J.-H. Hwang

Magnetic Properties of Graphitically Encapsulated Nickel Nanocrystals

Graphitically encapsulated ferromagnetic Ni nanocrystals have been synthesized via a modified tungsten arc-discharge method. By virtue of the protective graphitic coating, these nanocrystals are stable against environmental degradation, including extended exposure to strong acids. The magnetic properties of the encapsulated particles are characterized with regard to the nanoscale nature of the particles and the influence of the graphitic coating which is believed to be benign insofar as the intrinsic magnetic properties of the encapsulated nanocrystals are concerned. The Curie temperature of graphitically encapsulated Ni nanocrystals is the same as that of microcrystalline Ni. However, saturation magnetization, remanent magnetization, and coercivity of these particles are reduced, for a range of temperatures. The unique features are compared with those of unencapsulated nanocrystalline and coarse microcrystalline nickel particles.

Point defects and electrical properties of Sn-doped In-based transparent conducting oxides

Abstract In-based transparent conducting oxides (TCOs) share the prevailing defect structure of indium–tin oxide (ITO), i.e. electrons, isolated Sn In ⋅ donors, and neutral associates, believed to be (2Sn In ⋅ O i ″) x . The present work reviews the state of the literature, presents calculated Brouwer diagrams vs. oxygen partial pressure and vs. dopant concentration, and reports intermediate temperature electrical property data (thermopower, conductivity) vs. p O 2 and Sn concentration for three systems — polycrystalline bulk ITO, nanocrystalline ITO, and the recently reported ternary cation TCO, Ga 3− x In 5+ x Sn 2 O 16 . The influence of non-reduceable tin–oxygen complexes at high doping levels is identified for ITO. Ramifications for In-based TCO properties are discussed.

Experimental limitations in impedance spectroscopy: Part V. Apparatus contributions and corrections

Overlooked apparatus (cabling, leads, sample holder) contributions to experimental impedance spectra can be significant, especially at high frequencies, and can obscure the true sample response. Instrumental limitations are discussed and high-frequency artifacts arising from apparatus contributions are investigated as they pertain to accurate impedance measurements of materials systems. Remediation strategies are presented, including geometrical adjustments, lead shielding, and null-correction procedures.

A descriptive model linking possible formation mechanisms for graphite-encapsulated nanocrystals to processing parameters

New and modified mechanisms are proposed to account for detailed observations of carbon encapsulation of Fe, Ni, and Co nanocrystals. The mechanisms are based on aerosol and gas phase chemistry and on the catalytic effects of transition metals. Two parameters are found to qualitatively dominate production: the local-path carbon-to-metal ratio (LCM) and the global carbon-to-metal ratio (GCM). LCMs select which mechanisms are active along each pathway within the reactor. The GCM places bounds upon and determines the weighting between different LCMs and thus determines the distribution of different nanoscale products within the collected, macroscopic product. A two part processing parameter → mechanism → product map links the components. The generality of the model is discussed throughout with reference to related processes and the encapsulation of other materials.

Evaluating dielectric impedance spectra using effective media theories

The immittance spectra (i.e., impedance and modulus representations) are calculated for various effective medium theories, i.e., the Maxwell-Wagner (MW), Hashin-Shtrikman (HS), Bruggeman Asymmetric (BA) and Bruggeman Symmetric (BS) models, with emphasis on their individual microstructures. In addition the brick-layer (BL) model is also considered. The BL and MW-HS models yield similar single impedance arcs for a relatively low volume fraction conductive matrix (coating on the low conductivity phase). The BA model yields single impedance arcs different from the MW-HS models. The BL and MW-HS models yield virtually identical dual impedance arc behavior for a low volume fraction insulating matrix (coating on the high conductivity phase). At low volume fractions of insulating matrix, the low frequency arc due to the insulating material for the BA model is much smaller than for the MW-HS model. The BS model exhibits single impedance arc behavior when the volume fraction of conductor is above or near the percolation threshold and dual arc behavior somewhat below the percolation threshold. Equivalent circuits for these model materials are discussed, and application is made to experimental data for various electroceramic systems.

NANOPHASE NI PARTICLES PRODUCED BY A BLOWN ARC METHOD

Nanophase Ni particles (<10 nm in diameter) were produced by a blown arc method. A helium gas stream directed at the arc reduces the Ni vapor concentration and increases the quench rate. The helium gas velocity is the predominant factor influencing the size of the Ni particles. Gas velocities of 20 m/s and 56 m/s (at 26.6 kPa total helium pressure) resulted in Ni particle sizes of 13 nm and 7 nm, respectively.

Dielectric amplification in cement pastes

Dielectric amplification (dielectric constants >80) is observed in cement pastes at early ages. Standard nonlinear least squares fitting routines yield artificially large “capacitances” when constant phase elements are employed. Instead, capacitance vs. frequency analysis provides reliable evidence of dielectric amplification. A physical model system consisting of a polycarbonate box with electrodes at each end, divided into two compartments by a polycarbonate barrier with a single hole, and filled with electrolyte solution, simulates the impedance response in young cement pastes. The barrier represents hydration products whereas the hole represents the constriction between two adjacent capillary pores, i.e., the pore network remains percolated. Dielectric amplification is inversely proportional to barrier thickness, i.e., it decreases as the barrier (product phase) thickens. Impedance spectra from real pastes vs. water/cement (w/c) ratio and during freezing or solvent exchange (to preferentially reduce the conductivity of the capillary pores) exhibit significant dielectric amplification, even after freezing or exchange, suggesting that C-S-H gel also has a dielectrically amplified microstructure. Advanced Cement Based Materials 1997, 5, 41–48.

Phase stability of epitaxial KTa x Nb 1− x O 3 thin films deposited by metalorganic chemical vapor deposition

The phase stability of epitaxial KTa x Nb 1− x O 3 (0 ≤ x ≤ 1) thin films, with compositions over the entire solid solution range, was investigated. KTa x Nb 1− x O 3 thin films were deposited on (100) MgAl 2 O 4 substrates by metalorganic chemical vapor deposition. Films with compositions x ≤ 0.30 were orthorhombic, as determined by x-ray diffraction. Dielectric measurements at room temperature indicated the presence of morphotropic phase boundaries at x = 0.30 and at x = 0.74. Temperature-dependent measurements of the dielectric constant for KNbO 3 from 80 to 800 K indicated three structural phase transitions at 710, 520, and 240 K. For intermediate compositions, a decrease in the Curie and tetragonal–orthorhombic transition temperatures was observed with increasing Ta atomic percent, similar to the bulk phase equilibrium. In contrast to bulk materials, an increase in the orthorhombic–rhombohedral transition temperature with increasing x was observed for the films, resulting in the stabilization of a rhombohedral phase at room temperature for compositions 0.45 ≤ x ≤ 0.73. Differences between the phase stability for the thin films and bulk were attributed to lattice misfit strain.

Experimental limitations in impedance spectroscopy of materials systems

Using resistor-capacitor networks, sources of experimental artifacts in impedance spectroscopy were investigated, such sources include machine limitations, rig/cabling contributions at high frequencies, and artifacts due to high impedance reference electrodes and their geometrical placement. In the instance of electrode placement, computer simulations with a pixel-based model were in agreement with the experimental observations. Remedies for these artifacts such as rig shielding/grounding, geometrical adjustments, and null corrections are also discussed.