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Materials Chemistry and Physics 141 (2013) 549e552

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Effects of sintering temperature on microstructure and electricalproperties of 0.9Pb(Mg1/3Nb2/3)O3e0.1PbTiO3 modified with CuO

Methee Promsawat a, Jenny Y.Y. Wong c, Anucha Watcharapasorn a,b,Sukanda Jiansirisomboon a,b,*

aDepartment of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, ThailandbMaterials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, ThailandcDepartment of Chemistry and 4D LABS, Simon Fraser University, Burnaby, BC V5A 1S6, Canada

h i g h l i g h t s

� The grain size significantly increased with sintering temperature of 1050 �C.� Dielectric constant reached to the highest with sintering temperature of 1050 �C.� Ferroelectric properties were improved with sintering temperature of 1050 �C.� The optimum sintering temperature for the PMNT/1wt%CuO ceramic is 1050 �C.

a r t i c l e i n f o

Article history:Received 23 April 2012Received in revised form25 February 2013Accepted 24 May 2013

Keywords:CeramicsSinteringMicrostructureElectrical properties

* Corresponding author. Department of Physics andScience, Chiang Mai University, Chiang Mai 502941921x631; fax: þ66 53 943445.

E-mail addresses: m.promsawut@hotmail.com (M(J.Y.Y. Wong), anucha@stanfordalumni.org (A. Watcmu.ac.th, sukanda@chiangmai.ac.th (S. Jiansirisombo

0254-0584/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2013.05.060

a b s t r a c t

The structural, dielectric and ferroelectric properties of Pb(Mg1/3Nb2/3)0.9Ti0.1O3 (PMNT) ceramics pre-pared with CuO as a sintering aid at various sintering temperatures between 950 �C and 1150 �C areinvestigated. The lattice parameters slightly increase with the sintering temperature >1050 �C. A sig-nificant increase in the grain size is observed when the sintering temperature is increased from 1000 �Cto 1050 �C. The maximum dielectric constant reaches the highest value of w22,000 for the ceramicsintered at 1050 �C. For the ceramics sintered at >1050 �C, the temperature of maximum dielectricconstant and the diffuseness parameters tend to increase with the increasing sintering temperature. Theoptimal sintering temperature for this ceramic is 1050 �C, which displays significant improvements inferroelectric properties at room temperature, i.e. the increase in the remanent polarization and theferroelectric loop squareness.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

The complex perovskite solid solution 0.9Pb(Mg1/3Nb2/3)O3e

0.1PbTiO3 or Pb(Mg1/3Nb2/3)0.9Ti0.1O3 (PMNT) is a well-knownrelaxor ferroelectric material that has high dielectric permittivityand high electrostrictive coefficient around room temperature thusmakes PMNT an excellent material for applications in multilayercapacitors and electrostrictive actuators [1]. High electrical perfor-mances are obtained from high quality ceramics, which can only be

Materials Science, Faculty of00, Thailand. Tel.: þ66 53

. Promsawat), jyw2@sfu.cacharapasorn), sukanda.jian@on).

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fabricated at optimum sintering conditions. Therefore, optimizationof each parameter, such as sintering temperatures, dwell times, andheating or cooling rates is required to obtain high quality ceramics[2]. However, themajor drawback of PMNT is the volatility of PbO athigh sintering temperatures (>1200 �C) during the ceramic fabri-cation process, which degrades its performance [3]. In addition, thetoxicity of PbO is a potential threat to our health and environment[4]. Therefore, an advancement of the densification process at lowsintering temperatures for PMNT ceramics is needed.

Inmany studies, the usage of CuO as a sintering aid has shown toimprove ceramic densification in a wide range of dielectric andferroelectric materials [4e6]. Thus, it is proposed that the additionof CuO should also facilitate the densification of PMNT ceramics,thereby allowing lower processing temperatures, and improving itselectrical properties. Moreover, with the use of nano-sized CuOparticles which have large surface areas for the solid state reaction,the densification of the ceramics can be further improved.

Fig. 1. SEM images of the fractured surfaces of the PMNT/CuO ceramics sintered at (a) 950 �C, (b) 1000 �C, (c) 1050 �C, (d) 1100 �C and (e) 1150 �C. Arrows indicate the secondaryphases.

