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

doi:10.21203/rs.3.rs-40548/v1

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

https://doi.org/10.21203/rs.3.rs-40548/v1

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The powders of the Ba_0.75Sr_0.25TiO₃ (BST) nanoparticles were directly synthesized by milling of Ba(OH)₂·8H₂O, Sr(OH)₂·8H₂O 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 ^oC by them with adding 3 wt% sintering aids of Bi₂O₃ and Li₂CO₃. The sintering behavior of the BST nanoparticles by adding the aids of Bi₂O₃ and Li₂CO₃ 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 densification. These compounds such as Ba₂BiO₄ and SrBiO₄ appear at 800 ^oC, LiBa₄Bi₃O₁₁ and Sr_1.2Bi_0.8O₃ appear over 830 ^oC, and Bi_8.11Ba_0.89O_13.05 appears at 950 ^oC. 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 ^oC, 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 densification of the nanoparticles. The BST ceramics can have a peak dielectric constant >6500 at 53 ^oC, loss <0.025, and resistivity >10¹² W·cm when sintered at a temperature of ≥900 ^oC with 3 wt% sintering aids. They have a potential application for multiple layer ceramic capacitors (MLCC) with silver inner-electrodes.

Ceramics

BST

Sintering

Electrical property

Nanoparticles

Direct synthesis

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 − xSr_xTiO₃ (BST) ceramics have high performance in electrical devices. As a solid solution of two typical perovskite ferroelectrics BaTiO₃ and SrTiO₃, 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 filter, 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 ^oC 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 T⁴ 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-fired with the ceramics so we should adopt Pd or Pt as inner electrodes that can bear the sintering temperature above 1300 ^oC. If the dense ceramics can be sintered at the low temperature < 960 ^oC (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 efficiently controlled and the fine-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 Bi₂O₃ and Li₂CO₃ as sintering aids to sinter the BST ceramics at the low temperature < 960 ^oC for a potential application on silver inner-electrode multiple layer ceramic capacitors (MLCC). The sintering mechanism is also analyzed carefully.

To sinter the ceramics at low temperatures, the powders of BST nanoparticles should have fine grains to ensure their high activity. We directly synthesized them at room temperature, just similar to synthesize BaTiO₃ nanopaticles [15]. In this study, we give a typical example of BST as Ba_0.75Sr_0.25TiO₃ (also abbreviated as BST).

We adopted Barium octahydrate (Ba(OH)₂·8H₂O), strontium octahydrate (Sr(OH)₂·8H₂O), and tetrabutyl titanate (Ti(OC₄H₉)₄) as starting reagents to prepare the BST nanoparticles. We obtained the titanium solution by dissolving 34.0 g Ti(OC₄H₉)₄ into 50 ml absolute ethanol, the base slurry by ball milling 23.7 g Ba(OH)₂·8H₂O and 6.65 g Sr(OH)₂·8H₂O 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 Li₂CO₃ and Bi₂O₃ 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 ^oC.

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 field 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 ^oC 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).

