International Scientific Journals
of Scientific Technical Union of Mechanical Engineering "Industry 4.0"

Obtaining gas-tight joints between mullite articles was made by crystallization of yttrium alumosilicate glass. The glass joint layer softens at joining temperature (1450-1470^oC) to bond the mullite samples without additional pressure and then crystallized during cooling. The good adhesion of crystallized glass to mullite surfaces ensue the gas-tight joints with permittivity 2.5*10^-4 sccm/cm. The eutectic like crystallization of initial yttrium aluminosilicate glass into yttrium silicate and mullite lead to formation the nanocomposite structure which ensure the 4 point bending strength value near 105 MPa.

In the present paper authors investigated the effect of 2 wt % SiO₂ addition on the initial sintering stage of tetragonal zirconia nanopowder 3Y-TZP (ZrO₂-3 mol% Y₂O₃) which was manufactured by co-precipitation and mechanical mixing methods. It has been found that small amount SiO₂ has influenced on phase composition, crystallite size and agglomeration degree of zirconia nanopowders. The sintering behavior of 3Y-TZP with and without addition SiO₂ was investigated using the dilatometric data and analytical method for determining the sintering mechanism. The shrinkage behavior of all nanopowders was measured under constant rate of heating (CRH). The sintering mechanism on the initial sintering stage of 3Y-TZP changed from (VD) volume to (GBD) grain boundary diffusion by addition 2 wt% SiO₂ (3Y-TZP+2 wt% SiO₂) which have manufactured by co-precipitation method. The nanopowder 4PMM-3Y-TZP+2 wt% SiO₂ which has manufactured by mechanical mixing methods sintered according to (VD) volume diffusion mechanism.

It is known that the formation of oxide nanopowders takes place in the complex process including as non-isothermal and isothermal stages. In case of powders isothermal heat treatment led to realization of sequential process at transition from amorphous state to crystalline state, in particular, dehydration, crystallization, disintegration of linked nanoparticles in amorphous matrix and ensuing growth of crystals. In this work the questions of forming of particles and ceramic based on these nanoparticles are discussed. The influence of particles characteristics which synthesized at different temperatures on kinetics of isothermal sintering is estimated. It was shown that particles size and surface characteristics influence on kind of diffusion which is realized at ceramic sintering. The possible mechanisms of isothermal stage of nanoparticles and ceramics forming are discussed.

In the present study authors examined the sintering behavior on the initial sintering stage of fine tetragonal zirconia nanopowders (containing 3 mol % Y2O3) with and without mechanical activation in a ball mill. The initial sintering behavior was examined by constant rate of heating method (CRH) at different heating rates. We defined a role of the particles surface mechanical activation and its influence on the sintering mechanisms during the sintering of yttrium-stabilized zirconia nanopowders. We found that even a small mechanical action causes change initial characteristics of nanopowdres which can accelerate sintering processes. Increases of milling time causes change of sintering mechanism from volume (VD) to the grain-boundary (GBD).

The catalytic activity of material was tested by ESR spectroscopy and investigation of the formation of the super-anion radicals of oxygen on surface zirconia particles in dependence on dopant amount, pressure. UV-visible spectroscopy was used for estimation of optical properties of these materials. It was found the introduction of Y2O3 allows to creation of structure defects in partially stabilized zirconia, and its influence for tetragonal crystals is not monotonic in range 2-4 mol. %. It is connected with boundary of stable tetragonal phase at Y2O3 adding. The pressure is active factor of changing of surface state due to tetragonal – monoclinic transformation and may be used for improvements of catalytic properties of zirconia NPs.

A new approach to design of oxide ceramic-ceramic nanocomposites instead of industrial technical ceramics has been proposed. The use of mixing of liquid solutions of reagents instead of powders mixing allows entering in the matrix material the oxide dopants in supersaturated concentrations. The synthesized nanoparticles contain a potential for the formation of various structures. By the controlling of heating regimes during nanopowders sintering processes we can control the diffusion processes on the particles volume and boundaries. The decomposition of initial supersaturated solid solution during heat treatment (especially at fast sintering) can lead to formation of multilevel nanocomposite structure in the ceramic matrix with enhancing mechanical, electrical, optical and magnetic properties. It is established on the example of zirconia that even a slight amount of aluminum oxide, nickel oxide, zinc oxide leads to a significant change of the nanopowders and ceramics properties.