Influence of sintering conditions and doping on the dielectric relaxation originating from the surface layer effects in CaCu3Ti4O12 ceramics

Detailed investigations into the dielectric dispersion phenomenon in the giant dielectric constant material CaCu₃Ti₄O₁₂ (CCTO) around room temperature revealed the existence of two successive dielectric relaxations. In the temperature domain, a new dielectric relaxation was clearly observed around 250 K, in addition to the well-investigated dielectric relaxation close to 100 K. The effect of sintering and doping (La³⁺) on the strength of these dielectric relaxations were studied in detail. The sintering temperature as well as its duration was found to have tremendous influence on the dielectric relaxation that was encountered around 250 K. This Maxwell-Wagner (M-W) type of relaxation was found to be originating from the surface layer containing the Cu-rich phase, which was ascribed to the difference in the oxygen content between the surface and the interior of the sample. Interestingly, this particular additional relaxation was not observed in La_2/3Cu₃Ti₄O₁₂, a low dielectric constant member of the CCTO family, in which the segregation of Cu-rich phase on the surface was absent. Indeed the correlation between the new relaxation and the presence of Cu-rich phase in CCTO ceramics was further corroborated by the absence of the same after removing the top and bottom layers.     

Effects of excess Bi2O3 on grain orientation and electrical properties of CaBi4Ti4O15 ceramics

A CaBi₄Ti₄O₁₅ (CBTO) ceramic in which the Bi₂O₃ concentration was controlled from 0 to 10 wt% was fabricated using a solid-state reaction method. Structural analysis by field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) indicated differences in the preferred grain orientation and size of the plate-like grains according to the Bi₂O₃ concentration. The orientation of plate-like grains was also found to vary with the Bi₂O₃ concentration. There was no noticeable change trend of dielectric properties with different Bi₂O₃ concentrations. Relatively low dielectric constants (about 135) were exhibited by the CBTO ceramic with 1 wt% Bi₂O₃ and CBTO ceramic with 10 wt% Bi₂O₃ only, and similar values (about 150) were exhibited by the other ceramics. The dielectric loss exhibited a low value in the range of 0.01–0.09 for all samples (frequency range of 1–100 kHz). Regarding the ratio changes of the piezoelectric coefficient (d₃₃) and the ratio of a-axis orientation of plate-like grains, the trends of these two values were shown to be similar. These results suggest that the addition of Bi₂O₃ greatly influences the microstructure of CBTO ceramics, including the grain size and orientation of plate-like grains. In particular, the change in the preferred grain orientation is closely related to the change in the piezoelectric properties.     

Glassy properties of the relaxor ferroelectric strontium barium niobate at low temperatures

Relaxor ferroelectrics are known to show a low-temperature behaviour that quantitatively resembles the features of canonical glasses. In order to get a comprehensive characterisation of the responsible low-energy excitations, single crystals of strontium barium niobate were investigated below 10 K. The density of localised states was obtained from heat capacity, and their coupling to phonons from sound velocity and thermal conductivity. The presence of slowly relaxing systems could be estimated from heat release data, respectively.

In contrast with structural glasses, however, SBN and further relaxor ferroelectrics show an energy dependent density of states and an upper limit of the relaxation time spectrum. Considering these properties, which appear to be typical also for other glassy crystals, a modification of the standard tunneling model is proposed.     

Effect of sintering temperature on phase formation,microstructure and dielectric properties of nanogold modified barium titanate ceramics

In this work, BaTiO₃ ceramics modified with 0.5 mol% Au nanoparticles were fabricated by using a combination of the solid-state reaction and pressureless-sintering techniques. By employing a combination of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), Archimedes principle and dielectric measurement techniques, it was found that no phases other than tetragonal BaTiO₃ were observed in all ceramics. In contrast to the tetragonality, the relative density, grain size and maximum dielectric constant at Curie temperature of the ceramics were found to increase with sintering temperature. In addition, it has been found that, under suitable sintering temperature, dense perovskite nanogold modified BaTiO₃ ceramics with fine-grained microstructure (∼1 μm) and better dielectric properties than those of gold-free ceramics can be produced.     

