Low Degree Of Orientation Research Articles

Optical characterization of AlN films grown by plasma source molecular beam epitaxy

Thin films of AlN were grown on Si(111), sapphire (11̄02), and sapphire (0001) substrates by plasma source molecular beam epitaxy (PSMBE). Optical transmission spectra of PSMBE grown AlN/sapphire show neither significant band edge distortions nor deep level absorption in the spectral range 0.2–6.5 μm. Furthermore, there is no evidence of deep levels in the spectra of AlN/Si in the range 2.5–25 μm. The optical phonon modes of AlN/Si films were directly observed in infrared transmission and 80° angle of incidence reflectance spectra. Longitudinal optical (900 cm−1) and transverse optical (680 cm−1) modes are shifted toward each other comparable to previously reported bulk values. Optical spectra of thick textured film samples with low degree of orientation have phonon energies close to bulk values (probably due to strain relaxation).

Transfer of excitation energy in a three-dimensional-doped molecular crystal. V. Self-consistency of the temporal processes involved in energy transfer in photosynthetic units

Numerical experiments were carried out to determine the timewise self-consistency of different physical processes involved in the energy transfer in green plant photosynthetic units. A 6 × 6 × 6 array of chlorophyll-a with cubic lattice constants a = b = c = 20 A was chosen as a model of the thylakoid disc. Another model aggregate was obtained by substituting chlorophyll-b molecules for some of the chlorophyll-a molecules. In both models, a reaction center occupied a central site in the last xy plane. Two extreme arrangements were considered for the orientation of molecules. In one, the transition moments of all molecules were directed along the y-axis. The other had chlorophyll molecules randomly oriented. The four resulting model systems were used in our investigation on exciton generation, transport, decay by fluorescence, and trapping. All excitons were assumed to be generated by a 20 ms exposure to sunlight at high altitudes. The general trends noticed from these computations are as follows: The number of excitons generated is influenced by lattice disorders. Disorders also increase the time for the establishment of an equilibrium distribution. The decay of excitons by fluorescence is always a monotonic function of time. The energy transfer is adversely affected by a lower degree of orientation in the crystal: The trapping time increases with disorder. The number of trappings decreases with the onset of fluorescence of the host molecules and the trap. From these investigations, we also made three specific observations: (1) The efficiency of exciton utilization varies from 12% for a completely random arrangement of transition dipoles to 46% for a perfectly ordered arrangement. This agrees with the experimental efficiency, about 20%. (2) The number of excitons trapped varies from one to six. This tallies with the time scale of electron transfer along the Z-scheme that requires at least two excitons trapped in about 20 ms. Thus, the photon density and the exciton transfer rate are consistent with the rates of electron transfers. (3) The trapping rate also indicates that the thylakoid disc must resemble a considerably ordered system. © 1996 John Wiley & Sons, Inc.

Distribution of energy absorbed by purple bacteria between various paths of de-excitation

Polarized absorption, photoacoustic and fluorescence spectra of cells of the purple bacterium Rhodospirillum rubrum, embedded in isotropic and stretched polyvinyl alcohol films, were measured at 296 and 85 K. The existence of pools of bacteriochlorophyll molecules, characterized by various absorption and emission maxima and different orientations of their transition moments vdth respect to the membrane plane, was shown. The polarized spectra suggest that in addition to the light-harvesting LH1 B875 complexes, other antenna complexes (B806 and complexes absorbing in the long-wavelength region) are present in Rhodospirillum rubrum. The ratio between the yield of fluorescence polarized parallel and perpendicular to the direction of the electric vector of the exciting light (376 nm) and the film axis is about 1.8 for bacteriochlorophyll emission at 892 nm and about 0.14 for that at 928 nm. In the parallel component, 28% of the energy absorbed in the near-IR bands of bacteriochlorophyll is converted into heat and 72% is used for fluorescence emission and photochemistry, whereas in the perpendicular component, the corresponding values are about 60% and 40% respectively. We conclude that the mutually parallel bacteriochlorophyll antenna chromophores lose less energy by thermal dissipation than chromophores with a lower degree of orientation. This suggests a very efficient trapping in the reaction centres of that part of the excitation energy which migrates between chromophores having mutually parallel transition moments.

