Sol–gel production of zirconia nanoparticles with a new organic precursor
Introduction
The interest for nanostructured materials, which are synthesized from particles smaller than 100 nm, has been growing in the last decades. The interest has been stimulated by the large variety of applications in industries such as aerospace, steel, cosmetics, health, automotive, bioengineering, optoelectronics, computers, and electronics. Research to develop applications have resulted in technologies that make it possible to obtain multilayered films, porous pillars, thin films, nanocrystalline materials, nanopowders and clusters for e.g. paints, antiseptics, nanocomposites, drugs, biomedical implants and military components.
It is very well known that materials with nanoscale grain size show different properties from the same material in bulk form. These unique properties are related to the large number of surface or interface atoms. Nanostructured materials have good refractory properties, good chemical resistance, good mechanical resistance and hardness both at normal and high temperatures, they are especially amenable to sintering and reactions with different oxides. It has also been shown that the large number of surface atoms present in these materials influences the optical, electrical and magnetic properties.
In this article we focus on ceramic materials. Also for advanced ceramic components, the starting powder is of critical importance for optimization of the properties.
It is now well recognized that the mechanical, electrical, chemical as well as catalytic properties of zirconia can be improved by using nanopowders instead of conventional micron-sized zirconia. When synthesizing of conventional Zr based materials the medium size of the particles is normally in the region of , which is generally equivalent to . Particles with diameters ranging between 0.1 and are considered fine particles and are usually made up of –. Particles on a nanoscale, with dimensions ranging from 1 to 100 nm in at least one direction are of particular interest. Particles consisting of 200–300 atoms are designated clusters and their surface atoms can represent up to 80%–90% of the total number of the atoms in the particle.
Over the past several years, a number of techniques have been developed for the production of ceramic nanoparticles and include: laser ablation, microwave plasma synthesis, spray pyrolysis, plasma arc synthesis, hydrodynamic cavitation and gas condensation using either a physical evaporation source or chemical precursors. Yet other methods of obtaining nanoparticles have been used, such as wire explosion techniques (Sabari et al., 2004), the polymerizable complex method (Popa and Kakihana, 2003), flame synthesis of nanoparticles (Singhal et al., 2001), the sonochemical method (Liang et al., 2003), solid state reaction (Udomporn and Ananta, 2004), precipitation (Tai et al., 2001) and co-precipitation from a solution, and sol–gel synthesis.
The sol–gel method and co-precipitation from solutions form, together with oxidation–reduction reactions, hydrolysis, colloidal processes, and pyrolysis of organic–metallic complex substances, the chemical methods category (Laberty-Robert et al., 2001). Chemical methods have an important place among the experimental methods applied on pilot scale. This is so-called “soft chemistry” which uses relatively non-aggressive diluted solutions at moderate temperatures. The intense research and development work in this field has led to the availability of chemically pure, nanosized powders with a narrow size distribution. These materials are valuable, but difficult to handle and are prone to agglomeration when using conventional processing routes.
Section snippets
Chemical methods for producing nanoparticles
The most important chemical methods of obtaining nanopowders are the Pechini method, the co-precipitation method and the GN method. Yamahara et al. (2003), used all three methods to obtain 8YSZ (ZrO2 doped with 8 mol% Y2O3). In the Pechini method the zirconium salt is dissolved in distilled water after which citric acid (CA–C6H8O7) and ethylene glycol (EG–C2H6O2) are added to the solution. In the co-precipitation method a solution of 30% ammonium hydroxide is added dropwise to
Objective of this work
In this project, we attempted to obtain ultra fine zirconia powders, using a rapid, simple and cost-effective chemical method, replacing the traditional organic precursors in the sol–gel process with an alternative, which has not been tried out before. The aim was to synthesize the powder and characterize it using thermal analysis (TA), TEM electron microscopy, X-ray diffraction, and BET analysis.
Experiments
The organic precursors chosen for this research work was sucrose, C6H12O6 and pectin. Sucrose consists of one molecule of glucose and one molecule of fructose. C6H12O6 is the chemical formula for both glucose or fructose, but they have slightly different structures. Table sugar is nearly pure sucrose (around 99% sucrose). Pectin is present in ripe fruits and some vegetables. Pectin consists of a linear polysaccharide containing between 300 and 1000 monosaccharide units. Pectin is widely used in
Results and observations
The TA were used to determine the chemical and physical properties of the samples as a function of temperature or time based on the thermal effects that occur during heating or cooling (see Fig. 2). The thermal analyses were performed on dried ZrO2 gel using a Derivatograph Q 1500 (MOM Hungary) instrument which is based on the Pauli, Pauli and Erdey system.
Analysing the TG and TDG curves of the ZrO2 samples a 5% mass reduction occurs between 100 and 200 °C which can be due to elimination of the
Conclusions
We have shown that it is possible to produce fine-grained zirconium oxide using sucrose and pectin as polymerization agents in relatively simple conditions and at low costs.
The particles have practically uniform dimensions, and distinct forms, they do not easily adhere to each other and their dimensions are lower than 100 nm.
The process takes 40 h at most, as the charge (including the burning).
This reaction product may be used in synthesis processes because it requires lower temperatures and
Future research
We plan to use this reaction product in some synthesis processes, resulting in products with superior characteristics compared to equivalent products obtained from fine-grained materials now used industrially. One of the most interesting fields in which these nanoparticles can be used is in SOFCs as raw material for the electrolyte and for the anode.
Acknowledgements
The authors thank the Department of Material Science, Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, Cluj-Napoca, Romania, and Prototech A.S., Bergen, Norway for experimental support. The authors, also, wish to acknowledge the NFR (Norwegian Research Council) for funding.
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