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The combustion velocities of two fractions of mechanically activated Ni + Al mixtures are compared in this paper. The combustion of pressed samples and samples of bulk density is studied. The main aim of the study is to experimentally verify the main conclusions of the microheterogeneous model of gas-free combustion at the macrolevel using an activated Ni + Al mixture as an example. The relative elongation, the combustion velocity of the samples, the macrostructure, and phase composition of the synthesis products are studied. The combustion velocity is maintained with a change in the density of the samples and slightly increases with a decrease in the size of the composite particles. It is established that pressed samples during combustion elongate more strongly than samples from bulk density. Samples consisting of large particles elongate more strongly during combustion than samples consisting of smaller particles. The phase composition of combustion products depends on the fraction of composite particles and sample density. An explanation of the regularities observed in this study is proposed. [ABSTRACT FROM AUTHOR]

The microheterogeneous model makes it possible to describe the main transport properties of ion-exchange membranes using a single set of input parameters. This paper describes an adaptation of the microheterogeneous model for describing the electrical conductivity and diffusion permeability of a track-etched membrane (TEM). Usually, the transport parameters of TEMs are evaluated assuming that ion transfer occurs through the solution filling the membrane pores, which are cylindrical and oriented normally to the membrane surface. The version of the microheterogeneous model developed in this paper takes into account the presence of a loose layer, which forms as an intermediate layer between the pore solution and the membrane bulk material during track etching. It is assumed that this layer can be considered as a “gel phase” in the framework of the microheterogeneous model due to the fixed hydroxyl and carboxyl groups, which imparts ion exchange properties to the loose layer. The qualitative and quantitative agreement between the calculated and experimental concentration dependencies of the conductivity and diffusion permeability is discussed. The role of the model input parameters is described in relation to the structural features of the membrane. In particular, the inclination of the pores relative to the surface and their narrowing in the middle part of the membrane can be important for their properties.

Profiled ion-exchange membranes are promising for improving the parameters of reverse electrodialysis due to the reduction of pumping power and electrical resistance. The smooth commercial heterogeneous cation-exchange MK-40 and anion-exchange MA-41 membranes were chosen as the initial membranes. Profiled membranes with three different types of surface profiles were obtained by hot pressing the initial membranes. The bilayer membranes were made on the basis of single-layer profiled membranes by casting MF-4SK film on the profiled surfaces. The diffusion permeability of all types of single-layer and bilayer profiled membranes was higher than of the initial ones due to the appearance of large defects on their surface during pressing. The conductivity of the profiled membrane was lower in the diluted solution and higher in the concentrated solution than of the initial one for all samples except for the bilayer anion-exchange membrane. The conductivity of that sample was lower than that of the initial anion-exchange MA-41 membrane over the entire range of studied concentrations. The counter-ion transport numbers for all studied membranes were calculated based on the concentration dependences of conductivity and diffusion permeability of the membrane by the microheterogeneous model. The selectivity of single layer and bilayer profiled membranes became lower after their profiling due to the increase of the solution phases of membranes. The asymmetry of the current-voltage curves for all single-layer and bilayer profiled membranes was found. The application of the single layer and bilayer profiled membranes in reverse electrodialysis did not lead to an increase in power density.

This paper shows the possibility of using a microheterogeneous model to describe the properties of ion-exchange membranes and calculate the characteristics of a reverse electrodialyzer from the data obtained. We studied the properties of eight samples of heterogeneous cation exchange membranes (two samples of each type of membrane). The samples differed in the year of issue and storage conditions. It is shown that for heterogeneous ion-exchange membranes MK-40 and MA-41, the samples' properties can differ significantly. The counterions transport numbers calculated within the framework of the microheterogeneous model for Ralex membranes differ insignificantly. The counterion transport number in 1 mol/L sodium chloride solution is 0.96 for Ralex CM and 0.98 ± 0.01 for Ralex AMH. For the MK-40 membrane, the transport number in the same solution is 0.94 ± 0.04, and for the MA-41 membrane, it is 0.85 ± 0.1. The possibility of calculating the transport numbers and predicting the open-circuit voltage based on simple physicochemical measurements allows selecting the best membrane pairs for the reverse electrodialysis process. Comparison of the open-circuit potential value calculated using the obtained transfer numbers with experimental data showed that in the case of using Ralex membranes, the difference between the experimental and calculated values is 2%. The calculated value of the open circuit potential was 0.19 V/membrane pair or 1.69 V for the investigated reverse electrodialyzer with nine pair chambers.

