Low-temperature synthesis of nanoscale BaTiO3 powders via microwave-assisted solid-state reaction
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Microwave-assisted solid-state reactions (MSSR) are investigated for their ability to synthesize nanocrystalline BaTiO3 powders at low temperatures. Ba(OH)2·H2O and TiO2·xH2O are, respectively, initial precursors for Ba and Ti. In this study, these precursors were mixed according to their chemical stoichiometry and heated in a temperature range of 100–1000 °C by MSSR. Nanocrystalline BaTiO3 powders having an average size of 26 nm were produced by MSSR, even at 100 °C. The crystallization-related activation energy for the formation of BaTiO3 with the precursors by MSSR was ~ 9.6 kJ/mol, which is less than 1/10 of the value (120 kJ/mol) when the conventional solid-state reaction was applied. The nanostructural and physical features of the MSSR-based powders are compared with those prepared using the conventional solid-state reaction.
KeywordsBarium titanate (BaTiO3) Microwave-assisted solid-state reaction Activation energy Nanostructural features
Barium titanate (BaTiO3) is one of the most attractive ferroelectric materials having a ABO3-type perovskite structure. It is extensively used in electronic applications, including as multilayer ceramic capacitors, piezoelectric devices, thermistors with positive temperature coefficients, and semiconducting memory devices [1, 2, 3]. Various aspects of this material have been widely examined, including its synthesis, sintering, and nanostructural characteristics, to improve the dielectric properties of materials [3, 4, 5]. In particular, with the advent of nanotechnology, many researchers have investigated advanced synthesis methods to fabricate nano-sized BaTiO3 powders having fine features and high crystallinity. These methods include solid-state reactions, sol–gel processes, and hydrothermal methods [6, 7, 8].
Solid-state reactions comprise the most common powder manufacturing process, owing to the simplicity of the preparation process and the low cost of source materials. However, the process often requires high temperatures and long reaction times to achieve a complete reaction, and the powders exhibit a large size of more than 500 nm [6, 9]. In contrast, the sol–gel process is usually carried out at low temperatures, which can be used to synthesize nano-sized powders. However, it requires a rather complicated process to produce the sol-type precursors, and the cost of some of its source materials are high. Additionally, the hydrothermal method can be used to synthesize nano-sized powders having good crystallinity. However, this method has some drawbacks, including slow kinetics and a limited reaction temperature below 300 °C because of the limits of the apparatus (e.g., Teflon vessels) [10, 11, 12].
It has been shown that the application of microwaves has the potential to enhance crystallization and densification kinetics in materials-engineering processes [13, 14, 15]. Microwave-assisted processes have various advantages, such as high power density, controllable penetration depth, rapid heating, and uniform heating of processed materials [16, 17]. In particular, no thermal conductivity mechanism has been associated with microwave heating, because microwave energy is transferred directly via electromagnetic fields into the materials . Owing to these advantages, hybrid-type sol–gel processes and hydrothermal methods, including microwave irradiation, have been studied to control the crystallization of ceramic powders under various conditions.
Guo et al.  reported that 150-nm-sized BaTiO3 powders had been synthesized at a reaction temperature of 80 °C using the microwave-assisted hydrothermal method, which is considered preferable to the conventional hydrothermal method, owing to its time savings, low temperature, and easy application. Sun et al.  confirmed that the application of microwaves via the hydrothermal method can control the structural features of synthesized BaTiO3 powders with a short synthesis time. Malghe et al.  performed studies on the effects of microwaves when cubic-BaTiO3 powders were synthesized at 500 °C via the sol–gel method, reporting that it contributed to transformation to the tetragonal phase in a microwave field above 700 °C.
There have only been a few studies on the application of microwaves in the formation of ceramic powders via solid-state reactions. Precursors having polar-type bonds provide potential for future application in microwave-assisted solid-state reactions [22, 23]. There has been little investigation on the detailed kinetics relevant to the microwave contribution.
