Texture and Microstructural Influence on Piezoelectric Properties of Na0.5Bi0.5TiO3 Thin Films: A Lead Free Piezoelectric Material

Miriyala, Kumaraswamy and Ramadurai, Ranjit (2019) Texture and Microstructural Influence on Piezoelectric Properties of Na0.5Bi0.5TiO3 Thin Films: A Lead Free Piezoelectric Material. PhD thesis, Indian institute of technology Hyderabad.

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Among the lead free piezoelectric compounds Sodium Bismuth Titanate (Na0.5Bi0.5TiO3: NBT) show promising features that could replace the existing lead based piezoelectric materials in various applications. In this work, we report the effect of substrate temperature (400-650 °C), oxygen partial pressures (50-200mTorr) on the crystallographic orientations and microstructural evolution of NBT thin films grown on (111) Pt/TiO2/SiO2/Si (100) substrate by pulsed laser deposition (PLD) technique. The films grown at low partial pressures exhibits a preferred orientation along the <220> direction and at high pressures they exhibit polycrystalline nature. The variation of the growth temperature and oxygen partial pressures are allowed the microstructural features tuning from coarse faceted grains to fine spherical grains. The ferroelectric domain studies reveal that in case of fine spherical grain structure, microstructural features dominate the domain distribution and in case of coarse faceted grain structure, domain features are independent of its morphology. A control over the crystallographic orientations facilitate the tunability over the polarization vector components, which allows us to attain the dominating planar piezoresponse in case of highly oriented (<220>) samples. Further, in the case of polycrystalline films there are certain regions at which the ferroelectric domains are extending beyond the grain boundaries. Such an extension could plausibly arise due to the close crystallographic relation between the adjacent grains. The estimated grain boundary angles between the planes (-211), (-111) and (-311), (-111) are showing that there exist some of the planes that forms a low angle grain boundary (≤15°) with polarization components which are favoring the domain walls to cross over the adjacent grain boundaries in NBT polycrystalline films. Moreover from the empirical observation of surface roughness, when the surface roughness is larger than the average roughness (~ 2nm), the grain boundary acts as a physical boundary for the domain. Thus, the surface roughness also play a crucial role on defining the domain pattern by introducing a physical boundary for the coherent interaction of the polarization across grains. Fast Fourier Transform (FFT) spectrum analysis of the domain patterns confirmed that only highly oriented films possess periodic domain distribution. Moreover, the nano scale piezocoefficient values (d33) are increased from 16±0.4 to 30±0.46 pm/V with increasing the oxygen partial pressures and growth temperatures. Further the temperature dependent leakage current studies are performed for NBT thin films with different microstructures. For faceted grain structure, Schottky emission is dominated in the temperature range from 30°C to 120°C. Further in the temperature range 130-200 °C Ohmic conduction is dominated. In case of spherical grain structure 30-120 °C Poole-Frenkel emission and from 130-200 °C, Ohmic conductions are dominated. More over the highly oriented film with needle shaped grain structure exhibits Schottky emission in the temperature of 30-140 °C and from 150-200 °C, ohmic conductions was dominated. Further, we also performed the variation in the leakage currents by changing the oxygen partial pressures for a given temperature. The leakage current density under an applied electric field for the films grown at 650 ºC with various oxygen partial pressures. At a given field as the oxygen partial pressure increases, the leakage current density decreases. This is an indicative of the role of oxygen vacancies on leakage currents in NBT thin films. Hence, the optimized oxygen partial pressures could result in improved electrical properties in NBT thin films. Further, the microstructure influence on leakage currents studies reveal that coarse faceted columnar structure exhibit less leakage currents than the spherical grain structure. In addition, the phase transition studies indicates the plausible antiferroelectric transition at 120 °C and paraelectric transition in the range of 350-370 °C. In addition, the epitaxial NBT thin films are fabricated on LNO coated STO single crystal substrates with (100) (110) and (111) crystallographic orientations by pulsed laser deposition technique. The in- plane and out-of-lattice parameters and epitaxial strains are calculated from the reciprocal space map studies. Further, the FE-SEM studies shows that the surface energies of the substrate orientations play a crucial role in deciding the morphology of the respective film. We have observed a smooth 2D growth morphology in case of (100) orientation whereas (110) and (111) acquired 3D growth morphology. The ellipsometry studies reveal that the onset of the absorption is highly depends on the planar densities of the respective orientations. For all the three orientations, the calculated absorption coefficient values are in the range of 3.1-3.5 eV. The PFM studies reveals that the OP phase contrast is weak in case of (100) orientation and it is strong in case of (111) orientation. Further the IPPhase is strong in case of (100) than the (111) orientation. This is explained based on the polarization components that are participating in deriving the different types of domains in all three orientations. The single point piezoresponse measurements confirms that (111) oriented film is pocessing a large piezoelectric coefficient (d33) value of 63 pm/V compared with the other two orientations. The obtained d33 values for (100) and (110) are 42 pm/V and 51 pm/V respectively. This suggest that NBT (111) oriented film can be a potential candidate for replacing the lead based compounds for device applications.

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IITH Creators:
IITH CreatorsORCiD
Ramadurai, Ranjithttp://orcid.org/0000-0003-2991-0027
Item Type: Thesis (PhD)
Uncontrolled Keywords: Piezoelectric Material
Subjects: Materials Engineering > Materials engineering
Divisions: Department of Material Science Engineering
Depositing User: Team Library
Date Deposited: 25 Apr 2019 10:44
Last Modified: 21 Sep 2019 07:15
URI: http://raiith.iith.ac.in/id/eprint/5017
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