Dependence of the combustion of a Ti + C granular charge on a granule size is experimentally studied. It is revealed that the burning rate of a granular mixture of all fractions used in the work is higher than the burning rate of a bulk-density powder mixture. It is shown that, with a decrease in the granule size, the burning rate of the charge in the absence of gas decreases due to an increase in the number of boundaries between the granules per unit length of the sample. A strong influence of the nitrogen flow on the burning rate of both coarse and fine granules is established. It is shown that, in contrast to fine granules, an increase in the nitrogen flow rate of coarse granules up to 600 liters/h leads to a transition to convective combustion. The studies performed indicate that, despite the structural analogy between mechanically activated and granular mixtures, the relationship between the combustion time and the front transition time in granular mixtures is completely different. This means that the combustion of granular mixtures even in the absence of a gas flow cannot be explained within the framework of a microheterogeneous model. [ABSTRACT FROM AUTHOR]

The paper shows the possibility of using a microheterogeneous model to estimate the transport numbers of counterions through ion-exchange membranes. It is possible to calculate the open-circuit potential and power density of the reverse electrodialyzer using the data obtained. Eight samples of heterogeneous ion-exchange membranes were studied, two samples for each of the following types of membranes: Ralex CM, Ralex AMH, MK-40, and MA-41. Samples in each pair differed in the year of production and storage conditions. In the work, these samples were named “batch 1” and “batch 2”. According to the microheterogeneous model, to calculate the transport numbers of counterions, it is necessary to use the concentration dependence of the electrical conductivity and diffusion permeability. The electrolyte used was a sodium chloride solution with a concentration range corresponding to the conditional composition of river water and the salinity of the Black Sea. During the research, it was found that samples of Ralex membranes of different batches have similar characteristics over the entire range of investigated concentrations. The calculated values of the transfer numbers for membranes of different batches differ insignificantly: ±0.01 for Ralex AMH in 1 M NaCl. For MK-40 and MA-41 membranes, a significant scatter of characteristics was found, especially in concentrated solutions. As a result, in 1 M NaCl, the transport numbers differ by ±0.05 for MK-40 and ±0.1 for MA-41. The value of the open circuit potential for the Ralex membrane pair showed that the experimental values of the potential are slightly lower than the theoretical ones. At the same time, the maximum calculated power density is higher than the experimental values. The maximum power density achieved in the experiment on reverse electrodialysis was 0.22 W/m2, which is in good agreement with the known literature data for heterogeneous membranes. The discrepancy between the experimental and theoretical data may be the difference in the characteristics of the membranes used in the reverse electrodialysis process from the tested samples and does not consider the shadow effect of the spacer in the channels of the electrodialyzer.

The experimental results of the study of the concentration dependence of the electrical conductivity and diffusion permeability of ion-exchange membranes which are in contact with solutions of weak electrolytes (carboxylic acids) are provided for the first time. An attempt was made to correlate the observed properties of the membrane to their structure, based on the microheterogeneous model. Some features have been revealed that differentiate the behavior of membranes in solutions of strong and weak electrolytes. For example, for anion-exchange membranes, the dependence of the electrical conductivity on the equilibrium solution concentration disappears in the concentration above 0.05 M, and there is an insignificant increase in the electrical conductivity in lower concentrations. An explanation for the observed effects is suggested which takes into account that the electrical conductivity of the gel phase is much higher than the electrical conductivity of an electrically neutral solution. An equation is proposed which connects the electrical conductivity of a membrane with its ion-exchange capacity and mobility of the counterion in the gel phase. Recommendations on the practical aspect of electrodialysis technology for the treatment of weak electrolytes are given. From a fundamental point of view, it is concluded that for a full description of the transport-structural organization of ion-exchange membranes which are in a solution of a weak electrolyte, the knowledge of the concentration dependence of the electrical conductivity and diffusion permeability only is not enough, and other experimental methods are required. [ABSTRACT FROM AUTHOR]

