O. O. Shichalin

HfB2-SiC (45 vol %) ceramic material: Manufacture and behavior under long-term exposure to dissociated air jet flow

Ultra-high-temperature composite materials HfB2-SiC containing 45 vol % SiC were prepared by spark plasma sintering. The behavior of a sample of this composition under exposure to a subsonic jet of dissociated air of a high-frequency induction plasmatron was studied; the total time was more than 30 min. Under certain test conditions, some regions of the sample were found to experience a rapid increase in temperature up to 2700°C. So, most of the surface area of the sample experienced exposure to temperatures up to 2500–2700°C for more than 15–18 min, while the rest of the surface had a temperature of 1700–1800°C during almost the entire duration of the experiment. The joint use of optical microscopy, scanning electron microscopy (with EDX analysis), X-ray powder diffraction, and X-ray computed microtomography enabled us to study the microstructure and composition of a structurally complex oxidized layer.

HfB2-SiC (10–20 vol %) ceramic materials: Manufacture and behavior under long-term exposure to dissociated air streams

Ultra-high-temperature composite materials HfB2-SiC containing 10, 15, and 20 vol % SiC were prepared by spark plasma sintering. The behavior of the samples prepared under long-term exposure to subsonic dissociated airstreams of a high-frequency induction plasmatron was studied. The total test time per sample was 35–42 min. Under certain exposure conditions (which were dependent on the composition of a sample), some regions of the sample were found to experience a rapid increase in temperature up to 2700°C. These regions enlarged over time, so that most of the surface area of the sample experienced exposure to temperatures of up to 2500–2700°C for 19–38 min, while the rest of the surface had a temperature of up to 1800–1900°C during almost the entire duration of the experiment. The joint use of optical microscopy, scanning electron microscopy (with EDX analysis), and X-ray powder diffraction enabled us to study the microstructure and composition of a structurally complex oxidized layer.

Behavior of a sample of the ceramic material HfB2–SiC (45 vol %) in the flow of dissociated air and the analysis of the emission spectrum of the boundary layer above its surface

Ceramic HfB2–SiC samples with a silicon carbide content of 45 vol % have been obtained by spark-plasma sintering, their density and calculated porosity determined, and certain studies performed using IR spectroscopy, XRD, and other methods. The behavior of the HfB2–SiC (45 vol %) material has been studied using the VGU-4 high-frequency induction plasmatron with heating by a subsonic flow of dissociated air. It has been shown that the average temperature of the surface of the samples in the process of heating rises up to 2680–2690°C, which is connected with the formation on the surface, which has the temperature of ∼1700–1900°C, of local regions with the temperature of 2600–2700°C with gradually expanding areas. The total time during which the average surface temperature is higher than 2000°C is ∼30 min. The change in the composition of the gas phase above the sample surface in the course of the experiment was investigated using optical emission spectroscopy. Assumptions, which explain the changes in the concentration of boron and silicon under the action of a high-enthalpy flow, have been advanced. The elemental composition and the phase composition have been determined, and the microstructure of the surface and of various cuts of the samples have been studied.

Behavior of HfB2-SiC (10, 15, and 20 vol %) ceramic materials in high-enthalpy air flows

HfB2–SiC ceramic samples containing 10, 15, and 20 vol % silicon carbide were prepared by spark plasma sintering. The samples were characterized by X-ray powder diffraction, SEM, and other methods. Their densities and calculated porosities were determined. The behavior of the materials under heating by a subsonic dissociated air flow was studied on a VGU-4 high-frequency inductive plasmatron. The average surface temperatures of the 10 and 15 vol % SiC samples were shown to increase up to 2550–2675°C during heating, due to the generation of surface localities having temperatures of 2600–2700°C (the initial surface temperature was ~1700–1900°C) and the progressive growth of these regions in area. The overall time during which the average surface temperatures of these samples were higher than 2000°C, was about 31–32 min. For the 20 vol % SiC sample, heat removal (when the sample touched a water-cooled holder) was shown to influence the surface temperature and surface temperature distribution. The variation in gas-phase composition over the central area of the sample surface during an experiment was studied using emission spectroscopy. Explanations are proposed to the variation of boron and silicon concentrations in the course of exposure to high-enthalpy flows. The elemental and phase compositions were determined and the microstructures were studied on the surface and sections of samples after long-term (~40-min) exposure to high-enthalpy air flows.

