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.     

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.     

Small-angle neutron scattering study of the mesostructure of bioactive coatings for stone materials based on nanodiamond-modified epoxy siloxane sols

The structure formation of sol-gel-derived epoxy siloxane compositions with different ratios of the main precursors (R _{TEOS/EPONEX 1510} = 16/38, 27/27, 38/16 wt %) and with different concentrations of detonation synthesis nanodiamonds (c _DND = 0.05, 0.10, 0.20 wt %) has been investigated using small-angle neutron scattering (SANS). Based on the SANS data, it has been revealed that the synthesized epoxy siloxane xerogels are systems with a two-level fractal structure, in the formation of which the siloxane component plays a dominant role. It has been found that the fractal dimension D _m2 and the radius of gyration R _g2 of clusters in the epoxy siloxane compositions decrease with an increase in the content of the siloxane component. It has been established that the introduction of small additions of detonation synthesis nanodiamonds (less than 1 wt %) into the epoxy siloxane composition with an equal ratio of the main precursors R _{TEOS/EPONEX 1510} = 27/27 wt % leads to a transition from the two-level to three-level structure organization and affects the fractal dimension D _m and the radius of gyration R _g of the formed clusters.     

How xerogel carbonization conditions affect the reactivity of highly disperse SiO<Subscript>2</Subscript>–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).     

Complete inheritance of fractal properties during first-order phase transition

We present experimental evidence of complete fractal properties inheritance in the course of first-order phase transition from amorphous to monoclinic or tetragonal zirconia under hydrothermal conditions. This phenomenon takes place either under rapid microwave heating or conventional heating regardless of starting fractal dimension value. Exactly the same effect is observed for hafnia. The similarity of the local structures of amorphous and crystalline zirconia as well as relatively soft crystallization conditions could be the definite reasons for the conservation of the mesostructure in the course of phase transition.     

Spin correlations and magnetonuclear cross-correlation in Sm(Sr)-Mn-O perovskites in the low-temperature phase

The results of measurements of spin correlations and magnetonuclear cross-correlations (MNCCs) in the system ¹⁵⁴Sm_1−x Sr_xMnO₃ with the perovskite structure (x=0.25,0.4) in the low-temperature phase in magnetic fields 0<H<1 kOe are presented. These are the first measurements performed by the small-angle polarized-neutron scattering method (SAPNS). It is shown that ferromagnetic correlations with a scale of 180–250 Å and MNCCs characterizing the intercoupling of the magnetic and lattice subsystems on this scale exist in the system. It is found that in the low-temperature phase the system exhibits spin-glass properties. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 323–328 (25 February 1999)     

Magnetic Neutron Scattering in Reduced Graphene Oxide

JETP Letters - The scale of magnetic correlations in materials based on reduced graphene oxide (RGO) has been estimated for the first time using the method of small-angle scattering of polarized...