Thermal reactions of inorganic hydroxy-compounds under applied electric fields

The effect of electric fields on the thermal dehydration of amorphous hydrated iron(III) oxide under oxidizing, inert and reducing atmospheres was studied by X-ray diffractometry. In oxidizing and inert atmospheres, the transformation to-Fe₂O₃ was enhanced by electric fields, especially at the negative electrode. The crystallite size of the product was also greatest at the negative electrode. Both results are explained in terms of the migration of protons to the negative electrode, where they subsequently form water which acts as a nucleating agent for the crystalline phase. In reducing atmospheres the formation of Fe₃O₄ and FeO at the expense of Fe₂O₃ is facilitated by electric fields, particularly at the negative electrode. Possible reaction mechanisms are considered, and the role of protons in stabilizing defect-spinel intermediates and products is discussed.

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Zusammenfassung Der Einfluß eines elektrischen Feldes auf die thermische Dehydratisierung des amorphen hydratisierten Eisen(III)oxids in oxidierenden, inerten und reduzierenden Atmosphären wurde durch Röntgendiffraktometrie untersucht. In oxidierenden und inerten Atmosphären wurde die Umwandlung zu -Fe₂O₃ durch elektrische Felder gefördert, besonders an der negativen Elektrode. Die Form der Kristallite des Produkts war ebenfalls an der negativen Elektrode am größten. Beide Ergebnisse werden durch die Proteinwanderung zur negativen Elektrode erklärt, wo diese Wasser bilden, das für die kristalline Phase als Keimbildner fungiert. In reduzierenden Atmosphären wird die Bildung von Fe₃O₄ und FeO auf Kosten von Fe₂O₃ durch elektrische Felder besonders an der negativen Elektrode erleichtert. Mögliche Reaktionsmechanismen werden erörtert und die Rolle der Protonen bei der Stabilisierung defekt-spineller Zwischenstufen und Produkte besprochen.

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Résumé L'effet d'un champ électrique sur la déshydratation thermique de l'oxyde de fer(III) hydraté amorphe a été étudié par diffractométrie des rayons X, dans des atmosphères oxydante, inerte et réductrice. En atmosphères oxydante et inerte, le champ électrique facilite la transformation en -Fe₂O₃, en particulier à l'électrode négative. La taille des cristaux formés est également plus élevée à l'électrode négative où il se forme de l'eau qui agit comme agent de nucléation de la phase cristalline. En atmosphère réductrice, le champ électrique facilite la formation de Fe₃O₄ et FeO, aux dépens de Fe₂O₃, en particulier à l'électrode négative. On considère les mécanismes de réaction possibles et on discute le rôle des protons sur la stabilisation des produits et des intermédiaires défauts-spinelles.

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- (III) , . -Fe₂O₃ , , . , , . Fe₃O₄ FeO Fe₂O₃ . , .

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The author is indebted to P. J. Melling for assistance with the measurements in H₂/N₂ atmospheres.     

Thermal reactions of inorganic hydroxy-compounds under applied electric fields,II

The effect of electric fields on the thermal dehydroxylation of Mg(OH)₂ (brucite) and Al(OH)₃ (gibbsite) have been studied by thermogravimetry in a controlled inert atmosphere. Electric fields exert no beneficial effect on the reaction of gibbsite; in some cases the reaction is slightly retarded. By contrast, a small but significant beneficial effect is observed in brucite, in which the initiation temperature and activation energy is lowered at field strengths of about 10⁵ V/m. The difference in behaviour of the two hydroxides is attributed to differences in the mobility of anionic defects and oxygen-containing “proton-transfer complexes”. The transport of the various protonbearing species in an electric field is discussed.     

