Preparation and barrier property of poly(ethylene terephthalate)/clay nanocomposite using clay‐supported catalyst

Poly(ethylene terephthalate) (PET)/clay nanocomposite was prepared by the direct polymerization with clay‐supported catalyst. The reaction degree of catalyst against the cation exchange capacity of clay was 8 wt %. The intercalation of PET chains into the silicate layers was revealed by X‐ray diffraction studies. SEM morphology of the nanocomposite showed a good dispersion of clay‐supported catalyst, ranging from 30 to 100 nm. The intercalated and exfoliated clay‐supported catalyst in PET matrix was also observed by TEM. The improvement of O₂ permeability for PET/clay‐supported catalyst composite films over the pure PET is approximately factors of 11.3–15.6. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4875–4879, 2006     

Solution-processed semiconducting aluminum-zinc-tin-oxide thin films and their thin-film transistor applications

We prepared aluminum-zinc-tin-oxide (AZTO) thin films by the solution spin-coating method and investigated their physical and electrical properties according to different incorporated amounts of Al. AZTO films annealed at 400 °C were amorphous. Though SnO₂ crystallites were detected in films annealed at temperatures higher than 500 °C, the number of crystallites decreased as the Al content increased. Thin films had a smooth and uniform surface morphology with an optical transmittance value higher than 92% in the visible range. Electrical conductivity and its temperature dependence varied markedly according to the amount of Al incorporated in the film. We therefore systematically investigated activation energies for carrier transport for each film composition. Thin-film transistors (TFTs) were fabricated using solution-processed AZTO as an active channel layer. The effects of the amount of Al incorporated in the thin film on TFT characteristics were also evaluated. The best device performance was observed for a TFT with a 5 mol%-Al-incorporated AZTO channel. Field effect mobility, subthreshold swing, and on/off ratio were approximately 0.24 cm² V⁻¹ s⁻¹, 0.69 V/dec, and 1.03×10⁶, respectively.     

Synthesis and characterization of a series of bis(dimethyl-n-octylsilyl)oligothiophenes for organic thin film transistor applications

A series of bis-dimethyl-n-octylsilyl end-capped oligothiophenes consisting of two to six thiophene units has been synthesized and characterized to develop novel organic semiconductor materials. The UV–vis spectral data indicate that these silyl end-capped oligothiophenes have longer conjugation lengths as evidenced by the higher λ_max values than the corresponding unsubstituted thiophene oligomers. The thermal analyses indicate that the bis-silylated oligothiophenes show lower melting point (DSi-4T = 80 °C; DSi-5T = 115 °C; DSi-6T = 182 °C) than the corresponding dialkylated thiophene oligomers by 100 °C and hexamer DSi-6T exhibits a liquid crystalline mesophase at 143 °C. The α,ω-bis(dimethyl-n-octylsilyl)oligothiophenes (DSi-6T) have a remarkably high solubility in chloroform which are comparable to the corresponding α,ω-dihexyloligothiophenes. The remarkably increased solubility by these silyl end groups leads bis-silylated oligothiophenes to be applicable to solution processable devices for thin film transisitor (TFT) by utilizing a spin-coating technique. α,ω-Bis(dimethyl-n-octylsilyl)sexithiophene can be deposited as active semiconducting layer in thin film transistors, either by vacuum evaporation or by spin-coating. A high charge-carrier mobility has been obtained for both deposition techniques, μ = 4.6 × 10⁻² and 1.4 × 10⁻² cm² V⁻¹ s⁻¹, respectively.     

Finite element analysis of the bending behavior of a workpiece in equal channel angular pressing

For the first time, a detailed and systematic finite element study was carried to identify the parameters which cause the bending of the workpiece in equal channel angular pressing. These simulations were carried out by using commercial finite element code Abaqus with different materials behavior, processing parameters, and die geometries. The results showed that the optimal ways to reduce the bending of strain rate sensitive materials in ECAP without varying the strain homogeneity are the usage of lower processing speed and the increase in length of the die exit-channel.     

Characterization of NiCo composite silicides by 10 nm-Ni50Co50 alloy films with additional annealing

We fabricated 10 nm-Ni₅₀Co₅₀ alloy films into a single crystal and a polycrystalline silicon substrate, and applied silicidation annealing to these substrates at 600 °C to 1100 °C for 40 s. To test the thermal stability of the processed silicide layers, we examined the change in their physical properties after an additional 30-min annealing at the given rapid thermal annealing (RTA) temperatures. To characterize the physical properties of the silicide layers, we used a four-point probe, an x-ray diffractometer (XRD), a transmission electron microscope, a scanning electron microscope, an Auger electron spectroscope, and an atomic force microscope. The silicide layer formed only through RTA showed low resistance (20 Ω/sq) at up to 1100 °C and 900 °C for the single crystal and for the polycrystalline silicon substrate, respectively. Sheet resistance after the additional 30-min annealing was low, and did not differ significantly before and after the additional annealing for the single crystal substrate, but it became high at all temperatures for the polycrystalline substrate. The XRD confirmed the formation of the NiSi (or Ni(Co)Si) phase, in which there were no changes after the additional annealing. The thickness of the RTA-formed silicide layers varied from 11 nm to 13 nm, 20 nm and 28 nm, depending on whether the temperature was 700 °C or 1000 °C, for both the single and the polycrystalline substrates. The thickness of the silicide layers tended to increase from 22 nm to 25 nm, 48 nm and 82 nm after the additional 30-min annealing. Auger depth profiling also confirmed changes in thickness with the additional annealing. The surface roughness was no greater than 10 nm in all cases, even with the additional annealing. We verified that the nano-silicide layer formed with the proposed nano-NiCo alloy films satisfied the requirements for the nano-CMOS process.     

