Preparação e propriedades de cristais e de cerâmicos e perovesquites ferroeléctricas com camadas de bismuto

Doutoramento em Ciência e Engenharia de MateriaisImpulsionado pelo interesse em conhecer as propriedades intrínsecas dos compostos SrBi2Ta2O9 (SBT) e SrBi2Nb2O9 (SBN) que se apresentam como os materiais mais promissores para substituir o titanato de zircónio e de chumbo (PZT) nas memórias ferroeléctricas de acesso aleatório, surgiu a necessidade de monocristais destes compostos com dimensões e qualidade adequadas à medição de propriedades. No presente trabalho, fizeram-se crescer cristais simples de SBT e de SBN com qualidade e dimensões elevadas, usando um método de solução a alta temperatura, com um fluxo de Bi2O3 modificado com B2O3 e uma razão molar de 60/40 entre SBT (ou SBN) e fluxo (35 % em peso de Bi2O3 e 5 % em peso de B2O3). Primeiramente optimizaram-se as condições de processamento, testando-se diferentes perfis de temperatura para promover o crescimento e melhorar a qualidade dos cristais de SBT. As condições identificadas como óptimas foram usadas para fazer crescer cristais de SBN. Os cristais obtidos evidenciaram um hábito lamelar com morfologia de plaquetas com dimensões típicas de ∼ 7 × 5 × 0.2 e 5 × 5 × 0.4 mm3 para SBT e SBN, respectivamente. De acordo com as análises de topografia e de difracção de raios-X, ambos os cristais se apresentaram naturalmente orientados com a direcção [001] (eixo c) perpendicular à face de maior área do cristal e lados paralelos à direcção [110] da fase ortorrômbica (inclinação de 45º relativamente a ambos os eixos a e b). As primeiras medidas fiáveis sobre estrutura de domínios de cristais de SBT de boa qualidade foram realizadas no presente trabalho por microscopia de força piezoeléctrica. Ambos os domínios ferroeléctricos de 180º e ferroelásticos de 90º foram observados à temperatura ambiente após tratamento térmico dos cristais a 750 ºC, durante 10 horas. Os domínios apresentaram uma estrutura em “espinha” com paredes de domínios de 90º predominantemente planas. As paredes de domínios de 90º mostraram-se paralelas às arestas laterais do cristal [110], o que se coaduna com a orientação preferencial observada. A largura dos domínios de 90º situa-se entre 0,7 e 1,5 μm ao passo que a dos domínios de 180º varia entre 250 e 500 nm. A formação deste complexo padrão de domínios é atribuída a um processo de transição de fases em duas etapas, ou seja, a ocorrência não simultânea das transições de fase ferroelástica e ferroelétrica em SBT. A qualidade dos cristais de SBT e SBN foi também confirmada por medidas diélectricas, ferroelétricas e piezoeléctricas realizadas paralelamente ao eixo c (direcção [001]) e paralelamente ao plano ab (segundo a direcção [110]) demonstrando-se a elevada anisotropia das propriedades intrínsecas de ambos os cristais, i.e., a razão entre o valor médio de permitividade dieléctrica medida paralelamente ao plano ab e o valor medido paralelamente ao eixo c foi de cerca de 10 à temperatura de Curie, TC, diminuindo para 2 à temperatura ambiente. As baixas perdas dieléctricas acima e abaixo de TC (tanδ < 0.04) indicaram uma baixa concentração de defeitos nos cristais. Observaram-se ciclos de histerese saturados quando se aplicou um campo eléctrico alterno paralelamente ao plano ab do cristal SBT. A polarização espontânea segundo o eixo ferroeléctrico a foi estimada em cerca de ≈ 20 μC/cm2 para o SBT. Porém, no caso dos cristais de SBN, não foi possível obter ciclos de histerese saturados mesmo aplicando um campo eléctrico com o valor máximo de 100 kV/cm. O coeficiente piezoeléctrico d33 medido segundo a direcção [100] (eixo polar) é de ≈ 30 e de 62 pm/V para o SBT e o SBN, respectivamente. Os materiais ferroeléctricos com estrutura em camadas de bismuto (compostos BLSF) apresentam grande interesse para aplicações piezoeléctricas de elevada temperatura embora seja necessário prepará-los na forma