Showing papers in "Journal of Electroceramics in 1999"

FEATURE ARTICLE Conducting Polymer Composites

Abstract: Conducting polymer composites become increasingly important for technical applications. In this article, the resulting electrical properties of such materials are illustrated by a variety of experimental examples. It is shown that the combined mechanical, thermal and electrical interaction between the filler particles via their electrical contacts and the surrounding polymer host matrix are responsible for the properties of the composite material. A short review is given of the theoretical background for the understanding of the electrical transport in such materials. The arrangement of the filler particles and the resulting conductivity can be described either by percolation or by effective medium theories. It can also be related to different types of charge carrier transport processes depending on the internal composite structure. Special emphasis is given to the microstructure of the filler particles such as size, hardness, shape and their electrical and thermal conductivities. A detailed analysis of the physics of the contact spots and the temperature development during current flow at the contact is given. It is shown that the polymer matrix has a strong influence on the electrical conductivity due to its elastic properties and the response to external thermal and mechanical stimulation. Strong changes in the electrical conductivity of conducting polymer composites can be realized either by thermal stimuli, leading to a positive and negative temperature coefficient in resistivity, or by applying mechanical stress. By using nonlinear fillers an additional degree of functionality can be achieved with conducting polymers.

Brick Layer Model Analysis of Nanoscale-to-Microscale Cerium Dioxide

Abstract: The frequency-dependent impedance/dielectric behavior of the brick-layer model (BLM) was investigated vs. grain size and local parameters (resistivity, dielectric constant, and grain boundary width). The simulation shows a maximum in capacitance vs. grain size, governed by the grain boundary-to-grain interior resistivity ratio. The BLM was employed to analyze the 500 °C impedance behavior of polycrystalline cerium dioxide from the nano- (∼15 nm grain size) to the micro- (∼4 μm grain size) regime. The grain boundary resistivity is orders of magnitude larger than that of the grain interiors in the microcystalline specimen. This contrast is significantly smaller in the nanocrystalline specimens, suggesting enhanced conduction at grain boundaries. The limitations of the BLM for simulating the behavior of complex electroceramic microstructures are discussed.

Percolation Behavior of Conductor-Insulator Composites with Varying Aspect Ratio of Conductive Fiber

Abstract: The electrical properties of composite electroceramics are determined by the concentration, shape and distribution of filler phase in matrix. The carbon fiber-filled polymer was chosen as a model system and the electrical conductivity was measured as a function of carbon fiber content and the aspect ratio (AR) of the fibers to understand the percolation behavior of the composites. The composites of carbon fiber (1∼9 vol.%) and thermoplastic polymer were fabricated in a mold press with the aspect ratio of carbon fiber varying between 4 and 10. The percolation threshold volume concentrations (V c) of transition from the insulator to the conductor decreased as the fiber aspect ratio increased. With the fibers segregated at the polymer-polymer interfaces in the present study, V c values were much smaller than those with the fibers randomly distributed in the matrix shown in other studies. The inverse relation between V c and AR was found as expected. From the comparison with other experimental and simulated data, we concluded that the slope in 1/V c versus AR plot is a strong function of fiber segregation.

Effects of Cation Disorder on Oxygen Vacancy Migration in Gd2Ti2O7

Abstract: Atomistic simulations were used to calculate defect formation and migration energies for oxygen vacancies in the pyrochlore Gd2Ti2O7, with particular attention to the role of cation antisite disorder. Oxygen occupies two crystallographically distinct sites (48f and 8a) in the ordered material, but the 8b sites become partially occupied with disorder. Because cation and anion disorder are coupled, oxygen vacancy formation and migration energetics are sensitive to the configuration of the cation disorder. The VO8a vacancy and VO8a + O8bi Frenkel defects are energetically favored in the ordered material, but VO8a is favored at higher disorder. The VO8a + O8bi Frenkel is favored for some disorder configurations. Eight possible oxygen vacancy migration paths converge toward a unique migration energy as cation disorder increases, reflecting a reversion towards the fluorite structure. Oxygen vacancy migration is determined by O48f → O48f transitions along the shortest edges of the TiO6 octahedra. The transition V48a → V48f is also possible for low disorder, and can activate the V48f → V48f migration network by depositing vacancies there. The reverse transition may occur at very high disorder to retard ionic conduction, and is consistent with Frenkel defect stabilities. Local regions of ordered and disordered material both appear necessary to explain the observed trends in ionic conductivity.

