Preparation and properties of barium titanate nanopowder by conventional and high-gravity reactive precipitation methods

Scripta Materialia 49 (2003) 509–514 www.actamat-journals.com

Preparation and properties of barium titanate nanopowder by conventional and high-gravity reactive precipitation methods q Jian-Feng Chen a

a,*

, Zhi-Gang Shen a, Fang-Tao Liu a, Xiao-Lin Liu a, Jimmy Yun b

Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, College of Chemical Engineering, No. 15 Bei San Huan Dong Road, Beijing 100029, PR China b NanoMaterials Technology Private Ltd., Singapore 139944, Singapore Received 7 May 2003; received in revised form 4 June 2003; accepted 12 June 2003

Abstract Barium titanate powders have been prepared using a low temperature aqueous synthesis method in both conventional batch stirred tank reactor (BSTR) and continuous high-gravity reactive precipitation (HGRP) technique. The powders synthesized with HGRP technique show much smaller average particle size and exhibit higher specific surface area as compared to those prepared by conventional BSTR method. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Barium titanate; Synthesis; High-gravity reactive precipitation; Nanomaterials; Electroceramic

1. Introduction Ceramic materials based on Perovskite-like oxides are of significant interest because of their applications in electrical and electronic devices. Due to its high dielectric constant, barium titanate (BaTiO3 ) is probably one of the most studied compounds of such family and still represents

q The barium titanate synthesis by HGRP method in this paper has been applied for patent worldwide. * Corresponding author. Tel.: +86-10-64446466; fax: +86-1064434784. E-mail address: [email protected] (J.-F. Chen).

the basis for the preparation of multi-layer ceramic capacitors (MLCCs) and thermistors with positive temperature coefficient of resistivity [1,2]. In recent years, MLCCs have become smaller in size. It is desirable that the sintering temperature of the MLCC green body be as low as possible to reduce the cost of the internal electrode, which is usually made of a precious metal. Therefore, in order to achieve good sinterability, fine grained microstructure in the sintered bodies, high dielectric constant and reproducibility of the products, it is necessary to enhance the dielectric properties, which can be achieved by using uniform, wellcrystallized, sub-micrometer or nanoparticles as starting powders [3,4].

1359-6462/03/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S1359-6462(03)00361-0

510

J.-F. Chen et al. / Scripta Materialia 49 (2003) 509–514

BaTiO3 can be prepared by conventional calcination method, in which a certain amount of BaCO3 or BaO, and TiO2 are calcined and then pulverized. However, BaTiO3 powders prepared by this method consist of non-uniform and coarse particles. As a result, the probability of dielectric breakdown across a thin layer (
20–30 lm) is high. In order to eliminate these defects, a number of wet chemical synthesis techniques have been developed, including homogeneous precipitation [5], sol–gel [6], pyrolysis [7], hydrothermal processes [8], and techniques using microwave-assisted hydrothermal [9], combustion reaction [10], or solvothermal reaction [11]. However, even these synthesis routes suffer from various disadvantages, including inferior particle morphology, inconsistent dielectric properties, poor sinterability and non-uniform microstructure. These preparations are also uneconomical due to the high cost of the starting materials, high reaction temperature and pressure, and low yields. Furthermore, several reaction steps are required: calcination at high temperature to produce crystalline powders, extensive milling to reduce particle size, and complicated post-treatments to adjust the stoichiometry. Recently, Nanni and coworkers [12,13] reported a low-temperature aqueous synthesis (LTAS) method for the synthesis of crystalline barium titanate at a temperature as low as 85 °C under high pH (>13) conditions. This method requires a long digestion (aging) time of 6 h and produces agglomerated nanoparticles (
30 nm in diameter). The BaTiO3 powders produced using this method could not be sintered to high density unless being thermally treated at 950 °C for 15 h prior to pressing and sintering. To solve these problems associated with the LTAS method, a novel technique called highgravity reactive precipitation (HGRP) was employed in this study to carry out the reaction pathway as described in the LTAS method. Applications of HGRP technique in different chemical systems have produced small particles with narrow size distribution, as well as good morphology and stoichiometry control [14]. Another advantage of the HGRP technique is the ease of process scaling-up.

