Synthesis and characterizations of ultra-low sintering temperature BaTiO3/BaO–ZnO–Bi2O3–B2O3 glass ceramic composite
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Novel ultra-low temperature co-fired ceramic composites consisting of BaTiO3 and BaO–ZnO–Bi2O3–B2O3 (BZBB) glass were successfully produced under 500 °C, and their sintering behavior, phase composition, microstructure and dielectric properties as functions of sintering temperature were investigated. The XRD and SEM results showed that there was no reaction between the glass and the ceramic but the Bi24B2O39 was formed in all samples during sintering process. This indicates that the BZBB glass not only acts as sintering aids but also involves in crystal phase formation. The composites with 60–90 wt% of BZBB glass frit could be densified at a sintering temperature of 450 °C for 30 min, and exhibited widely adjustable dielectric constant (35–133) and acceptable dielectric loss (<0.012). In addition, the composites showed good compatibility with Ag electrode, which suggests their suitability for various dielectric applications with ultra-low sintering temperature.
Most of the known commercial ceramic materials with excellent dielectric properties usually need high sintering temperatures [1, 2]. So, it is important for them to reach ultra-low sintering temperatures (<650 °C) to enable further integration with plastic substrates, semiconducting devices and silver-based electrodes [3, 4, 5]. Additionally, an ultra-lower sintering temperature would decrease energy consumption and enable better control of volatile compounds [6, 7]. Barium titanate (BaTiO3), a well-known ferroelectric material, is widely used as a dielectric for multilayer ceramic capacitors, electro-optic devices and thermistor. However, the sintering temperatures of BaTiO3-based ceramics must be above 1300 °C, which is too high for ultra-low temperature co-fired ceramic processing . To our knowledge, it is very effective to lower sintering temperature of ceramics by adding some low-melting glass materials. Hence, various kinds of glass compositions, such as BaO–B2O3–SiO2 , ZnO–B2O3–SiO2 , and ZnO–B2O3–Li2O–Nb2O5–Co2O3 , have been used to decrease the sintering temperature of BaTiO3 ceramic. However, these studies have just regarded the glass as additive, and the sintering temperature of these glass compositions was still higher than 800 °C, which failed to meet the demand of ultra-low sintering temperature.
The development of ultra-low sintering temperature materials with a sintering temperature <650 °C is still in its infant stage and currently only a handful of materials with good properties are available for practical applications in this rapid and demanding sector of microelectronics. Recently, Yu and co-workers [11, 12] prepared a low softening point zinc borate (3ZnO–2B2O3) glass and studied the densification and dielectric properties of the SiO2-filled 3ZnO–2B2O3 glass/ceramic composites. They found that the composites could be densified at the sintering temperature of 650 °C for 30 min and the optimum content of SiO2 addition could improve the dielectric properties of composites. Chen et al.  reported that Al2O3 ceramics could be sintered at a rather low temperature of 450 °C with the addition of a large amount of B2O3–Bi2O3–SiO2–ZnO (BBSZ) glass, which still have applicable relative dielectric constant values and affordable losses. And they  also prepared BaTiO3 ceramic by adding 50–90 wt% BBSZ glass at an ultra-sintering temperature of 450 °C. Typically, these composites with an addition of 70 wt% BBSZ glass possessed the optimal dielectric properties which relative dielectric constant was 132 and dielectric loss was 0.0056 at 100 kHz.
In this work, the new low-melting BaO–ZnO–Bi2O3–B2O3 glass was added into BaTiO3 ceramic to prepare glass/ceramic composites at low sintering temperature (below 500 °C). Simultaneously, their densities, phase composition, microstructure evolution, dielectric properties and compatibility with Ag electrode have been investigated.
2 Experimental procedures
The nominal composition of the glass/ceramics was x BZBB–(1−x) BaTiO3 (wt%, where x = 0.5, 0.55, 0.6, 0.7, 0.8, 0.9). Powders of analytical reagent grade comprising Ba(OH)2 (≥99.0%), ZnO (≥99.0%), Bi2O3 (≥99.0%), and H3BO3 (≥99.0%) were used as starting materials to prepare the BZBB glass with a composition of 35 mol% Bi2O3, 16 mol% ZnO, 33 mol% B2O3, 16 mol% BaO. The well mixed powders were melted in a corundum crucible at 910 °C for 1 h. The melt was quenched in water to form amorphous glass. Subsequently, the obtained glass was ball-milled with deionized water for 24 h after being dried and crushed. Commercial BaTiO3 powders (>99.7%) with different amounts of BZBB glass were mixed in ethanol solution for 10 h with a small amount of polyethylene glycol as dispersant. After being dried, the mixed powders were pressed into Ф10 mm pellets without any binder. The pressed composite disks were sintered at 425–500 °C for 0.5 h with a heating rate of 3 °C/min and cooling in the furnace. As a comparison, pure BZBB disks were also fabricated in the same manner, excepting that their sintering temperature is 400 °C, which is under their deformation temperature.
