Felicia Gheorghiu - Academia.edu (original) (raw)

Papers by Felicia Gheorghiu

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Fe-doped BaTiO3 ceramics produced from powders prepared by solid state reaction at various proces... more Fe-doped BaTiO3 ceramics produced from powders prepared by solid state reaction at various processing
parameters (temperature, time) were investigated and discussed. X-ray diffraction analysis confirmed the
existence of the tetragonal phase in BaTi1−xFexO3−x/2 (x=0.005) ceramics. The microstructural characterization
reveals that a low calcination temperature leads to a higher densification of the bulk samples at around 96%
relative density with average grain sizes ~6 μm. The oxidation state of the metal ions in the sintered samples
analysed by means of X-ray photoelectron spectroscopy showed the presence of Ti4+2p3/2 and Ti4+2p1/2 peaks
in Fe-doped BaTiO3 and the lack of Ti3+ ions, which confirms the compositional quality of these ceramics. The
electron paramagnetic resonance measurements proved the incorporation of Fe ions as Fe3+ in the BaTiO3
lattice.

In the present work, it is shown how the controlled porosity can be exploited to obtain a comprom... more In the present work, it is shown how the controlled porosity can be exploited to obtain a compromise between a reduced permittivity down to a few hundreds and maintaining a high tunability level as in the dense material, to fulfill requirements for tunable applications. Nb-doped Pb(Zr,Ti)O3 ceramics with porosity in the range 5%-30% have been prepared by direct sintering method. X-ray diffraction analysis and Rietveld refinement indicated a co-existence of tetragonal and monoclinic phases in the porous ceramics. Dielectric properties revealed a gradual reduction in permittivity when increasing the porosity level, while maintaining low dielectric losses below 3%. The ferroelectric switching behavior is also influenced by the porosity level: a continuous reduction in the saturation and remnant polarization is observed with increasing porosity. The nonlinear dielectric properties of all the investigated ceramics preserve a high level of tunability in comparison with one of the dense material, irrespective of the porosity level, while zero field permittivity was decreased below 1000. An optimum behavior is found for the ceramic sample with 25% porosity, which shows a high tunability, smaller losses, and moderate dielectric constant (ε ~600).

In the present work, it was reported for the first time the new synthesis of Sm 2 NiMnO 6 double ... more In the present work, it was reported for the first time the new synthesis of Sm 2 NiMnO 6 double perovskite oxides by solegel auto-combustion method. The Rietveld analysis of the X-ray ceramics diffraction pattern recorded at room temperature for Sm 2 NiMnO 6 ceramics sintered at 1000 C/5 min from powders obtained at 700 C/7 h confirm the formation of the double perovskite with a monoclinic structure and the space group P2 1 /n. The HRTEM analysis shows clear lattice fringes that confirm a high crystallinity level of the material corresponding to the monoclinic structure. The non-linear dielectric character was checked for the first time in Sm 2 NiMnO 6 double perovskite and the results reveals a strong nonlinearity and a small hysteretic behaviour. The present structural, magnetic and dielectric data make the Sm 2 NiMnO 6 system due to its multiferroic character a promising candidate to different modern electronic devices applications.

The focus of the present paper is to describe the preparation procedure and to investigate the mi... more The focus of the present paper is to describe the preparation procedure and to investigate the microstruc-tural characteristics and the electrical properties of Co-doped PZT films deposited by rf-sputtering by using a " mixture " target system onto Au-electroded Al 2 O 3 ceramic substrates. The X-ray diffraction patterns of the Co-doped PZT thin films as a function of the annealing temperature confirmed the formation of pure per-ovskite phase started with temperatures of 600 °C, but a perfect crystallization was achieved at a temperature of ∼700 °C. The microstructures strongly depend on the thermal treatment temperature and indicated a dis-continuous surface without large pores and with a bimodal grain size distribution. The XPS analysis demonstrated that the dopant element is present mainly in its Co 2+ state. The macroscopic P(E) hysteresis loops were recorded in different locations of the films surface and demonstrated ferroelectric behaviour with a resistive leakage contribution.

