Hey，
looking for scientific pulication regarding colour change in vacuum hot-pressed MgO ceramics.
I have read professor Yi fang's ' Fabrication of translucent MgO ceramics using nanopowders ', he thinks that the surface of the as-hot pressed samples was black due to contamination from the graphite die. In our experiments, carbon contents in grey or black MgO ceramics prepared by vacuum hot-pressing were detected to be around 20 ppm by IGA. It's hard to believe so tiny carbon contents can change surface colour of hot-pressed MgO ceramics, so i would like to get more publication and explanations about the relation between carbon content and colour change phenomenon in oxide ceramics.
any work regarding similar phenomenon found in sintered MgO ceramics would be appreciated.
Hi Chen,
most probably it is carbon contamination, we observe the same in case of alumina made in the hot press or SPS in the range of 1200-1400°C. MgO was sintered with LiF and the heat treatment was carried out at a low temperature of 1100°C which makes classical substoichiometry (oxygen vacancies) improbable. I am not sure about the role of the Lithium fluoride (is there some intercalation ?).
If the ceramics are oxygen conductive you can anneal the carbon out in oxygen carefully.
It is well known that heating some oxides under vacuum or in a reducing atmosphere leads to color changes. For example, Ti3+ or/and oxygen vacancies turn white TiO2 into yellow, blue or black material.
In the case of MgO, vacancies and Mg+1 may, similarly, cause color changes. However, experimental proof is needed.
For reference regarding black TiO2:
http://pubs.rsc.org/en/content/articlelanding/2015/cs/c4cs00330f#!divAbstract
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