Journal of Materials Science and Engineering A 2 (1) (2012) 7-15
Characteristics of BaTiO 3 Particles Synthesized by Precipitation from Low-Temperature Aqueous Solutions
Hyeong Seok Lee 1, 2, Sang Man Koo 1 and Jung Whan Yoo 2
1. Department of Chemical Engineering, Hanyang University, Haengdang-dong, Seongdong-Gu, Seoul 133-791, Korea
2. Eco-Composite Materials Center, Korea Institute of Ceramic Engineering and Technology, 233-5, Gasan-Dong, Geumcheon-Gu, Seoul 153-801, Korea
Received: May 10, 2011 / Accepted: May 30, 2011 / Published: January 10, 2012.
Abstract: Crystalline BaTiO 3 powders were synthesized from aqueous solution of TiCl 4 and BaCl 2·2
H 2O in a strong alkaline solution at 60-100 °C by a precipitation method. The influence of parameters such as solution pH and reactant concentration on the particle size and morphology from low-temperature aqueous solutions was investigated. As the reactant concentration in the strong alkaline solution (pH > 13.0) incread, a smaller and more distinct spherical particles with narrow size distribution was formed. The well-defined spherical BaTiO 3 particles having an average crystallite size of 70 nm with average specific surface area of 80-90 m 2/g was successfully synthesized at 80 °C for 12 h in KOH solution of 1.5 mol/L (pH = 13.6).
Key words: BaTiO 3, precipitation, low-temperature, TiCl 4.
1. Introduction
Barium titanate (BaTiO 3), a perovskite-type electroceramic material, has been extensively studied and utilized due to its dielectric and ferroelectric properties [1, 2]. The main application is as a dielectric in multilayer ceramic capacitors (MLCCs) due to its high dielectric constant and low loss [3]. Recently, advances in microelectronics and communications have led to the miniaturization of MLCCs. Higher capacitance in smaller ca sizes requires a reduction of the thickness of the ceramic layers and the increa of the number of layers. To achieve this goal, powders with high qua
lity and small and uniform size are required. Barium titanate is conventionally produced by solid-state reactions between BaCO 3 and TiO 2 at high temperature (700-1,200 °C) [4, 5]. However, the conventional method suffers from interparticle sintering and contamination problems in the
Corresponding author: Hyeong Seok Lee, rearcher, rearch field: materials science. E-mail:
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calcinations and required milling steps, respectively. In order to overcome the drawbacks from the conventional solid-state reaction process wet-chemical synthesis methods have been widely ud to prepare fine barium titanate powders [1-3]. Among them, precipitation process are very attractive becau they avoid the preliminary preparation of a suitable solid (either amorphous or crystalline) or gel precursor as a titanium source [6-8]. In recent years, an increasing interest has been focud on the direct precipitation of BaTiO 3 in aqueous or mixed organic-aqueous media at temperatures < 100 °C and ambient pressure using solutions of organometallic or inorganic compounds. The size and morphology of the particles depends, in general, on the relative rates of nuclei formation and crystallite growth. Both nucleation and growth are very nsitive to temperature, concentration, and mixing conditions. A careful control of the above kinetic parameters should allow the synthesis of par
ticles with the desired size, morphology, and particle size distribution. In addition, the mixing conditions were
found to have a significant influence on the particle All Rights Rerved.
Characteristics of BaTiO3 Particles Synthesized by Precipitation from Low-Temperature
Aqueous Solutions
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size distribution and formation of condary phas [9]. To obtain results not dependent on a particular mixing condition, precipitation reactions were conducted by premixing the feedstock solutions at room temperature, where formation of BaTiO3 is kinetically inhibited, and then heating the precursor solution to the reaction temperature. Therefore, it needs to study thoroughly the nucleation and growth of BaTiO3 particles to control the processing parameters for affecting particle size and morphology.
In this paper, precipitation process was applied to prepare of fine and high purity BaTiO3 powders with narrow size distribution. Then, the results of a systematic investigation of the effect of solution p
H, feedstock concentration on the formation of BaTiO3 particles from the aqueous solution are discusd.
