scholarly journals Evolution of calcite microcrystal morphology during experimental dissolution

2021 ◽  
Vol 91 (3) ◽  
pp. 229-242
Author(s):  
MOHAMMED S. HASHIM ◽  
STEPHEN E. KACZMAREK
Keyword(s):  
Mass Balance ◽  
Laboratory Experiments ◽  
High Energy ◽  
Burial Diagenesis ◽  
Etch Pits ◽  
Equilibrium Conditions ◽  
Constant Degree ◽  
Dissolution Model ◽  
Sharp Edges ◽  
Far From Equilibrium

Abstract Phanerozoic limestones are composed of low-Mg calcite microcrystals (i.e., micrite) that typically measure between 1 and 9 μm in diameter. These microcrystals, which host most of the microporosity in subsurface reservoirs, are characterized by a variety of microtextures. Despite the overwhelming consensus that calcite microcrystals are diagenetic, the origin of the various textures is widely debated. The most commonly reported texture is characterized by polyhedral and rounded calcite microcrystals, which are interpreted to form via partial dissolution of rhombic microcrystals during burial diagenesis. A proposed implication of this model is that dissolution during burial is responsible for significant porosity generation. This claim has been previously criticized based on mass-balance considerations and geochemical constrains. To explicitly test the dissolution model, a series of laboratory experiments were conducted whereby various types of calcites composed of rhombic and polyhedral microcrystals were partially dissolved under a constant degree of undersaturation, both near and far-from-equilibrium. Our results indicate that calcite crystals dissolved under far-from-equilibrium conditions develop rounded edges and corners, inter-crystal gulfs (narrow grooves or channels between adjacent crystals), and a few etch pits on crystal faces—observations consistent with the burial-dissolution hypothesis. Crystals dissolved under near-equilibrium conditions, in contrast, retain sharp edges and corners and develop ledges and pits—suggesting that dissolution occurs more selectively at high-energy sites. These observations support the longstanding understanding that far-from-equilibrium dissolution is transport-controlled, and near-equilibrium dissolution is surface-controlled. Our results also show that while the rhombic calcite crystals may develop rounded edges and corners when dissolved under far-from-equilibrium conditions the crystals themselves do not become spherical. By contrast, polyhedral crystals not only develop rounded edges and corners when dissolved under far-from-equilibrium conditions but become nearly spherical with continued dissolution. Collectively, these observations suggest that rounded calcite microcrystals more likely form from a precursor exhibiting an equant polyhedral texture, rather than from a euhedral rhombic precursor as previously proposed. Lastly, the observation that calcite crystals developed rounded edges and corners and inter-crystal gulfs after only 5% dissolution indicates that the presence of such features in natural limestones need not imply that significant porosity generation has occurred.

The CVD Diamond Nucleation Mechanism on Si Overlaid with sp2 Carbon

1992 ◽  
Vol 242 ◽  
Author(s):  
Pehr E. Pehrsson ◽  
John Glesener ◽  
Arthur A. Morrish
Keyword(s):  
Carbon Fibers ◽  
Necessary Conditions ◽  
Cvd Diamond ◽  
High Energy ◽  
Nucleation Mechanism ◽  
Etch Pits ◽  
Diamond Nucleation ◽  
Spontaneous Nucleation ◽  
Cvd Growth ◽  
Saturated Surface

ABSTRACTRudder et al. [1] observed heavy (>109 cm-2) diamond nucleation on unscratched Si wafers overlaid with carbon fibers during CVD growth. We demonstrate that the nucleation occurs on the edges of etch pits and carbon-rich particles resulting from reaction between the fibers and the substrate. Both the etch pits and the particles satisfy what we consider to be two necessary conditions for ‘spontaneous’ nucleation; a carbon-saturated surface and high energy sites (unsatisfied valencies) at edges and steps.

Al-Sm Alloys Under Far-From-Equilibrium Conditions

2021 ◽  
pp. 85-91
Author(s):  
Can Okuyucu ◽  
Burçin Kaygusuz ◽  
Cemil Işıksaçan ◽  
Onur Meydanoğlu ◽  
Amir Motallebzadeh ◽  
...  
Keyword(s):  
Equilibrium Conditions ◽  
Far From Equilibrium

Integrated Testing of the Srl-165 Glass Waste Form

1986 ◽  
Vol 84 ◽  
Author(s):  
D.L. Phinney ◽  
F.J. Ryerson ◽  
V.M. Oversby ◽  
W.A. Lanford ◽  
R.D. Aines ◽  
...  
Keyword(s):  
Mass Balance ◽  
High Surface ◽  
High Energy ◽  
Leach Solution ◽  
Solution Chemistry ◽  
Waste Form ◽  
Depth Of Penetration ◽  
Glass Waste ◽  
Integrated Testing ◽  
Radionuclide Release

