A. P. Safronov

Self-stabilization of aqueous suspensions of alumina nanoparticles obtained by electrical explosion

The particle-size distribution, electrooptical potential, and acid properties of the surface of Al2O3 nanoparticles in aqueous media were studied by dynamic light scattering, electroacoustic spectral analysis, and potentiometric titration. The nanopowder with the mean particle size 22 nm was obtained by the oxidation of aluminum wire in the vapor phase using electrical explosion in oxygen-argon mixture. In spite of the presence of aggregates, aqueous suspensions of nanoparticles, as distinct from suspensions of micropowders, are stable without the addition of special stabilizers because of the spontaneous formation of a double electrical layer on their surface, the electrokinetic potential of which is positive and exceeds 30 mV. The most probable reason for electrostatic self-stabilization of suspensions of nanopowders is the adsorption of Al ions formed in the hydrolysis of aluminum nitrates, small amounts of which are produced in electrical discharges during preparation.

Influence of the degree of deaggregation of YSZ nanopowders in suspension on the process of electrophoretic deposition

Deaggregated nonaqueous suspensions of ZrO2 nanopowders stabilized by Y2O3 (YSZ) with an average diameter of particles of 11 nm obtained by laser evaporation and condensation are studied using dynamic light scattering (DLS). Two types of suspensions with an average size of aggregates of 108 and 66 nm are obtained by ultrasonic machining (USM) and centrifugation. It is shown that the combination of USM and centrifugation allows us to obtain narrow size distributions for the particles in YSZ suspensions. Coatings are obtained using electrophoretic deposition (EPD) from two types of YSZ suspensions with different degrees of deaggregation, and it is shown that no difference in the structure and morphology of suspensions is observed. It is established that, by EPT of the particles, a mechanism of electrochemical coagulation is realized at the electrode from the suspensions with different degrees of deaggregation.

Aggregation of air-dry alumina powder nanoparticles redispersed in an aqueous medium

The aggregation of air-dry Al2O3 nanopowders prepared by electrical wire explosion (EWE) and plasma deposition in a gas atmosphere was studied by dynamic laser light scattering during redispersion in water. The specific surface of nanopowders was from 9 to 38 m2/g. It was shown that aqueous suspensions of nanopowders stabilized by a double electric layer with an electrokinetic ξ-potential magnitude of 30–40 mV contain coexisting individual particles and their primary aggregates (35–50 wt %). The average size of the primary aggregate is approximately three times larger than the average size of an individual particle. The former probably consists of a central particle surrounded by the first coordination sphere. As the medium pH varies from 2.5 to 11.5, the electrokinetic potential of the Al2O3 nanoparticle suspension monotonically decreases from +40 to −40 mV. Near the isoelectric point (IEP), aggregation is significantly enhanced, which results in the formation of larger secondary aggregates of 103–104 individual particles. The use of Na citrate as an electrostatic stabilizer prevents the formation such large aggregates, and only an increase in the fraction of primary aggregates and a decrease in the number of individual particles occur near the IEP.

Stable suspensions of doped ceria nanopowders for electrophoretic deposition of coatings for solid oxide fuel cells

We have studied nonaqueous suspensions of Ce0.8Sm0.2O1.9 (CSO), Ce0.8Gd0.2O1.9 (CGO), and Се0.8(Sm0.75Sr0.2Ba0.05)0.2O2–δ (CSSBO) nanopowders produced by laser evaporation of a target. The nanoparticles were nearly spherical in shape and their average diameter was 9, 7, and 15 nm, respectively. Using ultrasonic processing, we obtained stable nanopowder suspensions in an isopropanol + acetylacetone mixed medium, investigated their particle size composition, evaluated their zeta potential as a function of pH, and obtained potentiometric titration curves. The starting nanopowder suspensions have been shown to be weakly acidic and have a rather high initial zeta potential. During titration of the nanopowder suspensions with 0.17 N KOH in isopropanol, no isoelectric point was observed. The maximum positive values of the zeta potential, favorable for electrophoretic deposition (EPD), were reached in weakly acidic media in the pH range 4–6. Using EPD, we obtained a coating from a stable self-stabilized CSSBO suspension (ζ = +31 mV, pH 4.0), which was then sintered in air at a temperature of 1400°C. Our results demonstrate that the starting nonaqueous suspensions of the CSO, CGO, and CSSBO nanopowders suit well for producing gas-tight, homogeneous solid oxide fuel cell coatings by EPD.

Electrophoretic deposition of Y2O3-stabilized ZrO2 nanoparticles on the surface of dense La0.7Sr0.3MnO3−δ cathodes produced by pyrolysis and solid-state reaction

La0.7Sr0.3MnO3−δ (LSM) cathode materials have been prepared through pyrolysis of liquid precursors and by solid-state reaction. We examined the effect of the cathode material synthesis procedure on the growth of thin films of a solid electrolyte based on Y2O3-stabilized ZrO2 (YSZ) nanopowder with an average geometric size of 10.9 nm on a dense surface of model cathodes by electrophoretic deposition. Using electron microscopy, BET surface area measurements, X-ray diffraction, thermal analysis, and dilatometry, we investigated the structure, specific surface area, phase composition, thermal properties, and sintering-induced volume changes of cathode materials differing in prior history. The results demonstrate that the starting cathode materials differed markedly in properties, but the cathodes produced by sintering them were identical in phase composition: they consisted entirely of a rhombohedral phase. The room-temperature electrical conductivity of the cathodes produced by solid-state reaction was 0.231 ± 0.01 S/cm, exceeding that of the cathodes produced through pyrolysis by an order of magnitude.

