Templated Grain Research Articles

Fabrication of textured lead-free strontium barium niobate (SBN61) bulk ceramics and their electrical properties

Sr0.61Ba0.39Nb2O6 (SBN61) ceramic fibers with a high [001] texture were fabricated by a combination of alginate gelation method and templated grain growth process using acicular Ba2NaNb5O15 (BNN) template particles. The rod-like BNN template particles were prepared by molten salt synthesis method with salt/powder ratio of 2 giving the highest aspect ratio. Fibers were drawn from alginate based slurries without or with 10wt% BNN template particles. Increasing sintering temperature and time were found to increase the texture. Textured SBN61 bulk ceramics with dimensions in cm scale were fabricated by wet packing a limited number of grain oriented green fibers and sintering the green bulk to obtain a dense, textured bulk ceramic through templated grain growth. A maximum texture fraction, f, of 0.91 was obtained, as measured by Lotgering factor. Increased remnant polarization and electromechanical properties were obtained as a result of this preferred grain orientation.

Textured Z-type hexaferrites Ba3Co2Fe24O41 ceramics with high permeability by reactive templated grain growth method

The c-axis oriented Ba3Co2Fe24O41 (Co2Z) ceramic with high permeability was successfully prepared by reactive templated grain growth (RTGG) method using reactive template BaFe12O19 (BaM) and complementary Ba2Co2Fe12O22 (Co2Y). The microstructure and magnetic property of the textured ceramic sample in different orientations were investigated. The sample presents both structural and magnetic anisotropic characteristics. The obtained (00h) textured Co2Z ceramic exhibits a favorable combination of easy magnetization plane orientation, high initial permeability of 19.0±1.2 (much higher than that from conventional ceramic method (CCM) (μi=12)), and high cut-off frequency (fr>1GHz), indicating the potential for high frequency application in novel electronic devices.

Effect of texturing on polarization switching dynamics in ferroelectric ceramics

Highly (100),(001)-oriented (Ba0.85Ca0.15)TiO3 (BCT) lead-free piezoelectric ceramics were fabricated by the reactive templated grain growth method using a mixture of plate-like CaTiO3 and BaTiO3 particles. Piezoelectric properties of the ceramics with a high degree of texture were found to be considerably enhanced compared with the BCT ceramics with a low degree of texture. With increasing the Lotgering factor from 26% up to 94%, the piezoelectric properties develop towards the properties of a single crystal. The dynamics of polarization switching was studied over a broad time domain of 8 orders of magnitude and was found to strongly depend on the degree of orientation of the ceramics. Samples with a high degree of texture exhibited 2–3 orders of magnitude faster polarization switching, as compared with the ones with a low degree of texture. This was rationalized by means of the Inhomogeneous Field Mechanism model as a result of the narrower statistical distribution of the local electric field values in textured media, which promotes a more coherent switching process. The extracted microscopic parameters of switching revealed a decrease of the critical nucleus energy in systems with a high degree of texture providing more favorable switching conditions related to the enhanced ferroelectric properties of the textured material.

Mechanistic interpretation of the reactive templated grain growth process of (Li, Ta, Sb) modified (K, Na)NbO3textured lead-free piezoceramics

Reactive templated grain growth (RTGG) is a promising synthetic method for textured ceramics especially for high performance lead-free piezoceramics. However, the sintering mechanism in a RTGG process remains unclear. In this study, we perform detailed structural and compositional characterization of KNN-based piezoceramics sintered at four elevated temperatures, i.e. 800 °C, 1100 °C, 1140 °C and 1190 °C, in order to precisely examine the phase and microstructure evolution during the RTGG processes. Upon increasing the temperature up to 1190 °C, the sintering system undergoes successive processes, including elemental interdiffusion between the matrix grains and NN templates, formation of a eutectic compound and hence melting of matrix grains, nucleation and epitaxial growth on template surfaces, and finally coarsening of epitaxial crystals by consuming the intermediate liquid phase. Finally, suggestions for rational design and fabrication of textured lead-free piezoceramics using RTGG are proposed.

Microstructural evolution of high purity alumina ceramics prepared by a templated grain growth method

Microstructural evolution of high purity alumina ceramics prepared by a templated grain growth method

層状チタン酸テンプレートを用いた反応性テンプレート粒子成長法によるBa<sub>1−x</sub>Ca<sub>x</sub>TiO<sub>3</sub>配向性セラミックスの作製

