1 Introduction

With the fast growth of microwave communication in the past decades, there is an ever-increasing demand for low-cost and high-performance dielectric ceramics. Which are key materials for the manufacturing of microwave components such as resonators, filters, oscillators, wave guides and antennas [1]. The properties required for high performance dielectric ceramics are high dielectric constant (εr), high quality factor (Q × f) and near zero temperature coefficient of resonant frequency (τf) [2]. Therefore, light weight and low cost ceramics attracting the component manufacturers’ attention and a large number of dielectric has been developed [3]. In the low-cost Bi2O3–TiO2 system, several compounds, such as Bi4Ti3O12, Bi2Ti2O7, Bi12TiO20 and Bi2Ti4O11, have been found and most of them have special electronic properties [4–6]. Furthermore, due to the low sintering temperature and high relative dielectric constant, these compounds have potential electronic devices applications.

Several researchers have attempted to prepare Bi2Ti4O11 ceramics and investigate their properties. The polymorphism and dielectric properties at 1 MHz of Bi2Ti4O11 was studied by Subbarao in 1962 [7]. Single crystal of Bi2Ti4O11 with platelike and prismatic shape were prepared by Barsukova et al. [8]. Using different staring materials and process, the microwave dielectric properties of Bi2O3–Bi2Ti4O11 composite ceramics were researched by Axelsson and co-workers [9]. Akimov studied the effect of pressing pressures and heating rate on the synthesis and sintering of Bi2Ti4O11. And found that the formation of Bi2Ti4O11 begins at 900 °C and follows with a diffusion mechanism [10]. However, Bi2Ti4O11 is one kind of meta-stable compounds, and it is difficult to synthesize the single phase by conventional oxides reaction method.

Moreover, Bi2Ti4O11 possess two phases, the high-temperature β-Bi2Ti4O11 and low-temperature α-Bi2Ti4O11 phase. Preparation of Bi2Ti4O11 without calcinations processes will lead to a weight loss greater than 1 % when the sintered temperature above 1,100 °C due to the volatilization of Bi2O3 [11]. And TiO2 is not soluble in the high-temperature form of Bi2Ti4O11 [12]. For these reasons, phase pure Bi2Ti4O11 is very difficult to prepare for short calcinations time and low temperature and the preparation processes on the dielectric properties of the ceramics are remains unknown. In order to prepare phase pure Bi2Ti4O11 powders, we attempted to increase the calcinations time at 1,000 °C by solid state reaction method and investigate the effects of sintering condition on microstructure and microwave dielectric properties of the ceramics.

2 Experimental

The powders of Bi2Ti4O11 were prepared by conventional solid state reaction method from high purity oxides of Bi2O3 (>99 %) and TiO2 (>99 %). Stoichiometric ratios of Bi2O3 and TiO2 were mixed and ball-milled in ethanol with zirconia balls for 6 h. After drying, the mixed powders were calcined in covered alumina crucible at 1,000 °C for 1–6 h in air, respectively. The heating and cooling rate of calcinations process was 5 °C/min. Ceramics were prepared from phase pure Bi2Ti4O11 powders which calcined at 1,000 °C for 6 h. The powders were remilled for 6 h and dried then press into pellets of 10 mm in diameter and 6 mm in thickness at 100 MPa. The pellets were sintered at 1,075–1,175 °C for 2 h in air with a heating rate of 5 °C/min.

Crystal structure of calcined powders and sintered ceramics were examined by X-ray diffraction (XRD, Rigaku, DMAX-RB, Japan) using Cu Kα radiation with a scan speed of 10 °C/min. The apparent densities of sintered samples were measured by Archimedes’ method. Microstructures of Bi2Ti4O11 ceramics were studied by scanning electron microscopy (SEM, JSM-6480LV). Microwave dielectric properties of ceramic samples were measured with a HP8720ES network analyzer at 3-8 GHz using Hakki-Coleman’s dielectric resonator method, as modified and improved by Courney and Kobayashi et al. [13–15].

3 Results and discussion

Figure 1 shows the XRD patterns of powders calcined with different holding time and ceramics sintered at various temperatures. When the mixed powders were calcined at 1,000 °C for 1 h, the reaction products consist of main phase of Bi2Ti4O11 (JCPDS, File No. 83-0672) and a small amount of second phase Bi2Ti2O7 (JCPDS, File No. 32-0118). The amount of Bi2Ti2O7 decreased with increasing holding time, and finally disappeared as the holding time is 6 h. Because the low melting point of Bi2O3 (860 °C), the calcinations process at 1,000 °C may be consider as the reaction of TiO2 and liquid Bi2O3, and the growth of Bi2Ti4O11 is diffusion-controlled reaction. According to Lu’s [16] study of interface compound of Bi2O3 and TiO2, Bi4Ti3O12 forms fast and then Bi2Ti4O11 arise between TiO2 and Bi4Ti3O12 when the heating time is 0.5 h, after that Bi2Ti2O7 appeared for longer holding time. However, both of Bi4Ti3O12 and Bi2Ti2O7 have not been detected in the calcined powders of Bi2O3 and TiO2, and pure phase of Bi2Ti4O11 can be obtained after 6 h calcination. Ceramics prepared using the above powders sintered at various temperature also show a single Bi2Ti4O11 phase, and no second phases was detected in the ceramics sintered at 1,075–1175 °C. Therefore, for preparing single phase Bi2Ti4O11 ceramics, the key process is obtain single phase Bi2Ti4O11 powders by controlling the calcining temperature and holding time.

