Mechanically activated ceria

The reduction of comercial and mechanically activatedCeO2 powder at 1200 °C in an argon atmosphere was examined by XRD. Mechanical activation of oxide powder was performed by milling in a vibratory mill for different lengths of time. During 30 min of milling it came to the greatest change in crystallite size, as well as of lattice distortion of CeO2 while after 60 min of activation effect of milling on the CeO2 properties was negligible. Fired CeO2 was partly reduced but firing of 60 min milled CeO2 produced only CeO2-x with lattice parameter a = 0,550 nm.Physical chemistry 2004 : 7th international conference on fundamental and applied aspects of physical chemistry; Belgrade (Serbia); 21-23 September 200

Synthesis of apatite-type Ce4.67(SiO4)3O via glycinenitrate combustion

The pure Ce4.67(SiO4)3O with the apatite-type of structure was obtained for the first time from cerium nitrate, glycine and tetraethyl orthosilicate (TEOS) through the self-combustion of the gel. The solution of the reactants with the molar ratio of TEOS to water 1: 42 and glycine to NO3 ¯ 1: 3 was transformed into gel. During combustion of the gel the ash containing nanostructured CeO2 and noncrystalline SiO2 was formed. The phase identification and the measurement of the crystallite size was done by XRD diffraction. Ce4.67(SiO4)3O was synthesized from the ash subsequently fired in argon at 1200 °C.Physical chemistry 2008 : 9th international conference on fundamental and applied aspects of physical chemistry; Belgrade (Serbia); 24-28 September 200

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

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

Preparation of basalt-based glass ceramics

Local and conventional raw materials-massive basalt from the Vrelo locality on Kopaonik mountain-have been used as starting materials to test their suitability for the production of glass-ceramics. Crystallization phenomena of glasses of the fused basalt rocks were studied by X-ray phase analysis. optical microscopy and other techniques. Various heat treatments were used, and their influences. on controlling the microstructures and properties of the products were studied with the aim of developing high strength glass-ceramic materials. Diopside CaMg(SiO3)(2) and hypersthene ((Mg,Fe)SiO3) were identifies as the crystalline phases. The final products contained considerable amounts of a glassy phase. The crystalline size was in range of 8-480 mum with plate or needle shape. Microhardness, crashing strength and wears resistence of the glass-ceramics ranged from 6.5-7.5, from 2000-6300 kg/cm(2) and from 0.1-0.2 g/cm, respectively

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

Composition of red mud and/or metakaolin-based modified geopolymers

There is potential use of red mud for synthesis of inorganic polymeric materials through a geopolymerization process as an alternative in the sectors of construction and building materials. By introducing of inorganic and organic modificators of microstructure (calcium hydroxide, bifunctional epoxy resins, or various types of alkoxylanes) during the geopolymer synthesis the enhanced values of ductility and strength can be obtained. Research was performed on alumosilicate material (red mud and metakaolin) and alkali activator raw mixture with defined quantity of modificator. The best sinthesys conditions were identified. Post-synthesis curing also play important role in obtaining of good-performing geopolymers. Characteristics of geopolymers were defined by measuring of compressive strength, N2-physisorption, as well as by SEM analysis, X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). In addition the lower value of zeta potential was identified as the assisting factor for the specific structure domains formation (within the certain range of pH) accompanied by the high compressive strength

Studies on Structural and Morphological Properties of Multidoped Ceria Ce0.8Nd0.0025Sm0.0025Gd0.005Dy0.095Y0.095O2-delta (x=0.2) as Solid Solutions

The nanopowdery solid solutions of multidoped ceria Ce0.8Nd0.0025Sm0.0025Gd0.005Dy0.095Y0.095O2 (delta) (x = 0.2) with the fluorite type crystal structure of CeO2 were synthesized for the first time. Two synthesis procedures were applied: the modified glycine-nitrate procedure (MGNP method) and room temperature self-propagating reaction (SPRT method). All nanopowders were characterized by XRPD analysis, Raman spectroscopy, low temperature nitrogen physisorption, TEM, and SEM methods. According to the XRPD and Raman spectroscopy results, single phase solid solutions of fluorite structure were evidenced regardless of the number of dopants and synthesis procedure. Both XRPD and TEM were analyses evidenced nanometer particle dimensions. The SPRT method results in obtaining sample with higher specific surface area, smaller crystallite and particles sizes, and the same values of the lattice parameter in comparison to pure CeO2. Raman spectroscopy was confirmed to the oxygen vacancies introduced into the ceria lattice when Ce4+ ions were replaced with cations (dopants) of lower valence state (3+), which may indicate the potential improvement of ionic conductivity. Additionally, the presence of oxygen vacancies in the lattice ceria, as well as very developed grain boundaries, gives a new possibility for potential application of obtained nanopowders in the area of room temperature ferromagnetism as spintronics

Synthesis of biomorphic SiC and SiO2 ceramics

Coniferous wood (fir) was transformed by pyrolysis into carbon preforms, which were subsequently converted into biomorphic ceramics by the pressure infiltration technique with colloidal silica. An in situ reaction between the silica and the carbon template occurred in the cellular wall at a high sintering temperature. Depending on the employed atmosphere, non-oxide (SiC) or oxide (SiO2) ceramics were obtained. The morphology of the resulting porous ceramics and their phase composition were investigated by scanning electron microscopy (SEM/EDX) and X-ray diffraction (XRD). The experimental results showed that the biomorphic cellular morphology of the wood maintained in both the SiC and silica ceramics, which consisted of only the beta-SiC phase and SiO2, respectively