Activity of counterions in hydrogels based on poly(acrylic acid) and poly(methacrylic acid): Potentiometric measurements

The behavior of partially ionized weakly crosslinked gels based on poly(acrylic acid) and poly(methacrylic acid) undergoing contraction in the presence of a low-molecular-mass salt is studied experimentally. The concentration dependences of the enthalpies of swelling of gels in water and aqueous solutions with potassium chloride concentrations of 1, 10, and 100 mmol/L are determined via the method of isothermal calorimetry. On the basis of the obtained data, the enthalpy parameter of interaction between a polymer network and a medium is estimated as a function of the amount of the salt. This parameter does not exceed 0.3 and monotonically decreases with an increase in the concentration of the salt. The Donnan potential of gels depending on the concentration of KCl in the external solution is measured via the method of potentiometry with the use of capillary electrodes, and the activity of potassium counterions in the medium of hydrogel is calculated. The main factor that causes contraction of polyelectrolyte gels in a solution of a low-molecular-mass salt is a decline in the activity of counterions that leads to a decrease in the osmotic pressure inside the gel. A decline in activity may be associated with both a reduction in the activity coefficient and ionic association processes in the hydrogel.

Interaction between Mo132 nanocluster polyoxometalate and solvents

The interaction between solid Mo132 nanocluster polyoxometalate with the structure of a keplerate and solvents of different natures (e.g., water, alcohols, hexane, and tetrachloromethane) is studied thermochemically. NMR spectroscopy is used to analyze interactions in solid polyoxometalate-organic compound systems in a gaseous medium. Data are collected on sorption interactions between a Mo132 nanocluster and camphor.

Effect of the nature of a polymer matrix on the enthalpy of adhesion interactions in composites filled with nickel nanoparticles

Interfacial interactions in composites filled with nickel nanoparticles are studied experimentally. The concentration dependences of the enthalpy of mixing of rubbery and glassy homo- and copolymers of different polarities with nickel nanoparticles coated with a carbon shell are measured via isothermal calorimetry. On the basis of these data, the values of adhesion and the structural contributions to the enthalpy of mixing are calculated. A decrease in the kinetic flexibility of macromolecular chains with an increase in the glasstransition temperature of the polymer causes a gain in the positive structural contribution to the enthalpy of mixing of the composite. The contribution of adhesion interaction is negative and predominantly determined by electrostatic forces of the induction type. The maximum enthalpy of adhesion is proportional to the squared dipole moment of a polymer unit and independent of the relaxation state of the polymer matrix.

Synthesis of Strontium Hexaferrite Nanopowder by the Laser Evaporation Method

Nanopowder of a magnetically hard material—strontium hexaferrite—has been synthesized using laser evaporation followed by annealing. The phase composition, magnetic properties, and dispersion characteristics of the nanopowder have been investigated. As was shown by electron microscopy studies, upon laser evaporation the powder particles have a regular spherical shape and the size of the main fraction is in the range of 5–50 nm. From the data of X-ray phase analysis, the synthesized powder contains a large amount of the amorphous phase, whereas magnetite (iron oxide) is the main crystalline component. From the data of magnetic measurements, the nanopowder has low saturation magnetization and there are no magnetically hard properties. The effect of thermal treatment on the nanopowder phase composition, dispersion, and magnetic properties has been studied. It has been demonstrated that heating up to 820°C results in the crystallization of the amorphous component of the nanopowder, which increases the ferrite phase content up to 90 wt %, and the material manifests magnetically hard. In this case the degree of dispersion decreases, but the powder remains nanosized.

Preparation of YSZ/Al2O3 composite coatings via electrophoretic deposition of nanopowders

Using electrophoretic deposition (EPD), we have produced YSZ individual ceramic coatings and YSZ/Al2O3 composite coatings for a wide range of applications in modern materials research. YSZ and Al2O3 nanopowders were prepared by high-energy physical dispersion techniques, namely, by a laser evaporation–condensation process and electroexplosion of wire, respectively. Stable nonaqueous suspensions for the EPD process have been prepared using YSZ and Al2O3 nanopowders with an average particle size of 11 and 22 nm, respectively. The YSZ/Al2O3 composite coating produced by sintering at 1200°C has been shown to have higher density in comparison with the YSZ individual coating produced at the same temperature. X-ray diffraction characterization showed that the YSZ/Al2O3 composite coating consisted of two crystalline phases: α-Al2O3 (corundum) (42 wt %) and cubic ZrO2〈Y2O3〉 (58 wt %). Quantitative analysis of electron micrographs of the surface of the films showed that the YSZ individual coating produced by sintering at 1200°C had a loose structure and contained pores (9%), as distinct from the composite coating, which had a dense, porefree grain structure.