Sintering, consolidation, reaction and crystal growth by the spark plasma system (SPS)

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Abstract

The graphite die set in spark plasma system (SPS) is heated by a pulse direct current. Weak plasma, discharge impact, electric field and electric current, which are based on this current, induce good effects on materials in the die. The surface films of aluminum and pure WC powders are ruptured by the spark plasma. Pure AlN powder is sintered without sintering additives in the electric field. The spark plasma leaves discharge patterns on insulators. Organic fibers are etched by the spark plasma. Thermosetting polyimide is consolidated by the spark plasma. Insoluble polymonomethylsilane is rearranged into the soluble one by the spark plasma. A single crystal of CoSb3 is grown from the compound powders in the electric field by slow heating. Coupled crystals of eutectic powder are connected with each other in the electric field.

Introduction

The spark plasma system (SPS) has been called spark plasma sintering (SPS) [1] or plasma activated sintering (PAS) [2]. Spark plasma system is an appropriate description, because the system enables the sintering of metals and ceramics, the consolidation of polymers, the joining of metals, crystal growth, and chemical reactions. It is suggested that the term spark plasma system includes spark plasma sintering (SPS), spark plasma consolidation (SPC), spark plasma joining (SPJ), spark plasma growth (SPG) and spark plasma reaction (SPR).

SPS was developed based on the idea of using the plasma on electric discharge machine for sintering metal and ceramics in the early 1960s by Inoue et al. They expected that sintering assisted by plasma could help realize advanced materials. The SPS equipment was patented in America [3], [4]. Few machines were only sold in America and Japan. In the late 1980s the patent expired, and various companies started to manufacture SPS equipment based on the original techniques.

SPS has been mainly characterized by spark plasma created by a pulse direct current during heat treatment of powders in a graphite die [1], [2], [5]. There are gaps between the two electrodes in the electric discharge machine, and high-energy plasma is generated there. The die alignment of SPS, however, does not allow any gaps, and high-energy plasma cannot be expected without such gaps. It is reasonable that the plasma of SPS has not been identified directly. Electric noise has been observed, and is thought to correspond to the plasma generation [6].

There are five expected merits of SPS [1], [2]: 1, generation of spark plasma; 2, effect of electric field; 3, effect of electric current in the conductor or skin current on the semiconductor and insulator; 4, Impact of spark plasma; 5, rapid heating and cooling. The first is considered as a means for the fabrication of new materials. The second and third effects had not been considered to be significant for sintering and reactions. The fourth effect is due to mechanical pressure caused by spark plasma, and does not seem to be serious. The fifth is the efficiency of heat-treatment. There are no insulators and heating elements of large heat capacity, and the graphite die is heated directly by an electric current. These conditions result in rapid heating and cooling.

Innovative materials have been obtained with ceramics, metals, polymers and semiconductors. Some of them cannot be prepared without SPS. Aluminum metal is solidified like other metals. Pure tungsten carbide and aluminum nitride powders can be sintered without additives. Discharge patterns are obtained on the surface of an insulator. Organic fibers are etched. Thermosetting polyimide is consolidated. The structure of insoluble polymonomethylsilane is rearranged into that of the soluble polymonomethylsilane. A single crystal of CoSb3 is grown from the compound powder by slow heating. Eutectics solidified from the eutectic powders keeps their original structures.

Section snippets

Sintering Al metal and alloy

Al particles are covered with aluminum oxide film, and cannot be sintered by normal and hot press sintering. The oxide film cannot be broken and/or removed by heat. The powders can be forged into a dense body under heating conditions.

The properties of the Al alloy (Si 25 wt.%, Fe 3.5 wt.%, Ni 3.5 wt.%, Mg 1 wt.%) sintered by SPS are shown together with that of the forged alloy in Table 1 [7]. The properties of these two are almost same. The oxide film of the Al alloy cannot be removed

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