Oxidative chemisorption of SO_2 on γ - Al_2O_3 and deposition of H_2-permselective SiO_2 films

The interaction of SO_2 with γ - Al_2O_3 and the deposition of H_2 permselective SiO_2 films have been investigated. The adsorption and oxidative adsorption of SO_2 on γ - Al_2O_3 have been examined at temperatures 500-700°C by Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). At temperatures above 500°C most of SO_2 adsorbed on the strong sites on alumina. The adsorbed SO_2 species was characterized by an IR band at 1065 cm^(-1). The equilibrium coverage and initial rate of adsorption decreased with temperature suggesting a two-step adsorption. When γ - Al_2O_3 was contacted with a mixture of SO_2 and O_2, adsorption of SO_2 and oxidation of the adsorbed SO_2 to a surface sulfate characterized by broad IR bands at 1070 cm^(-1), 1390 cm^(-1) took place. The results of a series of TGA experiments under different atmospheres strongly suggest that surface SO_2 and surface sulfate involve the same active sites such that SO_2 adsorption is inhibited by already formed sulfate. The results also indicate a broad range of site strengths.

The desorption of adsorbed SO_2 and the reductive desorption of oxidatively adsorbed SO_2 have been investigated by microreactor experiments and thermogravimetric analysis (TGA). Temperature programmed reduction (TPR) of adsorbed SO_2 showed that SO_2 was desorbed without significant reaction with H_2 when H_2 concentration was low while considerable reaction occurred when 100% H_2 was used. SO_2 adsorbed on the strong sites on alumina was reduced to sulfur and H_2S. The isothermal reduction experiments of oxidatively adsorbed SO_2 reveal that the rate of reduction is very slow below 550°C even with 100% H_2. The reduction product is mainly composed of SO_2. TPR experiments of oxidatively adsorbed SO_2 showed that H_2S arose from a sulfate strongly chemisorbed on the surface.

Films of amorphous SiO_2 were deposited within the walls of porous Vycor tubes by SiH_4 oxidation in an opposing reactants geometry : SiH_4 was passed inside the tube while O_2 was passed outside the tube. The two reactants diffused opposite to each other and reacted within a narrow front inside the tube wall to form a thin SiO_2 film. Once the pores were plugged the reactants could not reach each other and the reaction stopped. At 450°C and 0.1 and 0.33 atm of SiH_4 and O_2, the reaction was complete within 15 minutes. The thickness of the SiO_2 film was estimated to be about 0.1 µm. Measurements of H_2 and N_2 permeation rates showed that the SiO_2 film was highly selective to H_2 permeation. The H_2:N_2 flux at 450°C varied between 2000-3000.

Thin SiO_2 films were heat treated in different gas mixtures to determine their stability in functioning as high-temperature hydrogen-permselective membranes. The films were heat-treated at 450-700°C in dry N_2, dry O_2, N_2-H_2O, and O_2-H_2O mixtures. The permeation rates of H_2 and N_2 changed depending on the original conditions of film formation as well as on the heat treatment. Heating in dry N_2 slowly reduced the permeation rates of both H_2 and N_2. Heating in a N_2-H_2O atmosphere led to a steeper decline of H_2 permeability. But the permeation rate of N_2 increased or decreased according to whether the film deposition had been carried out in the absence or presence of H_2O vapor, respectively. Thermal treatment in O_2 caused rapid decline of the permeation rates of H_2 and N_2 in films that were deposited under dry conditions. The decline was moderate in films deposited under wet conditions.

I. Reactively sputtered Ti-Si-N thin films for diffusion barrier applications. II. Oxidation, diffusion and crystallization of an amorphous Zr_(60)Al_(15)Ni_(25) alloy.

Films of Ti-Si-N obtained by reactively sputtering a TiSi_2, a Ti_5Si_3, or a Ti_3Si target are either amorphous or nanocrystalline in structure. The atomic density of some films exceeds 10^23 at./cm^3. The room-temperature resistivity of the films increases with the Si and the N content. A thermal treatment in vacuum at 700 °C for 1 hour decreases the resistivity of the Ti-rich films deposited from the Ti_5Si_3 or the Ti_3Si target, but increases that of the Si-rich films deposited from the TiSi_2 target when the nitrogen content exceeds about 30 at. %.

