Initial Stage Of Precipitation Research Articles

A study on Z-phase nucleation in martensitic chromium steels

9–12% chromium martensitic steels are liable to the precipitation of Z-phase, Cr(V,Nb)N, after long time exposure at 550–650 °C. This complex nitride consumes vanadium nitrides and causes the creep strength of the material to fall drastically after several thousand hours of exposure. In this work, initial stages of precipitation of Z-phase have been studied and characterized using energy-filtered transmission electron microscopy (EFTEM). Vanadium nitrides were found to provide the most suitable nucleation site for Z-phase, since the misfit between the (0 0 1) planes of VN and Z-phase is very small. Furthermore, such a nucleation site would provide vanadium and nitrogen for the growth of Z-phase. The presence of niobium carbide has also been observed close to Z-phase nucleation sites, indicating niobium to be important for the nucleation and growth of Z-phase.

Entrainment, Mixing, and Microphysics in Trade-Wind Cumulus

The vertical evolution of microphysics in trade-wind cumuli (Cu) observed from the NCAR C-130 research aircraft during one flight of the RICO (Rain in Cumulus Over the Ocean) study is analyzed. Conditional sampling of > 200 Cu traversed on this flight is used to chose Cu for which the aircraft penetrated single and growing Cu turrets about 250-m below cloud top where maximum LWC is often found and where radar has often observed initial stages of precipitation. The vertical evolution of the sampled set of Cu was assumed to follow Lagrangian behavior. The entrainment rate, entrained parcel scales, mixing mechanisms, and effects on the droplet size distribution are measured and evaluated. A parcel model is applied over the 1100-m maximum Cu height of the traverses to determine the relationship between the observed large number of small droplets and the fewer ultra-giant sea-salt nuclei (UGN) in order to assess the role of these nuclei in evolving the size spectrum and in causing a growing “drizzle tail”. New insight on these topics is obtained by using the PVM (Particle Volume Monitor) probe to measure incloud microphysics with 10-cm resolution.The results include the following: Entrainment causes primarily dilution of the drops without significant size changes, thus either extreme inhomogeneous mixing or more likely homogeneous mixing resulting from mixing with cool and humid entrained air take place. The entrained parcels are surprisingly small following lognormal behavior and decaying rapidly upon entering the Cu, as a result super-adiabatic drops are not evident. The entrained parcels are consistent with the Bragg-scattering “mantle echo” often observed by radar in small Cu. The FSSP (Forward Scattering Spectrometer Probe) droplet spectra are nearly constant with height. These “self-preserving” spectra are a result of an approximate balance between dilution by entrainment of droplets originating at cloud base, droplet activation on entrained CCN (cloud condensation nuclei), and detrainment and coalescence losses. Sea-salt nuclei follow Woodcock’s wind dependence, and are shown with the parcel model to play an important role in forming the observed drizzle that increases with cloud height. Accretion is the dominant coalescence mechanism near cloud top in these Cu.

A novel approach to study the dynamic process of calcium carbonate crystal growth by microcalorimetric method

Through microcalorimetric analysis of the precipitation process of CaCO 3 in solutions, it is revealed that amorphous calcium carbonate (ACC) forms at the initial stage as a transient phase and finally transforms into crystalline phase. Due to the lowest dissolvability of calcite compares with other polymorphs, the transformation from the metastable phase to calcite is an endothermic process in this research. L-Asp can induce and stabilize the existence of vaterite, meanwhile, it can accelerate the initial stage of precipitation as an accelerator.

Mechanism of de-activation and clustering of B in Si at extremely high concentration

It is known that B deactivation and clustering occur in the presence of an excess of Si self-interstitials (Is). First principle calculations predicted the path of clusters growth, but the precursor complexes are too small to be visible even by the highest resolution microscopy. Channeling with nuclear reaction analyses allowed to detect the location of small B–Is complexes into the lattice formed as a consequence of the B interaction with the Is.In this work we extend this method to determine the complexes formed during the initial stage of B precipitation in Si doped at extremely high concentration (4at%) and subjected to thermal treatment. The samples were prepared by excimer laser annealing (ELA) of Si implanted with 1keV B. The thickness of the molten layer was 100nm and the B profile was boxlike with a maximum hole concentration of ∼2×1021cm−3. The electrical deactivation and carrier mobility of this metastable system has been studied as a function of subsequent annealing in the temperature range between 200 and 850°C. Channeling analyses have been performed to investigate the B lattice location at the initial stage of precipitation. The difference, with respect to previous investigations, is the very small distance (<1nm) between adjacent B atoms substitutional located in the lattice and the absence of Is that can be released during annealing, since the end of range defects were completely dissolved by ELA. In this way, information on the B complex evolution in a free-of-defects sample have been obtained.

