doc. Ing. Pavel Ctibor, Ph.D.

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.

Yttrium aluminum garnet (YAG) with cerium admixture (Ce:YAG) was plasma sprayed using two different devices - gas-stabilized plasma (GSP) torch and water-stabilized plasma (WSP) torch. Coatings on stainless steel as well as self-standing plates were produced. Besides microstructure and crystallographic characterizations, dielectric tests were performed on these coatings. They included capacitance (i.e. relative permittivity), loss tangent and volume resistivity measurements. After spraying, the YAG crystal phase was preserved without any decomposition, but an amorphous fraction was detected in the as-sprayed coatings deposited by both processes. The dielectric behavior of the coatings was influenced by imperfections like splat boundaries, pores and thin cracks. The Ce:YAG samples were successfully plasma sprayed by both spray techniques. Selected aspects of wear were measured and compared with a single-crystal. The dielectric properties are comparable with the single-crystal and highly promising, particularly the loss tangent with values so low that they were not found in any other as-sprayed ceramic coating.

The high pressure forming at 300 MPa and room temperature was applied before firing of pure lithium fluoride powder. This material has very low sintering temperature, only about one half of typical oxide-ceramic dielectrics. Despite this fact, the resulting fired bulk was tested as dielectrics and showed very interesting characteristics. The best sample, fired at 750 °C and for 8 hours, had relative permittivity 12.1 and loss tangent 0.0006, frequency-and temperature-independent up to at least 150 °C. These parameters are comparable with oxide ceramics, fired at temperatures over 1500 °C, as for example alumina or YAG.

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.

LaFeO 3 commercial powder was fired by spark plasma sintering. Microstructure with homogeneous grain size and certain porosity was produced. Microhardness of the compact was rather high. The dielectric measurements showed high permittivity, but very frequency dependent, associated also with a high loss tangent. Analysis of the data showed high contribution of grain boundaries to the total impedance. The results roughly agree with the literature.

Barium titanate coatings were, for the first time, sprayed by a high feed-rate plasma torch with water stabilization of the plasma. Two power levels of the torch were applied for spraying to cover steel substrates. Various substrate preheating levels from 125 C to 377 C were used to modify cooling conditions. Microstructure and phase composition including crystallinity quantification were observed. Dielectric measurements proved that the relative permittivity between 300 and 400 coatings is too temperature sensitive over 170 C but fits the requirements of the EIA temperature coefficient between room temperature and 170 C. Simultaneously, the loss tangent remains rather low, between 0.02 and 0.07, in a broad range of temperatures and frequencies. Annealing was performed in air to heal the oxygen deficiency, but only modified the microstructure insignificantly. The dielectric properties of as-sprayed and annealed samples were discussed, with the main finding that the temperature coefficient of permittivity was improved by annealing. This study contributes to the search for the suitability of plasma-sprayed BaTiO3 coatings for application in the electrical industry, namely by the optimization of conditions for high feed-rate spraying.

Yttrium oxide was sprayed by a plasma torch using a coarse thermal spray powder, which must be in size range of tens of micrometers to ensure good penetration into the plasma stream. Thick coatings on steel substrates were produced with two sprays systems facilitating gas stabilized plasma (GSP) and hybrid water-argon stabilized plasma (WSP-H) techniques. Additionally, an ultra-fine yttrium oxide powder was sprayed from a suspension. Hybrid water-argon stabilized plasma system was used for this purpose. Markedly thinner compact coatings were produced this way. All three sorts of plasma sprayed deposits were studied by the same methods. Dielectric properties were studied in a broad range of frequencies and temperatures. The microstructure aspects as well as crystallite size were analyzed and discussed in relation to electrical properties. All coatings exhibited stable dielectric parameters versus changing frequency and temperature, comparable with literature values for various samples. Concerning sintered bulks, and especially their thermal stability of capacitance, the plasma sprayed coatings were slightly worse. However due to shape and size variability of the plasma spraying are yttria coatings prospective for technical applications.

In this paper, we examined the dielectric properties of CaCu3Ti4O12 (CCTO) ceramics fabricated by various routes and discussed the most important conditions affecting their dielectric behavior. We prepared feedstock powder using a molten salt route and compared it with a commercial powder. Both powders were sintered using SPS. For some samples, annealing was applied after sintering. Other samples were obtained by high-pressure forming and conventional sintering, using both powders. Phase composition, porosity and microhardness were evaluated in comparison with the literature. The results showed that a sintering temperature just below or equal to 1000 C should be set for the SPS process. However, the best dielectric characteristics were obtained in samples prepared by high-pressure forming and conventional sintering, which showed a relative permittivity of 22,000 and a loss tangent of 0.13 at 1 MHz.

