Pulsed Electric Current Sintering of Transparent Alumina Ceramics

Aluminum oxide (Al2O3) commonly referred as to alumina is one of the most widely used as engineering oxide ceramics. From crystalline structure difference, there are many forms of Al2O3 (α, χ, η, δ, θ, γ and ρ), with α-Al2O3 being thermodynamically the most stable form. An example of α phase of Al2O3 is corundum or sapphire [1]. In the present chapter, α-Al2O3 is discussed and described as Al2O3. With a high melting temperature, chemical stability, Al2O3 is leading to applications as high-temperature components, catalyst substrates and biomedical implants. Al2O3 has excellent optical transparency and along with additives such as chromium and titanium, it is important as a sodium lamp (sapphire), a gem stone (sapphire and ruby) and a laser host (ruby).

oxides. Fundamentals of PECS were also discussed in the present chapter in order to understand PECS for transparent Al 2 O 3 .
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® (P 2 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 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 distributions with different conductivity materials have been reported in [18][19][20][21][22][23][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. Si 3 N 4 and Al 2 O 3 ), 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, Mo 2 C layer formed in Mo/C, Ti/C and Zr/C diffusion couples [28][29][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 Al 2 O 3 using the PECS method is still an on-going research area. Many papers on PECS of Al 2 O 3 powder focused on densification 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 -Position measurement system -Atmosphere control system -Water-cooling system -  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 Al 2 O 3 . The waveform of applied current is probably an important factor to the sintering process of Al 2 O 3 . An effect of two types of pulse current waveforms, inverter and pulsed DC, on sample temperature and densification of Al 2 O 3 powder by using the PECS process has been clarified in [33][34][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 Al 2 O 3 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 Al 2 O 3 sample is generated in PECS [33][34][35]. When PECS of Al 2 O 3 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][34][35][36][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 Al 2 O 3 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][34][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. 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 distribution 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][34][35][36][37]. The relative density as a function of the outside/inside sample temperature was discussed in [33][34][35][36][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.

Sintering of transparent polycrystalline alumina
Transparent polycrystalline Al 2 O 3 has increasingly become the focus of recent investigations primarily because of their unique combination of properties. Single crystals of Al 2 O 3 are highly transparent in visible and IR region. However polycrystalline Al 2 O 3 ceramics are usually opaque because of light scattering of pores and grain boundaries as well as impurities. High density is the most important factor to produce polycrystalline transparent ceramics, as well as grain size. Because of the high efficiency of pores for light scattering, transparency in polycrystalline materials requires extremely low level in porosity, less than 0.01 vol.%. Samples with such low porosity could only be produced under proper sintering conditions involving high temperatures and long sintering time. Residual porosity is much more important than grain boundaries for obtaining the transparency, even in crystallographically anisotropic materials in optical properties. The scattering efficiency for spherical pores, however, decreases dramatically when the pore size in the nanometric range could be achieved [38][39][40]. It is believed that nanostructured polycrystalline materials would possess higher transparency than ones with the micrometric grain size range. The sintering process at high temperature causes extensive grain growth and then seriously degrades the mechanical properties of the material. What is more important, the higher/bigger grain size larger than 410 μm leads to significant light scattering coming from the birefringence of coarse Al 2 O 3 grains [41].
Recently, fine-grained transparent polycrystalline Al 2 O 3 has attracted much attention due to its superior mechanical and optical properties. This material is prepared by sintering using HP and HIP at low temperature ranging from 1150 to 1400°C. The formation of nanostructure (< 1 μm) results in a significant improvement in both the mechanical strength and the optical transparency. It is reported that the mechanical strength of the fine-grained transparent Al 2 O 3 is reached up to 400 -600 MPa together with a high in-line transmission up to 60 % for visible light [41,45]. Thus far, the addition of small amount of MgO is known to suppress normal and abnormal grain growth.

Transparent Polycrystalline Al 2 O 3 Produced by using PECS
Many reports on PECS for sintering transparent Al 2 O 3 have been published as well as other transparent polycrystalline oxides. As process technology for ceramic powder is progressed, oxide ceramic powders with fine grain size and less agglomeration have been developed. Transparent polycrystalline Al 2 O 3 with fine grains have been able to be prepared with such advanced oxide powder by using PECS.
Recently transparent oxide ceramics with fine grains such as 300 nm have been reported with different the PECS techniques as well as Al 2 O 3 . Munir and his colleagues promote PECS with ultra-high pressure such as 500 MPa [82]. High-pressure PECS is effective for preparing highly transparent polycrystalline Al 2 O 3 [83], and also Y 2 O 3 -doped ZrO 2 [82] and Y 2 O 3 [84]. Highpressure PECS is very useful for eliminate closed pores. However sample size is likely limited in high-pressure PECS.
Kim et al. proposed slow-heating PECS for densifying Al 2 O 3 with less grain growth [57]. PECS with slow healing rate is available for not only Al 2 O 3 but also MgAl 2 O 4 [85]. Kim studied kinetics of densification and grain growth with stress rate in the point of view on "dynamic grain growth" [86]. He mentioned that slow stress rate in PECS is preferred in order to densification of Al 2 O 3 with less grain growth. On the other hand, Makino and his colleagues reported that transparent polycrystalline Al 2 O 3 can successfully obtained by PECS with fast heating rate such as 200 K/min [73]. However transparency of the sample with fast heating rate was not good in homogeneity. In order to densify Al 2 O 3 without significant grain growth, influences of heating rate is still in discussion. Goto and his colleagues reported PECS of transparent Lu 2 O 3 with two-step pressure profile [87]. Lu 2 O 3 is one of the candidates on laser host materials for high-power and ultra-short pulse lasers. However it is difficult for densification by conventional sintering. Taking account of advanced studies on transparent oxides given by Kim and Goto, a sintering profile is very important even in a process of PECS.
Thus PECS provides transparent polycrystalline oxides. Besides the oxides described here, there are many examples of transparent oxides sintered by using PECS. Table 1 shows a variety of transparent polycrystalline oxides prepared by using PECS.

Two-step PECS for transparent polycrystalline alumina
The authors study PECS with two-step temperature profile, that is, two-step PECS (referred as to TS-PECS), in order to fabricate transparent oxide ceramics with fine grains [74,75]. Figure  3 shows the sintering profile of TS-PECS with other PECS techniques. TS-PECS can provide well-transparent oxides with shorter sintering period in comparison with slow-heating PECS. Figure 4 shows appearance, fracture surface and density of polycrystalline Al 2 O 3 prepared by using TS-PECS with 1 st different temperature for 60 min and 1200°C for 20 min under 100 MPa in vacuum. A sample prepared by slow-heating PECS at 1200°C is shown for comparison. Importance of the 1 st step temperature can be understood in Figure 4. The sample sintered at 1000 o C in the 1 st step has high transparency and less grain growth. The meaning of the 1 st step is densification without significant grain growth. Sintering at 1000 o C can provide densification without grain growth, however, full densification cannot be achieved. In order to reach to the full densification of the sample, the 2 nd step with higher sintering temperature is necessary.     This is caused by the existence of macroscopic defects as large as a few tens micrometers. Figure 6 shows an optical microscopic image of the inside of the transparent Al 2 O 3 prepared by TS-PECS. Many black dots are observed in the sample. Figure 7    In particular e agglomeration of the initial particles is very important in even PECS for structural ceramics and transparent ceramics