填满型钨青铜结构SNN基无铅织构陶瓷的制备及电性能研究
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摘要
随着人类社会可持续发展的要求,为了尽可能解决铅基压铁电陶瓷广泛应用带来的环境污染问题,研究和开发无铅压铁电陶瓷材料己成为一项迫切的、具有重大社会和经济价值的课题。目前,对于无铅压铁电陶瓷的研究虽然已经取得了一定进展,但是无铅压铁电陶瓷的性能远不如铅基压铁电陶瓷。如果能将无铅压铁电陶瓷的微观结构调节与宏观结构设计相结合,则有可能最大程度地提高材料的电学性能。另外,在无铅压铁电材料体系中,钨青铜结构压铁电体由于具有较优的电学性质、电光性质、非线性光学性质以及变晶相界等特性,同时比较容易获得优质且具有大尺寸的晶体材料,因此被广泛应用于存储记忆、激光倍频、电光调制、超导湿度传感器和光学信息处理等领域。
其中,填满型钨青铜结构的Sr2NaNb5O15(SNN)基化合物具有优异的介电、铁电、光电和光折射性能,是无铅压铁电领域中最具有研究潜力的材料之一。但SNN基陶瓷材料存在着烧结温度过高、容易出现生长异常的过大尺寸晶粒和裂纹以及压电和介电性能相对较低等缺点,并且较难制备出具有良好致密性且性能较优的产品。本文在传统固相法的基础上采用二步烧结合成技术成功制备了致密性优良的SNN、Sr2KxNa1-xNb5O15(SKNN)、Sr2-xCaxNaNb5015(SCNN)等压铁电无取向陶瓷,并且从烧结工艺、配方设计和掺杂改性等方面进行系统研究,探讨材料组分、微观结构和宏观性能之间的关联性。具体研究过程为：首先研究工艺条件对体系相结构、微观形貌、密度以及介电和铁电性能的影响,筛选出最佳工艺条件；同时还进行组分设计,选择了K、Ca和Cu0分别作为取代和掺杂剂,对基础组分进行了A位取代和掺杂研究,以期达到控制异常晶粒生长、避免陶瓷表面裂纹出现、降低烧结温度、降低介电损耗、提高陶瓷致密度和优化电性能的目的。
通过以上微观组分研究为宏观结构设计(即陶瓷材料的织构化)提供较为理想的材料体系。然后,通过熔盐法采用SrNb2O6-Nb2O5-KCl体系制备纯四方相钨青铜结构、长径尺寸比适当、形貌非对称性高的棒状SKN粉体作为织构技术的模板晶粒,探讨SrNb2O6-Nb2O5-KCl体系中的反应机理和棒状SKN粉体的生长机制。同时研究棒状SKN晶种与SNN主晶相之间的结构匹配性,提出SNN晶体沿棒状SKN晶种的界面传质与取向附生机制。最后,通过模板晶种SKN和其他反应物混合,采用反应模板晶粒生长技术(RTGG)来制备SNN基无铅压铁电织构陶瓷。系统研究该织构过程中模板生长动力学、相形成及相演变、微观组织演变、织构历程、致密化行为以及液相烧结助剂对织构陶瓷烧结行为的影响规律；探讨晶粒的定向生长机制；提出SNN基陶瓷织构的反应机理；此外,还系统研究CuO掺杂、A位Ca取代与陶瓷的织构质量以及陶瓷电性能之间的关系；最终获得高取向度且具有高性能的SCNN无铅压铁电织构陶瓷。通过实验研究和理论分析,得到了以下一系列具有显著创新性的研究成果：
1.在传统固相法基础上采用二步烧结合成技术制备填满型钨青铜结构SNN基无铅压铁电陶瓷材料,成功解决该材料体系难以致密化的难题,并避免了异常晶粒和裂纹的出现,得到了具有较优性能的组分体系。通过研究工艺条件对SNN陶瓷的相结构、微观结构、密度、介电和铁电性能的影响,结果发现：当反应温度为1100℃时,SrNb206和NaNbO3开始反应生成SNN,当反应温度为1180℃时可获得纯相的SNN粉体,但是过高的烧结温度1360℃便会导致SNN相的分解。SNN相在高温下的分解会进一步导致陶瓷表面出现大面积的液相熔融区域和裂纹,从而降低致密度,并恶化所获SNN陶瓷的电性能。因此,制备致密化且具有较优性能的SNN无铅压铁电陶瓷的最佳制备条件是：预烧温度为1180℃,烧结温度为1340℃。此条件下获得的SNN陶瓷其相对密度最大,晶粒大小均匀,气孔数较少,电性能最佳：εr=1325,εm=2210,Tc=280℃,tanδ=0.023,Pr=11.71μC/cm2,Ec=16.15kV/cm。同时,还研究了A位K取代量对SKNN陶瓷的微观结构和电性能影响,结果表明：适当的K取代量能够促进陶瓷的致密化,使陶瓷晶粒生长饱满、晶界清晰,同时还可以提高SKNN陶瓷的介电性能。但是过高的K取代量会促使陶瓷在高温烧结过程中容易出现液相区域,从而导致陶瓷致密化变得越来越困难,陶瓷表面也伴随出现了比较严重的析晶现象,从而恶化陶瓷的电性能。因此,制备致密化且具有较优性能的SKNN无铅压铁电陶瓷其最佳K取代量的范围和制备条件是：x=0.05～0.10,预烧温度为1160℃,烧结温度为1320℃。
2.采用熔盐法发现在SrNb206-Nb2O5-KCl体系中可以获得纯四方相钨青铜结构、长径尺寸比适当、形貌非对称性高的棒状SKN粉体作为钨青铜结构陶瓷织构化的模板晶粒,提出了反应机理和生长机制。采用熔盐法在SrN2O6-Nb2O5-KCl体系中,当SrNb2O6与Nb2O5物质的量之比为1.0,熔盐KCl与氧化物质量之比为1.5时,在1150℃预烧6h时,获得了纯四方相钨青铜结构、长径尺寸比适当、形貌非对称性高的棒状SKN模板晶粒。此SKN模板无论在纯度还是形貌上其可重复性都很好,且尺寸较为均匀,可以很好地作为制备织构陶瓷的模板晶粒。熔盐法制备SKN模板的反应机理为：SrCl2与Nb205之间的反应比SrCl2与SrNb206之间的反应更容易进行。随着原料中Nb2O5含量的增加,SrCl2会和Nb205优先进行反应,从而减少了其与SrNb206反应生成杂相Sr2Nb207的机会。因此在SrNb2O6-KCl体系的基础上加入适量的Nb205就可以获得纯相的SKN。熔盐法获得的SKN模板其棒状形貌的生长机制为：在SrNb2O6-Nb2O5-KCl熔盐体系中SKN的形貌主要受两个过程的控制和影响：首先是反应过程,其次是晶体生长过程。反应过程与SrNb2O6在KCl中的溶解有关,如果反应物中SrNb2O6的含量比它在KCl中的溶解度高,部分未溶解的SrNb2O6粉体就会团聚在一起而无法在液相熔盐中得到充分分散。由于SrNb2O6既是生成SKN的反应物,又是SKN晶体形成的初始晶种,因此就导致了SKN形成时多个晶核位的共同存在。SKN的生长过程主要与钨青铜结构晶体的生长习性有关。对钨青铜结构晶体来说,其(001)面由于具有较高的表面能,生长速度比其他几个面要快,从而获得表面呈现台阶状的棒状形貌。
3.开展了棒状SKN晶种与SNN主晶相之间的结构匹配性研究,并提出了SNN晶粒沿棒状SKN晶种进行取向附延生长的机制。采用反应烧结技术在不同体系R0(不含有SKN晶种),R10(含有10mol.%棒状SKN晶种)和C10(含有10mol.%等轴SKN晶种)中,相形成过程均发生在致密化行为之前,且最高相对密度都达到了95%以上。另外,棒状SKN晶种和等轴SKN晶种的存在都可以促进SNN主晶相的形成,也可以促进陶瓷的致密化过程。在R10体系中,当烧结温度为1340℃时,陶瓷表面出现了尺寸较大(50～100μm),生长各向异性且形貌类似于棒状SKN晶种的SNN晶粒。在C10体系中,仅出现了尺寸在15μm左右的非等轴晶粒。而在R0体系中,并没有出现任何生长各向异性的陶瓷晶粒。R10体系中形貌类似于棒状SKN晶种的SNN晶粒的出现可用液相辅助传质模型来解释：随着烧结温度的升高,在SKN模板与SNN晶粒的交界处优先形成一层液相膜,液相膜的形成促进了K+、Na+在两物相中的相互扩散。这种K+、Na+的扩散导致SKN晶种的棒状形貌逐渐模糊并溶解消失,同时也促使液相区域的面积逐渐变大并形成棒状轮廓。钨青铜结构晶粒的自身生长习性、棒状轮廓液相区域的形成以及SKN晶种的棒状结构骨架都为生长各向异性的SNN晶粒的形成和生长提供有利因素,因而得到了形貌类似于棒状SKN晶种的SNN晶粒。说明棒状SKN模板与SNN晶体之间存在着很好的结构匹配性。
4.率先将RTGG技术用于制备填满型钨青铜结构的SKNN无铅压铁电织构陶瓷,获得最佳织构化条件。通过研究工艺条件对SKNN织构陶瓷相结构演变、织构度、致密化行为、微观结构演变和电性能的影响,结果发现：在RTGG技术制备织构陶瓷的过程中,随着烧结温度的升高和保温时间的延长,SKNN织构陶瓷的致密化程度及织构度均存在一极大值,超过极大值所对应的温度和保温时间,材料的致密化程度下降,织构度降低,且在陶瓷表面出现液相熔融迹象。获得了SKNN织构陶瓷的最佳织构化条件：烧结温度为1360℃,保温时间为6h,此时织构因子最大为82%。通过研究烧结温度和保温时间对SKNN织构陶瓷沿c轴方向和a/b轴方向电性能的影响规律发现：SKNN织构陶瓷其介电性能、铁电性能和压电性能都表现出明显的各向异性。当烧结温度为1360℃,保温时间为6h时,所获得的SKNN织构陶瓷沿c轴方向的常温介电常数εr,最大介电常数εm,剩余极化强度Pr和压电常数d33都明显高于a/b轴方向,且分别为a/b轴方向的1.42倍,4.20倍,5.67倍和5.38倍。另外,对比传统固相法(CMO)制备的SKNN无取向陶瓷的电性能与采用RTGG技术获得的SKNN织构陶瓷在c轴方向上的电性能发现：SKNN织构陶瓷沿c轴方向的各种电性能均明显高于无取向陶瓷。SKNN织构陶瓷其常温介电常数ε和最大介电常数εm,分别为无取向陶瓷的1.67和2.85倍。与SKNN无取向陶瓷相比,SKNN织构陶瓷的矫顽场略有降低,但其沿c轴方向剩余极化强度Pr为无取向陶瓷剩余极化强度Pr的2.94倍。SKNN织构陶瓷沿c轴方向的压电常数d33(86pC/N)为无取向陶瓷压电常数d33(46pC/N)的1.87倍,表明织构技术可以大幅度提高陶瓷的性能。
