Main features of SPS technology
SPS pulse current or the pressure, which has a greater impact on the sintering mechanism? The main features of SPS technology are direct Joule heating formed by pulsed DC current and mechanical pressure-assisted sintering. Figure 1 is a schematic diagram of spark plasma sintering. Compared with traditional methods (including hot pressing), one advantage of SPS is that the powder solidification time is shorter and the temperature is lower. In addition, even under extremely high thermal gradients, spark plasma sintering can obtain a uniform, crack-free microstructure. For example, ultra-high temperature advanced ceramic materials such as SiC and BC are difficult to achieve high density through traditional sintering even at high temperatures above 2200 °C without adding sintering aids. However, through SPS, it can achieve a density of more than 98% below 2000 °C, and the mechanical properties are higher than those of traditional sintering.
In terms of advanced functional ceramics, piezoelectric dielectric ceramic materials prepared by SPS have extremely high density. At the same time, the grain size can be adjusted over a large range (50 nm~50 mm) to obtain a higher piezoelectric constant and energy storage density, which effectively expands its application range. Advanced optical ceramic materials, such as transparent ceramics, require high temperatures and a lot of time to eliminate pores in the tissue due to traditional sintering. SPS can obtain transparent ceramic materials with high transparency and high mechanical properties in a few minutes through high pressure. The following will focus on the characteristics of the sintering process and the sintering mechanism research from the pulsed DC current and mechanical pressure in SPS.
Effect of SPS pulse current on the sintering process
Most SPS equipment uses pulsed direct current as the power source. Its effects on the sintering process are mainly as follows:
- Generation and action of plasma
- Effect of Joule heating on conductive and insulating samples
- Effect of electromagnetic field caused by pulsed direct current and effect of pulsed direct current duty cycle (0N/0FF rato) on the sintering process.
It does not mention or discuss the above aspects in detail in most studies on SPS. However, to truly understand the SPS sintering mechanism, the study of pulsed direct current is essential.
Generation and effect of plasma
In the study of pulsed DC current, an important role of pulsed DC is to generate plasma to purify the sample surface and grain boundaries. Omori noticed the discharge pattern on the surface of CeSiNO2, as shown in Figure 2. The surface shows a dendritic texture, which the researchers attributed to the limited location of plasma generation, which is concentrated only in the molten area. When the plasma increases the melt, this pattern is left on the surface. However, the study by Munir et al. shows that the existence of plasma must take into account other factors, such as the applied pressure and the sintering stage. So whether there is a plasma effect in SPS depends on the specific situation.
Heating and heat transfer caused by SPS pulse current
In addition to the plasma problem, heating and heat transfer caused by pulsed DC current are also very important. Because it affects the transport of current to matter and other intrinsic processes. At present, researchers have reported the existence of large temperature gradients in the mold in SPS experiments. Wang et al. and Matsugi et al. numerically simulated the temperature distribution under SPS conditions. A lot of simulation work was combined with experimental results to predict the actual temperature inside the SPS. It was proved that the actual temperature of the sample was higher than the measured temperature.
However, the results presented by conductive materials and non-conductive materials during SPS heating are completely different. When heats the conductive sample by Joule heating, the temperature inside the sintered sample is much higher than the external measured temperature. On the other hand, the hottest part of the non-conductive sample is the area close to the sintering mold. However, the measured temperature comes from the outside of the sintering mold. So the actual temperature of the sample is lower than the measured temperature.
Anselmi-Tamburini experimental analysis
Anselmi-Tamburini et al. used advanced modeling and experimental analysis to study the current distribution of two very different conductive materials, aluminum oxide and copper, under conventional SPS conditions. As shown in Figure 3, when heating aluminum oxide, since there is no current flowing through the sample, it does not cause the initial heating of the sample by the current, but by conduction through the mold. This is in stark contrast to the copper sample, where the current is passed directly through the copper sample at the start of heating, and Joule heating starts directly, resulting in a much greater current density in the sample than in the mold.
Boron carbide, as a conductive ceramic, cannot be fully dense by traditional sintering even at a high temperature of 2300℃. Ji et al. prepared fully dense boron carbide by SPS at 1700℃ and 80 MPa. The reason is that on the one hand, the densification mechanism of plastic deformation under pressure reduces the sintering temperature. On the other hand, the temperature measured during the SPS sintering of conductive samples is much lower than the internal temperature of the samples.
