Whenever I mill silica gel with other materials in stainless steel milling jar, the color of the powder goes from white (initial) to dark and darker color like yellow, dark yellow, brown, dark brown.
I did X-ray fluoresence on my samples with different milling duration and discovered that the metal composition for Si is decreasing (from 99.5% to 89%) while Cr, Fe and Mn is increasing after 9 hours of grinding (from 0% to 9%). I know my samples are contaminated with metal elements from stainless steel. However, at first glance, it doesn't look like the concentration is high enough to cause such significant color change in the powder (from white to dark brown). Could silica gel form any compound with these aforementioned metal element? Like maybe a composite? Or iron oxide is formed during the process as the reaction is performed in the air? But how could iron oxide form such easily just by grabbing oxygen from the air?
Dear Yongze Chen, three other things you should consider too. The frictional rise of temperature during milling and the presence of water (humidity) and oxygen since it is an open to air processing. These logically favor the formation of metal oxides and possibly metal complexes (ligation), leading to the noticed color changes. My Regards
I mean, depending on how hard you're milling, it's not at all unthinkable that you're "doping" the silica with the aforementioned metals, either in the bulk or just on the surface. If you're ball-milling then the likelihood goes up to me. In addition, you're creating fresh surfaces when you're milling and every metal in the first row react with air in one way or another, either forming a passivating oxide surface or forming other phases.
There are a lot of different possibilities here, so I recommend you carefully go through every parameter if you want to find the most likely explanation to this phenomenon.
It is a normal effect due to the release of metals from the jars to your sample. I do not think that compounds can form. To avoid this problem I suggest to make use of tungsten carbide or silicon nitride jars
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