N. F. Uvarov

The LiCl-KCl binary system

Liquidus temperatures in the LiCl-KCl system in the composition range from 0 to 100 mol % KCl have been measured for 16 samples using the methods of oscillation phase analysis and thermal analysis. The melting points of the components and the eutectic composition of the system have been refined.

Ion mobility and transport in β-PbF2 doped with alkaline-earth fluorides

We have studied ion mobility and conduction in (1 − x)PbF2 · xMF2 (M = Mg2+, Ca2+, Sr2+, Ba2+; 0.05≤x≤0.1) solid solutions by 19F and 207Pb NMR and impedance spectroscopy. The results are used to analyze the factors governing the structure of the fluoride sublattice and the nature and energetics of ionic motion in the temperature range 170–550 K. The solid solutions are shown to have high ionic conductivity (∼10−4 to 10−3 S/cm at temperatures above 470 K, with activation energies Ea ≤ 0.3−0.6 eV) and are, therefore, potentially attractive for engineering materials with tailored electrical properties.

Cathodic materials for intermediate-temperature solid oxide fuel cells based on praseodymium nickelates-cobaltites

A unique combination of methods (TPD of O2, thermogravimetry, isotopic heteroexchange of oxygen in different modes) was used to carry out detailed studies of oxygen mobility and reactivity in mixed praseodymium nickelates-cobaltites (PrNi1 − xCoxO3 + δ) and their composites with doped cerium dioxide (Ce0.9Y0.1O2 − δ) as promising cathodic materials stable towards the effect of CO2 in the intermediate-temperature region. It is shown that in the case of composites of PrNi1 − xCoxO3+δ-Ce0.9Y0.1O2 − δ synthesized using the Pechini method and ultrasonic treatment, stabilization of the disordered cubic perovskite phase due to redistribution of cations between the phases provides high oxygen mobility. Preliminary results on tests of cathodic materials of this type supported on planar NiO/YSZ anodes (H.C. Starck) with a thin layer of YSZ electrolyte and a buffer Ce0.9Y0.1O2 − δ layer showed that power density of up to 0.4 W/cm2 was reached in the region of medium (600–700°C) temperatures, which was close to typical values for fuel cells of this type with cathodes based on strontium-doped perovskites and their composites with electrolytes.

Mechanism of oxygen transfer in layered lanthanide nickelates Ln2 − xNiO4 + δ (Ln = La, Pr) and their nanocomposites with Ce0.9Gd0.1O2 − δ and Y2(Ti0.8Zr0.2)1.6Mn0.4O7 − δ solid electrolytes

The mechanism of oxygen transfer in layered nickelates having a Ruddlesden-Popper structure and their nanocomposites with Ce0.9Gd0.1O2 − δ (GDC) and Y2(Ti0.8Zr0.2)1.6Mn0.4O7 − δ (YTZM) solid electrolytes having fluorite and pyrochlore structures were studied by the oxygen isotope heteroexchange method in a flow and static reactor, thermoprogrammed desorption, and semiempirical interacting bonds method. The experimental heteroexchange data were adequately described by assuming that all atoms were equivalent in exchange in the bulk of layered nickelates, which was consistent with the cooperative oxygen migration model with fast exchange between the interstitial and regular positions. Strong interaction between the domains of the nickelate phases and solid electrolytes in nanocomposites, accompanied by a redistribution of cations between the phases, hindered the cooperative oxygen migration and led to a decrease in the diffusion coefficient as the exchange rate increased.

Methods of isotopic relaxations for estimation of oxygen diffusion coefficients in solid electrolytes and materials with mixed ionic-electronic conductivity

Theoretical basis of isotopic oxygen exchange in a gas-solid oxide system are considered. A generalized model is suggested that accounts for diffusion of tracer oxygen atoms and allows within a single approach to perform numeric analysis of isotope experiments implemented in reactors of various types and in different temperature modes. It is shown that when C18O2 is used as an isotopic reagent, the oxygen exchange rate on the metal oxide surface increases manifold (as compared to 18O2), which allows determining more precisely diffusion limitations in case of isotopic exchange of oxygen in the oxide bulk. Estimates of oxygen self-diffusion coefficients are obtained in dispersed systems based on doped cerium-zirconium oxides with a fluorite-type structure, doped lanthanum silicates with an apatite-type structure, and also mixed praseodymium nickelates-cobaltites and their composites with yttrium-doped ceria.

