Plate-like NaNbO3 Research Articles

Growth orientation control in rapid solid-state crystal growth of (K,Na)NbO3 single crystals using plate-like NaNbO3 single crystal particles

The rolling-extended orientation technique and plate-like NaNbO3 (NN) single crystal particles prepared by a single-step molten salt synthesis, both of which have been developed to fabricate (K, Na)NbO3 (KNN) textured ceramics, were utilized to control the growth orientation of KNN single crystals synthesized by a rapid solid-stated crystal growth (RSSCG) method. As the seed crystals, two kinds of NN single crystal particles were synthesized using pure NaCl and KCl-NaCl mixed molten salts. Plate-like KNN single crystals of about 1 cm squares with the upper and lower faces almost parallel to the (100) (001) planes were obtained with a probability exceeding 50% when NN single crystal particles were synthesized from mixed salts and were subsequently thermal-treated again in Na2CO3-NaCl mixed molten salts under appropriate conditions to remove the Bi element, which is known as the suppression factor of the crystal growth. The average crystal growth rate was 0.6–1.2 mm h−1. Controlling the growth orientation of KNN single crystals produced by the SSCG method using seed crystals other than KTaO3 single crystals was successfully accomplished for the first time.

Characteristics of NaNbO3 templates and improvement of electrical properties of textured KNLNS-BNKT ceramics

In this study, plate-like perovskite NaNbO3 (NN) templates were formed from Bi2.5Na3.5Nb5O18 (BiNN5) precursors via a molten salt method. The structure and microstructure of BiNN5 precursors and NN templates were studied. As a result, the platelike BiNN5 precursor particles have a bismuth layer perovskite structure, which is the basis for forming the synthesized plate-like NN particles that have a high aspect ratio with a width of 2-7 μm, a length of 5-12 μm and a thickness of 0.5-1 μm. From NN templates, we fabricated textured lead-free 0.98(K0.48Na0.48Li0.04)(Nb0.95Sb0.05)O3 − 0.02Bi0.5(Na0.4K0.1)TiO3 (KNLNS- BNKT) ceramic by the grain orientation technique. Experimental results show that the KNLNS-BNKT ceramics have a single-phase orthorhombic structure in which textured KNLNS-BNKT ceramics with 67% grain orientation. The electrical properties of the oriented ceramic are significantly improved compared to the non-oriented ceramic fabricated by the traditional method. Specifically, the electrical properties of the resulting KNLNS-BNKT oriented ceramics have a high dielectric constant (ε = 1323, an increase of 17%); large remnant polarization (Pr = 13.5 mC/cm2, an increase of 11.9%); high piezoelectric coefficient (d33 = 205 pC/N, an increase of 32%) and high electromechanical coupling coefficient (kp = 0.4, an increase of 11%).

Preparation technology and crystal structure analysis of NaNbO3 microcrystalline powders with different crystal orientations and morphologies by molten salt method

The different crystal orientations of NaNbO3 microcrystalline powders were synthesized by the molten-salt growth method with different precursors. The plate-like (001)-NaNbO3 orthogonal powders were synthesized by topochemical conversion using Bi2.5Na3.5Nb5O18 as precursor particles. The tabular NaNbO3 template particles were about 0.5–1 μm in thickness and about 15–25 μm in length. The rod-like (101)-NaNbO3 orthogonal powders were fabricated with Nb2O5 as a precursor. The preparation of Nb2O5 precursors required two processes: the molten salt method and the ion exchange method, respectively. The rod-like (101)-NaNbO3 orthogonal powders were about 0.6 μm in average diameter, and 8 μm in average length. The plate-like and rod-like orthogonal NaNbO3 powders had high aspect ratios. The diffraction peaks of plate-like NaNbO3 correspond to standards card JCPDS-33-1270, and its space group was Pbma. The crystal cell of Pbma NaNbO3 was a typical antiferroelectric structure. The diffraction peaks of rod-like NaNbO3 correspond to standard card JCPDS-86-0606, and its space group was P2lmc. The cell structure of P2lmc NaNbO3 was a typical ferroelectric structure.

Fabrication of textured (K,Na)NbO3-based ceramics at low sintering temperatures by using NaNbO3 templates prepared with hydrothermal method

Texturing is an effective approach to improve the performance of piezoelectric ceramics. The templates employed in this work were prepared using the hydrothermal method, in which plate-like hexagonal NaNbO3 (NN) particles were synthesized at 200 °C and converted to the perovskite structure through heat treatment at 550 °C. This is because the hydrothermal method represents the most promising approach to the low-cost, low pollution, and large-scale production of template particles for manufacturing textured ceramics. Textured Mn-doped (K,Na)NbO3-based piezoelectric ceramics with a vertical morphotropic phase boundary (MPB) were fabricated using the reactive template grain growth (RTGG) method. A good degree of orientation (66%) and a notable piezoelectric constant (320 pC N−1) were achieved after the addition of 10 mol% of the perovskite structure NN template and sintering at 1040 °C for 10 h. The SEM results revealed that the textured ceramics had a microstructure composed of rectangular grains that exhibited a brick wall-like alignment.

