JOURNAL OF MATERIALS SCIENCE 34 (1 9 9 9 ) 917 – 924 Dielectric properties of spark-plasma-sintered BaTiO 3 TOMONARI TAKEUCHI ∗ , E. B ´ ETOURN ´ E, MITSUHARU TABUCHI, HIROYUKI KAGEYAMA Osaka National Research Institute, AIST, Midorigaoka 1-8-31, Ikeda, Osaka, 563-8577 Japan E-mail: takeuchi@onri.go.jp YO KOBAYASHI Central Research Institute of Electric Power Industry, 11-1,2-chome, Iwado-Kita, Komae, Tokyo, 201-8511 Japan A. COATS, F. MORRISON, D. C. SINCLAIR, A. R. WEST Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen, AB24 3UE, UK Spark-plasma-sintering (SPS) has been applied to BaTiO 3 to prepare dense ceramics consisting of submicrometre-sized powder. Relatively dense (typically 97% of the theoretical X-ray density) pellets with an average grain size remaining similar to that of the starting powder, approximately 0.6 µm, were obtained by the SPS process. Fixed frequency (1 kHz) measurements show the room temperature permittivity of SPS ceramics to be relatively high, approximately 3500, and at least double the value of conventionally sintered ceramics, approximately 1500. Alternating current (a.c.) impedance spectroscopy measurements show that SPS is an effective process to reduce the influence of intergranular (grain boundary) effects on the permittivity and direct current (d.c.) resistance characteristics of BaTiO 3 ceramics substantially. C  1999 Kluwer Academic Publishers 1. Introduction Barium titanate (BaTiO 3 ) is a well known ferroelec- tric material with a high room temperature permit- tivity value, ≥1000, and will continue to be used in the manufacture of thermistors, multilayer capacitors and electro-optic devices into the next century [1]. Re- cently, it has been extensively studied particularly for its application as a capacitor material in down-sized portable machines and dynamic random access mem- ory (DRAM) devices [2–5]. For the above applications, BaTiO 3 ceramics with both high dielectric constants and high resistivity properties in small size are required. However, drastic decrease in electrical permittivity has been reported for BaTiO 3 ceramics with submicro- metre-sized grains [6–10]. This arises mainly because it is difficult to prepare dense BaTiO 3 ceramics with fine grains (<1 µm) via conventional sintering process. Oonishi et al. [11] prepared dense BaTiO 3 ceram- ics (more than 98% of the theoretical X-ray density) with submicrometre-sized grains (0.2–0.6 µm) by hot isostatic pressing (HIP). They applied 500 MPa to the powders during sintering, which is enough to decrease the c/a -ratio of the tetragonal unit cell, Curie temper- ature and permittivity of BaTiO 3 ceramics [12, 13]. Spark-plasma-sintering (SPS) is a process that makes use of microscopic electrical discharge between parti- cles under pressure (approximately 30 MPa) [14–16]. Although SPS is commonly used to produce dense metal and engineering ceramics, there are relatively few ∗ Author to whom correspondence should be addressed. reports on the application of this technique to produce dense ceramics of oxides for electrical ceramics. The SPS process enables a compact powder to be sintered under uniform heating to high density at relatively lower temperatures and in much shorter sintering periods, typ- ically a few minutes, compared with conventional sin- tering of cold-isostatic pressed (CIP) pellets [14–16]. The short sintering periods carried out for the SPS pro- cess would be advantageous in suppressing exaggerated grain growth. Many researchers who have studied the dielec- tric properties of BaTiO 3 ceramics report only fixed- frequency measurements, e.g. permittivity and/or dissipation factor (tan δ) at 1 kHz [1]. Polycrystalline materials commonly exhibit a variety of frequency- dependent effects associated with heterogeneities, such as grain boundaries or surface layers, in addition to the intrinsic properties of the materials concerned [17]. In order to characterize the electrical properties of poly- crystalline ferroelectric BaTiO 3 ceramics fully, Hirose and West have recently shown that a.c. impedance mea- surements should be made over a wide frequency range and the data analysed using a variety of formalisms [17]. In this paper, we try to produce dense pellets of BaTiO 3 with submicrometre grain size using the SPS process. The electrical properties of the SPS BaTiO 3 pellets were measured using a.c. impedance spec- troscopy and the results compared with those obtained for conventionally sintered BaTiO 3 ceramics. 0022–2461 C  1999 Kluwer Academic Publishers 917