Design of medium–temperature solid oxide fuel cells on porous supports of deformation strengthened Ni–Al alloy

An approach was developed towards design of medium-temperature solid-oxide fuel cells based on a deformation strengthened Ni-Al alloy. Methods of sintering were described that allowed obtaining layers of complex oxides with ionic and mixed conductivity and with regulated porosity in the range of 40–1%. Power density of a fuel cell on a metallic support reaches 500 mW/cm2 already at 700°C when humid H2 was used as fuel and air was used as an oxidant. Analysis of fuel cell cross-section after tests showed absence of fractures, flaking, and new phases with low conductivity, which proves good compatibility of all materials used in fuel cell design.

Ion Mobility and Electrophysical Properties of Potassium Hexafluorozirconate K2ZrF6

In our 19F NMR and impedance spectroscopy study, we investigated the mobility of fluoride ions (270-550 K) in polycrystalline K2ZrF6. A structural phase transition was found for K2ZrF6 in the temperature range of ∼513 to 518 K. The high-temperature β-K2ZrF6 modification is characterized by translational diffusion of the fluoride ions and uniaxial anisotropy of the 19F nuclear magnetic screening tensor. The electrophysical characteristics of K2ZrF6 were investigated in the temperature range of 300 to 560 K. It is established that the β-K2ZrF6 phase is characterized by high ionic (superionic) conductivity above 540 K (Σ ∼ 3.2× 10-4 S/cm, T=560 K).

Ionic mobility and conduction in the 50PbF2 · 30BiF3 · 20NaF solid solution studied by NMR and impedance spectroscopy

Ionic mobility and electrical transport in the 50PbF2 · 30BiF3 · 20NaF fluorite solid solution have been studied by 19F and 23Na NMR and impedance spectroscopy. Analysis of the NMR spectra allowed us to assess the temperature effect on the nature of ion motion in the fluorine and sodium sublattices, to identify the types of ionic mobility, and determine the corresponding temperature ranges. The results demonstrate that, at temperatures above 390 K, the dominant type of ion motion in 50PbF2 · 30BiF3 · 20NaF is the diffusion of fluoride and sodium ions. The ionic conductivity of the solid solution reaches 4.4 × 10−4 S/cm at 535 K, suggesting that the 50PbF2 · 30BiF3 · 20NaF solid solution can be used as a key component for engineering new functional materials.

Studies of oxygen transport mechanism in electrolytes based on doped lanthanum silicate with apatite structure using techniques of oxygen isotopic heteroexchange and impedance spectroscopy

The work presents the results of studying the mechanism of oxygen transport for a new promising class of oxygen-containing electrolytes based on lanthanum silicate with an apatite structure using impedance spectroscopy and isotopic oxygen heteroexchange. At 1000 K, in the case of samples with an optimum composition including codoped Fe and Al, σ ∼ 3 × 10−3 to 10−2 S/cm and D* reaches ∼10−8 cm2/s, which is close to the values of YSZ and Ce0.9Gd0.1O2 − δ (GDC). Lower energies of conductivity activation and oxygen diffusion for doped apatites (∼0.5–0.8 eV instead of ∼1 eV for GDC) and also equivalence as regards exchange of all oxygen atoms within apatite agree with the model, in which oxygen mobility is determined by a nonlinear cooperative migration process of oxygen atoms with fast exchange between interstitial and regular sites.

The effect of phase composition on the transport properties of composites La0.8Sr0.2Fe0.7Ni0.3O3 − δ-Ce0.9Gd0.1O1.95

Full conductivity, diffusion and oxygen exchange processes in composites (100 − x)La0.8Sr0.2Fe0.7Ni0.3O3 − δ−xCe0.9Gd0.1O1.95 (x is the volume fraction, 0 ≤ x ≤ 71.1%) at 700°C over the oxygen partial pressure range from 0.2 to 3 × 10−3 atm are studied by the electrical conductivity relaxation method. The composites’ conductivity was shown to decrease monotonically with the increasing of Ce0.9Gd0.1O1.95 fraction, while the oxygen chemical diffusion coefficient increased. The oxygen exchange constant is higher for the composites than for the individual phases of La0.8Sr0.2Fe0.7Ni0.3O3 − δ and Ce0.9Gd0.1O1.95. Possible reason of the dependence of the parameters Dchem and kchem on the temperature, oxygen pressure, and the composite composition is the effect of the interface on the oxygen transfer processes. Most effective oxygen transfer occurs in the composites whose composition approaches La0.8Sr0.2Fe0.7Ni0.3O3 − δ-Ce0.9Gd0.1O1.95 (x = 71%).