A batch of new cation-exchange membranes (CEM(n)) and three batches of used cation-exchange membranes (CEM(u)) at various use durations, from different electrodialysis (ED) units used in food industry, were characterized to evaluate the effects of prolonged use and organic fouling on their microstructure. Different characterization methods were experimented: ion-exchange capacity (IEC), thickness (T m ), conductivity (k m ), contact angle (θ), water content (W C ), volume fraction of the inter-gel solution ( f 2 ), tensile strength tests, FTIR, structural and elementary SEM analysis and morphological analysis by optical microscopy. Non-aggressive, easy and economic cleaning strategy was tested in the ex-situ static mode. Three solutions (35 g L −1 NaCl, reconstituted seawater and a water-ethanol mixture) were used as regenerative solutions, where the membranes were soaked from 2 to 120 h. The efficiency of the cleaning procedures were evaluated, for each sample of treated membrane, by following k m , IEC, θ, and f 2 as functions of the cleaning time in order to judge the recovery of their physicochemical properties. The water-ethanol cleaning method was found to be the most efficient for these membranes. This method leads, first, to significant increases in k m , IEC and f 2 (at least 47%, 26% and 63% , respectively after 120 h) which indicates the effectiveness of internal cleaning, and second, to a decrease of about 15% in θ (after 120 h), that confirms the stripping of a part of the external fouling layer. We have used the microheterogeneous model to demonstrate the progressive increase of f 2 as a function of the cleaning duration with the water-ethanol mixture. [ABSTRACT FROM AUTHOR]

The concentration dependences of the electrolyte sorption, specific electric conductivity, and diffusion permeability of MK-40 and Nafion-117 cation-Ion-exchange membranes in NaCl solutions have been obtained experimentally. Both the membranes contain macropores. The heterogeneous MK-40 membrane contains pores with a size of up to 1 μm between the particles of the ion Ion-exchange resin and polyethylene. The homogeneous Nafion-117 membrane has been subjected to a special thermal pretreatment which results in the formation of macropores. It has been shown for the first time that these experimentally found relationships can be quantitatively described using one set of structural and kinetic parameters of the microheterogeneous model. [ABSTRACT FROM AUTHOR]

The formation of organic colloidal particles in the pores and on the surface (fouling) of membranes used in the food industry is a significant constraint on the further development of membrane technology. A model to describe the effect of these particles on electric conductivity and diffusion permeability has been proposed. It is based on a microheterogeneous two-phase model constructed in terms of the concepts of nonequilibrium thermodynamics and effective medium theory. The model takes into account the presence of two phases: (i) the gel phase comprising a polymer matrix and fixed ions whose charge is compensated for by mobile ions and (ii) the electrically neutral solution filling the intergel spaces. Each of the phases is characterized by intrinsic thermodynamic and kinetic parameters. The model takes into account changes in the values of these parameters caused by the formation of organic nanoparticles in meso- and macropores (fouling). It is assumed that the formation of colloidal particles in the intergel solution leads to a decrease in the ion mobility. In addition, these particles are capable of deprotonating a portion of the fixed ions and thereby decreasing the exchange capacity of the membrane. A high degree of hydration of these particles is responsible for an increase in the volume fraction of intergel spaces. Selection of relevant model parameters provides good agreement between calculation and experimental results. [ABSTRACT FROM AUTHOR]

Immobilization of inorganic nanoparticles (like ZrO 2 , TiO 2 and SiO 2 ) allows essential improvement of proton-exchange membrane properties pertinent for fuel cell applications. We propose a model for describing the effect of nanoparticles on the membrane electrical conductivity, diffusion permeability and permselectivity. The basis is the microheterogeneous two-phase model, which, in turn, is built on the irreversible thermodynamics and effective medium approach. The model considers the presence of three different domains, each of them characterized by their own parameters (such as the diffusion coefficients): charged gel, electrically neutral solution filling the intergel spaces, and nanoparticles. The nanoparticles are localized in the meso- and macropores, which form the intergel solution. The comparison of calculations with literature experimental data shows a good agreement. With increasing content of nanoparticles, the membrane conductivity passes through a maximum, while diffusion permeability decreases and permselectivity increases. The effect is due to the fact that the nanoparticle replaces the electrically neutral solution, which occupies the inner part of macro- and mesopores. [ABSTRACT FROM AUTHOR]

