COMPOSITIONALLY COMPLEX PEROVSKITE OXIDES: DIELECTRIC, FERROELECTRIC, ELECTROCALORIC, AND ENERGY STORAGE PROPERTIES
Open Access
- Author:
- Son, Yeongwoo
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 26, 2024
- Committee Members:
- John Mauro, Program Head/Chair
Qiming Zhang, Outside Unit & Field Member
Susan Trolier-McKinstry, Chair & Dissertation Advisor
Jon-Paul Maria, Major Field Member
Clive Randall, Major Field Member - Keywords:
- High entropy perovskite oxide
Pulsed laser deposition
ferroelectricity
dielectric relaxation
relaxor ferroelectric
polarization stability
electrocaloric effects
energy storage
antiferroelectric - Abstract:
- This thesis investigates the structural, dielectric, ferroelectric, electrocaloric, and energy storage properties of high entropy-like formulations. Eight different high entropy perovskite oxides (HEPO) were explored in terms of synthesizability, phase development, and structural characteristics in both powders and thin films. Five different compositions with compositional disorder on the B-site of the perovskite structure; Pb(Hf0.2Zr0.2Ti0.2Nb0.2M0.2)O3 (M=Al, Cr, Fe, Mn, Sc) and three different compositions with compositional disorder on the A-site of the perovskite: (Bi0.2Ba0.2Sr0.2Ca0.2Na0.2)TiO3, (Bi0.2Ba0.2Sr0.2Ca0.2K0.2)TiO3, (Bi0.2Ba0.2Sr0.2Na0.2Pb0.2)TiO3. It was found that the Goldschmidt tolerance factor was an effective predictor of phase development. Metastable HEPO phases were accessible experimentally by pulsed laser deposition (PLD). It was shown that relative dielectric permittivity of HEPO films ranged from 180 to 2000 at frequencies from 100 Hz to 100 kHz at room temperature, depending on the composition. It was found that some of HEPO were prone to be electrically leaky due to charge hopping between multivalent ions. Configurational entropy induced dielectric relaxation below the temperature or the permittivity maximum (Tmax) at 105°C for the Al-based HEPO (AlP). A maximum polarization (Pmax) of 45.4 μC cm-2 and a remanent polarization (Prem) of 22.1 μC cm-2 were achieved for AlP at 10 kHz at an applied electric field of 1100 kV cm-1. However, the Sc-based HEPO (ScP) was found to be more like a normal ferroelectric with no observed dielectric relaxation below Tmax of 225°C. Pmax of 52.5 μC cm-2 and Prem of 24.6 μC cm-2 were obtained at 10 kHz at an applied field of 1100 kV cm-1. It was found that Tmax increases with an increase in the average B-site cation size, consistent with structural characteristics. Indirect measurements employing Maxwell relations were utilized to quantify the electrocaloric response of the films. Polarization-temperature curves (P-T curve) were extracted from temperature dependent Polarization-Electric field loops (P-E loop) from - 35°C to 275°C. In HEPO composition with multivalent ions such as Fe and Mn, bloated “banana”-like P-E loops were measured with increasing temperature, which was indicative of leakage. The temperature rise from the electrocaloric effect (ECE) of AlP was determined to be 7.39 K and 5.82 J Kg-1 K-1at 135°C for an applied electric field of 1100 kV cm-1. ScP exhibited an electrocaloric temperature change of 5.74 K and an entropy change of 5.77 J Kg-1 K-1 for an applied electric field of 1100 kV cm-1 at 50°C. Apparent negative temperature of Fe-based HEPO (FeP) were attributed to leakage-induced artifacts. Polarization stability was studied for HEPO films by employing a Positive-Up-Negative-Down (PUND) measurement protocol. By varying the pulse delay between measurement from 0.1 ms to 100,000ms, the polarization decay was estimated for ScP, AlP, and 2 % Nb doped-Pb(Zr0.52Ti0.48)O3 (PZT) films. It was found that the magnitude of polarization drop (∆dP = dP1 ms – dP100,000 ms) was approximately 3.2 μC cm-2 for ScP and 10.3 μC cm-2 for AlP, while that of PZT was only 1.5 μC cm-2. This result implied that Al breaks the long-range polar order, leading to rapid decay of polarization in AlP, compared to a normal ferroelectric such as PZT. In contrast, ScP underwent a relatively smaller drop of polarization, suggesting preservation of long-range polar order despite the existence of compositional disorder on the B site. A combination of PUND data and powder X-ray diffraction results suggests that ScP has field-induced phase transition from rhombohedral to tetragonal ferroelectric phases. The activation energy for the depolarization in AlP was estimated to be ≈ 87±5 meV for AlP, compared to ≈ 152±2 meV for PZT. By customizing the PUND profile to introduce 2 s of rest between measurement pulses, an enhanced ECE of 14.9 K was obtained for an AlP film at an applied electric field of 1120 kV cm-1. Finally, the energy storage properties of antiferroelectric (Pb0.87Sr0.05Ba0.05La0.02)(Zr0.52Sn0.40Ti0.08)O3 (M-PZO) thin film capacitors were explored. Excellent crystallinity (FWHM of 0.021°) could be obtained on SrRuO3/SrTiO3 substrates by pulsed laser deposition (PLD). The out of plane lattice parameter of M-PZO was determined to be 4.110±0.001 Å. The average maximum recoverable energy density was 88±17 J cm-3 with an efficiency of 85±6 % at 1 kHz and 80±15 J cm-3 with an efficiency of 92±4 % at room temperature. The capacitors could be cycled >106 at an applied electric field of 2 MV cm-1 before shoring significant loss of the energy storage density. This material is promising for high efficiency and low loss dielectric energy storage applications.