5. SKNN织构陶瓷的织构机制：随烧结温度的升高,SKNN织构陶瓷微观组织结构的变化分为三个阶段：烧结初期为固相反应阶段,表现为基体原料之间的相互交联以及SNN相与SKN模板之间的侵蚀反应；烧结中期为致密化阶段,表现为陶瓷表面气孔数的逐渐减少,陶瓷相对密度的急剧增大；烧结后期为织构化过程,表现为在较高的烧结温度下可以得到沿流延方向定向生长且具有类似于SKN模板棒状形貌的SKNN织构陶瓷。RTGG技术制备SKNN织构陶瓷的织构机理为：围绕在SKN棒状模板周围的SNN粉体与SKN棒状模板通过液相侵蚀生成的SKNN相具有类似于棒状模板的非等轴形貌,以此为织构行为产生的新模板,从而导致新模板周围的等轴SKNN晶粒在具有棒状形貌的非等轴SKNN模板晶粒周围进行趋向附延生长,从而获得织构形貌。
6.在SKNN无取向陶瓷中引入烧结助剂CuO,为进一步制备SKNN-CuO织构陶瓷确定最佳CuO含量。系统研究了CuO含量对SKNN陶瓷的相结构、显微结构、密度、介电性能和铁电性能的影响,提出了CuO掺杂的作用机制。结果发现：较低含量的CuO能完全进入晶格相,从而使所有组分都能获得纯相四方钨青铜结构的SKNN陶瓷,同时没有引起陶瓷晶格的畸变和相结构的转变。适当的CuO掺杂量可以提高陶瓷的致密化程度,但过多含量的CuO反而会导致陶瓷表面出现尺寸较大的晶粒从而降低陶瓷的致密度。另外,SKNN陶瓷的电性能与CuO的掺杂量密切相关。当CuO的掺杂量为0.015时,陶瓷具有较佳的电性能,此时陶瓷晶粒大小均匀,晶界清晰,陶瓷相对密度最大为99.6%,此时常温介电常数εr、最大介电常数εm和剩余极化强度Pr都获得最大值：εr=1160,εm=1573,Pr=4.82μC/cm2。同时发现在SKNN无取向陶瓷体系中,CuO同时起到液相助剂和受主掺杂的作用。
7.采用RTGG技术成功制备了SKNN-CuO织构陶瓷,提高了织构陶瓷的织构度,并进一步探讨了织构机制。系统研究了SKNN-CuO织构陶瓷在陶瓷织构过程中的烧结行为、致密化行为、相结构演变、微观结构演变以及模板生长过程中的织构度随烧结温度和保温时间的变化规律。结果表明：在RTGG-CuO织构陶瓷的相形成过程中,CuO的存在不仅促进了SNN相的形成,加快了陶瓷的致密化、降低了烧结温度,同时还提高了织构陶瓷的取向度,从而在较低的烧结温度1340℃保温6h便可获得织构因子高达86%的织构陶瓷。CuO促进SKNN陶瓷织构化的机制为：围绕在SKN棒状模板周围的SNN粉体与SKN棒状模板通过液相侵蚀生成的SKNN相具有类似于棒状模板的非等轴形貌,当反应体系中存在适量的CuO液相助剂时,会更利于Na+的扩散和具有棒状形貌的SKNN二次模板的形成,正是适量的液相使得等轴SKNN晶粒在具有棒状形貌的非等轴SKNN模板晶粒周围进行趋向附延生长的区域有所扩大,从而提高了陶瓷的织构度。进一步验证了织构机制。
8.研究了SKNN-CuO织构陶瓷其电性能的各向异性,并将SKNN-CuO织构陶瓷与SKNN织构陶瓷的性能进行对比分析。通过研究烧结温度和保温时间对SKNN-CuO织构陶瓷沿c轴方向和a/b轴方向各种电性能的影响规律发现：SKNN-CuO织构陶瓷其介电性能、铁电性能和压电性能具有显著的各向异性。SKNN织构陶瓷沿c轴方向的常温介电常数εr,最大介电常数εm,剩余极化强度Pr和压电常数d33都明显高于a/b轴方向。在最佳条件下获得的SKNN-CuO织构陶瓷在c轴方向上的各项电性能均明显高于SKNN-CuO无取向陶瓷。其沿c轴方向的常温介电常数εr和最大介电常数εm分别为无取向陶瓷的1.88和3.09倍。其沿c轴方向剩余极化强度pr为无取向陶瓷剩余极化强度pR的3.18倍。沿c轴方向的压电常数d33为无取向陶瓷压电常数d33的1.91倍。对比SKNN织构陶瓷与SKNN-CuO织构陶瓷的电性能可以看出,CuO虽然降低了烧结温度,并提高了织构陶瓷织构度。但是,引入CuO之后,织构陶瓷的最大介电常数,居里温度,剩余极化强度和压电常数都有所降低。说明CuO的引入无法同时兼顾提高织构度和优化电性能的双重目的。
9.通过A位取代,获得了同时具有高取向度和高性能的SCNN织构陶瓷,并成功实现了RTGG技术在其他钨青铜结构化合物体系的应用。首先选用传统固相法采用二步反应烧结技术成功制备出了不同Ca取代量的SCNN无取向陶瓷,并系统研究了Ca取代量对陶瓷的相结构、显微结构、密度、介电性能和铁电性能的影响。结果表明当Ca取代量为0.15时,获得了晶粒生长同性、尺寸大小均匀、致密性良好的陶瓷,其最佳电性能为：Tc=285℃,εm=1954,Pr=7.08μC/cm2。同时,采用RTGG技术成功制备了SCNN织构陶瓷,当烧结温度1340℃、保温6h时,便可获得织构因子高达88%的织构陶瓷。对比SKNN织构陶瓷、SKNN-CuO织构陶瓷与SCNN织构陶瓷发现,Ca取代A位Sr之后的织构陶瓷,其织构因子最大为88%,且其沿c轴方向的各项电性能均优于SKNN织构陶瓷和SKNN-CuO织构陶瓷。说明Ca的引入既可以很好地起到烧结助剂的作用,提高了织构陶瓷的致密度和取向度,同时也可以明显提高织构陶瓷的电性能。这个工作实现了RTGG技术在其他钨青铜结构化合物体系中的成功应用。
Due to the air pollution caused by the application of Pb-based materials, it is very important and necessary to study the lead-free piezoelectrics. For the polycrystalline ceramics, the physical properties are mainly controlled by the composition, processing conditions and the microstructure. Through microstructure control to make the ceramic grains grow along the preferred orientation (texturing ceramics) is an important approach to enhance the physical properties. In this thesis, Sr2NaNb5O15(SNN)-based ceramics with filled tungsten bronze structure were selected as the based system. It is well known that the tungsten bronze niobate ceramics are difficult simultaneously in obtaining high density and good electrical properties by traditional sintering methods. In order to solve the above problems, research on the micro structure design and macro structure design were carried out.
Firstly, for micro structure design, the two-step solid state reaction method and traditional sintering process were used to prepare random SNN-based ceramics. The effects of preparing conditions, effects of substitution of K for Na in A sites on the phase structure, microstructure and electrical properties were studied to improve the density, restrain the abnormal grain growth, reduce the sintering temperature and also optimize the electrical properties. Then the acicular Sr2KNb5O15(SKN) template was prepared by molten salt synthesis method in SrNb2O6-Nb2O5-KCl system. The reaction and growth mechanisms in SrNb2O6-Nb2O5-KCl system were also proposed. In addition, in order to discuss the structure matching between the acicular SKN template and the SNN phase, how the seeding SKN particles affect the phase formation, densification and microstructure development in SNN ceramics was also studied. Finally, the macro structure