Bulk Diffusion Control Research Articles

Analysis of lead pollution control in anode slime micromorphology evolution induced by Mn2+ ions for cleaner production of zinc electrolysis

To explore cleaner production technologies to control lead (Pb) pollution caused by anode slime is a challenging issue for zinc electrolysis industry. The influence of manganese ions (Mn2+) on anode slime microtopography evolution and its relationship with Pb release of lead-based anodes was studied in detail. Scaling analysis was first introduced to quantitatively evaluate anode slime micromorphology evolution, and reveal its intrinsic growth mechanism induced by Mn2+ ions through parameterizing the morphological information of atomic force microscopy (AFM). Low Mn2+ concentrations (1–3 g/L) were found to induce the initial anode slime micromorphology to be compact, of which growth was revealed to be controlled by surface diffusion alone corresponding the exponent α = 0.95. Results of Scanning electron microscopy (SEM) and inductively coupled plasma mass spectrometry (ICP-MS) extended to 240 h of electrolysis supported that anode slime micromorphology evolution in this case could contribute to inhibiting lead pollution, and the release of Pb from lead-based anodes into electrolytes was controlled at 0.5 mg/L. As the Mn2+ concentration increased to 10 g/L, the initial anode slime micromorphology was found that the evolution trend from smooth to rough. And the changes in the growth mechanism from the surface diffusion control to volume or bulk diffusion control were found, which appeared as two distinct hierarchical features corresponding to static exponents α = 0.92 and α’ = 0.55. In this case, excess Mn2+ ions inducing deteriorated anode slime micromorphology evolution was revealed, and large fluctuations of Pb2+ concentrations (1.2–1.5 mg/L) in electrolytes with prolonged electrolysis suggested that it was inadequate in inhibiting Pb2+ release. Scaling analysis for in-depth analysing and predicting the evolution trend of anode slime microtopography induced by Mn2+ ions has been established, thus new insights gained herein can contribute to exploring approaches to control lead pollution by cleaner production technologies during zinc electrolysis.

High-Temperature Static Coarsening of Gamma-Prime Precipitates in NiAlCr-X Single Crystals

The high-temperature coarsening behavior of γ′ precipitates in a series of NiAlCr, NiAlCrTi, NiAlCrW, and NiAlCrTa single-crystal alloys was determined at temperatures between 1158 K and 1473 K (885 °C and 1200 °C). For this purpose, samples were supersolvus solution treated, water quenched, and then subsolvus aged for times between 0.067 and 96 hours. All of the measurements revealed an r3 dependence of the average precipitate radius on time, thus suggesting bulk-diffusion control of the coarsening process. Coarsening kinetics were fastest for NiCrAl and slowest for NiCrAlTa. The observations were interpreted in terms of the classical Lifshitz–Slyosov–Wagner (LSW) theory modified to account for the finite volume fraction of particles, the composition of the precipitates, and the multicomponent nature of the alloys. By this means, an effective diffusivity for the coarsening process was determined and found to lie between 0.6 and 1.5 times that for the impurity diffusivity of chromium in nickel. Furthermore, the modified LSW theory in conjunction with experimental measurements suggested that the effective diffusivity controlling γ′ coarsening at high temperatures in multi-component nickel-base superalloys lay in the lower portion of this range.

Effective Hydrogen Diffusion in Aluminum Alloy 5083-H131 as a Function of Orientation and Degree of Sensitization

The effective hydrogen diffusivity in aluminum alloy (AA)5083- H131 (UNS A95083) for various orientations and degrees of sensitization (DoS) was investigated using AA5083/Pd bilayer membranes. Sensitization produced β-phase (Al3Mg2) precipitates. A modified Devanathan-Stachurski permeation method was used with hydrogen charging by open-circuit exposure to 0.01 M sodium hydroxide (NaOH) and detection via Pd hydride formation. The measured hydrogen diffusivity (DH,eff) in the longitudinal (L) or short transverse (S) direction was almost independent of AA5083 foil thickness, indicating bulk diffusion control. DH,eff was not sensitive to the DoS, ranging from 2 mg/cm2 to 49 mg/cm2. The average room temperature diffusivities of hydrogen, DH,eff (25°C), for the initial H charging cycle of as-sensitized foils, were determined by the slope (Lcorr2/6 vs. t) method to be 8.5 (±1.4) × 10−11 cm2/s along the S direction and 1.4 (±0.2) × 10−10 cm2/s along the L direction, with a 95% confidence level. By averaging individual measured data points, DH,eff (25°C) was found to be 1.0 (±0.3) × 10−10 cm2/s along the S direction and 1.5 (±0.4) × 10−10 cm2/s along the L direction. The effective diffusivity of hydrogen increased by one to two orders of magnitude during the second and third hydrogen charging cycles. This charge cycle dependency was interpreted to result from extensive hydrogen trapping in uncharged foils and remaining trap site occupancy after initial precharging. The implication of these results toward intergranular stress corrosion involving hydrogen-controlled cracking of inter-β ligaments between closely spaced, quickly dissolving β-phase precipitates on grain boundaries is discussed.

