Synthesis and thermodynamic evaluation of intermetallic Mg-Ni/Mg-Cu nanoscale powders
- PDF / 685,181 Bytes
- 8 Pages / 584.957 x 782.986 pts Page_size
- 49 Downloads / 170 Views
Nanometer-sized intermetallic Mg-Ni and Mg-Cu compound powders were prepared by a physical vapor deposition method (arc discharge) and characterized by means of x-ray diffraction and transmission electron microscopy. Based on an empirical specific heat equation, the effective heat of formation and its temperature dependence were calculated to explain phase formation in nanoscale powders of the binary Mg-Ni and Mg-Cu systems. It is shown that theoretic calculations are in good agreement with the experimental observations. I. INTRODUCTION
Mg-based alloys are considered to be the most promising materials for hydrogen storage owing to their high hydrogen storage capacity, light weight, the abundance of Mg in the earth’s crust, and low costs compared with other hydrogen-storage materials. Nanoscale materials can significantly improve the reaction kinetics, reduce the enthalpy of formation, and lower the hydrogen absorption and release temperatures through destabilization of the metal hydride and multiple catalytic effects in the system.1–3 Mg2Ni and Mg2Cu are typical candidates for Mg-based hydrogen storage alloys, and, thus, it is significant to investigate their formation and hydrogen storage properties in view of experimental and theoretical research. The equilibrium state of binary metal-metal systems is determined by thermodynamic functions such as free energy or chemical potential. However, materials rarely attain equilibrium state because dynamic factors have to be taken into account in some cases. For instance, it was always found that conventionally the largest heat of formation cannot accurately predict the certain compound phases formed in nanometer-scale multilayers by magnetron sputtering4 and ion implantation modification methods.5 Likewise, the formation of intermetallic compounds during an arc discharge process cannot be conventionally explained by the largest heat of formation rule in this work. By using thermodynamic data, Pretorius et al.6–10 first proposed the effective heat of formation (EHF) concept that can directly predict the primary phase formed and the sequence in phases’ formation for metalmetal thin-film systems. In addition, other groups also used the effective heat of formation to theoretically a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0311 J. Mater. Res., Vol. 24, No. 8, Aug 2009
http://journals.cambridge.org
Downloaded: 14 Mar 2015
predict the phase formation and the sequence in Zr-Si,11 In-Se,12 Zr-Al,13 and Ti-Al multilayer thin films.14 Until now, the temperature dependence of the effective heat of formation can seldom be explored due to a shortage of related thermodynamic data, which actually cannot be ignored in some cases. Huang et al.15 reported a method to estimate specific heat of intermetallic compounds and supplied a theory to consider the temperature dependence of effective heat of formation. In this work, Mg-Ni and Mg-Cu nanoscale powders were prepared by an arc discharge method, and the formed phases were determined by
Data Loading...