In this study, we investigated how microwave-assisted solid-state reaction (MSSR) can be applied to fabricate nanoscale BaTiO3 powders. The influence of the microwaves on the crystallization and growth of BaTiO3 powders was studied in terms of initial precursors bearing hydroxides and water. The conventional heating-related activation energy was estimated to examine kinetics for the formation of the crystalline BaTiO3 phase when the powders were prepared by MSSR. These results were compared with the values for BaTiO3 powders prepared by conventional solid-state reactions (CSSR).
2 Experimental details
In this study, two types of Ba and Ti precursors were used. Initial precursors for Ba-source included Ba(OH)2·H2O (BH, JUNSEI, Japan) and BaCO3 (BC, JUNSEI, Japan). The initial sources for Ti were TiO2·H2O (TH) and rutile-TiO2 (TO, JUNSEI, Japan). Source TH was an amorphous titania synthesized by adding NH4OH solution in a hydrolyzed TiOCl2 solution. The solution was produced by reacting it with titanium tetrachloride (TiCl4) and ice-cold de-ionized water at 60 °C for 1 h. The initial sources were made by mixing respective Ba and Ti precursors with a Ba/Ti mole ratio of 1:1. These mixed powders are BHTH (BH + TH), BHTO (BH + TO), BCTH (BC + TH), and BCTO (BC + TO). The four types of mixed powders were heat-treated at temperatures ranging from 100 to 1000 °C for 0–60 min via MSSR and CSSR. On the other hand, BaTiO3 powders were also prepared by hydrothermal synthesis to compare structural features with those of the powders prepared by MSSR method. Barium hydroxide octahydrate (Ba(OH)2·8H2O) and TH were used as source materials for Ba and Ti, respectively. Details of the hydrothermal process are listed in Ref. .
MSSR and CSSR processes were carried out at heating rates of 100 °C/min and 5 °C/min, respectively, and the samples were cooled naturally. The processing temperature was measured using a thermocouple placed near the sample within the hot zone in the furnace. The microwave heating furnace (Unicera Co., UMF-04), having a monomode reactor, was operated at a microwave frequency of 2.45 GHz and a power of 2 kW. The temperature was controlled using a micro-time slicing method of the microwave power. Conventional heating, however, was carried out with a power of 4 kW in an electrical furnace.
3 Results and discussion
3.1 Synthesis of BaTiO3 powders
In contrast, for powders prepared via CSSR, as shown in Fig. 1b, BaTiO3 phases started to form at ~ 600 °C. BaCO3 phases were dominantly present at this temperature. The BaTiO3 phase was produced at temperatures higher than 600 °C. As a result, in the temperature range of 700–1000 °C, BaTiO3 phase existed dominantly. Figure 1c, d show the variation in the nanocrystallite size of the BaTiO3 powders prepared using BHTH by MSSR and CSSR methods. The FWHM of diffracted peaks (101) is also illustrated in the figure. The nanocrystallite size of the powders synthesized at 100 °C by MSSR was 16 nm, and the size gradually increased to 27 nm with increasing synthesis temperature up to 600 °C. The value of FWHM gradually varied between 0.415 and 0.307 with temperature. On the other hand, in case of the powders prepared by the CSSR method, the nanocrystallite size increased from 42 to 58 nm at the reaction temperature ranging from 700 to 1000 °C. The value of FWHM gradually varied from 0.205 to 0.141 with temperature. To see the difference in how the precursors (e.g., BHTH) with hydroxide and/or water behave compared with those without polar-type bonds, BHTH, BHTO, BCTH, and BCTO were reacted at 400 °C for 60 min via MSSR.
When BH reacted with TH or TO, new compounds such as BaTiO3 and Ba2TiO4 phases were produced by the reaction between Ba and Ti ions, while a BaCO3 phase was also produced as a by-product. The presence of BaCO3 is likely to be attributed to the reaction of Ba ions with CO2 in air. On the other hand, when the Ba source material was of BaCO3 (BC), Ba and Ti ions reacted to a little degree at this temperature regardless of the type of Ti precursors. Instead, the phase transition related to the TiO2 structure occurred so that some of the source rutile-type phases became an anatase phase .