Table 1Chemical composition of secondary phases in PMNT/CuO ceramics.

Sinteringtemperature (�C)

Chemical composition (at%) Possiblephases

Pb Mg Nb Ti Cu O

950 2.68 1.01 2.31 0.22 27.89 65.88 Cu-rich1000 0.74 2.10 0.62 0.08 33.49 62.97 Cu-rich1100 28.11 e 4.77 0.78 0.65 65.69 Pb-rich1150 21.87 e 1.27 e e 76.86 Pb-rich

Note: The secondary phases indicated with arrows are revealed in Fig. 1(a), (b), (d)and (e), respectively.

M. Promsawat et al. / Materials Chemistry and Physics 141 (2013) 549e552550

Therefore, in this study, the PMNT ceramics are prepared with theaddition of CuO nanoparticles as the sintering aid, and the effects ofsintering temperature on the properties of the ceramics areinvestigated.

2. Experiment

PMNT powder was prepared by the columbite precursor method[7]. The columbite precursor (MgNb2O6) was prepared by mixingstoichiometric amounts of MgO (99.9%, Fluka) and Nb2O5 (99.9%,SigmaeAldrich) in ethanol, followed by ball-milling for 24 h usingZrO2 grinding medium. The slurry was dried at 120 �C and thepowder was calcined at 1000 �C for 4 h [3]. The columbite powderwas thenmixedandball-milledwith predetermined amounts of PbOand TiO2 powders (99.9%, SigmaeAldrich), and calcined at 850 �C for2 h. The calcined PMNT powders were added with 1 wt% of nano-sized CuO powder (99%, Nanostructured & Amorphous Materials)with particle size of w30e50 nm to form PMNT/1wt%CuO (PMNT/CuO) powder. Polyvinyl alcoholwas added to the PMNT/CuOpowderas a binder and then uniaxially pressed into pellet with underw5.5 MPa pressure. Each green body, which has 10 mm in diameterand2mmin thickness and thedensityw4.84 g cm�3,was sintered ina PMNTatmosphere at specific temperatures of 950,1000,1050,1100and 1150 �C for 2 h. Bulk density of the ceramics was determinedusing Archimedes’s method. Structural phase and compositionwerecharacterized using X-ray diffraction method (XRD, X-pert, Pan-alytical B.V., TheNetherlands) andenergy dispersive X-ray technique

(EDX, JSM-6335F, JEOL, Japan). Microstructure of the ceramics wasobserved using the scanning electron microscope (SEM, JSM-6335F,JEOL). Average grain size was determined using a mean linear inter-ceptionmethod from SEMmicrographs. In this method, a number ofstraight lines were drawn on each micrograph and interceptedlengths of grainswere obtained andaveraged.Dielectric constant ( 3r)anddielectric loss tangent (tan d)weremeasured using an LCRmeter(Hitester 3532-50, Hioki, Japan). Ferroelectric hysteresis (PeE) loopswere characterized using a computer-controlled modified SawyereTower circuit. Furthermore, the ferroelectric characteristics canbe assessed with the hysteresis loop squareness (Rsq) which canbe calculated from the empirical expression Rsq ¼ ðPr=PmaxÞþðP1:1Ec=PrÞ, where Pmax is the maximum polarization obtained atsomefinitefieldstrengthbelowdielectricbreakdownandP1:1Ec is thepolarization at the field equal to 1.1Ec [8].

Fig. 2. Powder XRD patterns of PMNT/CuO ceramics sintered at various temperatures.

Table 2Physical properties and lattice parameters of PMNT/CuO ceramics sintered at varioustemperatures.

Sinteringtemperature(�C)

Grain size (mm) Relative density (%)a Lattice parameter (�A)

950 2.0 � 0.4 96.78 � 0.01 4.0389 � 0.00031000 2.8 � 0.3 96.41 � 0.01 4.0388 � 0.00011050 7.1 � 1.6 94.59 � 0.02 4.0385 � 0.00021100 7.3 � 1.7 93.55 � 0.03 4.0417 � 0.00011150 10.5 � 1.8 92.83 � 0.03 4.0391 � 0.0001

a The theoretical density of the PMNT/1wt%CuO ceramic is 8.15 g cm�3 which wascalculated based on theoretical densities of PMN (8.17 g cm�3 [22]), PT (6.80 g cm�3

[23]) and CuO (6.51 g cm�3 [24]) obtained from powder diffraction standards.

