Quasielastic Neutron Scattering Technique Research Articles

Dynamics of polymer electrolyte with LiTFSI via Quasi-Elastic Neutron Scattering

Most lithium batteries offer a wide range of applications. However, safety issues are still an unresolved issue for several applications. To solve the safety issue of Li-ion batteries, solid polymer electrolyte is a promising candidate to replace commercial liquid electrolyte. A 4-arm star poly(ethylene oxide) polymer with LiTFSI salt as an electrolyte was studied. The dynamics of this polymer were explored with the Quasi-Elastic Neutron Scattering technique. Furthermore, the influence of temperature and Li salt concentration on the polymer dynamics was investigated. The dynamics of the polymer ends of the arm show much higher flexibility than the core parts making those types of polymers attractive for further studies in battery research.

The charge transport mechanism in Brønsted-acidic protic ionic liquid/water systems – An NMR and QENS study

In this study, a protic ionic liquid (PIL), 2-Sulfoethylmethylammonium triflate [2-Sema][TfO] is considered as a potential new proton conducting electrolyte for future polymer membrane fuel cells capable of ambient air operation above 100 °C. The proton dynamics of the PIL with residual water are examined as a function of the hydration level on different time scales using pulsed field gradient nuclear magnetic resonance (PFG-NMR) and quasi-elastic neutron scattering (QENS). The separation of the different contributing relaxation processes enables a quantification of the proton fractions for the underlying hopping or vehicular motions. The hopping motion of the water in the time scale of picosecond and the vehicular motion in the time scale of nanosecond are detected by means of QENS. Such dynamic processes can be well described by the Chudley-Elliot jump model. This emphasised the presence of fixed jump lenghts. In the timescale of millisecond, the cooperative transport of the active protons of the acidic SO3H group and of the H2O molecules, as well as the vehicular transport of the PIL cations are detected by NMR. The different diffusion coefficients obtained by the NMR and QENS techniques are discussed in detail.

Study on the dynamics of a vanadium doped LiFePO4 lithium‐ion battery using quasi‐elastic neutron scattering technique

AbstractThe investigation into the lithium diffusion in active electrode materials is especially interesting since it is directly connected with electrochemical performance of lithium‐ion battery. Quasi‐elastic neutron scattering (QENS) is a powerful experimental tool to study lithium diffusion dynamics. Here, Li0.99V0.01FePO4 (LFPV) cathode material is investigated using QENS technique. The QENS data showed that lithium diffusion can be well described in LFPV using a jump diffusion model. The Li self‐diffusion coefficient in LFPV material was estimated to be ~3 × 10−7 cm2/s at T = 600 K. This result is about 108 times faster than previous reports using electrochemical measurements, but similar to the molecular dynamic simulation. The slower Li ion diffusion might be due to the barrier at two phase boundary, grain boundary, and solid‐electrolyte interface.

Investigation of activation kinetics in geopolymer paste using quasielastic neutron scattering

Quasielastic neutron scattering (QENS) has been used to investigate the binding process of water molecules in pastes of calcium geopolymer prepared with low and high calcium fly ash contents, and at two different NaOH molarities, 10 and 14M. The in situ measurements were carried at ambient and elevated curing temperatures (60°C). By carefully monitoring the time evolution of the elastic peak intensity, we infer a gelation process, followed by polymerization and a hardening in the high calcium geopolymer paste at 60°C, in agreement with previously proposed geopolymerization model. This behavior was neither observed at ambient temperature in both low and high calcium geopolymer cement paste, within the precision of the neutron instrument. Our study clearly shows that a minimal amount of heat is necessary to form gelation and polymerization during the activation process. The activation of geopolymer paste with high NaOH molarity involves more chemically bound water molecules than that at lower activator concentration. This work shows that the QENS technique can be effectively used to characterize the alkali-activation kinetics in certain geopolymer pastes, and that temperature and molarity of the activator play a vital role in controlling the gel mechanism.

Temperature-Dependent Dynamics of Polyalkylthiophene Conjugated Polymers: A Combined Neutron Scattering and Simulation Study

The dynamics of conjugated polymers are known to influence the performance of optoelectronic devices. Polyalkylthiophenes are a widely studied class of conjugated polymers, which exhibit a glass transition around room temperature and consequently are sensitive to temperature variations. We studied the dynamics of two polyalkylthiophenes of different side chain lengths (hexyl and octyl) as a function of temperature, by comparing their quasi-elastic neutron scattering (QENS) with molecular dynamics simulations (MD). We found a good agreement between the simulated and experimental data within the explored time window (of ∼4 ns), demonstrating that the force fields used in MD simulations are appropriate and that the QENS technique can be used as a validation of such force fields. Using MD allows us to identify and to assign contributions to the QENS signal from different parts of the polymers and to determine the activation energies of the different motions.

