Metal-like peak stress behavior of yttrium-doped BaCeO3 ceramic

The high-temperature mechanical behavior of polycrystalline 5 at% yttrium-doped barium cerate with submicronic grain size (d = 0.5 µm) has been studied in compression between 1200 and 1300 °C at different initial strain rates. The true stress – strain curves display an initial peak stress followed by a softening stage and then by an extended steady-state stage; the magnitude of the peak stress is strongly dependent on strain rate and temperature. These characteristics are very similar to those found in metals and metallic alloys that exhibit dynamic recrystallization during creep at elevated temperatures. Microstructural observations by scanning and transmission electron microscopy have shown that the grain structure is progressively refined with increasing strain due to the strong interaction between dislocations and pre-existing twin boundaries, originated by the various crystal transformations that occur upon cooling from the sintering temperature. The empirical equations used in metals to describe the relationship between strain rate, peak stress, and peak strain are also valid in the present ceramic material.España , Ministerio de Ciencia e Innovación Project no. MAT2009-13979-C03-0

Materiales cerámicos tenaces a base de ZrO2: microestructura y propiedades mecánicas

El desarrollo de los materiales cerámicos estructurales está permitiendo disminuir la fragilidad inherente á los materiales cerámicos. En los compuestos tenaces a base de Zr02, se aprovecha la transformación martensítica tetragonal -^ monoclínica como mecanismo de aumento de la resistencia del material a la propagación de fisuras (comportamiento de curva-R). El objetivo es diseñar microestructuras con fase tetragonal metaestable capaces de nuclear la transformación martensítica bajo la acción de un campo de tensiones externo, evitando al mismo tiempo la transformación asistida térmicamente. El tamaño de las partículas tetragonales de Zr02 juega el papel más importante en este mecanismo de aumento de tenacidad. En este trabajo se revisan las características microestructurales de este tipo de materiales, y su influencia sobre las propiedades mecánicasThe development of new ceramics components has allowed decrease the brittleness associate to the traditional ceramic materials. In Zr02-containing ceramics, the martensitic tetragonal to monoclinic phase transformation is the mechanism controlling the «resistence-curve» behavior, wherein the crack resistence increases during the course of crack propagation. The tetragonal particles size play an important role in the mechanical properties of such transformation toughened ceramics. The goal is to design microstructures with metastable tetragonal Zr02 particles, which transform to monoclinic in the presence of a stress field. The microestructural aspects and mechanical properties of these ceramic materials are reviewed in this wor

Fabrication, microstructure and high-temperature plastic deformation of three-phase Al2O3/Er3Al5O12/ZrO2 sintered ceramics

The fabrication, microstructure and high-temperature creep behavior of chemically compatible, three-phase alumina/erbium aluminum garnet (Er3Al5O12, EAG)/erbia fully-stabilized cubic ZrO2 (ESZ) particulate composites with the ternary eutectic composition is investigated. The composites were fabricated by a solid-state reaction route of α-Al2O3, Er2O3 and monoclinic ZrO2 powders. The final phases α-Al2O3, EAG and ESZ were obtained after calcination of the powder mixtures at 1400 °C. High dense bulk composites were obtained after sintering at 1500 °C in air for 10 h, with a homogeneous microstructure formed by fine and equiaxed grains of the three phases with average sizes of 1 μm. The composites were tested in compression at temperatures between 1250 and 1450 °C in air at constant load and at constant strain rate. As the temperature increases, a gradual brittle-to-ductile transition was found. Extended steady states of deformation were attained without signs of creep damage in the ductile region, characterized by a stress exponent of nearly 2 and by the lack of dislocation activity and modifications in grain size and shape. The main deformation mechanism in steady state is grain boundary sliding, as found in superplastic metals and ceramics. In the semibrittle region, microcavities developed along grain boundaries; these flaws, however, did not grow and coalescence into macrocracks, resulting in a flaw-tolerant material. Alumina is the creep-controlling phase in the composite because of the grain boundary strengthening caused by the (unavoidable) Er3+- and Zr4+-doping provided by the other two phases.Ministerio de Ciencia e Innovación MAT2009-13979-C03- 0

Caracterización, integración y evaluación del esquema de bomba de calor a gas

En el presente trabajo se ha desarrollado un procedimiento simplificado para calcular las prestaciones de las demandas medias estacionales en todos aquellos sistemas hidrónicos que posean en sus esquemas una bomba de calor a gas. Para ello, se ha desarrollado una herramienta en Excel que simula de forma simplificada el comportamiento energéticos de edificios sujeto a condiciones climáticas concretas.In this work, a simplified procedure has been developed to calculate the performance of the average seasonal demands in all the hydronic systems that have a gas heat pump in their schemes. For this purpose, an Excel tool has been developed that simulates, in a simplified way, the energy behaviour of buildings subject to specific climatic conditionsUniversidad de Sevilla. Máster en Ingeniería Industria

