Preparation of AlON Powder by Carbothermal Reduction and Nitridation with Assisting by Silane Coupling Agent

In the preparation processes of aluminum oxynitride (AlON) powders by carbothermal reduction and nitridation, the homogeneity of mixed raw powders between Al₂O₃ and C is a critical factor by which the final composition and related properties of AlON transparent ceramic will be decided. In this paper, a silane coupling agent was used as a dispersant to optimize the distribution uniformity of raw material of Al₂O₃ and C, and the preparation of AlON powder with controllable composition and its distribution is investigated. The results show that the silane dispersant could effectively improve the distribution uniformity of raw material. The silane coupling agent contains functional groups of -SiH₃ and -C_nH_2n+1O. XPS showed that the silane could react with C and Al₂O₃ to form the Si-C bond and C-Al₂O₃ bond, respectively. The silane coupling agent provides a connected bridge for raw material powders. When the amount of the silane was 5 wt%, the mixed powder had a great distribution uniformity. The addition of silane coupling agent improved the reactivity of raw materials and decreased the synthesis temperature of AlON. The single-phase AlON powder was obtained after the Al₂O₃/C mixed powder was kept at 1670 °C for 30 min. Furthermore, the grain size of AlON powder was 100-200 nm with an AlN content of 27.5 mol%. With the increase of holding time to 4 h, the grain size increased to 15 μm, indicating that sintering between particles occurred, which may reduce the sintering activity of the powder.

Effects of Y2O3 and LiAl5O8 on the Microstructure and Optical Properties of Reactively Sintered AlON Based Transparent Ceramics

Sintering aid was very crucial to influence the microstructure and thus the optical property of the sintered optical ceramics. The purpose of this work was to explain the difference between the sintering aids of Li⁺ and Y³⁺ on Al₂₃O₂₇N₅ (AlON) ceramic via reaction sintering method. The effects of LiAl₅O₈ (LA) and Y₂O₃ on the sintering of Al₂O₃-AlN system were carefully compared, in terms of X-ray diffraction (XRD), microstructure, density, X-ray photoelectron spectroscopy (XPS) and optical transmittance. According to the XPS and XRD lattice analysis, the chemical structure of the materials was not obviously affected by different dopants. We firstly reported that, there was obvious volume expansion in the Y³⁺ dopped AlON ceramics, which was responsible for the low transparency of the ceramics. Obvious enhancements were achieved using Li⁺ aids from the results that Li: AlONs showing a higher transparency and less optical defects. A higher LA content (20 wt%) was effective to remove pores and thus obtain a higher transmittance (~86.8% at ~3.5 μm). Thus, pores were the main contributions to the property difference between the dopant samples. The importance of sintering aids should be carefully realized for the reaction sintering fabrication of AlON based ceramics towards high transparency.

Combustion Synthesis of Aluminum Oxynitride in Loose Powder Beds

This paper describes combusting loose powder beds of mixtures of aluminum metal powders and aluminum oxide powders with various grain sizes under various nitrogen pressure. The synthesis conditions required at least 20/80 weight ratio of aluminum metal powder to alumina powder in the mix to reach approximately 80 wt% of γ-AlON in the products. Finely ground fused white alumina with a mean grain size of 5 μm was sufficient to achieve results similar to very fine alumina with 0.3 μm grains. A lower nitrogen pressure of 1 MPa provided good results, allowing a less robust apparatus to be used. The salt-assisted combustion synthesis upon addition of 10 wt% of ammonium nitrite resulted in a slight increase in product yield and allowed lower aluminum metal powder content in mixes to be ignited. Increasing the charge mass five times resulted in a very similar γ-AlON yield, providing a promising technology for scaling up. Synthesis in loose powder beds could be utilized for effective production of relatively cheap and uniform AlON powder, which could be easily prepared for forming and sintering without intensive grounding and milling, which usually introduce serious contamination.

Insight the Luminescence Properties of AlON: Eu, Mg Phosphor under VUV Excitation

Owing to high quantum efficiency, adjustable composition and antioxidation properties of oxynitride phosphors, extensive investigations have focused on their photoluminescence properties under low-energy light excitation (UV or blue light). However, the vacuum ultraviolet (VUV) luminescence properties of oxynitride phosphors are rarely researched. Present work studies the structure and VUV luminescence properties of an oxynitride phosphor: AlON: Eu, Mg, which is synthesized by solid-state reaction. Under 147 nm excitation, it was found that AlON: Eu, Mg phosphor shows a blue emission band centered at about 470 nm. The first principle calculation is used to analyze the origin of the VUV absorption. Compared with BaMgAl₁₀O₁₇: Eu²⁺ phosphor, AlON: Eu, Mg phosphor shows better thermal stability.