r, it is possible that tantalum oxides are mixed into the main matrix at low concentrations such that diffraction technique can not detect them because there is a limit of detection in order to show the deflections in this case the corresponding oxides. The sample was also examined by X-ray diffraction at a milling time of 30 hours. The same phases of tantalum hydrides were conserved at this long milling time (30 h). Finally, at 20 h of milling or less, only tantalum hydrides phases were obtained. During the mechanical milling process, high energy is generated by the intense impact between the container walls, milling media and material. The experimental conditions were created (pressure and temperature) to break the chemical bonds of methanol, which is used as a control medium and dissociates into hydrogen, carbon and oxygen. The bonding energies between carbon-hydrogen, oxygen-hydrogen, and carbon-oxygen are 415, 460 and 352 kJ/mol, respectively. It should be mentioned that the values are mean energies because the energy of a given bond depends slightly on the other atoms bonded in a given compound. Although, it is very difficult to measure the local microscopic temperature during milling because of the dynamic nature of the milling process, it is assumed that the temperature in the milling system is higher than the binding energy of the methanol solvent. To dissociate into elemental oxygen, hydrogen and carbon, oxygen and hydrogen most likely remain as negative ions (O−, H−), which react quickly with the tantalum. The carbon probably remains in the elemental form and mixes with the other compounds without reacting with Ta or oxides, and carbon’s concentration is below the detection limit. Under these experimental conditions, hydride formation may be possible because elemental Ta, as well as the compounds subsequently formed during milling, act as catalysts that improve the reaction conditions, thereby causing dissociation of the control medium into its components, H, O and C. Another possibility for the formation of different phases of tantalum may be the reduction in particle size as a function of the milling time. Smaller particle sizes most likely facilitate the formation of tantalum hydrides, as shown in the diffractograms. However, this phenomenon could result from a combination of different events that occur during the milling process and methanol dissociation including high pressure originated between two colliding balls or a ball and the wall of the vial. On the other hand, the hydrogen content up to 2wt% was measured in the tantalum hydride phases. It is interesting to note that, in this investigation, the hydrogen absorption process was realized “in situ” during over a short milling time. By varying the milling time, different phases of tantalum oxide were formed and changed after 20 h or less of milling. It is also interesting to note that there is an alternative method for preparing some metal hydrides at low temperatures and normal pressures, which simultaneously confers a source of hydrogen via conversion of methanol or another solvent during mechanical milling process.
Figure 5 shows the thermogram of Ta powder hydrogenated from mechanical milling process. The spectrum corresponds to hydrogen desorption from the material after 20 hours of milling. The hydrogen in the structure of the tantalum material was reacted “in situ” during milling. Hydrogen liberation occurs from room temperature to 330˚C approximately. It is possible that there is a correlation between the two mass losses on the thermogram curve and hydride phases identified, unfortunately at this time we can not know which of the two hydrides are desorbed at low temperature. The shape of the curve implies that hydrogen diffuse with increasing temperature, the first desortion ramp was realized between 20˚C (ambient temperature) and 250˚C, the second hydrogen desorption event occurred between 250˚C and 330˚C. The percentage obtained directly in the thermogram corresponded to the hydrogen that was liberated by the tantalum hydride materials, which was 2.35% by weight. Furthermore, at a higher temperature of approximately 330˚C, we suppose that at this point of temperature the Ta element is recycled. On the other hand, at these conditions hydrogen is released according to thermogram and after the material increased in weight and moves to the top. In the TGA system the Ta element begin to react with oxygen and tantalum oxide (Ta2O5) was formed. The reaction takes place with oxygen, which is an impurity in nitrogen used as carrier gas into the TGA system, and the chemical reaction was carried out according the equation:. By XRD tantalum phase was identified without any other impurity which is illustrated in the Figure 5(B). On the other hand, under these experimental conditions, with 20 h of milling process used in this work, Ta hydrides were obtained. Based on these results, it can be confirmed that the hydride phases of tantalum were formed “in situ” during the milling process. The mechanical milling system has proven to be a useful tool for preparation of some hydrides materials as well as for the investigation of chemical transformations that can take place in some compounds under a solvent appropriate condition into the vial. When preparing intermetallic, alloys or metal hydrides each case is different according to the type of research that wants to perform, since each mechanical milling system are different. It is possible that the start of milling a thermal activation can be originated creating the conditions to dissociate the solvent and the same time reduction in particulate matter and therefore different chemical reactions and structural changes take place as a function of milling time perhaps this process be done instantly. In this case, we believe that no pressure is produced within the vial due to genera-
Figure 5. Thermogram of Ta after 20 h of milling process, indicating hydrogen liberation as a function of temperature (A) and XRD pattern (B) of Ta2O5 formation as by product after hydrogen desorption.
tion of hydrogen, oxygen and temperature, since the gases react as they are being produced during the milling time and temperature is dissipated as heat to the exterior through the container walls. Hydrogen storage in solid materials is the long-term goal in hydrogen technology. During the last two decades, a number of promising new lightweight materials have been developed and studied. However, no materials satisfying each of the main targets with respect to storage capacities thermodynamics and kinetics have been found, and further research in this field is required. These efforts will involve combinations of theory and experiments, development and improvement of novel methods for synthesis and finally, “in situ” methods and the development of strategies that combine different preparation techniques. Many research efforts are currently under way to develop new technologies for hydrogen storage. Metal hydrides are attractive candidates for a safe way to store hydrogen for a broad range of practical purposes, such as portable, mobile or static applications. However, more research is needed to develop metal hydrides that meet all industry requirements.
In conclusion, the tantalum hydrides were formed “in situ” during the mechanical milling process. Due to the high impact energy process within the mechanical milling system, the internal conditions into the sealed vial were created and methanol used as control agent was dissociated in carbon, hydrogen and oxygen. The oxygen and hydrogen were reacted with tantalum for producing the corresponding oxide and hydride phases, respectively after 5 to 20 h of milling. According to the SEM analysis tantalum hydride at nanometric particle sizes were obtained. Although the exact mechanism of milling phenomena should be determined on a case-by-case basis depending of the type of ball mill used, it appears that mechanical milling processes in tantalum powder was primarily driven by structural changes and high pressure and temperature by the components into the sealed vial during milling. The mechanical milling process can be used as an alternative method for some nanometric hydride preparations such as the tantalum hydride phases which were reproducible and readily formed. The TGA results, in the temperature interval programmed the percentage of hydrogen released was 2.1 ± 0.5 by weight.
We would like to thank the personnel of the scanning electron microscope, X-ray diffraction machine, and thermogravimetric analysis for their valuable support.