Intragranular carbon nanotubes in alumina-based composites for reinforced ceramics

The traditional methods for the synthesis of reinforced alumina-based matrix composites with carbon nanotubes (CNTs) have presented serious difficulties for obtaining well-dispersed and homogeneously distributed CNTs within the matrix. Besides this, the CNTs are typically found in the grain boundaries of the matrix. These features involve a non-optimal reinforcement role of the CNTs. With the aim of maximizing the efficiency of the reinforcement of the CNT, this work reconsiders a sol-gel-based procedure for ceramic composite fabrication with a two fold objective: to achieve a good dispersion of the CNTs and to promote the intragranular location of the CNTs. The mixing of precursors and CNTs has been developed under the presence of high-power ultrasounds, followed by a rapid in-situ gelation that “froze” the nanotubes inside the gel. The chemical and physical relationships between the ceramic matrix and the embedded reinforcing phase have been researched. First results confirm the success of the synthesis procedure for the preparation of alumina-based composite powders starting from a commercial boehmite sol and multiwalled carbon nanotubes. X-ray diffraction and Raman analyses confirmed the formation of the α-Al 2 O 3 and the persistence of the non-damaged nanotube structure. N 2 physisorption and electron microscopy were used to check the evolution of the nanostructure and to confirm the presence of intragranular carbon nanotube within the polycrystalline powder. Therefore, the alumina-based composite powder prepared by this new procedure is a good candidate for the preparation of reinforced ceramic matrix composites.Junta de Andalucía P12-FQM-107

Rietveld analysis and mechanical properties of in situ formed La-β-Al2O3/Al2O3 composites prepared by sol-gel method

In this work, the crystal evolution of α-Al2O3 composites reinforced with LBA platelets were monitored by XRD Rietveld. In addition, the mechanical properties of totally densified specimens were researched by Vickers and Knoop indentations. These composite materials were prepared by a sol-gel process from alumina seeded boehmite sol and lanthanum nitrate. X-ray diffraction data have been studied by Rietveld refinements and line profile analyses, paying attention to the LBA formation, the evolution of vol%, and crystallite size of the different phases. It has been observed that the appearance of the LBA phase happens at a lower temperature than in samples prepared by a conventional solid state reaction. Indentation tests revealed that the presence of LBA microplatelets in the sol-gel samples leads to a significant increase of their indentation fracture resistance, in comparison to the conventional samples

Reactive SPS for sol–gel alumina samples: Structure, sintering behavior, and mechanical properties

This work presents a fast and direct controlled routine for the fabrication of fully dense alumina based on the reactive spark plasma sintering (reactive-SPS) of boehmite (γ-AlOOH) nano-powders obtained by the sol–gel technique. The evolution of the transition aluminas during sintering has been studied. Some boehmite powders were seeded with α-Al2O3 particles prior to the gelation. Boehmite seeded powders exhibited a direct transition to α-Al2O3 at 1070 °C, enhancing the transformation kinetics and lowering the required temperature by more than 100 °C. For comparison, other samples were prepared by previously annealing the seeded and unseeded boehmite powders. Thus, α-Al2O3 powders were obtained and were sintered by standard-SPS. A detailed structural and mechanical characterization is presented, comparing the hardness and indentation fracture resistance for different grain sizes and porosities. Both the reactive-SPSed samples and the standard-SPSed samples showed a high hardness (18–20 GPa), whereas the reactive-SPSed samples exhibited a lower indentation fracture resistance due to a large grain size (∼10 μm). Improvements of this procedure for obtaining smaller grain size are discussed. In summary, the presented technique brings a revolutionary fast method for the fabrication of fully dense alumina, as this process reduces the time and temperature required for alumina densification.Ministerio de Ciencia e Innovación PGC2018-094952-B-I00Junta de Andalucía P12- FQM-1079, FQM393Universidad de Sevilla PGC2018-094952-B-I00Junta de Extremadura GR1808

