Undoubtedly throughout the last few decades, enormous dielectric properties of several ceramic materials have been successfully utilized in designing and fabricating devices intended to consumer as well as military electronics, telecommunication, energy transmission and computers based on their applicability. Ceramic material industry plays a pivotal role as the base for many other industries. And recently high dielectric constant containing ceramics have been preferably investigated to develop efficient capacitors. One of the most crucial perovskite ferroelectrics barium strontium titanate (BaxSr(1-x)TiO3, BST) has extensively been exploited due to its high dielectric constant, high tunability, low tangent and suitable phase transition temperature      . It was observed that improved dielectric and structural behavior can be achieved if some additives are incorporated in a specific way for better electrical ceramics like BST    and dielectric properties of BST can be improved either by controlling Ba/Sr proportion or by partially substitute isovalent or aliovalent cations for A-sites and/or B-sites of ABO3 perovskite system  .
The impact of rare earth metal oxide as dopant on dielectric behaviors and structure in BST has widely been investigated in recent years. Huang et al. summarized that Dy2O3-doped BST capacitor ceramics revealed high permittivity, low dielectric loss and high DC breakdown voltage  . According to Zhao et al. comprehensive characteristics were achieved in Y2O3 and Dy2O3-doped BST ceramics  . Besides, Li et al. reported that there was a change in substitution pattern of Y3+ ions for the host cations in perovskite system. When the doping concentration rose up to 0.5% (mole fraction), Y3+ ions showed preference to grasp the B-sites, resulting to the fluctuation of dielectric constant  . Also, Eswaramoorthi et al. concluded that with rise in Ga doping in BST thin films dielectric constant and dielectric loss of the products plummeted whereas surface morphologies improved  .
The influences of La2O3 doping on structure and properties of BST ceramics were also studied, Zhu et al. found that, crystalline phase of the BST perovskite glass-ceramics ascended with the rising of dopant (La2O3) concentration and microstructure showed uneven distribution of the crystals as dopant reached to a specific concentration. Conductivity and impedance both showed considerable change with change in dopant concentration  . In another work by Zhang et al. La2O3 incorporation in Ba0.4Sr0.6TiO3 glass ceramics revealed that dopant had a small effect on the dielectric constant but extensively affected the microstructure of the ceramics  . Zhang and Qu conducted a similar research on Ba0.74Sr0.26TiO3 where dopant La2O3 and Sb2O3 were used and results showed that dielectric constant, dielectric loss and phase transitions all were affected by dopant addition  . Although it was well observed that a small portion of dopant ions can surprisingly influence the dielectric behaviors of BST ceramics, La2O3 doped pure Ba0.3Sr0.7TiO3 ceramics have never been studied before.
This current study focuses on preparing La2O3 doped pure Ba0.3Sr0.7TiO3 lattice by conventional solid state reaction mechanism and characterizing its dielectric and several electrical properties against frequency and varied sintering temperature.
All the samples were prepared following the convenient solid-state reaction method. BaTiO3 (99.95%) and SrTiO3 (99.95%) were sampled in stoichiometric proportions to prepare a pure Ba0.3Sr0.7TiO3 (BST) matrix and doped with La2O3 (99.99%) with 0.05 g, 0.1 g and 0.15 g. All powder samples were wet-milled for 24 hours in pure alcohol media in a motor driven pot mill. After mixing, the mixture was removed with the help of a strainer and subsequently dried in an oven at 100˚C for 24 hours and later calcined at 800˚C for 8 hrs with a heating rate of 10˚C/min using air atmosphere. Polyvinyl alcohol (5% PVA solution) was mixed as binder with the calcined powder to provide some green strength for subsequent handling. Then, it was dried in an oven for 24 hours to evaporate excess moisture. The pellets were prepared using press machine at 25˚C with 0.8 g of powder for every sample to maintain uniformity in thickness and then compressed at a rate of 0.2 mm/min until a desired pelletizing pressure of 10 KN was reached. The pellets made placed in a micropyretic heater (high temperature furnace) for sintering. Sintering schedule was varied from batch to batch. Here, sintering was carried out at 1460˚C, 1480˚C and 1500˚C in an air atmosphere. However, for every sintering schedule all samples with similar composition were put in the furnace at the same time. X-ray diffraction experiments were performed in order to identify the main crystalline phase of Ba0.7Sr0.3TiO3 (BST) powders recording the diffractogram from 10˚ to 90˚ using Bruker’s D8 Advance X-ray diffractometer with CuKa radiation (λ = 1.5406 Å). The microstructure of the powders was studied by a JEOL FE-SEM-7600F Scanning Electron Microscopy (SEM). Dielectric properties of perovskite were carried out by using Keithley Electrometer and Hewlett Packart Impedance Analyzer (WAYNE KERR 6500B). For dielectric measurements, the pellet shaped samples were prepared. Later on the samples were well polished to remove any roughness and the surfaces of each pellet were coated with silver paste as contact material. Dielectric measurements have been done as a function of frequency in the range of 20 Hz to 1 KHz at room temperature.
3. Results and Discussion
3.1. Grain Size
Doping and sintering are two useful processes for tuning ceramic properties   . SEM images (Figure 1), with magnification × 30,000, of highest doping in
Figure 1. SEM images of 0.15 g La2O3 doped Ba0.3Sr07TiO3 sintered at 1500˚C.
Ba0.3Sr0.7TiO3 matrix at the highest sintering temperature (1500˚C) show that contraction in small grains takes place that causes movement of the grain boundary so the total boundary area decreases as the grain boundary contraction supports the grain growth  .
