Competitive gas-solid reactions realized by ball milling of Zr in ammonia gas
- PDF / 228,408 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 94 Downloads / 242 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Competitive gas-solid reactions realized by ball milling of Zr in ammonia gas Ying Chen and Jim S. Williams Department of Electronic Materials and Engineering, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, Australian Capital Territory 0200, Australia (Received 14 August 1995; accepted 8 January 1996)
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.
I. INTRODUCTION
To date a large number of solid-state reactions arising from high-energy ball milling have been extensively investigated.1,2 Recently, gas-solid reactions induced by ball milling between metal powders and a gaseous atmosphere have received much attention.3 In such cases, the reaction end products most usually have a nanocrystalline structure. Typical examples are (i) nitriding reactions during milling of metal powder in a nitrogen atmosphere at ambient temperature,4,5 (ii) hydriding reactions between a number of metals (Zr, Ti, Mg, etc. . .) and a pure hydrogen atmosphere,6 (iii) oxidation reactions that are so easily produced that they are often difficult to prevent in the milling process.7,8 The above reactions normally take place above a certain temperature and/or under high gas pressure by conventional thermally activated processes. By contrast, in the case of high energy milling, these reactions can occur at room temperature and under a very low gas pressure environment, as long as sufficient gas is available for complete chemical transformation.9 Indeed, the ballmilling technique is a very efficient method for inducing some gas-solid reactions such as nitridation of metals. However, just as for solid-state reactions, the detailed mechanisms involved in gas-solid reactions induced by ball milling are still not clear. During gas-solid reactions induced by ball milling, a reduction of gas pressure has often been observed.10,11 The decrease in pressure first indicates the adsorption of gas to the fresh surfaces of metal particles formed during the milling process. It is well known that highenergy ball impact results in a dramatic particle-size reduction, even down to nanometer sizes.12 Hence, new surfaces
Data Loading...
