J. Cent. South Univ. Technol. (2008) 15: 334-338

DOI: 10.1007/s11771-008-0063-2

Preparation and characterization of nanometer-sized barium titanate powder by complex-precursor method

SHI Ai-hua(石爱华)¹, YAN Wen-bin(颜文斌)¹, LI You-ji(李佑稷)¹, HUANG Ke-long(黄可龙)²

(1. School of Chemistry and Chemical Engineering, Jishou University, Jishou 416000, China;

2. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China)

Abstract: A new complex-precursor method was proposed to prepare nanometer-sized BaTiO₃ powder. Firstly, [Ti₂O(O₂)₂(ta)₂]^4- complex ions were prepared by the reaction of H₂O₂, Ti⁴⁺ and ta^3- (ta=[C₆H₆O₆N]^3-) with a desirable amount of surface active agent, and then the Ba₂[Ti₂O(O₂)₂(ta)₂]·2H₂O precursor was obtained by reaction between [Ti₂O(O₂)₂(ta)₂]^4- and Ba²⁺. Finally, the precursor was annealed at 800 ℃ for 2 h to obtain BaTiO₃ powder. The morphology, the particle size distribution, the purity and the molar ratio of Ba to Ti of BaTiO₃ powder were investigated systematically by TEM, XRD, IR, Raman and chemical analysis, respectively. The results show that the BaTiO₃ powders with the grain size of about 40 nm have a tetragonal crystalline structure at room temperature and a spherical morphology.

Key words: barium titanate; complex-precursor method; synthesis; nanometer-sized powder

1 Introduction

Barium titanate is considered an interesting material for the electronic ceramics because of its excellent electrical properties, such as high permittivity, low dielectric loss, ferroelectricity, piezoelectricity and electrical insulation. BaTiO₃-based materials are widely used in electronic industry for the application of thermistors, multilayer ceramic capacitors(MLCCS), memory materials, and so on. Recently, with the development and the demand of portable electronic devices, ultrafine BaTiO₃ powders less than 100 nm have been required to make progressive MLCCS with high densification and high capacity^[1-6]. BaTiO₃ is conventionally obtained by solid-state reaction(SSR) between barium carbonate (BaCO₃) and titania (TiO₂) at 1 200 ℃. However, BaTiO₃ powders prepared by SSR method always consist of non-uniform and sub-micrometer sized coarse particles, and even impurities^[7-11]. The distribution of particle size is not easy to be controlled, and the grinding process to reduce the size of the particles always results in chemical contamination. The need for nanometer-sized and pure powders has led to the development of many alternative chemical routes, such as thermal decomposition of co-precipitation, molten salt method, sol-gel technique, hydrothermal synthesis, and low-temperature aqueous synthesized(LTAS) method^[6-13]. However, to our knowledge, the complex-precursor method for the preparation of BaTiO₃ nanoparticles has not been reported.

In this study, a new complex-precursor method based on oxalic acid precipitation was proposed. This technique owns the following advantages: the whole process, with a simple experimental set-up and electrolyte system, can be performed under mild conditions (ambient temperature and atmosphere pressure). On the other hand, the synthesized BaTiO₃ exhibits uniform and fine particle sizes with a narrow size distribution.

2 Experimental

2.1 Preparation of Ba₂[Ti₂O(O₂)₂(ta)₂]·2H₂O precursor

BaCl₂?2H₂O was used as a source of Ba and TiCl₄ was used as a source of Ti in the present work. 200 mL solution containing 20 g nitrilotriacetic acid (N(CH₂COOH)₃, noted by H₃ta) and a little surfactant was mixed with 80 mL TiCl₄ solution under stirring. Then 10 mL H₂O₂ was added into the mixture solution. The solution color changed from achromatism to deep red. The mixed solution was stirred for 10 min to produce [Ti₂O(O₂)₂(ta)₂]^4- complex ions. Then 24 g BaCl₂?2H₂O was added into the above mixture solution. After BaCl₂?2H₂O was dissolved in the solution, NH₃?H₂O was dropped into the solution to adjust pH value to about 2.5 and stirred for further 30 min. The obtained mixture solution was stewed, filtrated and washed with deionized water to eliminate Cl^- ions. Then the mixture was washed with a desirable amount of absolute ethanol, and subsequently dried in a temperature range of 100-120 ℃. As a result, Ba₂[Ti₂O(O₂)₂- (ta)₂]·2H₂O precursor was obtained. The element content (mass fraction, %) of the precursor was analyzed and calculated to be C (17.4), H (1.5), N (3.4) and C (17.5), H (1.4), N (3.5), respectively.

