07) 1544–1548www.elsevier.com/locate/matlet

Materials Letters 61 (20

Microstructure and dielectric characteristics ofNi ion doped BaTiO3 nanoparticles

A. Jana a, T.K. Kundu b,⁎

a Department of Physics, Vidyasagar University, Midnapore-721102, W.B., Indiab Department of Physics, Visva-Bharati Santiniketan, Birbhum-731235, W.B., India

Received 5 February 2006; accepted 22 July 2006Available online 11 August 2006

Abstract

In this article, we report the effect of nickel oxide (NiO) addition on the microstructure and the dielectric properties of nano-barium titanate(BaTiO3). Nickel ion doped barium titanate nanoparticles have been synthesized through a chemical route using polyvinyl alcohol (PVA). Theconcentration of Ni2+ varies from 0 to 1.6 mol% in the specimens. The particle diameters in the specimen lie in the range of 24 nm to 35 nm. It isseen that the dielectric permittivity in doped specimens is enhanced compared to that of undoped BT ceramics. The dielectric permittivity showsmaxima in the specimen containing 0.6 mol% of dopant concentration. The results are explained in terms of the change in lattice parameter.c and.aof the specimens.© 2006 Elsevier B.V. All rights reserved.

Keywords: Nanoferroelectrics; BaTiO3; Nanostructured materials

1. Introduction

Barium titanate (BT) is one of the most common ferroelec-trics with a wide range of ferroelectric properties, such asspontaneous polarization, high dielectric permittivity, as well aspiezo- and pyroelectricity [1,2]. The ferroelectric properties ofBT can be efficiently controlled by doping with different dopingelements [3]. It is possible to tailor the parameters like maximumdielectric constant (.εm), transition temperature (.Tc) and d.ε/d.Tby a suitable doping. The report shows that the TiO6 octahedraare disturbed with B-site doping resulting in a broadening ofthe transition at .Tc [4]. Specifically doping with 3d transitionelements in BT stabilizes a different structural configuration inthe system [5]. Transition metal ions' substitution at the Ti sitewith oxygen vacancy compensation is confirmed, although theirbehaviour is less accurately reproduced.

Barium titanate has important applications in making diffe-rent types of multilayer capacitors where compositions aredoped heterogeneously to introduce compositional gradients

⁎ Corresponding author.E-mail address: tkkundu1968@yahoo.com (T.K. Kundu).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.07.075

that control their temperature dependence [6,7]. The incorpo-ration of the first series of transition metal impurities in BT isimportant for the use of cheaper metal electrode in multilayerBT-ceramic capacitors. Ni-electrode based multilayer ceramiccapacitors (MLC) have been successfully introduced in themarket since 1992. During the re-oxidation step for theprocessing of MLC, a small amount of NiO may dissolve intoBaTiO3. This causes a modification of the defect chemistry ofthe doped BT. Few reports are available on this issue where themicrostructural changes in NiO doped micron sized BT arediscussed [8–10]. The increased pressure for the miniaturizationof electronic devices has generated a need for nano-sized ferro-electric powders. The structure and dielectric property of ferro-electric materials are also greatly influenced by their particlesize [11–13]. Thus, it is important to examine the formation anddielectric properties of transition metal ion doped nano-sizedBT powder. We have reported previously on iron ion dopedbarium titanate nanoparticles whose dielectric behaviour wasvery uncommon and the permittivity showed a maximum at adopant concentration of 0.3 mol% [14]. In the present study, theeffect of NiO addition on the microstructure and the dielectricproperties of nano-BaTiO3 are investigated.

Fig. 1. XRD pattern of the pellet samples sintered at 1000 °C for 2 h.

