Self-propagating high-temperature synthesis of advanced ceramics MoSi2–HfB2–MoB

This study focuses on the investigation of the macrokinetic features of SHS (combustion synthesis) of elemental mixtures Mo–Hf–Si–B, in particular the mechanisms of structure and phase formation in the combustion front as well as the structure and properties of consolidated ceramics. Two routes for the fabrication of the composite SHS powder in system MoSi₂–HfB₂–MoB were used: (1) synthesis using Mo–Si–B and Hf–B mixtures followed by mixing of the combustion products and (2) synthesis using the four-component Mo–Hf–Si–B mixture. Dense ceramic samples with a homogeneous structure and low residual porosity (0.8–3.6%) were prepared by hot pressing of SHS powders. Although the particles size distribution and phase composition of SHS powders are similar for both synthesis routes, the structure and properties of both the composite SHS powders and hot-pressed ceramics differ considerably. Synthesis using the four-component Mo–Hf–Si–B mixture allows one to produce hierarchically ordered nanocomposite material with improved mechanical properties: hardness up to 17.6?GPa and fracture toughness up to 7.16?MPa?m^1/2.     

A stochastic model of disperse systems

We suggest a model of the structure of a disperse system that reflects the random character of the topology. We evaluate the effect of the stochastic nature of the topology on the effective conductivity. An asymptotic decrease in the conductivity with increase in the scale of the heat and mass transfer is established and explained.     

Influence of Radial Change in Gas Density on Nonlinear Hydrodynamic Effects in Its Flow Over a Rotating Disk

Journal of Engineering Physics and Thermophysics - A laminar boundary layer in a gas flow near a disk rotating at a constant angular velocity under the conditions of a significant radial...     

Comparative investigation of antibacterial yet biocompatible Ag-doped multicomponent coatings obtained by pulsed electrospark deposition and its combination with ion implantation

The aim of this work is to obtain antibacterial yet biocompatible coatings using pulsed electrospark deposition (PED). For this purpose new composite electrodes were fabricated from reaction mixtures Ti–C–20%Fe-10%Ca₃(PO₄)₂–3.4%Mg–X%Ag with different amount of antibacterial component (X = 0, 0.5, 1.0, 1.5 and 2.0 at% of Ag) using self-propagating high-temperature synthesis method. The electrodes consisted of TiC grains surrounded by TiFe₂ and TiFeP intermetallic matrix, CaO and MgO inclusions, and Ag-based phase. The influence of Ag content on the electrode mass transfer kinetics was studied by comparing the total substrate weight gain and electrode mass loss during PED. The structure, elemental composition, and surface roughness of coatings were studied by means of X-ray diffraction, scanning electron microscopy, and optical profilometry. The coatings were characterized in terms of Ag⁺ ion release, mechanical and electrochemical properties, as well as biocompatibility. The antibacterial characteristics of Ag-doped PED coatings were compared with those obtained by PED using Ag-free electrode and then implanted with Ag⁺ ions. The results indicated that an increase in the Ag content in electrode leads to a decrease in electrode erosion and substrate weight gain, but the efficiency of the PED process increases. Doping with a small amount of Ag (≤ 1 at%) resulted in 100% antibacterial effect against both gram-positive S. aureus and gram-negative E. сoli bacteria. In addition, the dynamics of МС3Т3-Е1 cell proliferation on the surface of PED coatings with 0.6–0.7 at% of Ag was similar to that in control samples, hereby indicating their biocompatibility. The coating biological characteristics were discussed based on the results of Ag⁺ ion release and electrochemical tests.     

Features of Synthesizing Ceramic Composites Discretely Reinforced by Carbon Fibers and SiC Nanowires Formed in situ in the Combustion Wave

Russian Journal of Non-Ferrous Metals - A new method is proposed for the engineering of SiC-based ceramic-matrix composite materials strengthened by discrete carbon fibers and single-crystal...     

Dynamics of phase formation during the synthesis of magnesium diboride from elements in thermal explosion mode

The influence of the heating rate of the Mg + 2B mixture on the dynamics of the phase formation during the thermal explosion in the helium medium is investigated by the time-resolved X-ray diffraction method. It is shown that the MgB₂ phase appears without the formation of intermediate compounds. The presence of impurity oxygen is a substantial factor affecting the formation kinetics of MgB₂. The oxide film on the surface of magnesium particles has no time to form with the heating rate of the charge mixture of 150–200°C/min. A result of this circumstance is the reaction diffusion mechanism of the Mg + 2B = MgB₂ reaction immediately after the melting of magnesium. Synthesis products mainly consist of MgB₂ and MgO traces at a level of 5%. The thermal explosion temperature is 1100°C. A comparatively thick oxide film which retards melt spreading and shifts the onset of the formation reaction of MgB₂ by 8–9 s grows on the magnesium surface at a heating rate of 30–50°C/min. Synthesis products contain MgB₂ and up to 15% MgO. The thermal explosion temperature is 1020°C in this case.     

Heretophase Ceramics in the Hf–Si–Mo–B System Fabricated by the Combination of SHS and Hot Pressing Methods

Russian Journal of Non-Ferrous Metals - This work is devoted to the fabrication of heterophase powder and consolidated ceramics based on hafnium and molybdenum borides and silicides by combining...