We have studied the effect of sodium fluoride as a sintering aid for β-sialons on the phase composition and physicomechanical properties of Si5AlON7 and Si4Al2O2N6. Two-step high-temperature firing of the β-sialons in the presence of NaF under a nitrogen atmosphere has been shown to cause no significant changes in the phase composition of the materials. The density and microhardness of the materials prepared using 0.5 and 5.0 wt

Изучено влияние фторида натрия как спекающей добавки для β-сиалонов на фазовый состав и физико-механические свойства Si 5 AlON 7 и Si 4 Al 2 O 2 N 6 . Показано, что двухстадийный высокотемпературный обжиг β-сиалонов в атмосфере азота с добавлением NaF не приводит к значимым изменениям в фазовом составе образцов. Плотность и микротвердость полученных с добавлением 0.5 и 5.0 мас. % NaF образцов оказываются ниже, чем для образцов без спекающих добавок, однако прочность на изгиб демонстрирует рост на величину до +14.3% для Si 5 AlON 7 при содержании NaF 0.5 мас. % и до +4.9% для Si 4 Al 2 O 2 N 6 при содержании NaF 5.0 мас. %.

A comparison infrared spectroscopic study of reactions under nitrogen in various batch compositions for the preparation of β-sialon Si 3 Al 3 O 3 N 5 from various crystalline nitrides (AlN and Si 3 N 4 ) and amorphous oxides prepared by the sol–gel process from silicon and aluminum alkoxides, and from mixed aluminosilicate xerogels nitrided by molecular nitrogen at various steps of the β-sialon synthesis. The IR absorption bands at ~1080−1040 and 580 cm –1 indicate that the sialons prepared from different batch compositions are isostructural to β-Si 3 N 4 , thereby verifying the formation of β-sialon of the tailored composition Si 3 Al 3 O 3 N 5 .

Ceramic samples based on Si3N4 were obtained and studied using a sintering additive in the MnO-TiO2 system of eutectic composition, which was accepted and applied to the surface of silicon nitride by the sol-gel method. Ceramic samples were obtained by hot pressing at 1650 °C and 1750 °C. Phase composition, thermo-physical and mechanical properties were studied. The microstructure of the resulting ceramics was also studied.

The conditions for β-SiAlON to be prepared by reacting highly dispersed mixed silicon and aluminum alkoxide xerogels with molecular nitrogen at 1600–1700°С are determined. The nitrogenation of mixed xerogels is shown to be due to the catalytic pyrolytic decomposition of hydrolysis products of silicon and aluminum alkoxides in a nitrogen atmosphere, namely, a mixture of alcohols that are adsorbed or chemisorbed on aluminosilicate xerogel particles. The pyrolysis of alcohols in a nitrogen atmosphere catalyzed by highly dispersed alumina yields hydrogen and carbon monoxide, which activate nitrogenation and affect the phase composition of the samples and change the sialon stoichiometry from β-Si3Al3O3N5 to the 15R polytype.

The compaction of initial powders, the bending strength, and the microhardness of 21R sialon ceramics obtained by hot pressing have been studied experimentally. It is found that the Sm2O3 sintering additive substantially reduces the annealing temperature and enhances the properties of the ceramics. The samples of 21R sialon ceramics without additives, which are prepared by annealing at 1950°C, have a density of 3.01 g/cm3, a bending strength of 240 ± 15 MPa, and a Vickers microhardness of 16.2 ± 0.4 GPa; the samples of 21R sialon ceramics with 2.5 wt % Sm2O3, which are annealed at 1750°C, are characterized by a density of 3.39 g/cm3, a bending strength of 315 ± 16 MPa, and a Vickers microhardness of 21.9 ± 0.2 GPa. It is shown that Sm2O3 reacts with 21R sialon in the temperature range of 1600–1700°C to form the 27R sialon and SmAlO3 impurity phases. In the samples annealed at 1750°C, the 27R sialon and Sm–sialon (Sm3Si2.5Al3.5O12.5N1.5) impurity phases are detected.