Conformational changes of amorphous poly(ethylene terephthalate) films during uniaxial stretching

AbstractRapid scanning Fourier transform infra‐red spectroscopy was used to follow the changes in the population of trans conformers and in the infra‐red dichroic functions of the bands characterizing the trans (T) and gauche (G) conformers and the chains in the amorphous regions during stretching of a solution‐cast amorphous poly(ethylene terephthalate) (PET) film A in the glassy state (67°C) and in the rubbery state (88°C). Stretching at 67°C showed a yield stress peak followed by strain softening in the stress‐strain curve. The population of trans conformers in the amorphous and crystalline regions, Ta + Tc, decreased in the pre‐yielding region and then increased continuously after yielding. The T conformers formed during post‐yield stretching showed high orientation, while the G conformers remained in low degree of orientation. Stretching at 88°C showed no pronounced yield stress peak in the stress‐strain curve, and the population of T conformers did not change until the beginning strain‐induced crystallization. A solvent‐cast PET film annealed at 67°C for 24 h, the film N, showed a sharp and high yield stress peak in the stress‐strain curve, and strain‐induced crystallization occurred at lower draw ratio as compared to the film A. The film A showed a non‐equilibrium high population of Ta conformers, part of which reverted to G conformers during sub‐Tg annealing.

Dendritic and axonal morphology of HRP‐injected neurons in the inferior colliculus of the cat

The dendritic and axonal morphology of neurons in the inferior colliculus of the cat was investigated after intracellular injection of HRP, in vivo. All injected axons gave off local collaterals, and most showed a widespread distribution and lacked a specific orientation. In contrast, the dendrites of injected neurons were distinguished by their degree of orientation and the direction of the longest axis of orientation. Dendrites showed a high, moderate, or low degree of orientation. Most highly oriented cells had their longest axis in the rostrocaudal direction with fewer in the mediolateral direction. In the central nucleus, only the rostrocaudally oriented cells correspond to the disc-shaped cells identified in Golgi preparations. Unlike most cells in our sample, the two cells that were disc-shaped had axons that were parallel to the orientation of the dendritic tree. In the dorsal cortex, rostrocaudally oriented cells also were found, but they had unoriented axons. In both the central nucleus and dorsal cortex, cells with a mediolateral axis of orientation or no specific orientation correspond to stellate cells and had axons with widespread local collaterals. These results suggest that an extensive network of local axon collaterals may contribute to neural processing within the inferior colliculus. In the central nucleus, local axons may establish connections within or across the fibrodendritic laminae. In the dorsal cortex, the local and afferent axons may form a complex reticular network. Finally, some injected cells had axons terminating locally and also entering the brachium of the inferior colliculus. This suggests that cells in the inferior colliculus may function as both interneurons and projection neurons.

Flow‐drawing of poly(ethylene terephthalate)

AbstractThe drawing characteristics of poly(ethylene terephthalate) (PET) yarns spun at 1000 and 3000 m/min and film cast at 10 m/min have been studied as functions of drawing temperature up to 200°C and drawing rate up to 50 min−1. The fiber spun at 1000 m/min and the film had very low degrees of pre‐orientation and flow‐drawing occurred 20 to 30°C above their respective glass transition temperatures, for which the drawing tension dropped to almost zero and the resulting drawn samples were found to be amorphous and isotropic. In these samples, flow‐drawing could be prevented if they were drawn at relatively low temperatures or high strain rates. The partially‐oriented yarn spun at 3000 m/min, which had higher pre‐orientation, did not exhibit flow‐drawing under the conditions used. Flow‐drawing apparently occurs due to molecular relaxation processes, which predominate if the drawing temperatures are high, the strain rates are low, and the PET precursor has a low degree of orientation.

