Plate-like NaNbO3 Research Articles

Orientation Control of (K,Na)NbO3 Ceramics Using NaNbO3 Particles Prepared by Single-Step Molten Salt Synthesis

An effective orientation control of (K,Na)NbO3 (KNN) ceramics using plate-like NaNbO3 (NN) particles prepared by single-step molten salt synthesis was accomplished. The factors inhibiting the grain orientation in KNN ceramics were identified. The mixture ratio of Bi2O3 in the starting materials used to synthesize the plate-like NN particles was optimized for the orientation control. In addition, by improving the sintering process, the textured ceramics with an orientation factor of 97% and a relative density of 96% were fabricated.

Role of Alumina in the Preparation of Platelike NaNbO3 Powder by Molten Salt Synthesis and Proposal of New Preparation Method

The preparation of platelike NaNbO3 grains via single‐step molten salt synthesis using Bi2O3, Na2CO3, Nb2O5, and NaCl as reactants was examined. When a new alumina crucible was used, platelike NaNbO3 grains were obtained, but a repeatedly used alumina crucible resulted in irregularly shaped NaNbO3 grains. When a platinum crucible was used, even NaNbO3 could not be obtained. Addition of alumina substrates and alumina granules to the reaction mixture in the platinum crucible resulted in the formation of platelike NaNbO3 grains and second‐phase grains. The second‐phase grains, which were composed of Al2O3, Bi2O3, Na2O, and Nb2O5 and had a pyrochlore structure, could be removed by sieving. The second phase acted as a scavenger for Bi2O3 and hence, the possibility of using another scavenger was attempted. The new scavenger was a mixture of Na2CO3 and Nb2O5, and using them, platelike NaNbO3 grains were successfully obtained with NaNb5Bi2O16 as a byproduct, which could then be removed by sieving.

Fabrication of textured KNNT ceramics by reactive template grain growth using NN templates

Lead-free non-stoichiometric (K0.470Na0.545)(Nb0.55Ta0.45)O3 (KNNT) textured ceramics were prepared by a reactive templated grain growth method using NaNbO3 (NN) templates. The Plate-like NaNbO3 (NN) templates were synthesized from bismuth layer-structured Bi2.5Na3.5Nb5O18 (BNN) particles by a topochemical microcrystal conversion (TMC) method. Using 5 wt% of NN templates, textured KNNT ceramics were fabricated, and their crystal structure, microstructure, dielectric and piezoelectric properties were compared with non-textured KNNT ceramics prepared by a conventional solid state reaction method. The textured KNNT ceramics exhibited high grain orientation and high dielectric constant. In addition, piezoelectric properties of textured KNNT ceramics were improved, giving a high piezoelectric coefficient d33 = 390 pC/N and piezoelectric coupling coefficient kp = 0.60.

Microstructure Development in -Oriented (K,Na)NbO3-based Ceramics

The development of grain orientation and the factors controlling the degree of orientation in (K,Na)NbO3-based ceramics were examined using two types of platelike NaNbO3 template powders and four types of matrix powders—KNbO3, NaNbO3, and (K0.5Na0.5)NbO3 powders, and a mixture of KNbO3 and NaNbO3 powders. The NaNbO3 template powders were prepared by single- and two-step molten salt synthesis (MSS). In the ceramic specimens, the template NaNbO3 grains changed to (K,Na)NbO3. The direction of material transport in this reaction determined the degree of orientation. The sintering characteristics of the matrix powders also contributed to the control of the degree of orientation.

Enhanced Ferroelectric and Piezoelectric Properties of Textured K<SUB>0.45</SUB>Na<SUB>0.55</SUB>NbO<SUB>3</SUB> Ceramics Prepared by Screen-printing Technique

Grain oriented K0.45Na0.55NbO3 (KNN) lead-free piezoceramics with sintering aid K4CuNb8O23 (KCN) were fabricated by screen-printing technique, using plate-like NaNbO3 as templates. The plate-like NaNbO3 template particles were synthesized from bismuth layer-structured Bi2.5Na3.5Nb5O18 precursors by the topochemical micro-crystal conversion method. The textured KNN ceramics showed brick-wall-like grains which aligned parallel to the screen-printing direction. The screen-printed KNN ceramics possessed both a high grain orientation (Lotgering factor f=95%) and a relative high density (92% of theoretical density). The orientation degree and electrical properties of textured (K0.45Na0.55)NbO3 ceramics exhibited anisotropic feature in the parallel (sp parallel to) and perpendicular (sp perpendicular to) plane. Compared with the random oriented ceramics with same composition, the dielectric constant e piezoelectric constant d(33), and electromechanical coupling coefficient k(p) were increased by about 75%, 44% and 42% in sp parallel to plane, respectively, and about 35%, 30% and 35% in sp perpendicular to plane, respectively. Screen-printing is a simple and effective method for the fabrication of grain oriented lead-free piezoelectric ceramics.

