Cristina Elena Ciomaga

Ferroelectric and dielectric properties of ferrite-ferroelectric ceramic composites

Particulate composites of ferrite and ferroelectric phases with xNiFe2O4 (NF) and (1 − x)Pb0.988(Zr0.52Ti0.48)0.976Nb0.024O3 (where x = 2, 10, 20, 30, 50, 70, and 100 wt. %) were prepared in situ by sol-gel method. The presence of a diphase composition was confirmed by X-ray diffraction while the microstructure of the composites was studied by scanning electron microscopy revealing a good mixing of the two phases and a good densification of the bulk ceramics. The dielectric permittivity shows usual dielectric dispersion behavior with increasing frequency due to Maxwell-Wagner interfacial polarization. AC conductivity measurements made in frequency range 1 Hz-1 MHz suggest that the conduction process is due to mixed polaron hopping. The effect of NF phase concentration on the P-E and M-H hysteresis behavior and dielectric properties of the composites was investigated. At low NF concentration a sharp ferro-paraelectric transition peak can be observed at around 360 °C while for higher NF concentrations a tre...

Oxygen deficiency and grain boundary-related giant relaxation in Ba(Zr,Ti)O3 ceramics

The influence of the oxygen vacancies on the dielectric response of BaZr0.10Ti0.90O3 ceramics prepared by solid-state reaction and sintered at 1400 °C for 2 h was investigated. The as-sintered ceramic exhibits a giant relaxation with a shift of the transition temperature from ∼85 °C to above 170 °C in the frequency ranges of 1 Hz–100 kHz, with high losses above unity and two components in the complex impedance plot. A complex dielectric relaxation response, with at least two thermally activated defect mechanisms with activation energies of ∼0.2 eV below the transition temperature and ∼0.7 eV for higher temperatures in the range of 85 °C–170 °C was determined. The observed giant relaxation is an extrinsic effect related to the oxygen deficiency, inhomogeneous distributed in the ceramic grain and not to the relaxor behavior of this system. After a post-annealing treatment at 1000 °C for 50 h, the dielectric response is completely changed: the permittivity vs. temperature dependences present maxima located a...

Low field permittivity of ferroelectric-ferrite ceramic composites: Experiment and modeling

The dielectric properties of xNiFe2O4-(1−x)Pb0.988(Zr0.52Ti0.48)0.976Nb0.024O3 ceramic composites with different volume filling factors (x = 2, 5, 10, 20, 30, 40, 50, 60, 70 wt. %) prepared by sol-gel method have been investigated in the frequency range (106 ÷ 109) Hz. A reducing effective permittivity with increasing the ferrite amount was observed as result of the “sum property.” The experimental results have been compared with the effective permittivity values predicted by effective medium approximation (EMA) models and finite element method (FEM) calculations. The comparison was aimed to evaluate the appropriateness of the results of the two approaches in describing the effective dielectric properties of the composite with contrasting permittivities and to understand the role of microstructural characteristics on the dielectric response in a broad filling factor range. The best description of the experimental data in overall filling factor range is given by the results of FEM calculations which used r...

Using multi-walled carbon nanotubes in spark plasma sintered Pb(Zr0.47Ti0.53)O3 ceramics for tailoring dielectric and tunability properties

The addition of small amounts (below 0.1 wt. %) of multi-walled carbon nanotubes (MWCNTs) to Pb(Zr0.47Ti0.53)O3 (PZT) ceramics prepared by spark plasma sintering is proposed as a method of tailoring the electrical properties, which are expected to be modified with respect to the pure PZT, both as result of the presence of 1-D conductive fillers in the ceramic product and via the microstructural modifications of ceramics induced during the sintering. The addition of even small amounts of carbon nanotubes strongly reduced the sinterability of PZT ceramics and resulted in the porous and fine-grained microstructures (relative density of 73% for a MWCNT addition of 0.5 vol. % by comparison with 91% in the pure PZT, produced in the same conditions). A monotonous decrease of permittivity with increasing the MWCNT level from ∼830 in pure PZT to ∼627 for x = 0.5 vol. %, at a fixed frequency f = 1kHz, and low dielectric losses below 2% have been observed. Tunability increases with respect to the values of dense PZT...

Experimental and analytical modeling of resonant permittivity and permeability in ferroelectric-ferrite composites in microwave range

The ferroelectric-magnetic composite materials are very suitable for designing and producing microwave devices and components, due to their dielectric and magnetic properties. For microwave antennas, sensors, resonators, oscillators, filters, phase shifters, attenuators, and amplifiers, the output and input impedances, the resonance frequencies, the quality factor (the losses), and the attenuation or the amplification can be very easily adjusted by modifying the dielectric permittivity or/and the magnetic permeability. This is possible by a right choice of the ratio between the ferroelectric and magnetic phases in the composite material. Therefore, it is highly important to accurately determine these characteristics of the composite material in microwave range.

