Chang Kyung Kim

Phase transitions and dielectric properties in A-site ion substituted (Na1/2Bi1/2)TiO3 ceramics (A=Pb and Sr)

The dielectric permittivity of A-site ion substituted (Na1/2Bi1/2)TiO3(A=Pb,Sr) solid solutions has been investigated with the aid of structural analysis. As the addition of PT increased, the first order characteristics of the phase transition appeared. In contrast, additions of ST made the solid solution exhibit relaxor ferroelectrics behavior. Microscopic structural analysis revealed that the dielectric properties were determined by cation ordering which controlled the translational symmetry and domain configuration. In the NBT–PT solid solution, A-site cations were randomly redistributed and macrodomains formed. However, the addition of ST distributed A-site cations with local ordering and divided the domains of NBT into micropolar regions. The contrasting effects of the addition of the Pb and Sr cations on the dielectric properties of NBT are discussed in terms of the polarizability and the tolerance factors as they relate to the perovskite structure.

Factors affecting initial permeability of Co-substituted Ni-Zn-Cu ferrites

Iron deficient compositions of (Ni/sub 0.2/Cu/sub 0.2/Zn/sub 0.6/)/sub 1.02x/Co/sub x/Fe/sub 1.98/O/sub 4/ (0/spl les/x/spl les/0.05) were prepared to investigate their initial permeability dependence on cobalt contents. Extrinsic factors such as grain size and sintered density change little in samples sintered at 900/spl deg/C, so their effects on permeability can be neglected. Intrinsic factors such as saturation magnetization, magnetocrystalline anisotropy (K/sub 1/) and magnetoelastic anisotropy (K/sub /spl sigma//) can not account for the variation of initial permeability with Co content. Measurement of thermoelectric power shows that the concentration of cation vacancies increases with Co content. Therefore, the local induced anisotropy increases by the ordering of Co ions via increased cation vacancy concentration. This increase in induced anisotropy results in the decrease of initial permeability.

Formation of gold nanoparticles embedded in a polyimide film for nanofloating gate memory

A monolayer of vertically aligned Au nanoparticles embedded in a dielectric film was fabricated by sandwiching a 3.4-nm-thick Au film between two polyimide (PI) precursor layers. Au formed uniform-sized nanoparticles on the PI precursor and coalesced into 10nm sized nanoparticles during imidization, forming a well-dispersed monolayer of Au nanoparticles embedded in PI. Capacitance-voltage measurement at 300K showed that the monolayer of Au nanoparticles functioning as a floating gate in Al∕PI∕Au nanoparticles/PI/Si metal-insulator-semiconductor-type capacitor exhibited a capacitance hysteresis of 3.4V at an applied voltage of 6V. The memory effect can be potentially utilized in next generation flash memories.

Microwave Dielectric Properties of (1-x)Ba5Nb4O15–xBaNb2O6 Mixtures

The mixture behavior and microwave dielectric properties of (1-x)Ba5Nb4O15–xBaNb2O6 ceramics were studied by means of X-ray powder diffraction and a network analyzer. 0.17Ba5Nb4O15–0.83BaNb2O6 samples sintered at 1250°C for 2 h, exhibit excellent microwave dielectric properties: dielectric constant (er)=35.2, quality factor (Q×f)=59300 GHz, and temperature coefficient of resonant frequency (τf)=0 ppm/°C. The er and τf of mixtures could be predicted by the appropriate mixing relation, but deviated slightly from calculated values. This was attributed to dissimilar microstructural grain morphologies of the Ba5Nb4O15 and BaNb2O6 end members. In the mixtures, higher Q×f was obtained rather than Ba5Nb4O15 and BaNb2O6, due to pore reduction. Also, the mixture behavior of low-fired Ba5Nb4O15–BaNb2O6 was discussed. A major contributor to microwave dielectric properties of low-fired samples is the low temperature phase, hexagonal BaNb2O6.

