Mohamad M. Ahmad

Aging-induced dielectric relaxation in barium titanate ceramics

In this letter the authors report on the aging effect on the phase transitions and dielectric properties of 1mol% Mn-doped BaTiO3 ceramics. Aging the ceramics in the ferroelectric state stimulates a resistance for the orthorhombic-tetragonal and tetragonal-cubic phase transitions, which leads to stabilization of the tetragonal phase over a wider temperature range. The dielectric constant of the aged sample shows dispersion, associated with a dielectric loss relaxation peak near the 100kHz frequency region, in contrast to the unaged one. The present results are discussed in light of the existing models for ferroelectric domain stabilization.

Structure, ionic conduction, and giant dielectric properties of mechanochemically synthesized BaSnF4

BaSnF4 fluoride ion conductor has been prepared by mechanochemical milling technique at room temperature. The as-prepared material crystallizes in the cubic fluorite-type structure due to disordering of metal cations. The cubic phase transforms to tetragonal structure by annealing at 460 K. The ionic conductivity of the cubic phase is two to three orders of magnitude lower than the annealed sample. The complex conductivity of the investigated materials has been analyzed by a power-law model in order to extract the hopping rates and the concentration of mobile ions, the parameters that control the overall conduction behavior. The estimated values of the concentration of mobile ions are found to be of the same order, indicating that the conduction process is controlled by the mobility of charge carriers. BaSnF4 has been found to belong to a new class of nonoxidic giant dielectric constant materials with a value of e′∼105, which is independent of temperature and frequency over wide ranges. It is found that t...

Dielectric behavior and ac conductivity study of NiO∕Al2O3 nanocomposites in humid atmosphere

Humidity sensing characteristics of NiO∕Al2O3 nanocomposites, prepared by sol-gel method, are studied by impedance spectroscopy. Modeling of the obtained impedance spectra with an appropriate equivalent circuit enables us to separate the electrical responses of the tightly bound chemisorbed water molecules on the grain surfaces and the loosely associated physisorbed water layers. Dependence of the dielectric properties and ac conductivity of the nanocomposites on relative humidity (RH) were studied as a function of the frequency of the applied ac signal in the frequency range of 0.1–105Hz. The electrical relaxation behavior of the investigated materials is presented in the conductivity formalism, where the conductivity spectra at different RHs are analyzed by the Almond-West formalism [D. P. Almond et al., Solid State Ionics 8, 159 (1983)]. The dc conductivity and the hopping rate of charge carriers, determined from this analysis, show similar dependences on RH, indicating that the concentration of mobile...

Giant dielectric constant in CaCu3Ti4O12 nanoceramics

Nanoceramics of CaCu3Ti4O12 (CCTO) were synthesized by mechanosynthesis and spark plasma sintering with grain size of 150–200 nm. Giant dielectric constant properties are observed in the CCTO nanoceramics due to internal barrier layer capacitance (IBLC) effects. Impedance spectroscopy data suggest that the presence of resistive grain boundaries in addition to resistive domain boundaries is the origin of the IBLCs in CCTO nanoceramics.

Hopping rates and concentrations of mobile fluoride ions in Pb1−xSnxF2 solid solutions

In the present paper, the ion dynamics and relaxation of fluoride ions in Pb(1-x)Sn(x)F(2) (with x=0.2-0.6) solid solutions, prepared by mechanochemical milling, are studied in the conductivity formalism over wide ranges of frequencies and temperatures. The conductivity spectra of the investigated materials are analyzed by the Almond-West (AW) power-law model. The estimated values of the hopping rates and the dc conductivity of different compositions are thermally activated with almost the same activation energy. The calculated values of the concentration of mobile ions, n(c), are almost independent of temperature and composition for x=0.2-0.4. The maximum value of n(c) is obtained for the x=0.6 sample, although it does not show the maximum conductivity. Therefore, the composition dependence of the ionic conductivity of these solid solutions could be explained based on the extracted parameters. The results presented in the current work indicate that the AW model represents a reasonable approximation of the overall frequency-dependent conductivity behavior of the investigated materials. The conductivity spectra at different temperatures for each composition are successfully scaled to a single master curve, indicating a temperature-independent relaxation mechanism. For different compositions, however, the conductivity spectra cannot be scaled properly, indicating composition-dependent relaxation dynamics.

Fluoride ion diffusion of superionic PbSnF4 studied by nuclear magnetic resonance and impedance spectroscopy

Superionic material PbSnF4 prepared by precipitation from aqueous solution was characterized by x-ray diffraction, DTA and 19 F nuclear magnetic resonance (NMR) techniques. The temperature and frequency dependence of the conductivity, the modulus and the dielectric properties were investigated by means of impedance spectroscopy. The Arrhenius plot of the dc conductivity shows a gradual slope change around 355 K from a high-activation-energy (Eσ = 30 kJ mol−1) region to a low-activation-energy region (Eσ = 22 kJ mol−1). At lower temperatures the real part of the ac conductivity exhibited a power-law behaviour as found in most ionic conductors. At high temperatures, on the other hand, a low-frequency dispersion of the conductivity was observed due to the space charge polarization, which resulted from the high ionic conductivity. An extremely large dielectric constant was observed with decreasing frequency due to the space charge polarization. A dielectric anomaly was also observed around the phase transition temperature. The modulus formalism was used to estimate the conductivity relaxation times and was compared with those from the line width transition of the 19 F NMR.

