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We investigate the influence of a wire-based impedance-matching network for an axial-mode helical antenna on the radiation pattern. This network, proposed in our previous work, comprises a single wire attached to the helix. We show that the matching wire, mounted on a tubular dielectric support and attached to the helix close to the reflector, does not degrade the antenna radiation pattern. We also investigate limitations of the equivalent thin-wire model proposed in our previous work.

Solid-state reaction between BaTiO3and Fe2O3was used to produce a multiferroic heterostructure composite. Commercial BaTiO3and Fe(NO3)3•9H2O were suspended in ethanol for 30 minutes in an ultrasound bath. The prepared mixture was thermally processed at 300°C for 6 h. Sintering at 1300 °C for 1 h resulted in a mixture of different phases, BaTiO3, BaFe12O19and Ba12Ti28Fe15O84, which were confirmed by x-ray powder diffraction. A dense microstructure with a small volume fraction of closed porosity was indicated by the scanning electron microscopy, while a homogeneous distribution of Fe ions over BaTiO3phase was visible from energy dispersive spectroscopy mapping. Doping of BaTiO3with Fe2O3resulted in formation of magnetic hexaferrite phases, as confirmed by dielectric measurements that showed a broadened maximum of the permittivity measured as a function of temperature. © 2021 Authors. Published by association for ETRAN Society.

Breakdown in oxygen, in external radio-frequency (RF) electric field is analyzed by employing a Monte Carlo simulation (MCS). Results were obtained for 13.56 MHz and distance between electrodes of 15 mm. Physical background of an oxygen RF breakdown is explained by observing time-resolved spatial distributions of electron concentration, mean energy, elastic scattering rate, ionization rate and attachment. The role of attachment is investigated in cases when these processes are included and when they are not. Especially influence of the attachment is highlighted by comparing oxygen and argon breakdown-voltage curves and spatial profiles. The electron losses induced by attachment extend the motion of the electron prebreakdown swarm much closer to electrodes to achieve a greater production; hence, spatial profiles at high values of the product pd where p is the pressure and d is the gap between electrodes, become more similar to those at the minimum of the breakdown curve. The most striking difference between the breakdown curves in argon and in oxygen is in the high increase of the breakdown voltage for high pd in oxygen.

Solid-state mechanical activation of MgO and α-Al2O3 powders was used to produce MgAl2O4. The cation site occupancy in the resulting MgAl2O4 spinel was investigated using different methods. Differential thermal analysis and thermal gravimetry showed that mechanical activation reduced the spinel formation temperature by around 200 °C, and the corresponding activation energy by about 25%. In addition, characteristic temperatures for evaporation of physisorbed water and decomposition of Mg(OH)2 shifted to lower values, and peaks were more pronounced. Raman spectra were used to characterize the degree of inversion as a function of the sintering temperature for all of the sintered specimens, indicating that the breaking point for ordering of the crystal structure was around 1500 °C for non-activated samples, and 1400 °C for activated samples. This is the peer-reviewed version of the article: Obradović, N., Fahrenholtz, W.G., Filipović, S., Marković, S., Blagojević, V., Lević, S., Savić, S., Đorđević, A., Pavlović, V., 2019. Formation kinetics and cation inversion in mechanically activated MgAl2O4 spinel ceramics. J Therm Anal Calorim. 140: 95-107. [https://doi.org/10.1007/s10973-019-08846-w] Published version: [https://hdl.handle.net/21.15107/rcub_dais_10027]

Sintering of pure cordierite 2MgO:2Al2O3:5SiO2, and cordierite with the addition of 5 mass % TeO2 was studied. Green bodies were prepared from powder mixtures mechanically activated in a high-energy planetary mill, shaped by uniaxial (20 MPa) and cold isostatic pressing (1000 MPa). The pressure-less sintering of these specimens was performed at 1350°C for 1 h. High relative density over 95% of the theoretical value was obtained through solid-state reaction and pressure-less sintering of powder activated for 40 min, and for the first time reported in the literature. Phase composition and microstructures of sintered samples were determined by XRD and SEM, coupled with EDS mapping. The real part of the complex relative permittivity of the samples was measured at 200 MHz. The loss tangent of all samples was below the resolution of the measurement setup. A strong correlation between the relative permittivity and the density agrees with previously published data. This is the peer-reviewed version of the article: Obradović, N., Pavlović, Vera, Kachlik, M., Maca, K., Olćan, D., Đorđević, A., Tshantshapanyan, A., Vlahović, B., Pavlović, Vladimir, 2018. Processing and properties of dense cordierite ceramics obtained through solid-state reaction and pressure-less sintering. Advances in Applied Ceramics. [https://doi.org/10.1080/17436753.2018.1548150]