design was carried out by mixing the SKN template and other reactant materials. The reactive templated grain growth (RTGG) method was employed to obtain the textured SNN-based ceramics. The preparing conditions on the phase formation, densification, micro structural evolution, texture development and electrical properties were discussed. The oriented grain growth mechanism and reaction mechanism were proposed. The sintering aids effects during sintering process were also discussed to improve the texture fraction. Through the above research work, the Sr1.85Ca0.15NaNb5O15(SCNN) textured ceramics with high texture quality and optimized electrical properties were obtained. The main conclusions obtained from experimental results and analyses were as follows:
1. The high-density tungsten bronze SNN-based ceramics without abnormal grain growth were successfully synthesized by two-step solid state reaction method. The phase evolution and reaction mechnism in SrNb2O6and NaNbO3solid solution were analyzed by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results showed that the Sr2NaNb5O15phase could exist stably in the temperature range of1100～1360℃, and would be decomposed to SrNb2O6and NaNbO3phases at temperatures above1360℃. It was also found that the sintering temperature could significantly affect the density, microstructure, and electric properties of SrNaNb5O15ceramics. When the sintering temperature was1340℃, the ceramic with a near-theoretical density and a uniform and fine-grained microstructure was obtained, exhibiting outstanding electrical properties:εm=2210, tan δ=0.023, Tc=280℃, Pr=11.71μC/cm2, and Ec=16.15kv/cm. Further increasing the temperature to1360℃led to the existence of some molten areas and cracks on the surface of obtained ceramics, which could significantly deteriorate the electric properties. In addition, K substitution for Na in A sites could accelerate the phase formation at lower temperatures. The lattice constant calculation indicated expansion of the unit cell and reduced the distortion of the crystal structure with K substitution due to the bigger ionic size of K+(1.64A) compared to that of Na+(1.39A). Electrical properties of Sr2KxNa1-xNb5O15ceramics greatly depended on the K content. Curie temperature Tc shifted downward, whereas the maximum dielectric constant εm and the degree of diffusion phase transition all increased initially and then decreased as K content increased, indicating that proper amount of K substitution with x between0.05and0.10could enhance the dielectric properties.
2. The well-developed acicular SKN templates were obtained by molten salt synthesis (MSS) method using SrNb2O6-Nb2O5-KCl system. The reaction and growth mechanisms were also proposed. When synthesized in molten KCl salt at1150℃for6h with the molar ratio of SrNb2O6and Nb2O5is1and the weight ratio of salt to oxide sources was1.5, pure SKN particles with well-developed acicular morphology were successfully obtained in this system. They were agglomerate-free and with proper scale in size range of5-30μm, making them to be the ideal template for fabricating textured ceramics. The reaction mechanisms were as follows:5SrNb2O6+2KCl=2Sr2KNb5O15+SrCl2(1) SrCl2+SrNb2O6+1/2O2=Sr2Nb207+Cl2↑(2) SrCl2+Nb2O5+1/2O2=SrNb2O6+Cl2↑(3) The reaction between SrCl2and Nb2O5(3) with lower reaction Ea than reaction (2) could limit the synthesis of Sr2Nb2O7to some extent. Then the pure SKN without any impurity was obtained. The growth mechanism was:the SKN morphology was controlled initially by the formation process and later by the growth process. The formation process was related to the solubility of SrNb2O6in KCl salt. If the SrNb2O6amount in precursor mixture was more than its solubility in KCl salt, they would not be well diffused in KCl salt liquid and clumped together to form the multiple nucleation sites for SKN crystal. The subsequent growth was in accordance with the growth habits of SKN crystals with tungsten bronze structure which growed faster along c direction than along any other axis direction.
