Lead-Free Piezoelectric Bulk Ceramics and Thin Films

In addition, the nonlinear optical and microwave dielectric properties of the thin films were investigated for nonlinear photonic and microwave tunable device applications. In addition, the outstanding performance of BNT ceramics and thin films for piezoelectric applications and their drawbacks were also discussed.

Chapter 2 presents a detailed overview of various experimental techniques for the synthesis and characterization of bulk BNT ceramics and thin films. The pure and

• Introduction to Ferroelectric and Piezoelectric Ceramics and Thin Films
• Synthesis and characterization techniques 39-78
• Structural and Dielectric Properties of Ce-BNT and Gd-BNT Doped Ceramics
• Dielectric, Piezoelectric and AC-Conductivity Studies of BNKT and BNT-KNNG Ceramics
• Nonlinear Optical and Microwave Dielectric Studies of BNT and BNT-KNNG Thin Films

Microwave dielectric properties of thin films evaluated using the split postdielectric resonator (SPDR) technique. Nonlinear optical and microwave dielectric studies of BNT and BNT-KNNG thin films and BNT-KNNG thin films.

Measured real and imaginary part of dielectric permittivity of BNT thin films for different pressures. c - f) Real part of the dielectric function of BNT thin films along with the fitted curve. g). Frequency variation of (a) εr and (b) tan of BNT films deposited under different pressures and measured at RT.

Figure no. Figure description Page no.

LIST OF TABLES

List of Symbols and Abbreviations Units of measurement

Electrical measurements

Optical measurements

Other parameters

FESEM Field emission scanning electron microscopy FETEM Field emission transmission electron microscope HRTEM High resolution transmission electron microscope SAED Selected area electron diffraction.

Introduction to Ferroelectric and Piezoelectric Ceramics and Thin Films

Introduction and motivation

It also describes some of the technologically critical piezoelectric bulk ceramics and thin films with their applications. The motivation behind the choice of lead-free ceramics and thin film, a brief overview, literature review and objectives for the work are given in detail.

Theory and fundamental concepts

• Dielectrics
• Piezoelectricity
• Pyroelectricity
• Ferroelectricity
• Perovskite structure and stability
• Relaxor ferroelectrics

The polarization rises to saturation with increasing field, i.e. of the maximum number of domains oriented in the direction of the field, which is known as saturation polarization (Ps). The atomic size of 'A' is larger than that of 'B.' A-site cations have 12-fold cuboctahedral coordination with anions, while B-site cations are in 6-fold coordination surrounded by octahedral oxygen (O) anions.

Figure 1.1: Schematic diagram of polarization mechanisms.

Lead-based piezoelectric ceramics

• Lead zirconium titanate - morphotropic phase boundary
• Doping of PZT

Doping can be done mainly by three types, such as isovalent, donor and acceptor doping at A and/or B sites [17]. Second, higher charge donor dopants such as La3+, Nd3+ and Bi3+ in the Pb2+ site and Nb5+, Sb5+ and Ta5+ in the Zr4+ sites are compensated by the formation of cation vacancies can be either A or/and B.

Figure 1.11: Phase diagram of PZT [44].

Lead-free piezoelectric ceramics

• Barium titanate (BaTiO 3 )
• Sodium potassium niobate (K 0.5 Na 0.5 NbO 3 )
• Bismuth sodium titanate (Bi 0.5 Na 0.5 TiO 3 )
• BNT thin films

The electrical properties of BNT thin films were extensively reported (at low frequency, ≤ 1MHz), in contrast to optical properties of the heterogeneous form, which are quite sparse. However, there are no reports available for the third-order nonlinear optical and microwave dielectric study of BNT thin films for high-frequency tunable devices.

Figure 1.12: (a) ε r (b) d 33 , and (c) k 33 as a function of Curie temperature of different piezoelectric ceramics, measured at room temperature [44]

Objective of the present work

The effect of oxygen partial pressure on the linear, nonlinear and microwave dielectric and nonlinear optical properties of pure BNT and its best thin film compositions will be identified and investigated in detail. To study the effect of oxygen partial pressure on the optical, dielectric and microwave dielectric properties of BNT thin films, they will be deposited by PLD.

Synthesis and characterization techniques

Synthesis of bulk BNT ceramics

• Conventional solid-state reaction method
• Stoichiometric weighing of starting materials
• Calcination
• Particle size reduction
• Pelletization
• Sintering

The flowchart of the different steps of the synthesis procedure by CSSR is shown in Figure 2.1. In general, the phase formation temperature of the sample is confirmed by differential scanning calorimetry and thermogravimetric analysis. As the initial particle size decreases, the sintering temperature decreases and the density of the samples improves.

The sample sintering process can be completed in three stages: (a) initial stage, (b) intermediate and (c) final stage as shown in Figure 2.4 [2].