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 SrBi_0.5³⁺Bi_0.5⁵⁺O₃ [17], and Ba₂BiO₄ is described as the probable end member (x = 0.67) of an oxygen-defect series the generic formula of Ba₂(Bi_1 − x³⁺Ba_x)Bi⁵⁺O_6−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 (Ba_0.75Sr_0.25TiO₃) 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 Sr_1.262Bi_0.737O₃ (pdf#48–0508) and LiBa₄Bi₃O₁₁ (pdf#46–0634) were indexed. The valence of Bi in Sr_1.262Bi_0.737O₃ is different from that in SrBiO₃. Only Bi⁵⁺ exists in Sr_1.262Bi_0.737O₃ but both Bi³⁺ and Bi⁵⁺coexists in SrBiO₃. Sr_1.262Bi_0.737O₃ can be expressed as perovskite style as SrSr_0.262Bi_0.737⁵⁺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 Sr_1.262Bi_0.737O₃. 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 Sr_1.262Bi_0.737O₃ that it does not good effect on the densification of the ceramic sintering. Furthermore, Sr_1.262Bi_0.737O₃ was rarely reported in the previous documents. LiBa₄Bi₃O₁₁ can be expressed as Ba₄LiBi⁵⁺Bi³⁺Bi⁵⁺O_12−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 Q_f as 35000–54000 GHz [19]. For the sample sintered at 850 ^oC, LiBa₄Bi₃O₁₁ was indexed as the main impurity phase and the intensity of Sr_1.262Bi_0.737O₃ decreased rapidly. In the sample sintered at 900 ^oC, LiBa₄Bi₃O₁₁ was also indexed as the main impurity phase and Sr_1.262Bi_0.737O₃ disappeared. In the sample sintered at 950 ^oC, Bi_8.11Ba_0.89O_13.05 (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. Bi_8.11Ba_0.89O_13.05 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⁺)	Sr_1.262Bi_0.737O₃ (Bi⁵⁺)	LiBa₄Bi₃O₁₁ (Bi³⁺, Bi⁵⁺)	Bi_8.11Ba_0.89O_13.05 (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 Sr_1.262Bi_0.737O₃ and Bi_8.11Ba_0.89O_13.05 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. Sr_1.262Bi_0.737O₃ and Bi_8.11Ba_0.89O_13.05 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 (T_c(BT)-T_c(ST))/100, where T_c(BT) = 125 ^oC and T_c(ST)=-125 ^oC, the dielectric peak of the Ba_0.75Sr_0.25TiO₃ 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 < 960^oC (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.

We directly synthesized BST nanoparticles with a grain size of ~ 15 nm by the milling of Ba(OH)₂•8H₂O, Sr(OH)₂•8H₂O and Ti(BuO)₄ in ethanol solution at room temperature. They have homogenous particle grains and high sintering activity. The BST nanoparticles can be sintered as dense ceramics with the density > 90% at a temperature as low ~ 900 ^oC with the sintering aids of Bi₂O₃ and Li₂CO₃ < 5 wt%. Several Bi-related secondary phases in the ceramics were involved during sintering. Sintering at 800 ^oC, two Bi-related perovskite phases, SrBiO₃ and Ba₂BiO₄ can be observed. It is realized that Ba₂BiO₄ is more difficult to form than SrBiO₃, although Ba content is much higher than Sr in BST nanoparticles. The relative content of Ba₂BiO₄ vs SrBiO₃ increases with the content of the sintering aids. They are both beneficial to the sintering densification. Sintering above 800 ^oC, SrBiO₃ disappears. LiBa₄Bi₃O₁₁ appears from 830 ^oC, Sr_1.2Bi_0.8O₃ appears from 830 ^oC and disappears above 850 ^oC, and Bi_8.11Ba_0.89O_13.05 only appears at 950 ^oC. LiBa₄Bi₃O₁₁ and Sr_1.2Bi_0.8O₃ can be regarded as perovskite phases as that of Ba₂BiO₄. Sintered at 950 ^oC, the most of alkaline earth cations in Bi-related perovskite impurity phases which driven the sintering densification of the ceramics were resorbed by the main lattice of BST, at the same time Bi₂O₃ phase was released again in the end. SrBiO₃, Ba₂BiO₄, and LiBa₄Bi₃O₁₁ might be taken effect on the sintering densification of the ceramics, in which Bi has complex valences. On the other hand, Sr_1.262Bi_0.737O₃ and Bi_8.11Ba_0.89O_13.05 might be taken little effect on the sintering densification of the ceramics, in which Bi has a single valence. Sintering at low temperature, the samples have low resistivities due to semiconducting secondary phases, pores, and their distribution on the grain boundary. Sintering at a temperature of ≥ 900 ^oC, the ceramics were fabricated with the density > 90%, resistivity > 10¹² Ω·cm, dielectric constant > 6500 at 53 ^oC, and loss < 0.025. The sintering temperature lower to ≤ 950 ^oC, 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.

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

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