Heloisa N. Bordallo

Nano-scale hydrogen-bond network improves the durability of greener cements.

More than ever before, the worlds increasing need for new infrastructure demands the construction of efficient, sustainable and durable buildings, requiring minimal climate-changing gas-generation in their production. Maintenance-free “greener” building materials made from blended cements have advantages over ordinary Portland cements, as they are cheaper, generate less carbon dioxide and are more durable. The key for the improved performance of blends (which substitute fine amorphous silicates for cement) is related to their resistance to water penetration. The mechanism of this water resistance is of great environmental and economical impact but is not yet understood due to the complexity of the cements hydration reactions. Using neutron spectroscopy, we studied a blend where cement was replaced by ash from sugar cane residuals originating from agricultural waste. Our findings demonstrate that the development of a distinctive hydrogen bond network at the nano-scale is the key to the performance of these greener materials.

Metamagnetism and soliton excitations in the modulated ferromagnetic Ising chain CoV2O6

We report a combination of physical property and neutron scattering measurements for polycrystalline samples of the one-dimensional spin-chain compound CoV2O6. Heat capacity measurements show that an effective S = 1/2 state is found at low temperatures and that magnetic fluctuations persist up to similar to 6T(N). Above T-N = 6.3 K, measurements of the magnetic susceptibility as a function of T and H show that the nearest-neighbor exchange is ferromagnetic. In the ordered state, we have discovered a crossover from a metamagnet with strong fluctuations between 5 K and T-N to a state with a 1/3 magnetization plateau at 2 < T < 5 K. We use neutron powder diffraction measurements to show that the antiferromagnetic state has incommensurate long-range order and inelastic time-of-flight neutron scattering to examine the magnetic fluctuations as a function of temperature. Above T-N, we find two broad bands between 3.5 and 5 meV and thermally activated low-energy features which correspond to transitions within these bands. These features show that the excitations are deconfined solitons rather than the static spin reversals predicted for a uniform ferromagnetic Ising spin chain. Below T-N, we find a ladder of states due to the confining effect of the internal field. A region of weak confinement below T-N, but above 5 K, is identified which may correspond to a crossover between two- and three-dimensional magnetic ordering. (Less)

Polymorphism of Paracetamol: A New Understanding of Molecular Flexibility through Local Methyl Dynamics.

This study focuses on the interplay of molecular flexibility and hydrogen bonding manifested in the monoclinic (form I) and orthorhombic (form II) polymorphs of paracetamol. By means of incoherent inelastic neutron scattering and density functional theory calculations, the relaxation processes related to the methyl side-group reorientation were analyzed in detail. Our computational study demonstrates the importance of considering quantum effects to explain how methyl reorientations and subtle conformational changes of the molecule are intertwined. Indeed, by analyzing the quasi elastic signal of the neutron data, we were able to show a unique and complex motional flexibility in form II, reflected by a coupling between the methyl and the phenyl reorientation. This is associated with a higher energy barrier of the methyl rotation and a lower Gibbs free energy when compared to form I. We put forward the idea that correlating solubility and molecular flexibility, through the relation between pKa and methyl rotation activation energy, might bring new insights to understanding and predicting drug bioavailability.

How mobile are protons in the structure of dental glass ionomer cements

The development of dental materials with improved properties and increased longevity can save costs and minimize discomfort for patients. Due to their good biocompatibility, glass ionomer cements are an interesting restorative option. However, these cements have limited mechanical strength to survive in the challenging oral environment. Therefore, a better understanding of the structure and hydration process of these cements can bring the necessary understanding to further developments. Neutrons and X-rays have been used to investigate the highly complex pore structure, as well as to assess the hydrogen mobility within these cements. Our findings suggest that the lower mechanical strength in glass ionomer cements results not only from the presence of pores, but also from the increased hydrogen mobility within the material. The relationship between microstructure, hydrogen mobility and strength brings insights into the materials durability, also demonstrating the need and opening the possibility for further research in these dental cements.

Hindered Water Motions in Hardened Cement Pastes Investigated over Broad Time and Length Scales

We investigated the dynamics of confined water in different hydrated cement pastes with minimized contributions of capillary water. It was found that the water motions are extremely reduced compared to those of bulk water. The onset of water mobility, which was modified by the local environment, was investigated with elastic temperature scans using the high-resolution neutron backscattering instrument SPHERES. Using a Cauchy-Lorenz distribution, the quasi-elastic signal observed in the spectra obtained by the backscattering spectrometer was analyzed, leading to the identification of rotational motions with relaxation times of 0.3 ns. Additionally, neutron spin echo (NSE) spectroscopy was used to measure the water diffusion over the local network of pores. The motions observed in the NSE time scale were characterized by diffusion constants ranging from 0.6 to 1.1 x 10(-9) m(2) s(-1) most likely related to water molecules removed from the interface. In summary, our results indicate that the local diffusion observed in the gel pores of hardened cement pastes is on the order of that found in deeply supercooled water. Finally, the importance of the magnetic properties of cement pastes were discussed in relation to the observation of a quasi-elastic signal on the dried sample spectra measured using the time-of-flight spectrometer.

Raman and Neutron Scattering Study of Partially Deuterated L‐Alanine: Evidence of a Solid‐Solid Phase Transition

Raman and neutron experiments using specific isotope labeling were combined in order to study the dynamics and structure of L-alanine. Inelastic neutron and Raman scattering data of C(2)H(4)(ND(2))CO(2)D are discussed in relation to the doubling of the lattice parameter a observed by means of neutron powder diffraction in C(2)D(4) (NH(2))CO(2)H. The major changes accompanying the phase transition are found in the vibrational frequencies involving the torsional vibration tau(CO(2)(-)), which is clearly affected by the hydrogen bonds between the protons of the ammonium group and the oxygen atoms of the carboxylate group. At lower temperatures the rearrangement of identifiable hydrogen bonds induces changes in the bending vibration delta(ND(3)), confirming some orientational disorder.

