Dr. Luminita AMARANDE

The synergistic piezoelectric and osteoconductive properties of barium titanate (BT) and hydroxyapatite (HA) could stir the development of a new generation of synthetic bone graft substitutes, with capability for rapid and safe osseointegration. The research focused on two concurrent approaches for coupling the BT and HA materials: (i) conventional sintering of BT-HA powder mixtures; and (ii) functionalization of pre-sintered BT with HA coatings using magnetron sputtering (MS). Irrespective of the BT/HA ratios ranging from 95/5 to 80/20 wt%, nanocrystalline or highly-crystallized nature of the powders, sub-micron- or micron-sized particle dimensions, and sintering temperature, it was observed that the BT-HA reactivity cannot be prevented above 800 degrees C. At higher temperatures in the range of 1000-1300 degrees C, HA undergoes decomposition and extensively reacts with BT, leading to the formation of several secondary phases such as CaTiO3, Ba2Ca(PO4)2, BaCa6(PO4)4O, BaCa(PO3)4, and beta-Ca2P2O7. As a consequence, the cytocompatibility assessed in fibroblast and osteoblast cell cultures, as well as the piezoelectric response, were significantly altered. Applying HA coatings by MS to the sintered BT ceramics successfully preserved their piezoelectric properties, while also providing an unaltered cytocompatible and osteogenic-prone surface. The HA coatings were fully crystallized at post-deposition annealing temperatures of 550 and 700 degrees C, achieving crystalline qualities comparable to HA powders sintered at 1100 and 1200 degrees C, respectively. No reactivity events between BT and HA were observed. Partial reactivity was only noticeable upon annealing at 1000 degrees C. Therefore, it is suggested that the HA coating of BT is effective in seamlessly coupling the piezoelectric and osteogenic properties of the two constituents without compromise.

Fine-tuning of grain sizes can significantly influence the interaction between different dielectric phenomena, allowing the development of materials with tailored dielectric resistivity. By virtue of various synthesis mechanisms, a pathway to manipulate grain sizes and, consequently, tune the material's dielectric response is revealed. Understanding these intricate relationships between granulation and dielectric properties can pave the way for designing and optimizing materials for specific applications where tailored dielectric responses are sought. The experimental part involved the fabrication of dense BCT-BZT ceramics with different grain sizes by varying the synthesis (conventional solid-state reaction route and sol-gel) and consolidation methods. Both consolidation methods produced well-crystallized specimens, with Ba0.85Ca0.15O3Ti0.9Zr0.1 (BCTZ) perovskite as the major phase. Conventional sintering resulted in microstructured and submicron-structured BCT-BZT ceramics, with average grain sizes of 2.35 mu m for the solid-state sample and 0.91 mu m for the sol-gel synthesized ceramic. However, spark plasma sintering produced a nanocrystalline specimen with an average grain size of 67.5 nm. As the grain size decreases, there is a noticeable decrease in the maximum permittivity, a significant reduction in dielectric losses, and a shifting of the Curie temperature towards lower values.

This work was devoted to the first multi-parametric unitary comparative analysis of a selection of sintered piezoceramic materials synthesised by solid-state reactions, aiming to delineate the most promising biocompatible piezoelectric material, to be further implemented into macro-porous ceramic scaffolds fabricated by 3D printing technologies. The piezoceramics under scrutiny were: KNbO3, LiNbO3, LiTaO3, BaTiO3, Zr-doped BaTiO3, and the (Ba0.85Ca0.15)(Ti0.9Zr0.1)O-3 solid solution (BCTZ). The XRD analysis revealed the high crystallinity of all sintered ceramics, while the best densification was achieved for the BaTiO3-based materials via conventional sintering. Conjunctively, BCTZ yielded the best combination of functional properties-piezoelectric response (in terms of longitudinal piezoelectric constant and planar electromechanical coupling factor) and mechanical and in vitro osteoblast cell compatibility. The selected piezoceramic was further used as a base material for the robocasting fabrication of 3D macro-porous scaffolds (porosity of similar to 50%), which yielded a promising compressive strength of similar to 20 MPa (higher than that of trabecular bone), excellent cell colonization capability, and noteworthy cytocompatibility in osteoblast cell cultures, analogous to the biological control. Thereby, good prospects for the possible development of a new generation of synthetic bone graft substitutes endowed with the piezoelectric effect as a stimulus for the enhancement of osteogenic capacity were settled.

