Surface Recession Research Articles

Collisional atomic mixing in polyatomic targets.

The balance equation derived by Sigmund, Oliva, and Falcone [Nucl. Instrum. Methods 194, 541 (1982)] for the evolution with ion fluence of the composition of irradiated polyatomic targets is generalized to include the effect of the implanted bombarding ions and the allowance for the target density to depend on depth. The model also includes atomic mixing, surface recession due to sputtering, and homogeneous target relaxation. As an additional feature in theoretical descriptions of collisional mixing, the need to introduce the effect of the surface in the relocation operator is stressed, and a model for the evaluation of this contribution is proposed. The relocation operators are evaluated in detail for power-law interatomic potentials. We give explicit expressions that are ready for use in numerical solutions of the coupled integro-differential equations that describe the concentration profiles. The potential of the formalism to treat a variety of topics in the interaction of energetic ions with matter is commented upon. In particular, we discuss the conditions for the balance equation to be linear, and results on the evolution with ion fluence of thin markers embedded in solids are compared with other theoretical predictions and with experimental data.

Mass transfer from an opaque swollen polymer measured by a projected fringe optical technique

Swollen polymer studies, pioneered by Dr. N. Macleod, have been widely used in mass transfer measurement and heat transfer estimation. In most previous studies holography has been used to measure the recession of a transparent polymer. An attempt has been made to extend the usage of the method by making surface recession measurements using a more robust optical system. The system utilises projected fringes which are observed on the surface of an opaque polymer. The polymer and swelling agent combination used here gives a working range of about 30 microns. This is somewhat greater than that required for measurement by holography but is still much less than the recessions encountered when naphthalene is used as the mass transfer surface. A recession of about 30 microns is unlikely to alter the flow conditions significantly during an experiment, especially if the flow is unconfined. The projected fringe method has enabled the measurement of surface recessions of the opaque swollen polymer in response to the impingement of an air jet with an accuracy of about 7 microns in a situation where the maximum recession is 30 microns. This demonstrates the feasibility of the technique. An improvement in accuracy is sought by the elimination of systematic errors. Alternative polymer and swelling agent combinations are also under investigation.

An update on the carbon-oxygen reaction

Some of the pertinent literature on the C-O 2 reaction, available over the past 25 years, is considered. Fundamental, specific reaction rates are available from microscopy studies on carbons of large crystallite size, excellent crystallite alignment, and high purity. Rates are determined in a straight-forward way by following recession of the active surface as a function of time. Problems of obtaining fundamental rates for gasification of carbons from measuring total active surface area and/or those fractions covered by both a stable complex and a fleeting oxygen complex during gasification are considered. Ways now appear to be available to make the necessary measurements for calculating fundamental rates using the latter approaches.

NUMERICAL SIMULATION OF METAL DRILLING WITH A CO2LASER BEAM

A numerical code that simulates the initiation and evolution of a crater in a metal plate exposed to a focused CO2 laser beam has been developed. The code treats a time-dependent, three-dimensional axisymmetric heat conduction problem in a workpiece with temperature-dependent properties that undergoes two phase transitions. The boundary conditions depend on the surface temperature, orientation, and recession speed, while the solution domain changes continuously as a result of material removal. Discretization is by the control volume approach. Simple one-dimensional cases that demonstrate the “positive feedback” nature of the problem are simulated and compared with analytical solutions. Thus the proper discretization requirements are evaluated in terms of the grid “gradient number” and the grid “speed number” Convergence of the simulations for a typical exposure of an aluminum plate is also presented.

Blanket peat erosion in a mid‐wales catchment during two drought years

AbstractThe rate of blanket peat erosion was measured at an upland site in central Wales during the 1983‐1984 drought years. Erosion pins, a peat surface sediment trap, and sediment sampling in the effluent stream, were used to estimate the rate of peat surface recession and the rate of organic sediment loss from the catchment. An overall rate of surface recession of 16 mm y−1 on exposed peat faces was recorded; this differed between faces of different aspects, with the greatest recession on southwest faces. Eroding peat surfaces exhibited maximum recession during the summer, but the peat surface sediment trap indicated that the highest rates of sediment loss from peat faces due to rain wash occurred during the autumn and early winter. Stream sediment sampling showed that the yield of organic sediment from the catchment was 34·4 t km−2 yr−1, with greatest losses also during the autumn and early winter. The evidence suggested that the surface recession, as measured on erosion pins, included a ‘wastage’ or shrinkage component, which possibly accounted for as much as 80 per cent of the apparent loss. Direct and circumstantial evidence suggested that peat wastage during the summer months was the most important agent of surface recession in the study period, which encompassed the two dry summers. Desiccation provided available sediment during the autumn, but organic sediment supply became limited as the winter progressed, despite the occurrence of frost heave.