A series of samples was sintered from nanometric BaTiO3 powder by spark plasma sintering (SPS). 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 BaTiO3 samples produced by SPS with or without additional heat treatment. Majority of our SPS fired samples had, even without subsequent thermal treatment, advantageous combination of high permittivity, DC resistivity and low loss factor, seldom reported for SPS technology. Samples were tested also in annealed state and compared with as-sintered ones.

This paper aims to study various physical properties of lithium silicate ceramics. The work starts with the synthesis followed by characterization of the compounds. The samples are synthesized by reaction between SiO2 and Li2CO3 powders, compacted by spark plasma sintering (SPS) and then characterized by XRD, SEM and other techniques. The electrical properties are studied by dielectric spectroscopy. Dielectric and conductive properties of lithium silicate depend on the composition and microstructure, which factors are interconnected with the processing parameters. Li2 SiO3 and Li2Si2O5 were the major components present in the SPS bulk. The best combination of dielectric parameters is relative permittivity 6.41 and loss tangent 0.001 at frequency 0.9 MHz for the sample sintered at 950 °C for 30 min. Relative permittivity as well as loss tangent was monitored to increase with temperature up to 250 °C.

Titanium dioxide (TiO2) was plasma sprayed using a high feed-rate spray system with hybrid water– argon stabilization torch (WSP-H). TiO2 powder agglom- erated from nanometric particles was used as the feedstock. The produced coating had thickness over 4 mm, to be comparable with bulk ceramic bodies. The deposit was removed from the substrate and annealed in air to reoxidize the oxygen deficient as-sprayed titania and to obtain material with a proper dielectric behavior. Besides the microstructure studies, radiofrequency dielectric spec- troscopy and microwave dielectric spectroscopy were in the focus of investigation. The approach to annealing of an extremely thick coating is studied: The significance of annealing of a large-area thick coating is discussed, as the role of grain interior and grain boundaries in the charge transport is influenced. Relative permittivity 94 and the Q*f product 12,800 GHz were reached at frequency 4 GHz, whereas the frequency response under 1 MHz was mapped continuously and the values (i.e., low and stable loss tangent) confirmed that a sprayed and air-an- nealed TiO2 is able to approach or also overperform dielectric parameters typical for TiO2 sintered bulk.

Tantalum pentoxide (Ta 2 O 5 ) was sintered with spark-plasma sintering (SPS) using a commercial powder with micometric particles composed of nanometric crystallites. The influence of the SPS process and subsequent annealing on the microstructure, phase composition and dielectric properties was studied. After the sintering, the product was oxygen deficient because of the low pressure in the sintering chamber. Evacuation was necessary for the process because of the carbon pistons and carbon die used for the powder compaction. After the subsequent annealing in air, the white color of Ta2O5 was restored, being a proper indication of a re-oxidation. Dielectric properties were studied in a broad range of frequencies and temperatures. As-sintered and also annealed samples exhibited a high relative permittivity in a range of 61–68, a loss factor of about 0.001 and a resistivity in the order of 10E11 Ohm m.

ZrB2 ceramics were prepared by boro/carbothermal reduction, using spark plasma sintering (SPS). This technology allows to control and monitor the ongoing reduction reaction during the sintering of ZrO2 and B4C powder mixtures with graphite, leading to the production of ZrB2. This work shows the effect of several parameters on the amount of produced ZrB2. The results are compared with literature data. Sintering temperatures in the range 1400 - 1800 °C were chosen for the experiment. For example, after sintering at a temperature of 1800 °C, with a hold time of 15 min, the total amount of ZrB2 produced from a powder mixture of stoichiometric ratio was more than 60 %. The porosity dropped with increasing sintering temperature to a final value of about 0.3 %. The number of pore sections per square unit dropped with the temperature as well. The final product is composed of very hard as well as rather soft components.

Strontium zirconate SrZrO3 is a multifunctional material studied in literature as a luminescent material, proton-conductor, thermal barrier coating, and dielectric ceramics. In our study this material was sprayed by a high feed-rate water-stabilized plasma torch WSP 500 at its standard electric power 150 kW. The as-deposited coatings exhibited lamellar microstructure with relatively high porosity over 13%. Archimedean (water immersion) specific weight was 4.54 g/cm3. Annealing was done in air at 1350 °C. Annealed coating exhibited an interesting response to UV light, including after-glow luminescence. After excitation by 225 nm light the sample exhibited after-glow exponential luminescence decay with 5 s characteristic lifetime. Diffused reflectance of the coatings was measured as well. The infrared reflectance is slightly lower after annealing, whereas in the ultraviolet band it is higher.