texturizada devido à sua elevada anisotropia. O presente trabalho estuda a possibilidade de usar os cristais de SBT como sementes para induzir a texturização de cerâmicos de SBT pela via de template grain growth (TGG). Produziram-se cerâmicos de SBT texturizados com propriedades dieléctricas e ferroeléctricas melhoradas, usando sementes anisométricas e de morfologia lamelar, com tamanho médio de ~ 40 × 40 × 8 μm3. Dispersou-se uma pequena quantidade de sementes anisométricas de SBT numa matrix de partículas finas de SBT contendo um excesso de Bi2O3 para formar fase líquida e alinharam-se essas sementes por prensagem unidireccional. Avaliaram-se os efeitos de vários parâmetros de processamento tais como o excesso de Bi2O3, as sementes de SBT, as condições de prensagem e de sinterização, tentando obter-se cerâmicos densos, com elevada textura e propriedades melhoradas. Correlacionou-se a evolução da microestructura dos cerâmicos com as condições de processamento recorrendo a uma análise estereológica. Demonstrou-se a existência de anisotropia nas propriedades dieléctricas e no ciclo de histerese e a sua dependência do grau de textura. Mediram-se propriedades dieléctricas e ferroeléctricas melhoradas segunda uma direcção perpendicular à da prensagem unidirecional, observando-se valores de permitividade e de polarização acima dos apresentados pelos cerâmicos sem sementes. Mostrou-se que esta melhoria de propriedades resultou da orientação do grão, da anisotropia de propriedades dos monocristais e do grau de textura dos cerâmicos. Apresentou-se um modelo para descrever a polarização espontânea máxima de cerâmicos de SBT com orientação de grão aleatória ou com textura, em função do grau de textura, usando uma análise de textura baseada na distribuição da orientação dos grãos grandes e anisométricos. Seleccionou-se a equação de March-Dollase para descrever os dados experimentais referentes à distribuição de orientação e discutiu-se a distribuição espacial do vector polarização em grãos lamelares de materiais BLSF. A aplicação do referido modelo aos cerâmicos texturizados de SBT permitiu a comparação dos valores de polarização espontânea previstos pelo modelo com os valores experimentais obtidos a partir do ciclo de histerese ferroeléctrica dos mesmos cerâmicos.The interest in the understanding of the intrinsic properties of SrBi2Ta2O9 (SBT) and SrBi2Nb2O9 (SBN), which are the most promising materials for substituting lead zirconate titanate in non-volatile ferroelectric random access memories, arouse the need of single crystals of these compounds with suitable size and quality for the properties measurement. In this work, high-quality SBT and SBN single crystals were successfully grown by a high-temperature selfflux solution method, using a B2O3 modified Bi2O3 flux and a molar ratio of 60/40 of SBT (or SBN) powder to flux (35 wt% Bi2O3 and 5 wt% B2O3). The processing conditions were optimized by testing different thermal profiles to increase the size and improve the quality of the grown SBT crystals. The optimized conditions were then applied for the growth of SBN crystals. The grown crystals showed a layered habit with a platelet morphology and typical sizes of ∼ 7 × 5 × 0.2 and 5 × 5 × 0.4 mm3 for SBT and SBN, respectively. According to x-ray diffraction and topography analyses, both crystals were naturally oriented with [001] direction (c-axis) perpendicular to the major face and edges parallel to [110] direction (45º to both a- and b-axes) of the orthorhombic phase. The first reliable measurements of the domain structure of high-quality SBT single crystals were performed in this work by piezoelectric force microscopy. Both ferroelectric 180º domains and ferroelastic 90º domains (twins) were revealed at room temperature after annealing the crystals at 750 ºC for 10 h. The coexisting domains form a well-defined “herringbone” structure with mostly flat 90º walls. The