3. The structure matching between the acicular SKN template and the SNN phase was studied. The liquid-phase-assisted growth mechanism was proposed. The phase formation, densification behavior and microstructure development of SNN ceramics in the10mol.%acicular SKN seed-containing system (R10), the10mol.%equiaxed SKN seed-containing system (CIO), and the unseeded system (R0) were discussed. The results showed that the acicular and isometric SKN seeds could not only accelerate the SNN phase formation but also promote the densification at lower temperatures. In R10system, some large anisotropic grains (>80μm) could be obtained at definite sintering temperature. In CIO system, only some small anisotropic grains (<20μm) were obtained. However, there were no such anisotropic grains obtained in the SKN-free R0system. The growth mechanism of the large anisotropic grain (>80μm) was summarized as liquid-phase-assisted growth mechanism, which was that the SKN seed prepared by molten salt synthesis method could give rise to the formation of a liquid phase and provide a structural framework for the growth of ceramic grains, leading to the existence of large anisotropic grains in the ceramic when sintered at1340℃.
4. The SKNN textured ceramics were prepared by reactive template grain growth method. The optimized preparing condition was obtained. The phase formation, texture fraction, microstructure development, densification behavior and electrical properties as a function of processing parameters such as sintering temperature and holding time were investigated in detail. With increasing the sintering temperature and prolonging the holding time, the texture fraction all increased at first and then decreased. The maximum of82%was obtained when sintered at1360℃for6h. The SKNN textured ceramics showed obvious anisotropy behavior in electrical properties:εr, εm, Pr and d33in c-axis direction samples of the SKNN textured ceramics were much higher than those in a/b-axis direction samples. Compared with the SKNN random ceramics and textured ceramics, it was also found that the electrical properties in c-axis direction samples of the SKNN textured ceramics were much higher than those of SKNN random ceramics:the εT, εm, Pr and d33were about1.67,2.85,2.94and1.87times as much as those of SKNN random ceramics, respectively. The above results showed that the reactive template grain growth could improve the electrical properties of ceramics obviously.
5. The oriented grain growth mechanism in SKNN textured ceramics was proposed. It was found the whole microstructure development process can be divided into3steps:(1) the phase formation stage,(2) the densification stage, and (3) the texture development stage. At the later stage of the SNN phase formation, the interdiffusion of ions led to the phase formation of SKNN, which was occurred between SNN particles and acicular SKN templates due to the chemical gradient. It was also reasonable to expect that some liquid phase around SKN templates caused by the coexistence of higher concentration of K+and Na+would accelerate the interdiffusion of ions. The obtained SKNN phase showed two different kinds of morphology:the SKNN particles formed around SKN templates showed acicular morphology; the SKNN particles far away from SKN templates showed equiaxed morphology. The stable acicular shape provided the structural framework for the anisometric SKNN grains that could serve as secondary templates because final SKNN phase and SKN template were all tetragonal tungsten bronze structure. So the formation of the texture microstructure in c-direction could continue on the original template particles by epitaxy after densification. Then the texture development was further driven when the samples were sintered at higher temperatures and longer holding times.
6. Introduction of CuO sintering aid could improve the electrical properties of SKNN random ceramics. The effects of CuO content on phase formation, microstructure, density and electrical properties were studied. It was found that lower amount of CuO could accelerate the SKNN phase formation and improve the densification. But higher amount of CuO led to the decrease in densification and the existence of some liquid phases. The SEM micrographs showed that with increasing CuO content the average grain size increased, but no abnormal grain growth could be found. The electrical properties of SKNN-CuO random ceramics depended greatly on the CuO amount. When CuO amount=0.015mol, the electrical properties of the ceramics were better:εr=1160, εm=1573and Pr=4.82μC/cm, showed that CuO served as both sintering aid and acceptor doping.
7. Templated grain growth method was employed to fabricate SKNN-CuO textured ceramics, further discussing the oriented grain growth mechanism. It was found that CuO could not only accelerate the SKNN phase formation but also improve the densification, lower the sintering temperature, and also improve the texture fraction. The maximum of86%was obtained when sintered at1340℃for6h. At the later stage of the SNN phase formation, the interdiffusion of ions led to the phase formation of SKNN, which was occurred between SNN particles and acicular SKN templates due to the chemical gradient. The existence of CuO liquid phase could also accelerate the interdiffusion of ions. The stable acicular shape provided the structural framework for the anisometric SKNN grains that could serve as secondary templates because final SKNN phase and SKN template were all tetragonal tungsten bronze structure. So the formation of the texture microstructure in c-direction could continue on the original template particles by epitaxy after densification. The texture development is further driven by the existence of liquid phase of CuO as the grain growth rapid along [001] direction was fastest. Then higher texture fraction could be obtained when introducing CuO sintering aid.
8. The SKNN-CuO textured ceramics showed obvious anisotropy behavior in electrical properties. er, em, Pr and d33in c-axis direction samples of the SKNN-CuO textured ceramics were much higher than those in a/b-axis direction samples. Compared with the SKNN-CuO random ceramics and textured ceramics, it was also found that the electrical properties in c-axis direction samples of the SKNN-CuO textured ceramics were much higher than those of SKNN-CuO random ceramics:the er, εm, Pr and d33were about1.88,3.09,3.18and1.91times as much as those of SKNN-CuO random ceramics, respectively. Compared with the electrical properties of SKNN-CuO textured ceramics and SKNN textured ceramics, it was found that sintering aid CuO could lower the εm, Pr and d33.
9. The SCNN textured ceramics with higher texture fraction and optimized electrical properties were successfully obtained. The phase structure, microstructure, and electrical properties of the Sr2-xCaxNaNb5O15ceramics as a function of Ca content were investigated. Pure tungsten bronze structure and fine-grained microstructure could be obtained in all ceramics. The lattice constant calculation indicated the crystal structure of Sr2-xCaxNaNb5O15ceramics slightly distorted from the tetragonal phase and became orthorhombic phase at room temperature with increasing Ca substitution. The smaller ionic radius of Ca2+(1.34A) compared to that of Sr2+(1.44A) also led to the shrinkage of the crystal structure. Dielectric spectra of all compositions displayed two phase transitions:the ferroelastic orthorhombic to ferroelectric tetragonal phase transition (Te) at lower temperatures, and the ferroelectric to paraelectric phase transition (Tc) at higher temperatures. Dielectric, ferroelectric and piezoelectric properties of the Sr2-xCaxNaNb5O15ceramics greatly depended on the Ca content. The optimized electrical properties could be obtained at x=0.15, which could be attributed to the most homogeneous microstructure, the highest density, and the changes in the crystal structure caused by Ca substitution. In addition, reactive template grain growth method was used to fabricate Sr1.85Ca0.15NaNbsO15(SCNN) textured ceramics. The maximum of88%was obtained when sintered at1340℃for6h. Compared with the electrical properties of SKNN textured ceramics, SKNN-CuO textured ceramics and SCNN textured ceramics, it was found that substitution of Ca for Sr in A sites could not only improve the texture fraction, but also improve the electrical properties.
