Core–Shell Structured Nanoenergetic Materials: Preparation and Fundamental Properties

Energetic materials, including explosives, pyrotechnics, and propellants, are widely used in mining, demolition, automobile airbags, fireworks, ordnance, and space technology. Nanoenergetic materials (nEMs) have a high reaction rate and high energy density, which are both adjustable to a large extent. Structural control over nEMs to achieve improved performance and multifunctionality leads to a fascinating research area, namely, nanostructured energetic materials. Among them, core–shell structured nEMs have gained considerable attention due to their improved material properties and combined multiple functionalities. Various nEMs with core–shell structures have been developed through diverse synthesis routes, among which core–shell structured nEMs associated with explosives and metastable intermolecular composites (MICs) are extensively studied due to their good tunability and wide applications, as well as excellent energetic (e.g., enhanced heat release and combustion) and/or mechanical properties. Herein, the preparation methods and fundamental properties of the abovementioned kinds of core–shell structured nEMs are summarized and the reasons behind the satisfactory performance clarified, based on which suggestions regarding possible future research directions are proposed.     

On the Time–Space Complexity of Geometric Elimination Procedures

In [25] and [22] a new algorithmic concept was introduced for the symbolic solution of a zero dimensional complete intersection polynomial equation system satisfying a certain generic smoothness condition. The main innovative point of this algorithmic concept consists in the introduction of a new geometric invariant, called the degree of the input system, and the proof that the most common elimination problems have time complexity which is polynomial in this degree and the length of the input. In this paper we apply this algorithmic concept in order to exhibit an elimination procedure whose space complexity is only quadratic and its time complexity is only cubic in the degree of the input system. Received: April 14, 1999; revised version: October 31, 2000     

Physicochemical Properties of Sb, Sn, Zn, and Sb–Sn System

The physicochemical properties, viscosity, density, and surface tension, were measured using the discharge crucible method (DC) on five alloys of Sb–Sn. The performance of the DC method was demonstrated with measurements on pure metals Sb, Sn, Zn, and comparisons with the corresponding literature data. The results reported in this study are for Sb–Sn alloys containing (10, 20, 25, 50, and 75) at% of Sb at 550 K to 850 K. The results show that all the physicochemical properties decrease with decreasing temperature and increase with increasing Sb content in the alloy. The experimentally measured surface-tension values are compared with the Butler model. Several models for viscosity are compared and discussed.     

Measurements of Microstructural, Mechanical, Electrical, and Thermal Properties of an Al–Ni Alloy

The Al–7.5 wt% Ni alloy was directionally solidified upwards with different temperature gradients, $G$ ( $0.86\,\text{ K}~{\cdot }~ \text{ mm}^{-1}$ to $4.24\,\text{ K}~{\cdot }~\text{ mm}^{-1})$ at a constant growth rate, $V$ ( $8.34\,\upmu \text{ m}~{\cdot }~\text{ s}^{-1})$ . The dependence of dendritic microstructures such as the primary dendrite arm spacing ( $\lambda _{1}$ ), the secondary dendrite arm spacing ( $\lambda _{2}$ ), the dendrite tip radius ( $R$ ), and the mushy zone depth ( $d$ ) on the temperature gradient were analyzed. The dendritic microstructures in this study were also compared with current theoretical models, and similar previous experimental results. Measurements of the microhardness (HV) and electrical resistivity ( $\rho $ ) of the directionally solidified samples were carried out. Variations of the electrical resistivity ( $\rho $ ) with temperature ( $T$ ) were also measured by using a standard dc four-point probe technique. And also, the dependence of the microhardness and electrical resistivity on the temperature gradient was analyzed. According to these results, it has been found that the values of HV and $\rho $ increase with increasing values of $G$ . But, the values of HV and $\rho $ decrease with increasing values of dendritic microstructures ( $\lambda _{1}, \lambda _{2}, R,$ and $d$ ). It has been also found that, on increasing the values of temperature, the values of $\rho $ increase. The enthalpy of fusion ( $\Delta {H}$ ) for the Al–7.5 wt%Ni alloy was determined by a differential scanning calorimeter from a heating trace during the transformation from solid to liquid.     