ferroelastic 90º walls were parallel to the single crystal edges [110], which agree with the observed preferential orientation. The width of 90º domains (twins) lies in the range of 0.7 - 1.5 μm, while that of 180º domains, which were oriented parallel to the [100] direction (polar axis), exhibited a periodicity of about 250 to 500 nm. Formation of the observed complex domain pattern was attributed to a two-stage process associated with the presence of separate ferroelastic and ferroelectric phase transitions in SBT. The high quality of the grown SBT and SBN single crystals was confirmed by dielectric, ferroelectric and piezoelectric measurements, which were performed in the ab-plane (along the [110] direction) and along the c-axis (the [001] direction), demonstrating the large anisotropy in the intrinsic properties of both crystals, i.e., the ratio between average permittivity along the [110] (ab-plane) and the [001] direction (c-axis) was about 10 at TC and decreased to ~ 2 at room temperature. The low dielectric losses above and below TC (tanδ < 0.04) indicate a low concentration of defects in the crystals. Saturated hysteresis loops were observed for switching in the ab-plane of the SBT single crystal and the spontaneous polarization along the ferroelectric a-axis was estimated to be PS ≈ 20 μC/cm2 for SBT. However, for SBN crystals, saturated hysteresis loops were not obtained for a maximum electric field of 100 kV/cm. The longitudinal piezoelectric coefficient d33 was measured along the [100] direction (polar-axis) in both crystals, and was estimated as ≈ 30 and 62 pm/V for SBT and SBN, respectively. Bi-layer structured ferroelectric (BLSF) materials like SBT present significant interest for high-temperature piezoelectric applications, though they are required to be prepared in a textured form due to their high anisotropy. This work studies the possibility of using the grown SBT crystals as seeds for the fabrication of textured SBT ceramics by templated grain growth (TGG). Seeded SBT ceramics with improved dielectric and ferroelectric properties were produced by using plate-like anisometric SBT templates with average sizes of ~ 40 × 40 × 8 μm3. A small amount of the anisometric SBT templates was distributed in a fine-grained matrix of SBT powder containing Bi2O3 excess as liquid phase, and then aligned by conventional uniaxial pressing. Several processing parameters, e.g., the Bi2O3 excess, the amount of templates, or the processing and sintering conditions including the uniaxial pressure, the sintering temperature and time, were examined in order to produce textured SBT ceramics with enhanced properties. The ceramics microstructure evolution was correlated with the processing parameters via a stereological analysis. Anisotropy in the dielectric and ferroelectric properties of the seeded SBT specimens and its dependence on the degree of texture were demonstrated. Enhanced properties were measured perpendicularly to the uniaxial pressing direction revealing permittivity and polarization values above those of unseeded SBT ceramics. Such improved properties were shown to result from the grain orientation, anisotropy of single crystal properties, and degree of texture of the sintered ceramics. A quantitative model was presented for predicting the maximum spontaneous polarization, PS, of randomly oriented and textured SBT ceramics as a function of the degree of texture, using a texture analysis accomplished via the orientation distribution of large anisometric grains. The March-Dollase equation was selected to fit the measured orientation distribution, and the spatial distribution of polarization vector in platelet grains of BLSF materials was discussed. The results were applied to the case of textured SBT ceramics, and the predicted PS values as a function of the degree of texture were compared with those measured from the hysteresis loops.FCT - SFRH/BD/6619/200