Initial sources having additional polar dipoles (e.g., water (H2O) and hydroxide (OH-)) were expected to have molecular oscillations under microwave irradiation [22, 28, 29]. Consequently, microwave-related energy is absorbed effectively in the materials, and it can decrease the conventional heating-related thermal energy barrier for the formation of a BaTiO3 structure. In Fig. 3, the activated state was set for the formation of BaTiO3, and, as a result, the initial source materials having water and/or hydroxide could overcome the energy barrier (ΔEa,1), even at 100 °C with the assistance of microwave (in Fig. 1a) energy. Here, the conventional heating-related energy at this temperature is represented by ΔE100 °C, and the energy, which was conceived from the electromagnetic field, is regarded to be approximately ΔEa,1. These approximated values were made based on the lowest temperature at which the formation of BaTiO3 was observed in Fig. 1a. On the contrary, in case of initial sources without water and hydroxide, the energy barrier of ΔEa,3 was higher than that of ΔEa,1, Therefore, the contribution from the microwave was negligible for overcoming ΔEa,3.
Similarly, TO had a lower reactivity than did TH by ΔEwater/hydroxide. Owing to the low reactivity and the lack of additional polar dipoles of TO, BHTO had insufficient energy to overcome the activation energy for the formation of BaTiO3, even at a reaction temperature of 400 °C (in Fig. 2).
3.2 Estimation of the crystallization activation energy for the formation of BaTiO3 phase
Crystallization activation energy of the MSSR and CSSR synthesis processes
Soaking time (min)
Synthesis temperature (°C)
The activation energy for a reaction time of 0+–10 min was ~ 9.6 kJ/mol for BHTH prepared by MSSR, which is far less than that of BHTH prepared by CSSR (120 kJ/mol). The activation energies for BHTH samples prepared by MSSR and CSSR for 10–60 min slightly decreased to 8.3 and 115 kJ/mol, respectively.
There have been few reports about the estimation of the crystallization energy of BaTiO3 based on Ba- and Ti- source materials having hydroxide or water. Therefore, it is not possible to make a direct comparison of our results with other research group’s work. According to Osman , the activation energy for the formation of the BaTiO3 phase based on the reaction between BaCO3 and TiO2 at temperatures of 970–1075 °C ranged from 183 to 197 kJ/mol. Additionally, Balaz et al.  reported that the activation energy for the formation of the BaTiO3 phase by the reaction between BaCO3 and TiO2 at a temperature of 620 °C was approximately 270 kJ/mol. The values (183 and 270 kJ/mol) for the formation of the crystalline BaTiO3 phase were much higher than 120 kJ/mol obtained in this study by CSSR for 0–10 min.
As in the case of BHTH, the energy barrier (ΔEa,2) was smaller than those (ΔEa,3) of initial sources without water and hydroxide. Thus, the activation energy for the formation of BaTiO3 from BHTH by CSSR in this study was lower than that for the reaction between BaCO3 and TiO2 at a temperature of 620 °C . Additionally, under microwave irradiation, BHTH prepared by MSSR had a significantly lower conventional heating-related activation energy value (9.6 kJ/mol) compared with BHTH prepared by CSSR. This difference indicated that the additional water molecules of the initial sources seemed to bring an excitation energy of ~ 110 kJ/mol to BHTH under microwave irradiation. Consequently, owing to the contribution of the microwave effect on the reaction, BHTH underwent an efficient synthesis process by MSSR resulting in the rapid synthesis of the BaTiO3 phase, even at a low temperature of 100 °C and a short reaction time of less than 1 min.
3.3 Microstructural and physical features of the BaTiO3 powders prepared by various synthesis methods
BaTiO3 powders having an average size of ~ 26 nm were formed at even 100 °C via MSSR. Ba(OH)2·H2O and TiO2·xH2O were used as Ba and Ti source materials, respectively. The crystallization kinetics of the BaTiO3 phase were examined by estimating the conventional heating-related activation energies. The values for the synthesis of BaTiO3 powders from Ba(OH)2·H2O and TiO2·xH2O by MSSR and CSSR were 9.6 and 120 kJ/mol, respectively. This result clearly shows that MSSR was an efficient synthesis process when initial source materials having water and/or hydroxide are used. Their additional polar molecules brought high reactivity under microwave irradiation.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1D1A1B03934622).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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