Comportamiento tribológico de materiales cerámicos de nitruro de silicio texturados

Se han fabricado materiales densos de nitruro de silicio (Si3N4) texturados incorporando semillas de β- Si3N4, favoreciendo su alineación mediante extrusión de las piezas en verde y prensado en caliente. Estos materiales presentan elevada anisotropía microestructural que conduce a un comportamiento anisótropo de sus propiedades, tanto mecánicas como tribológicas. Se han realizado ensayos de desgaste en seco mediante movimiento recíproco lineal de pares homólogos con geometría esfera-placa. El material texturado presenta mayor resistencia al desgaste que el de referencia, sin texturar, identificándose como principal mecanismo de desgaste la abrasión debida a microfractura y arranque de granos de Si3N4. En este material, además, se desarrolla un tercer cuerpo que protege la superficie del material

Understanding the active sites of boron nitride for CWPO: An experimental and computational approach

Hexagonal boron nitride (h-BN) has been explored as a catalyst for degrading persistent organic pollutants in wastewater by Catalytic Wet Peroxide Oxidation (CWPO). Herein, the superior activity of the h-BN on the phenol degradation (model pollutant) compared to other metal-free catalysts, such as carbon-based ones, and the lower selectivity to CO encourage the potential application of h-BN catalysts in CWPO processes. Through a combined density functional theory calculations, experimental reactions and catalyst characterization approach, a comprehensive study on the reaction mechanism has been conducted. According to this, only defected B atoms in the h-BN layer, protonated as B-(OH2)+, decompose the hydrogen peroxide into highly reactive hydroxyl radicals. The radical species diffuse towards inner h-BN regions and react with the phenol adsorbed by π-π interaction on the h-BN surface. Oxidation by-products cause carbonaceous deposits and progressive deactivation of the h-BN catalyst that can be directly regenerated by burning off in airThe authors thank the financial support by the Community of Madrid and the Government of Spain through the projects: S2018/EMT-4341 and RTI2018-095052-BI00 (MCIU/AEI/FEDER, UE), respectively. The work done at the University of Sevilla was funded by Spanish Ministerio de Ciencia e Innovación and EU-FEDER, grant PID2019-106871GB-I00, and the Junta de Andalucía-FEDER, grant: US-1381410. Also, G. Vega acknowledges the Community of Madrid for the Predoctoral contract PEJD-2018-PRE/AMB-9019, co-financed by the European Social Fund through the Youth Employment Operational Program and the Youth Employment Initiative (YEI) 2018. J. Carbajo thanks the financial support by the Government of Spain for a grant under the Juan de la Cierva_Incorporación programme (IJCI-2017-32682

Quemadores de gas cerámicos

The extended use of natural gas for domestic applications is spurring the continuous improvement in the gas boiler technology. Thus, compact boilers that are able to work in a wider power range, in order to satisfy the exact energy requirements of the users, are a common goal. This requires the development of gas burners capable of working in radiant mode for increasing the combustion efficiency besides reducing the emissions of hazardous gases. The ceramic materials are strong candidates for this application. They have the properties required for working in the severe conditions associated with the combustion of gases, i.e. high temperatures and the presence of corrosive chemical species. These work conditions produce a slow but continuous degradation of their properties and the ultimate burner failure. This paper discusses the different types of ceramic gas burners available, their working requirements, modes of operation and the possible degradation mechanisms.<br><br>El creciente uso del gas natural, como fuente de energía térmica en aplicaciones domésticas, está produciendo una continua evolución en la tecnología de este tipo de calderas. Estas son cada vez más compactas y capaces de trabajar en un mayor rango de potencias caloríficas, para satisfacer las demandas de energía requeridas, en cada momento, por los usuarios. Para ello, deben estar provistas con quemadores que puedan funcionar intermitentemente en modo radiante, lo que permite un aumento en la eficiencia del proceso de combustión y, además, una reducción en la emisión de gases nocivos. Los materiales cerámicos son unos buenos candidatos para fabricar estas placas radiantes de los nuevos quemadores de gas. En general, estos materiales poseen las propiedades adecuadas para funcionar en las severas condiciones que se producen en la combustión, caracterizadas por las altas temperaturas y la presencia de especies químicas corrosivas. Estas condiciones de trabajo producen una degradación lenta pero progresiva de sus propiedades y, con el tiempo, dan lugar al fallo del quemador. En este trabajo se presenta una revisión sobre los tipos de quemadores de gas cerámicos existentes, requisitos que deben cumplir, su modo de operación y los modos en que se degradan

