Competitive gas-solid reactions realized by ball milling of Zr in ammonia gas

Competitive gas-solid reactions have been observed during high-energy ball milling of zirconium metal powder under an ammonia atmosphere at room temperature. The milling processes were investigated by monitoring pressure changes and subsequently analyzing milled powders with x-ray diffraction, thermal analysis, and elemental composition analysis. It is shown that during an early stage of milling, while the pressure is decreasing, a hydridation reaction predominates and leads to the formation of ZrH₂ phase. During further milling the pressure increases again, and corresponds to a nitridation reaction between ZrH₂ and available nitrogen (either excess NH₃ or incorporated nitrogen in the powder) to form ZrN. Hydrogen gas is liberated and complete conversion to ZrN occurs after prolonged milling. The above reaction sequence was confirmed by the nitridation reaction of ZrH₂ ball milled in NH₃ atmosphere.