Esteban F. Aglietti

Physicochemical and thermal properties of mechanochemically activated talc

Abstract The progressive physicochemical and textural changes undergone by talc when submitted to mechanochemical treatment in a planetary mill are described. Crystallinity loss is observed together with particle agglomeration, while specific surface area increases in the early stages of the treatment, diminishing afterwards. The distortion of the basal planes of the crystal structure is greater than that of the planes nonparallel to the (001) ones. Differential thermal analysis reveals the formation of a noncrystalline metastable phase which exothermically crystallizes to enstatite at 830°C; at the same time, the endothermic effect at 1060°C, corresponding to the decomposition of talc into enstatite and SiO 2 , disappears. The formation of enstatite at 830°C from noncrystalline precursors is also observed by thermal treatment of a stoichiometric mixture Mg(OH) 2 ue5f8 SiO 2 gel.

Effect of starting powders on the sintering of nanostructured ZrO2 ceramics by colloidal processing

Abstract The effect of starting powders on the sintering of nanostructured tetragonal zirconia was evaluated. Suspensions were prepared with a concentration of 10 vol.% by mixing a bicomponent mixture of commercial powders (97 mol.% monoclinic zirconia with 3 mol.% yttria) and by dispersing commercially available tetragonal zirconia (3YTZ, Tosoh). The preparation of the slurry by bead-milling was optimized. Colloidal processing using 50 μm zirconia beads at 4000 rpm generated a fully deagglomerated suspension leading to the formation of high-density consolidated compacts (62% of the theoretical density (TD) for the bicomponent suspension). Optimum colloidal processing of the bicomponent suspension followed by the sintering of yttria and zirconia allowed us to obtain nanostructured tetragonal zirconia. Three different sintering techniques were investigated: normal sintering, two-step sintering and spark plasma sintering. The inhibition of grain growth in the bicomponent mixed powders in comparison with 3YTZ was demonstrated. The inhibition of the grain growth may have been caused by inter-diffusion of cations during the sintering.

Firing transformations of an argentinean calcareous commercial clay

Mineralogical transformations caused by firing are usually studied by XRD methods only semi-quantitatively. In this work the original mineral disappearance and the neo-mineralization were evaluated quantitatively. Furthermore an indirect non crystalline phase quantification was performed under 1100 oC was also carried out using the quartz content as internal standard. This study specifically discusses the behavior of an Argentinean white calcareous earthenware commercial when subjected to traditional ceramic firing, besides the technological importance of this particular material, it acts as a model for other clay based ceramic materials. Materials were subjected to thermal treatments between 700 oC and 1100 oC. A preliminary sintering characterization was carried out by contraction and porosity evolution. Simultaneous thermogravimetric and differential thermal analysis (TG-DTA) was carried out to elucidate the actual temperature at which the chemical changes occur. Finally, a quantitative analysis based on the Rietveld refinement of the X-ray diffraction patterns was performed to characterize the crystalline phases present in both the clay and in the materials obtained after different thermal treatments. The actual chemical reactions are proposed. The phases identified after firing at traditional working temperature (1040 oC) are quartz, plagioclase, and the Spinel type alumino-silicate, accompanied by the non-diffracting un-reacted metakaolin and some amount of amorphous glassy phase. At intermediate temperatures (900 oC) the presence of gehlenite was also detected. The carbonates (calcite and dolomite) presence and decomposition were also evaluated and demonstrated to determine the sintering characteristics of this material.

Mullite (3Al2O3·2SiO2) ceramics obtained by reaction sintering of rice husk ash and alumina, phase evolution, sintering and microstructure

Abstract The use of industrial waste (by-products) as raw materials in the ceramic industry has been under study for decades due to the economical, energy, tax and environmental advantages. The specificity of the waste requires a basic characterization and technology thereof. The applicability of rice husk ash (RHA), as silica (SiO2) source, in refractory and porous materials withpotential structural, insulating and/or filtering applications was carried out by characterizing the ceramicbehavior of stoichiometric mixtures of calcined alumina (Al2O3) and RHA. A reaction-sintering frameworkcan be defined in the (Al2O3–SiO2) system. The sinterability and conversion during the reaction sinteringprocesses were studied in order to obtain mullite (3Al2O3·2SiO2) ceramics. Also some microstructural fea-tures of the developed materials were studied in the 1100–1600ºC range. The mullitization was studiedquantitatively. Partial densification was achieved (30%) and highly converted materials were obtained. The developedmicrostructure consisted in a dense ceramic matrix with homogenous interconnected porosity, with anarrow pore size distribution below 20 μm. The developed material gives enough information for design-ing mullite ceramics materials with either porous or dense microstructures with structural, insulating orfiltering applications employing RHA as silica source and calcined alumina as the only other raw material.

SiCSi3N4 bonded materials by the nitridation of SiC and talc

Abstract Silicon carbide-based materials are used in many applications due to their good thermo-mechanical properties at high temperatures. The nitrogenous bond formed from nitriding reactions of silicate minerals seems to be an interesting method to obtain these materials. In this work, the reaction of SiC with talc and N 2 atmosphere is studied. Pellets and/or bars of SiC-talc mixtures were reacted within the range 1395–1650 °C. Reaction products were studied through XRD, textural properties, bending strength and microscopy. Reactions and mechanisms for this system are proposed. Silicon nitride is formed as principal bond phase. During reaction, volatilization of Mg takes place, where its loss is favoured at high temperatures and high N 2 flow rates. A loss of SiO(g) is also observed. Mechanical strength has a linear relation with the bond phase content.

