Gotowa bibliografia na temat „Lead based Piezoelectrics”

ABSTRACTLead free niobate solid solutions can exhibit piezoelectric properties comparable to that of lead zirconate titanate piezoelectrics in the vicinity of its morphotropic phase boundary (MPB). Here we describe how (Na,K)NbO3 and (Na,K)NbO3-LiTaO3 solid solution thin films can be grown epitaxially by the hydrothermal method at temperatures of 200 °C or below in water and be made ferro- and piezoelectrically active by a simple 2 step post growth treatment.

In this review, the crystal structure and the synthesis of the sodium potassium niobate ( K 0.5 Na 0.5 NbO 3) as a promising candidate for lead-free piezoelectrics are addressed. The sintering and the microstructure as prerequisites for obtaining ceramics with reliable and sufficiently high piezoelectric properties for selected applications are discussed.

BiFeO3–based ceramics possess an advantage over large spontaneous polarization and high Curie temperature, and are thus widely explored in the field of high–temperature lead–free piezoelectrics and actuators. However, poor piezoelectricity/resistivity and thermal stability of electrostrain make them less competitive. To address this problem, (1 − x) (0.65BiFeO3–0.35BaTiO3)–xLa0.5Na0.5TiO3 (BF–BT–xLNT) systems are designed in this work. It is found that piezoelectricity is significantly improved with LNT addition, which is contributed by the phase boundary effect of rhombohedral and pseudocubic phase coexistence. The small–signal and large–signal piezoelectric coefficient (d33 and d33*) peaks at x = 0.02 with 97 pC/N and 303 pm/V, respectively. The relaxor property and resistivity are enhanced as well. This is verified by Rietveld refinement, dielectric/impedance spectroscopy and piezoelectric force microscopy (PFM) technique. Interestingly, a good thermal stability of electrostrain is obtained at x = 0.04 composition with fluctuation η = 31% (Smax'−SRTSRT×100%), in a wide temperature range of 25–180 °C, which is considered as a compromise of negative temperature dependent electrostrain for relaxors and the positive one for ferroelectric matrix. This work provides an implication for designing high–temperature piezoelectrics and stable electrostrain materials.

Lead-free electroceramics represent an emerging area of research that has the potential to enable new green advances in electronics. Research has mainly focused on the development of new piezoelectric materials for replacing lead containing oxides exhibiting superior electromechanical behavior. Lead-free BiFeO3-based materials are not only the promising candidates to replace lead-based materials but also show intriguing properties which may inspire innovative material design for the next generation of lead-free piezoceramics. This review aims to highlight the current state of research and overlooked aspects in lead-free BiFeO3-based ceramics, which could be insightful in elucidating certain controversial issues. Current strategies to reduce high conductivity, influence of chemical heterogeneity on both functional properties and crystal structure, effective heat treatment procedures, and the role of pseudo-cubic structures on the enhancement of piezoelectric properties are subjects of highlighted within this review as they have a significant impact on the quality of BiFeO3-based lead-free piezoelectrics (but are often disregarded).

K0.5Na0.5NbO3-based ceramics, a promising group of lead-free piezoelectrics, are challenging to sinter dense while avoiding alkali evaporation. This work explores hybrid atmosphere processing, a new approach where reducing atmospheres is used during heating to avoid coarsening from alkali carbonates and hydroxides, and oxidizing atmospheres is used during sintering to avoid alkali evaporation. Discs of Li0.06(K0.52Na0.48)0.94Nb0.71Ta0.29O3 with 0.25 mol% Mn (KNNLTM) were sintered in air, N2, 9% H2 in N2, or 9% H2 in N2 during heating and air during sintering (hybrid atmosphere processing). The highest density was obtained by sintering in 9% H2 in N2, but resulted in high alkali loss and decomposition of the surface, followed by low piezoelectric response. However, with the hybrid H2/air processing it was possible to both avoid surface decomposition and leakage currently associated with alkali evaporation during sintering in H2, and to obtain a denser, more phase-pure and small-grained KNNLTM ceramic with a higher piezoelectric response than obtained by sintering in air or N2.

Piezoelectric actuators are indispensable over a wide range of industries for their fast response and precise displacement. Most commercial piezoelectric actuators contain lead, posing environmental challenges. We show that a giant strain (1.05%) and a large-signal piezoelectric strain coefficient (2100 picometer/volt) are achieved in strontium (Sr)–doped (K,Na)NbO 3 lead-free piezoceramics, being synthesized by the conventional solid-state reaction method without any post treatment. The underlying mechanism responsible for the ultrahigh electrostrain is the interaction between defect dipoles and domain switching. The fatigue resistance, thermal stability, and strain value (0.25%) at 20 kilovolt/centimeter are comparable with or better than those of commercial Pb(Zr,Ti)O 3 -based ceramics, showing great potential for practical applications. This material may provide a lead-free alternative with a simple composition for piezoelectric actuators and a paradigm for the design of high-performance piezoelectrics.

