Phase Structure, Microstructure and Dielectric Properties of(K0.5Na0.5)NbO3-LaFeO3 High-Temperature Dielectric Ceramics

Hualei Cheng,* Wancheng Zhou, Hongliang Du, Fa Luo, and Dongmei Zhu

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China

Boxi Xu

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798,Singapore

The (1-x)(K0.5Na0.5)NbO3-xLaFeO3 (abbreviated as (1-x)KNN-xLF, x = 0–0.03) ceramics were synthesized by the conven-tional solid-state sintering method. X-ray diffraction analysis shows that the phase structure of the (1-x)KNN-xLF ceramics trans-fers from orthorhombic to pseudocubic with increasing the LF content. The SEM studies reveal that a small amount of LF, as a

grain growth inhibitor, has an evident effect on grain size reduction. The (1-x)KNN-xLF (x = 0.02) ceramics show high permit-tivity maximum (near 2000) and low dielectric loss (<5%) in the temperature range of 100–400°C, and the capacitance variation(DC/C150°C) is keeping within �15%, indicating the potential application for the high-temperature capacitors.

Introduction

Multilayer ceramics capacitors (MLCCs) are indis-pensable components in contemporary electronicdevices.1 However, recently, many fields have expressedthe need for the MLCC which can be operated at thehigher temperatures (>200°C) than currently available,such as nuclear reactors, aerospace, and related industrialapplications.2–5 It is evident that the BaTiO3-based X8Rand X9R MLCCs will fail in the extreme environmentsbecause their ceiling temperatures are 150°C and 170°C,respectively.6–10 Large dielectric permittivity and rela-tively low permittivity variation caused by the dispersionphase at a broad temperature range make the high-tem-perature lead-free relaxor ferroelectric ceramics becomepromising candidates for high-temperature MLCC.11–30

Among various lead-free relaxor ferroelectric composi-tions, the (K0.5Na0.5)NbO3 (KNN)-based ceramics sys-tem has attracted great attention. KNN-ABO3 perovskitebinary compositions, such as BaTiO3,

20 SrTiO3,21,22

BiScO3,23,24 (Ba0.5Sr0.5)TiO3 (BST),25 Ba0.5Ca0.5TiO3

(BCT),29 and so on, have been extensively studied. Inaddition, our group has found that the KNN-basedrelaxor ferroelectrics ceramics show great potential forthe high-temperature MLCC application.23–27 Especially,the 0.9KNN-0.1(Ba0.5Sr0.5)TiO3 ceramics show a broaddielectric peak with permittivity maximum near 1500

and low dielectric loss (<4%) in the temperature rangeof 25–350°C.25 However, the dielectric permittivity andusage temperature range still need to be furtherimproved to satisfy the requirement for preparing thehigh-temperature MLCC.

To enhance the dielectric temperature stability ofKNN ceramic, the (Bi0.8La0.2)FeO3-modified KNNceramics were studied31 and the results revealed that (1-x)KNN-xBi0.8La0.2FeO3 ceramics own the orthorhombic-tetragonal morphotropic phase boundary (MPB) and therelaxor properties. Unfortunately, the degree of the relaxorbehavior is not significant. La3+ is one of frequentlyadopted dopants to help inducing the ferroelectric relaxorbehavior and the diffuseness degrees increase with increas-ing the La3+ content.32 To further increase the La3+ con-tent, lanthanum orthoferrite (LaFeO3, LF) as a new endmember is chosen to modify the KNN ceramics in thisarticle. LaFeO3 as an orthorhombically distorted perov-skite material has received extensive attention.33–36 Inaddition, the electric-field-induced long-range ferroelectricorder is suppressed and the good temperature stability isdeveloped in LaFeO3-modified PbTiO3

37 and Bi1/2(Na0.78K0.22)1/2TiO3

38 ceramics systems. The effects ofLa2O3 and Fe2O3 doping or their codoping on the sinter-ing, microstructure, and various electrical properties ofKNN ceramics have been investigated in the previousreport.39 However, the ceramics show poor densification,which affects the dielectric properties. In this work, thedense (1-x)(K0.5Na0.5)NbO3-xLaFeO3 ceramics were syn-thesized by the conventional solid-state sintering method,

*hualeicheng@163.com; duhongliang@126.com

© 2013 The American Ceramic Society

Int. J. Appl. Ceram. Technol., 1–8 (2013)DOI:10.1111/ijac.12179

and the phase structure, microstructure, and dielectricproperties of the ceramics were studied in detail. Wefound that (1-x)KNN-xLF (x = 0.02) ceramics show abroad permittivity maximum near 2000 and lowerdielectric loss (<5%) at broad temperature usage range(100–400°C), which indicates that this material may bepotential candidates for high-temperature MLCC.