Piezoelectric performance can be improved by using crystal-axis oriented materials because piezoelectric properties of the ferroelectric materials are dependent on the crystal-axis direction. In this study, we tried to fabricate Ba1−xCaxTiO3 (BCT) oriented ceramics by reactive template grain growth (RTGG) method using H1.07Ti1.73O4 layered titanate (HTO) plate-like particles as the reactive template. BCT ceramics with [110]-orientation were prepared using HTO-BaCO3-Ca(OH)2 (HBC) and HTO-BaCO3-Ca(OH)2-TiO2 (HBCT) reaction systems. The degree of orientation and the relative density of the BCT ceramics are dependent on the green sheet preparation conditions, such as dispersion condition and added binder amount, calcination temperature and starting material composition. In these reaction systems, firstly oriented BCT particles are formed by a topotactic reaction of HTO template particles with Ba and Ca matrix components, and then, BCT oriented ceramic is formed by the growth of the oriented BCT particles. BCT ceramics with higher density and higher orientation can be fabricated using HBCT reaction system than HBC reaction system, where the orientation degree is dependent strongly on the ratio of HTO template particle to TiO2 matrix particle.

Highly Textured BaTiO3 via Templated Grain Growth and Resulting Polarization Reversal Dynamics

Samples of bulk textured polycrystalline BaTiO3 ceramics were fabricated using a templated grain growth (TGG) approach in order to investigate effects of polycrystallinity and texture related to ferroelectric domain reversal under high‐power drive conditions. Barium titanate platelets were formed via two‐step topochemical conversion of bismuth titanate platelets grown via molten salt synthesis, then aligned via tape casting within a matrix of fine BaTiO3 powder. The coarse‐grained parts showed a high degree of crystallographic texture after sintering. Combined with ceramics of similar density and polycrystallinity, but random orientation and commercial single‐crystal specimens, this sample set enabled direct isolation of crystallographic texture and polycrystallinity as the primary variables for high‐power polarization reversal studies. These studies have also demonstrated a link between grain size and polarization reversal time that strongly suggests that grain boundaries serve effectively as nucleation sites during the ferroelectric switching process.

Highly textured Ba0.85Ca0.15Ti0.90Zr0.10O3 ceramics prepared by reactive template grain growth process

〈001〉 textured Ba0.85Ca0.15Ti0.90Zr0.10O3 (BCZT) lead-free ceramics were fabricated by reactive-templated grain growth (RTGG) method using plate-like BaTiO3 templates. Aligned plate-like BaTiO3 particles grew at the expense of equiaxed matrix powder with random orientation with increasing sintering temperature, resulting in development of texture. Highly textured (texture fraction f=0.98) BCZT ceramics were prepared at first with a RTGG process. Development of texture was investigated during sintering. The influence of templates content on texture fraction and electrical properties of the textured BCZT ceramics were also studied. The textured ceramics shows high piezoelectric coefficient (d33=455pC/N) and dielectric properties (εr~3500, tanδ~0.010).

Characterization of thick bismuth ferrite–lead titanate films processed by tape casting and templated grain growth

The templated grain growth technique was used to synthesise textured 60BiFeO3–40PbTiO3 (60:40BFPT). Both Aurivillius (Bi4Ti3O12, PbBi4Ti4O15) and perovskite templates (BaTiO3, SrTiO3) were used to prepare 60:40BFPT. Only BaTiO3 templates were found to successful impart a texture to the ceramic matrix. In the case of perovskite templates, ferroelectricity was evident from saturated polarisation hysteresis loops. Saturated polarisation loops were achieved due to the substitution of Ba2+ or Sr2+, which reduces the coercive field. SrTiO3 and BaTiO3 templated ceramics showed remanent polarisation of 30 and 36μC/cm2, respectively. Aurivillius templates did not generate ferroelectric materials. Because of their high chemical stability in this system, BaTiO3 templates appear to be the best candidate for fabricating textured BFPT by the reactive templated grain growth method.

Templated grain growth and piezoelectric properties of 〈001〉-textured PIN–PMN–PT ceramics

Abstract

Fabrication of textured CoFe2O4 ceramics by a novel RTGG method using rod-like α-FeOOH particles as templates

A textured CoFe2O4 ceramic was fabricated by reactive template grain growth (RTGG) method using rod-like α-FeOOH particles as templates. The template particles were synthesized by hydrothermal method, the particles showed good crystallization and rod-like shape with a diameter of 0.1–0.2 μm and an aspect ratio of 20–50. {ll0} oriented CoFe2O4 ceramic was obtained after a sintering process. The ceramic sintered at 1150 °C for 4 h showed dense and oriented structure with orientation degree of 0.89. Different magnetic hysteresis loops were observed along the in-plane and out-of-plane direction of the textured CoFe2O4, which was due to anisotropy properties of the sample. The textured ceramic exhibited an in-plane Ms enhancement (355 emu/cm3) compared to that of the non-textured ceramics (320 emu/cm3) in the same sintering conditions.

