The temperature dependence of the dynamics of water inside microporous activated carbon fibers (ACF) is investigated by means of incoherent elastic and quasi- elastic 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 (sim 12 and 18 {AA}, 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 Dr and relaxation time <{tau}_0> of the restricted water on pore size and temperature. The observed Dr values are tested against a proposed scaling law, in which the translational diffusion coefficient Dr of water within a nanoporous matrix was found to depend solely on two single parameters, a temperature independent translational diffusion coefficient Dc associated with the water bound to the pore walls and the ratio {theta} 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.
When water molecules are confined to nanoscale spacings, such as in the nanometer size pores of activated carbon fiber (ACF), their freezing point gets suppressed down to very low temperatures ($sim$ 150 K), leading to a metastable liquid state with
remarkable physical properties. We have investigated the ambient pressure diffusive dynamics of water in microporous Kynoltexttrademark ACF-10 (average pore size $sim$11.6 {AA}, with primarily slit-like pores) from temperature $T=$ 280 K in its stable liquid state down to $T=$ 230 K into the metastable supercooled phase. The observed characteristic relaxation times and diffusion coefficients are found to be respectively higher and lower than those in bulk water, indicating a slowing down of the water mobility with decreasing temperature. The observed temperature-dependent average relaxation time $<tau>$ when compared to previous findings indicate that it is the size of the confining pores - not their shape - that primarily affects the dynamics of water for pore sizes larger than 10 {AA}. The experimental observations are compared to complementary molecular dynamics simulations of a model system, in which we studied the diffusion of water within the 11.6 {AA} gap of two parallel graphene sheets. We find generally a reasonable agreement between the observed and calculated relaxation times at the low momentum transfer $Q$ ($Qle 0.9$ AA${^{-1}}$). At high $Q$ however, where localized dynamics becomes relevant, this ideal system does not satisfactorily reproduce the measurements. The best agreement is obtained for the diffusion parameter $D$ associated with the hydrogen-site when a representative stretched exponential function, rather than the standard bi-modal exponential model, is used to parameterize the self-correlation function $I(Q,t)$.
We present neutron scattering measurements of the phonon-roton (P-R) mode of superfluid 4He confined in 47 AA MCM-41 at T = 0.5 K at wave vectors, Q, beyond the roton wave vector (Q_R =1.92 AA$^{-1}$). Measurements beyond the roton require access to
high wave vectors (up to Q = 4 AA$^{-1}$) with excellent energy resolution and high statistical precision. The present results show for the first time that at T = 0.5 K the P-R mode in MCM-41 extends out to wave-vector Q$simeq$3.6 AA$^-1$ with the same energy and zero width (within precision) as observed in bulk superfluid 4He. Layer modes in the roton region are also observed. Specifically, the P-R mode energy, $omega_Q$, increases with Q for Q > QR and reaches a plateau at a maximum energy Q = 2{Delta} where {Delta} is the roton energy, {Delta} = 0.74 $pm$ 0.01 meV in MCM-41. This upper limit means the P-R mode decays to two rotons when its energy exceeds 2{Delta}. It also means that the P-R mode does not decay to two layers modes. If the P-R could decay to two layer modes, $omega_Q$ would plateau at a lower energy, $omega_Q$ = 2{Delta}L where {Delta}L = 0.60 meV is the energy of the roton like minimum of the layer mode. The observation of the P-R mode with energy up to 2{Delta} shows that the P-R mode and the layer modes are independent modes with apparently little interaction between them.
We report the diffusion of water molecules confined in the pores of folded silica materials (FSM-12 with average pore diameter of $sim$ 16 AA), measured by means of quasielastic neutron scattering using the cold neutron chopper spectrometer (CNCS). T
he goal is to investigate the effect of electric field on the previously observed fast component of nano-confined water. The measurements were taken at temperatures between 220 K and 245 K, and at two electric field values, 0 kV/mm and 2 kV/mm. Similar to the recently observed electric field induced enhancement of the slow translational motion of confined water, there is a an equally important impact of the field on the faster diffusion.
We report a study of the effects of pressure on the diffusivity of water molecules confined in single- wall carbon nanotubes (SWNT) with average mean pore diameter of 16 Angstroms. The measurements were carried out using high-resolution neutron scatt
ering, over the temperature range 220 < T < 260 K, and at two pressure conditions: ambient and elevated pressure. The high pressure data were collected at constant volume on cooling, with P varying from 1.92 kbar at temperature T = 260 K to 1.85 kbar at T = 220 K. Analysis of the observed dynamic structure factor S(Q, E) reveals the presence of two relaxation processes, a faster diffusion component (FC) associated with the motion of caged or restricted molecules, and a slower component arising from the free water molecules diffusing within the SWNT matrix. While the temperature dependence of the slow relaxation time exhibits a Vogel-Fulcher-Tammann law and is non-Arrhenius in nature, the faster component follows an Arrhenius exponential law at both pressure conditions. The application of pressure remarkably slows down the overall molecular dynamics, in agreement with previous observations, but most notably affects the slow relaxation. The faster relaxation shows marginal or no change with pressure within the experimental conditions.
Magnetic correlations in the paramagnetic phase of CaFe2As2 (T_N=172 K) have been examined by means of inelastic neutron scattering from 180 K (~ 1.05 T_N) up to 300 K (~1.8 T_N). Despite the first-order nature of the magnetic ordering, strong but sh
ort-range antiferromagnetic (AFM) correlations are clearly observed. These correlations, which consist of quasi-elastic scattering centered at the wavevector Q_{AFM} of the low-temperature AFM structure, are observed up to the highest measured temperature of 300 K and at high energy transfer (E> 60 meV). The L dependence of the scattering implies rather weak interlayer coupling in the tetragonal c-direction corresponding to nearly two-dimensional fluctuations in the (ab) plane. The spin correlation lengths within the Fe layer are found to be anisotropic, consistent with underlying fluctuations of the AFM stripe structure. Similar to the cobalt doped superconducting BaFe2As2 compounds, these experimental features can be adequately reproduced by a scattering model that describes short-range anisotropic spin correlations with overdamped spin dynamics.
Neutron scattering measurements of the magnetic excitations in single crystals of antiferromagnetic CaFe2As2 reveal steeply dispersive and well-defined spin waves up to an energy of 100 meV. Magnetic excitations above 100 meV and up to the maximum en
ergy of 200 meV are however broader in energy and momentum than the experimental resolution. While the low energy modes can be fit to a Heisenberg model, the total spectrum cannot be described as arising from excitations of a local moment system. Ab-initio calculations of the dynamic magnetic susceptibility suggest that the high energy behavior is dominated by the damping of spin waves by particle-hole excitations.