Experimental Procedure

(1-x)(K0.5Na0.5)NbO3-xLaFeO3 (abbreviated as (1-x)KNN-xLF, x = 0–0.03) ceramics were prepared by theconventional solid-state sintering method. Reagent-gradeoxide and carbonate powders of K2CO3 (99.0%),Na2CO3 (99.8%), Nb2O5 (99.9%), La2O3 (99.0%), andFe2O3 (98%) were used as the starting materials. Beforeweighed, these powders were first separately dried in anoven at 110°C for 5 h. They were milled for 24 h usingplanetary milling with zirconia ball media and alcohol,

then dried and calcined at 950°C for 5 h. After the calci-nations, these powders were ball-milled again for 12 h,dried, and pressed into disks of 12 mm in diameter and1 mm in thickness under 300 MPa using polyvinyl alco-hol (PVA) as a binder. And after burning off PVA, the pel-lets were sintered at 1150°C for 2 h in the sealed Al2O3

crucibles. The obtained samples were polished. Silver pastewas fired on both sides of the samples at 810°C for20 min as the electrodes for the sake of measurements.

The phase structures of the sintered ceramics wereexamined using x-ray powder diffraction analysis with aCu Ka radiation (Philips X-Pert ProDiffractometer,Almelo, and the Netherlands) at room temperatures. Themicrostructure evolution was observed using a scanningelectron microscopy (SEM) (model JSM-6360, JEOL,Tokyo, Japan). The dielectric spectrum measurementswere performed using the LCR meter (Agilent E4980,Agilent Technologies, Santa Clara, CA) with a heat rateof 3°C/min in a temperature range of 21–500°C and afrequency range of 1–1000 kHz.

(a) (b)

(c) (d)

Fig. 1. (a) XRD patterns of (1-x)KNN-xLF ceramics and the expanded XRD patterns in the 2h range of (b) 20–25° and (c) 40–50° (d)The variation of the lattice parameters and cell volumes as a function of LF.

2 International Journal of Applied Ceramic Technology—Cheng, et al. 2013

Results and Discussion

Figure 1a shows the XRD patterns of (1-x)KNN-xLF (x = 0–0.03) ceramics. As can be seen from these

patterns, all samples show a pure perovskite phase andno secondary phase could be certified. This indicates thata stable perovskite phase is obtained when KNN contentis >97 mol%. Figures 1b and c show the expanded

(a) (b)

(c) (d)

(e) (f)

Fig. 2. SEM micrographs of the thermally etched surface for (1-x)KNN-xLF ceramics, (a) x = 0, (b) x = 0.005, (c) x = 0.01, (d)x = 0.015, (e) x = 0.02, and (f) x = 0.03.

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XRD patterns of (1-x)KNN-xLF ceramics in the 2hrange of 20–25° and 40–50°. It can be clearly seen thatthe phase structures of the samples with x ≤ 0.01, similarto pure KNN ceramic, exhibit an orthorhombic structureat room temperature, as indicated by the splitting peaksof (100)/(010) at a 2h of ~22.5° and (200)/(020) charac-teristic peaks at a 2h of ~45.5°.40 With further increasingthe LF content (x ≥ 0.015), the ceramics are the pseu-docubic structure, where the split (200) and (100) peaksgradually emerge into a single peak. Figure 1d shows thevariation of the lattice parameters and cell volumes forthe (1-x)KNN-xLF ceramics. It can be seen that the lat-tice parameters (a � c > b) for an orthorhombic struc-ture decrease with increasing the LF content (x ≤ 0.01),and the lattice parameters for the pseudocubic structureincrease with further increasing the LF content