M. Promsawat et al. / Materials Chemistry and Physics 141 (2013) 549e552 551

3. Results and discussion

The SEM images of fractured surfaces of the PMNT/CuO ceramicssintered at various temperatures are shown in Fig. 1. The samplessintered at 950 �C and 1000 �C contains dark spots which is asecondary phase, as indicated with arrows in Fig. 1(a) and (b),

Fig. 3. (a) The temperature dependence of dielectric constant and loss tangent measured atPMNT/CuO ceramics sintered at various temperatures and the inset in (b) shows the loop t

respectively. From EDX analysis, this secondary phase is found to beCu-rich and the chemical composition is shown in Table 1. How-ever, the Cu-rich phase does not appear in the samples sintered at>1000 �C. Therefore, it is expected that the Cu2þ ion can completelyenter the PMNT lattice. For the samples sintered at �1100 �C, awhite phase is observed, as indicated with arrows in Fig. 1(d) and(e), respectively. This phase is found to be Pb-rich (see in Table 1).The grain size for samples sintered between 950 and 1000 �C isw2.0e2.8 mm, while it increases significantly to w7.1 mm whensintered at 1050 �C (Table 2). For the samples sintered at �1050 �C,the grain size appears to increase with higher sintering tempera-ture. It is believed that the significant increase in grain size isattributed to the effects of the liquid phase sintering process. Asseen in the samples sintered at �1100 �C, the Pb-rich phase is ex-pected to be a liquid phase since PbO has a low melting point(888 �C [9]). With the presence of this liquid phase, it helps facili-tate ion diffusion and enhances the grain growth mechanism dur-ing sintering process. Nevertheless, the Pb-rich phase was notobserved in the ceramics sintered at 1050 �C, whichmay be that thesmall amounts of liquid phase was evaporated at the final stage ofthe sintering process. For all the samples, the density of the sinteredceramics seems to decrease with higher sintering temperature, asthe result shown in Table 2. The decrease in density is believed to beattributed to the PbO evaporation, which is consistent with previ-ous other investigations [10e12].

The powder XRD patterns of the PMNT/CuO ceramics are shownin Fig. 2 which matches the XRD standard (ICSD file No. 99710) ofPbMg0.3Nb0.6Ti0.1O3 in the Pm3m cubic space group [13]. None ofthe secondary phases, either Cu-rich or Pb-rich, are present, whichmight be because the amounts of secondary phases are lower thanthe detection limit of the instrument. Based on the position of theXRD peaks, the lattice parameter a was calculated and listed inTable 2. The result shows that the increase in sintering temperaturefrom 950 �C to 1050 �C does not significantly change the latticeparameter of the samples, which the pseudo-cubic parameter a isfound to be w4.0385e4.0389�A. However, for the samples sinteredat �1100 �C, the parameter a increases to w4.0391e4.0417 �A. It isexpected that the Cu2þ ion can substitute the smaller Mg2þ ion onB-site of PMNT lattice due to the similar ionic radii and valency(rPb2þ ¼ 1.49, rMg2þ ¼ 0.72, rNb5þ ¼ 0.64, rTi4þ ¼ 0.605 andrCu2þ ¼ 0.73 �A [14]), leading to the expansion of the unit cell.

Temperature dependence of dielectric constant and loss tangentof the ceramics measured at a frequency of 1 kHz are shown inFig. 3(a) and the relevant values are listed in Table 3. The resultsshowed that the maximum dielectric constant, 3max, of w16,000,for the ceramic sintered at 950 �C, and did not significantly changewhen the sintering temperature increases to 1000 �C. The 3max

1 kHz and (b) the PeE hysteresis loops measured at room temperature at 50 Hz of theips of a positive field side.

Table 3Dielectric and ferroelectric properties of the PMNT/CuO ceramics sintered at varioustemperatures.

