Search

Field-assisted sintering techniques are at the forefront of advanced energy-saving procedures to create fully dense ceramic materials. At present, there is an extraordinary interest in understanding the sintering process and effectively controlling the final specimen’s microstructure. This latter aspect is critical, but unfortunately, the short sintering times and intense electric fields under non-equilibrium conditions prevent a rigorous prediction in most cases. This paper is a theoretical contribution to gain insight into an outstanding effect of applying intense electric fields: the formation of non-spherical grain structures. A phase field simulation approach has been carried out to reproduce and determine the origin of this effect. The final simulated microstructures are proved to be textured. These results are explained in the framework of a recent model and compared with experimental evidence reported in the literature.

Accurate determination of the structural disorder (i.e., cation misplacement, anionic and cationic vacancies, and reduced cations) of spinels is crucial to understanding the properties of this technologically attractive sub-family of ceramics, but it is very difficult to do in practice. Here, constrained–restrained Rietveld refinements of the experimentally measured X-ray diffraction patterns are proposed to tackle the challenging quantification of the structural disorder in spinels. First, it is demonstrated that the constraints–restraints to be imposed during the Rietveld refinements can be formulated by mathematical modelling through the linear inverse problem whose framework is first presented generically and then particularised to the different types of possible normal, inverse, and mixed spinels, namely, stoichiometric perfect spinels, stoichiometric and non-stoichiometric imperfect spinels, and non-stoichiometric imperfect spinels with oxidation state changes. And second, by way of example this type of constrained–restrained Rietveld refinement is successfully applied to the experimental quantification of the structural disorder in two custom-made spinels (i.e., zinc-ferrite and nickel-ferrite spinels).

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.

Machine learning-based algorithms have been widely applied recently in different areas due to its ability to solve problems in all fields. In this research, machine learning techniques classifying the Bravais lattices from a conventional X-ray diffraction diagram have been applied. Indexing algorithms are an essential tool of the preliminary protocol for the structural determination problem in crystallography. The task of reverting the obtained information in reciprocal lattice to direct space is a complex issue. As an alternative way to afford this problem, different machine learning algorithms have been applied and a comparison between them has been conducted. The obtained accuracy was 95.9% using 10-fold cross-validation (while the best result obtained so far has been 84%). A model based on Bragg positions was our unique predictor, allowing us to obtain the set of the interplanar lattice distances. Our model was successfully checked with a complex example. In addition, our procedure incorporates the following advantages: robustness versus imprecision in data acquisition and reduction of the amount of necessary input data. This is the first time so far that such classification has been carried out in true ab initio condition.

A powder metallurgy route combining high-energy ball-milling (HEBM) of elemental powders and reactive spark-plasma sintering (SPS) is proposed for the controlled fabrication of novel composites based on a Zr-Al intermetallic matrix hardened with a ceramic second-phase. As proof-of-concept, its suitability is demonstrated on ZrB2-hardened Zr3Al2. Specifically, commercially available powders of ZrH2, Al, and B were first combined in molar ratios of 2:1:1 to give an intermetallic-ceramic composite nominally formed by ∼76.8 vol.% Zr3Al2 plus 23.2 vol.% ZrB2, and were intimately mixed and mechanically activated by HEBM in the form of dry shaker milling for 30 min, next identifying by a dilatometric SPS test at 50 MPa pressure that the densification window of these composites is ∼975-1275 °C. Subsequent densification SPS tests at 50 MPa pressure in that temperature interval, and also at 1350 °C, plus the microstructural and mechanical characterisations of the resulting materials, established 1175 °C as the optimal SPS temperature. It was also identified that densification takes place by transient liquid-phase sintering with molten Al, and that it occurs gradually, not abruptly, because most molten Al disappears in a flash by reacting with Zr to form in situ the intermetallic. It is also shown that the combination of HEBM plus reactive SPS yields Zr3Al2+ZrB2 composites with fine-grained microstructures formed essentially by multitudinous ZrB2 nanograins dispersed within a matrix of submicrometre, or nearly submicrometre, Zr3Al2 grains. Importantly, these intermetallic-ceramic composites were found to be very hard (i.e., ∼11.5 GPa), attributable to the hardening provided by the ZrB2 nanograins, and fairly tough (i.e., ∼4.5 MPa·m1/2), and therefore potential candidate materials for a multitude of structural-tribological applications. Finally, implications for future study are discussed.

Boron carbide is among the most promising ceramic materials nowadays: their mechanical properties are outstanding, and they open potential critical applications in near future. Since sinterability is the most critical drawback to this goal, innovative and competitive sintering procedures are attractive research topics in the science and technology of this carbide. This work reports the pioneer use of the laser-floating zone technique with this carbide. Crystallographic, microstructural and mechanical characterization of the so-prepared samples is carefully analysed. One unexpected output is the fabrication of a B6C composite when critical conditions of growth rate are adopted. Since this is one of the hardest materials in Nature and it is achievable only under extremely high pressures and temperatures in hot-pressing, the use of this technique offers a promising alternative for the fabrication. Hardness and elastic modulus of this material reached to 52 GPa and 600 GPa respectively, which is close to theoretical predictions reported in literature.

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 atmosphere

A worked example of polytypism is presented, aimed at assisting undergraduates in the learning and instructors in the teaching of this topic. In particular, this crystallography concept, not necessarily obvious for beginners, is illustrated pedagogically using to that end the model case of the prolific polytypism of silicon carbide (SiC). On the basis of concepts that are easily assimilated by students (i.e. simple topological constraints) this article first presents a unified description of the polytypism phenomenon in SiC that allows one to understand without difficulty the existence of its numerous polytypic variants and how they develop. Then the various notations used to designate these different polytypes are described, and finally the crystal structures of the most common are discussed. This worked example is thus expected to contribute to motivating undergraduates in the study of a crystallography topic that often is not treated in sufficient depth in class.

This paper presents a novel methodology to calculate cation diffusion coefficients and activation energies in cubic Y2O3–ZrO2 by Molecular Dynamics. The calculation is based upon modulating the interaction potential to promote cation mobility within the lattice. The technique was calibrated by measuring static properties and oxygen self-diffusion characteristics, and then applied to cation diffusion. The respective activation energies and diffusion coefficients agree well with experimental findings. Preliminary results about grain boundary cation diffusion are presented for the first time as a proof of the potentiality of the procedure

El estudio de los perfiles de fisuras generados mediante ensayos Vickers en 4Y-PSZ muestra una configuración cuasi radial a cargas bajas, semicircular a cargas altas, pasando por una configuración tipo "ritíon" a cargas intermedias. A pesar de las diferencias morfológicas observadas, la longitud de las fisuras en superficie muestra una dependencia con la carga característica de fisuras semicirculares. Por otro lado, se observa bajo la imprenta una región inaccesible a las fisuras (corazón). Para explicar estos resultados, se propone un modelo que admite la existencia de una zona de transición de mayor tenacidad entre el corazón y la matriz elástica. El análisis efectuado, considerando el factor de intensidad de tensiones residuales "standard", proporciona valores de tenacidad en la zona de transición que llegan a superar en aproximadamente cuatro veces a la tenacidad del materia

El sistema no cristalino Se100-xBix se encuadra dentro de la familia de los calcogenuros amorfos, y ha sido muy poco estudiado dada su tendencia a la separación de fases en la región de composiciones ricas en Se. Las muestras estudiadas en este trabajo ha