The effective role played by graphene fillers for improving the tribological properties of ceramics

Nowadays, new materials with enhanced tribological properties are required to fulfil the new demanding working conditions of more efficient and eco-friendly industrial processes. To achieve this goal, we propose the addition of graphene nanofillers, a very attractive self-lubricating solid, to ceramic materials for improving their tribological responses. As proof of concept, here we present the role of the graphene nanoplatelets (GNPs) on the friction and wear resistance of two different graphene/ceramic nanocomposites, in particular, GNPs/silicon nitride and GNPs/silicon carbide materials. The tribological tests evidence that GNPs are ideal nanofillers to increase the wear resistance, especially under high contact pressures, because the nanoplatelets are exfoliated during the sliding motion and lead to the formation of a carbon-based protecting film; whereas the benefits on the friction depend on the tribo-testing conditions

Método del “Pulso Láser” para la medida de la difusividad térmica en materiales cerámicos

Among the several techniques available to measure the thermal conductivity/ diffusivity, the laser flash method stands out for its low run time (less than 1 hour per temperature) and the reduced sample size required. Nevertheless, the laser flash technique is not very accurate for porous materials due to laser transmission problems and the underestimation of the total sample thickness. The attaching to the porous sample of two thin foils of an opaque and high diffusivity material (Cu) can solve these problems. This type of assembly was used to measure diffusivity in a porous alumina material. Data obtained are compared with theoretical models, wich predict effective thermal conductivity of the porous material from the thermal conductivity of the dense material, the amount of porosity and the type of microstructure.<br><br>Entre las distintas técnicas de medida de la conductividad/difusividad térmica de materiales, destaca la técnica del pulso láser por la rapidez de medida de la difusividad térmica y la facilidad de preparación de las muestras. Sin embargo, cuando se mide la difusividad térmica en materiales porosos, aparecen problemas relacionados con la transmisión del haz láser y la subestimacion del espesor de la muestra. En este trabajo, se propone un método para evitar estos problemas que consiste en formar un sistema tricapa Cu/material poroso/Cu. Este método se ha verificado experimentalmente utilizando materiales de alúmina densa y porosa. Los valores experimentales para la alúmina porosa se han explicado a partir de modelos que estiman la conductividad térmica efectiva de un material poroso en función de la conductividad del material denso, la porosidad y el tipo de microestructura

Microstructural designs of spark-plasma sintered silicon carbide ceramic scaffolds

Concentrated ceramic inks based on β-SiC powders, with different amounts of Y2O3 and Al2O3 as sintering aids, are developed for the adequate production of SiC scaffolds, with different patterned morphologies, by the Robocasting technique. The densifi cation of the as-produced 3D structures, previously heat treated in air at 600 ºC for the organics burn-out, is achieved with a Spark Plasma Sintering (SPS) furnace. The effects of the amount of sintering additives (7 - 20 wt. %) and the size of the SiC powders (50 nm and 0.5 μm) on the processing of the inks, microstructure, hardness and elastic modulus of the sintered scaffolds, are studied. The use of nano-sized β-SiC powders significantly restricts the attainable maximum solids volume fraction of the ink (0.32 compared to 0.44 of the submicron-sized powders-based ink), involving a much larger porosity of the green ceramic bodies. Furthermore, reduced amounts of additives improve the mechanical properties of the ceramic skeleton; particularly, the stiffness. The grain size and specific surface area of the starting powders, the ink solids content, green porosity, amount of sintering additives and SPS temperatures are the main parameters to be taken into account for the production of these SiC cellular ceramics.Se han fabricado andamiajes de carburo de silicio (SiC) usando la técnica de “Robocasting”, a partir de tintas cerámicas conteniendo β-SiC y distintas cantidades de Y2O3 and Al2O3, como aditivos de sinterización. La densificación de las estructuras tridimensionales, previamente calcinadas a 600 ºC para eliminar los aditivos orgánicos, se realizó en un horno de “Spark Plasma Sintering” (SPS). Se analizó el efecto de la cantidad de aditivos de sinterización (7-20 % en peso) y del tamaño de partícula inicial del polvo de SiC (50 nm y 0.5 μm) en el procesado de las tintas, en la microestructura, la dureza y el módulo elástico de las estructuras sinterizadas. El uso de polvo nanométrico restringió significativamente la fracción máxima de sólidos alcanzable (0.32 frente a 0.44 de la tinta de SiC submicrométrico), lo que condujo a una mayor porosidad de los andamiajes en verde. Una reducción del contenido de aditivos produjo aumentos en la dureza y módulo elástico del esqueleto cerámico. El tamaño de partícula y la superficie específi ca del polvo de partida, el contenido en sólidos, la porosidad en verde, y cantidad de aditivos y temperatura de sinterización, fueron los parámetros esenciales para la producción de estas cerámicas celulares de SiC

