Search

Intense sources of very cold neutrons (VCNs) would be beneficial for various neutron scattering techniques and low-energy particle physics experiments. Binary clathrate hydrates hosting deuterated tetrahydrofuran (THF-d) and dioxygen show promise as potential moderators for such sources due to a rich spectrum of localized low-energy excitations of the encaged guest molecules. In this article, we present a reliable manufacturing technique for such hydrates. Neutron diffraction data confirm their clathrate structure as type II (CS-II), determine their purity, and cage occupancy. Furthermore, we present data on the thermal expansivity of THF-d– and THF-d–O2clathrates, drawing attention to them as an interesting case study for the complex structure and dynamics of this class of material. [ABSTRACT FROM AUTHOR]

Clathrate hydrates are crystalline compounds consisting of water molecules forming cages (so-called "host") inside of which "guest" molecules are encapsulated depending on the thermodynamic conditions of formation (systems stable at low temperature and high pressure). These icelike systems are naturally abundant on Earth and are generally expected to exist on icy celestial bodies. Carbon monoxide hydrate might be considered an important component of the carbon cycle in the solar system since CO gas is one of the predominant forms of carbon. Intriguing fundamental properties have also been reported: the CO hydrate initially forms in the sI structure (kinetically favored) and transforms into the sII structure (thermodynamically stable). Understanding and predicting the gas hydrate structural stability then become essential. The aim of this work is, thereby, to study the structural and energetic properties of the CO hydrate using density functional theory (DFT) calculations together with neutron diffraction measurements. In addition to the comparison of DFT-derived structural properties with those from experimental neutron diffraction, the originality of this work lies in the DFT-derived energy calculations performed on a complete unit cell (sI and sII) and not only by considering guest molecules confined in an isolated water cage (as usually performed for extracting the binding energies). Interestingly, an excellent agreement (within less than 1% error) is found between the measured and DFT-derived unit cell parameters by considering the Perdew-Burke-Ernzerhof (denoted PBE) functional. Moreover, a strategy is proposed for evaluating the hydrate structural stability on the basis of potential energy analysis of the total nonbonding energies (i.e., binding energy and water substructure nonbonding energy). It is found that the sII structure is the thermodynamically stable hydrate phase. In addition, increasing the CO content in the large cages has a stabilizing effect on the sII structure, while it destabilizes the sI structure. Such findings are in agreement with the recent experimental results evidencing the structural metastability of the CO hydrate. [ABSTRACT FROM AUTHOR]

Digital Light Processing (DLP) allows the fast realization of 3D objects with high spatial resolution. However, DLP is limited to transparent resins, and therefore not well suited for printing electrically conductive materials. Manufacturing conductive materials will significantly broaden the spectrum of applications of the DLP technology. But conductive metals or carbon‐based fillers absorb and scatter light; inhibiting thereby photopolymerization, and lowering resolution. In this study, UV transparent liquid crystal graphene oxide (GO) is used as precursor for generating in situ conductive particles. The GO materials are added to a photopolymerizable resin via an original solvent exchange process. By contrast to earlier contributions, the absence of drying during the all process allows the GO material to be transferred as monolayers to limit UV scattering. The absence of UV scattering and absorption allows for fast and high‐resolution 3D printing. The chosen resin sustain high temperature to enable an in situ efficient thermal reduction of GO into reduced graphene oxide (rGO) that is electrically conductive. The rGO particles form percolated networks with conductivities up to 1.2 × 10−2 S m−1. The present method appears therefore as a way to reconcile the DLP technology with the manufacturing of 3D electrically conductive objects. [ABSTRACT FROM AUTHOR]

The self-consistent-charges density-functional tight-binding (SCC-DFTB) method is used to investigate the behavior of ammonium cyanate in the solid state under high pressure. The results have shown that the transformation of ammonium cyanate into urea is strongly exothermic.

Confocal Raman microspectrometry is used to study the C-Br stretching band for the gauche end-group conformation of the 1,8-dibromooctane guest molecules, allowing an assessment of possible changes in the conformational properties of the guest molecules as a consequence of the exchange process. The wider implications are discussed in relation to fundamental aspects of the molecular transport process in the crystalline solid.

Three electron reduced Mn12 complexes are examined with the help of inelastic neutron scattering (INS) spectroscopy. The extent of electron reduction and the axial anisotropy parameter of the cluster display an approximately linear relationship.

