Material-length-scale-controlled nanoindentation size effects due to strain-gradient plasticity

This paper proposes a concept of strain-gradient plasticity that is based on characteristic material length scale. Experimental results for indentation size effect are correlated with the predictions from a dislocation-based strain-gradient plasticity model. Nanoindentations on single crystals of Al, Ag, Ni, polycrystalline Cu and poly-synthetically twinned (PST) lamellar α₂- and γ-TiAl are conducted with an atomic force microscope with an add-on force transducer from Hysitron, Inc. It is found that the indentation size effect is controlled by a characteristic material length scale $\text{l}$, which is a function of the Burger’s vector, the shear modulus, and a material reference stress. The material length scales are found to be in the order , which corresponds to the indentation size effect.     

Air-stable,non-volatile resistive memory based on hybrid organic/inorganic nanocomposites

A non-volatile memory element based on organic/inorganic nanocomposites is presented. The device can be operated in ambient conditions, showing high retention time and long-term life time. The formation/rupture of metallic filaments in the organic matrix is investigated by HR-XPS and ToF-SIMS analysis, and is demonstrated to be the driving mechanism for the resistive switching.     

Effect of heat-input on pitting corrosion behavior of friction stir welded high nitrogen stainless steel

In this study, different welding parameters were selected to investigate the effects of heat-input on the microstructure and corrosion resistance of the friction stir welded high nitrogen stainless steel joints. The results showed that, the welding speed had major influence on the duration at elevated temperature rather than the peak temperature. The hardness distribution and tensile properties of the nugget zones (NZs) for various joints were very similar while the pitting corrosion behavior of various NZs showed major differences. Large heat-input resulted in the ferrite bands being the pitting location, while tool wear bands were sensitive to pitting corrosion in the low heat-input joints. Cr diffusion and tool wear were the main reasons for pitting. The mechanisms of pitting corrosion in the NZs were analyzed in detail.     

Radical polymers improve the metal-semiconductor interface in organic field-effect transistors

Modifying the organic-metal interface in organic field-effect transistors (OFETs) is a critical means by which to improve device performance; however, to date, all of the interfacial modifying layers utilized in these systems have been closed-shell in nature. Here, we introduce open-shell oxidation-reduction-active (redox-active) macromolecules, namely radical polymers, in order to serve as interfacial modifiers in pentacene-based OFETs. Through careful selection of the chemistry of the specific radical polymer, poly(2,2,6,6-tetramethylpiperidine-1-oxyl methacrylate) (PTMA), the charge transport energy level of the interfacial modifying layer was tuned to provide facile charge injection and extraction between the pentacene active layer and the gold source and drain electrodes of the OFET. The inclusion of this radical polymer interlayer, which was deposited in through straightforward inkjet printing, led to bottom-contact, bottom-gate OFETs with significantly increased mobility and ON/OFF current ratios relative to OFETs without the PTMA interlayer. The underlying mechanism for this improvement in device performance is explained in terms of the charge transport capability at the organic-metal interface and with respect to the pentacene grain growth on the radical polymer. Thus, this effort presents a new, open-shell-based class of materials for interfacial modifying materials, and describes the underlying physics behind the practical operation of these materials.     

Influence of source/drain electrodes on external quantum efficiency of ambipolar organic light-emitting transistors

The influence of source/drain (S/D) electrodes on the external quantum efficiency (EQE) of ambipolar organic light-emitting transistors (OLETs) based on fluorene-type polymer films is investigated. The electrical properties and the maximum EQE value of the device with indium tin oxide (ITO) S/D electrodes are almost the same as those of the device with Ag S/D electrodes. A relatively high EQE of 1% is achieved regardless of the emission site for the OLET with ITO. In contrast, the EQE of the OLET with Ag is low when the emission occurs close to the S/D electrodes. The maximum EQE of the device with Ag is obtained when the emission is observed in the middle of the channel. It is found that the exciton quenching by Ag electrodes significantly influences the low EQE of the OLET with Ag electrodes. The achievement of high EQE regardless of the emission site is attributable to both better carrier injection and lower exciton quenching at the interface of S/D electrodes for the OLET with ITO.     

