A study of the structural and electrical properties of low-temperature-sublimed silicon films indicates that they are characterized by a high density of grain boundaries, hence crystal defects. A trapping model has been proposed to explain the experimentally observed temperature-dependencies of resistivity and carrier concentration of these films. The result shows that the defect density at the grain boundaries is of the order of 10¹² cmˉ², and that it is independent of the doping concentrations in the films. It has been shown that the thin-film metal-insulator-semi-conductor (MIS) structure can be reduced to a transmission line problem by expressing the equivalent capacitance of the structure as a series combination of the depletion capacitance and the insulator capacitance. The variations of both the capacitance and channel conductance of the MIS structure have been utilized to make notch filters in which the notch frequency can be varied over 200% by an external biasing voltage. In view of the need for maintaining a constant null depth in the semiconductor notch filter under various biasing potentials, a new notch network has been proposed in which the optimal notch condition could be maintained simply by designing the ratios of the lengths and widths of the MIS structure to the appropriate values. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Heat pulse experiments have been made on Al₂O₃ single crystals in the temperature range 3.8° K to 35°K with the aim of gaining further insight into the nature of heat transport in solids at low temperatures. Short heat pulses were produced by heating a thin metal film evaporated on to one end of the crystal. The thermal pulse arriving at the other end of the crystal was detected by an indium film thermometer placed in a coil connected to a sensitive radio-frequency bridge, so that the variation of resistance was finally displayed on an oscilloscope. The pulses received at low temperatures (3.8°K to 8°K) show two quite separate parts, an initial sharp rise followed by a slow rise, starting at a definite delay time corresponding to the phonon velocity in the medium. The results up to 18°K do not show appreciable variation in delay time, showing that the heat pulse propagation has not entered a second sound region. As the temperature increases, the amplitude of the initial phonon pulse decreases very much compared with the amplitude of the slow rise. Above 18°K, the small sharp rise can no longer be seen clearly so that the delay time is no longer well defined, and at 30°K only the slow rise is observed. It is found that the conventional theory of heat conduction is inadequate to interpret our results at low temperatures, as it fails to predict the finite delay of the initial rise of the received pulse. A phenomenological approach is taken, using a modified heat equation which has an electrical transmission line analogy. Using Laplace transforms, a solution is obtained and the results calculated with a computer are compared with the experimental curves. It is found that the pulse shape can be interpreted quite satisfactorily, especially at the lowest temperatures. The thermal diffusivity, D, for different temperatures is found, and the apparent thermal conductivity, K, is calculated and compared with Herman's (1955) results. The solution of the modified heat equation is also calculated for liquid He II at 0.25°K and compared with the heat pulses observed by Kramers et al (1954); very good agreement is obtained. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A study has been made of the properties of thin yttrium oxide dielectric films prepared by the electron beam evaporation of high purity Y₂O₃ powder. Films deposited on freshly cleaved NaCl crystals and on polished n-type silicon were examined in the electron microscope. The specimens were found to be polycrystalline, with a crystal size of the order of 100 Å. The structure was found to be essentially the same as found for bulk Y₂O₃. D.C. conduction measurements were made on films of various thicknesses. The characteristics were found to be bulk-limited, with the conductivity decreasing at lower pressures. An activation energy of 0.6 eV was found. The conduction mechanism was believed to be Poole-Frenkel emission of electrons from donor centers into the Y₂O₃ conduction band. The donor centers were believed to be interstitial yttrium atoms rather than oxygen vacancies because of the pressure dependence observed in conductivity. Step response measurements were made, and the results explained on the basis of a loss peak with a most probable relaxation time of 200 seconds. The relaxation of oxygen atoms dissolved in the anion defective Y₂O₃ lattice was assumed to be the mechanism. The results of step response and A.C. bridge loss measurements indicated that different relaxation mechanisms are dominant in different frequency ranges. Internal photoemission measurements were made on Al-Y₂O₃-Al sandwiches. The energy barrier between the electrodes was found to be trapezoidal, with barrier heights of 3.14 and 3.72 eV. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Nitric Oxide (NO) is produced in the vascular endothelium where it then diffuses to the adjacent smooth muscle cells (SMC) activating agents known to regulate vascular tone. The close proximity of the site of NO production to the red blood cells (RBC) and its known fast consumption by hemoglobin, suggests that the blood will scavenge most of the NO produced. Therefore, it is unclear how NO is able to play its role in accomplishing vasodilation. Investigation of NO production and consumption rates will allow insight into this paradox. DAF-FM is a sensitive NO fluorescence probe widely used for qualitative assessment of cellular NO production. With the aid of a mathematical model of NO/DAF-FM reaction kinetics, experimental studies were conducted to calibrate the fluorescence signal showing that the slope of fluorescent intensity is proportional to [NO]2 and exhibits a saturation dependence on [DAF-FM]. In addition, experimental data exhibited a Km dependence on [NO]. This finding was incorporated into the model elucidating NO2 as the possible activating agent of DAF-FM. A calibration procedure was formed and applied to agonist stimulated cells, providing an estimated NO release rate of 0.418 ± 0.18 pmol/cm2s. To assess NO consumption by RBCs, measurements of the rate of NO consumption in a gas stream flowing on top of an RBC solution of specified Hematocrit (Hct) was performed. The consumption rate constant (kbl)in porcine RBCs at 25oC and 45% Hct was estimated to be 3500 + 700 s-1. kbl is highly dependent on Hct and can reach up to 9900 + 4000 s-1 for 60% Hct. The nonlinear dependence of kbl on Hct suggests a predominant role for extracellular diffusion in limiting NO uptake. Further simulations showed a linear relationship between varying NO production rates and NO availability in the SMCs utilizing the estimated NO consumption rate. The corresponding SMC [NO] level for the average NO production rate estimated was approximately 15.1 nM. With the aid of experimental and theoretical methods we were able to examine the NO paradox and exhibit that endothelial derived NO is able to escape scavenging by RBCs to diffuse to the SMCs.

