**2. Fundamentals on PECS**

According to the open literatures [2, 3], the electric current activated/assisted sintering technology was pioneered by Duval d'Adrian in 1922 [4]. However, the first patent on pure direct current (DC) resistance sintering (RS) was proclaimed by Bloxam in 1906 [5, 6]. There‐ after, Taylor [7-9] developed the resistive sintering process consisting of capacitors, trans‐ formers and special switching devices. This process originated the electric discharge compaction (EDC) [10].

Inoue [11, 12] developed the first concept of the PECS technology in 1966. It introduced different electric current waveforms, i.e. low-frequency alternate current (AC), high-frequency unidirectional AC or pulsed DC. These sintering techniques were combined in one sintering process of electric-discharge sintering (EDS) [11], also known as spark sintering (SS). In SS process, a unidirectional pulsed DC or a unidirectional AC, is applied, then DC is eventually superimposed. This process led the development of current PECS technology, e.g. plasma activated sintering (PAS), spark plasma sintering (SPS), filed assisted sintering and plasma pressure compaction® (P2 C) [13].

In the late 1980s various companies started to manufacture PECS machines based on Inoue's patents. Since then, the number of the PECS applications has been extended further. In the early 1990s, Sumitomo Coal Mining Co. commercialized the new PECS apparatuses (2-20 kA DC pulse generators, 98-980 kN load cells) [14, 15]. The PECS process is schematically shown in Figure 1.

It simultaneously applies an electric current along with a uniaxial pressure in order to accelerate densification of powders with desired configuration [16]. The electric current delivered during PECS processes could in general assume different intensity and waveform which depend upon the power supply characteristics [2, 3, 14, 16].

The PECS process is characterized by the application of the pulsed electric current during sintering. The heating rate in the PECS process depends on the materials and shapes of the die/sample ensemble and on the electric power supply. Heating rates from 100 to 600 K/min can be obtained in the current PECS equipments. As a consequence, the PECS process can be in time ranges from a few ten seconds to minutes depending on the material and its size to be sintered, configuration and equipment capacity.

The temperature is measured either with a pyrometer focused on the surface of the graphite die or with the thermocouple inserted into the die. Usually, the measured temperature at the surface of the die (die temperature) is lower than that of the sample (sample temperature). The magnitude of this temperature difference depends on a number of factors such as thermal conductivity of the die and the sample, the heating rate used, the pressure used, how well the die is thermal insulated etc. [17]. The current and consequent temperature distributions within Pulsed Electric Current Sintering of Transparent Alumina Ceramics http://dx.doi.org/10.5772/59170 5

**Figure 1.** Schematic representation of the PECS process

oxides. Fundamentals of PECS were also discussed in the present chapter in order to under‐

According to the open literatures [2, 3], the electric current activated/assisted sintering technology was pioneered by Duval d'Adrian in 1922 [4]. However, the first patent on pure direct current (DC) resistance sintering (RS) was proclaimed by Bloxam in 1906 [5, 6]. There‐ after, Taylor [7-9] developed the resistive sintering process consisting of capacitors, trans‐ formers and special switching devices. This process originated the electric discharge

Inoue [11, 12] developed the first concept of the PECS technology in 1966. It introduced different electric current waveforms, i.e. low-frequency alternate current (AC), high-frequency unidirectional AC or pulsed DC. These sintering techniques were combined in one sintering process of electric-discharge sintering (EDS) [11], also known as spark sintering (SS). In SS process, a unidirectional pulsed DC or a unidirectional AC, is applied, then DC is eventually superimposed. This process led the development of current PECS technology, e.g. plasma activated sintering (PAS), spark plasma sintering (SPS), filed assisted sintering and plasma

In the late 1980s various companies started to manufacture PECS machines based on Inoue's patents. Since then, the number of the PECS applications has been extended further. In the early 1990s, Sumitomo Coal Mining Co. commercialized the new PECS apparatuses (2-20 kA DC pulse generators, 98-980 kN load cells) [14, 15]. The PECS process is schematically shown

It simultaneously applies an electric current along with a uniaxial pressure in order to accelerate densification of powders with desired configuration [16]. The electric current delivered during PECS processes could in general assume different intensity and waveform

The PECS process is characterized by the application of the pulsed electric current during sintering. The heating rate in the PECS process depends on the materials and shapes of the die/sample ensemble and on the electric power supply. Heating rates from 100 to 600 K/min can be obtained in the current PECS equipments. As a consequence, the PECS process can be in time ranges from a few ten seconds to minutes depending on the material and its size to be

The temperature is measured either with a pyrometer focused on the surface of the graphite die or with the thermocouple inserted into the die. Usually, the measured temperature at the surface of the die (die temperature) is lower than that of the sample (sample temperature). The magnitude of this temperature difference depends on a number of factors such as thermal conductivity of the die and the sample, the heating rate used, the pressure used, how well the die is thermal insulated etc. [17]. The current and consequent temperature distributions within

stand PECS for transparent Al2O3.

