X - R A Y N A N O P R O B E

S C I E N T I F I C H I G H L I G H T S

9 2 H I G H L I G H T S 2 0 2 2 I

Grain boundaries form as nanoporous metals coarsen

Nanoporous metals are prototypical bicontinuous microstructures that frequently undergo coarsening when annealed at elevated temperatures. The mechanical strength is highly dependent on the crystalline structure. X-ray Laue micro diffraction is used to identify that an in-grain orientation spread develops during coarsening in nanoporous gold, leading to the formation of a nanocrystalline- nanoporous structure.

Nanoporous metals are a bicontinuous two-phase mixture of metal and void phase created by metallic dealloying. The large interfacial area leads to exciting applications in battery electrodes, actuators and catalysts [1]. Nanoporous structures frequently undergo coarsening when annealed at elevated temperatures, altering the length scales in the structure and ultimately impacting optical, chemical and mechanical properties. Factors such as volume fraction and topological transitions can affect coarsening, which may then impact crystallographic evolution. Nanoporous gold (NPG) often serves as a prototype for studying nanoporous metals. Laue micro diffraction (µLaue) demonstrates that an in-grain orientation spread (grain boundaries) develops as NPG coarsens. Phase-field simulations of coarsening

using computationally generated (CG) and experimental structures is employed in combination with 3D electron backscatter diffraction (3D EBSD) to demonstrate that the formation of a significant number of these grain boundaries may happen from particle detachment and subsequent reattachment.

Prior to dealloying, all samples started as a Ag75Au25 alloy containing multiple grains with a mean grain size of 50 µm. All the samples are dealloyed and some are coarsened for 420 minutes at 300°C. The CRG- IF beamline BM32 with a polychromatic X-ray beam was used to perform µLaue measurements [2,3] on microcolumns fabricated from within as-dealloyed and coarsened NPG grains. In order to get orientation data that was not obscured by surface preparation artifacts, µLaue was employed on as-dealloyed and coarsened structures, while 3D EBSD could be used reliably for the coarsened structures only. Figure 84 shows a histogram of the uncorrelated misorientation angles, differences between orientations of all pairs of orientations, for the dealloyed and coarsened NPG sample. Any misorientations in as- dealloyed grains are very small, while the coarsened sample contained in-grain misorientations larger than 25°. The µLaue measurements demonstrate that the grain orientation of the original metallic sample is not preserved during annealing.

PRINCIPAL PUBLICATION AND AUTHORS

Solid-state compatibility of Ca:LaNbO4 with perovskite cathodes: Evidences from X-ray microspectroscopy, A. Chiara (a), G. Canu (b), A. Longo (c,d), C. Pipitone (a), A. Martorana (a), F. Giannici (a), Electrochim. Acta 401, 139495 (2022); https:/doi.org/10.1016/j.electacta.2021.139495 (a) Università di Palermo (Italy) (b) CNR-ICMATE, Genova (Italy) (c) CNR-ISMN, Palermo (Italy) (d) ESRF

REFERENCES

[1] K.D. Kreuer, Annu. Rev. Mater. Res. 33, 333-359 (2003). [2] A. Magrasó et al., Solid State Ion. 262, 382-387 (2014). [3] F. Giannici et al., Chem. Mater. 27, 2763-2766 (2015). [4] A. Chiara et al., ACS Appl. Mater. Interfaces 12, 55537-55553 (2020). [5] F. Giannici et al., ACS Appl. Mater Interfaces 9, 44466-44477 (2017). [6] F. Giannici et al., ACS Appl. Energy Mater. 2, 3204-3210 (2019).

Micro-X-ray absorption near-edge structure (XANES) spectra show a distinctive trend for each cation, and their ab-initio modeling allows to pinpoint the secondary phases brought about by the annealing. Iron maintains the perovskite octahedral coordination, shifting to higher Fe4+ valence in the interface reactivity zone due to higher Sr/La ratio. Calcium is incorporated in two different chemical environments after interdiffusion, either in a (Ca,Sr)2Nb2O7 pyrochlore, or in the A-site of the cathode perovskite as a dopant. Niobium also shows similar behaviour, diffusing out of LNC into (Ca, Sr)2Nb2O7 or in the B-site of the cathode. Figure 83 shows the dramatic change in the

XANES spectra of Nb5+ ions tetrahedrally coordinated in LNC or octahedrally coordinated in secondary phases (spots 1 to 8).

In summary, the presence of near-interface secondary products with variable compositions and morphologies attests to the limited chemical compatibility of LNC ceramics in contact with perovskite cathodes containing cobalt, iron and copper; on the other hand, Mn-containing LSM can be considered the most stable alternative. This highlights a major issue in the long-term operations of SOC devices based on these cathode/electrolyte couples.