A new method for determining the electrical conductivity of ion-exchange membranes was implemented with four commercial membranes (AMX, CMX, MK-40 and MA-41). It is based on lateral resistance measurements without direct contact between electrodes and membranes. The cell configuration made it possible to determine the membrane conductivity over a wide range of electrolyte concentrations (measurements were carried out in the range 10 −5 –5×10 −1 M). The structural parameters of the different membranes were inferred from AC conductivities and the microheterogeneous model. They were found in good agreement with literature results obtained by normal measurements (i.e. with current lines oriented normally to the membrane surface), thus confirming the reliability of the proposed method. The main advantage of our method is the possibility to characterize ion-exchange membranes even at low salt concentration unlike usual non-contact methods based on normal measurements. Counterion diffusion coefficients and transport numbers within the membrane gel phase were estimated. The inter-gel phase volume fraction of an anion-exchange membrane (AMX) was also determined for the first time from membrane DC conductivity inferred from streaming potential and streaming current measurements. An excellent agreement was found with structural parameters obtained from AC measurements. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the physicochemical, structural and mechanical properties of an anion-exchange membrane were investigated, during its lifetime in industrial electrodialysis for whey demineralization. Several analytical techniques permitted to disclose the membrane structure and to describe the evolution with time of the membrane properties. The homogeneous anion-exchange membrane, AMX-SB, is constituted of a semi-interpenetrating polymer network of poly(vinyl chloride) and functional poly(styrene-co-divinylbenzene). No significant loss of the ion-exchange capacity or degradation of the functional poly(styrene-co-divinylbenzene) chains were detected. However, fouling caused a decrease in the counter-ion mobility within the membrane, which produced a reduction of the electrical conductivity. A progressive loss of poly(vinyl chloride), which was degraded or washed out from the membrane during the cleaning-in-place process, was clearly evidenced. This led to the formation of non-charged pores available for electroneutral electrolyte solution and large molecules, such as lactose and proteins. The occurrence of such pores resulted in an increase in electrolyte permeability through the membrane and a rebound of the conductivity at the last stage of electrodialysis. The microheterogeneous model was applied to well account for these changes. By means of tensile strength tests, it was possible to investigate the mechanical properties of the membrane, which turned gradually from a rigid and tough material, to a rigid and brittle one, due to the loss of poly(vinyl chloride), leading to the end of the membrane lifetime. [Copyright &y& Elsevier]

Abstract: Important parameters such as ion exchange capacity, conductivity, permselectivity, quantity of sorbed electrolyte and water uptake of the Selemion AMV anion exchange membrane conditioned in KCl and NaCl solutions were determined in order to assess the applicability of the two-phase model of structure microheterogeneity to this case. In general, a good consistency between the experimental results and the theoretical approach was obtained. Electrical conductivity measurements allowed evaluating the volume fractions of two distinct internal phases inside the membrane. Aside from the chronopotentiometric behaviour of the AMV membrane, SEM and XPS techniques contributed to provide a better description of the overall membrane homogeneity. Influence of co-ions in relation with water uptakes on the conductivity and permselectivity of the studied membrane has been shown. An existence of partition equilibrium between the electrolytes sorbed in the membrane and the external solution confirmed in this work should be introduced within the microheterogeneous model. [Copyright &y& Elsevier]

When electrodialysis (ED) is applied to process streams that are known to foul and scale the system, it can be operated with an oscillating electric field. This procedure is known as pulsed electric fields (PEF) and has been proven to reduce the fouling susceptibility of the system. In literature, the suppression of fouling is attributed to a combination of three effects: the reduction of concentration polarisation, the promotion of electroconvection and the suppression of water splitting. However, these effects have yet to be substantiated quantitatively. Here we study how much of the fouling suppression during PEF can be attributed to the reduction of concentration polarisation. A Nernst–Planck and Kedem–Katchalsky modelling approach is adopted to simulate the evolution of concentration profiles of a ternary solution of sodium, chloride and dodecyl sulfate (DS) during PEF. The effect of the pulse parameters on the boundary layer concentration of sodium dodecyl sulfate (SDS) is studied along with an evaluation of the current efficiency and energy consumption. Our results illustrate the counterproductivity of low-frequency pulses and the trade-off between the current efficiency and fouling suppression but fail to explain the improvement when compared to the constant current operation. Fouling layer relaxation is put forth to complement to previously mentioned fouling suppression effects and is relevant for future research. [Display omitted] • Fouling reduction during pulsed electric field (PEF) ED is investigated. • Fouling behaviour of sodium dodecyl sulfate (SDS) during PEF is studied. • A Nernst–Planck model simulates the build up of SDS at the membrane surface. • The pulse frequency and the duty cycle affect both fouling and energy consumption. • The optimal frequency is system dependent. [ABSTRACT FROM AUTHOR]