Dense fine-grained Ba0.6Sr0.4TiO3 ceramics with submicronic grains sizes (GS) have been prepared using nanopowders synthesized via sol-gel route and consolidated by Spark Plasma Sintering (SPS). By changing SPS parameters, the GS was reduced from 214 nm to 74 nm. Diffuse ferroelectric-paraelectric phase transitions and low values of dielectric permittivity (<1000) at the Curie temperature (T-C similar to 280 K) were revealed by Impedance Spectroscopy in all sintered ceramics. The GS reduction from submicron to nanoscale range reflects in a gradually diminishment of dielectric constant, tunability, polarisation and storage energy properties. Raman spectroscopy investigations pointed out the presence of polar nanoclusters above the T-C. The short-range polar order is affected by the GS decrease, but becomes more thermally stable. The observed properties of Ba0.6Sr0.4TiO3 nanostructured ceramics are interpreted by considering the interplay between the GS reduction, the role of low-permittivity grain boundaries and the diffuse character of the ferroelectric-to-paraelectric transformation.

Novel (1-x) SrFe12O19 - x BNT-BT0.08 (x = 0; 0.5; 0.8; 1) nanocomposites were explored in this study. The samples were produced by sol-gel method and compacted by conventional sintering. The composition, morphology, local structure, dielectric and magnetic properties were investigated by X-ray diffraction, Transmission Electron Microscopy, Impedance Analysis, Mossbauer spectroscopy, and SQUID magnetometry. The desired composition and the presence of the magnetoplumbite SrFe12O19 and perovskite BNT-BT structures were verified by X-ray diffraction. Irregular morphology and large size distributions are evidenced in the electron microscopy micrographs. The reported room temperature dielectric constants in this study are the highest values obtained in multiferroic composites at room temperature: giant dielectric constants (similar to 1.3 x 10(6)) were obtained, relative to 0.13 x 10(4) in BNT-BT. The hyperfine parameters allowed the identification of the Wyckoff positions of the Fe ions corresponding closely to the theoretical case. The hard magnetic character of the SrFe12O19 phase is evidenced from the magnetic measurements. For the first time in multifermic composites, superdielectric characteristics are evidenced at room temperature.

Piezoelectric hard/soft effects of multivalence co-dopants (Sb and Mn) in correlation with their location in the lattice, were investigated in PZT ceramics, prepared by conventional ceramic technology, with the following compositions: Pb0.98Sr0.02 ((Ti0.49Zr0.51)(1-0.015-x)Mn0.015Sbx)O-3 with x = 0, 0.005, 0.01, 0.02, 0.03, where antimony was initially assumed to substitute for Ti/Zr ions. The antimony valence state was found to be +3 in all samples by X-ray Photoelectron Spectroscopy investigations. The Electron Paramagnetic Resonance spectra evidenced a steep enhancement of the Mn2+ concentration upon increasing antimony doping level, explained by a charge compensation mechanism, between the Sb3+ ions substituting Pb2+ at the A-sites and the Mn2+ ions, localized at the B-sites. The incorporation of Sb3+ at the A-site of the PZT lattice is also supported by the variation of the lattice parameters, determined by X-ray Diffraction, with the increasing Sb concentration. The investigation of the dielectric, electromechanical and ferroelectric properties evidenced a hard piezoelectric behavior, mainly attributed to the presence of large sized Mn2+ ions, localized at B-sites. Our results prove that the piezoelectric hard/soft response is decisively influenced by the interplay between multiple valence states and locations of the co-dopants, on one hand, and the charge compensation mechanisms, on the other hand. This provides indirect information about the location of some co-dopants which can substitute for both cationic sites in the PZT based ceramics.

Structural and electrical properties of epitaxial Pb(Zr0.2Ti0.8)O-3 films grown by pulsed laser deposition from targets with different purities are investigated in this study. One target was produced in-house by using high purity precursor oxides (at least 99.99%), and the other target was a commercial product (99.9% purity). It was found that the out-of-plane lattice constant is about 0.15% larger and the a domains amount is lower for the film grown from the commercial target. The polarization value is slightly lower, the dielectric constant is larger, and the height of the potential barrier at the electrode interfaces is larger for the film deposited from the pure target. The differences are attributed to the accidental impurities, with a larger amount in the commercial target as revealed by composition analysis using inductive coupling plasma-mass spectrometry. The heterovalent impurities can act as donors or acceptors, modifying the electronic characteristics. Thus, mastering impurities is a prerequisite for obtaining reliable and reproducible properties and advancing towards all ferroelectric devices.

Composites of BaTiO3 (BT) and doped-PZT (d-PZT) micro-particles randomly dispersed in a PDMS matrix were prepared by a molding process. The morpho-structural characterization, performed by SEM and XRD, showed ceramic micro-grains of cubic BT and orthorhombic d-PZT, randomly dispersed in the PDMS matrix. Polarization (P) as a function of the applied field (E) was measured for composite samples, as well as for polymer samples. Hysteresis loops typical for a dielectric material were obtained, but also atypical ones, especially for higher fraction of polymer in composite, lower fields and shorter measuring periods, as a result of the dielectric relaxation in polymer and the presence of interfacial polarization charges at the contact between polymer and ferroelectric. All these composites show very low polarizations (less than 0.2 mu C/cm(2) and 0.05 mu C/cm(2) the maximum and remnant polarization, respectively), caused by the very low dielectric constant of the polymer (less than 10), which drastically reduces, up to 100 times, the electric field effectively applied to the ferroelectric. Weak pyroelectric response was recorded on BT/PDMS, but a typical behavior of a pyroelectric detector was observed. A figure of merit of the material which exceeds 10(-4) m(2) /C was estimated.

