Microstructure and Dielectric Property of KNbO3 Ceramics with KVO3 Addition
Microstructure and Dielectric Property of KNbO3 Ceramics with KVO3 Addition
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添加KVO3的KNbO3陶瓷的微观结构和介电性能
DOI：
10.4028/www.scientific.net/amr.11-12.105
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发表时间：
2006
期刊：
影响因子：
--
通讯作者：
H. Ohsato
中科院分区：
文献类型：
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作者：
K. Kakimoto;K. Higashide;H. Ohsato
研究了KVO3对KNbO3陶瓷晶粒生长和介电性能的影响。少量KVO3的加入提高了煅烧起始混合物生成KNbO3的反应动力学。当KVO3含量达到0.10 wt%时，KNbO3陶瓷的平均晶粒尺寸急剧增大。当kvo3添加量为0.01 wt%时，KNbO3的平均晶粒尺寸约为1 μm，机电耦合比kp=0.33, kt=0.48。介质居里-魏斯分析还表明，少量KVO3的加入改变了居里温度下的相变扩散程度，降低了介质损耗正切。KNbO3是一种钙钛矿型铁电体，与BaTiO3相似，随着温度的升高而发生相变，但在室温以上的相变温度要高得多；也就是说，铁电（FE）正交到铁电四边形为225°C，铁电四边形到准电（PE）立方跃迁为435°C。因此，KNbO3及其近亲被认为是有前途的无铅候选者，可以在高温压电应用中使用。然而，通过普通的无压烧结制备致密的KNbO3陶瓷是困难的，因为(1)烧结过程必须在接近KNbO3的低熔点（1040°C）的温度下进行，导致氧化钾（K2O）大量汽化，限制了化学测量控制，(2)不稳定的二次相在暴露于水时容易形成并表现出潮解。这些严重的问题阻碍了KNbO3陶瓷的完全致密化。为了避免这一问题并提高压电性能，已经成功提出了几种可以与KNbO3形成固溶体的新型系统[2,3]。从KNbO3-KVO3伪二元体系的相图可以看出，随着KVO3含量[4]的增加，液相温度降低，KVO3可以作为一种助熔剂组分，适合于较大畴尺寸的KNbO3晶体的生长。这可能表明KVO3的加入可以改变KNbO3陶瓷的晶粒尺寸。一般来说，陶瓷的介电性能是由晶粒尺寸和化学成分控制的。然而，据我们所知，尚未对KNbO3体系的晶粒尺寸与介电性能之间的关系进行过研究。本研究的目的是研究KVO3对KNbO3陶瓷的加性效应，因为有报道称BaTiO3陶瓷的介电性能强烈依赖于晶粒尺寸，并得出室温下介电常数在晶粒尺寸约为1 μm时最大，这被认为是由铁电畴90的最大有效宽度[5]引起的。实验程序：样品制备。采用高纯度的K2CO3、Nb2O5和V2O5粉末源，分别在820℃和460℃下煅烧kbo3和KVO3前驱体。Advanced Materials Research Online: 2006-02-15 ISSN: 1662-8985, vol .11-12, pp 105-108 doi:10.4028/www.scientific.net/AMR.11-12.105©2006 Trans Tech Publications Ltd， Switzerland版权所有。未经Trans Tech Publications Ltd， www.scientific.net的书面许可，不得以任何形式或任何方式复制或传播本文的部分内容。（semanticscholar.org/20/03/20,09:20:04）根据KNbO3 + xKVO3的公式组成，其中x从0到10 wt%变化。称重后的粉末在丙酮中球磨24小时，然后通过300目筛网筛分，并单轴形成直径为12毫米的圆盘形状。在200 MPa下进行冷等静压，在1020℃下烧结。以上热处理均在富k2o气氛下进行。鉴定。用Cu - Ka辐射x射线粉末衍射（XRPD）在室温下鉴定了晶体的物相。采用全模式粉末分解（WPPD）方法对晶格参数进行细化。通过扫描电子显微镜（SEM）的显微照片来评估平均晶粒尺寸。对于电测量，银膏涂在样品的重叠表面作为电极。使用LCR计（NF ZM2355）获得了冷却过程中介电常数与温度的关系。在硅油浴中，施加5 kV/mm的直流电场，在135℃下进行压电测量。基于IEEE标准，利用阻抗分析仪（Agilent 4294A）采用共振-反共振法测定了平面模式（kp）和厚度模式（kt）的机电耦合系数。结果与讨论。图1为KNbO3 + x KVO3陶瓷的XRPD图。x=1.00 wt%及以下样品的衍射峰均属于KNbO3的正交对称（JCPDS 32-0822）。在实验误差范围内，进行高精度精化时，无法识别出晶格参数的明显变化。改进后的晶格参数为a=5.693(3), b=5.719(3), c=3.974(2) Å。另一方面，对于x=10.00 wt%的样品，K2O和KVO3相在XRPD模式中分别以27度和46度的衍射角（2θ）出现。该样品由于烧结后的膨胀而变形，这可能是由大量液相形成或剩余K2O相的吸收性引起的。而KVO3含量较低的其他样品在机械抛光过程中对水没有潮解现象。KNbO3 + xKVO3陶瓷的平均晶粒尺寸变化如图2所示。当KVO3含量增加到0.10 wt%时，晶粒尺寸从0.6 μm急剧增大到2.7 μm；当KVO3含量增加到0.10 wt%以上时，晶粒尺寸在3 μm左右基本饱和。少量KVO3的加入显著提高了焙烧后的knbo3的反应动力学。KNbO3 + x KVO3陶瓷的x=10.00 x=1.00 x=0.70 x=0.50 x=0.10 x=0.05 x=0.01 x=0 K2O KVO3在ns it y (ar b. un it s) 2θ（度）60 50 40 30 20 In ns it y (ar b. un it s) 106 AICAM 2005
