Improvement of n-type OTFT electrical stability by gold electrode modification

N-type organic thin film transistors (OTFT) containing modified gold electrodes have been fabricated to investigate the influence of the self assembled monolayer on the transistor characteristics. We report on the effect of drain/source modification by thiol derivatives on the performances, electrical parameters uniformity and electrical stability of C₆₀ transistors. In the literature, electrical instability is often attributed to organic semiconductor (OSC), OSC-insulator interface and insulator. We found here that OSC-metal interfaces affect dramatically the operational stability for bottom gate/bottom contact structure. These effects have been attributed to morphological evolution at the interface metal-OSC induced by the self-assembled monolayers.     

Coarsening of boron carbide grains during the infiltration of porous boron carbide preforms by molten silicon

This work investigates the coarsening of boron carbide grains during the infiltration of porous boron carbide preforms by molten silicon with respect to fabrication of reaction-bonded boron carbide ceramics. Experimental results reveal that the shape of boron carbide grains evolve from the irregular shape to faceted shape due to dissolution-precipitation during infiltration. For infiltration temperatures below 1750 °C, the boron carbide grains are irregular and exhibit an unimodal size distribution, which can be ascribed to the normal grain growth. The growth of the irregular grains follow a cubic law of diffusion control. In contrast, for infiltration temperatures above 1750 °C, the boron carbide grains become faceted and exhibit a bimodal size distribution, indicative of the typical abnormal grain growth. The abnormal growth of faceted grains is proposed to be controlled by coalescence-enhanced two-dimensional nucleation.     

A synchronous cellular automaton model of mass transport in porous media

In this work we present a fully synchronous coarse grained cellular automaton model for large-scale simulations at molecular level. The model is based on Margolus partitioning scheme, which was generalized as to describe quantitatively diffusion, adsorption and directed flow in porous media. Our aim is to create conceptually simple and computationally efficient framework to model the mass transport in porous materials with large representative volume. This work focuses on the fundamental aspects of the generalized Margolus cellular automaton. We exemplify the model by solving several diffusion problems, studying the monolayer adsorption, chromatography on disordered porous structures and chemical transformation in a system with phase separation. The results indicate that the model reflects the essential features of these phenomena. Absence of round-off errors, fully synchronous way of implementation, autonomous physically meaningful time scale and ease-to-handle boundary conditions make this model a promising framework for study various transport phenomena in porous structures.     

Evolutionary multi-objective optimisation with preferences for multivariable PI controller tuning

Multi-objective optimisation design procedures have shown to be a valuable tool for control engineers. They enable the designer having a close embedment of the tuning process for a wide variety of applications. In such procedures, evolutionary multi-objective optimisation has been extensively used for PI and PID controller tuning; one reason for this is due to their flexibility to include mechanisms in order to enhance convergence and diversity. Although its usability, when dealing with multi-variable processes, the resulting Pareto front approximation might not be useful, due to the number of design objectives stated. That is, a vast region of the objective space might be impractical or useless a priori, due to the strong degradation in some of the design objectives. In this paper preference handling techniques are incorporated into the optimisation process, seeking to improve the pertinency of the approximated Pareto front for multi-variable PI controller tuning. That is, the inclusion of preferences into the optimisation process, in order to seek actively for a pertinent Pareto front approximation. With such approach, it is possible to tune a multi-variable PI controller, fulfilling several design objectives, using previous knowledge from the designer on the expected trade-off performance. This is validated with a well-known benchmark example in multi-variable control. Control tests show the usefulness of the proposed approach when compared with other tuning techniques.     