Maintenance of Supersaturation II: Indomethacin Crystal Growth Kinetics Versus Degree of Supersaturation

This study compares the kinetics of crystal growth of indomethacin from supersaturated suspensions at varying degrees of supersaturation (2 ≤ S ≥ 9) in the presence of seed crystals of the γ-form of indomethacin, the lowest energy polymorph. At high S (6 ≤ S ≥ 9), the crystal growth was first order with rate coefficients (kG ) that were nearly constant and consistent with the value predicted for bulk-diffusion control. At lower S (<6), kG values were significantly smaller, decreasing approximately linearly with a decrease in S. The decline in kG at low S was attributed to a prolonged period during the initial stages of crystal growth in which surface integration was rate limiting. The apparent solubility of indomethacin after crystal growth for 3 days increased by ∼1.6-fold at both low (S = 2) and high (S = 6) degrees of supersaturation suggesting that a higher energy surface layer was deposited on the γ-form seed crystals during crystal growth. When growth experiments were repeated at low S in the presence of indomethacin seed crystals isolated from a previous crystal growth experiment (i.e., seed crystals having higher energy surface), kG matched the higher values observed for bulk diffusion-controlled crystal growth. Crystal growth experiments were also conducted at S < 1.6 using a constant infusion of an indomethacin solution in the presence of γ-form seed crystals to obtain kG under conditions where deposition of a higher energy surface could not occur. At these conditions, the smaller value of kG indicative of surface integration control was again observed and the apparent solubility of indomethacin after crystal growth matched that of the γ-form. A quantitative mechanistic understanding of the crystal growth kinetics of indomethacin derived from experiments at high and low S may be useful in future studies aimed at understanding the inhibitory effects of pharmaceutical excipients on the crystal growth of poorly soluble compounds and their utility in maintaining drug supersaturation during oral absorption.

Hydrogen Diffusivity During Corrosion of High-Purity Aluminum

The effective diffusivity of hydrogen in as-annealed (550°C for 5 h) 99.999% purity polycrystalline aluminum was determined using Al/Pd bilayer membranes following the procedure of K.R. Hebert. Bulk diffusion control was verified by the foil thickness variation method. The room temperature diffusivity of hydrogen was determined to be 1.6 ± 0.4 × 10−10 cm2/s during initial hydrogen charging in 0.01 M sodium hydroxide (NaOH) solution. The effective diffusivity of hydrogen increased by two orders of magnitude during the second hydrogen charging and approaches the literature for gas phase egress from precharged material. This difference was interpreted to be the result of extensive trapping in freshly annealed foils and high residual trap site occupancy after initial precharging. The activation energy for lattice hydrogen egress was analyzed through constant heating rate thermal desorption spectra. This activation energy was 17.0 ± 0.6 kJ/mol. The experimental results also demonstrated that the measured hydrogen diffusivity is sensitive to the NaOH concentration speculated to be related to its influence on hydrogen production rate, hydrogen overpotential, and solid-state hydrogen concentration.

Effects of surface coatings on the determination of Dchem and kchem in La 2NiO 4+δ by conductivity relaxation

In this work, we utilize surface modification to accurately determine oxygen diffusivity and surface exchange in La 2NiO 4 + δ by transient conductivity. To achieve ‘instantaneous’ gas exchange in the reaction cell, a total pressure change was adopted instead of exchange of gas mixtures at 1 atm. Conductivity relaxation measurements were performed in the temperature range of 600–900 °C and at oxygen partial pressures of 0.2, 1.0 and 1.9 atm. Due to the large uncertainties associated with deriving oxygen chemical diffusion and surface coefficients simultaneously, we limited the relaxation to pure bulk diffusion control by coating nano-grained La 2NiO 4 + δ particles on the sample surface. After determining D chem, k chem was derived by fitting the relaxation data of the uncoated sample. The transient experiments via both oxidation (a step change of pO 2 from 0.2 to 1.0 atm) and reduction (1.9 to 1.0 atm) give consistent results of D chem and k chem showing Arrhenius-type behaviour with activation energies of ~ 90 kJ/mol and ~ 150 kJ/mol, respectively.

