Citation: Ctibor, P.; Straka, L.; Sedlᡠcek, J.; Lukᡠc, F. Dielectric Properties of Compacts Sintered after High-Pressure Forming of Lithium Fluoride. Ceramics 2023, 6, 1913–1925. https://doi.org/10.3390/ ceramics6040118 Academic Editor: Gilbert Fantozzi Received: 28 July 2023 Revised: 14 September 2023 Accepted: 19 September 2023 Published: 22 September 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/). ceramics Article Dielectric Properties of Compacts Sintered after High-Pressure Forming of Lithium Fluoride Pavel Ctibor 1, * , Libor Straka 2 , Josef Sedlá ˇ cek 2 and František Luká ˇ c 1 1 The Czech Academy of Sciences, Institute of Plasma Physics, Za Slovankou 3, 182 00 Prague, Czech Republic; lukac@ipp.cas.cz 2 Faculty of Electrical Engineering,Czech Technical University, Technická 2, 160 00 Prague, Czech Republic; sedlacek@fel.cvut.cz (J.S.) * Correspondence: ctibor@ipp.cas.cz Abstract: High-pressure forming at 300 MPa and room temperature was applied before the sintering of a lithium fluoride (LiF) powder. The as-fired samples were tested as dielectrics and showed very interesting characteristics. The best sample, sintered at 750 ◦ C for 8 h, had a relative permittivity of 12.1 and a loss tangent of 0.0006, both of them frequency-independent and temperature-independent up to at least 150 ◦ C, and moreover, the volume DC resistivity was 27.4 × 10 12 Ωm at room temperature. These parameters are comparable with oxide ceramics, processed at temperatures over 1300 ◦ C, as for example, aluminum dioxide (Al 2 O 3 ) or Y 3 Al 5 O 12 (YAG). LiF material is advantageous because of its very low sintering temperature, which is only about one-half of typical oxide ceramic dielectrics. Keywords: high-pressure forming; lithium fluoride; dielectrics; optical properties 1. Introduction In recent years, low temperature cofired ceramics (LTCC) have become an attractive technology for electronic components and substrates that should be compact, light, and offer high speeds and functionality for portable electronic devices. For LTCC applications, the densification temperature of the dielectric ceramics (a material having lower dielectric loss than organic materials) must, however, be lower than the melting temperatures of metallic electrodes such as Ag (~961 ◦ C) [1]. The sintering temperature of most ceramics is over 1200 ◦ C and markedly exceeds the melting temperature of the electrodes. This factor precludes the co-firing. Microwave dielectric ceramics with low permittivity and low dielectric loss are usually oxides because of their low ionic polarizabilities [2]. In addition, the limited research on halides shows that they also exhibit low permittivity [2]. Within the family of halides, fluo- rides are typically the most lightweight. The low permittivity of fluorides is attributed [2] to the lower ionic polarizability of F − (1.62 Å 3 ) than O 2− (2.00 Å 3 ). Lithium fluoride, LiF, has been widely applied as an effective sintering additive or flux in many ceramic substances but only recently has the interest in the synthesis of pure LiF materials via conventional sintering and their dielectric properties begun. One serious problem with the sintering of pure lithium fluoride is the difficulty in its densification [2]. The sintering procedure is even more problematic for LiF than for alkaline earth fluorides. The highest relative density of LiF ceramics prepared by traditional sintering was only about 90% [2]. The reason for the low density of LiF compacts is the pore network that breaks up at the initial stage of sintering. Due to them, the air in the isolated pores balances the sintering pressure and prevents further densification. LiF has a melting point of approximately 845 ◦ C[3]. In connection with this, LiF is generally treated as an effective additive in achieving highly transparent ceramics, such as Y 2 O 3 -MgO, Y 3 Al 5 O 12 (YAG), and MgAl 2 O 4 [4], as well as dielectric materials such as MgO, CaWO 4 , or Li 2 TiO 3 . Its compatibility with silver electrodes suggests that Ceramics 2023, 6, 1913–1925. https://doi.org/10.3390/ceramics6040118 https://www.mdpi.com/journal/ceramics