Orientation and linear dichroism characteristics of porphyrin-DNA complexes.

The linear dichroism spectra of complexes of tetrakis(N-methyl-4-pyridinio)prophine (H2TMpyP) and its zinc(II) derivative (ZnTMpyP) with DNA oriented in a flow gradient have been investigated. The dichroism of H2TMpyP determined within the Soret band and the Qy band system is consistent with an intercalative conformation in which the plane of the porphyrin ring system is nearly parallel to the planes of the DNA bases. In the case of ZnTMpyP on the other hand, the porphyrin ring system is inclined at angles of 62-67 degrees with respect to the axis of the DNA helix. The pyridyl groups in both cases are characterized by a low degree of orientation with respect to the axis of the helix. In contrast to H2TMpyP which does not significantly affect the degree of alignment of the DNA in the flow gradient, the binding of ZnTMpyP causes a significant decrease (about 50% for a base pair/ZnTMpyP ratio of 20) in the intrinsic dichroism at 260 nm due to the oriented DNA bases; the binding of ZnTMpyP to DNA either gives rise to regions of higher flexibility or causes bends or kinks at the binding sites. Increasing the ionic strength has little influence on the linear dichroism of the ZnTMpyP-DNA complexes, but the number of molecules bound at intercalation sites diminishes in the case of the H2TMpyP-DNA complexes; the accompanying changes in the linear dichroism characteristics suggest that external H2TMpyP complexes are formed at the expense of intercalation complexes.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of Forming and Sintering Methods on the Microstructure and Electrical Conductivity of 8, 8"-Alumina Sintered Body

In the preparation of β, β″-alumina sintered body from its powders with the composition of O.87Na2O·O, 6MgO·5Al2O3 which were produced by a spray pyrolysis technique, the effects of forming and sintering conditions on the microstructure (duplex structure and grain orientation)and anisotropy of electrical conductivity of the sintered body were investigated. The density of green compact was dependent on the forming method and increased in the order uniaxial press(A) <uniaxial pressing after granulation(B)<isolatic pressing after granulation (C). On firing at 1560-1620°C for 5-90 min, the microstructure of sintered body tended to become a duplex structure due to abnormal grain growth as the density of green compact decreased. In green compact formed by method(B), the c-planes of β, β″-alumina particles oriented preferentially perpendicular to the press direction; the degree of orientation was increased markedly by firing. Forming method C gave green compact and sintered body with low degree of orientation, indicating that the dense compaction is essential to suppress the 1727preferential orientation of β, β″-alumina particles. The fraction of β″ phase in sintered body was higher as the grains grew into larger ones. The specific resistivity of sintered body in the press direction (pr) was generally larger than that in the perpendicular direction (pR). With increasing in the degree of particle orientation, pp increased markedly, whereas pa remained nearly constant.

Decay of the orientational light scattering by rayleigh particles

The decay of the orientationally induced variation in light scattering of a disperse system is treated in the Rayleigh approximation. In the general case of complete orientation the law of disorientation is obtained biexponentially. Monoexponentiality of the decay is to be expected only at low degrees of orientation.

The Degree of Common Orientation of Hydrometeors Observed by Polarization Diversity Radars

Abstract Measurements are presented of the extent to which a common alignment of hydrometeors exists in various forms of precipitation. The observations, obtained with circularly-polarized radars situated at Penhold, Alberta and Ottawa, Ontario, were made at wavelengths of 10.4 and 1.82 cm respectively. Good agreement between results obtained at these two wavelengths was found for observations made on similar classes of precipitation. The degree of preferred alignment is high (typically 75%) in moderate or heavy rain; the presence of ice-phase hydrometeors is usually associated with a low degree of orientation. The degree of orientation, in conjunction with other parameters, is useful for the identification of the precipitation particles. Limitations on its usefulness imposed by propagation effects and by multiple scattering are discussed briefly.