Orbital Degeneracy, Jahn–Teller Effect, and Superconductivity in Transition-Metal Chalcogenides

We have studied the electronic structure of FeSe_1?x Te _x and Ir_1?x Pt _x Te₂ using photoemission spectroscopy. For FeSe_1?x Te _x , angle-resolved photoemission results indicate that the Fe 3d yz/zx orbital degeneracy at ?? point and orbitally induced Peierls effect in the tetragonal lattice play important roles for the superconductivity. It is suggested that the Jahn-Teller instability of the yz/zx states couples with local lattice distortion derived from the Te substitution for Se and provides an inhomogeneous electronic state. Photoemission results of IrTe₂ with triangular lattice are also consistent with the orbitally induced Peierls scenario. The Pt substitution for Ir suppresses the static band Jahn?CTeller effect and induces an inhomogeneous electronic state in which orbital (or bond or nematic) fluctuations may help superconductivity through the Peierls effect.     

Structure,Composition, and Properties of Arc PVD Mo–Si–Al–Ti–Ni–N Coatings

Using an arc physical vapor deposition process, we have produced nanostructured Mo–Si–Al–Ti–Ni–N coatings with a multilayer architecture formed by Mo₂N, AlN–Si₃N₄, and TiN–Ni and a crystallite size on the order of 6–10 nm. We have studied the physicomechanical properties of the coatings and their functional characteristics: wear resistance, adhesion to their substrates, and heat resistance. According to high-temperature (550°C) wear testing and air oxidation (600°C) results, the coatings studied here are wearand heat-resistant under appropriate temperature conditions. Their properties are compared to those of Mo–Si–Al–N coatings.     

Time–dependent nature and origin of displacive transformation

We have investigated athermal and isothermal martensitic transformations (typical displacive transformations) in Fe–Ni and Fe–Ni–Cr alloys under pulsed and static magnetic fields and hydrostatic pressures in order to understand the time-dependent nature of martensitic transformation, that is, the kinetics of martensitic transformation. Also, we have calculated electronic structures of B₂ and ζ′₂ phases in AuCd by FLAPW and/or LAPW methods in order to understand the origin of B₂–ζ′₂ transformation. The following results were obtained. (i) The two transformation processes are closely related to each other, that is, the athermal process changes to the isothermal process under a hydrostatic pressure and the isothermal process changes to the athermal one under a magnetic field. (ii) These findings of (i) can be explained by the phenomenological theory, which gives a unified explanation for the two transformation processes previously proposed by our group. (iii) The calculation of the generalized susceptibility, x(q), for the B2 phase of AuCd shows that there exists a nesting vector of near 1/3<110>2Π/a as in the B2 phase of TiNi calculated previously. The density of states at the Fermi energy of the ζ′₂ phase is lower than that of the B2 phase, which is similar to the case of B2–R transformation in TiNi previously calculated.     

Thermodynamic Properties of Si–Ge and Si–Sn Melts

The mixing enthalpies of Si–Ge and Si–Sn liquid alloys were measured in an isoperibolic calorimeter. The results demonstrate that the formation of Si–Ge melts is accompanied by a small heat release, while the formation of Si–Sn melts is an endothermic process. Calculations of the Si activity in Si–Sn melts by Schroeder's equation indicate large positive deviations from Raoult's law.     

Effect of Milling Time on the Microstructure and Tensile Properties of Ultrafine Grained Ni–SiC Composites at Room Temperature

Bulk metallic nickel–silicon carbide nano-particle(Ni–Si CNP) composites, with milling time ranged from8 to 48 h, were prepared in a planetary ball mill and sintered using a spark plasma sintering(SPS)furnace. The microstructure of the Ni–Si CNP composites was characterized by transmission electron microscopy(TEM) and their mechanical properties were investigated by tensile measurements. The TEM results showed well-dispersed Si CNP particles, either within the matrix, between twins or along grain boundaries(GB), as well as the presence of stacking faults and twin structures, characteristics of materials with low stacking fault energy. Dislocation lines were also observed to interact with the Si CNP which were plastically nondeformable. A synergistic relationship existed between Hall–Petch strengthening and dispersion strengthening mechanisms, which was shown to greatly influence the mechanical properties of the Ni–Si CNP composites. Both the maximum yield and tensile strengths were found in the Ni–Si CNP composite with a milling time of 48 h, whereas the increased rate of strengths drastically decreased in material milled above 8 h due to the significant Si CNP agglomeration. The ball milling process resulted in the formation of nano-scale, ultra-fine grained(UFG) Ni–Si CNP composites when the milling time was extended for longer periods, greatly strengthening these materials. The sharp decrease in elongation percentages, however, should be comprehensively considered before irreversible inelastic deformation.     