Sinteringtemperature(�C)

Dielectricproperties

Tmax

(�C)d

(�C)Ferroelectric properties Loop

squareness(Rsq)

3max tan d Pr(mC cm�2)

Ec(kV cm�1)

950 16,094 0.0144 45 38 3.5 1.9 0.31000 15,884 0.0076 45 38 3.2 1.6 0.31050 22,381 0.1135 42 37 6.0 2.0 0.51100 18,508 0.1851 47 46 11.0 3.9 0.81150 16,982 0.1963 48 48 10.4 4.4 0.7

Note: 3max and tan d represent dielectric constant and dielectric loss tangent,respectively, measured at the temperature of a maximum dielectric constant (Tmax).Themeasurements were carried out at a frequency of 1 kHz. d represents diffusenessparameter as defined by the quadratic law [18]. The ferroelectric properties weremeasured at room temperature at a frequency of 50 Hz. Pr, Ec and Rsq represent theremanent polarization, coercive field and ferroelectric hysteresis loop squareness,respectively.

M. Promsawat et al. / Materials Chemistry and Physics 141 (2013) 549e552552

reaches the highest value of w22,000 with the sintering temper-ature of 1050 �C. This improvement in the dielectric properties isattributed to the significant increase in the grain size of the ce-ramics. The grain boundary, which is usually a non-polar region, isreduced with an increased grain size which results in an increase inthe dielectric constant. This is consistent with previous results re-ported by Tang et al. [15]. However, for the ceramics sintered at>1050 �C, the 3max appears to decrease while the loss tangent in-creases with higher sintering temperatures. Because at high tem-peratures, oxygen vacancies are created from the formation of thePb-rich phase and the evaporation of PbO, therefore this causesthe decrease in dielectric constant and the increase in loss tangent[16]. The temperature of maximum dielectric constant, Tmax, of theceramics sintered between 950 �C and 1000 �C did not change,while the ceramics sintered at 1050 �C has a slightly lower Tmax(Table 3). However, for the samples sintered at >1050 �C, the Tmaxtend to increase. This is attributed to the space-charge field and thelocked-in ferroelectric polarization contributions caused by thelead and oxygen vacancies, which are created due to the loss of PbOand the formation of the Pb-rich phase, resulting in the increase inphase transition temperatures, as has been discussed in the Ref.[17]. In order to determine the diffuseness parameter (d), the hightemperature slope of dielectric peak was fitted to the quadratic lawintroduced by Bokov and Ye [18] and the results are given in Table 3.For the samples sintered at temperature �1050 �C, the d parameterseems to decrease with the increasing sintering temperature, whilethe samples sintered at�1100 �C, the d parameter tends to increase.It is believed that the created lead and oxygen vacancies contributeto the decrease in the polar ordering which enhances the degree ofdiffuseness parameters [19].

The polarization versus electric field (PeE) hysteresis loops ofthe ceramics were measured at room temperature and the resultsare shown in Fig. 3(b). The ferroelectric properties, i.e. the rema-nent polarization (Pr), the coercive field (Ec) and the loop square-ness (Rsq), are tabulated in Table 3. For the samples sintered at�1100 �C, the hysteresis loops reveal that the samples areconductive. The lead and oxygen vacancies which act as the mobilecharge carriers cause this increase in leakage current as observed intheir hysteresis loops [16]. Moreover, the reduced sharpness of thehysteresis loop tips, as shows in the inset in Fig. 3(b), which isassociated with lower resistivity [20], also confirms the higherleakage current of the samples. The Pr, Ec and Rsq are 3.5 mC cm�2,1.9 kV cm�1 and 0.3, respectively, for the sample sintered at 950 �C.These values did not significantly change with the increase in

sintering temperature of 1000 �C. But, there is improvement in theferroelectric properties when the sintering temperature isincreased to 1050 �C, the Pr, Ec and Rsq are 6.0 mC cm�2, 2.0 kV cm�1

and 0.5, respectively. This improvement in ferroelectric propertiesis due to the large increase in grain size, which is consistent withprevious result reported by Eremkin et al. [21].