Piezoelectric microelectromechanical systems (MEMS) employing ferrroelectric thin films have been extensively studied. In this paper, materials issues of the piezoeletric films are presented. Temperature dependence of the electrical and electromechanical properties of Pb(ZrxTi1-x)O3 (PZT, x= 0.3, 0.52, and 0.7) thin films were measured using scanning probe microscopy in the temperature range from -100°C to 150°C. The field-induced displacement increased with increase of the temperature; however, their temperature dependence was relatively small, compared with that reported on bulk PZT ceramics. Thus far, the use of PZT film has been most widely studied for MEMS applications. However, the lead toxicity associated with PZT and other lead oxide-based ferroelectrics is problematic. Therefore, properties of the lead-free thin film piezoelectrics are also described in this paper. As candidate for the lead-free piezoelectrics, Bi4-xNdxTi3O12 (BNT) and Ba(Zr0.2Ti0.8)O3 (BZT) thin films are chosen. BNT and BZT films prepared by chemical solution deposition exhibit field-induced strain corresponding to 38 pm/V and 35 pm/V, respectively.

A theoretical model based on the constitutive equations of piezoelectrics and magnetostrictor is introduced to discuss the magnetoelectric (ME) coupling in freebody bilayer containing magnetostrictive and piezoelectric phases. The ME coupling at low frequency of Ni0.8Zn0.2Fe2O4–PZT bilayer have been studied by using the model and the corresponding material parameters of individual phases. The results show that the ME voltage coefficients can increase to a maximum at a given volume fraction of piezoelectric phase. An approximately linear increase of the maximum has been obtained with strengthening interface coupling. Analysis shows that large magnetostriction, appropriate volume fraction and ideal interface coupling are key ingredients for obtaining excellent ME performance.

Our 21st century faces to a “sustainable society”, which enhances (a) usage of non-toxic materials, (b) disposal technology for existing hazardous materials, (c) reduction of contamination gas, (d) environmental monitoring system, (e) new energy source creation, and (f) energy-efficient device development in the piezoelectric area. With reducing their size, the electromagnetic components reduce their efficiency drastically. Thus, piezoelectric transducers with much less losses are highly sought recently. Piezoelectric devices seem to be all-around contributors and a key component to the above mentioned five R&D areas. Some of the efforts include: (a) Since the most popular piezoelectric lead zirconate titante ceramics will be regulated in European and Asian societies due to their toxicity (Pb2+ ion), lead-free piezoelectrics have been developed. (b) Since hazardous organic substances can easily be dissolved by the ultrasonic irradiation in water, a new safe disposal technology using piezoelectric transducers has been developed. (c) We demonstrated an energy recovery system on a hybrid car from its engine’s mechanical vibration to the rechargeable battery. (d) Micro ultrasonic motors based on piezoelectrics demonstrated 1/20 reduction in the volume and a 20-time increase in efficiency of the conventional electromagnetic motors. This paper introduces leading piezoelectric materials, devices, and drive/control methods, relating with the above “sustainability” technologies, aiming at further research expansion in this area.

Our research investigates the correlations between domain texture and microstructural features, including crystallographic texture in bulk and thin film polycrystalline materials to understand the development of piezoelectric and other anisotropic properties in a number of rapidly evolving lead free piezoelectric materials. We investigate approaches to understanding polarization distributions by starting from polarization measurements. In addition, 2D and 3D microstructural simulations are carried out in all types of ferroelectrics to rationalize and then engineer their equilibrium and kinetic response. This paper discusses recent findings associated with bulk piezoelectricity, phase stability, and ferroelastic and ferroelectric domain motion for materials such as Ba(Ti0.8Zr0.2)O3-x(Ba0.7Ca0.3)TiO3 (BZT-BCT) and Bi0.5Na0.5TiO3 (BNT). Conventional and synchrotron-based x-ray diffraction, electron and optical microscopy and piezoelectric characterization techniques are employed to assess texture, both as a function of poling and temperature. The coupling between microstructure and the inherent directional biases fundamental to piezoelectric and ferroelectric performance enable consideration of orientation and anisotropy in systems with unique constraints.

In many commercial devices, there are a number of ways in which energy is wasted or dissipated. This waste energy can, in principle, be harvested by using the correct functional material. In the case of vibrational energy, the logical materials of choice are piezoelectric ceramics. However, all current commercial piezoelectrics contain lead oxide which is classed as a restricted material in environmental legislation. The main contenders for lead free piezoelectrics are based on K0.5Na0.5NbO3 (KNN) and Na0.5Bi0.5TiO3. The former however, has the advantage in that it is compatible with cheap Ni-based internal electrodes and thus it is feasible to manufacture low cost KNN based multilayer devices provided formulations do not deteriorate in the reducing condition required to suppress the formation of NiO. Consequently, KNN based lead free piezoelectric ceramics have been studied from the perspective of optimising their performance for multilayer actuators, potentially for energy harvesting applications. To this end, the defect chemistry of KNN has been investigated under different sintering conditions, dopants (acceptors: Mn2+, Ti4+, Sn4+ in KNN_50/50 ratio; Donor: Sr2+ in KNN_50/50 ratio; Ta5+ as an isovalent in KNN-51/49 ratio; and co-dopants: Bi3+ and Zr4+ in KNN_50/50 ratio) have been incorporated into KNN to enhance the piezoelectric performance and prototype multilayers of 10 and 16 layers with inner Pt electrodes have been fabricated to demonstrate the potential of 0.942KNN-0.058BNZ+ZrO2 for the fabrication of multilayer actuators. This lead free composition has the potential to replace PZT-4 and PZT-8 in piezoelectric devices for room temperature applications. To fabricate the multilayers, a novel Wet-Multilayer-Method (WMM) was also developed to overcome the issues of delamination during firing of MLCCs.

