Internal Front Research Articles

Growth of ordered lamellar precipitates during nitridation of Nb–10 at.% Ti at 1300°C

Growth of three morphologically distinct layers occurs during the parabolic nitridation of Nb–10 at.% Ti at 1300°C under 0.3 bar nitrogen atmosphere. The outermost layer is composed of δ-(Nb,Ti)N, the intermediate layer of β-(Nb,Ti)2N and the innermost layer corresponds to an internal precipitation zone with ordered lamellar precipitates of β-(Nb,Ti)2N+β-(Nb,Ti). The internal precipitation front advances with parabolic kinetics and the three layers grow by nitrogen inward diffusion. The interlamellar spacing at the precipitation front increases as the reaction time increases. The formation of the lamellar microstructure can be described as a discontinuous precipitation process and the interlamellar spacing can be considered as a measure of the distance over which lateral Nb–Ti interdiffusion occurs in the alloy.

Numerical solution of stefan problems by the method of green functions

A method for reducing a multidimensional Stefan problem to a system of Hammerstein integral equations is proposed. Application of the proposed method to numerical solution of one-dimensional nonstationary Stefan problems formulated for the cases of an internal phase front, coincidence of the phase front with the external boundary, and a movable external boundary is considered. The efficiency of the method is tested on an exactly solvable Stefan problem.

Investigation of the internal reflectance and prediction of infrared diffuse reflectance of the polymeric coating on aluminum substrate

This study employs the ray tracing method to develop and analyze mathematical formulae for the IR diffuse reflectance of the polymeric coating on a metal substrate. The effects of the thickness and the absorption property of the polymer film on the internal reflectance are also investigated. In addition, the diffuse reflectance of the coating/substrate system which is irradiated with a perfect diffuse source is formulated as well. Analysis results indicate that the internal reflectance of the internal front surface (polymer/air interface) is not a constant which depends on the film thickness and absorption property. Closely examining the internal multiple reflections between the front and the substrate surface reveals that the diffuse reflectance of the coating/substrate system can be obtained by summing the fractions of rays emerging from the front surface. By knowing the refractive index and the extinction coefficient of the polymer, the diffuse reflectance of the coating/substrate system can be calculated by the formulae presented here. In addition an alkyd resin coating/aluminum substrate system is also implemented to compare the experimental reflectances with the calculated ones. According to the comparisons the analysis and developed formulae are quite effective.

The German opposition to Hitler: the resistance, the underground, and assassination plots, 1938-1945

Between 1933 and 1945, more than 500,000 non-Jewish German civilians were imprisoned for so-called political crimes. Most of the resistance was, therefore, underground--within the religious, political, civilian, and even military communities. This history of the various segments of the German resistance movement covers groups and methods from underground newspapers--Rote Kapella, Internal Front, The Opponent, The Front Line--to conspiracy movements within unions. While emphasizing the active plots to either arrest or assassinate Hitler, the work embraces also the passive resistance seen in the Protestant and Catholic churches, the Kreisau Circle, trade unions, the foreign ministry and the civil service. The opposition's planned coup d'tat of 1938 is fully detailed, as well as the deep involvement of the Abwehr (military intelligence) in the plots against Hitler.

The Effect of Supersaturation on the Internal Oxidation of Binary Alloys

According to the theory of Bohm and Kahlweit ofthe internal oxidation of binary A-B alloys, theparabolic rate constant for the formation of reasonablystable internal BOν oxides as well as theconcentrations of O and B at the oxidation front arecontrolled only by the degree of supersaturationnecessary for the nucleation of new oxide particles. Theeffects of this factor on the previous parameters arecalculated for various values of the solubility product ofthe oxide and of the diffusion coefficients of O and B.Moreover, an alternative procedure for the calculationof the critical degree of supersaturation behind the precipitation front required for oxideprecipitation, which is a function of the concentrationof the reactants at the internal oxidation front, isproposed. A simple modification of Wagner's theory of internal oxidation is also presented, andits results are compared with those of the treatment byBohm and Kahlweit. Finally, the limitations of the twomethods are examined.

