Abstract
The main subject of this work is an investigation of the effects of heating rate and current on the crystallisation of amorphous precursors in spark plasma sintering (SPS). For this, dry gel of Al2O3-SiO2 with a molar ratio of 1:1, was synthesized and sintered in-situ by SPS, and also by hot pressing (HP) for comparison. Phase analysis showed that the only crystalline product in both cases was mullite, whose Al2O3 content was lower in the SPS specimens. The microstructures showed a low volume fraction of large mullite fibers in the SPS specimens, whereas a high volume fraction of fine equiaxed grains was present in the HP specimen. The main difference in microstructure between HP and SPS specimens could be explained in terms of the higher heating rate of the SPS specimens. The size of the SPS die also affected the size and aspect ratio of the mullite fibers produced, which might have been due to either the different electrical current required or a difference in specimen temperature profile.
Highlights
A key difficulty in producing fine grained ceramics is the attainment of full density without allowing excessive grain growth
The process resembles conventional hot pressing, but with much higher heating rate owing to direct heating of the graphite die or the sintering precursor itself by passing direct current (DC) through it
The main crystalline product found in all three samples is mullite
Summary
A key difficulty in producing fine grained ceramics is the attainment of full density without allowing excessive grain growth. In classical solid state synthesis, synthesis of MxV2O5 phases requires careful grinding of the corresponding powders and long treatments (e.g., 2 days) in sealed quartz tubes at 600 °C; in SPS, they are formed in only a few minutes This success is attributed to M atoms penetrating micro-crystals of V2O5 oxide at high speed, which shear the original crystal network and rebuild the crystal as one of the new products reported. In order to investigate the effect of SPS in systems where chemical reaction or crystallisation occurs in addition to the usual sintering mechanisms, the effect of SPS on an amorphous dry gel of Al2O3–SiO2 was chosen This is a well known system whose phase equilibria, reaction kinetics and grain growth have been studied widely in conventional conditions. The SPS die dimensions have been varied in an attempt to vary the electric current/field, which has been reported to be important in affecting grain growth in conventional sintering, for example [8,9,10]
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