Instalación de energía solar térmica para producción de ACS en una clínica

En el presente proyecto se realiza el diseño de una instalación para la producción de agua caliente sanitaria en una clínica especializada en la fecundación. Se explicará con detalle el diseño de dicha instalación, así como la selección de los equipos. Para ello incluye los siguientes documentos: memoria descriptiva, memoria de cálculo, pliego de condiciones, presupuesto y mediciones y planos Se pone de manifiesto la viabilidad e importancia de la utilización de energías renovables para los aspectos cotidianos, como el uso de agua caliente, fomentando así un desarrollo sostenible.This project involves the design of an installation for the production of sanitary hot water in a clinic specialising in fertilisation. The design of this facility will be explained in detail, as well as the selection of the equipment. This includes the following documents: descriptive report, calculation report, specifications, budget and measurements and plans. The feasibility and importance of the use of renewable energies for everyday aspects, such as the use of hot water, is highlighted, thus promoting sustainable development.Universidad de Sevilla. Grado en Ingeniería de las Tecnologías Industriale

Superplasticity in ceramics. Applications

Las características generales macroscópicas y microscópicas de la superplasticidad son analizadas en detalle. Mediante el análisis de resultados experimentales en policristales de circonia dopada con ytria, se propone una ecuación de la superplasticidad basada en la idea de una tensión umbral. Por último y basándose en el mecanismo microscópico de la superplasticidad, se describe una nueva técnica de uniones de piezas cerámicas.The macro and microscopic characteristcs of the superplasticity are analyzed in detail. From the analysis of the experimental results in Y-TZP and based on the threshold stress concept, a creep equation is proposed for the superplastic behavior of ceramics. Finally, it is described the superplastic junction of ceramics as a new application of the superplasticity

Alteración de las propiedades mecánicas por formación de precipitado en ZrO2 parcialmente estabilizado con Y2O3

Se ha estudiado el comportamiento mecánico a altas temperaturas del Zr02 monocristalino parcialmente estabilizado con 4. 7 mol% de Y 203, mediante ensayos de velocidad de deformación constante, a 1.4xl0·5 s· 1, en aire a 1400 °C. Las muestras fueron sometidas a tratamientos térmicos a 1600 °C antes de la deformación. Se ha realizado un estudio detallado de la interacción dislocación-precipitado mediante microscopía electrónica de transmisión, para explicar el comportamiento observado, proponiendo un modelo de deformación.Single crystals of partially-stabilized Zr02 contammg 4. 7 mol% Y 203 were deformed by constan! strain rates of 1.4x10·5 s·1, in air at 1400 °C. The samples were annealed at 1600 °C prior to deformation. TEM studies of the dislocation-precipitate interaction were performed to explain the observed behavior, and a model for the deformation is suggested

High-temperature mechanical behavior of polycrystalline yttrium-doped barium cerate perovskite

The high-temperature mechanical properties of the mixed ionic-electronic conductor perovskite BaCe0.95Y0.05O3-δ with average grain size of 0.40μm have been studied in compression between 1100 and 1300°C in air at different initial strain rates. The true stress-true strain curves display an initial stress drop, followed by an extended steady-state stage. As the temperature decreases and/or the strain rate increases, there is a transition to a damage-tolerant strain-softening stage and eventually to catastrophic failure. Analysis of mechanical and microstructural data revealed that grain boundary sliding is the primary deformation mechanism. The strength drop has been correlated with the growth of ultrafine grains during deformation, already present at grain boundaries and triple grain junctions in the as-fabricated material.Ministerio de Ciencia e Innovación MAT2009-13979-C03-0

Pure diffusional creep of NiO polycrystals

Compressive creep testing of polycrystalline NiO has been performed in air and argon, at temperatures between 1 100 and 1 500 °C (0.61 to 0.79 TM) under loads corresponding to initial applied stresses from 6 to 90 MPa. Nabarro creep was observed in the lower stress range; creep rate and oxygen diffusivity show a good correlation.Des essais de fluage en compression ont été faits sur des polycristaux de NiO, dans l'air et l'argon, à des températures entre 1 100 et 1 500 °C (0,61 à 0,79 TF) sous des charges correspondant à des contraintes initiales entre 6 et 90 MPa. Le fluage Nabarro a été observé à faible contrainte; la vitesse de déformation et la diffusivité de l'oxygène montrent une bonne corrélation