Sintering Behavior of BST Nanoparticles at Low Temperature and Electrical Properties of their Ceramics

The powders of the Ba 0.75 Sr 0.25 TiO 3 (BST) nanoparticles were directly synthesized by milling of Ba(OH) 2 ·8H 2 O, Sr(OH) 2 ·8H 2 O and Ti(BuO) 4 in ethanol at room temperature. They have homogenous grains of ~15 nm and the high sintering activity. The dense ceramics with the density >90% can be obtained at a sintering temperature of ≤ 950 o C by them with adding 3 wt% sintering aids of Bi 2 O 3 and Li 2 CO 3 . The sintering behavior of the BST nanoparticles by adding the aids of Bi 2 O 3 and Li 2 CO 3 is studied carefully. Several Bi-related compounds are involved in the sintering procedure at a different temperature. They enhance the mass transfer and promote the sintering densication. These compounds such as Ba 2 BiO 4 and SrBiO 4 appear at 800 o C, LiBa 4 Bi 3 O 11 and Sr 1.2 Bi 0.8 O 3 appear over 830 o C, and Bi 8.11 Ba 0.89 O 13.05 appears at 950 o C. The cation Bi in the ceramics has mixture valences of 3+ and 5+. It makes the ceramics as semiconducting state with the dark gray color and decreases the ceramics resistivities. With the sintering temperature increase, especially at 950 o C, the cation Bi tends back to single valence of +3 in the ceramics. The most of alkaline earth cations in Bi-related compounds will release and resorb into the lattice of BST and drive the densication of the nanoparticles. The BST ceramics can have a peak dielectric constant >6500 at 53 o C, loss <0.025, and resistivity >10 12 W·cm when sintered at a temperature of ≥ 900 o C with 3 wt% sintering aids. They have a potential application for multiple layer ceramic capacitors (MLCC) with silver inner-electrodes.


Introduction
With the development of electrical and electronic industry, higher performance of electrical devices with smaller size and lower cost are demanded. At the same time, more and more attention is paid to environmental protection during fabrication of these devices, such as no pollutant discharge, low energy consumption to control carbon emission, etc. Ba 1 − x Sr x TiO 3 (BST) ceramics have high performance in electrical devices. As a solid solution of two typical perovskite ferroelectrics BaTiO 3 and SrTiO 3 , BST can modulate its Curie temperature continuously by the compositions, and thus control its properties desirably [1]. With high dielectric permittivity, low loss, outstanding ferroelectricity, it has attracted attention for decades on the application of a high-voltage capacitor [2], tunable lter, detector [3][4], piezoelectrics [5], sensor [6], pressure transducer and actuator [7], optoelectronic device [8], etc. Especially, the high capacity capacitors based on BST ceramics are indispensable devices in the electronic and electrical circuit for energy storage with high power density [9]. Also the positive temperature resistance (PTCR) [10] based on semiconducting BST ceramics have applied widely for temperature sensors and current control devices. However, the high temperature over 1300 o C is often necessary to sinter the dense BST ceramics, and thus leads to abnormal graingrowth and properties deterioration [11]. To keep a high temperature for sintering, the energy consumed by the kiln is high because it is proportional to T 4 as Stefan-Boltzmann law which determines the blackbody radiation [12]. To obtain high performance of the ceramics and save much energy, it is necessary to lower the sintering temperature of the BST ceramics.
For the multiple layer ceramic devices (MLCD), the inner metal electrodes must be co-red with the ceramics so we should adopt Pd or Pt as inner electrodes that can bear the sintering temperature above 1300 o C. If the dense ceramics can be sintered at the low temperature < 960 o C (silver melting point), the relatively cheap electrodes Ag can be suitable and the total price of the electrical devices can be decreased distinctly.
To control the microstructures and the properties, the ceramics should be sintered at the low temperature.
At this time, the graingrowth during sintering can be e ciently controlled and the ne-grain sized ceramics would be obtained. We can decrease the depth of the single layer of MLCD, increase the number of the layers, and thus improve the properties especially as breakdown voltages of the ceramics.
There are several documents on the low temperature sintering of BST ceramics. Unfortunately, they documented the deteriorated properties of the ceramics or did not report them [13,14]. In this study, we directly synthesized high active BST nanoparticles at room temperature. Adopt Bi 2 O 3 and Li 2 CO 3 as sintering aids to sinter the BST ceramics at the low temperature < 960 o C for a potential application on silver inner-electrode multiple layer ceramic capacitors (MLCC). The sintering mechanism is also analyzed carefully.