Restraints in low dimensional organic semiconductor devices at high current densities

The understanding of the charge carrier transport in electronic materials is of crucial interest for the design of efficient devices including especially the restraints that arise from device miniaturization. In this work the performance of organic thin-film and single crystal field-effect transistors with the same active material was studied in detail focusing on the high current density regime, where a pronounced non-hysteretic maximum in the transconductance was found. Interestingly, in this operation mode for both, thin films and single crystals, comparable densities of free and gate-induced charge carriers were estimated. Kelvin probe microscopy was used to measure the contact potential difference and the electrical field along the transistor channel during device operation exhibiting the formation of local space charges in the high current density regime.     

Synthesis and characterizations of oligomeric (4,4′-dialkyl-2,2′-bithiazole) carboxylic acids and their LB-film behavior

The synthesis of series of 4,4′-dialkyl-2,2′-bithiazole oligomers, (ABTz)_1,2, and their 5-carboxylic acid derivatives is described. The Langmuir–Blodgett film-forming behavior of the several of the bithiazole dimer carboxylic acids was explored. The compounds form LB films with areas per molecule increasing from 32 to 40 Å² as the length of the alkyl side chain increases from ethyl to hexyl.     

Permeation of gases in industrial porous catalysts

An experimental method for determination of porous solid texture was proposed and verified. The method is based on unsteady permeation of single gases in a porous medium. Transport parameters characterizing porous materials were determined by fitting the experimental data to the theoretical transient responses by minimizing the objective function. Confidence limits of the transport parameters were evaluated on the base of the Beale criterion. Results were discussed in terms of different mechanisms of mass transport. Advantages of this technique were demonstrated for three porous catalysts of different textures.     

Interface channels accelerating ion transport through alkaline earth metal carbonate - Gd0.1Ce0.9O1.95 heterostructure

Designing the interface proton channel between different phases to accelerate the proton ion transport is an effective way to realize the high proton conduction for the low-temperature ceramic fuel cell (CFC). Cerium based materials coated with molten carbonate has been widely demonstrated for high performance CFCs. Here, we prepared alkaline earth metal carbonate - Gd_0.1Ce_0.9O_1.95 (GDC) heterostructure composites in various compositions by precipitation method using NH₄HCO₃ and NaHCO₃ as the deposit. The samples prepared using NH₄HCO₃ as the electrolyte, the cell can deliver an even higher power output of 811 mW cm⁻². The results are much higher than that reported in the literature for the GDC electrolyte fuel cells. The ion conduction on the interface between GDC and solid carbonate particles is proposed. The ionic conductivity is determined to be 0.13 S cm⁻¹ at 500 °C; while GDC as reported in literature is 0.005 S cm⁻¹ at the same temperature. This proposed solid carbonate-GDC heterostructure method has succeeded in enhancing ionic conductivity and the CFC performance, which presents a new way to develop high proton conducting materials and advanced ceramic fuel cells at low temperatures (<550 °C).     

Kinetics of the abnormal austenite–ferrite transformation behaviour in substitutional Fe-based alloys

The kinetics of the recently discovered abnormal austenite (γ)–ferrite (α) transformation of substitutional Fe–Co and Fe–Mn alloys were measured by dilatometry and compared with the, also measured, corresponding normal transformation behaviour. A phase transformation model, involving site saturation, interface-controlled continuous growth and incorporating an impingement correction, has been employed to extract the migration velocity of the γ/α interface. It was found that the normal transformation process could be well described assuming a constant nucleus density and interface migration velocity. The thus assessed misfit-accommodation, deformation Gibbs energy is of the same order of magnitude as the chemical Gibbs energy change driving the transformation. A large austenite grain size was shown to be the precondition for the occurrence of abnormal kinetics. The abnormal transformation process involves the occurrence of additional peaks in the transformation rate for the first stage of the transformation. An autocatalytic type of nucleation was successfully incorporated in the above model to describe the occurrence of the repeated nucleation during the abnormal transformation.