ZnO Grain Boundaries: Electrical Activity and Diffusion

Abstract: The operation and performance of electroceramics are commonly dependent on the characteristics of electrically active grain boundaries. To date, our understanding of the role of specified additives and heat treatments on the grain boundary properties remains underdeveloped. We describe efforts directed towards improving our understanding by (a) fabrication and analysis of individual boundaries, (b) improved control and simplification of boundary chemistry (c) systematic investigation of properties (e.g., I-V, DLTS, DO and DM) as a function of boundary structure and chemistry and (d) development of appropriate energy band, defect and diffusion models. Following this approach, preliminary results suggest that lattice defects play critical roles in controlling both the electrical and diffusive properties of the boundaries while the additives appear to act in supportive manner by activating the key lattice defects particularly with respect to the electrical activity of the boundaries.

Optimized PZT Thin Films for Pyroelectric IR Detector Arrays

Abstract: A planar multi target sputtering technology was used to deposit highly (111) oriented Pb(Zr x Ti1−x )O3 (PZT) thin films with x ranging from 0–0.6. The preparation of a stable Pt/ZrO2 electrode is described and analyzed in terms of stress and stress-temperature behavior. The PZT films with low Zr content are under compressive stress after deposition. The dielectric constant and loss peaks occur at a composition close to the morphotropic phase boundary. Films on the tetragonal side of the phase diagram with a Zr content up to about 25% exhibited a strong self polarization and strong voltage shifts in the C(V) curves. High pyroelectric coefficients of >2×10−4 C/(m2K) have been measured on these films without additional poling. The self polarization fades out with increasing Zr content. The low values of the pyroelectric coefficient for the PZT film with 60% Zr is discussed in terms of the possible crystallographic variants after distortion and the tensile stress state during the phase transition. Based on the systematic study of stress and electrical properties of PZT films with a wide range of composition presented in this paper, films with a Zr content up to about 25% turned out to give the best properties for the use in pyroelectric detector arrays.

Characterization and Electrochemical Studies of LiMn2O4 Cathode Materials Prepared by Combustion Method

Abstract: A combustion method using urea as a fuel has been developed for the synthesis of the spinel LiMn2O4 around 500°C. Physical features of the products were identified by X-ray photoelectron spectroscopy, X-ray diffractometry, Raman scattering and FTIR spectroscopy. Cells were fabricated with Li//LiMn2O4 and C//LiMn2O4 in nonaqueous organic electrolyte and their performances were studied. A kinetic profile for diffusion of Li ions in the composite matrix was developed and tested.

Oxygen Diffusion in Single- and Poly-Crystalline Zinc Oxides

Abstract: 18O diffusion coefficients were measured in zinc oxide ceramics using a secondary ion mass spectrometer. The results are interpreted as indicating extrinsic behavior. The values of the lattice diffusion coefficients with higher valence dopants compared with zinc ions are greater than lower valence dopant such as lithium ions. Using the data at deeper depth, the grain boundary diffusivity of oxide ions was also evaluated. Although the lattice diffusion coefficients varied by two orders of magnitude, the products of grain boundary width and grain boundary diffusion coefficient were less sensitive to the type of dopants.

