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Strontium (Sr) doped Calcium Copper Titanate Ca1−xSrxCu3Ti4O12 with two values x = 0 and x = 0.05 commonly known as CSCTO ceramics have been obtained by using the semi wet route synthesis method. Rietveld refinement shows that the powders crystallize in the cubic perovskite related structure with Im3 space group. Scanning Electron Micrograph (SEM) analysis shows that the average grain size of ceramics becomes larger when Sr doping is considered. Both DC and AC electrical conductivity are investigated throughly in the temperature and frequency ranges between [373–653 K] and [20 Hz–1MHz], respectively. The values of the average activation energies for CSCTO-0 (x = 0) and CSCTO-5 (x = 0.05) were found to be 612 meV and 576 meV, respectively. In Fact, Sr doping has an effect on the broadening of the impurity band that can lead to a dramatic decrease in the activation conduction energy of CSCTO ceramics. A systematic study of AC electrical conductivity reveals that the predominant conduction mechanism existing in these ceramics is generated by large polaron hopping process. This mechanism has also been identified by the modulus analysis and confirmed by comparing the hopping polaron size with the lattice parameter.

The Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) ceramics were successfully prepared by the sol-gel process and sintered at 1420 °C. The effect of sintering times (2, 4 and 6 h) on structural, microstructural, electric properties, energy storage, and electrocaloric effect was systematically examined. X-ray diffraction (XRD) results show that all the samples crystallize in the pure perovskite structure. The morphotropic phase transition from the tetragonal to the orthorhombic phase (T-O) was identified and confirmed by Rietveld refinement. The BCZT ceramic sintered at 1420 °C for 4 h possesses a good relative density of 98% and exhibits optimal properties with a high dielectric permittivity (er,max) ~16310, a large electrocaloric effect coefficient (ξ) ~ 0.244Kmm/kV and a high energy storage efficiency of 63.65%.

International audience; A series of Pb-free ferroelectric materials Ba0.9Sr0.1Ti1-xSnxO3 (BSTS-x) with 0

In the present work, an attempt has been made to study theoretically and experimentally the AC electrical conduction mechanism in disordered semiconducting materials. The key parameter considered in this analysis is the frequency exponent s ( ω , T ) = ( ∂ ln ( σ A C ( ω , T ) ) ∂ ln ( ω ) ) T , where σ AC is the AC electrical conductivity that depends on angular frequency ω and temperature T. In the theoretical part of this work, the effect of the barrier hopping energy, the polaron radius and the characteristic relaxation time is considered. The theoretical models of Quantum Mechanical Tunneling (QMT), Non overlapping Small Polaron Tunneling (NSPT), Overlapping Large Polaron Tunneling (OLPT) and Correlated Barrier Hopping (CBH) are considered to fit experimental data of σ AC in p -CuIn 3 Se 5 (p-CIS135) in the low temperature range up to 96 K. Some important parameters, as the polaron radius, the localization length and the barrier hopping energies, are estimated and their temperature and frequency dependence discussed.

By using the semi-wet synthesis method, lead-free ferroelectric materials Ba0.9Sr0.1Ti1-xSnxO3 with x = 0, 0.02, 0.05, and 0.10 (abbreviated as BSTS) were prepared and their structural, electric and electrocaloric properties were investigated. The X-ray diffraction (XRD) patterns show that the samples calcined at 950 °C have well crystallized into perovskite structure suggesting the substitution of Ti4+ by Sn4+ in BST lattice. With increasing content of Sn, the enhancement of the dielectric permittivity was observed for (0 ≤ x ≤ 0.05) and the ferroelectric transition temperature (TC) was found to shift towards the room temperature (TC = 20 °C for x = 0.10). Direct measurements of the electrocaloric effect (ECE) were performed on all samples by using the high-resolution calorimeter. It is found that Ba0.9Sr0.1Ti0.95Sn0.05O3 exhibits a high ECE temperature change of ΔTEC = 0.188 K at an applied electric field of only 7 kV/cm. Impedance spectrum analysis of all the samples performed in the temperature range of 300–360 °C reveals the existence of two relaxation contributions related to the grain and grain boundaries that are well separated in frequency. Activation energies of conduction and relaxation processes were deduced for both contributions in order to determine the conduction mechanism of the studied compositions.

