Terminology related to SPS technology:
1) SPS(Spark Plasma Sintering):
The original SPS technology was innovated in Japan in 1960s. After going through 2nd and 3rd generations, substantial enhancement was made by Dr. M. Tokita to be more practical and suitable for actual use in the R&D of new materials in 1990s.
After 2001, the 4th generation of SPS systems entered the stage. Now, with very speedy progress, we have just started supplying its 5th generation systems which are applicable to actual production processes in versatile industries.
SPS can be also called as PECS(Pulse Electric Current Sintering) technology in a broad sense. It is a part of single axis pressurizing sintering. But it also utilizes ON-OFF of direct pulse current which brings about spark electric discharge in the initial stage with the effect of cleansing and activation of the grain surface of the materials. In the middle and late stage of SPS process, effect of heat diffusion and electric field diffusion by means of joule heating and electromagnetic energy appear, which promotes the progress of densification remarkably.
With SPS, we can perform “Rapid Sintering”, “Reaction Sintering” and “Microstructure Control Sintering” through self-heating from inside of powder materials themselves as well as rapid increase of temperature.
2) Electrical Discharge, Spark Discharge:
The phenomenon of electrical discharge is basically the breakdown of insulation. It undergoes the stages of “Undercurrent”, “Corona discharge”, “Spark Discharge” and “Arc discharge”. Under the conditions of electrical discharge, liquid and even solid bodies of all materials will be evaporated. They are further decomposed into molecules and atomic elements and will become plasma (ionization) state. Generally, duration of spark discharge is about 10^-7 to 10^-5 sec. and electrical gradient is 10^5 to 10^6V/cm. Also it is said that current density will become such high level as 10^6 to 10^9A/cm^2, which brings about extremely high energy release per unit time with mechanical pressure, high temperature, electromagnetic waves, light and sounds. The rate of energy variability dƐ/dt (Ɛ: discharge energy, t:time), and the current variability dI/dt (I: discharge current) of spark discharge is highest amongst all electrical discharging stages.
3) Functionally Graded Materials:
It is generally called as “FGMs” as the abbreviated expression. This concept was borne out by the researchers of materials in Sendai area of Japan (north east) in around 1984 to 85. It is now considered as the “Oneiric New Material”. The degree of research level of Japan in this field is the highest in the world. The FGMs are materials of which composition, structure and physicality are changed continuously or in respective stages. It shows totally different characteristics from uniform material of original state.
Supposing we have a plate which has two faces i.e. observe and reverse sides. The one side has characteristics of metal and the other side ceramic, for instance. If we can control it to create the state of the border not distinctive but very smoothly changing from one to the other, it is very effective in various applications.
Let’s take an example of a space shuttle. They used to put ceramic panels on the outside walls of the shuttle in order to avoid melting of body metal by high temperature caused by high speed friction in the aerosphere. But as far as we put ceramic tiles by adhesives, its strength can’t be kept ideally, because the strength of such friction becomes enormously high so that ceramic tiles would be removed away as we saw accidents took place in the past.
However, if ceramic is melt into metal body, strength of ceramic walls against the friction will be remarkably increased to avoid abruption.
Under the circumstances, the national project of FGMs has started recently in Japan receiving assistance of Science and Technology agency of Education ministry. Later on, this project was taken up by the Ministry of Education, Culture, Sports Science and Technology itself as the project of primary importance for which maximum budget would be allowed to support its progression. Also it is taken up as the national project by the Ministry of Economy, Trade and Industry in its subordinate organization NEDO (New Energy and Industrial Technology Development Organization) Currently, the world wide researchers are starting to take actions in this field positively.
4) Nano-phase materials:
“Nano” means 1/1,000,000,000. So 1 nm (nanometer) is 1/1,000 micron meter, which is the realm of ultimate fineness. In case of some materials such as metals or ceramics, we observe special characteristics of physicality such as super plasticity, high strength, high tenacity, high electronic properties etc. if their individual unit cell of crystal is divided into nano-meter size.
In the field of sintering technology, following materials are called as “Nano-phase materials” or otherwise as “nano-materials”:
i. Material sintered from powder of which grain is nano-size.
ii. Material powder has diameter of ten to hundreds micron meter. But its grain has crystal
structure of nano-size.
iii. Composite materials having dispersed phase of nano-size.
5) Temperature Gradient Sintering:
In case of a conductor with resistivity”α”, Resistance (R) is proportional to its length (L) and is inversely proportional to its cross sectional area(S). R=α(L/S)
On the other hand, the relation between “workload (W)”, “time (t)” and “electric current (I)” is formulated as:
W=12 x R x t.
Consequently, if we apply electric current to graphite dies having different cross sectional areas, smaller part generates more heat than larger one. Utilizing this nature, we can apply high end and low end of ideal sintering temperature against materials such as metals or ceramics simultaneously. This is a unique method available only with SPS and is especially effective for the creation of FGMs.
6) Size effect and Shape effect of SPS:
In case of SPS, pulse current flows intensively through the selective area. Therefore, more pulse current tends to flow through the center and/or the edge area of pressed powder or sintered materials rather than the other area.
On the other hand, pressing power and current distribution during sintering is very much different between shapes with or without corners.
Therefore, the optimum sintering conditions between small job (20 to 30mm dia.) and large ones (50 to 200mm dia.) are different from one to another. This is called “Size and Shape effect of SPS”.
7) Bulk Density:
This means the density per unit volume when powder materials are filled into prescribed cubic capacity.
8) Relative Density:
This is the rate of “measured density”/“theoretical density” expressed in (%). The latter is the density when the powder material has become completely dense. This ratio is referred to find the extent of sintering or state of pores.
9) Grain Growth:
Grains of material themselves bond with each other and grow their size with higher density and progress the sintering process. In solid phase sintering, this means the growth of powder crystals itself. In liquid phase sintering, it means the growth of solid phase crystal of original material.
Powder materials are arranged in the graphite dies with receiving pressure in the first stage. While sintering proceeds, their location shifts gradually due to densification and shrinkage reaction. This is called as “Rearrangement of grains”.
11) Plastic Flow:
This is the deformation of solid or grain materials due to change of plasticity.
12) (Crystal) Grain Boundary:
There are border(s) between different grains having different crystal directions within the same sintered body of poly crystal having same composition and same crystal structure.
This is the movement of atomic element which comprises the crystal, through vacancy of solid body.
14) van der Waals’ force:
This is attraction force among grains and/or particles of powder materials, which may be caused by densification.
15) SPS sintering temperature:
In the sintering processes, this is the temperature of the last process, with which dense sintering result can be obtained. In the SPS, we measure the temperature of outer walls of graphite dies, which is called “Sintering Temperature”. This is different from the inner temperature of the actual sintering operation itself. In general, this “Sintering Temperature” is lower than inner temperature for about 100 to 250 deg. C.
16) Sintering Time:
In the conventional sintering process, this means the time to keep the maximum temperature with which required result of sintering can be obtained. The time to heat the temperature to the target level is called “Heating-Up Time” while that to cool down the temperature is called “Cooling Time”.
On the other hand, in case of SPS, we occasionally call “Sintering Time” which includes both of heating and keeping time together since they are comparatively much shorter than those of conventional sintering methods. This feature leads to lower running cost of SPS.
This is the ratio of total cubic volume of open as well as closed pores in the total volume of a porous medium.