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 (Ba_xSr_(1-x)TiO₃, BST) has extensively been exploited due to its high dielectric constant, high tunability, low tangent and suitable phase transition temperature [1] [2] [3] [4] [5] . 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 [6] [7] [8] 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 [9] .

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 [10] . According to Zhao et al. comprehensive characteristics were achieved in Y₂O₃ and Dy₂O₃-doped BST ceramics [11] . 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 [12] . 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 [13] .

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 [14] . In another work by Zhang et al. La₂O₃ incorporation in Ba_0.4Sr_0.6TiO₃ glass ceramics revealed that dopant had a small effect on the dielectric constant but extensively affected the microstructure of the ceramics [15] . Zhang and Qu conducted a similar research on Ba_0.74Sr_0.26TiO₃ where dopant La₂O₃ and Sb₂O₃ were used and results showed that dielectric constant, dielectric loss and phase transitions all were affected by dopant addition [16] . Although it was well observed that a small portion of dopant ions can surprisingly influence the dielectric behaviors of BST ceramics, La₂O₃ doped pure Ba_0.3Sr_0.7TiO₃ ceramics have never been studied before.

This current study focuses on preparing La₂O₃ doped pure Ba_0.3Sr_0.7TiO₃ 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 Ba_0.3Sr_0.7TiO₃ (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 Ba_0.7Sr_0.3TiO₃ (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.

Barium strontium titanate (BST), Ba_0.3Sr_0.7TiO₃ 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

(0.1g La₂O₃ doped Ba_0.3Sr_0.7TiO₃) 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.

The authors declare no conflicts of interest regarding the publication of this paper.

Dewanjee, S., Das, R.C. and Miah, M.Y. (2020) Preparation and Characterization of La₂O₃ Doped Ba_0.3Sr_0.7TiO₃ Perovskite Ceramics of Varied Sintering Temperature and Doping. Advances in Materials Physics and Chemistry, 10, 253-262. https://doi.org/10.4236/ampc.2020.1011019