Mechanical characterization of sol-gel alumina-based ceramics with intragranular reinforcement of multiwalled carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs) have been widely considered for mechanical reinforcement of ceramic matrix composites. Nevertheless, the efficiency of this reinforcement strategy is under debate due to fabrication issues, such as a good homogenization or the location of the MWCNTs inside the matrix composite. Regarding this, the intragranular location of the MWCNTs has been deemed a crucial feature for optimizing the reinforcement compared to the typical intergranular placement achieved by conventional procedures. Recently, the sol-gel method has been reconsidered, as it promotes the intragranular placement of the MWCNTs. This work presents the mechanical characterization of these composites synthesized by the sol-gel method, where crack-bridging has been revealed as toughening mechanism. Finally, the conventional use of the bibliographical Young's modulus of pure alumina for the estimation of the fracture toughness is discussed, obtaining significant improvements of the fracture toughness when indentation measurements are treated by considering elastic moduli obtained by nanoindentation.Ministerio de Ciencia e Innovación PGC2018-094952-B-100Junta de Andalucía P12-FQM-107

Selective and rapid detection of acetone using aluminum-doped zno-based sensors

We report the preparation and characterization of pure and doped ZnO nanoparticles with 1%, 3%, and 5% aluminum (AZO) using a sol-gel method followed by annealing at 400 °C for 2 h. The structural and morphological properties of the AZO nanoparticles were analyzed using X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM) techniques, and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive Spectrometry (EDS). Optical and specific area properties were investigated by photoluminescence (PL) and N2 physisorption measurements. The results showed that pure and doped AZO nanoparticles crystallize under a hexagonal wurtzite structure and exhibit spherical shapes with nanometric dimensions. TEM and SEM images revealed that the pure and Al-doped ZnO were round nanoparticles with a size smaller that 100 nm. FTIR measurements were conducted to investigate the presence of Al-O stretching vibrations, which served as an indication of aluminum incorporation into the ZnO lattice. The results confirmed the successful integration of aluminum into the ZnO structure. Additionally, XPS measurements were performed to examine the elemental composition of the AZO samples. The presence of Zn 2p peaks in all AZO samples, along with the presence of Al 2p peaks in the Al-doped ZnO structures, provided further evidence for the successful incorporation of Al ions into the ZnO lattice. The PL spectra revealed the presence of various defects (oxygen vacancies, interstitials) in the structure of pure and doped ZnO. Moreover, we fabricated gas sensors by spray-coating the AZO nanoparticles on alumina substrates equipped with interdigitated gold electrodes. The sensors demonstrated linear responses to gas concentration in the range of 5 to 50 ppm, with high sensitivity and good reproducibility, particularly for A1ZO (1% Al-doped ZnO), which exhibited the highest response (~12) at 300 °C under 10 ppm of acetone. Furthermore, A1ZO demonstrated excellent selectivity to acetone compared to other volatile organic compounds (VOCs) gases. Our findings highlight the potential of aluminum-doped ZnO nanoparticles as a promising material for enhancing the sensing properties of acetone gas sensors. Graphical Abstract: [Figure not available: see fulltext.]This work is financially supported by the Tunisian Ministry of Higher Education and Scientific Research (PRF 2019-D4P2), the European Regional Development Fund (ERDF), and the Walloon Region of Belgium through the Interreg V France-Wallonie-Vlaanderen program, under PATHACOV project, and the Micro + project co-funded by the European Regional Development Fund (ERDF) and Wallonia, Belgium (No. 675781-642409). In addition, this work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding Contract UIDB/04650/2020. J.P.B.S. also expresses gratitude to FCT for the contract under the Institutional Call to Scientific Employment Stimulus – 2021 Call (CEECINST/00018/2021).Peer reviewe

Síntesis, caracterización estructural y propiedades mecánicas de cerámicas avanzadas preparadas mediante sol-gel