The obtained precursor was annealed at 800 ℃ for  2 h to obtain nanometer-sized BaTiO₃ powder. The main reaction processes can be suggested as follows:

2TiCl₄+2H₃ta+2H₂O₂+3H₂O→

H₄[Ti₂O(O₂)₂(ta)₂]·2H₂O+8HCl     (1)

H₄[Ti₂O(O₂)₂(ta)₂]·2H₂O+2BaCl₂→

Ba₂[Ti₂O(O₂)₂(ta)₂]+4HCl+2H₂O    (2)

Ba₂[Ti₂O(O₂)₂(ta)₂]+O₂→BaTiO₃+CO+CO₂+NO₂+H₂O

 (3)

2.2 Characterization of precursor and BaTiO₃ powder

The complex precursors were analyzed by FT-IR spectrum (Impact 400, US) via KBr pressed disc method. DTA/TG (WCT-2C, Netzsch STA) analysis was performed by using alumina as the reference material. The crystalline structure was identified by X-ray diffraction analysis (Y-2000, Bruker, Germany) with   Cu K_α as radiation at tube voltage of 30 kV, pipe flow of 20 mA and scanning speed of 0.2 (?)/s. The morphology and particle size of the prepared BaTiO₃ samples were observed by a transmission electron microscope(TEM) (JEM-1000X, Japan).

2.3 Purity analysis of BaTiO₃ powder

The purity of the prepared BaTiO₃ powder was investigated by analyzing the molar ratio of Ba to Ti in the powder with chemical analysis method. A small quantity of BaTiO₃ powder was dissolved into concentrated hydrochloric acid, and then H₂O₂ complex compound titanium (Ⅳ) and (NH₄)₂SO₄ (1 mol/L) were added. The mixture was filtrated, washed and annealed to obtain BaSO₄. The content of BaO was calculated by the obtained BaSO₄. The concentrated H₂SO₄ (30 mL), the concentrated HCl (40 mL) and (NH₄)₂SO₄ (10 g) were added into filtrate, then the mixture was boiled to  remove H₂O₂. Subsequently, Ti(Ⅳ) was deoxidized to Ti(Ⅲ) by aluminum piece with titration by using iron ammonium sulfate as standard solution. The content of Ti was calculated and converted into content of TiO₂. The content of BaTiO₃ is equal to the sum of TiO₂ and BaO content. So the molar ratio of Ba to Ti can be calculated.

3 Results and discussion

3.1 Analysis of annealing process

The element analysis of precursor shows that the experimental value of the synthesized compound is consistent with the theoretical value, indicating that the intermediate product is Ba₂[Ti₂O(O₂)₂(ta)₂]·2H₂O. The infrared spectrum of the complex compound precursor is shown in Fig.1. The antisymmetric stretching vibration of complex compound carboxyl is present at 1 588 cm^-1, corresponding to its symmetric stretching vibration at   1 418 cm^-1. Compared with the shaking frequency of free nitrilotriacetic acid, it is found that the shaking frequency of carboxyl shifts to lower frequency, implying that the coordination exists between nitrilotriacetic acid and titanium atom by carboxyl. The O—O bond in superoxide radicle of complex compound shows stretching vibration at 875 cm^-1. Meanwhile, symmetric and antisymmetric stretching vibrations of Ti(O₂) are at 602 and 520 cm^-1, respectively. The above spectra are in agreement with the reported IR spectrum of titanium superoxide^[14-16].

Fig.1 FT-IR spectrum of Ba₂[Ti₂O(O₂)₂(ta)₂]·2H₂O precursor

The synthetic reaction of complex compound precursor and the related thermal decomposition processes are complicated. The analytical results of thermogravimetry and differential thermal analysis of complex compound precursor are shown in Fig.2. It is found that the synthetic reaction of peroxo tris(hydro- xycarbonylmethyl)amine precursor includes three main processes, namely, water loss, decomposition of organic complex compound and formation of BaTiO₃. The precursor shows one endothermic peak at 97 ℃ due to water loss, and one endothermic peak at 675 ℃ properly for the phase change of formed complex oxides^[17-19]. The actual productive rate (53.8%) is basically consistent with the theoretical productive rate (54.3%), which indicates that the precursor was completely decomposed. The experimental results show that precursor is basically decomposed by annealing at 700 ℃. By increasing the temperature, the grain size of BaTiO₃ is decreased and then increased, and grain size reaches the minimum when the temperature is 800 ℃. Setting the calcination temperature as 800 ℃, the grain size of BaTiO₃ is gradually decreased and then increased with prolonging the heating time, and reaches the minimum and the maximum at 2 and 3 h, respectively. In order to prepare BaTiO₃ powder with the smallest size and decompose the precursor entirely, the heating process for precursor is chosen as 800 ℃ for 2 h in this work.