Differential scanning calorimetry (DSC, Model STA449C, Netzsch) was used to analyze the crystallization behavior of the BZBB glass. For DSC experiments, approximately 10 mg of samples were fired in flowing air from 50 to 650 °C at a heating rate of 10 °C/min. The bulk densities of sintered samples were measured by the Archimedean immersion method using water as medium. The temperature-dependent shrinkage behavior of samples was characterized by using thermomechanical analyzer (Netzsch DIL402EP, Germany). The crystal structures of these samples were analyzed by X-ray diffraction diffractometer (XRD, Model D8-Advance, Bruker, Germany) at room temperature. Microstructures of these samples were studied by field emission scanning electron microscopy (FE-SEM, Model S-4800, Hitachi, Japan) with an energy-dispersive spectrometer (EDS). For electrical measurements, these samples were polished by using an automatic polishing machine (Model UNIPOL-802, Shenyang Kejing Auto-instrument Company, China) to achieve parallel, smooth faces, and silver electrodes of 10 mm in diameter were sputtered on both faces through magnetic sputtering. The measurements of dielectric constant and dielectric loss from 1 kHz to 100 MHz were performed using a Precision multifunction LCR meter (Model HP4292A, Agilent) at room temperature.
3 Results and discussion
In order to assess the sintering behavior of the composite ceramics, the DSC curves and dilatometric curves of BZBB/BaTiO3 composite are shown in Fig. 1. It is apparently observed that a weak endothermic peaks located at 353 °C in the DSC curve of the BZBB glass (Fig. 1a), which can signify its deformation temperature. The deformation temperature is a reference of softening and sintering temperatures for the BZBB glass. Thus, the softening temperature of BZBB glass is about 353 °C. With further increasing temperature up to 509 °C, there is only one exothermic peak, which indicates presence of a single crystalline phase and is consistent with the XRD results in Fig. 3. It is also noted that the 60 wt% BaTiO3–40 wt% BZBB glass composite displays similar deformation and crystallization temperature compared to the BZBB glass sample. Figure 1b shows the shrinkage behavior of BaTiO3 ceramic with 60–80 wt% of BZBB glass. The onset shrinkage temperature for three samples is during 355–363 °C, which is similar to the deformation temperature of the glass, as shown in DSC curve. With the increase of glass content, the shrinkage rate increases from 7.7 to 14.6%. Additionally, the slope of the shrinkage rate shows that the densifications of samples occur more rapidly within a relative short temperature range. It is clear that high addition of BBSZ glass could promote the densification of the samples at even low temperatures and short dwell time. Considering that the results are very similar, the shrinkage behavior of the samples with other addition of glass is not shown here.
The bulk density of samples with different addition of BZBB glass sintered at 425–500 °C for 0.5 h is shown in Fig. 2a. Although the composition of samples is different, the bulk density curves have a similar tendency with sintering temperature. As can be seen, the bulk density gradually increases and then slightly decreases with the increase of sintering temperature, and the maximum bulk density is achieved at 450 °C in all samples. The degradation of bulk density at temperatures above 450 °C is perhaps because the viscosity of glass phase would reduce quickly and the gas could be wrapped inside the samples forming closed pores before they were exhausted . Figure 2b illustrates the bulk density, relative density, and theoretical density of the composites with different glass content sintered at their suitable sintering temperature (450 °C). Evidently, both bulk density and relative density increase gradually with the increase of glass content. When the glass concentration is >60 wt%, bulk density and relative density exhibit high value over 5.22 g/cm3 and 83%, respectively. The dashed line denotes the theoretical density of samples which is calculated by the measured density of BZBB glass (6.93 g/cm3) and BaTiO3 powder (5.85 g/cm3). It is noted that the bulk density of samples with 80–90 wt% glass is even higher than their calculated theoretical density, which may be attributed to the precipitation of an extra phase from glass.
