There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Electroactive phases of poly(vinylidene fluoride) : determination, processing and applications

In this article, several applications of nanomaterials in food packaging and food safety are reviewed, including: polymer/clay nanocomposites as high barrier packaging materials, silver nanoparticles as potent antimicrobial agents, and nanosensors and nanomaterial-based assays for the detection of food-relevant analytes (gasses, small organic molecules and food-borne pathogens). In addition to covering the technical aspects of these topics, the current commercial status and understanding of health implications of these technologies are also discussed. These applications were chosen because they do not involve direct addition of nanoparticles to consumed foods, and thus are more likely to be marketed to the public in the short term.

Applications of nanotechnology in food packaging and food safety: Barrier materials, antimicrobials and sensors

Summary Although MXene nanosheets have attracted significant scientific and industrial attention, these materials are highly susceptible to oxidation, which leads to their chemical degradation and loss of functional properties in a matter of days. Here we demonstrate an effective method to prevent the oxidation of colloidal Ti3C2Tx MXene nanosheets by using sodium L-ascorbate as an antioxidant. The success of the method is evident in the stable morphology, structure, and colloidal stability of Ti3C2Tx. Even in the presence of water and oxygen, the electrical conductivity of Ti3C2Tx nanosheets treated with sodium L-ascorbate was orders of magnitude higher as compared with untreated ones after 21 days. This resistance to oxidation also persists in the dried state. We propose that the sodium L-ascorbate protects the edges of the nanosheets, restricting water molecules from otherwise reactive sites; this is supported by molecular dynamics simulations that show association of the ascorbate anion with the nanosheet edge.

Antioxidants Unlock Shelf-Stable Ti3C2Tx (MXene) Nanosheet Dispersions

A hierarchically structured nitrogen-doped porous carbon is prepared from a nitrogen-containing isoreticular metal-organic framework (IRMOF-3) using a self-sacrificial templating method. IRMOF-3 itself provides the carbon and nitrogen content as well as the porous structure. For high carbonization temperatures (950 °C), the carbonized MOF required no further purification steps, thus eliminating the need for solvents or acid. Nitrogen content and surface area are easily controlled by the carbonization temperature. The nitrogen content decreases from 7 to 3.3 at % as carbonization temperature increases from 600 to 950 °C. There is a distinct trade-off between nitrogen content, porosity, and defects in the carbon structure. Carbonized IRMOFs are evaluated as supercapacitor electrodes. For a carbonization temperature of 950 °C, the nitrogen-doped porous carbon has an exceptionally high capacitance of 239 F g–1. In comparison, an analogous nitrogen-free carbon bears a low capacitance of 24 F g–1, demonstrating...

In Situ One-Step Synthesis of Hierarchical Nitrogen-Doped Porous Carbon for High Performance Supercapacitors

Ti3C2Tx belongs to the family of MXenes, 2D materials with an attractive combination of functional properties suitable for applications such as batteries, supercapacitors, and strain sensors. However, the fabrication of devices and functional coatings based on Ti3C2Tx remains challenging as they are prone to chemical degradation by their oxidation to TiO2. In this paper, we examine the oxidation of Ti3C2Tx in air, liquid, and solid media via conductivity measurements to assess the shelf life of Ti3C2Tx MXenes. The oxidation of Ti3C2Tx was observed in all the media used in this study, but it is fastest in liquid media and slowest in solid media (including polymer matrices). We also show that the conventional indicators of MXene oxidation, such as changes in color and colloidal stability, are not always reliable. Finally, we demonstrate the acceleration of oxidation under exposure to UV light. The storage and dispersion of MXene films in media influence their oxidation behavior. A team led by Miladin Radovic and Micah Green at Texas AM the oxidation was the slowest in solid media and the fastest in liquid media. In aqueous dispersions, Ti3C2Tx oxidized in 2 weeks, along with a sharp decrease in electrical conductivity; however, the dispersion retained its dark color and colloidal stability. Ti3C2Tx could be well preserved in ice, whereas its polymers were not effective barriers to decrease oxidation.

/pdf/oxidation-stability-of-ti-3-c-2-t-x-mxene-nanosheets-in-4p8qh7tp1o.pdf

Oxidation stability of Ti 3 C 2 T x MXene nanosheets in solvents and composite films

With an ever-increasing need for thin, flexible and functional materials in electrochemical systems, the layer-by-layer (LbL) technique provides a simple and affordable route in creating new, active electrodes and electrolytes. The LbL technique, which is based upon the alternate adsorption of oppositely charged species from aqueous solution, possesses unprecedented control of materials selection (e.g. polyelectrolytes, clays, nanoparticles, proteins), materials properties (e.g. conductivity, glass-transition temperature) and architecture (e.g. blends, stratified-layers, pores). These advantages make LbL assemblies excellent candidates for use in proton-exchange membrane and direct methanol fuel-cells, batteries, electrochromic devices, solar cells, and sensors. This review addresses the design of LbL films for electrochemical systems and recent progress.

Electrochemically enabled polyelectrolyte multilayer devices: from fuel cells to sensors

Poly(ethylene oxide) (PEO) is a key material in solid polymer electrolytes, biomaterials, drug delivery devices, and sensors. Through the use of hydrogen bonds, layer-by-layer (LBL) assemblies allow for the incorporation of PEO in a controllable tunable thin film, but little is known about the bulk properties of LBL thin films because they are often tightly bound to the substrate of assembly. The construction technique involves alternately exposing a substrate to a hydrogen-bond-donating polymer (poly(acrylic acid)) and a hydrogen-bond-accepting polymer (PEO) in solution, producing mechanically stable interdigitated layers of PEO and poly(acrylic acid) (PAA). Here, we introduce a new method of LBL film isolation using low-energy surfaces that facilitate the removal of substantial mass and area of the film, allowing, for the first time, the thermal and mechanical characterization that was previously difficult or impossible to perform. To further understand the morphology of the nanoscale blend, the glass transition is measured as a function of assembly pH via differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The resulting trends give clues as to how the morphology and composition of a hydrogen-bonded composite film evolve as a function of pH. We also demonstrate that LBL films of PEO and PAA behave as flexible elastomeric blends at ambient conditions and allow for nanoscale control of thickness and film composition. Furthermore, we show that the crystallization of PEO is fully suppressed in these composite assemblies, a fact that proves advantageous for applications such as ultrathin hydrogels, membranes, and solid-state polymer electrolytes.

Jodie L. Lutkenhaus

Papers

Antioxidants Unlock Shelf-Stable Ti3C2Tx (MXene) Nanosheet Dispersions

In Situ One-Step Synthesis of Hierarchical Nitrogen-Doped Porous Carbon for High Performance Supercapacitors

Oxidation stability of Ti 3 C 2 T x MXene nanosheets in solvents and composite films

Electrochemically enabled polyelectrolyte multilayer devices: from fuel cells to sensors

Elastomeric Flexible Free-Standing Hydrogen-Bonded Nanoscale Assemblies