The prime objective of this dissertation is to design, synthesize and characterize lead-free piezoelectric based magnetostrictive components based magnetoelectric (ME) composites that exhibit self-bias characteristics and high amplitude of ME coupling. The secondary goal of this thesis was to lay down the foundation for nanoscale and flexible magnetoelectric devices.
Self-biased ME effect was investigated in lead-free three-phase laminate composites. This effect is characterized by non-zero remanent ME responses at zero magnetic bias field (Hbias). It was revealed that the self-biased ME effects can be observed in three-phase laminate composites consisting of piezoelectric material and two dissimilar magnetostrictive materials. On applying Hbias to the laminates in bending mode configuration, the ME responses were found to exhibit hysteretic behaviors with remanent ME responses. The shape of hysteretic ME response could be controlled by adjusting the magnetic interactions and piezoelectric properties. Further, converse magnetoelectric (CME) responses in bending-mode three-phase laminates exhibited hysteretic behaviors with similar magnitudes during Hbias sweep as it was generated directly by applying ac voltage (Vac) without any external Hbias.
Lead-free (1 - x) [0.948 K0.5Na0.5NbO3 - 0.052 LiSbO3] - x Ni0.8Zn0.2Fe2O4 (KNNLS-NZF) compositions were synthesized for optimizing ME properties of particulate composites. Island-matrix microstructure was developed to improve the magnitude of ME coupling effect by "

overcoming the problems found in conventional particulate composites. The structure lead to improvement of ME coefficient with maximum magnitude of 20.14 mV/cm ae as well as decrease of optimum Hbias of < 500 Oe in the composition of 0.7 KNNLS - 0.3 NZF particulate composites.
Room-temperature ME phase diagram of (1 - x) BaTiO3 - x BiFeO3 materials (BT - x BFO, x = 0.025 - 1.0) was investigated for designing compositions suitable for thin film devices. The BT - x BFO compositions in narrow range of x = 0.71 - 0.8 were found to exhibit good piezoelectric, dielectric and magnetic properties simultaneously. The room temperature ME coefficient was found to be maximum with high magnitude of 0.87 mV/cmOe in the optimized composition of x = 0.725.This composition was found to consist of local monoclinic distortions with average rhombohedral symmetry as confirmed by detailed structural analysis through Raman spectroscopy and atomic pair distribution functions (PDFs).
MnFe2O4 (MFO)-Ni core-shell nanoparticles were synthesized and characterized for developing tunable devices such as memristor. The MFO nanoparticles synthesized by solvothermal method exhibited diameter of 200 nm, mean primary particle size of 15 nm, high saturation magnetization of 74 emu/g and coercivity of 89 Oe. Ni encapsulation on MFO nanoparticles was performed by aqueous ionic coating method. Ni shells with uniform thickness of 1 nm were coated on MFO nanoparticles by this method.
In order to develop future nanoscale dual phase energy harvesters and magnetic field sensors, vertically-aligned piezoelectric nanorods were synthesized. In the initial attempt, Pb(Zr0.52Ti0.48)O3 (PZT) was used to verify the feasibility of developing one dimensional (1D) piezoelectric nanostructures with controlled diameter and height. For the 1D nanostructure, well-ordered anodic aluminum oxide (AAO) templates were prepared by two step aluminum anodizing. The PZT nanorods were synthesized by vacuum infiltration of PZT precursor solutions and exhibited uniform diameter of 90 nm and aspect ratio of 10 with vertical in respect to the Pt-Si substrate. The piezo-response of PZT nanorods showed good magnitude owing to the reduced clamping effect from the substrate.
Attempt towards the development of flexible tunable devices that possess magnetic field sensing and actuation ability was made in the later part of the thesis. The electroactive polymeric actuators in the form of Polypyrrole (PPy) / Au / Polyvinylidene fluoride (PVDF) / Au / Polypyrrole (PPy) were synthesized and the process flow was optimized. Pore size and thickness of PVDF layer was adjusted by changing the solvent, viscosity and drying temperature. Different types of electrolyte solutions were investigated to improve the strain and response time. The actuators exhibited high deflection of 90 % with fast response of 50% deflection per second. Dual-functional structure in the form of  PPy-MFO / Au / PVDF / Au / PPy-MFO was developed by PPy polymerization including MFO nanoparticles via cyclovoltammetric method.
Ph. D.