Corundum (α-alumina) is a suiTab. material for usage in various industry fields owing to its chemical stability, electrical and mechanical features. It is known that properties of ceramics could be modified by addition of different oxides, as well as by changing the consolidation parameters. In this respect, alumina was doped with 1 wt.% of Cr2O3, Mn2O3 and NiO, followed by 1 hour of mechanical activation in a high-energy planetary ball mill. A sensitive dilatometer was used for sintering of powder mixtures up to 1400 oC and recording the obtained dilatation. The final density varied between cca. 1.9 and 3.3 g/cm3. Microstructural changes were detected by SEM measurements. Changes in electrical permittivity and loss tangent were associated with the preparation conditions (types of additives, duration of mechanical activation). For a given mixture, the sintering increases the relative permittivity and decreases losses, exhibiting the optimal values of 8.32 and 0.027, respectively, for the sample activated 60 minutes and sintered, with the addition of MnO2. Mechanical measurements indicate significant differences in strength with the addition of different transition metal oxides. Samples with Mn and Ni, activated and sintered, with strength of 121 and 86 MPa, respectively, have a significantly higher tensile strength than the other tested samples, due to their more compact microstructures. [Project of the Serbian Ministry of Education, Science and Technological Development, Grant no. OI 172057, Grant no. III 45007 and Grant no. III 45019]

Alumina-based compounds have many technological applications and attract great attention even after decades of investigation, due to their good chemical, electrical, and mechanical properties. A mixture of several alumina modifications (α-, κ-, γ-Al2O3) doped with 1 wt% of Mn2O3 was used for this experiment. The powder was mechanically activated for 60 min in an ethanol medium. After compaction, green bodies were sintered in the temperature range from 1200 to 1400 °C. Microstructures of the obtained specimens were investigated by scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS). The influence of morphological changes on electrical and mechanical properties was examined in detail. We have developed a test fixture and corresponding software for measurement of the relative dielectric permittivity and the loss tangent of ceramic specimens. This new method overcomes the problems of dimensions and shape of samples, as well as the problem of applying silver paste. The accuracy is around 2% for ε'r and 0.003 for tanδ. We demonstrate that the mechanical activation (MA) has the dominant influence on lowering the characteristic temperatures and microstructure development, which further leads to increased permittivity and tensile strength. This is the peer reviewed version of the paper: Filipović, S., Obradović, N., Marković, S., Mitrić, M., Balać, I., Đorđević, A., Pavlović, V.B., 2019. The effect of ball milling on properties of sintered manganese-doped alumina. Advanced Powder Technology 30, 2533–2540. [https://doi.org/10.1016/j.apt.2019.07.033] Published version: [https://vinar.vin.bg.ac.rs/handle/123456789/8441]

Steatite, as ceramic with composition predominantly resting on magnesium silicate, was produced from economic resources – talc, aluminosilicate clays, and either BaCO3 or feldspar as flux. Titanium dioxide was a doping agent. Four steatite mixtures were mechanically activated in a planetary ball mill for 30, 45 or 60 min, prior to the thermal treatment. Two-step sintering with initial phase set at 1350 °C and holding period conducted at 1250 °C was applied to initiate diffusion and prevent grain growth. Thereby, a high density ceramic material with low-porous submicron structure was acquired. The effects of TiO2 addition on densification, microstructure, and dielectric characteristics of steatites were monitored. The thermal stability of green mixtures was tested by differential thermal and thermogravimetric analyses. Changes in crystallinity and mineral phase composition were observed by the X-ray diffraction technique. Microstructural visualization with spatial arrangements of individual chemical elements on surface of the sintered ceramics was acquired by scanning electron microscopy accompanied with EDS mapping. In order to test the possibility of employment of the obtained steatites in insulation materials, electrical measurements were conducted by recording variations of the dielectric constant and loss tangent as a function of alternations in the mix-design and the mechanical activation period. This is the peer reviewed version of the following article: Terzić, A., Obradović, N., Kosanović, D., Stojanović, J., Đorđević, A., Andrić, L., Pavlović, V.B., 2019. Effects of mechanical-activation and TiO2 addition on the behavior of two-step sintered steatite ceramics. Ceramics International 45, 3013–3022. [https://doi.org/10.1016/j.ceramint.2018.10.120] Published version: [https://hdl.handle.net/21.15107/rcub_dais_4635]