(x ≥ 0.015). At the same time, the cell volumes of the(1-x)KNN-xLF ceramics decrease gradually with increasingthe LF content. Thereby, it can be concluded that theadditions of LF give rise to a small shrinkage of thecell volume and induce a structure phase transformation.This can be explained as follows: the replacement ofFe3+ for Nb5+ does not cause any evident change becausethey have similar ionic sizes (ionic radii: 0.064 nm and0.065 nm for Nb5+ and Fe3+). In view of the radius,it is evident that La3+ cannot enter into B-site ofperovskite, but can occupy A-site of the ceramics. Theoccupation of La3+ at A-site tends to shrink the latticeowing to the formation of A-site cation vacancies anda smaller ionic size of La3+ than that of K+ and Na+

(K+: 0.164 nm and Na+: 0.139 nm). Similar phenomenahave been reported in (La, Ta)-modified KNNceramics.32

Figure 2 shows the SEM micrographs of the ther-mally etched surface for (1-x)KNN-xLF ceramics. Itshould be noted in Fig. 2 that the quasi-cubic grain sizedistribution with clear grain boundary can be seen fromall the ceramics. For the undoped sample (x = 0), aninhomogeneous grain size distribution can be seen inFig. 2a, and the average grain size is about 10 lm. Fur-ther increasing the LF content (x ≥ 0.005), the grainsizes become fine and more uniform. The average grainssize is 2.5 lm for the composition (x = 0.005),~1.5 lm for the composition (x = 0.01 and 0.015), and~1 lm for the composition (x = 0.02 and 0.03). Detailsof the average grain size G and relative density q of thematerials studied are summarized in Table I. By compar-ison, the grain size variations are similar to the previousreport,39 whereas, the densities show the differentchange. In addition, it is clearly found that the density

Table I. Average Grain Size (G), Relative Density(q), Transition Temperature of Tetragonal-Cubic(Tc), Transition Temperature of Orthorhombic-Tetragonal (TO-T) and Maximum er Values at Tc

of (1-x)KNN-x LF Ceramics

KNN-x LFG(lm)

q(%)

Tc

(°C)To-T

(°C) emax (Tc)

x = 0 ~10 92 408 204 6749x = 0.005 ~2.5 93 381 175 6700x = 0.01 ~1.5 95 366 167 6136x = 0.015 ~1.2 90 391 197 2016x = 0.02 ~1.0 89 375 167 1918x = 0.03 ~0.7 85 373 150 1672

(a) (b)

Fig. 3. Temperature dependence of dielectric permittivity for (1-x)KNN-xLF ceramics measured at 10 kHz.

4 International Journal of Applied Ceramic Technology—Cheng, et al. 2013

increase significantly with increasing the LF contentwhen x ≤ 0.01; whereas the density become slightlydecrease with further increasing the LF content whenx ≥ 0.015. It can be thought that a change in phasestructure may make the mass transportation more diffi-cult in compact structures, as KNN ceramics changefrom an orthorhombic to a pseudocubic structure withincreasing the LF content.

The temperature dependence of dielectric permittivity(er) of (1-x)KNN-xLF at a frequency of 10 kHz is plottedin Fig. 3. Pure KNN ceramic has two phase transitionsabove the room temperature, corresponding to theferroelectric orthorhombic-tetragonal polymorphic phasetransition (TO-T) at ~200°C and the tetragonal-cubictransition (Tc) at ~420°C.

41 It can be observed in Fig. 3

that all the samples investigated in this work, similar topure KNN ceramic, have two phase transitions above theroom temperature. Differently, both of these phase transi-tions shift to the lower temperatures and the highest per-mittivity values emax of (1-x)KNN-xLF ceramics at Tc

decrease with increasing the LF content. The details of thephase transition temperatures and the emax values (at Tc)of the materials are listed in Table I. The broader permit-tivity peaks are observed in (1-x)KNN-xLF (x ≥ 0.01),which can be called diffuse phase transition. These similarphenomena can be found in KNN-(Bi0.8La0.2)FeO3

ceramics31 and La2O3 doping or Fe3+-La3+ codopingceramics.32,37–39 In these researches, the broader permit-tivity peaks and the decreased emax values at Tc of theceramics are due to the diffuse phase transition. Thechange mechanism on the phase transition temperatures isdue to the heterogeneous doping. More interesting, thebroader permittivity peaks feature at a broad temperaturerange is intensified when the LF content is >0.015. Partic-ularly, 0.98KNN-0.02LF ceramics exhibit a very stabletemperature dependence of dielectric permittivity withpermittivity maximum near 2000 in the temperaturerange of 100–400°C, which is higher than that of theprevious report due to the enhanced densities.39