Pore-size dependence and characteristics of water diffusion in slitlike micropores.

The temperature dependence of the dynamics of water inside microporous activated carbon fibers (ACF) is investigated by means of incoherent elastic and quasielastic neutron-scattering techniques. The aim is to evaluate the effect of increasing pore size on the water dynamics in these primarily hydrophobic slit-shaped channels. Using two different micropore sizes (∼12 and 18 Å, denoted, respectively, ACF-10 and ACF-20), a clear suppression of the mobility of the water molecules is observed as the pore gap or temperature decreases. This suppression is accompanied by a systematic dependence of the average translational diffusion coefficient D(r) and relaxation time 〈τ(0)〉 of the restricted water on pore size and temperature. The observed D(r) values are tested against a proposed scaling law, in which the translational diffusion coefficient D(r) of water within a porous matrix was found to depend solely on two single parameters, a temperature-independent translational diffusion coefficient D(c) associated with the water bound to the pore walls and the ratio θ of this strictly confined water to the total water inside the pore, yielding unique characteristic parameters for water transport in these carbon channels across the investigated temperature range.

The application of quasi-elastic neutron scattering techniques (QENS) in surface diffusion studies

Neutron scattering techniques such as quasi-elastic neutron scattering, QENS, have proven to be well-suited tools for studying structure and dynamics of surface adsorbed molecules. In contrast to many more widely used surface science techniques neutron scattering allows the microscopic characterization of samples under a wide range of thermodynamic conditions, as the samples are not constrained to ultra high vacuum environment. Moreover, neutron scattering allows the separation of coherent and incoherent scattering, giving access to different diffusive mechanisms such as single particle diffusion, mass transport, rotations, or vibrations. In this paper we will review recent progress and the state-of-the-art in neutron scattering experiments on surface adsorbed molecules in the sub-monolayer coverage range with a specific emphasis on studies of carbon and other high surface density substrates. We will also cover recent progress in theoretical modeling, since the usefulness of neutron scattering data on surface dynamics can be strongly enhanced by computational modeling, such as molecular dynamics (MD) simulations and the development of analytical models.

Molecular motion in restricted geometries

Molecular dynamics in restricted geometries is known to exhibit anomalous behaviour. Diffusion, translational or rotational, of molecules is altered significantly on confinement in restricted geometries. Quasielastic neutron scattering (QENS) offers a unique possibility of studying molecular motion in such systems. Both time scales involved in the motion and the geometry of motion can be studied using QENS. Molecular dynamics (MD) simulation not only provides insight into the details of the different types of motion possible but also does not suffer limitations of the experimental set-up. Here we report the effect of confinement on molecular dynamics in various restricted geometries as studied by QENS and MD simulations. An example where the QENS technique provided direct evidence of phase transition associated with change in the dynamical behaviour of the molecules is also discussed.

QENS 2002 held in Potsdam

QENS 2002, organized from September 4–7, 2002 in Potsdam by Ruep Lechner, Amaud Desmedt, Thomas Gutberlet, and Jorg Pieper of Hahn-Meitner-Institut Berlin, was the sixth meeting of an international conference series on dynamic structure research of complex systems using quasielastic neutron scattering (QENS). QENS 2002 focused on disordered condensed matter, including plastic and liquid crystals, ionic conductors, inclusion compounds, glasses, polymers, liquids, melts, solutions, micro-emulsions, micelles, artificial and biological membranes, proteins, and DNA. An effort was made to bring together results, not only from QENS techniques, but also from light scattering, NMR, dielectric spectroscopy, as well as Monte Carlo and molecular dynamics (MD) computer simulation, whenever such results could be related with those from QENS experiments.