Deformation—corrosion interactions in selected advanced high temperature alloys

The present work deals with investigations on the deformation—corrosion interactions of the three advanced high temperature alloys MA 956, Nicrofer 45 TM, and HR3C. The technical background relates to the corrosion conditions to be expected for heat exchanger tubes in coal gasification plants. The atmosphere typical for a dry-feed entrained slagging gasifier, was composed of CO, H2, CO2 and H2S, with medium S- and very low O-potentials of about 10−11 and 10−28 bar, respectively, at a test temperature of 600°C. The gas has been used in a non-equilibrium condition corresponding to fast gas quenching from about 1300°C (gasifier) to 400–600°C (tube walls). Possible deformation of the technical components have been taken into account by superimposed creep strains. The results have shown a very acceptable corrosion resistance for all alloys. For 10 000 h exposure the internal attack remained clearly below 100 μm. The external corrosion products were composed of sulphides whereas a sulphide—oxide mixture formed internally. An interesting observation concerns the fact that for longer exposure times an increasing fraction of Cr2O3 formed at the internal corrosion front. This was probably the reason for the sub-parabolic corrosion kinetics. For superimposed creep, the type and morphology of the overall attack did not change. Differences for stress-free and stressed samples however, concerned enhanced local attack. The critical creep value for the initiation of strain-assisted corrosion was about 1–2%. Approximately 2% or more strain was needed for the corrosion to clearly exceed that of stress-free samples. The mechanism of corrosion enhancement was characterized by the formation of thin cracks in the compact internal corrosion zone whereas the external sulphides did not play a role in loss of protection. Corrosion areas which formed at the crack tip, composed of oxide-sulphide mixtures enabled the cracks to extend further into the alloy under simultaneous corrosion—creep interaction at the crack tip. Crack filling occurred mainly by iron sulphides and therefore restoration of protection was not possible.

From Ultracompact to Extended H II Regions

view Abstract Citations (208) References (73) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS From Ultracompact to Extended H II Regions Garcia-Segura, Guillermo ; Franco, Jose Abstract The dynamical evolution of H II regions and wind-driven bubbles in dense clouds is studied. In particular, we address two different issues: (1) the conditions under which ultracompact H II (UCHII) regions can reach pressure equilibrium with their surrounding medium (and thereby stall their expansion) and (2) the appearance of a powerful dynamic instability in expanding H II regions. At pressure equilibrium, the ionized regions become static, and as long as the ionization sources and the ambient gas densities remain about constant, the resulting UCHII regions are stable and long-lived. The equilibrium sizes and densities, Rs,eq ∼3 X 10-2F⅓48T⅔H II, 4P-⅔7 pc and ni,eq ∼4 × 104P7T-1H II, 4 cm-3 (where Fβ8 is the photoionizing flux in units of 1048 s-11, P7 is the pressure in units of 10-7 dyne cm-2, and TH II,4 is the ion temperature in units of 104 K), are similar to those actually observed in UCHII regions. Similarly, ultra- compact wind-driven bubbles can reach pressure equilibrium, and the resulting final sizes are similar to those of UCHII'S. The same is true for a combined ultracompact structure consisting of an interior wind- driven cavity and an external H II region. For nonmoving stars in a constant-density medium, the lifetimes for all types of ultracompact objects only depend on the stellar lifetimes. For cases with a density gradient, depending on the core size and slope of the density distribution, some regions never reach the static equilibrium condition. A powerful dynamic instability appears when cooling is included in the neutral gas swept up by an H II region or a combined wind-H II region structure. This instability was first studied by Giuliani and is associated with the thin-shell instability described by Vishniac. The internal ionization front exacerbates the growth of the thin-shell instability, creating a rapid shell fragmentation, and our numerical simulations confirm the linear analysis of Giuliani. The fragments tend to merge as the evolution proceeds, creating dense and more massive clumps, and are slowly eroded by ionization fronts. Thus the resulting structures have a variety of shapes, sizes, densities, and lifetimes. Intriguing features such as "elephant trunks" and cometary-like globules can easily be explained as a result of this instability. Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177769 Bibcode: 1996ApJ...469..171G Keywords: ISM: H II REGIONS; HYDRODYNAMICS; ISM: BUBBLES; ISM: CLOUDS full text sources ADS | data products SIMBAD (4)