Esta tesis doctoral comprende una serie de artículos enfocados en la fabricación de materiales cerámicos (principalmente alúmina, pero también circona) reforzados con nanofases de carbono (grafeno y nanotubos de carbono). La base de la tesis consiste en la preparación de cerámicas avanzadas mediante la técnica sol-gel, y su caracterización estructural y mecánica. En concreto, la investigación se centra en la preparación de cerámicas de alúmina reforzadas con diferentes fases carbonosas, como nanotubos de carbono o grafeno. El objetivo era determinar las condiciones de preparación de los compuestos cerámicos que conduzcan a la mejora de ciertas propiedades mecánicas, como la reducción de la inherente fragilidad de las cerámicas. El trabajo experimental incluye la síntesis de polvos cerámicos mediante el método sol-gel y otras técnicas convencionales, así como formas tradicionales y avanzadas de sinterización. Se realizaron caracterizaciones estructurales de las muestras en diferentes etapas y se midieron sus propiedades mecánicas. Para todo esto, se empleó una amplia variedad de técnicas, que incluyen rutas óptimas de procesamiento sol-gel para maximizar la dispersión de nanofases de carbono, densificación mediante Sinterización de Chispa de Plasma (SPS), microscopía electrónica de barrido y transmisión (SEM y TEM), difracción de rayos X, espectroscopía Raman, indentación Vickers y nanoindentación. Además, también se consideró un enfoque de simulación estructural para explorar y discutir, desde un punto de vista geométrico, la distribución espacial de la fase de refuerzo dentro de la matriz y la viabilidad de lograr distribuciones perfectas de alótropos de carbono nanoestructurados incrustados en una matriz cerámica en función del contenido de carbono. Los resultados experimentales y teóricos sugieren que una ruta basada en sol-gel de boehmita seguida de una sinterización reactiva-SPS es un método efectivo, económico y rápido para producir piezas de α-alúmina de alta densidad. Además, también se observó que las cantidades de nanofases de carbono típicamente utilizadas por la comunidad científica para reforzar matrices cerámicas no pueden generar dispersiones homogéneas de las fases de refuerzo, lo que hace imposible lograr aumentos significativos en las propiedades mecánicas de los compuestos cerámicos completamente densificados. Además, se encontró que la microestructura de las cerámicas era crucial para sus propiedades mecánicas. En general, esta investigación ha contribuido a la comprensión del candidato sobre la preparación, caracterización microestructural y mecánica, y simulación estructural de compuestos de matriz cerámica avanzada.This doctoral thesis comprises a series of articles focused on the fabrication of ceramic materials (mainly alumina, but also zirconia) reinforced with carbon nanophases (graphene and carbon nanotubes). The backbone of the thesis consists of the preparation of advanced ceramics by the sol-gel technique and their structural and mechanical characterization. Specifically, the research focuses on the preparation of alumina ceramics reinforced with different embedded phases as carbon nanotubes or graphene. The objective was to determine the preparation conditions of ceramic composites that lead to the improvement of certain mechanical properties, such as the reduction of inherent ceramic fragility. The experimental work included the synthesis of ceramic powders via sol-gel and other conventional techniques, traditional and advanced forms of sintering, structural characterization of the samples at different stages, and the measurement of mechanical properties. For all this, a wide variety of techniques have been employed, including optimal sol-gel processing routes to maximize the dispersion of carbon nanophases, densification via Spark Plasma Sintering (SPS), scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction, Raman spectroscopy, Vickers indentation and nanoindentation. Aditionally, an structural simulation approach was also considered in order to explore and discuss, from a geometrically point of view, the spatial distribution of the reinforcing phase within the matrix, and the viability of achieving perfect distributions of embedded nanostructured carbon allotropes in a ceramic matrix as a function of the carbon iv content. The experimental and theoretical results suggest that a boehmite-based sol-gel route followed by a reactive-SPS sintering is an effective, cheap and fast method for producing α-Al2O3 with high density. Moreover, it was also observed that the amounts of carbon nanophases typically used by the research community to reinforce the ceramic matrix cannot lead to homogeneous dispersions of the reinforcing phases, making impossible to achieve significant increases in the mechanical properties of fully densified ceramic compounds. Furthermore, it was found that the microstructure of the ceramics was crucial to their mechanical properties. Overall, this research has contributed to the doctoral candidate’s understanding of the preparation, microstructural and mechanical characterization, and structural simulation of advanced ceramic matrix composites

The dispersion of carbon nanotubes in composite materials studied by computer simulation of Small Angle Scattering