General model for thermochemical ablation into a vacuum

A model is presented for the prediction of ablation rates and surface temperatures of laser-hardening materials subjected to high heating rates. The model incorporates translational nonequilibrium gas flow at the surface, sonic outflow constraint at the equilibrium gas state away from the surface, general thermochemistry, and in-depth heat conduction. The effects of surface irreyersibilities and sonically limited transfer rates are incorporated into a new code that generates input for modified versions of existing ablation-conduction programs. Comparison of the predictions with experimental data for surface temperature and recession confirms the accuracy of the model for carbon phenolic at irradiances from 0.3 to 5 kW/cm.

The influence of CO ice on the activity and near-surface differentiation of comet nuclei

The role of unclathrated CO ice in the activity and near-surface differentiation of comet nuclei is investigated. A computer program used in the authors' previous work (Fanale and Salvail 1987, Icarus 72, 535–554) has been adapted to model a comet nucleus composed of a homogeneous mixture of H 2O ice, CO ice, and dust in a Halley-type orbit. The difference in the volatility of CO and H 2O ices results in greatly different gas production rates, and recession of the CO ice surface occurs, forming a porous surface layer of H 2O ice and dust. Temperatures of H 2O and CO ice surfaces, fluxes of CO and H 2O gases and, depths of the CO ice are presented as functions of orbital position. Also considered are the pressures of subsurface CO as they relate to the stability of the overlying H 2O ice and dust mantle and the possibility of dust or H 2O ice grain emission. Possible pressures are in the range 3 × 10 2 dyn cm −2 to 1 × 10 6 dyn cm −2. These are insufficient to fragment annealed H 2O ice, yet capable of overcoming the lithostatic pressures of many meters of unannealed H 2O ice and emitting free surface grains in the 1-μm range. However, they are comparable to the inferred range of ice-poor mantle strength and may account for episodic regional changes in activity. It has been found that CO ice should always exist within 3 m from the surface and that CO gas production exceeds H 2O gas production beyond 2.5 AU. This has implications for planned in situ elemental analysis of nuclei. CO gas production from “new” comets can be as much as 3 orders of magnitude greater before perihelion. After perihelion there is little difference in CO production for “new” and periodic comets. The results have been found not to depend significantly on the permeability of the water ice mantle or the estimated variability of the mass fraction of CO.

Contact interaction between two half-spaces in the presence of a surface recess partially filled with an incompressible liquid

A study has been made on the contact interaction between two elastic isotropic homogeneous half-spaces in the presence of a recess on one of them, which is partially filled with an incompressible liquid. With this recess shape, a solution has been obtained in closed form. Results are presented illustrating the effects of the recess and the liquid in it on the contact characteristics.

ESCA study of poly(vinylidene fluoride), tetrafluoroethylene-ethylene copolymer and polyethylene exposed to atomic oxygen

The ESCA (electron spectroscopy for chemical analysis) spectra of films of poly(vinylidene fluoride) (PVDF), tetrafluoroethylene-ethylene copolymer (TFE/ET) and polyethylene (PE) exposed to atomic oxygen (O( 3P)), in or out of the glow of a radio-frequency O 2 plasma, were compared. ESCA spectra of PE films exposed to O( 3P) in low Earth orbit (LEO) on the STS-8 Space Shuttle were also examined. Apart from O( 3P)-induced surface recession (etching), the various polymer films exhibited surface oxidation, which proceeded towards equilibrium saturation oxygen levels. The maximum surface oxygen uptakes for in-glow or out-of-glow exposures were in the order: PE > TFE ET > PVDF ; for PE itself, the oxygen uptakes were in the order: in glow > out of glow > LEO. Given prior ESCA data on poly(vinyl fluoride) and polytetrafluoroethylene films exposed to O( 3P), the extent of surface oxidation is seen to decrease regularly with increase in fluorine substitution in a family of ethylene-type polymers.

Etching of plasma‐polymerized tetrafluoroethylene, polytetrafluoroethylene, and sputtered polytetrafluoroethylene induced by atomic oxygen [O(3P)

