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[4] viXra:2610.0031 [pdf] submitted on 2026-10-09 00:06:20
Authors: Roberto Köferstein, Stefan G. Ebbinghaus
Comments: 42 Pages. The paper was published in: J. Eur. Ceram. Soc. 43 (2023) 6137—6144. (Open Access, DOI: 10.1016/j.jeurceramsoc.2023.06.040)
Magnetoelectric (NiFe2O4)0.3-(Sr0.5Ba0.5Nb2O6)0.7 composites with addition of LiNbO3 as sintering additive were prepared by a classical mixed-oxide method. XRD patterns of ceramics sintered between 1000 and 1200 °C show the desired Sr0.5Ba0.5Nb2O6 and NiFe2O4 phases. SEM investigations confirm the 0-3 connectivity of the composite ceramics. The addition of 10 and 20 mol% LiNbO3 improves the densification of the composite ceramics and leads to an increase of the size of the Sr0.5Ba0.5Nb2O6 grains. Magnetic measurements show hystereses with low coercivities. Dielectric measurements were carried out depending on temperature and frequency. The samples with the LiNbO3 addition show significantly higher resistivity values (sDC). Magnetoelectric measurements were carried out in dependence of the magnetic DC-field, temperature, and frequency. The maximum magnetoelectric coefficient (aME) rises with the addition of LiNbO3 from 180 to 803 µV Oe-1 cm-1 (@ 900 Hz). Temperature dependent measurements show a continuously decreasing of aME with lower temperature.
Category: Chemistry
[3] viXra:2610.0030 [pdf] submitted on 2026-10-09 00:06:02
Authors: Roberto Köferstein, Florian Oehler, Stefan G. Ebbinghaus
Comments: 40 Pages. The paper was published in: Mater. Chem. Phys. 278 (2022) 125616. (Open Access, DOI: 10.1016/j.matchemphys.2021.125616)
Magnetoelectric (Sr0.5Ba0.5Nb2O6)1-x-(CoFe2O4)x (x = 0.2-0.6) composites were prepared by a one-pot soft-chemistry synthesis using PEG400. Calcining at 700 °C resulted in nanocrystalline composite powders (dcryst. = 24-30 nm) which were sintered between 1050 and 1200 °C to ceramic bodies with relative densities up to 98 %. SEM investigations confirm the formation of composite ceramics with a 0-3 connectivity and variable grain sizes from 0.2 to 3.6 µm for sintering up to 1150 °C, while sintering at 1200 °C leads both to a change in the microstructure and a considerable grain growth. Magnetic measurements at 300 K reveal ferrimagnetic behaviour with saturation magnetization values smaller than bulk CoFe2O4 and coercivities between 790 and 160 Oe. Temperature-dependent impedance spectroscopy showed that the relative permittivities decrease both with rising frequency and CoFe2O4 fraction. The frequency dependence of the impedance can be well described using a single RC circuit. Magnetoelectric measurements show the presence of pronounced field hystereses. The maximum magnetoelectric coefficient (aME) depends both on the CoFe2O4 fraction (x) and sintering temperature. The composite with x = 0.3 exhibits the largest aME value of 37 µV Oe-1 cm-1 (@ 900 Hz). With rising frequency of the AC driving field aME increases up to 300-400 Hz and is nearly constant until 1 kHz.
Category: Chemistry
[2] viXra:2610.0029 [pdf] submitted on 2026-10-09 00:05:47
Authors: Roberto Köferstein, Stefan G. Ebbinghaus
Comments: 42 Pages. The paper was published in: , J. Solid State Chem. 278 (2022) 123564. (Open Access, DOI: 10.1016/j.jssc.2022.123564)
Nano-crystalline Sr0.5Ba0.5Nb2O6 powders with addition of LiNbO3 or LiF as sintering additives were prepared by a soft-chemistry synthesis using polyethylene glycol. Calcination at 600 °C results in nanocrystalline powders (dcryst. ~ 30 nm) which were sintered between 1000 and 1300 °C to ceramic bodies. By addition of 10 and 20 mol% LiNbO3, the sintering temperature was reduced by about 200 K and the activation energy of the initial stage of sintering decreases from 386 to 271 kJ mol-1. The sintering aid improves the grain growth and dense ceramic bodies were obtained after sintering at 1125 °C for 1 h. A higher LiNbO3 content favors the formation of a pillar-like microstructure. XRD patterns of ceramics sintered above 1000 °C show only reflections of the Sr0.5Ba0.5Nb2O6 phase indicating an incorporation of LiNbO3 in the Sr0.5Ba0.5Nb2O6 structure. Dielectric measurements reveal a diffuse phase transition and a relaxor-like behavior. The phase transition temperature (Tm) depends on the sintering conditions and is between 117 and 127 °C for 10 mol% LiNbO3, which is very close to pure Sr0.5Ba0.5Nb2O6, while addition of 20 mol% shifts Tm to around 200 °C and the transition becomes slightly more diffuse. The optical band gap of the samples is ~ 3.3 eV and depends slightly on the sintering conditions. We also tried LiF as sintering aid, but this leads to the formation of considerable amounts of secondary phases in the ceramics and relative densities of only 82 %.
Category: Chemistry
[1] viXra:2610.0023 [pdf] submitted on 2026-10-06 20:18:35
Authors: R. Köferstein, J. Jacobs, S. G. Ebbinghaus
Comments: 40 Pages. The paper was published in: J. Eur. Ceram. Soc. 45 (2025) 117567. (Open Access, DOI: 10.1016/j.jeurceramsoc.2025.117567).
Lix(Sr0.5Ba0.5)1-x/2Nb2O6 ceramics were prepared by a solid state synthesis and sintered at 1300 °C. XRD investigations reveal single phase samples up to a substitution limit of x = 0.275. For higher lithium contents, the appearance of LiNbO3 as secondary phase was observed. The density of the ceramics increases with lithium content. The microstructures show globular grains which turns to pillar-like grains with rising lithium substitution. Dielectric investigations confirm relaxor ferroelectric behavior and the diffuse phase transition is shifted towards higher temperatures with increasing lithium content from 129 °C (x = 0) to 221 °C (x = 0.275). The diffuseness coefficient increases with the lithium content. From high-temperature XRD measurements the length of the tetragonal cell parameter c decreases with increasing temperature, reaches a minimum at about 150-230 °C (depending on x) and afterward increases with increasing temperature. Additionally, a significant shift of the Nb(2) position with temperature can be observed.
Category: Chemistry
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