In the present study, the electrical properties of BiFeO 3 ceramic specimens prepared by solid-st... more In the present study, the electrical properties of BiFeO 3 ceramic specimens prepared by solid-state sin-tering method by using two thermal treatment strategies are comparatively investigated. The room temperature XRD pattern shows perovskite single-phase, in the limit of XRD accuracy, for BiFeO 3 ceramic prepared by single-step method. For two-step sintering method sample small amounts of secondary Bi 2 Fe 4 O 9 phases were identified. The ceramics show a non-homogeneous microstructure, consisting of ceramic grains with irregular morphology and interconnected porosity mainly in the grain boundary regions in the case of two-step sintering sample. The most interesting feature is the conduction anomaly observed on the conductivity in the low-frequency range close to dc-conductivity. The Arrhenius plot of the dc-conductivity determined at the lowest frequency vs. 1/T shows two distinct linear regions separated by the mentioned temperature range of (189e244) K, for which the dc conductivity could not be determined from the present impedance spectroscopy data only. It is clear that in the mentioned temperature range (for both samples), a conduction anomaly takes place.

The nonlinear dielectric properties of BiMn x Fe 1Àx O 3 ceramics with compositions x = 0–0.30 we... more The nonlinear dielectric properties of BiMn x Fe 1Àx O 3 ceramics with compositions x = 0–0.30 were investigated at room temperature. The present results demonstrate a high dielectric tunability over quite a large range of electric field biases, which increases with Mn addition. The experimental data are discussed in terms of the Johnson model combined with Langevin terms that describe the extrinsic contributions to the nonlinear e(E) dependences. The weights of these contributions are different for various Mn additions.

In the present study, particles with different Bi2Fe4O9 micro/nanostructures with a few particula... more In the present study, particles with different Bi2Fe4O9 micro/nanostructures with a few particular morphologies (flower-like nanoplatelets, hierarchical microstructures, perfectly square platelets single crystals, etc.) obtained under specific hydrothermal synthesis conditions were investigated. The role of the processing
parameters (such as NaOH concentration, reaction temperature, and reaction duration time) on the phase formation mechanism and on the microstructural characteristics was investigated. All the Bi2Fe4O9 morphologies showed orthorhombic symmetry with space group Pbam. The photocatalytic properties and magnetic behavior as a function of the micro/nanostructural characteristics of various Bi2Fe4O9 powders were determined. In the presence of Bi2Fe4O9, a degradation rate of Rose Bengal in the range of 52–61% was determined after 180 min under UV light irradiation (¼254 nm). Magnetic activity with
antiferromagnetic behavior and a transition at 240K slightly dependent on the microstructures was found. The role of Bi2Fe4O9 microstructures in the photocatalytic activity and magnetic properties was discussed.

The nonlinear dielectric properties of BiFeO3 ceramics as a function of temperature were investig... more The nonlinear dielectric properties of BiFeO3 ceramics as a function of temperature were investigated. The present results demonstrate a high dielectric tunability in quite low range of the bias electric fields over a broad temperature interval around the room temperature. The non-linear permittivity-field response results from multiple contributions, whose weights are different at various temperatures

Stoichiometric films were prepared by rf-magnetron sputtering from a Pb(Zr 0.54 Ti 0.46)O 3 ceram... more Stoichiometric films were prepared by rf-magnetron sputtering from a Pb(Zr 0.54 Ti 0.46)O 3 ceramic target onto Au-electroded substrates of alumina. During deposition the substrate holder was kept at a temperature of 300 • C. Post-deposition heat treatment in air at 650 • C was carried to promote the full crystallization and to result in pure perovskite PZT phase. SEM–EDX measurements for the films were performed both on surface and on cross-section. The impedance spectroscopy data demonstrates that the films have rather good dielectric properties and low losses. The recorded P–E loops prove their macroscopic ferroelectric characteristics, while piezoresponse force microscopy experiments confirm a nanoscale switching mechanism based on domain nucleation-growth.