Figure 4 shows the temperature dependence ofdielectric loss (tan d) of (1-x)KNN-xLF at a frequency of10 kHz. It is found from Fig. 4 that the tan d of pureKNN ceramic possess lower dielectric loss (tan d < 5%)at the high temperature from 21 to 400°C. After addingLF into the KNN, the tan d of all the ceramics is lowerat a broad temperature usage range. Particularly,0.98KNN-0.02LF ceramics show a low tan d (<3%) inthe 21–350°C range and low dielectric loss (<5%) in thetemperature range of 21–400°C; the tan d of the (1-x)KNN-xLF (x ≥ 0.005) ceramics increase rapidly whenthe temperature is beyond 400°C owing to the conduc-tive losses.

Figure 5 shows the temperature coefficient of capac-itance (TCC) curves of (1-x)KNN-xLF (x ≥ 0.015)ceramics. TCC is equal to (C-C150°C)/C150°C, where Cand C150°C represent the capacitance value at measuringtemperature and 150°C, respectively. It is found that theTCC curves vary obviously with the content of LF. Bothof the capacitance variation (DC/C150°C) for the sampleswith x = 0.015 and x = 0.02 are smaller than �15%from 100°C to 400°C. The flat TCC curves and slightcapacitance variation may attribute to the diffuse phasetransition of relaxor ferroelectrics. The more diffuse ofthe ceramics is, the broader dielectric peak and the lowerpermittivity variation are. The similar phenomena hasalso been observed in other KNN systems, such as,BiScO3-(K0.5Na0.5)NbO3

23,24 and KNN-(Ba0.5Sr0.5)

Fig. 4. Temperature dependence of dielectric loss for (1-x)KNN-xLF ceramics measured at 10 kHz.

Fig. 5. Temperature coefficient of capacitance (TCC) curves of(1-x)KNN-xLF (x ≥ 0.015) ceramics as a function of LF. Theinset is the temperature coefficient of permittivity as a function of LF.

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TiO3.25 The stable dielectric permittivity (near 2000),

low dielectric loss (<5%), and little capacitance variation(within �15%) at a broad temperature usage range(from 100 to 400°C), indicate that the 0.98KNN-0.02LF ceramics may have great advantages for high-

temperature MLCC applications. The inset is the relativepermittivity data of all compositions measured at10 kHz are used to determine the temperature coefficientof permittivity (TCe) in accordance with the followingequation42:

(a) (b)

(c) (d)

(e) (f)

Fig. 6. Temperature dependence of dielectric permittivity of (1-x)KNN-xLF ceramics under various measuring frequencies.

6 International Journal of Applied Ceramic Technology—Cheng, et al. 2013

TCs ¼ ð 1

e300Þ½� ðe200 � e400Þ

200�

where e200, e300, and e400 are the relative permittivity at200°C, 300°C, and 400°C, respectively. For high-tem-perature capacitor applications, it is desirable to haveTCe close to zero for optimum performance. In the inset,it is found that the magnitudes of TCe decrease and areclose to zero with increasing the LF content, which alsoconfirm the potential application for the high-tempera-ture MLCC applications.

