A. S. Lysenkov

Effect of dopant concentration on the phase composition and luminescence properties of Eu2+- and Ce3+-doped AlONs

Herein we report the synthesis of aluminum oxynitride (Al5O6N) doped with Eu2+ and Ce3+ and examine the effects of the dopant concentration and the nature of the rare-earth-containing precursor (europium and cerium oxides, Eu(acac)3, and Ce(acac)3) on its properties. At dopant concentrations above the solubility limit of rare-earth aluminates (EuAl12O19 and CeAl11O18) in aluminum oxynitride, we observed the formation of aluminum nitride and aluminum oxide (α-Al2O3) as individual phases. The concentration of the latter phase was higher when the rare-earth oxides were used. The emission intensity in the cathodoluminescence spectra of the samples was shown to have a maximum at Eu2+ and Ce3+ concentrations from 0.1 to 0.5 at %. Concentration quenching of luminescence was observed.

Influence of WSi2 content and additions of magnesium alumosilicates on oxidation and strength properties of MoSi2-WSi2 composites

Samples of MoSi2-WSi2 composite materials with additions of magnesium alumosilicates are obtained by solid phase sintering at 1650–1800°C in an argon atmosphere of powdered mixtures of initial components and disintegrated cast samples of 70% MoSi2–30% WSi2 (Mo0.7W0.3Si2), synthesized in an SHS reactor. The influence of the content of tungsten and oxide additive on oxidation of samples in air at 750°C for 100 h is studied. It is found that 70% MoSi2–30% WSi2 composite materials are less exposed to oxidation than MoSi2 and WSi2, and addition of 20 vol % magnesium alumosilicates decreases the rate of low temperature oxidation.

Effect of hot pressing temperature on the microstructure and strength of hydroxyapatite ceramic

A hot pressing method was applied for production of a dense ceramic with homogeneous structure and enhanced strength characteristics. Comparative investigations were carried out for properties of a ceramic obtained by traditional sintering in air and sintering under pressure in a temperature range of 900–1300°C. The hot pressing made it possible to decrease the temperature of achieving the vitrified state by at least 200°C. The microhardness of such materials at 1000°C is 4.2 GPa at an average crystal size of 80–150 nm and open porosity of 4%.

Hot-pressed ceramic SiC–YAG materials

Dense SiC-based ceramic materials containing yttrium aluminum garnet (YAG) as an oxide sintering aid have been prepared by hot pressing in the temperature range 1750–1850°C. As a result of melting, the oxides fill spaces between the SiC particles, contributing to the densification of the material and mass transport during the hot pressing process. The present results demonstrate that relatively small amounts of the oxides (≤5 wt %) are needed to ensure a high degree of densification of the SiC–YAG materials. The best physicomechanical properties are offered by the SiC + 3 wt % YAG material sintered at a temperature of 1850°C: ρ = 3.24 ± 0.01 g/cm3, П = 1.1 ± 0.1%, σb = 640 ± 10 MPa; KIc = 6.4 ± 0.2 MPa m1/2, Еel = 410 ± 20 GPa, and HV = 26.0 ± 0.2 GPa. This material experiences predominantly intercrystalline fracture.

Radioluminescent properties of Eu2+-doped aluminum oxynitride

X-ray luminescence (XRL) and pulsed cathodoluminescence (PCL) spectra of Eu2+-doped aluminum oxynitride Al5O6N (prepared using the standard sol-gel technique) have been investigated. The radioluminescence spectra, concentration dependences of light yield and time parameters of luminescence process have been measured and discussed. Some peculiarities of luminescence spectra as well as the possible excitation mechanism of AL5O6N with Eu2+ dopants are discussed too.

