Photo-modulable Molecular Transport Junctions based on Organometallic Molecular Wires

International audiencePhoto-modulable molecular transport junctions are developed via on-wire lithography-fabricated nanogaps functionalized with a dithienylethene unit bearing two ruthenium fragments. A reversible and repeatable bi-state conductive switching upon alternate irradiation of UV and visible light can be distinctly observed. Theoretical calculations further suggest that bi-directional isomerization is due to the ruthenium moieties that modulate judiciously the electronic coupling between the photochromic part and the metal electrodes, and that the differences in electronic structure between the two isomers (open and closed states) are responsible for conductivity switching

CRITICAL COOLING RATES FOR GLASS FORMATION OF ZR-AL-CU-NI AND MG-NI-ND ALLOYS

Master'sMASTER OF SCIENC

Reactivity of Al/CuO nanothermite composites with fluoropolymers

Aluminum/copper oxide (Al/CuO) nanothermite is a promising metal fuel due to its high combustion enthalpy and excellent gas-producing characteristics. Fluoropolymers are energetic binders that can potentially be used to make moldable reactive composites without compromising the ignitability of thermites for structural applications. Herein, we report the reactivity of Al/CuO nanothermite composites with two processable fluoropolymers, THV, and Viton. The reactivities of the prepared composites were investigated from pressure measurement. The corresponding reaction mechanism was also studied by post-reaction products analysis. The results showed that both THV and Viton composites, even those containing up to 30 wt.% of fluoropolymer, were ignitable. However, the reactivities of the prepared composites were observed to reduce to different extent depending on the type of fluoropolymers used. The reduced reactivity was ascribed to the subdued nanothermite reaction due to the consumption of copper oxide and aluminum by reacting with the fluoropolymers. Among the two fluoropolymers studied, the THV composite exhibited a relatively higher reactivity. The ignition delay and combustion temperature of the THV composite were further determined

Combustion of fluoropolymer coated Al and Al–Mg alloy powders

This work presents an experimental investigation of the combustion characteristics of micron scale aluminum and aluminum-magnesium alloy powders coated with a thin layer of fluoropolymer. Burn times of the coated powders ignited by CO₂ laser were estimated from the time resolved emission signals recorded by photomultiplier tubes. Both fluoropolymer coated powders recorded reduced burn times. This result is likely associated with the lowered diffusion barrier in the fluoropolymer coated particles due to the gasification of oxide shell in the presence of fluorinated species from the decomposing fluoropolymer. Combustion temperatures determined using two-color pyrometry and optical spectroscopy were consistently higher for the fluoropolymer coated powders in comparison with that of the pristine. The reactivity of Al and Al-Mg alloy powders as assessed by constant volume explosion experiments was improved due to the fluoropolymer coating. Dust clouds of fluoropolymer-coated samples could achieve higher burning velocity as estimated from the experimental pressure traces using a semi-empirical correlation for dust explosions. A plausible mechanism responsible for the improvement in metal combustion due to the incorporation of fluoropolymer was proposed

Nanoenergetic Composites with Fluoropolymers: Transition from Powders to Structures

Over the years, nanoenergetic materials have attracted enormous research interest due to their overall better combustion characteristics compared to their micron-sized counterparts. Aluminum, boron, and their respective alloys are the most extensively studied nanoenergetic materials. The majority of the research work related to this topic is confined to the respective powders. However, for practical applications, the powders need to be consolidated into reactive structures. Processing the nanoenergetic materials with polymeric binders to prepare structured composites is a possible route for the conversion of powders to structures. Most of the binders, including the energetic ones, when mixed with nanoenergetic materials even in small quantities, adversely affects the ignitability and combustion performance of the corresponding composites. The passivating effect induced by the polymeric binder is considered unfavorable for ignitability. Fluoropolymers, with their ability to induce pre-ignition reactions with the nascent oxide shell around aluminum and boron, are recognized to sustain the ignitability of the composites. Initial research efforts have been focused on surface functionalizing approaches using fluoropolymers to activate them further for energy release, and to improve the safety and storage properties. With the combined advent of more advanced chemistry and manufacturing techniques, fluoropolymers are recently being investigated as binders to process nanoenergetic materials to reactive structures. This review focuses on the major research developments in this area that have significantly assisted in the transitioning of nanoenergetic powders to structures using fluoropolymers as binders