2. Experimental section 2.1. Powder synthesis All reagents of chemical grade (i.e. BaCl2  H2 O, TiCl4 , NaOH) were used without further purification. All solutions were filtered through 0.45 lm pore size membranes to remove the particulate impurities before use. For the conventional batch stirred tank reactor (BSTR) synthesis method, a 1.0 l reactor was used and protected with nitrogen to avoid CO2 contamination from air. The mixed solution of barium and titanium chlorides was added into the batch reactor, which was previously filled with NaOH solution preheated to 90 °C, at a constant flow rate under stirring. After that, the solution was sitting there for aging for 3 min. For the HGRP synthesis, the key part of the rotating packed bed (RPB, Higee machine) is a packed rotator. More details about this equipment can be seen in our previous paper [14]. The entire experimental setup was made of titanium. In a typical run, a mixed BaCl2 –TiCl4 solution, and a NaOH solution were simultaneously and continuously pumped from their storage tanks into different slotted pipe distributors. The RPB provided intense mixing for the reaction to yield the white precipitate of BaTiO3 . The flow rates of the mixed chlorides and the NaOH solutions were kept constant at 40 and 35 l h1 respectively. The BaTiO3 precipitate was formed instantaneously. The entire precipitation process lasted 15 min and was carried out in air at 85 °C, which was maintained by a constant-temperature circulator. For both synthesis routes, the total concentration of BaCl2 and TiCl4 in the feeding solution was kept at 1.0 mol l1 , and the molar ratio [BaCl2 ]/ [TiCl4 ] was 1.07. NaOH was used in excess to maintain a high pH value (pH > 13) throughout the reaction for achieving the complete precipitation of the BaTiO3 . After the reaction was finished, the BaTiO3 suspension was filtered and the solid was then washed with de-ionized water to remove the side products. The powders were then dried at 100 °C.

J.-F. Chen et al. / Scripta Materialia 49 (2003) 509–514

2.2. Powder characterization The mean particle size and particle size distribution (PSD) of BaTiO3 spherical particles were determined by a laser particle size analyzer (Malvern Zetasizer 3000 HS) and transmission electron microscopy (TEM, Hitachi H-800). The sample size for determining the average particle size and the size distribution from TEM was about 300 particles. The specific surface areas (SBET ) was measured by a gas absorption analyzer (micromeritics ASAP-2010M). Phase identification was conducted using a X-ray diffractometer (Shimadzu XRD-6000) with CuKa radiation at a scan speed of 4° min1 . The synthesized powders were dispersed (ultrasonic bath, 250 W) in 2 wt.% PVA solutions and then degassed, dried and pressed in cylindrical moulds. The dried slip samples were sintered at 1300 °C for 2 h with dwell periods at 600 °C for PVA binder burnout. Densities of sintered samples were measured by the ArchimedesÕ method. The values obtained are the average of three samples. Dielectric measurements were carried out on an automated system equipped with a temperature control box and a LCR meter (Hewlett-Packard 4192 A). to determine the relative dielectric constants, a sintered pellet was coated with Ag metal electrode on both sides, and the capacitances were evaluated at a frequency of 1 kHz and at temperatures ranging from )50 to 150 °C. To examine the microstructures, the sintered pellets were fractured and the surfaces were examined by scan-

511

ning electron microscopy (SEM, Cambridge S250MK3). The barium and titanium contents of the powders were assayed by gravimetric and titration techniques respectively. Approximately 1 g of the dry powder was dissolved in 25 cm3 of hot concentrated sulfuric acid. The formed solution was diluted with cold de-ionized water and left to stand for 12 h. The resulting white precipitate was filtered, rinsed with 0.01 mol l1 HCl solution, burnt in a crucible at 1000 °C for 30 min, and eventually weighed as BaSO4 . To determine the content of titanium, 30 cm3 of concentrated HCl solution was added to the above filtrate. The solution was then heated to 75 °C, and 3 g of high purity aluminum metal was added under stirring until totally dissolved. After being cooled to below 50 °C, 5 cm3 of saturated NH4 SCN solution was added as an indicator, and the final solution was then titrated with 0.1 mol dm3 FeNH4 (SO4 )2 .