AbstractIntegrated testing of the important components of a glass waste form waste package has been performed in order to gain a better understanding of the processes of radionuclide release and transport in the near field environment. Based upon an interpretation of the depth of penetration of hydrogen in reacted SRL-165 glass we have modeled the radionuclide release from the glass as a combined process of (1) the diffusive exchange of alkalis and boron in the glass for hydrogen species in the solution (D=10−16 cm2/s) and (2) surface dissolution. Surface dissolution controls the release of components not exchanged by diffusion and takes place at a rate of 1.5-3.0 μm/yr. Subsequent to release the radionuclides may remain in the leach solution, diffuse into the tuff, or precipitate as secondary phases. Precipitation is particularly important for plutonium and americium. Diffusive transport of radionuclides through the tuff takes place at an extremely slow rate, D=10−16 cm2/s. As such, the mass of radionuclides incorporated in the tuff by diffusion during the tests is inconsequential relative to that in the leach solution (with the exception of plutonium) and can be ignored in mass balance calculations. Mass balance calculations based upon the release of radionuclides by surface dissolution of the glass waste form are in good agreement with observed solution chemistry when allowances are made for a pulse of dissolution early in the tests. This pulse may be due to either the rapid dissolution of high-energy surface features early in the inLegrated tests, or an initially high surface dissolution rate that decreases with time as silica saturation is approached [1], or a combination of the two.

scholarly journals Non-Equilibrium Ionization in Puppis A

1983 ◽  
Vol 101 ◽  
pp. 245-252
Author(s):  
P. F. Winkler ◽  
C. R. Canizares ◽  
B. C. Bromley
Keyword(s):  
High Resolution ◽  
Spectroscopic Data ◽  
Supernova Remnant ◽  
Optical Data ◽  
Equilibrium Conditions ◽  
X Ray ◽  
Flux Measurements ◽  
Far From Equilibrium ◽  
Einstein Observatory ◽  
Non Equilibrium

High resolution X-ray spectroscopy of the brightest knot of emission in the Puppis A supernova remnant shows that it is made up of ionizing plasma, far from equilibrium. Flux measurements in several X-ray lines enable us to determine the non-equilibrium conditions: electron temperature, ion populations, and time since the knot was heated by the supernova shock. Imaging and spectroscopic data from the Einstein Observatory together suggest that this knot is a cloud of density about 10 cm−3 which has recently been shocked to a temperature 7 × 106 K. Radio and optical data on the region appear consistent with this picture.

scholarly journals How and why kinetics, thermodynamics, and chemistry induce the logic of biological evolution

2017 ◽  
Vol 13 ◽  
pp. 665-674 ◽  
Author(s):  
Addy Pross ◽  
Robert Pascal
Keyword(s):  
Present Report ◽  
Biological Evolution ◽  
Evolutionary Process ◽  
Second Law ◽  
Equilibrium Conditions ◽  
Chemical Systems ◽  
Dynamic Kinetic Stability ◽  
Far From Equilibrium ◽  
Self Organisation

Thermodynamic stability, as expressed by the Second Law, generally constitutes the driving force for chemical assembly processes. Yet, somehow, within the living world most self-organisation processes appear to challenge this fundamental rule. Even though the Second Law remains an inescapable constraint, under energy-fuelled, far-from-equilibrium conditions, populations of chemical systems capable of exponential growth can manifest another kind of stability, dynamic kinetic stability (DKS). It is this stability kind based on time/persistence, rather than on free energy, that offers a basis for understanding the evolutionary process. Furthermore, a threshold distance from equilibrium, leading to irreversibility in the reproduction cycle, is needed to switch the directive for evolution from thermodynamic to DKS. The present report develops these lines of thought and argues against the validity of a thermodynamic approach in which the maximisation of the rate of energy dissipation/entropy production is considered to direct the evolutionary process. More generally, our analysis reaffirms the predominant role of kinetics in the self-organisation of life, which, in turn, allows an assessment of semi-quantitative constraints on systems and environments from which life could evolve.

scholarly journals Planetary Nebulae with Massive Nuclei

1983 ◽  
Vol 103 ◽  
pp. 230-230
Author(s):  
R. Tylenda
Keyword(s):  
Thermal Structure ◽  
Planetary Nebulae ◽  
General Rule ◽  
Central Star ◽  
Time Dependent ◽  
Equilibrium Conditions ◽  
Ngc 7027 ◽  
Different Temperatures ◽  
Far From Equilibrium ◽  
Central Stars

Massive central stars (M > 1 Mo) of planetary nebulae burn nuclear fuel on a time scale of hundreds or tens of years which is shorter than the recombination time in a typical planetary nebula. Consequently the ionization and thermal structure of a nebula with such a nucleus is expected to be far from equilibrium conditions. The greatest chance of observing such a nebula is when the central star cools down to the white dwarf region. Time-dependent photoionization models suggest the following non-equilibrium effects to be expected at this stage. Firstly, the nebula shows a double shell structure, i.e. a bright, inner ring is surrounded by a faint, extended halo best seen in the HI lines and infrared lines from low-ionization species, such as (Ne II) 12.8 μ. Secondly, the low-excitation emission ((O II), (Ne II), (S III)) is enhanced relative to the high-excitation ((O III), (Ne III), (S III)). Thirdly, different modifications of the Zanstra method result in significantly different temperatures for the central star with a general rule that THI > THeII > THeII/HI The He II Zanstra method gives the most reliable result. Fourthly, the electron temperature derived from the (O III) lines is appreciably higher than that obtained from the (N II) lines. It is suggested that NGC 7027 and NGC 2440 possess massive central stars and that the above time-dependent effects are currently observed in these nebulae.