Effect of external stress on discontinuous precipitation on {100} planes in Cu–5.7 wt% Ag single crystals

The application of external stress during ageing induces the oriented formation of disc-shaped aggregates consisting of rod-shaped Ag precipitates and a Cu matrix on {100} planes in single crystals of a Cu–5.7 wt% Ag alloy. Tensile stress along the [001] axis induces preferential formation of the aggregates perpendicular to the [001] axis and compressive stress induces the same parallel to the stress axis. The stress-orienting effect is determined during the initial stage of precipitation, when the formation of the aggregates starts on dislocations. The experimental result can be understood if the misfit strain of the aggregates in a direction perpendicular to the disc plane is larger than that in the parallel direction. This idea is supported by length-change measurements during ageing.

Microstructural Changes at the Initial Stage of Precipitation in an Aluminum&amp;ndash;Silicon Alloy

In this study, we use a transmission electron microscope (TEM) to observe the microstructure of an Al–Si alloy from immediately after quenching to the initial stage of precipitation. Results reveal the presence of patches immediately after quenching that are thought to be Si clusters, and these are coherent with the {111}matrix. At the initial stage of aging, Si crystals with a plate-like structure are observed, contradicting the round shape previously described. We propose that the Si clusters that appear immediately after quenching act as nuclei for the plate-like Si crystals. As the aging time increases, these increase in length and thickness, becoming thick plate-like Si phase precipitates.

Al-1.2%Si合金の析出初期段階の微細組織変化

Al-1.2%Si合金の析出初期段階の微細組織変化

Effect of Temperature on the Formation of Microporous PVDF Membranes by Precipitation from 1-Octanol/DMF/PVDF and Water/DMF/PVDF Systems

The effects of precipitation temperature on the morphology and crystal structure of PVDF membranes formed in a wet phase inversion process were studied. The liquid−liquid demixing line (binodal) and gelation phase boundaries of the systems were determined over the temperature range 25−85 °C. As the temperature was raised, the crystallization-induced gelation line was found to approach and/or intersect the binodal, leading to a favored situation for liquid−liquid demixing to dominate the initial stage of precipitation. This was well demonstrated in the cellular asymmetric morphology as observed by SEM and AFM imaging. The top surface of this membrane was composed of stacks of crystalline lamellae, with interlamellar distance estimated to be 13−20 nm by AFM. At lower temperatures (e.g., 25 °C), the membrane solution precipitated into a uniform morphology composed of spherical crystallites that exhibited the “β” form (type I) crystal structure, as was evidenced by the WAXD analysis. By contrast, when PVDF wa...