Barium titanate coatings were first time sprayed by a high feed-rate plasma torch with water stabilization. Two power levels of the torch were applied. Two substrate materials were used—stainless steel and carbon steel. Various substrate preheating levels from 125 to 450 C were applied to change the cooling conditions. Microstructure, phase composition including crystallinity quantification, surface roughness and microhardness were evaluated. Dielectric measurements proved that coatings with medium-level relative permittivity and with stable loss factor course versus frequency of the AC electric field were fabricated by spraying. The cold substrates provided coatings with low reflectivity and visibly darker surface compare to the hot substrates. The coatings exhibited good dielectric properties - on the top of an expectable range for plasma-sprayed BaTiO3. This study is targeted to coatings prospective for electrical industry, namely via optimization of conditions at the high feed-rate spraying.

Tantalum pentoxide Ta2O5 was sintered by spark plasma sintering (SPS) using a commercial micrometric powder. Influence of the SPS process and subsequent annealing on microstructure, phase composition and dielectric properties was studied. After sintering the product is oxygen-deficient because of low pressure in the sintering chamber. The evacuation is necessary for the process because of carbon pistons and carbon die used for powder compaction. After subsequent annealing in air the color of the product Ta2O5 is restored that is proper indication of the re-oxidation. Dielectric properties were studied in a broad range of frequencies. Both as-sintered and annealed samples exhibit high relative permittivity about 61 to 68, loss factor about 0.001 and resistivity in order 10E11 Ωm.

Several silicate materials were plasma sprayed and characterized by the authors in recent years from the point of view of their chemical and phase compositions, microstructure and mechanical as well as thermal properties. The present work is concerned with selected dielectric properties of these deposits. Synthetic mullite and steatite as well as natural olivine forsterite were plasma sprayed using the water - stabilized plasma system (WSP®). The deposits were striped-out, ground and polished to produce samples in form of plates with a smooth surface. Part of samples was later annealed in air. These samples-after coverage by metal electrodes functioning as monoblock capacitors were tested in the alternative low voltage electric field to measure capacity and loss factor in the frequency range from 200 Hz to 1 MHz. Relative permittivity was calculated from the measured capacity. Volume resistivity was measured in the direct electric field. It is shown that the relative permittivity of as-plasma sprayed silicates is less stable compared to bulk in the whole studied frequency range. However, thermal annealing modifies the structure much closer to the sintered bulk which is also reflected in dielectric properties. Insulating ability of plasma sprayed silicates with and without annealing is discussed in consequence with chemical changes and phase changes induced by annealing.

A multifunctional material, strontium zirconate, SrZrO 3 , studied in literatures as a dielectric ceramics, thermal barrier coating, proton- conductor, and luminescent material was sprayed by a water-stabilized plasma torch WSP 500. Stainless steel and plain carbon steel were used as substrates. Coatings with thickness of 1 to 2 mm were produced, whereas the substrates were preheated over 450 °C. The torch working at 150 kW was able to spray SrZrO 3 with a spray rate of 10 kg per hour. Microstructure, phase composition, dielectric properties, and optical band gap were also investigated.

A novel thermal barrier coating (TBC) material, strontium zirconate SrZrO3, was sprayed by a high feed-rate water-stabilized plasma torch WSP 500. Stainless steel coupons were used as substrates. Coatings with a thickness of about 1.2 mm were produced, whereas the substrates were preheated over 450 °C. The torch worked at 150 kW power and was able to spray SrZrO3 with a high spray rate over 10 kg per hour. Microstructure and microhardness, phase composition, adhesion, thermal conductivity and thermal expansion were evaluated. The coating has low thermal conductivity under 1 W/m K in the interval from room temperature up to 1200 °C. Its crystallite size is slightly over 400 nm and thermal expansion 12.3 µm K−1 in the similar temperature range.

An extremely thick TiO2 deposit was plasma sprayed using a water-stabilized spray system. Natural rutile TiO2 was used as feedstock powder. Reaching the required enormous thickness, 15 mm, was fast thanks to a high feed-rate WSP gun. The massive deposits were cut into slices aligned parallel with the substrate and studied. Their microstructure is lamellar with defects typical for plasma sprayed ceramics. Differences between the near-substrate slices, medium space slices and near-surface slices of the deposit were found namely in physical properties. Besides the microstructure, dielectric spectroscopy and electrochemical impedance spectroscopy were in the focus of investigation. The role of the grain interior and grain boundaries in the charge transport and the role of dipoles of various sizes are different near the substrate, in the middle and near the surface. The deposit was annealed to remove the oxygen sub-stoichiometry of the semiconductive as-sprayed titania. Various differences are present also in the annealed deposit since they are inherited from the as-sprayed state, which was not completely overlaid by annealing.

