Fabrication and characterisation of KNN-based multifunctional ceramics for hybrid energy harvesters

Ye, Guoyang (2022). Fabrication and characterisation of KNN-based multifunctional ceramics for hybrid energy harvesters. University of Birmingham. Ph.D.

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Abstract

There is an increasing demand for transforming from traditional energy resources into renewable, environmental energies. Owing to the excellent coupling of mechanical – electrical effect, piezoelectric ceramics have devoted considerable attention as a promising candidate for use in energy harvester by converting mechanical energy into electricity. Ferroelectric materials are the subset of piezoelectric materials due to switchable polarisation direction. Recently, the built-in field in ferroelectric ceramics can promote the separation of photo-induced charges and produces voltages higher than its bandgap (bulk photovoltaic effect), exceeding the Shockley-Queisser limit. Hence, it is possible to produce photovoltaic-ferroelectric hybrid energy harvesters with improved performance and applicability by using photovoltaic-ferroelectrics. For many years, Pb(Zr\(_{1-x}\)Ti\(_x\))O\(_3\) (PZT) have been identified as a leading ferroelectric ceramic. However, due to concerns from lead pollution, the development of lead-free alternatives is required. Sodium potassium niobate (K\(_x\)Na\(_{1-x}\)NbO\(_3\), KNN)- based ceramics is one of the leading replacements for PZT ceramics being large owing to their high Curie temperature (410°C) and outstanding piezoelectric properties (465 pC/N). However, the KNN usually possess poor light absorb ability due to wide bandgap. Thus, this aim of this project is to explore a way to enhance the light absorbing ability and avoid the decreasing of polarisation of 5KNN and investigate the effect of multifunctional materials on the performance of hybrid prepared energy harvester.

This dissertation includes three parts. As KNN-based ceramics always suffer poor sinterability and exhibit poor piezoelectric properties, in the first part, the research of this project is mainly concentrated on the fabrication of 5KNN (K\(_{0.5}\)Na\(_{0.5}\)NbO\(_3\)) ceramics. In this study, a new sintering method, two-step sintering (TSS), was adopted to improve the sinterability by suppressing the grain growth and improving the density. The maximum density achieved by optimised TSS conditions was 95% of theoretical. This compared favourably, however, to the 91% achieved by single-step sintering (SSS). 5KNN ceramics densified by optimised TSS exhibited enhanced piezoelectric, dielectric properties, energy conversion efficiency and energy density. The typical dielectric, piezoelectric and ferroelectric property values of ε\(_r\)≈294, d\(_{33}\)=101 pC/N, k\(_p\)=0.33, P\(_r\)=9.8 μC/cm\(^2\) and ε\(_r\)≈337, d\(_{33}\)=122 pC/N, k\(_p\)=0.36, P\(_r\)=12 μC/cm\(^2\) have been observed for the 5KNN ceramics densified by SSS and TSS, respectively. Moreover, the correlation between grain size, density and functional properties were also studied.

In the second part, the dependence of sintering mechanism, phase structure and functional properties of the 5KNN ceramics on BNNO addition were investigated. The doping with Ba and Ni creates an oxygen-deficient environment, which caused the ex-solution of Ni at high sintering temperature even after a short soak time. Compared with 5KNN ceramics, all doped 5KNN ceramics exhibited denser, finer and more homogenous microstructures. The phase transition diagrams of (1-y) 5KNN - y BNNO (y≤0.10) have been successfully established by employing Raman spectroscopy and X-ray diffraction. It revealed that the (1-y) 5KNN - y BNNO systems evolved from orthorhombic (y=0, 5KNN) phase to tetragonal phase with a raise in y. At elevated temperatures up to 500°C, the KNN-based ceramics underwent an additional phase transition from orthorhombic\(\rightarrow\)tetragonal\(\rightarrow\)cubic phase, and the Curie temperature decreased from ~415°C in y=0 composition (5KNN) to ~168°C in y=0.10 composition (5KNN-10BNNO). The highest room temperature functional properties have been detected for y=0.02 compositions. The typical dielectric, piezoelectric and ferroelectric property values of ε\(_r\)≈742, d\(_{33}\)=132 pC/N, k\(_p\)=0.42, P\(_r\)=14 μC/cm\(^2\) and ε\(_r\)≈337, d\(_{33}\)=122 pC/N, k\(_p\)=0.36, P\(_r\)=12 μC/cm\(^2\) were observed for the 5KNN-2BNNO and 5KNN composition, respectively. However, with further increases in the dopant level, the 5KNN-based ceramics exhibited enhanced conductivity, relaxing dielectric properties and degraded piezoelectric properties on account of the high concentration of oxygen-vacancy and phase transition that developed. Moreover, a continuous bandgap reduction was also found in all doped compositions.

In the third part, energy harvesters were designed, prepared and tested using 5KNN and 5KNN-2BNNO piezoelectric elements. The performance comparison of the energy harvesters suggested that functional elements with higher electromechanical coupling coefficient and energy density may be preferred for the selection of piezoelectric materials, while higher polarisation and narrower bandgap may be preferred for the selection of multifunctional materials. The highest open-circuit voltage (1.77 V) and power density (268 μW/cm\(^3\)) were observed on 5KNN-2BNNO hybrid energy harvester under vibration at 10 Hz, combined with illumination. Results from the prepared 5KNN-2BNNO hybrid energy harvester indicated the potential of the novel 5KNN-2BNNO multifunctional materials for energy harvesting applications.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):