AbstractA study was carried out of the surface recession (etching) of thin films of plasma‐polymerized tetrafluoroethylene (PPTFE), polytetrafluoroethylene (PTFE), and ion‐beam sputter‐deposited polytetrafluoroethylene (SPTFE), exposed to atomic oxygen [O(3P)] downstream from a nonequilibrium radio‐frequency O2 plasma. At 22°C, the etch rates for PTFE, SPTFE, and PPTFE were in the ratio of 8.7 : 1.0. A thin, conformal coating of PPTFE (etch rate of 0.3 nm/h at 22°C) was found to protect an underlying, cast film of a reactive polymer, cis‐1,4‐polybutadiene (etch rate of 0.13 mg/cm2 h ≡ 1300 nm/h at 22°C), against O(3P) attack for the time required to fully etch away the PPTFE coating. From ESCA analysis, PTFE exhibited only minor surface oxidation (uptake of 0.5 atom % O) upon etching, its F/C ratio decreasing slightly from 2.00 to 1.97; PPTFE exhibited considerable surface oxidation (uptake of 5.9 atom % O) and a decrease in F/C ratio from 1.30 to 1.23; and SPTFE exhibited a surface oxidation (uptake of 2.2 atom % O) intermediate between those of PTFE and PPTFE, with a decrease in F/C ratio from 1.73 to 1.67. The O(3P)‐induced etching of PTFE had an activation energy of 3.8 Kcal/mol, but no activation energy value was obtained for PPTFE which gave a nonlinear Arrhenius plot apparently because of thermally induced thinning above 50°C.

Rates of air pollution induced surface recession and material loss for a cathedral in belgium

Runoff water samples were taken at the St. Rombout's Cathedral (Mechelen, Belgium), which was constructed with sandy limestones of Balegem and Gobertingen. Gypsum appeared to be the principal deterioration compound. The mean annual surface recession from the cathedral was calculated to be around 20 μm. Yearly several tons of stone material are flushed away from the cathedral with the rain water.

GaAs device passivation using sputtered silicon nitride

This letter describes the passivation of GaAs field-effect transistors (FET’s) using sputtered silicon nitride films. A low-power reactive sputtering process for silicon nitride film deposition was developed and applied to both metal-semiconductor FET (MESFET) and high electron mobility transistor (HEMT) passivation. It is shown that the shifts in the FET parameters such as threshold voltage, breakdown voltage, and transconductance, after nitride deposition, depend not only on the stress states of the nitride films but also on the surface composition and recess geometry of the passivated surfaces. With proper deposition parameters, the sputtered silicon nitride film can be successfully used for both MESFET and HEMT passivation with only minor degradation (less than 10%) of the device performance.

Acid rain damage to carbonate stone: A quantitative assessment based on the aqueous geochemistry of rainfall runoff from stone

AbstractAn onsite experimental procedure was used to identify and quantify acid rain damage to carbonate stone, based on the change in rain runoff chemical composition. Observed changes in runoff from stone are attributed to the interaction of acidic species present in the rain with the stone surface. Onsite data obtained during the summer and fall of 1984 at three locations in the northeastern United States indicate that carbonate stone surface recession is related to acid deposition. Although the study is continuing, current data are not adequate to distinguish differences in the acid‐stone reaction between limestone and marble nor among the three study sites. A single linear relation between carbonate stone surface recession (the dependent variable) and hydrogen ion loading to the stone surface (the independent variable) appears to describe the data set.

Corrosion of SiC Ceramics by Na2SO4

Previous work has shown that SiC ceramics are corroded by alkali halide vapor in an oxidizing atmosphere. The present work was conducted to determine if Na{sub 2}SO{sub 4} vapor caused similar corrosion. Specimens were exposed to Na{sub 2}SO{sub 4} vapor in an oxidizing atmosphere at 1,200{degree}C for 575 h. As a result, the specimens were covered with a silicate glass corrosion product, and surface recession had occurred. Corrosion was attributed to chemical alteration of the normally protective SiO{sub 2} film by Na{sub 2}O, which allowed oxidation of the SiC.

Reactions of atomic oxygen (O( 3P)) with various polymer films

Thin films of various polymers were exposed at ambient temperature to ground-state oxygen atoms (O( 3P)) downstream from a radio-frequency O 2 plasma or glow discharge. The polymer films examined were a polyimide (Kapton), poly(ethylene terephthalate), polystyrene, high- and low-density polyethylenes, poly(vinyl fluoride), poly(vinylidene fluoride), alternating tetrafluoroethylene-ethylene copolymer, polytetrafluoroethylene, and tetrafluoroethylene-hexafluoropropylene copolymer. The relative etch rates for the various polymer films exposed ‘out of the glow’ to O atoms with 0·04 eV translational energy are compared with corresponding literature values obtained from low-pressure, oxygen discharge studies involving exposure ‘in the glow’ and from the Space Shuttle STS-8 flight (with O atoms in low Earth orbit having collisional energy of ≈ 5 eV). The etch rate for Kapton exposed ‘out of the glow’ to 0·04-eV O atoms is also compared with etch rates obtained from various neutral and ion beam experiments (with O O + having impact energies ranging from 0·14 to 800 eV). The etch rate data for Kapton fit reasonably well a logarithmic plot, with positive slope, of reaction probability versus O( 3P) impact energy. ESCA spectra of Kapton before and after reaction with O( 3P) indicated steady-state competition between surface recession (etching) and oxidation.