In conclusion, combined micro structure design and macro structure design, the ceramics with high electrical properties and good texture quality were successfully textured by templated grain growth method, realizing the application of templated grain growth method in filled tetragonal tungsten bronze structure compounds.
其中,填满型钨青铜结构的Sr2NaNb5O15(SNN)基化合物具有优异的介电、铁电、光电和光折射性能,是无铅压铁电领域中最具有研究潜力的材料之一。但SNN基陶瓷材料存在着烧结温度过高、容易出现生长异常的过大尺寸晶粒和裂纹以及压电和介电性能相对较低等缺点,并且较难制备出具有良好致密性且性能较优的产品。本文在传统固相法的基础上采用二步烧结合成技术成功制备了致密性优良的SNN、Sr2KxNa1-xNb5O15(SKNN)、Sr2-xCaxNaNb5015(SCNN)等压铁电无取向陶瓷,并且从烧结工艺、配方设计和掺杂改性等方面进行系统研究,探讨材料组分、微观结构和宏观性能之间的关联性。具体研究过程为：首先研究工艺条件对体系相结构、微观形貌、密度以及介电和铁电性能的影响,筛选出最佳工艺条件；同时还进行组分设计,选择了K、Ca和Cu0分别作为取代和掺杂剂,对基础组分进行了A位取代和掺杂研究,以期达到控制异常晶粒生长、避免陶瓷表面裂纹出现、降低烧结温度、降低介电损耗、提高陶瓷致密度和优化电性能的目的。
通过以上微观组分研究为宏观结构设计(即陶瓷材料的织构化)提供较为理想的材料体系。然后,通过熔盐法采用SrNb2O6-Nb2O5-KCl体系制备纯四方相钨青铜结构、长径尺寸比适当、形貌非对称性高的棒状SKN粉体作为织构技术的模板晶粒,探讨SrNb2O6-Nb2O5-KCl体系中的反应机理和棒状SKN粉体的生长机制。同时研究棒状SKN晶种与SNN主晶相之间的结构匹配性,提出SNN晶体沿棒状SKN晶种的界面传质与取向附生机制。最后,通过模板晶种SKN和其他反应物混合,采用反应模板晶粒生长技术(RTGG)来制备SNN基无铅压铁电织构陶瓷。系统研究该织构过程中模板生长动力学、相形成及相演变、微观组织演变、织构历程、致密化行为以及液相烧结助剂对织构陶瓷烧结行为的影响规律；探讨晶粒的定向生长机制；提出SNN基陶瓷织构的反应机理；此外,还系统研究CuO掺杂、A位Ca取代与陶瓷的织构质量以及陶瓷电性能之间的关系；最终获得高取向度且具有高性能的SCNN无铅压铁电织构陶瓷。通过实验研究和理论分析,得到了以下一系列具有显著创新性的研究成果：
1.在传统固相法基础上采用二步烧结合成技术制备填满型钨青铜结构SNN基无铅压铁电陶瓷材料,成功解决该材料体系难以致密化的难题,并避免了异常晶粒和裂纹的出现,得到了具有较优性能的组分体系。通过研究工艺条件对SNN陶瓷的相结构、微观结构、密度、介电和铁电性能的影响,结果发现：当反应温度为1100℃时,SrNb206和NaNbO3开始反应生成SNN,当反应温度为1180℃时可获得纯相的SNN粉体,但是过高的烧结温度1360℃便会导致SNN相的分解。SNN相在高温下的分解会进一步导致陶瓷表面出现大面积的液相熔融区域和裂纹,从而降低致密度,并恶化所获SNN陶瓷的电性能。因此,制备致密化且具有较优性能的SNN无铅压铁电陶瓷的最佳制备条件是：预烧温度为1180℃,烧结温度为1340℃。此条件下获得的SNN陶瓷其相对密度最大,晶粒大小均匀,气孔数较少,电性能最佳：εr=1325,εm=2210,Tc=280℃,tanδ=0.023,Pr=11.71μC/cm2,Ec=16.15kV/cm。同时,还研究了A位K取代量对SKNN陶瓷的微观结构和电性能影响,结果表明：适当的K取代量能够促进陶瓷的致密化,使陶瓷晶粒生长饱满、晶界清晰,同时还可以提高SKNN陶瓷的介电性能。但是过高的K取代量会促使陶瓷在高温烧结过程中容易出现液相区域,从而导致陶瓷致密化变得越来越困难,陶瓷表面也伴随出现了比较严重的析晶现象,从而恶化陶瓷的电性能。因此,制备致密化且具有较优性能的SKNN无铅压铁电陶瓷其最佳K取代量的范围和制备条件是：x=0.05～0.10,预烧温度为1160℃,烧结温度为1320℃。
2.采用熔盐法发现在SrNb206-Nb2O5-KCl体系中可以获得纯四方相钨青铜结构、长径尺寸比适当、形貌非对称性高的棒状SKN粉体作为钨青铜结构陶瓷织构化的模板晶粒,提出了反应机理和生长机制。采用熔盐法在SrN2O6-Nb2O5-KCl体系中,当SrNb2O6与Nb2O5物质的量之比为1.0,熔盐KCl与氧化物质量之比为1.5时,在1150℃预烧6h时,获得了纯四方相钨青铜结构、长径尺寸比适当、形貌非对称性高的棒状SKN模板晶粒。此SKN模板无论在纯度还是形貌上其可重复性都很好,且尺寸较为均匀,可以很好地作为制备织构陶瓷的模板晶粒。熔盐法制备SKN模板的反应机理为：SrCl2与Nb205之间的反应比SrCl2与SrNb206之间的反应更容易进行。随着原料中Nb2O5含量的增加,SrCl2会和Nb205优先进行反应,从而减少了其与SrNb206反应生成杂相Sr2Nb207的机会。因此在SrNb2O6-KCl体系的基础上加入适量的Nb205就可以获得纯相的SKN。熔盐法获得的SKN模板其棒状形貌的生长机制为：在SrNb2O6-Nb2O5-KCl熔盐体系中SKN的形貌主要受两个过程的控制和影响：首先是反应过程,其次是晶体生长过程。反应过程与SrNb2O6在KCl中的溶解有关,如果反应物中SrNb2O6的含量比它在KCl中的溶解度高,部分未溶解的SrNb2O6粉体就会团聚在一起而无法在液相熔盐中得到充分分散。由于SrNb2O6既是生成SKN的反应物,又是SKN晶体形成的初始晶种,因此就导致了SKN形成时多个晶核位的共同存在。SKN的生长过程主要与钨青铜结构晶体的生长习性有关。对钨青铜结构晶体来说,其(001)面由于具有较高的表面能,生长速度比其他几个面要快,从而获得表面呈现台阶状的棒状形貌。
3.开展了棒状SKN晶种与SNN主晶相之间的结构匹配性研究,并提出了SNN晶粒沿棒状SKN晶种进行取向附延生长的机制。采用反应烧结技术在不同体系R0(不含有SKN晶种),R10(含有10mol.%棒状SKN晶种)和C10(含有10mol.%等轴SKN晶种)中,相形成过程均发生在致密化行为之前,且最高相对密度都达到了95%以上。另外,棒状SKN晶种和等轴SKN晶种的存在都可以促进SNN主晶相的形成,也可以促进陶瓷的致密化过程。在R10体系中,当烧结温度为1340℃时,陶瓷表面出现了尺寸较大(50～100μm),生长各向异性且形貌类似于棒状SKN晶种的SNN晶粒。在C10体系中,仅出现了尺寸在15μm左右的非等轴晶粒。而在R0体系中,并没有出现任何生长各向异性的陶瓷晶粒。R10体系中形貌类似于棒状SKN晶种的SNN晶粒的出现可用液相辅助传质模型来解释：随着烧结温度的升高,在SKN模板与SNN晶粒的交界处优先形成一层液相膜,液相膜的形成促进了K+、Na+在两物相中的相互扩散。这种K+、Na+的扩散导致SKN晶种的棒状形貌逐渐模糊并溶解消失,同时也促使液相区域的面积逐渐变大并形成棒状轮廓。钨青铜结构晶粒的自身生长习性、棒状轮廓液相区域的形成以及SKN晶种的棒状结构骨架都为生长各向异性的SNN晶粒的形成和生长提供有利因素,因而得到了形貌类似于棒状SKN晶种的SNN晶粒。说明棒状SKN模板与SNN晶体之间存在着很好的结构匹配性。
4.率先将RTGG技术用于制备填满型钨青铜结构的SKNN无铅压铁电织构陶瓷,获得最佳织构化条件。通过研究工艺条件对SKNN织构陶瓷相结构演变、织构度、致密化行为、微观结构演变和电性能的影响,结果发现：在RTGG技术制备织构陶瓷的过程中,随着烧结温度的升高和保温时间的延长,SKNN织构陶瓷的致密化程度及织构度均存在一极大值,超过极大值所对应的温度和保温时间,材料的致密化程度下降,织构度降低,且在陶瓷表面出现液相熔融迹象。获得了SKNN织构陶瓷的最佳织构化条件：烧结温度为1360℃,保温时间为6h,此时织构因子最大为82%。通过研究烧结温度和保温时间对SKNN织构陶瓷沿c轴方向和a/b轴方向电性能的影响规律发现：SKNN织构陶瓷其介电性能、铁电性能和压电性能都表现出明显的各向异性。当烧结温度为1360℃,保温时间为6h时,所获得的SKNN织构陶瓷沿c轴方向的常温介电常数εr,最大介电常数εm,剩余极化强度Pr和压电常数d33都明显高于a/b轴方向,且分别为a/b轴方向的1.42倍,4.20倍,5.67倍和5.38倍。另外,对比传统固相法(CMO)制备的SKNN无取向陶瓷的电性能与采用RTGG技术获得的SKNN织构陶瓷在c轴方向上的电性能发现：SKNN织构陶瓷沿c轴方向的各种电性能均明显高于无取向陶瓷。SKNN织构陶瓷其常温介电常数ε和最大介电常数εm,分别为无取向陶瓷的1.67和2.85倍。与SKNN无取向陶瓷相比,SKNN织构陶瓷的矫顽场略有降低,但其沿c轴方向剩余极化强度Pr为无取向陶瓷剩余极化强度Pr的2.94倍。SKNN织构陶瓷沿c轴方向的压电常数d33(86pC/N)为无取向陶瓷压电常数d33(46pC/N)的1.87倍,表明织构技术可以大幅度提高陶瓷的性能。