4. Conclusions

The PMNT/CuO ceramics were prepared by the solid state re-action at various sintering temperatures. The pseudo-cubic latticeparameter increases slightly with higher sintering temperatures upto 1100 �C. For all samples, the density tends to decrease with theincreasing sintering temperature. There is a significant increase ingrain size when sintered at 1050 �C, and it continues to increaseslightly as the sintering temperature is further increased. Themaximum dielectric constant reaches the highest value ofw22,000for the ceramics prepared at 1050 �C. For the ceramics sintered at>1050 �C, the temperature of maximum dielectric constant and thediffuseness parameters tend to increase with the increasing sin-tering temperature. Significant improvements in the ferroelectricproperties i.e. the increase in the remanent polarization and theloop squareness, is observed for the sample sintered at 1050 �C. Thedielectric and ferroelectric properties of the PMNT/CuO ceramicsindicate that the ceramics sintered at 1050 �C exhibited the bestproperties suitable for multilayer capacitor applications.

Acknowledgments

This work is financially supported by the Thailand ResearchFund (TRF) and the National Research University Project underThailand’s Office of the Higher Education Commission (OHEC). TheFaculty of Science and the Graduate School, Chiang Mai University,are also acknowledged. MP would also like to thank the financialsupport from the TRF through the Royal Golden Jubilee Ph.D.Program.

References

[1] T.R. Shrout, A. Halliyal, Am. Ceram. Soc. Bull. 66 (1987) 704e711.[2] S. Ananta, N.W. Thomas, J. Eur. Ceram. Soc. 19 (1999) 629e635.[3] S. Ananta, Mater. Lett. 58 (2004) 2781e2786.[4] D. Lin, K.W. Kwok, H.L.W. Chan, J. Appl. Phys. 102 (2007) 1e6.[5] I.W. Chen, J. Phys. Chem. Solids 61 (2000) 197e208.[6] P. Dai, H.A. Mook, G. Aeppli, S.M. Hayden, F. Dogan, Nature 406 (2000)

965e968.[7] S.L. Swartz, T.R. Shrout, J. Mater. Res. 17 (1982) 1245e1250.[8] B.M. Jin, J. Kim, S.C. Kim, Appl. Phys. A 65 (1997) 53e56.[9] S. Fushimi, T. Ikeda, J. Am. Ceram. Soc. 50 (1967) 129e132.

[10] J.P. Guha, J. Mater. Sci. 36 (2001) 5219e5226.[11] J. Ryu, J.J. Choi, H.E. Kim, J. Am. Ceram. Soc. 84 (2001) 902e904.[12] A. Udomporn, S. Ananta, Mater. Lett. 58 (2004) 1154e1159.[13] J.C. Bruno, A.A. Cavalheiro, M.A. Zaghete, M. Cilense, J.A. Varela, Mater. Chem.

Phys. 84 (2004) 120e125.[14] R.D. Shannon, J. Appl. Phys. 73 (1003) 348.[15] X.G. Tang, H.L.W. Chan, J. Appl. Phys. 97 (2005) 034109.[16] S.B. Majumder, D.C. Agrawal, Y.N. Mohapatra, R.S. Katiyar, Integr. Ferroelectr.

29 (2000) 63e74.[17] K. Okazaki, K. Nagata, J. Am. Ceram. Soc. 56 (1973) 82e86.[18] A.A. Bokov, Z.G. Ye, Solid State Commun. 116 (2000) 105e108.[19] K.M. Lee, H.M. Jang, J. Am. Ceram. Soc. 81 (1998) 2586e2596.[20] G.H. Haertling, J. Am. Ceram. Soc. 82 (1999) 797e818.[21] V.V. Eremkin, V.G. Smotrakov, S.I. Shevtsova, A.I. Sokallo, A.E. Panich,

L.P. Yurchenko, V.A. Mikheev, I.P. Bykov, Inorg. Mater. 45 (2009) 1197e1201.[22] A. Verbaere, Y. Piffard, Z.G. Ye, E. Husson, Mater. Res. Bull. 27 (1992) 1227e

1234.[23] H. Cheng, J. Ma, Z. Zhao, D. Qiang, Y. Li, X. Yao, J. Am. Ceram. Soc. 75 (1992)

1123e1128.[24] H.E. Swanson, E. Tatge, Natl. Bur. Stand. (U.S.) Circ. 539 (1953) 47e49.