The structure of the Nafion ionomer used in proton-exchange membranes of H(2)/O(2) fuel cells has long been contentious. Using a recently introduced algorithm, we have quantitatively simulated previously published small-angle scattering data of hydrated Nafion. The characteristic 'ionomer peak' arises from long parallel but otherwise randomly packed water channels surrounded by partially hydrophilic side branches, forming inverted-micelle cylinders. At 20 vol% water, the water channels have diameters of between 1.8 and 3.5 nm, with an average of 2.4 nm. Nafion crystallites (approximately 10 vol%), which form physical crosslinks that are crucial for the mechanical properties of Nafion films, are elongated and parallel to the water channels, with cross-sections of approximately (5 nm)(2). Simulations for various other models of Nafion, including Gierke's cluster and the polymer-bundle model, do not match the scattering data. The new model can explain important features of Nafion, including fast diffusion of water and protons through Nafion and its persistence at low temperatures.

Parallel cylindrical water nanochannels in Nafion fuel-cell membranes.

We used 600-femtosecond electron pulses to study the structural evolution of aluminum as it underwent an ultrafast laser–induced solid-liquid phase transition. Real-time observations showed the loss of long-range order that was present in the crystalline phase and the emergence of the liquid structure where only short-range atomic correlations were present; this transition occurred in 3.5picoseconds for thin-film aluminum with an excitation fluence of 70 millijoules per square centimeter. The sensitivity and time resolution were sufficient to capture the time-dependent pair correlation function as the system evolved from the solid to the liquid state. These observations provide an atomic-level description of the melting process, in which the dynamics are best understood as a thermal phase transition under strongly driven conditions.

/pdf/an-atomic-level-view-of-melting-using-femtosecond-electron-3dgv8l1htf.pdf

An Atomic-Level View of Melting Using Femtosecond Electron Diffraction

Probing surface and interface morphology with Grazing Incidence Small Angle X-Ray Scattering

A novel and general approach to scaling diffraction intensities is presented. The method minimizes the disagreement among multiple measurements of symmetry-related reflections using a stable refinement procedure. The scale factors are described by a flexible exponential function that allows different scaling corrections to be chosen and combined according to the needs of the experiment. The scaling model presented here includes: scale and temperature factor per batch of data; temperature factor as a continuous function of the radiation dose; absorption in the crystal; uneven exposure within a single diffraction image; and corrections for phenomena that depend on the diffraction peak position on the detector. This scaling model can be extended to include additional corrections for various instrumental and data-collection problems.

https://journals.iucr.org/a/issues/2003/03/00/we0011/we0011.pdf

Multiparametric scaling of diffraction intensities

Lattice-mismatch strains are widely known to control nanoscale pattern formation in heteroepitaxy, but such effects have not been exploited in colloidal nanocrystal growth. We demonstrate a colloidal route to synthesizing CdS-Ag2S nanorod superlattices through partial cation exchange. Strain induces the spontaneous formation of periodic structures. Ab initio calculations of the interfacial energy and modeling of strain energies show that these forces drive the self-organization of the superlattices. The nanorod superlattices exhibit high stability against ripening and phase mixing. These materials are tunable near-infrared emitters with potential applications as nanometer-scale optoelectronic devices.

/pdf/spontaneous-superlattice-formation-in-nanorods-through-2rsmjc893v.pdf

Spontaneous superlattice formation in nanorods through partial cation exchange.

It has been noted that the family of plane electromagnetic waves and the "electromagnetic universe" of Bertotti--Robinson exhaust the entire class of conformally flat Einsteirr-Maxwell spaces. In the formalism of Newman--Penrose a family of exact solutions of the Einstein--Maxwell equations of the type of Bertotti--Robinson is obtained with a cosmological term belonging to the degenerate type D in the algebraic classification of Petrov and describing the space--time generated by a covariantly constant, nonisotropic electromagnetic field.

Dorothée Sainte‐Marie Lorrain

Papers

X-Ray Diffraction in Crystals, Imperfect Crystals, and Amorphous Bodies