Synthesis of BaTiO₃/Ag nanocomposites using a sonochemical process and low-temperature sintering

ABSTRACT

A barium titanium oxide/silver (BaTiO₃/Ag) nanocomposite powder was synthesized using a sonochemical process. By irradiating an ethanol solution of BaTiO₃ and silver oxide (Ag₂O), Ag₂O thermally decomposed at 40°C and Ag-loaded BaTiO₃ nanocomposites powder was obtained. Nanocomposites containing 30 vol% Ag sintered at 1000°C showed near densities and higher dielectric constants compared to those of BaTiO₃ sintered at 1300°C. Moreover, percolation did not occur because the Ag nanoparticles were uniformly dispersed and loaded onto the surface of BaTiO₃. This sonochemical method is expected to be a new process for obtaining uniformly dispersed and loaded nanocomposite powders without percolation, even in large amounts.

KEYWORDS:

• Nanocomposites
• sonochemical process
• low-temperature sintering
• MLCC
• barium titanium oxide

1. Introduction

Barium titanium oxide (BaTiO₃) is a perovskite-type crystal not found in nature. Since the discovery of the ferroelectric BaTiO₃, its characteristics, and applications have been extensively studied [1]. BaTiO₃ has cubic, tetragonal, orthorhombic, and rhombohedral crystal structures. In particular, the tetragonal structure is crucial for the ferroelectric properties of BaTiO₃. In the tetragonal structure, the Ti⁴⁺ exists in an off-center position, and the resulting spontaneous polarization in the crystal causes ferroelectricity [2].

Multilayer ceramic capacitors (MLCCs) are applications of BaTiO₃. MLCCs have a layered structure comprising dielectric and internal electrodes. This characteristic structure enables a large capacitance and downsizing of products [3,4]. Moreover, by compositing with metal, the dielectric constant and capacitance increase according to the effective dielectric field theory [5] and internal electrode effects [6], respectively. However, the manufacture of MLCC requires complicated processes and high sintering temperatures above 1200°C [4].

From a practical perspective, its mechanical and electrical properties are important, with the nanocomposite technique being an effective method for these properties [7–10]. Several studies have been conducted on BaTiO₃ nanocomposites. For example, the fracture strength and toughness increases when silicon carbide is dispersed in BaTiO₃ [11]. Moreover, the nanocomposites of BaTiO₃ and nickel improve the dielectric constant [6,12,13].

Although nanocomposites exhibit many attractive characteristics, their synthesis requires multistep, high-temperature, and long-term processes. In general, nitrates, which lead to NOx generation, are required when synthesizing nanocomposites of metals and ceramics [14–17]. Sonochemical processes are effective in overcoming these disadvantages. Ultrasound has a frequency above 20 kHz and cannot be heard by humans [18]. Cavitation bubbles are generated when liquids are irradiated with ultrasound. The repeated rapid contraction and expansion caused by cavitation generates high temperature (~5000 K) and high pressure (~100 atm) hot spots [19]. Hot spots can result in both physical and chemical effects. Physical effects include crushing and mixing caused by microjets, while chemical effects include the induction of radical reactions and thermal decomposition [20]. Sonochemical processes with these effects have been used to synthesize nanoparticles [21–23] and nanocomposites [24–26].

In this study, we synthesized BaTiO₃/Ag nanocomposites using ultrasonic irradiation. Silver oxide (Ag₂O) was used as the starting material. Noble oxides such as Ag₂O are readily reduced by heating without emitting toxic by-products, which can occur at room temperature under ultrasonic irradiation [27]. It is expected that dispersing silver (Ag) nanoparticles enables low-temperature sintering by liquid phase diffusion and its high thermal conductivity, resulting in high dielectric constant [28]. Therefore, we attempted to synthesize dielectrics for a chip capacitor with a high dielectric constant by sintering BaTiO₃/Ag nanocomposites at temperatures lower than 1200°C.