Multilayer Ceramic Capacitors with Thin (Ba,Sr)TiO3 Layers by MOCVD

Abstract: Barium strontium titanate ((Ba,Sr)TiO3; BST) thin films were prepared on platinum-coated MgO substrates at 650°C by metalorganic chemical vapor deposition (MOCVD) Perovskite single phased BST thin films were obtained Dielectric constant at 1 kHz–100 mV was 1000 Multilayer ceramic capacitor with twelve BST dielectric layers of 026 μm thick was formed on the MgO substrate Capacitance and dissipation factor (tanδ) at 1 kHz–100 mV were 32 nF and 15% respectively Capacitance per unit volume of 33 μF/mm3 provided 10 to 20 times larger volumetric efficiency than the conventional multilayer ceramic capacitors Temperature coefficient of capacitance was −4000 ppm/°C The leakage current at 1 V was 23×10-9 A that yielded an acceptable CR product of 128 MΩ-μF MOCVD was proposed as one of the promising manufacturing technologies for multilayer ceramic capacitors of high performance with sub-micron thick dielectric layers

Properties of pyrochlore ruthenate cathodes for intermediate temperature solid oxide fuel cells

Abstract: Powders of Bi2Ru2O7.3, Pb2Ru2O6.5 and Y2Ru2O7 were prepared and their reactivity with Ce0.9Gd0.1O1.95 (CGO) electrolyte powders examined. Bi2Ru2O7.3 and Pb2Ru2O6.5 reacted with CGO but Y2Ru2O7 appeared stable when heated in contact with CGO powder at 900C for 24 h. Symmetrical electrodes of Y2Ru2O7-x were deposited on CGO ceramic pellets, either by tape-casting or by electrostatic assisted chemical vapor deposition (EACVD) techniques, and cathode resistivities determined by impedance spectroscopy. Undoped Y2Ru2O7 electrodes exhibited very high area specific resistivities (ASR) at 627C (∼4000 Ωcm2), but by doping with SrO the resistivity was reduced almost 100×to 47 Ωcm2. The behavior of the Y2Ru2O7 cathodes was interpreted in terms of available oxygen ion transport data for the Gd2Ti2O7 series, and it was concluded that optimization of pyrochlore ruthenate compositions should be possible to improve further the oxygen reduction behavior of ruthenate cathodes for intermediate temperature solid oxide fuel cells.

Direct-Write Fabrication of Integrated, Multilayer Ceramic Components

Abstract: The need for advanced electronic ceramic components with smaller size, greater functionality and enhanced reliability requires the ability to integrate electronic ceramics in complex 3-D architectures. However, traditional tape casting and screen printing approaches are poorly suited to the requirements of rapid prototyping and small-lot manufacturing. To address this need, a direct-write approach for fabricating highly integrated, multilayer components using a Micropen to deposit slurries in precise patterns is being developed at Sandia. This approach provides the ability to fabricate multifunctional, multimaterial integrated ceramic components (MMICCs) in an agile and rapid way. Commercial ceramic thick-film pastes can be used directly in the system, as can polymer thick-film pastes (PTF). The quality of printed components depends on both the rheology and drying behavior of the pastes. Pastes with highly volatile solvents are inappropriate for the Micropen. This system has been used to make integrated passive devices such as RC filters, inductors, and voltage transformers.

Doping Mechanism and Permittivity Correlations in Nd-Doped BaTiO3

Abstract: BaTiO3 forms an extensive range of solid solutions with Nd2O3 by means of the double substitution mechanism: Ba + Ti ⇒ 2Nd, as shown by both a phase diagram study and Rietveld refinement using powder neutron diffraction data. The solid solutions have the general formula Ba1-xTi1-xNd2xO3Odxd0.12 at 1300°C and 1300°C and O dxd at 1400°C. With increasing x, the symmetry changes from tetragonal to cubic at x∼ 0.09. The sharp permittivity maximum at ∼127°C in stoichiometric BaTiO3 broadens very rapidly with increasing x and gradually moves to lower temperatures: this appears to be because, with substitution of Nd onto Ti sites, formation of the ferroelectric domains is increasingly difficult because of the presence of dipole-inactive Nd3+ ions on the Ti sites.