On the occurrence of a metastable tetragonal t′-phase in a ZrO2−13.6 mole % MgO ceramic and its microscopic thermal evolution

The time-differential perturbed angular correlation technique has been used to investigate the thermal behavior of a ZrO 2 −13.6 mole % MgO ceramic between room temperature and 1423 K. Two different quadrupole hyperfine interactions corresponding to a tetragonal structure have been found to result on cooling the ceramic from the single-phase cubic field. One of them agrees with that depicting the pure t -ZrO 2 tetragonal phase and the other one has been interpreted as describing a high-MgO-content nontransformable t –ZrO 2 phase. As temperature increases, the latter gives rise to a similar but fluctuating interaction related to the oxygen vacancies mobility and which shows a thermal behavior analogous to that already reported for the stabilized cubic ZrO 2 . Above 1100 K these dynamic t -sites transform into pure tetragonal ones which behave ordinarily, suffering the t → m phase transition when cooling to room temperature. Differences found between TDPAC results and information drawn from other techniques are discussed.

Influence of the zirconia transformation on the thermal behavior of zircon–zirconia composites

During a heating–cooling cycle, zirconia (ZrO2) undergoes a martensitic transformation from monoclinic to tetragonal structure phases, which presents special hysteresis loop in the dilatometry curve at temperatures between 800 and 1100xa0°C. Monoclinic zirconia (m-ZrO2) particles reinforced ceramic matrix composites not always present this behavior. In order to elucidate this fact a series of zircon–zirconia (ZrSiO4–ZrO2) ceramic composites have been obtained by slip casting and characterized. The final properties were also correlated with the zirconia content (0–30xa0vol.%). The influence of the martensitic transformation (m–t) in well-dispersed zirconia grains ceramic composite on the thermal behavior was analyzed. Thermal behavior evaluation was carried out; the correlation between the thermal expansion coefficients with the zirconia content showed a deviation from the mixing rule applied. A hysteresis loop was observed in the reversible dilatometric curve of composites with enough zirconia grains (≥10xa0vol.%). Over this threshold the zirconia content is correlated with the loop area. The transformation temperatures were evaluated and correlated with the zirconia addition. When detected the m–t temperature transformation is slightly influenced by the zirconia content (due to the previously evaluated decrease in the material stiffness) and similar to the temperature reported in literature. The reverse (cooling) transformation temperature is strongly decreased by the ceramic matrix. The DTA results are consistent with the dilatometric analysis, but this technique showed more reliable results. Particularly the endothermic m–t transformation temperature showed to be easily detected even when the only m-ZrO2 present was the product of the slight thermal dissociation of the zircon during the processing of the pure zircon material.

Quantitative firing transformations of a triaxial ceramic by X-ray diffraction methods

Fil: Conconi, Maria Susana. Provincia de Buenos Aires. Gobernacion. Comision de Investigaciones Cientificas. Centro de Tecnologia de Recursos Minerales y Ceramica. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - la Plata. Centro de Tecnologia de Recursos Minerales y Ceramica; Argentina

Fabrication of Dense ZrO2/CNT Composites: Influence of Bead-Milling Treatment

Highly concentrated zirconia-carbon nanotube (CNT) water suspensions were prepared using an advanced milling technique. The bead-milling operation parameters were optimized for this system and used to prepare zirconia-stabilized water-based suspensions with different CNT contents. The effects of different milling conditions were studied. The particle dispersion was evaluated by SEM observations on dried suspension. Green’s density and SEM observations of compacts were used to follow the colloidal dispersability of the composites. Materials of tetragonal zirconia and CNTs were prepared with a high concentration of CNTs (1, 5, and 10xa0wt pct CNT). The homogeneous dispersion and distribution of the fibers in the bulk material after slip casting of the suspension were examined. The samples were sintered using spark plasma sintering (SPS) at 1473xa0K (1200xa0°C) and finally, fully dense materials were obtained. The mechanical properties were evaluated using the Vickers indentation technique.

Non isothermal kinetic study of the aluminium titanate formation in alumina-titania mixtures

Aluminum titanate (Al2TiO5) is a high refractoriness material with excellent thermal shock resistance. Hence it is suitable for several applications at elevated temperatures where insulation and thermal shock resistance are required. Such as components of internal combustion engines, exhaust port liners, metallurgy, and thermal barriers. The thermal instability of Al2TiO5 at high temperature is another characteristic of this material that has been studied and controlled by the incorporation of several additives. The Al2TiO5 formation from pure oxides presents an endothermic peak in the differential thermal analysis (DTA). The thermodynamic temperature is 1280 oC. But experimentally, as in every other DTA experiment, these peaks strongly depend on the heating rate: this fact has been extensively employed for the kinetic study of transformation processes and the mechanism determination of chemical reactions. Both activation energies (Ea) and nucleation rates can be obtained from these experiments. The present work reports the formation Ea of Al2TiO5 prepared from pure oxides at air atmosphere by the Kissinger DTA based methods. Previously the particle size distribution of the starting powders together with X-ray diffraction analysis of the starting powders and the resulting materials was carried out. The properties of the Al2TiO5 formation were grouped into two groups corresponding to the low and high heating rates, below and over 5 K/min. Ea values were obtained after the Avrami (n) constant evidenced that the crystallization mechanism is strongly related to the heating rate, even in the wide range studied which includes the technological ones(0.5-40 K/min).