Thermal reactions of inorganic hydroxy-compounds under applied electric fields,I

A method which has been developed for thermogravimetric study of solids in the presence of applied electric fields has been used in a study of kaolinite dehydroxylation in a controlled inert atmosphere. Electric fields of 10⁵ V/m lower the initiation temperature for dehydroxylation by as much as 60 in some cases; the activation energy for dehydroxylation is reduced by 3–12 kcal/mole. The rate constants for electrolysed samples (based arbitrarily on a first-order law) are increased by electrolysis but this effect falls off at higher temperatures as the normal thermal processes begin to predominate. The effect of the field on the various processes of ionic migration in the lattice is discussed.

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Zusammenfassung Eine zur thermogravimetrischen Analyse von Festkörpern unter Anwendung elektrischer Felder entwickelte Methode wurde zum Studium der Dehydroxylierung von Kaolinit in einer geregelten inerten Atmosphäre herangezogen. Elektrische Feldstärken von etwa 10⁵ V/m setzten die Anfangstemperatur der Dehydroxylierung in einigen Fällen sogar um 60 C herab; die Aktivierungsenergie der Dehydroxylierung wird um 3 bis 12 kcal/mol herabgesetzt. Die — willkürlich aufgrund einer Gesetzmäßigkeit erster Ordnung berechneten — Geschwindigkeitskonstanten der elektrolysierten Proben werden durch die Elektrolyse erhöht, doch fällt dieser Effekt bei höheren Temperaturen, wo er von den normalen thermischen Vorgängen überlagert wird, weg.Die Wirkung des elektrischen Feldes auf die verschiedenen Vorgänge der Ionenwanderung im Gitter wird erörtert.

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Résumé On a mis au point une méthode pour l'étude thermogravimétrique des solides sous l'action d'un champ électrique et on l'a utilisée pour suivre la déshydroxylation de la kaolinite en atmosphère inerte contrÔlée. L'application de champs électriques d'environ 10⁵ V/m abaisse la température initiale de la déshydroxylation de 60 C dans certains cas et l'énergie d'activation est réduite de 3 à 12 kcal/mol. On observe l'augmentation des constantes de vitesse des échantillons soumis à l'action du champ électrique (par rapport à une loi choisie arbitrairement du 1^er ordre) mais cet effet cesse aux températures élevées où les processus thermiques normaux deviennent prédominants.On discute l'influence du champ électrique sur les processus de migration d'ions dans la grille.

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. , 10⁵ V/m, , 60C; 3–12 /. ( ) , , .

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This work was supported by an S.R.C. Research Grant. The author is indebted to Dr W.T. Raines and Dr G. M. Fryer for helpful discussion.     

Devitrification of aluminosilicate glasses under applied electric fields

Journal of Thermal Analysis and Calorimetry - Electric fields have been shown by a combined DTA-electrolysis technique to decrease the temperature of the exothermic devitrification of...     

Devitrification of aluminosilicate glasses under applied electric fields

The effect of electrolysis on the devitrification of aluminosilicate glasses containing 2–10 wt. % Ti, V, Fe, Co and Ni was studied by DTA. In all cases the devitrification temperature is lowered under electrolysis by an amount greater than that which would be achieved by independently electrolysing or doping with the transition metal ion. The relative effect of the different transition metal ions on devitrification is explained in terms of the strengths of the respective metal-oxygen bonds and the octahedral/tetrahedral site preferences of the ions. The electrolysis mechanism is similar to that of the undoped base glass, but also includes migration of the transition metal ions to the cathode and the possibility of interaction between these species and the residual protons of the base glass.

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Zusammenfassung Der Einfluß der Elektrolyse auf die Entglasung von Alumosilikatglas-Arten mit 2 bis 10 Gew.% Ti-, V-, Fe-, Co- und Ni-Gehalten wurde mittels DTA untersucht. In allen Fällen wird die Entglasungstemperatur stärker herabgesetzt als bei unabhängig durchgeführter Elektrolyse oder Zusatz des Übergangs-Metall-Ions. Die relative Wirkung der verschiedenen Übergangs-Metall-Ionen wird aufgrund der Stärke der betreffenden Metall-Sauerstoff-Bindungen und des Vorrangs der tetraedrischen bzw. oktaedrischen Stellen der Ionen erklärt. Der Mechanismus der Elektrolyse ist ähnlich wie bei Gläsern ohne Zusatz, umfaßt aber auch die Wanderung des Übergangs-Metall-Ions zur Kathode und die Möglichkeit der Aufeinanderwirkung dieser Arten und der Restprotone des Grundglases.

