Structural and optical properties of ZnO nanoparticles can be fine tuned by a novel variant of milling performed at cryogenic temperatures. In this study intrinsic defect centers such as oxygen and zinc vacancies are characterized using electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. Three different surface defects with different g factors were identified by EPR for which the spectral intensities change upon decreasing the crystal size. EPR and PL intensities revealed a linear correlation giving detailed information about optical and electronic properties of ZnO. The core-shell model established from optical emission and EPR suggests distinguished electronic states in the band gap belonging to negatively charged Zn vacancies and positively charged oxygen vacancies. This model indicates a correlation between red emission and positively charged oxygen vacancies, which lead to a possible transition from a typical n-type to a p-type ZnO semiconductor.
Elemental aluminum (Al) powders reinforced with 1–10 wt% of ultra-dispersed nanodiamond (UDD) powders were ball-milled in a SpexTM Mixer/Mill between 0 and 120 min followed by consolidation and sintering. X-ray diffraction analyses on the ball-milled powders revealed only α-Al peaks, whereas Al4C3 phase was identified along with α-Al in all sintered composites. Increasing the addition of nanodiamond to Al-matrix resulted in improved hardness of both ball-milled and sintered composites. The wear resistances of the Al-UDD composites were significantly improved with increasing UDD contents. Under similar load and sliding conditions, the wear resistance of Al matrix composite containing 10 wt% nanodiamond enhances about 40 times when compared with unreinforced aluminum.
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