Pb0.98Bi0.02Zr0.5Fe0.1Nb0.09Ti0.31O3 (PBiZFNT) powder obtained by a solid state reaction technique was uniaxial pressed and sintered at 1200 degrees C in order to fabricate dense thin disk ceramics with different thickness (200 mu up to 600 mu m). The pyroelectric response was influenced by the thickness of the ceramics. The thin ceramic disks of 250 mu m thickness show better pyroelectric signal (approximate to 35 mV) which recommend them for potential use in laser energy meters.

Ba0.75Sr0.25TiO3 (BST) and PbZr0.68Fe0.14Nb0.14Ti0.04O3 (PZFNT) ceramic pellets were obtained by ceramic technology and their structural, ferroelectric and pyroelectric properties were investigated. The relative density of BST and PZFNT is about 93% and 90%, respectively, with an average grain size of 102 mu m and 6.45 mu m. Both materials have similar room temperature dielectric constants (similar to 2000), but PZFNT shows higher remnant polarization (similar to 15 mu C/cm(2)) and better pyroelectric properties (similar to 1.69 . 10(-4) C/m(2)K), which recommend it for pyroelectric detectors, infrared radiation- and laser pulse energy-meters.

Lead zirconate titanate doped with iron and niobium (PZFNT) was prepared by conventional processing technique, solid state synthesis method. The influence of dopants on the microstructure, ferroelectric and pyroelectric properties was investigated. XRD data reveals a perovskite structure near to the lead zirconate phase. The relative density of PZFNT is approximate 90%, with average grains size of 6.45 mu m.

Ceramic compounds of ferroelectric lead zirconate titanate (PZT) and ferroelectric barium titanate (BT) were prepared from nanopowders corresponding to the formula (1-x)PZT.xBT with x = 0; 0.1; 0.2 ... 0.9; 1. The samples were investigated by X-ray diffraction and electron microscopy and the piezoelectric properties were determined by means of an impedance analyzer. The sintering process is complex since samples densify at different temperatures depending on the composition resulting in different morphostructures. Accordingly, the piezoelectric properties show unusual behavior discussed in terms of different grain size and composition. A remarkably high enhancement of the relative dielectric constant, epsilon(r) coupled with maxima in both, piezoelectric charge constant d(33) and the piezoelectric voltage constant g(33), is observed for x = 0.4 while the values of the coupling coefficient kp are still high similar to those of pure PZT.

Pure BT and BT doped with 1 and 2 mol% Zr, with orthorhombic-tetragonal (O-T) phase transitions near room temperature were prepared, with different grain sizes, by conventional ceramic method, at various sintering temperatures. Intrinsic and extrinsic effects of Zr addition on structural, dielectric, piezoelectric and ferroelectric properties of BT ceramics, near O-T phase transition were studied. In coarse grained ceramics, the intrinsic effects manifested in low field measurements (dielectric and piezoelectric constants), were reduced by Zr addition, while the extrinsic effects which control the high field response (remanent polarization and coercive field) were significantly enhanced. Instead, in fine grains ceramics, of either pure or doped BT, the extrinsic effects related to increased domain wall and grain boundaries density were dominant in both low and high field measurements, overlapping Zr effects. The results were explained in terms of crystal anisotropy correlated with grain size effects.

The effect of 1 and 2 mol % V addition on sintering conditions, structural and functional properties of NbeLi co-doped PZT ceramics, prepared by conventional ceramic method, was systematically studied, by different experimental techniques. The reported results show that the increasing amount of vanadium, as a substitute for niobium, decreases the optimal sintering temperature with 100-150 degrees C, but slightly reduces the piezoelectric, ferroelectric, and electromechanical properties of these ceramics. In addition, the presence of vanadium increases the lattice tetragonality and grain size, promoting a harder piezoelectric behaviour, mostly in the absence of niobium. EPR spectra exhibit well-defined hyperfine splitting characteristics for V4+ state, dispersed in the lattice as VO2+ ion. Additionally, complex paramagnetic defects (charged oxygen vacancies and associated defects) are evidenced for 2 mol % vanadium addition. The results were explained by assuming that vanadium promotes the B-site Li substitution, compensated by oxygen vacancies, while niobium facilitates the A-site Li substitution with lead vacancies formation and partial reduction of V5+ to V4+. Thus, indirect information about lithium location is provided. (C) 2016 Elsevier B.V. All rights reserved.

