The acute Zn concentration sensitivity of the reaction between Sn-based solders and Cu substrate is reported and explained in this article. Three Sn-xZn solders (x = 0.5, 0.7, and 2 wt%) were reacted with Cu substrates at 250 °C for 2–10 min. A slight variation in the Zn concentration changed the reaction product formed at the interface. When the Zn concentration was low (x = 0.5 wt%), the reaction product was Cu6Sn5. When the Zn concentration was slightly increased to 2 wt%, the reaction product became Cu5Zn8. When Zn concentration was in-between (x = 0.7 wt%), Cu6Sn5 and CuZn co-existed. The above findings are explained using the Cu–Sn–Zn phase diagram. The implication is that the type of compound forms at the interface can be controlled by adjusting the Zn concentration of the Sn-based solders.

Carbon nanofibers (CNFs) were synthesized by both high-density plasma-enhanced chemical vapor deposition (CVD) and thermal CVD. The growth in the former was carried out in an inductively coupled plasma (ICP) reactor. The multilayer loop structure, which was reported to be found on both the inner and outer surfaces of cup-stacked-type CNFs grown using thermal CVD only after heat treatment above 1500 °C, was observed in the as-grown CNFs only on the outer surface using ICP-CVD. The dangling bonds caused by plasma etching and the bonding between edge carbon atoms aided by the high-density plasma are considered the main reasons of the formation of multilayer loops.

Our research is focused on developing computational capabilities for the prediction of properties of energetic materials associated with performance and sensitivity. Additionally, we want to identify and characterize the dynamic processes that influence conversion of an energetic material to products upon initiation. We are attempting to achieve these goals through the use of standard atomistic simulation methods. In this paper, various theoretical chemistry methods and applications to energetic materials will be described. Current capabilities in predicting structures, thermodynamic properties, and dynamic behavior of these materials will be demonstrated. These are presented to exemplify how information generated from atomistic simulations can be used in the design, development, and testing of new energetic materials. In addition to illustrating current capabilities, we will discuss limitations of the methodologies and needs for advancing the state of the art in this area.

The instability of interfaces in an ultrafine TiAl-(γ)/Ti3Al-(α2) lamellar structure by straining at room temperature has been investigated using in situ straining techniques performed in a transmission electron microscope. The purpose of this study was to obtain experimental evidence to support the creep mechanisms based upon the interface sliding in association with a cooperative movement of interfacial dislocations, which was proposed previously to rationalize a nearly linear creep behavior of ultrafine lamellar TiAl alloys. The results reveal that the sliding and migration of lamellar interfaces can take place simultaneously as a result of the cooperative movement of interfacial dislocations, which can lead to an adverse effect in the performance of ultrafine lamellar TiAl alloy.

Fully dense aluminium nitride (AlN) ceramics consolidated by aqueous slip casting were obtained after sintering at 1750 °C for 2 h. The obtained samples were characterized by Vickers hardness (1000 Hv), flexural strength (200 MPa) and thermal conductivity (115 W/mK). YF3 and CaF2 were used as sintering additives in total amounts ranging from 5 to 7 wt% in YF3/CaF2 weight ratios of 1.25, 1.5, and 2. The compatibility between the thermochemically treated AlN powder and the sintering additives in the aqueous suspensions was investigated by rheological and zeta potential measurements. Highly concentrated (50 vol%) and well-dispersed suspensions could be prepared and used to consolidate homogeneous green bodies by slip casting. The phosphate species used to protect the surface AlN particles against hydrolysis could no longer be detected at temperatures higher than 1400 °C. X-ray diffraction results revealed that all the new crystalline phases formed upon sintering belong to the system Al–Y–O–Ca, but their specific compositions as well as the thermal properties of the sintered materials were shown to depend on the total amount and ratio of sintering aids.