Preparation of Platelike NaNbO3 Particles by Single‐Step Molten Salt Synthesis

This article reports single‐step molten salt synthesis to prepare single‐crystal, platelike NaNbO3 particles without the Bi2O3 byproduct. In the previous method of two‐step molten salt synthesis, platelike Bi2.5Na3.5Nb5O18 particles have been prepared by heating a mixture of Bi2O3,Na2CO3, and Nb2O5 in molten NaCl; the particles are then converted to platelike NaNbO3 particles by reaction with Na2CO3 in molten NaCl. The obtained NaNbO3 particles are polycrystalline, and Bi2O3 particles form as a byproduct. In the present method, single‐crystal, platelike NaNbO3 particles without the Bi2O3 byproduct are obtained by heating mixtures of Bi2O3, Na2CO3, and Nb2O5 with molar ratios of 5:7:10 (the composition of Bi2.5Na3.5Nb5O18) and 5:5:8 (the composition of Bi2.5Na2.5Nb4O15) in molten NaCl at 1225°C for 12 h. During heating, platelike Aurivillius (Bi2.5Na3.5Nb5O18 and Bi2.5Na2.5Nb4O15) particles are formed first and these then react with NaCl to form platelike NaNbO3 particles, BiCl3, and Na2O. BiCl3 evaporates from the system and Na2O is washed out after heating. Thus, single‐phase NaNbO3 is obtained. Sluggish conversion from the Aurivillius phases to NaNbO3 is responsible for the formation of single‐crystal particles.

Preparation of (h00)-Oriented K0.5Na0.5NbO3 Ceramics and Its Electrical Properties

Textured (K0.5Na0.5)NbO3 (KNN) ceramics was fabricated via the reactive templated grain growth (RTGG) method with the plate-like NaNbO3 (NN) particles as templates. High orientation degree (Logering's factor) of 84% calculated from XRD patterns was obtained. The microstructure of the surface of KNN ceramics is observed. The dielectric and piezoelectric properties of the non-textured and textured KNN ceramics have been studied. The TO-T and TC of the textured KNN ceramics is 168°C and 400°C respectively which decreased slightly compared with the non-textured KNN ceramics. The d33 calculated from the unipolar strain-electric field (S-E) hysteresis loops was 69 and 117 pm/V for non-textured and textured KNN ceramics respectively.

Structure Control of NaNbO3 Template for Textured Ceramics Synthesized by Two-Step Molten Salt Method

In this paper, through control the precursor's structure with layer morphology, the plate-like particles with different edge length and thickness can be syntheszed by the two step molten salt (MSS) method. The results of SEM showed that the NaNbO3 particles were characterized by platelet-like morphology with an edge length of 10∼20 μm and a thickness of 1∼4 μm. The results of XRD indicated the NaNbO3 particles synthesized had a strong orientation in {0 0 l} crystal planes. The plate-like NaNbO3 particles we synthesized are a suitable template candidate to fabricate textured ceramics by TGG or RTGG method, especially to the textured potassium sodium niobate lead-free piezoelectric ceramics.

Synthesis of polycrystalline platelike NaNbO3 particles by the topochemical micro-crystal conversion from K4Nb6O17 and fabrication of grain-oriented (K0.5Na0.5)NbO3 ceramics

Topochemical microcrystal conversion (TMC) method is a powerful tool to synthesize platelike microcrystal particles with a regular-perovskite crystal structure, which is difficult to be fabricated by conventional flux techniques. By using the TMC method, polycrystalline rectangular-platelike NaNbO3 particles with a orthorhombic perovskite structure were able to be synthesized from platelike precursor particles of layer-structured K4Nb6O17 at 1000°C in molten NaCl-salt. The TMC-synthesized NaNbO3 particles preserved the shape of precursor particles, and had a thickness of about 1 micron and a width of 5-10 microns. However TMC-synthesized platelike NaNbO3 particles had a polycrystalline morphology having a preferred pseudo-cubic {100} orientation. Oriented particulate layer X-ray diffraction (OPL-XRD) analysis revealed that, during the TMC reaction, the crystallographic {010} plane of K4Nb6O17 is converted to the most of {001} plane of polycrystalline NaNbO3 particles in spite of polycrystalline morphology. Using the polycrystalline platelike NaNbO3 particles as a template in the reactive templated grain growth method (RTGG), {001} grain-oriented (K0.5Na0.5)NbO3—1 mol% CuO ceramics having a {001} orientation degree (Logering’s factor) of 45% could be fabricated. The result indicates that not only single crystalline particles, which were generally used, but also the polycrystalline particles can be act as template in the RTGG process. The availability of polycrystalline particles will give a new design of synthesizing templates for texturing of various kinds of perovskite crystal-structured ceramics.