Dynamic X-ray Diffraction Studies Research Articles

A cell forin situdynamic X-ray diffraction studies: application to the dehydration of zeolite SrX

A new compact cell forin situsynchrotron X-ray diffraction studies of adsorption or catalysis reactions has been designed to enable high performance whilst minimizing difficulties in use. It includes a system for simple insertion and alignment of capillary samples and an original miniaturized furnace providing excellent temperature homogeneity along the sample (temperature gradient less than 1 K over a 6 mm zone at 530 K). The efficiency of this setup is illustrated byin situcharacterization of a zeolite (SrX) at four hydration levels.

Self-propagating high-temperature synthesis of a zirconium diboride–alumina composite: a dynamic X-ray diffraction study

A time-resolved X-ray diffraction study during the formation of a zirconium diboride (ZrB2)–alumina (Al2O3) composite from zirconium dioxide, Al and boron oxide by self-propagating high-temperature synthesis (SHS) has been carried out. During the formation of the composite, solid solutions of Al in ZrB2 form, with a slight change in the lattice parameters of ZrB2, during the SHS reaction and finally form a ZrB2–Al2O3 composite powder. Dendritic growth of ZrB2 was also observed during the synthesis.

Thermodynamic and Kinetic Study on Phase Behavior of Binary Mixtures of POP and PPO Forming Molecular Compound Systems

The phase behavior of PPO (sn-1,2-dipalmitoyl-sn-3-oleoylglycerol) and POP (sn-1,3-dipalmitoyl-sn-2-oleoylglycerol) binary mixture was examined using differential scanning calorimetry, conventional and synchrotron radiation X-ray diffraction (XRD), and highly pure samples. A molecular compound, βC, was formed at the 1:1 concentration ratio of PPO and POP, giving rise to two monotectic phases of PPO/compound and compound/POP in a juxtapositional way. βC has a long spacing value of 4.1 nm of double chain length structure. In the PPO/compound region, the DSC melting peak increased with increasing PPO concentration, whereas the DSC melting peak increased with increasing POP concentration in the compound/POP region. The melting point of βC was lowest at 31.2 °C. Time-resolved XRD study unveiled the formation of molecular compounds in metastable forms, αC and β‘C, having the same PPO/POP concentration ratio as βC. αC and β‘C exhibited monotectic phases with corresponding metastable forms of the pure components. In αC, two hexagonal packing XRD short spacing peaks were obtained for αC, corresponding to a differently packed hexagonal subcell of two leaflets. A structure model of the PPO/POP molecular compound is proposed, involving separation of palmitoyl chain leaflet and palmitoyl−oleoyl mixed-acid chain leaflet in the double chain length structure. This model justifies the structure of βC of SOS/SSO proposed by Engstrom (Engstrom, L. J. Fat Sci. Technol. 1992 94, 173−181), in which the saturated and unsaturated chains are packed in the same leaflet of the double chain length. The present work additionally proved that the formation of the molecular compound was present in the αC and β‘C metastable forms, as directly proved by dynamic X-ray diffraction study using synchrotron radiation.

Dynamic X-Ray Diffraction Study of an Unreduced Iron Oxide Catalyst in Fischer-Tropsch Synthesis

As a result of a previous in situ DXRD study of Fischer-Tropsch synthesis over an unsupported, reduced iron catalyst, a similar study has been undertaken with an unreduced iron oxide catalyst. At 543 K and with H2/CO = 3.2, the initial α-Fe2O3 is immediately reduced to magnetite (Fe3O4), which is in turn, slowly converted to χ-Fe2.5C via a shrinking core process. Catalytic activity appeared to be directly correlated with the formation of the χ-carbide and it is postulated that this is the catalytically active species. Significantly lower deactivation rates are observed with the unreduced iron catalyst, and it is concluded that this is due to the direct formation of χ-carbide in unreduced iron catalysts, which precludes the ϵ′-carbide → χ-carbide transformation and avoids the production of the nucleate carbon arising from that transformation.

Dynamic X-ray diffraction study of an unsupported iron catalyst in Fischer-Tropsch synthesis

In situ, Dynamic X-Ray Diffraction (DXRD) has been used to correlate changes in bulk catalyst composition with Fischer-Tropsch activity over unsupported iron catalysts. It was found that the specific iron carbide phase formed was a function of particle size and temperature, with ε′-Fe 2.2C formed over ξ-Fe 2.5C for smaller particle sizes and lower temperatures. While catalytic activity initially increased with the increase in carbide formation, deactivation was observed to be coincident with the ε′ → ξ iron carbide transformation. This and subsequent hydrogen etching experiments indicate that the carbon formed from this transformation may act as nucleation sites for subsequent carbon deposition via the Boudouard reaction. All of the data obtained in this study are consistent with the competition model of Fischer-Tropsch synthesis, but while the carbide model does not appear to be a viable mechanism, the carbide itself may act as the catalyst.

