Review paperSynthesis of SrAl2O4 and Sr3Al2O6 at high temperature, starting from mechanically activated SrCO3 and Al2O3 in blends of 3:1 molar ratio
Introduction
Strontium is found mainly as SrSO4. This material is processed to obtain strontium carbonate, strontium nitrate [1], [2], strontium oxide [3], or strontium chloride [4]. The strontium carbonate is widely used in several applications such as the fabrication of TV screens and in the manufacture of permanent magnets [5], [6]. Recently, strontium aluminates have been extensively studied due to their wide range of applications.
When strontium aluminates are doped with Eu and Dy ions, phosphorescence properties are obtained [7], [8], [9]. Depending on the doped crystal structure of the strontium aluminate matrix, different colors could be reflected [10], [11]. The phosphorescence properties attained in this way are useful for applications such as watches fabrication, luminescent instruments, tools used in the dark, etc., where luminescent coatings produce required light for better vision [12], [13].
Strontium aluminates can be also used in the fabrication of CO2 sensors for the chemical or metallurgical industries, instead of sensors based on alkaline metals, as they can respond to temperatures of up to 1100 °C, for any change in the partial pressure of CO2. This is due to their high properties such as chemical stability, mechanical resistance, corrosion resistance, and thermal and conductivity values [14], [15]. For these reasons, they are highly employed also in the refractories industry.
The conventional process for obtaining strontium aluminates consists in mixing SrCO3 and Al2O3 in stoichiometric amounts, then using temperatures greater than 1150 °C to attain SrCO3 decomposition into SrO, forming in this way aluminates type compounds [1], [3]. Temperatures can vary between 1200 and 1600 °C, with holding times of up to 144 h for solid state reactions.
A method of preparation of the starting materials by mechanical activation has proven to be a powerful tool for enhancing reactive sintering process, as less processing time and lower temperatures are required. Heinicke [16], [17] define mechanical activation as a branch of chemistry, related to the physical and chemical transformations of the solids through the use of mechanical energy. One of the main effects of mechanical activation is the reduction in particles size, which has an important impact on the reaction rate [18]. As of the heat generated during milling, this treatment increase chemical reactivity of the solid not only by reducing particles size but also by promoted structural changes (lattice disorder, atomic mobility, point defects increase, etc.), generating high energy reaction zones, fractures and new surfaces [16], [19], [20], [21].
Mechanochemical processing, which was initially intended to enhance ceramic strengthened alloys [4], has been successfully used to synthesize a wide range of nanosized ceramic powders, not only using high purity materials, but also starting from low grade materials. Chen et al. [22] used the high-energy ball milling treatment to prepare SrAl2O4. The milling was performed between 5 and 30 h. Other researchers observed that SrAl2O4 is the stable phase between 1000 and 1100 °C, so the transformation of this phase into pure compounds requires calcinations temperatures higher than 1250 °C, for completion [23].
In this study, SrCO3 was ball milled between 30 and 180 min before mixing with Al2O3, in order to obtain pure Sr3Al2O6 by solid-state reaction. The effects of the ball milling treatment on the microstructure of SrCO3 and the formation of Sr3Al2O6 were investigated using XRD and SEM techniques. The kinetics of the reactions was studied using thermogravimetric analysis at constant temperature, so it was possible to measure activation energy of different processes that occurred, as it will be discussed later.
Section snippets
Experimental
Powders of commercial Al2O3 (>99.9 purity) and SrCO3 (>99.9 purity) with an average particle size of 50 μm, were used as starting materials. SrCO3 was mechanically activated in a planetary mill (Analizette, Fritsch) for 30, 60, 90, 120, 150 and 180 min. The particle size distribution was measured using a Coulter LS100Q particle size analyzer.
For the analysis of the effect of mechanical activation on crystallite size, the most common technique employees the Scherrer equation. For determining
Mechanical activation of SrCO3
In a previous work [26], samples of SrCO3 were mechanically activated and the particle size distribution for the samples was analyzed. It was determined that over 45 min of milling, the particles size distribution did not change. Therefore, it is assumed that mechanical activation is effective after this period of time, so 45 min could be taken as the period of time at which mechanical activation starts. This is called 0 min activation time. The average size distribution of the particles is shown
Conclusions
Strontium aluminates of the SrAl2O4 and Sr3Al2O6 type were synthesized by solid-state reaction. Starting from mechanically activated SrCO3 in blends 3:1 molar with Al2O3. X-ray diffraction and SEM analyses confirmed the presence of strontium aluminates phases, demonstrating that mechanical activation is a useful method to obtain SrAl2O4 and Sr3Al2O6 by solid-state reaction, which can shorten processing times compared to conventional processes. Kinetics measurements allowed to determine the main
Acknowledgement
The financial support from the CONACyT is gratefully recognized (project ECO-2006-C01-52822).
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