Procesado y caracterización microestructural, mecánica y eléctrica de compuestos cerámica-grafeno

El desarrollo de nuevos materiales avanzados es esencial para superar los desafíos científicos y tecnológicos a los que se enfrenta la sociedad actual. Con el auge del grafeno en la última década, surgen nuevas oportunidades para la Ciencia e Ingeniería de los Materiales de obtener materiales compuestos con grafeno con propiedades mejoradas. Destaca el uso de las nanoestructuras basadas en grafeno como segunda fase para fabricar materiales compuestos de matriz cerámica multifuncionales y con altas prestaciones mecánicas. Esta Tesis recoge el trabajo de investigación desarrollado con el objetivo de ampliar el conocimiento existente hasta la fecha de compuestos cerámicos de circona tetragonal estabilizada con 3 %mol de Y2O3, una cerámica tenaz de alto interés tecnológico, con dos tipos de nanoestructuras basadas en grafeno: nanoplaquetas de grafeno y óxido de grafeno reducido. Para ello, se ha llevado a cabo un estudio sistemático de los compuestos, desde su fabricación -comparando diferentes técnicas de procesado de polvos y sinterización- hasta la caracterización de sus propiedades microestructurales, mecánicas y eléctricas. Se han sinterizado en horno convencional compuestos con diferentes contenidos de nanoplaquetas y se han obtenido materiales con una distribución isótropa de éstas en la matriz cerámica y conductividades eléctricas comparables a las de compuestos similares sinterizados por técnicas más sofisticadas como la sinterización por descarga eléctrica pulsada, más comúnmente conocida como Spark Plasma Sintering (SPS). Se ha optimizado la técnica de homogeneización de polvos mediante la comparación de cuatro rutinas distintas de procesado de polvos para la fabricación de compuestos con 10 %vol. de nanoplaquetas sinterizados por SPS. El compuesto con el mejor comportamiento eléctrico se ha obtenido mediante la aplicación de molienda de alta energía con molino planetario de bolas en seco. Esta técnica ha permitido exfoliar y reducir el tamaño lateral de las nanoplaquetas facilitando su distribución en la matriz cerámica. Los compuestos con óxido de grafeno se han preparado a partir de dos técnicas de procesado de polvos -una coloidal y otra que combina el uso de ultrasonidos con molienda de alta energía en medio húmedo-. La consolidación de los polvos se ha llevado a cabo por SPS, lo que ha permitido obtener compuestos de sinterización a alta temperatura. El grado de reducción se ha evaluado cuantitativamente mediante espectroscopía Raman. Los mejores resultados en términos de conductividad eléctrica se han conseguido al aplicar la ruta con molienda en húmedo, gracias a la mejor distribución del óxido de grafeno reducido en la matriz y al mayor grado de reducción del óxido de grafeno alcanzado en estos compuestos. Se ha evaluado el comportamiento frente a la fractura de los compuestos con óxido de grafeno reducido y nanoplaquetas exfoliadas. En concreto, se ha analizado la resistencia frente a la propagación lenta de fisuras (curvas R) en estos materiales a partir de ensayos de flexión en tres puntos, utilizando el método indirecto de la complianza. Este método ha sido validado para el compuesto con la mejor respuesta mecánica (el compuesto con 2,5 %vol. de óxido de grafeno reducido) a partir de la comparación directa de las curvas R obtenidas tanto por el método indirecto de la complianza como por la medida de la longitud de la fisura real (método óptico). Mientras que las nanoplaquetas exfoliadas no aportan un refuerzo significativo a la matriz de circona como consecuencia de sus pequeñas dimensiones, la incorporación de un 2,5 %vol. de óxido de grafeno reducido al compuesto promueve un comportamiento frente a la fractura mejor que el de la cerámica monolítica cuando el plano principal de las láminas de la nanoestructura se encuentra orientado perpendicularmente al frente de grieta

Unravelling the optimization of few-layer graphene crystallinity and electrical conductivity in ceramic composites by Raman spectroscopy

Zirconia composites with few-layer graphene (FLG) were prepared by two powder processing routines -ultrasonic agitation or planetary ball milling- and spark plasma sintered at 1250 and 1300 °C. An in-depth study of the crystallinity of FLG, in terms of presence and nature of defects, was performed by Raman spectroscopy, revealing enhanced FLG crystallinity after sintering. This enhancement was more noticeable in the composites sintered at the highest temperature, with lower amount of structural defects and amorphous carbon. However, remaining amorphous carbon was detected in the composites prepared by planetary ball milling even after sintering at the highest temperature, resulting in lower electrical conductivities. Optimum results in terms of electrical conductivity were achieved for the composites prepared by ultrasonic agitation and sintered at 1300 °C, with electrical percolation limit below 2.5 vol% FLG and high electrical conductivity (678 S/m for 5 vol% FLG), as result of the enhanced FLG crystallinity after sintering.Ministerio de Ciencia, Innovación y Universidades PGC 2018- 101377-B-100Ministerio de Asuntos Económicos y Transformación Digital BES-2016- 078711Universidad de Sevilla USE-18740-