Static and dynamic deformation mechanisms of a high-density polyethylene having rownucleated crystalline texture of a-axis orientation as observed by orientation distribution function of crystallites

The rheo-optical properties of a high-density polyethylene having a row-nucleated crystalline texture of a-axis orientation were investigated in the machine direction (MD) and transverse direction (TD) of fabricating the specimen. The tensile complex dynamic compliance function J∗ displays two dispersions designated as the α 2 and α 2 mechanical retardation processes with activation energies of about 21 and 35 kcal mol −1, respectively, for both MD and TD specimens, and the complex dynamic stress-optical coefficient function M∗ exhibits a single discrete dispersion designated as the α optical retardation process with an activation energy of about 24 kcal mol −1 for both specimens over the same frequency and temperature ranges as those covered for the dynamic mechanical measurements. The real and imaginary components of M∗ converge from positive values to almost zero at the highest frequencies and lowest temperatures covered for both specimens, with the exception of the real component of the MD specimen which converges to a small but definitely negative value. The analysis of this anomalous behaviour of the MD specimen in terms of the optical retardation time spectrum has ascribed the negative convergence to a negative form birefringence that arises from a dynamic lamellar splaying or bending process responding in phase with the dynamic bulk stress. The dynamic X-ray diffraction studies have revealed the α optical dispersion and possibly the α mechanical dispersion to be mostly attributed to the crystal orientation dispersions involving intralamellar shearing processes, i.e. detwisting and tilting processes of the lamellar segments for the MD and TD specimens, respectively. The optical dispersion, if any, associated with the lamellar splaying or bending process may be expected at higher frequencies or lower temperatures than those covered here to assign the β mechanical dispersion to the interlamellar shearing process.

Dynamic X-ray Diffraction Studies for Assigning the Alpha and Beta Mechanical Dispersions of Spherulitic Poly-alpha-olefines

Dynamic X-ray Diffraction Studies for Assigning the Alpha and Beta Mechanical Dispersions of Spherulitic Poly-alpha-olefines

Repetitive propagation of action potentials destabilizes the structure of the myelin sheath. A dynamic x-ray diffraction study

Time courses of myelin lattice swelling in toad sciatic nerves preexposed to different treatments were determined by x-ray diffraction using a one-dimensional position-sensitive detector. In the nerves supramaximally stimulated for 1 h at 200 Hz, the subsequent process of myelin swelling occurred 45.0 +/- 7.3 min (n = 24) sooner than in resting controls. Sciatic nerves incubated for 1 h in a Ringer's solution deprived of divalent cations (Ca++ and Mg++) exhibited a kinetics of swelling similar to that shown by the stimulated nerves, that is, 52.5 +/- 14.2 min (n = 6) sooner than controls preincubated for the same time in normal Ringer's solution (with divalent cations). The fact that both pretreatments supramaximal stimulation and removal of divalent cations from the perfusion solution produced a similar effect; namely, a decrease of the myelin lattice stability against swelling in distilled water, suggests that the repetitive propagation of action potentials could modify the ionic composition at either the intraperiod channel or the paranodal axoglial junction complexes.

A dynamic x-ray diffraction study of anaesthesia action. Changes in myelin structure and electrical activity recorded simultaneously from frog sciatic nerves treated with [formula omitted

Changes induced in the structure and electrical activity of myelin were recorded simultaneously from frog sciatic nerves treated with n- alkanes . The results suggest that the effect of n- alkanes seems to be two-fold: (a) there is an initial reversible phase, in which a significant modification of the X-ray diffraction patterns, concomitant with the continuous fall of the action potential, is observed; (b) there is a final phase which is irreversible. This occurs some time after the complete abolition of the electrical activity. At this stage, further changes of the X-ray diffraction patterns are detected, the most significant of them being in the n- pentane-treated myelin, and consist of an increase in the membrane bilayer thickness.

A dynamic X-ray diffraction study of anesthesia action. Thickening of the myelin membrane by [formula omitted

The structural changes induced in the myelin sheath by n- pentane nerve impulse blockage were studied by small-angle X-ray diffraction using a linear position-sensitive detector. The results show that the thickness of the myelin period lattice increases from 170 to 180 Å during n- pentane treatment.