Water splitting by means of semiconducting photoelectrodes and solar light represents a promising alternative to conventional fossil fuel economy. In this process the photoactive electrode absorb sunlight directly thus initiating the photochemical reaction which create excess electrons in the conduction band of the semiconducting electrode. Titanium doped iron oxide seems to be a promising semiconducting material for photoelectrodes. Consequently, we investigated the effect of Ti doping on the structure, electrical and photoelectrochemical properties alpha-Fe2O3. The Ti doped alpha-Fe2O3 were prepared by a slightly modified mixed oxide route, consisting in a prolonged mixing of the raw materials in a high energy planetary ball mill until the particles decreased to the nanometric sizes. Optimum results were obtained for samples doped with 5 at. % titanium and sintered at 1200 degrees C. Photocurrents as high as 8.4 mA/cm(2), for illumination from a 300 W xenon lamp, were recorded for such samples. It is hoped that such photoelectrodes show promise for water splitting.

A new noniterative method, for determining the dielectric, piezoelectric and elastic constants, in complex form, for piezoceramic materials, in the thickness extensional mode, was proposed. This method is very flexible, as it is based on the standard approach to determine the elastic stiffness, followed by the measurement of the impedance at two frequencies to calculate the dielectric and piezoelectric constants, by solving a system of two equations. The new method was tested on materials with various thickness coupling factors (k(t) = 4.5-60%) and mechanical quality factors (Q(m) = 20-1600), proving very good accuracy for all of them. The accuracy of standard method was also evaluated for the same materials. (C) 2014 Elsevier Ltd. All rights reserved.

Effects of Ni, Nb and Mn co-doping on microstructural, dielectric and electromechanical properties of lead titanate (PT) dense ceramics, obtained as a result of decreasing lattice anisotropy with increasing amount of co-dopants, were investigated. Different values of the average valence (AV) of Ni-Nb co-dopants were obtained by mutual valence compensation between these acceptor donor ions of various relative concentrations. Significantly higher amounts of Mn2+ (resulted by reduction of Mn4+ in presence of donor co-dopants) were found in materials with Ni-Nb AV >4+. These materials show higher coupling factors, correlated with higher mechanical quality factors, compared to the rest of materials. Hard-like piezoelectric behavior was manifested in materials with Ni-Nb AV 4+, and without manganese, turned out to have a soft-like behavior. Electromechanical properties of PT ceramics are discussed in terms of average valence of Ni-Nb co-dopants and multivalence state of manganese. (C) 2013 Elsevier Ltd. All rights reserved.

A high performance piezoceramic material of PZT type, doped with 6 atomic % W, was prepared in order to be used as sensing element for a vibration transducer used to equilibrate the motor-fan group of auto vehicles. The piezoelectric parameters of material showed high values which recommended it for the vibration transducer. The transducer consists of a ring shaped active element sandwiched between two steel blocks. The transducer works in quasistatic regime, at frequencies much lower than the resonance frequency, being tightly screwed in the measurement system by a metallic cover attached at the back end.

A new noniterative method, for determining the dielectric, piezoelectric and elastic constants, in complex form, for piezoceramic materials, in the radial mode, was proposed. This method uses the standard procedure to determine the elastic compliance and Poisson factor and the measurement of admittance at two frequencies to calculate the dielectric and piezoelectric constants, by solving a system of two equations. The accuracy of the new method was determined for materials with different planar coupling coefficients (k(p) = 2.5-57%) and mechanical quality factors (Q(m) = 20-3000). This method proved to be very accurate for all materials especially for those with large coupling factors. The accuracy of standard method was also evaluated for the same materials. (C) 2011 Elsevier Ltd. All rights reserved.

Three ceramic magnetic compositions within the CuZnTi ferrite system, having the Curie temperatures centered around 60 degrees C, 80 degrees C and 100 degrees C, respectively were investigated in order to determine the time-temperature stability of their main magnetic properties. Rates of changes of 33, 66 and 100 ppm/h of the maximum slopes of permeability were found for the samples aged at 60 degrees C, 80 degrees C and 100 degrees C, respectively. The samples aged at temperatures higher than 200 degrees C show a rather sudden decrease of the maximum slope of permeability, but no shift of the Curie temperature and the working point, corresponding to the temperature where the slope has the maximum value. The results are discussed in terms of the migration processes of the cations, especially Cu2+, from metastable positions on which they were frozen during the rapid cooling of the samples from the sintering temperature, to the more stable ones, namely the octahedral sites into the spinel lattice.

