Suresh M Chathoth

Abstract: We used small-angle neutron scattering (SANS) and neutron contrast variation to study the structure of four nanoporous carbons prepared by thermo-chemical etching of titanium carbide TiC in chlorine at 300, 400, 600, and 800 °C with pore diameters ranging between �4 and �11 �. SANS patterns were obtained from dry samples and samples saturated with deuterium oxide (D2O) in order to delineate origin of the power law scattering in the low Q domain as well as to evaluate pore accessibility for D2O molecules. SANS cross section of all samples was fitted to Debye�Anderson�Brumberger (DAB), DAB�Kirste�Porod models as well as to the Guinier and modified Guinier formulae for cylindrical objects, which allowed for evaluating the radii of gyration as well as the radii and lengths of the pores under cylindrical shape approximation. SANS data from D2O-saturated samples indicate that strong upturn in the low Q limit usually observed in the scattering patterns from microporous carbon powders is due to the scattering from outer surface of the powder particles. Micropores are only partially filled with D2O molecules due to geometrical constraints and or partial hydrophobicity of the carbon matrix. Structural parameters of the dry carbons obtained using SANS are compared with the results of the gas sorption measurements and the values agree for carbide-derived carbons (CDCs) obtained at high chlorination temperatures (>600 °C). For lower chlorination temperatures, pore radii obtained from gas sorption overestimate the actual pore size as calculated from SANS for two reasons: inaccessible small pores are present and the model-dependent fitting based on density functional theory models assumes non-spherical pores, whereas SANS clearly indicates that the pore shape in microporous CDC obtained at low chlorination temperatures is nearly spherical.

Abstract: The microscopic diffusivity of methane (CH4) confined in nano-porous carbon aerogel was investigated as a function of added carbon dioxide (CO2) and nitrogen (N2) pressure using quasi-elastic neutron scattering (QENS). In the range of the external pressure of 1�2.5 MPa, the self-diffusivity of methane was found to increase with CO2 pressure and remain practically unchanged in the N2 environment. Increasing mobility of methane with CO2 pressure suggests that the adsorbed CH4 molecules become gradually replaced by CO2 on the surface of carbon aerogel pores, whereas the presence of N2 does not induce the replacement. The molecular mobility of the methane, with or without added carbon dioxide and nitrogen, is described by the unrestricted diffusion model, which is characteristic of methane compressed in small pores. On the other hand, both nitrogen and carbon dioxide molecules in carbon aerogel, when studied alone, with no methane present, follow a jump diffusion process, characteristic of the molecular mobility in the densified adsorbed layers on the surface of the aerogel pores.

Abstract: We report a quasielastic neutron scattering study in the temperature range of 290 to 350 K of a room temperature ionic liquid, [bmim+ ][Tf 2 N� ], in the bulk form and confined in the 8.8 ± 2.1 nm diameter pores of a mesoporous carbon matrix. In both bulk and confined liquids, our measurements, which are sensitive to the dynamics of the hydrogen-bearing cations, detect two distinct relaxation processes related to the diffusion of the cations. We have found that the cations that do not become immobilized near the pore walls exhibit an enhanced rather than suppressed diffusivity compared to the cation diffusivity in bulk liquid. Our results provide first experimental observation of molecular diffusion in a room temperature ionic liquid in confinement which is faster than diffusion in the bulk liquid.

Abstract: Quasielastic neutron scattering (QENS) has been used to investigate microscopic dynamics in the glass-forming Ni80P20, Pd40Ni40P20 and Pd43Ni10Cu27P20 melts. These melts are characterized by a high-packing fraction that is similar at their liquidus temperatures. Increasing the number of components in these melts increases the viscosity at their liquidus temperature. However, the fragility of these melts did not show a composition dependence. Atomic dynamics in these liquids agree well with mode-coupling theory (MCT) predictions. From the MCT analysis of the QENS data the critical packing fractions for the microscopic glass-transition (varphic) were obtained. The values obtained for varphic are well within the MCT theoretical predictions for hard-sphere liquids.

Abstract: Microscopic dynamics of water confined in nanometer and sub-nanometer pores of carbide-derived carbon (CDC) were investigated using quasielastic neutron scattering (QENS). The temperature dependence of the average relaxation time, langle�rangle, exhibits super-Arrhenius behavior that could be described by Vogel-Fulcher-Tammann (VFT) law in the range from 250 K to 190 K; below this temperature, langle�rangle follows Arrhenius temperature dependence. The temperature of the dynamic crossover between the two regimes in water confined in the CDC pores is similar to that observed for water in hydrophobic confinement of the larger size, such as 14 � ordered mesoporous carbon (CMK) and 16 � double-wall carbon nanotubes. Thus, the dynamical behavior of water remains qualitatively unchanged even in the very small hydrophobic pores.