Dynamics of zirconium oxide thin-film growth and ion-beam etching.

Some aspects of the dynamics of growth and ion-beam etching of dielectric zirconium oxide films have been deduced from in situ measurements by ellipsometry, photometry, and ion scattering spectroscopy for the cases of vapor-deposited films produced with or without ion assistance. These measurements confirm that the porous microstructure exhibited by an evaporated film can be modified significantly by ion bombardment during growth, since ion and surface-atom recoil implantation lead to film densification. The ellipsometric data for ion-beam etching have been compared with a dynamical collision cascade model which has been used recently to describe ion-assisted thin-film densification. The model predicts the time evolution of the atomic densities and stoichiometry near the surface and considers the diffusion of atoms from overdensified and nonstoichiometric regions in terms of random-walk theory. The measured growth of the surface damage layer, which is formed during etching of a porous film, as well as the surface recession rate, can be explained quantitatively by the model.

Surface recession and backface temperature of laser-irradiated opaque slabs

T HE purpose of this work is to derive broadly applicable equations of immediate relevance for investigating continuous-wave/laser-radiation interactions with material targets that vaporize without first melting. The procedure is based on the heat-balance integral method and emphasizes the use of nondimensional variables. Specifically, the author demonstrates how to solve the differential equations that describe the front-surface recession and the backface-temperature evolution in a burnthrough mode.

Recession of a coal face exposed to a high temperature

A one-dimensional, transient model is developed to describe drying, pyrolysis, endothermic gasification and spalling(thermomechanical failure) of a wet coal face exposed to a high temperature. Such a situation occurs, for example, at the roof of an underground coal gasification (UCG) cavity. Emphasis is placed on thermochemistry, and rock mechanics are simplified by use of two parameters, a failure length and temperature, which measure the strength of the coal. Drying of the coal and convection of the evaporated water are modeled as a Stefan problem, and reaction of this water vapor with carbon at the free surface is described by an asymptotic solution in the limit of a large activation energy for this reaction. Thus, evaporation and gasification effects become analytic boundary conditions on the numerical solution to the transient heat penetration in the dry coal. Pyrolysis is treated numerically as the release of a single component in the dry coal according to one-step Arrhenius kinetics. Both surface gasification and spalling are shown to be of importance for typical UCG conditions, and their relative interaction can provide an explanation for UCG field test observations. The model, in particular the perturbation solution developed for the surface recession rate due to gasification, has applications in pyrolyzing ablative and related systems.

Sputtering of multicomponent materials: The diffusion limit

Bombardment of systems such as binary alloys normally results in the preferential sputtering of one component combined with a subsurface depletion of the same component. Driving forces for preferential loss include mass differences, chemical binding differences, and Gibbsian or similar segregation. We are here concerned with the simpler profiles which are set up in the absence of segregation. This problem has most commonly been reduced to one of conventional diffusion, governed by relocation and surface recession but not conserving lattice sites. More recently, a ballistic treatment was introduced which in its most complete form allowed for relocation, surface recession, and conservation of lattice sites. We show that the ballistic treatment can be re-expressed in a diffusion limit such that conservation of lattice sites is retained and that, moreover, a closed-form steady-state solution exists. It is further shown that the diffusion limit satisfactorily defines the various diffusion coefficients, the steady-state surface compositions, the sputtering yields, and the overall profile shape.

Extracellular compartments in matrix morphogenesis: collagen fibril, bundle, and lamellar formation by corneal fibroblasts.

The regulation of collagen fibril, bundle, and lamella formation by the corneal fibroblasts, as well as the organization of these elements into an orthogonal stroma, was studied by transmission electron microscopy and high voltage electron microscopy. Transmission and high voltage electron microscopy of chick embryo corneas each demonstrated a series of unique extracellular compartments. Collagen fibrillogenesis occurred within small surface recesses. These small recesses usually contained between 5 and 12 collagen fibrils with typically mature diameters and constant intrafibrillar spacing. The lateral fusion of the recesses resulted in larger recesses and consequent formation of prominent cell surface foldings. Within these surface foldings, bundles that contained 50-100 collagen fibrils were formed. The surface foldings continued to fuse and the cell surface retracted, forming large surface-associated compartments in which bundles coalesced to form lamellae. High voltage electron microscopy of 0.5 micron sections cut parallel to the corneal surface revealed that the corneal fibroblasts and their processes had two major axes at approximately right angles to one another. The surface compartments involved in the production of the corneal stroma were aligned along the fibroblast axes and the orthogonality of the cell was in register with that of the extracellular matrix. In this manner, corneal fibroblasts formed collagen fibrils, bundles, and lamellae within a controlled environment and thereby determined the architecture of the corneal stroma by the configuration of the cell and its associated compartments.