5. SKNN织构陶瓷的织构机制：随烧结温度的升高,SKNN织构陶瓷微观组织结构的变化分为三个阶段：烧结初期为固相反应阶段,表现为基体原料之间的相互交联以及SNN相与SKN模板之间的侵蚀反应；烧结中期为致密化阶段,表现为陶瓷表面气孔数的逐渐减少,陶瓷相对密度的急剧增大；烧结后期为织构化过程,表现为在较高的烧结温度下可以得到沿流延方向定向生长且具有类似于SKN模板棒状形貌的SKNN织构陶瓷。RTGG技术制备SKNN织构陶瓷的织构机理为：围绕在SKN棒状模板周围的SNN粉体与SKN棒状模板通过液相侵蚀生成的SKNN相具有类似于棒状模板的非等轴形貌,以此为织构行为产生的新模板,从而导致新模板周围的等轴SKNN晶粒在具有棒状形貌的非等轴SKNN模板晶粒周围进行趋向附延生长,从而获得织构形貌。
6.在SKNN无取向陶瓷中引入烧结助剂CuO,为进一步制备SKNN-CuO织构陶瓷确定最佳CuO含量。系统研究了CuO含量对SKNN陶瓷的相结构、显微结构、密度、介电性能和铁电性能的影响,提出了CuO掺杂的作用机制。结果发现：较低含量的CuO能完全进入晶格相,从而使所有组分都能获得纯相四方钨青铜结构的SKNN陶瓷,同时没有引起陶瓷晶格的畸变和相结构的转变。适当的CuO掺杂量可以提高陶瓷的致密化程度,但过多含量的CuO反而会导致陶瓷表面出现尺寸较大的晶粒从而降低陶瓷的致密度。另外,SKNN陶瓷的电性能与CuO的掺杂量密切相关。当CuO的掺杂量为0.015时,陶瓷具有较佳的电性能,此时陶瓷晶粒大小均匀,晶界清晰,陶瓷相对密度最大为99.6%,此时常温介电常数εr、最大介电常数εm和剩余极化强度Pr都获得最大值：εr=1160,εm=1573,Pr=4.82μC/cm2。同时发现在SKNN无取向陶瓷体系中,CuO同时起到液相助剂和受主掺杂的作用。
7.采用RTGG技术成功制备了SKNN-CuO织构陶瓷,提高了织构陶瓷的织构度,并进一步探讨了织构机制。系统研究了SKNN-CuO织构陶瓷在陶瓷织构过程中的烧结行为、致密化行为、相结构演变、微观结构演变以及模板生长过程中的织构度随烧结温度和保温时间的变化规律。结果表明：在RTGG-CuO织构陶瓷的相形成过程中,CuO的存在不仅促进了SNN相的形成,加快了陶瓷的致密化、降低了烧结温度,同时还提高了织构陶瓷的取向度,从而在较低的烧结温度1340℃保温6h便可获得织构因子高达86%的织构陶瓷。CuO促进SKNN陶瓷织构化的机制为：围绕在SKN棒状模板周围的SNN粉体与SKN棒状模板通过液相侵蚀生成的SKNN相具有类似于棒状模板的非等轴形貌,当反应体系中存在适量的CuO液相助剂时,会更利于Na+的扩散和具有棒状形貌的SKNN二次模板的形成,正是适量的液相使得等轴SKNN晶粒在具有棒状形貌的非等轴SKNN模板晶粒周围进行趋向附延生长的区域有所扩大,从而提高了陶瓷的织构度。进一步验证了织构机制。
8.研究了SKNN-CuO织构陶瓷其电性能的各向异性,并将SKNN-CuO织构陶瓷与SKNN织构陶瓷的性能进行对比分析。通过研究烧结温度和保温时间对SKNN-CuO织构陶瓷沿c轴方向和a/b轴方向各种电性能的影响规律发现：SKNN-CuO织构陶瓷其介电性能、铁电性能和压电性能具有显著的各向异性。SKNN织构陶瓷沿c轴方向的常温介电常数εr,最大介电常数εm,剩余极化强度Pr和压电常数d33都明显高于a/b轴方向。在最佳条件下获得的SKNN-CuO织构陶瓷在c轴方向上的各项电性能均明显高于SKNN-CuO无取向陶瓷。其沿c轴方向的常温介电常数εr和最大介电常数εm分别为无取向陶瓷的1.88和3.09倍。其沿c轴方向剩余极化强度pr为无取向陶瓷剩余极化强度pR的3.18倍。沿c轴方向的压电常数d33为无取向陶瓷压电常数d33的1.91倍。对比SKNN织构陶瓷与SKNN-CuO织构陶瓷的电性能可以看出,CuO虽然降低了烧结温度,并提高了织构陶瓷织构度。但是,引入CuO之后,织构陶瓷的最大介电常数,居里温度,剩余极化强度和压电常数都有所降低。说明CuO的引入无法同时兼顾提高织构度和优化电性能的双重目的。
9.通过A位取代,获得了同时具有高取向度和高性能的SCNN织构陶瓷,并成功实现了RTGG技术在其他钨青铜结构化合物体系的应用。首先选用传统固相法采用二步反应烧结技术成功制备出了不同Ca取代量的SCNN无取向陶瓷,并系统研究了Ca取代量对陶瓷的相结构、显微结构、密度、介电性能和铁电性能的影响。结果表明当Ca取代量为0.15时,获得了晶粒生长同性、尺寸大小均匀、致密性良好的陶瓷,其最佳电性能为：Tc=285℃,εm=1954,Pr=7.08μC/cm2。同时,采用RTGG技术成功制备了SCNN织构陶瓷,当烧结温度1340℃、保温6h时,便可获得织构因子高达88%的织构陶瓷。对比SKNN织构陶瓷、SKNN-CuO织构陶瓷与SCNN织构陶瓷发现,Ca取代A位Sr之后的织构陶瓷,其织构因子最大为88%,且其沿c轴方向的各项电性能均优于SKNN织构陶瓷和SKNN-CuO织构陶瓷。说明Ca的引入既可以很好地起到烧结助剂的作用,提高了织构陶瓷的致密度和取向度,同时也可以明显提高织构陶瓷的电性能。这个工作实现了RTGG技术在其他钨青铜结构化合物体系中的成功应用。
Due to the air pollution caused by the application of Pb-based materials, it is very important and necessary to study the lead-free piezoelectrics. For the polycrystalline ceramics, the physical properties are mainly controlled by the composition, processing conditions and the microstructure. Through microstructure control to make the ceramic grains grow along the preferred orientation (texturing ceramics) is an important approach to enhance the physical properties. In this thesis, Sr2NaNb5O15(SNN)-based ceramics with filled tungsten bronze structure were selected as the based system. It is well known that the tungsten bronze niobate ceramics are difficult simultaneously in obtaining high density and good electrical properties by traditional sintering methods. In order to solve the above problems, research on the micro structure design and macro structure design were carried out.
Firstly, for micro structure design, the two-step solid state reaction method and traditional sintering process were used to prepare random SNN-based ceramics. The effects of preparing conditions, effects of substitution of K for Na in A sites on the phase structure, microstructure and electrical properties were studied to improve the density, restrain the abnormal grain growth, reduce the sintering temperature and also optimize the electrical properties. Then the acicular Sr2KNb5O15(SKN) template was prepared by molten salt synthesis method in SrNb2O6-Nb2O5-KCl system. The reaction and growth mechanisms in SrNb2O6-Nb2O5-KCl system were also proposed. In addition, in order to discuss the structure matching between the acicular SKN template and the SNN phase, how the seeding SKN particles affect the phase formation, densification and microstructure development in SNN ceramics was also studied. Finally, the macro structure design was carried