BiFeO 3 is one of the few single-phase multiferroics, showing antiferromagnetic and ferroelectric... more BiFeO 3 is one of the few single-phase multiferroics, showing antiferromagnetic and ferroelectric ordering. Since the dielectric properties in the ceramic state of the pure BiFeO 3 were rather poor and in order to stabilize the perovskite state and to induce ferromagnetism at room temperature, it was adopted the strategy of doping with rare earth or forming solid solutions. Substituting Fe with Mn in BiFeO 3-based compounds is supposed cause better properties in terms of leakage current density and also to induce changes in the magnetic order of the system. In the present paper, the effect of Mn substitution on the dielectric and magnetic properties of the BiFe 1−x Mn x O 3 ceramics has been studied. Homogeneous samples from microstructural point of view were obtained for all the compositions analyzed. The magnetic properties are strongly affected by the presence of Mn ions. The possible origin of these behavior is discussed in terms of phase purity, grain size and grain boundary phenomena. The extrinsic properties are impossible to be fully controlled by normal ceramic processing. By controlling the extrinsic contributions to the dielectric properties, the ceramic system might be a valuable multiferroic material for magnetoelectric applications. At room temperature the ceramic is a multiferroic, since it is ferroelectric and magnetically-ordered.

Solid solutions of (1−x)BiFeO3–xBaTiO3 (0≤x≤0.30) have been prepared by a two-step solid state re... more Solid solutions of (1−x)BiFeO3–xBaTiO3 (0≤x≤0.30) have been prepared by a two-step solid state reaction. By comparison with other reported methods, a higher electrical homogeneity of the ceramic bodies
resulted in better dielectric properties, with a single-component impedance plot, small losses and permittivities below 240. The maximum magnetization is observed for x = 0.05, which might represent the proper range of compositions for multiferroism at room temperature. In order to better understand the composition-dependent magnetic properties in correlation with the expected Fe2+/Fe3+ ratio fluctuations in the solid solutions, a detailed X-ray Photoelectron Spectroscopy (XPS) analyses were performed on the surfaces and on the fractured sintered bodies.

A series of compounds with the general formula (La 1 - x Nd x) 2 Ti 2 O 7 (0.0 < x< 1.0) has been... more A series of compounds with the general formula (La 1 - x Nd x) 2 Ti 2 O 7 (0.0 < x< 1.0) has been prepared by the sol–gel method. The decomposition of the gel was characterized by thermo-gravimetric analysis coupled to mass spectrometry, indicating the reaction is achieved above 850C. The lattice parameters versus x show an expected decrease in the a and b parameters while c and the b angle remain almost unchanged with respect to the monoclinic symmetry conserved for the full solid solution. Dielectric, piezoelectric and ferroelectric properties were measured on the entire series.

Solid solutions of (1-x)BiFeO 3 –xBaTiO 3 (0 ≤ x ≤ 0.30) were prepared by a two-step solid state... more Solid solutions of (1-x)BiFeO 3 –xBaTiO 3 (0 ≤ x ≤ 0.30) were prepared by a two-step solid state reaction procedure. After pre-sintering and sintering at 800 • C/1h and slow cooling, pure single phase compositions were obtained for all the analyzed mixtures in the limit of XRD accuracy, including for x = 0. A gradual attenuation of the rhombohedral distortion with the increase of BaTiO 3 content was found. Small dielectric losses (tan δ < 3 %) and permittivities in the range of (180, 240) were found for all the compositions at room temperatures. The magnetic investigations shown a composition-dependent ferromagnetism with a maximum magnetization and coercivity for the composition x = 0.05 and perfect antiferromagnetic behavior for the BiFeO 3 ceramics. The complex impedance is sensitive to the magnetic field at small temperatures below 300 K, with a stronger variation in the range of fields of (0, 2 T) and a tendency of saturation at higher fields, demonstrating a kind of magnetoelectric coupling, with both magnetocapacitance and magnetoresistence phenomena.