Diffusion-induced grain-boundary migration during internal reduction of chromium-doped Al2O3

Abstract Diffusion-induced grain-boundary migration is observed during internal reduction of chromium-doped A12O3. Chemical diffusion of oxygen along grain boundaries is fast compared with volume diffusion; therefore the internal reduction front penetrates from the grain boundaries into the grains, leading to chromium precipitation and formation of a thin, coherently strained chromium-depleted oxide layer. Migration of the grain boundary to either side of the boundary occurs, while the global energy in the overswept area decreases owing to oxygen potential relaxation, precipitate coarsening and elastic strain relaxation. The role of multiple factors, namely the chemical composition of the starting mixed oxide, chemical composition inhomogeneities, crystallographic orientation of adjacent grains and of the boundary plane, kinetics and mechanism of the internal reduction itself, on the direction and velocity of boundary migration is experimentally studied and interpreted.

Internal oxidation of NiAl and NiAlSi alloys at the dissociation pressure of NiO

NiAl and NiAlSi alloys were internally oxidized at temperatures of 1073–1273 K by the Rhines Pack method. For the NiAl alloy, the oxidation process follows parabolic law and the oxidation front was flat with severe integranular oxidation occurring at 1073 K and extensive grain boundary sliding at 1273 K. As for NiAlSi alloys, the oxidation rate increased with increase of Si content at 1073 K but the rate decreased at higher temperatures due to total or partial continuous oxide layer formation at the internal oxidation front. The depth of intergranular oxidation was also greatly reduced. For all samples, nickel was found to be transported out to the surface with the amount proportional to the Si content. Lattice diffusion (Nabarro-Herring creep) was believed to be the main cause for nickel transport in the NiAl alloy while dislocation pipe diffusion is the mechanism for NiAlSi alloys.

Electrochemical oxidation of CoO-ZrO 2(CaO) lamellar eutectic structures to form aligned Co 3O 4-ZrO 2(CaO) material: Mechanism and crystallography of the reaction

Oriented lamellar eutectic composites of CoO and ZrO 2(CaO) platelets were oxidized in oxygen at 880°C to form Co 3O 4 between the zirconia plates. The relatively rapid longitudinal oxidation of CoO can be described by an electrochemical local cell model whereby oxygen ions are conducted from the external surface to the internal oxidation front through the zirconia electrolyte. The crystallographic relationship between the Co 3O 4 and ZrO 2(CaO) resulting phases is the same as that found for CoO and ZrO 2(CaO) in the starting material.

The Mediterranean water tongue and its chlorofluoromethane signal in the Iberian Basin in early summer 1989

Hydrographic observations from the Iberian Basin demonstrate the variability of water masses in upper and intermediate layers. The surveyed area embraces the internal front between water masses from higher latitudes and the Mediterranean outflow, exhibits several isolated Mediterranean eddy (meddy) structures at middepth, and displays the virtual source region for the Mediterranean Water (MW) tongue off the Portuguese continental slope. The description is enhanced by additional chlorofluoromethane measurements, which show anomalously high concentrations at middepth, due to mixing of MW with the overlying Atlantic waters in the Gulf of Cadiz. The geostrophic stream function shows several meddylike features that not only are remarkably extended in the depth range of the MW, but are also correlated with surface height anomalies.

Oxidation kinetics of fayalite (Fe2SiO4)

Single crystals of fayalite (Fe2SiO4) have been oxidized either in the hematite or the magnetite stability field to investigate the kinetics and mechanisms of oxidation. For samples heated in air at 770° C, a two-phase region composed of fine-grained iron oxide and silica phases formed as the reaction front moved into the sample, and an iron oxide layer formed external to this two-phase region. The presence of the single-phase oxide layer coating the specimens indicates that oxidation occurs by the migration of iron from the fayalite to the gas-solid interface rather than by the movement of oxygen in the opposite direction. For oxidation in air, the kinetics followed a parabolic growth law, with the rate of oxidation limited by the diffusion of iron from the internal reaction front to the gas-solid interface through the iron oxide. When fayalite was oxidized in the magnetite stability field, using a CO/CO2 gas mixture at 1030° C, oxidation was controlled by the reaction at the gas-solid interface, yielding an oxidation rate considerably slower than that predicted for diffusion-controlled growth of the oxide layer.

