Citation: Ctibor, P.; Sedlᡠcek, J.; Straka, L.; Lukᡠc, F.; Neufuss, K. Dielectric Spectroscopy of Calcium Titanate Processed by Spark Plasma Sintering. Materials 2023, 16, 975. https://doi.org/10.3390/ ma16030975 Academic Editor: Mattia Biesuz Received: 8 December 2022 Revised: 12 January 2023 Accepted: 17 January 2023 Published: 20 January 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Dielectric Spectroscopy of Calcium Titanate Processed by Spark Plasma Sintering Pavel Ctibor 1, * , Josef Sedlá ˇ cek 2 , Libor Straka 2 , František Luká ˇ c 1 and Karel Neufuss 1 1 Institute of Plasma Physics of the Czech Academy of Sciences, Za Slovankou 1782/3, 182 00 Prague, Czech Republic 2 Faculty of Electrical Engineering, Czech Technical University, Technicka 2, 166 27 Prague, Czech Republic * Correspondence: ctibor@ipp.cas.cz; Tel.: +420-266053717 Abstract: Calcium titanate (CaTiO 3 ) powder was compacted by spark plasma sintering (SPS). The resulting products were subjected to the phase stability study and dielectric characterization. The change in temperature of SPS between 1100 ◦ C and 1250 ◦ C had a clear and straightforward effect on density, porosity, relative permittivity, loss tangent, and DC resistivity. Since the SPS itself introduces certain oxygen deficiency into Ti-perovskites, all samples were annealed after SPS. However, this post-processing did not mask the effects of the SPS regime. Optical reflectance measurements were completed to compare and quantify the sample coloration and support the dielectric results with corresponding optical band gap estimations. Subtle changes in the CaTiO 3 crystal lattice arrangement, completed between 1150 ◦ C and 1250 ◦ C and documented in the literature for conventionally sintered samples, could not be confirmed for SPS-prepared calcium titanate. The novelty of this research work is in producing very stable dielectric ceramics and an indication of the SPS processing parameters suitable for this. The best sample showed at 1 MHz frequency the combination of relative permittivity 370, loss tangent 0.008, and DC resistivity 3 × 10 12 Ωm. Keywords: calcium titanate; spark plasma sintering; permittivity; dielectrics; band gap 1. Introduction Calcium titanate (CaTiO 3 ) with a perovskite structure is a ceramic material often exploited in the fabrication of electronic devices. A powder neutron diffraction study [1] suggested that there may be as many as three phase transitions in CaTiO 3 : (i) from cubic to body-centered tetragonal at 1580 K (i.e., 1307 ◦ C), (ii) to a possible centered orthorhombic phase at 1500 K (i.e., 1227 ◦ C), and (iii) to the low temperature primitive orthorhombic phase at 1380 K (i.e., 1107 ◦ C). All of the phases are related to the perovskite structure via small distortions of ions from their ideal perovskite positions. Between 1150 ◦ C and 1250 ◦ C the tetragonal phase exists [2]. However, the tetragonal phase appears metrically cubic within the resolution of the diffractometer [2]. Calcium titanate has been produced using soft chemistry such as sol–gel, hydrother- mal, and coprecipitation methods [3–6]. Monolithic ceramics can be densified by sintering, which is a technique that influences the microstructures and properties of materials. The general concept of sintering is an interface elimination process by atomic transport that reduces high surface energy. In this way, the total free energy of the system is reduced [7]. The conventional sintering procedure applies a constant heating rate in a furnace with a holding time at the stabilized sintering temperature [8]. Spark plasma sintering (SPS) enables the very rapid fabrication of bulk ceramic ma- terials. It is a consolidation technique combining pulsed electric currents with uniaxial pressure-induced compaction. Heating rates, applied pressures, and pulsed current pat- terns are the main factors responsible for the enhancement of densification kinetics and conservation of the fine-grained structure of materials [9]. The SPS was used for the pro- cessing of dielectric ceramics, although it brought a structural disorder [10] that affected Materials 2023, 16, 975. https://doi.org/10.3390/ma16030975 https://www.mdpi.com/journal/materials