2. Materials and methods

2.1. Materials

BaTiO₃ (Purity 99.9%, Wako Pure Chemical Industries, Ltd.) and Ag₂O (Purity 99%, Koujundo Chemical Laboratory Co., Ltd.) were used to synthesize the BaTiO₃/Ag nanocomposites. Ethanol (C₂H₅OH) was used as the solvent. Dotite (HUJIKURA KASEI Co., Ltd.) was used as the Ag electrode to measure the dielectric constant.

2.2. Preparation of BaTiO₃/Ag nanocomposites

Figure 1 shows the method used to prepare the BaTiO₃/Ag nanocomposites. BaTiO₃ and Ag₂O were added to 100 mL C₂H₅OH in a flask. The volume of Ag was 10 or 30% of that of BaTiO₃. The mixture was sonicated (44.13 kHz, 100 W) in air for 1 or 3 h using an ultrasound reactor (Honda Electronics Co., Ltd.). During sonication, the flask was placed in a constant temperature water bath maintained at 40°C using a low temperature circulator (CTP-3000, TOKYO RIKAKIKAI Co., Ltd.). The sonicated sample was dried under vacuum (30°C) to obtain the BaTiO₃/Ag nanocomposite powder.

Figure 1. Procedure to synthesize BaTiO₃/Ag nanocomposites.

2.3. Forming and sintering pellets of BaTiO₃/Ag nanocomposites

The nanocomposite powder was formed into pellets via uniaxial pressing at 200 MPa. The diameter of each pellet was 8 mm. Pellets were sintered at 1000 or 1100°C (5°C/min temperature ramp rate) for 3, 6, 9, and 12 h in air in an electric furnace. To compare with the effect of the BaTiO₃/Ag nanocomposites, the BaTiO₃ pellets were formed and sintered under the same conditions and at 1300°C for 6 h.

2.4. Characterization

The using powder X-ray diffraction (XRD) patterns of the BaTiO₃/Ag nanocomposites were recorded using a RINT 2000 diffractometer (Rigaku Corporation). The surface morphologies of the BaTiO₃/Ag nanocomposites and pellets were observed by field-emission scanning electron microscopy (FE-SEM, Model-S-48 Hitachi High-Technologies Corporation). The density of the pellets was measured using Archimedes’ method (n = 3).

2.5. Measurement of the dielectric constant

One side of the sintered pellets was coated with dotite to synthesize the Ag electrodes. An electrode was synthesized by sintering the pellets at 300°C for 1 h with a temperature ramp rate of 10°C/min in an electric furnace. The same procedure was performed on the other side of the pellets. The dielectric constants of the prepared pellets were measured using a precision impedance analyzer (Agilent 4294A, Agilent Technologies Ltd.). The temperature range of this measurement was 40 to 150°C and the frequency was 100 kHz.

3. Results and discussion

3.1. Sample characterization

The XRD patterns of BaTiO₃/Ag nanocomposites containing 10 vol% Ag are shown in Figure 2(a). For the sample sonicated for 1 h, certain peaks were assigned to Ag. This suggests that the thermal decomposition of Ag₂O progressed. Ultrasound irradiation in ethanol produces OH radicals, which generates reducing agent of Ag₂O. This chemical equation is shown below.

$\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{O}\mathrm{H}+\mathrm{u}\mathrm{l}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{s}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}\to \mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\cdot +\mathrm{O}\mathrm{H}\cdot$

$\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{O}\mathrm{H}+\mathrm{O}\mathrm{H}\cdot \to \mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{O}\cdot +{\mathrm{H}}_2\mathrm{O}$

$2\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{O}\cdot +2\mathrm{A}{\mathrm{g}}_2\mathrm{O}\to 2\mathrm{A}\mathrm{g}+2\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$

Figure 2. XRD patterns of BaTiO₃/Ag nanocomposites containing (a) 10 vol% Ag and (b) 30 vol% Ag.

