铌酸钾钠无铅压电陶瓷的水热制备和放电等离子烧结

1、铌酸钾钠无铅压电陶瓷的水热制备和放电等离子烧结        【中文摘要】压电陶瓷是重要的功能材料,广泛用于谐振器、滤波器、传感器等多种功能器件。半个多世纪以来,占主导地位的压电陶瓷是以锆钛酸铅(PZT)为代表的含铅压电陶瓷。但是,PZT中铅的含量超过60%,在其生产和废弃处理过程中会对人体和环境造成很大危害。基于环境保护的考虑,人们一直在寻找可以替换PZT的无铅压电陶瓷。碱金属铌酸盐是重要的无铅压电陶瓷体系。其中,最有代表性的铌酸钾钠NaNbO_3-KNbO_3二元固溶体的研究始于二十世纪五十年代,但由于同时期发

2、现的PZT陶瓷具有更优异的压电性能,因此多年来铌酸钾钠无铅压电陶瓷并未成为主流的压电陶瓷研究对象。直到2004年,日本的Saito等学者在Nature杂志上报道了可与传统PZT媲美的(K_(0.5)Na_(0.5)NbO_3基无铅压电陶瓷材料,铌酸钾钠无铅压电陶瓷才重新成为研究热门。近几年来,关于铌酸钾钠无铅压电陶瓷的研究报道非常多,但主要都是集中在成分的优化与探索方面,比如掺杂Li,Ta,Sb等元素来进步其压电性能。这些研究所采用的都是传统的固相反应合成工艺。固然工艺简单,但制备的陶瓷粉末颗粒粗大、成分不均匀,且轻易在球磨粉碎过程中引进杂相,易造成陶瓷块体的性能劣化及工艺不稳定。而水热法是一

3、种基于水溶液反应的化学合成方法,已广泛商用于传统的PZT、BaTiO_3等功能陶瓷粉末制备产业。近年来更是有报道指出,由水热法制备的BaTiO_3粉末烧结成压电陶瓷块体样品性能非常优异。而关于水热合成铌酸钾钠无铅压电陶瓷体系的报道非常少,且研究不够充分,尤其关于具有最优压电性能的(K_(0.5)Na_(0.5)NbO_3陶瓷粉末的研究更是凤毛麟角。因此,本论文主要研究了用水热法合成铌酸钾钠无铅压电陶瓷粉末,并用SPS放电等离子烧结将制备的粉末烧结成块体,并测试了其介电、压电和铁电性能。研究工作的主要结论如下:(1)利用水热法成功合成了铌酸钾钠无铅压电陶瓷粉末,系统研究了反应温度、反应时间、起始

4、碱溶液浓度、Nb_2O_5加进量对于合成产物的影响。实验发现,进步反应温度有利于天生单一的钙钛矿相陶瓷粉末,但是延长反应时间并不能使天生的多相钙钛矿陶瓷粉末转化为单相。碱溶液的起始浓度,尤其是KOH/NaOH的比例对于合成粉末的成分及形貌有较大影响。实验结果表明,水热合成接近KNbO_3-NaNbO_3固溶体任意一真个单相钙钛矿相都比较轻易,但是成分接近(K_(0.5)Na_(0.5)NbO_3时就天生两相。(2)初步研究了添加剂对于水热合成铌酸钾钠无铅压电陶瓷粉末的影响。通过起始溶液中添加聚乙烯醇(PVA)和乙二胺四乙酸(EDTA),在KOH/NaOH起始浓度比例为4.5/1.5,碱总浓度6

5、mol/L,水热反应温度为220和反应时间为24h的条件下,成功合成了钙钛矿相铌酸钾钠陶瓷粉末。实验结果表明,添加剂可抑制水热反应中某些晶面的生长,并对天生物的形貌产生很大的影响。(3)利用乙醇-水混合溶剂热法,成功合成了铌酸钾钠无铅压电陶瓷粉末,并研究了乙醇添加量、NaOH/KOH比例、起始碱溶液浓度对于合成产物的影响。实验发现,乙醇的添加,进步了反应釜中的压力,更有利于天生单相,但同时改变了其成分组成。结果表明,合成成分接近KNbO_3-NaNbO_3固溶体任意一真个单相钙钛矿相都比较轻易,但是成分接近(K_(0.5)Na_(0.5)NbO_3时就天生两相。(4)成功采用了SPS放电等离子

