Phase assembly and electrical conductivity of spark plasma sintered CeO2–ZrO2 ceramics

Ce x Zr1−x O2 (x = 0.10, 0.16 and 0.33) nanocrystalline powders were obtained by a two-step synthesis technique and sintered by spark plasma sintering (SPS). As consequence of the reduction of Ce4+ to Ce3+ species by carbon in the graphite environment in SPS, phase assemblies including tetragonal, monoclinic and pyrochlore phases were generated in the ceramics during the sintering process. The electrical conductivity was highly dependent on phase assembly and atmosphere (N2, H2 and O2). A significant decrease in the activation energy was noticed in the ceramics with high pyrochlore content when measuring the conductivity in H2 atmosphere, consequence of the strong reduction promoted in these ceramics during the measurement. Equal conduction behavior with similar activation energy was observed in all the ceramics when measuring in O2 atmospherePeer reviewe

Determinación de la composición de fases en circona mediante un procedimiento polimórfico simple

In this work, we propose a polymorphic method for the quantitative analysis in zirconia. This procedure uses a square system of equations relating the intensities of a peaks groups set in the diffraction pattern with the weight fractions of the different phases present in the sample. In this system it is used as unknowns, besides the relative abundance of the phases, a group of parameters that incorporates the contribution from the tails of the groups adjacent to the intensity of a selected group. The described procedure has been applied to a simulated diffractogram corresponding to a composition of zirconia of 50% of cubic phase, 25% of tetragonal phase and 25% of monoclinic phase, and also to a sample of zirconia processed by the sol-gel method.<br><br>En este trabajo se propone un método polimórfico para el análisis cuantitativo de fases en la circona. Este procedimiento emplea un sistema de ecuaciones compatible y determinado que relaciona las intensidades de un conjunto de grupos de picos del espectro de difracción con las fracciones en peso de las diferentes fases presentes en la muestra. En dicho sistema, además de las abundancias relativas de las fases, se utilizan como incógnitas un conjunto de parámetros que dan cuenta de la aportación de las colas de los grupos de picos adyacentes al grupo de picos considerado. Este procedimiento se ha aplicado a un difractograma simulado correspondiente a una circona con 50% de fase cúbica, 25% de fase tetragonal y 25% de fase monoclínica y también a una muestra en polvo de circona fabricada mediante técnicas sol-gel

Aplicación del método de Rietveld al análisis cuantitativo SiC sinterizado en fase líquida

Accurate X-ray quantitative analysis in SiC-based ceramics is a difficult task owing to the strong overlap among the Bragg reflections of the different polytypes. In relation to this point, the Rietveld method can be used as a powerful tool in order to solve this problem. In this study we have applied this procedure to determine the weight fractions of the phases in a liquid– phase-sintered SiC sample. It is shown that the consideration of preferred orientation effects is also indispensable to obtain the accurate proportion of the phases.<br><br>La determinación de las fracciones en peso de las fases mediante difracción de rayos X es enormemente complicada en cerámicos a base de SiC debido al intenso solapamiento entre las reflexiones Bragg de los diferentes politipos. No obstante, el método de Rietveld constituye una herramienta poderosa para resolver este problema. En este trabajo se utiliza el método de Rietveld al objeto de determinar las fracciones en peso de las diferentes fases para una muestra de SiC sinterizado en fase líquida (LPS SiC). Nuestros resultados ponen también de manifiesto la necesidad de incorporar correcciones debidas a orientación preferencial para efectuar un análisis cuantitativo preciso

Sintering kinetics, defect chemistry and room-temperature mechanical properties of titanium nitride prepared by spark plasma sintering

Fully dense titanium nitride polycrystals have been prepared by spark plasma sintering. The kinetics of the sintering process and the optimized conditions for SPS processing have been put forward. Microstructural analyses of the resulting samples have unambiguously shown the coexistence of titanium as Ti, Ti and Ti, thus driving the presence of cation vacancies. This fact is a new ingredient which is shown to influence the mechanical properties of this strategic ceramic.The authors want to acknowledge the financial support provided by the research project MAT2015-71411-R awarded by the Spanish authorities. JMM wants to acknowledge the support provided by the Regional Government ‘Junta de Andalucía' through his contract in frame of the ‘Plan de Empleo Joven’ program. BMM expresses her recognition to the Spanish Ministry of Science for her postdoctoral contract through the ‘Juan de la Cierva-incorporación' program at the University of Zaragoza, Spain

Additive-free superhard B4C with ultrafine-grained dense microstructures

A unique combination of high-energy ball-milling, annealing, and spark-plasma sintering has been used to process superhard B4C ceramics with ultrafine-grained, dense microstructures from commercially available powders, without sintering additives. It was found that the ultrafine powder prepared by high-energy ball-milling is hardly at all sinterable, but that B2O3 removal by gentle annealing in Ar provides the desired sinterability. A parametric study was also conducted to elucidate the role of the temperature (1600-1800°C), time (1-9min), and heating ramp (100 or 200°C/min) in the densification and grain growth, and thus to identify optimal spark-plasma sintering conditions (i.e., 1700°C for 3min with 100°C/min) to densify completely (>98.5%) the B4C ceramics with retention of ultrafine grains (~370nm). Super-high hardness of ~38GPa without relevant loss of toughness (~3MPam1/2) was thus achieved, attributable to the smaller grain size and to the transgranular fracture mode of the B4C ceramics. © 2013 Elsevier Ltd.Peer Reviewe

Spinel ferrite catalysts for CO2 reduction via reverse water gas shift reaction

The production of CO via Reverse Water Gas Shift (RWGS) reaction is a suitable route for CO2 valorization. In this study a series of modified spinels AB2O4 (A site = Ni, Zn and Cu and B site=Fe) are investigated as RWGS catalysts and their structure-to-function relationships derived from the changes on the A-site cation are rationalized. For all ferrite systems, the RWGS reaction the process main activity and selectivity is governed by the B-site cation, but the variations on the A-site metals determines catalysts' structural features and stability in the reaction. Among the catalyst series, superior RWGS performance displayed the ferrites modified with Cu and Ni associated to the greater oxygen vacancy population for those spinels enabled by the partial allocation on Fe3+ cations into the tetrahedral sites.Financial support for this work was obtained from Junta de Andalucía via grant US-1263288 and Spanish Ministerio de Ciencia e Innovación (MICIN) (Grant number RTI2018-096294-B-C33) all co-financed by FEDER funds from the European Union. Dr. González-Castaño acknowledges the Spanish Ministry of Universities and the European Union (Next Generation EU) for the excellence Maria Zambrano fellowship.Peer reviewe