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Low Temperature Sintering andDielectric Properties of BaTiO3 CeramicsIncorporating Nano-Sized PowdersGang Liu a , Yun Jiang a & Tim W Button aa School of Metallurgy and Materials , University of Birmingham ,Edgbaston, Birmingham, B15 2TT, UKPublished online: 13 Oct 2011.

To cite this article: Gang Liu , Yun Jiang & Tim W Button (2011) Low Temperature Sintering andDielectric Properties of BaTiO3 Ceramics Incorporating Nano-Sized Powders, Ferroelectrics, 421:1,72-81, DOI: 10.1080/00150193.2011.594350

To link to this article: http://dx.doi.org/10.1080/00150193.2011.594350

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Ferroelectrics, 421:72–81, 2011Copyright © Taylor & Francis Group, LLCISSN: 0015-0193 print / 1563-5112 onlineDOI: 10.1080/00150193.2011.594350

Low Temperature Sintering and DielectricProperties of BaTiO3 Ceramics Incorporating

Nano-Sized Powders

GANG LIU,∗ YUN JIANG, AND TIM W BUTTON

School of Metallurgy and Materials, University of Birmingham, Edgbaston,Birmingham, B15 2TT, UK

The densification of nano-sized BaTiO3 (BT) using LiF and LiF-BaCO3 as sinteringaids was investigated. The density of the sample containing 1 wt.% LiF and 1 mol%BaCO3 sintered at 900◦C could achieve 5.67 g/cm3 (95% of the theoretical density).The temperature-dependence of the dielectric properties was measured from −20 to125◦C. The results showed that using 1 wt.% LiF and 1 mol% BaCO3 as a sintering aid,the BT ceramic sintered at 900◦C for 1.5 h could obtain very high room temperaturepermittivity (εr ∼5382 at 10 kHz) and with relatively small variation with temperature,which is suitable for low temperature cofired ceramic (LTCC) applications.

Keywords BaTiO3; Low-temperature; LiF; Capacitors

1. Introduction

The recent rapid developments in microelectronics and communication systems have led tothe miniaturization of dielectric components [1, 2]. During the past decades, low temper-ature co-fired ceramics (LTCC) technology has become a significant breakthrough whichwas utilized to improve the volume efficiency through integrating passive components suchas capacitors, inductors and resistors [3]. Barium titanate (BT) is a very famous ferroelec-tric material and widely used in the fabrication of multilayer ceramic capacitors due toits attractive dielectric properties [4–6]. However, the high sintering temperature of BT-based ceramics, more than 1300◦C, has become an apparent barrier for LTCC processing.Therefore, in order to achieve the miniaturization goal, there is a requriement to developlow-temperature sintered BT-based ceramics.

The improvement of ceramic densification at low-temperature can be fulfilled by eitherchemical processing such as adding flux or physical modifications such as using startingmaterials with smaller particle size. Adding flux is a much more effective way to decreasethe sintering temperature [3, 7]. Many investigations have already been done on the low-temperature sintering of BaTiO3-based ceramics [8–10]. The typical fluxes used includeZnO, CuO, LiF, and CdO. Usually the fluxes must meet the requirement that they shouldlower the sintering temperature markedly without deterioration of the dielectric propertiesof the host material [10]. Among the fluxes mentioned above, LiF is a very effective sinteringaid for BT-based dielectrics, which hence has attracted attention and also been utilized as

Received in final form August 12, 2010.∗Corresponding author. E-mail: liugangxjtu@gmail.com

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a flux agent for SrTiO3 and other peroskites [7]. LiF utilized as a flux in BT-based ceramicwas first investigated by Walker et al [11], who pointed out that the addition of 3 wt.% LiFcould lower the sintering temperature by 500◦C. Haussonne’s research work showed that apseudocubic perovskite phase formed during sintering BT with 1–2 wt.% LiF and excessBaCO3, which resulted in the improved densification behaviour [12]. The investigationby Wang et al [7] revealed that the low-temperature densification behavior exhibited byLiF-fluxed BT in the presence of excess BaCO3 was clearly due to two eutectic liquids thatformed at ≈610 and 700◦C.

Though there are many reported investigations using LiF as flux in BT-based ceramic,few are about nano-sized BT powders. Nano-sized powders have recently attracted muchattention and can lead to significant improvement in key properties. Therefore, in the currentstudy, commercial nano-sized BT powder has been employed and the effect of LiF on thedensification behavior and dielectric properties has been investigated.

