Dielectric investigations of BiFeO3 ceramics

In this paper we present the results from the investigation of the dielectric permittivity of BiFeO3 ceramics, prepared by mechanochemical synthesis method in a broad frequency and temperature range. The dielectric permittivity is mainly caused by high conductivity, which is suppressed in the frequencies above 1 MHz. The investigated ceramics showed conductivity activation energy E/k=11280±12 K, and σ0=54161±800 S. The plots of M* revealed conductivity mechanism with τ0=1.12·10^-13 s, and E/k=9245 K

Electrical conductivity and dielectric relaxation in Ag1-xLixNbO3

The broadband electrical properties of Ag1xLixNbO3 (ALNx) ceramics (x 0.1) together with AgNbO3 (AN) crystals were studied over a wide temperature interval of 20–800 K. For ALNx with x 0.05, a very diffused ferroelectric phase transition was observed. The position of the dielectric permittivity maximum in this phase transition is strongly frequency-dependent and is described well by the Vogel–Fulcher law. The freezing temperature decreases when the lithium concentration increases. Below the ferroelectric phase transition temperature, the dielectric dispersion is mainly caused by ferroelectric domain dynamics. Moreover, for ALN3 and ALN5 ceramics at very low temperatures (below 100 K), behavior typical of dipolar glasses is observed. At higher temperatures (above 650 K for ALN5), electrical conductivity effects become important. The DC conductivity increases with temperature according to the Arhenius law and the activation energy is highest in the antiferroelectric phase. Moreover, the activation energy is strongly dependent on the lithium concentration and it is greatest when x = 0.02

Peculiar Bi-ion dynamics in Na1/2Bi1/2TiO3 from terahertz and microwave dielectric spectroscopy

Dynamics of the main dielectric anomaly in Na1/2Bi1/2TiO3 (NBT) was studied by time-domain THz and microwave spectroscopy, using also previously published data and their new overall fits. Above the dielectric maximum temperature Tm ~ 600 K, the response consists of coupled sub-THz oscillator and a relaxation mode, assigned to strongly anharmonic Bi-ion vibrations and hopping, whose slowing down explains the paraelectric-like permittivity increase to Tm. Below Tm, the main relaxation continues slowing down and additional relaxation, assigned to quasi-Debye losses, appears in the 10^11 Hz range. The oscillator hardens on cooling and takes over the whole oscillator strength. The permittivity decrease below Tm is caused by the reduced strength of the relaxations due to dominance of the rhombohedral phase within the coexistence region with the tetragonal phase. The anharmonic dynamics of Bi is supported by previous structural studies. NBT represents a hybrid between standard and relaxor ferroelectric behaviour

Computational electromagnetic analysis of partially-filled rectangular waveguide at X-band frequencies

In this paper, the full-wave analysis of the direct scattering problem in the rectangular waveguide is derived. The numerical calculation results of the scattering characteristics are presented. To show the advantage of our proposed model, we compared direct problem results with three-dimensional simulation software Ansys HFSS calculations. An excellent agreement is observed when compared these two approaches

INFLUENCE OF DOPANTS ON BARIUM BISMUTH TITANATE ELECTRICAL PROPERTIES

The Aurivillius structure has capability to host ions of different size, so a large number of different dopants can be accommodated in the BaBi4Ti4O15 (BBT) lattice. It was detected that various substitutions of Bi3+ and Ti4+ ions can affect the change of microstructure and electrical properties of barium bismuth titanate ceramics. Doping of BBT ceramics is very important due to possibility to obtain materials with required characteristics. In this work, pure and niobium and lanthanum doped barium bismuth titanate powders were prepared by conventional solid state method, according to formulas BaBi4-xLaxTi4O15 and BaBi4Ti4-5/4xNbxO15 (x=0.05). Obtained powders were uniaxially pressed and sintered at different temperature depending on the composition. The influence of dopant type on structure change, grain size reduction and microstructure development was analyzed. XRD measurements showed formation of orthorhombic BBT crystal structure without presence of secondary phase in doped samples. Dopants had influence on shifting of temperature phase transition peaks to the lower temperatures, broadening of ε - T curves and increasing relaxor behavior of phase transition. Temperature dependence of the electrical conductivity pointed out that niobium as a donor dopant decrease conductivity and lanthanum as a isovalent dopant increase conductivity of BBT ceramics. Obtained results were analyzed in the frame of the influence of the grain and grain boundaries contribution to the dielectric behavior through impedance spectroscopy