This research study describes the gas separation performance of different sets of Polyethyleneoxide-block-Polysulfone-block-Polyethyleneoxide/poly(ether-block-amide) (PEO-b-PSU-b-PEO/Pebax) composite membranes for CO2 separation from N2 and CH4. Gas permeation properties of the membranes prepared were studied at ambient temperature (21oC) and 8bar. The dependence of gas flux and selectivity on pressure was explored for different pressures from 2.5 bar up to 19 bar also at ambient temperature. Pebax/PAN composite membranes have a range of CO2 permeance of 4467±274 GPU, 365±64 GPU for CH4 and 152±17 GPU for N2 with CO2/N2 and CO2/CH4selectivities ranges of 30 and 13 respectively. Improved CO2/N2 and CO2/CH4 selectivities coupled with flux reduction were obtained from the introduction of PEO-b-PSU-b-PEO films on Pebax/PAN composite membranes. The gas flux reduced to about one-tenth of Pebax/PAN membrane’s while the selectivities obtained for all the PEO-b-PSU-b- PEO/Pebax/PAN membrane range from 33 to 60 for CO2/N2 and 17 to 33 for CO2/CH4 as the thickness of PEO-b-PSU-b-PEO top layer increases.

Previously, we found that 5-25 μg/ml safrole oxide could inhibit apoptosis and dramatically make a morphological change in human umbilical vein vascular endothelial cells (HUVECs). But the possible mechanism by which safrole oxide function is unknown. To answer this question, in this study, we first investigated the effects of it on the activity of nitric oxide synthetase (NOS), the expressions of Fas and integrin β4, which play important roles in HUVEC growth and apoptosis, respectively. The results showed that, at the low concentration (10 μg/ml), safrole oxide had no effects on NOS activity and the expressions of Fas and integrin β4. Then, we investigated whether HUVECs underwent differentiation. We examined the expressions of neuron-specific enolase (NSE) and neurofilament-L (NF-L). Furthermore, we analyzed the changes of intracellular reactive oxygen species (ROS). After 10 h of treatment with 10 μg/ml safrole oxide, some HUVECs became neuron-like cells in morphology, and intensively displayed positive NSE and NF-L. Simultaneously, ROS levels dramatically decreased during HUVECs differentiation towards neuron-like cells. At the low concentration, safrole oxide induced HUVECs differentiation into neuron-like cells. Furthermore, our data suggested that safrole oxide might perform this function by depressing intracellular ROS levels instead of by affecting cell growth or apoptosis signal pathways.

The objective of this research was to study the catalytic graphitization of cellulose fibers coated with iron oxide nanoparticles. Bleached cellulose fibers and iron oxide nanoparticles coated cellulose fibers were pyrolyzed at five elevated temperatures. The crystallographic structures of carbon-encapsulated iron oxide nanoparticles were then investigated by the following techniques: Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Raman Spectroscopy, Transmission Electron Microscopy (TEM), and Selected-Area Electron Diffraction (SAED). The graphitization of cellulosic fibers was enhanced by the presence of iron oxide nanoparticles. Moreover, iron oxide nanoparticles deposited on cellulosic fiber samples pyrolyzed above 800°C produced graphitic structures. TEM and XRD were performed to identify and characterize the phase transitions of carbon-encapsulated iron oxide nanoparticles after pyrolysis treatment at four temperatures: 500°C, 800°C, 1000°C, and 1600°C. TEM of samples pyrolyzed at or above 800°C showed resulting units were core-shell structures consisting of dark grains and a light matrix with graphitic structure.

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