In this paper, we simulate the changes in the structure and transport properties of an anion-exchange membrane (CJMA-7, Hefei Chemjoy Polymer Materials Co. Ltd., China) caused by its modification with a perfluorosulfonated ionomer (PFSI). The modification was made in several stages and included keeping the membrane at a low temperature, applying a PFSI solution on its surface, and, subsequently, drying it at an elevated temperature. We applied the known microheterogeneous model with some new amendments to simulate each stage of the membrane modification. It has been shown that the PFSI film formed on the membrane-substrate does not affect significantly its properties due to the small thickness of the film (≈4 µm) and similar properties of the film and substrate. The main effect is caused by the fact that PFSI material "clogs" the macropores of the CJMA-7 membrane, thereby, blocking the transport of coions through the membrane. In this case, the membrane microporous gel phase, which exhibits a high selectivity to counterions, remains the primary pathway for both counterions and coions. Due to the above modification of the CJMA-7 membrane, the coion (Na+) transport number in the membrane equilibrated with 1 M NaCl solution decreased from 0.11 to 0.03. Thus, the modified membrane became comparable in its transport characteristics with more expensive IEMs available on the market. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we have measured the conductivity of two cation-exchange membranes (Nafion®117 and CM2), as a function of methanol content (X MeOH) and lithium chloride concentration in the external solution. The experimental data are treated and analyzed using the microheterogeneous model, leading to the determination of volume fractions of different structural regions: the gel phase containing the polymer chains with fixed sites and only counter-ions, the inter-gel phase formed by the electroneutral electrolyte solution filling central regions of the pores, and the hydrophobic domains of polymer chains free from functional sites. The variations of the membrane conductivity and of the inter-gel volume fraction (f 2) versus X MeOH are explained by different physicochemical interactions between water, methanol, counter-ions, functional sites, and membrane material. The decrease in the membrane conductivity is attributed to the diminution of the dielectric constant in presence of methanol, and so to the formation of ion-pairs between counter-ions and functional sites in the gel phase and between counter-ions and co-ions in the inter-gel phase. We have confirmed the higher acidity of the functional groups in Nafion compared to the CM2 membrane. For low X MeOH values, the decrease of f 2 versus X MeOH is attributed to the transfer of methanol molecules from the inter-gel phase to the gel phase in order to maintain the solubility of the different ions (functional sites and counter-ions). For high X MeOH values, due to the formation of ion-pairs, the gel phase becomes almost uncharged, and so methanol molecules come back to the inter-gel phase, leading to an increase of the f 2 values. [Copyright &y& Elsevier]

Gasless combustion of a mechanically activated Ni-Al mixture is experimentally studied; the process is compared with the characteristics of combustion of a nonactivated Ni-Al mixture. Mechanical activation of the Ni-Al mixture is shown to form layered conglomerates consisting of numerous layers of the initial components. As a result, the structure of the medium becomes very similar to a cellular structure, which forms the basis for advanced microheterogeneous models of gasless combustion. A relation between the local (microscopic) and global (macroscopic) parameters of gasless combustion is found. Modification of the microstructure of the initial medium by means of mechanical activation allows obtaining products whose microstructure differs significantly from the microstructure of products obtained from nonactivated mixtures. [ABSTRACT FROM AUTHOR]

The paper shows the possibility of using a microheterogeneous model to estimate the transport numbers of counterions through ion-exchange membranes. It is possible to calculate the open-circuit potential and power density of the reverse electrodialyzer using the data obtained. Eight samples of heterogeneous ion-exchange membranes were studied, two samples for each of the following types of membranes: Ralex CM, Ralex AMH, MK-40, and MA-41. Samples in each pair differed in the year of production and storage conditions. In the work, these samples were named "batch 1" and "batch 2". According to the microheterogeneous model, to calculate the transport numbers of counterions, it is necessary to use the concentration dependence of the electrical conductivity and diffusion permeability. The electrolyte used was a sodium chloride solution with a concentration range corresponding to the conditional composition of river water and the salinity of the Black Sea. During the research, it was found that samples of Ralex membranes of different batches have similar characteristics over the entire range of investigated concentrations. The calculated values of the transfer numbers for membranes of different batches differ insignificantly: ±0.01 for Ralex AMH in 1 M NaCl. For MK-40 and MA-41 membranes, a significant scatter of characteristics was found, especially in concentrated solutions. As a result, in 1 M NaCl, the transport numbers differ by ±0.05 for MK-40 and ±0.1 for MA-41. The value of the open circuit potential for the Ralex membrane pair showed that the experimental values of the potential are slightly lower than the theoretical ones. At the same time, the maximum calculated power density is higher than the experimental values. The maximum power density achieved in the experiment on reverse electrodialysis was 0.22 W/m2, which is in good agreement with the known literature data for heterogeneous membranes. The discrepancy between the experimental and theoretical data may be the difference in the characteristics of the membranes used in the reverse electrodialysis process from the tested samples and does not consider the shadow effect of the spacer in the channels of the electrodialyzer. [ABSTRACT FROM AUTHOR]