Sinterización de magnesia con diferentes concentraciones de óxidos de nanopartículas Fe₂O₃, AI₂O₃ y SiO₂ respectivamente mediante láser pulsado.

Se realizó un estudio acerca del proceso de sinterización de óxido de magnesio (MgO) con adiciones de 3, 5 ,7 y 10 % en peso de nanoparticulas de óxido de hierro (Fe2O3), óxido de aluminio (Al2O3) y óxido de silicio (SiO2) así como los producidos pellets mediante compactación uniaxial. Los pellets fueron posteriormente irradiados con dos longitudes de onda 532 y 1064 nm correspondientes al láser pulsado Q switch Nd:YAG. Al material obtenido se le realizó una caracterización de morfología y microestructura, con técnicas de Difracción de Rayos X (DRX), Microscopia Electrónica de Barrio (MEB), Espectroscopia de Fotoelectrones Emitidos por Rayos X (XPS). Los parámetros del experimento comprenden dos fluencias de energía 0.8 y 1.7 J/cm2 para la longitud de onda de 532 nm, mientras que para la longitud de onda de 1064 nm se emplearon la fluencia de energía de 2.5 y 3.0 J/cm2. Sólo en el caso de las nanopartículas de óxido de silicio se emplearon las fluencias de energía de 3.8 y 4.7 J/cm2. Por otra parte se analizaron distintas velocidades de traslación 110, 250, 400 y 550 μm/s así como los tiempos de irradiación para cada una de ellas. Se evaluaron los parámetros antes mencionados al momento de hacer incidir la energía del láser sobre los pellets refractarios. Se observó que los pellets mostraron cambios en las distintas intensidades y cantidad de planos de reflexión, cambios de estado químico para cada una de las nanopartículas adicionadas a la matriz, así como nuevas fases correspondientes a distintas espinelas, en el caso de las nanopartículas de óxido de hierro (Fe2O3) se obtuvo la espinela magnesioferrita (MgFe2O4), para las nanopartículas de óxido de aluminio (Al2O3) se obtuvo magnesio-alúmina (MgAl2O4) y finalmente para las nanopartículas de óxido de silicio (SiO2) la presencia de la espinela forsterita (Mg2SiO4). Los cambios morfológicos son apreciables en las micrografías en donde se observa el crecimiento de cuellos entre los granos, disminución de las porosidades y formación de las nuevas fases en los límites de grano. Con esto, se evidencia que es posible lograr la sinterización de materiales cerámicos empleando nuevas tecnologías, como lo es el láser. En este trabajo de tesis, se describirán las principales contribuciones que se aportaron, mediante el desarrollo del trabajo y evaluación de distintos parámetros, a este amplio tema de estudio

Effect of high Al2O3 content on the microstructure and electrical properties of Co- and Ta-doped SnO2 varistors