Due to its low resistivity and excellent thermal stability, IrO2 has attracted attention as an alternative for electrode material in ferroelectric integrated circuit applications. Oriented growth of IrO2 electrode film was investigated with the goal to control the texture of the PZT thin film. IrO2 films were prepared by DC reactive sputtering. PZT film was prepared by RF magnetron sputtering single target deposition method. The whole layer stack was grown onto amorphous thermal oxide of a silicon wafer. The results indicate that IrO2 thin film was preferentially (200) oriented when a TiO2 seeding layer was used. The orientation relationships along the whole PZT(111)/IrO2(200)/TiO2(200)/Ti structure was discussed.

The application area of piezoceramic materials and devices based on such materials is continuously extended. The working conditions for such devices and structures may be sometimes very hard such as low and high temperatures, nuclear radiation, high pressure and so on. Such conditions may affect to some extent the transducer performances due to the changes of the basic properties of piezoceramic materials used as sensors. In this regard we have investigated the changes of the main piezoelectric properties of two typically hard and soft piezoceramics within a large temperature range from 2 to 600 K. All parameters measured showed a rather steady increase or decrease with increasing temperature. The results were discussed in terms of intrinsic and domain wall contribution to the dielectric and piezoelectric response of materials.

Multiferroic composite materials, consisting of piezoelectric and piezomagnetic phases, show electric polarization in the presence of a magnetic field and magnetization on applying an electric field, i.e. piezomagnetic phase causes polarization while piezoelectric phase causes changes in magnetization. The figure of merit of such multiferroic particulate composites is the magnetoelectric voltage coefficient representing the amount of polarization induced by a given magnetic field. In the present investigation we prepared and studied the properties of bulk particulate composites made from cobalt ferrite (COF) and a soft piezoelectric material (PZT). Composite samples, according to the formula xCOF center dot(1-x)PZT with 0 <= x <= 1 were prepared by mixing the COF and PZT powders, pressing them as discs and then sintered at 1200 degrees C for 3 hours. Structural, magnetic, piezoelectric and magnetoelectric characteristics of the sintered composites were determined as a function of the mass fraction of COF into PZT and magnetic field. The ME coefficient shows a maximum value of about 70 mV/cm.Oe for composites containing 40 % COF and 60 % PZT, in a DC magnetic field of 3 kOe at a frequency of 1 KHz of the ac magnetic field. Some of the dielectric and piezoelectric properties of the composite samples, as a function of the mass fraction x, were also determined.

Lead titanate (PT) ceramics co-doped with 8 to 13 mol% Ni-Nb, by substituting titanium ions, were prepared by conventional ceramic method and the influence of Ni-Nb doping level on their mopho-structural, electromechanical and dielectric properties was investigated. Dense tetragonal PT ceramics were obtained for 8 to 12 mol% Ni-Nb co-doping. Lattice anisotropy decreased from 1.053 to 1.046 with increasing doping level. Large thickness coupling factors of 0.4 were obtained for 8, 11, and 12 mol% Ni-Nb. Large electromechanical anisotropy of 10 was obtained for 8 mol% Ni-Nb. It was found that Ni-Nb co-doping has both softening and hardening effects as a function of donor or acceptor behavior of different amounts of Ni-Nb. For 13 mol% Ni-Nb, segregation of a small amount of a secondary phase occurs, lattice constants and tetragonality significantly decrease and the resulting ceramic becomes very hard.

In order to determine the effect of sample shape, compacting pressure and green density gradients on the final shape and density of sintered bodies a study was made on a PZT type powder, pressed within a large range of shapes from long cylinders up to thin discs and at pressures ranging between 40 and 180 MPa. Important shape distorsion were found for both long cylinders and thin discs but surprisingly the final density was not essentially changed even if the green compacted density showed important changes with in the samples. Differences in the shrink age of diameter and thickness brought about by sintering for samples with different shapes were found.

A high quality PZT type material was designed and produced in order to be used as sensor for a high internsity siren. Two disks from this materials were assembled into a parallel bimorph type transducer with an intermediate metallic plate. This assembly behaves like an acoustic resonator in the high frequency audio range. The fundamental of the flexural mode for a free resonator is taken into account Its frequency dependence on the geometry of the system and on its material properties is computed using the results for a circular plate with clamped edges and an observation on the equivalence between the movement of a particular part of a resonator in a stationary state and the movement of that pari separated from the resonator but with the same border conditions. Acoustical and impedance measurements were made in order to appreciate the influence of the metallic plate on the frequency. In order to increase the acoustic emission a frontal cardboard horn was added to the resonator.

