Nicolás M. Rendtorff

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.

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

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).

Densification of lightweight aluminum borate ceramics by direct sintering of milled powders.

ABSTRACT Dense aluminum borate (Al18B4O33) materials were processed by simple milling-sintering of aluminum borate powders obtained by reacting calcined alumina and fine boric acid at temperatures of around 600–800ºC. The effect of milling on the grain size and sintering behavior of the aluminum borate powders was also determined. Sintering began at around 1000°C and was limited by the thermal decomposition (T > 1300 °C) of the borate in to alumina and boron oxide, which volatilize at this temperature. Sub-angular and medium spherical sintered grain microstructures were developed. A hardness of 6 GPa and bulk density below 2.5 g/cm3 were achieved. The results are accomplished encouraging the structural applications of borate materials.

Comparative evaluation of properties of a clay based ceramic shaped via four techniques

The results of qualitative and quantitative properties of clay based ceramic are presented in this work. Four different shaping methods and sintering temperatures were used to understand their influence in the final properties of a ceramic material formulated using kaolinite clay and calcined alumina. This material can be used as a structural ceramic for different applications, and there is no pre-established relation between the forming method and the final sintered properties. Forming methods used to prepare the samples were uniaxial pressing (a batch process that allows application in dry samples), extruding (a continuous process that requires moisture), slip casting (a process that allows to shape complex ceramic ware), and lamination (a batch process that requires moisture). Sintering temperatures were in the range of 1100 and 1400 °C. In order to compare how properties behave as the shaping method and sintering temperature change, textural properties, shrinkage, porosimetry, phase composition and mechanical strength were evaluated and analyzed. Scanning electron microscopy and microtomography were employed for analyzing and comparing the developed microstructures. Differences in the resulting properties are explained in terms of the developed crystalline phases and microstructure.

Rapid prototyping of a complex model for the manufacture of plaster molds for slip casting ceramic

Computer assisted designing (CAD) is well known for several decades and employed for ceramic manufacturing almost since the beginning, but usually employed in the first part of the projectual ideation processes, neither in the prototyping nor in the manufacturing stages. The rapid prototyping machines, also known as 3D printers, have the capacity to produce in a few hours real pieces using plastic materials of high resistance, with great precision and similarity with respect to the original, based on unprecedented digital models produced by means of modeling with specific design software or from the digitalization of existing parts using the so-called 3D scanners. The main objective of the work is to develop the methodology used in the entire process of building a part in ceramics from the interrelationship between traditional techniques and new technologies for the manufacture of prototypes. And to take advantage of the benefits that allow us this new reproduction technology. The experience was based on the generation of a complex piece, in digital format, which served as the model. A regular 15 cm icosahedron presented features complex enough not to advise the production of the model by means of the traditional techniques of ceramics (manual or mechanical). From this digital model, a plaster mold was made in the traditional way in order to slip cast clay based slurries, freely dried in air and fired and glazed in the traditional way. This experience has shown the working hypothesis and opens up the possibility of new lines of work to academic and technological levels that will be explored in the near future. This technology provides a wide range of options to address the formal aspect of a part to be performed for the field of design, architecture, industrial design, the traditional pottery, ceramic art, etc., which allow you to amplify the formal possibilities, save time and therefore costs when drafting the necessary and appropriate matrixes to each requirement.

Thermal Shock Behavior of Zircon Based Refractories

In service refractory materials are submitted to local temperature gradients that originate thermal stresses causing a thermal shock (TS) damage to the material. Practical tests for evaluating the thermal shock resistance (TSR) determine the variation or change of some characteristic property of the test sample like E (elastic module) or the strength before and after quenching. In this work, the microstructure and thermal shock behavior of Zircon based refractories are analyzed. Several compositions (eight), from pure Zircon to 70 % of Zircon were studied. The main structural and mechanical properties of the refractories were characterized, as modulus of rupture, elastic modulus, porosity, and microstructure. The dynamic elastic modulus E of the refractories was measured by the excitation technique. The TS behavior was evaluated by measuring the decrease in E and the modulus of rupture, before and after a quenching test. The influence of the presence of other phases was also analyzed. Refractories showed Zircon as the main crystalline phase. In some materials, m-ZrO2 appears coming from Zircon dissociation. The thermal shock behavior of refractory of high Zircon content is typical of the brittle ceramic materials. Materials showed a relation between elastic module and strength. Dynamic elastic modulus measurements have shown to be suitable for evaluation the TSR for these materials.