Sea-Fue Wang

Unipolar resistive switching characteristics of ZnO thin films for nonvolatile memory applications

Highly (002)-oriented and columnar-grained ZnO thin films were prepared by radio frequency magnetron sputtering at room temperature. The Pt∕ZnO∕Pt devices exhibit reversible and steady bistable resistance switching behaviors with a narrow dispersion of the resistance states and switching voltage. Only a low forming electric field was required to induce the resistive switching characteristics. The resistance ratios of high resistance state to low resistance state were in the range of 3–4 orders of magnitude within 100cycles of test. It was also found that the conduction mechanisms dominating the low and high resistance states are Ohmic behavior and Poole-Frenkel emission, respectively.

Structure property relationships in core-shell BaTiO3-LiF ceramics

A sintering, microstructural development and dielectric property study of BaTiO 3 –LiF ceramics was performed to assess the potential application of low-fired multilayer capacitors. Not only does LiF allow for sintering below 1000 °C, it also allows for the manipulation of dielectric properties and interfaces within BaTiO 3 –LiF ceramics. Using mixing laws, a model of the dielectric properties of the core-shell microstructures is presented that agrees well with the observed experimental data.

Intrinsic photocatalytic oxidation of the dye adsorbed on TiO2 photocatalysts by diffuse reflectance infrared Fourier transform spectroscopy

Abstract Photocatalytic oxidation of aqueous pollutants by semiconductor photocatalysts was found efficient. The overall process by which the heterogeneous photocatalysis proceeds includes: the sequence of the adsorption of reactants, surface reaction and the desorption of final products. As a result, factors such as, the presence of oxygen concentration, pH values of the aqueous solution, pore properties for photocatalyst particles all determine the rate of photodegradation. This study is to concentrate on the photocatalytic mechanisms of the intrinsic reaction occurred on the photocatalysts and the adsorbed dye in air atmosphere. Additionally, the photocatalytic activities of adsorbed dyes prepared under different pH values were also examined and the solid-state results were compared with the aqueous systems at the same pH conditions. Dark adsorption experiments at different pH conditions showed that the saturation amount of dyes adsorbed on the catalysts differs significantly. However, the solid-state photodegradation rates of adsorbed dyes on TiO 2 at various pH values only showed slightly different, which is opposite to the results obtained from the aqueous systems. This evidence reveals that the external and internal mass transport processes are rate-controlling steps that restricted the photodegradation reaction of aqueous dyes at different pH conditions. Furthermore, this investigation supports a proposed direct photocatalytic mechanism for aqueous systems that the photocatalytic oxidation always begins with the adsorption process and the adsorbed dye will then be attacked by the excited hole–electron pairs and hydroxyl radicals from TiO 2 surface to produce final products.

Deposition and characterization of TiNi-base thin films by sputtering

Abstract Chemical compositions, crystallography, microstructure and phase transformation behavior of TiNi-base alloy films grown by R.F. magnetron sputtering have been investigated. Effects of alloying elements such as Cu and Fe on film crystallography and properties are studied. Due to the non-equilibrium characteristic of sputter deposition, as-deposited TiNi-base films prepared are in a non-equilibrium, amorphous state. To yield crystalline structures and desirable phase transformation behaviors, post-deposition annealing in vacuum at 600°C for 1 h is needed. Phase transformation behavior is evaluated by a differential scanning calorimeter and results indicate a typical single-stage transformation in annealed binary films (Ti-48.9 at.%Ni and Ti-41.8Ni). For the Ti-36.6Ni-10.4Cu film, a two-stage transformation is revealed. The single-stage transformation takes place between the B2 parent phase and M (monoclinic) martensite or R (rhombohedral) phase, while the two-stage transformation occurs in a sequence of B2 phase to O (orthorhombic) phase to M phase on cooling and vice versa on heating. Yet, an imperfect phase transformation is found in the Ti-45.4Ni-6.3Fe film for the temperature range examined. Transformation temperatures as well as energies involved in the transformations are in general relatively lower than those of the target. Transformation peak-temperature hystereses of the film are also smaller than that of the target, implying a beneficial characteristic of fast response for shape memory effect.

Effect of glass composition on the densification and dielectric properties of BaTiO3 ceramics

An ongoing research goal of thick film capacitors and multilayer capacitors is to lower the firing temperature of the dielectrics. This paper presents the results of using three simple glass systems including PbO–B2O3, PbO–SiO2, and Bi2O3–B2O3 as sintering aids for hydrothermal synthesized BaTiO3. Glasses with different ratios of the modifier/glass former were employed. Effects of adding these glass systems on the BaTiO3 ceramics sintered at 850°C were investigated through measuring and analyzing the density, grain size and dielectric property. It was found that BaTiO3 sintered with glasses composed of 90 mol% PbO–10 mol% SiO2 or 90 mol% PbO–10 mol% B2O3 to 60 mol% PbO–40% B2O3 are helpful to reduce the firing temperature for typical thick film and MLCC applications. They possess high dielectric constant (≈1650) due to their high densification characteristics with the grain size of ≈0.7 μm. In addition, glasses composed of 90 mol% Bi2O3–10 mol% B2O3 to 40 mol% Bi2O3–60 mol% B2O3 are beneficial for thin dielectric layer applications, on account of the high sintering density and a small grain size of ≈0.1 μm with an acceptable K value.

