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Spark plasma sintering

Technologies for preparing metals, ceramics, nanomaterials, amorphous materials, composites, gradient materials, etc

Spark Plasma Sintering SPS ）The process is to load metal and other powders graphite In the mold made of materials such as Lower die punch A new kind of high-performance material is prepared by applying a specific sintering power source and pressing pressure to the sintered powder through discharge activation, thermoplastic deformation and cooling Powder metallurgy sintering Technology.

Spark plasma sintering is characterized by sintering under pressure, pulse current Generated Plasma And the pressure during sintering is conducive to reducing the sintering temperature 。 At the same time, the characteristics of low voltage and high current can make the powder Rapid sintering compact.

Chinese name: Spark plasma sintering
Foreign name: Spark Plasma Sintering

Abbreviation: SPS
Nature: A new technology for preparing functional materials

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preface

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along with High tech industry The development of, new materials Especially new Functional materials Type and requirement With the continuous increase, new functions of materials call for new preparation technologies.

Spark Plasma Sintering (SPS) is a new technology for preparing functional materials organization structure Controllable Energy saving and environmental protection And other distinctive features, which can be used to prepare Metallic materials 、 ceramic material , composite materials, can also be used to prepare nanometer Bulk materials, amorphous bulk materials, gradient materials, etc.

Development and application

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SPS technology is to directly connect powder particles pulse current For heating sintering, it is also called plasma Activated sintering Or plasma assisted sintering（ plasma Activatedsintering PAS or plasma assistedsintering PAS).

As early as 1930, American scientists put forward the principle of pulse current sintering, but it was not until 1965 that the 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 problems existing in this technology at that time production efficiency Low level problems, so SPS technology has not been 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 industrial production SPS third generation product with 10~100t sintering pressure and 5000~8000A pulse current. Recently, a large SPS device with a pressure of 500 t and a pulse current of 25000 A has been developed.

In recent years, many foreign universities and scientific research institution SPS sintering system is equipped one after another, and SPS is used for research and development of new materials. 1998 Sweden Purchase SPS sintering system carbide 、 oxide 、 Bioceramics And other materials.

In recent three years, China has also carried out research on the preparation of new materials by SPS technology, and introduced several SPS sintering systems, mainly for sintering nanometer material and ceramic material 。 SPS, as a new technology of material preparation, has attracted extensive attention at home and abroad.

fundamental of sintering

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Plasma and Plasma processing technology

SPS is sintered by discharge plasma. Plasma It is a state of matter under high temperature or specific excitation. It is the fourth state of matter except solid, liquid and gas. Plasma is Ionized gas , from a large number of positive and negative charged particle and Neutral particle It is a kind of criterion that constitutes and shows collectivity Neutral gas 。

Plasma is a dissociated high-temperature conductive gas, which can provide a state of high reactivity. Plasma temperature 4000~10999 ℃, gas state Molecules and atoms In a highly activated state, and Plasma gas 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 PBD, plasma and Ion beam etching Etc. At present, plasma is mostly used for oxide coating and plasma etching, and it is used to prepare high-purity carbide and nitrides The powder also has certain applications. Another potential application area It is in the sintering of ceramic materials.

The methods of generating plasma include heating, discharge and light excitation. The plasma generated by discharge includes DC discharge, RF discharge and Microwave discharge Plasma. SPS uses DC discharge plasma.

SPS device and sintering principle

SPS device mainly includes the following parts: Axial pressure Device; water-cooling punch pin Electrode; Vacuum Chamber ； Atmosphere control system (vacuum, argon); DC pulse and cooling water Displacement measurement temperature measurement , and security, etc control unit 。

SPS is similar to hot pressing (HP), but its heating mode is completely different. It is a pressurized sintering method that uses the on-off DC pulse current to directly electrify sintering. The main function of on-off DC pulse current is to generate discharge plasma and discharge Shock pressure , Joule heat and electric field Diffusion 。 During SPS sintering, the electrode is connected with DC pulse current Instantaneous discharge Plasma So that each particle in the sintering body can produce itself evenly Joule heat And activate the particle surface. Similar to self heating reaction synthesis (SHS) and microwave sintering, SPS is to effectively utilize the self inside the powder fever Sintering under action.

