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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]

ABSTRACT The present study deals with the optimization of polyimide (PI) mechanical properties, obtained by Spark Plasma Sintering (SPS), by using a method combining Design of Experiments (DOE) with physical, structural, and mechanical characterizations. The effects of SPS parameters such as temperature, pressure, dwell time, and cooling rate on the density, mechanical properties, and structure of PI were investigated. The experimental results revealed that the mechanical properties of the material were optimized by raising the sintering temperature up to 350°C. The optimized SPS processing parameters were a temperature of 350°C, a pressure of 40 MPa, and a dwell time of 5 min. Under these conditions, a relative density of 99.6% was reached within only a few minutes. The corresponding mechanical properties consisted of Young's modulus of 3.43 GPa, a Shore D hardness of 87.3, and a compressive strength of 738 MPa for a maximum compressive strain of 61.8%. Moreover, when working at 320°C and at 100 MPa, an increase in the dwell time was necessary to enhance the properties. Contrary to the other parameters, the cooling rate appeared to be a non-significant parameter. Finally, correlations between the PI structure and the mechanical properties were made to demonstrate the densification mechanisms. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41542. [ABSTRACT FROM AUTHOR]

ABSTRACT This article presents two high-temperature thermoplastic powders which were sintered by spark plasma sintering in order to get homogeneous mechanical properties. Dense polyimide (PI) and polyetheretherketone (PEEK) specimens were obtained at temperatures as low as 320°C for PI and 200°C for PEEK, respectively. Relative densities higher than 99% were reached for both materials. In order to characterize their properties, in situ measurements with compression and hardness tests were carried out on sintered samples. This method allowed to obtain polymeric materials with improved mechanical properties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40783. [ABSTRACT FROM AUTHOR]

This study deals with the understanding of the sintering mechanisms that occur during consolidation of an ultra‐high‐performance polymer: poly(ether‐ether‐ketone). In particular, we investigated the effects of uniaxial pressure during spark plasma sintering (SPS) processing. Glass‐transition temperature (Tg) measurements under loading, stress–strain curves and scanning electron microscopy analysis allowed us to determine the role of pressure intensity and temperature of application on macromolecular chain mobility in both the bulk and the surface of the particles. First, a loss of chain mobility in the bulk of the particles was observed under high pressure. Second, it was shown that high pressure applied at low temperature leads to friction effects between the particles which enhances chain mobility in the particle shell through a local melting phenomenon. These experimental conditions favor the healing of particles and high cohesion was then observed in the resulting sintered samples. The level of friction is enhanced when pressure is applied at a low temperature since the powders are still in a glassy state. Finally, the achievement of cohesive grain boundaries was found to be related to the location and conformation of chain ends. Good healing between particles can only occur if a sufficient number of chain ends are available at the surface of the particle shell. We showed that the native powder state plays a significant role. The direct use of as‐received powder leads to final material exhibiting good cohesion whereas pretreatments of the native powder are highly detrimental. It should be noted that this processing does not affect the high initial degree of crystallinity because no bulk melting is observed during consolidation by sintering. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47645. Influence of location and conformation of chain ends on polymer cohesion (compression test). [ABSTRACT FROM AUTHOR]