This thesis is concerned with the fabrication and characterisation of lead free piezocomposites and transducers for use in high frequency medical ultrasound imaging applications. A water based gel casting and micro moulding approach has been developed to fabricate 1-3 composites with a random pillar structure in the lead free and lead based piezoelectric material. High frequency transducers incorporating the random composites as the active components have been fabricated, characterised and demonstrated in real tissue imaging environments. A water based gel casting system has been used incorporating Hydantoin Epoxy resin, amine hardener (Bis (3-aminoproply) amine) and dispersant. Viscosities of the 50BCZT and PZT systems were minimised by the addition of 2.4 and 1 wt% of dispersant respectively. The highest values of piezoelectric and dielectric properties corresponded to 50BCZT samples fabricated with a gel casting slurry incorporating 30 wt% resin and sintered at 1425 °C, with d33 and kp values of 330 pC/N and 0.43, respectively. 1-3 composites were successfully fabricated from the BCZT and PZT bristle block structures and only one resonance peak corresponding to the thickness mode was observed. PZT composites offered generally higher thickness coupling coefficients than 50BCZT composites, where the highest value of 0.78 was measured for samples sintered at temperature 1425 °C. Focused PZT, focused 50BCZT, unfocussed PZT and unfocussed 50BCZT transducers were successfully fabricated using the composites with randomised structure, and have operating frequencies of 35, 40, 50 and 35 MHz respectively.

The review of current technologies for measurement of gas velocity in stack flow applications is undertaken and it is shown that the ultrasonic time-of-flight method is the most suitable and offers a number of advantages over alternatives. Weakness of current piezoelectric based transducers are identified as the inability to operate at temperatures above 400 °C due to limitation of piezoelectric materials used, and a case for development of an alternative high temperature material is put forward. A novel and highly enhanced, lead free piezoelectric material, suitable for continuous operation at temperatures above 400 °C has been engineered for ultrasonic gas velocity sensor applications. Structural modification of pure bismuth titanate (Bi4Ti3O12) or BIT compound, through multi-doping at the Ti-site, has been found to enhance piezoelectric properties accompanied with a mild reduction in Curie temperature, Tc. Initially, compounds doped with tungsten and chromium were found to increase the piezoelectric coefficient (d33) from around 5 pC N¯¹ in pure bismuth titanate, to above 20 pC N¯¹ in doped compounds. This increase is attributed to lower conductivity and improved poling conditions. Further increases in d33 (up to 35 pC N¯¹) were then realised through controlled grain growth and reduction in conductivity for niobium, tantalum and antimony doped compounds. The Curie temperature of the material with best properties is found to be 667 °C, which is a slight reduction from 675 °C for pure bismuth titanate ceramic. The enhancements in modified bismuth titanate achieved in present work allow the material to be considered as suitable for high temperature ultrasonic transducer applications. Integration of bismuth titanate material into a working high temperature transducer is then considered and the investigation of suitable, high temperature bonding method is undertaken. It is shown that reactivity of bismuth titanate with the titanium based fillers makes brazing unsuitable as a bonding method between piezo-ceramics and stainless steel. A novel assembly method, using liquid gallium as an electrically conductive bond, and a mechanical restraint for the piezo actuator is then presented as an alternative with the potential to reduce the negative effects of differences in thermal expansion coefficients between constituents of the transducer assembly.

This PhD project is focused on three lead-free ferroelectric solid solutions, which are specifically Na0.5Bi0.5TiO3-KNbO3(NBT-KN), Na0.5Bi0.5TiO3-NaNbO3(NBT-NN) and Na0.5Bi0.5TiO3-BaTiO3(NBT-BT), to evaluate the effects of composition, electric field and temperature on structural and electrical properties. Novel observations of both reversible and irreversible electric field-induced phase switching were made in both NBT-KN and NBT-NN ceramics. The NBT-KN solid solution is the primary focus of this thesis. All compositions were observed to be cubic in the as-sintered, unpoled state. However, a well-defined ferroelectric hysteresis P-E loop was obtained for compositions with low KN contents, indicating that an irreversible phase transition from a weak-polar relaxor ferroelectric (RF) to a long-range ordered metastable ferroelectric (FE) state had occurred during the measurement procedure. Both the unpoled and poled ceramic powders were examined using high resolution synchrotron XRD. For the poled state, a rhombohedral R3c structure was identified for compositions with low KN content, confirming the occurrence of the irreversible electric field-induced structural transformation from cubic to rhombohedral. In contrast, a cubic structure was retained for high KN contents, giving rise to reversible phase switching evidenced by constricted P-E hysteresis loops. Similar behaviour was observed for NBT-NN system. An 'in-situ' electric field poling experiment was conducted using high energy synchrotron XRD. In certain NBT-KN compositions the structural transformation, from cubic to mixed phase cubic+rhombohedral and finally single phase rhombohedral, occurred progressively with increasing cycles of a bipolar electric field. Similar behaviour was observed for NBT-NN compositions having low NN contents. Furthermore, the distributions of domain orientation and lattice strain over a range of orientations relative to the poling direction were determined for NBT-KN, NBT-NN and NBT-BT ceramics exhibiting the rhombohedral phase. By combining the structural information with the results of dielectric and ferroelectric measurements, a phase diagram was constructed to illustrate the influence of temperature and composition on the stability of the metastable ferroelectric and relaxor ferroelectric states for the NBT-KN system. Furthermore, the phase transition temperatures obtained from dielectric measurements were correlated with the ferroelectric and thermal depolarisation characteristics for each of the NBT-KN, NBT-NN and NBT-BT systems.

