Field-Assisted Sintering Technology / Spark Plasma Sintering

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Field-Assisted Sintering Technology/Spark Plasma Sintering (FAST/SPS) is a unique technique for the synthesis and sintering of inorganic materials at high temperature that combines uniaxial pressure with a pulsed direct current. The voltage is low (below 10 V), but the current is high (typically from 1 to 10 kA), leading to efficient Joule heating as the current flows through the graphite dies where the inorganic powder is placed. These conditions allow to achieve heating rates up to 1000 K/min, although typically in the range 50 – 200 K/min, which are much higher than the heating rates (around 20 K/min) of other conventional sintering techniques such as pressureless sintering and hot pressing. The isothermal holding time is strongly reduced and just in a few minutes the sample is fully densified, meanwhile other techniques require several hours during the dwell time for the same purpose. Consequently, FAST/SPS leads to fully densify materials in just a few minutes, limiting – or even inhibiting - the grain growth and thus, tailoring the microstructure. Typically, graphite tools are used to contain the powders due to the high electrical and thermal conductivity as well as the high melting point (sintering up to approx. 2500 K). On the other hand, the graphite tools limit the maximal uniaxial pressure (typically up to 75 MPa) and require to use protective atmosphere such as vacuum or argon. Nevertheless, non-graphite tools are an alternative to increase the uniaxial pressure at expenses of reducing the maximal temperature. FAST/SPS is a versatile technique that leads to consolidate a manifold of materials, microstructures and structures. Metals and ceramics can be sintered, without any restriction due to their intrinsic electrical or thermal features, including intermetallics, superalloys, refractory metals, ultra-high temperature ceramics (UHTCs), transparent ceramics, carbides, nitrides, borides, oxides, MAX phases, and non-equilibrium materials. Importantly, synthesis can be also performed during the sintering process, which is called reactive sintering, developing fully dense compounds that cannot be produced by other methods. Furthermore, due to the characteristics of the heating conditions, the microstructure can be controlled to confer different properties. Regarding the structures, porous architectures with controlled porosity, bulk materials, ceramic matrix composites and joining of metals and ceramics can be obtained by FAST/SPS, avoiding the chemical reaction between different components due to the short thermal cycles (minutes instead hours).