Sol–gel synthesis of forsterite nanopowders with narrow particle size distribution
Introduction
Nanoparticles offer a host of attractive properties which includes increased strength, high hardness, high diffusion rates, and reduced sintering times in comparison to those associated with coarser particles [1], [2], [3].
Forsterite (FS, Mg2SiO4) is a member of olivine family of crystals in the magnesia–silica system [4], [5]. In recent years, FS has gained popularity due to its wide range of applications. For instance, in electronics it is used as an active medium for microwave and tunable laser applications owing to its high melting point at 1890 °C, low dielectric permittivity, chemical stability, and excellent insulation properties even at high temperatures [4], [5], [6]. For SOFC (solid oxide fuel cells) applications, FS is an important material since its linear thermal expansion coefficient perfectly matches the other cell components and it exhibits very high stability in fuel cell environments [7]. Besides FS exhibits good biocompatibilty [8], [9] and fracture toughness (KIC = 2.4 MPa m1/2) (superior to the lower limit reported for cortical bone [10], [11], [12]) thereby making it an excellent biomaterial.
Despite the growing demand; the synthesis of pure nanocrystalline FS with controlled particle size has remained a challenge. This is mainly because the reaction of the starting oxides to form silicate is generally slow due to the relatively low diffusivity which results in the formation enstatite (MgSiO3) and/or MgO instead of FS. Hence very high processing temperature of 1200–1600 °C is required [13] resulting in coarse grained powders. Many alternative powder processing techniques have been reported for the synthesis of pure FS, for example mechano-thermal synthesis [14], mechano-chemical synthesis [15] and sol–gel techniques [12], [16], [17]. Although the techniques adopted in the above works [14], [15], [16], [17] reported FS crystallite size in the range of 10–90 nm (this was calculated using X-ray diffraction (XRD) technique), no analysis was reported on the particle size distribution of the synthesized powder. Considering the calcination temperature (typically 800–1000 °C) and time (2–3 h) applied in these processing methods, we believe that the particles could have been subjected to aggregation (i.e. particle sintering) due to high surface energy. This can broadly distribute the particle size range from nano to submicronic scale. Keeping the above points in view, in the present work FS nanopowder is synthesized using a simple sol–gel based method. Besides the simplicity, the advantage of this technique is that the synthesized FS powder (∼27 nm) is narrowly distributed and controlled over a size range of 5–90 nm.
Section snippets
Material and methods
Magnesium nitrate hexahydrate (Mg (NO3)2·6H2O) (MNH) (Junsei chemicals Co., Ltd., S. Korea) and Tetra ethyl ortho-silicate (TEOS) (Acron Organics, USA) were used as starting magnesium and silicon precursors. About 8.5 g of TEOS was dissolved in 300 ml of 1 M HNO3. To this solution 20.5 g of MNH was added and stirred for 2 h at room temperature. The pH of the solution was measured to be about 4. This stirred solution was kept in dry oven and aged at 65 °C for 12 h to form a highly viscous gel. This
Results and discussion
Fig. 1 illustrates XRD patterns of calcined gel of forsterite powder at 800 °C for 30 min. It should be noted that the sintering time adopted here is much lower that the techniques reported earlier [12], [16], [17]. The crystallite size (using Eq. (1)) calculated from XRD for five powder batches were found to be 22 ± 7 nm, respectively. The crystallite size obtained from XRD was in close proximity to PSD analysis (Fig. 2) which showed skewed distribution ranging from 5–90 nm with particle size
Conclusions
In this study, the synthesis of a nanosized FS powder via sol–gel method is reported using Mg and Si based precursors. This process showed that high purity product of nano-FS powders could be obtained by this simple process. Particle distribution analysis showed a skewed distribution plot where the particle size was centered at ∼27 nm. TEM analysis showed the nanoparticles to be of prolate spheriodal structure. XRD analysis revealed the powder to be pure and crystalline. FT-IR results showed SiO4
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