The effect of KVO3 addition on the grain growth and dielectric properties of KNbO3 ceramics has been investigated. Small addition of KVO3 enhanced the reaction kinetics of the calcined starting mixture to form KNbO3. The mean grain size of the KNbO3 ceramics showed a sharp increase with KVO3 content up to 0.10 wt%. The 0.01 wt% KVO3–added KNbO3 showed an average grain size of around 1 μm, and demonstrated the enhanced electromechanical coupling ratios of kp=0.33 and kt=0.48. Dielectric Curie-Weiss analysis also indicated that the degree of the phase transition diffuseness at Curie temperature was modified and dielectric loss tangent was reduced by a slight amount of KVO3 addition. Introduction KNbO3 is a perovskite ferroelectrics showing a sequent phase transition similar to BaTiO3 with temperature, but shows much higher transition temperatures above room temperature; i.e., 225°C for ferroelectric (FE) orthorhombic to FE tetragonal and 435°C for FE tetragonal to paraelectric (PE) cubic transitions [1]. Therefore, KNbO3 and its relatives are considered to be promising lead-free candidates which can service at high temperature in piezoelectric applications. However, the preparation of dense KNbO3 ceramics is difficult by ordinary pressure-less sintering, because (1) the sintering process must be carried out at temperature close to the low melting point (1040°C) of KNbO3, resulting in the massive vaporization of potassium oxide (K2O) to limit the stoichiometric control, and (2) unstable secondary phases tend to form and show deliquescence at times when exposed to water. Such serious problems prevent KNbO3 ceramics from full densification. To avoid this problem and to enhance the piezoelectric performance, several kinds of novel systems that can form solid solution with KNbO3 have been proposed successfully [2,3]. According to the phase diagram of KNbO3-KVO3 pseudobinary system, the liquidus temperature was lowered with increasing KVO3 content [4], and it is known that KVO3 can work as a flux component suitable for the growth of KNbO3 crystals with larger domain sizes. This may indicate that KVO3 addition can modify the grain size of KNbO3 ceramics. In general, the dielectric properties of ceramics are controllable by grain size as well as by chemical composition. To the best of our knowledge, however, the study on the relationship between grain size and dielectric properties has never been conducted on KNbO3 system. The aim of this study is to investigate the additive effect of KVO3 to KNbO3 ceramics, since it is reported that the dielectric property of BaTiO3 ceramics strongly depends on the grain size, and concluded that the dielectric constant at room temperature has a maximum for a grain size of about 1 μm, which was considered to be resulted from the maximum effective width of ferroelectric 90 domains [5]. Experimental Procedure Sample Preparation. KNbO3 and KVO3 precursors were calcined at 820 and 460°C, respectively, using high-purity powder sources of K2CO3, Nb2O5 and V2O5. These precursors were mixed to obtain Advanced Materials Research Online: 2006-02-15 ISSN: 1662-8985, Vols. 11-12, pp 105-108 doi:10.4028/www.scientific.net/AMR.11-12.105 © 2006 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-20/03/20,09:20:04) the