Optimization of an organic photovoltaic device via modulation of thickness of photoactive and optical spacer layers

We examine the modulation effects of thicknesses of both a photoactive layer (a bulk-heterojunction (BHJ) of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM)) and an optical spacer of a transparent metal oxide, for power conversion efficiency optimization of organic photovoltaic devices. The redistribution of the optical intensity at the photoactive layer via the thickness modulation of both layers is taken into account, to produce three-dimensional (3D) plots as a function of both layer thicknesses of 0 to 400 nm range (5 nm step), for the device efficiency optimization. The modulation pattern of absorption is produced in the 3D plot as scanning the thicknesses of both layers as a result of modulation of interference between incoming and reflected light, which can be secured by changing the effective optical path length between two electrodes of a photovoltaic device. It is also seen that the case of inserting the spacer of the higher refractive index demands finer adjustment of the spacer layer thickness to achieve the optimum device efficiency.In addition, the series resistance of the photoactive layer of the thickness range of 0 to 70 nm is taken into account to provide the 3D plots as a function of the scanned thicknesses of both layers. Inclusion of the series resistance of the photoactive layer, which is also the function of its thickness, in the simulation, indicates that the series resistance can influence qualitatively the dependence of power conversion efficiency (PCE) on the thicknesses of both layers. We also find that minimization of series resistance, e.g., by device annealing, allows not only the relevant voltage to increase but also the optimum thickness of the photoactive layer to increase, leading to more absorption of light.     

Effect of oxygen vacancy and Mn-doping on electrical properties of Bi4Ti3O12 thin film grown by pulsed laser deposition

An amorphous Bi₄Ti₃O₁₂ phase was formed when films were grown at <400 °C while Bi₂Ti₂O₇ and Bi₂Ti₄O₁₁ transient phases were developed when films were grown at 400–500 and 600 °C, respectively. A homogeneous Bi₄Ti₃O₁₂ crystalline phase was formed in the film grown at 700 °C. The high leakage current density (5 × 10^?7 A cm^?2 at 0.2 MV cm^?1) of the film grown at 300 °C under 100 mTorr oxygen partial pressure (OPP) decreased to 2 × 10^?8 A cm^?2 for the film grown at 200 mTorr OPP, due to the decreased number of intrinsic oxygen vacancies. However, when OPP exceeded 200 mTorr, the electrical properties were deteriorated due to the formation of oxygen interstitial ions. Mn-doping at a suitable level improved the electrical properties of the films by producing extrinsic oxygen vacancies that reduced the number of intrinsic oxygen vacancies. Schottky emission was suggested as the leakage current mechanism of the Bi₄Ti₃O₁₂ film.     

Design optimization of extruded preform for hydroforming processes based on ideal forming design theory

The conventional practice to predict preform shapes in hydroformimg processes based on finite-element analysis and/or experiment is an iterative procedure and requires many trials. In this paper, a computationally efficient direct design method, which effectively improves the design procedure, was introduced. The direct design method based on ideal forming theory, which was successfully applied for the design of flat blanks for stamping processes, was extended for the design of non-flat preform for tube hydroforming processes. A preform optimization methodology for non-flat blank solutions was proposed based on the penalty constraint method for the cross-sectional shape and length of a tube. The hybrid membrane/shell method was employed to capture thickness effect while maintaining membrane formulation in the ideal forming theory. Several classes of examples were analyzed to verify the current formulation.     

The effect of elastic misfit strain on the morphological evolution of γ’-precipitates in a model Ni-base superalloy

The morphological evolution of coherent γ’-precipitates in a Ni-23.4Co-4.7Cr-4Al-4.3Ti (wt%) alloy has been observed under systematically varying heat-treatment conditions. By deeply etching the matrix and observing under a scanning electron microscope, the precipitate morphologies are determined accurately. By quenching after solution treatment and aging isothermally, high initial precipitate density is obtained, and the cuboidal precipitates cluster and align to form regularly spaced arrays. The dimensionality of the aligned structure increases with increasing precipitate volume fraction as the aging temperature decreases. By directly cooling to an aging temperature after the solution treatment intermediate precipitate density is obtained. During the isothermal aging, the cuboidal precipitates split into octets and clusters of several pieces which disintegrate subsequently into aligned cuboids. Upon further aging the coherency is broken. When directly cooled to aging temperatures very close to the solvus temperature, the dispersed precipitates grow dendritically along the 〈111〉 directions. Various thermodynamic and kinetic theories for the elastic strain effect stemming from the lattice misfit agree quantitatively with the observed clustering, alignment, and splitting, but presently there is no complete theory which accurately describes the observed morphologies.