Dislocation imprint by cold sintering to tailor ferroelectric polarization

Reducing energy cost when fabricating new and increasingly demanded functional materials for efficiently powering modern equipment and electronics, is one of the main concerns of materials scientists. Here we want to use a newly-developed cold sintering process for fabricating new energy-related ferroelectric-based materials with modified functionalities. The main goal of our project is to exploit the mechanisms underlying this low temperature (< 300 °C), pressure- and chemical solution-assisted technique to engineer intrinsic defects, dislocations and other discontinuities where charge and strain accumulate, to tailor the functional responses of ferroelectrics. We will explore the full potential of designing advanced ferroelectric ceramics through dislocation imprint by cold sintering, thus, novel materials’ nanostructures that are difficult to achieve by ‘normal’ sintering procedures due to inter-diffusion of species and high mobility of defects, causing their properties to be significantly different. Furthermore, standard sintering requires very high temperature processing (above 1000 °C) and shows several drawbacks, e.g., high energy consumption and potential harmful element volatilization; thus negative impact on the environment, exaggerated grain growth, defects dissolution, restrictions in materials design when combining materials of different nature. Our current knowledge on the cold sintering of ferroelectric ceramics implies pressure-dissolution process as the driving force for cold sintering – we use hydroxides mixtures that melt up to 300 °C, flooding the particles. Aided by applied pressure (up to 600 MPa) particles start to dissolve at the contacts and the pressure forces them together. The sintering process is complete when the grains are interlocked and cemented and the liquid is forced out or trapped in the matrix. The densification and the contacts between the grains are strongly marked by the pressure-dissolution process, forming stylolites or toothed surfaces with imprinted grains, numerous defects in the form of edge or screw dislocations and slip planes are frozen-in, numerous nm-sized closed pores, as well as networks of open channels, healed cracks, and so on mark the uniqueness of the cold sintering process. Samples appear dark, indicating reduced states of ionic species and/or oxygen vacancies. All the mentioned microstructural properties and induced defects of various dimensionalities strongly influence the size, shape, pinning and movement of domain walls that are inherent to ferroelectrics and thus bring unique functionalities we want to explore here. We want i) to understand how we can adapt the sintering process to controllably change the defect states in the materials; ii) to study defects, especially dislocations, at the atomic level, i.e. the distribution of charges and stresses in their vicinity and their interaction with domain walls, which is the key to understand and tailor the electromechanical responses. We will implement the 4D electron microscopy technique with a high-resolution detector that offers unique image capture, which allows to detect shifts in electron densities, understanding the local polarization distribution; iii) to study the connection between the analysed and tailored defects and functional properties by electrical conductivity and dielectric losses in different atmospheres, and polarization and strains measurements under electric field. This tells us a lot on the nature of the defects and how to tailor them for a targeted ferroelectric function especially related to electromechanical energy conversion and energy storage.