Si3N4–TiN ceramic composites have been produced by hot-pressing mixtures of silicon nitride and metallic titanium powders at temperatures from 1600 to 1800°C in a nitrogen atmosphere. We have examined the influence of the concentration and morphology of metallic Ti particles in the starting mixture and synthesis conditions on the microstructure, phase composition, and mechanical strength of the Si3N4–TiN ceramic composites. The results demonstrate that the sintering process ensures complete titanium nitridation, leading to the formation of a nonstoichiometric titanium nitride with the composition TiN0.9. The Si3N4–TiN composites prepared from the mixtures containing 5–30 wt % Ti have densities in the range 3.02–3.41 g/cm3, water absorption from 0.01 to 0.14%, open porosity from 0.03 to 0.44%, and bending strength from 250 to 584 MPa. The Si3N4–TiN ceramics prepared using calcium aluminates as sintering aids consist of dense intergrowths of silicon nitride crystallites, which ensures increased strength of the materials. Moreover, the samples containing 25–30 wt % TiN offer high electrical conductivity.

Specific features of the synthesis of sialons (SiAlON) in the Si–Al–O–N system by firing at 1600– 1700°C in a nitrogen atmosphere of mixtures of different initial compositions corresponding to the same composition of the final product Si3Al3O3N5 have been studied. β-Sialon was synthesized from reaction mixtures based on crystalline silicon and aluminum nitrides and amorphous highly dispersed hydrated oxides— (1) AlN + SiO2(sol), (2) Si3N4 + Al2O3(sol), and (3) AlN + Si3N4 + Al2O3(sol)—obtained by applying sols based on silicon or aluminum alkoxides (based on the β-sialon stoichiometry) to the surface of nitride powders by the sol-gel method. It has been demonstrated that β-Si3Al3O3N5 can form in the reaction at 1600°C of highly dispersed mixed xerogels made of silicon and aluminum alkoxides—(4) (SiO2 + Al2O3) (mixed sol)—with molecular nitrogen without the participation of the initial nitrides. The influence of the starting composition of xerogel mixtures and synthesis conditions on the phase composition and morphological features of sialons obtained in one stage of calcination in a nitrogen atmosphere has been studied by thermogravimetry, X-ray powder diffraction analysis, and electron microscopy. It has been shown that the proposed reactions of the formation of sialons involving crystalline nitrides, xerogels, and molecular nitrogen are related to reduction–nitridation reactions, in which the reducing agents are organic products of hydrolysis of silicon and aluminum alkoxides and their pyrolytic decomposition in a nitrogen atmosphere.

To obtain silicon and aluminium oxonitrides, the transformations of xerogels and nitrides during firing in a nitrogen atmosphere were studied by thermal and x-ray phase analysis. It was established that the interaction between nitrides and xerogels in a nitrogen atmosphere with the formation of oxonitrides is due to reduction-nitriding reactions. The CO and hydrogen which are formed during the pyrolysis of adsorbed and chemisorbed alcohols (which are the products of the hydrolytic decomposition of silicon and aluminium alkoxides in xerogels) is a reducing agent for the finely dispersed oxides. The effect of the composition of the initial mixtures on the phase composition of the final product (SiAlON) and the morphological features of the powders obtained in one stage of firing in a nitrogen atmosphere is shown.

