Complex Heterogeneous Systems Research Articles

NMR studies of non-freezing water in randomly packed beds of porous particles

NMR water proton transverse relaxation time distributions are shown to provide valuable information about the distribution of ice and nonfreezing water in complex heterogeneous systems. In a model system consisting of randomly packed Sephadex beads the NMR data confirm that ice inside the porous Sephadex beads coexists as a two phase system with unfrozen dextran gel. The NMR relaxation technique is shown to provide complementary information to both differential scanning calorimetry and cryomicroscopy.

Effect of ion implantation on the chemical composition and structure of surface layers of heat-resistant alloys

We present a critical analysis of literature data devoted to the effect of ion implantation on the physicochemical state of surface layers of heat-resistant materials. We pay special attention to the problem of choosing the type of ions to be implanted, the bombardment conditions, and the finishing heat treatment with the goal of achieving the optimal level of operating properties. We show that for complex heterogeneous systems, the optimal treatment conditions can be estimated by methods of local equilibrium thermodynamics, chemical kinetics, and the theory of interaction of fast ions with solids using the basic principles of bulk alloying of heat-resistant alloys. We consider the characteristics features of continuous and pulsed implantation in heat-resistant materials. We formulate the requirements for techniques for investigating the chemical composition and the structural phase state of the near-surface regions of the bombarded targets. Analysis of the literature data on the effect of the bombardment conditions on the structure of the metals has shown that the data are ambiguous, that different authors use different terminology, and that the question of the phenomenology of the “long-range effect” is under dispute.

Certain regularities of phase formation and structure evolution during hot-pressing of ultrafine powders

We studied the processes of structure evolution and phase formation in the material during hot-pressing of ultrafine (highly dispersed) composite powders of the [Si3N4-10% Y2O3] system. Densification of the powder occurs under the conditions facilitating chemical interaction in the complex heterogeneous system. The sintering mechanism of UFCP consists of a stage of self-activated rapid grain growth in the solid phase mainly in the 1120–1150°C range and a stage of liquid phase sintering in the 1150–1650°C range under the conditions of varying composition, viscosity, and content of the liquid phase. It was established that densification is initiated by a low-melting yttrium silicate type liquid phase that disappears after participating in the process leading to the formation of a high-melting liquid phase having a composition close to that of yttrium-silicon oxynitrides. The revealed regularities in the behavior of UFCP during the process of hot-pressing offer the possibility of controlling the structure evolution in them purposefully in order to obtain the required set of properties. The results of our studies are being used for obtaining engineering components.

Asymptotic analysis of a heterogeneous renewable complex system with random environments

This paper deals with an asymptotic analysis of a complex renewable system with N heterogeneous elements looked after by n repairmen. Each element and the repair facility are assumed to operate in independent random environments governed by ergodic Markov chains. The operating and repair times of an element are supposed to be exponentially distributed random variables with parameter depending on the index of the element, the state of the corresponding random environment, and the indices of the failed elements. The repair is carried out according to a First Come, First Served (FCFS) discipline. Assuming that the repair rates are many times greater than the corresponding failure rates (“fast” repair), it is shown that the time to the first system failure converges in distribution, under appropriate norming, to an exponentially distributed random variable. Some numerical examples illustrate the effectiveness of the method proposed by comparing the approximate characteristics to the exact ones.

Voltammetry of heterogeneous metal complex systems: Theoretical analysis of the effects of association/dissociation kinetics and the ensuing lability criteria

Some basic elements are developed for the theory of the voltammetric response of a heterogeneous metal complex system, i.e. a system in which the effective stability of the complex species varies with the metal-to-ligand ratio. The treatment accounts for (1) a Freundlich-type binding characteristic, (2) differences between the diffusion coefficients of complexed and uncomplexed metal species, and (3) the kinetics of the complex association/dissociation reactions, but it is limited to cases with (i) an overall excess of ligands, (ii) a non-variant association rate constant, so that the stability distribution is completely reflected by the dissociation rate constant distribution. Lability criteria are derived and separated into homogeneous and heterogeneous factors. Various aspects are further explained and illustrated by some numerically simulated polarographic waves.

A Conjugate Gradient Finite Element Model of Flow for Large Multiaquifer Systems

A quasi three‐dimensional finite element model based on the conjugate gradient technique is developed and applied to the analysis of subsurface flow occurring in complex heterogeneous multiaquifer systems. Transient leakage across the aquitards is fully taken into consideration using the convolution approach originally derived by Herrera (1970). The convolution integrals enhance the favorable features of the modified conjugate gradients (MCG), which prove to be a very efficient tool for the solution of the integrodifferential model. The high performance of MCG results from two combined factors: (1) the diagonal dominance of the assembled finite element matrix, which is emphasized by the integral contributions; (2) the essential reducibility of the matrix for a wide range of time step values, which is due to the virtual decoupling of the aquifer equations. The preconditioning of the conjugate gradient scheme is therefore improved, and a good convergence occurs after a number of iterations smaller than (N/n)½, N and n being the overall number of nodes and aquifers, respectively. The computational cost of the model is generally low, and its relative efficiency tends to increase for small to moderate time steps and for complex multiaquifer systems where several interconnected aquifers are simulated.

