|FR | ENGL|
|Oxide powder synthesis by the combustionn route|
|A.M. . Segadães
University of Aveiro
As technological innovations appear at an ever-increasing pace, sustaining competitive advantage depends all the more on the timely recognition and proper introduction of new technologies. Very traditional methodologies have slowly but steadily been replaced by new techniques that are more precise, thorough or faster. Industrial sectors that have been regarded as very traditional, heavy set, fields of research, have seen within the past few decades, gigantic technological advances, particularly through the developments in powder production and processing routes. Time and added-value are the key words these days and powder technology research is ready to welcome alternative clean techniques that simplify and further the knowledge needed in this field. Even cost is becoming less critical and mostly it is traditional inertia that delays the adoption of a different methodology.
In advanced ceramics technology (superconductors, multilayer capacitors, gas sensors, solid oxide fuel cells, high speed cutting tools, to name a few), powders are the starting material for the production of components either by sintering or hot pressing. The characteristics of the resulting microstructure (e.g. grain and pore size, secondary phase distribution, element homogeneity) determine most of the properties of the final ceramic body. In other words, the quality of sintered components is strongly influenced by the characteristics of the starting powder.
Thus, ceramic science and engineering research work normally start with the necessary synthesis of the powder with the desired purity and stoichiometry. Powder synthesis methods are usually selected in response to specific requirements. Obviously, any method may be advantageous in some applications and disadvantageous in others, as different synthesis methods lead to different particle morphology and specific surface area, which, in turn, determine the powder properties.
The synthesis of the majority of powders is carried out by solid state reaction (ceramic route) among the individual component oxides, at elevated temperatures (Figure 1). Sometimes the production of such multicomponent oxides may require repeated grinding and firing steps in order to achieve the desired phase. The resulting material may still contain secondary phases and unreacted components and, frequently, present contamination from grinding.
Wet-chemistry synthesis methods, which start with the preparation of a homogeneous liquid solution of cation ingredients , produce ceramic powders with high sinterability, high surface area, well defined chemical compositions and homogeneous distribution of the elements. Although powder synthesis can be achieved at low temperatures, difficulties in up-scaling, expensive starting materials and sophisticated techniques, can be technological obstacles to the reproducibility, cost and reliability of the desired powders. Despite those limitations, in the sol-gel technique of powder synthesis (Figure 1) the effect of the variables is well documented enabling the manipulation of the microstructure and the properties of the final product, and the method has been successfully used to prepare a variety of powders .
In recent years, combustion synthesis has come up as a quick, straightforward preparation process (Figure 1) to produce homogeneous, very fine and crystalline multicomponent oxide ceramic powders, without the intermediate decomposition and/or firing steps [3-14]. The combustion synthesis of oxide powders stems from the 30-odd years old SHS technique (Self-propagating High-temperature Synthesis) introduced by Merzhanov et al., which has generated so much interest, both academic and practical, that quickly became an extensively and intensively studied discipline by chemists, physicists, mathematicians, chemical engineers and materials scientists alike. Today, efforts are focused on net-shape production of components (i.e. simultaneous synthesis and sintering) and the original concept of SHS has expanded spontaneously to include any self-sustaining combustion-like process that yields useful products.
The SHS powder synthesis method exploits an exothermic, usually very rapid and self-sustaining chemical reaction between the desired metal nitrates and a suitable organic fuel (e.g. urea). Its key feature is that the chemical reaction is ignited at low temperature and the heat required to drive it and accomplish the compound synthesis is provided by the reaction itself and not by an external source. While redox reactions such as this are exothermic in nature and often lead to explosion if not controlled, the combustion of metal nitrates-urea mixtures usually occurs as a self-propagating and non-explosive exothermic reaction.
By simple calcination, the metal nitrates can, of course, be decomposed into metal oxides upon heating to or above the phase transformation temperature. A constant external heat supply is necessary in this case, to maintain the system at the high temperature required to accomplish the appropriate phase transformation. In the combustion synthesis, the energy released from the exothermic reaction between the nitrates and the fuel, which is usually ignited at a temperature much lower than the actual phase transformation temperature, can rapidly heat the system to a high temperature and sustain it long enough, even in the absence of an external heat source, for the synthesis to occur.
