Effect of crack width on chloride diffusion coefficients of concrete by steady-state migration tests

The purpose of the present study is to explore the diffusion characteristics of cracked concrete according to the width of cracks. Major test variables include crack width, concrete strength, fly ash addition, and maximum aggregate size. The diffusion characteristics have been measured by steady-state migration test. The present study indicates that the diffusion coefficients do not increase with increasing crack widths up to the so-called “threshold crack width.” The threshold crack width for diffusion is found to be around 55–80 μm. Above this threshold value, the diffusion coefficients start to increase with crack width. A composite model with the introduction of “crack geometry factor” was derived to identify the diffusion coefficient in cracked concrete. It was shown that the crack geometry factor ranges from 0.067 to 0.206. Finally, the effects of concrete strength, fly ash addition and maximum aggregate size on diffusion coefficients are also discussed.     

Defect controlled diffusion of lithium ions in Mn doped V2O5 for potential applications as cathode material

With a motivation to unravel the effect of cation (Mn) doping-induced modifications in structure, charge transfer resistance, and Li-ion diffusion in V₂O₅ a systematic study using X-ray diffraction (XRD), Raman Spectroscopy and electrochemical impedance spectroscopy (EIS) has been employed using three electrodes configuration. Structural investigations using XRD suggest the selective diffusion of Mn ion towards the c-axis at a low doping percentage. Raman spectroscopy suggests the shift in 994 cm^-1 modes which substantiates the uniaxial diffusion of Mn ions. Nyquist plots show that interfacial charge transfer resistance is highest for the lowest doping i.e., 1% Mn-doped V₂O₅ and exhibits the lowest diffusion coefficient as compared to other doped V₂O₅ samples. Specific capacitance calculated from cyclic voltammetry is found to be highest for the 4% Mn-doped V₂O₅ sample. Moreover, diffusion of Lithium ions improves with an increase in doping concentration due to higher concentration of defects as evident from Δd/d and Nelson–Riley factor (NRF) for pure V₂O₅ and Mn-doped V₂O₅.     

Effects of ionic migration on the concentrations and mass transfer rate in the diffusion layer of dissolving metals

The transport processes occurring within the diffusion layer of dissolving anodes are analysed with the help of a mathematical model which takes into account mass transfer by both diffusion and ionic migration in the presence of a supporting electrolyte. The steady-state transport equations are solved for the ionic concentrations and potential difference as a function of distance within the diffusion layer, metal-ion charge and diffusivity, supporting electrolyte concentration and metal dissolution rate. Upon normalization of the variables, a dimensionless group is obtained. This group includes the transport properties of the system and shows the inter-relationship between them. The anodic dissolution of Cu in HClO₄ was chosen to test some of the predictions of the system. The measured metalion concentrations were much less, while the potential gradient was much higher, than predicted. This is explained on the basis of ionic interactions which operate at higher concentrations. It is shown, both theoretically and experimentally, that in this strong acid medium the concentration of hydrogen ions decreases in the diffusion layer of a dissolving anode due to ionic migration of the hydrogen ions in accord with the prevailing potential gradient.     

Evaluation of steady-state flow curves from model molecular weight distributions

Graessley's theory of entanglement was applied to several model distributions. The distribution functions chosen were such that the differential weight distributions were triangular with respect to a log molecular weight axis. The molecular weight level, breadth of the molecular weight distribution, skewness, and blending of simple distributions were studied in their effect on the steady-state flow curve. The governing factor for the shape of the reduced flow curve was shown to be M?_z+1·M?_z/M?_w². Other general features of the flow curve predicted by Graessley's theory were discussed.     

Self-healing and conductivity of chitosan-based hydrogels formed by the migration of ferric ions

The currently reported self-healing hydrogels have problems of low mechanical strength, single performance, and poor self-healing efficiency, which greatly limit their applications. Here, through adding N-carboxyethyl chitosan to acrylic-Fe³⁺ system, the self-healing physically crosslinked hydrogels were prepared via in situ free radical polymerization, which have excellent self-healing ability and mechanical properties. The maximum tensile strength and elongation at break of the hydrogels can reach up to 280 KPa and 1900%, respectively. Owing to the reversibility of coordination, self-healing efficiency of the hydrogels can reach 98% in 2.5 h. Moreover, the hydrogels also have good conductivity due to the migration of Fe³⁺. This strategy can broaden the applications of chitosan-based self-healing hydrogels. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47885.     

Influence of concentrations of copper,levelling agents and temperature on the diffusion coefficient of cupric ions in industrial electro-refining electrolytes

Few data are available for diffusion coefficients measured in industrial copper electrolytes. In the present work the influence of copper concentration (19.9–58.1 g dm⁻³), temperature (20–60°C) and concentrations levelling agents i.e. animal glue (0–5 mg dm⁻³) and thiourea (0–5 mg dm⁻³) on diffusion coefficients of copper was studied in industrial copper refinery electrolytes. Chronoamperometry at ultramicroelectrodes was used as an electrochemical technique. Apparent bulk diffusion coefficients were calculated on the basis of the theory of electrochemical nucleation on disc-shaped ultramicroelectrodes. Increasing copper concentration decreased the apparent bulk diffusion coefficient of copper and diffusion coefficients followed the Arrhenius temperature relationship. The experimental activation energy was 26.8 kJ mol⁻¹. The influence of levelling agents on diffusion coefficients was not strong in the studied concentration range of animal glue and thiourea.     