A study was made on the temperature behavior of the main piezoelectric parameters, namely the electromechanical coupling factor k(p), the mechanical quality factor Q, the dielectric permittivity (real and imaginary part), the loss tangent t(g) and the piezoelectric charge constants 6133 and d(31) of a soft type piezoelectric material. It was evidenced that temperatures under 150 degrees C do not influence these parameters, which means that every transducer made from this type of material may be successfully used up to this temperature. Between 150 degrees C and 250 degrees C, the piezoelectric properties undergo more or less important changes, mainly due to the depoling effect. After that, at temperatures over 250 degrees C, they degrade very rapidly, tending to zero.

Barium titanate was prepared by the advanced procedure of mechanochemical synthesis from barium and titanium oxides after 50 hours of milling. The X-ray examination of the milled powders showed the phase evolution during synthesis and the perovskite phase was initiated after 5 hours and was completed after 50 hours. Sintered samples from these nanopowders show different dielectric properties for fine and coarse grained ceramics. Samples with an average grain size of 2 mu m showed the highest dielectric constant of 5800 at room temperature. A sharp maximum of the dielectric constant with values of about 16000 was recorded at the Curie temperature.

The temperature behavior of the main piezoelectric parameters, such as the electromechanical coupling factor k(p), the mechanical quality factor Qm, the dielectric permittivity epsilon (real and imaginary part), the loss tangent tan delta, the piezoelectric charge constants d(33) and d(31) and the voltage constants 933 and 931 of a soft type piezoelectric material was investigated. The results showed that temperatures under 150 degrees C do not influence these parameters, which indicate that every transducer made from this type of material may be successfully used up to this temperature. Between 150 degrees C and 250 degrees C, the piezoelectric properties undergo more or less important changes, mainly due to the depoling effect and above that, at temperatures over 250 degrees C, they degrade very rapidly, tending to zero.

New PZT type ceramic materials, doped with Ni, were prepared by the usual solid state reaction. The best composition proved to be the one containing 6% atomic Ni and having the following chemical formula: Pb0.95Bi0.03Nb0.02Zr0.51 Ti0.43Ni0.06O3. Rectangular samples of this powder were pressed and sintered at 1200 OC for 4 hours and then it was cut in thin slices used to construct an efficient bimorph type ventilator to be used in miniaturized devices for cooling. Miniature ventilators made of PZT ceramics are preferred due to their smallness, high electromechanical efficiency and lack of rotating parts and last but not least of contiguous electrical contacts. We made such a device by using two thin ceramic PZT plates of the material with the best piezoelectric properties, assembled in a flexural oscillator through a proper bimorph association. The emerging doublet behaves itself very much like a bimetal system and possesses a good developed flexural mode. Attaching a metallic elastic plate to the bimorph produces a highly efficient miniature ventilator. A theoretical quantitative study of this flexural bimorph system with elastic plate attached was proposed.

A study was made to determine the effect of sample shape, compacting pressure and green density gradients on the final shape and density of sintered bodies. Experiments were carried out on a PZT type powder, pressed with a large range of shapes from long cylinders up to thin discs, at pressures ranging between 40 and 180 MPa. Important shape distorsion were found for both long cylinders and thin discs but surprisingly the final density was not essentially changed even if the green compacted density showed important changes within the samples. Differences in shrinkage of diameter and thickness brought about by sintering for samples with different shapes were found.

The new advanced procedure of mechanochemical synthesis was used to prepare barium titanate from barium and titanium oxides. The X-ray examination of the powders milled for different times showed the phase evolution during synthesis. The perovskite phase was initiated after 5 hours of milling and was completed after 50 hours. Sintered samples from these nanopowders show different dielectric properties depending on their morphostructure. Thus for samples with an average grain size of 2 mu m we obtained the highest dielectric constant of 5 800 at room temperature. A sharp maximum of the dielectric constant with values of about 16 000 was recorded at the Curie temperature.

Accuracies of three methods used to determine the mechanical qualily-factors of-piezoceramic resonators were evaluated and discussed-for the specific case of five materials with different mechanical quality factors (between 20 and 3000) and electromechanical coupling coefficients (between 2.5% and 60%). The mechanical quality factor Q(m) was calculatedfor both radial and thickness modes. The three methods are: the equivalent circuit, the band width and the iterative method. The last two methods proved to be very accurate for all materials (the error of Q(m) is less than 0.1%), but for low Q(m) materials the second method requires to redefine the involved frequencies. The first method is less accurate, especially for materials with low mechanical quality and electromechanical coupling factors or large difference between the mechanical quality factors of the resonance and antiresonance bands (the error of Q(m) is more than 10-20%).