4 Sintering Techniques of Materials

**2. Fundamentals on PECS**

compaction (EDC) [10].

pressure compaction® (P2

in Figure 1.

C) [13].

which depend upon the power supply characteristics [2, 3, 14, 16].

sintered, configuration and equipment capacity.

the sample inside are very important to the homogeneity of density and grain size distribution of the product. Locally dense parts, at the beginning of current flow in particular, may result in locally overheating or even melting [16]. Experimental evidence of temperature distribu‐ tions with different conductivity materials have been reported in [18-24]. It has been verified that the electrical properties of the sample influence significantly the temperature distributions inside the die as well as sample inside. Thus, in a nonconductive sample (i.e. Si3N4 and Al2O3), larger thermal gradients has been sometimes observed than in the case of a conductive one (i.e. Ti and Ni), indicating that the temperature distribution within the nonconductive sample is not as homogeneous as within a conductive sample.

Current understanding of the effects of pulse current waveform on compact density in a PECS process is still incomplete. The pulse current did not affect significantly the PECS of cast-iron powder [20] and Ni-20Cr powder [21]. In the PECS process of Al powder, densification behavior is independent of pulse frequency ranging from 300 Hz to 20 kHz [25]. The applied current, however, can significantly affect the growth of the product layer in chemical reaction between Mo and Si plates [26, 27]. With the PECS process, the pulse DC current affected the growth of Nb-C system, Mo2C layer formed in Mo/C, Ti/C and Zr/C diffusion couples [28-30]. Inoue claimed that there was a frequency-dependent effect in his patent [31]. The densification rate of Fe and Ni based alloy processed by pulsed current was about 5% faster than by direct current [32].

However, the sintering mechanism of insulating oxides such as Al2O3 using the PECS method is still an on-going research area. Many papers on PECS of Al2O3 powder focused on densifi‐ cation and grain growth behavior by investigating effects of various parameters such as particle size, heating rate, sintering time, pressure and sintering temperature during the PECS process. Influences of the sintering parameters on densification and grain growth are not clear yet.

There are no reports about pulse current waveform effects on sintering behavior of Al2O3. The waveform of applied current is probably an important factor to the sintering process of Al2O3. An effect of two types of pulse current waveforms, inverter and pulsed DC, on sample temperature and densification of Al2O3 powder by using the PECS process has been clarified in [33-35]. The magnitude of the voltage peaks increased with an increase of the "OFF" time relative to the "ON" time for all of pulse power generator. Maximum voltage value of the inverter generator was higher than that of the pulsed DC generator. PECS with the inverter generator had higher sample temperature than that with the pulsed DC generator.

In PECS of Al2O3 powder, the electric current would be mostly applied to the punches and graphite die for heating up to sintering temperature. The average peak height of the 12/2 pulsed DC pattern is lower than that of the 2/6 pulsed DC pattern as well as lower than that of 40/10 inverter and 10/20 inverter pattern at the same die temperature. The inverter-type PECS had a higher voltage applied to the graphite die than the pulsed DC-type ones at the same die temperature. When the number of the "OFF" pulses increased as in the 10/20 inverter or the 2/6 pulsed DC pattern, the peak height of voltages of the "ON" pulses must have increased to keep the output power constant.

Temperature difference in Al2O3 sample is generated in PECS [33-35]. When PECS of Al2O3 sample with ϕ15 in diameter and 3 mm in thickness was conducted, temperature of sample outside was 20 - 30 K higher than that of the inside sample. The difference of inside/outside temperature using pulsed DC was approximately 10 K lower compared to the inside/outside temperature using the inverter. PECS with an inverter had a higher sample temperature than that with a pulsed DC power generator and it also higher than the die temperature. When the die temperature is increased, the temperature difference between the die surface and the sample also increases.

The sample temperature would be strongly affected by the applied current profile during the PECS process. The current flow should be strongly dependent on the characteristics of the different elements which compose the system (powder, punches, die) and, particularly, their electrical and thermal characteristics. For an insulating material, the applied current does not flow through the sample when a pulse current power is applied to the die-sample, but could only flow from one punch to the other punch via the die. The current forms a magnetic field in the near surface of punches and the die inside where is close to the sample surface, and this magnetic field affects current density [19, 33-37]. The highest current density should be located close to the sample surface as can be illustrated in Figure 2. The temperature distribution is closely related to the current distribution because the heat transfer is generated by the flow of current at the graphite die and the punches. Thus, during the PECS process, the Al2O3 powder must be sintered by the heat transferred from the die inside close to the sample surface and punches by means of heat conduction. Given that heat generation and transfer lead to a temperature distribution, temperature outside is higher than that inside the sample [33-35]. In the punch-compression direction, the die temperature is lower than the sample because the punches are in contact with water-cooled jacket and the die is cooled by radiation from the die outer surface.