On the prediction of crystal shape distributions in a steady-state continuous crystallizer

Zhang and Doherty [2004. Simultaneous prediction of crystal shape and size for solution crystallization. A.I.Ch.E. Journal 50, 2101-2112] have provided a one-dimensional analysis of crystallization based on the assumption that the relative face-specific growth rates of a (2-D) crystal are independent of supersaturation and hence invariant with time. Subsequent work by these authors [Zhang, Y., Sizemore, J.P., Doherty, M.F., 2006. Shape evolution of 3-dimensional faceted crystals. A.I.Ch.E. Journal 52, 1906-1915) consider shape evolution of single three-dimensional crystals with morphological changes. In this work, we present a multidimensional population balance approach accounting for dependence of the relative face-specific growth rates on supersaturation, a situation more commonly encountered. For example, Joshi and Paul [1974. Effect of supersaturation and fluid shear on habit and homogeneity of potassium dihydrogen phosphate crystals. Journal of Crystal Growth 22, 321-327] and Mullin and Whiting [1980. Succinic acid crystal-growth rates in aqueous solution. Industrial & Engineering Chemistry Fundamentals 19, 117-121] report face-specific growth rates with different dependence on the supersaturation. Thus it has been observed that there exists significantly different crystal shapes in a crystallizer [Yang, G., Kubota, N., Sha, Z., Louhi-Kultanen, M. Wang, J., 2006. Crystal shape control by manipulating supersaturation in batch cooling crystallization. Crystal Growth and Design 6, 2799-2803]. Consequently, the population of crystals at any instant will have widely varying crystal shapes and sizes depending upon the initial crystal shape and size distribution. Computations are presented for the shape distributions of the crystal population emerging from a steady-state continuous crystallizer for two cases: (1) feed without crystals including nucleation for the formation of new crystals, and (2) feed with seed crystals of known shape, with suppressed nucleation. In the range of mean residence times investigated, the calculated crystal volume distributions for the first case show geometrically dissimilar shapes without morphological variations. However, in the second case, because the feed crystals of the chosen shape were susceptible to morphological changes, the volume distributions display this feature with shape and size distributions for each of a number of different morphologies. By varying operating conditions such as the flow rate, the inlet supersaturation, and the shapes of feed crystals, the proposed model can clearly be used to manipulate the crystal shape and size distributions and their morphologies.     

Chemotaxis in shear flow: Similarity solutions of the steady-state chemoattractant and bacterial distributions

When chemotactic bacteria are exposed to a concentration gradient of chemoattractant while flowing along a channel, the bacteria accumulate at the interface between the chemoattractant source and bacterial suspension. Assuming that the interface is no-slip, we can apply the shear flow approximation near the no-slip boundary and solve a steady-state convection-diffusion model for both chemoattractant and bacterial concentrations. We suggest similarity solutions for the two-dimensional problem and identify a critical length scale η_c for bacteria chemotaxis in a given concentration gradient. The analysis identifies three dimensionless groups representing, respectively, chemotactic sensitivity, the chemotaxis receptor constant, and the bacteria diffusion coefficient, which typically show coupled effects in experimental systems. We study the effect of the dimensionless groups separately and provide understanding of the system involving shear flow and chemotaxis.     

Numerical and experimental simulations of diffusion charging of non-spherical aerosol particles by dense bipolar ions

Numerical and experimental simulations have been conducted for the time history of the diffusion charging process on the surface of aerosol particles by dense bipolar ions under continuum conditions. The range of conditions treated in the numerical simulations include positive-negative ion diffusion coefficient ratio from 0 to 1, aerosol particle radius from 0.1 to 10 μm, Debye ratio R_p/λD from 0 to 1 (equivalent to maximum charge density up to N₁ = 10¹² cm⁻³ for an ion temperature of 300 K), the major-to minor axis ratios of prolate spheroids, L, from 1 to 100. The experimental simulation was conducted by using a conductive dummy particle suspended by a thin shielded wire, and the charged particle deposition current flux was measured and the bipolar environments. Then the effect of particle surface charges was simulated by imposing an electric potential on the dummy particles. The results show that, (1) for small ion density (R_{p/λD 10⁻²}); the present results are in good agreement with model of Chang et al. (1978, 1983). (2) the aeroso particle charging speed and charging limit increase with increasing Debye ratio; (3) for larger Debye ratio, bipolar charging is faster than unipolar charging; (4) the effect of particle shape L is observed to be significantly influenced by Debye ratios: (5) the charging limit of the aerosol particle increases with L.     

