Spark plasma sintering (SPS) is a new pressure-assisted sintering process that quickly gained popularity with researchers looking for ways to consolidate materials with nanoscale or simply very fine grain sizes, or with other nonequilibrium microstructures.
The term Spark Plasma Sintering (SPS) is generally used to identify a sintering technique involving the contemporaneous use of uniaxial pressure and high-intensity, low-voltage, pulsed current. In general terms, SPS can be considered a modification of hot pressing, where the furnace is replaced by the mold containing the sample, that is heated by a current flowing directly through it and eventually through the sample. However, since there is no general agreement on the details of the technique, an unequivocal definition of the SPS and its associated procedures cannot be defined (Grasso et al., 2009; Orrù et al., 2009). The name SPS itself has been often disputed as, despite several attempts, the presence of plasma and electric discharges during the process it has never been proved unequivocally.
SPS has been receiving growing attention in the last two decades due to its remarkable effectiveness, allowing to obtain fast sintering and densification, particularly in the case of materials considered hard to sinter, such as extremely refractory materials, metastable phases or nanomaterials. For some of these materials, SPS has already become the sintering technique of choice.
SPS belongs to a quite extensive group of techniques involving the use of electric current for the sintering of materials. Some of these techniques have been introduced, and found widespread application, since the beginning of the last century, particularly for metallic materials. More details on this historical evolution can be found in recent reviews (Olevsky and Dudina, 2018; Guillon et al., 2014; Munir et al., 2011; Orrù et al., 2009). The most distinctive characteristics of SPS, represented by the use of pulsed DC, appeared originally in a patent by Inoue in the mid ‘60 of the last century (Inoue, 1966), while the term SPS was introduced later on, by the Japanese producers of the first commercial machines. In the beginning, the technique found application mostly in Japan and in a few other far-east countries. The diffusion in western countries, mainly in research institutions, started in the mid ‘90 of the last century and spread rapidly in the industrial environment.
Machines similar to SPS, but using electric currents presenting a time pattern different from pulsed DC have also been proposed over the years. In particular, the use of plain DC or AC has been suggested. Although there is no generally recognized nomenclature, such techniques are usually referred to as ‘Field Assisted Sintering’ (acronym FAST) (Orrù et al., 2009). However, a thorough investigation on the role played by the current pattern in determining the characteristics of the sintering process has never been presented. Available indications are suggesting that the current pattern might play a significant role only in a few specific cases, while the main characteristics remain quite similar. For this reason, the coupled acronym SPS/FAST is often used in recent literature to indicate all pressure-assisted, current-assisted, rapid sintering techniques.
The Spark Plasma Sintering (SPS) method is an effective technique for the compaction of powder materials. A main characteristic of this method is the direct heating of the pressing tool and/or the sample by pulsed direct electrical current with low voltage. This results in high heating rates and allows for short treatment times in order to obtain highly compacted sinter bodies. The material transport (e.g. by diffusion) occurring during the sintering process can also be used for performing chemical reactions. Especially the conditions during the SPS process allow the use of the method also as an alternative synthesis route for intermetallic compounds, of which, some can be obtained only with difficulties by other techniques.
The MPI for Chemical Physics of Solids was the first institute in where an SPS setup had been installed. Two SPS apparatus are available, one of them being installed inside an Argon-filled glove box, which make it very useful for the compaction and/or synthesis of air/moisture sensitive samples. Both machines allow for external forces up to 50 kN, direct current up to 1500 A and a voltage limit of 25 V with typical pulse length of 3 ms. Within the Dresden SPS facilities we have established close cooperations with the Fraunhofer IFAM and IKTS institutes. Currently, there are 5 SPS machines of different size and capabilities in these facilities.
An example of a successful chemical synthesis in an SPS process are the clathrate phases K8-δSi46 (δ=0÷1.5), Rb6Si46, Rb11Si136, Cs8Si136, K8Sn44, Na2K6Si46, and Na8(Si,Ge)46. They were obtained via a solid-state electrochemical variant of the redox reaction by using binary or ternary alkali-metal Zintl phase precursors . Also the single step synthesis of binary titanium oxides and substituted derivatives, starting from rutile and elemental titanium, shows the relevance of the diffusion controlled reaction during SPS experiments . Based on our experience, several compounds were successfully synthesized, e.g. transition-metal oxides, binary and ternary borides, silicides, germanides.
Usually, SPS is carried out in four main stages. The first stage is performed to remove gases and create vacuum. Then pressure is applied in the second stage followed by resistance heating in the third stage and finally cooling in the fourth stage. When a spark discharge appears in a gap or at the contact point between the particles of a material, a local high-temperature state of several to ten thousands of degrees centigrade is generated momentarily.
This causes evaporation and melting on the surface of powder particles in the SPS process, and necks are formed around the area of contact between particles. The application of pressure and current, in addition to the high-localized temperatures generated through resistance pulse heating, improves heating rates and reduces sintering time and temperature leading to the consolidation of nanopowders without excessive grain growth. On the other hand, SPS is not only a binderless process, but also does not require a precompaction step. The powder is directly filled into a graphite die through which current is passed and pressure is applied leading to a fully dense material with superior mechanical properties.