Crystallization and grain growth characteristics of yttria-stabilized zirconia thin films grown by pulsed laser deposition

Knowledge about the crystallization and grain growth characteristics of metal oxide thin films is essential for effective microstructural engineering by thermal post-annealing and the integration to Si-based miniaturized electroceramic devices. Finite size and interface effects may cause fundamentally different behavior compared to three dimensional macroscopic systems. This work presents a comprehensive investigation of the crystallization kinetics and microstructural evolution upon thermal post-annealing of amorphous 200 nm and 1.2 μm thin films of 8 mol% yttria-stabilized zirconia grown by pulsed laser deposition (PLD) using ex- and in-situ X-ray diffraction, Raman spectroscopy, and electron microscopy techniques. The layers exhibit a remarkably low crystallization temperature of 200–250 °C while exposure to energetic electrons induces the formation of randomly dispersed ~ 20 nm sized crystallites already at ambient temperature. The isothermal amorphous to crystalline phase transformation kinetics can be described quantitatively by the Johnson–Mehl–Avrami–Kolmogorov model. They reveal characteristics of a three dimensional growth under cation bulk diffusion control with heterogeneous nucleation that changes from continuous to instantaneous initial seeding at temperatures above 300 °C. Large (> 100 nm) equiaxed grains are formed rapidly without a stabilization of transient nanocrystals during the thermally induced phase transformation. A stagnation of normal grain growth resulting in a logarithmic normal size distribution is observed once the average grain dimensions approach the film thickness. The results on the crystallization and grain growth of the PLD-grown YSZ films are evaluated with regards to the fabrication of YSZ solid electrolyte membranes for Si-supported micro solid oxide fuel cells and gas sensors.

Oxygen Permeation Through Cobalt-Containing Perovskites: Surface Oxygen Exchange vs. Lattice Oxygen Diffusion

The oxygen permeation fluxes from to across cobalt-containing perovskite ceramic membranes and were measured by gas chromatography as functions of oxygen chemical potential gradient, temperature, thickness, and catalytic activity on the surface. Power indexes for uncatalyzed and for were obtained when vs. was plotted as a straight line. The results clearly indicate an overall permeation process controlled by both surface oxygen exchange and bulk oxygen diffusion for uncatalyzed and Application of a thin layer of catalytically active on the feeding-gas surface of under the condition of a fixed atm and a varied not only increases remarkably the overall oxygen flux, but also changes a mixed control to a bulk diffusion control. This enables evaluation of the bulk transport properties of the mixed conductors. A coat of on the permeate side has little catalytic effect, especially at low range, due to the formation of a poorly conducting brownmillerite phase. The results explicitly show a higher activation energy for the surface exchange kinetics than for the ambipolar transport in the mixed conductors. The mechanism of the surface exchange is discussed, and an analytic expression that agrees well with the experimental results is obtained. © 2001 The Electrochemical Society. All rights reserved.

Particle coarsening behavior of α-β titanium alloys

A systematic study was undertaken to develop particle coarsening relationships in terms of time, temperature, and volume percents of phases and to determine the rate-controlling mechanisms for the coarsening of α-β Ti-Mn and Ti-V alloys. This investigation is confined to equiaxed-type α+ β microstructures. For a given alloy anc te nperature, the coarsening of particles refers to the growth of particles under essentially constant volume percents of the component phases. The coarsening exponents increased from 0 24 to 0.29 and from 0.28 to 0.32 for Ti-V and Ti-Mn systems, respectively, when the temperature was increased from 973 to 1073 K. In addition, it was found that the activation energy chances as the temperature is increased from 973 to 1108 K for the Ti-Mn system. Based on these results, it is suggested that the rate-controlling mechanism for particle coarsening changes from mixed (bulk + grain-boundary) diffusion control toward bulk diffusion control when the temperature is increased from 973 to 1108 K for the Ti-Mn system. Similarly, for the Ti-V system, it is suggested that the rate-controlling mechanism changes from grain-boundary diffusion control toward mixed diffusion control when the temperature is increased from 973 to 1073 K. These results clearly indicate that the rate-controlling mechanism depends on temperture as well as the diffusivities of the alloying elements.

Isothermal particle growth in two-phase titanium alloys

Isothermal particle growth studies were carried out on several Ti−Mn and T−V alloys consisting of varying amounts of α and β phases at 973 K. It was found that the particle growth kinetics of the α and the β phases could be represented by simple equations in terms of time and volume percents of the phases. The growth process is presented as a two-way diffusion process where the solvent and the solute atoms move in opposite directions resulting in conversion of α to β and β to α leading to the growth of the particles. The Lifshitz-Wagner-Slyozov theory, which considers only solute diffusion for growth of particles, is slightly modified to incorporate the diffusion of both solute and solvent into the growth equation. The observation of the growth kinetics indicated that, under identical conditions, the growth of α particles in a β matrix was faster than the growth of β particles in an α matrix. Furthermore, for identical conditions, the growth kinetics of Ti−Mn alloys are faster than those of the Ti−V alloys. While the faster kinetics are consistent with higher bulk interdiffusivities of the Ti−Mn system, the magnitude of the difference is much smaller than that expected from the bulk interdiffusivity considerations. In addition, it was found that the growth exponent,n, values are in between the theoretically predictedn values for grain boundary diffusion control and bulk diffusion control. Based on these observations, it is suggested that a mixed mode diffusion mechanism consisting of bulk diffusion as well as grain boundary diffusion is controlling the growth process at 973 K. Details of the investigation and various growth models for α-β titanium alloys are presented.

