Capacitive characteristics and electrosorption of hydrogen in microporous aсtivated carbon fibers

The possibility of hydrogen electrosorption in microporous activated carbons for acidic and alkaline electrolytes is shown. The electrochemical behavior of activated carbon fiber CH-900-20 has been investigated to find out the main factors, which influence the charging processes of microporous electrodes. The following methods were used: cyclic voltammetry, electrochemical impedance spectroscopy, chronoamperometry, and chronopotentiometry. A comparative analysis of the influence of the electrolyte type and electrode structure on the capacitance performances of carbon has been carried out. It is assumed that there is a potential barrier at the interface between micropores and macropores. The features of the process of hydrogen electrosorption in carbon micropores have been analyzed in detail. A possible mechanism of electrosorption in micropores that decreases the capacitance of the electric double layer with a corresponding drop of the electronic conductance of activated carbons in acidic electrolytes has been suggested. The influence of temperature on the kinetics of hydrogen electrosorption has been shown and the activation energy of this process has been estimated. An assumption has been made about the influence of the semiconductor conductivity of activated carbons on the capacitive and resistive characteristics of the electrodes.     

The influence of processing methods on the dielectric properties of BaTi1-xGdxO3-x/2 - Based materials

In this study, the intermediate rare-earth oxide Gd₂O₃ (Gd) was substituted in different amounts (x = 0.2–2 mol%) for the formulation of BaTi_1-xGd_xO_3-x/2 (BTG_x) dielectric materials. The effect of B-site substitution was confirmed by the additional Raman active A_1g octahedral peak at ~835cm^-1 strengthened at x ≥ 0.4 mol%. Additionally, properties of 0.9BTG_0.007-0.1BA dielectric ceramics were analysed based on the influence of various processing methods as a function of sintering temperature. The focal samples were labelled Method-A (direct-mix) and Method-B (indirect-mix). As the sintering temperature (1075–1200 °C) increased, the 1 kHz response of the ε–T curves of Method-A samples transformed from a single peak to broad-narrow double peaks of high dielectric loss tangent (tan δ). Nonetheless, samples of Method-B possessed a clearly defined transmission electron microscopy (TEM) core-shell structure, flattened double-peak ε-T curves, optimised dielectric properties (ε = ~1563–1851 and tan δ < 1.5% at room temperature), and a wide-ranging temperature behaviour that meets the X8R dielectric standards (ΔC/C_25°C < ±15%). The maximum dielectric breakdown strength of Method-B samples reached ~131 kVcm, while the energy storage density was ~0.726 J/cm³ at a maximum efficiency of ~80% at 1100 °C. Thus, exhibiting good potentials for balancing temperature stability with energy storage applications.     

DC conductivity and AC impedance of Mn doped magnesia alumina spinel (MgAl2-2xMn2xO4) over a large temperature range

MgAl_2-2xMn_2xO₄ (MAMO) with x = 0-0.12 was synthesized in a single-phase form by solid-state reaction. XRD analysis showed that the samples had the cubic center structure of the Fd-3 m space group. Electrical properties of the samples were studied over the temperature range of 300 K∼1073 K. The results showed that the DC conductivity (σ_DC) increased from 10⁻¹¹S/cm at 300 K (MAMO, x = 0) to 10^-3S/cm at 1073 K (MAMO, x = 0.12). The equivalent circuit of the complex impedance spectra suggested that the relaxation of charge carriers was of non-Debye type. The conduction was mainly caused by grain boundaries and the capacitance was mainly attributed to polarization. The complex permittivity values (ε’ and ε’’) were increased by two orders of magnitude with the increase in Mn content and temperature over the measured frequency range (1 Hz-1 MHz). Therefore, doping with Mn could be applied to modify the electrical properties of MAMO at high temperature.     

Multiferroic behaviour in ‘Bi’ doped solid solution SmFeO3-BaTiO3 perovskite system

The solid solution of (Sm_0.75Bi_0.25FeO₃)_0.5 (BaTiO₃)_0.5 perovskite system is developed through conventional solid state reaction route. Prepared compound is thoroughly analyzed for its multipurpose use by studying its multiferroic character. The XRD spectra verifies the synthesized material is crystallize in tetragonal structure (space group = P4mm). The identification of the involved elements and their actual oxidation states are inspected through X-ray photoelectron spectroscopic (XPS) technique. Dielectric studies reveal the material has high dielectric constant at room temperature for possible storage devices. The relaxation process in the system is related to the short-range portability of charge transporters as studied from modulus spectra. Material can be beneficial for memory devices according to the room temperature multiferroic studies.     

