Development and application of the hottest spark p

2022-10-03
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Development and application of spark plasma sintering technology

key words: spark plasma sintering; development; Application

Abstract: discharge plasma sintering (SPS) is a new material preparation technology with fast, low temperature, energy saving and environmental protection. This paper summarizes the development and application of SPS at home and abroad, and introduces the principle, characteristics and application of SPS in the preparation and processing of new materials

1 preface

with the development of high-tech industry, the types and demands of new materials, especially new functional materials, are increasing. The new functions of materials call for new preparation technologies. Spark plasma sintering (SPS) is a new technology for preparing functional materials. It has the distinctive characteristics of fast heating up, short sintering time, controllable microstructure, energy saving and environmental protection. It can be used to prepare metal materials, ceramic materials, composites, nano block materials, amorphous block materials, gradient materials, etc

2 the development and application of SPS at home and abroad

SPS technology is to directly apply pulse current between powder particles for heating and sintering, so it is also called plasma activated sintering PAS or plasma assisted sintering PAS in some literatures. As early as 1930, American scientists proposed the principle of pulse current sintering, but it was not until 1965 that pulse current sintering technology was applied in the United States, Japan and other countries. Japan obtained the patent of SPS technology, but failed to solve the problem of low production efficiency of this technology at that time, so SPS technology was not popularized and applied

in 1988, Japan developed the first industrial SPS device and popularized it in the field of new material research. After 1990, Japan launched the third generation SPS products that can be used in industrial production, with a sintering pressure of 10 ~ 100t and a pulse current of 5000 ~ 8000A. Recently, a large SPS device with a pressure of 500t and a pulse current of 25000A has been developed. Because SPS technology has the advantages of fast, low temperature, high efficiency and so on, many foreign universities and scientific research institutions have successively equipped SPS sintering system in recent years, and used SPS for the research and development of new materials [3]. In 1998, Sweden purchased SPS sintering system and carried out a lot of research work on carbides, oxides, bioceramics and other materials [4]

in recent three years, China has also carried out research on the preparation of new materials using SPs technology [1, 3], and introduced several SPS sintering systems, which are mainly used to sinter nano materials and ceramic materials [5 ~ 8]. As a new technology of material preparation, SPS has attracted extensive attention at home and abroad

3sps sintering principle

3.1 plasma and plasma processing technology

sps uses discharge plasma for sintering. Plasma is a state of matter under high temperature or specific excitation. It is the fourth state of matter except solid state, liquid state and gas state. Plasma is an ionized gas, which is taken out and tested by a device with a large number of positive and negative charged particles and neutral particles, and shows collective behavior

plasma is a dissociated high-temperature conductive gas, which can provide a state of high reaction activity. Plasma temperature is 4000 ~ 10999 ℃, its gaseous molecules and atoms are in a highly activated state, and the degree of ionization in the plasma gas is very high. These properties make plasma a very important material preparation and processing technology

plasma processing technology has been widely used, such as plasma CVD, low temperature plasma PVD, plasma and ion beam etching. At present, plasma is mostly used in oxide coating and plasma etching, and also has some applications in the preparation of high-purity carbide and nitride powders. Another potential application of plasma is in the sintering of ceramic materials

the methods of generating plasma include heating, discharging and optical excitation. The plasma produced by discharge includes DC discharge, RF discharge and microwave discharge plasma. SPS uses DC discharge plasma

3.2sps device and basic principle of sintering

sps device mainly includes the following parts: axial pressure device; Water cooled punch electrode; Vacuum chamber; Atmosphere control system (vacuum, argon); DC pulse power supply and control units for cooling water, displacement measurement, temperature measurement and safety. The basic structure of SPS is shown in Figure 1

sps is similar to hot pressing (HP), but the heating method is completely different. It is a pressurized sintering method that uses on-off DC pulse current to directly energize and sinter. The main functions of on-off DC pulse current are to generate discharge plasma, discharge shock pressure, Joule heat and electric field diffusion [11]. During SPS sintering, the pulse current can merge and clear the pressure plate through the powder particles; The spring particles that can meet the requirements of testing deformation and displacement are shown in Figure 2. What will cause errors in S? During PS sintering, the discharge plasma generated instantaneously when the electrode is connected with DC pulse current makes each particle in the sintering body generate Joule heat evenly and activate the particle surface. Similar to self heating reaction synthesis (SHS) and microwave sintering, SPS is sintered by effectively utilizing the self heating effect inside the powder. This direct discharge heating method has high thermal efficiency, and the dispersion distribution of discharge points can realize uniform heating, so it is easy to prepare homogeneous, dense and high-quality sintered bodies. SPS sintering process can be regarded as the result of particle discharge, conductive heating and pressurization. In addition to the two factors that promote sintering, heating and pressurization, in SPS technology, the effective discharge between particles can produce local high temperature, which can locally melt the surface and peel off the surface materials; The sputtering and discharge impact of high-temperature plasma remove the impurities on the surface of powder particles (such as removing surface oxides) and adsorbed gases. The function of electric field is to accelerate the diffusion process

