Reduction of tantalum chloride by magnesium during reaction milling

Abstract

Recent studies have demonstrated the application of 345 mechanical alloying to chemical refining [1-3]. The reduction of oxides and halides by milling with 335' strong reducing elements, such as Ca or Mg, has been reported for a number of metals and alloys, E 325including titanium [2] and rare earth permanent 2 magnet alloys [4, 5]. The reduction reaction may K occur in a steady-state manner during milling or by ~ 315 unstable thermal combustion of the reactants [6-10]. Schaffer and McCormick [9] have shown 3o5 that combustion occurs during milling if the temperature rise during ball/powder collision events 295 exceeds the ignition temperature of the reactant mixture. In this letter we report a study of the reaction of tantalum chloride with magnesium during mechanical milling. The starting materials used for milling were TaC1 s (FLUKA, 99.9%) and Mg (CERAC, 99.9%, -325 mesh). A total of 5 g of powder, including a 10% stoichiometric excess of Mg, and a ball to powder 600 mass ratio of 7:1 was used. Milling was carried out using a Spex 8000 mixer/mill with a hardened steel 500vial. The milling media consisted of hardened steel .c balls and the effect of ball diameter (d = 9.5, 6.4, 4.8 E 400 and 3.2 mm) was examined. The vials were loaded E and sealed under high-purity argon in a controlled'~ 300atmosphere glove-box. The reaction was followed by c monitoring the vial temperature during the course of .O 20O milling using a type K thermocouple attached to its ~" -~ looouter surface. Samples were taken from the vial at different stages of milling and the structures were o " analysed by X-ray diffraction (XRD) using a Sie3 mens D5000 Diffractometer with a C u K , monochromatic radiation. The morphology of the powders was examined in a Philips EM 430 transmission electron microscope (TEM) with an EDAX 9900 energy dispersive spectroscopy (EDS) system attached. Measurements of vial temperature during milling are shown in Fig. 1for the four different ball sizes used in the study. At short milling times, the vial temperature increased steadily with milling, due to the mechanical energy dissipated in the system. At a critical time, tig , a n abrupt increase in temperature occurred, which indicated the onset of combustion. The ignition time for combustion increased with decreasing ball size as shown in Fig. 2. The structural evolution during milling with 3.2 mm balls was followed by X-ray diffraction measurements (Fig. 3). The diffraction pattern after milling for 1 h is shown in Fig. 3a. Only peaks corresponding to Mg were observed, indicating the