Synthesis and characterization techniques
2.1 Synthesis of bulk BNT ceramics
Several methods have been developed for the synthesis of polycrystalline samples with good purity, homogeneity and finer particle size, such as conventional solid-state reaction (CSSR) method, sol-gel method, co-precipitation method, and wet (semi-wet) chemical method, etc. In this thesis, we have employed a conventional solid-state reaction method for the synthesis of BNT based ceramics.
2.1.1 Conventional solid-state reaction method
The CSSR method is the most versatile technique and widely used for the synthesis of polycrystalline samples. The main advantage of the CSSR provides a wide range of selection starting materials (like oxides, carbonates, etc.), thermodynamically
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stable, environment-friendly, cost-effectiveness, the reaction takes place without any solvents, the structurally pure final product and simplicity. In this method, two main steps are involved that are the uniform mixing of starting materials for better homogenization and heat treatment process at high temperatures for phase formation. The starting materials do not react with each other at room temperature, sufficient temperature with proper heating and cooling rates are required for the phase formation and densification. Various steps were involved in this method which is explained in the following sections. The flowchart of the various steps of the synthesis procedure by CSSR is shown in Figure 2.1.
Figure 2.1: Flowchart of the various steps of the synthesis procedure in a conventional solid-state reaction.
2.1.2 Stoichiometric weighing of starting materials
The high purity of starting materials (> 99.99%) is very important to get the desired phase. The starting materials of Bi0.5Na0.5TiO3 ceramics are weighed (AG135,
Mettler Toldo), accuracy ~0.01 mg) as per the stoichiometric ratio using the following equation.
π΅π2π3+ ππ2πΆπ3+ 4 πππ2 β 4 (π΅π0.5ππ0.5πππ3) + πΆπ2 (2.1) 2.1.3 Uniform mixing of starting materials
The weighed powders were uniformly mixed using a planetary ball mill (Pelverisette 6, Fritsch GmbH, Germany) in a zirconium jar with zirconia balls of 5 mm diameter, where distilled water used as milling media. The powders were milled at a speed of 120 rotations per minute for 5h. Figure 2.2 shows photograph of the planetary ball mill was used in the present study.
Figure 2.2: Photograph of the planetary ball mill.
2.1.4 Calcination
To obtain the desired phase formation, the samples must be heated to an appropriate temperature, called the calcination temperature. Calcination is a thermal treatment of mixed starting materials at lower temperatures prior to the sintering. The calcination process produces a new solid phase and eliminates unwanted reagents like carbonates, and nitrides, sulfates, etc. In this process, the calcination temperature, duration
and atmospheres are important parameters that play a key role in the phase formation of the ceramics. Generally, the phase formation temperature of the sample is confirmed from differential scanning calorimetry and thermogravimetric analysis. The phase purity of the calcined powder sample can be identified from the X-ray diffraction pattern.
2.1.5 Particle size reduction
The surface to a volume ratio is higher for smaller size particles as compared to the larger particles, where the smaller size particles possess high chemical reactivity. As the initial particle sizes decrease, the sintering temperature reduces and improves the density of the samples. Therefore, it is important to decrease the particle size after the calcination process. In this study, we have employed the planetary ball mill twice; the first time is for the uniform mixing of the initial reagents and the second time is for the particle size reduction. All the processing parameters (milling time, rotations per minute, ratio of balls with respect to powders) are optimized to obtain smaller particle sizes without any secondary phase formation.
2.1.6 Pelletization
After the process of ball milling, the slurry was dried at 120 oC for 24 h to obtain the fine powders. The organic binder polyvinyl alcohol was added to the fine powders and pressed into cylindrical shaped discs of 10 mm in diameter and 1 mm in thickness using KBr hydraulic press (M-20, Technosearch Instruments, India). The applied pressure on the upper part of the die is given by the following equation,
D KL a
x P
P
ο 4
exp
β
= (2.2)
where Px is the pressure gradient; Pa is the applied pressure, ΞΌ is the friction coefficient, K is a constant, L and D are the length and diameters of the die, respectively [1]. In this study, the pressure was applied in uniaxial pressing to prepare the green cylindrical discs.
2.1.7 Sintering
Sintering is the process of compacting and forming a solid mass of material by heat and/or pressure without melting the material. The main purpose of sintering is densifying the ceramics and reducing porosity. Density and porosity are very important parameters, which can directly impact the dielectric, ferroelectric and piezoelectric properties of the samples. Hence, it is important to optimize the sintering temperature to achieve maximum density and minimum porosity in the material. In this thesis, the calcination and sintering temperature of prepared samples are optimized by using conventional furnace (LHT 04/18, Nabertherm GmbH, Germany; Temperature range:
25oC-1800oC). Figure 2.3 shows a photograph of the conventional furnace that was used in the present study.
Figure 2.3: Photograph of the conventional furnace.
Basically, the sintering process can be categorized into two types: (a) solid-state sintering and (b) liquid phase sintering. In solid-state sintering, the densification occurs by changing in the particle shape without any rearrangement, while some dopants are added to the material to form a liquid phase during the sintering process which enhances densification in the liquid phase sintering. In this study, we have employed the solid-state sintering process for all the prepared samples. The sintering process of the samples can be completed in three stages: (a) initial, (b) intermediate and (c) final stage as shown in Figure 2.4 [2]. In the first stage, surface smoothening of grains (or particles), neck growth and rounding of interconnected pores take place. The relative density of the green discs reaches around 60-65% at the end of the first stage. The grain growth and rapid densification occur in an intermediate stage. A number of transport and diffusion mechanisms such as surface diffusion, lattice diffusion, grain boundary diffusion are involved in this stage, which leads to the densification by neck growth between the particles. The relative density increases from 65% to 90% of theoretical density at the end of the intermediate stage.
Figure 2.4: (a) Initial (b) intermediate and (c) final stages of the sintering process.
The final stage of sintering is much slower than the initial and intermediate stages.
In this stage, isolated pores in the grain boundaries and within the grains are removed through the process of grain boundary diffusion and lattice diffusion, respectively. The maximum removal of porosity is possible when all of the void spaces are connected to fast
and short diffusion paths along the grain boundaries. The final stage of sintering begins around 90 - 95 % of theoretical density.