SPS sintering process can be regarded as the result of particle discharge, conductive heating and pressurization. In addition to heating and pressurization, which are two factors promoting sintering, in SPS technology, effective discharge between particles can produce local high temperature, which can make the surface locally melt and peel off surface materials; High temperature plasma sputtering and discharge shock can remove impurities (such as surface oxides) and adsorbed gases on the surface of powder particles. The role of electric field is to accelerate diffusion process 。

Technological advantages

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SPS The advantages of the process are obvious: uniform heating, fast heating speed, low sintering temperature, short sintering time, high production efficiency, fine and uniform product structure, and can maintain the natural state , can get high Densification And can be sintered Gradient material And complex workpieces. Compared with HP and HIP, SPS devices are easy to operate and do not require special skills. It is reported in the literature that a piece of ZrO2 (3Y) with a diameter of 100mm and a thickness of 17mm is produced/ stainless steel Gradient material（ FGM ）The total time used is 58 min, of which Heating time 28min、 holding time 5 min and cooling time 25 min. Compared with HP, the sintering temperature of SPS technology can be reduced by 100~200 ℃.

application

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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. In addition to preparing materials, SPS can also connect materials, such as MoSi2 and stone mill, ZrO2/Cermet/Ni, etc.

In recent years, the research on preparing new materials with SPS at home and abroad mainly focuses on: ceramics cermet 、 Intermetallic compound , composite materials and functional materials. Among them, functional materials are the most studied, including Thermoelectric material 、 magnetic material Functional gradient materials, composite functional materials and nano functional materials. Preparation of SPS Amorphous alloy 、 shape memory alloy 、 diamond And so on, and achieved good results.

Functionally graded material

Functionally graded material （ FGM ）The composition of sintering temperature Different, using traditional Sintering method difficult to One time firing 。 Using CVD PVD Preparation by equal methods Gradient material The cost is very high, and it is difficult to realize industrialization. Stepped stone grinding mould is adopted current density Different, so it can generate temperature gradient 。 Gradient generated in stone grinding mould by SPS Temperature field It can be sintered in a few minutes Distribution ratio Different gradient materials. At present, the gradient materials successfully prepared by SPS include: stainless steel/ZrO2; Ni/ZrO2； Al/ high polymer ； Al/ Plant fibre ； PSZ/T equigradient material.

In SHS, the electric field is large Activation effect In particular, the field activation effect can enable the materials that could not be synthesized before to be successfully synthesized, expand the composition range, and control the composition of the phase porous material , further processing and improvement are needed Densification 。 Utilize electric field similar to SHS Activation SPS technology for ceramics, composites and Gradient material The synthesis and densification of 65 nm Nanocrystalline , one less densification process than SHS. Large size FGM At present, the larger FGM system prepared by SPS is ZrO2 (3Y)/stainless steel disc, whose size has reached 100mm × 17mm.

Using ordinary sintering and hot pressing WC Additives must be added to the powder, and SPS makes it possible to sinter pure WC. Properties of WC/Mo gradient materials prepared by SPS Vickers hardness (HV) and fracture toughness They reach 24Gpa and 6Mpa · m1/2 respectively, greatly reducing the heat expansion mismatch between WC and Mo Thermal stress Cracking caused by.

Thermoelectric material

Due to thermoelectric conversion high reliability , pollution-free and other characteristics, recently thermoelectric converter It has aroused great interest and has studied many Thermoelectric conversion Material Science. through bibliography retrieval It was found that in the study of functional materials prepared by SPS Thermoelectric material There are many researches.

