Narrow sintering temperature
window for(K,Na)NbO3-bad lead-free
piezoceramics caud by compositional gregation
Jian Fang1,Xiaohui Wang*,1,Ruzhong Zuo2,Zhibin Tian1,Caifu Zhong1,and Longtu Li1
1State Key Laboratory of New Ceramics and Fine Processing,Department of Materials Science and Engineering,Tsinghua University, 100084Beijing,P.R.China
2Institute of Electro Ceramics and Devices,School of Materials Science and Engineering,Hefei University of Technology, 230009Hefei,P.R.China
Received20April2010,revid1December2010,accepted3December2010
Published online12January2011
Keywords ceramics,KNbO3,NaNbO3,piezoelectric materials,gregation,sintering
*Corresponding author:e-mail*****************.edu,Phone:þ8601062792689,Fax:þ8601062792689
逼和In this paper,a typical Li-and Ta/Sb-modified alkaline niobate-bad ceramics prepared by conventional sintering were investigated in terms of different sintering temperatures and dwelling times.Compositional gregation phenomenon is found to exist in the alkaline niobate-bad ceramics sintered at the temperatures above their melting points.As a result, microstructures of the ceramics become nonuniform,and electrical properties of them are degraded.Results indicate that compositional gregation is also a factor in the narrow sintering temperature window for alkaline niobate-bad ceramics.
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1Introduction In recent years,various attempts have been made to arch for alternative lead-free piezoelectric ceramics and considerable progress have been achieved [1–4].Among them,Li-and Ta/Sb-modified(K,Na)NbO3 (KNN)ceramics have been considered as promising candidates for lead-free piezoelectric ceramics becau of their excellent electrical properties[1].However,limited by poor sinterability and performance repeatability,such materials have not been widely ud in piezoelectric devices. So far,the KNN-bad ceramics are mostly prepared within a narrow processing window in terms of the sintering temperature.Moreover,the sintering temperatures of the ceramics in air are clo to their melting points[5,6].Slight fluctuation of sintering temperature may result in deterio-rated
performance[7–9].At one time,the volatilization of alkali-metal elements was considered as a main cau resulting in abnormal grain growth and compositional inhomogeneity,and many rearchers had done much good work on the problem[10–13].However,the reported quantitative chemical analys are not consistent with each other,and the problem of poor performance repeatability caud by high-temperature instability has not been solved. Hence,the volatilization of alkali-metal elements may not be the unique factor causing the narrow sintering temperature window for alkaline niobate-bad ceramics.
Compositional gregation is a common physical phenomenon during the process of liquid-pha sintering. The objective of the paper is to reveal that compositional gregation is also a factor causing the high-temperature instability of KNN-bad piezoceramics.The microstruc-tures and properties of a typical Li-and Ta/Sb-modified KNN ceramics sintered under different sintering tempera-tures were studied,and the pha diagram for the solid-solution KNbO3–NaNbO3system was ud as an auxiliary theoretical analysis tool.Additionally,the lected-area diffraction patterns(SADPs),high-resolution transmission electron microscopy(HRTEM),and distribution of chemical elements were studied as further evidences.
2Material and methods A typical Li-and Ta/Sb-modified KNN composition,0.9525(Na0.5K0.5NbO3)–
0.0475Li(Ta0.4Sb0.6)O3(KNN-LTS),was prepared by a conventional solid-state method from high-purity alkali carbonates and oxides of Nb,Sb,and Ta(>99.9%).After being mixed according to the nominal composition,the powder mixtures were calcined twice at8508C for5h.
Phys.Status Solidi A,1–4(2011)/DOI10.1002/pssa.201026500
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The calcined powders were then milled again for24h with a PVB binder(0.5wt%).After compaction,disc specimens were sintered isothermally at different tempera-tures for various dwelling time at a heating rate of38C/min in a aled crucible to minimize the evaporation of alkali metals.For
electrical measurements,silver paste was screen printed on both sides of the disc samples.Electric poling was conducted at1108C in a silicone oil bath by applying a dc field of 2kV/mm for30min.For microstructure measure-ments,ceramics were polished and etched at10008C for1h.