Figure 3 depicts the XRD patterns of BaTiO3 ceramics with different BZBB glass contents sintered at 450 °C for 0.5 h. It can be observed that there are distinct peaks related to BaTiO3 structure (dashed lines) and a certain amount of amorphous glass phases which background distributing at 20°–35° in all samples. The clear existence of BaTiO3 phase indicates that it may not be heavily involved in the process of crystallization driven by the glass at such a lower sintering temperature. Besides, an extra crystalline phase also appears in all the samples. In order to confirm it, the XRD graph of BZBB glass sintered at 450 °C for 0.5 h is also shown in Fig. 3. The extra crystalline phase could be identified as Bi24B2O39 (PDF-#29-0227), which is consistent with earlier reports by He et al.  and Rejisha et al.  in the similar glass system. It is worth noting that the height of Bi24B2O39 peak increases with the glass concentration, while the intensity of the BaTiO3 peaks decreases. Therefore, the results suggest that the BZBB glass not only acts as sintering aids but also involves in crystal phase formation. And the bulk density of samples with 80–90 wt% glass is higher than the theoretical value as shown in Fig. 1b, which may be caused by the formation of Bi24B2O39 phase with a larger density.
The backscattered electron images (BEI) and energy dispersive spectrometer (EDS) spectra of composites with 60 and 90 wt% addition of BZBB glass are shown in Fig. 4. It can be seen that three phases are co-existed in all samples from the backscattered electron images. The EDS analysis indicates that the dark area represents the BaTiO3 phase, the light grey refers to the BZBB glass, and the white area is denoted as Bi24B2O39 phase. It also can be observed that BaTiO3 particles are uniformly distributed in the composite which illustrates the dielectric powders are well covered and evenly diffuse into the melted glass during sintering process. It means that BaTiO3 can stably exist in the glass phase, which is advantage to the regulation on the dielectric properties of the glass/ceramic composite. Meanwhile, the content of BaTiO3 phase decreases while the flower-like Bi24B2O39 phase increases with the increase of glass concentration, which could further confirm the XRD observation. Moreover, densest surface morphology and lowest porosity indicates that a dense glass/ceramic composite has been successfully obtained at such a lower temperature.
In order to investigate the phase composition dependence on the sintering temperature, the crystal structures of these samples have been analyzed by X-ray diffraction diffractometer. Figure 5 illustrates the XRD patterns of the 60 wt% BZBB glass-40 wt% BaTiO3 composites sintered at 425, 450, 475, 500 °C for 0.5 h, respectively. The XRD pattern of the sample sintered at 425 °C reveals that there is only BaTiO3 phase in the sample. When the sintering temperature is above 450 °C, the second phase Bi24B2O39 mentioned above appears in samples and the intensity of the diffraction peak increases with the temperature. There is no significant change in the height of the diffraction peak of BaTiO3 peaks for all the samples. Thus, it can be proposed that the BaTiO3 would not react chemically with the BZBB glass matrices during the sintering procedure.
To further study the densification behavior, the microstructural changes of the sintered composites have been analyzed by SEM. Figure 6 shows the typical SEM images of fractured surfaces of 60 wt% BZBB glass-40 wt% BaTiO3 samples as functions of sintering temperature. It appears that sintering temperature has an important effect on the sintering behavior of glass/ceramic composites. As seen in Fig. 6a, a distinct porosity microstructure is observed in the specimen sintered at 425 °C, which suggests the densification process of specimen is not completed. As the temperature rises up to 450 °C, the pores in the sample disappear and specimen reveals a dense microstructure (seen in Fig. 6b). However, further increase in sintering temperature would deteriorate the microstructure, and the amount of the pores of sample distinctly increase, as shown in Fig. 6c, d. The variation in microstructure is in well agreement with the trend of bulk density as shown in Fig. 2a. It was notable that the BaTiO3 was hardly found in the fracture section, which was attributed to the BaTiO3 particles that were covered by a large amount of glass phase. The microstructure change could be explained by the densification process. In the process of sintering, the BZBB glass begins to wet and form liquid which spreads across the BaTiO3 particles as the sintering temperature rising to the softening point. The melting glass prefers to occupy the lowest energy configuration, such as smaller grains and pores. Then the grains are rearranged and resulted in a more compact structure due to the forces induced by the capillary liquid bridges among small grains after the liquid layers touch each other . Nevertheless, excessive sintering temperature leads to a large amount of glass generates the liquid phase, they will leave behind large pores that are difficult to remove . Thus, the suitable choice of sintering temperature is very important for the preparation of glass/ceramic composites.
It is well known that the densification of glass/ceramic composites with a relatively large amount of glass can be described by conventional three-stage liquid phase sintering process, i.e., particle rearrangement, dissolution and precipitation, and solid state sintering. According to the reactivity between glass and ceramic, the densification of glass/ceramics composites can be further classified as nonreactive, partially reactive, and completely reactive systems . Based on the above discussion of bulk densities, XRD and SEM results, the densification and microstructure evolution may be attributed to a nonreactive type during the sintering process, which has been given in other literatures [12, 21]. For this work, a high BZBB glass content in composites is chosen to prevent direct contact between the BaTiO3 particles in the early stage of sintering. The BZBB glass just plays a binder role during the sintering process. Thus, the sintering model in this paper is believed to be three-stage sintering process: glass redistribution and grains rearrangement, closure of pores, and viscous flow.