Macroscopic ferroelectricity and piezoelectricity in nanostructured BiScO3–PbTiO3 ceramics

3 pages, 4 figures.-- PACS: 77.80.Bh; 73.61.Ng; 77.84.Dy; 77.80.Fm; 81.20.EvWe have studied the macroscopic electrical properties of highly dense, nanostructured ceramics of BiScO3–PbTiO3 with high Curie temperature and piezoelectric activity. Materials were processed by spark plasma sintering of nanocrystalline powder obtained by mechanosynthesis. Results indicate that the nanostructured material still presents the ferroelectric transition above 700 K. Ferroelectric switching is unambiguously demonstrated. Furthermore, ceramic disks were poled and their radial piezoelectric resonance was excited, which has not been achieved in nanostructured BaTiO3 ceramics.Funded by MEC (Spain) through the MAT2007-61884 and MAT2008-02003/NAN projects. H.A. and T.H. thank the financial support by MEC (JdC Programme) and MAEAECI, respectively. Collaboration between ICMM and CEMES is framed within the ESF COST Action 539 ELENA.Peer reviewe

Point defect engineering of high temperature piezoelectric BiScO3-PbTiO3 for enhanced voltage response

BiScO-PbTiO is the most promising system among high sensitivity piezoelectric BiMO-PbTiO perovskite solid solutions with high Curie temperature, which are under extensive investigation for expanding the operation temperature of state of the art Pb(Zr,Ti)O (PZT) up to 400 °C. The viability of these alternative materials requires the development of specific point defect engineering that allow a range of piezoelectric ceramics comparable to commercial PZTs to be obtained, optimized for the different applications. A distinctive feature of BiMO-PbTiO systems is the simultaneous presence of both Bi and Pb at the A-site of the perovskite. This enables the possibility of introducing charged point defects without incorporating new chemical species, just by defining an A-site non–stoichiometry. In this work, we present a comprehensive study of the effects of Bi substitution for Pb, along with the formulation of Pb vacancies for charge compensation. Results indicate an overall lattice stiffening that yields reduced polarizability and compliance, and dominates over a limited enhancement of the ferroelectric domain wall dynamics, so as a largely enhanced voltage response is obtained. Specifically, BiScO-PbTiO with the A-site non-stoichiometry is shown to be very suitable as the piezoelectric component of magnetoelectric composites for magnetic field sensing.Funded by Spanish MINECO through the MAT2014-58816-R and MAT2011-23709 projects. Technical supports by Ms. I. Martínez and Ms. M. M. Antón are also acknowledged

Point defect engineering of high temperature piezoelectric BiScO3-PbTiO3 for high power operation

[EN] BiScO3-PbTiO3 is used as a model system of BiMO3-PbTiO3 perovskite solid solutions with enhanced electromechanical response at ferroelectric morphotropic phase boundaries, and high Curie temperature to demonstrate specific point defect engineering for high power operation. The objective is to obtain a range of piezoelectric ceramics comparable to hard Pb(Zr,Ti)O3 materials, optimized for the different applications. In this work, a comprehensive study of Mn substitution for Sc is provided. Care is taken to isolate the effects of the point defects from those of concomitant structural and microstructural changes that have been previously described after MnO2 addition. Results strongly suggest that Mn substitution results in the formation of (MnSc'-VO••) dipolar complexes that effectively clamp domain walls. This is the same mechanism responsible of hardening in Pb(Zr,Ti)O3. Indeed, Bi0.36Pb0.64Sc0.36-x MnxTi0.64O3 with x=0.02 is shown to be a high sensitivity piezoelectric with strongly reduced losses, suitable for high power operation between 200 and 400 ºC.Funded by Spanish MINECO through the MAT2014-58816-R and MAT2011-23709 projects. Technical supports by Ms. I. Martínez and Ms. M. M. Antón are also acknowledged.Peer Reviewe

Mechanosynthesis and Multiferroic Properties of the BiFeO3-BiMnO3-PbTiO3 Ternary System along its Morphotropic Phase Boundary

International audienceA highly topical set of materials are those ABO3 perovskite oxides, in which multiferroicity is chemically engineered by placing ferroelectrically and magnetically active cations in A-and B-site, respectively. This is the case of the BiFeO3 and BiMnO3 perovskites, and also of the solid solutions they form with PbTiO3. Interest in these binary systems is fostered by the presence of distinctive morphotropic phase boundaries (MPBs); multiferroic in the case of BiFeO3-PbTiO3, for which a high magnetoelectric response has been anticipated. Here, new compositions belonging to the ternary system BiFeO3-BiMnO3-PbTiO3, and specifically along the line that joins the former MPBs, have been prepared by mechanosynthesis to accomplish a thorough analysis of their multiferroic nature. Nanocrystalline powders with perovskite-type structure were obtained in the entire range of compositions, which all exhibited polymorphic phase coexistence allowing a line of MPBs to be established. The variation of the perovskite structural characteristics along this line has been defined, and correlated with those of the magnetic and electrical properties. A set of novel and promising multiferroic materials has been found for BiFeO3 rich compositions

Multiferroism and enhancement of material properties across the morphotropic phase boundary of BiFeO3-PbTiO3

