Insight into the evolution mechanism of carbon film and Eu valence in carbon coated BaMgAl10O17: Eu2+ phosphor annealed in air

Performing carbon coating on the surface of phosphors has been proven to be an effective strategy to enhance the oxidation resistance, which is an important factor to achieve stable luminescent devices. Therefore, a good understanding of the protection mechanism favors a continuous improvement of oxidation resistance of phosphors. In present paper, the evolution of the carbon layer, Eu valence (Eu2+/Eu3+), and luminescent properties for the C coated BaMgAl10O17: Eu2+ phosphor when annealed at high temperature is investigated carefully. Decrease of carbon layer promotes the appearance color transition from black to white as the annealing temperature rises to 1000 degrees C in air. As expected, the decrease of carbon layer will enhance the luminescence intensity, but risk the possible oxidation of Eu2+ to Eu3+, which inhibits the blue emission ascribed to Eu2+. The results indicate that luminescence intensity of phosphor is dependent on the synergistic effect of carbon thickness and Eu2+/Eu3+ ratio. Additionally, a reduction reaction of Eu3+ to Eu2+ is observed in C coated BaMgAl10O17: Eu2+ phosphor when annealed at high temperature, which also contributes to the higher luminescence intensity.