2. Experiment
Titanium tetrachloride (99.9% TiCl4, Aldrich Chemical Co.) and barium chloride dihydrate (99% BaCl2·2H2O, Showa Chemical Co.) as titanium and barium sources, respectively, were ud as starting materials to prepare BaTiO3 powders. Titanium tetrachloride was taken into the reactor which had been cooled below -5 °C under stirring and then dilute HCl solution of 1.0 mol/L was slowly added to the reactor in order to minimize the explosive generation of orthotitanic acid [Ti(OH)4]. Barium chloride dihydrate was dissolved in deionized water at room temperature. The clear feedstock solution was prepared by mixing the aqueous solution of TiCl4 and BaCl2·2H2O to obtain a molar ratio of Ba:Ti = 1:1. The concentration of the feedstock solutions ranged from 0.1 mol/L to 0.5 mol/L. The homogeneous precursor suspensions were coprecipitated by addition of the feedstock solution of 50 mL into KOH solution of 100 mL under vigorous stirring at room temperature and ambient pressure. The pH of the precursor suspension was controlled by changing the concentration of KOH solution from 0.5 mol/L to 1.5 mol/L. After the coprecipitation, the pH of precursor suspension was recorded. The precursor suspension was transferred to a 200 mL Teflon-li
法国红酒ned stainless steel vesl. In order to proceed to precipitation reaction, the aled vesl was placed in an oven at constant temperature for a given reaction time. After precipitation reaction, the BaTiO3 precipitates were filtered using deionized water and membrane filter with a porosity of 0.1 m to completely remove impurities from the precipitates. Then, the BaTiO3 precipitates were dried in a drying oven at 50 °C for 24 h.
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BaTiO3 powders were examined by powder X-ray diffractometer (Rikagu D/Max P/N: 3 kW/40 kV, 45 mA) with CuKα(λ = 1.54056 Հ) over range of 2θ, 20°-80° at a scan speed of 3°/min, with 0.03°/sampling step. The shapes of the prepared particles were obrved using transmission electron microscope (TEM; JEOL, EM-2000 EX II). After drying the BaTiO3 powder at 200 °C for 20 h or more, the specific surface area was determined using ASAP 2010 (Micromeritics) according to the BET method. Particle size and morphology were characterized by scanning electron microscopy (SEM; Models XL30 and DX-41, Philips Electronic Instruments, Mahwah, NJ). The amount of Ti4+ and Ba2+ remaining in the supernatant liquid was analyzed by Inductively Coupled Plasma-Optical Emission Spectroscopy ICP-OES (Perkin Elmer Optima 3200).
3. Results and Discussion
In order to investigate the formation of BaTiO3 particles in the aqueous solution at temperature below 100 °C and ambient pressure, firstly, there is need to determine the appropriate reaction temperature and time. Then, the precipitation reaction was conducted at three different temperatures as a function of reaction time using aqueous solution of TiCl4 and BaCl2·2H2O in strong alkaline solution (pH > 13) [3]. To synthesize BaTiO3 powders by precipitation, the precursor suspensions prepared by adding the feedstock solution of 0.5 mol/L into a KOH solution of 1.5 mol/L were reacted at temperatures of 60, 80, and 100 °C for 12 h,
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Characteristics of BaTiO 3 Particles Synthesized by Precipitation from Low-Temperature
Aqueous Solutions
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respectively. Fig. 1 illustrates the variation of (110) plane XRD intensity for synthesized BaTiO 3 powders, which is cloly related to crystallinity [10], with respect to the reaction temperature vs. time. As shown in this figure, XRD intensity for barium titanate incread as the temperature ris fro
m 60 °C to 80 °C irrespective of the reaction time. It means that the formation rate of BaTiO 3 particles is rather fast 80 °C than 60 °C. In the current material system, as the reaction temperature ris, the crystallization process of BaTiO 3 particles was accelerated, but was not significantly accelerated at reaction temperature between 80 °C and 100 °C.
On the other hand, XRD intensity for barium titanate
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incread gradually up to 130 min past the beginning of the reaction at all temperatures. However, the changes were not conspicuous after 130 min elapd. In addition, Fig. 2 shows the TEM photographs of the BaTiO 3 particles obtained for different reaction times at 80 °C after 130 min elapd. From the TEM obrvations, the particles formed for 130 min appear to have a broad size distribution of about 100-200 nm, whereas the particles formed for 720 min appear to
have a narrow size distribution of below 100 nm with average specific surface area of 80-90 m 2/g.时间的说说
According to above the results, for a fixed reaction temperature (80 °C) and reaction time (12 h), the influence of reaction parameters, such as solution pH
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Fig. 1 XRD intensity changes for (110) plane of BaTiO 3 powders as a function of reaction time for 0.5 mol/L feedstock solutions with 1.5 mol/L KOH solution at 60 °C, 80 °C, and 100 °C.
