method has been used to solve the reaction-diffusion equations. Enrichment of Cu and depletion of Au have been observed near to the boundaries.

In this work we compare two different detection schemes that are sensitive to the focus shift of a probe beam due to induced surface curvature. The technique on which both detection schemes are based is called ThERM (Thermal Expansion-Recovery Microscopy) and allows the retrieval of the thermal diffusivity at microscopic levels, hence mapping such magnitude over a sample surface. The induced thermal expansion defocuses the probe beam due to the surface deformation (curvature). The dependence of the defocusing with the pump modulation frequency yields the thermal diffusivity of the sample at the impinging location. The explored depth is controlled by the pump beam size. By scanning both beams, a complete map of the thermal diffusivity can be retrieved.

During the last years carbon-based nanostructures (such as, fullerenes, carbon nanotubes and graphene) have been object of intense investigations. The great interest in these nanostructures can be attributed to their remarkable electrical and mechanical properties. Their inorganic equivalent structures do exist and are based on boron nitride (BN) motifs. BN fullerenes, nanotubes and single layers have been already synthesized. Recently, the fracture patterns of single layer graphene and multi-walled carbon nanotubes under stress have been studied by theoretical and experimental methods. In this work we investigated the fracturing process of defective carbon and boron nitride nanotubes under similar stress conditions. We have carried out fully atomistic molecular reactive molecular dynamics simulations using the ReaxFF force field. The similarities and differences between carbon and boron nitride fracture patterns are addressed.

A dislocation-density based crystalline plasticity and specialized finite-element formulations were used to study the behavior of energetic crystalline aggregates. The energetic crystalline material studied was RDX (cyclotrimethylene trinitramine) with a polymer binder and different void porosities. The aggregate was subjected to different dynamic pressures, and the analyses indicate that maximum temperature increases, constrained dislocation densities, and plastic strain accumulations occurred around the void peripheries, which affected overall deformation behavior. These regions of extreme temperature rise and thermal decomposition can result in hot spot formation.

Nematic liquid crystals (NLCs) under micron-range confinement exhibit a rich defect phenomenology that can be used to extract elastic (Frank moduli) material parameters of critical importance for next generation electro-optical devices. In this work we develop a model to predict defect-driven textural transformations that arise when a NLC is confined to a circular capillary. In the initial transformation stage an unstable disclination defect of strength +1 nucleates in the axis of the capillary and quickly branches into two stable +1/2 disclination defects. The model includes: (1) the Kirchhoff branch balance equation which predicts the splitting of a +1 into two +1/2 wedge disclinations; (2) the curvature of the +1/2 disclination lines as a function of elastic properties. This model shows that by increasing the ratio of tension strength to bending stiffness, the branch point angle increases, but the final defect distance decreases; and (3) the aperture branching angle of the +1/2 lines as a function of the elastic properties and the magnitude of the curvature at the branch point. These three predictions form the basis for the evaluation of the Frank elastic moduli on NLCs. The key advantage of the implemented methodology is to use time-dependent textural transformations under micron-range capillary confinement to extract elastic parametric data needed to further develop NLCs in functional and structural application.

By using a thermodynamic model of nanocrystalline alloys the grain size effect on the solubility of carbon in α-iron is calculated. More specifically the enrichment at grain boundaries is predicted to result in a solubility enhancement. An experimental setup is devised to measure carbon solubility in nanocrystalline iron powder in equilibrium with graphite. At 390 °C a solubility of 0.514 at% is determined for nanocrystalline iron with a grain size of 23 nm.

The transient lattice changes on the Ag(111) crystal due to acoustic wave propagation after excitation with femtosecond pulses was studied by means of time resolved X-ray diffraction. The lattice disorder after UV irradiation is detected by changes of the XRD rocking curve shift, broadening, and total diffraction intensity as a function of time. We have observed a blast force formed within two picoseconds after fs UV irradiation. Experimental results show an initial lattice contraction followed by lattice expansion that propagates with sound velocity.

We have analyzed the hopping movement of a new ionic solid electrolyte by calculating defect formation energies and activation barriers. The role of the lattice during diffusion was established. Thermodynamic properties were determined by means of first principles and phonon calculations at working temperatures. The new solid electrolyte, an antiperovskite, Li3-2xMxAO (in which M is a higher valent cation like Ca2+ or Mg2+ and A is a halide like Cl- or Br- or a mixture of halides), was studied either pure or doped. Moreover, we present experimental ionic conductivity data for these novel solid state ionic conductors for the doped and the pure solid electrolyte from room temperature and up to ∼253 °C. In this paper, we compare the ionic conductivity of the latter solid electrolyte with other fast ionic conductors.

In this work a survey of possible optical stimulation processes in irradiated KCl:Eu with a focusing on photo-transfer thermoluminescence (PTTL) effects are shown. For different wavelengths in the range from 180 to 800 nm a cycle of measurements was performed, each comprising of a TL measurement after light irradiation, a TL measurement after beta irradiation for reference purposes and a PTTL measurement. The latter was obtained by applying first a beta irradiation, then a partial readout up to a certain end temperature followed by a monochromatic light irradiation of a specific wavelength and finally a TL measurement. This procedure was repeated for different partial readout end temperatures. From the results the existence of at least four different photo-transfer processes, induced by 310, 245 and 550 nm light are deduced. The photo transfer process induced by an approximate value of 245 nm produced a TL glow peak not seen before in beta or light induced TL. Furthermore it was observed that some of the TL peaks created by light of 240 and 260 nm were strongly sensitized after a beta irradiation and a partial readout.

CdTe is well known as an excellent photovoltaic material for high efficiency solar cell applications because it has a direct band-gap, low fabrication cost and high optical absorption coefficient. However, the nonradiative recombination and low average minority carrier lifetime caused by the defects in CdTe solar cells limit its efficiency. So far, grain boundaries (GB) have been considered to be the major origin of the nonradiative recombination. However, we show that CdTe grains contain many dislocations that could limit device efficiency. Scanning transmission electron microscopy (STEM) was used to determine the atomic structure of intrinsic and extrinsic stacking faults and their terminating partial dislocation cores. Z-contrast images are sensitive to atomic number and are able to distinguish Cd and Te atomic columns. Unpaired Cd and Te atomic columns were found to form the partial dislocation cores, suggesting the presence of dangling bonds. These defects are likely to be electrically active, and may be the origin of the low minority carrier lifetime.

High electron mobility transistors (HEMTs) based on AlGaN/GaN hetero-structures are promising for both commercial and military applications that require high power, high voltage, and high temperature operation. Reliability and radiation effects of AlGaN-GaN HEMTs need to be thoroughly studied before they are successfully deployed in potential satellite systems. A few AlGaN HEMT manufacturers have recently reported encouraging reliability, but long-term reliability of these devices under high electric field operation and extreme space environments still remains a major concern. A large number of traps and defects are present in the bulk as well as at the surface, leading to undesirable characteristics including current collapse. The present study is part of our investigation to study traps and defects in the AlGaN HEMT devices using micro-analytical techniques before and after they are life-tested.