Availableonlineatwww.sciencedirect.col RECTO State ELSEVIER Solid State Sciences 5(2003)125-131 ciences On the preparation of bio-, nano-and structural ceramics and composites by spark plasma sintering Mats Nygren* Zhijian Shen Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University 10691 Stockholm, Sweden Dedicated to Sten Andersson for his scientific contribution to Solid State and Structural Chemistry bstract park plasma sintering(SPS) is a comparatively new technique. It allows very fast heating and cooling rates, very short holding times, and the possibility to obtain fully dense samples at comparatively low sintering temperatures, typically a few hundred degrees lower than in normal hot pressing. During recent years, a wide variety of materials, e.g., ceramics, composites, cermets, metals and alloys, have been successfully compacted by the SPs process. This and other processing techniques that use direct current and electrically conducting pressure dies are briefly described, but the focus of our presentation are on how the kinetics of densification and grain growth can be manipulated by the use of the SPs technique so as to yield various materials that hold significant fundamental and technological interest. e 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved Keywords: Sintering; Grain growth; Nano-materials; Laminates; Composites; FGM; SPS 1. Introducti sintering process. This procedure is often referred to in the literature as a"single pulse cycle process", with a typical The idea to compact metallic materials by an electro- 30-60 ms pulse current of x1000 A intensity during 60- discharge process was originally proposed in the 1960s [1 90 s In a few cases the pulsed dC procedure is repeated Based on this concept, three sintering processes have been during the sintering process, and in this case the procedure developed and commercialised during recent years, they Is referred to as a"multiple pulse cycle process";(ii)In the are named"Spark Plasma Sintering"(SPS)[21,"Plasma electroconsolidation process, various types of currents(DC, Activated Sintering(PAS)[3 and"Electroconsolidation pulsed DC, or AC)can be used, i. e, the system is designed [4]. These processes are similar to conventional hot pressing to be flexible in terms of power supply. These processes have in that the precursors are loaded in a die and a uniaxial now been developed beyond the production of small objects pressure is applied during the sintering. However, instead of with simple shapes, as continuous production of compacts of using an external heat source, a current(DC, pulsed DC, or complex geometry and of pieces with diameters larger than AC)is allowed to pass through the electrically conducting 150 mm has been achieved. Despite the fact that a uniaxial pressure die and, in appropriate cases, also through the pressure is applied, green bodies of complex geometry can sample. This implies that the die itselfacts as heat source and be exposed to a "pseudo-isostatic"pressure when embedded characteristics of the three processes are as follows: () In as pressure-transmitting medium inside the de ates that act that the sample is heated from both outside and inside. The in free-flowing electrically conducting particul the SPs process a pulsed dC (3.3 ms pulses of 0.5 to 10 Common to these three processes is the possibility of kA intensity)is applied from the beginning to the end of using very fast heating rates(up to 600 C/min or more) the sintering cycle; (i)In the PAS process a pulsed DC is and very short holding times(minutes), and the ability to normally applied at room temperature for a short period of obtain fully dense samples at comparatively low sintering time, and then a continuous DC during the remainder of the temperatures, typically a few hundred degrees lower than in normal hot pressing. Four factors that contribute to the fast densification process can be discerned: (i)Rapid heat transfer;(ii) The application of a mechanical pressure exceeding that used in normal hot pressing processes 1293-2558/03A-see front matter o 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved. i:10.1016/1293-2558(02)00086-9Solid State Sciences 5 (2003) 125–131 www.elsevier.com/locate/ssscie On the preparation of bio-, nano- and structural ceramics and composites by spark plasma sintering Mats Nygren ∗, Zhijian Shen Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, 106 91 Stockholm, Sweden Received 21 June 2002; accepted 24 August 2002 Dedicated to Sten Andersson for his scientific contribution to Solid State and Structural Chemistry Abstract Spark plasma sintering (SPS) is a comparatively new technique. It allows very fast heating and cooling rates, very short holding times, and the possibility to obtain fully dense samples at comparatively low sintering temperatures, typically a few hundred degrees lower than in normal hot pressing. During recent years, a wide variety of materials, e.g., ceramics, composites, cermets, metals and alloys, have been successfully compacted by the SPS process. This and other processing techniques that use direct current and electrically conducting pressure dies are briefly described, but the focus of our presentation are on how the kinetics of densification and grain growth can be manipulated by the use of the SPS technique so as to yield various materials that hold significant fundamental and technological interest.  2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Sintering; Grain growth; Nano-materials; Laminates; Composites; FGM; SPS 1. Introduction The idea to compact metallic materials by an electro￾discharge process was originally proposed in the 1960s [1]. Based on this concept, three sintering processes have been developed and commercialised during recent years; they are named “Spark Plasma Sintering” (SPS) [2], “Plasma Activated Sintering” (PAS) [3] and “Electroconsolidation” [4]. These processes are similar to conventional hot pressing in that the precursors are loaded in a die and a uniaxial pressure is applied during the sintering. However, instead of using an external heat source, a current (DC, pulsed DC, or AC) is allowed to pass through the electrically conducting pressure die and, in appropriate cases, also through the sample. This implies that the die itself acts as heat source and that the sample is heated from both outside and inside. The characteristics of the three processes are as follows: (i) In the SPS process a pulsed DC (3.3 ms pulses of 0.5 to 10 kA intensity) is applied from the beginning to the end of the sintering cycle; (ii) In the PAS process a pulsed DC is normally applied at room temperature for a short period of time, and then a continuous DC during the remainder of the * Corresponding author. E-mail address: mats@inorg.su.se (M. Nygren). sintering process. This procedure is often referred to in the literature as a “single pulse cycle process”, with a typical 30–60 ms pulse current of ∼1000 A intensity during 60– 90 s. In a few cases the pulsed DC procedure is repeated during the sintering process, and in this case the procedure is referred to as a “multiple pulse cycle process”; (iii) In the electroconsolidation process, various types of currents (DC, pulsed DC, or AC) can be used, i.e., the system is designed to be flexible in terms of power supply. These processes have now been developed beyond the production of small objects with simple shapes, as continuous production of compacts of complex geometry and of pieces with diameters larger than 150 mm has been achieved. Despite the fact that a uniaxial pressure is applied, green bodies of complex geometry can be exposed to a “pseudo-isostatic” pressure when embedded in free-flowing electrically conducting particulates that act as pressure-transmitting medium inside the die. Common to these three processes is the possibility of using very fast heating rates (up to 600 ◦C/min or more) and very short holding times (minutes), and the ability to obtain fully dense samples at comparatively low sintering temperatures, typically a few hundred degrees lower than in normal hot pressing. Four factors that contribute to the fast densification process can be discerned: (i) Rapid heat transfer; (ii) The application of a mechanical pressure exceeding that used in normal hot pressing processes; 1293-2558/03/$ – see front matter  2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. doi:10.1016/S1293-2558(02)00086-9