Monolayer-mediated patterning of integrated electroceramics

Abstract: Integrated electroceramic thin-film devices on semiconducting or insulating substrate materials offer a wide variety of attractive attributes, including high capacitance density, nonvolatile memory, sensor/actuator ability, and other unique electrical, electromechanical, magnetic and optical functions. Thus the ability to pattern such electroceramic thin films is a critical technology for future device realization. Patterned oxide thin-film devices are typically formed by uniform film deposition followed by somewhat complicated post-deposition ion-beam or chemical etching in a controlled environment i.e., a subtractive method. We review here an upset technology, a different way of patterning, by an additive approach, which allows for the selective deposition of electroceramic thin layers without such post-deposition etching. In this method, substrate surfaces are selectively functionalized with hydrophobic self-assembled monolayers to modify the adhesion of subsequently deposited solution-derived electroceramics. The selective functionalization is achieved through microcontact printing (μ-CP) of self-assembled monolayers of the chemical octadecyltrichlorosilane on substrates of current technical interest. Subsequent sol-gel deposition of ceramic oxides on these functionalized substrates, followed by lift-off from the monolayer, yields high quality, patterned oxide thin layers only on the unfunctionalized regions. A variety of micron-scale dielectric oxide devices have been fabricated by this method, with lateral resolution as fine as 0.5 μm. In this paper, we review the monolayer patterning and electrical behavior of several patterned electroceramic thin films, including Pb(Zr,Ti)O3 [PZT], LiNbO3, and Ta2O5. A multilevel example is also given which combines selective MOCVD deposition of metal electrodes and sol-gel patterned PZT for Pt//PZT//Pt//Si(100) ferroelectric memory cells.

Orientation Control of ZnO Thin Film Prepared by CVD

Abstract: The orientations of ZnO films parallel and perpendicular to the surface of the substrate were investigated as functions of the deposition temperature and substrate material. The degree of orientation increased with increasing deposition temperature and became perfectly oriented at a characteristic temperature. At a deposition temperature of 620°C, polycrystalline films were obtained on polycrystalline Al2O3; substrates. (001) oriented films were obtained on fused silica and (100) rutile substrates. Epitaxially grown (110) and (001) oriented films were obtained on various kinds of single crystal substrates. The difference between the (110) and (001) orientations was explained by the lattice mismatch between the films and the substrates. Epitaxial growth of films exhibiting two directions was observed when the two equivalent directions of lattice mismatch existed. These results show the possible formation of various types of the crystallographic relationships between the grains in the film.

Microstructure and Phase Development of Buried Resistors in Low Temperature Co-Fired Ceramic

Abstract: Embedded resistor circuits have been generated with the use of a Micropen system Ag conductor paste (DuPont 6142D), a new experimental resistor ink from DuPont (E84005-140), and Low Temperature Co-fired Ceramic (LTCC) green tape (DuPont A951). Sample circuits were processed under varying peak temperature ranges (835 C-875 C) and peak soak times (10 min-720 min). Resistors were characterized by SEM, TEM, EDS, and high-temperature XRD. Results indicate that devitrification of resistor glass phase to Celcian, Hexacelcian, and a Zinc-silicate phase occurred in the firing ranges used (835-875 C) but kinetics of divitrification vary substantially over this temperature range. The resistor material appears structurally and chemically compatible with the LTCC. RuO{sub 2} grains do not significantly react with the devitrifying matrix material during processing. RuO{sub 2} grains coarsen significantly with extended time and temperature and the electrical properties appear to be strongly affected by the change in RuO{sub 2} grain size.

Chemical Variations in Barium Titanate Powders and Dispersants

Abstract: Commercial raw materials for multilayer ceramic capacitors—barium titanate (BT) and ammonium polyacrylate (APA) dispersant—were examined for lot-to-lot variations which cause poor reproducibility in BT slips and in capacitor chips. Two lots of BT supplied by a commercial source were different with respect to surface species examined by Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, electrokinetic sonic amplitude and total carbon analysis. It was found that there was more APA dispersant chemisorption on BT when the BT surface was more hydroxylated and less carbonated. For the dispersant, the two lots differed in pH and ammonium ion content. The amount of APA dispersant adsorbed on BT depended more on the ceramic powder surface species than on the chemical differences in APA lots.