(a) 130min (b) 480min (c) 720min
Fig. 2 TEM photographs of BaTiO 3 powders prepared for various reaction times at 80 °C for 0.5 mol/L feedstock solution with 1.5 mol/L KOH solution.
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Characteristics of BaTiO3 Particles Synthesized by Precipitation from Low-Temperature
Aqueous Solutions
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and feedstock concentration, which is affected in formation rate of BaTiO3 particles in the aqueous solution is investigated. The XRD patterns of the BaTiO3 powders synthesized at 80 °C for 12 h after adding feedstock solution of 0.5 mol/L into various concentrations of KOH solution (0.5-1.5 mol/L) are shown in Fig. 3. As shown in Figs. 3a-3c, the product obtained at KOH solution of 0.5 mol/L was TiO2 (Anata, JCPDS No.:21-1272) as well as products obtained at KOH solutions of 1.0 mol/L and 1.2 mol/L were amorphous except for minor barium carbonate pha [BaCO3 (JCPDS No.:44-1484)]. Barium titanate (JCPDS No.: 31-0174) began to form above 1.3 mol/L KOH solution (pH > 13.
2), but shown weak diffraction peaks for the BaCO3 pha (Figs. 3d-3f). The crystal structure of BaTiO3 powders obtained was found to be a cubic irrespective of KOH concentrations. In addition, as the concentration of KOH solutions (i.e., solution pH) increas, the peak intensity involving formation rate of barium titanate increas as well as the peak intensity of minor barium carbonate decreas. The differences in the XRD intensities imply that the solution pH (i.e., the concentration of OH-) plays an important role in barium titanate formation kinetics.
A small aliquot of the precursor suspensions precipitated in various concentrations of KOH solution at room temperature was collected, and then the supernatant liquid was parated from the precursor suspension by centrifugation. Fig. 4 illustrates the amount of Ti4+ and Ba2+ remaining in the supernatant liquid by ICP-OES analysis. The amount of titanium ions remaining in the supernatant liquid is very low (Ti4+ < 0.1 ppm) regardless of KOH concentration (i.e., solution pH). However, the amount of barium ions remaining in the supernatant liquid dramatically decreas as the concentrations of KOH solution increa from 0.5 mol/L to 1.4 mol/L and in particular, the amount of barium ions remaining, when KOH concentration was 1.5 mol/L (pH = 13.6), is lowest. It is obvious that the titanium ion in the aqueous TiCl4 solution completely converted into the amorphous
Fig. 3 XRD patterns of BaTiO3 powders prepared by reaction at 80 °C for 12 h after adding a 0.5 mol/L feedstock solution into various KOH concentrations: (a) 0.5 mol/L, (b) 1.0 mol/L, (c) 1.2 mol/L, (d) 1.3 mol/L, (e) 1.4 mol/L, and (f) 1.5 mol/L.
precipitates immediately after addition into the KOH solution (independent on solution pH in the range of our experiments). But, the barium ion in the aqueous BaCl2 solution was fairly dependent on the KOH concentrations (solution pH) to convert the amorphous precipitates.
The SEM micrographs of BaTiO3 powders synthesized from various KOH concentrations are shown
安身立命的意思in Fig. 5. As shown in this figure, particles of barium titanate prepared from 1.0 mol/L KOH solution (Fig. 5a) have agglomerated by amorphous phas, as identified by XRD. With the increasing KOH concentrations from 1.3 mol/L to 1.5 mol/L, the particle morphology is more uniform and spherical, as showed in Fig. 5. Especially, when KOH concentration was 1.5 mol/L (Fig. 5d), the particle size and morphology have shown spherical particles with narrow size distribution of about 70 nm. In addition, it can be found that the particle sizes revealed from the TEM obrvations (Fig. 2c) correspond to the SEM measurements (Fig. 5d).
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