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Résumé On a étudié par ATD l'effet de l'électrolyse sur la dévitrification de verres types aluminosilicates contenant 2 à 10 % en poids de Ti, V, Fe, Co et Ni. Dans tous les cas la température de la dévitrification est abaissée d'une manière plus importante sous électrolyse que si celle-ci est effectuée indépendamment ou que par dopage par des ions des métaux de transition. L'effet relatif des divers ions des métaux de transition s'explique en termes de forces des liaisons respectives du métal et de l'oxygène ainsi que par la préférence des ions pour les sites octaédriques ou tétraédriques. Le mécanisme de l'électrolyse est similaire pour les verres sans additifs. Il comprend cependant aussi la migration de l'ion du métal de transition vers la cathode et la possibilité de l'interaction de ces espèces avec les protons résiduels du verre de base.

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, 2–10 Ti, V, Fe, Ni, . , , . - / . , , .

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We are indebted to Professor J. F. Duncan for advice and helpful criticism, Mr. R. A. Kennerley for the use of the thermal analyser, Dr. G. Challis for the electron microprobe analyses and Mr. G. D. Walker for the scanning electron microscopy.     

Crystallite growth in magnesium oxide under applied electric fields

An electrolysis cell is described for use in conjunction with a high-temperature X-ray furnace, which permits X-ray diffraction studies to be made of solids during the course of their high temperature electrolysis. The technique is applied to an investigation of the effect of electric fields on crystallite growth in MgO at 600°C in air, argon and reducing atmospheres. In all cases crystallite growth is greatest at the positive electrode face, suggesting that the charge carriers are anions. The efficiency of the electrolysis process depends both on the concentration of charge carriers and on the concentration of anion vacancies by which the charge carriers migrate. The defect concentration is increased under reducing conditions. In MgO samples containing a high concentration of protons, the conduction is protonic, with the greatest crystallite growth occurring at the cathode due to the catalytic action of the water vapour formed in that region by recombination of protons with hydroxyl ions.     

Polyaniline and its modification for electroresponsive material under applied electric fields

Polyaniline (PANI) and its various copolymers were synthesized using different monomers such as o‐methyl‐, o‐methoxy‐, o‐ethyl‐, o‐ethoxyaniline and sodium diphenylamine sulfonate by the chemical oxidation polymerization as air‐stable organic conducting polymers, and adopted as one of the most potential materials of electrorheological (ER) fluids, especially for the anhydrous system. A relatively low density, a controllable conductivity, and thermal stability are advantages of the PANI based ER system compared with other ER materials. An FT‐IR analysis was adopted to confirm their synthesis and a scanning electron microscopy (SEM) analysis indicated the shape of PANI derivatives was irregular. The effect of functional group of PANI on electric and electroresponsive properties of poly(aniline‐co‐ethoxyaniline) (COPA) particles were examined. A universal scaling equation of the yield stress was applied to these ER fluids and it was found that all data were collapsed successfully onto a single curve regardless of monomer type of the polyaniline synthesized. Copyright © 2005 John Wiley & Sons, Ltd.     