process. Influences of the sintering parameters on densification and grain growth are not clear

There are no reports about pulse current waveform effects on sintering behavior of Al2O3. The waveform of applied current is probably an important factor to the sintering process of Al2O3. An effect of two types of pulse current waveforms, inverter and pulsed DC, on sample temperature and densification of Al2O3 powder by using the PECS process has been clarified in [33-35]. The magnitude of the voltage peaks increased with an increase of the "OFF" time relative to the "ON" time for all of pulse power generator. Maximum voltage value of the inverter generator was higher than that of the pulsed DC generator. PECS with the inverter

In PECS of Al2O3 powder, the electric current would be mostly applied to the punches and graphite die for heating up to sintering temperature. The average peak height of the 12/2 pulsed DC pattern is lower than that of the 2/6 pulsed DC pattern as well as lower than that of 40/10 inverter and 10/20 inverter pattern at the same die temperature. The inverter-type PECS had a higher voltage applied to the graphite die than the pulsed DC-type ones at the same die temperature. When the number of the "OFF" pulses increased as in the 10/20 inverter or the 2/6 pulsed DC pattern, the peak height of voltages of the "ON" pulses must have increased to

Temperature difference in Al2O3 sample is generated in PECS [33-35]. When PECS of Al2O3 sample with ϕ15 in diameter and 3 mm in thickness was conducted, temperature of sample outside was 20 - 30 K higher than that of the inside sample. The difference of inside/outside temperature using pulsed DC was approximately 10 K lower compared to the inside/outside temperature using the inverter. PECS with an inverter had a higher sample temperature than that with a pulsed DC power generator and it also higher than the die temperature. When the die temperature is increased, the temperature difference between the die surface and the

The sample temperature would be strongly affected by the applied current profile during the PECS process. The current flow should be strongly dependent on the characteristics of the different elements which compose the system (powder, punches, die) and, particularly, their electrical and thermal characteristics. For an insulating material, the applied current does not flow through the sample when a pulse current power is applied to the die-sample, but could only flow from one punch to the other punch via the die. The current forms a magnetic field in the near surface of punches and the die inside where is close to the sample surface, and this magnetic field affects current density [19, 33-37]. The highest current density should be located close to the sample surface as can be illustrated in Figure 2. The temperature distribution is closely related to the current distribution because the heat transfer is generated by the flow of current at the graphite die and the punches. Thus, during the PECS process, the Al2O3 powder must be sintered by the heat transferred from the die inside close to the sample surface and punches by means of heat conduction. Given that heat generation and transfer lead to a temperature distribution, temperature outside is higher than that inside the sample [33-35]. In the punch-compression direction, the die temperature is lower than the sample because the

generator had higher sample temperature than that with the pulsed DC generator.

yet.

6 Sintering Techniques of Materials

keep the output power constant.

sample also increases.

**Figure 2.** Current flows and distributions in the PECS die/punch/sample system for various pulse waveforms.

The ON/OFF pulse patterns and power generator frequency could also affect the sample temperature. In the case of inverter power waveform, high voltage with high frequency at long "OFF" time flows into die and punch gives higher heat transfer to heat sample than that of other pulse patterns. The difference in sample temperature using pulse current waveform inverter and pulsed DC could be explained by the skin effect as shown in Figure 2. In the punch/die/sample system, the back color area shows the current concentration during the PECS process. In both cases, the heat is generated only in the conductive die and the distribu‐ tion of the heat generation does not change drastically with the electric conductivity of sample. Electric current distribution is the main cause of the temperature gradient between the sample and the external surface of the die, together with radiation heat from the die surface. When the high frequency current (inverter generator) is applied to punches and graphite die, the current density near the inner surface of the punches/die should be higher than that at its center. In contrast, at low frequency (pulsed DC generator), current density would be uniformly distributed across graphite die and punches. This difference suggests that higher temperature could be achieved due to higher applied voltages, and the required energy for heating a sample with an inverter is higher than that with pulsed DC generator [19, 33-37]. The relative density as a function of the outside/inside sample temperature was discussed in [33-37]. These results show a consistent relative density increase trend with an increase in the sample temperature, independent of the applied pulse current waveforms and ON/OFF patterns. It was also revealed that the average grain size increases with an increase in sample temperature even in different pulse current waveforms and ON/OFF patterns. Densification and grain growth were predominated by sample temperature. The pulse electric current waveform had effects on the sample temperature, but did not have direct influence on the densification, grain growth and homogeneity of the sample sintered by the PECS process.
