In situ transmission electron microscopy as a toolbox for the emerging science of nanometallurgy

Abstract

Potential applications of nanomaterials range from electronics to environmental technology, thus a better understanding of their manufacturing and manipulation is of paramount importance. The present study demonstrates a methodology for the use of metallic nanomaterials as reactants to examine nanoalloying in situ within a transmission electron microscope. The method is further utilised as a starting point of a metallurgical toolbox, e.g. to study subsequent alloying of materials by using a nanoscale-sized chemical reactor for nanometallurgy. Cu nanowires and Au nanoparticles are used for alloying with pure Al, which served as the matrix material in the form of electron transparent lamellae. The results showed that both the Au and Cu nanomaterials alloyed when Al was melted in the transmission electron microscope. However, the eutectic reaction was more pronounced in the Al-Cu system, as predicted from the phase diagram. Interestingly, the mixing of the alloying agents occurred independently of the presence of an oxide layer surrounding the nanowires, nanoparticles, or the Al lamellae while performing the experiments. Overall, these results suggest that transmission electron microscope-based in situ melting and alloying is a valuable lab-on-a-chip technique to study the metallurgical processing of nanomaterials for the future development of advanced nanostructured materials.

Fig. 1. The sketch illustrates the preparation of the sample prior to the heating experiments. The sample is electropolished, sectioned using a scalpel, and transferred to a chip where NW or NP solutions are added. Note: this methodology is a modified, but new version of a previously reported sample preparation method for MEMS/TEM analysis.

Fig. 2. BFTEM micrographs featuring an Al thin foil resting on a SiN substrate. The Al foil appears as a translucent, ghostly veil against the darker gray of the substrate. The brighter area on the image indicates a hole present on the SiN substrate. In (a) and (b), a change in shape can be observed within the region highlighted by a blue circle. This alteration in shape was the result of melting and served as a temperature calibration.

Fig. 3. Results of melting and alloying for the Al–Cu system. The same region of the Al–Cu sample before and after alloying is shown. In the foreground are the Cu NWs, and in the background is the sample. (a) and (b) Show BFTEM micrographs. (c) and (d) Display the corresponding EDS maps of the sample.

Fig. 4. Results of melting and alloying for the Al–Au system. HAADF STEM micrographs and EDX maps before and after melting are shown in the insets (a)–(d), respectively. Some Au NP clusters (highlighted by the red arrows) are no longer present after the melting experiment and small nanometric precipitates formed after solidification, which are presented in the inset (e). Note that the speckled pattern in the dark areas is an artifact of the EDX mapping.

Fig. 5. Annealing of the Al–Cu system at 440 °C after solution treatment (spike annealing at 537 °C). The image series (a) display HAADF micrographs taken at different times. Image series (b) show the sequence with a high-pass filter applied in the Velox software. (c) and (d) Show the EDX maps of the composition/matrix composition before and after the heat treatment. At the start of the annealing treatment, a different structure compared to the dendritic-like seen in Fig. 3 is observed, followed by the spheroidization and coarsening of Al₂Cu-type precipitates (verified with SAED pattern of the precipitates, see insert in series (a) on the right).

Fig. 6. Annealing of the Al–Au system at 460 °C after melting. The image series (a) display HAADF micrographs taken at different times, starting with the as-alloyed state (0 s). (b) and (c) Show the EDX maps of the matrix composition before and after the heat treatment, respectively. At the start of the annealing treatment, precipitates are sporadically present in the Al matrix, followed by the formation of Al₂Au-type precipitates and EDS (indicated by the SAED pattern of the precipitates, see insert in c).

Fig. 7. Calculated phase diagram of Al–Cu (a and b) and Al–Au (c and d), using the thermochemical software FactSage 8.2. (a) and (b) display the Al–Au diagram with two different regions, (a) presenting the limit solubility of the liquid phase at 660 °C, and (b) showing the limit of solubility in the solid phase. (c) and (d) Show the Al–Cu, whereas (c) depicts the solubility of Au in the liquid phase, and (d) in the solid phase. The numbers indicate the solubility limits at the corresponding annealing temperatures.

Fig. 8. HAADF image illustrating the formation of plate-like Al₂Au structures during a heat treatment for 3600 s hour at 250 °C.