5978.2:
Fundamental Research on Ceramic Nanoparticles (Project of the TOP NANO 21 Innovation Committee)
Abstract
A fundamental research program on ceramic nanoparticles is proposed. Today it is well understood that nanosize particles have distinctly different properties than meso– and macro size materials. The coordination number of atoms and thereby their physical and chemical properties are often different from that of corresponding atoms in the bulk, i.e. inside the particles. As a result, the particle melting point decreases, light absorption increases, and electromagnetic and other properties change, compared with those of the bulk. Fundamental research is now needed to understand basic phenomena at the nano level so new products and applications may emerge including new medicine or medical devices, cosmetics, low temperature catalysts, advanced fuel cells, self-cleaning surfaces to just name a few.
The core scientific team consists of Prof. Baiker (D-CHEM) for catalysis, Prof. Gauckler (D-WERK) for ceramics, Prof. Koumoutsakos (D-INFK) for computational simulation, Prof. Pratsinis (D-MAVT) for processing plus Prof. Schlapbach (EMPA) and his plasma team at Thun. Prof. Baiker will investigate the fundamentals of catalytic activity and selectivity of novel mixed oxide nanostructured catalysts with a Ph.D. student using nanoparticles made by flame spray pyrolysis by a Ph.D. student under the direction of Prof. Pratsinis. Prof. Gauckler will use the size dependent melting of small particles and thin layers to sinter nm-scaled oxide powders to full density at unusual low temperatures thus avoiding substantial grain growth in the final stage of sintering of ceramics with two Ph.D. students. In addition, with another Ph.D. student he will tailor additives in nanoparticle suspensions, the specifically adsorbing molecules and the solvent to the nano–particles, to achieve a stable particle suspension with the highest solids loading and to process it to a stable particle network. Prof. Koumoutsakos will develop simulators of molecular dynamics focusing on nanofluidics for understanding the effect of curvature on nanoparticle properties (i.e. Kelvin effect) and their interaction with other particles with one post-doctoral fellow (50 %), in collaboration with Prof. Pratsinis. Prof. Pratsinis will support the modelling efforts by Prof. Koumoutsakos with one post-doctoral Research Associate and those at EMPA with another post-doctoral fellow (50%). Furthermore, he will administer the whole program with a Project Leader Research Associate who also will be involved together with one Ph.D. student in synthesis of non-agglomerated particles (e.g. by nozzle quenching). Prof. Schlapbach and Dr. Leparoux from EMPA Thun will guide one post-doctoral Research Associate to understand the inductively coupled plasma processing of nanoparticles and its modelling in collaboration with Prof. Pratsinis for industrial design and manufacture of application-tailored nanopowders.
This cluster of activities will hold a monthly seminar series to bring together the entire cluster membership encouraging and preparing for a strong presence in international conferences. This will create synergism between existing ETHZ and EMPA research programs that will translate to even stronger research accomplishments and scientific leadership that should attract industry and even venture capital. This project will support the research of 6 Ph.D. students and 4 post-doctoral research associates for 21 months.
Deposition of Noble Metals in Carrier Oxides – Catalysts for Hydrogenation and CO Oxidation
(Baiker / Pratsinis)
The aim of the project is to use flame spray pyrolysis (FSP) to produce heterogeneous catalysts for hydrogenation reactions and catalytic oxidation of CO in the presence of hydrogen. Two systems will be investigated. Nanosized gold on titania and other oxides has recently been found to exhibit excellent activity for low temperature CO oxidation even in the presence of hydrogen (Maciejewski et al., 2001). This process is of crucial importance for the development of fuel cells. Platinum and Palladium on carrier oxides such as silica and titania are highly active for the catalytic hydrogenation of alkenes. This is one of the largest industrial processes in organic chemistry.
Both systems are composed of noble metal nanoparticles dispersed on an oxide carrier. The influence of the major preparation parameters on metal particle size and morphology is investigated. The performance of these materials for the low temperature oxidation of CO is measured in a fixed bed microreactor setup. Major process parameters include the chemical nature of the precursor, composition of the solvents used to bring the substances into the flame, and inherent spray parameters. Changing dispersion gas to liquid feed ratio, composition of the dispersion gas and nozzle size can be used to adjust the flame spray. This leads to considerable control over crystallinity, specific surface area and metal dispersion.
Electrical Properties of Nanoscaled Mixed Electronic/Ionic Conductors
(Gauckler)
In order to lower the sintering temperature of nanoscaled oxide powder particles we intend to apply the concept of ”activated sintering” which is well characterized also as ”size dependent surface melting”. In short words: Small particles or thin layers of a material melt at lower temperatures when their dimensions reach the nm scale (see e.g. Kofman et al., 1999; Lai et al., 1996). The melting point then scales with:
Where r is the dimension of the thin metal oxide layer on the nanometer sized particles, T its melting temperature and T is the melting temperature of the bulk material, b is some constant. As thin (preferably 1-2 nm thick) layer we intend to use a transition metal oxide such as Co3O4; Fe2O3; TiO2; NiO; ZnO. This oxide is chemically grafted in a precipitation process onto the metal oxide particles such as CeO2; Ce0.8Gd2-x; Zr1-xYxO2-x/2 which have diameters in the 20-40 nm range. The additives have to be chosen this way that they lower the high surface energy of the base material. In earlier work (see paragraph 3 and 3.1) we observed a drastic lowering of the sintering temperatures (several 100 °C) for such systems compared to sintering and melting of the same composition when using larger powder particles with thicker surface layers. The effect was 10-100 times more pronounced compared to just choosing smaller particles for sintering!
In case small metal oxide particles (20-40 nm) and thin surface layers are used (1-3 nm corresponding to 1…3 cat. % of additive) then the additive spreads as amorphous oxide grain boundary layer. This amorphous layer enables rearrangement of the particles as well as fast materials transport for sintering. In addition, when choosing particles in the nm-range, the amorphous layer also is assumed to act like a liquid in the extremely small capillaries (pores between the bulk powder particles) exerting a high densifying capillary pressure. The amount of additive is sufficient to densify the ceramic at rather low temperature resulting in nm small grain size. The amorphous films form a 3D percolating network of an electronic conductor in the otherwise ionic conducting ceramic (e.g.) Ce0.8Gd0.2O1.9. Therefore the ceramic becomes mixed electronic (grain boundary film) and ionic conducting (O2- ions in the grains) at the same time which is of great advantage in case of an electrode in solid oxide fuel cells. This will be studied and modeled by electrical impedance spectroscopy.
As these amorphous films are not stable at temperatures higher than the sintering temperature, the grain boundary material (the electronic conducting phase) can be brought into solid solution of the base material. This solid solution formation can be controlled by post sintering eat treatments. Herewith we may be able to tune the charge carrier density for both conduction mechanisms. Preliminary sintering results were published for the system Ce0.8Gd0.2O1.9 – CoO as well as electrical impedance measurements showing mixed electronic and ionic conductivity after solid solution annealings for one example only (Kleinlogel and Gauckler, 2000a, b).