Pavel Ctibor

The composite cermet material based on BaTiO ₃ ceramics doped with Fe was compacted using the field‐assisted sintering technology (FAST). Certain oxidation of Fe and also the presence of carbon, used in sintering equipment, were detected in the sintered material. The microstructure showed well separated two components and certain porosity. Dielectric parameters were measured between 30°C and 150°C and between frequencies 100 Hz and 20 MHz. Combination of dielectric parameters at 1 MHz frequency, that is, relative permittivity 5 200, loss tangent 0.051 and thermal coefficient of capacitance 715 ppm/°C made this composite promising among the addressed category of materials. The mechanisms of polarisation and conduction and their thermal activation were discussed.

Titanium dioxide coatings (TiO₂) were sprayed using a water-stabilized plasma gun (WSP) to form robust self-supporting bodies with the character of a ceramic disc capacitor (CDC). Agglomerated nanometric powder was used as feedstock. Argon was applied for powder feeding as well as coating–cooling to minimize the influence of ambient air. Stainless steel was used as a substrate, and the coatings were released after cooling. A more than three-millimeter-thick self-supporting TiO₂ plate was observed using HR-TEM and SEM. Porosity was studied by image analysis on polished sections. Thermal post-treatment on the coating was conducted at a rather low temperature of 500 °C. The results of the subsequent dielectric measurement showed high permittivity, but this was strongly frequency-dependent and accompanied by a progressively decreasing loss tangent. On the other hand, the plasma-sprayed TiO₂ exhibited persistent DC photoconductivity under and after illumination with a standard bulb.

Commercially available LaFeO₃ powder was processed using the spark plasma sintering (SPS) technique. The results of the dielectric measurement showed high permittivity, but this was strongly frequency-dependent and was also accompanied by a high loss tangent. The chemical purity of the powder and changes induced by the SPS process influenced the stability of the dielectric parameters of the bulk compacts. A microstructure with a homogeneous grain size and a certain porosity was produced. The microhardness of the sintered LaFeO₃ was rather high, about 8.3 GPa. All the results are in reasonable agreement with the literature related to the production of LaFeO₃ using different techniques. At frequencies as low as 100 Hz, the material behaved like a colossal permittivity ceramic, but this character was lost with the increasing frequency. On the other hand, it exhibited persistent DC photoconductivity after illumination with a standard bulb.

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 × 10¹² Ωm at room temperature. These parameters are comparable with oxide ceramics, processed at temperatures over 1300 °C, as for example, aluminum dioxide (Al₂O₃) or Y₃Al₅O₁₂ (YAG). LiF material is advantageous because of its very low sintering temperature, which is only about one-half of typical oxide ceramic dielectrics.

In this work, Eu₂O₃-doped (CaCu₃Ti₄O₁₂)_x of low dielectric loss have been fabricated using both conventional (CS) and microwave sintering (MWS), where x = Eu₂O₃ = 0.1, 0.2, and 0.3, respectively. According to X-ray diffraction (XRD) and scanning electron microscope (SEM) reports, increasing the concentration of Eu³⁺ in the CCTO lattice causes the grain size of the MWS samples to increase and vice versa for CS. The X-ray photoelectron spectroscopy (XPS) delineated the binding energies and charge states of the Cu²⁺/Cu⁺ and Ti⁴⁺/Ti³⁺ transition ions. Energy dispersive spectroscopy (EDS) analysis revealed no Cu-rich phase along the grain boundaries that directly impacts the dielectric properties. The dielectric characteristics, which include dielectric constant (ε) and the loss (tan δ), were examined using broadband dielectric spectrometer (BDS) from 10 to 10⁷ Hz at ambient temperature. The dielectric constant was >10⁴ and >10² for CS and MWS samples at x > 0.1, respectively, with the low loss being constant even at high frequencies due to the effective suppression of tan δ by Eu³⁺. This ceramic of low dielectric loss has potential for commercial applications at comparatively high frequencies.

Calcium titanate (CaTiO₃) 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 CaTiO₃ 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 × 10¹² Ωm.

Various kinds of natural garnets belonging to the almandine type (3FeO·Al₂O₃·3SiO₂) were sprayed by plasma spray technique to build coatings on metallic substrates. The experimental garnet powders came from different mines in the Czech Republic and Mongolia. After coating and cooling the substrates were removed. In this way, self-supporting plates were obtained and further studied with microscopy, X-ray diffraction, and dielectric spectroscopy. Mechanical properties were in our focus as well. Microhardness was measured on cross sections dedicated to microstructure observation. Wear resistance in wet conditions was tested in a slurry. Reflectance was measured applying visible and infrared (VIS-NIR) radiation. Dielectric properties of coatings were studied at low voltage capacitance, loss tangent and also under direct current (DC) resistance. The results show that garnet minerals are interesting candidates for various optical and electronic applications; they have similar dielectric behavior as, for example, aluminum oxide or similar high-purity synthetic oxides, and, simultaneously, they have extraordinarily low reflectance in VIS-NIR radiation. The differences between natural powders and resulting coatings are discussed in connection with their chemical and phase compositions.