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          The development and application of Spark plasma sintering
Beijing University of New Functional Materials of Ministry of Education Key Laboratory of Jiuxing, LIU Ke-high, Zhou Meiling

"Keywords": spark plasma sintering; development; application

"Summary": spark plasma sintering (SPS) is a fast, low-temperature, energy saving, environmentally friendly materials preparation technology. This paper reviews the SPS development and application at home and abroad, introduces the principle characteristics of the SPS and in preparation of new materials processing applications.

1 Introduction

With the high-tech industries, new materials, especially the types of new functional materials and increasing demand, new features call for a new material preparation techniques. Spark plasma sintering (Spark Plasma Sintering, referred to as SPS) is a preparation of functional materials, new technology, it has a fast heating rate, sintering time is short, the organizational structure of control, environmental protection and other distinctive characteristics, can be used to prepare metal, ceramic materials, composite materials, can also be used to prepare nano-bulk materials, amorphous bulk materials, gradient materials.

2 Development and application of international SPS status

SPS technology is directly between the powder particles pass into the heating pulse current sintering, so in some literature also known as plasma activated sintering or plasma-assisted sintering (plasmaactivatedsintering-PAS or plasma-assistedsintering-PAS) [1,2]. As early as 1930, U.S. scientists proposed a pulse current sintering theory, but until 1965, before the pulse current sintering technology in the United States, Japan and other countries are applied. Japan won the SPS technology patents, but were unable to solve the technical problem of the existence of low productivity and therefore promote the use of SPS technology has not been.

1988, Japan developed the first industrial-type SPS units, and research in the field of new materials to promote the use. 1990, Japan launched the SPS can be used for industrial production of third generation products, with 10 ~ 100t of sintering pressure and pulse current 5000 ~ 8000A. Recently developed a pressure up to 500t, a large pulse current of 25000A SPS units. As the SPS technique is a rapid, low-temperature, high efficiency, etc., in recent years, many foreign universities and research institutions have been equipped with the SPS sintering system, and the use of SPS for research and development of new materials [3]. In 1998, Sweden purchased SPS sintering system, carbides, oxides, ceramics and other materials of biological research more [4].

China also launched nearly three years with the SPS technology to prepare new materials research [1,3], the introduction of several sets of SPS sintering system, mainly used to sinter nano-materials and ceramic materials [5-8]. Preparation of SPS as a new material technology, has attracted widespread attention at home and abroad.

3 SPS sintering theory

3.1 plasma and plasma processing technologies [9,10]

SPS was carried out using discharge plasma sintering. Plasma is excited by high temperature or a particular state of matter, is in addition to solid, liquid and gaseous beyond the fourth state of matter. Plasma is ionized gas, by the large number of positive and negative charged particles and neutral particles, and show a quasi-collective of the neutral gas.

High-temperature plasma is conductive gas dissociation, can provide high reactivity state. Plasma temperature 4000 ~ 10999 ??, the gas molecules and atoms in a highly activated state, and the plasma gas within the ionization level is high, these properties make plasma has become a very important material preparation and processing technology.

Plasma processing technology has been more applications, such as plasma CVD, plasma PBD and plasma and ion beam etching. Oxide coating is currently used for plasma, plasma etching in the preparation of high purity carbide and nitride powder also has a certain application. The plasma is another promising application area is in the sintered ceramic materials [1].

Into the plasma production methods, including heating, discharge and light incentives. Discharge produced plasma, including DC discharge, radio frequency discharge and microwave discharge plasma. SPS is the use of DC discharge plasma.

3.2 SPS sintering equipment and the basic principles of

SPS device includes the following components: axial pressure device; water punch electrode; vacuum chamber; climate control system (vacuum, argon); DC pulse and cooling water, displacement measurement, temperature measurement, and safety control unit . The basic structure of SPS is shown in Figure 1.