Microstructures were obrved by SEM(Leo-1530, Oberkochen,Germany).TEM bright-field images,SADPs, and HRTEMs were obrved by FE-TEM(JEM-2010F, Tokyo,Japan),and the distribution of chemical elements was determined by its EDS microanalysis appendix.The piezo-electric coefficient d33was measured by a quasistatic piezoelectric coefficient testing meter(PM300, PIEZOTEST,London,UK)and planar electromechanical coupling factor K p were tested by the resonance-antireso-nance method with an impedance analyzer(HP4192A,HP, Santa Clara,USA).Dielectric properties at room tempera-ture were determined by a capacitance meter(HP4278A,HP, Santa Clara,USA)at1kHz and the bulk densities were obtained by Archimedes’method.
3Results and discussion Figure1shows the micro-graphs of the KNN-LTS ceramics sintered under different conditions.Figures1a–c show that the microstructures of the ceramics sintered at11058C are almost uniform.With the increa of dwelling time,grains gradually grow but almost no large agglomerate-like areas or abnormal grains can be found.However,as shown in Figs.1d–f,the microstr
uctures of the ceramics sintered at11208C are completely different. Large agglomerate-like areas andfine matrix grains together constitute the microstructures of them.Furthermore,with the increa of dwelling time,the microstructure nonuniformity becomes more rious and agglomerate-like areas or abnormal grains become larger.In liquid-pha sintering, angular grains often grow abnormally[14].Hence,it can be considered that the nonuniform microstructures of the ceramics sintered at11208C are caud by the appearance of liquid pha,and hardly any liquid pha exists in the ceramics sintered at10508C.
a型血女人的性格The properties of the KNN-LTS ceramics are shown in Table1.The electrical properties of the ceramics sintered at a lower temperature(ceramics1,2,3)are much better than that of samples sintered at a higher temperature(ceramics4,5,6, 7),although they have similar densities.Furthermore,for the ceramics sintered below11058C,the electrical properties increa with increasing relative densities.By increasing dwelling time or sintering temperature,the relative densities of the ceramics sintered above11208C can be enhanced to some extent,however,the performance does not improve. Considering the clo sintering temperatures,dwelling times and relative densities of the ceramics,it is difficult to explain the phenomena only by using the volatilization of alkali-metal elements.Thus,there may be other factors that riously influence the per
formance of the KNN-LTS ceramics sintered above a certain temperature.And taking the above microstructure analys into account,the appear-ance of liquid pha may be a factor.
There is no pha diagram for the KNN-LTS ceramics, but the pha diagram for KNbO3-NaNbO3system[15]can be ud as an auxiliary theoretical analysis tool.There are two obvious characteristics about the KNN system,one is a flat solid–liquid pha line,and the other is a relative low melting point of about11108C around Na/K¼50/50,for that the sintering temperatures of most KNN-bad ceramics
2J.Fang et al.:Narrow sintering temperature window for(K,Na)NbO3-bad lead-free piezoceramics
大学生打工
Figure1SEMoftheKNN-LTSceramicsisothermallysinteredat11058Cfor0min(a),20min(b),and6h(c);11208Cfor0min(d),20min (e),and6h(f).
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reported [16,17]are around 11008C.As a result,rious compositional gregation is easy to occur in
KNN-bad ceramics.Figure 2shows the sketch of compositional gregation phenomenon in KNN-bad ceramics.When the KNN-bad ceramics with ‘‘a’’composition sintered at a temperature above the melting point (for example,11208C),compositional gregation will inevitably appear in the sintered ceramic since the sintered ceramic is impossible to reach a thermal balance at the cooling rates of most ordinary sintering process.Hence,the sintered ceramic will not be an ‘‘a’’compositional sample,but a liquid ‘‘b’’and solid ‘‘c’’mixed compositional sample.Besides,the Na/K atomic
ratio of liquid ‘‘b’’is less than 1,but that of solid ‘‘c’’is bigger than 1.
