M Noren, Z Shen/Solid State Sciences 5(2003)125-131 (iii) The use of fast heating and cooling rates; (vi) The use of a pulsed direct current, implying that the samples are also △-3YZo2 exposed to an electric field It is generally accepted that application of mechanical pressure is helpful in removing pores from compacts and 0.8 enhancing diffusion. The heat transfer from the die to the 8 compact is very efficient in these processes, because the die 9 medium when used)acts as heating element, so that the ther- 20.7 mmm mal energy necessary for compaction is efficiently transmit- ted to the sample. However, it is frequently argued that the improved densification rates stem mostly from the use of DC pulses of high energy. Thus it was originally claimed by the process inventors that the pulses generate spark and 400 001400 even plasma discharges between the powder particles, which Temperature(C) is why the processes were named spark plasma sintering and plasma activated sintering. During the initial part of the sin- △-3Y-Z2 of cleaned and activated surfaces is expected to enhance he grain boundary diffusion which, together with the pro- posed spark discharges and/or plasma processes, will pro- and grain growth [2, 3). Murir and Smaltzrield argue that the 2 0.002 application of an electric field enhances grain growth and erefore also densification 5] 001 Whether plasma is generated has not yet been confirmed. specially when non-conducting ceramic powders are com- 0.000 pacted. It has, however, been experimentally verified that the densification is enhanced by the use of DC pulses [6], and 200 this family of techniques is accordingly named pulsed elec Temperature (C tric current sintering(PECS)[7-9]or electric pulse assisted consolidation(EPAC)[10] Fig. I Recorded sintering curves for Zno, ZrO(doped with 3% yttria), and Various materials have been compacted by spark plasma a-A1,03 powders, using a heating rate of 100oC/min Relative density(a) sintering. The number of publications on this topic has in- and shrinkage rate(b). Note that the densification is completed within a few creased dramatically during the last few years. It is not our minutes aim to give a comprehensive review of these technique and their application in this article. We will rather focus 1250C, respectively. As seen in this figure, the main part our presentation on how the kinetics of densification and of the densification occurs within a period of 3 min, whereas grain growth can be manipulated by the use of one of these conventional hot-press and pressure-less sintering processes techniques, namely SPS, to yield: (i) Dense compacts con- typically require hours of heat treatment at substantially taining nano-sized grains; () Fully compacted bodies con- higher temperatures to yield fully densified compacts taining metastable constituent(s); (iii) Laminated structures The normalized shrinkage rate is plotted versus the (iv) very rapid formation of tough interlocking microstru temperature in Fig. Ib. The shrinkage rate curves exhibit tures in Si3 N4-based ceramics maximums around 700. 1100 and 1150C. for ZnO. ZrO and Al2O3, respectively and these maximum rates are one or two orders of magnitude faster than those observed during 2. Preparation of nano-ceramics conventional pressure-less sintering [11, 12] As shown in a recent article concerning SPS processing Shrinkage curves of the oxides Zno, ZrO(doped with of Al2O3, the kinetics of grain growth is quite fast at high 3 mol%Y203), and Al2O3, compacted by SPs under a sintering temperatures, for both gain-boundary migration pressure of 50 MPa and a heating rate of 100C/min are and grain-boundary diffusion are enhanced. It is, however, shown in Fig. la. Using these sintering conditions, shrinkage possible to determine a temperature"window, within which starts at600, 800 and 900C for ZnO, ZrO2 and Al2O3, fully dense compacts can be obtained without or with only and fully dense compacts are obtained at 850, N 1100, and very limited grain growth [ 12]126 M. Nygren, Z. Shen / Solid State Sciences 5 (2003) 125–131 (iii) The use of fast heating and cooling rates; (vi) The use of a pulsed direct current, implying that the samples are also exposed to an electric field. It is generally accepted that application of mechanical pressure is helpful in removing pores from compacts and enhancing diffusion. The heat transfer from the die to the compact is very efficient in these processes, because the die itself (and the electrically conductive pressure-transmitting medium when used) acts as heating element, so that the ther￾mal energy necessary for compaction is efficiently transmit￾ted to the sample. However, it is frequently argued that the improved densification rates stem mostly from the use of DC pulses of high energy. Thus it was originally claimed by the process inventors that the pulses generate spark and even plasma discharges between the powder particles, which is why the processes were named spark plasma sintering and plasma activated sintering. During the initial part of the sin￾tering process, the spark and/or plasma discharges are said to clean the surfaces of the powders from adsorbed species such as CO2 and H2O. In subsequent stages, the presence of cleaned and activated surfaces is expected to enhance the grain boundary diffusion which, together with the pro￾posed spark discharges and/or plasma processes, will pro￾mote transfer of material and thus facilitate densification and grain growth [2,3]. Murir and Smaltzrield argue that the application of an electric field enhances grain growth and therefore also densification [5]. Whether plasma is generated has not yet been confirmed, especially when non-conducting ceramic powders are com￾pacted. It has, however, been experimentally verified that the densification is enhanced by the use of DC pulses [6], and this family of techniques is accordingly named pulsed elec￾tric current sintering (PECS) [7–9] or electric pulse assisted consolidation (EPAC) [10]. Various materials have been compacted by spark plasma sintering. The number of publications on this topic has in￾creased dramatically during the last few years. It is not our aim to give a comprehensive review of these techniques and their application in this article. We will rather focus our presentation on how the kinetics of densification and grain growth can be manipulated by the use of one of these techniques, namely SPS, to yield: (i) Dense compacts con￾taining nano-sized grains; (ii) Fully compacted bodies con￾taining metastable constituent(s); (iii) Laminated structures; (iv) Very rapid formation of tough interlocking microstruc￾tures in Si3N4-based ceramics. 2. Preparation of nano-ceramics Shrinkage curves of the oxides ZnO, ZrO2 (doped with 3 mol% Y2O3), and Al2O3, compacted by SPS under a pressure of 50 MPa and a heating rate of 100 ◦C/min are shown in Fig. 1a. Using these sintering conditions, shrinkage starts at ∼ 600, ∼ 800 and 900 ◦C for ZnO, ZrO2 and Al2O3, and fully dense compacts are obtained at ∼ 850, ∼ 1100, and (a) (b) Fig. 1. Recorded sintering curves for ZnO, ZrO2 (doped with 3% yttria), and α-Al2O3 powders, using a heating rate of 100 ◦C/min. Relative density (a) and shrinkage rate (b). Note that the densification is completed within a few minutes. ∼ 1250 ◦C, respectively. As seen in this figure, the main part of the densification occurs within a period of 3 min, whereas conventional hot-press and pressure-less sintering processes typically require hours of heat treatment at substantially higher temperatures to yield fully densified compacts. The normalized shrinkage rate is plotted versus the temperature in Fig. 1b. The shrinkage rate curves exhibit maximums around 700, 1100, and 1150 ◦C, for ZnO, ZrO2 and Al2O3, respectively and these maximum rates are one or two orders of magnitude faster than those observed during conventional pressure-less sintering [11,12]. As shown in a recent article concerning SPS processing of Al2O3, the kinetics of grain growth is quite fast at high sintering temperatures, for both gain-boundary migration and grain-boundary diffusion are enhanced. It is, however, possible to determine a temperature “window”, within which fully dense compacts can be obtained without or with only very limited grain growth [12]