Boosting electric-field-induced strain of dual templates-textured (Na1/2Bi1/2)TiO3-based lead-free piezoceramics by polarization coupling

The templated grain-growth method is an effective technique to fabricate textured (Na1/2Bi1/2)TiO3-based ceramics for enhancing the electric-field-induced strain response, where the commonly used templates are plate-like (Na1/2Bi1/2)TiO3 (NBT) or NaNbO3 (NN). However, the textured samples prepared by one of the templates are usually not up to expectations due to the chemical composition deviations or lattice distortions. Herein, we propose a new approach to boost the texturing effect by integrating NBT-textured grains and NN-textured grains into one <001>-oriented (Na1/2Bi1/2)TiO3-26SrTiO3 ceramic using dual templates (both NBT and NN templates). Benefiting from the complementary effect and polarization coupling effect of these two different types of textured grains, the sample achieves a 56% increase in d33* (650 pm/V at 60 kV/cm) with preferred temperature stability, while showing the same relaxor ferroelectric characteristic as the randomly oriented counterpart. The finding is expected to provide a novel paradigm for lead-free piezoceramics with excellent properties.

Orientation factor and piezoelectric property in textured (K,Na)NbO3 system ceramics with minimal templates

To lower the sintering temperature and decrease the weight ratio of templates required for fabricating textured (K,Na)NbO3 (KNN) system ceramics, the method of preparing platelike NaNbO3 (NN) single-crystal particles for the templates was improved. The NN crystal particles synthesized by the single-step molten salt method using a mixed salt were treated by ultrasonic vibration and subsequently screened. Using these improved templates, we developed the fabrication method for the textured KNN system ceramics by which the weight ratio of the templates required for the orientation control can be reduced to 1/10 of the conventional weight ratio. The effects of decreasing the weight ratio of templates on sintering temperature, orientation factor, and piezoelectric properties were investigated. In addition, the effects of adding Bi2O3 to KNN calcined powder were also studied to suppress abnormal grain growth, which appears in the textured samples fabricated using smaller amounts of the templates.

Enhanced piezoelectric properties and temperature-insensitive strain behavior of -textured KNN-based ceramics

In this study, <001>-textured 0.99(K0.5Na0.5)0.95Li0.05Nb0.93Sb0.07O3−0.01CaZrO3 [abbreviated as 0.99KNLNS-0.01CZ] lead-free ceramics were prepared by templated grain growth (TGG) using plate-like NaNbO3 templates and sintered by a two-step sintering process with different soaking time. All textured samples with high Lotgering factor (f >85%) presented orthorhombic and tetragonal coexisting phase, and the proportion of orthorhombic phase was varied with prolonged soaking time. A large piezoelectric constant d33 (~ 310 pC/N) was obtained in the textured samples with a 12h soaking time, which was almost twice larger compared to the randomly oriented one. Furthermore, the field-induced piezoelectric strain coefficient d33*(~ 440pm/V) of the textured ceramics with 6h soaking time was larger than the value of randomly oriented one (~ 298pm/V) at room-temperature. Enhanced piezoelectric response and good temperature stability prove that <001>-textured 0.99KNLNS-0.01CZ ceramics are promising candidates in the field of lead-free piezoelectric materials.

Revealing the hydrothermal crystallization mechanism of ilmenite-type sodium niobate microplates: the roles of potassium ions

The roles of K ions in the hydrothermal crystallization process of plate-like NaNbO3 microcrystals were revealed.

Orientation control of (K,Na)NbO3 ceramics using platelike NaNbO3 templates prepared by single-step molten salt synthesis with mixed salt

In order to shorten the firing time for sintering textured (K,Na)NbO3 system ceramics, the fabrication method of the single-step molten salt synthesis for preparing platelike NaNbO3 single-crystal particles was improved. An uneven shape was formed on the surfaces of platelike NaNbO3 single-crystal particles obtained by single-step molten salt synthesis in the mixed salts of KCl and NaCl. Using these single-crystal particles as the templates, textured (K,Na)NbO3 ceramics were fabricated. The sinterability of the textured ceramics and their piezoelectricity were compared between two kinds of templates synthesized in the mixed salt and in the conventional NaCl salt. The diffusivity of K atoms into these templates from the matrix powder of (K,Na)NbO3 during firing was also compared.