Mechanical Properties and Microstructure of Diamond–SiC Nanocomposites

A bulk composite material close in hardness to diamond was fabricated from nanocrystalline diamond and SiC. The mechanical properties and microstructure of the composite were studied. Young's modulus of the composite is found to be notably lower than the one following from the additivity rule, which is attributable to the influence of structural defects present in the interfacial zone between SiC and diamond. SiC consists of nanometer-scale grains near the interface and submicron grains in the pores.     

DFT + U Analysis of Structural,Electronic, and Magnetic Properties of Mn–As–Sb Ternary Systems

Electronic and magnetic investigations have been carried out on Mn–As–Sb ternary systems and their two end members, MnAs and MnSb, using ab initio techniques based on density functional theory plus U (DFT + U). Although the electronic structures of pure compounds have been extensively studied by first-principles calculations, there have been no reports of first-principles calculations of the evolution of the electronic structure with the composition. In order to get accurate results for these kinds of systems including Mn 3d electrons, we varied the U parameter from 0 to 10 eV for use in local density approximation (LDA) + U approach. The computed structural, electronic, and magnetic properties of MnX (X = As, Sb) are observed to display strong correlation with experimental data. In particular, the best agreement with the experimentis obtained within the LDA + U in which on-site Coulomb interaction parameter U _eff for Mn is taken as 3.0 eV. Next, we have studied the energetic, electronic, and magnetic properties of alloys and we have also investigated the effects of compositional disorder in both hexagonal and orthorhombic structures. Several important properties of these materials were established. The improvement achieved with the ab initio LDA + U method and the agreement with the experimental spectra are discussed.     

Modeling and Optimization of Properties of Domestic Mullite–Corundum Composites

High Temperature - Mathematical modeling of spectral-kinetic, thermal, and electrophysical characteristics, which are difficult to determine experimentally, has been carried out based on the...     

Corrosion and Catalytic Properties of Co–Mo–Re Electrolytic Coatings

Materials Science - We study the corrosion properties of Co–Mo–Re electrolytic alloys deposited from citrate electrolyte with pH 3.5 and 6.3. It is shown that these coatings have high...     

John Bardeen and the Theory of Superconductivity: A Study of Insight,Confidence, Perseverance,and Collaboration

A biographical narrative of John Bardeen’s study of superconductivity from the 1930s through 1957, when he and his postdoc Leon Cooper and his graduate student J. Robert Schrieffer put forth BCS theory. This paper, adapted from L. Hoddeson and V. Daitch [1].     

Structural, Magnetic and Electrical Properties of Cu Substituted Mn–Zn Soft Nanoferrites

Soft nanoferrites of nominal composition Mn_0.5Cu _x Zn_0.5?x Fe₂O₄ with 0.0??x??0.35 were prepared by chemical co-precipitation method. The formation of single phase spinel structure with different compositions, sizes and macrostructure were confirmed by X-ray diffraction patterns and scanning electron microscopic (SEM) measurements. The lattice parameter decreased with increase in Cu²⁺ content. The crystallite size of the powder samples varied from?14 to 27?nm. The theoretical density increased with increase in Cu²⁺ content. Room temperature saturation magnetization was measured as a function of copper content. The saturation magnetization (Ms) and Bohr magneton (?? _B) increases up to x=0.25 due to increased A?CB interactions in the AB₂O₄ type spinel nanoferrites. Dielectric permittivity, dielectric loss tangent and complex impedance plots were studied in the frequency range 20?Hz?C5?MHz. Loss peak occurs for all the studied compositions and shifts towards low frequency with increased Cu²⁺ content. Complex impedance spectroscopic studies confirmed that conduction in the samples is due to grain boundaries. The high values of DC electrical resistivity support this result.     