In the present investigation we studied the effect of lead content on a hard type ceramics starting from lack to excess PbO. Specimens of Pb-x(Mn0.017Sb0.033Zr0.48Ti0.47)O-3 with 0.96 <= x <= 1.06 were prepared by conventional oxide mixing technique and sintered at temperatures between 1280 and 1350 degrees C. In samples with lower Ph concentration the pyrochlore phase appeared, while in those with higher Pb an excess of PbO phase was detected. These phases proved detrimental for the properties of sintered ceramics. The highest values of piezoelectric properties were observed only for samples with an excess lead of about 2% sintered at 1340 degrees C, indicating that it represents the optimum amount of PbO. For such samples, the density p was 7.77 g/cm (3). the remnant polarization P, was 37 mu C/cm(3) the planar coupling coefficient k(P) was 0.57 and the charge constant d(33) was 340 C-P/N. (c) 2007 Elsevier Ltd. All rights reserved.

The effect of Fe and Ni doping on piezoelectric properties of a soft type piezoelectric material was investigated. The materials composition was as follows: Pb0.95Bi0.03Nb0.02Zr0.51Ti0.49-xMxO3, where M stands for the transitional metals Fe or Ni and x = 0, 0.02, 0.04, 0.06, 0.08 and 0.10. The materials were prepared by the conventional ceramic technique. X-ray diffractograms showed that compounds were completely formed and they were situated in the nanometric range with an average crystallite size of about 95 urn. The optimum amounts of doping for both types of transitional elements were situated somewhere around x = 0.06 with better results for nickel doped samples. The maximum density for 0.06 nickel doped material was 7.87 and 7.80 g/cm(3) for iron doped one. The electromechanical coupling factor k(p) for 0.06 nickel doped samples was 0.665 while for the correspondingly iron doped ones it was 0.638. The relative dielectric constant was about 4050 for nickel doped samples and 3400 for iron doped ones. The corresponding values for the charge constant d(33) were 625 and 530 pm/V, respectively. These results were discussed in terms of the positions occupied by Ni and Fe into the lattice, the type of vacancies created by this and the shift of the morphotropic phase boundary. (c) 2005 Published by Elsevier Ltd.

Effect of Bi additives on the piezoelectric properties of PT ceramics having the formula (Pb1-3x/2Bix(Ti0.98Mn0.02)O3, with x = 0,04, 0.06, 0.08 was investigated. The samples were prepared by conventional ceramic technique, using p.a. purity raw materials. The Mixed powders were sintered at temperatures between 1050-1300 degrees C. Poling was done in fields of about 70 kV/cm. Density and coupling factors of the samples were determined as a function of sintering temperatures, and doping level. Temperature dependence of the main piezoelectric characteristics was also investigated for temperatures as high as 500 degrees C. A Curie point of 450 degrees C, a low dielectric constant (< 140) and a large electromechanical anisotropy), (the radial mode is nearly inexistent, at room temperature and the thickness coupling factor is about 0.4) were found. These enhanced properties make such ceramics very attractive for high temperature and high frequency transducers applications.

Powders of barium titanate (BT) and lead zirconate-titanate (PZT) were mixed together to form solid solutions in the ceramic system (1 - x)BT (.) xPZT, where 0 <= x <= 1. The mixed compounds were synthesized by sintering pressed samples of the mixed powders at high temperatures between 1100-1400 degrees C, for 3 hours. Dielectric and piezoelectric properties of the sintered samples were investigated at room temperature. X-ray diffractograms showed that monophase ceramics with perovskite structure were formed along the whole compositional range. The dielectric permittivity epsilon(r) increased with increasing amount of PZT, reaching a maximum value of about 6900 for composition with x = 0.6, and then suddenly decreases up to lower values of less than 2000. The piezoelectric properties (electromechanical coupling coefficient k(p), voltage and charge constants g and d respectively) behave as follows: at first there is a slight decrease of their values for all compositions with x = 0.6. Such a behavior could possible be explained in terms of the structural changes brought about by the incorporation of PZT phase into BT one. The mechanical mixture of BT and PZT particles of similar dimensions gives rise to both new compositions as well as to distinct particles of BT or PZT of different dimensions under the influence of temperature, whose properties may differ from the initial ones so that their properties can be either the result of a size effect or the compositional one, but most probably the combination of both.

Lead titanate (PT) is well known as a good piezoelectric material with high Curie temperature (similar to 490 degrees C) and low dielectric constant (similar to 200). Pure lead titanate cannot be sintered as ceramic bodies, due to its high crystalline anisotropy (c/a=1.064), but dense ceramic bodies can be obtained if small additions of other elements are used. In this regard we investigated the effect of Y addition on the structure and piezoelectric properties of PT ceramics, with general formula (Pb1-3x/2Yx)(Ti0.98Mn0.02)O-3, and x=0.04, 0.06, 0.08 and 0.10. Density and coupling factors were determined as a function of sintering temperatures, and doping level. Temperature dependence of the electromechanical behaviour was also investigated for temperatures as high as 500 degrees C. The Y modified PT ceramics have low dielectric constant ( 450 degrees C), and large electromechanical anisotropy.