Texture development and enhanced electromechanical properties in -textured BNT-based materials

Grain-oriented lead-free piezoceramics 0.83(Bi0.5Na0.5)TiO3-0.17(Bi0.5K0.5)TiO3 was synthesized with different grain orientation along the <00l> crystallographic orientation using plate-like NaNbO3 templates. The effects of degree of texturing on the microstructure, phase transition temperature and resulting changes in piezoelectric and strain response of ceramics were explored. The electromechanical properties increased monotonously with increasing the degree of texturing. A high <00l> texture degree of 91% was obtained, giving rise to larger electromechanical properties of d33=217pC/N, kp=35%, S=0.246%, and Smax/Emax=410pm/V in textured samples relative to the conventional polycrystalline counterparts. Furthermore, the grain oriented ceramics exhibited improved temperature stability as compared to its randomly oriented form. This work opens the possibility of achieving BNT-based high performance ceramics.

Electric properties of textured (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3 thick film prepared by screen printing method

Textured (K0.44Na0.52Li0.04) (Nb0.86Ta0.10Sb0.04)O3 thick film was fabricated by the screen printing method with plate-like NaNbO3 particles as template. Thick film with 75% grain orientation was prepared. Remnant polarization and coercive field observed from the P–E loops of textured thick film were 3.6 μC cm−2 and 21 kV cm−1, respectively. Textured (K0.44Na0.52Li0.04) (Nb0.86Ta0.10Sb0.04)O3 thick film exhibited diffusion behaviour by analysing the temperature dependence of permittivity and loss tangent. The result of leakage current density showed a conduction mechanism of Schottky emission. Piezoelectric (PZT) properties of the thick film were characterized by the relationship of unipolar strain and applied electric field and the PZT constant \(\phantom {\dot {i}\!} {d}_{{33}}^{{\ast }}\) of textured thick film reached to 150 pm V−1. Nonlinear PZT property of the thick film was investigated by Rayleigh law.

Synthesis of anisotropic NaNbO3 seed crystals and fabrication of textured (K0.5Na0.5)NbO3-based ceramics

High aspect ratio patelike NaNbO3 particles with pure perovskite structure have been successfully synthesized by topochemical microcrystal conversion (TMC) from plate-like precursor particles of the layer-structured Bi2.5Na3.5Nb5O18. By changing the Bi2.5Na3.5Nb5O18/Na2CO3 ratio, large and thin NaNbO3 particles with a thickness of approximately 0.5 μm and a width of approximately 20 μm were obtained. The obtained NaNbO3 particles is quite suitable for fabricating textured (K0.5Na0.5)NbO3-based ceramics. Using the fine platelike NaNbO3 particles as templates, dense -oriented (K0.5Na0.5)NbO3-0.5 mol %MnO2 ceramics with high texture quality (Lotgering factor F 001 = 87 %) and excellent piezoelectric properties were produced by templated grain growth. Compared with randomly oriented ceramics, textured samples show greatly enhanced properties. The room-temperature strain S, the piezoelectric coefficient d 33 * and d 33 reach up to 0.093 %, 233 pm/V and 195pC/N, respectively, which are all about 1.5 times larger than those of non-textured ceramics.

Effect of Different Templates on Electrical Properties of Textured KNN-Based Ceramics

Platelike and perovskite structure NaNbO3 (NN) with orthorhombic, SrTiO3 (ST) with cubic and BaTiO3 (BT) with tetragonal were used as templates to fabricate grain-oriented 0.94(K0.5Na0.5)NbO3-0.06LiSbO3 (0.94KNN-0.06LS) ceramics by templated grain growth (TGG). The effects of different templates on the grain orientation, ferroelectric, dielectric, piezoelectric properties and strain behavior of 0.94KNN-0.06LS ceramics were systematically investigated. Textured ceramics with high orientation factor were obtained successfully when the platelike NN was used as a template, while high degree of grain orientation cannot be obtained with ST and BT template. The templated growth of <00l> oriented grains on the NN template resulted in textured ceramics with brick significantly high <00l> texture degree corresponding to 87% Lotgering factor at 5 mol% NN templates. Textured ceramics with NN templates exhibited excellent piezoelectric properties as compared to the random counterparts due to high degree of grain orientation, while ceramics used ST templates shown excellent strain behavior.

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.

Effect of different templates on structure evolution and large strain response under a low electric field in -textured lead-free BNT-based piezoelectric ceramics

<00l>-textured 0.83Na0.5Bi0.5TiO3–0.17Bi0.5K0.5TiO3 (BNT–BKT) ceramics with high degree of texturing (>80%) were synthesized by templated grain growth using different perovskite plate-like BaTiO3, SrTiO3 and NaNbO3 as template particles to develop lead-free piezoelectric materials with an excellent actuating performance. The effects of different template particles addition to BNT–BKT ceramics on the grain orientation, structure evolution, phase stability and macroscopic functional properties was systematically investigated. BaTiO3 template particles addition to form <00l>-textured BNT–BKT–BT textured ceramics suppresses an electric-field-induced long-range ferroelectric order, giving rise to a high field-induced Smax/Emax of 710pm/V under a relatively low driving field of 45kV/cm compared with existing lead-free Bi-perovskite ceramics. In particular, temperature-dependent strain response reveals that <00l>-textured BNT–BKT–5BT ceramics exhibit not only large Smax/Emax at a low driving field of 45kV/cm at room temperature but also enhanced the temperature stability of Smax/Emax featured by large Smax/Emax of 517pm/V at high temperature.