Dielectric properties of Pb(Mg1/4Zn1/4Nb1/2)O11/4

Polycrystalline samples of Pb(Mg_1/4Zn_1/4Nb_1/2)O_11/4 have been synthesized by high temperature columbite precursor solid state reaction technique. Using X-ray diffraction (XRD) technique, compound formation in single phase cubic structure was observed and XRD analysis provided preliminary structural data. Detailed studies of dielectric properties of the compound reveal that this compound has high dielectric constant and diffuse phase transition in a wide range of temperatures around the Curie temperature. The charge deficiency of the compound presumably gets compensated in the high temperature columbite precursor process of sample preparation which is supported by single phasic form of the material.     

Evolution of microstructure and dielectric properties of (LiCe)-doped Na0.5Bi2.5Nb2O9 Aurivillius type ceramics

Aurivillius type (NaBi)_0.5?x(LiCe)_xBi₂Nb₂O₉ ceramics were prepared by the standard ceramics route. The single crystal structural ceramics were achieved for all compositions and lattice distortion was decreased by (LiCe) dopants. The temperature dependent dielectric properties revealed that all compositions possess a high Curie-temperature (>780 °C). A modified Curie–Weiss relationship is used to study the diffuseness behavior of a ferroelectric phase transition indicating the degree of diffuseness of NBN-based ceramics increased with (LiCe) modifications. The degradation of resistance implied a plausible model that Ce⁴⁺ ions entered into the B-site of the pseudo-perovskite structure and acted as acceptor doping. Further investigation demonstrated that both electrical conduction and dielectric relaxation processes were associated with the oxygen vacancies produced by the substitution of Nb⁵⁺ ions by the Ce⁴⁺ ions.     

The effects of oxygen vacancies on the electrical properties of W,Ti doped CaBi2Nb2O9 piezoceramics

Different type doped CaBi₂Nb₂O₉ (CBN) ceramics were prepared by a conventional solid state sintering method. The number of oxygen vacancies were decreased or increased by the introduction of W⁶⁺ and Ti⁴⁺ doping in CBN ceramics. The influence of W⁶⁺, Ti⁴⁺ and W⁶⁺/Ti⁴⁺ dopants on the microstructures and electrical properties of CBN-based ceramics was investigated. The voids and spherical morphology in the SEM image of W, Ti co-doped ceramics indicate that W, Ti co-doping could change the microstructure of CBN-based ceramics. Impedance analysis results show that the electrical properties of CBN-based ceramics have a close relationship with the number of oxygen vacancies. W doping and W, Ti co-doping decrease the oxygen vacancies, as a result, the resistance and piezoelectric properties were increased and the frequency dispersion of dielectric properties were restrained.     

Acoustic properties of multiferroic PbFe1/2Ta1/2O3

The longitudinal acoustic wave velocity and attenuation in PbFe_1/2Ta_1/2O₃ ceramics have been measured by pulse-echo technique in the temperature range from 4.2 to 530 K. The anomalies observed in the sound velocity and attenuation behavior versus temperature were correlated with Burns temperature, temperature range of the coexistence of relaxor ferroelectric and antiferromagnetic states, and a suggested second antiferromagnetic phase transition at low temperatures.     

Effect of sintering temperature on the structural, dielectric and ferroelectric properties of tungsten substituted SBT ceramics

Structural, dielectric and ferroelectric properties of tungsten (W) substituted SrBi₂(Ta_1−xW_x)₂O₉ (SBTW) [x=0.0, 0.025, 0.05, 0.075, 0.1 and 0.2] have been studied as a function of sintering temperature (1100-1250 °C). X-ray diffraction patterns confirm the single-phase layered perovskite structure formation up to x=0.05 at all sintering temperatures. The present study reveals an optimum sintering temperature of 1200 °C for the best properties of SBTW samples. Maximum T_c of ∼390 °C is observed for x=0.20 sample sintered at 1200 °C. Peak-dielectric constant (ε_r) increases from ∼270 to ∼700 on increasing x from 0.0 to 0.20 at 1200 °C sintering temperature. DC conductivity of the SBTW samples is nearly two to three orders lower than that of the pristine sample. Remnant polarization (P_r) increases with the W content up to x≤0.075. A maximum 2P_r (∼25 μC/cm²) is obtained with x=0.075 sample sintered at 1200 °C. The observed behavior is explained in terms of improved microstructural features, contribution from the oxygen and cationic vacancies in SBTW. Such tungsten substituted samples sintered at 1200 °C exhibiting enhanced dielectric and ferroelectric properties should be useful for memory applications.     