Thermal stability of sputtered copper films containing dilute insoluble tungsten: Thermal annealing study

This paper describes studies on the thermal annealing behavior of Cu films with 2.3 at.% W deposited on Si substrates. The magnetron cosputtered Cu films with insoluble W were vacuum annealed at temperatures ranging from 200 to 800 °C. Twins were observed in focused ion beam and transmission electron microscopy images of as-deposited and 400 °C annealed pure Cu film, and these twins were attributed to the intrinsic low stacking fault energy. Twins in pure Cu film may provide an additional diffusion path during annealing for copper silicide formation. The beneficial effect of W on the thermal stability of Cu film was supported by the following observations: (i) x-ray diffraction studies show that Cu 4 Si was formed at 530 °C in Cu-W film, whereas pure Cu film exhibited Cu 4 Si growth at 400 °C; (ii) shallow diffusion profiles for Cu into Si in Cu-W film through secondary ion mass spectroscopy analyses, and the high activation energy needed for the copper silicide formation from the differential scanning calorimetry study; (iii) addition of W in Cu film increases the stacking fault energy and results in a low twin density.

Liquid-phase sintering and chemical inhomogeneity in the BaTiO 3 –BaCO 3 –LiF system

An ongoing goal of multilayer capacitor research is to lower the firing temperature of the dielectric. This paper gives a detailed study of sintering BaTiO 3 with LiF flux, which lowers the firing temperature through liquid-phase sintering. A detailed set of experiments is discussed concerning microstructural evolution and corresponding dielectric properties under a number of processing variables, including amount of LiF, sintering temperature, and particle size. Different scales of chemical inhomogeneity were observed in this system, which reflect two underlying mechanisms: solution reprecipitation with limited grain growth at low temperatures, which resulted in distinct core–shell structures, and flux volatility, which gave rise to microscopic chemical inhomogeneity at higher sintering temperatures.

Hexagonal Ba(Ti1−xMnx)O3 ceramics: Microstructural evolution and microwave dielectric properties

Up to date, little work has been done to explore the possible application of the high-temperature 6H-hexagonal BaTiO3 polymorph (h-BaTiO3). In this letter, the feasibility for use of h-BaTiO3 ceramics as microwave dielectric resonators was evaluated through the Mn substitution in B sites. Based on the overall performance, the best results were acquired for the h-Ba(Ti0.65Mn0.35)O3 (er:41.2, Q∙fr:14300, and τf:18.6ppm∕°C) and h-Ba(Ti0.50Mn0.50)O3 ceramics (er:32.8, Q∙fr:6,450, and τf:3.6ppm∕°C). These results suggest the hexagonal Ba(Ti1−xMnx)O3 ceramics are suited for the dielectric resonator applications.

Effects of Metallo-Organic Decomposition Agents on Thermal Decomposition and Electrical Conductivity of Low-Temperature-Curing Silver Paste

Six low-temperature-curing silver pastes were prepared from silver flake, α-terpineol and various metallo-organic decomposition (MOD) compounds. The thermal decomposition behaviors of the pastes were determined. The microstructures and resistivities of screen-printed films on alumina substrate after thermal treatment were characterized and discussed. Results indicated that 2-ethylhexanoate possesses the lowest decomposition temperature (190.3 °C) among the MOD agents studied, and it forms silver particles to promote the linking of silver flake powders and thus reduces the resistivity to <13 µΩcm at a temperature as low as 200 °C.

Thermoplastic polyurethane/clay nanocomposite foam made by batch foaming

In this study, the foaming of nonwoven fabrics and bulk polymers was investigated. Four different polymers that are commonly used for textiles including polyethylene terephthalate, thermoplastic polyurethane, thermoplastic polyether ester elastomer and polypropylene were foamed by batch foaming. Among the polymers, thermoplastic polyurethane seems to be the most promising material since it possesses the highest cell density and smallest cell size. In addition, nanoclay was added to thermoplastic polyurethane to promote cell nucleation and to increase the cell density. The results showed that well dispersed nanoclay in thermoplastic polyurethane served as an excellent nucleation agent and the cell structure was improved. The cell size of thermoplastic polyurethane nanocomposite foam decreased to 1 µm while the cell density increased to 3 × 1011 cells/cm3.