| Author: | Yang, Wentao |
| Title: | Study on the energy storage property of high entropy ceramics with perovskite structure from bulks to thin films |
| Advisors: | Zheng, Guangping (ME) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Energy storage Dielectrics Ceramic materials -- Electric properties Thin films Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xix, 198 pages : color illustrations |
| Language: | English |
| Abstract: | Energy storage technologies are playing an important role in the effective implementation of renewable energies. The dielectric capacitor serves as one of the main energy storage devices, which is a widely used component in pulse power systems. Although the dielectric capacitor exhibits a fast charge-discharge process with large high-power density, its small energy storage density has yet to be improved. High entropy ceramics (HECs) with perovskite structure can accommodate multiple cations at A or/and B-site, exhibiting a single phase with specific dielectric properties that could be tuned to improve the energy storage density of dielectrics. In this thesis, bulk and thin-film HECs with chemical disorders at A-site are successfully prepared to study their lattice distortion and dielectric and ferroelectric properties. Meanwhile, the enhanced energy storage properties are achieved in HEC-based solid solutions by coupling the chemical disorder at A-site with Sc-doping at B-site or relaxor compositions at the morphotropic phase boundary (MPB). Moreover, based on the analysis of the pair distribution functions as measured by neutron total scattering, the mechanisms of enhanced energy storage properties of HECs are discussed. Firstly, it is found that lattice distortion, a crucial characteristic in crystalline materials with chemical disorders, is caused by the mismatches in the radius and valences of different ions, thereby affecting the electrical and dielectric properties of HECs. Since hetero-valent substitutions at one lattice site could cause charge re-distribution, it creates more oxygen vacancies to achieve internal charge neutrality. Lattice distortion can give rise to the migration of oxygen vacancies, which can be detected by dynamic mechanical analysis. Hence, the relation between lattice distortion and the relaxation of oxygen vacancies for HECs is investigated. Compared with other characterizations methods, it is demonstrated that the activation energy of relaxation of oxygen vacancies can be used as a quantitative measurement of lattice distortion in HECs with perovskite structure. Secondly, (Bi0.2Na0.2K0.2La0.2Sr0.2)TiO3 (BNKLST) HEC with single-phase perovskite structure is found to show reduced lattice parameters and a dense nanostructure, which are conducive to a high electric breakdown field ( as high as 180 kV/cm) and low leakage current density(≤10-6 A/cm2) of the HEC. Compared to those of conventional ceramics Bi0.5Na0.5TiO3 (BNT), the polarization-electric field loops of BNKLST are slim, demonstrating that BNKLST has a maximum recoverable energy density of 0.959 J/cm3 with an efficiency of 67%. Meanwhile, the HEC shows good thermal stability at 40-200 ˚°C with a high energy storage efficiency of 85.1-95.2%. The local structure of BNKLST is revealed for the first time through analyzing the pair distribution functions measured from neutron scattering experiments, which shows a tetragonal phase due to local lattice distortion. The degree of AO12 dodecahedron and BO6 octahedral distortions is directly related to polarization fluctuation. Thirdly, scandium ion (Sc3+) is chosen to study the effects of B-site doping on dielectric and ferroelectric properties of BNKLST HECs. The single-phase perovskite structure of Sc-doped BNKLST (BNKLST-xSc) could be maintained with the atomic content of Sc3+ up to x=0.3. Although the introduction of Sc3+ could reduce the dielectric constant, the grain size and dielectric relaxation behavior are enhanced (x ≤ 0.2) due to lattice expansion and inconsistency of polar regions in BNKLST-xSc. In addition, the dielectric and mechanical losses for BNKLST-xSc (x ≤ 0.2) are much small, which is favorable for energy storage efficiency. It is found that BNKLST-0.2Sc has the maximum recoverable energy density of 1.094 J/cm3 with an efficiency of 80%, mainly resulting from the increase in weakly polar nano-regions. Fourthly, a coupled mechanism is designed to optimize energy storage performance, that is coupling 0.94(Bi0.5Na0.5TiO3)-0.06BaTiO3 (0.94BNT-0.06BT) in an MPB composition with BNKLST HEC in the highly chemical disordered system. The introduction of BNKLST into the solid solutions gradually changes the phase composition with the appearance of a non-polar cubic phase, thereby reducing the dielectric property and dielectric relaxation behavior. While the temperature range of polar nano-regions (PNRs) at an ergodic state could be extended, which is beneficial for thermal stability of energy storage. Particularly, 0.7(0.94BNT-0.06BT)- 0.3BNKLST shows a recoverable energy density of 2.24 J/cm3 at an electric field of 140 kV/cm, owing to the enhanced Ti-O bond vibrations and low concentration of oxygen vacancies. More importantly, based on the pair distribution function analyzed by neutron total scattering experiments, the polarization mechanism is clarified where the addition of HECs could reduce the overall octahedral tilting with less distortion. Lastly, the influences of dimensions and microstructures of HECs on their energy storage properties are studied. Thin-film (1-x)(0.96BNT-0.04BT)-xBNKLST solid solutions with x=0, 0.3, 0.5 and 1 are prepared by spin-coating method. The addition of BNKLST into 0.96BNT-0.04BT could generate dense nanostructures with small grain sizes, which could lead to ultra-high electric breakdown fields with small remanent polarization. Meanwhile, the introduction of BNKLST HECs could enhance the piezoelectricity because of the induced nanodomains with small energy barriers. As a result, the 0.5(BNT-4BT)-0.5BNKLST thin film has an excellent recoverable energy density of 16.92 J/cm3 with an efficiency of 59.4% at an electric field of 1500 kV/cm. |
| Rights: | All rights reserved |
| Access: | open access |
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