We investigate the lead free ferroelectric perovskite ceramic, (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT), prepared by sol-gel method. The as-synthesized precursor powder is calcined at 1000 °C for 4 h in order to obtain single-phase perovskite structure. X-ray diffraction (XRD) analysis at room temperature shows the formation of single-phase compound with a tetragonal structure refined in P4mm space group. The dielectric measurements in a frequency range of [100 Hz-1 MHz] and in a temperature range [35–400 °C] in the ceramic sintered at 1350 °C for 2 h, reveal a high maximum dielectric constant ∼5200 (100 Hz) at 107 °C, which is associated with ferroelectric paraelectric phase transition. The sol-gel method has displayed a very important advantages comparing to the conventional solid-state method. Special emphasis was done to characterize diffuse phase transition (DPT) that occurs close to ∼107 °C. We have shown that the real part of the permittivity close to DPT is well described by Santos–Eiras phenomenological model. Impedance spectroscopy analysis over the frequency range of 100 Hz to 1 MHz illustrated mainly bulk contribution up to 300 °C while grain and grain-boundary effects were present above 300 °C using the impedance spectroscopy technique.

We investigate the relaxation and conduction mechanism of Ba0.90Sr0.10TiO3 (BST) ceramic, synthesized by the solid state reaction method. The dielectric and relaxation properties are analyzed in the temperature range of 380–450 °C with alternative current in the frequency range of 20 Hz-1MHz. Variation of dielectric constant, e′, with temperature shows a normal ferroelectric transition at Tc = 95 °C with a weak degree of diffuseness. The modified Cole-Cole equation is used to describe all contributions to the relaxation mechanism. The frequency exponent m(ω,T) deduced from experimental data of the dielectric loss (e″) as m ( ω , T ) = ( ∂ ln e ″ / ∂ ln ω ) T shows a temperature and frequency dependence. Two conduction process are observed: non-overlapping small-polaron tunneling (NSPT) at low frequencies and overlapping large polaron tunneling (OLPT) at high frequencies. The analysis of Nyquist plots reveals also the presence of two contributions, who which the activation energies have been calculated.

Substitution of Pb with Li and K in the Pb2KNb5O15 phases leads to a new composition with chemical composition Pb2−x K1+x Li x Nb5O15 which crystallizes with tetragonal tungsten bronze-type structure. Ferroelectric ceramics with different compositions were synthesized using solid-state reaction and complex dielectric permittivity measurements in these compounds were performed in a frequency and temperature range of 20 Hz–1 MHz and from 25 to 550 °C, respectively. Special attention was paid to the diffuse phase transition (DPT) that occurs close to the Curie temperature. The empirical equation proposed by Santos–Eiras for a phenomenological description of the temperature dependence of the dielectric permittivity ( $$\varepsilon_{\text{r}}^{{\prime }}$$ ) peak is used to calculate some characteristic parameters of DPT. From the results, it must be assumed that these compounds show a diffuse phase transition with non-relaxor behavior. A basic phase diagram showing the evolution of T m function of composition x is deduced from this study.

We studied the structural and dielectric properties of new Tetragonal Tungsten Bronze (TTB) ceramics Pb 1.85 K 1.15 Li 0.15 Nb 5 O 15 that was synthesized by solid-state reaction. We pay a special attention to the diffuse phase transition (DPT) that occurs close to 425 °C. Using dielectric measurements in a frequency range of 10 Hz–1 MHz and in the temperature range 30–560 °C, we have shown that the real permittivity close to DPT is well described by Santos-Eiras phenomenological model. Space-charge polarization, relaxation phenomena and free charges conductivity have been analyzed using dielectric spectroscopy impedance and modulus characterization. Cole-Cole plots show a non-Debye (polydispersive) type relaxation. In paraelectric phase the Arrhenius activation energy was determined as E τ = 0.72 eV. We demonstrated that frequency dependence of ac conductivity at different temperatures obeys the Jonscher's universal law: σ ac = σ dc + A( ω ) n .

Ba1-xMgx(Ti0.9Zr0.1)O3 (x = 0.01 and 0.02) ceramics were prepared using the conventional solid state reaction. Rietveld refinement performed on X-ray diffraction patterns indicates that the samples are tetragonal crystal structure with P4mm space group. By increasing Mg content from 1 to 2% the unit cell volume decreased. Likewise, the grains size is greatly reduced from 10 μm to 4 μm. The temperature dependence of dielectric constants at different frequencies exhibited typical relaxor ferroelectric characteristic, with sensitive dependence in frequency and temperature for ac conductivity. The obtained activation energy values were correlated to the proposed conduction mechanisms.