Doutoramento em Ciência e Engenharia de Materiais
K0.5Na0.5NbO3 (KNN), is the most promising lead free material for substituting lead zirconate titanate (PZT) which is still the market leader used for sensors and actuators. To make KNN a real competitor, it is necessary to understand and to improve its properties. This goal is pursued in the present work via different approaches aiming to study KNN intrinsic properties and then to identify appropriate strategies like doping and texturing for designing better KNN materials for an intended application. Hence, polycrystalline KNN ceramics (undoped, non-stoichiometric; NST and doped), high-quality KNN single crystals and textured KNN based ceramics were successfully synthesized and characterized in this work. Polycrystalline undoped, non-stoichiometric (NST) and Mn doped KNN ceramics were prepared by conventional ceramic processing. Structure, microstructure and electrical properties were measured. It was observed that the window for mono-phasic compositions was very narrow for both NST ceramics and Mn doped ceramics. For NST ceramics the variation of A/B ratio influenced the polarization (P-E) hysteresis loop and better piezoelectric and dielectric responses could be found for small stoichiometry deviations (A/B = 0.97). Regarding Mn doping, as compared to undoped KNN which showed leaky polarization (P-E) hysteresis loops, B-site Mn doped ceramics showed a well saturated, less-leaky hysteresis loop and a significant properties improvement. Impedance spectroscopy was used to assess the role of Mn and a relation between charge transport – defects and ferroelectric response in K0.5Na0.5NbO3 (KNN) and Mn doped KNN ceramics could be established. At room temperature the conduction in KNN which is associated with holes transport is suppressed by Mn doping. Hence Mn addition increases the resistivity of the ceramic, which proved to be very helpful for improving the saturation of the P-E loop. At high temperatures the conduction is dominated by the motion of ionized oxygen vacancies whose concentration increases with Mn doping. Single crystals of potassium sodium niobate (KNN) were grown by a modified high temperature flux method. A boron-modified flux was used to obtain the crystals at a relatively low temperature. XRD, EDS and ICP analysis proved the chemical and crystallographic quality of the crystals. The grown KNN crystals exhibit higher dielectric permittivity (29,100) at the tetragonal-to-cubic phase transition temperature, higher remnant polarization (19.4 μC/cm2) and piezoelectric coefficient (160 pC/N) when compared with the standard KNN ceramics. KNN single crystals domain structure was characterized for the first time by piezoforce response microscopy. It could be observed that <001> - oriented potassium sodium niobate (KNN) single crystals reveal a long range ordered domain pattern of parallel 180° domains with zig-zag 90° domains. From the comparison of KNN Single crystals to ceramics, It is argued that the presence in KNN single crystal (and absence in KNN ceramics) of such a long range order specific domain pattern that is its fingerprint accounts for the improved properties of single crystals. These results have broad implications for the expanded use of KNN materials, by establishing a relation between the domain patterns and the dielectric and ferroelectric response of single crystals and ceramics and by indicating ways of achieving maximised properties in KNN materials. Polarized Raman analysis of ferroelectric potassium sodium niobate (K0.5Na0.5)NbO3 (KNN) single crystals was performed. For the first time, an evidence is provided that supports the assignment of KNN single crystals structure to the monoclinic symmetry at room temperature. Intensities of A′, A″ and mixed A′+A″ phonons have been theoretically calculated and compared with the experimental data in dependence of crystal rotation, which allowed the precise determination of the Raman tensor coefficients for (non-leaking) modes in monoclinic KNN. In relation to the previous literature, this study clarifies that assigning monoclinic phase is more suitable than the orthorhombic one. In addition, this study is the basis for non-destructive assessments of domain distribution by Raman spectroscopy in KNN-based lead-free ferroelectrics with complex structures. Searching a deeper understanding of the electrical behaviour of both KNN single crystal and polycrystalline materials for the sake of designing optimized KNN materials, a comparative study at the level of charge transport and point defects was carried out by impedance spectroscopy. KNN single crystals showed lower conductivity than polycrystals from room temperature up to 200 ºC, but above this temperature polycrystalline KNN displays lower conductivity. The low temperature (T < 200 ºC) behaviour reflects the different processing conditions of both ceramics and single crystals, which account for less defects prone to charge transport in the case of single crystals. As temperature increases (T > 200 ºC) single crystals become more conductive than polycrystalline samples, in which grain boundaries act as barriers to charge transport. For even higher temperatures the conductivity difference between both is increased due to the contribution of ionic conduction in single crystals. Indeed the values of activation energy calculated to the high temperature range (T > 300 ºC) were 1.60 and 0.97 eV, confirming the charge transport due to ionic conduction and ionized oxygen vacancies in single crystals and polycrystalline KNN, respectively. It is suggested that single crystals with low defects content and improved electromechanical properties could be a better choice for room temperature applications, though at high temperatures less conductive ceramics may be the choice, depending on the targeted use. Aiming at engineering the properties of KNN polycrystals towards the performance of single crystals, the preparation and properties study of (001) – oriented (K0.5Na0.5)0.98Li0.02NbO3 (KNNL) ceramics obtained by templated grain growth (TGG) using KNN single crystals as templates was undertaken. The choice of KNN single crystals templates is related with their better properties and to their unique domain structure which were envisaged as a tool for templating better properties in KNN ceramics too. X-ray diffraction analysis revealed for the templated ceramics a monoclinic structure at room temperature and a Lotgering factor (f) of 40% which confirmed texture development. These textured ceramics exhibit a long range ordered domain pattern consisting of 90º and 180º domains, similar to the one observed in the single crystals. Enhanced dielectric (13017 at TC), ferroelectric (2Pr = 42.8 μC/cm2) and piezoelectric (d33 = 280 pC/N) properties are observed for textured KNNL ceramics as compared to the randomly oriented ones. This behaviour is suggested to be due to the long range ordered domain patterns observed in the textured ceramics. The obtained results as compared with the data previously reported on texture KNN based ceramics confirm that superior properties were found due to ordered repeated domain pattern. This study provides an useful approach towards properties improvement of KNN-based piezoelectric ceramics. Overall, the present results bring a significant contribution to the pool of knowledge on the properties of sodium potassium niobate materials: a relation between the domain patterns and di-, ferro-, and piezo-electric response of single crystals and ceramics was demonstrated and ways of engineering maximised properties in KNN materials, for example by texturing were established. This contribution is envisaged to have broad implications for the expanded use of KNN over the alternative lead-based materials.