Thermodynamic Stability and Interfacial Impedance of Solid-Electrolyte Cells with Noble-Metal Electrodes

Abstract: In solid-electrolyte cells, the electrode-electrolyte interfacial stability and impedance are found to be dependent on temperature, atmosphere, current density, microstructure and the process history of the cell. The modifications induced by temperature and oxygen pressure on the impedance spectra of Pt/Yttria-stabilized zirconia (YSZ) and Pd/YSZ interfaces have been studied. The interfacial impedance was controlled by adsorption/desorption of oxygen with a Langmuir-type dependency. When the surface coverage was small, the interfacial impedance decreased with increase in temperature and \(P_{O_2 }\). In certain temperature and \(P_{O_2 }\) regimes and depending on the process history, the metal electrode formed stable oxygen-containing species. In this region, the interfacial impedance increased markedly and its \(P_{O_2 }\) dependence also changed. Anodic and cathodic currents altered the local thermodynamic conditions at the charge-transfer sites and accordingly influenced the interfacial impedance. The concentration of oxygen-containing species and the interfacial microstructure are shown to influence the shape of the impedance response. Pt was found to form a neck at the YSZ electrolyte and Pd did not. The electrode polarization in the case of Pt/YSZ interface corresponded to one impedance-response arc signifying charge-transfer resistance at the three-phase boundary (TPB), gas/Pt/YSZ interface. For the Pd/YSZ interface, the electrode polarization corresponded to two impedance-response arcs at low \(P_{O_2 }\). The high-frequency response is related to charge transfer at the TPB and the low frequency to the gas-phase mass transfer.

Structure and Electrical Properties of Ni-substituted Lanthanum Gallate Perovskites

Abstract: Crystal structure, electrical conductivity and thermal expansion of materials in the system La0.9Sr0.1Ga1–yNiyO3 (LSGN) with 0 < y ≤ 0.5 have been studied as a function of Ni content, temperature and Po 2. The materials have an orthorhombic structure at low dopant content and a hexagonal structure for higher Ni content. The σ(Po 2, T) results show increasing electronic conductivity at high Po 2 with increasing Ni content. At low Po 2 the conductivity fits a model for predominant ionic conductivity. AC impedance spectroscopy on an electron blocking cell of the form M/LSG/LSGN/LSG/M was used to isolate the ionic conductivity in the y=0.1 and 0.2 materials. The materials were found to have appreciable ionic conductivity in air with a similar magnitude and activation energy to the electrolyte materials. An analysis of the low frequency impedance of the blocking cell provided values for D at 800°C of the order of 9 × 10−5 cm2/s. An evaluation of the so called chemical capacitance enabled determination of the electron density and mobility. The calculated electronic mobility of 3.8 × 10−3 cm2/Vs and activation energy of 0.14 eV for the y = 0.2 material are in excellent agreement with expectations of an electronic transport model involving electron hopping within the Ni impurity band. The thermal expansion coefficients of the Ni doped materials were determined as a function of temperature and dopant level. The presence of mixed conductivity suggests that this material may be useful as an electrode for a lanthanum gallate based fuel cell or other electrochemical device.

Chemical solution deposition of epitaxial La1-x(Ca, Sr)xMnO3 thin films

Abstract: We report on the epitaxial growth of magnetoresistive La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 thin films by chemical solution deposition. Thin films were prepared by spin-coating of single-crystal LaAlO3 (100) substrates with precursor solutions of different concentrations and crystallized at 850°C. The structure of the thin film was found to be influenced by the concentration of the spin-coating solution. The thin film structure and epitaxy was clearly improved by reducing the concentration of the precursor solution. All thin films displayed excellent electrical properties such as a low resistivity and very high metal-insulator transition temperatures T MI .