Structure and Nonlinear Electrical Properties of Ni–Fe–Mg–O Epilayers

NiFe₂O₄-based epilayers grown on MgO substrates and exhibitingS-type negative-resistance behavior were characterized by x-ray diffraction, Mössbauer spectroscopy, and electron probe x-ray microanalysis. The current–voltage characteristics of the epilayers were measured at different temperatures in air, water, and liquid nitrogen, and the activation energy for conduction was evaluated. The near-surface and interfacial regions of the films were shown to differ in the nature of disordering as a result of the through-thickness variation in composition.     

Investigation of Structural, Magnetic and Magnetotransport Properties of Electrodeposited Co–TiO2 Nanocomposite Films

Nanocomposite Co?CTiO₂ thin films were prepared by simultaneous electrodeposition of Co and TiO₂ on a Cu substrate from a solution based on Co sulfate in which TiO₂ nanoparticles were suspended by stirring. We investigated the influence of the TiO₂ nanoparticles concentration in the bath on the morphology, composition, magnetic and magnetotransport properties of the films. The Co?CTiO₂ thin films were characterized by using scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction analyses, and their magnetic properties were evaluated by using an induction type device with data acquisition system and a torque magnetometer. The current in-plane transport properties of the films have been investigated. The results showed that the films were composed of a Co metal matrix containing embedded TiO₂ nanoparticles and cobalt hydroxide which is formed simultaneously with cobalt metal deposition. The amount of TiO₂ in the film increases with the rising concentration of TiO₂ nanoparticles in the plating bath. This complex structure favored the increase of the magnetoresistance. The Co?CTiO₂ nanocomposite films (containing about 1.3 at.% Ti) exhibit a giant magnetoresistance contribution of 47.6 %. From the magnetic measurements, we have found that the saturation magnetization, the magnetic susceptibility, and the effective magnetic anisotropy constant decrease with the increasing content of TiO₂ in the thin layer. The easy magnetization axis direction changes from in-plane to almost perpendicular-to-plane, with increasing TiO₂ nanoparticles content in the film. The existence of a giant magnetoresistance effect in Co?CTiO₂ is very promising for potential applications in spintronics.     

Space–time VMS computation of wind-turbine rotor and tower aerodynamics

We present the space–time variational multiscale (ST-VMS) computation of wind-turbine rotor and tower aerodynamics. The rotor geometry is that of the NREL 5MW offshore baseline wind turbine. We compute with a given wind speed and a specified rotor speed. The computation is challenging because of the large Reynolds numbers and rotating turbulent flows, and computing the correct torque requires an accurate and meticulous numerical approach. The presence of the tower increases the computational challenge because of the fast, rotational relative motion between the rotor and tower. The ST-VMS method is the residual-based VMS version of the Deforming-Spatial-Domain/Stabilized ST (DSD/SST) method, and is also called “DSD/SST-VMST” method (i.e., the version with the VMS turbulence model). In calculating the stabilization parameters embedded in the method, we are using a new element length definition for the diffusion-dominated limit. The DSD/SST method, which was introduced as a general-purpose moving-mesh method for computation of flows with moving interfaces, requires a mesh update method. Mesh update typically consists of moving the mesh for as long as possible and remeshing as needed. In the computations reported here, NURBS basis functions are used for the temporal representation of the rotor motion, enabling us to represent the circular paths associated with that motion exactly and specify a constant angular velocity corresponding to the invariant speeds along those paths. In addition, temporal NURBS basis functions are used in representation of the motion and deformation of the volume meshes computed and also in remeshing. We name this “ST/NURBS Mesh Update Method (STNMUM).” The STNMUM increases computational efficiency in terms of computer time and storage, and computational flexibility in terms of being able to change the time-step size of the computation. We use layers of thin elements near the blade surfaces, which undergo rigid-body motion with the rotor. We compare the results from computations with and without tower, and we also compare using NURBS and linear finite element basis functions in temporal representation of the mesh motion.