An Experimental Study of Heat and Mass Transfer During Drying of Packed Beds

An experimental study has been conducted to provide a data base for drying packed beds of granular, nonhygroscopic materials. Experimental results for drying rate, saturation distribution, temperature distribution, and surface saturation are reported for drying glass beads under carefully documented drying conditions. Capillary pressure for both imbibition and drainage was measured for the glass beads, whose size ranged from 65 μm to 450 μm. The drying results demonstrate that, contrary to available model predictions, porous materials do not necessarily exhibit saturation gradients that always increase with distance from the drying surface. Under certain conditions the capillary potential is sufficient to create an internal drying front. The measurements of surface saturation are the first to be reported. They are utilized to speculate on the reasons for the failure of drying models to compare well with experiment without adjusting the convective heat or mass transfer coefficients.

Simulation of ozone uptake distribution in the human airways by orthogonal collocation on finite elements

Ozone transport in a rigid single-pathway anatomic model of the lung was analyzed by a stable convergent numerical algorithm, the method of orthogonal collocation on finite elements. The simulations predicted the dynamic behavior of gas phase concentration profiles for both ozone and an insoluble inert gas. An internal quasi-stationary diffusion front was observed during early inspiration for both gases. In addition, the absorptive distribution of ozone in lung airways was computed as a total dose as well as a tissue dose. The total dose of ozone decreased along the airway path from the mouth. However, the tissue dose of ozone increased along the conducting airways, reached a maximal dose in the terminal bronchioles, and decreased sharply in the respiratory airways.

Oxidation Behavior of SiC‐Whisker‐Reinforced Alumina‐Zirconia Composites

During high‐temperature oxidation in air, SiC‐whisker‐reinforced Al2O3—ZrO2 composites degrade by the formation of a whisker‐depleted mullite‐zirconia scale. The reaction kinetics have been studied as a function of time and temperature for composites with whiskers preoxidized for different times. The evolution of the microstructure has been investigated by optical, scanning and transmission electron microscopy. Possible reaction mechanisms have been discussed. A model compatible with our observations on Al2O3—ZrO2—SiC and the results reported in the literature for Al2O3—SiC whisker composites is proposed: The oxidation occurs at an internal reaction front. Oxygen diffuses along dislocations and grain boundaries through the mullite scale to react at this front with silicon carbide, thereby forming amorphous silica and graphite. Silica penetrates grain boundaries and further reacts with alumina and zirconia to form mullite and zircon, while the second reaction product, graphite, is oxidized into carbon monoxide when the reaction front moves deeper into the sample.

Semigeostrophic Flow over Orography in a Stratified Rotating Atmosphere. Part III. Evaluation of a Lee Cyclogenesis Mechanism

Three-dimensional, adiabatic, inviscid flow over orography is examined by means of a semigeostrophic model expressed in isentropic coordinates. A nondimensional mountain height ε/D ≲ 0.5, based on the deformation depth D ∼ 3 × 103 m, and a Rossby number Ro ≲ 0.3, based on the mountain breadth L ≲ 3.5 × 105 m and a constant Coriolis parameter, f, provide constraints on the flow field. Vortex tube stretching is evaluated as a mechanism for lee cyclogenesis. It is shown that the convergence of both planetary and geostrophic relative vorticity filaments enhances geostrophic cyclonic vorticity in a semi-geostrophic model. In contrast, ageostrophic cyclonic vorticity is weakened by convergence. These features are illustrated in a numerical simulation of flow over an isentropic isolated mountain. The initial state is characterized by an isolated region of ageostrophic cyclonic vorticity in the lee of the obstacle, accompanied by convergence. A potential vorticity disturbance, associated with an internal cold front and a two-dimensional upper-level jet, is advected over the isolated obstacle. In principle, the coupling of ascending motion ahead of the disturbance with descending motion in the lee provides vortex tube stretching. It is shown that this mechanism does not initiate lee cyclogenesis in either a quasi-geostrophic or semigeostrophic model with isentropic boundaries. In particular, horizontal convergence and vertical stretching are significantly diminished by the intrusion of weakly stratified air into the lee. Additionally, the ageostrophic cyclonic vorticity present in the initial state is not effectively enhanced by vortex tube stretching, according to the ageostrophic vorticity theorem for semigeostrophic flow. The absence of both blocking by the obstacle and potential temperature gradients along the lower boundary is suggested as a possible reason for the failure of vortex tube stretching to initiate lee cyclogenesis in the model presented here.