Low-temperature oxidation of MoSi2–Si3N4 composites

Composites in the MoSi2 + Si3N4 system containing 1.0, 2.5, and 5.0 wt % Si3N4 were synthesized by hot pressing at the temperature of 1650°C and the pressure of 30 MPa. Silicon nitride powders of two types of were used as reinforcing additives: one with isometric equiaxed crystals and one with fibrous structure. The samples were characterized by microstructural and phase analyses; the relative density and the flexural strength were determined. Composites with the flexural strength of up to 410 MPa were synthesized. The resistance to low-temperature oxidation of MoSi2 + Si3N4 composites in air at the temperature of 750°C was studied. The increase in the resistance to oxidation of composites with silicon nitride in air compared to pure molybdenum disilicide for both types of silicon nitride powders was established. For all the composites obtained, the parabolic oxidation rate constants were calculated.

Radiation-Induced Effects in Ce 3+ - and Eu 2+ -Doped Al 5 O 6 N

Polycrystalline Al5O6N samples have been prepared by sol–gel synthesis followed by carbothermal reduction and nitridation. X-ray luminescence (XRL) spectra and thermoluminescence (TL) curves of Ce3+- and Eu2+-doped aluminum oxynitride have been measured in the temperature range 5–380 K and analyzed. Working TL peaks have been detected in the range 150–250 K for the Al5O6N:Ce3+ sample and in the range 140–200 K for the Al5O6N:Eu2+ samples. There are also lower temperature peaks: in the ranges 10–60 and 20–100 K. The XRL spectra of the samples have been measured at temperatures of 8 (for the first time) and 300 K.

Effect of Si additions on the microstructure and mechanical properties of hot-pressed B4C

Dense B4C-based materials containing up to 4.9 wt % Si have been produced by hot pressing in the temperature range 1600–1700°C. In this process, the silicon melts to form a liquid phase, which improves the sinterability of the material. The boron carbide partially dissolves in the liquid Si to form silicon carbide between the B4C grains. The relative density, bending strength, Vickers hardness, and fracture toughness of the materials obtained in this study are 99.0 ± 0.1%, 584 ± 12 MPa, 39.4 ± 0.1 GPa, and 5.3 ± 0.2 MPa m1/2, respectively. The materials experience predominantly transcrystalline fracture.

Synthesis of aluminum oxynitride (AlON) and study of the properties of ceramics based on it

Aluminum oxynitride Al23O27N5 was synthesized by solid phase synthesis from a stoichiometric mixture of aluminum nitride and aluminum hydroxide at 1850°C in a nitrogen atmosphere. Aluminum oxynitride ceramic was prepared from the synthesized powders by hot pressing without sintering additives. The obtained samples were characterized by scanning electron microscopy and X-ray phase analysis; the mechanical properties of the ceramic samples were measured.

Luminescent properties of Eu2+ in AlON, SiAlON, Ca-SiAlON oxynitrides

The X-ray luminescence (XRL) properties of γ-AlON (γ-Al5O6N), β-SiAlON (Si6–nAlnOnN8–n, where n = 2 and 3) and Ca-α-SiAlON (Cam/2Si12−(m+n)Alm+nOnN16−n) oxynitride samples doped by Eu2+ were investigated. The strong correlation of luminescence spectra parameters of Eu2+ luminescence centers in dependence of oxynitride samples composition is observed. With fitting procedure the composition of XRL spectra with complex profile of investigated oxynitride samples of γ-AlON, β-SiAlON and Ca-α-SiAlON was identified and discussed.The X-ray luminescence (XRL) properties of γ-AlON (γ-Al5O6N), β-SiAlON (Si6–nAlnOnN8–n, where n = 2 and 3) and Ca-α-SiAlON (Cam/2Si12−(m+n)Alm+nOnN16−n) oxynitride samples doped by Eu2+ were investigated. The strong correlation of luminescence spectra parameters of Eu2+ luminescence centers in dependence of oxynitride samples composition is observed. With fitting procedure the composition of XRL spectra with complex profile of investigated oxynitride samples of γ-AlON, β-SiAlON and Ca-α-SiAlON was identified and discussed.