out by mixing the SKN template and other reactant materials. The reactive templated grain growth (RTGG) method was employed to obtain the textured SNN-based ceramics. The preparing conditions on the phase formation, densification, micro structural evolution, texture development and electrical properties were discussed. The oriented grain growth mechanism and reaction mechanism were proposed. The sintering aids effects during sintering process were also discussed to improve the texture fraction. Through the above research work, the Sr1.85Ca0.15NaNb5O15(SCNN) textured ceramics with high texture quality and optimized electrical properties were obtained. The main conclusions obtained from experimental results and analyses were as follows:
1. The high-density tungsten bronze SNN-based ceramics without abnormal grain growth were successfully synthesized by two-step solid state reaction method. The phase evolution and reaction mechnism in SrNb2O6and NaNbO3solid solution were analyzed by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results showed that the Sr2NaNb5O15phase could exist stably in the temperature range of1100～1360℃, and would be decomposed to SrNb2O6and NaNbO3phases at temperatures above1360℃. It was also found that the sintering temperature could significantly affect the density, microstructure, and electric properties of SrNaNb5O15ceramics. When the sintering temperature was1340℃, the ceramic with a near-theoretical density and a uniform and fine-grained microstructure was obtained, exhibiting outstanding electrical properties:εm=2210, tan δ=0.023, Tc=280℃, Pr=11.71μC/cm2, and Ec=16.15kv/cm. Further increasing the temperature to1360℃led to the existence of some molten areas and cracks on the surface of obtained ceramics, which could significantly deteriorate the electric properties. In addition, K substitution for Na in A sites could accelerate the phase formation at lower temperatures. The lattice constant calculation indicated expansion of the unit cell and reduced the distortion of the crystal structure with K substitution due to the bigger ionic size of K+(1.64A) compared to that of Na+(1.39A). Electrical properties of Sr2KxNa1-xNb5O15ceramics greatly depended on the K content. Curie temperature Tc shifted downward, whereas the maximum dielectric constant εm and the degree of diffusion phase transition all increased initially and then decreased as K content increased, indicating that proper amount of K substitution with x between0.05and0.10could enhance the dielectric properties.
2. The well-developed acicular SKN templates were obtained by molten salt synthesis (MSS) method using SrNb2O6-Nb2O5-KCl system. The reaction and growth mechanisms were also proposed. When synthesized in molten KCl salt at1150℃for6h with the molar ratio of SrNb2O6and Nb2O5is1and the weight ratio of salt to oxide sources was1.5, pure SKN particles with well-developed acicular morphology were successfully obtained in this system. They were agglomerate-free and with proper scale in size range of5-30μm, making them to be the ideal template for fabricating textured ceramics. The reaction mechanisms were as follows:5SrNb2O6+2KCl=2Sr2KNb5O15+SrCl2(1) SrCl2+SrNb2O6+1/2O2=Sr2Nb207+Cl2↑(2) SrCl2+Nb2O5+1/2O2=SrNb2O6+Cl2↑(3) The reaction between SrCl2and Nb2O5(3) with lower reaction Ea than reaction (2) could limit the synthesis of Sr2Nb2O7to some extent. Then the pure SKN without any impurity was obtained. The growth mechanism was:the SKN morphology was controlled initially by the formation process and later by the growth process. The formation process was related to the solubility of SrNb2O6in KCl salt. If the SrNb2O6amount in precursor mixture was more than its solubility in KCl salt, they would not be well diffused in KCl salt liquid and clumped together to form the multiple nucleation sites for SKN crystal. The subsequent growth was in accordance with the growth habits of SKN crystals with tungsten bronze structure which growed faster along c direction than along any other axis direction.