Varistor behavior shows signifcant diferences when the addition levels of diferent dopants like In2O3, Cr2O3, and Al2O3 are changed, thus stimulating current investigations on the SnO2–Co3O4–Ta2O5 ceramic system. In this contribution, the infuence of high additions of Al2O3 on the microstructure, structure and the electrical properties of SnO2–Co3O4–Ta2O5 varistors ([98.95-X]%SnO2–1% Co3O4–X% Al2O3–0.05% Ta2O5, where X=0, 0.05, 0.1, 1 or 2 mol%) is investigated. Characterization techniques such as thermal analysis, scanning electron microscopy and X-ray difraction with Rietveld refnement were used for specimen analysis. The endothermic peaks in the ceramic system containing Al2O3 additions between 0.05 and 1% are ascribed to the formation of the Co2SnO4 and CoAl2O4 spinel type phases. Doping the ceramic system with 1 and 2 mol% Al2O3 leads to the formation of 1.163 and 3.449%, respectively, of the spinel phase Al2CoO4, which acts as a grain growth inhibitor because grain size decreases in about 16% for both addition levels. The apparent grains homogeneity and narrowest monomodal grain size distribution for the specimens with 2 mol% Al2O3 confrm the inhibitory role. With the lowest level of Al2O3 (0.05 mol%) the nonlinearity coefcient reaches a maximum, after which it decreases and fades at the highest alumina level. A remarkable decrease of about 50% in the leakage current from the reference specimen´s value to that of the one with 0.05 mol% Al2O3 concurrently with an increase in about 40% in the nonlinearity coefcient favors the potential use of alumina in the SnO2–Co3O4–Ta2O5 ceramic system.El comportamiento del varistor muestra diferencias significativas cuando los niveles de adición de diferentes dopantes como In2O3, Cr2O3 y Al2O3 se modifican, lo que estimula las investigaciones actuales sobre el sistema cerámico SnO2 – Co3O4 – Ta2O5. En esta contribución, el Influencia de altas adiciones de Al2O3 en la microestructura, estructura y propiedades eléctricas de SnO2 – Co3O4 – Ta2O5 varistores ([98.95-X]% SnO2–1% Co3O4 – X% Al2O3–0.05% Ta2O5, donde X = 0, 0.05, 0.1, 1 o 2% mol) se investiga. Técnicas de caracterización como análisis térmico, microscopía electrónica de barrido y difracción de rayos X con refinamiento de Rietveld. fueron utilizados para el análisis de muestras. Los picos endotérmicos en el sistema cerámico que contienen adiciones de Al2O3 entre 0.05 y 1% se atribuyen a la formación de las fases de tipo espinela Co2SnO4 y CoAl2O4. Dopando el sistema cerámico con 1 y 2 mol% de Al2O3 conduce a la formación de 1.163 y 3.449%, respectivamente, de la fase de espinela Al2CoO4, que actúa como un inhibidor del crecimiento del grano porque el tamaño del grano disminuye en aproximadamente un 16% para ambos niveles de adición. La aparente homogeneidad de los granos. y la distribución de tamaño de grano monomodal más estrecha para las muestras con 2% en moles de Al2O3 confirman el papel inhibidor. Con el nivel más bajo de Al2O3 (0.05 mol%) el coeficiente de no linealidad alcanza un máximo, después de lo cual disminuye y se desvanece a El nivel más alto de alúmina. Una disminución notable de aproximadamente el 50% en la corriente de fuga del valor de la muestra de referencia al de aquel con 0.05 mol% Al2O3 concurrentemente con un aumento de aproximadamente 40% en los favores coeficientes no lineales El uso potencial de alúmina en el sistema cerámico SnO2 – Co3O4 – Ta2O5

Desarrollo de un refractario de MgO dopado con η-Al2O3

Se estudia el efecto de las nanopartículas de α-Al2O3 (hasta 5 % en peso) sobre las propiedades físicas, mecánicas y térmicas, así como sobre la evolución microestructural de un refractario de magnesia densa. Se utilizan temperaturas de sinterización a 1300°C, 1500°C y 1600°C. Las propiedades físicas de interés fueron la densidad y la porosidad aparentes, que se evaluaron mediante el método de Arquímedes. Las propiedades térmicas se examinaron mediante calorimetría diferencial de barrido. El comportamiento mecánico se estudió mediante pruebas de resistencia a la trituración en frío y micro dureza. Finalmente, la microestructura y las características cualitativas mineralógicas se estudiaron mediante microscopía electrónica de barrido y difracción de rayos X, respectivamente. El aumento de la temperatura de sinterización dio como resultado una densidad mejorada y una porosidad aparente reducida. Sin embargo, a medida que aumentaba el contenido de nanopartículas de α-Al2O3, la densidad y la micro dureza disminuían. Las observaciones microestructurales mostraron que la presencia de nanopartículas de α-Al2O3 en la matriz de magnesia indujo la formación de espinela de magnesio-aluminato (MgAl2O4), que mejoró la resistencia mecánica más significativamente a 1500°C

Effect of Mineral Aggregates and Chemical Admixtures as Internal Curing Agents on the Mechanical Properties and Durability of High-Performance Concrete

Abstract: In the present work, the effect of mineral aggregates (pumice stone and expanded clay aggregates) and chemical admixtures (superplasticizers and shrinkage reducing additives) as an alternative internal curing technique was investigated, to improve the properties of highperformance concrete. In the fresh and hardened state, concretes with partial replacements of Portland cement (CPC30R and OPC40C) by pulverized fly ash in combination with the addition of mineral aggregates and chemical admixtures were studied. The physical, mechanical, and durability properties in terms of slump, density, porosity, compressive strength, and permeability to chloride ions were respectively determined. The microstructural analysis was carried out by scanning electronic microscopy. The results highlight the effect of the addition of expanded clay aggregate on the internal curing of the concrete, which allowed developing the maximum compressive strength at 28 days (61 MPa). Meanwhile, the replacement of fine aggregate by 20% of pumice stone allowed developing the maximum compressive strength (52 MPa) in an OPC-based concrete at 180 days. The effectiveness of internal curing to develop higher strength is attributed to control in the porosity and a high water release at a later age. Finally, the lowest permeability value at 90 days (945 C) was found by the substitutions of fine aggregate by 20% of pumice stone saturated with shrinkage reducing admixture into pores and OPC40C by 15% of pulverized fly ash. It might be due to impeded diffusion of chloride ions into cement paste in the vicinity of pulverized fly ash, where the pozzolanic reaction has occurred. The proposed internal curing technology can be considered a real alternative to achieve the expected performance of a high-performance concrete since a concrete with a compressive strength range from 45 to 67 MPa, density range from 2130 to 2310 kg/m3, and exceptional durability (< 2000 C) was effectively developed