An iterative method is presented for the characterization of lossy piezoelectric materials in the thickness resonant mode, which allows to calculate the frequencies at which the electrical impedance is to be measured. This new method considerably reduces the measurement time by separating the programs for data acquisition and for processing thus simplifying the measurement procedure. The accuracy of the method was tested for several piezoceramic materials, covering a wide range of values of the thickness coupling factor and mechanical quality factor. The new method proved to be as accurate as other iterative methods, but it is easier to apply. (C) 2002 Elsevier Science Ltd. All rights reserved.

An iterative method is presented for the characterization of lossy piezoelectric materials in the radial resonant mode, which provides a new formula to calculate the frequencies at which the electrical admittance is to be measured. This new method considerably reduces the measurement time by separating the programs for data acquisition and for calculus. The accuracies of the material constants for several soft and hard piezoceramic materials, covering a wide range of values of the planar coupling factor and mechanical quality factor, were tested and it was evidenced that they depended on the type of the piezoelectric material. The new method proved to be as accurate as, or in some cases even more accurate than, the other iterative methods. This new method can also be applied whenever the standard method does not allow the determination of the material constants. (C) 2002 Elsevier Science Ltd. All rights reserved.

Effect of 1-4 wt.% excess PbO, added to the calcined powder, on the structure and dielectric and piezoelectric properties of Bi-doped lead titanate ceramics was investigated. The ceramics containing 4, 6 and 8 mol% Bi were prepared by the usual ceramic technique, using high purity oxides. The calcined powder of each composition was next intimately mixed with 1, 2, 3 and 4 wt.% PbO and then sintered at 1200-1300 degreesC for 3 h. The dielectric piezoelectric and electromechanical properties were determined as a function of excess PbO. Addition of PbO up to 2 wt.% was found to improve the densification, by the presence of the PbO liquid phase during sintering. For all Bi doping levels, the dielectric constant and the thickness coupling factor reach maxima (representing an increase of 2.5-8% for dielectric constant and 7-8% for thickness coupling factor) at about 2 wt.% PbO addition. The planar coupling factor is very low, less than 0.01, regardless the PbO content. The disipation factor slightly decreases with increasing PbO content and the mechanical quality factor of the thickness mode was about 300. The experimental results were discussed in terms of the mechanisms of densification in the presence of the liquid phase and of lead vacancy concentration. (C) 2002 Elsevier Science Ltd. All rights reserved.

Densification and piezoelectric performances of some fast fired lead zirconate-titanate ceramics were investigated in comparison with those made by conventionally sintering technique. Micro and nano PZT powders were used as initial powders for the compacts subjected to the Cast and conventionally sinterings. Density maxims of about 98 % of the theoretical density, were obtained for nanopowder compacts fast fired at 1350 degreesC for 20 minutes. The piezoelectric properties of the fast fired and conventionally sintered compacts were determined. Significant increase for d(33) piezoelectric charge constant was observed in the case of fast fired samples compared to conventionally sintered ones. The electromechanical planar coupling factor k(p) was also increased From 0.62 to 0.65 for fast fired samples. The densification mechanism was discussed in terms of a predominantly transient stresses induced during the initial stage of densification by the rather high temperature gradient of the fast fired samples.

Lead titanate (PT) is well known as a good piezoelectric material with a high Curie temperature (similar to 490 degreesC) and a low dielectric constant (similar to 200). But pure lead titanate cannot be sintered as ceramic bodies, for practical purposes, due to its large crystal anisotropy (tetragonality c/a=1.064). However, dense PT ceramic bodies can be obtained if small additions of other elements are used. Ca and Sm modified PT were succesfully tried, but such dopings proved rather expensive for mass production. Therefore some other doping elements must be tried. In this regard we investigated the effect of Bi and Y additives on the piezoelectric properties of PT ceramics. The general formula of our Bi, Y doped lead titanate ceramics was: (Pb1-3x/2Mex)(Ti0,98Mn0,02)O-3, with Me=Bi,Y and x=0.04, 0.06, 0.08. The modified PT ceramics were prepared by conventional ceramic technique, using p.a.purity raw materials. The mixed powders were sintered at temperatures between 1000-1300 degreesC. Poling was done in fields of about 70kV/cm. Density and coupling factors of the samples were determined as a function of sintering temperatures, and doping level. Temperature dependence of the main piezoelectric characteristics was also investigated for temperatures as high as 500 degreesC. It was found that Bi,Y modified lead titanate ceramics have a very high Curie point (T-c>450 degreesC), a low dielectric constant (<140) and a large electromechanical anisotropy (the radial mode is nearly inexistent, at room temperature and the thickness coupling factor is about 0.4), which make them competitive to the rare-earth doped PT ceramics, for high temperature and high frequency transducers applications.