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Notes: Abstract Rock fracture enhances permeability and provides pathways through which fluids migrate. During contact metamorphism, fluids contained in isolated pores and fractures expand in response to temperature increases caused by the dissipation of heat from magmas. Heat transport calculations and thermomechanical properties of water-rich fluids demonstrate (1) that thermal energy is a viable mechanism to produce and maintain pore fluid pressure (Pf) in a contact metamorphic aureole; (2) that the magnitude of Pf generated is sufficient to propagate fractures during the prograde thermal history (cause hydrofracture) and enhance permeability; and (3) that Pf-driven fracture propagation is episodic with time-scales ranging from years to thousands of years. Because Pf dissipation is orders of magnitude faster than P, f buildup, Pf oscillations and cyclical behaviour are generated as thermal heating continues. The Pf cycle amplitude depends on the initial fracture length, geometry and the rock's resistance to failure whereas the frequency of fracture depends on the rate of heating. Consequently, oscillation frequency also varies spatially with distance from the heat source.Time series of fluid pressures caused by this process suggest that cyclical fracture events are restricted to an early time period of the prograde thermal event near the intrusive contact. In the far field, however, individual fracture events have a lower frequency but continue to occur over a longer time interval. Numerous fracture cycles are possible within a single thermal event. This provides a provisional explanation for multiple generations of veins observed in outcrop. P f cycling and oscillations may explain several petrological features. If pore fluids are trapped at various positions along a pressure cycle, the large amplitude of Pf variations for small fractures may account for different pressures recorded by fluid inclusions analysed from a single sample. Pf oscillations, during a single thermal episode, also drive chemical reactions which can produce complex mineral textures and assemblages for discontinuous reactions and/or zoning patterns for continuous reactions. These can mimic polymetamorphic or disequilibrium features.Temporal aspects of fracture propagation and permeability enhancement also constrain the likely timing of fluid flow and fluid-mineral interactions. These data suggest that fluid flow and fluid-mineral reactions are likely to be restricted to an early period in the prograde thermal history, characterized by high Pf coincident with relatively high temperatures, fracture propagation and consequent increases in permeability. This early prograde hydration event is followed by diffusional peak metamorphic reactions. This relationship is evident in the complex mineralogical textures common in some metamorphosed rocks.

Notes: The structure of YBa2Cu3Ox thin films and YBa2Cu3Ox/PrBa2Cu3Ox superlattice films grown on (00L) SrTiO3 substrates has been studied using x-ray diffraction and transmission electron microscopy. The films consist of four symmetry-equivalent domains related by mutually perpendicular (hh0) and (hh¯0) twin boundaries. One twin orientation is often highly favored over the other. We have correlated this in-plane symmetry breaking with small miscuts of the substrate surface towards the [HH0] direction and propose that interfacial steps act as nucleation sites for twins. Thus, by controlling the surface normal, a technique is described for producing films containing aligned twin boundaries.

Notes: Residual compressive stress due to plume-induced energetic particle bombardment in CeO2 films deposited by pulsed-laser deposition is reported. For laser ablation film growth in low pressures, stresses as high as 2 GPa were observed as determined by substrate curvature and four-circle x-ray diffraction. The amount of stress in the films could be manipulated by controlling the kinetic energies of the ablated species in the plume through gas-phase collisions with an inert background gas. The film stress decreased to near zero for argon background pressures greater than 50 mTorr. At these higher background pressures, the formation of nanoparticles in the deposited film was observed. © 1999 American Institute of Physics.

Notes: We have determined the stability line in the 1/T−log[P(O2)] phase space for the synthesis of Nd1+xBa2−xCu3Oy (NdBCO) films. A systematic study of Tc, Jc, and ρ(T) dependence on oxygen partial pressure and temperature for the deposition of thin NdBCO films grown by pulsed-laser deposition was performed. The conditions for optimal NdBCO film growth were determined by varying oxygen partial pressure from 0.02 to 400 mTorr, and substrate temperature between 730 and 800 °C. The results show that the best NdBCO films are obtained at oxygen pressures in the range of 0.2–1.2 mTorr, depending on the substrate temperature. This is more than two orders-of-magnitude lower than the correspondent oxygen pressure appropriate for YBa2Cu3O7−δ film growth. © 1999 American Institute of Physics.

Notes: Symmetric superlattice structures consisting of alternating atomic-scale layers of KTaO3 and KNbO3 with variable periodicity were grown on KTaO3 substrates by pulsed laser deposition. The in-plane structure of KNbO3 closely matches that of the KTaO3 substrate, resulting in KTaO3/KNbO3 heterostructures that are uniformly strained in-plane without misfit dislocations. This strain imposes an in-plane KNbO3 lattice spacing identical to that of the KTaO3 substrate for the temperature range 30 °C〈T〈700 °C, and a tetragonal-to-tetragonal transition is observed whose phase transition temperature Tc depends on the KNbO3 layer thickness. The in-plane strain results in a significant increase in this ferroelectric-paraelectric Tc for superlattices with relatively thick KNbO3 layers (Tc=535 °C for a 17 nm thick layer, as compared to 435 °C for bulk KNbO3) and for K(Nb0.5Ta0.5)O3 random-alloy thin films. As the superlattice period decreases, a reduction of Tc is observed. For superlattices with periodicities of 50 Å or less, the Curie temperature is identical to that of the K(Ta0.5Nb0.5)O3 random-alloy film, indicating significant long-range ferroelectric coupling across the KTaO3 layers. © 1998 American Institute of Physics.

Notes: Photochemical vapor deposition of polycrystalline GaAs on synthetic fused silica is reported here. A Hg-Xe arc lamp is used as the light source with triethylgallium and arsine serving as the reactants. We report, for the first time, on a GaAs deposition process using the above-mentioned light source and reactants which is completely controlled by the light source with no deposition occurring in the absence of light. GaAs thin films of thicknesses up to 1.6 μm have been deposited. X-ray diffraction, energy-dispersive spectrometry, and optical transmittance are used to analyze these films.