Anomalous diffusion of iodine ions into polypropylene implanted with F+ and I+ ions

The diffusion of K and I atoms from 5 M water solution of KI into polypropylene implanted with 150 keV F⁺ and I⁺ was studied at boiling temperature. Standard Rutherford back scattering (RSB) technique was used for the determination of concentration depth profiles of diffused atoms. It was found that the I and K atoms penetrate the polymer surface layer affected by ion implantation and they are captured by trapping centers produced by electronic energy losses of implanted ions. The uptake of K atoms is several times higher than that of I atoms, and the relative K concentration exceeds the KI stoichiometric value. © 1995 John Wiley & Sons, Inc.     

Change in distributions of particle positions by Brownian diffusion in a non-uniform external field

General expressions for the particle position distribution in non-uniform external fields are provided based on an investigation of distribution function. For a case of uniform or vanishing external fields, the particle position distribution with time results in a normal distribution with a variance as obtained for pure Brownian motion. To account for the non-uniformity of external fields, simple linearization methods were applied and the change in particle position distribution was investigated. A procedure to determine time evolution of change in variance of particle position distribution in non-uniform external fields is presented. For any non-uniform external field described by an analytic function, this simple procedure can accurately calculate the spreading characteristics of nanoparticles in the diffusional regime. This approach is used to predict particle diffusional spreading in a coaxial DMA, and the results are seen to closely match Monte Carlo simulations.     

Studies in the adsorption and diffusion of ions in chitosan

Chitin was obtained from shell of the tail portions of the prawn Nephrops norvegicus (L) and was deacetylated to give chitosan. The adsorption and diffusion of copper (II) ions were studied and the diffusion of the acid dye CI Acid Orange 10 through chitosan films containing different amounts of chelated copper was investigated. It was found that with increasing copper content the rate of diffusion through the membrane decreased.     

A direct method for determining chloride diffusion coefficient by using migration test

Calculation of the effective diffusion coefficient is proposed using an analytical model based on Nernst-Planck equation, on electroneutrality and on measurements of current intensity. To validate this approach, migration tests were carried out on different w/c mortar samples. In steady state, the resulting effective diffusion coefficients are larger than those usually obtained with models based on Nernst-Planck equation without electroneutrality. An explanation of these differences and the importance of electroneutrality are discussed in the present work.     

Determination of diffusion coefficients of depositing ions in molten chlorides by transient electrochemical techniques

The electrochemical reduction of Pb(II), Zn(II) and Mg(II) ions at glassy carbon and tungsten electrodes in molten KCl–LiCl eutectic was studied by linear sweep voltammetry (LSV), convolutive potential sweep voltammetry (CPSV), chronopotentiometry and chronoamperometry. In the case of lead deposition, the initial nucleation stage was found to influence the shape of the voltammetric current peak. CPSV was consequently believed to be a more reliable method than LSV for the calculation of the diffusion coefficient of the Pb(II) ion. Similar complications were not observed for zinc and magnesium since the nucleation overpotentials for these metals were significantly lower than for lead. On the other hand, lithium codeposition made it difficult to interpret the zinc and especially the magnesium convolution voltammograms. Chronopotentiometry yielded practically identical results for D _Pb(II) as the voltammetric techniques. However, due to a substantial residual current, the Sand equation was not obeyed for the Zn(II) and Mg(II) ions. Determination of diffusion coefficients from single potentiostatic current transients and the Cottrell equation was not found to be a very reliable method. Empirical expressions for the temperature dependence of D in the range 400–500 °C were calculated for all three ions.     

Field-assisted diffusion of potassium ions in sodium silicate glass

The kinetics of interaction between the 20Na₂O · 80SiO₂ glass and the potassium nitrate melt is investigated in the presence of a dc electric field and without it. The interdiffusion coefficients and effective electrical mobilities of potassium ions, as well as the parameters of the temperature dependence of the electrical mobility, are determined. It is established that the models describing the field-assisted diffusion processes in the framework of the Nernst-Einstein equation are not applicable to analyzing the interactions responsible for the considerable changes in the chemical composition of the diffusion zone.     

Calculation of diffusion turbulent flame from instantaneous parameters

Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 24, No. 2, pp. 90–92, March–April, 1988.     

Hydrogenation of nitrotoluenes controlled by the electrochemical potential of the catalyst

The hydrogenation of 2-nitrotoluene, 2,4-dinitrotoluene and 2,4,6-trinitrotoluene on Pd/C, Pt/C and Ni/SiO₂ catalysts was investigated in a liquid phase batch reactor at pressures between 0.4 and 2.5 MPa. The reaction was controlled by regulating the substrate supply in such a way that the electrochemical potential of the catalyst and, consequently, the surface concentration of active hydrogen was kept at a constant level. The observed hydrogenation rate dependence on the concentration of active surface hydrogen shows that the hydrogen activation is at least one of the rate determining steps. Additionally, controlling the reaction by the potential of the catalyst allows one to prevent dangerous feed concentrations and fast deactivation, too.