SPS and hot pressing (HP) are similar, but different heating methods, it is a use pass - the direct power-off DC pulse current pressure sintering sintering. Pass - off DC pulse current primary role is to generate discharge plasma, discharge impact pressure, Joule heat and electric field diffusion [11]. SPS sintering powder particles by pulse current shown in Figure 2. In the SPS sintering process, the electrode leads to the instant when the DC pulse current generation of discharge plasma, so that all particles within the sintered body itself produces Joule heat evenly and make the particle surface activation. And self-heating reaction synthesis (SHS) and microwave sintering method is similar, SPS is the effective use of powder within the self-heating effect and the sintering. SPS sintering process can be seen as a particle discharge, conductive heat and pressure combined result. In addition to heat and pressure to promote the sintering of these two factors, the SPS technique, the effective discharge between the particles can produce local temperature, the surface can make partial melting, peeling off the surface of the material; high temperature plasma discharge sputtering and the impact of removal of the powder particle surface impurities (such as the place to surface oxides, etc.) and adsorbed gases. The role of the electric field is to accelerate the diffusion process [1,9,12].

4 SPS process advantages

SPS process advantages are clear: even heating, warming fast, low sintering temperature, sintering time is short, high efficiency, even small organizations can maintain the natural state of raw materials, can get high density materials, graded materials can be sintered and complex parts [3,11]. Compared with HP and HIP, SPS device is simple, does not require special skilled. [11] reported that the production of a diameter of 100mm, 17mm thick of ZrO2 (3Y) / stainless steel graded material (FGM) with the total time is 58min, which heat up time 28min, cooling time and holding time 5min 25min. Compared with HP, SPS technology can reduce the sintering temperature of 100 ~ 200 ?? [13].

5 SPS in Preparation of materials

Present in foreign countries, especially Japan carried out more with the SPS preparation of new materials research, some products have been put into production. SPS types of materials can be processed as shown in Table 1. In addition to preparation of materials outside, SPS materials can also be connected, such as connection MoSi2 and stone [14], ZrO2/Cermet/Ni [15].

In recent years, preparation of new materials at home and abroad with the SPS study focused on: ceramics, cermets, intermetallics, composites and functional materials and so on. One of the most functional materials, thermoelectric materials, including his [16], magnetic materials [17], functionally graded materials [18], composite functional materials [19] and nano-functional materials [20]. Preparation of the SPS amorphous alloy, shape memory alloys [21], diamond, also made attempts to obtain better results.

5.1 Functionally Graded Materials

Functionally graded materials (FGM) is the gradient of the composition, the sintering temperature of different layers, using a conventional sintering methods are difficult to burn. Using CVD, PVD, etc. Preparation of gradient materials, high cost, it is difficult to achieve industrialization. With ladder-like stone mold, because mold the upper and lower ends of the current density is different, so you can produce the temperature gradient. The use of SPS in the stone mold temperature gradient generated, only a few minutes to a good chemical composition of different sintering of graded materials. Successful preparation of the current SPS gradient materials: stainless steel / ZrO2; Ni/ZrO2; Al / polymer; Al / plant fiber; PSZ / T and other graded material.

In the self-propagating combustion synthesis (SHS), the electric field has a greater effect and role of activation, particularly in fields that previously could not activate the effect of synthetic materials can be successfully synthesized, expanding the composition range, and can control the phase composition, but has been the porous material, need to be further processed to improve density. Activation of the electric field is similar to SHS using the SPS technology, ceramics, composites and graded materials synthesis and densification at the same time, the nanocrystals obtained 65nm, less than one SHS densification process [22]. SPS can be prepared using large-size FGM, the current SPS preparation of a larger size of the FGM system is ZrO2 (3Y) / stainless steel disc, the size has reached 100mm ?? 17mm [23].

WC with ordinary powder sintering and hot pressing must be additive, and SPS makes it possible to sinter pure WC. SPS prepared with WC / Mo FGM Vickers hardness (HV) and fracture toughness were achieved 24Gpa and 6Mpa ?? m1 / 2, WC and Mo significantly reduced due to thermal expansion mismatch caused by thermal stress-induced cracking [24 ].

5.2 Thermoelectric Materials

As the hot conversion of high reliability, no pollution, thermoelectric converters recently aroused great interest, and study a number of thermoelectric conversion materials. The literature search found that the SPS preparation of functional materials research, more research on thermoelectric materials.