In addition to the sonochemical reduction shown above, thermal decomposition may be also considered. OH radicals are also generated during ultrasound irradiation of water, but the decomposition of Ag₂O by ultrasound does not occur in water. The thermal decomposition temperature of Ag₂O about 280°C, so it is considered that thermal decomposition by hot spots produced by ultrasound irradiation is dominant reaction in this study [19,27]. For the sample sonicated for 3 h, the peaks assigned to Ag₂O disappeared, indicating that Ag₂O completely decomposed. Peaks assigned to silver acetate (CH₃COOAg) were observed in the samples sonicated for 3 h. Ultrasonic irradiation promoted the reaction between C₂H₅OH and Ag₂O, so that Ag₂O was reduced and C₂H₅OH was oxidized simultaneously. When C₂H₅OH is oxidized, acetic acid is formed via acetaldehyde. It was assumed that CH₃COOAg was formed by the reaction of acetic acid with silver oxide [29].

Figure 2(b) shows the XRD patterns of the BaTiO₃/Ag nanocomposites containing 30 vol% Ag. Similar to the nanocomposites containing 10 vol% Ag, peaks assigned to Ag₂O (1 h), Ag (1, 3 h), and CH₃COOAg (3 h) are observed. The diffraction peaks of the BaTiO₃/Ag nanocomposites containing 30 vol% Ag are higher in intensity than those of the BaTiO₃/Ag nanocomposites containing 10 vol% Ag. This is attributed to an increase in the amount of Ag₂O.

The morphologies and particle size distributions of the BaTiO₃ and Ag nanoparticles of BaTiO₃/Ag nanocomposites are shown in Figures 3 and 4, respectively. The small Ag nanoparticles were uniformly dispersed and loaded onto the BaTiO₃ surface. Hot spots produced by ultrasonic irradiation were generated randomly in the C₂H₅OH solution; thus, the heterogeneous nucleation of Ag on the surface of BaTiO₃ was promoted while uniformly dispersing it. The size of the Ag nanoparticle increased with increasing Ag₂O amount and ultrasonic irradiation time. This may have been due to an increase in the amount of reacted Ag₂O.

Figure 3. SEM image and particle size distribution of BaTiO₃.

Figure 4. SEM images and particle size distributions of Ag nanoparticles in BaTiO₃/Ag nanocomposites.

3.2. Characterization of sintered BaTiO₃/Ag nanocomposites

Figure 5 and Tables 1 and 2 show the relative densities of the sintered BaTiO₃/Ag nanocomposites synthesized from the samples sonicated for 3 h and BaTiO₃. The relative densities were calculated using the theoretical densities. The theoretical densities of BaTiO₃ and the 10 and 30 vol% Ag BaTiO₃/Ag nanocomposites were 6.08, 6.19, and 6.40 g/cm³, respectively. Figure 5 shows that the BaTiO₃/Ag nanocomposite containing 10 vol% Ag sintered at 1100°C for 6 h has almost the same relative density as BaTiO₃ sintered at 1300°C for 6 h; therefore, nanocompositing is an effective method to achieve low-temperature sintering.

Figure 5. Reaction-time dependence of the relative densities of sintered BaTiO₃/Ag nanocomposites.

Table 1. Relative densities (%) and porosities (%) of BaTiO₃/Ag nanocomposites sintered at 1000°C.

	10 vol% Ag		30 vol% Ag		BaTiO3
	Relative density	Porosity	Relative density	Porosity	Relative density	Porosity
0 h	51.1	48.9	56.6	43.4	47.9	52.1
3 h	70.0	30.0	69.6	30.4	61.6	38.4
6 h	72.9	27.1	71.4	28.6	65.6	34.4
9 h	75.9	14.1	75.0	15.0	68.5	31.5

Table 2. Relative densities (%) and porosities (%) of BaTiO₃/Ag nanocomposites sintered at 1100°C.

