Discharge plasma sintered nanocrystalline materials

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Nanocrystalline material A material consisting of crystals of nanoscale size (1~10nm). Because the crystal is extremely fine, the grain boundary may account for 50% or more of the whole material. The arrangement of atoms is different from the ordered crystalline state and the disordered amorphous state (glassy state). Its properties are also different from those of crystals or non-crystals of the same composition.

Nanocrystalline materials have incomparable performance than conventional materials due to their ultrafine characteristics. Nanocrystalline metal powders are insulators due to the conversion effect of metal insulators due to size. Nanometer powder can be used in thick film technology to prepare long and thin conductive tunnel, and can also be used as the dispersion phase of high dispersion material based on ceramic or polymer which is widely used in high frequency technology field. Porous nanomaterial sintered body has strong activity and is suitable for use as catalyst and high power capacitor.

There are usually two ways to prepare nanocrystals: one is to change the bulk amorphous into the bulk nanocrystals by the amorphous crystallization method, or to obtain the bulk nanocrystals by various deposition techniques (PVD, CVD, etc.); the other is to change the bulk nanocrystals into the bulk nanocrystals. The second way is by sintering the nano powder to form a mass material. Sintering densification of nanometer powder is a key process in the synthesis of nanometer block materials. Due to the particle surface, interface effect and small size effect of nanomaterials, the driving force of grain growth is very large. Therefore, how to control grain growth in sintering process by controlling sintering process has become a key issue whether nanomaterials can be obtained.

The preparation mechanism of SPS nanoblock materials
Discharge plasma sintering (SPS) technology is a new hot pressure sintering method, which originated in the United States in the 1930s and spread to Japan in the late 1960s. In the early 1990s, the technology gained renewed interest with extensive research into new materials, especially nanomaterials. In addition to the characteristics of hot pressing sintering, the main characteristics of this sintering method are that each particle inside the sintered body is heated uniformly and the surface of the particle is activated by the discharge plasma generated instantaneously, so it has very high thermal efficiency and can densify the sintered body in a relatively short time.

It is generally believed that pulsed current sintering may have the following densification pathways:
(1) Intergrain discharge (especially at the early stage of sintering) causes instantaneous local high temperatures up to thousands or even tens of thousands of degrees, causing evaporation and melting on the grain surface, and forming a “neck” at the grain contact point, thus directly promoting the sintering of materials;

(2) Under the action of pulse current, the grain surface is easy to be activated, and various diffusion effects are strengthened, thus promoting the densification process;

(3) The discharge impact pressure produces the phenomenon of high-speed diffusion, which further accelerates the sintering densification process.

It is difficult to ensure the nanometer size of grain and meet the requirement of complete compactness in the preparation of nanometer materials by the traditional methods of hot pressing sintering and hot isostatic pressing. Using SPS technology, because of its rapid heating and short synthesis time, can obviously inhibit grain coarsening. Using SPS technology, because of its rapid heating and short synthesis time, can obviously inhibit grain coarsening. SPS can quickly cool this feature is used to control the reaction mechanism of sintering process, and avoid some unnecessary reaction, defects and the structure of the powder could be preserved in the block after sintering material, in a broader sense, it is advantageous to the synthesis of dielectric materials, especially for the preparation of nanometer materials.

SPS is a new rapid sintering technology, which has been widely used in the preparation of various materials. The use of pulsed electric furnace and the use of die and head as heating element are its prominent characteristics. The pulsed current can promote the sintering of conductive powder, and its mechanism still needs further study. For non-conductive powders, many experiments show that the densification process is mainly completed by the heat transfer between the die and the head. SPS have a fast heating rate, and the mold and head act as heating elements. The efficient transfer of heat makes rapid sintering of SPS possible. SPS process can effectively inhibit crystal growth and is expected to provide a new way for the preparation of nanoblock materials. At the same time, the material, shape and size of the die and the head directly affect the temperature distribution in the die. By designing the material and shape of the die, the temperature distribution can be effectively controlled, so as to provide an effective means for the preparation of heterogeneous composite materials and complex special-shaped workpiece.

Learn More : Spark Plasma Sintering Furnace