Current distribution in samples and graphite molds
For non-conductive samples, since the sintering current will not pass through the insulator material, the process of local discharge excitation plasma to promote sintering will be difficult to achieve. At this time, to better understand the mechanism of spark plasma sintering. It is essential to explore the current distribution in the sample and graphite mold during the sintering process.
Munir et al. used a combination of simulation calculation and experimental analysis to study the distribution of sintering current and temperature under spark plasma sintering conditions. As shown in the figure below, the distribution of the sintering current is directly related to the conductivity of the sample. For the non-conductive sample Al2O3, the sintering current bypasses the sample and passes through the graphite mold. For the conductive sample Cu, most of the sintering current passes through the sample. From the axial (Z-axis) point of view, the density of the sintering current in the Cu sample is significantly higher than that in the Al2O3 sample.
Radial distribution of sintering current
Figure 2 shows the radial distribution of sintering current in Al2O3 and Cu samples. In the non-conductive sample Al2O3, the sintering current mainly exists in the graphite mold and hardly flows inside the sample. The maximum sintering current is located near the inner surface of the graphite mold. In the graphite mold, the gradient of the sintering current from the inside to the outside is large. In the conductive sample Cu, the density of the sintering current in the sample is significantly higher than that in the graphite mold. There is a sintering current gradient inside the sample, and a large amount of sintering current is concentrated on the side of the sample. In the graphite mold, the gradient of the sintering current change is small.
(a) Al2O3 and (b) Cu samples
The above results show that during the spark plasma sintering process, the distribution of sintering current in conductive samples and non-conductive samples is significantly different. For the former, the sintering current mainly flows in the sample. For the latter, the sintering current mainly flows in the graphite mold. Therefore, when preparing non-conductive samples by spark plasma sintering, heat the samples with the graphite mold.
Axial distribution of sintering current
Figure 3 is the axial current distribution curve (along the z-axis pressure direction) of spark plasma sintering conductive and non-conductive materials. As we can see from the figure, the current density in the non-conductive Al2O3 sample is lower than that in the conductive Cu sample. The maximum current density is at the exposed graphite pressure head at both ends.
The temperature distribution during sintering significantly affects the microstructure and properties of the final sintered sample.
Anselmi-Tamburini et al. simulated and calculated the axial temperature distribution of non-conductive Al2O3 material as a function of sintering time. As shown in Figure 4. They also obtained similar results by simulating the axial temperature distribution of conductive Cu material. This shows that whether it is spark plasma sintering conductive or non-conductive materials, heat is mainly generated in the graphite mold. The conductivity of the sample has no obvious effect on the distribution of heat.
Figure 5 is the radial temperature distribution curve of non-conductive Al2O3 and conductive Cu samples (current 1000A, sample thickness 3mm) calculated by Anselmi-Tamburini et al. through simulation. From the figure, we can see that the radial temperature is not stable. There is an obvious temperature gradient from the center of the sample to the outer surface of the mold, and the temperature gradient of non-conductive materials will be larger.
As an advanced rapid sintering technology, spark plasma sintering in the processing and synthesis of new materials due to its unique sintering mechanism. Such as the preparation of intermetallic compounds, amorphous alloys, gradient functional materials, etc. It can also be used to regulate the pore structure of porous materials and maintain the nanostructure in bulk materials. Its characteristics of a fast heating rate and short sintering time effectively inhibit the coarsening of the structure in the traditional sintering process. At the same time, it can also avoid some side reactions caused by long sintering times.
You might be interested in:
