Aging behaviours of CuO modified BaTiO3 ceramics

Abstract

In order to reduce the sintering temperature and enhance the piezoelectricity, a small amount of CuO was added into BaTiO₃ during sintering. The aging behaviours of these CuO modified BaTiO₃ ceramics were studied systematically. For unpoled ceramic, the P–E loop changes gradually from a normal single-loop curve to a double-loop one with aging. Meanwhile, the S–E loop develops gradually from a normal strain curve to the one with almost zero strain under electric field below 10 kV mm^–1. For poled ceramic, it builds a much stronger and more stable internal bias after aging. The P–E loops shift significantly along the electric field and the strain loop shows high asymmetry with one side of high linearity. A dipolar defect model was proposed to explain these aging behaviours. The defect dipoles consisting of Cu²⁺ occupying B site and oxygen vacancy can reorientate gradually with time to align with the spontaneous polarisation. Then the aligned defect dipoles could provide a strong restoring force on the domain wall motion and thus lead to all the observed aging behaviours.     

Evaluation of dielectric breakdown of BaTiO3 by novel indentation method

Succinct evaluation of dielectric breakdown strength of dielectric materials has been hampered by extrinsic parameters like size/thickness dependence, electrode configuration, and defect concentration which all mask the materials’ intrinsic value. A novel indentation method to prepare arch-shaped indents on the surface of BaTiO₃ green stacks has been developed to overcome this problem. Utilizing this method enables simple and reliable sample preparation for investigation of dielectric breakdown behavior. Theoretical and experimental investigations reveal vanishing electrode edge effects governed by the improved distribution of electric field due to the arch-shaped indents. Thus, the novel indentation method bears advantages compared to measuring DBS of disk-shaped samples and allows for narrowing influences of extrinsic parameters. By obtaining breakdown channels close to the center of the indent, this technique additionally features the possibility of localizing the breakdown event.     

Grain size effect on piezoelectric and ferroelectric properties of BaTiO3 ceramics

A series of dense barium titanate (BaTiO₃, BTO) ceramics with different grain sizes (GS) were prepared by two-step sintering method. The effect of GS on piezoelectric coefficient (d₃₃) and planar electromechanical coupling factor (k_p) displayed a trend similar to that on relative permittivity (ɛ′). The values of d₃₃, k_p, and ɛ′ increased significantly with decreasing GS, reaching maximum values (ɛ′ = 6079, d₃₃ = 519 pC/N and k_p = 39.5%) at approximately 1 μm, and then decreased rapidly with further decreasing GS. The results revealed that high-performance BTO ceramics could be effectively prepared by controlling GS. Polarization–electric field hysteresis loops and temperature dependence of ɛ′ were also investigated.     

Enhanced dielectric and ferroelectric properties of BaTiO3 ceramics prepared by microwave assisted radiant hybrid sintering

Structural, dielectric and ferroelectric properties of polycrystalline BaTiO₃ (BTO) ceramics prepared with hybrid sintering i.e., microwave assisted radiant heating (MARH) are reported. It is observed that the permittivity (ε) and true switched ferroelectric charge density (Q_SW) of BTO ceramics can be enhanced by employing MARH. An enhancement of 58% in ε and 17% in Q_SW is observed for the BTO sample prepared with 30% microwave power applied during MARH as compared to the conventional radiant heating. The results are explained in terms of microstructure resulting from the microwave assisted sintering.     

Enhanced grain growth and dielectric properties in aerosol deposited BaTiO3

Piezoelectric ceramics are envisioned as cell stimulating materials for in-vivo, load-bearing applications. To compensate for their brittle nature developing ceramic films on medically accredited metals is a promising approach. However, high temperature consolidation is often required to achieve highly dense ceramics with suitable functional properties, which can compromise the metal substrate integrity. With aerosol deposition highly dense thick films can be produced at room temperature. Still, an annealing step is required to enhance the functional properties of piezoelectric ceramics. Thermal annealing of dense, aerosol deposited BaTiO₃ thick films on 304SUS stainless steel gave a clear enhancement of the dielectric properties. An increase in saturation polarization and the adoption of ferroelectric hysteresis at 750 °C coincided with a significant reduction in mechanical properties. The simultaneous appearance of grain growth and diffusion of chromium from the substrate at 750 °C suggests that chromium acts as a sintering aid.     

Dielectric properties and electrocaloric effect of yttrium-modified BaTiO3 ceramics

The electrocaloric effect (ECE) is a promising candidate to replace the vapor-compression cooling technology, which has reached its end of improving the energy utilization efficiency. In the present work, the Y-modified BaTiO₃ ceramics with nominal compositions of Ba(Ti_1-xY_x)O₃ (abbreviated as BT-100xY, where x = 0.0125, 0.025, 0.0375, 0.050 and 0.0625) have been prepared through the conventional solid-state reaction sintering method. The dielectric properties and electrocaloric effect of BT-100xY ceramics have been investigated in detail. The XRD patterns indicate that all the BT-100xY ceramics possess pure perovskite structure without secondary phases. The temperature dependence of dielectric permittivity reveals that the BT-1.25Y, 2.5Y, 3.75Y and 5.0Y are normal ferroelectrics, and the BT-6.25Y is a relaxor ferroelectric. The ECE is calculated through the indirect equation based on Maxwell relation. The BT-2.5Y exhibits the largest ΔT = 1.26 K and the largest ΔS = 1.68 J/kg · K among all the BT-100xY ceramics, and the BT-2.5Y also exhibits the largest ΔT/ΔE = 0.296 × 10^?6 K · m/V and the largest ΔS/ΔE = 0.394 × 10^?6 J · m/kg · K?V. The ECE in our work is comparable with or even larger than that of BaTiO₃-based ceramics previously reported, which indicates that the BT-100xY ceramics are promising ECE materials.     

A comparative study of the structure,defects, optical,dielectric, and magnetic properties of GdMnO3 multiferroic ceramics synthesized by solid-state reaction and sol-gel methods

In this work, GdMnO₃ ceramics were synthesized by solid state reaction and sol-gel methods, and the structure, defects and optical, dielectric and magnetic properties of the synthesized samples were comparatively investigated. The samples synthesized by different methods show a single phase structure without any detectable impurities. The SEM results suggest that the particle size of the specimen obtained by the solid phase route is on the micron scale, while that of the specimen fabricated by the sol-gel route is on the nanometer scale. Compared with the ceramic fabricated by solid-state reaction technology, the specimen synthesized by sol-gel technique possesses lower oxygen vacancies and Mn²⁺ concentration, and Mn³⁺ concentration. The positron annihilation analyses show that the cation vacancy concentration of the specimen synthesized by the solid phase approach is higher than that of the specimen synthesized via the sol-gel approach. The compound obtained by the solid phase reaction has better dielectric properties than that obtained with the sol-gel method. The magnetic transition temperature and the effective magnetic moment are influenced by the Mn ion valence state in GdMnO₃. The stronger magnetization of the ceramic synthesized via the sol-gel approach is associated with the lower concentration of cation vacancies.     

Enhanced dielectric permittivity of BaTiO3/epoxy resin composites by particle alignment

Barium titanate (BaTiO₃)/epoxy resin composites with a novel structure, in which the BaTiO₃ particles were directionally aligned in the epoxy resin matrix, were fabricated using the ice-templating method. The effects of the filler particle alignment and the filler fraction on the dielectric permittivity as well as the dielectric loss of the composites were studied. The results show that the aligning filler particles can significantly improve the dielectric permittivity while maintaining the dielectric loss compared with the traditional composite structure (homogeneously distributed). Due to the feasibility of the enhancement of the dielectric properties of the composites, the particle alignment that is achieved via the ice-templating method can be used in the field of high energy density capacitors.     

Optical properties of multi-stacked BaTiO3/SrTiO3 thin films synthesized via chemical method

In this work, the BaTiO₃ and SrTiO₃ thin films as well as BaTiO₃/SrTiO₃ lamellar composites are synthesized via sol-gel spin-coating method. The formation of corresponding phases from their sols is investigated by virtue of X-ray diffraction on their powders, which confirms the formation of tetragonal structure for BaTiO₃, but cubic structure for SrTiO₃. The field emission scanning electron microscopy images show that crack-free films with different morphologies are formed in each sample. Likewise, by changing periodicity of the samples, the morphology of the composite samples is changed. As the number of layers increases from 1 to 20, the band gap reduces from 4.38 eV to 4.10 eV for BaTiO₃ samples and from 4.13 eV to 3.80 eV for SrTiO₃ samples confirmed by UV–Vis spectra. The band gap of periodicity = 1 sample is higher than that of BaTiO₃, while band gaps of periodicity = 2 and 5 composites mount between those of BaTiO₃ and SrTiO₃. In addition, the refractive indices of multi-stacked composites are about 0.2 lesser than refractive indices of BaTiO₃ sample in high wavelengths. The periodicity dependence of optical frequency dielectric constant, dielectric loss, impedance, Urbach tail, extinction coefficient, and electric modulus of multi-stacked composites are also studied.     

Blacklight sintering of BaTiO3 ceramics

Blacklight sintering has been used for rapid densification of ceramics. Compared to other methods, neither electrodes nor complex sample holders and dies are required, resulting in an effective technique for energy-efficient sintering. Still, the question remains whether excellent functional properties can be obtained by this method. Therefore, blacklight-sintered ferroelectric BaTiO₃ was investigated. Gradient-free microstructures and phase purity could be reached. Enhanced electrical conductivity was determined, indicating changes in defect chemistry and, most likely, an increase in oxygen vacancy concentration. Furthermore, measurements of ferroelectric properties confirmed leaky ferroelectric behavior, with migration and pinning of oxygen vacancies affecting the polarization loops. An additional short annealing step in an oxygen-rich atmosphere led to di/ferroelectric parameters equivalent to those of a conventionally sintered reference sample. Thus, the fabrication of ferroelectric ceramics by blacklight sintering is possible if the quenching effect caused by the freeze-in of defects during rapid cooling is considered.     

Study of structural,dielectric, ferroelectric and magnetic properties of vanadium doped BCT ceramics

In this paper, the structural, microstructural, dielectric, ferroelectric and magnetic properties of vanadium-doped Barium Calcium Titanate (BCT) ceramics have been reported. All the ceramic samples were prepared by the conventionalsolid-state reaction method. By Rietveld refinement of X-Ray diffraction patterns, we confirm the pervoskite structure of all the prepared samples. A decrease has been observed in lattice parameters (a and c) and lattice volume on the substitution of Ti⁴⁺ ions by vanadium ions in BCT lattice. SEM micrographs revealed change in microstructure from irregular, large grain porous structure to a regular small and compact structure. The dielectric constant data above T_c has been fitted with both Curie-Weiss Law and Power Law. The dielectric behavior of the presently studied system is more accurately explained by Power law. Feeble ferromagnetism has also been observed with vanadium substitution in BCT ceramics.     

Preparation of BaTiO3 nanopowders by the solution combustion method

Barium titanate was synthesized by the method of exothermal combustion in solutions using barium nitrate, titanium dioxide (TiO₂) and titanyl nitrate (TiO(NO₃)₂) as sources of titanium and barium and reducing agents such as glycine (C₂H₅NO₂), carbamide (СН₄N₂O) and glycerol (C₃H₅(OH)₃). In an effort to form a barium titanate phase, the materials synthesized using titanium dioxide were subjected to additional calcination at 800 °С. With titanyl nitrate the use of glycine and carbamide enabled carrying out a single-step synthesis of barium titanate. The obtained materials have pseudo-cubic lattice and are characterized by high stability of properties in a wide range of temperatures and frequencies of the electromagnetic field.     

Fabrication of bulk piezoelectric and dielectric BaTiO3 ceramics using paste extrusion 3D printing technique

A simple and facile method was developed to fabricate functional bulk barium titanate (BaTiO₃, BT) ceramics using the paste extrusion 3D printing technique. The BT ceramic is a lead-free ferroelectric material widely used for various applications in sensors, energy storage, and harvesting. There are several traditional methods (eg, tape casting) to process bulk BT ceramics but they have disadvantages such as difficult handing without shape deformation, demolding, complex geometric shapes, expansive molds, etc. In this research, we utilized the paste extrusion 3D printing technique to overcome the traditional issues and developed printable ceramic suspensions containing BT ceramic powder, polyvinylidene fluoride (PVDF), N,N-dimethylformamide (DMF) through simple mixing method and chemical formulation. This PVDF solution erformed multiple roles of binder, plasticizer, and dispersant for excellent manufacturability while providing high volume percent and density of the final bulk ceramic. Based on empirical data, it was found that the maximum binder ratio with good viscosity and retention for desired geometry is 1:8.8, while the maximum BT content is 35.45 vol% (77.01 wt%) in order to achieve maximum density of 3.93 g/cm³ (65.3%) for 3D printed BT ceramic. Among different sintering temperatures, it was observed that the sintered BT ceramic at 1400°C had highest grain growth and tetragonality which affected high performing piezoelectric and dielectric properties, 200 pC/N and 4730 at 10³ Hz respectively. This paste extrusion 3D printing technique and simple synthesis method for ceramic suspensions are expected to enable rapid massive production, customization, design flexibility of the bulk piezoelectric and dielectric devices for next generation technology.     

Enhancing properties of lead-free ferroelectric BaTiO3 through doping

The substitution on either the A- or B-site of ferroelectric perovskites by aliovalent elements has a profound influence on their properties. Donor doping “softens” ferroelectrics, whereas acceptor doping “hardens” them. The charge compensation mechanisms are reviewed, as well as the models describing their effects. The focus of this review is doped-BaTiO₃, a model lead-free ferroelectric. The effects of aliovalent doping on its dielectric, ferroelectric, and piezoelectric properties are reviewed and illustrated in the case of Cu (acceptor) doping.     

Effect of yttrium (Y) substitution on the structure and dielectric properties of BaTiO3

As the rate of application of multilayer ceramic capacitors (MLCCs) in small electronic devices increases, the use of the raw material barium titanate (BaTiO₃) with a small particle size and excellent dielectric properties becomes needed. Due to the size effect, small-sized BaTiO₃ generally has a cubic phase structure with a low dielectric constant, which limits its use in MLCCs. We report the preparation of small cubic phase Y-doped BaTiO₃ (BYT) nanoparticles by a hydrothermal method and the preparation of highly dielectric tetragonal phase BYT ceramics based on this method. XRD and Raman analysis showed that the BYT nanoparticles are in substable cubic phases. The particle size of the BYT nanoparticles, measured by TEM, XRD, and BET, was approximately 35 nm. The dielectric properties of the BYT ceramics were tested by an impedance analyzer, and the dielectric constant of the BYT ceramics was 7547 when the Y³⁺ doping amount was 0.5 mol%. In addition, the substitution mechanism of Y³⁺ doping in BaTiO₃ crystals was proposed from XPS and EPR analysis. The results demonstrate for the first time that the 50 nm cubic phase BaTiO₃ powder can meet the needs of next-generation high-capacity MLCCs. This work provides a reference for small cubic phase BaTiO₃ as a dielectric material for high-capacity MLCCs.     

Synthesis,phase composition and structural and conductive properties of ferroelectric microparticles based on ATiOx (A = Ba,Ca, Sr)

The paper presents the results of a study of structural, optical and conductive properties of ferroelectric microparticles based on ATiO_x (A = Ba, Ca, Sr) obtained by solid-phase synthesis. According to the data obtained, as a result of the synthesis microparticles of dendroid form were obtained. During the study, the dependences of the change in the phase composition and crystallographic characteristics on the choice of component A in the structure of a ferroelectric are established. Studies of conductive and optical characteristics of investigated microparticles were conducted. During tests of conductive characteristics of microparticles, the nonlinearity of the current – voltage characteristics was established due to the presence of impurity inclusions in the structure and a high concentration of vacancy and dislocation defects, which have a significant effect on the ballistic nature of charge transfer.     

Effects of grain size and temperature on the energy storage and dielectric tunability of non-reducible BaTiO3-based ceramics

BaTiO₃-based ceramics with various grain sizes (136–529 nm) are prepared through a chemical coating method followed by sintering in a reducing atmosphere. Effects of grain size and temperature on electric properties, energy-storage properties, and dielectric tunability are studied via Current-Field (J-E) curves, ferroelectric hysteresis loops, Capacitance-Voltage (C–V) curves and Thermally stimulated depolarization currents (TSDC). At all temperatures, fine-grain ceramics yield a lower energy density but a higher energy efficiency under the same electric field, owing to a lower ferroelectric contribution. Meanwhile, fine-grain ceramics exhibit a higher maximum energy density due to their higher breakdown strength. Fine-grain ceramics with the grain size of 136 nm have the maximum energy density of 0.41 J/cm³ under the breakdown strength of 75 kV/cm, the corresponding efficiency is 81%. C–V curves show that fine-grain ceramics have better bias-field stability. According to TSDC results, fine-grain ceramics exhibit fewer oxygen vacancies and a higher relaxation activation energy.     

Enhancement in magnetic,piezoelectric and ferroelectric properties on substitution of titanium by iron in barium calcium titanate ceramics

Lead free polycrystalline ceramics with compositional formula Ba_0.90Ca_0.10Ti_1–3x/4Fe_xO₃ (BCT), (x=0.000, 0.005, 0.010, 0.015 and 0.020) were prepared by the solid state reaction method. The effect of substitution of Fe³⁺ ion at Ti-site on the ferroelectric and piezoelectric properties of BCT ceramics was studied. Remanant polarisation (P_r) and saturation polarisation (P_s) show an increasing trend while the reverse trend was observed for coercive field (E_c) with Fe³⁺ substitution. The values of the piezoelectric charge coefficient (d₃₃) and piezoelectric coupling coefficient (k_p) was found to increase with increase in Fe content. Ceramic sample with x=0.02 was found to have a maximum value of d₃₃ (130 pC/N) and k_p (29%). The prepared ceramic samples show magnetic properties as confirmed by recorded MH loops. On substitution of Fe³⁺ ions, the ferroelectric BCT ceramics show clear transformation of the diamagnetic nature of BCT ceramics to weak ferromagnetism.     

Preparation and characterisation of BaTiO3 nanopowder by microwave assisted microemulsion method

Through the microwave assisted reverse microemulsion method, BaTiO₃ nanopowders are prepared in 10 min by selecting water/OP-10/hexanol/cyclohexane reverse microemulsion system, Ba(NO₃)₂ and tetrabutyl titanate as reactants. The effect of reaction temperature, reaction time, concentration of reactants and molar ratio of water and surfactant on the reaction was studied, and the prepared powders were characterised by X-ray diffraction, TEM and X-ray fluorescence analysis. The results show that cubic phase BaTiO₃ powders of particle size about 50–80 nm with uniform distribution and good dispersion were prepared under atmospheric pressure in 10 min at 65°C. The method has characteristics of lower reaction temperature, shorter reaction time, smaller particle size, narrow particle size distribution and good particle dispersion.     

LCD-SLA 3D printing of BaTiO3 piezoelectric ceramics

Stereolithography-based 3D printing is a promising method to produce complex shapes from piezoceramic materials. In this study, LCD-SLA 3D printing was used to create lead-free piezoceramics based on barium titanate (BaTiO₃, BT). Three types of BT powders (micron, submicron and nanoscale) were tested in LCD-SLA 3D printing, and a technique for the preparation of a ceramic slurry suitable for LCD-SLA printing has been developed. Using TGA-DSC analysis, the thermal debinding parameters to obtain crack-free samples were determined, followed by further sintering and the study of the piezoelectric properties (ε_r = 1965, d₃₃ = 200 pC/N, tan = 1,7 %). The results of the study demonstrate high potential for the production of complex piezoceramic elements that can be used in aviation, in particular, aviation radio equipment; in the marine industry for transceiver modules of hydroacoustic antennas; and in the nuclear industry for pressure control sensors in the steam–water path.