Spectrophotometric variable‐concentration kinetic experiments applied to inorganic reactions

The dependence of the observed rate constant of inorganic substitution reactions on the concentration of nucleophilic reagents was obtained by single variable‐parameter kinetic runs. The experiments were carried out spectrophotometrically, varying the concentration of the nucleophile inside the reaction vessel. Software and apparatus were developed for an easy and rapid performance. The method gives accurate results and a saving in time by a factor of up to 100 compared to conventional methods. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 497–502, 2003     

Electrohydrodynamics of a drop under nonaxisymmetric electric fields

We consider the electrohydrodynamics of a spherical drop in a nonaxisymmetric electric field, which can be approximated by the sum of a uniform field and a linear straining field. We obtain the analytic solution of the three-dimensional flow fields inside and outside a drop for the Stokes flow regime by using Lamb's general solution and the leaky dielectric model. With the analytic solution, the dielectrophoretic migration velocity of a drop is obtained as a function of the type and the frequency of the imposed electric field. The direction of drop motion is found to be parallel to the dielectrophoretic force. The analytic solution is also used to investigate the characteristics of the interfacial flow under various nonaxisymmetric electric fields. While investigating the interfacial flow, we find a surface vortex structure under certain nonaxisymmetric electric fields, which is found to be related to the chaotic mixing inside the drop. Finally, we consider the chaotic features of three-dimensional flows inside the drop under static nonaxisymmetric electric fields.     

Thermal decomposition reactions of amminecobalt(III)complexes

Simultaneous TG-DTG-DTA studies under non-isothermal conditions on [Co(NH₃)₆]Cl₃, [Co(NH₃)₅]Cl₂ and [Co(NH₃)]₂(C₂O₄)₃.4H₂O complexes have been carried out in air and argon atmospheres in the temperature range 293–1273 K. All the dissociation processes occur in three main stages. The kinetics of thermal decomposition of the complexes have been evaluated from the dynamic weight loss data, to determine the most probably mechanisms of the stages on the basis of statistical analysis. The decomposition of the compounds was controlled by diffusion and phase boundary reactions except stage III of the oxalate complex in argon (random nucleation). The activation energiesE _a of the particular stages of the thermal decomposition were calculated.

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Zusammenfassung Simultane TG-DTG-DTA-Untersuchungen an den Komplexverbindungen [Co(NH₃)₆]Cl₃, [Co(NH₃)₅Cl]Cl₂ und [Co(NH₃)₆]₂(C₂O₄)₃.4H₂O unter nichtisothermen Bedingungen wurden in Luft und Argonatmosphäre bei 293–1273 K durchgeführt. Die Zersetzung läuft in jeweils drei Stufen ab. Für die kinetische Auswertung der thermischen Zersetzung der Komplexverbindungen aus den dynamischen Gewichtsabnahmekurven wurde der wahrscheinlichste Mechanismus der einzelnen Stufen mit Hilfe von statistischen Analysen ermittelt. Die Zersetzung der Komplexverbindungen wird meist durch Diffusions- und Phasengrenzreaktionen kontrolliert, nur bei der 3. Stufe des Oxalatkomplexes in Argon herrscht statistische Keimbildung. Die AktivierungsenergienE _a der einzelnen Zersetzungsstufen werden berechnet.

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293–1273 — [Co(NH₃)₆]Cl₃, [Co(NH₃)₅Cl]G₂ [Co(NH₃)₆]₂(C₂O₄)₃.4H₂O. . , . , III , . _a .

     

Water absorption in polyethylene under external electric fields

Monte Carlo simulations of the solubility and structure of water in polyethylene in thermodynamic equilibrium with liquid water were performed in external fields ranging from 2 x 10(5) to 4 x 10(9) V/m. For a given equilibrium temperature and pressure, the water solubility decreases at higher fields. This occurs since it is energetically favorable for water molecules to be in the pure water phase than in the polyethylene matrix at high field strengths, and results in an increased density in the water phase. However, fields relevant to high voltage conduction (in the absence of defects that can lead to large local field strengths) do not change the solubility. In addition, at large fields the number of water clusters decreases for all cluster sizes. The rate of decrease is highest for large clusters, and a larger fraction of water molecules exist as monomers in the polyethylene matrix at high fields. Large fields also cause alignment of the water molecules, which leads to more clusters with linear topologies and hence an increase in the cluster radius of gyration.     

