Exploration of the Cold Sintering Process for the Functional Metal Oxide Electroceramics
Open Access
- Author:
- Tsuji, Kosuke
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 05, 2021
- Committee Members:
- Clive A Randall, Dissertation Advisor/Co-Advisor
Clive A Randall, Committee Chair/Co-Chair
Susan E Trolier-Mckinstry, Committee Member
James Hansell Adair, Committee Member
Michael T Lanagan, Outside Member
John C Mauro, Program Head/Chair - Keywords:
- Cold Sintering
Ceramics
Ferroelectrics
Piezoelectrics - Abstract:
- Lowering the sintering temperature of electroceramics proves beneficial in numerous applications, such as co-firing with metal electrodes, suppressing volatilization of some cationic ions, or synthesizing complex composite structures, etc. The cold sintering process (CSP), a recent discovery, is one of the sintering techniques for low-temperature densifications. It uses uniaxial pressure and an appropriate solvent, promoting the pressure-solution creep between ceramic particles; densification is then completed efficiently at low temperatures in a short time. In this study, the CSP was used to densify three different types of electroceramics, yttrium-stabilized Bi2O3 for oxygen ion conductors, BaTiO3 for capacitors, and (K0.5,Na0.5)NbO3 for lead-free piezoelectrics. Yttrium-doped bismuth oxide (BYO) is known for its high ionic conductivity of oxygen. Lowering the sintering temperature of this material might be useful to synthesize the bi-layer structure of SOFC electrolyte, which provides much higher energy efficiency. It is demonstrated that the CSP enabled densification of BYO ceramics at 300 °C. The relative density reached was ~ 90 % of the theoretical density (T.D.). The as-sintered BYO ceramic had a total conductivity > 1 mS/cm at 500 °C. The conductivity slightly increased after the annealing process. The scanning and transmission electron microscopy analysis revealed that a metastable secondary phase formed adjacent to BYO grains in the as-CSP specimens. Once annealed, the secondary phase was recrystallized and there was an associated total conductivity improvement. The CSP work was then extended to BaTiO3. Dense nanocrystalline BaTiO3 ceramics are prepared at 300 ºC, under a uniaxial pressure of 520 MPa for 12 h using a molten hydroxide flux (NaOH-KOH). The average grain sizes are 75-150 nm depending on the flux amount. The dielectric permittivity is 700 – 1800 at room temperature at 106 Hz, with a dielectric loss, tan δ ~ 0.04. The differences in permittivity and phase transition behavior are explained in terms of the intrinsic ferroelectric size effect of the BaTiO3. The nanocrystalline BaTiO3 ceramics still show a macroscopic ferroelectric switching via a hysteresis loop. The obtained dielectric & ferroelectric properties are comparable to reported values for nanocrystalline BaTiO3, but at this time, these are achieved with the lowest processing temperatures ever used. A similar approach was taken for the densification of the KNN piezoelectric ceramics. The final densities were all over 92% with the use of a NaOH-KOH transient sintering aid at approximately 200 °C, at 400 MPa within the total 2 hours of sintering time. The grain size was ~200 nm; grains were limited in coarsening at isothermal holds at these temperatures. The insulation resistance and low field dielectric permittivity were similar to conventional sintered KNN ceramics. High electric field strengths can be applied ~ 100 kV/cm, without high losses or electric breakdown, suggesting a reasonable dielectric strength. Despite these observations, these high fields did not yield high piezoelectric performance. All previous poling strategies were considered to aid the poling of the domains, but the piezoelectric properties remained low (d33 < 30 pC/N). A Rayleigh analysis showed difficulties in moving the domains, as reflected in the lower non-linear Rayleigh coefficients. A thermal annealing process improves the non-linear coeffecients along with the piezoelectric coefficients and the establishment of higher remanent polarization. These results suggest that the presence of defects in the core-region of the grain limits the domain alignment process and thereby the piezoelectric properties obtained under the present CSP. This is supported by TEM observation showing a high concentration of defects in the core of grains.