The Reversible Phase Transition and Dielectric Properties of BaNb2O6 Polymorphs

Barium metaniobate, BaNb2O6 with polymorphic orthorhombic and hexagonal forms was prepared by solid-state reaction. Hexagonal BaNb2O6 is the low temperature phase, and can be transformed to the orthorhombic phase over a broad temperature range above 1150°C. It was observed that the hexagonal-to-orthorhombic phase transition is reversible due to the structural similarity of these two polymorphs. The difference in transition kinetics and associated mechanisms could be confirmed from the analysis of time dependence of transformed phase. We also found the dielectric properties of each phase to be significantly different. In particular, orthorhombic BaNb2O6 has good microwave dielectric properties: Q×f=43,000 GHz, er=30, τf=-45 ppm/°C.

The catalytic effect of Pt nanoparticles supported on silicon oxide nanowire

3.5 nm sized Pt nanocatalysts supported on silicon oxide nanowires (SiO(X)NWs) were fabricated by decorating the SiO(X)NWs with Pt nanoparticles using a simple physical deposition system without any surface pretreatment. High curvature of the nanowire surface together with the weak metal-substrate interaction helped to maintain discrete particle morphology with spherical shapes during deposition. Catalytic efficiency of the SiO(X)NWs coated with Pt nanoparticles was demonstrated through reduction of methylene blue in the presence of sodium borohydride. It was demonstrated that the highly curved nanowire surface allowed the Pt nanoparticles to be loaded with increased particle density, providing a larger surface area for the catalytic reaction. It was also shown that a simple heat treatment in vacuum improves the effectiveness of the Pt nanoparticles as a catalyst without loss of catalytic activity when used repeatedly. We expect that this metal nanoparticle-decorated nanowire can be easily extended to other heterogeneous reactions and can also be used as a template for building three-dimensional hierarchical nanostructures.

Microstructural evolution of duplex grain structure and interpretation of the mechanism for NiO scales grown on pure Ni- and Cr-doped substrates during high temperature oxidation

Abstract Duplex layers are a very important class of film microstructures and form under a wide variety of processing conditions such as oxidation (sulfidation), sputtering, evaporation and casting on a large number of substrates. In this paper, the available models for duplex layer formation are criticized in detail and the basic assumptions which are based upon them are examined. Then, mechanisms of formation of the duplex layers based on microstructural observations in pure Ni and Ni–1at% Cr systems during high temperature oxidation are addressed.

Monolayer CoPt magnetic nanoparticle array using multiple thin film depositions

A monolayer of L10 CoPt nanoparticles was prepared on a polyimide film by depositing Co–Pt and annealing the resulting film stack at 650–800°C. To obtain well-separated magnetic nanoparticles avoiding substantial coarsening often seen on annealing of thicker films, a multiple deposition of thinner Co–Pt alloy films with intentionally increased Co composition was employed. The multiple depositions relying on heterogeneous nucleation on the preexisting CoPt nanoparticles allowed the average particle size to increase from 4to7nm. The monolayer of 7nm sized CoPt nanoparticles exhibited a coercivity of 9.8kOe with a remanence ratio of 0.72.

Calculation of the contribution to grain boundary diffusion in ionic systems that arises from enhanced defect concentrations adjacent to the boundary

The enhancement of the concentration of a defect in the space-charge region near a grain boundary in an intrinsic ionic system was used to specify the change in the corresponding diffusion coefficient as a function of distance, x, normal to the boundary. This zone of enhanced and continuously varying diffusivity serves as a vehicle for enhanced transport along the boundary in addition to the expected contribution arising from the different structure at the boundary core. A two-dimensional diffusion equation was established for concentration c(x,y,t) in the space-charged region, solved numerically using the Crank–Nicolson finite-difference method, and integrated normal to the boundary to provide the average concentration gradient c(y,t) along the boundary—the solute distribution usually measured by experiment. The present gradients were analyzed in the same fashion as experimental data using a recent solution to the conventional model for the grain–boundary diffusion problem in which the interface is trea...

Oxidation of Amorphous Co75.26−xFe4.74(BSi)20+x Magnetic Alloy

AbstractThe oxidation characteristics of the Co-rich amorphous magnetic alloy,