Colossal permittivity in (Li + Nb) co-doped Fe2O3 ceramics

(Li + Nb) co-doped (Li + Nb)_xFe_2-xO₃ (with x = 0.0005, 0.005, 0.05, and 0.1) ceramics were prepared by sol-gel method. Their structural, dielectric, humidity, and magnetic properties were investigated. Colossal permittivity (~10⁴) was approached or achieved in all doped samples even with a very small doping level of x = 0.0005. The colossal permittivity behavior is composed of two dielectric relaxations with the low-temperature one being a polaron relaxation due to electrons hopping between Fe³⁺ and Fe⁴⁺ ions and the high-temperature one being a Maxwell-Wagner relaxation caused by humidity-sensing properties.     

High-temperature dielectric properties of Gd2CuO4 ceramics

A sample of Gd₂CuO₄ (GCO) has been prepared through the solid state reaction technique. Dielectric properties of this material have been measured in detail as functions of temperature (between 285 and 450 K) and frequency (20 Hz-10 MHz). A step-like increase below 330 K and a broad peak around 360 K were observed in the real part of the permittivity (ε′) which were found to be originated from the oxygen vacancy hopping motions that cause a dipolar relaxation, followed by a Maxwell-Wagner relaxation as the hopping carriers are blocked by the interfaces and surfaces of the sample.     

Effect of B-site isovalent doping on electrical and ferroelectric properties of lead free bismuth titanate ceramics

In the present work, zirconium modified bismuth titanate ceramics have been studied as potential lead-free ferroelectric materials over a broad temperature range (RT – 800 °C). Polycrystalline samples of Bi₄Ti_3−xZr_xO₁₂ (x=0.2, 0.4, 0.6) (BZrT) with high electrical resistivity were prepared using the solution combustion technique. The effect of Zr doping on the crystalline structure, ferroelectric properties and electrical conduction characteristics of BZrT ceramics were explored. Addition of zirconium to bismuth titanate enhances its dielectric constant and reduces the loss factor as it introduces orthorhombic distortion in bismuth titanate lattice which is exhibited by the growth along (0010) lattice plane. Activation energy due to relaxation is found to be greater than that due to conduction thus confirming that electrical conduction in these ceramics is not due to relaxation of dipoles. Remanent polarization of the doped samples increases as the Zirconium content increases.     

Dielectric and AC conductivity behavior of BaBi2Nb2O9 ceramics

The complex dielectric and AC conductivity response of BaBi₂Nb₂O₉ relaxor ferroelectric ceramics were studied as a function of frequency (100 Hz-10 MHz) at various temperatures. The observed dielectric behavior was characterized by two types of relaxation processes which were described by the ‘universal relaxation law’. The frequency dependence of conductivity which showed a classical relaxor behavior followed the Jonscher's universal law σ(ω)=σ₀+Aωⁿ. The exponent n exhibited a minimum in the vicinity of temperatures of dielectric anomaly while the pre-factor A showed a maximum. The temperature dependence of n followed the Vogel-Fulcher relation with activation energy of about 0.14 eV.     

Enhanced ferroelectric,piezoelectric and dielectric properties of (1-x)CaBi2Nb2O9-xBaZr0.2Ti0.8O3 high-temperature piezoelectric composite ceramics

Among Aurivillius layer-structured materials, CaBi₂Nb₂O₉ is a best potential candidate for ultrahigh-temperature applications because of its highest Curie temperature of about 940 °C. In this paper, (1-x)CaBi₂Nb₂O₉-xBaZr_0.2Ti_0.8O₃ composite ceramics were prepared by conventional solid-state sintering method. The dielectric results show that the introduction of BaZr_0.2Ti_0.8O₃ not only increases the permittivity of the material, but also reduces its dielectric loss. The optimum electrical properties were obtained in the x = 0.01 sample with piezoelectric coefficient (d₃₃) of 15.1 pC/N and high ferroelectric remnant polarization (P_r) of 9.9 μC/cm². Furthermore, the composite samples show good thermal depoling performance, the d₃₃ of the x = 0.01 sample is 13.8 pC/N, which is about 91% of the initial value after depoling at 800 °C. Therefore, (1-x)CaBi₂Nb₂O₉-xBaZr_0.2Ti_0.8O₃ is one of the candidates for high temperature piezoelectric materials.     