(1) the composition gradient of thermoelectric material's efficiency is currently an effective way to improve the hot one. For example, the composition gradient ??FeSi2 is a more promising thermoelectric materials can be used between 200 ~ 900 ?? for thermoelectric conversion. ??FeSi2 not toxic in the air, good oxidation resistance and higher electrical conductivity and thermoelectric power. Hot materials, the higher the quality factor (Z = ??2/k??, where Z is the quality factor, ?? is the Seebeck coefficient, k is thermal conductivity coefficient, ?? is the resistivity), the higher its thermal efficiency. Tests showed that the use of SPS preparation of graded composition ??FeSix (Si content of the variable), the thermoelectric performance than ??FeSi2 greatly improved [25]. An example of this well Cu/Al2O3/Cu [26], MgFeSi2 [27], ??Zn4Sb3 [28], tungsten silicide [] 29].

(2) for thermoelectric cooling semiconductor material is not only the traditional strength and durability is poor, and mainly the growth of single-phase preparation, production cycles, high costs. In recent years, some manufacturers in order to solve this problem, the use of sinter production of semiconductor materials, refrigeration, although improved mechanical strength and improved material utilization, but the performance is far below the single crystal thermoelectric performance of semiconductors, semiconductor manufacturing is now using SPS cooling material can be prepared in minutes complete semiconductor material, and crystal growth has to ten hours. SPS preparation of semiconductor thermoelectric material has the advantage that can be directly processed into wafers, no single growth method as cutting, material savings, improved production efficiency.

Hot and cold - below the sintering performance of semiconductor crystal growth prepared by performance. Are used for thermoelectric cooling semiconductor materials is the main component of Bi, Sb, Te and Se, the highest Z-value of 3.0 ?? 10-3 / K, and the preparation of thermoelectric semiconductors with SPS Z value has reached 2.9 ~ 3.0 ?? 10-3 / K, almost equal to the performance of single crystal semiconductors [30]. Table 2 is the SPS and other methods of production BiTe material comparison.

5.3 Ferroelectric Materials

With the SPS sintered PbTiO3 ferroelectric ceramics, in 900 ~ 1000 ?? sintering 1 ~ 3min, the average particle size after sintering <1??m, the relative density of more than 98%. Because fewer holes in ceramics [31], so the basic dielectric constant between 101 ~ 106HZ does not vary with frequency.

Preparation of ferroelectric materials with SPS Bi4Ti3O12 ceramics, sintered in a coarse grain elongation and at the same time, the rapid densification of ceramic. SPS is easy to get with a good degree of grain-oriented samples, preferred orientation of grains can be observed electrical properties of ceramic Bi4Ti3O12 strong anisotropy [32].

Preparation of ferroelectric Li with SPS replacement IIVI semiconductor ZnO ceramics, the ferroelectric phase transition temperature Tc up to 470K, while the previously cold sintered ceramic only 330K [34].

5.4 Magnetic Materials

With the SPS sintered Nd Fe B magnetic alloy, sintering at higher temperatures if you can get a high density, but the sintering temperature is too high will cause the temperature is too high will result in ??-phase and grain growth, magnetic properties deteriorate. If the sintering at lower temperatures, although maintain good magnetic properties, but the powder was not fully compacted, so to study in detail the relationship between density and performance [35].

SPS sintered magnetic material when the sintering temperature with low holding time shorter process advantages. Nd Fe Co VB heat at 650 ?? 5min, can be almost fully dense sintered into a massive magnet and found no grain growth [36]. Prepared 865Fe6Si4Al35Ni with SPS and MgFe2O4 composite (850 ??, 130MPa), with high saturation magnetization Bs = 12T and a high resistivity ?? = 1 ?? 10-2?? ?? m [37].

Previously prepared by rapid solidification of soft magnetic alloy ribbons has exceeded several tens of nanometers of small grains, but not prepared alloy block, application is limited. And now prepared using SPS block the magnetic properties of magnetic alloy has reached the amorphous and nanocrystalline soft magnetic strip organizational performance [3].