Temperature-Dependent Tunability in the Paraelectric State of BaTiO3-Based Solid Solutions

The electric field dependence of the dielectric constant (dc-tunability) as a function of composition at a few temperatures was investigated for BaTiO 3 -based solid solutions. Dense and homogeneous BaZr x Ti 1 − x O 3 ceramics (ferroelectric: x = 0.10 and x = 0.15 and relaxor: x = 0.18) with high dielectric constant and low losses were prepared by solid state reaction and their dc-tunability characteristics were investigate at a few temperatures from 20 to 70°C, including the Curie range. The nonlinear ϵ (E) dependences were described using the Johnsons approach plus an additional term. A model of random uncorrelated non-interacting dipolar units in a double well potential was employed for the ϵ (E) in the paraelectric state of the composition x = 0.18. For this composition, the average dipole moment of the nanopolar regions as a function of temperature was determined.

Engineering magnetoelectric composites towards application as tunable microwave filters

Tunable microwave devices are widely used in radar, telecommunication and radio frequency measurement systems. In recent years, a promising technology for tunable microwave devices, which combines the advantages of using the magnetic and electrical properties of materials as well as their magnetoelectric coupling, has been developed. In the present work, xMnFe2O4-(1 − x)Pb0.988(Zr0.52Ti0.48)0.976Nb0.024O3 ferrite-ferroelectric composites have been prepared and their functional properties were investigated, in order to be designed as components in tunable microwave devices. Structural, microstructural, dielectric and magnetic investigations confirmed the formation of composite ceramics with valuable dielectric and magnetic properties at room temperature. The complex permittivity and permeability of the investigated magnetoelectric ceramics, measured in the frequency range of 1 MHz to 1 GHz, show as a combination of the properties of Pb(Zr,Ti)NbO3 ferroelectric and magnetic MnFe2O4 phases. Experimental measurements and the numerical simulations were done using a high frequency structure simulator (HFSS) which revealed that the investigated composite disk placed on a microstrip line acts as an electromagnetic resonator and it could be consider as a filter with tunable stop-band capabilities controlled by an external magnetic field. It was found that the central stop-band frequency of the designed filter can be tuned with 18 MHz by changing the magnetic field from 0 to 2 kOe.

Improvement of impedance spectroscopy methods: resonance analysis of samples

An impedance spectroscopy method, using an Agilent E4991A RF Impedance/Material Analyzer, was employed for the investigation of radiofrequency and microwave dielectric characteristics for some ceramic materials with high dielectric permittivity. Experimental observations recorded single, double and even triple resonances in the permittivity spectra, while for the selected samples significantly different values of permittivity from the ones found in existing literature were directly measured and observed in the frequency range of 100 MHz–1 GHz. The specified material analyzer is designed to work within certain permittivity-frequency domains; working outside these limits leads to significant errors when carrying out direct measurements of permittivity. Having observed that the resonance measurements are not associated with the intrinsic material properties but with the sample resonances, we proposed a dielectric resonance cavity model for sample purposes and we determined the dielectric permittivity at different resonance frequencies. A scaling procedure, based on the computed values of permittivity, was applied in order to correct the directly measured permittivity spectra where the frequency was outside the resonance domains. The corrected data are in good agreement with those measured using other techniques and suggest that the proposed procedure extends the spectroscopy method currently used for high permittivity measurements. The sources of error for the proposed method were investigated.

Dielectric and phonon spectroscopy of Nb-doped Pb(Zr1-yTiy)O3-CoFe2O4 composites

Broad-band dielectric and phonon response of Nb-doped (1-x)Pb(Zr1-yTiy)O3-xCoFe2O4 composites with x = 10%–30% was investigated between 0.1 MHz and 100 THz. At room temperature, a broad distribution of relaxation times causes a constant dielectric loss below 1 GHz. Above room temperature, a strong Maxwell-Wagner relaxation process dominates below 1 GHz due to the conductivity of CoFe2O4 (CF). Two additional relaxation processes are seen between 1 GHz and 1 THz. The lower-frequency one, coming from domain wall motion, disappears above TC ≈ 650 K. The higher-frequency component slows down on heating towards TC, because it is the central mode, which drives the ferroelectric phase transition. Time-domain THz transmission and infrared reflectivity spectra reveal a mixture of polar phonons from both ferroelectric Nb-doped Pb(Zr,Ti)O3 (PZTN) and magnetic CoFe2O4 (CF) components, while the micro-Raman scattering spectra allow to study phonons from both components separately. Similar temperature behavior of phonon...