DIELECTRIC INVESTIGATIONS OF BARIUM TITANATE DOPED WITH DEFERENT CONCENTRATION OF LANTHANUM OR ANTIMONY

The dielectric properties of barium titanate doped with different concentration lanthanum or antimony are investigated. There are samples with 0.3 mol%, 0.5 mol% lanthanum and 0.1 mol%, 0.5 mol% antimony concentrations. All samples were prepared by Pechini procedure. Dielectric measurements in the temperature range of 170 K to 420 K and frequency range of I kHz to I MHz show three phase transitions, which temperatures matches those of bulk barium titanate. Curie temperature shifts to the lower temperatufe when lanthanum or antimony concentration grows. At low frequencies, the samples present conductivity Conduction-free loss spectra are calculated using a compact solution of the Kramers-Kronig transformation and an approximation based

DIELECTRIC INVESTIGATION OF BARIUM TITANATE WITH 0.5% ANTIMONY

Barium titanate (BaTiO:) is versatile electroceramic that exhibits high permittivity (e') making it desirable material for capacitor and others widespread applications. On heating, it undergoes a ferroelectric/paraelectric phase transition to the cubic polymorph at a Curie temperature Tc of 130C, at which e' passes through a maximum e'max and typically reaches values of -10000 in undoped ceramic samples. The phase transition is first order, and the peak in e' is correspondingly sharp [1]. For many years dopants have been used to modify the electrical properties of BaTiO3-based ceramics. For example, isovalent dopants are commonly used to alter Z. and the lower temperature orlhorhombic-hetragonal and rhombohedral/orthorhombic phase transition temperatures. In this way, the temperature of e 'max may pe modified and in some cases lead to diffuse phase transition-type behaviour. We prepared 0.5% Sb-dopped BaTiO3 sample. In the present study, the real (e ) and imaginary (e ") part and of dielectric permiittivity were investigated in the frequency range of 20 Hz to 1.0 MHz at temperature range of 1 20 K to 460 K. How we see, the temperature dependence of the dielectric permittivity is not typical for,barium titanate. But from Cole-Cole fit parameter relaxation time we see three phase transitions approximately ar T=207 K, 298 K and 273 K. In this study also will be presented barium titanate Sb-dopped with different concentration: O.1% and 0.3%

Crossover from Ferroelectric to Relaxor Behavior in Ba1−xCaxTiO3 (x = 0.17) System

The dielectric properties of Ba1xCaxTiO3 (x = 0.17) ceramics were studied in a wide frequency range of 20 Hz–53 GHz. Di used ferroelectric phase transition was revealed close to 339 K in the dielectric properties of ceramics. The behaviour of distributions of relaxation times in vicinity of the ferroelectric phase transition temperature is also typical for order-disorder ferroelectric phase transition. However, at lower temperatures (below 200 K), the most probable relaxation increased according to the Arrhenius law. At lower temperatures the maximum of the imaginary part of dielectric permittivity versus temperature strongly shifted to higher temperatures when the frequency increased (from 125 K at 1.21 kHz to 300 K at 33 GHz). This behaviour was attributed to the dynamics of Ti ions. The origin of the crossover from ferroelectric to relaxor behaviour of Ba1xCaxTiO3 (x = 0.17) ceramics is discussed in the paper

Influence of dopants on barium bismuth titanate electrical properties

The Aurivillius structure has capability to host ions of different size, so a large number of different dopants can be accommodated in the BaBi4Ti4O15 (BBT) lattice. It was detected that various substitutions of Bi3+ and Ti4+ ions can affect the change of microstructure and electrical properties of barium bismuth titanate ceramics. Doping of BBT ceramics is very important due to possibility to obtain materials with required characteristics [1]. In this work, pure and niobium and lanthanum doped barium bismuth titanate powders were prepared by conventional solid state method, according to formulas BaBi4-xLaxTi4O15 and BaBi4Ti4-5/4xNbxO15 (x=0.05). Obtained powders were uniaxially pressed and sintered at different temperature depending on the composition. The influence of dopant type on structure change, grain size reduction and microstructure development was analyzed. XRD measurements showed formation of orthorhombic BBT crystal structure without presence of secondary phase in doped samples. Dopants had influence on shifting of temperature phase transition peaks to the lower temperatures, broadening of ε - T curves and increasing relaxor behavior of phase transition (Table). Temperature dependence of the electrical conductivity shown at Fig. pointed out that niobium as a donor dopant decrease conductivity [2] and lanthanum as a isovalent dopant increase conductivity of BBT ceramics. Obtained results were analyzed in the frame of the influence of the grain and grain boundaries contribution to the dielectric behavior through impedance spectroscopy