Figure 2.1: Flowchart of the various steps of the synthesis procedure in a conventional solid-state reaction

Thin-film preparation

• Preparation of BNT target
• Pulsed laser deposition (PLD)
• Description of PLD
• Film growth
• Substrate temperature
• Energy of the deposition flux
• Deposition rate, vacuum quality and background gas

Film growth and quality mainly depends on some fundamental parameters, such as substrate temperature, deposition flux kinetic energy, deposition speed, vacuum quality and background gas [4]. It occurs when the cohesive energy of the atoms within the film is greater than the cohesive energy between the film and the atoms on the surface. It occurs when there is greater cohesive energy between the film and surface atoms than the cohesive energy of the film atoms.

The kinetic energy of deposited fluxes can be controlled by the introduction of the gas.

Figure 2.5: Schematic diagram of the PLD process.

Characterization techniques

• Thermal analysis
• X-Ray diffraction
• Density measurement
• Scanning electron microscopy
• Field emission transmission electron microscopy
• Dielectric measurements and impedance spectroscopy
• Ferroelectric measurements
• Piezoelectric measurements .1 Polling
• Piezoelectric coefficients
• Atomic force microscope
• Optical characterization .1 Linear optical properties
• Nonlinear optical characterization
• Microwave dielectric characterization

In the present thesis, the FETEM instrument (2100F, JEOL) is used to study the morphology, and the crystallographic structure of the prepared samples was analyzed using a bright-field TEM image, high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). in some of the papers. The complex admittance (Y*), complex permittivity (ε*) and complex modulus (M*) of the sample can be estimated using impedance values ​​by following expressions,. It can be estimated by the composition, shape and volume of the material.

The photograph of the atomic force microscope was used in this work as shown in Figure 2.22.

Figure 2.8: Photographic image of the DSC-TGA system.

Structural and Dielectric Properties of Ce-BNT and Gd-BNT Doped Ceramics

Introduction

To the author's knowledge, there is no systematic study on the dielectric and AC conductivity analysis of Ce and Gd doped BNT ceramics. In the present chapter, our aim is to study the effect of Ce+3-BNT (BNTC) and Gd+3 -BNT (BGNT) doped ceramics on their structural, morphological, dielectric properties. The first part describes the pure BNT with the effect of different sintering temperatures on structural, microstructural and dielectric properties.

The second and third parts investigate the substitution of Ce and Gd in BNT in structural, microstructure, dielectric and AC conduction studies, respectively.

Sample preparation of Ce and Gd doped BNT ceramics

In addition, BNT ceramics with relaxor behavior have attracted attention in industrial applications due to their higher dielectric constant over a wide temperature range and diffuse phase transition [12]. The origin of the relaxor behavior may be due to the compositional disorder that arises from the exchange of different ions occupying equivalent crystallographic sites [13]. Lead-free piezoelectric ceramics doped with rare earths have been observed to exhibit relaxor behavior by inducing crystallographic distortion with improved dielectric properties [14].

Further, these samples were sintered in the range from 1050 to 1150 ºC for 3 hours to achieve maximum density.

Results and discussion: Effect of sintering temperature on the densification of BNT ceramics

Furthermore, as the sintering temperature increased (>1100 oC), the dielectric properties decreased due to lower density and increased porosity. εr was found to decrease (466 at 10 MHz) with increasing frequency due to a decrease in polarization. Initially, the dissipation factor was found to decrease up to 100 kHz and then increase with frequency (10 MHz) due to the increase in ionic conductivity in the sample.

This is because the molecules cannot orient themselves at room temperature.

Figure 3.1: DSC and TGA curves of the BNT precursor powder.

Results and discussion: Ce doped BNT ceramics

The relative density of the samples improved with increasing Ce concentration, which is a consequence. The dielectric properties of BNTC ceramics were measured in the temperature range from 30 oC to 400 oC at 1 MHz and are shown in Figure 3.12. In addition, with increasing x concentration, the value of γ decreased due to the replacement of Ce3+ in place of Na+.

From Figure 3.16, RH was shown to decrease while WH increases with an increase in temperature and is attributed to the increase in disorder in the system.

Figure 3.9: Rietveld refined XRD pattern of the BNTC ceramics for (a) x = 0.00, (b) 0.005, (c) x = 0.015, (d) x = 0.025, (e) x = 0.035 and (f) magnified image of (110) diffraction peak of all BNTC samples

Results and discussion: Gd doped BNT ceramics

These optical modes are divided into longitudinal (LO) and transverse (TO) components due to the electrical structure with the polar character of a grating. He observed that the intensity of the peaks found increased with Gd concentration due to the increase in disorder in the A zone of the BGNT ceramics. The RH values ​​decreased and the WH values ​​increased with temperature, which is attributed to the increased disorder in the BGNT system.

The reduction in average hopping length and hopping energy with the Gd concentration signifies the formation of additional localized states due to the incorporation of Gd into the BNT system.