Graphene nanoplatelets for electrically conductive 3YTZP composites densified by pressureless sintering

3 mol% yttria tetragonal zirconia polycrystalline (3YTZP) ceramic composites with 2.5, 5 and 10 vol% graphene nanoplatelets (GNP) were pressureless sintered in argon atmosphere between 1350 and 1450 °C. The effects of the GNP content and the sintering temperature on the densification, microstructure and electrical properties of the composites were investigated. An isotropic distribution of GNP surrounding ceramic regions was exhibited regardless the GNP content and sintering temperature used. Electrical conductivity values comparable to the ones of fully dense composites prepared by more complex techniques were obtained, even though full densification was not achieved. While the composite with 5 vol% GNP exhibited electrical anisotropy with a semiconductor-type behaviour, the composite with 10 vol% GNP showed an electrically isotropic metallic-type behaviour.Ministerio de Economía y Competitividad MAT2015-67889-

Spark Plasma Sintered Zirconia Ceramic Composites with Graphene-Based Nanostructures

The addition of graphene-based nanostructures (GBNs) can improve the inherent fragility of ceramics and provide them with improved electrical and thermal conductivities. However, both the starting material (ceramic matrix and GBNs) and the processing/sintering approach are crucial for the final composite microstructure and properties. This work focuses on the influence of the content and dimensions of the GBN filler (10 and 20 vol%; 3 and ~150 layers), the powder-processing conditions (dry versus wet), and the homogenization method (ultrasound sonication versus high-energy planetary ball milling) on GBN/tetragonal zirconia (3YTZP) composites. The microstructure and electrical properties of the spark plasma sintered (SPS) composites were quantified and analyzed. The highest microstructural homogeneity with an isotropic microstructure was achieved by composites prepared with thicker GBNs milled in dry conditions. A high content (20 vol%) of few-layered graphene as a filler maximizes the electrical conductivity of the composites, although it hinders their densification.Ministerio de Economía y Competitividad MAT2015-67889-P

Microstructure, interfaces and properties of 3YTZP ceramic composites with 10 and 20 vol% different graphene-based nanostructures as fillers

The graphene family comprises not only single layer graphene but also graphene-based nanomaterials (GBN), with remarkably different number of layers, lateral dimension and price. In this work, two of these GBN, namely graphene nanoplatelets (GNP) with n∼15–30 layers and few-layer graphene (FLG) with n < 3 layers have been evaluated as fillers in 3 mol% yttria stabilized tetragonal zirconia (3YTZP) ceramic composites. Composites with 10 and 20 vol% GNP or FLG have been fabricated by wet powder processing and spark plasma sintering (SPS) and the influence of the content and number of layers of the graphene-based filler has been assessed. For both graphene-based fillers, an intermediate zirconia oxycarbide has been detected in the grain boundaries. The lower stacking degree and much more homogeneous distribution of the FLG, revealed by transmission electron microscopy (TEM), can improve load transfer between the GBNs and the ceramic matrix. However, high FLG contents lower densification of the composites, due partly to the larger FLG interplanar spacing also estimated by TEM. The hardness (both Vickers and nanoindentation) and the elastic modulus decrease with increased GBN content and with improved graphene dispersion. The FLG greatly inhibit the crack propagation that occur perpendicular to their preferential orientation plane. The composites with thinner FLG have higher electrical conductivity than those with GNP. The highest electrical conductivity is achieved by composites with 20 vol% FLG in the direction perpendicular to the compression axis during sintering, σ⊥ = 3400 ± 500 Sm-1.Ministerio de Economía y Competitividad MAT2015-67889-

Flexure Strength and Fracture Propagation in Zirconia Ceramic Composites with Exfoliated Graphene Nanoplatelets

In this work, the flexure strength and fracture propagation mechanisms in yttria tetragonal zirconia (3YTZP) dense composites with 1 and 5 vol.% exfoliated graphene nanoplatelets (e-GNP) were assessed. The composite powders were processed by dry planetary ball milling to exfoliate the as-received GNP, and then densified by spark plasma sintering (SPS). The hardness and Young’s modulus were measured by Vickers indentation and the impulse-echo technique, respectively. Flexural strength and modulus were estimated by four-point bending tests. Finally, cracks originated by Vickers indentations were analyzed by scanning electron microscopy (SEM). The Raman spectra and SEM observations showed a reduction in the number of graphene layers and most remarkably in the lateral size of the e-GNP, achieving a very homogeneous distribution in the ceramic matrix. The hardness, elastic modulus, and flexural strength of the 3YTZP matrix did not vary significantly with the addition of 1 vol.% e-GNP, but they decreased when the content increased to 5 vol.%. The addition of e-GNP to 3YTZP increased its reliability under bending, and the small lateral size of the e-GNP produced isotropic fracture propagation. However, the energy dissipation mechanisms conventionally attributed to the larger GNP such as fracture deflection or blocking were limited.Ministerio de Economía y Competitividad MAT 2015-67889-PMinisterio de Ciencia, Innovación y Universidades PGC 2018–101377–B-10