Capillary electrophoretic separation of dsDNA under nonuniform electric fields

Improved sensitivity for the analysis of DNA by capillary electrophoresis has been achieved, based on simultaneous increases in optical path length and injection volume. To increase the optical path length, bubble cells with diameters ranging from 150 to 450 microm have been fabricated and tested. In terms of resolution and sensitivity, a bubble cell of 300 microm diameter is appropriate when using 75-microm capillaries. To allow greater injection volumes, we performed on-line concentration of DNA in the presence of electroosmotic flow (EOF) using 2.0% poly(ethylene oxide) (PEO). With a 300-microm bubble cell, a 170-fold improvement in the sensitivity for the 89-bp fragment has been accomplished when injecting about 0.33 microL DNA. In the presence of the bubble cell, the resolution for the large fragments improves while that for the small ones (<124 base pair) decreases. The effect of bubble cells was further investigated by conducting DNA separation in the absence of EOF, showing that improvements in resolution are mainly due to increased migration differences when DNA migrated at low electric field strengths in the bubble region. We have suggested that such an effect is more profound using shorter capillaries, leading to complete separation of phiX 174 RF DNA-Hae III digest in 2 min.     

Model of a confined spherical cell in uniform and heterogeneous applied electric fields

Cells exposed to electric fields are often confined to a small volume within a solid tissue or within or near a device. Here we report on an approach to describing the frequency and time domain electrical responses of a spatially confined spherical cell by using a transport lattice system model. Two cases are considered: (1) a uniform applied field created by parallel plane electrodes, and (2) a heterogeneous applied field created by a planar electrode and a sharp microelectrode. Here fixed conductivities and dielectric permittivities of the extra- and intracellular media and of the membrane are used to create local transport models that are interconnected to create the system model. Consistent with traditional analytical solutions for spherical cells in an electrolyte of infinite extent, in the frequency domain the field amplification, G(m) (f) is large at low frequencies, f<1 MHz. G(m) (f) gradually decreases above 1 MHz and reaches a lower plateau at about 300 MHz, with the cell becoming almost "electrically invisible". In the time domain the application of a field pulse can result in altered localized transmembrane voltage changes due to a single microelectrode. The transport lattice approach provides modular, multiscale modeling capability that here ranges from cell membranes (5 nm scale) to the cell confinement volume ( approximately 40 microm scale).     

Molecular dynamics simulations of ion transport through carbon nanotubes. III. Influence of the nanotube radius, solute concentration, and applied electric fields on the transport properties

The present investigations continue previous research on transport in aqueous ionic solutions through carbon nanotubes. Specifically, the effects of the nanotube radius, solute concentration, and applied external electric fields on the transport properties are investigated in terms of mobilities, currents, and pairing times of the solute ions. The simulated transport features are corroborated with general theoretical results of nanofluidics (such as the linear log-log regime of the nanochannel conductance as function of the solute concentration and the current-voltage curve of the channel). Discontinuities in the partial ionic currents are explained on the basis of a recent theoretical model of quantized ionic conductance in nanopores, developed by Zwolak et al. Correlations between the structural and dynamic properties are established, linking causally the highly structured spatial density profiles, the ion pairing phenomenon and the ionic currents.     

Tailoring the structure of S-PEEK/PDMS proton conductive membranes through applied electric fields