Sn-119 Mössbauer study of dilute tin-lead alloys

M{umlt o}ssbauer spectroscopy has been performed using the {sup 119}Sn resonance on a variety of low-concentration tin in lead samples, ranging from 1.3 at.thinsp{percent} to 18.5 at.thinsp{percent} tin in lead. We observe three basic resonances, which we identify as dispersed tin in lead (I), precipitated {beta} (white) tin in lead (II), and precipitated {alpha} (gray) tin in lead (III), where the latter is only observed below 286 K. Contrary to earlier published results that reported an anomalous falloff in the elastic fraction for dispersed tin in lead, we find only the typical temperature dependence. The M{umlt o}ssbauer temperature is found to be 104 K for the dispersed state, as found from a Debye model analysis based on the transmission integral. Both this M{umlt o}ssbauer temperature and the isomer shift relative to a CaSnO{sub 3} source agree with earlier published values within experimental error. For the precipitated state (II), it is found in the initial stages of precipitation both the isomer shift and the measured M{umlt o}ssbauer temperature are close to that of the dispersed state (I). As the sample is annealed or cold worked, the precipitates grow and both the isomer shift and the characteristic M{umlt o}ssbauer temperature approach thatmore » of {beta}-Sn, 140 K. The precipitation-dissolution reaction is traced using a sample containing 18.5 at.thinsp{percent} tin in lead by annealing at progressively higher temperatures and following changes in the isomer shift. Dissolution is found to begin as low as 360(+15/{minus}5)thinspK. A significant result that has not been reported by earlier investigators of this system is the observation using M{umlt o}ssbauer spectroscopy of the {alpha} phase of tin (III). Furthermore, the formation of this phase takes place without the usual long incubation period required when a lead host matrix is not used. Moreover, a technique for curve fitting such M{umlt o}ssbauer spectra based on an analytic expansion of the transmission integral is demonstrated that isolates each component in the unresolved resonance. We find that the M{umlt o}ssbauer effect technique can be used for assessing the phase diagram in systems such as the Sn-Pb alloy, and our results indicate that the accepted phase diagram may need refinement. {copyright} {ital 1998} {ital The American Physical Society}« less

Microstructural Development of Coherent Tetragonal Precipitates in Magnesium‐Partially‐Stabilized Zirconia: A Computer Simulation

A new computer simulation technique for modeling morphological pattern formation during nucleation, growth, and coarsening of coherent misfitting particles is developed. Microstructure evolution during precipitation of tetragonal phase from Mg‐partially‐stabilized cubic zirconia is investigated. Our computer simulation shows that during the initial stage of precipitation, the tetragonal particles formed by homogeneous nucleation display strong alignment along certain crystallographic directions, forming the so‐called “tweed” pattern. During subsequent growth of the spatially correlated nuclei, an alternating band structure is observed, with each individual band consisting of lens‐like‐shaped tetragonal phase particles of the same orientation variant dispersed in the cubic matrix. The particles in the neighboring bands are twin‐related. The microstructures obtained in our computer simulation seem to agree well with the experimental observations.

Precipitation and relaxation in strained Si1−yCy/Si heterostructures

We have studied the thermal stability of Si1−yCy/Si (y=0.007 and 0.014) heterostructures formed by solid phase epitaxial regrowth of C implanted layers. The loss of substitutional C was monitored over a temperature range of 810–925 °C using Fourier transform infrared absorbance spectroscopy. Concurrent strain measurements were performed using rocking curve x-ray diffraction to correlate strain relaxation with the loss of substitutional C from the lattice. Loss of C from the lattice was initiated immediately without an incubation period, indicative of a low barrier to C clustering. The activation energy as calculated from a time to 50% completion analysis (3.3±5 eV) is near the activation energy for the diffusion of C in Si. Over the entire temperature range studied, annealing to complete loss of substitutional C resulted in the precipitation of C into β-SiC. The precipitates are nearly spherical with diameters of 2–4 nm. These precipitates have the same crystallographic orientation as the Si matrix but the interfaces between the Si and β-SiC precipitates are incoherent. During the initial stages of precipitation, however, C-rich clusters form which maintain coherency with the Si matrix so the biaxial strain in the heterostructure is partially retained.

The morphological stability of lateral growth in solid-solid phase transformation during thin-film interdiffusion in Al/Cu bimetal films

This article describes the phenomenon of morphological instability in solid-solid phase transformations during thin-film interdiffusion, specifically related to the initial stages of precipitation when phase growth occurs along the interface between thin films. The experimental observations that revealed this effect will be presented, and a working hypothesis will be discussed. Experimental observations suggest that the ledge mechanism of growth is present in this system but does not inhibit the formation of interfacial instabilities. It is proposed that morphological stability for solid-phase growth occurring during thin-film interdiffusion can be treated by the inclusion of a solute-source term into the two-dimensional perturbation approach generally used to study unstable growth morphologies. Experimental observations that provide qualitative and semiquantitative support for the solute-source model are also presented.