Strong phase-change magnetoelectric responses have been anticipated by a first-principles investigation of phases in the perovskite BiFeO 3-BiCoO3 solid solution, specifically at the morphotropic phase boundary (MPB) between the multiferroic rhombohedral and tetragonal polymorphs. This might be a general property of multiferroic MPBs and a novel promising approach for room temperature magnetoelectricity, which requires the identification of suitable material systems. We present here a comprehensive description of the electrical and electromechanical properties across one such system; the perovskite BiFeO3-PbTiO3 solid solution. All the temperature dependence of dielectric permittivity, ferroelectric hysteresis loops, and piezoelectric coefficients have been obtained, and are discussed in relation to the previously reported perovskite structural evolution. Results show ceramic materials to be very promising for ferroelectric random access memories (remnant polarization as high as 63 μC cm-2 with a comparatively low coercive field of 4.5 kV mm-1 for MPB compositions) and high temperature electromechanical transduction (crystal piezoelectric coefficient of 87 pC N-1 with a Curie temperature above 873 K). Moreover, the occurrence of phase changes between the monoclinic and tetragonal polymorphs under high electric fields is indicated, while the canted antiferromagnetic character of the phases involved is corroborated. © 2014 AIP Publishing LLC.Funded by MINECO (Spain) through the MAT2011-23709 project. Dr. H. Amorín thanks financial support by MICINN Ramón y Cajal Programme (RYC-2008-03247). Ms. C. Correas and Ms. C. M. Fernández-Posada also thank the specific financial support of FPI Programme (BES-2008-005409 and BES-2012-053017, respectively).Peer Reviewe

Wide-range magnetoelectric response on hybrid polymer composites based on filler type and content

Abstract: In order to obtain a wide-range magnetoelectric (ME) response on a ME nanocomposite that matches industry requirements, Tb0.3Dy0.7Fe1.92 (Terfenol-D)/CoFe2O4/P(VDF-TrFE) flexible films were produced by solvent casting technique and their morphologic, piezoelectric, magnetic and magnetoelectric properties investigated. The obtained composites revealed a high piezoelectric response (≈-18 pC.N-1) that is independent of the weight ratio between the fillers. In turn, the magnetic properties of the composites were influenced by the composite composition. It was found that the magnetization saturation values decreased with increasing CoFe2O4 content (from 18.5 to 13.3 emu.g-1) while the magnetization and coercive field values increased (from 3.7 to 5.5 emu.g-1 and from 355.7 to 1225.2 Oe, respectively) with increasing CoFe2O4 content. Additionally, those films showed a wide-range dual-peak ME response at room temperature with the ME coefficient increasing with weight content of Terfenol-D, from 18.6 mV.cm-1.Oe-1 to 42.3 mV.cm-1.Oe-1.FCT-Fundação para a Ciência e Tecnologia—for financial support in the framework of the Strategic Funding UID/FIS/04650/2013 and under project PTDC/EEI-SII/5582/2014. Pedro Martins, Silvia Reis and Marco Silva also acknowledge support from FCT (SFRH/BPD/96227/2013, SFRH/BDE/406 and SFRH/BD/70303/2010 grants, respectively). The authors acknowledge funding by the Spanish Ministry of Economy and Competitiveness (MINECO) through the project MAT2016-76039-C4-3-R (AEI/FEDER, UE) (including the FEDER financial support). Financial support from the Basque Government Industry Department under the ELKARTEK program is also acknowledged. The authors thank INL, International Iberian Nanotechnology Laboratory, Braga, Portugal, for offering access to their instruments and expertiseinfo:eu-repo/semantics/publishedVersio

Fine-grained high-performance Ba0.85Ca0.15 Zr0.1Ti0.9O3 piezoceramics obtained by current-controlled flash sintering of nanopowders