Figure 6 shows the temperature dependence of thedielectric permittivity of (1-x)KNN-xLF ceramics undervarious measuring frequencies. For the samples withx = 0, no obvious frequency dispersion is observed;meanwhile, no diffuse phase transition can be found.These results indicate that pure KNN ceramic is a nor-mal ferroelectric. The strong frequency dispersion of thedielectric permittivity can be seen for the samples withx = 0.005 and 0.01, but the diffuse phase transition isnot obvious, so, we think that the samples withx = 0.005 and 0.01 only show a “relaxorlike” character-istic. Further increasing the LF content, these sampleswith x ≥ 0.015 show both the diffuse phase transitionand the frequency dispersion of the dielectric permittiv-ity. Therefore, the (1-x)KNN-xLF ceramics (x ≥ 0.015)are indeed lead-free relaxor ferroelectrics. Based on abovediscussions, it can be concluded that (1-x)KNN-xLFceramics show a transition from a normal ferroelectric torelaxor ferroelectric with increasing the LF content, andthis phenomenon may be caused by many reasons. Inperovskite-type compounds, the relaxor behavior appearswhen at least two cations occupy the same crystallo-

graphic site A or B. A cationic disorder induced byB-site substitution is always regarded as the main deriva-tion of relaxor behavior. However, according to previousreport by Guo et al.,43 the relaxor behavior in KNN-based ceramics should attribute to a cationic disorderinduced by both A-site and B-site substitutions. In thesolid solution of (1-x)KNN-xLF, K+, Na+, and La3+ ionsoccupy A sites of ABO3 perovskite structure because oftheir large ionic radius, and Nb5+ and Fe3+ ions occupyB-sites, where Nb5+ and Fe3+ also possess differentvalences and ionic radius. Therefore, there are the ran-dom fields caused by the ion disorder and which hinderthe long-range dipole alignment and then lead to therelaxor behavior.

Figure 7a shows the P-E hysteresis loops (at 1 Hz)of (1-x)KNN-xLF ceramics measured at room tempera-ture. One can see that the pure KNN ceramic has wellsaturated loop, whereas that the P-E hysteresis loopsbecome slimmer with increasing the LF content. It iswell known that another typical characteristic of relaxorferroelectrics is a slim P-E loop. A slimmer hysteresisloop observed for (1-x)KNN-xLF further confirm therelaxor behavior in (1-x)KNN-xLF ceramics. Figure 7bgives the coercive field 2EC value and the remanentpolarization 2Pr value at room temperature as a functionof LF. As seen from Fig. 7b, the 2EC value increaseswith increasing the LF content, whereas the 2Pr valueexhibits a converse varying trend. The decrease in 2Prvalue confirming that the additions of LF would weakenthe ferroelectric properties of the ceramics, because theadditions of LF give rise to a small shrinkage of the cellvolume and it changes the structure of KNN fromorthorhombic to pseudocubic symmetry, thereby reducingthe ferroelectricity.

(a) (b)

Fig. 7. (a) P-E hysteresis loops (at 1 Hz) of KNN-xLF ceramics measured at room temperature. (b) Coercive field 2EC value and the rem-anent polarization 2Pr value at room temperature as a function of LF.

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Conclusion

In conclusion, this work has demonstrated the devel-opment of new lead-free relaxor ferroelectrics forhigh-temperature MLCC. The results are summarized asfollows:1. The (1-x)KNN-xLF ceramics were synthesized by the

conventional solid-state sintering. The phase structureand microstructures of the ceramics change withincreasing the LF content.

2. The temperature stability of KNN ceramics is furtherimproved with increasing the LF content. (1-x)KNN-xLF (x = 0.02) ceramics show high permittivity max-imum (near 2000), low dielectric loss (<5%), and lit-tle capacitance variation (DC/C150°C ≤ �15%) inthe temperature range of 100–400°C, which indicatethe potential application for high-temperatureMLCC.

3. The diffuse phase transition and frequency dispersionof the dielectric constant, which are two typical char-acteristics of relaxor ferroelectrics, are observed in the(1-x)KNN-xLF ceramics (x ≥ 0.015). These resultsindicate that (1-x)KNN-xLF ceramics (x ≥ 0.015)are high-temperature lead-free relaxor ferroelectricceramics.

Acknowledgments

This work was supported by the Doctorate Founda-tion of Northwestern Polytechnical University (NoCX201108), the National Natural Science Foundation ofChina (Grant No. 51072165), and the fund of StateKey Laboratory of Solidification Processing in NWPU(No. KP200901 and SKLSP201104).

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