Spark Plasma Sintering as a high-tech approach in a new generation of synthesis of nanostructured functional ceramics

The results of Spark Plasma Sintering (SPS) synthesis of different types of ceramic materials for various industrial applications are presented. A high quality of ceramics is achieved through the originality of the developed approach based on combining SPS technology with other methods of inorganic synthesis, for instance, with sol–gel technology. The suggested approach enables one to synthesize, at the first stage, nanostructured powders of inorganic materials, whose subsequent consolidation by the SPS method ensures the formation of nanostructured ceramics with unique physicochemical characteristics and properties.

Wollastonite ceramics with bimodal porous structures prepared by sol–gel and SPS techniques

In this work, we suggest a novel synthetic route combining sol–gel and SPS for the fabrication of porous ceramics based on wollastonite with bimodal pore size distributions (100–500 nm and 1–500 μm) and high mechanical strength (Youngs modulus from 72.5 to 172 MPa). We studied peculiarities in the formation of the biporous silicate framework using two types of pore forming additives (templates) of various natures, shapes and sizes, introduced during different stages of sol–gel and SPS synthesis: organo-nonorganic (polymer latex of “core–shell” type) and nonorganic (carbonaceous filler). The influence of carbonaceous template content (5% or 25%) on the structural and mechanical properties of ceramics synthesized by spark plasma current was described. The original approach presented here provides formation of wollastonite ceramics with high mechanical strength and a biporous structure that is similar to the texture of bone tissue and is capable of executing its main functions. The obtained ceramics meet all the requirements of the bioceramics class demanded by modern medicine.

Study of the Thermal Behavior of Wedge-Shaped Samples of HfB 2 –45 vol % SiC Ultra-High-Temperature Composite in a High-Enthalpy Air Flow

By spark plasma sintering, HfB2–45 vol % SiC ultra-high-temperature ceramic was prepared, from which wedge-shaped samples were cut. The behavior of the samples was examined in a flow of dissociated air produced by an induction plasmatron, where the surface temperature of the leading edges of the samples reached ~2700°C. The dependence of the temperature distribution gradient on the distance from the leading edges of the samples was experimentally investigated. For the samples after the experiments, the elemental and phase compositions were determined, and features of the surface microstructure in various regions of the sample and on its polished surface were studied.

Preparation of porous SiC-ceramics by sol–gel and spark plasma sintering

A method for the production of porous SiC-ceramics from finely dispersed starting system SiO2–C by means of single-stage carbothermal process involving spark plasma sintering at moderate temperatures (≤1800 °C) is proposed. Synthesis of finely-dispersed SiO2–C system was performed by sol–gel as follows: tetraethoxysilane was hydrolyzed in the presence of polymeric source of carbon (phenolformaldehyde resin) with the subsequent drying and thermal treatment under inert atmosphere at 850 °C. The density of the obtained ceramic samples was found to vary within the range 1.43–1.84 g·cm−3 depending on the experimental conditions (the estimated porosity amounted to 42–55%). Ultimate compression strength was measured to be 94–279 MPa. Thermal behavior of materials in air flow up to 1400 °C, as well as microstructure and volumetric pore distribution (X-ray computerized microtomography), were studied.Graphical Abstract

Transparent 4 at% Nd3+:Y3Al5O12 ceramic by reactive spark plasma sintering

Neodymium doped yttrium aluminum garnet 4 at% Nd3+:Y3Al5O12 transparent ceramic was first fabricated by reactive spark plasma sintering using commercially available α-Al2O3, Y2O3, Nd2O3 powders as starting materials. Optimized sintering conditions were included two-steps: 1000 °C/3 min/100 °C min−1 and 1350 °C/5 min/30 °C min−1. 4 at% Nd3+:Y3Al5O12 ceramic had a homogenous microstructure with average grain size of 710 nm, and exhibited over 75 % transmittance in the visible wavelength range.