5.5 Nanomaterials

Preparation of dense nano-materials more and more attention. Use of traditional hot-pressing and hot isostatic pressing and sintering and other methods to prepare nano-materials, it is difficult to ensure that can simultaneously achieve nanometer-sized grains and fully dense requirements. Using SPS technology, due to fast heating, sintering time is short, can significantly inhibit the grain coarsening. For example: The average particle size of 5??m TiN powder by SPS sintering (1963K, 196 ~ 382MPa, sintering 5min), receive an average grain size of TiN 65nm dense entity [3]. [3] cited the examples of the SPS sintering grain growth inhibition by the maximum, the obtained sintered body loose and no significant grain growth.

In the SPS sintering, although the applied pressure is small, but the role of stress in addition to activation capabilities will lead to lower Q, the discharge because of the role, but also make leaving the grain to be activated to further reduce the Q value, which will promote the crystal grains grow, so this regard, the use of SPS sintering nano-materials have certain difficulties.

But in fact have been successfully fabricated an average particle size of TiN 65nm dense entity instance. In [38], amorphous powder prepared by SPS sintering of Fe90Zr7B3 20 ~ 30nm nano-magnetic materials. In addition, the grain has been found with the SPS sintering temperature is relatively slow [7], the mechanism of nano-materials prepared by SPS and the grain growth and further research is needed.

5.6 Preparation of amorphous alloys

Preparation of the amorphous alloy, to select alloy composition in order to ensure the formation of amorphous alloys have very low critical cooling rate, resulting in a high glass-forming ability. Mainly in the preparation process in metal casting method and the water quenching method, the key is to control the rapid cooling and non-homogeneous nucleation. As the preparation of amorphous alloy powder technology is relatively mature, so many years, the use of amorphous powder at below their crystallization temperatures warm extrusion, rolling temperature, impact (explosion) and isostatic sintering curing methods to prepare large amorphous alloy block, but there are many technical problems, such as amorphous powder, the total is higher than the static hardness of powder, which suppress the poor performance, its overall performance and spin-quenched amorphous ribbon prepared by similar, it is difficult as a high-strength structural material used [39]. Visible with ordinary bulk amorphous materials prepared by powder metallurgy there are many technical problems.

SPS sintering technology as a new generation is expected to make progress in this regard, the literature [40] the use of SPS sintering system by mechanical alloying of amorphous Al-based powder to take to get a massive wafer samples (10mm ?? 2mm), magnetic amorphous alloy is 375MPa 20min insulation under 503K when prepared, containing amorphous and crystalline phase and residual phase of Sn. The crystallization temperature of amorphous phase is 533K. [41] using pulsed current 423K and 500MPa were prepared Mg80Ni10Y5B5 bulk amorphous alloy, the analysis mainly amorphous phase. Amorphous alloy Mg alloy and pure magnesium than A291D has a higher corrosion potential and lower corrosion current density, amorphous improved corrosion resistance of magnesium alloys. From a practical point of view, you can use SPS Sintering bulk amorphous alloys. Therefore, the use of advanced technology for the SPS Preparation of bulk amorphous alloys is necessary.

6 Summary and Outlook

Spark plasma sintering (SPS) is a low-temperature, short-term rapid sintering method can be used to prepare metal, ceramics, nano-materials, amorphous materials, composite materials, gradient materials. Promote the use of SPS in the new field of materials research and production play an important role.

SPS's basic theory is not entirely clear, the need for a lot of research to improve practice and theory, SPS need to increase the equipment's versatility and pulse current capacity to do larger size products; particular need to develop a fully automated production of SPS system to meet the complex shape, three-dimensional high-performance products and production needs of functionally gradient materials [42].

For actual production, the need to develop technology for SPS powder material, but also need to develop mold than the currently used materials (graphite) a higher intensity, better reuse of new mold materials to improve the carrying capacity and lower mold dies costs.

In the process, the mold temperature and the need to establish the actual temperature of the workpiece temperature relationships in order to better control product quality. In the SPS product performance testing, the need to establish standards and corresponding methods.

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