Jose Luis Rodríguez-Galicia

Precursor mixtures composed of fly ash, BaCO3, SrCO3, Al2O3 and ZrO2, were subjected to attrition milling for 0-8 h and then uniaxially pressed and sintered at 900-1500 °C/5 h, for the in situ solid state synthesis of composites with nominal Ba0.75Sr0.25Al2Si2O8 (SBAS)/ZrO2 mass ratios of: 1) 90/10, 2) 70/30, and 3) 50/50. Mechanical activation, combined with the likely generation of a considerable amount of transient liquid during sintering of the composites, notably enhanced the reactivity of the precursor mixtures. ZrO2 decreased the conversion from the hexagonal (Hexacelsian) into the monoclinic (Celsian) phases of SBAS in the composites, which became more pronounced when the content of ZrO2 was increased in the materials. Nearly full conversions could be achieved at temperatures as low as 1100 °C, by mechanically activating the precursor mixtures for times that increased with increasing content of ZrO2 in the materials. An increment in the time of mechanical activation of the precursor mixtures, as well as in their ZrO2 content and in the sintering temperature, increased the mechanical properties of the synthesized materials. Thus, the best mechanical properties were obtained for composition 3 milled for 8 h and sintered at 1500 °C. Mezclas precursoras de cenizas volantes, BaCO3, SrCO3, Al2O3 y ZrO2, fueron activadas mecánicamente en un molino de atrición por 0-8 h y luego prensadas uniaxialmente y sinterizadas a 900-1500 °C/5 h, para la síntesis in situ por reacción en el estado sólido de compósitos con relaciones nominales Ba0.75Sr0.25Al2Si2O8 (SBAS)/ZrO2 en masa de: 1) 90/10, 2) 70/30, y 3) 50/50. La activación mecánica, combinada con la probable generación de una cantidad considerable de líquido transitorio, incrementó notablemente la reactividad de las mezclas precursoras. La ZrO2 disminuyó la conversión de la fase hexagonal (Hexacelsiana) a la monoclínica (Celsiana) del SBAS, lo cual fue más pronunciado cuando el contenido de ZrO2 se incrementó en los compósitos. Fue posible obtener conversiones casi completas a temperaturas tan bajas como 1100 °C, activando mecánicamente las mezclas precursoras por tiempos que se incrementaron con el incremento en el contenido de ZrO2 en los materiales. El incremento en el tiempo de activación mecánica de las mezclas precursoras, así como en su contenido de ZrO2 y en la temperatura de sinterización, incrementó las propiedades mecánicas de los materiales sintetizados. Así, las mejores propiedades mecánicas correspondieron a la composición 3 molida por 8 h y sinterizada a 1500 °C

Ceramic composites with Ba0.75Sr0.25AlSi2O8 (SBAS)/Al2O3 mass ratios of: 1) 90/10, 2) 70/30, and 3) 50/50, were in situ synthesized at 900-1500 °C/5 h from mixtures of fly ash, BaCO3, SrCO3 and Al2O3. The green mixtures were mechanically activated for 0, 4 and 8 h in an attrition mill. As a result, the solid state reactions were faster and occurred at lower temperatures. Only the SBAS and Al2O3 phases were obtained at 1300-1500°C, with the SBAS present in composition 1 achieving full conversion from its hexagonal (Hexacelsian) into its monoclinic (Celsian) form, with or without milling. The higher nominal SBAS content of composition 1 facilitated in it the mentioned conversion, in comparison with the other two studied compositions, which required to be mechanically activated for times that increased with increasing Al2O3 content, in order to attain in them similarly high Hexacelsian to Celsian conversions. The mechanical properties of the synthesized materials increased with increasing milling time, sintering temperature and Al2O3 content. Thus, the best mechanical properties were obtained for composition 3 milled for 8 h and sintered at 1500 °C. Compósitos cerámicos con relaciones Ba0.75Sr0.25AlSi2O8 (SBAS)/Al2O3 en masa de: 1) 90/10, 2) 70/30, y 3) 50/50, fueron sintetizados in situ a 900-1500 °C/5 h usando mezclas de cenizas volantes, BaCO3, SrCO3 y Al2O3 . Las mezclas en verde fueron activadas mecánicamente por 0, 4 y 8 h en un molino de atrición. Como resultado, las reacciones en el estado sólido fueron más rápidas y ocurrieron a menores temperaturas. A 1300-1500°C sólo se obtuvo las fases SBAS y Al2O3 , con el SBAS presente en la composición 1 transformado completamente de su forma hexagonal (Hexacelsiana) a la monoclínica (Celsiana), con o sin molienda. El mayor contenido nominal de SBAS en esa composición facilitó dicha conversión, en comparación con las otras dos composiciones estudiadas, las cuales requirieron ser activadas mecánicamente por tiempos que se incrementaron con el incremento en su contenido de Al2O3 , para que pudiesen alcanzar conversiones Hexacelsiana a Celsiana similarmente altas. Las propiedades mecánicas de los materiales sintetizados se incrementaron con el incremento en el tiempo de molienda, en la temperatura de sinterización y en el contenido de Al2O3. De esta manera, las mejores propiedades mecánicas correspondieron a la composición 3 molida por 8 h y sinterizada a 1500 °C

In the present work, the characterization of monolithic materials formulated at different weight concentrations was conducted; employing two of the ceramic materials most used in the refractory industry, zircon and alumina. These monolithic materials were fabricated using colloidal techniques, specifically plaster casting mold, in order to obtain pieces with a higher particle consolidation and density, reducing porosity to lower values than the obtained using traditional shaping process of these materials. The monoliths were obtained employing two ceramic powders with different average particle size and morphology to achieve better packing in the green body. This characterization was carried out, firstly, determining the particle size of the raw materials by laser diffraction and the evaluation of particle morphology by scanning electron microscopy. Aqueous suspensions were formulated by containing both ceramic materials, which were dispersed with Tamol 963, and analyzed by rheometric techniques. Subsequently, bars were manufactured having the following dimensions; 4 mm wide, 3 mm thick and 45 mm in length, according to ASTM C1161-02cc, to be characterized microstructural and mechanically, also was observed the fracture habit after the mechanical test. As a final result, the materials formulated at higher alumina content showed higher density values, reaching 94.95% of the theoretical density, also showed a higher thermal expansion coefficient and high rupture modulus, reaching up to 600 MPa and Young modulus of 230 GPa. From the microstructure characterization it was observed that alumina matrix shows a transgranular fracture across the grains and zircon particles exhibited intergranular fracture among the grain boundaries.