Design of ceramics with predetermined microstructure is based on the arrangement of concordant structure transformations under sintering of selected phase components, whose evolution determines the formation of required microstructure elements, their crystal morphology, scale, content, and distribution throughout the volume. Such elements may include grains, intergrain and interphase boundaries, elements of substructure responsible for the appearance of intragrain boundaries,such as dislocation, twinning, domain, and interphase ones, and pores of different types. Knowledge of these mechanisms is the base for design of novel materials an dimprovement of the currentones. The grain structure is a basic material structure component. Based on our research (using TEM with microdiffraction and SEM with microanalysis) and literature data, general regularities of grain structure formation in ceramics are considered in the course of primary (PR) and collective (CR) recrystallization and during formation of composites under sintering. It’s worth noting that recrystallization in ceramics was first studied by G.V. Samsonov at our Institute. PR in ceramics can occur under different conditions of loading such as free sintering, hot pressing including that under highpressure, uniaxial compression, rolling, friction, etc. The structural mechanisms of the PR evolution in ceramics are similartometals. The formation of grain structure in composites is determined by structure transformations due to the contact interaction of phase components; transformations in the grain volume of initial phases (plastic fragmentation, polymorphic transformations, decomposition of solid solutions, twinning, domainization, etc.); transformations in grain boundaries (phase transitions, boundary faceting, etc.). All of them lead to refining grain structure due to the formation of new grains and saturation of the initial ones with internal boundaries resulting from substructure formation [1].

In the present work, we report on structure formation of ultradispersed diamond through detonation synthesis. In particular, based on the present data, we can state that ultradispersed diamond of detonation (UDD) synthesis is characterized by a rail substructure formation, which is typical for diamonds of dynamic and static synthesis formed from graphite by the martensitic mechanism. The formation of UDD is characterized by a two-stage process: the first stage is the formation of graphite from detonation products, and the second stage is the transformation of graphite into diamond by the martensitic mechanism. A microlamellar substructure in the rails of diamonds formed from the original graphite is due to the presence of basic packing defects that is characteristic for covalent crystals with a wurtzite lattice deformed at high pressures and temperatures typical for lonsdaleite. The presence of lonsdaleite in UDD also indicates its formation in the process of synthesis of diamond from graphite through the martensitic transformation.

The paper presented some results from the electron microscopic study of structure formation processes in boron carboxynitride (BNCO) during its synthesis on the basis of initial boron oxide and melamine in the form of a powder and a coating on single-crystal diamond particles. Structural studies were performed by transmission electron microscopy (TEM) including high-resolution TEM in combination with elemental microanalysis. Turbostratic BNCO with a different degree of ordering was established to be a major component during the synthesis of a single-phase powder in the temperature region T = 700–1200°C. At a temperature of 1200°C, the formation of an ordered phase occurred in the particles together with inclusions shaped as nanotubes, onions, and prisms with a size of up to 100 nm. BNCO precipitations on diamond particles were monolithic and have continuous interfaces with diamond. The sintering ( р = 7.7 GPa, Т = 1700°C) of BNCO as a single phase and in composition with diamond (both particles with a coating and in the form of a powder) was accompanied by the formation of the sphalerite phase with a graphite-like residue. The sphalerite phase was a major component of sintered samples only when diamond powders with a grit size of 0.1/0 were used.

Results of an investigation by scanning electron microscopy methods of the microstructure of self-reinforced aluminum nitride obtained on the basis of aluminum nitride powder and containing 3 mass.% oxygen by plasma-chemical synthesis in the temperature range 1700-2000 °C are presented. Initial aluminum nitride was represented by the wurtzite (2H) phase. Samples were obtained by free sintering in a nitrogen atmosphere. It was established that, during sintering of AlN in the indicated temperature range, three microstructural types of the material and six types of interfaces (three types of intergranular and three types of intragranular ones) formed. The features of the microstructure of the materials are fully determined by the development of intergranular crystal-oriented polytype transitions of 2H AlNmultilayer polytypes (MP) in sintering. The sequence of successive structural transformations that determine the development of polytype transitions was established. 1. Formation of initial 2H AlN grains of the solid solution 2H AlN-O. The substitution of nitrogen by oxygen takes place. 2. The development of isomorphous delamination of the solid solution in every grain with the precipitation of an interlayer enriched in oxygen. 3. In interlayers, polytypes consisting of a series of polytypes with different number of layers (MP) form. Such an interlayer has developed base surfaces and propagates from one boundary to another in the grain, which determines the formation of a special structural state of fragments of boundaries, that border interlayers in the direction . 4. The high mobility of the indicated fragments of boundaries determines their break-off from the common boundary and formation of a grain nucleus of anisometric (plate-like) shape. This process begins already at a sintering temperature Tsint. = 1800 °C, and, at Tsint. = 2000 °C, polycrystals practically entirely consist of grains of plate-like shape. In this case, with increase in the sintering temperature, the aspect ratio (the length-to-width ratio) of such grains rises. Keywords: aluminum nitride, polytype transformations, grains, microstructure, boundaries, self-reinforcement.

