Gabi Klein

Size effects, including the occurrence of superparaelectric phases associated with small scale, are a significant research topic for ferroelectrics. Relevant phenomena have been explored in detail, e.g. for homogeneous, thin ferroelectric films, but the related effects associated with nanoparticles are usually only inferred from their structural properties. In contrast, this paper describes all the steps and concepts necessary for the direct characterization and quantitative assessment of the ferroelectric properties of as-synthesized and as-received nanoparticles. The method adopted uses electrical polarization measurements on polymer matrix composites containing ferroelectric nanoparticles. It is applied to ten different BaTiO3 particle types covering a size range from 10 nm to 0.8 μm. The influence of variations of particle characteristics such as tetragonality and dielectric constant is considered based on measurements of these properties. For composites containing different particle types a clearly differing polarization behaviour is found. For decreasing particle size, increasing electric field is required to achieve a given level of polarization. The size dependence of a measure related to the coercive field revealed by this work is qualitatively in line with the state of the knowledge for ferroelectrics having small dimensions. For the first time, such results and size effects are described based on data from experiments on collections of actual nanoparticles.

Different types of tetragonal BaTiO3 particles were synthesized in a wet chemical process including an autoclave treatment. The synthesis and autoclaving medium was varied from isopropyl alcohol to water in different ratios, leading to a size variation of the particles from 38 to 145 nm. By applying combustion analysis of carbon and hydrogen (C/H analysis) released from the samples, it can be demonstrated that the amount of hydroxyl impurities incorporated into the crystal lattices of the particles increases with the water content of the synthesis medium. C/H analysis is a suitable tool for this characterization task as it allows discriminating different sources of these elements, i.e., different impurities. By comparing the self-synthesized particles with commercial ones, it can be concluded that the particle size has the dominant influence on the tetragonality, if the amount of internal hydroxyl groups is relatively low. In contrast, if the amount of water originating from the internal OH groups is elevated (greater than ~0.4 wt%) an impairment of the tetragonality is caused.

Low temperature processable materials with high dielectric constants are required for application on flexible organic substrates, for example, in printed electronics. To date, mainly organic polymers with embedded functional particles have been investigated for this purpose. For the first time, we present a printable dielectric composite material composed of ferroelectric high permittivity particles (BaTiO3) bonded by a mainly inorganic sol-gel derived network. The exemplary optimization of the properties by varying the sol-gel precursor illustrates the potential of sol-gel chemistry for printable functional materials. An operational gravure printed capacitor including printed silver electrodes is presented. The measured dielectric constants are among the highest reported in literature for low temperature cured films with moderate dissipation factors. Besides these promising dielectric properties, this composite film shows a ferroelectric response.

Dispersions of three different types of zirconia nanoparticles were treated in a stirred media mill. The deployed surface modifier was present during milling and it established separating mechanisms between the particles. The combination of mechanical deagglomeration and chemical surface modification results in stable zirconia colloids with average particle sizes down to 9 nm. In addition to deagglomeration, the milling treatment also causes comminution of nanoparticles. This was indicated for the two coarser types of the examined particles, by increasing surface areas and decreasing primary crystallite sizes. Transmission electron microscopy of the particles confirmed the creation of smaller primary crystallites and a minority of small fragments, but the majority of particles do not undergo comminution into halves or fragments with similar size. Changes of the particles’ phase composition, wear of milling media, amorphization of the particles to a small extent, and leaching of Y2O3 dopant have been observed as side effects in the process and are characterized quantitatively. This work describes a process for nanoparticle deagglomeration and preparation of high quality colloids, and informs about occurring side effects, including approaches for their minimization.