Magnesium aluminate, MgAl2O4 and other alumina-based spinels are ceramics with high hardness, high melting point and mechanical strenght. Spinels can also be used as dielectrics in microwave applications. The goal of this study was to examine the effects of mechanical activation and sintering temperatures on physico-chemical properties of spinel. MgAl2O4 was produced by solid state reaction between MgO and α-Al2O3. The starting powders were mixed by ball milling to homogenize without significant particle size reduction. Mechanical activation of mixed powders was performed in a high-energy planetary ball mill in air for 1 h. Powders were compacted at 300 MPa. Heat treatments were performed in air, at temperatures ranging from 1200 to 1600 °C with 2 h dwell time, to determine the amount of spinel formation as a function of temperature. Phase composition and microstructure of initial powders and heated samples were determined by means of X-ray diffraction, particle size analysis, and scanning electron microscopy. The influences of milling and consolidation parameters were studied by electrical measurements and mechanical characterization. The main conclusion of this study was that mechanical activation for 60 min initiated a mechano-chemical reaction, resulted in spinel formation at much lower temperatures than within non-activated powders, and indicated that final sintering stage started at much lower temperatures for activated powders. Changes in microstructure parameters, as a consequence of mechanical treatment and subsequent heating of investigated powder mixtures, strongly affect electrical and mechanical properties of the final ceramics. This is the peer-reviewed version of the article: Obradović, Nina, W. G. Fahrenholtz, Suzana Filipović, Darko Kosanović, A. Dapčević, Antonije Đorđević, Igor Balać, and Vladimir B. Pavlović. 2019. “The Effect of Mechanical Activation on Synthesis and Properties of MgAl2O4 Ceramics.” Ceramics International, [https://doi.org/10.1016/j.ceramint.2019.03.095] Published version: [https://hdl.handle.net/21.15107/rcub_dais_5763]

The focus of the study was on providing insights into interconnections between sintering and development of the crystalline microstructure, and consequently variations in dielectric behavior of four steatites fabricated from a low-cost raw material, i.e. talc. The changes, induced by the alternations of the binders (bentonite, kaolin clay) and fluxing agents (BaCO3, feldspar), were monitored in the temperature range 1000 degrees to 1250 degrees C in which complete densification and re-crystallization of the investigated structures were accomplished. The critical points in the synthesis of steatite materials were assessed by instrumental analyses. Crystallinity changes and mineral phase transition during sintering were monitored by X-ray diffraction technique. Microstructural visualization of the samples and the spatial arrangements of individual chemical elements were achieved via scanning electron microscopy accompanied with EDS mapping. The thermal stability was observed on the green mixtures using differential thermal and thermo gravimetric analyses. Electrical measurements recorded variations of the dielectric constant (epsilon(r)) and loss tangent (tan delta) as a function of the sintering temperature. The investigation highlighted critical design points, as well as the optimal combinations of the raw materials for production of the steatite ceramics for advanced electrical engineering applications.

The aim of this study is to explore the influence of the processing route on the structural and physical properties of bulk MgTiO3 ceramics. Commercially available MgO and TiO2 powders were mechanically activated in a planetary ball mill. Green bodies were formed by an isostatic pressure of 300 MPa. The sintering of these samples was done either by the Two-Step Sintering (TSS) approach or by conventional pressureless sintering followed by Hot Isostatic Pressing (post-HIPing). The first set of compacts was sintered by TSS in air at 1300 degrees C for 30 min and the next step was performed at 1200 degrees C for 20 h. The density of the obtained samples after the two-step sintering reached almost 90% of the theoretical density (%TD). The second set of compacts was sintered at 1400 degrees C for 30 min in air. The samples without open porosity were post-sintered by the HIP at 1200 degrees C for 2 h in an argon atmosphere at a pressure of 200 MPa. The density significantly increased up to 96%TD. The differences between samples prepared by these two techniques were also analyzed by XRD and SEM. The lattice vibration spectra were obtained using Raman spectroscopy and they indicate a high degree of lattice disorder, as well as high values of the oxygen vacancy concentration. Electrical characteristic were established in the frequency range from 10 kHz to 10 GHz. The choice of the processing route had considerable influence on structural and physical properties of MgTiO3 ceramics. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

The aim of this paper is to determine the influence of the position of the electrodes on the range of a plasma jet, for specific experimental conditions, by using time-resolved optical emission spectroscopy. The optimal position of the electrodes is determined for a fixed gas flow rate and applied excitation voltage. We characterize the helium plasma jet for different distances from the end of the glass tube, showing detailed results for four different electrode positions from the jet nozzle (7, 15, 30 and 50mm). It was found that at the distance of 15mm, the length of the plasma jet is at its maximum. The highest speeds of the plasma package travelling outside the glass tube of the atmospheric plasma jet are obtained for the same electrode configuration (15mm from the jet nozzle). With the electrodes positioned at smaller distances from the nozzle, the plasma plume was much shorter, and at the larger distances the plasma did not even leave the glass tube.

We study development of plasma packages in atmospheric pressure plasma jet from their formation as a discharge close to the instantaneous cathode, following their motion between and inside the electrodes up to their emergence at the edge of the glass tube and formation of a plasma bullet. Inside both electrodes, plasma is concentrated close to the walls and is bright, while outside it is located at the axis. This paper opens issues of the geometry of electrodes, fields, and atomic processes, allowing some predictions to be made about pertinent mechanisms.