O niobato de sódio e de potássio, K0.5Na0.5NbO3 (KNN), é o material isento de chumbo mais promissor para substituir o titanato zirconato de chumbo (PZT), que ainda é o líder de mercado utilizado para sensores e actuadores. Para tornar o KNN verdadeiramente competitivo, é necessário compreender e melhorar as suas propriedades. Esse objectivo é perseguido no presente trabalho através de diferentes abordagens, visando o estudo das propriedades intrínsecas do KNN e a subsequente identificação de estratégias apropriadas, como por exemplo a dopagem e a texturização, para desenhar melhores materiais à base de KNN para as aplicações pretendidas. Assim, no presente trabalho, fabricaram-se e caracterizaram-se cerâmicos de KNN dopado e não dopado, de KNN não estequiométrico e de KNN texturizado. Adicionalmente cresceram-se e caracterizaram-se cristais simples de KNN de elevada qualidade. Os cerâmicos de KNN (não dopado, dopado com Mn e não-estequiométrico (NST)) foram preparados pelo método convencional de mistura de óxidos, tendo-se subsequentemente medido as suas propriedades eléctricas e analisadas as respectivas estruturas e microestruturas. No caso dos cerâmicos dopados com Mn bem como no dos cerâmicos NST verificou-se existir uma estreita janela de composição monofásica associada à dopagem e não estequiometria na posição-A. Nos cerâmicos NST a variação da razão (A/B) influencia o ciclo de histerese da polarização ferroeléctrica (P-E), verificandose a obtenção de respostas dieléctricas e piezoeléctricas melhoradas para pequenos desvios da estequiometria (A/B = 0.97). No que se refere ao KNN dopado com Mn, quando comparado com o KNN não dopado cujos ciclos de histerese são não saturados, verificou-se que a dopagem no lugar B conduz a uma curva (P-E) mais saturada e a uma melhoria significativa de propriedades. Usou-se a espectroscopia de impedância para esclarecer o papel do Mn, tendo-se estabelecido uma correlação entre defeitos/transporte de carga e a resposta ferroeléctrica do K0.5Na0.5NbO3 (KNN) e do KNN dopado com Mn. À temperatura ambiente a condução eléctrica no KNN, associada ao transporte por buracos, é minimizada pela dopagem com Mn. A adição de Mn incrementa assim a resistividade do cerâmico, o que permite melhorar a saturação do ciclo (P-E). A temperaturas elevadas a condução passa a ser dominada pela movimento de lacunas de oxigénio ionizadas cuja concentração aumenta com a dopagem com Mn. Preparam-se também cristais simples de KNN recorrendo-se a um método de fluxo de alta temperatura. Usou-se um fluxo modificado com adição de B2O3 para crescer cristais a uma temperatura relativamente baixa. Caracterizou-se a qualidade química e cristalográfica dos cristais por análise de DRX, EDS e ICP. Os cristais obtidos exibiram propriedades com valores elevados, designadamente uma permitividade dieléctrica de 29,100 à temperatura de transição da fase tetragonal para fase cúbica, uma polarização remanescente 19,4 μC/cm2 e um coeficiente piezoeléctrico de 160 pC/N, valores estes superiores aos dos cerâmicos convencionais de KNN. Usou-se pela primeira vez a microscopia de força piezoeléctrica para caracterizar a estrutura de domínios dos monocristais de KNN. Foi possível observar que os cristais simples de KNN orientados segundo <001>, evidenciaram um padrão de estrutura de domínios, com domínios de 180º dispostos paralelamente e domínios de 90º dispostos em zig-zag. Com base na comparação entre cristais e cerâmicos de KNN é possível sustentar-se que a presença nos cristais simples de um tal padrão de domínios ordenados com longo alcance, ausente nos cerâmicos, é responsável pelas propriedades melhoradas dos cristais simples de KNN. Espera-se que os presentes resultados, ao estabelecerem uma relação entre o padrão de estrutura de domínios, uma espécie de impressão digital, e a resposta dielétrica e ferroelétrica dos cristais simples e ao indicarem vias para se atingirem propriedades maximizadas em materiais de KNN, venham a ter fortes implicações na expansão do uso dos materiais de KNN. Caracterizaram-se também os monocristais ferroeléctricos de KNN por espectroscopia de Raman, obtendo-se pela primeira vez evidências que permitem a atribuir a estrutura cristalina de simetria monoclínica ao KNN. As intensidades dos fonões A′ , A" e A' + A" foram calculadas teoricamente e comparadas com os dados experimentais em função da rotação de cristal, o que permitiu a determinação precisa dos coeficientes do tensor Raman para modos (non-leaking) em KNN monoclínico. No contexto da literatura este estudo confirma que a atribuição da simetria monoclínica é mais adequada do que a ortorrômbica. Este estudo constitui ainda uma base para a avaliação não-destrutiva da distribuição de domínios por espectroscopia Raman em materiais ferroelétricos isentos de chumbo, à base de KNN e com estruturas complexas. Procurando aprofundar a compreensão do comportamento eléctrico dos cristais simples e dos cerâmicos de KNN, com o objectivo de desenhar materiais com propriedades optimizadas, realizou-se um estudo comparativo ao nível de defeitos e transporte de carga, usando-se para tal a espectroscopia de impedância. Os monocristais apresentam menor condutividade do que os materiais policristalinos homólogos para temperaturas até 200 ºC ao passo que, acima desta temperatura, são os materiais policristalinos quem apresenta menor condutividade. O comportamento de baixa temperatura (T < 200 ºC) reflecte as diferentes condições de processamento dos cerâmicos e dos cristais, que são responsáveis pelo menor teor de defeitos transportadores de carga no caso dos cristais simples. À medida que a temperatura aumenta, (T > 200 ºC) os monocristais tornam-se agora mais condutores do que as amostras policristalinas nas quais as fronteiras de grão actuam como barreiras ao transporte de carga eléctrica. Para temperaturas ainda mais elevadas a diferença de condutividade entre cristais e cerâmicos é incrementada devido à contribuição da condução iónica nos cristais. Efectivamente, para a gama de temperatura elevada (T > 300 ºC),calcularam-se valores da energia de activação de 1,60 e 0,97 eV que confirmam um transporte de carga associado a condução iónica e a lacunas de oxigénio ionizadas, em cristais simples e em cerâmicos, respectivamente. Sugere-se assim que, dependendo da aplicação em em vista, os cristais, com baixo teor de defeitos e propriedades electromecânicas melhoradas serão uma escolha indicada para aplicações a temperaturas próximas da temperatura ambiente ao passo que, para altas temperaturas, os cerâmicos, menos condutores, serão a opção mais indicada. Com o objectivo de desenhar as propriedades dos materiais policristalinos de KNN na mira de um desempenho semelhante ao dos cristais simples, prepararam-se e estudaram-se as propriedades de cerâmicos de (K0.5Na0.5)0.98Li0.02NbO3 (KNNL) com orientação (00l), usando cristais simples de KNN como partículas modelo para produzir cerâmicos texturizados por crescimento de grão modelado ( do inglês “template grain growth”). A escolha dos cristais simples como partículas modelo baseou-se no facto destas possuírem boas propriedades, aqui usadas como ferramenta indutora de melhores propriedades nos cerâmicos de KNN. A análise DRX revelou que os cerâmicos preparados com partículas modelo evidenciavam uma estrutura monoclínica à temperatura ambiente e um fator de Lotgering (f) de 40 %, o que confirma o desenvolvimento de textura cristalográfica. Estes cerâmicos texturizados apresentam um padrão de domínios ordenado com longo alcance que consiste em domínios de 90º e de 180º, semelhante ao observado nos cristais simples. Observaram-se valores elevados de constante dieléctrica (13017 na transição de fase C/T), de polarização ferroelétrica (2Pr = 42,8 μC/cm2) e de coeficiente piezoelétrico (d33 = 280 pC/N ) nos cerâmicos KNNL texturizados, quando comparados com os cerâmicos não orientados. Sugerese que esta resposta eléctrica se deve ao padrão de domínioordenados, observado nas amostras texturizadas. Os resultados obtidos, quando comparados com dados anteriormente reportados para cerâmicos de KNN texturizados confirmam a superioridade das propriedades obtidas, que se atribui à estrutura de domíneos observada. Este estudo fornece uma abordagem que pode ser de grande utilidade para a melhoria das propriedades dos cerâmicos piezoelétricos à base de KNN. Globalmente considerados, os presentes resultados configuram um importante contributo para o conjunto dos conhecimentos sobre as propriedades do niobato de sódio e de potássio: demonstrou-se que existe uma relação entre o padrão de estrutura de domínios e a resposta dieléctrica, ferroeléctrica e piezoeléctrica de cristais simples e de cerâmicos de KNN e apontou-se uma via para a melhoria das propriedades dos cerâmicos através da texturização. Prevê-se assim que este contributo tenha um impacto significativo na viabilização do uso generalizado do KNN em detrimento dos materiais à base de chumbo.