Effect of field effect on sintering
In addition, a series of effects caused by the change of electromagnetic field caused by pulsed DC current during SPS sintering have also attracted a lot of attention. That is the so-called field effect. Salamon et al., and Tan et al. observed the phenomenon of pressure oscillation during SPS sintering. It was attributed to the change of magnetic field generated by pulsed DC current. It affected the alternating change of magnetic force on the copper plate used to conduct current in the SPS equipment. Grasso et al. reported that the electric field strength depends on the ratio of the outer diameter to the inner diameter of the mold. It significantly affects the electric field strength passing through the sintered sample by the duty cycle (ON/OFF ratio) of the pulse current.
It shows the finite element simulation results of the sample electric field in Figure 4. Under the condition that the sintering temperature reaches 1300℃, the smaller the mold thickness, the higher the electric field strength passing through the sample (white represents a higher electric field strength). According to their conclusions, it can significantly increase the sample electric field by reducing the mold wall thickness or extending the OFF time of the DC pulse current. Holland et al. described how the polarization of the dielectric material affects the field strength on the particle surface and interface in the initial stage of sintering through a numerical model.
There is currently a lack of direct evidence for the role of field effects
Although researchers believe that the “field effect” exists and plays an important role in SPS. However, there is still a lack of direct evidence of its effect on the microstructure and properties of the material. At the same time, Anselmi-Tamburini et al. discussed an important fact that contains the sample in a highly conductive mold. Usually made of high-density graphite, and acts as an electrical load parallel to the sample itself. Therefore, although the power supply used in SPS can generate voltages up to tens of volts. But even if the sample is not conductive, only a small part of the voltage (only a few millivolts) actually exists in the entire sample. Only in very rare cases does the sample experience a strong current flow. Most of the current flow through the mold, except when the resistance of the sample is lower than that of the mold.
In the current SPS literature on the “field effect”, the role of processes related to current and electric field effects on the densification process may be overestimated. Under certain specific conditions, they may play an important role in specific materials. But at the current stage of understanding, it is difficult to point out that one of these effects is the reason for the significant enhancement of densification.
Effect of pulse mode on sintering
In addition to the study of the effect of pulsed DC current, scholars have also studied the nature of pulsed DC current, that is, the pulse mode (ON/OFF setting).
Densification and grain growth are mainly determined by sample temperature
Dang et al. studied the effect of pulse current waveform on the SPS sintering behavior of alumina. The results show that it mainly determines the densification and grain growth by the sample temperature and is not directly dependent on the pulse mode. Xie et al. studied the effect of pulse frequency on the SPS sintering process of pure aluminum powder and obtained the same results. When sintering the samples at pulse frequencies of 10 kHz, 40 kHz, and 300 kHz and under direct current (0 Hz), we find that there was no effect on the relative density, resistivity, and tensile properties of the sintered materials.
The maximum intensity of the pulse
Maniere et al. studied the current pulse characteristics and their changes over time under different temperatures, pulse mode materials, and total electrical power conditions. The results show that the maximum intensity of the pulse increases with the increase of the pulse step opening time (ON time) and the decrease of the vacancy time (OFF time). The pulse mode can play a greater role when the electrical contact between the punch and the die is at a very low pressure (less than 10 MPa). The study of Grasso et al. also confirmed this result.
Effect of pulse mode on heating efficiency
On this basis, Tan et al. systematically studied the effect of pulse mode on heating efficiency. The results showed that the lower the pulse duty ratio (ON/OFF ratio), the higher the heating efficiency. As shown in Figure 5, when SPS sintering titanium dioxide powder with different pulse duty ratios, it obtained samples with different densities and grain sizes at the same output power, which also confirmed the effect of pulse mode on heating efficiency.
Summary of the influence of SPS pulse current
In summary, the influence of SPS pulse current on the mechanism of the SPS sintering process and the latest conclusions of current research are as follows:
- Generate plasma to purify local surface and grain boundary: It does not generate the plasma in all SPS, which is related to the applied pressure and sintering stage.
- Pulsed direct current Joule heating and heat transfer: It concentrates the current in the material with low resistance. That is, in direct Joule heating of conductive materials, the internal temperature of the material is higher than the external temperature (measured temperature). The opposite is true for insulating materials.
- Field effect: The field effect exists, but the influence on the sintering mechanism of the material is not clear.
- The influence of pulse frequency and duty cycle of pulsed direct current on heating efficiency: The pulse frequency has no obvious effect on heating efficiency. The lower the duty cycle, the higher the heating efficiency.
Mechanical pressure
It can divide the influence of mechanical pressure on the SPS process into many aspects. Among them, the influence of pressure on the sintering process has been studied by many researchers. On the other hand, the pressure application time and rate are often overlooked. This section summarizes the influence of pressure application time and rate on the sintering process based on the explanation of the influence of pressure.
In terms of the influence of pressure, the maximum pressure is limited by the standard setting range of graphite molds (0~150 MPa). However, the SPS process has a great influence on the sintering temperature, densification, grain size, and mechanical properties.