2. Experimental

The commercial nano-sized BaTiO3 powder (HPB-1000, TPL Inc., USA) was utilized asthe raw material. High-purity LiF (Aldrich, UK) and BaCO3 (Fluka, Switzerland) were usedas the sintering additives. 5 wt% (w.r.t. BaTiO3 powders) phosphate esters were dissolvedin ethanol to serve as surfactant.

The nano-BaTiO3 powders and appropriate amount of additives (all based on BT) weregradually added into the solvent under constant stirring. Then milling was carried out for

Figure 1. Nano-sized BaTiO3 powder used in this investigation.

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Figure 2. Shrinkage behavior of pure and different amount of LiF doped BT, measured under aheating rate of 5◦C/min.

18 hours using zirconia balls as a ball-milling media. The mixture was dried subsequently inoven at 90◦C overnight. After deagglomeration using an agate mortar and sieving through a250 micron mesh, the mixed powder with appropriate amount was die-pressed into pelletsof 13 mm in diameter and 2 mm in thickness under a pressure of 150MPa. The green palletswere subsequently sintered at various temperatures in air.

Figure 3. XRD patterns of pure BT sintered at 1325◦C and LiF-doped BT sintered at 850 oC.

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The linear shrinkage was measured using a NetzschTM model TASC 414/3 pushroddilatometer. The density of the sintered samples were measured by the Archimedes method.Crystallographic and phase analyses for the BT powder and sintered samples were per-formed using an X-ray diffractometer (Philips X’pert) with monochromatic Cu Kα radi-ation. The detection normally ranges from 2θ = 10o to 80o. Identification of crystalline

Figure 4. Temperature and frequency-dependence dielectric properties of BT samples with differentamounts of LiF: (a) dielectric constant; (b) dielectric loss.

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Figure 5. Shrinkage behavior of doped BT with different amounts of LiF and 1mol.% of BaCO3.

phases were carried out by comparison of the XRD patterns with JCPDS standards. Thedielectric properties were measured by a HP-4194A impedance analyzer with an environ-mental chamber. The microstructures of the samples were analyzed by scanning electronmicroscopy (SEM) (JSM 7000, Jeol, Tokyo,Japan).

Figure 6. Shrinkage rates of samples with different LiF concentration.

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3. Results and Discussions

A SEM micrograph of the BT powders used in this study is shown in Fig. 1 and it can beseen that the powders have a near-spherical morphology with a primary particle size around50 nm. Fig. 2 shows the shrinkage behavior of BT samples with various amounts of LiF.All the samples shown here were heated to the target temperatures under a heating rate of5◦C/min. The densification of the pure BT sample is completed at about 1300◦C. For thedoped BT samples, they were only heated to 1000◦C. The sample with 1wt.% LiF exhibitsvery limited densification. With the increasing amount of LiF in BT, the densification of thesamples is improved. Higher addition amount of LiF leads to larger shrinkage and higherdensification.

Figure 3 shows the XRD patterns of sintered samples with and without LiF. Thecrystalline phase of pure BT is tetragonal indicated by the peak splitting at (200)/(002). BTsamples with LiF addition do not exhibit this phenomenon, which indicates that the maincrystalline phase in LiF doped BT samples is cubic. Moreover, there is no secondary phaseobserved.

Figure 4 illustrates the temperature-dependence of the dielectric properties of pure BTsintered 1325◦C for 1.5h and samples with different amounts of LiF sintered at 850◦C for 1.5hours. As shown in the inset pictures, the pure BT exhibits a very large dielectric constantand low dielectric loss. Due to the very limit shrinkage, samples with 1 wt.% LiF exhibitvery low dielectric constant. The densification of 3 wt.% LiF sample is nearly completed,however, the dielectric constant is relatively low and the dielectric loss is relatively high.The 2 wt.% LiF sample show the lowest dielectric loss and the highest dielectric constant.Therefore, the optimum addition of LiF as a single sintering aid for this nano-sized BTpowder is around 2 wt.%, which could lead to an acceptable dielectric properties.

Figure 7. XRD patterns of LiF-doped BT with excess BaCO3 sintered at 900 oC.

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All studies agreed that Ba/Ti > 1 reduces the sintering temperature and increases thebulk density of the ceramic. According to the report by Wang et al [7], BT with 2 mol%BaCO3 was a very good composition for BT sintered with LiF. However, BaCO3 was foundin the initial nano-sized BT powder according to our former study [6], so here the additionamount of BaCO3 starts from 1 mol%.

