Probing crystallisation of a fluoro-apatite - mullite system using neutron diffraction

Real-time small angle neutron scattering and wide angle neutron scattering studies were undertaken concurrently on a glass ionomer of nominal composition 4.5(SiO2)-3(Al2O3)-1.5(P2O5)-3(CaO)-2(CaF2). Neutron studies were conducted as a function of temperature to investigate the crystallisation process. No amorphous phase separation was observed at room temperature and the onset of crystallisation was found to occur at 650°C, which is 90°C lower than previously reported. The first crystalline phase observed corresponded to fluorapatite; it was only upon further heating was the mullite phase became present. The crystallite size at 650°C was found to be ~115Å and the result was consistent across all measurements

Structure and purity of single-walled carbon nanotubes

International audiencePulsed neutron diffraction has been used to characterize the microscopic structure and purity of single walled carbon nanotubes samples produced by arc discharge. We employed a time of flight diffractometer whose performance in measuring the microscopic structural properties of light-mass materials is well known and recognized. The extended Q-range of the instrument allows for a direct inversion of the data to determine the radial distribution function of the carbon atoms. This is compared with the corresponding function produced by computer simulation. In addition, the absolute calibration of the neutron diffraction data evidences anomalies in the diffraction spectra of the carbon nanotubes, especially at the level of the total scattering section, that could not be observed in previous neutron scattering experiments. These are attributed to the presence of a substantial amount of spurious carbonaceous material that was not quantitatively detected with more conventional diagnostic techniques

The temperature dependent structure of liquid 1-propanol as studied by neutron diffraction and EPSR simulations

The structure of liquid 1-propanol is investigated as a function of temperature using neutron diffrac- tion together with Empirical Potential Structure Refinement modelling. The combined diffraction and computer modelling analysis demonstrates that propanol molecules form hydrogen bonded clusters with a relatively wide size distribution, which broadens at lower temperatures. We find that the clus- ter size distribution is well described by a recently proposed statistical model for branched H-bonded networks [P. Sillr\ue9n, J. Bielecki, J. Mattsson, L. B\uf6rjesson, and A. Matic, J. Chem. Phys. 136, 094514 (2012)]. The average cluster size increases from ∼3 to 7 molecules, whilst the standard deviation of the size distribution increases from 3.3 to 8.5 as the temperature is decreased from 293 to 155 K. The clusters are slightly branched, with a higher degree of branching towards lower temperatures. An analysis of the cluster gyration tensor (Rmn) reveals an average elongated ellipsoidal shape with axes having proportions 1:1.4:1.9. We find that the average radius of gyration has a cluster size dependence consistent with that of fractal clusters, Rg ∝ n1/D , with a fractal dimension D ≈ 2.20, which is close to D = 2.00 expected for an ideal random walk or D = 2.11 expected for reaction limited aggregation. The characteristic angles between the H-bonded OH-groups that constitute the clusters show only a weak temperature dependence with O–H\ub7 \ub7 \ub7O angles becoming more narrowly distributed around 180◦ at lower temperatures

The temperature dependent structure of liquid 1-propanol as studied by neutron diffraction and EPSR simulations

The structure of liquid 1-propanol is investigated as a function of temperature using neutron diffrac- tion together with Empirical Potential Structure Refinement modelling. The combined diffraction and computer modelling analysis demonstrates that propanol molecules form hydrogen bonded clusters with a relatively wide size distribution, which broadens at lower temperatures. We find that the clus- ter size distribution is well described by a recently proposed statistical model for branched H-bonded networks [P. Sillr\ue9n, J. Bielecki, J. Mattsson, L. B\uf6rjesson, and A. Matic, J. Chem. Phys. 136, 094514 (2012)]. The average cluster size increases from ∼3 to 7 molecules, whilst the standard deviation of the size distribution increases from 3.3 to 8.5 as the temperature is decreased from 293 to 155 K. The clusters are slightly branched, with a higher degree of branching towards lower temperatures. An analysis of the cluster gyration tensor (Rmn) reveals an average elongated ellipsoidal shape with axes having proportions 1:1.4:1.9. We find that the average radius of gyration has a cluster size dependence consistent with that of fractal clusters, Rg ∝ n1/D , with a fractal dimension D ≈ 2.20, which is close to D = 2.00 expected for an ideal random walk or D = 2.11 expected for reaction limited aggregation. The characteristic angles between the H-bonded OH-groups that constitute the clusters show only a weak temperature dependence with O–H\ub7 \ub7 \ub7O angles becoming more narrowly distributed around 180◦ at lower temperatures

Modeling the structure of complex aluminosilicate glasses: the effect of zinc addition

An empirical potential structure refinement of neutron and X-ray diffraction data combined with extended absorption fine structure evidence has been applied to the investigation of two distinct sets of complex aluminosilicate glasses containing different quantities of zinc. Data come from (i) neutron and X-ray total scattering experiments, which have been performed at the ISIS neutron spallation source (SANDALS beamline) and at the European Synchrotron Radiation Facility (ID11 beamline), and (ii) EXAFS experiments which have been performed at the European Synchrotron Radiation Facility (BM23 beamline). By careful examination of the modeled ensemble of atoms, a wide range of structural information has been extracted: coordination numbers, bond distances, cluster sizes, type of oxygen sharing, and the preference of large cations to adopt a charge-compensating role. The first series of glasses, which is characterized by a fixed network modifier element content (i.e., Na), shows how the introduction of Zn at the expense of Si and Al network forming elements does not significantly alter the polymerization degree, as a result of its dominant 4-fold coordination. In the case of the second series, which is characterized by fixed network forming element content (i.e., Si and Al), it is shown how the replacement of a network modifier element (i.e., Ca) with the introduction of Zn does not change the propensity of Zn to be mainly 4-fold coordinated by promoting the network. Where appropriate the experimental results have been compared with classical theoretical approaches such as stoichiometric models based on Zachariasen's rules and computational routines