Effect of Liquid Phase and Vaporization on the Formation of Microstructure of Pr Doped ZnO Varistor

Abstract: Our previous works and our recent data were summarized to discuss the effect of liquid phase formation and vaporization of the components on the densification, grain growth and change of the microstructure of Pr doped ZnO ceramics in air In the ZnO-Pr2O3 binary system, eutectic liquid forms at 1382± 5°C° and significant vaporization of the components occurred above the eutectic temperature Below eutectic temperature (1350°C), only 01 mol % of Pr doping into ZnO brought about the grain growth of ZnO, however further addition of Pr brought about the suppression of the grain growth Comparing the grain size distribution at the surface and inside part of Pr doped ZnO ceramics, it was clarified that wider grain size distribution was observed at the surface than inside part At the temperature just below the eutectic (1370°C), abnormal grain growth was also observed Depth profile of Pr content indicated that no concentration gradient was observed below eutectic temperature (1350°C), however above eutectic temperature (1500°C), condensation of Pr was observed at the surface Grain growth rate as well as weight loss rate were drastically increased between 1350 and 1370°C, which suggested that formation of liquid phase accelerate the grain growth rate and weight loss rate Consequently the microstructure of Pr doped ZnO ceramics was formed by both effects on the formation of liquid phase and on the vaporization of the components

Implementation of GaAs Monolithic Microwave Integrated Circuits with On-Chip BST Capacitors

Abstract: GaAs microwave monolithic ICs with novel on-chip BST (Ba X Sr1 −X TiO3) capacitors are demonstrated. MOD (Metal Organic Decomposition) technique was employed to make the BST thin film. The fabricated BST capacitor has the dielectric constant as high as 300, which is 50 times higher than the conventional Si3N4 one. The implemented GaAs MMIC with on-chip BST capacitor provides both high gain and low power dissipation characteristics. These MMIC can be packaged in a compact outline with reduced pin counts. BST capacitors also show the suppressed harmonics due to their low-pass filtering characteristics of the frequency roll-off around 2 GHz. The present technology will reduce the system size for a variety of the mobile communication systems.

ICTS Measurements of Single Grain Boundaries in ZnO:rare-earth Varistor

Abstract: Properties of single grain boundaries in ZnO:rare-earth varistors were examined by the isothermal capacitance transient spectroscopy (ICTS) method. Micro-electrodes prepared by photolithography were used for measuring the behavior of single junctions in ZnO varistors. From current-voltage measurements, it was found that the non-linear exponents of single junctions varied from 3 to 14. Interface state levels existed at 0.9 eV below the conduction band edge for every junction. On the other hand, the interface state density varied from junction to junction and the non-linearity was shown to increase with increasing interface state density.

Microassembly of a piezoelectric miniature motor

Abstract: An assembly method for the fabrication of a piezoelectric miniature motor is presented. The fabrication is done by placement and soldering of piezoceramic elements onto a silicon substrate. The assembly of the miniature motor demands high precision (+/-5

Numerical Calculation of Electrical Conductivity of Porous Electroceramics

Abstract: A numerical method for the calculation of the electrical conductivity of porous ceramics for gas sensing applications is developed, which takes into account detailed microstructural features by mapping a mesoscopic irregular resistor network onto the microstructure. The overall conductance of the ceramic sample is obtained by solving the Kirchhoff equations for the irregular network using an efficient iterative algorithm. The method is designed to handle the widely varying conductivities of different microstructural components present in ceramic gas sensors. The evolution of the macroscopic conductance of the model systems during a phase field simulation of sintering is obtained and several characteristic stages are distinguished. The potential applications of the method in computer aided microstructural optimization for ceramic gas sensors is discussed.

Physical Properties and Electrochemical Features of Lithium Nickel-Cobalt Oxide Cathode Materials Prepared at Moderate Temperature

Abstract: In this paper, we report the synthesis, the physical properties and the electrochemical features of the lithium nickel-cobalt oxide cathode materials prepared by a combustion method at moderate temperature. Structural properties were investigated by X-ray diffraction, Raman scattering and FTIR. Spectroscopic measurements show unambiguously that the final product is identified as a modified-spinel structure (Fd3m space group) with the stoichiometric formula Li2[Ni0.5Co0.5]2O2. Electrochemical cells Li//Li2[Ni0.5Co0.5]2O2 were fabricated using an organic electrolyte and their performances were tested. For a modified-spinel Li2[Ni0.5Co0.5]2O4 structure the chemical diffusion coefficient of Li ions is around 10−10 cm2s, which is lower than for a layered LiNi0.5Co0.5O4 host matrix.

