Why is aluminium oxide powder white in spite of diffuse reflection. On the contrary, Aluminum massive has metal luster while it becomes gray due to diffuse reflection. what's more, I have carried out an experiment, finding aluminum powder have a pretty low reflection of light form 200nm to 500nm compared with aluminium oxide powder. Where did optical energy projected to metal powder? transform to intrinsic energy? I want to consult somethiong about crystal optics, thank u!
The light diffusion regime depends on the size of particles in the powder. The differences you have seen between Al and AlOx may be due to a difference in the average particle size between them. Just consider that particles with an average size much smaller than the incoming wavelength will mainly dissuse light with a raileigh mechanism, which means mostly isotropic diffusion (white aspect), while particles with size comparable or even larger than the incoming wavelength will diffuse light in the Mie regime, which is anisotropic (gray aspect and reduction in the measured reflectivity).
Therefore, you should get an estimation of the average particle size in all your powder samples.
Chen, Zhi-Zhan, Er-Wei Shi, Wen-Jun Li, Yan-Qing Zheng, Ji-Yong Zhuang, Bing Xiao, and Lian-An Tang. "Preparation of nanosized cobalt aluminate powders by a hydrothermal method." Materials Science and Engineering: B 107, no. 2 (2004): 217-223.
Dear Maohang Zhang;
You have to study the electronic structures, structural relaxation, and energetics of intrinsic vacancies and interstitials in Al2O3. In the attached study detailed principles of the calculation of intrinsic vacancies and interstitials in Al2O3. it was found that In intrinsic point defects, extra levels appeared in the bandgap.Article First-Principles Calculations of Intrinsic Defects in Al2O3
Giuseppe Curro I have tested the particle size of different powder, but no evidence was found to prove the influence of paritilce size on powder color. However, my test proved powder( metal and oxide) size effected light asorbtion.
Dear Maohang Zhang
Perhaps I was not so clear in my former answer. Particles of Aluminum o r Alumina (or even defectivity modified AlxOy) DO NOT absorb "visible" light, but have a marked absorption only in UV range, around 250 nm wavelength. When you see some powder is white, that means all the visible light (from violet to red) is reflected in a diffuse manner, as for milk, flour, lime and many other powders. When you see gray it means two possible causes: an overall large absorption is occurring , involving all the visible range with the same weight , or a substantial part of the light is diffused at angles far from back-reflection.
By the way, white and grey are not "colors" , they are "fully homogeneous" mixtures of all visible colors, just with different overall intensity at the observing point.
This answer helps me understand this question, thank you Professor Giuseppe Curro ! If I have any further questions, please allow me to continue to consult you
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