G. P. Kopitsa

Oxygen Nonstoichiometry of Nanocrystalline Ceria

Correlation relations between oxygen nonstoichiometry and particle size in nanocrystalline CeO2 − x are revised. The ceria unit cell parameter is shown to increase from 0.5410 to 0.5453 nm as the particle size decreases from 23 to 2.3 nm. The CeO2 − x critical particle size where cerium(IV) is completely reduced to cerium(III) is calculated as 1.1–1.3 nm.

Mesostructure, fractal properties and thermal decomposition of hydrous zirconia and hafnia

By use of small angle and ultra small angle neutron scattering techniques it was established that amorphous xerogels of hydrous zirconia and hafnia possess fractal properties in a wide range of scales, and the fractal dimension of these materials can be intentionally modified by changing their precipitation pH. It was found that the changes in fractal dimension and specific surface area of hydrous zirconia and hafnia xerogels are governed by the changes in the adsorption of anions onto their surface. It was demonstrated that particle size and specific surface area of ZrO2 and HfO2 nanopowders prepared by thermal decomposition of hydrous zirconia and hafnia xerogels depends strongly on the mesostructure and synthesis conditions of these xerogels.

Hydrothermal Growth of Ceria Nanoparticles

Peculiarities of CeO2 nanoparticle coarsening during hydrothermal treatment in a neutral medium are determined by small-angle neutron scattering, powder X-ray diffraction, low-temperature nitrogen adsorption, and transmission electron microscopy. Coherent intergrowth of individual CeO2 crystallites is the main scenario of nanoparticle coarsening.

Specifics of high-temperature coarsening of ceria nanoparticles

Particle coarsening trends at high temperatures (200 to 700 °C) in CeO2 nanoparticles having various chemical histories are determined by powder X-ray diffraction (XRD), low-temperature nitrogen adsorption, transmission electron microscopy, and small-angle neutron scattering (SANS). Intergrowth of individual CeO2 crystallites is the main scenario of nanoparticle coarsening.

How xerogel carbonization conditions affect the reactivity of highly disperse SiO2–C composites in the sol–gel synthesis of nanocrystalline silicon carbide

A transparent silicon polymer gel was prepared by sol–gel technology to serve as the base in the preparation of highly disperse SiO2–C composites at various temperatures (400, 600, 800, and 1000°C) and various exposure times (1, 3, and 6 h) via pyrolysis under a dynamic vacuum (at residual pressures of ~1 × 10–1 to 1 × 10–2 mmHg). These composites were X-ray amorphous; their thermal behavior in flowing air in the range 20–1200°C was studied. The encapsulation of nascent carbon, which kept it from oxidizing in air and reduced the reactivity of the system in SiC synthesis, was enhanced as the carbonization temperature and exposure time increased. How xerogel carbonization conditions affect the micro- and mesostructure of the xerogel was studied by ultra-small-angle neutron scattering (USANS). Both the carbonization temperature and the exposure time were found to considerably influence structure formation in highly disperse SiO2–C composites. Dynamic DSC/DTA/TG experiments in an inert gas flow showed that the increasing xerogel pyrolysis temperatures significantly reduced silicon carbide yields upon subsequent heating of SiO2–C systems to 1500°C, from 35–39 (400°C) to 10–21% (1000°C).

Combined SANS and SAXS study of the action of ultrasound on the structure of amorphous zirconia gels.

In the present work, we have studied for the first time the combined effect of both sonication and precipitation pH on the structure of amorphous zirconia gels synthesized from zirconium(IV) propoxide. The techniques of small-angle neutron and X-ray scattering (SANS and SAXS) and low temperature nitrogen adsorption provided the integral data on the changes in the microstructure and mesostructure of these materials caused by ultrasonic (US) treatment. Amorphous ZrO2·xH2O synthesized under ultrasonic treatment was found to possess a very structured surface, characterized by the surface fractal dimension 2.9-3.0, compared to 2.3-2.5 for the non US-assisted synthesis, and it was also found to possess a higher specific surface area, while the sizes of the primary particles remain unchanged.

Evolution of composition and fractal structure of hydrous zirconia xerogels during thermal annealing

The mesostructure of amorphous hydrous zirconia xerogels and the products of their heat treatment was studied for the first time using powder X-ray diffraction and small-angle neutron scattering (SANS). The samples prepared at low and high pH values have fundamentally different phase compositions and structures. The high-temperature annealing of hy drous zirconia xerogels is useful for manufacturing materials with controlled surface fractal dimensions.

Effect of high intensity ultrasound on the mesostructure of hydrated zirconia

We report structural changes in amorphous hydrated zirconia caused by high intensity ultrasonic treatment studied by means of small-angle neutron scattering (SANS) and X-ray diffraction (XRD). It was established that sonication affects the mesostructure of ZrO2×xH2O gels (i.e. decreases their homogeneity, increases surface fractal dimension and the size of monomer particles). Ultrasound induced structural changes in hydrated zirconia governs its thermal behaviour, namely decreases the rate of tetragonal to monoclinic zirconia phase transition.

Investigation of the evolution of the hydrated zirconia mesostructure at different stages of heat treatment

The mesostructure of amorphous xerogels based on hydrated zirconia ZrO2 and its evolution at different stages of heat treatment have been investigated using the small-angle neutron scattering method. A correlation function of the nuclear scattering amplitude density has been obtained from experimental neutron scattering cross sections, and the characteristic radii of ZrO2 nanoparticles and their specific surface areas have been determined. The influence of the annealing temperature on the fractal properties of the zirconia surface has been elucidated.

Study of the effect of methods for liquid-phase synthesis of nanopowders on the structure and physicochemical properties of ceramics in the CeO 2 –Y 2 O 3 system

Two alternative chemical synthesis methods—cryotechnological coprecipitation of hydroxides and cocrystallization of salts—were used for preparing (CeO2)1–x(Y2O3)x nanopowders (x = 0.10, 0.15, 0.20) with a mean coherent scattering domain size of ~7–11 nm and Ssp = 2.1–97.5 m2/g. From these nanopowders, ceramic nanomaterials with mean coherent scattering domain sizes of ~61–85 nm were synthesized. It was studied how the phase composition, microstructure, and electrical transport properties of the produced samples depend on the Y2O3 content of a CeO2-based solid solution and on the synthesis method. It was shown that, in the series (CeO2)1–x(Y2O3)x (x = 0.10, 0.15, 0.20), the solid solution (CeO2)0.90(Y2O3)0.10 has the highest ionic conductivity with the ion transport number ti = 0.73 (600°C). In its physicochemical characteristics, this ceramic can be used as a solid electrolyte of intermediate-temperature fuel cells.