compositions according to the formula of KNbO3 + xKVO3, where x is varied from 0 to 10 wt%. The weighed powders were ball-milled for 24 h in acetone, followed by the sieving through a 300-mesh screen and by the uniaxially forming of a disk shape with 12 mm diameter. The disk was cold-isostatic-pressed under 200 MPa and sintered at 1020°C. All the above heat treatments were carried out under K2O-rich atmosphere. Characterization. The crystal phase was identified at room temperature by X-ray powder diffraction (XRPD) using Cu Ka radiation. The lattice parameter was refined by a whole pattern powder decomposition (WPPD) method. The mean grain size was evaluated from the micrographs taken by a scanning electron microscope (SEM). For electric measurement, silver paste was painted on the lapped surfaces of the specimens as electrodes. The dielectric constant was obtained as a function of temperature during the cooling process using an LCR meter (NF ZM2355). Specimens for the piezoelectric measurements were poled at 135C in a silicon oil bath by applying a dc electric field of 5 kV/mm. The electromechanical coupling factors for planar (kp) and thickness (kt) modes were determined from the resonance-antiresonance method on the basis of IEEE standards using an impedance analyzer (Agilent 4294A). Results and Discussion Grain Growth. Figure 1 shows the XRPD patterns of KNbO3 + x KVO3 ceramics. All the diffraction peaks observed in the specimens with x=1.00 wt% or less belong to an orthorhombic symmetry of KNbO3 (JCPDS 32-0822). A clear change in the lattice parameter could not be recognized in the range of the experiment error, when the high accuracy refinement was carried out. The refined lattice parameters were a=5.693(3), b=5.719(3) and c=3.974(2) Å. On the other hand, K2O and KVO3 phases additionally appear at diffraction angles (2θ) of 27 and 46 degrees in the XRPD pattern, respectively, for the sample with x=10.00 wt%. This sample was deformed considerably due to swells after the sintering, which may have been caused by a massive liquid-phase formation or the absorbency of the remained K2O phase. However, the other specimens with lower KVO3 contents demonstrated no deliquescence against water in the mechanical polishing. The variation of the mean grain size measured in the KNbO3 + xKVO3 ceramics is shown in Fig. 2. The mean grain size showed a sharp increase from 0.6 to 2.7 μm with KVO3 content up to 0.10 wt%, then it was almost saturated at the size around 3 μm with increasing the KVO3 content above 0.10 wt%. Small addition of KVO3 significantly enhanced the reaction kinetics of the calcined K N b O 3 Fig. 1. XRPD patterns of of KNbO3 + x KVO3 ceramics x=10.00 x=1.00 x=0.70 x=0.50 x=0.10 x=0.05 x=0.01 x=0 K2O KVO3 In te ns it y ( ar b. un it s) 2θ (degree) 60 50 40 30 20 In te ns it y ( ar b. un it s) 106 AICAM 2005
影响因子：
4
作者：
Guo, YP;Kakimoto, K;Ohsato, H
通讯作者：
Ohsato, H