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Journal of Alloys and Compounds 472 (2009) 516–520

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Journal of Alloys and Compounds

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Electrical properties of copper and titanium-codoped zinc ferrites

Hsing-I. Hsiang ∗, Yi-Lang LiuParticulate Materials Research Center, Department of Resources Engineering, National Cheng Kung University, Tainan, Taiwan, ROC

a r t i c l e i n f o

Article history:Received 5 November 2007Received in revised form 5 May 2008Accepted 6 May 2008Available online 18 June 2008

Keywords:Zinc ferritesElectrical propertiesDielectric properties

a b s t r a c t

In this study, the effects of Cu and Ti substitution on the sintering behavior, substitution mechanism,resistivity and dielectric properties of (Zn)(Cux Tix Fe1.98−2x)O3.97 were investigated using X-ray diffrac-tometer, scanning electron microscopy, and dilatometer. The results indicate that codoped Cu2+ and Ti4+

can effectively promote zinc ferrite densification. For the samples with x = 0–0.05, Cu2+ and Ti4+ ions dis-solved into the spinel structure and mainly occupied the octahedral sites, which resulted in an increasein resistivity. For the specimen with x = 0.1 and 0.2, the Cu-rich and Zn-rich precipitates occurred at thegrain boundary, leading to a decrease in resistivity. The variations in dielectric constant for samples withvarious x values as a function of frequency can be explained using the space charge polarization arisingfrom the differences between the conductivity of the various phases present.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Nonmagnetic ferrites such as Zn ferrite and CuZn ferrite havebeen widely used in the electronics industry. Recently, multilayerchip LC filters have been developed as a promising electromagneticinterference (EMI) device [1]. They are made with a cofired multi-layer structure of ferrite, dielectric and internal conductors. One ofthe most important processes in manufacturing defect-free mul-tilayer chip LC devices involves capacitor and inductor materialscofiring at a low temperature. Mismatched densification kinet-ics and severe chemical reactions between the different materialscould generate undesirable defects such as delamination, cracksand camber in the final products [2,3]. Nakano et al. [4] reportedthat a nonmagnetic CuZn ferrite can act as an intermediate layerat the interface between the dielectric and NiCuZn ferrite layer toprevent the above defects in multilayer chip LC devices. The induc-tance value in a multilayer chip inductor quickly declines as thecurrent exceeds the rated current (dc superposition characteris-tic). This is caused by the magnetic saturation due to the closedmagnetic path in the magnetic body. Tsuzuki [5] reported that anexcellent dc superposition characteristic can be obtained when anonmagnetic ferrite (CuZn ferrite) layer is sandwiched betweenthe NiCuZn ferrite layers. For electronic application, it is impor-tant for a nonmagnetic ferrite (zinc ferrite) to be densified at a lowtemperature and have a high insulation resistivity.

Ahmed et al. [6] observed that the addition of Cu can effec-tively decrease the densification temperature of NiZn ferrite. Rao

∗ Corresponding author. Tel.: +886 6 2757575x62821; fax: +886 6 2380421.E-mail address: hsingi@mail.ncku.edu.tw (H.-I. Hsiang).

et al. [7] investigated the influence of the Ti substitution concen-tration on the dc resistivity and dielectric properties of NiZn ferriteand observed that the resistivity increased with increasing Ti4+

ion addition. However, to the best of our knowledge, the Cu andTi-codoping effect on the sintering and electric properties of zincferrite has not yet been reported. In this study, the effects of xvalues on the sintering behavior, substitution mechanism, resis-tivity and dielectric properties of (Zn)(Cux Tix Fe1.98−2x)O3.97 wereinvestigated using X-ray diffractometer (XRD), scanning electronmicroscopy (SEM), and dilatometer.

2. Experimental

Ferrite powders of composition (Zn)(Cux Tix Fe1.98−2x)O3.97 with x = 0–0.2 wereprepared from reagent-grade ZnO, CuO, TiO2 and Fe2O3, mixed and then calcined at750 ◦C for 2 h. The powders were then milled for 24 h using Y-TZP balls. The pow-ders were dried in an oven and PVA was then added for granulation. The powderswere compacted using a cold isostatic press at 150 MPa. These specimens were thendebindered at 500 ◦C and sintered at 1000 ◦C for 2 h. Thermal shrinkage was mea-sured using a dilatometer (Netzsch, DIL 420C). The densities of the sintered sampleswere determined using the Archimedean method. The microstructure was observedusing scanning electron microscopy (Hitachi, S4100) and the distribution of ele-ments was measured using electron probe microanalysis (EPMA) (JEOL, JXA-8900R).The crystalline phase identification was determined using X-ray diffractometry(Siemens, D5000) with Cu K� radiation. The electrical resistivity of the samples wasmeasured using the two-probe dc technique (Keithley, Multimeter-2001). Dielec-tric constant was measured using a HP4284A LCR meter over a frequency range of100 Hz to 1 MHz.