Composite membranes were formed composed of proton conductive sulfonated poly(ether ether ketone) (S-PEEK) particles dispersed in a non-proton conductive polymeric matrix, a cross-linked poly(dimethyl siloxane) (PDMS). The structure of the composites was controlled by applying electric fields to suspensions of S-PEEK particles in the liquid PDMS precursor, followed by thermally initiated cross-linking polymerization to fix the field-induced structure. The effects of the electric field on membrane structure, proton conductivity, methanol permeability, and water swelling were examined. Under certain conditions, the applied electric field induced the S-PEEK particles to form long chains across the liquid PDMS prepolymers. The degree of particle chaining was a function of the electric field frequency, magnitude, and application time. The S-PEEK particle chaining resulted in an improvement of the membrane conductivity, water uptake ability, and dimensional stability in comparison to membranes containing randomly distributed particles. The particle chaining also increased the methanol permeation across the composite membranes, but the selectivity of the membranes for protons over methanol increased sharply because the increase in proton conductivity was much larger relative to the methanol permeability increase. The membranes also display anisotropic swelling behavior in water that may prove advantageous for enhancing mechanical stability in fuel cells undergoing humidity cycling. The present study demonstrates a novel fabrication approach that can be used to control the structure of a variety of types of composite membranes to enhance performance for fuel cell applications.     

Thermal decomposition reactions and kinetic parameters of nitritobismuthates(III)

Thermal studies have been carried out on nitritobismuthates(III) of the silver group having the general formula M₂Ag[Bi(NO₂)₆] where M = Cs, Rb, K, and the sodium group having the formula M₂Na[Bi(NO₂)₆] where M = Cs, Rb.Typical thermal curves, temperatures of the endothermic peaks and percentage weight loss, calculated from the TG curve, are shown. On the basis of the results of thermal and X-ray analysis the mechanisms of the thermal decomposition reaction were determined for nitritobismuthates(III).The thermal curves were used to calculate kinetic parameters, activation energy E_a and order of reaction n by Zsako's and Coats and Redfern's methods. A comparison of the thermal stabilities of the salts under study was made. Within the same group of nitritobismuthates(III), the activation energy and the decomposition temperatures of the salts increases when the difference between the radii of outer sphere cations increases.     

Heat capacity of superfine oxides of iron under applied magnetic fields

Heat capacity is one of the most characteristic and important properties when the peculiarities of magnetic nanosystems are studied. In these systems the magnetic ordering becomes obvious due to the thermal effects such as heat capacity anomalies. It was considered earlier that heat capacity change under magnetic fields applied is slight and it cannot be taken into account in thermodynamic calculations. However the experimental heat capacity data for ferrofluids under magnetic fields applied show that field and temperature heat capacity dependences have a complicated behavior and in magnetic fields an essential heat capacity change takes place. On the other hand in the literature the contradictory data about heat capacity of nanoparticles appear. According to some papers nanoparticles heat capacity can exceed heat capacity of a bulk material a few times.     

Deformation,scattering, and birefringence of flexible polymer chains under external forces or electric fields

The effect of a tensile stress or an electric field on the conformation of a flexible polymer chain has been studied by combining theory with numerical simulation. In the presence of such external agents, the macromolecule experiences the action of two opposite forces at the chain ends. Two models are considered: the Gaussian bead-and-spring chain, and the freely jointed chain with segments of fixed length. From simulated Brownian trajectories we calculate steady-state properties of the polymer under the continuing action of the external forces. Thus, we compute the chain deformation and expansion, measured by the square radius of gyration, and analyze the influence of the external force on low-angle scattering of radiation. The effect of the link orientation in the optical anisotropy or birefringence is also analyzed. From existing theories, we predict very simple relationships between expansion, low-angle scattering, and birefringence, valid for Gaussian chains of any length, and for long freely jointed chains. The simulation results confirm those relationships. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 689–697, 1997     

The effect of electric fields on blue phase III with negative dielectric anisotropy

In order to further understand the structure of the blue phase III (BPIII) phase of chiral liquid crystals, we have observed the effect of electric fields on the reflectivity of BPIII phases in which the liquid crystal has negative dielectric anisotropy. The reflectivity increases dramatically with increasing field, in contrast to the behaviour for materials of positive dielectric anisotropy. We compare the data with cubic domain and double-twist models for BPIII structure and we find that the data best agree with the double-twist model and possibly with an O⁵ cubic domain model.