(1) The composition gradient of thermoelectric materials is one of the effective ways to improve thermoelectric efficiency. For example β FeSi2 is a promising thermoelectric material, which can be used for thermoelectric conversion between 200~900 ℃. β FeSi2 is not toxic, and it is very good in the air Oxidation resistance , and there are higher conductivity And thermoelectric power. Thermoelectric quality factor Higher (Z= α 2/k ρ， Where Z is the quality factor, α Is Seebeck coefficient, k is thermal conductivity coefficient, ρ Material resistivity ）, its thermoelectric conversion efficiency The higher. The experiment shows that the composition gradient prepared by SPS β FeSix( Si content Variable), ratio β The thermoelectric properties of FeSi2 are greatly improved. Examples of this are Cu /Al2O3/Cu[26],MgFeSi2[27], β Zn4Sb3 [28], tungsten silicide [29], etc.

(2) For Thermoelectric refrigeration Tradition of semiconductor material Not only strength and Durability Poor, and it is mainly prepared by single-phase growth method, with long production cycle and high cost. In recent years, in order to solve this problem, some manufacturers have adopted sintering method to produce semiconductor cooling materials mechanical strength And improve the utilization rate of materials, but the thermoelectric performance is far from reaching Single crystal semiconductor Now SPS is used to produce semiconductor refrigeration materials, and complete semiconductor materials can be prepared in a few minutes crystal growth But it takes more than ten hours. SPS preparation semiconductor thaermoelectric material Its advantage is that it can be directly processed into Wafer It does not need the cutting processing like the unidirectional growth method, which saves materials and improves the production efficiency.

Hot pressing and Cold pressing -The properties of sintered semiconductors are lower than those prepared by crystal growth method. Currently used for thermoelectric cooling semiconductor material The main components of are Bi, Sb, Te and Se, the highest Z value It is 3.0 × 10/K, while the Z value of thermoelectric semiconductor prepared by SPS has reached 2.9 ～ 3.0 × 10/K, almost equal to the performance of single crystal semiconductor. Table 2 shows the comparison of BiTe materials produced by SPS and other methods.

Ferroelectric material

When sintering ferroelectric ceramics PbTiO3 with SPS, sintering at 900 ～ 1000 ℃ for 1 ～ 3min, average sintering time particle size ＜1 μ m ， relative density Over 98%. Since there are fewer holes in ceramics HZ between Dielectric constant Basically does not change with frequency.

Prepared with SPS Ferroelectric material When Bi4Ti3O12 ceramics are sintered, the grains of the sintered body elongate and coarsen at the same time, the ceramics are rapidly densified. It is easy to get grains with SPS Orientation degree Good sample, grains can be observed preferred orientation The electrical properties of Bi4Ti3O12 ceramics anisotropy 。

Ferroelectric Li Replacement IIVI Semiconductor Prepared by SPS ZnO Ceramics, ferroelectric Phase transition temperature Tc is increased to 470K, while previously cold pressed sintered ceramics were only 330K [34].

magnetic material

Sintering Nd Fe B with SPS Magnetic alloy If sintered at a higher temperature Densification , but sintering temperature Too high will lead to too high temperature will lead to α Phase sum grain growth , magnetic properties deteriorate. If sintered at a lower temperature, the powder can not be fully compacted although it can maintain good magnetic properties. Therefore, the relationship between density and performance should be studied in detail.

SPS is sintering magnetic material It has the advantages of low sintering temperature and short holding time. Nd Fe Co V B can be sintered into nearly completely dense blocks after holding at 650 ℃ for 5min magnet No grain growth was found. The composite of 865Fe6Si4Al35Ni and MgFe2O4 prepared by SPS (850 ℃, 130MPa) has high saturation magnetization Bs=12T and high resistivity ρ= 1×10Ω·m。

Previously used Rapid solidification Preparative Soft magnetic alloy Thin tape, although it has reached dozens nanometer But it can not be made into alloy block, so its application is limited. But now the blocks prepared by SPS Magnetic alloy Its magnetic properties have reached the soft magnetic properties of amorphous and nanocrystalline ribbons [3].

nanometer material

More and more attention has been paid to the preparation of dense nanomaterials. Utilize traditional Hot pressing sintering and HIP sintering It is difficult to meet the requirements of both nanometer sized grains and complete compactness when nano materials are prepared by such methods. With SPS technology Heating speed Fast, short sintering time, can significantly inhibit grain coarsening. For example: use Average particle size Is 5 μ After SPS sintering (1963 K, 196 ～ 382 MPa, sintering for 5 min), the TiN dense body with an average grain size of 65 nm can be obtained. Some examples are cited in the literature to illustrate the process of SPS sintering grain growth Under the maximum inhibition, the sintered body produced has no porosity and obvious grain growth.