Experimental observations on the nucleation and growth of δ′ (Al3Li) in dilute Al-Li alloys

The influence of nucleation temperature and pre-aging thermal history on δ′ (Al3Li) precipitate distributions was studied in a binary Al-8.2 at. pct Li alloy. The distribution of δ′ in the microstructure is found to be homogeneous and insensitive to variations in pre-aging thermal history when nucleation occurs at large undercoolings (ΔT > 90 °C). When nucleation occurs at low undercoolings (90 °C > gDT > 40 °C), nucleation is homogeneous but the precipitate number density is influenced by preaging thermal history. At very small undercoolings (ΔT < 40 °C) nucleation next to dislocations is favored unless large numbers of excess vacancies are present, in which case homogeneous nucleation can be more prevalent than nucleation next to dislocations. The early stages of δ′ growth are seen to be consistent with bulk diffusion control. Enhanced growth rates are observed when the material contains a higher concentration of excess vacancies. The retention of this enhanced growth for long periods of time suggests that vacancies can be trapped within the structure for long times despite their mobility. Morphological instabilities are found to develop during δ′ particle growth. These instabilities occur at very small particle sizes and cause the normally spherical particles to tend toward a dendritic morphology.

Generalised coarsening theory in binary alloys

Previous coarsening theories are generalised to include both reaction and diffusion control as well as a nonstationary size distribution function. An initial coarsening rate proportional to t1/2 is found, that changes after a characteristic transition time to the previously found t1/3 law. This agrees with an increase of the diffusion path within the matrix, favouring bulk diffusion control more and more. This and a sharpening of the precipitate size distribution with time, which is predicted by this theory was experimentally observed and cannot be explained by the classical Lifshitz-Slyozov-Wagner (LSW) theory. Moreover the model takes care of a dependence of the specific interface energy sigma on the precipitate size. This leads to modifications for early annealing times, if there is no minimum of total energy at a finite precipitate size. In case of the existence of a total energy minimum at a finite size, a cessation of coarsening and a delta function appears for the equilibrium size distribution function.

Influence of oscillating external pressure on gas-phase reactions in porous catalysts

It is shown that the effectiveness factor of an isothermal, first order, gas-phase catalytic reaction may be increased by operation under conditions of oscillating total pressure. The mathematical analysis considers two models. An Eulerian model, based on the bulk diffusion control and high permeability limit of the dusty gas model, is used to study the simple isomerization reaction. To extend the results to polymerization and cracking reactions which experience volume change, a Lagrangian model which neglects diffusion is developed. Solutions from both models agree in the limit of large Thiele modulus. It is in this limit that the pressure oscillation technique shows greatest promise for improving catalyst performance. Perhaps the most important conclusion of the present analysis is that pressure oscillations also show promise as a means to control the selectivity of complex reactions.

Sintering behavior of a reactively bonded thick film gold ink

The sintering behavior of thick film reactively bonded Au films was studied using electrical re-sistance measurement and scanning electron microscopy. Isothermal firings were conducted at 350‡ to 850‡C for times up to three hours. The sintering process consists of three portions: 1) Burnoff of the binding resin at 350‡ to 450‡C. 2) A second stage involving neck growth between the gold particles with an activation energy of ∼ 15 Kcal/mole and a time exponent of 5.5 to 6.5 corresponding to surface diffusion control and 3) a third stage of densificat ion by pore annihila-tion and grain growth. The activation energy in stage III of 35–45 Kcal/mole agrees closely with bulk diffusion control. Electrical resistance change appears to be a useful method for studying particularly the initial stages of sintering.

Pressure gradients in porous catalyst pellets in the intermediate diffusion regime

Conditions under which pressure gradients might be expected in porous catalyst pellets are investigated and a priori bounds on the magnitude of the pressure variations are obtained for the limiting cases of bulk diffusion control and Knudsen diffusion control.Two previous investigations of the effect of neglecting pressure gradients in calculating the effectiveness factor are compared critically, and a third procedure is suggested which avoids inconsistencies inherent in the earlier work and appears to give a better approximation.

On the limit of bulk diffusion control and high permeability in porous catalyst pellets

Passage to the limit of bulk diffusion control and infinite permeability in porous catalyst pellets presents certain problems. These are analysed and, using equations based on the dusty gas model, it is shown that the proper limiting formulation agrees only in very special circumstances with the form of equations customarily used.