Fig.2 TG-DTA diagrams of Ba₂[Ti₂O(O₂)₂(C₆H₆O₆N)₂]·2H₂O precursor

3.2 Crystal structure and morphology of BaTiO₃ powder

Fig.3 shows the typical XRD pattern of BaTiO₃ powders annealed at 800 ℃ for 2 h. In Fig.3, only peaks assigned to BaTiO₃ are observed, revealing that the product is pure BaTiO₃. Splitting peak for (2 0 0)/(0 0 2) (2θ≈45?), (210)/(201) (2θ≈51?), (112)/(211) (2θ≈56?), (202)/(220) (2θ≈66?), (202)/(220) (2θ≈66?) and (103)/  (301)/(310) (2θ≈75?) imply that the primary phase is

Fig.3 X-ray diffraction pattern of nano-BaTiO₃ powders

tetragonal perovskite structure. The mean particle size of the BaTiO₃ powders is evaluated from the broadening line of X-ray diffraction peaks by the Scherrer equation:

                                 (4)

where  D is the primary particle size, λ is the wavelength of Cu K_α radiation (0.154 nm), θ is the diffraction angle and β is the full wave at half maximum(FWHM) of the peak. The mean particle sizes of the calcined BaTiO₃ at 800 ℃ are calculated to be about 36 nm.

According to the crystallography, Raman active modes for tetragonal BaTiO₃ (P4mm) are 4E (TO+LO)+ 3A1(TO+LO)+1B1(TO+LO). Fig.4 shows the Raman spectrum of BaTiO₃ calcined at 800 ℃. These absorption peaks at 276, 304, 523 and 718 cm^-1 are observed. Thus, the Raman spectra confirm that the structure of BaTiO₃ particles is tetragonal phase, which is in good agreement with the result of XRD pattern.

Fig.4 Raman spectrum of BaTiO₃ powder

One of the TEM images of the prepared     BaTiO₃ powders is shown in Fig.5. The TEM image shows that the average size of BaTiO₃ is about 40 nm with spherical shape and uniformity. This is in accordance with the calculated size (36 nm) from XRD pattern. Compared with the previous researches, the experimental results indicate that the surfactant has evident effect on particle size and morphology in reaction system. As shown in Fig.6, the possible reason is that the surfactant changes the surface charge distribution of the deposition ions in solution, and then a mini-reaction environment is formed. The special environment has the capacity to separate the ions from each other. So the Ba₂[Ti₂O(O₂)₂(C₆H₆O₆N)₂]·2H₂O precursor is not reunited to a certain degree. The surface performances of BaTiO₃ are evidently improved and agglomeration is obviously induced.

Fig.5 TEM image of nano-BaTiO₃ powder

3.3 Physical properties of BaTiO₃ particles

The molar ratio of Ba to Ti of the prepared BaTiO₃ powder is 1.000±0.005 with a purity of 99.9%, which reaches the primary standard of high purity barium titanate for electronic industry (HG/T3587-1999).

The BaTiO₃ particles obtained under the optimum synthesis condition were cold isostatically pressed, sintered and measured for the physical properties. The density of the BaTiO₃ ceramics sintered at 1 200 ℃ for  2 h is 5.86 g/cm³, which means 97.6% of the theoretical

density. The dielectric constant measured in the ceramics is 4 800 at room temperature and is much higher than that of hydrothermally prepared BaTiO₃ powders^[20]. The significantly high value of dielectric constant may be mainly due to uniform nano-sized particles distribution with low aggregation.

4 Conclusions

1) BaCl₂?2H₂O, TiCl₄, nitrilotriacetic acid and surfactant are used as raw materials to prepare BaTiO₃ nano-powders by a complex-precursor method. High- purity nanometer-sized BaTiO₃ powders can be synthesized by the complex-precursor method.

2) The complex-precursor method can be used as an alternative technique for preparation of nanometer-sized BaTiO₃ powders. The potential advantages of this technique are simple setup and mild conditions.

3) The average size of prepared BaTiO₃ powders is about 40 nm. The density of the BaTiO₃ ceramics prepared by the synthesized BaTiO₃ powders is 5.89 g/cm³, and the dielectric constant measured in the ceramics is about 4 800 at room temperature.

Fig.6 Synthesis mechanism of Ba₂[Ti₂O(O₂)₂(ta)₂] ·2H₂O precursor

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(Edited by YANG Hua)

Foundation item: Project(06JJ50150) supported by the Hunan Provincial Natural Science Foundation of China

Received date: 2007-12-10; Accepted date: 2008-03-18

Corresponding author: YAN Wen-bin, Professor; Tel: +86-743-8563911; E-mail: jishouyanwenbin@163.com