Experiments
Anselmi-Tamburini et al. pointed out that the influence of applied pressure on the final density of nano-zirconia ranges from 20 to 150 MPa. However, at a sintering temperature of 1200℃ and a holding time of 5 min, the crystallite size was not affected. Anselmi-Tamburini et al. also sintered nano-zirconia and nano-cerium oxide ceramics at a pressure of 1 GPa and extremely low temperature and overcame the harmful effects of powder agglomeration. Shen et al. obtained nano-alumina at different pressures (50MPa, 100MPa, and 200MPa), and the hardness and fracture toughness at all pressures were similar.
In addition, to break through the pressure limitation of graphite molds, Balima et al. designed and built a new system in a belt SPS device by using a large-capacity chamber and different metal calibers, making the pressure as high as 6 GPa. One application direction of high-pressure SPS (HP-SPS) is to prepare transparent ceramic materials.
Grasso et al. obtained transparent pure alumina (transmittance 64%) with an average grain size of 200 nm at a pressure of 500 MPa. Zhang et al. obtained transparent (transmittance 68%) yttrium oxide at a pressure of 300 MPa. Sokol et al. obtained transparent magnesia quartz at a pressure of 350 MPa, with a transmittance of up to 84%.
In terms of sintering theory, pressure has both intrinsic and extrinsic effects on sintering. Fundamentally, the former involves an increase in chemical potential, which affects diffusion-related mass transfer, as shown in the formula:
μ1=μ0i – σnΩ1
Where: μ1 is the chemical potential of the particle interface under stress. μ0i is the standard chemical potential. σn is the normal stress at the interface. Ω1 is the atomic volume of the diffusing substance.
Pressure has an intrinsic effect on other processes.
These include viscous flow, plastic flow, and creep. Externally, pressure affects particle rearrangement and the destruction of agglomerates in the powder. The latter plays an important role in the consolidation of nanopowders. Makino et al. studied the effect of pressure on the sintering of ultrafine α-alumina powders under SPS conditions. The inhibition of grain growth is enhanced when the powder is sintered under high pressure.
In terms of the timing of pressure application, Guillard et al. studied the role of SPS parameters in the densification process of SiC. Applying the pressure at two different temperatures of 1000 ℃ and 1800 ℃ as shown in Figure 5. The results show that the effect of pressure depends on the temperature at which the pressure is applied. In a similar study, Chaim and Shen showed that the timing of pressure application did not affect density when sintering Nd-yttrium-aluminum garnet (YAG) nanopowders. However, the effect on grain size is extremely complex. When applying the pressure at the final sintering temperature if the temperature is below 1375 ℃, the grain size is smaller. However, when sintered at higher temperatures, the grain size was larger. When applying the pressure at lower temperatures (1200°C), the grain size was independent of the sintering temperature.
Pressure and temperature difference between mold and die
Another aspect of the pressure effect is related to the temperature difference between the sample and the die. Grasso et al. showed that an increase in pressure leads to a significant decrease in the temperature difference between the two locations shown in Figure 7. They attributed this to a decrease in the electrical and thermal contact resistance of the punch-die interface. This is also caused by the Poisson deformation of the punch with higher pressure.
In addition, Xu et al. also studied the rate of pressure application. In the densification process of yttria-stabilized zirconia (YSZ), we found that the rate of pressure application had no significant effect on the densification rate and final density of the sample.
Summary of the influence of mechanical pressure
Summarize the effect of mechanical pressure on the sintering mechanism during SPS and the current research conclusions as shown.
- Strengthening mass transfer during sintering: The greater the pressure, the lower the densification temperature, the finer the grains, the higher the mechanical strength, and the better the performance.
- Particle rearrangement and destruction of powder agglomeration: Enhanced inhibition of grain growth in nanopowder sintering.
- Effect of pressure application timing on density and grain size: No effect on density, but the effect on grain size is extremely complex.
- Temperature difference between sample and mold: The greater the pressure, the lower the contact resistance and the smaller the temperature difference.
- Effect of pressure application rate on densification: No significant effect.
This article is excerpted from：
Rapid Sintering Techniques of Advanced Ceramic Materials: A Review
TAN Hua, NAN Bo, MA Weigang, GUO Xin, LIU Jing, YUAN Qi, YANG Tingwang, LU Wenlong, ZANG Jiadong, LI Haoyu, YAN Wenchao, ZHANG Shengwei, LU Ya, ZHANG Haibo
(1. State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology. 2. Guangdong Huazhong University of Science and Technology Industrial Technology Research Institute. 3. Shenzhen Geekvape Technology Co., Lad,.)