The basis of the combustion synthesis technique comes from the thermochemical concepts used in the field of propellants and explosives. The need of a clear indication of the effective constitution of a fuel-oxidizer mixture led Jain et al.  to devise a simple method of calculating the oxidizing to reducing character of the mixture. The reaction releases the maximum energy when the redox mixture is stoichiometric, that is, when the valencies of the elements are balanced, irrespective of whether they are present in the oxidizer or the fuel components of the mixture. Thus, the method consists on establishing a simple valency balance and the assumed valencies are those presented by the elements in the usual products of the combustion reaction, which are CO2, H2O and N2. The extrapolation of this concept to the combustion synthesis of ceramic oxides  means that metals should also be considered as reducing elements with the valencies they have in the corresponding oxides.Experimentally, the valency balance is used to calculate the appropriate amounts of the selected starting materials (i.e. the cation precursors and the fuel) in the stoichiometry needed to obtain the desired oxide composition. This concept is particularly useful when thermodynamic calculations are difficult to carry out for lack of the relevant parameters and it has been shown  that there is a direct correlation between results derived from the valency balance and those based on heats of formation or bond energies.
When both the precursor salts and the fuel are water soluble, a good homogenization can be achieved in the solution. Thus, the reactants are first dissolved or melted, usually in a wide-mouth vitreous silica basin (300 cm3), by heating up to ~300°C on a hot-plate inside a fume-cupboard, under ventilation (Figure 2). This temperature is enough to trigger the decomposition of the salts and the subsequent explosive combustion of the fuel, with evolution of large amounts of gas and strong incandescence, often with the appearance of a flame which can reach temperatures in excess of 1000°C. The energy released from the exothermic reaction results in the rapid formation (1-2 minutes) of a dry oxide powder with a very fluffy, foamy macrostructure and the desired stoichiometry. Given that the metal cations were mixed in a boiling liquid that sustains the homogeneity of the mixture and the reaction is very fast, the resulting powder is also homogeneous.
Figure 2: Combustion synthesis: typical stages
In spite of the short reaction time, the high temperature reached is usually enough to promote the particle crystallization and X-ray diffraction of the as-prepared powders usually shows good crystallinity and no trace of the reactants (full conversion). The thinness of the original liquid foam causes the oxide flakes to be very fine, with the typical morphology of glass shards (Figure 3), and TEM studies (Figure 3) have shown crystallite sizes in the nanometer range, leading to high BET specific surface areas (typically, 2-12 m2/g, i.e. one order of magnitude higher than the values obtained with more conventional synthesis techniques).
Figure 3: Morphology of combustion powders
The extensive work already carried out [7-10, 12-14] shows that the valency based molar proportions of reactants proposed by the propellant chemistry can be conveniently used to successfully produce multicomponent oxide powders, with good compositional control and atomic level homogeneity. The variety of compounds produced has called for some adaptations of the combustion technique, to encompass the use of odd cation precursors  and the need for combustion aids .
Like various other methods that have been proposed and used to prepare ceramic powders, the combustion synthesis route enables the synthesis at low temperatures and the products obtained are in a finely divided state with large surface areas. Unlike the former, combustion synthesis offers as added advantages the simplicity of experimental set-up, the surprisingly short period mediating between the preparation of the reactants and the availability of the final product, the savings in external energy consumption and the equally important potential of simplifying the processing prior to forming, providing a simple alternative to other elaborate techniques.
The submicron features, high specific surface area and narrow particle size distribution typical of combustion powders, associated to the peculiar glass shard morphology, suggest that these powders might yield lower packing densities and, being more reactive than powders produced by conventional routes, might display poor densification behaviour and/or abnormal grain growth during sintering. It is speculated that those peculiar characteristics of the combustion powders might trigger specific sintering/densification mechanisms that require further investigation.
|Issues | Table of Content|