Hydrogen-assisted cracking studies of 4340 steel by using the optical method of caustics

The optical method of caustics is used to measure the effect of hydrogen on the localized deformation within the fracture process zone (PZ). In particular, the role of the PZ in hydrogen-assisted cracking (HAC) of 4340 steel in hydrogen gas at 1 atm is examined. No change in the caustic diameter was detected prior to crack initiation but an “anomalous” enlargement of the caustic diameter as a function of crack growth was observed as a result of crack tunneling. The “center” of the caustic follows the position of the internal crack front, thus providing a simple means to monitor the growth of internal cracks. It is demonstrated that the technique provides a sensitive means for detecting HAC initiation (the smallest crack depth change detected is 35 μm) and insight into the three-dimensional character of the PZ.

Reduction of NiO Platelets in a NiO/ZrO2(CaO) Directional Composite

Oriented lamellar eutectic composites of NiO and ZrO2(CaO) platelets were reduced in CO2/CO gaseous atmospheres to form metallic Ni between the zirconia plates. The relatively rapid longitudinal reduction of NiO is described by a local cell electrochemical model whereby oxide ions are conducted from the Ni/NiO internal reaction front to the external surface through the zirconia electrolyte, while electrons are passed through the Ni product. Both the product morphologies and the reaction kinetics are consistent with the model. Apparent “lateral’reduction of NiO platelets indeed corresponds to longitudinal reduction for a platelet inclined to the external surface. Some plastic deformation of the product zone permits partial accommodation of the volume change upon reduction of NiO to Ni.

World Trade Patterns: an Empirical Study of Revealed Comparative Advantage Approach

The theory of international trade emphasises that trade of an economy is determined by the factors arising from internal supply and external demand. The factors on the internal supply front constitute the elements like, cost of production, behaviour of internal demand, tariff, taxes, subsidies and overall considerations of comparative cost advantages. Factors on the external demand side include such variables as the price, quality, marketing of the products, trade and production policies of the buyer countries and the mutual trade agreements and relations between and/or among the supplier and demanding countries. Over a period of time it was found that the non-oil developing countries were facing severe constraints to augment their exports particularly in the developed market economies. These constraints were quite often mainly due to comparative cost disadvantages, lack of competitiveness, lack of commodity correspondence and so on. This leads us to examine the state of comparative advantage across products vis-a-vis products' competitiveness and commodity correspondence over time. In the context of the above, the first section examines the structure of world trade, whereas section second reveals the state of comparative cost advantage. Sections third and fourth look into the nature of the country's competitiveness and complementarities across the various products. The commodity correspondence ratio that reflects the future prospects of trade cooperation is dealt in section five, whereas, policy prescription in order to enhance trade cooperation has been discussed in section six—the concluding remarks.

Internal oxidation of Ag-in alloys: Stress relief and the influence of imposed strain

The kinetics of internal oxidation of silver-indium alloys containing 3.5, 5.9, and 9.8 at.% In in air at temperatures 773 to 973 K were established by TGA with no load applied to the specimens. Silver nodules free of oxide particles were observed to form at the surface during internal oxidation. The volume of these silver nodules was comparable to the total volume increase caused by internal oxidation. The alloys were also creep tested during oxidation in air at creep rates varying from 10−7 to 5×10−5 s−1 at 773, 873, and 973 K. The parabolic rate constants kp for the internal oxidation of the solute were determined from the measured widths of the internal oxidation zones. A small increase in kp was observed with increased strain rate. The large volume change associated with internal oxide formation resulted in a stress gradient between the stress-free surface and the internal oxidation front which is under a high compressive stress. Stress relief occurred by transport of silver to the surface. A Nabarro-Herring creep type mechanism based on lattice diffusion of Ag cannot account for the high rate of silver transport to the surface. Pipe-diffusion controlled creep is proposed as the mechanism of stress accommodation by silver diffusion.