Structural and functional properties of BaTiO₃ porous ceramics produced by using pollen as sacrificial template

Abstract

The present paper reports the dependence of structural and functional properties on the porosity level with values down to 50% in BaTiO₃ ceramics. Micro-porosity (pore size below 15 μm) with (0–3) connectivity has been produced by using dried pollen particles as sacrificial template. The properties of such porous ceramics are mostly affected by porosity, but also by possible small doping with foreign ions resulted from the template and by small variations of Ba/Ti stoichiometry at the ceramic-pore interfaces, as observed by a shift of the Curie temperature towards lower values when porosity level increases. The dielectric relaxation evidenced a few processes characterised by different activation energies which seem to be not affected by porosity, i.e. they might be assigned to the ceramic part and not to the ceramic-pore interfaces. When increasing porosity, a regular tilting of P(E) loops and increasing of coercivity and reducing polarization were observed, as result of reduction of active ferroelectric component and to the field inhomogeneity in such ceramics.

Introduction

Ferroelectric ceramics are multifunctional materials with a broad range of applications in multiple technological fields, due to their special dielectric, electromechanical, tunability and thermal properties. Following the need of improving or adapting their properties for specific devices, various strategies of changing composition (by doping, forming solid solutions or composites), the crystalline symmetry e.g. using phase superpositions (or morphotropic phase boundary) for enhancing properties, changing grain size (scaling properties) or ceramic grain shape/orientation (texturing) have been employed. There is still a constant need for exploring other tools enabling the control of their functional properties. Even if dense compact electroceramics are usually preferred in the large majority of typical applications (i.e. porosity is detrimental), there are cases in which the presence of porosity, especially in some specific configurations, might be beneficial for tailoring functional properties. Exemplary cases include increasing the figure of merit for piezoelectrics and improving the acoustic impedance matching between a ceramic and measurement media in ultrasonic applications such as underwater sonars or ultrasonic medical imaging (echography) [1,2], enhancing the pyroelectric figure of merit [[3], [4], [5]], reducing permittivity while maintaining or even increasing tunability [[6], [7], [8]], improving energy harvesting capabilities [9,10] or modifying the ferroelectric switching properties [[11], [12], [13], [14]]. Among the various types of porous electroceramics, BaTiO₃-based materials have been less investigated and only recently they started to be considered [[3], [4], [5],7,10,11,14,15], mostly because they are an environmentally friendly alternative to the performant Pb-based ferroelectrics. The main types of processing, allowing the fabrication of ceramics with controlled porosity are: (i) partial sintering, (ii) sacrificial fugitive templating, (iii) replica templating or (iv) direct foaming [16]. In the case (ii), appropriate amounts of pore-forming agents are mixed with ceramic raw powders and after that, by evaporation or burning out before or during sintering, pores replicating their size, shape and distribution may be created inside the ceramic body. As sacrificial pore-forming agents, various types of polymer beads, organic fibers, starch or other saccharides, carbonaceous species [16] and biomaterials as seeds [17], fibers, micro-organisms [18] or pollen [19,20] have been employed.

In the present work, dried pollen particles were employed as fugitive sacrificial template for inducing close (0–3) type micro-porosity in BaTiO₃ ceramics. The role of pollen particles addition on phase formation, microstructures and on the dielectric and ferroelectric properties are reported.