In SPS sintering, although the applied pressure is small, in addition to the effect of pressure, the activation capacity Q will be reduced, the grains will also be activated due to the effect of discharge Q value Further reduction will promote grain growth. Therefore, it is difficult to prepare nanomaterials by SPS sintering.

But in fact, it has been successfully prepared Average particle size Is an example of a 65nm TiN dense entity. In the literature, amorphous powders were sintered by SPS to prepare 20 ～ 30nm Fe90 Zr 7B3 nanometer magnetic material. In addition, it has also been found that the grains along with SPS sintering temperature The change is relatively slow, so the mechanism of SPS preparation of nanomaterials and grain growth Further research is needed on the impact of.

Preparation of amorphous alloy

stay Amorphous alloy In the preparation of Amorphous formation Critical cooling rate So as to obtain extremely high amorphous forming ability. The preparation process mainly includes metal Casting method and Water quenching method The key is Rapid cooling And control Heterogeneous nucleation 。 Because the technology for preparing amorphous alloy powder is relatively mature, for many years Crystallization temperature Go down Warm extrusion 、 Warm rolling , shock (explosion) solidification and Isostatic pressure Sintering and other methods are used to prepare bulk amorphous alloys, but there are many technical problems. For example, the hardness of amorphous powders is always higher than that of static powders, so the compaction performance is poor. Its comprehensive performance is similar to that of amorphous ribbons prepared by spin quenching, and it is difficult to be used as high strength Structural materials use. Visible common Powder metallurgy There are many technical problems in preparing bulk amorphous materials.

SPS, as a new generation sintering technology, is expected to make progress in this regard Mechanical alloying The prepared amorphous Al based powder obtained a block disc sample (10mm × 2mm) Amorphous alloy It is prepared at 375MPa and 503K for 20min, containing Amorphous phase And crystalline phase and residual Sn Phase. Its amorphous phase Crystallization temperature It's 533K. Prepared by pulsed current at 423K and 500MPa Mg 80Ni10Y5B5 bulk amorphous alloy, which is mainly amorphous phase after analysis. Amorphous Mg alloy has higher corrosion potential And lower corrosion current density Amorphization improved magnesium alloy Corrosion resistance. In practice, bulk amorphous alloys can be prepared by SPS sintering method. Therefore, it is necessary to use advanced SPS technology to study the preparation of bulk amorphous alloys.

Summary and Prospect

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Spark plasma sintering (SPS) is a kind of low-temperature, short-term Rapid sintering Method, which can be used to prepare metals, ceramics, nano materials Amorphous material 、 compound material 、 Gradient material Etc. SPS will be popularized and applied in the research and development of new materials Production field Play an important role in.

SPS Basic theory At present, it is not completely clear, and a lot of practice and theoretical research are needed to improve it. SPS needs to increase the versatility of equipment and the capacity of pulse current in order to make larger products; especially Needs development Fully automated SPS for production system To meet complex shapes, high-performance products and 3D Functionally gradient material Production needs.

For actual production, it is necessary to develop powder materials suitable for SPS technology, and also to develop powder materials that are better than those currently used Die material （ graphite ）Higher intensity and repetition Usage Better new mold materials to improve the carrying capacity And reduce mold costs.

In terms of process, it is necessary to establish the temperature difference relationship between the mold temperature and the actual temperature of the workpiece, so as to better control product quality 。 In SPS product performance testing On the other hand, it is necessary to establish corresponding standards and methods.

At present, the main domestic manufacturers of spark plasma sintering system (SPS) are:

Shanghai Chenxin Electric Furnace Co., Ltd

Shandong Guanghe Information Engineering Co., Ltd

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