It is well known that the higher sintering temperature of the ceramics brings in the more energy consumption. To keep a certain sintering temperature, the energy consumed by the kiln is proportional to T4 as Stefan-Boltzmann law which determines the blackbody radiation. To decrease the sintering temperature and save energy, we need fine grain size nanoparticles of the BST ceramic powders to ensure their high activity. We directly synthesized them at room temperature. The XRD pattern of them is shown in Fig. 1. It is confirmed that the as-prepared nanoparticles have perfect perovskite structures as indexed according to PDF#31–0174. The very weak peak of the impurity phase at 24 o is indexed as BaCO3. The slight amount of BaCO3 in the as-prepared samples possibly derives from the raw material barium hydroxides, which can react with CO2 in the air during its storage. The peaks shift left as a comparison of the theoretical positions, and thus the lattice cell expanded because of hydrogen interstitial and nanoscale size effect. The nanoscale size effect also broadens the peaks of the nanoparticles as we discussed previously [15].
The SEM image of the as-prepared powders is shown in Fig. 2. The average grain size can be estimated at ~ 15 nm. It is following the calculated result by Scherrer’s Equation from the XRD peak broadening effect. The nanoparticles agglomerated severely for their high activity. Some sintering aids are necessary to sinter the ceramics at low temperatures because we can hardly eliminate the pores in the inner of the agglomerations.
The ceramic samples sintered at 800 oC with different contents of sintering aids are shown in Fig. 3. The sample with 2 wt% sintering aids has a white color. Others have a gray color and get darker and darker with the content of sintering aids. The sintering shrinkages and relative densities of the samples are shown in Fig. 4. It reaches the maximum of both shrinkages and the density with 4 wt% sintering aids. This means that the sintering process at 800 oC can be promoted with the sintering aids lower than 4 wt%, and above that, it can be repressed mildly.
The SEM of the samples sintered at 800 oC for 8 h with different contents of sintering aids are shown in Fig. 5. It is shown that the grain size of the ceramics decreases with the contents of the sintering aids. Meanwhile, the size of the pores in ceramics also decreases. It means that during the sintering process, the mass transfer and the followed expansion of the grain boundary are repressed by the increase of the aids content, and thus the graingrowth is depressed to obtain the fine grain size ceramics. Opposite to above, the density of the ceramics increases with the content of sintering aids at the beginning. Therefore, it is a benefit for the elimination of the pores and the densification of the ceramics if the sintering aids increase. The mass transfer promotes the densification in the traditional sintering mechanism, but the repressed mass transfer yields the promoted densification in this system when the sintering aids increase. It is a contradiction with the traditional sintering mechanism. There must be some new one except for traditional graingrowth, such as, suppose a certain force to close the distance between two nanoparticles to drive the sintering. In Fig. 4, the density and the sintering shrinkage of the samples increases with the content at first, reaches a maximum at 4 wt%, and then decreases slightly after that. To clarify the supposition, the XRD patterns of the ceramic samples added with 3 and 5 wt% sintering aids are shown in Fig. 6. There are two impurity phases indexed in the patterns, main impurity phase: SrBiO3 (PDF#48–0321) and other: Ba2BiO4 (PDF#46–0088). They both are distorted perovskite structures and display superconductivity [16]. SrBiO3 can be expressed as SrBi0.53+Bi0.55+O3 [17], and Ba2BiO4 is described as the probable end member (x = 0.67) of an oxygen-defect series the generic formula of Ba2(Bi1 − x3+Bax)Bi5+O6−δ as double perovskite structure [18]. The peak of SrBiO3 is much stronger than that of Ba2BiO4 as shown as the inset of Fig. 6. It is realized that Ba2BiO4 is more difficult to form than SrBiO3, although Ba content is much higher than Sr in our BST (Ba0.75Sr0.25TiO3) nanoparticles. The relative content of Ba2BiO4 vs SrBiO3 increases with the content of the sintering aids. The mismatched ratio of Ba2BiO4 vs BST is higher than that of SrBiO3 vs BST because Bi and Ba have a bigger ion radius than Ti, Ba2BiO4 has bigger perovskite cell than SrBiO3 and they both have bigger cell than BST. For all of them are perovskite structure, the Bi-related perovskites grow epitaxially on the surface of BST nanoparticles, and close them together to make the sintering densification without mass transfer from one BST nanoparticle to another.
To study the influence of the temperature on the sintering procedure, the sample added with 3 wt% sintering aids is sintered at different temperatures from 800 to 950 oC. The ceramic samples are shown in Fig. 7. They have a gray color with a sintering temperature of 800 ~ 900 oC. The color of them gets darker and darker till 850 oC, becomes shallow at 900 oC, and then changes to yellow at 950 oC. The sintering shrinkages and relative densities of the samples are shown in Fig. 8. They both increase with the sintering temperature approximately. The density can reach > 90% above 900 oC.
The SEM of the samples sintered at different temperatures is shown in Fig. 9. It is shown that the grain size of the ceramics increases with the temperature. Because the sintering can be promoted with the temperature, the mass transfer is a run-up to make graingrowth.