Fig. 1
figure 1

XRD patterns of Bi2Ti4O11 powders and ceramics: (a), (b), (c) powders calcined at 1,000 °C for 1, 4, and 6 h; (d), (e), (f) ceramics sintered at 1,075, 1,125 and 1,175 °C, respectively

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Densities of Bi2Ti4O11 ceramics sintered at different temperature for 2 h in air are shown in Fig. 2. All the ceramics samples sintered at 1,075–1,175 °C reach above 95 % of the theoretical density. They increase with the increasing of sintering temperature and then decline after reaching a maximum at 1,125 °C. The maximum density of 6.033 g/cm3 (96 % of the theoretical density) can be obtained for samples sintered at 1,125 °C, which suggests that the Bi2Ti4O11 ceramics can be sintered at a low temperature.

Fig. 2
figure 2

Densities of Bi2Ti4O11 ceramics as a function of sintering temperature

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Surface SEM photographs of Bi2Ti4O11 ceramics sintered at different temperature are shown in Fig. 3. All samples sintered at the temperature in this study present two different sizes of prismatic shape crystals. The sizes of those grains increased with increasing sintering temperature and the smaller size crystals transform into larger ones. In early studies of single crystal of Bi2Ti4O11 by hydrothermal method, Barsukova found that platelike crystal of Bi2Ti4O11 appeared when Bi2O3 excesses in the Bi2O3–TiO2 system and prismatic crystal obtained in other conditions [8]. However, in this study, single phase Bi2Ti4O11 powders were used to prepare ceramics by solid reaction method, and the crystals in the ceramics exhibit only prismatic shape except that they have different sizes.

Fig. 3
figure 3

Surface SEM photographs of Bi2Ti4O11 ceramics sintered at different temperature: a 1,075 °C, b 1,100 °C, c 1,125 °C, d 1,150 °C, e 1,175 °C

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Effect of sintering temperature on microwave dielectric properties of Bi2Ti4O11 ceramics is shown in Fig. 4. Both of the dielectric constant (εr) and quality factor (Q × f) increase below 1,125 °C, and then decreased with increasing of the sintering temperature. These results are similar to the effect of sintering temperature on the density of Bi2Ti4O11 ceramics. However, temperature coefficient of resonant frequency (τf) has negative value and decreasing with the sintering temperature near linearly. This result illuminates that increasing sintering temperature will bring on the increase of τ f values for Bi2Ti4O11 ceramics. As the sintering temperature is 1,125 °C, Bi2Ti4O11 ceramics exhibits a microwave dielectric properties: εr = 51.2, Q × f = 3,050 GHz and τf = −297 ppm/°C. In previous study, Fukuda et al. [17] reported that Bi2Ti4O11 ceramic have εr = 53.2, Q × f = 4,500 GHz and τf = −550 ppm/°C . However, the XRD patterns of Bi2Ti4O11 they provided presents a trace of Bi2Ti2O7 phase. The values of εr and Q × f obtained in this study are somewhat lower and temperature coefficient of resonant frequency is higher than their result. The reasons may be the different calcinations and sintering temperature and during times.

Fig. 4
figure 4

Microwave dielectric properties of Bi2Ti4O11 ceramics as a function of sintering temperature from 1,075 to 1,175 °C

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4 Conclusions

Single phase Bi2Ti4O11 powders can be synthesized by controlling the calcining temperature and holding time through solid-state reaction method. It was found that second phase Bi2Ti2O7 appeared when the holding time below 4 h, and the single phase of Bi2Ti4O11 powders can be obtained by calcining the mixture of stoichiometric Bi2O3 and TiO2 at 1,000 °C for 6 h. Bi2Ti4O11 ceramics sintered at different temperature present two different sizes of prismatic shape crystals, and smaller ones grow into larger sizes ones when raising the sintering temperature. The sintering temperatures have a significant effect on microwave dielectric properties of Bi2Ti4O11 ceramics. Both of ε r values and Q × f values increase with increasing sintering temperature, and present a maximum value of 51.2 and 3,050 GHz at 1,125 °C, respectively. Increasing sintering temperature will bring on the increase of τ f values for Bi2Ti4O11 ceramics. The microwave dielectric properties of Bi2Ti4O11 ceramic sintered at 1,125 °C are: εr = 51.2, Q × f = 3,050 GHz and τf = −297 ppm/°C.