1545A. Jana, T.K. Kundu / Materials Letters 61 (2007) 1544–1548

2. Experimental

The composition of the specimens is represented asBa(Ti1−.xNi.x)O3 where .x=0, 0.03, 0.06, 0.10, and 0.16 assummarized in Table 1. The specimens are prepared by amodified sol gel route. The precursor solution was synthesizedfrom barium acetate, tetraisopropyl orthotitanate (C12H28O4Ti)and nickel nitrate. The composition of the samples in mol% isgiven in Table 1.Weighted amount of tetraisopropyl orthotitanatewas hydrolyzed in a solution of ethyl alcohol and acetic acid.Another solution comprising of proportional amounts of bariumacetate and distilled water was poured into the first solution.Finally the solution of nickel nitrate was added to the abovesolution that results in mixture ‘A’. In the next stage we haveintroduced a polymer (PVA) additive to encapsulate the cationspecies in divided groups. Solution ‘A’ was poured into aseparately prepared transparent PVA solution with 5:3 volumeratios. The details of the preparation route were explained in ourprevious reports [14]. The solutions were allowed to form intoclear gel at room temperature. The gels were dried in an oven at200 °C for 24 h and calcined at a temperature of 750 °C for 2 h forthe precipitation of the crystalline phases. The calcined powderswere isostatically pressed into pellets under pressure of 5 bars for5 min. Finally the disks were sintered in air at 1000 °C for 2 h.

The structure of the sample was analyzed by X-ray diffractionusing a PhilipsX-ray diffractometer (model PW1710)with filteredCuKα radiation of wavelength .λ=0.154 nm. Average value ofparticle diameters (.D) was calculated from widths,Δ2.θ1 / 2, in theX-ray diffraction peaks with the Debye Scherer relation [15].Transmission electron microscopy (TEM 200 CX) was employedto investigate the morphology and particle diameters in the spe-cimens. We have used the capacitance method for the measure-ments of permittivity. LCR meter (Agilent-4284A) with a testfixture (parts No. 16047A) was used for the measurement of thecapacitance of the materials. A sample holder containing twocopper leads separated at 1 mm apart inside a glass tube evacuatedto a pressure level of 10−3 Torr was used for the present case. Thesamples with 10 mm diameter and 1 mm thickness coated withsilver paint (supplied byAcheson ColloidenB.V., Netherlands) onboth the surfaces were connected to the copper leads of the sampleholder. Finally the sample holder was inserted inside an ovenwhose temperature could be controlled. The stray capacitance wassubtracted every time to extract the correct capacitance values.

3. Results and discussions

The X-ray powder diffraction patterns of the specimens sintered at1000 °C are shown in Fig. 1. The patterns are identical with at least six

Table 1Composition, crystal structure and crystalline size (.d) in BaTiO3

Specimen no. .x Crystal structure Particle diameter (nm)

1 0.0 Tetragonal BT 242 0.03 Tetragonal BT 253 0.06 Tetragonal BT 254 0.10 Tetragonal BT 335 0.16 Tetragonal BT 35

distinct peaks. Interplanar spacings (.dhkl) are calculated from the peaksand these are shown in the same figure. The peaks of the diffractogramare successfully indexed with the tetragonal structure of BaTiO3

crystal. Few peaks, marked as ‘*’, are also present in the specimenswhich are fitted with crystalline barium carbonate. The intensity of thepeaks is very weak in specimen 1. The lines of barium carbonate are

Fig. 2. Transmission electron micrograph for specimen no. 4.

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most intense in specimen 3 that is doped with 0.6 mol% of NiO. With afurther increase of the dopant concentration in specimens 4 and 5 theamount of BaCO3 decreases. But the specimens do not show any phaseinvolving the NiO compound. Thus it appears that the Ni2+ ions withthe given concentration are totally soluble in BT lattice and the dopantinfluences the growth of a secondary phase in a peculiar way. The Ni2+

ions may substitute Ti4+ as reported previously [5,16]. Particle sizes(coherently diffracting domain size) in the specimens were measuredby Reitveld method. Table 1 also shows that particle sizes of BaTiO3 liein the range from 24 nm to 35 nm. Close examination of Fig. 1 revealsthat the intensity of BaTiO3 reflection gradually increases with theincreasing concentration of NiO. This fact is also evident from Table 1showing that the particle size value of BaTiO3 phase graduallyincreases with increasing mol% of NiO.

