Barium titanate nanometric polycrystalline ceramics fired by spark plasma sintering Pavel Ctibor a,n , Josef Sedlacek b , Vasyl Ryukhtin c , Jakub Cinert a,b , Frantisek Lukac a a Institute of Plasma Physics, ASCR, Za, Slovankou 3, 182 00 Praha 8, Czech Republic b Department of Electrotechnology, Faculty of Electrical Engineering, Czech Technical University, Technická 2,166 27 Praha 6, Czech Republic c Nuclear Physics Institute, ASCR, Husinec-Rez 250 68, Czech Republic article info Article history: Received 20 June 2016 Received in revised form 14 July 2016 Accepted 14 July 2016 Available online 16 July 2016 Keywords: BaTiO 3 Spark plasma sintering Electrical properties abstract A series of samples was sintered from nanometric BaTiO 3 powder by spark plasma sintering (SPS). Porosity on ultra-fine size scales, hardly accessible for quantification by conventional techniques, was measured by small angle neutron scattering (SANS). Band gap was estimated from diffuse reflectance measurements. Sintered tablets were polished, sputtered by aluminum to create electrode system and tested in DC and AC electric fields. The results were correlated with the SPS processing parameters and are discussed in connection with other BaTiO 3 samples produced by SPS with or without additional heat treatment. Majority of our SPS fired samples had, even without subsequent thermal treatment, ad- vantageous combination of high permittivity, DC resistivity and low loss factor, seldom reported for SPS technology. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. 1. Introduction Spark plasma sintering enables very rapid fabrication of bulk ceramic materials. It is an emerging consolidation technique which combines pulsed electric currents and uniaxial pressure induced compaction. Heating rates, applied pressures and pulsed current patterns are the main factors responsible for the en- hancement of densification kinetics and conservation of the sub- micron-scale structure of the materials. Barium titanate, BaTiO 3 , is a multifunctional oxide that exhibits complex phase appearance. Due to its high relative permittivity, BaTiO 3 is used in multilayer ceramic capacitors, dynamic random access memories, piezoelectric sensors, thermistors and resistors with high positive thermal coefficients [1,2]. Between 120 °C (393 K) and 1457 °C (1730 K) BaTiO 3 has a cubic perovskite structure that consists of corner linked oxygen octahedra con- taining Ti 4 þ , with Ba 2 þ . Cooling below 120 °C results in small displacements in the positions of the cations in the unit cell re- sulting in polar ferroelectric phase existing in the temperature interval between 5 °C (278 K) and 120 °C [3]. Suitability of the SPS technique for preparing of ferroelectric ceramic disks was up to now relatively seldom reported. Samples of BaTiO 3 with different grain size ranging from 80 nm to several micrometers were prepared by controlling sintering conditions at the SPS process [4]. Investigation of the grain size dependence of dielectric properties showed that the relative permittivity at room temperature decreased with the diminishing grain size – from 4000 for 1 mm grain size to about 1500 for 100 nm grain size. The SPS process, when operated under a reducing atmosphere, is able to create such defects as Ti 3 þ associated with oxygen vacancies [5]. We have investigated earlier barium titanate sample sintered by SPS at 1300 °C [6]. The difference in both observed sample or- ientations (i.e. parallel and perpendicular to the pressure direction at SPS) was higher relative permittivity and simultaneously lower loss factor in the pressure direction. In our paper [6] it was at- tributed to certain anisotropy of the porosity that should mean more flat pores in the pressure direction and therefore denser structure with easier polarization. Detailed microstructure in- vestigation on this sample was however not performed. The re- ported “colossal permittivity” [7] values (over 200,000), elsewhere called “supercapacitor features“ [5], was confirmed also with our results [6]. The volume resistivity of our selected SPS barium ti- tanate sample was relatively low, 5 Â 10 5 Ω m [6]. Other authors produced BaTiO 3 by SPS at 1200 °C and performed post-annealing step at 800 °C for 12 h in air [5]. Such a post-annealing is con- sidered to be necessary to restore the stoichiometry [8,9]. Idea of our present paper to use an ultra fine powder with a narrow size distribution, lower the SPS temperature even down below 1000 °C and test it in the as-sintered conditions. For detecting of pores with size of about tens of nanometers up to several micrometers, small angle neutron scattering (SANS) is Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ceramint Ceramics International http://dx.doi.org/10.1016/j.ceramint.2016.07.104 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. n Corresponding author. E-mail address: ctibor@ipp.cas.cz (P. Ctibor). Ceramics International 42 (2016) 15989–15993