3. Results and discussion

Fig. 1 shows the XRD patterns for the general formula (Zn)(Cux

Tix Fe1.98−2x)O3.97 ferrite samples with various x values. For sampleswith x ≤ 0.1, the crystalline structure remained a cubic spinel struc-

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Fig. 1. XRD patterns for the general formula (Zn)(Cux Tix Fe1.98−2x)O3.97 ferrite sam-ples with various x values.

ture with no other phases observed. However, a secondary phase,ZnO, was observed for the sample with x = 0.2.

The variation in the lattice parameter as a function of the x valueis shown in Fig. 2. The lattice parameter increased as the x valueincreased from 0 to 0.05. This is in good agreement with the obser-vation that the lattice parameter of Cu1+xTixFe2−2xO4 increasedwith increasing x value reported by Patil et al. [8]. Tawfik [9]investigated the effect of CuO addition on the lattice parameterin Ni0.65Zn0.35CuxFe2−xO4 and observed that the lattice parame-ter decreased with increasing Cu2+ content in spite that Cu2+ ion(0.073 nm) has an ionic radius greater than Fe3+ ion (0.0645 nm).These phenomena can be explained using defect chemistry. In gen-eral Cu2+ and Ti4+ ions have a strong tendency to occupy octahedralsites, while Zn2+ ions prefer to occupy tetrahedral sites and Fe3+ ionsare distributed between the two sites for a spinel structure [10–12].Therefore, Cu2+ and Ti4+ ions will occupy octahedral sites and cre-ate oxygen vacancies and iron vacancies, respectively, according tothe following defect reaction equation:

2CuO → 2Cu′Fe + Vo•• + 2Ox

o (1)

3TiO2 → 3TiFe• + V′′′

Fe + 6Oxo (2)

The oxygen vacancy concentration increases when the Cu2+ ionssubstitution amount is increased, resulting in the lattice parameterof Ni0.65Zn0.35CuxFe2−xO4 decreasing with increasing Cu2+ content.For the Cu2+ and Ti4+-codoped zinc ferrite, the neutrality approxi-mation can be written as follows:

[Cu′Fe] + 3[V′′′

Fe] = [TiFe·] + 2[Vo · ·] (3)

This neutrality approximation predicts that the defect concentra-tion due to Cu2+ substitution, [Cu′

Fe], increases linearly with thatcreated by Ti4+, [TiFe

•]. In the case of the (Zn)(Cux Tix Fe1.98−2x)O3.97

Fig. 2. Variation in the lattice parameter as a function of the x value.

Fig. 3. Dilatometric analyses results of samples with various x values.

samples, the molar quantities of doped Cu2+ and Ti4+ were equal.The Cu′

Fe and TiFe• defect species present at equal molar quanti-

ties are expected to completely compensate for each other withoutcreating extra oxygen and iron vacancies for charge compensation.In addition, the ionic radius of Cu2+ and Ti4+ are both larger thanthat of Fe3+, which results in the lattice parameter of (Zn)(Cux TixFe1.98−2x)O3.97 increasing with increasing x value. Further increasein x value up to 0.1, the lattice parameter decreased. This may beexplained by the fact that a certain number of Fe3+ ions start tooccupy tetrahedral sites, which leads to Zn2+ ions on the tetrahedralsites being expelled and ZnO precipitation as the x value increasesabove 0.1. The replacement of Zn2+ ions on the tetrahedral sitesby Fe3+ ions, which have an ionic radius smaller than that of Zn2+

(Fe3+: 0.049 nm, Zn2+: 0.06 nm), results in the decrease in latticeparameter.

Fig. 3 demonstrates that the onset of shrinkage occurring atlower temperatures and larger shrinkages were observed for sam-ples with larger x values. The relative densities of samples withvarious x values sintered at 1000 ◦C are shown in Fig. 4. With theexception of samples with x = 0 and 0.01, the relative densitiesfor all samples reached above 90%. Note that the relative densi-ties increased with increasing x value. These results indicate thatcodoped Cu2+ and Ti4+ can effectively promote zinc ferrite densifi-cation.

Fig. 5 shows the microstructures of samples with various x val-ues sintered at 1000 ◦C. For the samples with x values = 0–0.1, thegrain sizes are all about 0.3–1 �m and no other secondary phasewas detected. In the case of the sample with x value = 2.0, the grainsizes are about 1.5–2 �m and a large amount of secondary phaseprecipitates were observed in the grain boundary or triple junctionregions. The precipitates can be identified using the EPMA map-ping method. The result is shown in Fig. 6. It was found that theprecipitates occurring at the grain boundary and existing in a triple

Fig. 4. Relative densities of samples with various x values sintered at 1000 ◦C.

