Jan. 25, 21
Spark plasma sintering (SPS) is a rapid sintering method, which uses axial force and pulse current to heat and solidify powder at high speed under low atmospheric pressure. This heating method can apply a very high heating temperature and greatly increase the density (see the figure below), keeping the inherent properties of nano powder in its fully dense product.
The basic structure of SPS system is shown in the figure below. The system mainly includes vertical pressure system, water cooling system, atmosphere control system, pulse current generator and controller. Under the combined action of axial pressure and pulse current, the temperature of powder materials stacked in the sintering mold cavity rises rapidly to 1000 ~ 2500 ℃ higher than the ambient temperature, so that high quality sinter can be produced in a few minutes.
SPS process is based on the phenomenon of electric spark discharge. High energy and low voltage pulse current can generate high temperature between local particles instantaneously, resulting in spark plasma discharge, so as to produce the best thermal and ionization diffusion. SPS sintering temperature ranges from low temperature to over 2000 ℃, which is 200 ℃ to 500 ℃ lower than conventional sintering. The sintering mechanism of SPS mainly includes the following processes
1. Plasma generation
It was initially studied by Inoue and the inventor of SPS process that high temperature spark or even plasma was produced between particle contacts by pulse current, which is also the reason why the process was named. In the later research, Inoue et al thought that the "pulse pressure" caused by the contact ionization of particles caused by spark discharge promoted the diffusion of atoms at the contact. Groza et al. Proposed that pulsed current has cleaning effect on the surface of particles, but there is no oxidation between particles.
Whether plasma is produced or not has not been directly confirmed by experiments. Therefore, there is no conclusive evidence to prove the formation of plasma in SPS process. The occurrence of plasma discharge is still controversial, but it is generally believed that the discharge will occur at the micro level.
2. Electroplastic effect
It is believed that a large number of free electrons with directional drift will be generated when pulsed current passes through metal materials. When the drift electron group frequently strikes the dislocation, an electron wind force similar to the applied stress will be generated on the dislocation segment, which promotes the dislocation to move on its slip surface. At the same time, when the pulse current is applied, the electric energy, thermal energy and stress are instantaneously input into the material. The random thermal motion of the atom can obtain enough kinetic energy to leave the equilibrium position under the instantaneous impact force of the pulse current. The diffusion ability of the atom is strengthened, and the dislocation is easier to slip and climb, thus improving the plasticity of the metal.
3. Joule heating
Joule heating due to current passing through the particles helps to weld the particles under mechanical pressure. The strong Joule heating effect on the surface of conductive particles usually reaches the boiling point, resulting in local evaporation or cleaning of the powder surface.
4. Pulse current
In the sintering process, the pulse current will produce: spark plasma, spark shock pressure, Joule heating and electric field diffusion effect.
Compared with the traditional electric sintering process, the surface of powder particles in SPS process is easier to purify and activate. At the micro level, atoms, dislocations and vacancies are easier to move, and at the macro level, materials are easier to deform. In a short time, it is easier to obtain high quality sinter at lower temperature. The figure below shows how the pulse current flows through the powder particles in the SPS sintering mold.
SPS process is a kind of electric sintering technology, which applies the pulse current from pulse generator to the particle powder (see the figure below). In addition to the above factors, the effective discharge between the powder particles also promotes the sintering.
The high temperature sputtering phenomenon caused by spark plasma and spark shock pressure eliminates the adsorbed gases and impurities on the surface of powder particles.
5. Mechanical pressure
When the spark discharge occurs in the gap or at the contact point between the material particles, a local high temperature state (discharge column) of tens of thousands of degrees centigrade is generated instantaneously. This results in evaporation and melting on the surface of the powder particles during SPS, and a "neck" is formed around the contact area between the particles. The figure below shows the basic mechanism of spark plasma neck formation.
Figure a shows the initial behavior of the neck formation due to the spark in the plasma. The heat immediately transfers from the center of the spark discharge column to the surface of the sphere and diffuses, which makes the intergranular bonding part cool rapidly. As shown in Figure B, it shows several necks, and the pulses cause spark discharge one after another between the particles. Even for a single particle, with repeated discharge, the number of neck forming positions between adjacent particles increases. Fig. C shows the state of SPS sintering grain boundary, which has plastic deformation after further development of sintering.
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