Experimental Procedure
To sinter the ceramics at low temperatures, the powders of BST nanoparticles should have ne grains to ensure their high activity. We directly synthesized them at room temperature, just similar to synthesize BaTiO 3 nanopaticles [15]. In this study, we give a typical example of BST as Ba 0.75 Sr 0.25 TiO 3 (also abbreviated as BST). We adopted Barium octahydrate (Ba(OH) 2 ·8H 2 O), strontium octahydrate (Sr(OH) 2 ·8H 2 O), and tetrabutyl titanate (Ti(OC 4 H 9 ) 4 ) as starting reagents to prepare the BST nanoparticles. We obtained the titanium solution by dissolving 34.0 g Ti(OC 4 H 9 ) 4 into 50 ml absolute ethanol, the base slurry by ball milling 23.7 g Ba(OH) 2 ·8H 2 O and 6.65 g Sr(OH) 2 ·8H 2 O in 100 ml ethanol for 4 h. We added the titanium solution into the base slurry in the jar and then resealed for another 18 h milling at the rate of 200 rpm. After that, we obtained the white slurry and then the BST nanoparticles by followed air-dry. To study the role of the sintering aids, we added the mixture of Li 2 CO 3 and Bi 2 O 3 to the as-prepared BST powders with different contents from 2 to 5 wt%, dry pressed them into disks, and then sintered at different temperatures from 800 to 950 o C.
All the samples were analyzed through X-ray diffraction (XRD, X'Pert-Pro MPD, Panalytical Ltd., Holland) using CuKα radiation (40 kV, 30 mA) and eld emission scanning electron microscopy (FE-SEM, supra55, Zeiss, Germany). After both surfaces of the ceramic samples were coated with silver paste and sintered at 800 o C for 10 min, the dielectricities of them were measured by Precision LCR Meter (Keysight Technologies, America, E4980A), and the I-V characteristics by High Resistance Meter (Keithley, America, 2400).

Results And Discussion
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 T 4 as Stefan-Boltzmann law which determines the blackbody radiation. To decrease the sintering temperature and save energy, we need ne 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 con rmed 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 BaCO 3 . The slight amount of BaCO 3 in the as-prepared samples possibly derives from the raw material barium hydroxides, which can react with CO 2 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 o C 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 o C 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 o C 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 ne 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 bene t for the elimination of the pores and the densi cation of the ceramics if the sintering aids increase. The mass transfer promotes the densi cation in the traditional sintering mechanism, but the repressed mass transfer yields the promoted densi cation 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 rst, 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: SrBiO 3 (PDF#48-0321) and other: Ba 2 BiO 4 (PDF#46-0088). They both are distorted perovskite structures and display superconductivity [16]. SrBiO 3 can be expressed as SrBi 0.5 3+ Bi 0.5 5+ O 3 [17], and Ba 2 BiO 4 is described as the probable end member (x = 0.67) of an oxygen-defect series the generic formula of Ba 2 (Bi 1 − x 3+ Ba x )Bi 5+ O 6−δ as double perovskite structure [18]. The peak of SrBiO 3 is much stronger than that of Ba 2 BiO 4 as shown as the inset of Fig. 6. It is realized that Ba 2 BiO 4 is more di cult to form than SrBiO 3 , although Ba content is much higher than Sr in our BST (Ba 0.75 Sr 0.25 TiO 3 ) nanoparticles. The relative content of Ba 2 BiO 4 vs SrBiO 3 increases with the content of the sintering aids.
The mismatched ratio of Ba 2 BiO 4 vs BST is higher than that of SrBiO 3 vs BST because Bi and Ba have a bigger ion radius than Ti, Ba 2 BiO 4 has bigger perovskite cell than SrBiO 3 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 densi cation without mass transfer from one BST nanoparticle to another.
To study the in uence of the temperature on the sintering procedure, the sample added with 3 wt% sintering aids is sintered at different temperatures from 800 to 950 o C. The ceramic samples are shown in 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 e cient sintering impurities involved at a higher temperature than 800 o C. 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 o C, Bi-related perovskite impurity phases as SrBiO 3 [20]. Therefore, most of Bi-related perovskite impurity phases drove the densi cation of the ceramic sintering. The alkaline earth cations in them were resorbed by the main lattice of BST, and Bi 2 O 3 phase were released again. The sintering mechanism of BST nanoparticles driven by Bi-related perovskite is illustrated in Fig. 12 Sr 2+ , as shown as small blue circles) immigrated to Bi 2 O 3 (as shown as orange circles) and partial of Bi 3+ is oxidized to Bi 5+ by oxygen in the air. They make epitaxial growth of a layer of Bi-related perovskite along a xed 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 Bi 2 O 3 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 densi cation 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 insu ciently. Figure 13 showed SEM of the samples sintered at 950 o C 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 su cient sintering further. Therefore, the sintering mechanism illustrated in Fig. 12 was con rmed.
The I-V curves of the samples sintered at 800 o C with different contents of the sintering aids are shown in

Conclusions
We directly synthesized BST nanoparticles with a grain size of ~ 15 nm by the milling of Ba(OH) 2  Sintering at a temperature of ≥ 900 o C, the ceramics were fabricated with the density > 90%, resistivity > 10 12 Ω·cm, dielectric constant > 6500 at 53 o C, and loss < 0.025. The sintering temperature lower to ≤ 950 o C, the performance of the ceramics will be increased, the energy consumption during ceramic sintering can be decreased and the cost of the devices also will be lowered. The ceramics have a potential for MLCC with silver inner-electrodes. Relative density and sintering shrinkages of the samples sintered at 800 oC for 8 h with different contents of sintering aids