Lead-based piezoelectric materials, such as PbZrxTi1-xO3 (PZT), have attracted considerable attention and have been widely used in actuators, sensors and transducers due to their excellent electric properties. However, considering the toxicity of lead and its oxides, environmentally friendly lead-free piezoelectric materials are attracting more attention as potential replacements for PZT. Among them, Bi0.5Na0.5TiO3 (BNT)-based materials exhibit good electrical properties and electromechanical coupling response. In this work, the 0.97Bi0.5Na0.5TiO3-0.03BiAlO3 (BNTBA) thin films (~120 nm thickness) were successfully prepared using the pulsed laser deposition (PLD) method on Pt/TiO2/SiO2/Si substrates. The effects of substrate temperature, oxygen pressure, laser repetition rate, and post-annealing treatment were investigated. X-ray diffraction (XRD) and scanning electron microscope (SEM) are used to study the structure of the films and the ferroelectric and dielectric properties are measured. The results show that it is necessary to introduce excess sodium and bismuth to compensate for their evaporation in further thermal treatment. The values of remnant polarization increase from 8.7 μC/cm2 to 12.3 μC/cm2 with the introduction BiAlO3. The dielectric constant increases from 600-550 to 710-600 and the dielectric loss increases from 4.2% to 6.7% at higher frequency when the oxygen pressure increases from 20 Pa to 30 Pa.
Blybaserade piezoelektriska material, såsom PbZrxTi1-xO3 (PZT), har väckt stor uppmärksamhet och har använts i stor utsträckning på grund av deras utmärkta elektriska egenskaper. Men med tanke på toxiciteten hos bly och dess oxider lockar miljövänliga blyfria piezoelektriska material mer uppmärksamhet från forskare som potentiella utbyten för PZT. Bland dem uppvisar Bi0.5Na0.5TiO3 (BNT) -baserade material bra elektriska egenskaper och elektromekanisk kopplingssvar. I detta arbete framställdes 0,97Bi0.5Na0.5TiO3-0.03BiAlO3 (BNTBA) tunna filmer (~ 120 nm tjocklek) med användning av pulserad laseravsättningsmetod på Pt / TiO2 / SiO2 / Si-substrat. Effekterna av substrattemperatur, syretryck, laserrepetitionshastighet och efterglödande behandling undersöktes. Röntgendiffraktions (XRD) och skanningelektronmikroskop (SEM) används för att studera filmens struktur och de ferroelektriska och dielektriska egenskaperna mäts. Resultaten visar att det är nödvändigt att införa överskott av natrium och vismut för att kompensera för deras avdunstning vid vidare termisk behandling. Värdena för återstående polarisation ökar från 8,7 μC / cm2 till 12,3 μC / cm2 med introduktionen BiAlO3. Den dielektriska konstanten ökar från 600-550 till 710-600 och den dielektriska förlusten ökar från 4,2% till 6,7% vid högre frekvens när syretrycket ökar från 20 Pa till 30 Pa.