Doping of La3+ to BST in this case act as introduction of impurities, this impurities lower the melting point of the resulting ceramic, so, grain boundary shrinkage ensued the grain growth  . Several research results suggested that the mobility of grain boundary is reduced by the incorporation of soluble impurities   .
X-ray diffraction (XRD) was carried out for (Ba, Sr)TiO3 doped with La2O3. Results showed that perovskite, (Ba, Sr)TiO3 doped with 0.1 g La2O3 had given no extra peak at 1460˚C (Figure 2), indicating no phase shifting has been observed as compared to standard BST. But, one new peak was found (indicated by arrow in (Figure 3) for the highest doping (0.15 g La2O3) supported by other studies where extra peaks were found for pure La2O3 when doping exceeded the maximum limit  .
3.3. Dielectric Constant
Dielectrc constant for La2O3 doped (0.05 gm, 0.1 gm and 0.15 gm) in 20 gm
Figure 2. XRD patterns of La2O3 doped Ba0.3Sr0.7TiO3 calcined at 1460˚C.
Figure 3. XRD patterns of La2O3 doped Ba0.3Sr0.7TiO3 calcined at 1500˚C.
Ba0.3Sr0.7TiO3 (BST) sample sintered at 1460˚C, 1480˚C and 1500˚C has been estimated and the result is shown in Figures 4-6.
Dielectric constant of the samples is higher at lower frequencies and decreases with increase of frequency by approaching approximately a more or less constant value. Dielectric constant is determined by the polarization of dielectric which depends on ionic, electronic and dipole polarization. Electronic polarization occurs at very short interval of time. But, when high frequency is applied electrons do not get enough time for polarization  . Again, dielectric constant depends on thermal motion and thermal expansion of dielectric. High temperature helps to orient the dielectrics for polarization up to an optimum temperature. On the other hand, successive increase of temperature decreases the number of dielectric in the effective length. Therefore it can be summarized as that, dielectric constant value decreases with increase of both of sintering temperature and frequency. As a consequence in the current research, dielectric constant is found maximum for the sample (0.1 gm La2O3 doped Ba0.3Sr0.7TiO3) at 100 Hz sintered at 1460˚C (Figure 7).
Figure 4. The variation of dielectric constant with frequency of Ba0.3Sr0.7TiO3 doped with varying amount of La2O3 sintered at 1460˚C.
Figure 5. The variation of dielectric constant with frequency of Ba0.3Sr0.7TiO3 doped with varying amount of La2O3 sintered at 1480˚C.
Figure 6. The variation of dielectric constant with frequency of Ba0.3Sr0.7TiO3 doped with varying amount of La2O3 sintered at 1500˚C.
Figure 7. Variation of dielectric constant with Sintering temperature of Ba0.3Sr0.7TiO3 with varying amount of La2O3 at frequency 100 Hz.
3.4. AC Conductivity
The electrical conductivity in dielectric materials is primarily due to hopping of electrons. Also, it is between ions within a same element present above monovalent state  . The charges tend to relocate under the impact of the field resulting in conductivity  . In our research, the ac conductivity may be ascribed to formation of Ti3+ ions which are generated due to partial reduction of Ti4+ ions during the sintering process. Conductivity is found maximum at lowest sintering temperature (1460˚C) and optimum doping (0.1 g) and decreases with rising sintering temperature Figure 8. This is because of densification of Perovskite increases with sintering temperature and that consequently lower the conductance  .
3.5. Impedance Analysis
The sophisticated impedance mechanism grants us to differentiate between the attribution of intra-grain (bulk), inter-grain (grain boundary) and the electrode effect to conduction  . Impedance plots show expected result that supports the conduction property of the composites. From Figure 9, it is visible that the
Figure 8. Temperature dependent conductance of Ba0.3Sr0.7TiO3 with varying amount of La2O3 at frequency 100 Hz.
Figure 9. Temperature dependent impedance of Ba0.3Sr0.7TiO3 with varying amount of La2O3 at frequency 100 Hz.
lowest impedance is found for optimum doping (0.1 g) which showed the highest conductance.
3.6. Capacitance Analysis
From Figure 10, it can be seen that the highest amount of capacitance is obtained for 0.1 gm La2O3 doped Ba0.3Sr0.7TiO3 sintered at 1460˚C. Here, capacitance went down with rising sintering temperature for all doping. That is in similar trend as AC conductivity and follows the same principle as mentioned  .
Barium strontium titanate (BST), Ba0.3Sr0.7TiO3 perovskite has been successfully synthesized by solid state reaction method. La3+ doped to BST and subsequently sintered at different temperatures to alter or add properties to BST. Doping and sintering helped to grain growth of perovskite. Dielectric constant gets lower with rising of frequency, as electrons do not get enough time to polarize at high frequency. In this study dielectric constant is found maximum for the sample
Figure 10. Temperature dependent capacitance of Ba0.3Sr0.7TiO3 with varying amount of La2O3 at frequency 100 Hz.
(0.1g La2O3 doped Ba0.3Sr0.7TiO3) at 100 Hz sintered at 1460˚C. The electrical conductivity, primarily for hopping of electrons between ions of the same element present above monovalent state in this studied dielectric, is found maximum for optimum doping of 0.1 g La2O3 and vice-versa for impedance analysis.
We are grateful to Bangladesh Council of Scientific and Industrial Research (BCSIR), Dhaka, Bangladesh for their support for successful completion of the various experiments. We are also thankful to Dr. Md Fakhrul Islam, Professor Department of GCE, BUET, Dhaka for his support.
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