Bismuth Pyrochlore-Based Thin Films for Dielectric Energy Storage

Open Access
Author:
Michael, Elizabeth Kathleen
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 11, 2015
Committee Members:
  • Susan E Trolier Mckinstry, Dissertation Advisor
  • Clive A Randall, Committee Member
  • Thomas Nelson Jackson, Committee Member
  • Thomas R Shrout, Committee Member
  • Michael T Lanagan, Committee Member
Keywords:
  • dielectric energy storage
  • pyrochlore
  • thin film
  • bismuth zinc niobate
  • nanocomposite
  • flexible electronics
Abstract:
The drive towards the miniaturization of electronic devices has created a need for dielectric materials with large energy storage densities. These materials, which are used in capacitors, are a critical component in many electrical systems. Here, the development of dielectric energy storage materials for pulsed power applications, which require materials with the ability to accumulate a large amount of energy and then deliver it to the system rapidly, is explored. The amount of electrostatic energy that can be stored by a material is a function of the induced polarization and the dielectric breakdown strength of the material. An ideal energy storage dielectric would possess a high relative permittivity, high dielectric breakdown strength, and low loss tangent under high applied electric fields. The bismuth pyrochlores are a compositionally tunable family of materials that meet these requirements. Thin films of cubic pyrochlore bismuth zinc niobate, bismuth zinc tantalate, and bismuth zinc niobate tantalate, were fabricated using a novel solution chemistry based upon the Pechini method. This solution preparation is advantageous because it avoids the use of teratogenic solvents, such as 2-methoxyethanol. Crystalline films fabricated using this solution chemistry had very small grains that were approximately 27 nm in lateral size and 35 nm through the film thickness. Impedance measurements found that the resistivity of the grain boundaries was two orders of magnitude higher than the resistivity of the grain interior. The presence of many resistive grain boundaries impeded conduction through the films, resulting in high breakdown strengths for these materials. In addition to high breakdown strengths, this family of materials exhibited moderate relative permittivities of between 55 ± 2 and 145 ± 5, for bismuth zinc tantalate and bismuth zinc niobate, respectively, and low loss tangents on the order of 0.0008 ± 0.0001. Increases in the concentration of the tantalum end member increased the dielectric breakdown strength. For example, at 10 kHz, the room temperature breakdown strength of bismuth zinc niobate, Bi1.5Zn0.9Nb1.5O6.9, was 5.1 MV/cm, while the 10 kHz breakdown strength of bismuth zinc tantalate, Bi1.5Zn0.9Ta1.5O6.9, was 6.1 MV/cm. This combination of a high breakdown strength and a moderate permittivity led to a high discharged energy storage density for all film compositions. For example, at a measurement frequency of 10 kHz, bismuth zinc niobate exhibited a maximum recoverable energy storage density of 60.8 ± 2.0 J/cm3, while bismuth zinc tantalate exhibited a recoverable energy storage density of 60.7 ± 2.0 J/cm3. Intermediate compositions of bismuth zinc niobate tantalate were explored to maximize the energy storage density of the substitutional solid solution. At an optimized concentration of ten mole percent tantalum, the maximum recoverable 10 kHz energy storage density was ~66.9 ± 2.4 J/cm3. These films of bismuth zinc niobate tantalate (Bi1.5Zn0.9Nb1.35Ta0.15O6.9) sustained a maximum field of 5.5 MV/cm at 10 kHz, and demonstrated a relative permittivity of 122 ± 4. The films maintained a high energy storage density above 20 J/cm3 though temperatures of 200°C. The second major objective of this work was to integrate complex oxides processed at temperatures below 350°C onto flexible polyimide substrates for potential use in flexible energy storage applications. Nanocomposite films consisting of a nanocrystalline fluorite related to delta-bismuth oxide in an amorphous matrix were prepared by reducing the citric acid concentration of the precursor solution, relative to the crystalline films. These solutions were batched with the composition Bi1.5Zn0.9Nb1.35Ta0.15O6.9. The nanocomposite had a relative permittivity of 50 ± 2 and dielectric losses on the order of 0.03 ± 0.01. For measurement frequencies of 1 kHz and 10 kHz, the nanocomposite demonstrated a breakdown strength of 3.8 MV/cm, and a room-temperature energy storage density of approximately 40.2 ± 1.7 J/cm3. To determine the suitability of the nanocomposite films for use in flexible applications, free-standing flexible nanocomposite films underwent repetitive compressive and tensile bending around a minimum bend diameter of 7 mm, which corresponded to a strain of 0.10%. After bending the films 30,000 times, the energy storage density of the films was unchanged, indicating that nanocomposite bismuth zinc niobate tantalate films may be suitable for flexible energy storage applications. To demonstrate the broader applicability of the nanocomposite approach to developing energy storage dielectrics at low processing temperatures, films of nanocomposite lead titanate, Pb1.1TiO3.1, were deposited using an inverted mixing order solution preparation, and annealed at a maximum temperature of 400°C. X-ray diffraction indicated the presence of nanocrystalline ordering, and transmission electron microscopy confirmed the nucleation of isolated nanocrystals of lead oxide in an amorphous lead titanate matrix. The lead titanate nanocomposite demonstrated a relative permittivity of 33 ± 1 and a low loss tangent of 0.0008 ± 0.0001. The 10 kHz breakdown strength of the material was 5.0 MV/cm, resulting in an energy storage density of 28.4 ± 1.2 J/cm3.