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Fig. 5. Microstructures of samples with various x values sintered at 1000 ◦C (a) x = 0, (b) x = 0.01, (c) x = 0.02, (d) x = 0.05, (e) x = 0.1, and (f) x = 0.2.

junction can be identified as Cu-rich and Zn-rich phases, respec-tively. The Zn-rich phase occurred due to the substitution of Fe3+

for Zn2+ on the tetrahedral sites can be assigned as ZnO on thebasis of XRD and EDS results. The Cu-rich phase segregated nearthe grain boundary areas during cooling from the sintering tem-perature due to the large elastic strain associated with more Cu2+

substitution for Fe3+ on the octahedral sites can be relaxed by segre-gation at the grain boundary region as suggested by Fujimoto [13].However, in this study the Cu-rich phase could not be detected byXRD because its content may be below the XRD detection limit. Thiswill be further characterized using TEM in the future.

The electric resistivity of samples with various x values mea-sured at room temperature is shown in Fig. 7. Initially, the resistivityincreased rapidly as the x value was increased from 0 to 0.05. Thedecrease in resistivity was observed as the x value was increasedto 0.1–0.2. The change in resistivity with x value can be explainedby the Verwey mechanism, which consists of electron exchangesbetween ions having multiple valence states at equivalent crystal-lographic sites, such as the electron exchange between Fe2+ andFe3+ ions on octahedral sites. For samples with x = 0–0.05, Cu2+ andTi4+ ions dissolved into the spinel structure and mainly occupiedthe octahedral sites, which led to the substitution of Fe3+ ions. The

substitution ions, Cu2+ and Ti4+ ions, which do not participate inthe electronic exchange dilute the Fe2+ and Fe3+ ion concentrationon octahedral sites and also hinder electron hopping between Fe2+

and Fe3+ ions. An increase in resistivity is therefore expected withthe increase in x values. As x value increased up to 0.1, the resis-tivity declined due to the occurrence of the low resistivity phases,Cu-rich and Zn-rich phase, at grain boundary.

The variations in dielectric constant for samples with various xvalues as a function of frequency are shown in Fig. 8. For the samplewith x = 0, the dielectric constant decreased rapidly with increas-ing frequency up to 10 kHz and beyond that remained constant.A barrier-layer structure with semiconducting areas encircled byinsulating layers can be used to explain the very large dielectricconstant of zinc ferrite (x = 0) at low frequency. This behavior is char-acterized by the space charge polarization arising from differencesbetween the conductivity of the various phases present. At lowfrequency electron hopping occurs between Fe3+ and Fe2+ on theoctahedral sites. The electrons reach the grain boundary throughhopping and are piled up at the grain boundaries, which results inthe interfacial polarization. However, as the frequency is increased,the probability of electrons reaching the grain boundary decreases,which results in a decrease in the interfacial polarization. Therefore,

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Fig. 6. EPMA mapping result of the sample with x = 0.2 sintered at 1000 ◦C.

Fig. 7. Electric resistivity of samples with various x values measured at room tem-perature.

the dielectric constant decreases with increasing frequency. As thex value increased (up to 0.05), the resistivity increased, which leadsto the decrease in space charge polarization. Therefore, the slope ofthe dielectric constant variation with frequency decreased as the x

Fig. 8. Variation in dielectric constant for samples with various x values as a functionof frequency.

value increased (up to 0.05). As the x value increased (x ≥ 0.1), theresistivity declined resulting in an increase in the amount of spacecharges existing in the grains, which leads to the increase in theinterfacial polarization.

4. Conclusions

A nonmagnetic ferrite (Zn)(Cux Tix Fe1.98−2x)O3.97, with highinsulation resistance was developed in this work. The codoped Cu2+

and Ti4+ can effectively promote zinc ferrite densification. In sam-ples with x = 0–0.05, Cu2+ and Ti4+ ions dissolved into the spinelstructure and occupied mainly the octahedral sites, resulting in anincrease in resistivity. However, as the x value increased above 0.1,the resistivity decreased due to the occurrence of Cu-rich and Zn-rich precipitates at the grain boundary. The dielectric responses ofthese samples can be described using an interfacial polarizationmodel. Increasing the resistivity due to the Cu and Ti-codoping inzinc ferrite resulted in a decrease in the amount of space chargesexisting in the grains, leading to a decrease in the space chargepolarization.

Acknowledgments

This work was financially co-sponsored by the Ministry ofEconomic Affairs of the Republic of China through contract (96-EC-17-A-08-S1-023) and National Science Council of the Republicof China (NSC94-2216-E-006-026).

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