Crystallite orientation in polycrystalline graphites made from glass-like carbons under high pressure

The preferred orientation of crystallites was investigated on polycrystalline graphites made from the glass-like carbons with spherical particles under a pressure of 5 kbar at 1300 to 2000‡ C. The glassy carbon spheres 40 to 70 Μm diameter which were pre-heated at 1000‡ C under normal pressure, gave sintered discs with a bulk density of 1.5 g cm−3 and a relatively high degree of orientation. However, spheres of the same type pre-heated at 2000‡ C gave the high bulk density of 1.9 g cm−3 and a very low degree of orientation. The carbon beads 40 to 70 Μm diameter gave a relatively low degree of orientation and pre-heating at 2000‡ C reduced the degree very remarkably. Carbon beads with a smaller particle size, <20 Μm, gave a lower degree of orientation. Pre-heating of the initial carbon of small particle size at temperatures as high as 2000‡ C resulted in a low degree of preferred orientation of the crystallites in the sintered discs, the temperature dependence being the strongest.

Sintering and Graphitization of Various Garbons under Pressure

By the heat treatments at 1700°-2000°C under 5 kbar, the sintered graphite disks were prepared from various starting materials, such as graphitizing polyvinylchloride coke, graphitized fine powder, non-graphitizing phenol resin char and carbon beads, fluid and gilsonite cokes with onion-like texture, and the powder of PAN fibre.From the polyvinylchloride coke and PAN fibre, the disks with high degrees of graphitization and orientation were obtained. The graphitized fine powders, phenol resin char, 1000°C-pre-heated carbon beads and fine powders of fluid coke gave the sintered disks with similar values of orientation degree, which being a little lower than that of the polyvinylchloride coke. The fluid coke and gilsonite coke gave relatively low degree of orientation. The 2000°C-pre-heated carbon beads gave the disks with high degree of graphitization, very low degree of orientation and high bulk density.

Effect of Reactor Irradiation on the Tensile Strength of Uniaxially Oriented Polyethylene Terephthalate

A series of uniaxially oriented polyethylene terephthalate fibers irradiated in a nuclear reactor have been investigated. On the basis of tensile studies the ultimate tensile strength is found to decrease as the dose of reactor irradiation increases. However, the ultimate tensile strength is found to increase when the degree of molecular orientation increases. But after the samples have received a relatively high dose of irradiation the tensile strength approaches practically zero. The x-ray diffraction scans of the nonirradiated samples with low degrees of orientation indicate very little crystallinity, whereas the very definite diffraction peaks associated with the irradiated and oriented samples indicate a definite trend toward crystallinity.

Deformation mechanism of cellulose gels. II. Course of crystallite orientation compared to that required by Kratky's theory

AbstractThe paper offers a verification by x‐ray experiments of Kratky's theory of affined transformation (affine Raumverzerrung), aiming at an explanation of the mechanism of deformation of swollen cellulose gels upon stretching. Isotropic model filaments of various degrees of swelling were stretched to various extents and x‐ray photographs were then taken. Following Kratky, the intensity distribution along the sickles of two paratropic planes of the ribbon‐shaped crystallites, the lamellar plane, A0 and the A3 plane (perpendicular to the latter) were measured and the average orientation, expressed in terms of the orientation factor, fx, was calculated. The superposition of the (021) interference on that of the A3 sickle, which had been neglected by Kratky, was accounted for. The experimental results are in conformity with certain characteristic features of the theory; (a) the orientation of the A0 planes advances more rapidly than that of the A3 planes; (b) regardless of the initial degree of swelling of the isotropic filament, the average orientation is actually a univocal function of the elongation, va. On the other hand, the rate of orientation appears to be much greater than that required by theory; the average orientation, expressed in terms of the orientation factor, increases almost twice as rapidly. It is shown, moreover, that, apart from low degrees of orientation, Kratky's theory fails to explain the velocity function of the relative rotation of the crystallites, as derived from the experimental data according to a procedure proposed by Kratky. It is concluded that further work is required to elucidate the mechanism of deformation.