Reactions responsible for mass loss during pressure less sintering of Si3N4 ceramics with LiYO2 additive

The reactions were investigated with emphasis on the mass loss during pressureless sintering of Si3N4 ceramics without powder bad. A Si3N4 powder compacts with LiYO2 additive were heat-treated at different temperatures under the pressure of N2 of 0.1 MPa.. The mass loss increased with increasing temperature as well as with increasing additive content. It is expected that the major mass loss should be caused by volatilisation of lithia due to its high vapour pressure. However, the results of the chemical analysis of the samples are in contradiction with this assumption.Physical chemistry 2004 : 7th international conference on fundamental and applied aspects of physical chemistry; Belgrade (Serbia); 21-23 September 200

The effect of Y2O3 addition on thermal shock behavior of magnesium aluminate spinel

The effect of yttria additive on the thermal shock behavior of magnesium aluminate spinel has been investigated. As a starting material we used spinel (MgAl2O4) obtained by the modified glycine nitrate procedure (MGNP). Sintered products were characterized in terms of phase analysis, densities, thermal shock, monitoring the damaged surface area in the refractory specimen during thermal shock and ultrasonic determination of the Dynamic Young modulus of elasticity. It was found that a new phase between yttria and alumina is formed, which improved thermal shock properties of the spinel refractories. Also densification of samples is enhanced by yttria addition

Phase evolution of Si3N4 with Mg-exchanged zeolite additive

This paper deals with the chemical reactions and phase evolution during pressureless sintering of Si3N4 with Mg-exchanged zeolite as sintering additive. The XRD data show that the first MgO-rich liquid forms at as low as 1250 °C, which as a consequence results in a significant reduction of sintering temperature. At 1300 °C the β−Si3N4 phase had disappeared indicating its dissolution into the liquid phase. At 1500 °C the secondary phase had been completely converted into a glassy phase, however, no phase transformation from α−Si3N4 to β−Si3N4 had taken place. Thus the composition of this material consists of α−Si3N4 embedded in magnesium silicon oxynitride glassy phase.Physical chemistry 2006 : 8th international conference on fundamental and applied aspects of physical chemistry; Belgrade (Serbia); 26-29 September 200

Synthesis of crystaline silicon oxynitride composites

Silicon oxynitride / silicon nitride (Si2N2O/Si3N4) ceramics have been prepared from Si3N4 powder and amorphous silica (SiO2) by hot pressing at different temperature. It was found that material sintered at lower temperature exhibit fine composite structure composed of equiaxed α-Si3N4 grains and Si2N2O crystals. At higher temperature the growing of Si2N2O particles as well as phase transformation from α-Si3N4 to β-Si3N4 phase take place.Physical chemistry 2006 : 8th international conference on fundamental and applied aspects of physical chemistry; Belgrade (Serbia); 26-29 September 200

Crystal structure analysis of Nd-doped ceria solid solutions

This paper deals with Nd-doped ceria solid solutions: Ce1-xNdxO2-d with "x" ranging from 0 to 0.25. Six different powders were synthesized by applying the method based on selfpropagating room temperature reaction (SPRT) between metallic nitrates and sodium hydroxide. The method is known to assure very precise stoichiometry of the final product in comparison with a tailored composition. Rietveld refinement was employed to get structural information on the synthesized powder. An increase of Nd ion concentration increases the unit cell parameters and average bond distances. We have shown that all obtained powders were solid solutions with a fluorite-type crystal structure and all powder particles were of nanometric size (about 3 nm)

Cerium oxide based nanometric powders: synthesis and characterization

Nanometric powders of solid solutions of cerium oxide were obtained by a modified glycine nitrate procedure. Solid solutions of the host compound CeO2 with one or more dopants in the lattice were synthesized. Rare earth cations (Re=Yb, Gd and Sm) were added to ceria in total concentration of x= 0.2 that was kept constant. The criterion in doping was to keep the value of lattice parameter of ceria unchanged. The lattice parameters were calculated by using the model that takes into account the existence of oxygen vacancies in the structure

Thermal shock resistance of ceramic fibre composites characterized by non-destructive methods

Alumina based ceramic fibres and alumina based ceramic were used to produce composite material. Behaviour of composite ceramics after thermal shock treatments was investigated. Thermal shock of the samples was evaluated using water quench test. Surface deterioration level of samples was monitored by image analysis before and after a number of quenching cycles. Ultrasonic measurements were done on samples after quench tests. Dynamic Young modulus of elasticity and strength degradation were calculated using measured values of ultrasonic velocities. Strengths deterioration was calculated using the non-destructive measurements and correlated to degradation of surface area and number of quenches. The addition of small amount of ceramic fibres improves the strengths and diminishes the loss of mechanical properties of samples during thermal shock experiments

The effects of detergent, sodium tripoly-phosphate and ethoxyled oleyl-cetyl alcohol on metabolic parameters of the fungus trichothecium roseum link

The degradation of detergents that are dispersed in water and soil partially depends on the metabolic activities of fungi. Among the fungi that have this ability, Deuteromycetes are particularly noted for their biochemical characteristics. Taking this into account, it was of interest to analyze the influence of detergent and its main compounds, ethoxyled oleylcetyl alcohol (AOC) and sodium tripoly-phosphate (TTP), on the metabolism of the fungus Trichothecium roseum. Our results revealed that both detergent and AOC had an inhibitory effect on the bioproduction of free organic acids, while TTP stimulated their production. Also, detergent inhibited the bioproduction of basic amino acids, with the exception of alanine. In addition, detergent applied at 1% concentration inhibited the bioproduction of proteins and the total biomass of the fungus, while AOC and TTP inhibited the production of proteins, but stimulatedl the production of Trichothecium

Kinetics of the alpha-beta Phase Transformation in Seeded Si3N4 Ceramics

The alpha-beta phase transformation in Si3N4 was studied for seeded samples using all Y2O3-Al2O3 mixture as a sintering aid. The concentration of beta-seeds varied from 0 to 5 wt.%. The results showed that alpha-beta phase transformation followed the first-order reaction. The calculated activation energy for seeded samples was in the range from 348 to 3 78 KJ/mol, indicating boundary reaction as the rate limiting step

Synthesis of fibrous SiC from natural precursor

Fibrous magnesio-silicate (mountain leather asbestos) of Serbian origin was used as Si precursor for the synthesis of SiC by carbothermal-reduction process [1, 2]. As a reducing agent, sugar (saccharose) was used. Formation of SiC was confirmed by XRD analysis and optical microscopy images. Results showed that obtained SiC possess fibrous morphology. Due to the experimental procedure MgF2 is also formed, which is known as an excellent additive for sintering of non-oxide ceramics.Physical chemistry 2006 : 8th international conference on fundamental and applied aspects of physical chemistry; Belgrade (Serbia); 26-29 September 200