Results of an electron microscopic investigation of particles of lonsdaleite powder with additives of cubic diamond and polycrystalline specimens based on it at Р = 7,7 GPa in the temperature range 1700―1900 °С are presented. Lonsdaleite particles are characterized by a predominantly ternary texture [1120]l of different degree of perfection. Structural mechanisms of transformations in lonsdaleite particles, which cause the formation on nanograined structure in sintering, have been established. The initial stage is the mechanical dispersion of particles and dispersion as a result of plastic faulting deformation, which promotes their fragmentation without breakdown of continuity. The indicated processes lead to the destruction of texture in particles. Beginning from 1700 °С, the lonsdaleitecubic diamond phase transformation with the orientation ratio (111)dII (001)l occurs. It is realized within rods as elements of the substructure of lonsdaleite. At 1900 °С, the size of detected elements of the structure (grains) is 5―15 nm. Beginning from 2000 °С, the self-association of such grains into aggregates with sizes up to 70 nm and the subsequent process of coalescence of grains in aggregates with the formation of the monocrystalline state occur. The next stage of formation of the granular structure is caused by the formation of grain boundaries and development of collective recrystallization. After sintering at Т = 2100 °С, the grain size in specimens does not exceed 100 nm. It has been established that the transformation in lonsdaleite proceeds by structural mechanisms that are characteristic of wurtzite modifications of boron nitride and silicon carbide (strong disordering in the direction of the basal axis, plastic faulting deformation, and formation of multilayer polytypes during the hexagonal-to-cubic phase transformation). Keywords: lonsdaleite, particles, sintering, fragmentation, grains, coalescence.

The results have been reported of electron microscopic studies, which confirm the existence of a contact interaction of particles of cubic diamond and wurtzitic boron nitride at pressure 7.7 GPa and temperature 1700–1900°C to form an interlayer (transition zone) of diamond-like BN. It has been established that in the transition zone there is carbon, whose quantitative portion decreases as it passes from the region that contacts with diamond into the matrix component based on dense BN phases.

Nowadays paroxysmal AV nodal reentrant tachycardia (AVNRT) is one of the most widespread arrhythmias. In most cases AVRNT is a recurrent process, and it worsens the life quality of such patients, reduces their workability and increases the incidence of applying for medical help. Thus AVNRT today is of special attention among investigators. The interest of clinicians to the problem of cardiac arrhythmias is associated with permanent dissatisfaction with the results of antiarrhythmic therapy and also with the rapid development of the surgical methods of treatment, i.e. the use of radio frequency catheter ablation.

The ordering in carbon formed as a result of annealing of pine wood shavings in argon at T = 1900 and 2250°C has been studied by high-resolution transmission electron microscopy. It is established that the ordering involves graphene layers, which serve as the basis for the formation of monolithic structural elements in the form of ribbons. Graphene layers are bent to various degrees and their spacing in ribbons varies within 0.34–0.38 nm. The coarsening of ribbonlike structural elements takes place via the formation of new graphene layers from the matrix component.

A principle new approach to the non-metal nitride powders activation is proposed in this paper. This approach consists in hydrogen-thermal treatment (HTT) of precursor non-metal nitrides powders at 500–1,150°C in presence of hydrogen and catalysts. The goal of hydrogen-thermal treatment of powders is to extract the irrelevant impurities, to grind the powder particles and to improve the powder structure. Evolution of AlN and Si3N4 powders structure in HTT at different temperatures (500–1,150°C) has been studied. HTT of non-metal nitrides powders was shown to favour the removal of sorbed impurities from the particle surface and to decrease in starting temperature of powder sintering

Diamond single crystals with decahedral and icosahedral faceting and dimensions of 50–80 nm have been obtained by high-temperature high-pressure treatment of an ultradisperse detonation-synthesized diamond (UDD) powder. It is concluded that the growth of diamonds is related to the oriented association of initial UDD nanoparticles, the driving force of which is the tendency of the system to the state of minimum surface energy. The crystal habit is determined by the crystal morphology of UDD nanoparticles, which possess regular polyhedral shapes and flat facet morphology.

In present work we investigate the treatment of UDD powder in a hydrogen atmosphere and during sintering of the powder both in the as-delivered condition and after annealing in hydrogen. Features of the evolution of structural reconstruction in the system of UDD particles during sintering were elucidated.

The high pressure sintering process of nanocrystalline diamond powder was studied. The influence of the liquid phase on the base of boron oxide was analyzed. The mechanism of cooperative-diffusive coalescence, which acts during sintering of ultradisperse diamond powders, is proposed.

Deformation structures formed in diamond grains during polycrystalline sample sintering at 7.7 GPa were studied using TEM. A number of deformation features were observed in diamond during sintering in the temperature range 700–2500 °C. Based on these data a sequence for the structure formation processes in diamond grains under P-T treatment was ascertained.

Oxygen-doped AlN polycrystals were investigated by transmission electron microscope, and different stages of multilayered polytype formation during pressureless sintering were fixed. The decomposition of polytypes was found to take place during quasihydrostatic compression of samples at high temperatures. A model of multilayered polytypes appearing is proposed. The process of polytype formation is represented as isostructural delamination of the AlN-O solid solution at the expense of oxygen extraction on stacking faults, causing the oxygen-rich interlayer formation.