Figure 7 shows the dielectric properties of composites with 50–90 wt% of BZBB glass sintered at different temperatures. In comparison, the pure BZBB glass sintered at 400 °C for 30 min obtained εr of ∼28 and tanδ of ∼0.009 at 100 MHz. It is observed that the dielectric constants of composites reveal a trend of increasing firstly and then decreasing with the increase of glass content at each fixed sintering temperature from Fig. 7a. Obviously, the decrease of dielectric constant may be mainly due to that the BBSZ glass possesses lower dielectric constant. It was reported that the dielectric properties of the composites depended on the bulk densities, microstructures and dielectric properties of each phase . Thus, it is reasonable for composites with 50 and 55 wt% BZBB glass to have such lower dielectric constant owing to their poor densification degree as shown in Fig. 2. Besides, as seen in Fig. 7b, the changes of dielectric losses of composites are also associated with bulk density and microstructure variation. It is also found that the dielectric property depends not only on the amounts of glass addition, but also on the sintering temperature. As glass <60 wt%, the dielectric constant increases with increase of the sintering temperature. This could be attributed to the densification of the composites was greatly improved with raising sintering temperature. As glass ≥60 wt%, however, the dielectric constant firstly increases and then decreases with an increase in the sintering temperature. The increase in the permittivity of the samples is mainly due to the increase in densification. However, the samples will be burned out as the sintering temperature rises, which will result in the reduction of dielectric constant. The Additionally, the dielectric loss of composites decreases with the increasing of glass content and composites with 60–90 wt% BZBB glass have the dielectric loss below 0.012, which may be attributed to the addition of low loss component BZBB glass and dense microstructure. Interestingly, as shown in Table 1, glass/ceramic composites with 60 wt% BZBB addition sintered at 450 °C has a εr of 133 at 100 MHz, which is 4 times higher than that of BaO–B2O3–SiO2/BaTiO3 glass ceramics sintered at 900 °C in other literature . The volume resistivity (ρv) of different glass addition is also presented in Table 1. The results show that glass addition markedly improve the volume resistivity for this glass/ceramic composites. As we know, the combination of high dielectric constant, low loss and the volume resistivity is of technical significance.
Dielectric properties and volume resistivity of composites with different addition of glass sintered at 450 °C
ρv (Ω cm)
0.006 × 1012
0.03 × 1012
1.25 × 1012
2.71 × 1012
4.36 × 1012
7.73 × 1012
The chemical compatibility between these composites and silver electrode was investigated by co-firing the BaTiO3 with 60 wt% BZBB glass with Ag electrode. Figure 8 shows the XRD pattern, SEM, and EDS results of the co-fired sample sintered at 400 °C for 1 h. Obviously, there is no formation of another new phase except for Ag, BaTiO3, and Bi24B2O39 from the XRD patterns (Fig. 8a), which indicates that no chemical reaction has taken place between the glass ceramic matrix and Ag. This observation is also confirmed by SEM micrographs and EDS analysis (Fig. 8b, c). The results show that the glass ceramics sheet matches Ag electrodes well and Ag won’t diffuse into the composite. In conclusion, BaTiO3/BZBB composites could be selected as suitable candidates for ultra-low temperature co-fired ceramic application because of its ultra-low sintering temperature, adjustable dielectric properties, and good compatibility with Ag electrode.
A series of glass/ceramic composites based on BaO–ZnO–Bi2O3–B2O3/BaTiO3 with an ultra-low sintering temperature of 425–500 °C were prepared by solid state reaction. The sintering behavior, phase composition, microstructure, dielectric properties and compatibility with Ag electrode were investigated. The XRD, SEM and EDS results display that BaTiO3 particles could be uniformly distributed in the glass and only crystallization phase Bi24B2O39 could be found. The densification and the microstructure evolution could be attributed to a nonreactive type where the glass played a binder role during the sintering process. Results showed the dielectric properties of composites are strongly dependent on the glass content and sintering temperature. Typically, with the amount of BZBB glass increasing from 60 to 90 wt%, the dielectric constant increased from 35 to 133 at the frequency of 100 MHz. A better compatibility with Ag electrode suggests that the composite could be a promising candidate for ultra-low temperature sintering applications.
This work was financially supported by the National Natural Science Foundation of China (No. 51502349) and the National Natural Science Foundation of China (No. 51672310).
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