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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. [ABSTRACT FROM AUTHOR]

Magnesium titanate ceramics were prepared by reactive spark plasma sintering (SPS) at 1200 °C for 5 min. Prior to sintering, MgO and TiO 2 powders were mixed by high energy ball milling (HEBM) for 15, 30, or 60 min. The effect of milling time on phase composition was analyzed by X-ray diffraction (XRD) for milled powders and sintered specimens. The morphology of the sintered ceramics was investigated by scanning electron microscopy (SEM), while elemental distribution was determined by energy dispersive spectroscopy (EDS). The presence of the MgTi 2 O 5 phase was detected in XRD and was confirmed by EDS analysis. Microcracking was observed for specimens prepared with longer milling times and attributed to the coefficient of thermal expansion (CTE) mismatch dominantly due to the anisotropic crystal structure of MgTiO 3 phase. The sample milled for 15 min showed the highest Vickers hardness due to less or no microcracking and nearly full density. The dielectric measurements in the range of 10 Hz–250 MHz demonstrated extremely high dielectric permittivity, as high as 104 at 1 kHz. An increase in loss tangents was observed due to oxygen vacancy formation, which was promoted by the vacuum environment during SPS. Oxygen vacancy-related dielectric relaxation was also detected and explained. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract Steatite, as ceramic with composition predominantly resting on magnesium silicate, was produced from economic resources – talc, aluminosilicate clays, and either BaCO 3 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 TiO 2 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. [ABSTRACT FROM AUTHOR]

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 (BaCO 3 , feldspar), were monitored in the temperature range 1000° to 1250 °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 (ε r ) and loss tangent (tan δ) 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. [ABSTRACT FROM AUTHOR]