Due to environmental concerns, extensive research has been carried out to develop high-performance lead-free piezoceramics capable of replacing commercial lead-based materials. The lead-free (Ba0.7Ca0.3)TiO3 -Ba(Zr0.2Ti0.8)O3 system has emerged as a candidate for room temperature transducer applications because a high piezoelectric charge coefficient is achieved in this system for compositions at the morphotropic phase boundary. However, conventional ceramic processing of these eco-friendly piezoceramics demands high energy consumption because long-lasting, high-temperature heat treatments are needed, which often lead to microstructural degradation that compromises the material reliability. Field-assisted flash sintering has started to be explored since the application of an adequate electric field was shown to significantly reduce the sintering time and temperature, thereby controlling grain growth. In this work, Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramics are obtained by current-controlled flash sintering of mechanosynthesized nanopowders. Exhaustive control of the sintering parameters allows tailoring of the microstructure, which allows dense fine-grained flash-sintered ceramics exhibiting a high electric field-induced strain response to be obtained.This work was supported by the Agencia Estatal de Investigación (AEI), Spain, projects PGC2018-099158-B-I00 and PID2021–122708OB-C33. S. L-B. thanks Agència de Gestió d′Ajuts Universitaris i de Recerca (AGAUR), Catalonia, Spain, for the FI-SDUR contract (2020 FISDU 00489). The authors acknowledge the ESRF (The European Synchrotron) for provision of synchrotron radiation facilities, and we would like to thank the BM25 (SpLine) staff for assistance in using the beamline.Peer ReviewedPostprint (published version

High-sensitivity piezoelectric perovskites for magnetoelectric composites

© 2015 National Institute for Materials Science. A highly topical set of perovskite oxides are high-sensitivity piezoelectric ones, among which Pb(Zr,Ti)O3 at the morphotropic phase boundary (MPB) between ferroelectric rhombohedral and tetragonal polymorphic phases is reckoned a case study. Piezoelectric ceramics are used in a wide range of mature, electromechanical transduction technologies like piezoelectric sensors, actuators and ultrasound generation, to name only a few examples, and more recently for demonstrating novel applications like magnetoelectric composites. In this case, piezoelectric perovskites are combined with magnetostrictive materials to provide magnetoelectricity as a product property of the piezoelectricity and piezomagnetism of the component phases. Interfaces play a key issue, for they control the mechanical coupling between the piezoresponsive phases. We present here main results of our investigation on the suitability of the high sensitivity MPB piezoelectric perovskite BiScO3-PbTiO3 in combination with ferrimagnetic spinel oxides for magnetoelectric composites. Emphasis has been put on the processing at low temperature to control reactions and interdiffusion between the two oxides. The role of the grain size effects is extensively addressed.This work has been funded by the Spanish MINECO through projects MAT2011-23709 and AIB2010PT-00332. Collaboration between ICMM and CEMES is framed within the COST Action MP0904. Serviciencia S L (Spain) participation in the design and built-up of a novel magnetoelectric measurement system is acknowledged. HA thanks the Ramón y Cajal Programme for financial support.Peer Reviewe

Solvent-Free Design of Biobased Non-isocyanate Polyurethanes with Ferroelectric Properties

[EN] Increasing energy autonomy and lowering dependence on lithium-based batteries are more and more appealing to meet our current and future needs of energy-demanding applications such as data acquisition, storage, and communication. In this respect, energy harvesting solutions from ambient sources represent a relevant solution by unravelling these challenges and giving access to an unlimited source of portable/renewable energy. Despite more than five decades of intensive study, most of these energy harvesting solutions are exclusively designed from ferroelectric ceramics such as Pb(Zr,Ti)O3 and/or ferroelectric polymers such as polyvinylidene fluoride and its related copolymers, but the large implementation of these piezoelectric materials into these technologies is environmentally problematic, related with elevated toxicity and poor recyclability. In this work, we reveal that fully biobased non-isocyanate polyurethane-based materials could afford a sustainable platform to produce piezoelectric materials of high interest. Interestingly, these non-isocyanate polyurethanes (NIPUs) with ferroelectric properties could be successfully synthesized using a solvent-free reactive extrusion process on the basis of an aminolysis reaction between resorcinol bis-carbonate and different diamine extension agents. Structure-property relationships were established, indicating that the ferroelectric behavior of these NIPUs depends on the nanophase separation inside these materials. These promising results indicate a significant potential for fulfilling the requirements of basic connected sensors equipped with low-power communication technologies.Spanish Ministerio de Economía y Competitividad (MINECO) forfinancial support through projects MAT2017-88788-R and MAT2016-76851-R. This project has received funding from European Union’s Horizon 2020 research and innovation programmeunder the Marie Skłodowska-Curie grant agreement no.955700