3. The structure matching between the acicular SKN template and the SNN phase was studied. The liquid-phase-assisted growth mechanism was proposed. The phase formation, densification behavior and microstructure development of SNN ceramics in the10mol.%acicular SKN seed-containing system (R10), the10mol.%equiaxed SKN seed-containing system (CIO), and the unseeded system (R0) were discussed. The results showed that the acicular and isometric SKN seeds could not only accelerate the SNN phase formation but also promote the densification at lower temperatures. In R10system, some large anisotropic grains (>80μm) could be obtained at definite sintering temperature. In CIO system, only some small anisotropic grains (<20μm) were obtained. However, there were no such anisotropic grains obtained in the SKN-free R0system. The growth mechanism of the large anisotropic grain (>80μm) was summarized as liquid-phase-assisted growth mechanism, which was that the SKN seed prepared by molten salt synthesis method could give rise to the formation of a liquid phase and provide a structural framework for the growth of ceramic grains, leading to the existence of large anisotropic grains in the ceramic when sintered at1340℃.
4. The SKNN textured ceramics were prepared by reactive template grain growth method. The optimized preparing condition was obtained. The phase formation, texture fraction, microstructure development, densification behavior and electrical properties as a function of processing parameters such as sintering temperature and holding time were investigated in detail. With increasing the sintering temperature and prolonging the holding time, the texture fraction all increased at first and then decreased. The maximum of82%was obtained when sintered at1360℃for6h. The SKNN textured ceramics showed obvious anisotropy behavior in electrical properties:εr, εm, Pr and d33in c-axis direction samples of the SKNN textured ceramics were much higher than those in a/b-axis direction samples. Compared with the SKNN random ceramics and textured ceramics, it was also found that the electrical properties in c-axis direction samples of the SKNN textured ceramics were much higher than those of SKNN random ceramics:the εT, εm, Pr and d33were about1.67,2.85,2.94and1.87times as much as those of SKNN random ceramics, respectively. The above results showed that the reactive template grain growth could improve the electrical properties of ceramics obviously.
5. The oriented grain growth mechanism in SKNN textured ceramics was proposed. It was found the whole microstructure development process can be divided into3steps:(1) the phase formation stage,(2) the densification stage, and (3) the texture development stage. At the later stage of the SNN phase formation, the interdiffusion of ions led to the phase formation of SKNN, which was occurred between SNN particles and acicular SKN templates due to the chemical gradient. It was also reasonable to expect that some liquid phase around SKN templates caused by the coexistence of higher concentration of K+and Na+would accelerate the interdiffusion of ions. The obtained SKNN phase showed two different kinds of morphology:the SKNN particles formed around SKN templates showed acicular morphology; the SKNN particles far away from SKN templates showed equiaxed morphology. The stable acicular shape provided the structural framework for the anisometric SKNN grains that could serve as secondary templates because final SKNN phase and SKN template were all tetragonal tungsten bronze structure. So the formation of the texture microstructure in c-direction could continue on the original template particles by epitaxy after densification. Then the texture development was further driven when the samples were sintered at higher temperatures and longer holding times.
6. Introduction of CuO sintering aid could improve the electrical properties of SKNN random ceramics. The effects of CuO content on phase formation, microstructure, density and electrical properties were studied. It was found that lower amount of CuO could accelerate the SKNN phase formation and improve the densification. But higher amount of CuO led to the decrease in densification and the existence of some liquid phases. The SEM micrographs showed that with increasing CuO content the average grain size increased, but no abnormal grain growth could be found. The electrical properties of SKNN-CuO random ceramics depended greatly on the CuO amount. When CuO amount=0.015mol, the electrical properties of the ceramics were better:εr=1160, εm=1573and Pr=4.82μC/cm, showed that CuO served as both sintering aid and acceptor doping.
7. Templated grain growth method was employed to fabricate SKNN-CuO textured ceramics, further discussing the oriented grain growth mechanism. It was found that CuO could not only accelerate the SKNN phase formation but also improve the densification, lower the sintering temperature, and also improve the texture fraction. The maximum of86%was obtained when sintered at1340℃for6h. At the later stage of the SNN phase formation, the interdiffusion of ions led to the phase formation of SKNN, which was occurred between SNN particles and acicular SKN templates due to the chemical gradient. The existence of CuO liquid phase could also accelerate the interdiffusion of ions. The stable acicular shape provided the structural framework for the anisometric SKNN grains that could serve as secondary templates because final SKNN phase and SKN template were all tetragonal tungsten bronze structure. So the formation of the texture microstructure in c-direction could continue on the original template particles by epitaxy after densification. The texture development is further driven by the existence of liquid phase of CuO as the grain growth rapid along [001] direction was fastest. Then higher texture fraction could be obtained when introducing CuO sintering aid.
8. The SKNN-CuO textured ceramics showed obvious anisotropy behavior in electrical properties. er, em, Pr and d33in c-axis direction samples of the SKNN-CuO textured ceramics were much higher than those in a/b-axis direction samples. Compared with the SKNN-CuO random ceramics and textured ceramics, it was also found that the electrical properties in c-axis direction samples of the SKNN-CuO textured ceramics were much higher than those of SKNN-CuO random ceramics:the er, εm, Pr and d33were about1.88,3.09,3.18and1.91times as much as those of SKNN-CuO random ceramics, respectively. Compared with the electrical properties of SKNN-CuO textured ceramics and SKNN textured ceramics, it was found that sintering aid CuO could lower the εm, Pr and d33.
9. The SCNN textured ceramics with higher texture fraction and optimized electrical properties were successfully obtained. The phase structure, microstructure, and electrical properties of the Sr2-xCaxNaNb5O15ceramics as a function of Ca content were investigated. Pure tungsten bronze structure and fine-grained microstructure could be obtained in all ceramics. The lattice constant calculation indicated the crystal structure of Sr2-xCaxNaNb5O15ceramics slightly distorted from the tetragonal phase and became orthorhombic phase at room temperature with increasing Ca substitution. The smaller ionic radius of Ca2+(1.34A) compared to that of Sr2+(1.44A) also led to the shrinkage of the crystal structure. Dielectric spectra of all compositions displayed two phase transitions:the ferroelastic orthorhombic to ferroelectric tetragonal phase transition (Te) at lower temperatures, and the ferroelectric to paraelectric phase transition (Tc) at higher temperatures. Dielectric, ferroelectric and piezoelectric properties of the Sr2-xCaxNaNb5O15ceramics greatly depended on the Ca content. The optimized electrical properties could be obtained at x=0.15, which could be attributed to the most homogeneous microstructure, the highest density, and the changes in the crystal structure caused by Ca substitution. In addition, reactive template grain growth method was used to fabricate Sr1.85Ca0.15NaNbsO15(SCNN) textured ceramics. The maximum of88%was obtained when sintered at1340℃for6h. Compared with the electrical properties of SKNN textured ceramics, SKNN-CuO textured ceramics and SCNN textured ceramics, it was found that substitution of Ca for Sr in A sites could not only improve the texture fraction, but also improve the electrical properties.
In conclusion, combined micro structure design and macro structure design, the ceramics with high electrical properties and good texture quality were successfully textured by templated grain growth method, realizing the application of templated grain growth method in filled tetragonal tungsten bronze structure compounds.
引文
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