Using Taguchi method to obtain the optimal design of heat dissipation mechanism for electronic component packaging

Packaging technology developments in semiconductor chips are moving towards miniaturization, thinner products, lighter weights, and higher performance. However, in the process of packaging, warpage and residual stress have always been major problems, such as pin deviation, breakage, and weak signals. Further, the distinctive properties of the numerous materials that comprise a semiconductor chip demand different molding temperatures; thus, excessive internal thermal stresses are produced within the packaging structure which ultimately results in colloid warpage. This study used a 3D coordinate measuring machine to determine the levels of warpage produced in electronic packaging products and to verify the amount of warpage simulated by the finite element method. Then, Taguchi method was also utilized to analyze and discuss the four critical control factors namely: (1) shape of the heat sink; (2) thickness of molding; (3) molding temperature; and (4) thickness of soldering tin. Thus, the minimum thermal stress for electronic packaging components was obtained, which meant the optimal parameter combination for the packaging was a triangle-shaped heat sink, with a molding compound of 1.175 mm thick, a molding temperature of 170 °C, and a soldering tin that was 0.03 mm thick.     

The dielectric constant and quality factor calculation of the microwave dielectric ceramic solid solutions

The dielectric constant of the microwave dielectric ceramic solid solution is usually predicted by the Clausius-Mosotti equation but the quality factor (Q) cannot be precisely calculated. In this paper, it finds that the dielectric constant of the solid solutions also could be well calculated by the Maxwell-Wagner formula, and that the Q of solid solutions can be precisely calculated, by assuming a solid solution as a two or more materials’ mixture.     

Prediction of ultra-high ON/OFF ratio nanoelectromechanical switching from covalently-bound C60 chains

Applying a first-principles computational approach, we have systematically analyzed the effects of [2+2] cycloaddition oligomerization of fullerene C₆₀ chains on their junction electronic and charge transport properties. For hypothetical infinite C₆₀ chains, we first establish that the polymerization can in principle increase conductance by several orders of magnitude due to the strong orbital hybridizations and band formation. On the other hand, our simulations of the constant-height scanning tunneling microscope (STM) configuration shows that, in agreement with the recent experimental conclusion, the junction electronic structure and device characteristics are virtually unaffected by the C₆₀ chain oligomerization. We further predict that the switching characteristics including even the ON/OFF-state assignment will sensitively depend on the substrate metal species due to the Fermi-level pinning at the substrate-side contact and the subsequent energy level bending toward the STM tip-side contact. We finally demonstrate that a force-feedbacked nanoelectromechanical approach in which both of the C₆₀–electrode distances are kept at short distances before and after switching operations can achieve a metal-independent and significantly improved switching performance due to the Fermi-level pinning in both contacts and the large intrinsic conductance switching capacity of the C₆₀ chain oligomerization.     

Preparation of Silica–Alumina Hollow Spheres with a Single Surface Hole by Co-axial Microchannel

Si/Al composite hollow spheres with a surface hole were prepared with the co-axial microchannel in a one-step method. It is easy to use the technique for size control and continuous operation. At Si/Al ratio between 4 and 5, a hole forms on the surface, due to the fast gelation process and high viscosity of the sol. Scanning electron microscopy, nitrogen adsorption–desorption isotherms, and mercury intrusion method are used to characterize the samples. The hole size is 40–150 μm and the particle size is 450–600 μm. The size can be adjusted by the flow rate of the oil phase.     

Investigation of thermal and magnetic behaviour of mixed valence iron hydroxyphosphate from Fe3(PO4)2(OH)2 lipscombite systems

Iron hydroxyphosphates are promising candidates for applications as cathode materials for Li-ion batteries, for example and present low production cost and reduced toxicity. In this work, a new fast method is proposed for the synthesis of mixed valence iron hydroxyphosphate from Fe₃(PO₄)₂(OH)₂ lipscombite systems. The as-obtained products were characterized by X-ray diffraction, FT-IR spectroscopy, scanning electron microscopy, and magnetic measurements were carried out. Thermal treatment performed at 300 °C and 500 °C for 4 h indicate the formation of other lipscombite phases in accordance with thermogravimetric analysis where two weight loss steps were observed at ∼200 °C and ∼450 °C. A complex magnetic behaviour is observed with antiferromagnetic interactions at T_N ∼140 K for the pristine lipscombite Fe₃(PO₄)₂(OH)₂ compound related to the strong magnetic interactions in the 3D crystal structure with small values for Fe-Fe distances and super-exchange contribution through PO₄ tetrahedra.