I have synthesized alumina nano-powder using surfactant and calcination in 550 celsius. it is extremely light and fine but when I press the powder between my fingers I sense some rough particles. unfortunately when I use this powder for powder coating this rough particles can be seen on the surface.
what is this impurities and how i can remove it?
is it due to impurities in row materials (Al(no3)3 and urea) or duo to the calcination temperature? I increased the temperature up to 950 but the problem was not solved.
I think increasing the temperature is not a good option. That may lead to particle coarsening.
Maybe due to the impurities in starting material. Ensure the purity.
If the rough particles are alumina, you can try with ball milling.
Dear Sharifi-viand
I don't think so nitrate is stable at that temperature, may be you can increase the holding time in the furnace. And also for determining the residuals you should also apply FTIR analyze.
I think what you feel is coming from agglomerated particles. The finer particles always tend to agglomerate, so may be sieving solve your problem or you can apply ultrasonication after sintering.
Your raw materials are Al nitrate and Urea and that leads to the calcined product light and fine, if you are doing it by some wet chemical synthesis. The rough particles are probably gamma alumina particles formed due to agglomeration of fine precursor materials. You can go for phase analysis (XRD), FTIR or EDX to analyze the composition. To avoid rough particles in the calcined product, you can go for milling of the synthesized material after drying around 2000C and then go for calcination.
Generally nitrate route with urea does not have a considerable impurity as the sources are chemically pure ingredients. Basically the reaction between these two are producing aluminium hydroxide precursor which on calcination converts to alumina. But at 550 C corundum phase is hard to form. (check thru phase analysis). The roughness in the calcined particles is due to temperature effect that causes agglomeration among the nano particles. You will get a wide difference between specific surface area and particles size of the calcined powders. But these are loose agglomerates and is obvious. Even after drying you may get these hard agglomerates. Can use slow and controlled drying and calcination to reduce the agglomeration effect. Best way is to introduce some milling process (soft milling or de-agglomeration) after calcination to get best effect of the nano particles formed.
For urea based system dry above 150 to 200 deg C. At low T it may be hygroscopic.
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