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It is well known that the presence of LiF as sintering aid to process transparent polycrystalline Y 3 Al 5 O 12 (YAG) ceramics by the Spark Plasma Sintering (SPS) technique induces three positive effects: a better particle rearrangement, a higher densification rate and the elimination of carbon contamination leading to the improvement of the material optical properties. However, the reaction mechanisms responsible of these effects still need to be clarified. Two YAG powders presenting different characteristics were selected to highlight the possible interaction between impurities and the sintering aid. In order to facilitate the determination of the LiF action during processing and reveal the involved phenomena, an excessive quantity of 5 wt% LiF was added. Samples were sintered at different temperatures between 800 °C and 1450 °C under 50 MPa. XRD, SEM and EDX techniques were used to characterise the microstructural development of ceramics and to establish the mode of action of LiF. The results highlighted that the role of LiF is mainly dependent on the purity and the phase composition of starting powders and that the SPS cycle must be adapted to the sintering aid characteristic temperatures. [ABSTRACT FROM AUTHOR]

The present work aims to establish a correlation between the characteristics of YAG and Er:YAG commercial powders produced by two different synthesis routes and sintered ceramic microstructures and their optical aspect by taking into account the influence of pressure applied during the Spark Plasma Sintering (SPS) process. Physical and chemical characteristics of the powders were compared using various techniques such as SEM, XRD, laser diffraction and chemical analyses. Their behaviours were evaluated through a rheological study, compressibility tests and dilatometry cycles using SPS. This paper pinpoints the most important powder features which influence the optical quality of YAG and Er:YAG ceramics. The optical quality is mainly affected by the porosity, related to powder characteristics that affect particle rearrangement, densification and grain growth. The applied pressure induces microstructural heterogeneities depending on the starting material used and resulting in core-shell aspects of sintered ceramics. [ABSTRACT FROM AUTHOR]

Transparent MgAl 2 O 4 spinel ceramics were processed from sub-micrometric commercial powder by applying a two-step procedure: pressureless sintering under vacuum followed by hot isostatic pressing. To limit grain growth and to avoid secondary reactions or impurities, no additives or sintering aids were added to the powder. First, pressureless sintering at 1500 °C during 2 h under vacuum led to opaque samples due to a high level of porosity. To improve the optical quality of the MgAl 2 O 4 ceramics and the in-line transmission in the visible range, a post-treatment by hot isostatic pressing was applied. Highly transparent ceramics were obtained after a post-treatment at 1800 °C for 10 h with an in-line transmission of 81% at 400 nm and 86% from 950 to 3000 nm for a thickness of 2 mm (98.8% of the theoretical transmission). [ABSTRACT FROM AUTHOR]

Abstract: Uniaxial compressive strength of cement paste increases with increasing loading rate. Two experimental campaigns are described: 2-day-old cement paste samples are tested by using quasi-static stress rates. 6-month-old and oven-dried cement paste samples are tested by using quasi-static and high-dynamic strain rates. As for the analysis of the former tests, we develop a nonlinear viscoelastic–brittle model. It explains our test data very well, and it suggests that strength decreases with decreasing loading rate, because the duration of the test increases and this provides the possibility for creep-related damage mechanisms to reduce the strength of the material. As for the analysis of the latter tests, we develop an elasto-brittle model considering crack propagation, at the Rayleigh wave speed, in loading direction. The model is free from fitting parameters and explains our measurements very well, indicating that high-dynamic strength increase is a structural effect. [Copyright &y& Elsevier]