We should consider another important factor as some efficient sintering impurities involved at a higher temperature than 800 oC. To clarify them, the XRD patterns of the ceramic samples sintered at different temperatures were analyzed carefully. They are shown in Fig. 10. All patterns showed the perfect perovskite structure of BST. To research the impurity phases in them, the patterns were zoomed-in along the Y axle as shown in Fig. 11. As analyzed as previously, in the sample sintered at 800 oC, Bi-related perovskite impurity phases as SrBiO3 and Ba2BiO3 were indexed. In the sample sintered at 830 oC, new impurity phases of Sr1.262Bi0.737O3 (pdf#48–0508) and LiBa4Bi3O11 (pdf#46–0634) were indexed. The valence of Bi in Sr1.262Bi0.737O3 is different from that in SrBiO3. Only Bi5+ exists in Sr1.262Bi0.737O3 but both Bi3+ and Bi5+coexists in SrBiO3. Sr1.262Bi0.737O3 can be expressed as perovskite style as SrSr0.262Bi0.7375+O3. Sr2+ also can jam into the B-site of ABO3-typed perovskite as discussed previously as partial Ba2+ in Ba2BiO4. In this sample, although Ba2BiO4 is still indexed, SrBiO3 disappears to turn to Sr1.262Bi0.737O3. The grain size of the sample sintered at 830 oC increased, but the relative density of it decreased as a comparison of the sample sintered at 800 oC. It may be caused by a single valence of Bi in Sr1.262Bi0.737O3 that it does not good effect on the densification of the ceramic sintering. Furthermore, Sr1.262Bi0.737O3 was rarely reported in the previous documents. LiBa4Bi3O11 can be expressed as Ba4LiBi5+Bi3+Bi5+O12−δ and thus has a tetrad perovskite structure. It can be sintered at 870 ~ 910 oC as microwave ceramics with dielectric constant as 38 ~ 44 and Qf as 35000–54000 GHz [19]. For the sample sintered at 850 oC, LiBa4Bi3O11 was indexed as the main impurity phase and the intensity of Sr1.262Bi0.737O3 decreased rapidly. In the sample sintered at 900 oC, LiBa4Bi3O11 was also indexed as the main impurity phase and Sr1.262Bi0.737O3 disappeared. In the sample sintered at 950 oC, Bi8.11Ba0.89O13.05 (pdf#45–0289) was indexed and the intensity of LiBa4Bi3O11decreased rapidly. In the samples sintered at all of our temperatures, a trace of Ba2BiO4 always existed. Bi8.11Ba0.89O13.05 can be considered as Ba-doped Bi2O3 which has not a perovskite structure. It was reported as a secondary phase in the ceramics of the binary system of BaTiO3 and Bi2O3 [20]. Therefore, most of Bi-related perovskite impurity phases drove the densification of the ceramic sintering. The alkaline earth cations in them were resorbed by the main lattice of BST, and Bi2O3 phase were released again.
Table 1
The impurity phases indexed in the ceramics sintered at different temperature
Sintering
temperature
|
SrBiO3
(Bi3+, Bi5+)
|
Ba2BiO4
(Bi3+, Bi5+)
|
Sr1.262Bi0.737O3
(Bi5+)
|
LiBa4Bi3O11
(Bi3+, Bi5+)
|
Bi8.11Ba0.89O13.05
(Bi3+)
|
800 oC
|
X*
|
X
|
|
|
|
830 oC
|
|
X
|
X
|
X
|
|
850 oC
|
|
X
|
X
|
X
|
|
900 oC
|
|
X
|
|
X
|
|
950 oC
|
|
X
|
|
X
|
X
|
* red “X” expresses the main impurity phase |
The impurity phases indexed in the ceramics sintered at different temperatures are listed in Table 1. Red “X” clearly showed the main impurity phase indexed in the ceramics sintered at a different temperature. The valent states of Bi were also listed in each impurity phases. Overall, three phases of impurities involved here, SrBiO3, Ba2BiO4, and LiBa4Bi3O11 might be taken effect on the densification of ceramic sintering, in which Bi has complex valences. The impurities as Sr1.262Bi0.737O3 and Bi8.11Ba0.89O13.05 might be taken little effect on it, in which Bi has a single valence.