The technique was developed, the installation was done, and the conditions for the production of Si3N4 and Si2N2O by the method of gas-phase pyrolysis of hexamethyldisilazane (CH3)3-Si-NH-Si-(CH3)3 (HMDS) were experimentally studied. In the experiments, two different methods of inputting the raw material were used—the input of a vapor–gas mixture (bubbling feeder with heating to supply the HMDS vapor in a stream of carrier gases) and input as a gas-droplet stream (pneumatic nozzle). The effect of gas-dynamic synthesis conditions at temperatures up to 1100°C on the properties of silicon oxonitride and silicon nitride nanopowders was studied. The influence of the conditions of mixing the reactants, the volume ratio of nitrogen/ammonia, and the content of HMDS in the vapor–gas mixture on the yield of products was shown. The dependences of the degree of conversion of the feedstock on the gas flow rate and the concentration of ammonia in the gas phase were obtained. The optimal conditions for the pyrolysis process were found: temperature, the ratio of the components of the gas mixture, the conditions of mixing, and the contact times of the phases. X-ray amorphous Si3N4 and Si2N2O powders with particle sizes of 50–200 nm and a specific surface area of up to 15 m2/g and powders of alpha modification of silicon nitride Si3N4 in the form of threadlike crystals with a particle diameter of 50–200 nm were obtained.

The phase composition and morphological features of си-sialons doped with REE (Eu, Tb, Ce) were studied by nitriding the mixed sol prepared on the basis of silicon and aluminum alkoxides without introducing the corresponding nitrides. It was established that as a result of nitriding of a doped REE mixed Si-Al sol at 1650 °C, the β-SiAlON phases with a hexagonal crystal structure corresponding to β-Si3Al3O3N5 are formed. At a firing temperature of 1700 °C crystalline sialon types corresponding to the χ-phase with a monoclinic structure (Si2Al3O7N and Si6Al10O21N4) are formed. The effect of the addition of red-earth elements on the morphology of crystalline particles of the resulting sialon phases is shown.

The aim of this work was to obtain Si3N4/SiC composites, where silicon nitride served as the matrix, and the silicon carbide content varied from 3 to 50wt.%. To reduce the sintering temperature (hot pressing) of the ceramic, it is used a sintering additive of calcium aluminate eutectic. The charge was prepared as follows: the silicon nitride powder was mixed with 10 wt.% sintering calcium aluminates, then the charge was added over 100 % of silicon carbide in an amount of 3, 5, 7, 10, 15, 20 and 50 wt.% in a planetary mill. Ceramic materials were hot pressed at 1660 degrees C, the pressure was 30 MPa for 60 minutes in N-2 atmosphere. The 10 wt.% SiC composite has highest properties: 3.16 g/cm(3) density, 650 MPa bending strength, 22 GPa microhardness.

The Si3N4-TiN composites were obtained by hot pressing at 1600–1800 °C in a nitrogen atmosphere from mixtures of powders of silicon nitride and metallic titanium. The influence of particle morphology, Ti content in the initial charge and synthesis conditions on the microstructure, phase composition, mechanical strength of the samples of ceramic composites Si3N4-TiN was studied. It was established that in the process of sintering, complete nitriding of titanium occurs with the formation of a non-stoichiometric nitride of composition TiN0.9. Samples of composites Si3N4-TiN, obtained from a mixture containing 5-30 % Ti, have a density of 3.02-3.41 g/cm3, water absorption 0.01-0.14 %, open porosity 0.03-0.44 % and flexural strength from 250 to 584 MPa. It is shown that ceramics Si3N4-TiN with a sintering additive of calcium aluminates is characterized by a dense intergrowth of silicon nitride crystals, which provides an increase in the strength of the samples.

The solid solutions based on Zirconia doped with indium oxide and praseodymium oxide with different concentration was obtained by the method of inverse heterophase co-deposition. The physico-chemical properties of obtained samples were examined. The highest properties have been achieved in samples of the composition Zr1-xRxO2-δ (R=23 mol .% In). This composition has a density of 5.38 g/cm3; open porosity is 0 %, the bending strength is 203 MPa.

The X-ray luminescence (XRL) properties of gamma-AlON (gamma-Al5O6N), beta-SiAlON (Si6-nAlnOnN8-n, where n = 2 and 3) and Ca-alpha-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 gamma-AlON, beta-SiAlON and Ca-alpha-SiAlON was identified and discussed.