Electromechanics—coupling of mechanical forces with others—exhibits a continuum-to-discrete spectrum of properties. In this chapter, classical and newer analysis techniques are developed for devices ranging from inertial sensors to scanning probes to quantify limits and sensitivities. Mechanical response, energy storage, transduction and dynamic characteristics of various devices are analyzed. The Lagrangian approach is developed for multidomain analysis and to bring out nonlinearity. The approach is extended to nanoscale fluidic systems where nonlinearities, fluctuation effects and the classical-quantum boundary is quite central. This leads to the study of measurement limits using power spectrum and, correlations with slow and fast forces. After a diversion to acoustic waves and piezoelectric phenomena, nonlinearities are explored in depth: homogeneous and forced conditions of excitation, chaos, bifurcations and other consequences, Melnikov analysis and the classic phase portaiture. The chapter ends with comments on multiphysics such as of nanotube-based systems and electromechanobiological biomotor systems.

Abstract High strain actuation can be obtained by engineering the crystallographic texture of piezoelectric ceramics. Single crystal piezoelectrics have significantly improved strain performance because crystal orientation can be controlled during use — their highly perfect lattice greatly simplifies the crystal orientation. Single crystals have properties significantly better than those of randomly oriented ceramics, but they are expensive and difficult to produce. Highly textured ceramics offer an alternative means of producing easily aligned crystals, as the majority of the crystallites in the ceramic share a common crystallographic orientation. Textured piezoelectric ceramics are expected to provide improved properties compared to untextured ceramics, with lower processing costs than single crystals. NexTech Materials is developing textured piezoelectrics in PMN-PT system via Templated Grain Growth (TGG). In the TGG process, anisotropic template particles are oriented in a fine particle matrix in the green state. During heat treatment, the crystals consume the surrounding matrix, resulting in a textured polycrystal. A TGG system requires both matrix and template particles. The matrix must provide significant driving force for densification and grain growth. Templates must be large, anisotropic, and crystallographically isostructural with the matrix material. Anisotropic template shape is difficult to achieve in perovskite-based piezoelectrics, which have highly symmetric (cubic) crystal structures at their processing temperature. In addition, the matrix environment often contains highly corrosive lead based liquid phases that make chemical stability difficult to achieve. Characterization of textured samples involves texture evaluation and phase analysis via XRD, and SEM analysis of template growth and chemical stability as well as piezoelectric performance measurement The development and characterization of the TGG components and their integration will be discussed.