Quantitative Surface Analysis of NBS Standard Materials and Mt. St. Helens Ash by Electron Spectroscopy for Chemical Analysis

Abstract Results are presented which develop a quantitative method of surface analysis by ESCA for complex heterogeneous systems. Calibration and application of the method to determination of surface weight percentages are discussed. Mt. St. Helens Ash is used to authenticate the method; results agree with bulk analysis to ±20%. Results from NBS standard materials are used to establish detection limits of 0.1% by weight for most materials. The minimum detectable concentration of surface segregated species can be as low as 200–300 ppm. Results from the NBS standards establish a surface composition for these species, and allow deduction about particle morphology; organics and volatile metals are likely to be surface segregated and mineral species are segregated to the particle core.

Reply by the authors to discussion by H. Pascal and D. Rankin

The interesting comment by Pascal and Rankin discusses, inter alia, the physical basis of a equation proposed by Schlumberger. This equation expresses formation traveltimes (e.g., inverse phase velocities) derived from measurements made by Schlumberger’s electromagnetic propagation tool (EPT) as the volume weighted sum of the traveltimes of the formation constituents. Pascal and Rankin interpret this equation as implying a layered medium consisting of layers of different dielectric properties. There is no doubt that the Schlumberger traveltime equation is correct for such a layered medium if the direction of wave propagation is normal to the direction of the layering planes. This does not preclude the possibility, however, that the Schlumberger equation may provide an adequate description, at microwave frequencies, of the composite traveltimes of more complex heterogeneous systems.

Calorimetric investigation of glass transitions of two component polymeric blends

Using the scanning calorimetry a study has been made of specific heat changes occurring in polymeric systems in the temperature interval from −150 to +200°, while varying the degree of compatibility of the components. Quantitative parameters for glass transition in these systems have been analyzed and the results used to determine the phase state of the components and relations between the phase state and the ratio of components. When the blends contain thermodynamically incompatible components.(NK-SKN-40, NR-SKF-32 and PVC-SKN-18) there appear to be only two glass transition regions, corresponding to glass transition of the initial components, whose parameters remain constant over a wide range of compositions. From results obtained light has been shed on the character of phase interaction in the systems in question. In the systems PVC-SKN-40 and SKD-SKMB phase combination takes place in amorphous regions of the components, which is reflected in a single glass transition temperature T g. Heterogeneity of these systems is marked by crystallization of one of the components. PVC and SKN-26 form a complex heterogeneous system in which interaction between the components leads to a situation where a combined phase is present in the latter system in addition to phases of the initial polymers.

Surface ionization and complexation at the oxide/water interface II. Surface properties of amorphous iron oxyhydroxide and adsorption of metal ions

The site-binding model for the electrical double layer of hydrous oxides reported in a previous paper is applied to the adsorption of metal ions from dilute solution and to complex heterogeneous systems, i.e., amorphous iron oxyhydroxide. More than one stoichiometric surface reaction is usually needed to describe the adsorption behavior of dilute heavy metal ions. If mass law equations for surface reactions of metal ions are corrected for effects of the electrostatic field at the interface, the calculated adsorption density depends upon the type of surface species formed. It is shown that calculations with surface reactions involving hydrolytic complexes of metal ions, e.g., Pb(II), Cd(II), Cu(II), Ag(I), are more consistent with experimental adsorption data than complexation by bidentate surface sites. A table of intrinsic surface complexation constants for various metal ions and oxide substrates is presented. Similar to results reported earlier for major electrolyte ions, the stability constants of surface complexes of heavy metal ions with the silica surface are significantly less than for other oxide surfaces. Empirical surface parameters for model calculations with iron oxyhydroxide are derived and the results are compared with experimental adsorption data for Cu(II) and Ag(I).

Parameters for Measuring the Activity and Viability of Activated Sludge

Abstract Conventional wastewater parameters are accepted as inadequate estimates of the condition of activated sludge but numerous other indices have been suggested as specific measurements of the activity and viability of the biomass. Literature in the related fields of microbiology and biochemistry were reviewed in order to select the most appropriate activity parameters for application to a heterogeneous biological material. Modified analytical methods were applied to a well-controlled biological system containing a single predominant bacterial species to evaluate the relative merit of each as an indicator of viability and activity. The potential of each parameter in a complex heterogeneous system was determined by monitoring each index in a bench activated sludge system. The predominant culture studies indicated that the ATP content of the biomass and the dehydrogenase activity were potential indicators of cell viability in a simple system. However, in the complex activated sludge system, only the ATP content showed significant correlation to the organic carbon removal rate.

The calculation of the energy of absorbed γ-radiation in heterogeneous macrosystems

The Monte Carlo method has been used to determine the fraction of γ-ray energy absorbed by the components of complex heterogeneous systems consisting of a cylindrical water-equivalent body which is being irradiated, cylindrical γ-ray sources arranged either inside or outside this body and the components of various accessory. structures. The results are used to find the various energy efficiency coefficients for γ-radiation m these systems, which are models for radiochemistry installations. The results of some suitably devised experiments agree well with the calculated data. A programme has been compiled which enables the energy efficiency of any macrosystem of a similar type to be found.

Calculation of ?-ray energy absorption in heterogeneous macro-systems

By means of the Monte-Carlo method, a determination was made of the fraction of ψ-ray energy absorbed in the elements of a complex heterogeneous system formed by a cylindrical, irradiated water-equivalent mass, cylindrical ψ-ray sources located inside or outside the mass, and units of auxiliary equipment. From the results, various energy efficiencies (e.e) were determined depending on the ψ radiation from particular systems which were models of radiochemical equipment. Results from corresponding experiments were in good agreement with the calculated data. The program enabled one to find the e.e. for arbitrary macro-systems of a similar type.