REPOSITORY > RESULTS

Abstract

Fabrication of electroceramics usually requires high sintering temperatures of 800 to 1400 °C, which can unintentionally lead to problems such as volatilization of species, formation of secondary phases and excessive grain growth. These problems can cause inhomogeneous microstructures, non-stoichiometric chemical compositions and limited material functionality. Therefore, there is a need to develop processes that significantly reduce sintering temperatures.
In this dissertation, I present a low-temperature sintering process, the Cold Sintering Process (CSP), which enables the processing of ferroelectric perovskite ceramics at temperatures as low as 300 °C. Uniaxial pressures of up to 650 MPa and the incorporation of liquid phase additives are used to improve sintering. The focus of this research is primarily on two ferroelectric ceramic systems, BiFeO3-based (BFO) and (K0.5Na0.5)NbO3-based (KNN) ceramics, which are environmentally friendly alternatives to conventional lead-based ferroelectrics, but are difficult to produce as single-phase and with homogeneous microstructure using conventional sintering methods.
Initially, I explored cold sintering of the BFO and KNN materials with different liquid-phase components. The best results were obtained with a NaOH/KOH mixture in a 1:1 molar ratio dissolved in water, which led to the most uniform microstructure and improved materials properties. The results regarding the influence of NaOH/KOH ratio and the quantity of additives on the microstructure and properties of BFO-based material are described in detail in the first article of the thesis. The article suggests the optimal additive ratio and concentration, leading to high remanent polarization and strain responses to the electric field. The second article summarizes the effects of processing conditions such as pressure, temperature, additive’s amount, and post-annealing on KNN-based ceramics properties. Under optimal CSP conditions, KNN exhibited a high relative density of up to 98%, although it required elevated pressure, which led to significant structural deformation.
Research of the two cold-sintered ceramics systems showed that the electrical conductivity of BFO was 100 times lower than that of its conventionally sintered counterpart, effectively reducing leakage currents — a common undesired effect with conventional BFO. It is noteworthy that the cold-sintered BFO did not require post-annealing, while KNN had to be post-annealed at 500 °C in an O2 environment to reduce the electrical conductivity. Furthermore, CSP enabled to successfully mitigate excessive grain growth and porosity in KNN, eliminating a major limitation in obtaining dense ceramics with conventional sintering. Both, BFO and KNN exhibited remarkable dielectric breakdown strength, with above 200 kV/cm for BFO and up to 170 kV/cm for KNN, making both materials systems promising candidates for energy storage applications.
My last research part was on cold sintering of KNN-BFO multiferroic composites. These composites exploit the functional advantages of both phases by adopting a high dielectric constant of KNN and increased remanent polarization of BFO, overcoming the limitations of the individual phases. The results are summarized in the third published article of this thesis.