Zirconium-tin titanate ceramics (Zr,Sn)TiO4 was prepared by spark plasma sintering (SPS) technique. Resulting samples, low-height cylinders, were subjected to microstructural observations, immersion measurements of density and porosity and X-ray diffraction phase analysis. Dielectric parameters – relative permittivity and loss factor – were tested in the frequency range from 80 Hz to 1 MHz and volume DC resistivity was measured at 100 volts.

This paper studies calcium titanate (CaTiO3) dielectrics prepared by plasma spray technology. A water stabilized plasma gun (WSP) as well as a widely used gas stabilized plasma gun (GSP) were employed in this study to deposit three sample sets at different spray conditions. Prepared specimens were annealed in air at atmospheric pressure for 2 h at various temperatures from 530 to 1170 °C. X-ray diffraction (XRD), Raman spectroscopy and porosity measurements were used for sample characterization. Dielectric spectroscopy was applied to obtain relative permittivity, conductivity and loss factor frequency dependence. Band gap energy was estimated from reflectance measurements. The work is focused on the explanation of changes in microstructure and properties of a plasma sprayed deposit after thermal annealing. Obtained results show significant improvement of dielectric properties after thermal annealing.

In article an attempt has been made to investigate the dielectric strengths of selected ceramic coatings namely alumina (Al2O3), yttria (Y2O3), YSZ (ZrO2-7% Y2O3) and ferroelectric BST (Ba0.68Sr0.32TiO3) deposited using a atmospheric plasma spray process. Samples of each material were deposited under three different spray distances. The hardness, moduli of elasticity, dielectric strengths of the deposits were evaluated. The microstructures and phases were analyzed using optical microscopy and X-ray diffraction analysis respectively. The results show the significant influence of spray distance on dielectric strengths of the coatings.

Previously published results on electrical and mechanical properties of BaTiO3 coatings prepared by atmospheric plasma spraying showed anomalies in their dielectric response. This paper provides a study of electrical and mechanical properties of BaTiO3 coatings after thermal posttreatment. The spraying was carried out by a direct current gas-stabilized plasma gun. BaTiO3 was fed into the plasma jet as a feedstock powder prepared by reactive sintering of micrometer-sized powders of BaCO3 and TiO2. In the next step the coatings were annealed in air. Microstructure and phase composition are reported and discussed in relation to electric and mechanical properties. Dielectric properties are reported for the radio frequency (RF) range.

Natural diopside CaMgSi2O6 was processed by plasma spray technique into self-supporting plates.The experimental powder for spraying came fromas-mine natural diopside and was crushed and sieved to the proper size for plasma spraying with a water-stabilized plasma torch (WSP). These deposits were further studied with optical microscopy, SEM, X-raydiffraction, thermal analysis,mechanical tests and dielectric spectroscopy. Microhardness was measured on cross-sections prepared for microstructure observation. Dielectric properties of capacitor samples prepared from the self-supporting sprayed deposits were measured at low voltage.The results showed that diopside is an interesting candidate for various electrical applications as it has similar dielectric behavior as alumina or silicates.Thermal annealing of the SSP was done. Despite crystallization of theamorphous fraction (presentafterspraying)intoa fine-grained structure,annealing worsened slightly dielectric responses of diopside. Differences between diopside and other silicates sprayed by the author sinthepast are discussed.

Self-supported CaTiO3 coatings prepared by plasma-spraying technology and effect of annealing on dielectric properties and microstructure of these coatings are studied in this work. A water stabilized plasma gun WSP (IPP ASCR, Prague) as well as a widely used conventional gas stabilized plasma gun GSP were employed. Prepared samples were annealed in air atmosphere up to 1170 C. The work is focused on dielectric properties of these specimens. Relative permittivity and loss factor were measured in a frequency range of 30 Hz–30 MHz. Microstructure was studied using light microscopy, microhardness measurements, X-ray diffraction – including high-temperature XRD measurements, and X-ray photoelectron spectroscopy. The work studies the influence of annealing temperature on dielectric properties, crystalline structure and porosity of plasma deposited calcium titanate. Results obtained show significant improvement in CaTiO3 coating properties after thermal annealing.