Ti_(34)Si_(23)N_(43) deposited from the Ti_5Si_3 target is an excellent diffusion barrier between Si and Cu. This film is a mixture of nanocrystalline TiN and amorphous SiN_x. Resistivity measurement from 80 K to 1073 K reveals that this film is electrically semiconductor-like as-deposited, and that it becomes metal-like after an hour annealing at 1000 °C in vacuum. A film of about 100 nm thick, with a resistivity of 660 µΩcm, maintains the stability of Si n+p shallow junction diodes with a 400 nm Cu overlayer up to 850 °C upon 30 min vacuum annealing. When used between Si and Al, the maximum temperature of stability is 550 °C for 30 min. This film can be etched in a CF_4/O_2 plasma.

The amorphous ternary metallic alloy Zr_(60)Al_(15)Ni_(25) was oxidized in dry oxygen in the temperature range 310 °C to 410 °C. Rutherford backscattering and cross-sectional transmission electron microscopy studies suggest that during this treatment an amorphous layer of zirconium-aluminum-oxide is formed at the surface. Nickel is depleted from the oxide and enriched in the amorphous alloy below the oxide/alloy interface. The oxide layer thickness grows parabolically with the annealing duration, with a transport constant of 2.8x10^(-5) m^2/s x exp(-1.7 eV/kT). The oxidation rate is most likely controlled by the Ni diffusion in the amorphous alloy.

At later stages of the oxidation process, precipitates of nanocrystalline ZrO_2 appear in the oxide near the interface. Finally, two intermetallic phases nucleate and grow simultaneously in the alloy, one at the interface and one within the alloy.

I. Studies on bacteriophage DNA's and minicircular DNA's in M. lysodeikticus and E. coli 15. II. Flow dichroism of DNA solutions

Part I

Chapter 1.....A physicochemical study of the DNA molecules from the three bacteriophages, N1, N5, and N6, which infect the bacterium, M. lysodeikticus, has been made. The molecular weights, as measured by both electron microscopy and sedimentation velocity, are 23 x 10⁶ for N5 DNA and 31 x 10⁶ for N1 and N6 DNA's. All three DNA's are capable of thermally reversible cyclization. N1 and N6 DNA's have identical or very similar base sequences as judged by membrane filter hybridization and by electron microscope heteroduplex studies. They have identical or similar cohesive ends. These results are in accord with the close biological relation between N1 and N6 phages. N5 DNA is not closely related to N1 or N6 DNA. The denaturation T_m of all three DNA's is the same and corresponds to a (GC) content of 70%. However, the buoyant densities in CsCl of Nl and N6 DNA's are lower than expected, corresponding to predicted GC contents of 64 and 67%. The buoyant densities in Cs₂SO₄ are also somewhat anomalous. The buoyant density anomalies are probably due to the presence of odd bases. However, direct base composition analysis of N1 DNA by anion exchange chromatography confirms a GC content of 70%, and, in the elution system used, no peaks due to odd bases are present.

Chapter 2.....A covalently closed circular DNA form has been observed as an intracellular form during both productive and abortive infection processes in M. lysodeikticus. This species has been isolated by the method of CsC1-ethidium bromide centrifugation and examined with an electron microscope.

Chapter 3.....A minute circular DNA has been discovered as a homogeneous population in M. lysodeikticus. Its length and molecular weight as determined by electron microscopy are 0.445 μ and 0.88 x 10⁶ daltons respectively. There is about one minicircle per bacterium.