Interstitial precipitation in Fe-Cr-Al alloys

Two separate stages of precipitation have been identified during the aging of ternary Fel8Cr3Al and Fel8Cr5Al alloys at temperatures in the vicinity of 475 °C. The first stage involves the formation of interstitial precipitates resulting from C and N impurities; the second and slower stage is the formation of the Cr-rich α′ phase. Transmission electron microscopy (TEM) results show that carbonitride precipitation occurs preferentially at dislocations, stacking faults, and grain boundaries, and also uniformly through the matrix. Aging for times in excess of 400 hours at 475 °C promotes coarsening of the heterogeneous precipitates and dissolution of the uniformly distributed matrix particles. A resistometric analysis shows that the kinetics of the initial stages of precipitation can be described by a (time)2/3 relation. This kinetic behavior is explained in terms of stress-assisted diffusion in the highly stressed matrix resulting from coherency strains accompanying carbonitride precipitation. Experimental values of the activation energy for the first stage reaction correlate closely with those reported for the interstitial diffusion of C and N in alpha iron.

The Early Stages of Precipitation in Al-Li Alloys

The initial stages of precipitation in supersaturated binary Al-Li solid solution alloys are discussed. Results of experimental investigations referring mostly to ageing at or near room temperature are reviewed and compared with predictions of theoretical treatments. In alloys containing more than about 7 at.% Li a fast continuous ordering (spinodal ordering) takes place before the somewhat slower decomposition. There are indications that the decomposition starts with a homogeneous nucleation in the ordered matrix. In artificially aged samples further precipitation takes place during the subsequent room temperature storage by nucleation within the disordered matrix phase

Initiation of ordering in an AuCu alloy

Planar misfit between fcc and fct and elastic anisotropy in a cubic crystal were examined to understand the crystallographic relationship between the ordered domain and the matrix in the ordering of an AuCu alloy. The results of these calculations did not indicate any definite conjugate plane between the ordered phase and surrounding matrix. A model modified from the crystallization phenomena in amorphous alloys was proposed to understand the ordering in the AuCu alloy. According to the model, the degree of order can be varied from the surface of the ordered domain to the matrix with some area of transitional region, i.e. without any defined interface. This leads to the conclusion that the idea of a specific habit plane is meaningless for understanding the initiation of the AuCu ordering. The validity of the model was supported by the direct observation of the initial stage of precipitation in an Al-Li alloy.

Analysis of the real-time measurement of the pH of rainfall at Padova, Italy: Seasonal variation and meteorological aspects

For some years, the free acidity in rainfall has been measured. Sampling was carried out on a weekly basis and in real time, in order to determine the effects of precipitation that are masked by dry deposition or post-precipitation equilibrium values. First, three classes of rainfall were established, MA, MB and MN (i.e. mainly acid, mainly alkaline and mainly neutral), based on the mean pH value during rainfall. A second classification, based on the trend of the pH curve at the initial stage of precipitation, was made: IA, IB and IC (i.e. initial stage more acid than the mean value, more alkaline, constant). The second grouping was made because the greatest variation in the pH value during rainfall occurs in the initial stage. Surprisingly, when a seasonal analysis was made, the MA class was much less frequent than expected, and was found to be lowest in winter (when SO 2 emission is highest) and highest in autumn. The MB class of rainfall was found most frequently in spring, whereas the MN class was most frequent in summer, when precipitation is mainly in the form of showers or thunderstorms, brought about by local conditions of atmospheric instability. Rainfall is often more acidic initially due to the presence of highly soluble anthropogenic emissions, but as it continues, its pH value may change because of the buffering action of soil particles and Saharan dust, which have slower kinetic reactions. It would seem, therefore, that a short-lasting drizzle would be more damaging, as it is in itself more acidic, and because it activates any already existing dry deposition. When the taking of acidity measurements is delayed, the resulting pH represents the equilibrium value rather than the real degree of rainfall acidity. Consequently, the real effect that precipitation has on an exposed surface is not clear. Analysis of the acidity roses showed that the nearby towns of Treviso and Venice, with its industrial zone, have an unfavourable contribution on a local scale. Under certain, well-defined circumstances, it is possible to forecast the relationship between pH and meteorological conditions, on the basis of three different systems of classifying weather types (i.e. Borghi and Giuliacci; U.K. Meteorological Office; Aerospace Science Division). This approach was useful in demonstrating the relationship between acid rain and physical processes, and meteorological conditions involved in the formation and transport of air masses. This approach also proved particularly useful in determining the alkalizing role of both the Bora (which transports soil particles) and the Scirocco (carrying Saharan dust) winds.