Effect of Nb-doping on phase transitions and selected properties of (Ba0.90Pb0.10)TiO3 ceramics

Dielectric, pyroelectric characteristics as well as the changes of electric conductivity were investigated for Nb₂O₅-doped (Ba_0.90Pb_0.10)TiO₃ ceramics. The influence of this dopant on the ceramic microstructure and the lattice parameters were also studied. The analysis of the results of electric conductivity and Seebeck coefficient has led to the conclusion that Nb⁵⁺ ions reducing the conductivity by about two orders play the role of donors. The correlation between investigated electric characteristics and the dopant concentration was confirmed. Due to these findings some progress in understanding the influence of Nb-dopant was achieved.     

Influence of zirconium doping on structure,microstructure, dielectric and impedance properties of strontium bismuth niobate ceramics

Efforts have been made in this work to enhance the dielectric properties of SrBi₂Nb₂O₉ (SBN) by partial substitution of Zr⁴⁺ for Nb⁵⁺. Systematic investigations on structure, microstructure, dielectric and impedance properties of the SrBi₂(Nb_2−(4/5)xZr_x)O₉ [where, x = 0, 0.1 and 0.2] ceramic samples were carried out to understand the effect of substitution of Zr⁴⁺ for Nb⁵⁺ in SrBi₂Nb₂O₉. The X-ray diffraction (XRD) investigations indicated that the lattice volume of SrBi₂(Nb_{2− (4/5)x}Zr_x)O₉ with x = 0.1 and 0.2 decreases compared to SBN. The SEM investigations revealed an increase in the size of grains and the change on shape of grains to elongated plate shaped structure with the increase of x (x = 0.1 and 0.2) in SrBi₂(Nb_2−(4/5)xZr_x)O₉. Higher Curie temperature and enhanced peak dielectric constant at the Curie temperature were observed for both the SrBi₂(Nb_2−(4/5)xZr_x)O₉ with x = 0.1 and 0.2 ceramic samples compared to SBN. Among the investigated compositions the higher Curie temperature and enhanced peak dielectric constant at the Curie temperature was observed for SrBi₂(Nb_2−(4/5)xZr_x)O₉ with x = 0.1.     

Electric-field influence on the neutron diffuse scattering near the ferroelectric transition of Sr0.61Ba0.39Nb2O6

ABSTRACT

Uniaxial relaxor ferroelectric Sr_0.61Ba_0.39Nb₂O₆ single crystal has been investigated in the vicinity of its phase transition using neutron scattering and dielectric spectroscopy. A global-type thermal hysteresis is evidenced by both techniques in the ferroelectric phase and up to about 15 K above T_c. In addition, a part of the transverse neutron diffuse scattering in the 001 Brillouin zone, presumably related to static nanodomain structure, can be suppressed by prior poling the crystal in electric field of 3 kV/cm. The remaining part of the transverse neutron diffuse scattering and the real part of permittivity show a similar temperature dependence. The temperature position of the maximal scattering intensity T_max depends significantly on the scattering wave vector. T_max shifts monotonically to higher temperature with the increasing wave vector in all investigated cooling and heating regimes. It is concluded that the critical fluctuations have space correlations which depend on frequency and wave vector.     

Structure and ferroelectric properties of BaBi3.8M0.2(Ti3.8Nb0.2)O15 (M=Mg, Ca, Sr and Ba) ceramics

Bismuth layer structured compounds BaBi_3.8M_0.2(Ti_3.8Nb_0.2)O₁₅, with M=Mg, Ca, Sr and Ba, were synthesized through a modified chemical route. Phase and structure of the compounds were analyzed by X-ray diffraction. The structure of Mg²⁺ based compound was orthorhombic, while it was tetragonal for the other three compositions. Experimental results indicate that Mg²⁺ prefers to substitute in the Bi₂O₂ layer. The room temperature permittivity was maximum for Ba²⁺ based compound (∼432). The piezoelectric coefficient was significantly enhanced in Mg²⁺ based compound. The Curie temperature was found to decrease gradually in the order Mg>Ca>Sr>Ba based compound. There was a marked improvement in the relaxor behavior and remnant polarization for Ba²⁺ based compound. The structural changes and dielectric, ferroelectric and piezoelectric properties of all the compounds was discussed and co-related.