Superionic PbSnF4: A giant dielectric constant material

In this letter, the authors report on the giant dielectric constant properties in a hitherto unexplored material, the superionic conductor lead tin fluoride PbSnF4. Despite its superionic behavior, PbSnF4 is surprisingly found to exhibit extraordinary high dielectric constant (e′∼105). e′ is almost independent of temperature and frequency over wide ranges. The authors find that the high value of e′ originates from an internal effect, such as internal barrier layer capacitor, rather than from a surface barrier layer capacitor formed at the electrode/sample interface.

Dielectric relaxation properties of Pb1?xSnxF2 solid solutions prepared by mechanochemical milling

The mechanochemical milling technique is employed to synthesize Pb1?xSnxF2 solid solutions (with x = 0.2?0.6) at room temperature. The obtained samples with x = 0.2?0.5 show x-ray powder diffraction patterns similar to the ?-PbF2 cubic structure, with the crystallite size in the range 25?29?nm. The ionic conductivity of the investigated materials, determined from impedance spectroscopy measurements, shows a maximum at x = 0.5, reaching a value of 2.28 ? 10?3?S?cm?1 at room temperature. The electrical relaxation dynamics are studied in the electric modulus formalism in the frequency range 50?Hz?1?MHz over a wide temperature range. The conductivity relaxation time, for different compositions, is thermally activated with the same activation energy of the dc conduction. Scaling of the modulus spectra at different temperatures and compositions indicates that the relaxation dynamics are independent of temperature but depend on the microscopic structure and/or the concentration of mobile charge carriers. The frequency dependence of the dissipation factor, tan ?, shows a dielectric relaxation process, most probably originating from interfacial polarization.

Estimation of the concentration and mobility of mobile Li+ in the cubic garnet-type Li7La3Zr2O12

Li7La3Zr2O12 (LLZ) lithium ion conductors with the garnet-like structure are promising candidates for applications in all solid-state lithium ion batteries. Due to the complexity of the structure and the distribution of Li+, it was difficult to get information on the true concentration of mobile Li+, nc, and their mobility. In this report, we estimate for the first time the values of nc from the analysis of the conductivity spectra at different temperatures. We found that only a small fraction of Li+ contributes to the conduction process. nc is found to be independent of temperature with an average value of 3.17 × 1021 cm−3, which represents 12.3% only of the total Li+ content in LLZ garnets. Comparing the conduction parameters of LLZ with Li6BaLa2Ta2O12 (LBLT) and Li5La3Ta2O12 (LLT) garnets indicates that the mobility of Li+ plays a prominent role in the conductivity enhancement in LLZ garnet materials. The diffusion coefficient of LLZ at room temperature has a value of 1.33 × 10−8 cm2 s−1, which is comparable with other fast Li+ conductors.

Grain size effect on the giant dielectric constant of CaCu3Ti4O12 nanoceramics prepared by mechanosynthesis and spark plasma sintering

In the present work, CaCu3Ti4O12 (CCTO) nanoceramics with different grain sizes were prepared by spark plasma sintering (SPS) at different temperatures (SPS-800, SPS-900, SPS-975, and SPS-1050) of the mechanosynthesized nano-powder. Structural and microstructural properties were studied by XRD and field-emission scanning electron microscope measurements. The grain size of CCTO nanoceramics increases from 80 nm to ∼200 nm for the ceramics sintered at 800 °C and 975 °C, respectively. Further increase of SPS temperature to 1050 °C leads to micro-sized ceramics of 2–3 μm. The electrical and dielectric properties of the investigated ceramics were studied by impedance spectroscopy. Giant dielectric constant was observed in CCTO nanoceramics. The dielectric constant increases with increasing the grain size of the nanoceramics with values of 8.3 × 103, 2.4 × 104, and 3.2 × 104 for SPS-800, SPS-900, and SPS-975, respectively. For the micro-sized SPS-1050 ceramics, the dielectric constant dropped to 2.14 × 104. The dielectric behavior is interpreted within the internal barrier layer capacitance picture due to the electrical inhomogeneity of the ceramics. Besides the resistive grain boundaries that are usually observed in CCTO ceramics, domain boundaries appear as a second source of internal layers in the current nanoceramics.In the present work, CaCu3Ti4O12 (CCTO) nanoceramics with different grain sizes were prepared by spark plasma sintering (SPS) at different temperatures (SPS-800, SPS-900, SPS-975, and SPS-1050) of the mechanosynthesized nano-powder. Structural and microstructural properties were studied by XRD and field-emission scanning electron microscope measurements. The grain size of CCTO nanoceramics increases from 80 nm to ∼200 nm for the ceramics sintered at 800 °C and 975 °C, respectively. Further increase of SPS temperature to 1050 °C leads to micro-sized ceramics of 2–3 μm. The electrical and dielectric properties of the investigated ceramics were studied by impedance spectroscopy. Giant dielectric constant was observed in CCTO nanoceramics. The dielectric constant increases with increasing the grain size of the nanoceramics with values of 8.3 × 103, 2.4 × 104, and 3.2 × 104 for SPS-800, SPS-900, and SPS-975, respectively. For the micro-sized SPS-1050 ceramics, the dielectric constant dropped to 2.14 × 104. The...