Use of light scattering techniques to study the kinetics of precipitation of mineral acid casein from skim milk

SummaryTwo light scattering methods for monitoring the precipitation of casein from skim milk (9% w/w) with mineral acid are presented. Experiments were carried out at precipitation temperatures in the range 25–53 °C and precipitation pH values in the range 3·9–5·1. The initial stages of precipitation were explored using a light fibre optics system attached to a closed-loop acid/milk mixing system. The later stages of particle formation and growth were examined using a laser doppler anemometer attached to an open-pipe flow system. It was found that there was a lag initially, followed by a period of rapid particle growth. During a third phase a dynamic equilibrium between particle growth, break-up and shrinkage was established. A model for isoelectric casein precipitation based on these observations is proposed and discussed.

Studies of the Hydrous Tin(IV) Oxide Ion Exchanger. V. The Sodium Isotopic Exchange Rate between the Exchanger in the Na+ Form and Aqueous Solutions

Abstract The sodium isotopic exchange rate between the hydrous tin(IV) oxide ion exchanger in the Na+ form and aqueous solutions was determined radiochemically. When the dried exchanger was used, the rate of the first step was controlled by the rate of the equilibration of water in the exchanger with the solutions, while the second step was found to be controlled by the diffusion of sodium ions in the particles (particle diffusion). When the exchanger had been sufficiently immersed in solutions in advance, the particle diffusion controlled the rate from the beginning. From the dependency of the rate of particle diffusion on the apparent particle size and the specific surface area of the exchanger, and on the sodium-ion concentrations in the solutions, the process controlling the rate is considered to be the diffusion in the primary particles, which are formed at the initial stage of precipitation and which aggregate to form the hydrous tin(IV) oxide gel. The diffusion constants and their activation energy (35±2 KJ mol−1) were also evaluated.

Excess nitrogen and discontinuous precipitation in nitrided iron-chromium alloys

FeCr (1.88 and 3.61 wt % Cr) specimens were gas nitrided (10 vol % NH3/90 vol % H2 gas mixture at 833 K). The structure after nitriding was studied employing metallographic methods (optical microscopy, hardness measurements), scanning electron microscopy and X-ray diffractometry (residual macrostress determination and phase identification). Further the weight and thickness increases of the specimens during nitriding were determined. Excess nitrogen uptake, discontinuous precipitation and void formation were observed. The amount of excess nitrogen observed in an initial stage of precipitation corresponds well with the predicted amount applying thermodynamics of stressed solids. The occurrence of a discontinuous precipitation reaction at later stages of precipitation was accompanied by the loss of excess nitrogen from the supersaturated ferritic matrix. In this way void formation and changes in the residual macrostress distribution within the specimen during nitriding can be explained. As a result a model is presented for the description of the nitriding behaviour of Fe-Cr alloys.

Extensive Submarine Lithification in a Cave in the Belize Barrier Reef Platform

ABSTRACT Extensive deposits of submarine cements discovered in a submerged Pleistocene cave in the outer barrier-reef platform off Belize, Central America, occur in an unusual setting--on and within countless calcareous projections formed by serpulid worms on the ceiling of the cave. The cements are predominantly dense submicrocrystalline and microcrystalline to porous chalky or submicrosucrosic magnesium calcite (approximately 15 mole % MgCO3), and they contain minor amounts of acicular aragonite, which generally forms a lining inside the aragonitic serpulid tubes. Radiocarbon dates, three-year artificial substrate experiments, and evidence of the ceiling's original relief indicate a low rate of accumulation for the serpulid-cement projections. The characteristic 20-60 µm peloidal texture associated with magnesium calcite submarine cements is well developed here and there within the projections. These peloids are physiochemical precipitates, having formed during the development of the submarine cement deposits, which accumulated preferentially on substrates that have a slow rate of accumulation and that experience little or no disturbance of their sediment cover. Artificial substrates placed on the ceiling of the cave for three years revealed that the initial stage of precipitation of magnesium calcite from seawater is in the form of a submicrocrystalline magnesium calcite dust.