A framework is presented for modeling the nucleation in the constitutionally supercooled liquid ahead of the advancing solid/liquid interface. The effects of temperature gradient, imposed velocity, slope of liquidus, and initial concentration have been taken into account in this model by considering the effect of interface retardation, which is caused by solute buildup at the interface. Furthermore, the effect of solute concentration on the chemical driving force for nucleation has been considered in this model. The model is used for describing the nucleation of Al–Si and Al–Cu alloys. It was found that the solute of Si has a significant impact on the chemical driving force for nucleation in Al–Si alloys whereas Cu has almost no effect in Al–Cu alloys.

We studied the deformation mechanisms and mechanics during indentation of polycrystalline gold thin films at depths below 100 nm. The measured material hardness decreased from 2.1 ± 0.1 to 1.7 ± 0.1 GPa after annealing for 4 h at 177 °C. Upon closer inspection, the hardness trends in the gold thin films were discovered to vary according to the indentation depth. At nanometer depths, the material hardness was quantified using multiple parameters, some of which were independent of the area calibration for the tip. The annealed specimen was very “hard” at low indentation depths, relatively soft at moderate indentation depths, and finally harder until the grain-size limit was reached. The as-deposited specimen demonstrated a relatively continuous harness trend as function of indentation depth, exhibiting monotonic convergence to Hall–Petch limited behavior. Discrete displacement jump events (excursions or “pop-ins”) were frequently observed for the annealed specimen but not for the as-deposited specimen. Variation in hardness, excursion activity, and displacement during the hold at maximum load was observed according to the applied loading, which was parametrically varied at constant strain rates. Hardness results are explained in terms of the population and evolution of defects present within the specimens. The population of point defects is also influential, and critical thermal fluctuations, as well as the thermally activated process of diffusion, are believed to influence hardness at the specimen’s free surface and further into its volume. After converging to a monotonic trend (proper tip engagement), the modulus of the gold was measured to be 106.0 ± 12.9 and 101.3 ± 6.0 GPa for the respective Au/Cr/Si specimens. These values exceeded predictions from the aggregate polycrystalline material theory, a representation used to estimate results for anisotropic single crystals. Exaggerated modulus measurements are explained as the result of the contribution of modulus mismatch with the substrate, pileup at the indentor tip, residual stress in the films, and crystallographic anisotropy of the gold.

Two isothermal sections of the Co–Cu–Ti ternary system at 973 and 1123 K were experimentally determined using the diffusion triple technique together with scanning electron microscopy and electron probe microanalysis. The solubility of Cu (substituting Co) in CoTi increased from 22.8 at.% at 973 K to 28.1 at.% at 1123 K while that of Co (substituting Cu) in CuTi decreased from 11.1 to 8.8 at.% accordingly. In addition, the solubility limits of the third element in the binary compounds CoTi2, CuTi2, Cu4Ti3, and Cu3Ti2 were remarkable. Besides the solubility change, we found the Cu2Ti phase was stable at 1123 K but disappeared at 973 K. A ternary compound “m” with a composition range covering Co10Cu57Ti33 was detected at both isothermal sections. An invariant reaction Cu4Ti3 + CoTi ↔ CuTi + m at a temperature between 973 and 1023 K was deduced. Further investigations are necessary to confirm the reactions among Cu, Cu4Ti, Cu2Ti, Cu3Ti2, and “m” between 1023 and 1123 K.

A variety of carbon nano- and submicro-structures with spectacular morphologies such as spaghetti-like, dendritic, and segmented carbon fibers; carbon pillars; and single-walled carbon nanotubes (SWNTs) was selectively synthesized by the alcohol chemical vapor deposition (CVD) method. The phase structure and morphologies were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and Raman spectroscopy. The carbon structures could be controlled by adjusting the deposition position and the growth temperature. The formation mechanism of these carbon structures was discussed on the basis of the experimental results. The various CVD products obviously imply that the growth mechanism for our alcohol CVD process evolves from catalytic growth mode to pyrolytic carbon deposition mode. The obtained various carbon nano- and submicro-structures may be promising for applications in functional nanodevices.