3. Results and discussion Fig. 1 shows the TEM micrographs of barium titanate particles obtained by BSTR and HGRP processes. It appears that the average particle sizes of BaTiO3 powders synthesized by both methods are nanosized (<100 nm). The X-ray diffraction (XRD) patterns (Fig. 2) revealed that all samples are crystalline and all the main diffraction peaks correspond to the peaks of cubic barium titanate

Fig. 1. TEM micrographs of BaTiO3 powders obtained from HGRP technique (a) and conventional BSTR route (b).

512

J.-F. Chen et al. / Scripta Materialia 49 (2003) 509–514

starting reactants with higher purity, as well as utilizing a closed RPB system where the reaction atmosphere can be controlled. The PSDs, measured by the laser particle size analyzer, are shown in Fig. 3. Although the HGRP powders were finer than the BSTR powders, both exhibited a certain degree of agglomeration. For the HGRP powders the agglomeration factor (dm50 =dBET ) [16] is 3.2, as compared to 2.6 for the BSTR powders. The typical powder characteristics for BSTR and HGRP powders are listed in Table 1. The stoichiometries (Ba/Ti molar ratio) nominally expected from the concentrations of the reagents and those calculated from experimental analysis showed a close correspondence for both powders within the limits of experimental error. The HGRP powders have a significantly higher measured surface area, which indicates a smaller primary particle size. This was also confirmed by the TEM observations. The sintered densities of the dry pressed titanate powders were reasonably good, with the HGRP powders showing a slightly higher relative density

o

BaTiO3 * BaCO3 o

o

o

HGRP powder

o

Intensity

o o

o

*

o

o

o

o

o

BSTR powder o

o

20

30

40

50

60

70

2θ Fig. 2. XRD patterns of BaTiO3 powders obtained from HGRP technique and conventional BSTR route.

(JCPDS no. 05-0626). Only small amounts of an amorphous unidentified phase and BaCO3 (61.5%) were detected in the powders synthesized by the HGRP technique. These undesired components could be reduced or eliminated by using

Size distribution(s) % in class

% in class

Size distribution(s)

(a)

(b) 30

40 20

20 10

200

400

600 800 Diameter (nm)

200

1000

400 600 800 Diameter (nm)

1000

Fig. 3. PSD of BaTiO3 particles obtained by HGRP technique (a) and BSTR method (b).

Table 1 Various powder characteristics for BaTiO3 produced via HGRP and BSTR techniques Powder

Ba/Ti

DTEM (nm)

DPS (nm)

SBET (m2 g1 )

DBET (nm)

HGRP sample BSTR sample

0.997 1.004

60 115

152 267

22.1 10.7

45 93

DTEM is the average particle size estimated from the TEM pictures; DPS , the average particle size measured by laser particle size analyzer; DBET , the average particle size calculated from the specific surface area by the use of the relation DBET ¼ 6=qSBET , where SBET is the specific surface area and q is the theoretical density of barium titanate (6.05 g cm3 ) [15].

J.-F. Chen et al. / Scripta Materialia 49 (2003) 509–514

Fig. 4. SEM micrographs of the fracture surface of sintered BaTiO3 ceramics sample derived from HGRP powders.

of 92.8% compared to 91.1% for the BSTR powders. These results are in good agreement with the literature values [17]. The microstructures of the fracture surfaces (Fig. 4) were similar for both samples, exhibiting small, uniform grain sizes ranging between 2 and 3 lm, with a small quantity of trapped pores. The variation of relative dielectric constant and dissipation factor as functions of temperature are shown in Fig. 5. A high relative permittivity of 2500 with low dissipation factor of 0.02 was observed at room temperature for sintered tablets derived from the HGRP technique, while a relative permittivity of 2200 with low dissipation factor of 0.02 was observed for the corresponding sample

0.20

HGRP-DC HGRP-DF BSTR-DC BSTR-DF

5000

0.15

4000 3000

0.10

2000 0.05

Dissipation factor(DF)

Dielectric constant(DC)

6000

1000 0

-50

0

50

temperature/ oC

100

150

0.00

Fig. 5. Variation of relative dielectric constant and dissipation factor for HGRP and BSTR derived BaTiO3 pellets with temperature.