Effect of mineral aggregates and chemical admixtures as internal curing agents on the mechanical properties and durability of highperformance concrete

In the present work, the effect of mineral aggregates (pumice stone and expanded clay aggregates) and chemical admixtures (superplasticizers and shrinkage reducing additives) as an alternative internal curing technique was investigated, to improve the properties of highperformance concrete. In the fresh and hardened state, concretes with partial replacements of Portland cement (CPC30R and OPC40C) by pulverized fly ash in combination with the addition of mineral aggregates and chemical admixtures were studied. The physical, mechanical, and durability properties in terms of slump, density, porosity, compressive strength, and permeability to chloride ions were respectively determined. The microstructural analysis was carried out by scanning electronic microscopy. The results highlight the effect of the addition of expanded clay aggregate on the internal curing of the concrete, which allowed developing the maximum compressive strength at 28 days (61 MPa). Meanwhile, the replacement of fine aggregate by 20% of pumice stone allowed developing the maximum compressive strength (52 MPa) in an OPC-based oncrete at 180 days. The effectiveness of internal curing to develop higher strength is attributed to control in the porosity and a high water release at a later age. Finally, the lowest permeability value at 90 days (945 C) was found by the substitutions of fine aggregate by 20% of pumice stone saturated with shrinkage reducing admixture into pores and OPC40C by 15% of pulverized fly ash. It might be due to impeded diffusion of chloride ions into cement paste in the vicinity of pulverized fly ash, where the pozzolanic reaction has occurred. The proposed internal curing technology can be considered a real alternative to achieve the expected performance of a high-performance concrete since a concrete with a compressive strength range from 45 to 67 MPa, density range from 2130 to 2310 kg/m3, and exceptional durability (< 2000 C) was effectively developed

Desarrollo de materiales refractarios y cerámicos mejorados aplicando nuevas técnicas de sinterización

Resumen del póster presentado al II Congreso Anual Internacional de Estudiantes de Doctorado, celebrado en la Universidad Miguel Hernández (Elche) del 3 al 4 de febrero de 2022.Los principales problemas de los materiales cerámicos y refractarios son sus pobres propiedades mecánicas (i. e., tenacidad y resistencia mecánica) y el deterioro en presencia de materiales fundidos (i. e. corrosión). Por este motivo, han sido numerosas las investigaciones que han tenido por objetivo mejorar las propiedades mecánicas y químicas de los materiales refractarios y cerámicos mediante la adición de segundas fases que sirvan como obstáculos en la propagación de la grieta o de la corrosión. Así pues, en esta investigación se estudian los cambios morfológicos y microestructurales de refractarios de Al2O3 y MgO, empleando nanopartículas metálicas o de otros óxidos empleando nuevas técnicas de sinterización, como el láser. Se ha visto que en el caso del Al2O3, el empleo de nanopartículas metálicas (molibdeno, hierro, etc.) adecuadamente dispersas podría conducir a significativas mejoras de la tenacidad a la fractura de estos refractarios. En el caso de los materiales de base MgO, se ha podido constatar una mejora de las propiedades del refractario frente a la corrosión añadiendo nanopartículas de óxidos (ZrO2) en diferentes cantidades. El estudio de estos sistemas junto con el empleo de nuevas técnicas de sinterización constituirá el cuerpo de la tesis doctoral, lo que conducirá a una mejora de las propiedades en servicio de los materiales refractarios y cerámicos de uso común en la industria.Peer reviewe

Zr-Based Biocomposite Materials as an Alternative for Fluoride Removal, Preparation and Characteristics

The development of biocomposite materials used as adsorbents to remove ions in aqueous media has become an attractive option. The biomasses (base materials) are chemically treated and impregnated with metal cations, becoming competitive for fluoride-capture capacity. In this research, Valence orange (Citrus sinensis) and Red Delicious apple (Malus Domestica) peels were modified by alkaline treatment, carboxylation, and impregnation with zirconium (Zr). These materials were characterized morphologically and structurally to understand the modifications in the treated biomasses and the mechanism of fluoride adsorption. The results show changes in surface area and composition, most notably, an increment in roughness and Zr impregnation of the bioadsorbents. After batch experimentation, the maximum capacity of the materials was determined to be 4.854 and 5.627 mg/g for the orange and apple peel bioadsorbent, respectively, at pH 3.5. The experimental data fitted the Langmuir model, suggesting that chemisorption occurs in monolayers. Finally, the characterization of the bioadsorbents in contact with fluoride allowed the replacement of OH species by fluoride or the formation of hydrogen bonds between them as an adsorption mechanism. Therefore, these bioadsorbents are considered viable and can be studied in a continuous system

