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焼結の専門用語集

SPS TECHNICAL TERMS

1. SPARK PLASMA SINTERING
Spark Plasma Sintering frequently called SPS was first born and developed in Japan during the 1960's. This first generation of SPS was further developed till Sumitomo Coal Mining Corporation, Ltd introduced the third generation which brought the technology to the application field.
The current fourth generation was introduced in 2001 as the Pulse Electric Current Sintering (PECS).
SPS is one type of axial pressure sintering technique. The principle of sintering with SPS is that a large pulse ON-OFF direct electric current is applied to the powder being sintered. The initial electrical current contact with powder particles causes an electric discharge. This latter is believed to clean up the particle surface and render them more active toward sintering reaction. The current loss at grain boundaries generates Joule heat which diffuses throughout the powder particles and enhances the densification of their structure.
2. Electric Discharge, Spark Discharge
Electric discharge is an electric breakdown phenomenon. In the process of the breakdown, first a dark current is formed. This latter develops to a crona which in turn develops to a spark discharge and then to an arc discharge. Matter from the body being discharged sublimates to form a gas. The gas dissociates to form a plasma of atoms and molecules. The discharge liftetime is about 10-7-10-5 sec, an electrical potential gradient is about 105-106V/cm . The current density reaches instantenous values of 106-109A/cm2 . This large energy release may induce large mechanical pressure, plasma formation, etc...
3. Functionally Gradient Materials
Generally called FGM. They were first presented between 1984-1985 by a researcher group at Senday city. The compositon, microstructure, properties etc.. of these materials are continously or gradually controlled attributing them new functions that cannot be found in ordinary materials. For example a plate made of one side from a metal and of the other side from a ceramic. The metal and ceramic side are joined in a way so that no clear boundaries do exist.
4. Nano-phase materials
Nano is an expression of part per billion size. One nanometer (1nm) is one meter (1m) divided by one billion. Or one maicrometer (1μm) divided by one thousand. If the particles forming metals, ceramics etc..are nanometer down sized, superplastic, high-intense, high ductile and high electronic materials can be fabricated.
Nano-phase materials are also called nano-materials.
5. Temperature Gradient Sintering
The resistivity (α) of a conducting body having an electrical resistance (R) is porportional the body length(L) and inversily proportional to its surface cross section(S). R=α(L/S). The work (W) performed by the current(I) at time(t) is expressed by W=I2.R.t=I2.α.(L/S). That is, if a current pulse is applied to a graphite die having different cross sections, the cross section with smaller surface will generate more heat than that the larger one. Cross section surface can be used to control the sintering temperature of the body at a desired point.
6. SPS size and shape effect
Theoritically, the electrical pulse current tends to flow and concentrate in the more conducting part of the body. The pulse current flowing through the body center of the pressed powder and that flowing at the powder surface due to skin effect is generally larger than that flowing through other parts of the body. Also, the pressure transmitted to the body being sintered through the die wall resistance depends on the shape of the particles and particle boundaries. All these factors effect the pulse current and make it non uniform. This non uniformity is enhanced the body size. The the non uniformity in a sample with φ20-30mm is different from that of 50-200mm. All these factors we call as SPS size and shape effect.
7. Bulk density
Is the ratio of the body weight to the body apparent volume.
8. Relative density
Is the ratio between the theoritical density and the measured (bulk) density. Theoritical density is calculated for particles perfectely densified. That is ratio of the mass of a unit crystal structure to its volume.
9. Grain growth
Powder particles intereact between each other leading to a growth in the particle size itself and hence to the desificiation and sintering of the powder. In solid particles, the growth is the crytalline particle size growth itself. In liquids, it is the growth of mother solid phase.
10. Particle rearrangment
In SPS, powder filled in the die is pressed and arranged along the pressure axis. As the sintering proceeds, particles densification and shrinkage occur, leading to particle movement along and out of pressure axis. This is termed rearrangement.
11. Plastic flow
Solid and particles deform due plastic flow.
12. Grain boundary
Is the boundary between particles forming the polycrystal.
13. Diffusion
Atoms forming the structure of the crystal move in the solid from space point to another.
14. Van der Wall's force
Is the attracting force working between small particle cluster composing the solid body.
15. SPS Sintering Temperature
Is the temperature allowing enough densification of the sintered body during the sintering process. In SPS, the temperature at the surface of the graphite die is measured. This temperature is called the sintering temperature. But it is not the real temperature of the powder being sintered inside the die. Generally, SPS sintering temperature is measured to be between 100-250℃ lower than the temperature of the powder being sintered.
16. Sintering time
In conventional sintering techniques, it is the holding time at the highest temperature at which the desired propertie can be achieved. The time requied to heat up from room temperature till the highest temperature is the heatging up time. The time to cool down the body being sintered is the cooling down time. In SPS, the heating up and holding time are short so that the total of both represents the sintering time.
17. Porosity
Represents the ratio of the closed and open pores volume to that of the volume of the porous body.
18. Open and closed pores
Open pores are pores with outmost surface open and linked to the surrounding media. Contrarely, closed pores are pores with outmost surface closed and unlinked to the surrounding media.
19. Creep
It is the tendency of a solid matter to permanently deform or move slowly under a constant external stress. It is a plastic deformation of the solid matter.
20. Green body
It is the state of the pressed powder at the preheating stage.

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