Diphase magnetoelectric composites of CoFe 2 O 4 –Pb(ZrTi)O 3 were prepared by citrate–nitrate c... more Diphase magnetoelectric composites of CoFe 2 O 4 –Pb(ZrTi)O 3 were prepared by citrate–nitrate combustion technique by using Pb(Zr,Ti)O 3 template powders obtained by the mixed oxide method. Pure diphase powder composites with a good crystallinity were obtained after calcination. The composition and purity were maintained after sintering at temperature of 1100 • C/2 h, which ensured limited reactions at interfaces, while by sintering at 1250 • C/2 h, some small amounts of secondary phases identified as nonstoichiometric ZrO 2−x resulted. The method allowed to produce diphase ceramics with homogeneous microstructures and a very good mixing of the two phases. The dielectric and magnetic investigation at room temperature confirmed the formation of composite ceramics with both dielectric and magnetic properties at room temperature, with permittivity and magnetization resulted as sum properties from the parent Pb(Zr,Ti)O 3 and ferrite phases.

Di-phase magnetoelectric composites formed by magnetic CoFe 2 O 4 and ferroelectric Nb–Pb(Zr,Ti)O... more Di-phase magnetoelectric composites formed by magnetic CoFe 2 O 4 and ferroelectric Nb–Pb(Zr,Ti)O 3 were prepared by citrate–nitrate combustion method technique by using Nb–Pb(Zr,Ti)O 3 template powders prepared by mixed oxides. Pure di-phase powder composites with a good crystallinity were obtained after the calcination step. The purity was maintained by sintering at temperature of 1100 • C/2 h, which ensured limited reactions at interfaces, while by sintering at 1250 • C/2 h, small amounts of secondary phases resulted. The method allowed to produce di-phase ceramics with homogeneous microstructures and a very good mixing of the two phases. The dielectric and magnetic investigations at room temperature confirmed the formation of composite ceramics with both dielectric and magnetic properties at room temperature, with permittivity and magnetization resulted as sum properties from the parent Pb(Zr,Ti)O 3 and ferrite phases.

The frequency-dependence of the complex permittivity of some Ba(Zr,Ti)O3 ferroelectric ceramics w... more The frequency-dependence of the complex permittivity of some Ba(Zr,Ti)O3 ferroelectric ceramics was investigated, with the aim to evidence different behaviors related to microstructural characteristics. Ferroelectric BaZr0.10Ti0.90O3 sintered at two temperatures gave different types of impedance spectra due to their differences in the microstructures. The multiple components in the complex impedance diagram result from the grain boundary phenomena, i.e. from local differences in the conductivity and dielectric properties of the ceramic grain bulk and ceramic grain boundary.

The aim of this work is to study the behavior of the perovskite-type complex oxides A 2 MnMoO 6 o... more The aim of this work is to study the behavior of the perovskite-type complex oxides A 2 MnMoO 6 obtained by the citrate combustion synthesis method. We have prepared our samples of nanosized perovskite-type complex oxides A 2 MnMoO 6 (A = Ca, Sr and Ba) by citrate combustion method. The starting materials were CaCO 3 , SrCO 3, BaCO 3 (treated with HNO 3 to form active nitrates) , (NH 4) 6 Mo 7 O 24 · 4H 2 O, Mn powder and citric acid in stoichiometric ratio. The precursors were presintered and sintered at different temperatures. The obtained powders were firstly characterised by infrared spectrophotometry. The infrared spectra have revealed several novel aspects of the structure. The position of bands indicates that all nanosized materials are polymetalic oxides exhibiting perovskite-type structure. The complex impedance properties have also been determined and they are discussed in relationship with the composition of the samples.