A novel low-temperature (900 °C) chemical gelation method was developed to synthesize spherical and nonagglomerated Ce3+-doped yttrium aluminum oxide particles (YAG:Ce3+). This represents a process with a much lower processing temperature than current solid-state reaction processes (1400 °C). Characterization of the particles via x-ray diffraction and thermoanalytical methods showed that calcination at 900 °C for 2 h allowed direct crystallization from the amorphous phase, inferring that this process allows homogeneous mixing and increased precursor reactivity. Electron microscopy results showed that the spherical particles (∼100 to ∼3 μm) were the flocks of crystallites. The crystallite sizes (Rietveld refinement) grew linearly from 27 nm (900 °C) to 114 nm (1300 °C). The surface area decreased from 40 m2/g (900 °C) to 5 m2/g (1300 °C) because of the coagulating and growing of crystallites to bigger grains at 1300 °C. Single-crystal nanoparticles (around 100 nm) were obtained with this process and their atomic structures were revealed via high-resolution transmission electron microscopy.

Quasi-elastic neutron spectroscopy was used to study the changes in the water content of hydrated tricalcium silicate cement paste with decreasing relative humidity (RH). The structurally bound water was divided into water bound in Ca(OH)2 and in calcium-silicate hydrate, or C–S–H-gel, utilizing the inelastic vibrational modes of Ca(OH)2. The quasi-elastic line was analyzed in terms of free and constrained water, and both were observed to decrease as the pores empty of pore-water with drying. An inelastic line related to translational vibrations of immobile water molecules was also extracted from the spectra, and its intensity was found to decrease with lower RH.

Nanoparticles of iron carbides (Fe3C and χ-Fe2.5C) wrapped in multilayered graphitic sheets were synthesized by a developed method in which an electric plasma was generated in an ultrasonic cavitation field containing thousands of tiny activated bubbles in liquid ethanol. Annealing changed the phase composition, structure, and size of the carbon nanocapsules as most of the iron carbides decomposed into the α-Fe phase and graphite. Powder samples annealed at 873 and 973 K have maximal saturation magnetization values equal to 80.6 and 83.4 A m2/kg, respectively, which is approximately 40% of the value of bulk iron. Using this method, it will be possible to synthesize nanoparticles of a metal of choice encapsulated by graphite shells by selecting appropriate materials for the ultrasonic tip and electrodes.

A new method was developed allowing large superconducting YBa2Cu3O7 (YBCO) monoliths having complex shapes to be obtained. This method consists of joining two or more YBCO monoliths, and it is based on the interfacial melting induced by metallic Ag thin foils inserted between YBCO pellets. Studies of the microstructure and the superconducting properties of the joints obtained by using this technology have shown that a perfect interface can be obtained without agglomerations of non-superconducting phases and with a critical current density as high as that of the original blocks. No evidence of Ag precipitates was detected either at the interface or into the YBCO solid matrix, suggesting a migration of Ag. For a better understanding of the interface growth mechanism, we studied the influence of the cooling rate. The knowledge on the Ag diffusion process has enabled us to propose a model for the growth mechanism of the YBCO/Ag/YBCO interfaces.

The molecular dynamics method has been used to simulate mode I cracking in body-centered-cubic iron. Close attention has been paid to the process of the atomic configuration evolution of the cracks. The simulation shows that at low temperatures, partial dislocations are emitted before the initiation of crack propagation, subsequently forming the stacking faults or multilayer twins on {112} planes, and then brittle cleavage and extended dislocation nucleation are observed at the crack tip accompanied by twin extension. These results are in agreement with the experimental observation that twinning and fracture processes cooperate at low temperatures. Furthermore, an energetics analysis has been made on the deformation behavior observed at the crack tip. The effect of temperature on the fracture process is discussed. At the higher temperature, plastic deformation becomes easier, and crack blunting occurs. With increasing temperature, the fracture resistance increases, and the effect of the lattice trapping can be weakened by thermal activation.