Zr-Based Biocomposite Materials as an Alternative for Fluoride Removal, Preparation and Characteristics

The development of biocomposite materials used as adsorbents to remove ions in aqueous media has become an attractive option. The biomasses (base materials) are chemically treated and impregnated with metal cations, becoming competitive for fluoride-capture capacity. In this research, Valence orange (Citrus sinensis) and Red Delicious apple (Malus Domestica) peels were modified by alkaline treatment, carboxylation, and impregnation with zirconium (Zr). These materials were characterized morphologically and structurally to understand the modifications in the treated biomasses and the mechanism of fluoride adsorption. The results show changes in surface area and composition, most notably, an increment in roughness and Zr impregnation of the bioadsorbents. After batch experimentation, the maximum capacity of the materials was determined to be 4.854 and 5.627 mg/g for the orange and apple peel bioadsorbent, respectively, at pH 3.5. The experimental data fitted the Langmuir model, suggesting that chemisorption occurs in monolayers. Finally, the characterization of the bioadsorbents in contact with fluoride allowed the replacement of OH species by fluoride or the formation of hydrogen bonds between them as an adsorption mechanism. Therefore, these bioadsorbents are considered viable and can be studied in a continuous system

Alumina/molybdenum nanocomposites obtained by colloidal synthesis and spark plasma sintering

Alumina/molybdenum nanocomposites were prepared by colloidal synthesis from alumina powder and molybdenum (V) chloride using ethanol as dispersion medium. Modified alumina was calcined at 450 °C in air atmosphere to remove chlorides, and then treated in a tubular furnace at 850 °C under Ar/H2 to reduce the MoO3 formed in the previous stage and obtain Al2O3 with molybdenum nanoparticles on the surface. Three different molybdenum contents were proposed (1, 5 and 10 wt % Mo), and pure alumina was used as reference, that were sintered by spark plasma sintering (SPS) under vacuum atmosphere at 1400 °C for 3 min with an applied pressure of 80 MPa. Composites were characterized by microstructure, hardness, toughness, and three-point bending test. The presence of molybdenum nanoparticles resulted in a fine-grained structure promoted by the presence of molybdenum at grain boundaries and triple points, as well as by the utilization of the SPS equipment. Hardness is at least a 20% greater and fracture toughness 30% larger in the composites than in the monolithic alumina.Daniel Fernández-González acknowledges the grant (Juan de la Cierva-Formación program) FJC2019-041139-I funded by MCIN/AEI/10.13039/501100011033 (Ministerio de Ciencia e Innovación, Agencia Estatal de Investigación). The authors also acknowledge the financial support received from FICYT (IDI/2021/000106). Juan Piñuela Noval acknowledges the Programa “Severo Ochoa” of Grants for Research and Teaching of the Principality of Asturias for the funds received for the elaboration of the Ph. D. Thesis (Ref: BP20 041).Peer reviewe

MgO refractory doped with ZrO2 nanoparticles: Influence of cold isostatic and uniaxial pressing and sintering temperature in the physical and chemical properties

The chemical environment and the internal conditions of the furnaces and ladles are extremely aggressive for the refractories, so metallurgical industries demand refractory linings with greater durability and resistance to avoid unforeseen stoppages and to reduce the changes of the furnace lining. Therefore, the current work aims to evaluate the impact of the additions of ZrO2-nanoparticles (1, 3, and 5 wt. %) in magnesia-based bricks. A comparative study of the physical and chemical properties in bricks obtained using two cold pressing techniques (uniaxial and isostatic pressing) and two sintering temperatures (1550 and 1650 &deg;C) was carried out. The microstructure and crystalline phase characteristics obtained after the heat treatments and the slag corrosion test was studied using scanning electron microscopy/electron dispersive X-ray spectroscopy (SEM/EDX) and X-ray diffraction (XRD). The results reveal that the sample with 5 wt. % of ZrO2 nanoparticles (obtained by cold isostatic pressing and sintering at 1650 &deg;C) has the lowest porosity and greatest resistance to penetration of blast furnace slag