Microstructure, interfaces and properties of 3YTZP ceramic composites with 10 and 20 vol% different graphene-based nanostructures as fillers

The graphene family comprises not only single layer graphene but also graphene-based nanomaterials (GBN), with remarkably different number of layers, lateral dimension and price. In this work, two of these GBN, namely graphene nanoplatelets (GNP) with n~15–30 layers and few-layer graphene (FLG) with n < 3 layers have been evaluated as fillers in 3¿mol% yttria stabilized tetragonal zirconia (3YTZP) ceramic composites. Composites with 10 and 20¿vol% GNP or FLG have been fabricated by wet powder processing and spark plasma sintering (SPS) and the influence of the content and number of layers of the graphene-based filler has been assessed. For both graphene-based fillers, an intermediate zirconia oxycarbide has been detected in the grain boundaries. The lower stacking degree and much more homogeneous distribution of the FLG, revealed by transmission electron microscopy (TEM), can improve load transfer between the GBNs and the ceramic matrix. However, high FLG contents lower densification of the composites, due partly to the larger FLG interplanar spacing also estimated by TEM. The hardness (both Vickers and nanoindentation) and the elastic modulus decrease with increased GBN content and with improved graphene dispersion. The FLG greatly inhibit the crack propagation that occur perpendicular to their preferential orientation plane. The composites with thinner FLG have higher electrical conductivity than those with GNP. The highest electrical conductivity is achieved by composites with 20¿vol% FLG in the direction perpendicular to the compression axis during sintering, s¿¿=¿3400¿±¿500¿Sm-1.Peer ReviewedPostprint (published version

Synthesis and Characterization of a Nearly Single Bulk Ti2AlN MAX Phase Obtained from Ti/AlN Powder Mixture through Spark Plasma Sintering

Article number 2217MAX phases are an advanced class of ceramics based on ternary carbides or nitrides that combine some of the ceramic and metallic properties, which make them potential candidate materials for many engineering applications under severe conditions. The present work reports the successful synthesis of nearly single bulk Ti2AlN MAX phase (>98% purity) through solid-state reaction and from a Ti and AlN powder mixture in a molar ratio of 2:1 as starting materials. The mixture of Ti and AlN powders was subjected to reactive spark plasma sintering (SPS) under 30 MPa at 1200 ◦C and 1300 ◦C for 10 min in a vacuum atmosphere. It was found that the massive formation of Al2O3 particles at the grain boundaries during sintering inhibits the development of the Ti2AlN MAX phase in the outer zone of the samples. The effect of sintering temperature on the microstructure and mechanical properties of the Ti2AlN MAX phase was investigated and discussed.Agencia Nacional de Investigación y Desarrollo de la Gobierno de Chile (ANID)1120092

Graphene nanoplatelets for electrically conductive 3YTZP composites densified by pressureless sintering

3 mol% yttria tetragonal zirconia polycrystalline (3YTZP) ceramic composites with 2.5, 5 and 10 vol% graphene nanoplatelets (GNP) were pressureless sintered in argon atmosphere between 1350 and 1450 °C. The effects of the GNP content and the sintering temperature on the densification, microstructure and electrical properties of the composites were investigated. An isotropic distribution of GNP surrounding ceramic regions was exhibited regardless the GNP content and sintering temperature used. Electrical conductivity values comparable to the ones of fully dense composites prepared by more complex techniques were obtained, even though full densification was not achieved. While the composite with 5 vol% GNP exhibited electrical anisotropy with a semiconductor-type behaviour, the composite with 10 vol% GNP showed an electrically isotropic metallic-type behaviour

Enhancing the electrical conductivity of in-situ reduced graphene oxide-zirconia composites through the control of the processing routine

Graphene oxide (GO) was mixed with 3 mol% yttria tetragonal zirconia polycrystal (3YTZP) using two powder processing routines: a colloidal method in an aqueous solution and a combination of ultrasonication with high-energy planetary ball milling in wet conditions. Highly densified 3YTZP composites with reduced GO (rGO) were consolidated by Spark Plasma Sintering. The in-situ reduction of GO was successfully achieved during the high-temperature sintering process and a detailed study of the restoration of the graphene structure in the sintered composites has been made by Raman spectroscopy. Although no differences between the composites prepared by the two processing methods were found in the distribution of the rGO throughout the 3YTZP matrix for high rGO contents (i.e. the composites with 5 and 10 vol% rGO), a better distribution of the graphene phase was found in the composites with 1 and 2.5 vol% rGO prepared by planetary ball milling. This result, together with a better reduction of the GO in these composites, led to the obtaining of rGO/3YTZP composites with a better behavior in terms of electrical conductivity: an electrical percolation threshold below 2.5 vol% rGO and a high electrical conductivity value (~610 S/m for 10 vol% rGO)