Hydration water in macromolecules of biological interest: An incoherent quasi elastic neutron scattering

We report a detailed dynamical study on a hydrated protein, Lysozyme comparing its dynamical features with those obtained in the case of aqueous solutions whose solute is a synthetic polymer, Poly(ethylene glycol), PEG600. The noteworthy feature of this kind of analysis is that it is yet reasonable to apply the same theoretical model to the experimental data, even if the complexity of the solute molecules, in passing from the polymer to the protein, increases dramatically. The experimental measurements performed by Incoherent Quasi Elastic Neutron Scattering technique (IQENS) were carried out by using hydration conditions in H 2O and D 2O in order to clearly evaluate the different contributions of the protein/polymer matrix from that of hydration water. A clear slowing down in the dynamics of the latter with respect to the bulk water is revealed in both the cases.

The quantum dynamics of proton transfer in the hydrogen bond

AbstractDouble proton transfer in the hydrogen bonds of carboxylic acid dimers has been investigated via measurements of the dispersion of the proton spin‐lattice relaxation time, T1, using magnetic field‐cycling NMR. A plot of the spin‐lattice relaxation rate, T1−1, as a function of magnetic field, provides a direct measurement of the motional spectrum and hence of the proton transfer rate. In the materials studied, these NMR experiments are optimised for the low temperature region where the dynamics are dominated by incoherent quantum tunnelling. Measurements of the proton transfer rate at higher temperatures have been made using quasi‐elastic neutron scattering (QENS) and these facilitate the characterisation of the transition between quantum and classical dynamics in the intermediate temperature region. The role of tunnelling in the excited vibrational states of the double minimum potential (DMP) which characterises the system is revealed in these measurements and has been analysed according to the behaviour of a particle in a DMP which is coupled to a bath of phonons. The complementarity between the NMR and QENS techniques is emphasised and discussed, and the dynamic range accessible to the field‐cycling NMR technique is explored through studying a range of related materials.

Rotational diffusion of methane molecules adsorbed on MgO(100).

Rotational diffusion in the methane monolayer adsorbed on MgO(100) is studied using the quasielastic neutron-scattering technique, between 20 and 50 K. The CH 4 molecules execute isotropic rotations above 40 K. At 20 and 30 K, the rotations might be intermediate between the free-rotor case and rotations around the twofold axis perpendicular to the surface in the dipod-down configuration. The rotational activation energy deduced from the temperature dependence of the diffusion coefficient is 2.7±0.3 meV

NMR relaxation study of tetrapentylammonium ions in acetonitrile solutions

NMR measurements of the spin-lattice relaxation rates of the 'H, 13C, and 14N nuclei and spin-echo self-diffusion coefficients are reported for tetrapentylammonium bromide in acetonitrile solutions Over a wide range of temperatures and concentrations. The results are discussed in terms of overall and segmental motions of the aliphatic chains of the organic ions. Order parameters characterizing the geometrical constraints upon local segmental motions are given also. Cationic self-diffusion coefficients are found to differ from those obtained with quasi-elastic neutron scattering techniques (QENS). The difference may be explained in terms of fast and slow modes contributing to the diffusion processes and their selective detection with QENS and NMR techniques.

The dynamics of water molecules in ionic solution. I. The application of quasi-elastic neutron scattering to the study of translational diffusive proton motion

The use of quasi-elastic neutron scattering (QENS) as a probe for the translational diffusive motion of water protons in aqueous ionic solutions is considered. A scattering law is given that takes into account the exchange of water molecules between the primary hydration shell of an ion and the remaining water. The restrictions on the experimental parameters are then discussed and the choice of a back-scattering spectrometer with energy resolution of around 1 mu eV is justified. The effect that a typical instrumental resolution function and multiple scattering of neutrons in an aqueous solution have on the shape of the derived scattering law is described, and the sensitivity of the technique to the magnitude of the cation-proton binding time tau 1 is examined. It is shown that the QENS technique will enable the proton population to be observed in 'fast exchange' if tau 1 or approximately=5*10-9 s.

Recent results in the investigation of dynamical disorder in plastic crystals by incoherent quasielastic neutron scattering

The Incoherent Quasielastic Neutron Scattering technique is a powerful tool for the study of the dynamical orientational disorder in molecular crystals. It is illustrated by a series of recent experiments. Each case provides an example of analysis on the basis of a particular model. First, usual models based on isotropic rotational diffusion or on reorientations by jumps about one or two axes are illustrated. Then, with octaphenylcylotetrasiloxane, we show how the same molecular motion can appear under different aspects, depending on the time scale of the experiment. We describe a model corresponding to a uniaxial whole-molecule rotation about one axis together with large-amplitude oscillations. Finally, an example is given where a low-frequency vibrational excitation significantly contributes to the quasielastic scattered intensity.