Synthesis of high aspect ratio (K, Na)NbO₃ plate-like particles and study on the synthesis mechanism.

Plate-like NaNbO3 (NN) templates have been successfully synthesized via a two-step molten salt method using K2CO3 as a raw material at 970 °C, which is below the topochemical reaction temperature using Na2CO3 as the raw material (higher than 1000 °C). The synthesized plate-like NN particles have a higher aspect ratio with a thickness of 0.5-1 μm, a width of 5-10 μm and a length of 15-25 μm. In this process, we found that Na2CO3 or K2CO3 plays an important role in removing the (Bi2O2)(2+) layers in the substitution process and the final composition is decided by the type of molten salt. Using KCl as molten salt and Na2CO3 or K2CO3 as raw material, the (Bi2O2)(2+) layers could be removed by Na2CO3 or K2CO3 and K(+) ions can be incorporated into the template during the topochemical reaction and thus (K, Na)NbO3 (KNN) plate-like particles can be fabricated. The growth process is reasonably elucidated by this growth mechanism.

Evolution of textured microstructure of Li-doped (K,Na)NbO3 ceramics prepared by reactive templated grain growth

(K0.45Na0.55)0.94Li0.06NbO3 (KNLN) ceramics with high texture degree (Lotgering factor f=94%) were prepared by reactive templated grain growth (RTGG) using plate-like NaNbO3 (NN) templates. XRD patterns and SEM micrographs of the samples fabricated at different stages of RTGG were investigated. The XRD results revealed that NN templates reacted with KNLN matrix particles to form a solid solution at the first stage of RTGG. Then the significant template growth could only start when the sample’s relative density was higher than 90%. SEM observations revealed that the templated grains showed cubic-like feature instead of plate-like feature of NN template particles finally. The process of texture development and mechanism of template growth were discussed in detail. The piezoelectric coefficient d33 and electromechanical coupling factor kt of textured ceramics were 70% and 80% higher than that of nontextured ceramics with the same composition, respectively.

One‐Step Surfactant‐Free Hydrothermal Synthesis of Platelike Sodium Niobate Template Powders

A one‐step surfactant‐free hydrothermal route is developed to prepare platelike NaNbO3 template powders. At optimal KOH concentration, pure platelike NaNbO3 with rhombohedral structure (width and thickness of 20 and 2 μm, respectively) is obtained at 200°C for 16 h. After calcination at 600°C for 4 h, the structure of the hydrothermally synthesized NaNbO3 changes from rhombohedral to orthorhombic, whereas the initial platelike morphology is maintained. Such characteristics in terms of phase structure, elemental composition, and morphology render our hydrothermally synthesized NaNbO3 suitable for textured ceramic fabrications.

The preparation and piezoelectric property of textured KNN-based ceramics with plate-like NaNbO3 powders as template

In this work, the platelike NaNbO3 (NN) particles were obtained via a new method using K2CO3 as raw materials. The synthesized platelike NN particles have high aspect ratio with a thickness of 0.5–1μm, a width of 5–8μm and a length of 10–18μm. The 〈001〉 textured lead-free piezoelectric 0.92(Na0.5K0.5)NbO3–0.06BaZrO3–0.02 (Bi0.5Li0.5)TiO3 (0.92KNN–0.06BZ–0.02BLT) ceramics were fabricated by template grain growth (TGG) in combination with tape casting using NN as a template. High 〈001〉 orientation degree of 86% was achieved at 1125°C for 2h. The textured ceramics exhibited excellent piezoelectric performance with the piezoelectric coefficient d33=271pC/N, Curie temperature Tc=251°C, strain levels of 0.13% at 40kV/cm and remnant polarization Pr=18.5μC/cm2.

Preparation and photocatalytic properties of platelike NaNbO3 based photocatalysts

Two kinds of plate-like NaNbO3 were separately prepared by the one- and two-step molten salt processes via topochemical micro-crystal conversion methods. Meanwhile, the composite photocatalysts were obtained via heating the mixture of corresponding NaNbO3 powders and urea. Their photocatalytic activities were evaluated from the photodegradation of Rhodamine B under full arc and visible light irradiation of Xe lamp. The sample containing NaNbO3 prepared by the one-step molten salt process and carbon nitride displays the highest activity. The enhancement of photocatalytic activity was attributed to the surface properties and the state of the carbon nitride.