The aim of this work is to understand the behaviour of the perovskite-type complex oxides Ca 2-x ... more The aim of this work is to understand the behaviour of the perovskite-type complex oxides Ca 2-x La x MnMoO 6 obtained by the combustion synthesis method. We have prepared our samples of nanosized perovskite-type complex oxides Ca 2-x La x MnMoO 6 (x=0.75; 0.9; 1.0) by combustion method. The starting materials were CaCO 3 , La 2 O 3 , (NH 4) 6 Mo 7 O 24 4H 2 O, Mn powder and citric acid in stoichiometric ratio. The precursors were pre-sintered and sintered at different temperatures. The obtained powders were firstly characterized by infrared spectrophotometry. The infrared spectra have revealed several novel aspects of the structure. The position of bands are attributed around 800 cm-1 and in the ranges of 670-590 cm-1 , 445-431 cm-1 , respectively: the results indicate that all nanosized materials have perovskite-type structure complex oxides. The complex impedance properties have also been determined and they are discussed in relationship with their compositions. The Ca 2-x La x MnMoO 6 double perovskites are interesting systems for important potential applications. The research for A 2 BB'O 6 type double perovskites (where A is an alkaline-earth or rare-earth ion, and B and B' are different transition metal cations) is due to the possible technological applications of these materials in many devices [1, 2, 3]. The studies of the crystal structures of the ALaMnMoO 6 (A = Sr, Ba) double perovskites show that the Sr compound is monoclinic with space group P2 1 /n (Glazer a-a-c +) tilting and an undistorted double perovskite structure –pseudocubic-(space group Fm3m) for BaLaMnMoO 6 [4]. The XRD pattern of almost pure powder sample BaLaMnMoO 6 indicated an ordered Mn and Mo cations [5]. The CaLaMnMoO 6 double perovskite was studied by powder neutron diffraction and the results are: monoclinic structure with P21/n space group (Glazer a + b-b-) tilting and the values of the parameters of the unit cell: a = 5.56961(9), b = 5.71514(9), c = 7.9358(1)Å and β = 90.043(1)°. Compared to ALaMnMoO 6 (A = Sr, Ba), CaLaMnMoO 6 has the largest octahedral distortion, as calculated from the structural data [6]. In this paper, synthesis, characterization and dielectric properties of Ca 2-x La x MnMoO 6 are reported. Experimental procedure Sample preparation For the preparation by sol-gel combustion method, high-purity starting materials as CaCO 3 (Sigma Aldrich), La 2 O 3 (Merck), Mn (Sigma Aldrich) were dissolved in HNO 3 (20%) and (NH 4) 6 Mo 7 O 24 4H 2 O (Sigma Aldrich) was dissolved in deionized water. The precursor solutions were mixed and after that heated and stirred. The right amount of citric acid (Sigma Aldrich) is added as chelating agent (citric acid: mixed oxide = 4:1) at 70°C and the resulting mixture was stirred for 20 min. A slightly yellow gel was obtained after concentrating the solution by slow evaporation at 80°C-90°C. The gel was then dried in air up to 300 ± 3°C and after the combustion process the pre-sintering thermal treatment was carried out in air at 350°C, 500°C and 750 ± 3°C with 7 h plateau. The samples were slowly cooled, then ground, shaped by uniaxial pressing at 150 kPa into pellets of 10mm diameter and 2mm thickness and sintered in air with a heating rate of 10°C/min at 900°C for 7 h. The samples were slowly cooled, then re-ground, pressed with radial gradient into pellets of 10 mm diameter and 2 mm thickness and sintered in air, with a heating rate of 5°C/min at 1100°C for 4 h. Sample characterization IR spectra were obtained using a JASCO 660 PLUS spectrophotometer with wave number range 4000 – 400 cm-1 used to complete the structure studies. The samples were mixed with KBr in the mass ratio 0.04:1, and then compacted into pellets with a thickness of 0.5-0.75 mm and a diameter of 13 mm under a pressure of 0.3 GPa in atmospheric air. X-ray diffraction measurements at room temperature, used to investigate the purity of the perovskite phase corresponding to the Ca 2-x La x MnMoO 6 powders were performed with a SHIMADZU XRD 6000 diffractometer using Ni-filtered Cu Kα radiation (λ = 1.5418 Å), with a scan step of 0.02° and a counting time of 1 s/step, for 2θ∈(20 – 80)°. The impedance spectroscopy measurements were performed on parallel-plate configuration of the sintered ceramic disks by using a dielectric holder with guard ring. The complex impedance in the frequency domain of 20 ÷ 210 6 Hz at room temperature was determined by using an impedance bridge type Agilent E4980A. Results and discussion IR spectra of the powder samples of Ca 2-x La x MnMoO 6 are shown in function of La content in the figure 1 in the 1000 – 400 cm-1 range of wavenumber. The formation of the perovskite phase was watched by the FT infrared method and then this was characterized by XRD method.