Raman spectroscopy and inelastic neutron scattering study of crystalline L-valine

Information about the strength of the hydrogen bond interactions in crystalline L-valine was obtained by Raman and inelastic neutron scattering spectroscopies. Raman studies were carried out from room temperature up to 423 K in order to examine both the external and the internal vibrations in L-valine. The structure seems to be stable above room temperature since no indication of a phase transition or clear indication of reorientation of L-valine molecules was observed. From the inelastic neutron scattering measurements, made between 270 and 320 K, for fully hydrogenated and partially deuterated L-valine, a better description of the low frequency modes was possible. Moreover it was possible to establish that the phase transition at low temperatures is related to the activation of an infrared mode in the Raman spectra. Additionally the low temperature phase of L-valine was further characterized by means of specific heat measurements between 5 and 350 K.

Molecular flexibility and structural instabilities in crystalline l-methionine.

We have investigated the dynamics in polycrystalline samples of l-methionine related to the structural transition at about 307K by incoherent inelastic and quasielastic neutron scattering, X-ray powder diffraction as well as ab-initio calculations. l-Methionine is a sulfur amino acid which can be considered a derivative of alanine with the alanine R-group CH3 exchanged by CH3S(CH2)2. Using X-ray powder diffraction we have observed at ~190K an anomalous drop of the c-lattice parameter and an abrupt change of the β-monoclinic angle that could be correlated to the anomalies observed in previous specific heat measurements. Distinct changes in the quasielastic region of the neutron spectra are interpreted as being due to the onset and slowing-down of reorientational motions of the CH3-S group, are clearly distinguished above 130K in crystalline l-methionine. Large-amplitude motions observed at low frequencies are also activated above 275K, while other well-defined vibrations are damped. The ensemble of our results suggests that the crystalline structure of l-methionine is dynamically highly disordered above 275K, and such disorder can be linked to the flexibility of the molecular thiol-ether group.

Concrete and Cement Paste Studied by Quasi-Elastic Neutron Scattering

Abstract In a world where the effects of climate change on weather patterns is accepted as real and serious, the problem of decreasing the production of carbon dioxide is perceived as increasingly important. The cement industry produces 5–7% of the world’s carbon dioxide emission and its survival will depend on improvements in the production of concrete which will be both more durable and require less carbon dioxide per unit of manufacture than the currently produced concrete. The durability of concrete is related to its ability to limit fluid transmission and knowledge of how to reduce the rate at which water will be transmitted through cement paste is critical to improving durability. However, because of the complex chemical and physical nature of cement pastes, understanding water mobility is a great challenge. Many techniques are not applicable simply because they are not sensitive to the range of size from angstroms to microns and the extent of water interaction with the cement where water can either be chemically bound at hydroxyls or physically free in large pores. In this review paper, we present the most up to date results on the physical chemistry of the water/ cement paste interactions studied by quasi-elastic neutron scattering. These results bring new insight to the mobility of water in the gel pores, the small pores (radius less than 50Å) that control the rate of water transmission in the cement pastes from which high quality concrete will be made.

Decisions on the European Spallation Source.

To the Editor — Theodore Roosevelt in his dry and direct style of speaking once said, “In any moment of decision, the best thing you can do is the right thing, the next best thing is the wrong thing, and the worst thing you can do is nothing.” These are striking words to keep in mind when it comes to decisions on scientific investments with enormous impact over several decades. To comprehend the importance of bringing a large scientific infrastructure to life, one only needs to consider the benefits and discoveries that were brought about by other European scientific facilities. A striking example is of course the European particle research institution CERN, whose investigation into the understanding of matter and the origins of the Universe also led to the development of innovative technology such as the World Wide Web. A decision to establish another European scientific facility finally appears on the horizon, the long-awaited European Spallation Source (ESS), a next-generation, high-flux neutron source for Europe1. There are three candidate sites to host the ESS. What is needed now is a decision to build it. The key advantage of the ESS for scientific research is that it uses a highpower accelerator to propel protons at a target to produce well-defined pulses of neutrons. The peak flux of the neutron pulse is much higher than that achieved in reactor-based sources, such as the Institut Laue-Langevin (ILL) in Grenoble and the Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II) in Munich. Indeed, reactor sources have reached their limit in terms of the maximum neutron flux that can be delivered for science, and spallation sources are the only viable opportunity available to produce higher neutron fluxes. The high neutron fluxes and the advanced neutron methods available at the ESS will allow scientists to look deep inside the structure of materials with unrivalled spatial and temporal resolution. The scientific knowledge gained from research at the ESS will not only help us to satisfy human scientific curiosity but will also contribute to the solution of complex problems ranging from efficient energy production and storage to medical advances, and will unlock the secrets of new materials and novel states of matter. Europe can pride itself in having the most sophisticated and well-equipped neutron science community in the world, operating the world’s most powerful research reactors (ILL and FRM II) and the world’s most successful spallation source (ISIS in the UK), as well as several powerful and innovative national and regional facilities. However, the opportunities opened by megawatt spallation sources such as the proposed ESS have not failed to be noticed by the United States and Japan, and indeed these countries have now forged ahead of Europe. In Japan the new spallation source J-PARC is close to performing its first user experiments, while in the Oak Ridge National Laboratory, Tennessee, the Spallation Neutron Source (SNS) is operating and instrumenting its first target station and has began to construct a second.