The sintering mechanism of BST nanoparticles driven by Bi-related perovskite is illustrated in Fig. 12. For the high vapor pressure of Bi2O3 near its melting point (817 oC), it is easily vaporized to fill the interspaces among BST nanoparticles shown as big white circles. The alkaline earth cations (Ba2+ or Sr2+, as shown as small blue circles) immigrated to Bi2O3 (as shown as orange circles) and partial of Bi3+ is oxidized to Bi5+ by oxygen in the air. They make epitaxial growth of a layer of Bi-related perovskite along a fixed direction shown as blue lines in the circles on the surface of BST nanoparticles. At this moment, the surfaces of BST nanoparticles near the epitaxial layer carry the negative charges and produce surface charges gathering because of the immigration out of cations. The opposite sides of it carry positive charges for the polarization of the surface charges. The condition of the epitaxial layer is just opposite to that of BST nanoparticles as shown in Fig. 12 (a). Driven with the electrostatic force, the nanoparticles of BST rotate in the same direction and close together as shown in Fig. 12 (b). Vaporized Bi2O3 from Bi-related perovskites, the most of alkaline earth cations in them are resorbed by the main lattice of BST to drive the nanoparticles close together much more as shown in Fig. 12 (c). At the end of the time, the BST nanoparticles emerged into a single particle as shown in Fig. 12 (d). The densification of the ceramic sintering is driven without mass transfer from one BST nanoparticle to another, and the dense ceramics can be obtained at a low temperature. The mechanism can be considered as the mergence of small particles driven by the intermediate impurity phase of Bi-related perovskite during the sintering.
The small particles merged into a large grain can be observed in the sample sintered insufficiently. Figure 13 showed SEM of the samples sintered at 950 oC for 2 h with 5 wt% sintering aids. It is observed that the small particles merge into a large grain in Fig. 13 as the condition illustrated in Fig. 12. The small particles can be faintly observed in a large one at left lower in Fig. 13 and also in other large grains. They can grow to normal grains under the sufficient sintering further. Therefore, the sintering mechanism illustrated in Fig. 12 was confirmed.
The I-V curves of the samples sintered at 800 oC with different contents of the sintering aids are shown in Fig. 14. For the samples behave impure resistances, the curves are not linear. The average resistivities of the samples were estimated as shown in Fig. 15. The leakage currents or conductivities as reciprocal of the resistivities of the samples are related to the conductive secondary phases, pores, and distribution of them in the ceramics. SrBiO3 with superconductivity can behave conductive secondary phase. In Ba2BiO4, a full Bi3+ -Bi5+ separation occurs as a result of the simultaneous presence of large Ba2+cation both in dodecahedral and octahedral coordination. Barium in an octahedral site breaks the Bi3+-O-Bi5+ bonding, stabilizes insulating properties, and decreases the conductivity of Ba2BiO4. These enlighten us that the content of SrBiO3 in ceramics is a crucial factor to influence on the resistivities of the samples in Fig. 15. In these samples, both the contents of SrBiO3 and Ba2BiO4 increase with the content of sintering aids, but the content of Ba2BiO4 increases much rapidly. If a sample has the maximum content of SrBiO3, and it can have the minimum resistivity, such as the sample with 4 wt% sintering aids.
The I-V curves of the samples sintered at different temperatures with 3 wt% sintering aids are shown in Fig. 16. The average resistivities of the samples were estimated as shown in Fig. 17. As we mentioned previously, the LiBa4Bi3O11 phase can be fabricated as a microwave capacitor, and thus it may have a high resistivity. Sr1.262Bi0.737O3 and Bi8.11Ba0.89O13.05 have a single valence of Bi ion in them, and they may not behave a good conductivity. The resistivity of the sample increases with the sintering temperature except the sample sintered at 850 oC.
Because of the decrease of the content of the conductive phase of SrBiO3 and the percentage of pores in the ceramics with the sintering temperature, the resistivity increases with the sintering temperature. For the sample sintered at 850 oC, it has a strange low resistivity. Checked the SEM of it as shown in Fig. 9, much more secondary phase than other samples were observed. The secondary phases behave as irregular particles differently from normal BST grains. Although we cannot confirm the secondary phase belongs to amorphous or what kind of crystalline, it leads to the low resistivity of the ceramics.
The temperature dependences of the dielectricities of the samples sintered at 900 oC and 950 oC are shown in Fig. 18. Both of the measured samples behave the typical temperature dependence of the dielectricity of the BST ceramics. They both have a dielectric peak at 53 oC. Assumption the shift efficient of dielectric peak as 2.5 oC/% at Curie point because of (Tc(BT)-Tc(ST))/100, where Tc(BT) = 125 oC and Tc(ST)=-125 oC, the dielectric peak of the Ba0.75Sr0.25TiO3 ceramics should locate at 57.5 oC. It is near the experimental value of 53 oC. The peak of the dielectric constant of the sample sintered at 900 oC ~ 6800 with a loss of 0.01, and that of the sample sintered at 950 oC~ 6300 with a loss of 0.009, respectively. All of their average resistivity > 1012Ω•cm and thus we achieved the goal of the sintering at the low temperature < 960oC (silver melting point) for a potential on silver inner-electrodes MLCC. The dielectric constant of the sample sintered at 900 oC is higher than that sintered at 950 oC for the results of the grain size effect on the dielectricity. For the BaTiO3 based ceramics, the dielectric constant reaches their maximum when their grain size ~ 0.5 µm. As we showed SEM of them in Fig. 9, the sample sintered at 900 oC is just meeting the condition. For the low resistivity of other samples, the dielectricities of others were not measured successfully.