Figure 8. Temperature-dependence dielectric properties of the samples with excess BaCO3 anddifferent amounts of LiF: (a) permittivity; (b) dielectric loss, at 10 kHz.

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Figure 9. SEM photos of samples with same excess BaCO3 and different amounts of LiF sinteredat 900◦C for 1.5 h: (a) 1wt.% LiF, (b) 2 wt.%, (c) 3 wt.% LiF.

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Figure 5 shows the shrinkage behavior of BT samples with excess BaCO3 (1 mol.%).The samples doped with LiF and BaCO3 can be densified at lower temperature, nearly500◦C below the sintering temperature of pure BT (Fig. 2). With the increase amount ofLiF, the shrinkage of samples starts at lower temperature, especially from the sample with1 wt.% LiF to that with 2 wt.% LiF. When the doping concentration of LiF increasesfrom 2 wt.% to 3 wt.%, the change is not that distinct. The density of the sample (1 wt.%LiF+1 mol% BaCO3) sintered at 900◦C can reach 5. 67 g/cm3, which is nearly 95% of thetheoretical density.

Figure 6 displays the shrinkage rates of the samples shown in Fig. 5. The increaseof LiF addition leads to a decrease with temperature at which the maximum shrinkagerate occurs, when the amount of LiF increases from 1 wt.% to 2 wt.%, the temperaturedecreases by 45◦C. However, when LiF amount increases from 2 wt.% to 3 wt%, a smalldecrease is observed. Fig. 7 shows the XRD patterns of samples with 1 mol.% of BaCO3

and different amounts of LiF sintered at 900◦C for 1.5 h. The main crystalline phase of allsamples shown in Fig. 7 is cubic BaTiO3 indicated by no peak splitting at (200). Besidesthe main crystalline phase, the cubic BT, the second phase is also observed in the samples.The BaTiO0.95F0.05 phase is the second phase in both samples containing 1 and 2 wt.%LiF. However, with the increase of LiF doping concentration, the diffraction intensity ofthis phase weakens markedly. When the doping concentration of LiF reaches 3 wt.%,diffraction peaks comparing to this phase can not be found and BaCO3 appears as thesecond phase. The differences in the second phase including the type and concentrationmay contribute to the dielectric properties Fig. 8 illustrates the temperature-dependencedielectric properties of the samples with 1 mol% of BaCO3 and different amounts of LiFsintered at 900◦C for 1.5 h. For all samples distinct peaks in the dielectric constant and losswere observed at temperature around 70◦C, consitant with a phase change in the material.For the 2 wt.% and 3 wt.% doped materials the dielectric constant increases rapidly as thetemperature approaches 120◦C, indicative of approaching the curie temperature. Samplesdoped with 1 wt.% LiF show a restively high dielectric constant and small variation overthe temperature range from -20 to 125◦C compared to the other two samples. Therefore,it could be considered for the fabrication of LTCC. However, the dielectric loss of thesethree samples is an order of magnitude higher than that of pure BT shown in Fig. 4 (b) insetphoto, which could be caused by the undesired second phase.

The microstructures of these three samples are shown in Fig. 9. They all show veryhomogenous grain size, which contributes much to the higher dielectric constant exhibitedin Fig. 8 (a).

Meanwhile, the grain size of the sintered samples increase slightly as the dopingconcentration of LiF goes up from 1 wt.% to 3 wt.%, which is in agreement with theshrinkage results shown in Fig. 5. The density of 1 wt.% LiF and1 mol% BaCO3 samplecan reach 5.67 g/cm3, 95% of the theoretical density.

4. Conclusions

The densification behavior and the dielectric properties of BaTiO3 with and without sinter-ing aids were investigated. Higher additions of LiF lead to larger shrinkage, which is helpfulin improving the densification of BT. For the nano-sized BT powder utilized in this study,using LiF as a single sintering aid, the optimum addition is around 2 wt.%, which couldlead to an acceptable dielectric property. The introduction of BaCO3 into the LiF doped BTcan effectively improve the densification. The density of the sample (1 wt.% LiF+1 mol%BaCO3) sintered at 900◦C can reach 5.67 g/cm3 (95% of the theoretical density). This

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sample has a high room temperature dielectric constant of 5382 (at 10 kHz) and relativelysmall variation with temperature, which could be considered for the fabrication of LTCC.Further optimization of varying the doping regime could lead to improved behavior.

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