Although numerous efforts have been made to reinforce ceramic materials by adding a nanostructured phase like carbon nanotubes (CNT), the appearance of aggregates during the manufacturing processes continues to be a problem. Given the size of the CNT (nm–μm), techniques such as Small Angle Scattering (SAS) can be a useful tool to study the formation of the aggregates and to quantify the degree of homogenization of the nanophase in the matrix. In this work, systems with different concentrations of CNT have been simulated in different states of aggregation, starting from a perfectly homogeneous dispersion of individualized CNT, and progressively aggregated. A Guinier regime appears in the scattering signal in the range of low values of the modulus of the scattering vector, q, as the aggregation occurs. Two parameters from the intensity curve are proposed to quantify the quality of the dispersion of the nanophase in the ceramic matrix.Peer reviewe

Fabrication of Porous Alumina Structures by SPS and Carbon Sacrificial Template for Bone Regeneration

In this work, a procedure for fabricating porous alumina with the use of a carbon sacrificial template has been tested in order to optimize the fabrication of porous structures mimicking the porosity and mechanical properties of the human cortical bone. Two different sources of sacrificial carbon were used and compared, and different sintering and calcination routes were considered. The porosity of the alumina structures studied by Hg porosimetry revealed that the amount of porosity and the size and shape of the pores are still below the required values, although some acicular pores were clearly observed by SEM. Moreover, measured mechanical properties (Young’s modulus) remained below that of the bone, suggesting the need for further consolidation treatments. In summary, these encouraging results drive the optimization of future fabrication routes.This research was funded by Project PGC2018-094952-B-I00 (INTRACER); by FEDER/Ministerio de Ciencia e Innovación—Agencia Estatal de Investigación is acknowledged, project P20_01121 (FRAC); and by Consejería de Transformación Económica, Industria, Conocimiento y Universidades (Junta de Andalucía). Special action I.9 from the VI-PPITUS (Universidad de Sevilla). M.G.-S. acknowledges European Social Fund from the Empleo Juvenil European Plan.Peer reviewe

Structural and mechanical optimization of porous alumina structures fabricated by carbon sacrificial template

The Taguchi method is used to optimize the manufacture of porous alumina made through reactive spark plasma sintering and carbon sacrificial template. The goal is to design a new versatile procedure that allows the fabrication of porous alumina with tailored physical properties. The structural and mechanical properties taken as target parameters were the subtle combination of porosity and Young’s modulus of the human cortical bone: typical pore size ¿100μm, and Young’s modulus in the range of 3–30 GPa. The input factors of the Taguchi method are wt.% of carbon, sintering time, calcination heating rate, and final heat treatment. Hg porosimetry, electron microscopy, uniaxial compression and computer aided tomography were used for the characterization of the porosity, pore size distribution, pore interconnectivity, and Young’s modulus. Finally, according to the conclusions of the Taguchi analysis, the parameters of the process were changed for the fabrication of the new samples with optimized properties. Highly porous structures with 90% interconnectivity, Young modulus of 5.5 ± 1.1 GPa, and compression strength of 49 ± 20 MPa, were obtained, successfully emulating the targeted properties

The Possible Detriment of Oxygen in Creep of Alumina and Zirconia Ceramic Composites Reinforced with Graphene

This paper aims to give an answer to the following question: is the oxidation of graphene a critical issue for high-temperature plasticity in graphene-reinforced ceramics? To give a convincing reply, we will focus on two very different graphene-based ceramic composites: reduced graphene oxide (rGO)-reinforced alumina ( -Al2O3) and reduced graphene oxide (rGO)-reinforced yttria tetragonal zirconia (t-ZrO2). The processing of the powders has been made using a colloidal route, and after that, a spark plasma sintering process was performed in order to densify the samples. Creep tests were performed at temperatures between 1200–1250 C in an argon atmosphere. The microstructure obtained by SEM of the sintered and tested specimens was characterized quantitatively to elucidate the deformation mechanism. Raman spectroscopy was carried out to check the integrity of the graphene. The average grain size was in the order of 1 m and the shape factor was 0.7 for all the studied materials. The integrity of the graphene was checked before and after the creep experiments. The careful analysis of the creep tests shows that graphene oxide or its reduced version are not efficient phases for creep resistance improvement in general, contrary to what is reported elsewhere. However, the results permit the suggestion of a creep improvement in nanocomposites at a very high temperature regime due to an enhanced reactivity of oxygen between carbon and alumina interfaces. In the case of zirconia, the results give us the conclusion that the oxidation of graphene is a highly detrimental issue regarding the improvement of high-temperature plasticityPeer reviewe