6、烧结工艺,将水热制备的铌酸钾钠粉末烧结成相对密度大于99%的压电陶瓷块体,并对其物相结构和微观形貌进行了观察,测试了其压电、铁电和介电性能。尽管实验制备的水热粉末为非单相的钙钛矿相,但是经SPS放电等离子烧结后,样品显示为单一的正交钙钛矿相。经空气中退火处理后,样品呈现良好的铁电性和压电性。压电常数为135pC/N,在未掺杂的KNN陶瓷中处于较高水平。并且,样品显示出比较大的剩余极化强度P_r=26.2C/cm2和机械品质因数Q_m150,大约分别是由固相反应法制备的粉末经SPS烧结后样品的两倍。');【Abstract】 Piezoelectric ceramics, the imp

7、ortant functional materials, could be used as resonators, filters, transducers and so on. For more than half a century, lead-containing piezoceramics, represented by PZT, have always been taking the dominant role. However, PZT contains more than 60% of lead, which is harmful for humanity as well as

8、environment during the manufacture and abandonment processes. In the view point of environment protection, people are always seeking for lead-free substitute.Alkaline niobate is important lead-free piezoelectric ceramics system, the most representative of which is NaNbO_3-KNbO_3 solid state solution

9、. Although the research of NaNbO_3-KNbO_3 started in the 1950s, the PZT ceramics, which were discovered almost at the same time, showed much better piezoelectric performance. As a result, the alkaline niobate did not drawn much attention until 2004, when Saito et al. reported (K_(0.5)Na_(0.5)NbO_3 b

10、ased ceramics with outstanding piezoelectric properties comparative to PZT ceramics. For recent years, the (K, Na)NbO_3 based ceramics has been a hot research topic, and relative reports appeared in large amount, which mainly concerned improvement of piezoelectric performance by means of doping with

11、 other elements, such as Li, Ta, Sb and so on. However, most investigators adopted conventional synthesis process of solid state reaction. Despite the *ness, the disadvantages of this process include inducing coarse grains distribution, inhomogeneous composition, and easy introduction of impurities

12、during ball-milling process, which could deteriorate piezoelectric properties of the ceramics, and cause instability of production process. On the contrast, the hydrothermal method is a chemical synthesis process based on water solution, and has been widely used for the manufacture of commercial PZT

13、, BaTiO_3 and other functional ceramics powders. And recent report indicated that the BaTiO_3 ceramics sintered from hydrothermal powders exhibited much more superior piezoelectric performance. Nevertheless, there were little reports concerning hydrothermal synthesis of (K, Na)NbO_3 powders. The pre

14、sent study is focused on the hydrothermal synthesis of (K, Na)NbO_3 powders and subsequent spark plasma sintering of these powders. The dielectrics, piezoelectric and ferroelectric properties of the obtaioned ceramics samples were also investigated.The major conclusions are as follows:The (K, Na)NbO

15、_3 powders were successfully prepared by a hydrothermal method, and the influences of reaction temperature, reaction time, alkality of starting solution and amount of Nb_2O_5 were systematically investigated. It was revealed that improvement of reaction temperature was in favor of synthesis of singl

16、e-phase perovskite, but prolonging reaction time could not convert two-phase perovskites into a single one. The alkality of starting solution, especially the KOH/NaOH ratio, had profound effects on the composition as well as microstructure of resultant powders. It was easy to synthesize single-phase

17、 NaNbO_3-KNbO_3 powders with compositions near either end members, but two-phase perovskites appeared around (K_(0.5)Na_(0.5)NbO_3.The influences of additives on the hydrothermal synthesis of (K, Na)NbO_3 powders were studied preliminarily. Polyvinyl alcohol (PVA) and ethylene diamine tetra acetic a

18、cid (EDTA) were added to the starring solution. When KOH/NaOH ratio was 4.5/1.5, the total alkality was 6 mol/L, the reaction temperature was 220, and the reaction time was 24 hours, (K, Na)NbO_3 powders with perovskite phase were prepared. The result indicated that additives hindered growth of some

19、 crystal face, so the microstructure of resultant powders was changed a lot.The (K, Na)NbO_3 powders were successfully prepared using a alcohol-water solvothermal synthesis method, and the influences of alcohol amount, KOH/NaOH ratio and alkality of starting solution were investigated. The addition

20、of alcohol improved the pressure of reaction vessel, which is in favor of synthesis of one single phase; however, compositions were changed at the same time. It was easy to synthesize single-phase NaNbO_3-KNbO_3 powders with compositions near either end members, but two-phase perovskites appeared around (K_(0.5)Na_(0.5)NbO_3.Lead-free KNN ceramics with relative densities more than 99% have been prepared using hydrothermal precursors and spark plasma si