Graphene nanoribbon ceramic composites

By unzipping multi-walled carbon nanotubes (MWCNTs) it is possible to obtain graphene nanoribbons (GNRs) that could then be used as fillers in ceramic composites. Here we report the fabrication of silicon nitride (Si3N4) ceramics with different contents of GNRs by spark plasma sintering. The GNR fillers confer electrical conductivity to the Si3N4 composites, following a semiconducting-like behavior at relatively low volume filler concentrations (0.04). In addition, a toughening effect, produced by GNRs bridging the cracks was observed. GNRs appear to be an efficient alternative to graphene-based composites, useful in the fabrication of novel multifunctional ceramic composites. (C) 2015 Elsevier Ltd. All rights reserved

Processing route to disentangle multi-walled carbon nanotube towards ceramic composite

Multi-walled carbon nanotubes were highly aggregated into ropes after their synthesis by chemical vapour deposition and, therefore, two different methods for disentangling the bundles of nanotubes were studied. One method compared the use of mild and vigorous mechanical treatments in ethanol and the other one employed dispersants in aqueous media. For comparison purposes and according to their different exfoliating behaviour, sodium dodecyl sulphate and gum arabic were selected as dispersants. The results evidenced that mechanical sonication was insufficient for disentangling the ropes, whereas, the combined action of mild sonication in an ultrasonic bath with the addition of gum arabic to an aqueous suspension containing nanotubes improved the exfoliating performance. Stable suspensions of unbundled multi-walled carbon nanotubes were obtained adding only 0.05 wt% of gum arabic with a dispersant/MWNTs concentration ratio of 0.25. These values corresponded to a reduction in the dispersant concentration between 1 to 2 orders of magnitude compared to those commonly employed. In addition, a processing route for manufacturing dense and homogenous silicon nitride composites using spark plasma sintering with 1.8 vol% of multi-walled carbon nanotubes almost free of organics was developed without nanotubes degradation and aggregation.Peer reviewe

Study of the curvature of green pieces in stoneware ceramic tiles

Ceramic tiles undergo some deformations on the production line. After pressing, tiles are flat but, in subsequent stages of the production line, surfaces are wetted producing temperature and water gradients across their thickness. These gradients generate curvature in the tiles and therefore deformations in the pieces. The aim of the present work has been to measure the curvature and the mechanical properties of the tiles as a function of the water absorption for two types of ceramic pastes.<br><br>Las baldosas cerámicas sufren deformaciones durante su proceso en línea de producción. Cuando las baldosas salen de la prensa presentan una superficie plana, pero en las siguientes etapas de la línea de producción la superficie de las muestras se humedece, produciéndose gradientes de temperatura y humedad que dan lugar a una deformación por curvatura de la pieza. El objetivo de este trabajo es estudiar dicha curvatura y las propiedades mecánicas de las baldosas en función de la humedad absorbida en dos tipos de pastas cerámicas