scholarly journals Nonlinear Pulsations of Luminous Blue Variables

2000 ◽  
Vol 176 ◽  
pp. 109-112
Author(s):  
Ernst A. Dorfl ◽  
Michael U. Feuchtinger ◽  
Alfred Gautschy
Keyword(s):  
Stability Analysis ◽  
Linear Stability ◽  
Linear Stability Analysis ◽  
Stellar Atmospheres ◽  
Radial Pulsation ◽  
Equilibrium Conditions ◽  
Stellar Radius ◽  
Luminous Blue Variables ◽  
Mass Ratios ◽  
Far From Equilibrium

AbstractWe present the results of various nonlinear radial pulsation calculations of LBVs with different luminosity to mass ratios. Depending on the stellar parameters, most of these objects undergo strongly non-adiabatic pulsations which can also modify their stellar atmospheres. In some cases part of the kinetic energy of the motions is transferred by shock waves into the atmospheric layers leading to an overall decrease or increase of the stellar radius. This results in different mean density accompanied by a change of the pulsational behavior which can be compared to the results of a linear stability analysis. We also find that regular pulsations can occur around a different equilibrium than the initial configuration which has been determined by stellar evolution calculations. In cases of even higher luminosity to mass ratios the linear stability analysis predicts several unstable modes and the nonlinear computations can exhibit rather irregular light curves. Due to the large non-adiabaticity of the pulsation the corresponding velocity curves still remain more regular, but show increasing amplitudes until the energy stored is released. The pulsations are far from equilibrium conditions and may also lead to a pulsationally driven mass loss for certain stellar parameters.

Defect-Selective Etching of Icosahedral Boron Arsenide (B12As2) Crystals in Molten Potassium Hydroxide

2011 ◽  
Vol 1307 ◽  
Author(s):  
C.E. Whiteley ◽  
A. Mayo ◽  
J.H. Edgar ◽  
M. Dudley ◽  
Y. Zhang
Keyword(s):  
Potassium Hydroxide ◽  
High Energy ◽  
Selective Etching ◽  
Etch Pits ◽  
X Ray ◽  
Etch Pit ◽  
Dislocation Densities ◽  
Boron Arsenide ◽  
Defect Densities ◽  
Molten Nickel

ABSTRACTThe present work reports on the defect-selective etching (DSE) for estimating dislocation densities in icosahedral boron arsenide (B12As2) crystals using molten potassium hydroxide (KOH). DSE takes advantage of the greater reactivity of high-energy sites surrounding a dislocation, compared to the surrounding dislocation-free regions. The etch pits per area are indicative of the defect densities in the crystals, as confirmed by x-ray topography (XRT). Etch pit densities were determined for icosahedral boron arsenide crystals produced from a molten nickel flux as a function of etch time (1-5 minutes) and temperature (400-700°C). The etch pits were predominately triangle shaped, and ranged in size from 5-25μm. The average etch pit density of the triangle and oval etch-pits was on the order of 5x107cm-2 and 3x106cm-2 (respectively), for crystals that were etched for two minutes at 550°C.

Wet Etching Methods for Perovskite Substrates

1999 ◽  
Vol 587 ◽  
Author(s):  
Victor Leca ◽  
Guus Rijnders ◽  
Gertjan Koster ◽  
Dave H. A. Blank ◽  
Horst Rogalla
Keyword(s):  
Ph Value ◽  
Substrate Surface ◽  
High Energy ◽  
Etching Time ◽  
Epitaxial Thin Films ◽  
Etch Pits ◽  
High Quality ◽  
Oxide Electronics ◽  
Force Microscopy ◽  
Atomically Flat

AbstractIn oxide electronics substrates with atomically flat terraces are a request for growing high-quality epitaxial thin films. In this paper results on chemical etching of some substrates with perovskite, ABO3, structure (e.g., SrTiO3, LSAT - the (LaAlO3)0.3(Sr2AlTaO6)0.35 solid solution, and NdGaO3) are presented. In order to obtain high quality substrates, different etchants (NH4F + HF, HCl + NH4Cl, and HCl + HNO3) with various pH values have been studied. From Atomic Force Microscopy (AFM), in air, we conclude that, irrespective of the etchant that has been used, a substrate surface with a BOx terminated layer and atomically flat terraces without etch pits could be obtained. The pH-value and temperature of the etchant and the etching time, however, influence significantly the surface quality. Reflection high energy electron diffraction (RHEED) patterns confirmed the AFM results.

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