4sps process advantages

sps process advantages are very obvious: uniform heating, fast heating up speed, low sintering temperature, short sintering time, high production efficiency, fine and uniform product structure, can maintain the natural state of raw materials, can obtain high density materials, can sinter gradient materials and complex workpiece, etc. Compared with HP and hip, SPS device is simple to operate and does not need special skilled technology. Literature [11] reports that the total time for producing a piece of ZrO2 (3Y)/stainless steel gradient material (FGM) with a diameter of 100mm and a thickness of 17mm is 58min, including the heating time of 28min, the holding time of 5min and the cooling time of 25min. Compared with HP, the sintering temperature of SPS technology can be reduced by 100 ~ 200 ℃ [13]

5Application of SPS in material preparation

at present, more research on the preparation of new materials with SPS has been carried out abroad, especially in Japan, and some products have been put into production. The types of materials that SPS can process are shown in Table 1. In addition to preparing materials, SPS can also connect materials, such as MoSi2 and graphite [14], zro2/cermet/ni, etc. [15]

in recent years, the research on the preparation of new materials with SPS at home and abroad mainly focuses on: ceramics, cermets, intermetallic compounds, composite materials, nano materials and functional materials. Among them, functional materials are the most studied, including thermoelectric materials [16], magnetic materials [17], functional gradient materials [18], composite functional materials [19] and nano functional materials [20]. The preparation of amorphous alloy, shape memory alloy [21] and diamond by SPS has also been tried, and good results have been achieved

5.1 functionally graded materials

the composition of functionally graded materials (FGM) is gradient changing, and the sintering temperature of each layer is different, so it is difficult to be sintered at one time by using the traditional sintering method. Using CVD, PVD and other methods to prepare gradient materials, the cost is very high, and it is difficult to achieve industrialization. Using a stepped graphite mold, because the current density at the upper and lower ends of the mold is different, a temperature gradient can be generated. Using the gradient temperature field generated by SPS in the graphite mold, gradient materials with different composition ratios can be sintered in only a few minutes. At present, the gradient materials successfully prepared by SPS include: stainless steel/zro2; Ni/ZrO2; Al/polymer; Al/plant fiber; Psz/ti graded materials

in self propagating combustion synthesis (SHS), the electric field has a large activation effect and role, especially the field activation effect can make the materials that cannot be synthesized before can also be successfully synthesized, expand the composition range, and control the composition of the phase, but the porous materials are obtained, which need further processing to improve the density. Using SPs technology similar to SHS electric field activation, the synthesis and densification of ceramics, composites and gradient materials are carried out at the same time, and 65nm nanocrystals can be obtained, which is one less densification process than SHS [22]

large-size FGM can be prepared by SPS. At present, the larger FGM system prepared by SPS is ZrO2 (3Y)/stainless steel disc, and the size has reached < 100mm × 17mm[23]。

additives must be added when using ordinary sintered and hot pressed WC powder, and SPS makes it possible to sinter pure WC. The Vickers hardness (HV) and fracture toughness of wc/mo gradient materials prepared with SPS reached 24GPa and 6Mpa · m1/2 respectively, which greatly reduced the cracking caused by thermal stress due to the mismatch of thermal expansion between WC and Mo [24]

5.2 thermoelectric materials

due to the high reliability and pollution-free characteristics of thermoelectric conversion, thermoelectric converters have attracted great interest recently, and many thermoelectric conversion materials have been studied. Through literature search, it is found that in the research of preparing functional materials by SPS, there is more research on thermoelectric materials

(1) composition gradient of thermoelectric materials is one of the effective ways to improve thermoelectric efficiency at present. For example, component gradient β FeSi2 is a promising thermoelectric material, which can be used for thermoelectric conversion between 200 ℃ and 900 ℃. β FeSi2 is non-toxic, has good oxidation resistance in the air, and has high conductivity and thermoelectric power. The higher the quality factor of thermoelectric materials (z= α 2/k ρ, Where Z is the quality factor of cosmetics and wine packaging, α Is Seebeck coefficient, K is thermal conductivity, ρ Is the resistivity of the material), and its thermoelectric conversion efficiency is also higher. The experiment shows that the composition gradient prepared by SPS is β FeSix (variable Si content), ratio β The thermoelectric performance of FeSi2 is greatly improved [25]. Examples of this are cu/al2o3/cu[26], Mg fesi2[27], β Zn4sb3[28], tungsten silicide [29], etc

(2) traditional semiconductor materials used for thermoelectric refrigeration not only have poor strength and durability, but also are mainly prepared by unidirectional growth method, with long production cycle and high cost. In recent years, in order to solve this problem, some manufacturers have used sintering method to produce semiconductor refrigeration materials. Although the mechanical strength and material utilization rate have been improved, the thermoelectric performance is far from that of single crystal semiconductors. Now, using SPs to produce semiconductor refrigeration materials, a complete semiconductor material can be prepared in a few minutes, while the crystal growth method takes more than ten hours. The advantage of SPS preparation of semiconductor thermoelectric materials is that it can be directly processed into wafers without cutting processing like unidirectional growth method, which saves materials and improves production efficiency

the properties of semiconductors prepared by hot pressing and cold pressing sintering are lower than those prepared by crystal growth method. The main components of semiconductor materials currently used in thermoelectric refrigeration are Bi, Sb, TE and Se, with the highest Z value of 3.0 at present ×/K. The Z value of thermoelectric semiconductors prepared by SPS has reached 29 ~ 3.0 ×/K. Almost equal to the performance of single crystal semiconductor [30]. Table 2 shows the comparison between SPS and other methods to produce bite materials

5.3 iron

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