Chemically fabricated magnetic quantum dots of InP:Mn

Quantum dots of InP:Mn are chemically prepared by following hot colloidal nanochemistry with starting precursors that obviate the need for external surfactant. These quantum dots are uniform spheres with 3-nm diameters; they are crystalline, photoluminescent, and magnetic. The crystallographic and optical properties are similar to those of undoped InP nanocrystallites, while the magnetism is consistent with the ferromagnetic response observed in a class of diluted magnetic semiconductors. Because of the ultrafine sizes, the sample shows superparamagnetic behavior, whereas ferromagnetic hysteresis loops are clearly seen below the blocking temperature. Structural characterization and analysis confirm that the magnetism in these quantum dots is not due to segregated binary MnP or MnO phases and that they truly represent a homogeneous dilute magnetic semiconductor.     

Solving the Ising model exactly on a 5×5×4 lattice using the Connection machine

We implement a recently proposed exact method for solving distrete statistical models for the 3-dimensional Ising model with open boundary conditions. Our computations were done on the Connection machine because the problem maps very naturally onto massively parallel architectures. We explicitly calculate the number of states of the system at each energy for systems of size 5×4×L _z, forL _z5. On serial or vector computers, the time for the computation scales with the volumeV likeV2 ^LLy . On the Connection Machine, the calculation can be spread across the processors. This decreases the computation requirements by a factor equal to the number of processors. We describe the method, its implementation on the Connection Machine both in PARIS and in FORTRAN, and our results. We also state the requirements for solving larger systems using this method.On leave from Physics Department, San Francisco State University, San Francisco, California, and Supercomputer Computations Research Institute, Florida State University, Tallahassee, Florida.     

Topological properties of the one dimensional exponential random geometric graph

In this article we study the one‐dimensional random geometric (random interval) graph when the location of the nodes are independent and exponentially distributed. We derive exact results and limit theorems for the connectivity and other properties associated with this random graph. We show that the asymptotic properties of a graph with a truncated exponential distribution can be obtained using the exponential random geometric graph. © 2007 Wiley Periodicals, Inc. Random Struct. Alg., 2008     

Effect of Microwave and Conventional Modes of Heating on Sintering Behavior,Microstructural Evolution and Mechanical Properties of Al-Cu-Mn Alloys

In this research, we intended to examine the effect of heating mode on the densification, microstructure, mechanical properties, and corrosion resistance of sintered aluminum alloys. The compacts were sintered in conventional (radiation-heated) and microwave (2.45 GHz, multimode) sintering furnaces followed by aging. Detailed analysis of the final sintered aluminum alloys was done using optical and scanning electron microscopes. The observations revealed that the microwave sintered sample has a relatively finer microstructure compared to its conventionally sintered counterparts. The experimental results also show that microwave sintered alloy has the best mechanical properties over conventionally sintered compacts. Similarly, the microwave sintered samples showed better corrosion resistance than conventionally sintered ones.     

Combined effects of viscous dissipation and MHD on free convection flow past a semi-infinite vertical plate with variable surface temperature in the presence of heat source

An unsteady MHD laminar viscous dissipative fluid flow past a semi-infinite vertical plate with variable surface temperature in the presence of heat source is considered in the present analysis. The present approach transforms the governing boundary layer equations into nondimensional form using the appropriate nondimensional quantities, which is valid in the free convection region. The resulting governing equations are solved numerically using the Crank–Nicolson method, an efficient implicit finite-difference scheme. Numerical results are obtained and presented in the form of local as well as average shearing stress, local and average heat transfer rate, velocity and temperature during the transient period. The present results are compared with the available results in the literature and are found to be in an excellent agreement.     

Mössbauer Studies of Core-Shell FeO/Fe3O4 Nanoparticles

Physics of the Solid State - FeO/Fe3O4 nanoparticles were synthesized by thermal decomposition. Electron microscopy revealed that these nanoparticles were of the core-shell type and had a spherical...     

Structural and compositional analyses of nanocrystalline diamond/β-SiC composite films

Nanocrystalline diamond/β-SiC composite films are synthesized by microwave plasma chemical vapor deposition using a gas mixture of H₂, CH₄, and tetramethylsilane (Si(CH₃)₄, TMS) in a single process step. Structural and compositional analyses revealed that the films consist of a mixture of diamond and β-SiC nanocrystalline phases in a desired volume fraction combinatorial form. Transmission electron microscopy analysis confirmed the X-ray diffraction results and showed that the major diffraction lines corresponded to a two-component nanocrystalline composite film. Infrared spectroscopic analysis showed that the content of β-SiC in the films can be increased by increasing the TMS concentration. This correlated very well with electron probe microanalysis and Rutherford backscattering analysis that showed an almost linear correspondence of β-SiC content in the films with the TMS concentration in the gas phase. The phase purity of the diamond crystallites decreased with increase in the β-SiC content in the films, as shown by micro Raman scattering studies. Smooth surface morphologies are measured for these films by using atomic force microscopy; the root mean square roughness was 12 ± 1 nm. The β-SiC volume fraction (vol. %) was identified as an important compositional factor to determine any mechanical and frictional properties of these films. PACS 68.55.-a; 68.55.Nq; 68.60.-p     

Density Functional Theory Study of Mn+1AXn Phases: A Review

M_n+1AX_n phases (MAX phases for short with M: transition metal, A: A group elements, X: C or N, and n = 1–3) have attracted considerable attention due to the unique combination of the ceramic- and metal-like properties. The density functional theory (DFT) has emerged as a powerful theoretical approach that complements experimental testing and serves as a predictive tool in the identification and characterization of MAX phases. After the beginning with a brief introduction of the MAX phase and DFT, we review the DFT study on this class of materials, including crystal structure, electronic structure, point defects, lattice dynamics, and related properties, phase stability, compressibility, and elastic properties. Comparison between the theoretical values and available experimental ones shows that they are in decent agreement for most part, especially in the lattice constants, elastic properties, and compressibility. This article is concluded with an outlook of future research on DFT study of MAX phases, major challenges to be met and possible solutions in some cases.     

Effect of couple stresses on the flow in a constricted annulus

The flow of an incompressible couple stress fluid in an annulus with local constriction at the outer wall is considered. This configuration is intended as a simple model for studying blood flow in a stenosed artery when a catheter is inserted into it. The effects couple stress fluid parameters α and σ, height of the constriction (ε), and ratio of radii (k) on the impedance and wall shear stresses are studied graphically. Graphical results show that the resistance to the flow as well as the wall shear stress increases as the ratio of the radii increases and decreases as the couple stress fluid parameters increases.     

Dispersion and phase separation of carbon nanotubes in ultrathin polymer films

The inner structure and nanoscale distribution of the stiffness was studied for polymer-single-wall carbon nanotube composites. Dispersion of nanotubes in a polystyrene and polyurethane polymer matrix was achieved by a proper choice of the organic solvent (NMP) and sonification of polymer/SWNT solutions. Ultrathin nanocomposite films were prepared through a dip-coating procedure and possessed a noticeable degree of nanotube orientation in the direction of the applied shear force. Peculiarities of the phase separation in the films were studied by atomic force microscopy (with application of force modulation mode to map the nanotube distribution within the polymer matrix) and Raman spectroscopy.