A. V. Ragulya

Synthesis and sintering of nanocrystalline barium titanate powder under nonisothermal conditions. I. Control of dispersity of barium titanate during its synthesis from barium titanyl oxalate

We have studied the process of synthesis of barium titanate powder from barium titanyl oxalate precursor under nonisothermal conditions. We studied the effect of the heating rate on the specific surface area of the powder. We have established the advantages of nonisothermal conditions compared with isothermal conditions, especially the possibility of flexible control over the barium titanate grain size.

Sintering of Refractory Compound Nanocrystalline Powders. Part 1. Storage and Preliminary Heat Treatment of Titanium Nitride Nanocrystalline Powders

Several approaches are considered for storing and processing finely-dispersed oxygen-free refractory compound powders using the example of titanium nitride nanopowder. The change in composition of nanopowders protected by a surfactant and without protection from oxidation during storage in air, and also during processing in gas atmospheres (nitrogen, hydrogen) and in a vacuum is studied.

Synthesis and Sintering of Nanocrystalline Barium Titanate Powder under Non-Isothermal Conditions. Part 6. Structure, Grain Boundaries, and Dielectric Properties of Barium Titanate Obtained by Various Sintering Methods

The grain boundary structure of barium titanate obtained by controlled-rate sintering and high-pressure sintering (HPS), and the dependence of dielectric properties on grain size and consolidation method were studied. It was shown that sintering without the application of pressure leads to a diffusion-controlled formation of equilibrium grain boundaries with minimal impedance factor, which minimally decrease the dielectric constant of the ceramics. HPS results in the formation of non-equilibrium grain boundaries which have a large free volume, and which substantially decrease the dielectric constant. The Curie-Weiss constant was analyzed from the viewpoint of a matrix structural model, and a «brick-wall» model.

Sintering of Refractory Compounds Nanocrystalline Powders. Part 2. Non-Isothermal Sintering of Titanium Nitride Powder

New approaches to the sintering of nanocrystalline powders of refractory compounds are proposed. Titanium nitride-based ceramics with a grain size of 50 nm and nanohardness 28.8 ± 2.47 GPa was obtained by sintering at a controlled rate of densification. Investigation of evolution of the block structure in the material during sintering enabled further optimization of the procedure and reduction of the sintering temperature to 1150°C.

Synthesis and Sintering of Nanocrystalline Barium Titanate Powder under Nonisothermal Conditions. Part V. Nonisothermal Sintering of Barium Titanate Powders of Different Dispersion

A comparative study has been made of the processes involved in the consolidation of nanosized barium titanate powders by nonisothermal sintering at a linear heating rate, by rate-controlled sintering, and high-pressure sintering (up to 5 GPa). The use of linear heating and high-pressure has been found to be ineffective for obtaining nonporous ceramics (residual porosity of about 2%) and for miniziming grain growth. The application of external pressure does not prevent coalescent grain growth controlled by surface diffusion. When rate-controlled sintering is employed a densification/grain growth optimum can be attained with a relative density of 99.9% of the theoretical value and a grain size of about 100 nm.

Friction and Wear of the TiB2–30 vol.% B4C Composite Consolidated in Spark Plasma Sintering

The tribotechnical properties of the TiB2–30 wt.% B4C composite material in pair with VK6 hard metal are investigated. The material demonstrates high wear resistance under dry friction conditions: the intensity of linear wear of the friction pair does not exceed 1.83 μm/km, the friction coefficient of the friction pair is 0.5. The analysis of 3D profiles of the sample surface shows the effect of smoothing of the surface of the ceramic composite by transfer of hard metal submicron particles on its surface and formation of tribofilm.

Production of Products of Various Shapes From Si3N4-Based Refractory Compounds by Spark Plasma Sintering

The possibility of manufacturing ball-, cylinder-, and hollow cylinder-shaped products based on Si3N4 and TiN nanocomposites by spark plasma sintering (SPS) is demonstrated. The features of the product properties associated with different conductivity of composites. The mechanical properties of ball- and hollow cylinder-shaped materials measured: HV ~ 11.5 GPa, KIc ~ 4.1 MPa · m1/2 (for Si3N4); and HV ~ 12.7 GPa, KIc ~ 4.8 MPa · m1/2 (for Si3N4–TiN).

Friction and Wear of TiN–Si3N4 Nanocomposites Against ShKh15 Steel

Spark plasma sintering is used to produce dense silicon nitride nanoceramics with titanium nitride additions and nanofiber-strengthened composites. The sintered nanocomposites demonstrate quite high mechanical properties (HV ≈ 14–16 GPa, KIc≈ 4.8 MPa ∙ m1/2) and low dry friction coefficient (f ≈ 0.65–0.68) and mass wear (~0.2–0.4 mg/km) against ShKh15 steel. The nanocomposite strengthened with 3.8 wt.% silicon nitride nanofibers shows the most balanced mechanical and tribotechnical characteristics.

Thermochemical microwave treatment of refractory nanopowders

Titanium nitride and silicon nitride nanopowders are subjected to microwave treatment to stimulate reduction and nitration reactions. It is established that oxygen content of the nanopowders after microwave treatment is 30–60% lower than that of the starting powders. It is also studied how the efficiency of thermochemical treatment and the particle-size distribution depend on the amount of the phase that absorbs microwave energy.

Electrical aspects of the spark-plasma sintering of TiN–TiB2 composite. I. Effect of electrical parameters of spark-plasma sintering on the structure and properties of TiN–TiB2 composite

The electrical aspects of spark-plasma sintering (SPS) of TiH2–BN and Ti + BN powder mixtures for the production of a TiN–TiB2 ceramic composite material are studied, and the dependence of its properties on direct-current density at the initial stage of sintering is established. To determine the direct-current density, a method for calculating the effective cross-sectional area Seff of the die–sample circuit is proposed. At the initial stage of sintering, the major portion of the current flows through the graphite die because of the resistance of the contacts and the presence of α-BN dielectric. At the final stage of sintering, the current flows mainly through the sintered sample because of the synthesis and densification of the TiN + TiB2 composite and, consequently, the abrupt increases in the conductivity of the sintered sample. Increasing the initial direct-current density during the sintering of TiHx hydride samples leads to an increase in relative density (conductivity), microhardness, fracture toughness, and abrasive wear resistance.