Chapter 4.....Several strains of E. coli 15 harbor a prophage. Viral growth can be induced by exposing the host to mitomycin C or to uv irradiation. The coliphage 15 particles from E. coli 15 and E, coli 15 T^- appear as normal phage with head and tail structure; the particles from E. coli 15 TAU are tailless. The complete particles exert a colicinogenic activity on E.coli 15 and 15 T^-, the tailless particles do not. No host for a productive viral infection has been found and the phage may be defective. The properties of the DNA of the virus have been studied, mainly by electron microscopy. After induction but before lysis, a closed circular DNA with a contour length of about 11.9 μ is found in the bacterium; the mature phage DNA is a linear duplex and 7.5% longer than the intracellular circular form. This suggests the hypothesis that the mature phage DNA is terminally repetitious and circularly permuted. The hypothesis was confirmed by observing that denaturation and renaturation of the mature phage DNA produce circular duplexes with two single-stranded branches corresponding to the terminal repetition. The contour length of the mature phage DNA was measured relative to φX RFII DNA and λ DNA; the calculated molecular weight is 27 x 10⁶. The length of the single-stranded terminal repetition was compared to the length of φX 174 DNA under conditions where single-stranded DNA is seen in an extended form in electron micrographs. The length of the terminal repetition is found to be 7.4% of the length of the nonrepetitious part of the coliphage 15 DNA. The number of base pairs in the terminal repetition is variable in different molecules, with a fractional standard deviation of 0.18 of the average number in the terminal repetition. A new phenomenon termed "branch migration" has been discovered in renatured circular molecules; it results in forked branches, with two emerging single strands, at the position of the terminal repetition. The distribution of branch separations between the two terminal repetitions in the population of renatured circular molecules was studied. The observed distribution suggests that there is an excluded volume effect in the renaturation of a population of circularly permuted molecules such that strands with close beginning points preferentially renature with each other. This selective renaturation and the phenomenon of branch migration both affect the distribution of branch separations; the observed distribution does not contradict the hypothesis of a random distribution of beginning points around the chromosome.

Chapter 5....Some physicochemical studies on the minicircular DNA species in E. coli 15 (0.670 μ, 1.47 x 10⁶ daltons) have been made. Electron microscopic observations showed multimeric forms of the minicircle which amount to 5% of total DNA species and also showed presumably replicating forms of the minicircle. A renaturation kinetic study showed that the minicircle is a unique DNA species in its size and base sequence. A study on the minicircle replication has been made under condition in which host DNA synthesis is synchronized. Despite experimental uncertainties involved, it seems that the minicircle replication is random and the number of the minicircles increases continuously throughout a generation of the host, regardless of host DNA synchronization.

Part II

The flow dichroism of dilute DNA solutions (A₂₆₀≈0.1) has been studied in a Couette-type apparatus with the outer cylinder rotating and with the light path parallel to the cylinder axis. Shear gradients in the range of 5-160 sec.^-1 were studied. The DNA samples were whole, "half," and "quarter" molecules of T4 bacteriophage DNA, and linear and circular λb₂b₅c DNA. For the linear molecules, the fractional flow dichroism is a linear function of molecular weight. The dichroism for linear A DNA is about 1.8 that of the circular molecule. For a given DNA, the dichroism is an approximately linear function of shear gradient, but with a slight upward curvature at low values of G, and some trend toward saturation at larger values of G. The fractional dichroism increases as the supporting electrolyte concentration decreases.

Investigation of strong earthquake ground motion

The pattern of energy release during the Imperial Valley, California, earthquake of 1940 is studied by analysing the El Centro strong motion seismograph record and records from the Tinemaha seismograph station, 546 km from the epicenter. The earthquake was a multiple event sequence with at least 4 events recorded at El Centro in the first 25 seconds, followed by 9 events recorded in the next 5 minutes. Clear P, S and surface waves were observed on the strong motion record. Although the main part of the earthquake energy was released during the first 15 seconds, some of the later events were as large as M = 5.8 and thus are important for earthquake engineering studies. The moment calculated using Fourier analysis of surface waves agrees with the moment estimated from field measurements of fault offset after the earthquake. The earthquake engineering significance of the complex pattern of energy release is discussed. It is concluded that a cumulative increase in amplitudes of building vibration resulting from the present sequence of shocks would be significant only for structures with relatively long natural period of vibration. However, progressive weakening effects may also lead to greater damage for multiple event earthquakes.

The model with surface Love waves propagating through a single layer as a surface wave guide is studied. It is expected that the derived properties for this simple model illustrate well several phenomena associated with strong earthquake ground motion. First, it is shown that a surface layer, or several layers, will cause the main part of the high frequency energy, radiated from the nearby earthquake, to be confined to the layer as a wave guide. The existence of the surface layer will thus increase the rate of the energy transfer into the man-made structures on or near the surface of the layer. Secondly, the surface amplitude of the guided SH waves will decrease if the energy of the wave is essentially confined to the layer and if the wave propagates towards an increasing layer thickness. It is also shown that the constructive interference of SH waves will cause the zeroes and the peaks in the Fourier amplitude spectrum of the surface ground motion to be continuously displaced towards the longer periods as the distance from the source of the energy release increases.