This study examines the photocatalytic activity of coatings produced by atmospheric plasma spraying (APS). The applied spraying tools were gas-stabilized (GSP) and water-stabilized (WSP) plasma guns. The TiO2-Fe2O3 powder with and without Na2SiO3 additive was produced as agglomerates suitable for feeding into the plasma jet. The coatings are analyzed by scanning electron microscopy, X-ray fluorescence and X-ray diffraction. Photocatalytic degradation of butane under UV and under visible light, as well as the growth kinetics of the products - carbon dioxide and carbon monoxide - is quantified for all coatings and compared with a pure TiO2 coating. The coatings show a lamellar structure, as it is typical for this process. However, their porosity is rather high. Anatase titania from the feedstock powder is converted into rutile phase in all coatings; whereas the presence of FeTiO3 is detected in the coating without Na2SiO3 made by WSP and metallic iron in the GSP coating. The coating from powder with Na2SiO3 admixture is partly amorphous. This coating is photocatalytically active especially under UV, whereas under visible light it is overcome by the WSP coating. The GSP coating has the best photocatalytic activity under both different radiations.

Yttrium–aluminum garnet doped with Ce was plasma sprayed using two different processes – gas-stabilized plasma torch and water-stabilized plasma torch. Coatings on various substrate materials (stainless steel, ceramics, YAG undoped crystals), as well as self-standing plates, were obtained. The coatings adhered on materials with relatively large variety of thermal expansion coefficient. Besides microstructural, crystallographic, and thermal-stability investigations, numerous optical tests were performed. They included cathodoluminescence (CL), UV-VIS-NIR reflectance, and response of the Ce:YAG on light with various wave lengths. After spraying, the desired YAG crystalline phase sustained without any decomposition, but an amorphous fraction was present in both types of coatings. Selected coatings were heat-treated to crystallize fully and change their optical properties. Minor amorphous fraction crystallized at 930 °C. The heat-treated coatings exhibited higher CL and also larger visible emission when illuminated with a 366 nm lamp. Microhardness of the coatings was tested as well and proved the mechanical similarity of both coating types and difference from the single crystal. The optical responses of the coatings were influenced by imperfections like splat boundaries, pores, and thin cracks, which were healed only partly by heat treatment. However, the Ce:YAG was first plasma sprayed and moreover produced by both spray techniques without an irreversible loss of the desired garnet phase.

A natural diopside CaMgSi2O6 was formed by plasma spray technique into coatings on magnesium substrates as well as self-standing plates. The powder for spraying came from as-mined natural diopside and was crushed and sieved to the proper size for plasma spraying by a water-stabilized plasma torch (WSP). By removing the substrates after cooling, self-supporting plates were obtained. These deposits as well as thinner coatings on Mg alloy were studied with optical microscopy, SEM, X-ray diffraction, thermal analysis, mechanical tests and dielectric spectroscopy. Microhardness was measured on cross-sections prepared for microstructure observation. Wear resistance at wet conditions was tested as well. Dielectric properties of monoblock capacitors prepared from the self standing deposits were measured at low voltage. The results showed that diopside is interesting candidate for various electrical applications; it has similar dielectric behavior as alumina or silicates. The differences between diopside and other silicates sprayed in the past are discussed in connection with their chemical and phase compositions.

This paper presents a study of dielectric properties, namely the relative permittivity and loss factor dependence on the frequency of a weak electric field. Perovskite CaTiO3 was studied in the form of coatings and self-supporting plates made by plasma spraying. A conventional gas stabilised plasma gun (GSP) as well as a water stabilised plasma gun (WSP) were employed. It was observed that plasma sprayed titanates exhibit a strong relaxation of permittivity and loss factor decrease with a frequency rise. These properties are influenced by spray technique and spraying parameters, but the relaxation character in general is preserved in all cases. The volume resistivity of the samples was studied as well. Several aspects of the structural features of plasma deposits, especially the phase composition, porosity character, and their influence on dielectric properties are discussed.

This paper presents a comparison of properties of BaTiO3 ceramics prepared by two different production methods:gas-stabilized plasma spraying (GSP) and spark plasma sintering (SPS).Samplesof both materials were evaluated by various techniques,the goal being to detect theCurie temperature of the ferroelectric transformation between the tetragonal and the cubic phase.All tests,resonant ultrasound spectroscopy,dielectric measurements,differential scanning calorimetry and temperature-resolvedX-ray diffraction (XRD), used in combination,proved the absence of this transformation in the case of GSPcoating up to 500 °C. Similarly,the tetragonal-to- orthorhombic transition temperature is shifted downwards,this transition probably taking place in a small fraction of the volume of coating. The SPS samples exhibit several anomalies, such as astrong anisotropy of relative permittivity, but their phase transformations were detected in the usual temperature ranges.

Dielectric properties of plasma sprayed silicates