513

from the BSTR route. These results showed that both Perovskite powders produced by BSTR and HGRP routes exhibit similar dielectric behavior upon sintering. The above results indicated that HGRP synthesis of barium titanate holds obvious advantages over the conventional BSTR route. During the HGRP process, fluids entering the RPB were divided into very fine droplets, threads, and thin films due to the high-gravity environment. This results in intense micromixing between the mixed chlorides and sodium hydroxide solutions, and favors the generation of numerous local sites with supersaturated concentrations, therefore precipitating barium titanate powders with narrow and uniform size distribution.

4. Conclusion Barium titanate powders have been produced using both a conventional BSTR reactor and a novel continuous technology called HGRP. Both techniques could synthesize spherical, nanometer sized primary particles around 60 nm for HGRP and 115 nm for the BSTR process. Both powders showed a certain amount of aggregation after re-dispersing by ultrasonication. The sintering behaviors and dielectric properties of BaTiO3 ceramics made of powders prepared from both methods were found to be similar. Since the synthesis of nanomaterials by HGRP has the advantages of low cost and no negative scale-up effects (several commercial production lines of nanosized calcium carbonate by HGRP have been put into operation successfully), it can thus be concluded that the preparation of nanosized barium titanate by HGRP has a very promising prospect on the industrial scale.

Acknowledgements This work was supported by National Natural Science Foundation of China (no. 20236020), the key projects of Science and Technology Research of the Ministry of Education of China (grant no. Key 0202), the National High Technique Program

514

J.-F. Chen et al. / Scripta Materialia 49 (2003) 509–514

(‘‘863’’ program) of China (grant no. Key 2001AA325014), the Talent Training Program of the Beijing City (grant no. 9558103500), and the Fok Ying Tung Foundation (grant no. 81063). We would like to thank Prof. Shi Yong-Xi for her help in TEM and SEM analyses.

References [1] Hennings D, Metzmacher C, Schreinemacher B. J Am Ceram Soc 2001;84:179. [2] Buscaglia M, Buscaglia V, Viviani M, Nanni P, Hanuskova M. J Eur Ceram Soc 2000;20:1997. [3] Chen R, Cui A, Wang X, Gui Z, Li L. Mater Lett 2002;38:314. [4] Fukai K, Hidaka K, Aoki M, Abe K. Ceram Int 1990;16:285. [5] Kim S, Lee M, Noh T, Lee C. J Mater Sci 1996;31:3643.

[6] Cheung M, Chan H, Zhou Q, Choy C. NanoStruct Mater 1999;11:837. [7] Potdar H, Deshpande S, Date S. Mater Chem Phys 1999;58:121. [8] Um M, Kumazawa H. J Mater Sci 2000;35:1295. [9] Choi G, Kim H, Cho Y. Mater Lett 1999;41:122. [10] Duran P, Capel F, Tartaj J, Gutierrez D, Moure C. Solid State Ion 2001;141:529. [11] Bocquet J, Chhor K, Pommier C. Mater Chem Phys 1999;57:273. [12] Nanni P, Leoni M, Buscaglia V, Aliprandi G. J Eur Ceram Soc 1994;14:85. [13] Viviani M, Lemaitre J, Buscaglia M, Nanni P. J Eur Ceram Soc 2000;20:316. [14] Chen J, Wang Y, Guo F, Wang X, Zheng C. Ind Eng Chem Res 2000;39:949. [15] Urek S, Drofenik M. J Eur Ceram Soc 1998;18:282. [16] Bowen P, Donnet M, Testino A, Viviani M. Key Eng Mater 2002;213:21. [17] Liu S, Abothu I, Komarneni S. Mater Lett 1999;38: 344.