Effect of sintering temperature on the transport properties of La2Ce2O7 ceramic materials

La₂Ce₂O₇ (LCO) based materials are of a paramount importance since they can be utilized for ammonium production, thermal barrier application, catalysts, hydrogen production and solid oxide fuel cells (SOFCs). In this work, a nano crystalline LCO powder was prepared using glycine-nitrate combustion method and then its properties were comprehensively characterized. The structural analysis of the synthesized LCO was carried out using conventional X-ray diffraction (XRD) and Raman spectroscopy. In a disordered phase, LCO is a biphasic mixture composed of C- and F-type phases. Densification studies were performed by sintering LCO pellets at different sintering temperatures. A densification of ≥95% was observed in all the samples with a very little variation. Sintering temperature had a marked effect on the electrical conductivity of LCO. The LCO sintered at 1100 °C showed the highest conductivity (3.68 mS/cm at 700 °C in air). The electrical conductivity was found to be decreasing with an increase in sintering temperature from 1100 to 1400 °C. To understand the behavior, the analysis of distribution function of relaxation times (DFRTs) utilized for correct separation of grain and grain boundary resistances. The presence of C- and F- type phases calculated from Raman spectra plays a crucial role in deciding conduction behavior of LCO. The results suggest a strong relationship between history of the ceramics preparation and their electrical properties.     

Impedance matching optimization of SiCf/Si3N4–SiOC composites for excellent microwave absorption properties

SiC fiber reinforced ceramic matrix composites (SiC_f-CMCs) are considered to be one of the most promising materials in the electromagnetic (EM) stealth of aero-engines, which is expected to achieve strong absorption and broad-band performance. Multiscale structural design was applied to SiC_f/Si₃N₄–SiOC composites by construction of micro/nanoscale heterogeneous interfaces and macro double-layer impedance matching structure. SiC_f/Si₃N₄–SiOC composites were fabricated by using SiC fibers with different conductivities and SiOC–Si₃N₄ matrices with gradient impedance structures to improve impedance matching effectively. Owing to its unique structure, SiC_f/Si₃N₄–SiOC composites (A3-composites) achieved excellent EM wave absorption performance with a minimum reflection coefficient (RC_min) of ?25.1 dB at 2.45 mm and an effective absorption bandwidth (EAB) of 4.0 GHz at 2.85 mm in X-band. Moreover, double-layer SiC_f/Si₃N₄–SiOC with an improved impedance matching structure obtained an RC_min of ?56.9 dB and an EAB of 4.2 GHz at 3.00 mm, which means it can absorb more than 90% of the EM waves in the whole X-band. The RC is less than ?8 dB at 2.6–2.8 mm from RT to 600 °C in the whole X-band, displaying excellent high-temperature absorption performance. The results provide a new design opinion for broad-band EM absorbing SiC_f-CMCs at high temperatures.     

Electrical properties of calcia-stabilised zirconia ceramics

The electrical properties of cubic, calcia-stabilised zirconia ceramics, Ca_xZr_1-xO_2-x: 0.12 ≤ x ≤ 0.18 have been investigated using impedance spectroscopy to separate bulk, grain boundary and electrode contact impedances. The most appropriate equivalent circuit to characterise the bulk response required inclusion of a dielectric component, represented by a series RC element, in parallel with the oxide ion conductivity represented by a parallel combination of a resistance, capacitance and constant phase element. The dielectric component may be attributed to defect complexes involving immobile oxygen vacancy pairs whereas long range conduction involves single oxygen vacancies.     

Application of the cold sintering process to the electrolyte material BaCe0.8Zr0.1Y0.1O3-δ

This paper describes and discusses the application of the original sintering process named cold sintering to the electrolyte material BaCe_0.8Zr_0.1Y_0.1O_3-δ to enhance its densification at a temperature below that needed in a conventional sintering. This new technique enables the acceleration of the densification resulting in a more compacted microstructure with an unexpected high relative density of 83 % at only 180 °C. A subsequent annealing at 1200 °C further enhances the densification which reaches 94 %. The electrochemical performance of CSP sintered ceramics was investigated and optimized by varying different process parameters. The comparison with the conventional sintered material reveals an increase of the total conductivity by mostly increasing the grain boundary one. This result emphasizes the benefits of CSP to not only reduce the sintering temperature but also to enhance the electrochemical properties.