Combustion characteristics of fluoropolymer coated boron powders

The problem of sluggish combustion reactivity of Boron (B) can be overcome by inclusion of fluoropolymers. In this paper, three commercial fluoropolymers; PVDF (59 wt% F), Viton (66 wt% F) and THV (72 wt% F) coated (Ca. 4 wt%) B powders (Ca. 1-µm in size) were prepared and their combustion characteristics have been investigated. Among the fluoropolymer-coated powders, THV coated B provided the highest average improvement in heat of oxidation, reactivity in terms of pressure generation and combustion temperature followed by Viton and PVDF coated B. The sequence of reactivity enhancement has been explained by the variances in gasification efficiencies of the boric oxide shell, induced by the thermal decomposition of the respective fluoropolymer coatings. THV with higher fluorine and lower hydrogen content, supposedly promotes better gasification of the boric oxide shell by releasing more fluorine rich alkanes/alkenes during thermal decomposition. However, PVDF predominantly produces HF, which apparently less effective in gasification of the boric oxide shell, resulting in limited improvement of the measured properties

Effect of Ag-doping on crystal structure and high temperature thermoelectric properties of c-axis oriented Ca3Co4O9 thin films by pulsed laser deposition

Ag-doped Ca3Co4O9 thin films with nominal composition of Ca3−xAgxCo4O9 (x = 0∼0.4) have been prepared on sapphire (0 0 0 1) substrates by pulsed laser deposition (PLD). Structural characterizations and surface chemical states analysis have shown that Ag substitution for Ca in the thin films can be achieved with doping amount of x ≤ 0.15; while x > 0.15, excessive Ag was found as isolated and metallic species, resulting in composite structure. Based on the perfect c-axis orientation of the thin films, Ag-doping has been found to facilitate a remarkable decrease in the in-plane electrical resistivity. However, if doped beyond the substitution limit, excessive Ag was observed to severely reduce the Seebeck coefficient. Through carrier concentration adjustment by Ag-substitution, power factor of the Ag-Ca3Co4O9 thin films could reachAccepted versio

Vanadium pentoxide-based cathode materials for lithium-ion batteries : morphology control, carbon hybridization, and cation doping

Vanadium pentoxide (V2O5) is a promising cathode material for high-performance lithium-ion batteries (LIBs) because of its high specific capacity, low cost, and abundant source. However, the practical application of V2O5 in commercial LIBs is still hindered by its intrinsic low ionic diffusion coefficient and moderate electrical conductivity. In the past decades, progressive accomplishments have been achieved that rely on the synthesis of nanostructured materials, carbon hybridization, and cation doping. Generally, fabrication of nanostructured electrode materials can effectively decrease the ion and electron transport distances while carbon hybridization and cation doping are able to significantly increase the electrical conductivity and diffusion coefficient of Li+. Implementation of these strategies addresses the problems that are related to the ionic and electronic conductivity of V2O5. Accordingly, the electrochemical performances of V2O5-based cathodes are significantly improved in terms of discharge capacity, cycling stability, and rate capability. In this review, the recent advances in the synthesis of V2O5-based cathode materials are highlighted that focus on the fabrication of nanostructured materials, carbon hybridization, and cation doping

Nanothermite composites with a novel cast curable fluoropolymer

Structural nanoenergetic (SNE) composites consisting of nanothermites dispersed in a polymer binder are attractive for combustion applications due to their substantially higher volumetric energy, density, and tailorable energy release characteristics. Cast curing is an extensively utilized, safe, and economical method for processing energetic materials into structural composites possessing macroscale geometries. The use of cast curing as a route for processing SNE composites is limited by the lack of suitable binder systems. The widely used cast curable binder such as hydroxyl terminated polybutadiene (HTPB) if used for this purpose will render the SNE composites thermally non-ignitable. Fluoropolymers have emerged as energetic binders for aluminum containing nanothermites due to their capability to induce pre-ignition reaction and sustain the ignitability of composites. Herein, we report the synthesis of a novel cast curable fluoropolymer (FP) with 58 wt% fluorine content produced through the cationic ring opening polymerization of 3-Perfluorohexyl-1, 2-epoxypropane. We demonstrate the utility of the synthesized FP as a cast curable binder for aluminum/copper oxide (Al/CuO) nanothermite to prepare SNE composites. The SNE composites were cast cured as cylindrically shaped specimens with the FP content ranging from 30 to 50% by weight. All the SNE composites were observed to be thermally ignitable even under an inert environment, suggestive of the unique role of FP on promoting the ignitability of SNE composites. SNE composite containing 40 wt% FP recorded the highest energetic performance in terms of heat of reaction (880 cal/g) and pressure generation characteristics (pressurization rate = 3.81kPsi/s; time to maximum pressure = 340 ms). Additionally, the 40 wt% FP SNE composite generates time averaged combustion temperature of 2530 K as estimated by spectroscopic technique