Figure 3.17: Rietveld refined XRD pattern of the BGNT ceramics (x = 0.00 to 0.06), sintered at 1150 ºC for 3 h

Conclusions

Dielectric, Piezoelectric and AC-Conductivity Studies of BNKT and BNT-KNNG Ceramics

Introduction

14] reported that the piezoelectric properties were strongly dependent on the differences in ion size, atomic weight, and electronegativity of the A-site and B-site ions of the ABO3 perovskites. Furthermore, considerable effort has also been made with the addition of doping agents either in the A or B site of BNT ceramics to improve the ferroelectric and piezoelectric properties [15-21]. However, there are only few reports available on the effect of A-site substitution in BNT ceramics to study its effect on the dielectric and piezoelectric properties.

In this chapter, our aim is to study the effect of K and KNNG on the structural, morphological, electrical, dielectric and piezoelectric properties of BNT ceramics and to address the problems related to compaction, leakage current and difficulties in the polling process.

Sample preparation of K doped BNT and BNT-KNNG ceramics

Moreover, this study also reveals the diffuse phase transition relaxor behavior present in the system near the transition temperature. Furthermore, studies on transport mechanisms in BNT-based ceramics are scarce, even though the volatility of A-site cations and their deficiencies are speculated to be responsible for the leakage current. This chapter attempts to understand the leakage current mechanism that occurs via space charge limited conduction (SCLC) and is associated with the deep acceptor traps.

For the electrical measurements, a silver paste was applied to both surfaces of the sintered samples and dried at 150oC for 10 minutes to remove the moisture before electrical measurements were taken.

Results and discussion of K doped BNT ceramics

1 MHz. The r values ​​were found to increase with K concentration up to x = 0.2 and beyond that, it decreased. At higher concentration of x (> 0.2), the dielectric permittivity values ​​decreased and the loss tangent increased mainly due to the distortions of the crystal cells and also led to the decrease of the net dipole moment. The increase of γ with K substitution may be due to the strong distortion of the BNT system.

Furthermore, it also shows that the conduction is due to the thermal movement of oxygen vacancies and residual cations in the grain boundary of the BNKT ceramic [50].

Figure 4.1: (a) XRD pattern of Bi 0.5 (Na (1-x) K x ) 0.5 TiO 3 ceramics in the 2θ range of 20 − 80 0 and (b) Splitting of (021)) and (122) peaks in the 2θ range of 39.2 − 41.5 0 and 56.5 −

Results and discussion of BNT-KNNG composite ceramics

The increase/decrease in lattice parameters was attributed to the incorporation of KNNG into the BNT matrix. The increase of γ with the replacement of Nb may be due to a strong deformation in the BNT system. The RH values ​​decreased, while the WH values ​​increased with temperature, which is attributed to the increase of disorder in the BNT-KNNG system.

Similarly, the addition of KNNG (0.01) to BNT effectively increased the electromechanical coupling factor (k33) and piezoelectric load coefficient (d33) to 0.477 and 108 pC/N due to the incorporation of KNNG compound into the BNT system at x= 0.01. Electromechanical coupling factor mainly depends on their resonance frequencies (fr and fa), there is variation (f) and they vary from material to material.

Figure 4.15: Rietveld refined XRD patterns of BNT-KNNG composites for (a) x = 0, (b) x = 0.005, (c) x = 0.01 and (d) x = 0.02

Conclusions

Nonlinear Optical and Microwave Dielectric Studies of BNT and BNT-KNNG Thin Films

Introduction

For the deposition of thin films, the composition (x = 0.01) with the best dielectric and piezoelectric properties was chosen. Microwave dielectric and nonlinear optical properties of films have been widely reported for microwave communications, radio frequency and photonic devices due to their fast response, lower operating voltage, better compatibility and greater nonlinearity. Nevertheless, limited studies are available on microwave dielectric and nonlinear optical properties of BNT-based thin films.

Therefore, the present chapter provides the influence of O2 partial pressure on microwave dielectric and nonlinear optical properties of BNT and BNT-KNNG thin films systematically and is divided into two parts: (i) BNT thin films and (ii) BNT- KNNG composite thin film.

Deposition of thin films

Further, polyvinyl alcohol was added to them and pressed into BNT and BNT-KNNG targets using KBr Press (M-20, Technosearch Instruments, India) in the form of a circular disk with a diameter of 20 mm and a thickness of 4 etc. The deposition of BNT and BNT-KNNG thin films was performed on Pt(111)/Ti/SiO2/Si and quartz substrates by PLD for electrical and optical studies, respectively. The films were deposited using a KrF excimer laser with 248 nm wavelength, 5 Hz pulse repetition rate and 225 mJ pulse energy.

To extract the electrical properties, Al is deposited on the surface of the films (Ag/BNT and BNT-KNNG/Pt(111)/Ti/SiO2/Si) as electrodes using thermal vaporizer (Lab Coater Auto 500, Hind High Vacuum, India).

Results and discussions of BNT thin films