The TEM bright-field image,SADPs,and HRTEM of the ceramic sintered at 11208C for 20min are shown in Fig.3.As shown in Fig.3a,amorphous rings can be found in SADP 1,2,and 3,revealing the existence of liquid pha.Furthermore,the intensity of the amorphous ring gradually weakens and that of the reciprocal lattice gradually strength-ens from SADP 1to SADP 3,suggesting gregation of liquid pha and solid pha.In accordance with that,the HRTEM also reveals a clear solid/liquid interface,as shown in Fig.3b.In order to identify the distribution of chemical elements of a single grain,the grain in Fig.3a underwent energy-dispersive spectroscopy (EDS)analysis from A point to B point,and the results are shown in Fig.4.It can be en that the contents of Nb,Sb,and Ta decrea g
中国风水罗盘radually to a low level after a certain distance from the grain boundary to the grain core,exhibiting a ‘‘core–shell’’-like structure.However,the distributions of Na and K are very different.From the grain boundary to the grain core,the content of Na increas gradually to a high level after a certain distance,but that of K first increas,then decreas,and finally increas to a high level.Furthermore,the Na/K atomic ratio changes from a value less than 1to a value greater than 1,and finally fluctuates around 1.All the above results suggest that the grain is partially melting (the edge region of the grain)and undergoes a compositional paration,exhibiting a core–shell-like structure.
Phys.Status Solidi A (2011)
3
Table 1Properties of the amic sintering conditions d 33K p (%)e T 33 e 0tan d relative density (%)11100–3h 26543.415700.028894.721105–2h 26646.016380.028195.831105–6h 29646.020340.027297.241120–20min 20940.511510.029294.551120–4h 23343.312290.032695.761
120–6h 20141.412850.027896.27
1130–4h
下水管道安装
190
39.0刑罚种类
1281
0.0283
95.9
Figure 2(online colour at:)Sketch of com-positional gregation in KNN-bad
萎凋是什么意思
虚拟语气的用法
ceramics.
Figure 3TEM bright-field image and its corresponding SADPs of the ceramics sintered at 11208C for 20min (a),and the HRTEM of its corresponding solid/liquid interface (b).
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Taking the previous experimental results into account,it can be concluded that the melting point of the KNN-LTS ceramics may be between1105and11208C.And for ceramics sintered at11208C or higher temperatures, compositional gregation takes place,leading to the appearance of liquid pha and compositional inhomogen-eity.The appearance of liquid pha plays an important role in the formation of nonuniform microstructures of KNN-bad ceramics,which will induce aggregations of manyfine matrix grains and even abnormal grains,as shown in Figs.1d–f.In addition,compositional gregation will result in a large deviation from the original stoichiometric proportion,and conquently may lead to a rious degradation in performance,as shown in Fig.4and Table1.
4Conclusions Compositional gregation exists in the KNN-LTS ceramics sintered above their melting points. It leads to the appearance of liquid pha and compositional inhomogeneity,resulting in nonuniform microstructures, and degraded electrical properties accordingly.All the results indicate that compositional gregation is also a factor in the narrow sintering temperature window for alkaline niobate-bad ceramics.The suppression of com-positional gregation could be an effective solution to the problem of poor performance repeatability,and a great help to the fabrication of high-performance KNN-bad ceramics.
Acknowledgements The work was supported by National Science Fund for distinguished young scholars(No.50625204),Science Fund for Creative Rearch Groups(No.50921061), Ministry of Sciences and Technology of China through National Basic Rearch Program of China(973Program,2009CB623301), Outstanding Tutors for Doctoral Disrtations of S&T project in Beijing(No.YB20081000302)and Tsinghua University Initiative Scientific Rearch Program.
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4J.Fang et al.:Narrow sintering temperature window for(K,Na)NbO3-bad lead-free piezoceramics
Figure4(online colour at:)EDS line profiles of the composition distribution across the grain in Fig.3a from A to B,and the Na/K atomic ratio across the grain.
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