The oxidation of both single crystal and relatively pure polycrystalline silicon carbide, between 973 and 2053 K, resulted in the formation of cristobalite, quartz, or tridymite, which are the stable crystalline polymorphs of silica (SiO2) at ambient pressure. The oxide scales were found to be pure SiO2 with no contamination resulting from the oxidizing environment. The only variable affecting the occurrence of a specific polymorph was the oxidation temperature. Cristobalite was formed at temperatures ≥1673 K, tridymite between 1073 and 1573 K, and quartz formed at 973 K. The polymorphs were determined using electron diffraction in a transmission electron microscope. These results were further confirmed using infrared and Raman spectroscopies. Cristobalite was observed to grow in a spherulitic fashion from amorphous silica. This was not the case for tridymite and quartz, which appeared to grow as oriented crystalline films. The presence of a thin silicon oxycarbide interlayer was detected at the interface between the SiC substrate and the crystalline silica using x-ray photoelectron spectroscopy.

The microstructure of Ba[Mg1/3(NbxTa(4−x)/4)2/3]O3 ceramics (x = 1, 2, 3, and 4) was investigated. Ordered structure and domain boundary were observed by high-resolution transmission electron microscopy. The disordered structure of 0.41 nm lattice modulation and the 1:2 ordering structure of 0.71 nm lattice modulation in the [111] direction are both formed in Ba[Mg1/3(NbxTa(4−x)/4)2/3]O3 ceramics. Except for the incoherent interface between disordered and 1:2 ordering domain, the coherent interface between two 1:2 ordering domains can also be observed in the Ba[Mg1/3(NbxTa(4−x)/4)2/3]O3 ceramics. In the Ba[Mg1/3(Nb1/4Ta3/4)2/3]O3 ceramic, the formation of an extra ordering structure (lattice modulation of 1.24 nm in the [111] direction) on the antiphase domain boundary with proper Nb substitution contributes to lower dielectric loss due to the stabilization of the domain boundary.

Homogeneous ∈-iron nitride powder particles of the compositions Fe3N1.00, Fe3N1.22, Fe3N1.30, and Fe3N1.39 were investigated using selected-area electron diffraction. The superstructure reflections, due to N ordering on the octahedral interstitial sites of a hexagonal close-packed (hcp) arrangement of Fe, indicate a hexagonal supercell with a′ = 31/2ahcp, c′ = chcp for all cases; P6322 space group symmetry for Fe3N1.00, Fe3N1.30, and Fe3N1.39; and lower symmetry (P312) for Fe3N1.22 and some of the Fe3N1.30 particles. Recognition of the occurrence of double diffraction is essential for correct interpretation of the diffraction patterns. Additionally, diffuse scattering by some of the Fe3N1.39 grains was observed.

The effect of stress, resulting from the presence of hydrogen, on the aluminum-induced crystallization of hydrogenated amorphous silicon films was studied. Layered thin films of hydrogenated and unhydrogenated amorphous silicon and aluminum, deposited by sputtering, were used to study this effect. The stress of the deposited films was determined by measuring the radius of curvature of c-Si substrates before and after deposition of the films. It was observed that unhydrogenated amorphous silicon films exhibit a high compressive stress compared with hydrogenated ones. The amount of stress is shown to decrease with increasing hydrogen content. It was also observed that aluminum always provides tensile stress. After the initial stress measurements, all the samples were annealed for 30 min at temperatures between 200 °C and 400 °C. X-ray diffraction was used to determine the crystallinity of the silicon films. The results of the study show that the temperature at which crystallization of amorphous silicon is initiated is lower for films with a lower initial stress.

Bulk Ta4AlC3, a new layered-ternary carbide in the Ta–Al–C system, was synthesized and characterized. Transmission electron microscopy investigations on this new phase are reported here. Selected area electron diffraction and convergent beam electron diffraction analyses indicated that this ternary carbide crystallized with the space group P63/mmc. Atomic-scale microstructures of Ta4AlC3 were achieved by means of high-resolution transmission electron microscopy and Z-contrast scanning transmission electron microscopy. The experimental crystal structural parameters agreed well with the theoretical values obtained using density-functional calculations.