Structures and Electrical Properties of Textured Ca0.85(LiCe)0.075Bi4Ti4O15 Ceramics Prepared by the Reactive Templated Grain Growth

Textured Ca0.85(LiCe)0.075Bi4Ti4O15 ceramics were prepared by reactive templated grain growth method using Bi4Ti3O12 templates. The highly oriented Ca0.85(LiCe)0.075Bi4Ti4O15 ceramics (f = 98%) were formed in situ topotaxially on the Bi4Ti3O12 templates, which exhibit high piezoelectric properties (d33 = 33 pC/N) compared with the randomly ceramics in Aurivillius-type bismuth layer structured ferroelectrics. The d33 value of textured ceramics has excellent temperature stability when the temperature is below the 500°C. High Tc and DC resistance also illustrate that the textured Ca0.85(LiCe)0.075BiTi4O15 ceramics can be a promising high-temperature piezoelectric application.

Hydrothermal synthesis of plate-like sodium niobate particles

Sodium niobate (NaNbO3) particles with perovskite structure and plate-like morphology were successfully prepared through calcining the plate-like hexagonal NaNbO3 particles that were synthesized by hydrothermal method. A single phase of hexagonal NaNbO3 was obtained under the condition of 200°C and [OH−]=1.0mol/L, and the particle size is about 15μm in diameter and 2μm in thickness. Through researching the effects of reaction temperature and concentration of OH−, the best condition to prepare the hexagonal NaNbO3 plate-like particles was investigated. After further calcination treatment, the particles were completely transformed into perovskite structure with the orthorhombic unit cell without morphology change. Compared with the traditional high temperature molten salt method, this work provides a simpler way to prepare NaNbO3 template for templated grain growth (TGG) or reactive templated grain growth (RTGG) texture techniques.

(Li, Ta, Sb) modified (K, Na)NbO3 ceramics as high temperature dielectric materials

A high temperature dielectric material of (Li, Ta, Sb) modified (K, Na)NbO3 (KNLNTS) ceramics with excellent temperature stability was prepared by a reactive templated grain growth (RTGG) method. The effects of sintering temperature on the phase structure, microstructure and electric properties of the ceramics were investigated. The XRD and SEM results revealed that the structure and microstructure of the ceramics could be controlled by changing the sintering temperature. Multiphase and cation disordering in the ceramics due to compositional inhomogeneity resulted in relaxor behavior, thus improving the temperature stability of the ceramics greatly. The KNLNTS ceramics sintered at 1150°C exhibit high permittivity maximum (~2000) and low dielectric loss (<4%) at a temperature range of 120–335°C, and the capacitance variation (ΔC/C185°C) maintains within ±15% at this temperature range, showing that KNLNTS ceramics is a promising candidate for high temperature MLCCs applications.

Na0.5Bi0.5TiO3–BaZrO3 textured ceramics prepared by reactive templated grain growth method

In this work 0.94Na0.5Bi0.5TiO3–0.06BaZrO3 (NBT–BZ) ceramics were prepared by a reactive templated grain growth (RTGG) method using 15wt% NBT template. The as synthesized NBT–BZ textured ceramics were sintered at 1150°C for different soaking times (2–15h) and their crystal structure, microstructure, dielectric, ferroelectric and field-induced strain properties were investigated and compared. Results show that all textured NBT–BZ ceramics have (h00) preferred orientation, which increases with increase in the sintering soaking time. No significant change in the depolarization temperature was observed in NBT–BZ textured ceramics with change in the sintered time. However, the field induced strain response increased by 20% when the sintering time was increased from 2h to 15h.

Modified hydrothermal synthesis and structural characterization of monoclinic (KxNa1−x)NbO3 (0.05≤x≤0.15) rods

Monoclinic (KxNa1−x)NbO3 (0.05≤x≤0.15) rods were prepared successfully by the hydrothermal synthesis method combined with dissolution–precipitation and subsequent heat-treatment processes. At first, the primary precipitate of the niobate precursor was obtained using Nb2O5, KOH and NaOH as the initial reactants, and deionized water as the solvent for the hydrothermal synthesis. Interestingly, when the obtained precipitate was dissolved in 90°C of deionized water and then the solution kept standing till ambient temperature, the secondary precipitate of rod-like niobate (KxNa6−x)H2Nb6O19·2H2O was formed. After centrifugation, washing, and the subsequent heat treatment of the secondary precipitate sample, the (KxNa1−x)NbO3 rods of 1–60μm in length and 1–5μm in width were finally prepared. The surface morphology and phase structure of the prouducts were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively, and the chemical composition of the prouducts were identified by means of energy-dispersive X-ray spectroscopy(EDS). Furthermore, fourier transform infrared spectroscopy (FT-IR) and high-resolution transmission electron microscopy (HRTEM) were employed to investigate the fine structures of the prouducts. This research provides a new approach for the preparation of one-dimensional (KxNa1−x)NbO3 material and template grains for the fabrication of lead-free textured alkali niobate piezoelectric ceramics.