The grain size of the nanocrystalline ceramics was also estimatedusing transmission electronmicroscope (TEM). Typicalmicrograph fromspecimen 4 is shown in Fig 2. There is a distribution of particle sizes in thespecimens. Average particle diameters were calculated from the micro-graphs. Particle diameters extracted from TEM are summarized inTable 1. The results estimated from TEM reasonably agree with thoseobtained from XRD. Here, the polymer precursor can very efficientlyadjust the size of the grain. The PVA molecules adsorb Ba2+ and Ti4+

cations over the molecular PVA surfaces in a specific template structure.Despite the organic counterpart disintegrates a refined template species isretained in local structures. A reaction rate limited growth of Ba2+, Ti4+

and O2− thus occurs in divided groups of small templates.The variation of dielectric constant (.ε) with temperature for

specimen 3 at different frequencies ranging from 100 Hz to 1 MHzis shown in Fig. 3. A similar feature is observed for other specimens.Ferroelectric transition in the specimen is not observed if the signalfrequency is below 200 kHz. Also Fig. 3 shows a strong dielectric

Fig. 3. Variation of dielectric constant with temperature of specimen 3 atdifferent frequencies ranging from 100 Hz to 1 MHz. Inset shows the variationof dielectric constant with frequency.

Fig. 4. (A): Temperature dependence of dielectric constant (at 500 kHz) of thesamples. (B): Temperature dependence of dielectric loss (at 500 kHz) of thesamples.

dispersion with frequency. The variation of dielectric constant withfrequency in the range from 100 Hz to 1 MHz is shown in the inset.This indicates the presence of space charge characteristics in thissystem. The transition metal ions preferably substitute on the Ti sites.The incorporation of Ni2+ ions is compensated by the formation ofoxygen vacancies. Thus the presence of space charge polarization inthis system is quite natural. The effect of space charge polarizationdecreases with the increase of the signal frequency. It is seen that thispolarization is greatly minimized above 300 kHz. The dielectricconstant (.ε) and dielectric losses (tan.δ) of the specimens measured at500 kHz as a function of temperature are shown in Fig. 4(A) and (B). In

Fig. 5. (a): Inverse permittivity with temperature (fitted with Curie–Weiss law)for specimen no. 1. (b): Inverse permittivity with temperature (fitted with Curie–Weiss law) for specimen no. 3.

Fig. 6. Variation of.εm with mole % of the dopant.

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Fig. 4(A), it is seen that the dielectric permittivity in doped specimensis enhanced compared to that of undoped BT ceramics. It is evidentfrom Fig. 4(B) that dielectric losses remain always less than 1.Dielectric behaviour of the specimens was measured both in heating

Table 2The parameters from Curie–Weiss law fitting

Specimen no. 1 2 3 4 5

.C (105 °C) 0.58 0.49 0.39 0.24 0.21Δ.Tm=.Tdev−.T.εm (°C) 0 0 0 0 20.Tc (°C) 125 120 108 120 116

and cooling cycle. The nature of .ε vs. .T curves obtained in both thecycles is similar. The value of permittivity in cooling cycle remainslower by 5–7% compared to the value in heating cycle. Roomtemperature dielectric permittivity of the specimens measured at500 kHz ranges from 150 to 380 and loss is around 0.05. It is reportedearlier that room temperature.ε of 0.3–0.5 μm sized BT ceramics wasequal to 2000 when measured at 10 kHz [2,17]. Similarly few reports[18,19] show that .ε=230 and 700 for 25 nm and 40 nm grain sizedspecimens. Dielectric constants of the present set of specimens arelower than the values of containing micron-sized crystallites but closeto the values of BT nanoparticles.

As the signal frequency is 500 kHz, the enhancement of dielectricconstant shown in Fig. 4(A) is not probably caused by the effect spacecharge polarization. The dielectric permittivity data at a frequency of

Fig. 7. The variation of the lattice parameters.c (○) and.a (▵) of the samples as afunction of dopant concentration.