Effect of Silica Doping on the Electrical Conductivity of 3 mol % Yttria-Stabilized Tetragonal Zirconia Prepared by Colloidal Processing

Abstract: Silica-doped (SiO2 = 0 - 1.0 wt%) 3Y-TZP (3 mol % yttria-doped tetragonal zirconia polycrystal) ceramics are prepared from hetero-coagulated aqueous suspension by colloidal processing. Consolidation of the suspension was carried out by pressure filtration at 10 MPa followed by cold isostatic pressing (CIP) at 400 MPa. Consolidated compacts are densified to a relative density over 99% by sintering at 1573 K for 2 h. The formation of glass pockets at grain boundary multiple junctions was observed by SEM for ≥0.5 wt % silica-doped samples. Electrical conductivity measurements were performed to evaluate the modification of grain-boundaries by silica. The apparent grain boundary conductivity decreased with an increase in silica content and became nearly constant above 0.3 wt % of silica, while the bulk conductivity was constant with silica content.

Electrolytic Formation and Morphology of Biomimetic Apatite Coatings

Abstract: Calcium-deficient apatite has been cathodically deposited on SUS stainless steel from an acidic calcium phosphate solution. The resulting apatite porous coatings were characterized morphologically, compositionally and structurally. The porous coatings were modified into dense coatings by immersion in an aqueous supersaturated calcium phosphate solution.

The structure and defect chemistry of non-stoichiometric nickel molybdates

Abstract: A series of materials represented by the formula Ni1;δMo1−δ/3O4, where -1/5 ≤ δ ≤ 1/3, were prepared by calcination of layered ammonium nickel molybdates having a general formula (NH4)H2xNi3-xO(OH)(MoO4)2, where 0 ≤ x ≤ 3/2. Phase determination using high temperature X-ray diffraction studies showed that the variable stoichiometry of the precursor phase that allowed for Ni/Mo ratios between 0.75 and 1.5 led to the formation of a single phase of the form Ni1;δMo1−δ/3O4 following calcination. AC impedance spectroscopy was used to investigate the electronic conductivity of the materials. The defect chemistry of these ternary ionic materials was modeled to correlate the electronic conductivity with the structure.

Valence band structure of ZnO (10(1)over-bar0) surface by cluster calculation

Abstract: Discrete variational (DV) Xα cluster method has been employed in calculating electronic structures of ZnO. Electronic structures of the bulk and the non-polar surface model clusters are calculated with inclusion of electrostatic potentials in the bulk and near the surface, and the electronic origins of experimental spectra and chemical bonds at the surface are examined in detail. The valence band structure constructed by Zn-3d and O-2p bands is much influenced by electrostatic potentials in ZnO. It is found that the reduction of an electrostatic potential near the surface gives rise to the difference of the valence band structures between in the bulk and at the surface. The calculated density of states at the non-polar surface of ZnO, where the Zn-3d and O-2p bands are more widely separated than in the bulk, is in good agreement with the experimental UPS. In addition, a Zn-O bond at the surface is found to show stronger covalency than that in the bulk, as a result of the change of the valence band structure due to the effect of the electrostatic potential.

High Transition Temperature Superconducting Quantum Interference Devices: Basic Concepts, Fabrication and Applications

Abstract: The performance of superconducting quantum interference devices (SQUIDs) and SQUID based magnetometers made from thin films of high transition temperature superconductors (HTS) has greatly improved since the discovery of HTS more than a decade ago. This fact is related to a steady improvement in the fabrication technology for HTS thin films and Josephson junctions. The state-of-the-art in HTS SQUID fabrication, device concepts and applications is briefly reviewed.