Abstract: The diffusion of methane confined in nano-porous carbon aerogel with the average pore size 48 � and porosity �60% was investigated as a function of pressure at T = 298 K using quasi-elastic neutron scattering (QENS). The diffusivity of methane shows a clear effect of confinement: it is about two orders of magnitude lower than in bulk at the same thermodynamic conditions and is close to the diffusivity of liquid methane at 100 K (i.e. �90 K below the liquid�gas critical temperature TC � 191 K). The diffusion coefficient (D) of methane initially increases with pressure by a factor of �2.5 from 3.47 ± 0.41 � 10�10 m2 s�1 at 0.482 MPa to D = 8.55 ± 0.33 � 10�10 m2 s�1 at 2.75 MPa and starts to decrease at higher pressures. An explanation of the observed non-monotonic behavior of the diffusivity in the confined fluid is based on the results of small-angle neutron scattering experiments of the phase behavior of methane in a similar carbon aerogel sample. The initial increase of the diffusion coefficient with pressure is explained as due to progressive filling of bigger pores in which molecular mobility in the internal pore volume is less affected by the sluggish liquid-like molecular mobility in the adsorbed phase. Subsequent decrease of D, is associated with the effect of intermolecular collisions, which result in a lower total molecular mobility with pressure, as in the bulk state. The results are compared with the available QENS data on the methane diffusivity in zeolites, metal organic frameworks, and porous silica as well as with the molecular dynamics simulations of methane in nano-porous carbons and silica zeolites.

Abstract: Quasielastic neutron scattering has been used to investigate atomic motion in a very fragile binary metallic melt and a multicomponent bulk glass-forming metallic melt. Both melts show a breakdown of the Stokes�Einstein relation and display a change in the slope of In�D dependence on In(η/T). We also observed that the values for the exponent in the fractional Stokes�Einstein relation are not in the commonly observed range for Cu46Zr42Al7Y5 melts. At low temperatures, the deviation from the Stokes�Einstein law is very significant and can be expressed in the form of a power law with exponent ξ = �1.82±0.08. The change in the slope is found to be associated with a change in friction coefficient while increasing the packing density of the melt. The abrupt change in the value of friction coefficient is independent of packing density, but it occurs at a common value of ζ = (3.2±0.1)�10�12�kg�s�1 in these melts.

Abstract: We report unexpectedly strong variations in the relaxational dynamics in a glass-forming metallic melt while microalloying. Analysis of quasielastic neutron scattering data revealed that changes in the values of stretching of the self-correlation function and the temperature dependence of self-diffusivity showed an Arrhenius to non-Arrhenius transition. The intermediate structure of the melts did not show any prepeak or indication of short range order. These observations are correlated with the enhanced glass-forming ability of metallic melts on microalloying.

Abstract: We have investigated thermal expansion and surface tension of highly doped Si and Ge melts under microgravity conditions. The experiments were conducted in the TEMPUS facility on-board of a Zero-G aircraft. The thermophysical properties were quantified by analyzing the images of levitated droplets. Both Si and Ge are metallic in their solid state at the doping (P and Sb) of 1�1019�atoms�cm�3. However, thermal expansion and surface tension of highly doped Si and Ge melts did not show significant changes in comparison with undoped samples.

Abstract: We have investigated microscopic dynamics in the bulk glass-forming Cu46Zr42Al7Y5 melts using quasielastic neutron scattering (QENS). Self-correlation functions show a fast β and a slow α-relaxation process. α relaxation exhibits stretching and is independent of momentum transfer and temperature. The melt is characterized by low packing density and high viscosity. The dynamics observed and the critical packing density derived from the QENS data are in good agreement with mode-coupling theory predictions for hard-sphere liquids. However, we can show that viscosity dominates over packing density in determining the atomic dynamics and glass-forming ability of the metallic melt.