1548 A. Jana, T.K. Kundu / Materials Letters 61 (2007) 1544–1548

500 kHz was also fitted with the Curie–Weiss law. The Curie–Weisslaw can be represented as

1=e ¼ ðT � T0Þ=C ð1Þwhere .C is the Curie constant. Two typical plots of 1 /.ε vs. .T fromspecimens 1 and 3 respectively are shown in Fig. 5(a) and (b). Here it isseen that the data fit well to the Curie–Weiss law. Only a deviation fromthe law is seen in specimen 5 that contains 1.6 mol% of NiO. In Table 2the parameters.C are given. A.C of 1.56×105 °C was obtained earlier forpure BaTiO3 crystal [20]. A lower value of.C in the present case repre-sents a flat response of.εwith temperature. But the existence of any spacecharge polarization in the system is not clear from this analysis.

The enhancement of dielectric constant and their dependence ondopant concentration is more clearly represented in Fig. 6. It shows thevariation of room temperature dielectric permittivity maxima (.εm) as afunction of Ni content. The curve has a maximum at a dopantconcentration of 0.6 mol%. This type of dependence of .ε on dopantconcentration is quite uncommon. Dielectric constant depends uponparticle sizes in the specimens [12,13]. It decreases with the decrease ofparticle diameter. In the present set of specimens the particle diameterlies in the range from 24 nm to 35 nm. Average particle diameter inspecimen 1 and specimen 3 is 24 nm and 25 nm respectively. Thus, thepresent enhancement of .ε is not a result of the change in particlediameter. Even if the enhancement is caused by grain size of BT,.ε willincrease linearly from specimen 1 to specimen 5. But the results of.εmvs. concentration of NiO show a maximum at 0.6 mol% of dopantconcentration. We have observed a similar feature in Fe2+ ion dopedBT nanoparticles. The results were explained in terms of the change incrystalline structure of the specimens. In the present case, the latticeparameters .a and .c were obtained by employing Reitveld's powderstructure refinement method. Fig. 7 shows the variation of the latticeparameters .c and .a with the dopant concentration. The parameter .cinitially increases with the increase of dopant concentration reaching amaximum of 0.4007 nm at 0.6 mol% of the NiO and it decreases furtherwith the increase of the dopant concentration. Thus the variation of.εmwith composition can be explained in terms of the change in latticeparameter.c of the specimens.

The ferroelectric to paraelectric transition temperatures (.Tc) for thepresent set of specimens are given in Table 2. The transitiontemperatures lie in the range of 108 °C to 125 °C. For undopedspecimen 1 a phase transition is observed at 125 °C and the transitionoccurs at 108 °C for specimen 3. The composition dependence of.Tc inμm sized BT was reported earlier [2,15]. The dielectric behaviour inthose cases was explained in terms of the change in crystalline structureof doped BT specimens. To put a similar explanation here wehave calculated .c/.a ratio from Fig. 7. The calculation shows thevalues of.c/.a=1.008, 1.003, 1.007, 1.005 from specimens no. 1, 3, 4 and5 respectively. The lowest value of .c/.a=1.003 is obtained fromspecimen 3 that shows the lowest transition temperature (.Tc=108 °C).Thus the variation of.Tc with dopant concentration is also explained interms of the change in crystal tetragonality in the specimens.

4. Conclusion

Nickel ion doped barium titanate (BaTiO3) nanoparticles havebeen synthesized through a chemical route using polyvinyl

alcohol (PVA). The concentration of Ni2+ varies from 0 to1.6 mol% in the specimens. The particle diameters in thespecimen lie in the range 24 nm to 35 nm. The polymer precursorcan very efficiently adjust the size of the grain as it serves as asurfactant to encapsulate the cationic species in divided groupsduring the reaction. It is seen that the dielectric permittivity indoped specimens is enhanced compared to that of undoped BTceramics and ferroelectric–paraelectric phase transition temper-ature (.Tc) depends upon the dopant concentration in thespecimens. The dielectric permittivity shows maxima in thespecimen containing 0.6 mol% of dopant concentration. Theenhancement is caused due to unusual change in crystallinestructure of the specimens. The lattice parameters.c, obtained byemploying Reitveld's powder structure refinement method,initially increases with the increase of dopant concentrationreaching a maximum of 0.4007 nm at 0.6 mol% of the NiO and itdecreases further with the increase of the dopant concentration.

Acknowledgements

This work is financially supported by DST, Govt. of Indiathrough NSTI scheme.

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