Preparation and enhanced electrical properties of grain-oriented (Bi1/2Na1/2)TiO3-based lead-free incipient piezoceramics

(Bi1/2Na1/2)TiO3 (BNT) based lead-free incipient piezoceramics are promising candidates for actuator applications due to their giant electromechanical strains originating from the reversible relaxor-ferroelectric phase transition. To decrease the electric field required for this large strain, the templated grain growth method was employed to prepare 〈100〉 oriented 0.91Bi1/2Na1/2TiO3–0.06BaTiO3–0.03AgNbO3 piezoceramics using plate-like BNT templates. Textured samples provided a high unipolar strain of 0.38% and a corresponding large signal piezoelectric coefficient, d33∗ of 766pm/V at 5kV/mm, which are 78% higher than the values of the randomly oriented ones. The enhanced electric-field-induced strain at relatively lower field was attributed primarily to the facilitated phase-transition to form a long range ferroelectric order along the 〈100〉 direction. It was also found that the textured piezoceramics exhibited significantly reduced frequency dependence in the unipolar strain behavior at room temperature, resulting from the decreased electric field required for the relaxor-ferroelectric phase transition.

Synthesis and characterization of Na0.5Bi0.5TiO3 platelets with preferred orientation using Aurivillius precursors

Plate-like Na0.5Bi0.5TiO3 (NBT) templates with perovskite structure were successfully synthesized by a topochemical microcrystal conversion (TMC) method from Aurivillius precursors, Bi4Ti3O12 (BIT) and Na0.5Bi4.5Ti4O15 (NBIT) platelets. Both NBT platelets and their precursors showed a high aspect ratio between an average size of 10–20μm and thickness of 0.5–1.0μm. The clear lattice fringe and the diffraction spots of BIT, NBIT and NBT platelets obtained from high-resolution transmission electron microscopy (HRTEM) indicated good crystallinity. The NBT platelets showed a high degree of preferred pseudocubic [001] orientation with a large Lotgering factor of 0.87 which exhibited good potential as suitable templates for synthesizing textured lead free piezoelectric ceramics, e.g. Na0.5Bi0.5TiO3–xBaTiO3, via the templated grain growth process.

High Thermoelectric Performances in Co-oxides Processed by a Laser Floating Zone Technique

Cobaltite ceramics are very promising materials for new thermoelectric devices, as they exhibit high thermopower and relatively low electrical resistivity, as well as high chemical stability at high temperatures. They show anisotropic thermoelectric properties linked to their layered structures. As a consequence, different texturing methods developing oriented grains, such as sinter-forging, and template grain growth processes have been successfully used to improve the performances of these materials.There are some other less used methods, like the directional growth from the melt, which have already shown their applicability to this kind of compounds. One of these methods is the laser floating zone (LFZ) melting technique. This process has demonstrated its suitability in the Co-oxide based thermoelectric materials processing in the last years in our laboratories.In this work, two examples of the versatility and usefulness of the LFZ technique will be shown. The first one will be based on the processing of the well known Ca3Co4O9 ceramic material. In this case, a very dense and homogeneous material is obtained in two steps (growth and annealing), reaching PF values as high as 0.42 mW/K2m at 800°C. In the second case, a new and well textured thermoelectric composite (Bi2Ca2Co1.7Ox + Ca3Co4O9) is obtained, reaching PF values of 0.31 mW/K2m at 650°C.

Synthesis of (111) textured lead-free piezoelectric ceramics through template grain growth process

P devices in consumer applications are being required to eliminate the lead due to its toxic nature to human health. This has led to significant emphasis on the development of lead-free piezoceramic materials that can match performance of current lead-based compositions. However the performance of all the known lead free remains inferior. A promising approach for enhancing the piezoelectric shear d15 response of lead-free piezoceramics is to synthesize the (111) grain oriented material by using the template particles with high (111) crystallographic orientation. In this work, we report the synthesis of (111) oriented BaTiO3 (BT) template particles and their application towards deriving the (111) textured leadfree piezoelectric ceramics. BT particles were synthesized by two-step molten salt synthesis method (MSS) from plate-like Ba6Ti17O40 (B6T17) precursor through topochemical microcrystal conversion (TMC) method. In order to identify the largest developed planes of the TMC-synthesized BT particles, X-ray diffraction measurement technique (OPL-XRD) was applied. The prepared BT particles were embedded in the slurry and then subjected to the tape casting. During casting, the templates are aligned by shear force in the preferred orientation. The samples were found to exhibit higher intensity for the (111) Bragg reflection in the XRD spectra suggesting good orientation of BaTiO3 particles along the direction. These (111) oriented BT templates were next used for synthesizing (111) grain oriented lead-free piezoelectric ceramics.