Abstract: Liquid germanium exhibits a change in the bonding character from being more covalent to more metallic while heating. We used quasielastic neutron scattering to measure the absolute value of self-diffusion coefficients in this liquid. Compared to other monoatomic liquids, such as liquid Ni or Ti, the self-diffusivity is an order faster near the melting temperature and shows a non-Arrhenius-like behavior. Above 1325 K, the activation energy for self-diffusion is low and obeys Stokes�Einstein relation. Even though the packing density of liquid germanium is less than that of simple metallic melts such as Pb or Sn, the temperature dependence of self-diffusivity does not exhibit D�Tn(n � 2) form, which is observed for uncorrelated binary collisions of hard-spheres.

Abstract: Self diffusion of Ni and Ce in Al-Ni-Ce and Al-Ni-La melts has been investigated by quasi-elastic neutron scattering and by the long-capillary technique showing comparable results for both methods. The investigated temperature range reached from the liquidus temperature up to 1795 K. Self diffusion of Ni in Al-Ni-(rare earth element) melts is slower than in binary Al-Ni melts and scales with the amount of the rare earth element. Diffusion coefficients calculated with help of the Stokes-Einstein equation from experimental viscosity data from literature support the diffusion coef-ficient measurements only in the case of Ce additions. For La additions the calculated diffusion coefficients increase.

Abstract: We present experimental results on the fast β and slow α relaxational dynamics observed through incoherent quasielastic neutron scattering from Pr60Ni10Cu20Al10 melts. The density correlation function, measured over a range of temperatures, shows a clear two-step relaxation process. The critical or crossover temperature, Tc, evaluated from the asymptotic scaling function of the mode-coupling theory was found to be at 620±5�K. The correlation decay is not exponential, being extended over a far wider time range. In addition, the stretched exponent βq was found to be independent of temperature and momentum transfer (q). The self-diffusivity is an order lower compared with that observed in simple metallic liquids at their melting temperature.

Abstract: In the liquid state, glass-forming Ni59.5Nb40.5 and Ni60Nb34.8Sn5.2 alloys exhibit an extraordinarily high packing fraction. The self-correlation functions measured using quasielastic neutron scattering clearly show the slowing down of microscopic dynamics with an increase in packing fraction. The self-diffusivity in liquid Ni60Nb34.8Sn5.2 decreases by about 2 orders of magnitude within a temperature range of 360 K. For these highly fragile systems, the critical packing fraction obtained form the analysis of incoherent data is in excellent agreement with the prediction made by mode-coupling theory. Our results provide the first experimentally observed value for the critical packing fraction in glass-forming metallic liquids.

Abstract: We have investigated density, thermal expansion, and surface tension of Si, Ge, and Si�Ge alloys in the melt and undercooled state under microgravity conditions. The experiments were conducted in the TEMPUS facility on board a Zero-G aircraft. The density of the liquid alloys as a function of composition show a nonideal behavior. Thermal expansion coefficients were found to be in the order of 10�4�K�1 and was highest for Si75Ge25 melt. The surface tension is lowered with the addition of 25�at.�% Si in Ge. The further addition of Si increases the surface tension almost linearly with composition.

Abstract: Properties of mass transport in liquid Al80Cu20 were measured over a broad temperature range of more than 500 K by means of oscillating cup viscometry and quasielastic neutron scattering. The shear viscosity and the coefficient of the Cu self-diffusion exhibit an Arrhenius-type temperature dependence. The activation energy for the viscous flow is 2.4 times smaller than that of the Cu self-diffusion. Below 1400 K, the Cu self-diffusion becomes increasingly smaller than expected from the viscosity data rescaled via the Stokes�Einstein relation.

Abstract: We use inelastic neutron scattering and molecular dynamics simulation to investigate the chemical short-range order (CSRO), visible through prepeaks in the structure factors, and its relation to self-diffusion in Al�Ni melts. As a function of composition at 1795�K, Ni self-diffusion coefficients from experiment and simulation exhibit a nonlinear dependence with a pronounced increase on the Al-rich side. This comes along with a change in CSRO with increasing Al content that is related to a more dense packing of the atoms in Ni-rich Al�Ni systems.

Abstract: We investigated the self-diffusion of Ni in liquid Ni, Ni80P20, Pd40Ni40P20, and Pd43Ni10Cu27P20 at temperatures up to 1795�K with incoherent, quasielastic neutron scattering. Values of measured self-diffusion coefficients vary over the accessible temperature ranges as a function of composition only within 10%. Although mixing has a drastic effect on the liquidus temperature and the undercooling capabilities, a relation between these properties and the atomic diffusion in the liquid is not observed. Apparently, diffusive motion is governed by the packing fraction of the atoms, that is very similar in these dense liquids.