	10 vol% Ag		30 vol% Ag		BaTiO3
	Relative density	Porosity	Relative density	Porosity	Relative density	Porosity
0 h	51.8	48.2	56.2	43.8	47.9	52.1
3 h	84.3	15.7	84.6	15.4	82.3	17.7
6 h	88.4	11.6	86.9	13.1	79.3	20.7
9 h	90.5	9.5	83.5	16.5	79.6	20.4

The relative densities of the nanocomposites containing 10 vol% Ag were higher than these of the nanocomposites containing 30 vol% Ag at both sintering temperatures (1000 and 1100°C, Tables 1 and 2, respectively). There are two reasons for the decrease in the density of the nanocomposites containing 30 vol% Ag. First, Ag leaked onto the pellet surface because of the high temperature and long sintering time. Ag was observed on the pellet’s surface of the 30 vol% Ag pellets sintered at 1100°C for 12 h. It is conceivable that Ag melted and moved to the pellet surface. Second, CH₃COOAg was formed by ultrasonic irradiation (Figure 2(b)). Figure 6 shows the SEM images of the BaTiO₃/Ag nanocomposite containing 30 vol% Ag. The rod-shaped CH₃COOAg [30] that is present before sintering (Figure 6(a)) thermally decomposed after sintering (Figure 6(b)). As a result, it was inferred that rod-shaped vacancies formed in the nanocomposite, resulting in the density decrease.

Figure 6. SEM images of the BaTiO₃/Ag nanocomposite containing 30 vol% Ag (a) Before and (b) after sintering at 1100°C for 6 h.

Figure 7 shows the SEM images of the 10 or 30 vol% Ag pellets sintered at 1100°C for 6 h and BaTiO₃ pellets sintered at 1300°C for 6 h. The particle size of the BaTiO₃/Ag nanocomposites was smaller than that of BaTiO₃. It is assumed that the particle growth of BaTiO₃ was suppressed by nanocompositing owing to the pinning effect of the Ag nanoparticles [31,32].

Figure 7. SEM images of the BaTiO₃/Ag nanocomposite with different amount of Ag and BaTiO₃.

3.3. Mechanism of sintering nanocomposites

Figure 8(a) shows a schematic of the sintering of BaTiO₃. Similar to general sintering, it proceeds through solid-phase diffusion. During sintering, particle growth progresses, and necks or pores form. The solid-phase diffusion rate is very slow; therefore, high-temperature and long-term reactions are required to synthesize densely sintered bodies [33,34]. In addition, smaller initial particle size results in higher relative density after sintering [35]. In this study, BaTiO₃/Ag nanocomposites sintered body have higher relative density than raw BaTiO₃ sintered body. BaTiO₃ is one of the ceramics and hard, so it is difficult to downsize the BaTiO₃ particles by ultrasound irradiation. Thus, it is considered that initial particle size of BaTiO₃ does not change with or without ultrasound irradiation in this study. We think that relative density increased by compositing with Ag nanoparticles. Detailed discussion is shown below.

Figure 8. Schematic of the sintering of (a) BaTiO₃ and (b) BaTiO₃/Ag nanocomposites.

A schematic of the sintering of the BaTiO₃/Ag nanocomposites is shown in Figure 8(b). Densely sintered BaTiO₃/Ag nanocomposites were obtained by low-temperature sintering. This was attributed to the melting and then diffusion of Ag, during sintering (melting point: 962°C). Thus, BaTiO₃/Ag nanocomposites were sintered in the liquid phase where rapid diffusion enables sintering at a lower temperature [36]. Moreover, the higher thermal conductivities of Ag (429 W m⁻¹ K⁻¹) compared with BaTiO₃ (6 W m⁻¹ K⁻¹) contributed to low-temperature sintering. Therefore, it was inferred that the addition of Ag caused faster thermal conduction, which progressed quickly to the interior of the pellet, resulting in low-temperature sintering. The growth of BaTiO₃ particles was suppressed by the pinning effect of the Ag nanoparticles. However, Ag nanoparticles exist at the BaTiO₃ particle interface, and the presence of Ag in the liquid state causes particle rearrangement. Thus, a dense and uniformly sintered body was obtained via low-temperature sintering. Furthermore, in general, contact flattening effects also contribute to densification in liquid phase sintering [37]. Therefore, contact flattening of BaTiO₃ by dissolution and precipitation in molten silver rarely occurs, and it is assumed that particle rearrangement is the main cause of densification.