Our recent Communication suggested that ammonia in aqueous solution may preferentially destabilize large cages in methane clathrate hydrates. A Comment favored ammonia incorporation instead, but it did not accurately describe our proposed mechanism and relied primarily on studies conducted in different chemical systems and/or which used other preparation methods. [ABSTRACT FROM AUTHOR]

The inter-cage hopping in a type II clathrate hydrate with different numbers of H2 and D2 molecules, from 1 to 4 molecules per large cage, was studied using a classical molecular dynamics simulation at temperatures of 80 to 240 K. We present the results for the diffusion of these guest molecules (H2 or D2) at all of the different occupations and temperatures, and we also calculated the activation energy as the energy barrier for the diffusion using the Arrhenius equation. The average occupancy number over the simulation time showed that the structures with double and triple large-cage H2 occupancy appeared to be the most stable, while the small cages remained with only one guest molecule. A Markov model was also calculated based on the number of transitions between the different cage types. [ABSTRACT FROM AUTHOR]

The incorporation of ammonia inside methane clathrate hydrate is of great interest to the hydrate chemistry community. We investigated the phase behavior of methane clathrate formed from aqueous ammonia solution. Ammonia's presence decreases methane occupancy in the large cages, without definitive Raman spectroscopic evidence for its incorporation inside the structure. [ABSTRACT FROM AUTHOR]

NH4(C6H5)4B represents a prototypical system for understanding aromatic H bonds. In NH4(C6H5)4B an ammonium cation is trapped in an aromatic cage of four phenyl rings and each phenyl ring serves as a hydrogen bond acceptor for the ammonium ion as donor. Here the dynamical properties of the aromatic hydrogen bond in NH4(C6H5)4B were studied by quasielastic incoherent neutron scattering in a broad temperature range (20≤=T≤=350 K). We show that in the temperature range from 67 to 350 K the ammonium ions perform rotational jumps around C3 axes. The correlation time for this motion is the lifetime of the “transient” H bonds. It varies from 1.5 ps at T=350 K to 150 ps at T=67 K. The activation energy was found to be 3.14 kJ/mol, which means only 1.05 kJ/mol per single H bond for reorientations around the C3 symmetry axis of the ammonium group. This result shows that the ammonium ions have to overcome an exceptionally low barrier to rotate and thereby break their H bonds. In addition, at temperatures above 200 K local diffusive reorientational motions of the phenyl rings, probably caused by interaction with ammonium-group reorientations, were found within the experimental observation time window. At room temperature a reorientation angle of 8.4°±2° and a correlation time of 22±8 ps were determined for the latter. The aromatic H bonds are extremely short lived due to the low potential barriers allowing for molecular motions with a reorientational character of the donors. The alternating rupture and formation of H bonds causes very strong damping of the librational motion of the acceptors, making the transient H bond appear rather flexible. [ABSTRACT FROM AUTHOR]

In the perchloric acid clathrate hydrate HClO4·5.5H2O, the perchlorate anions are contained inside an aqueous host crystalline matrix, positively charged because of the presence of delocalized acidic protons. Our experimental results demonstrate that the microscopic mechanisms of proton conductivity in this system are effective on a time scale ranging from nanosecond to picosecond. In the present paper, we discuss more specifically on the relaxation processes occurring on a nanosecond time scale by combining high-resolution quasielastic neutron scattering and 1H pulse-field-gradient nuclear magnetic resonance experiments. The combination of these two techniques allows us to probe proton dynamics in both space and time domains. The existence of two types of proton dynamical processes has been identified. The slowest one is associated to long-range translational diffusion of protons between crystallographic oxygen sites and has been precisely characterized with a self-diffusion coefficient of 3.5×10-8 cm2/s at 220 K and an activation energy of 29.2±1.4 kJ/mol. The fastest dynamical process is due to water molecules’ reorientations occurring every 0.7 ns at 220 K with an activation energy of 17.4±1.5 kJ/mol. This powerful multitechnique approach provides important information required to understand the microscopic origin of proton transport in an ionic clathrate hydrate. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Confocal Raman microspectrometry is used for probing the kinetic aspects of the guest exchange process in a urea inclusion compound. The transport of the new pentadecane guest molecules along the tunnel has displayed a linear dependence on time, with the rate of the process in the region of 70-100 nm [s.sup.-1].

A series of quasielastic neutron scattering (QENS) experiments were carried out on cement pastes with water/cement ratio varying to investigate the interaction of water in the gel pores with the cement paste made from Portland cement. The results reveal that to a first approximation the QE response can be explained in terms of the diffusion of water within the gel pores within the paste and that QENS can be used to examine the interactions of the pore water to the pores structure in the paste.

A study is conducted to demonstrate that confocal Raman microspectrometry can be used successfully as an in situ probe of the transport process, yielding information on the spatial distribution of guest molecules and its time dependence. The focus is on the system in which the original and new guest molecules are 1,8-dibromoctane (DBO) and pentadecane (PD).