Figure 2; Variation of transmittance of NBT–BF solid solutions as a function of temperature at 250 kHz. The basic mechanism for piezoelectricity is asymmetry in the unit cell of materials. It is necessary to develop lead-free piezoelectric materials with properties similar to those of the PZT 1 system.
These phase transitions are important for the electrical properties of the material because only non-centrosymmetric space groups can exhibit ferroelectricity. Phase transitions in ferroelectric materials are one of the most important parameters that must be controlled to tailor the properties to meet specific applications. Phase transitions are one of the main determinants of the material's dielectric properties.
The Curie temperature is the temperature at which the relative permittivity of the material is greatest. Bismuth sodium titanate is a relaxing ferroelectric in the rhombohedral phase, and it exhibits diffuse phase transitions between each of the phases. The width of the phase transition depends mostly on the amount of disorder in the structure.
This broad peak can be thought of as a global average of the peaks generated by the disordered individual.
Effect of dopants
- Barium doping in BNT
- Strontium doping in BNT
- Doping that causes peak suppression in BNT
- Lead doping in BNT
- Other dopants in BNT
The disordered material basically functions as a composite matrix, with each section having an effect on the overall properties of the matrix.29. The broad peak is a result of the MPB behavior along with increased disorder created by the Ba 2+ ions replacing the Bi 3+ and Na + ions at the A site. Strontium doping in BNT has a well-defined effect on the material's dielectric properties.
Sr2+ ions can replace Bi3+ and Na+ ions in the A site of the perovskite because the average valence between Bi and Na ions is 2+. The polar units in the matrix can assume any of the six directions, unless the material is pushed in a specific direction. This net polarization is the cause of the weak polar structure found in the 26-50 mol.
Increasing the Sr content also decreases lattice distortion of the material; this behavior is predicted by the bond valence model discussed later in the text. The peak temperature drop has been shown to decrease in a very predictable linear fashion.22 Figure 6 shows the linear nature of the drop in peak temperature of BNT as the amount of strontium increases. Tin is used as a substitute for zirconium or titanium at the B site in PZT to stabilize the antiferroelectric phase of the material.
Extra bismuthions were added because it was concluded that the ferroelectricity of BNT was mainly caused by the Bi3+ ions at the A site of the structure. Lead Pb2+ doping at the A site in BNT is useful because it increases the relative permittivity and enhances the response of the interphase transition that occurs near 200°C. BNT modified by lead has also been modified at the A site with dopants such as K+, La3+ at the A site and at the B site with Zr. The addition of K+ ions to BNT with lead causes a lengthening of the intermediate phase, and an increase in the relative permittivity.
Lanthanum additions shift all phase transitions to lower temperatures and reduce the relative permittivity. Compounds such as Bi0.5 K0.5 TiO3 (BKT) and KNbO3 (KN) are of interest because of the MPB behavior they exhibit. The solid solution of BNT-BKT has been found to have an MPB between the rhombohedral and tetragonal phases in the range of 16–20 mol% BKT.39 This is significant due to the enhanced electrical properties that MPB compositions exhibit.
3.Statement of Problem
Experimental Procedure
- The raw materials used for synthesis of BNT
- Preparation of iron nitrate soution
- Preparation of Ti-Oxynitrate solution
- Powder preparation by combustion synthesis
- Calcination
- Phases in calcined powder
- Compaction into pellets
- Sintering of pellets
5 grams of iron nitrate was dissolved in 50 ml of distilled water. The volume was then made up in a 250 ml volumetric flask and the flask was shaken. The solution was filtered through Whatman-42 filter paper to remove any impurities and the clear solution was collected. Of this clear solution, 0.3 ml was taken separately into two 50 ml beakers. To this, 2-3 drops of AAC buffer were added to obtain a precipitate. This is then filtered through Whatman-40 filter paper along with hot water washing. Then the collected precipitate was placed in a pre-weighed platinum crucible and stored in an oven. baked at 1000oC for 1 hour. After firing, the platinum crucible was weighed again and the differences from the previous weights now represent the amount of iron oxide present in 50 ml of iron nitrate solution. H2SO4 and 70 ml (NH4)2SO4 and heated until a clear and transparent solution was obtained. The solution was then cooled and its volume doubled. Now NH4OH was added to obtain precipitation.
The pH was maintained at 10. The solution was allowed to settle and then decantation was carried out until all the ammonia was removed. Then it was filtered and to 10 ml of the solution was added NH 4 OH to get a precipitate. Then filtration was done and the residue was taken in a pre-weighed platinum crucible and fired at 900oC for 1 hour. The crucible was then weighed and the difference between previous weight and new weight gives. amount of titanium dioxide present in 10 ml of the solution.
PW1830 diffractometer, Philips, Netherlands). This is done to know the different phases present in the calcined powder. The calcined powder was mixed with 3% PVA solution (for binding). It was mixed in an agate mortar and allowed to dry. After drying, it was scraped and ground into a fine powder. The different composition powders were individually packaged after being weighed. powder was then pressed into pellets by uniaxial compaction with a load of 3 tons. The sintered pellets were taken for SEM analysis. The pellets were sputtered in a sputtering unit. They were then loaded for analysis. This analysis helps us know the complete microstructure of the sintered sample.
Results and Discussions
- XRD analysis
- XRD plot of 0.92 BNT- 0.08 BF prepared at different pH
- XRD plot of 0.94 BNT- 0.06 BF prepared at different pH
- XRD of 0.96BNT -0.04BF prepared at different pH
- XRD of all samples at pH 7
- Density of sintered pellets
- SEM of calcined powder and sintered pellets of 0.92BNT-0.08BF
- SEM of calcined powder and sintered pellets of 0.94BNT-0.06BF
Density was measured according to Archimedes' principle. From the relative density of 0.94BNT-0.08BF, it was found to reach 90-95% at 1150oC. While in other samples the relative density varies from 85-90%. From the SEM observation, it was found that the powder is mostly agglomerated and flaky. Figure 12 shows the 0.92BNT-0.08BF sample prepared at pH 7. Figure 13 shows the SEM micrograph of the sintered pellets at 1150oC The SEM micrograph shows that a significant amount of voids appeared in the sintered sample.
Figure 14 is the SEM of the powder sample produced at 900 oC. Powders are agglomerated but spherical in shape.
Conclusion
Yao, “Some effects of different additives on dielectric and piezoelectric properties of (Bi1/2Na1/2)TiO3-BaTiO3 morphotropic-phase-boundary composite,” Mater. Kuharuangrong, "Effect of La and K on the microstructure and dielectric properties of Bi0.5Na0.5TiO3-PbTiO3," J. Takenaka, "Large piezoelectric constant and high curie temperature of lead-free piezoelectric ceramic ternary system based on titanate -bismuth-sodium Bismuth Potassium Titanate- Barium Titanate Near the Morphotropic Phase Boundary,” Jpn.
Takenaka, "Morphotropic phase boundary and electrical properties of bisumuth sodium titanate - potassium niobate solid solution ceramics," Jpn. Yao, “Effect of Sn doping on the phase transition behavior of antiferroelectric lead zirconate titanate,” Mater.