3.4. Evaluation of electric characteristics

Figure 9 and Table 3 show the dielectric constants of the samples sintered for 6 h. The dielectric constants of BaTiO₃ and BaTiO₃/Ag nanocomposites containing 10 vol% Ag increased with an increase in the sintering temperature. Additionally, the dielectric constants of the samples sintered at 1000°C increased with increasing Ag content. The dielectric constant of the BaTiO₃/Ag nanocomposites containing 30 vol% Ag sintered at 1000°C is 9.2 × 10³, which is larger than that of BaTiO₃ sintered at 1300°C (6.2 × 10³). This indicates that the nanocomposite fabrication technique is effective for lowering the sintering temperature and increasing the dielectric constant. The reason for the increase in dielectric constant with the addition of Ag nanoparticles can be attributed to the effective dielectric field theory and the contribution of the internal electrode effect. First, the dispersion of Ag nanoparticles in BaTiO₃ is assumed to create an effective dielectric field around the Ag nanoparticles based on the Maxwell-Wagner effect. Therefore, it is presumed to increase the dielectric constant of sintered body. Second, the dispersed Ag nanoparticles in BaTiO₃ behave like internal electrodes in MLCC. Thus, it is assumed that the addition of Ag nanoparticles increased the electrode area, resulting in an increase in the dielectric constant. Moreover, the BaTiO₃/Ag nanocomposites containing 30 vol% Ag maintained their insulation. Thus, the nanocomposites prepared using sonochemical processes can uniformly disperse highly concentrated particles. Percolation occurred in the BaTiO₃/Ag nanocomposites containing 30 vol% Ag sintered at 1100°C. It may be speculated that the high-temperature and long sintering time caused the melting and diffusion of Ag which resulted in the formation of conductive paths.

Figure 9. Temperature dependence of dielectric constant of 6 h sintered BaTiO₃/Ag nanocomposites and BaTiO₃.

Table 3. Dielectric constants of BaTiO₃/Ag nanocomposites and BaTiO₃ (sample sintered for 6 h).

	BaTiO3	10 vol% Ag	30 vol% Ag
1000°C	1.7 × 10^3	2.5 × 10^3	9.2 × 10^3
1100°C	2.8 × 10^3	2.9 × 10^3	Percolation
1300°C	6.2 × 10^3	-	-

In Figure 9, peak shifts were observed. It is known that the particle size of BaTiO₃ and residual stress can change the Curie temperature of sintered body [38,39]. One of the reasons for the peak shift is that the pinning effect of Ag nanoparticles inhibits grain growth, resulting in a decrease in grain size and residual stresses. In the sintering of BaTiO₃ at 1000°C, the densification did not progress sufficiently due to the lower sintering temperature, causing a change in the particle size and accumulating residual stresses which is assumed to be the peak shift.

4. Conclusion

In this study, we successfully synthesized BaTiO₃/Ag nanocomposite powders near room temperature without producing hazardous waste. Under ultrasonic irradiation, hot spots formed, and Ag₂O decomposed. As a result, the Ag nanoparticles were uniformly dispersed onto the surface of BaTiO₃ particles. The melting and high thermal conductivity of Ag led to a lower sintering temperature of BaTiO₃/Ag nanocomposites than that of BaTiO₃. Sintered BaTiO₃/Ag nanocomposites containing highly concentrated Ag nanoparticles provided electrical insulation because the Ag nanoparticles were uniformly dispersed. The low-temperature sintered BaTiO₃/Ag nanocomposites have a higher dielectric constant than BaTiO₃ based on the effective dielectric field theory and internal electrode effects by dispersing Ag nanoparticles. Currently, the synthesis of a high-performance SDGs-oriented material is required. The synthesis of nanocomposite powders by sonochemical processes and their sintering at low temperatures are expected to achieve high performance and SDGs.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by JSPS KAKENHI Grant Number JP 22H02117.