Characterization of nanomaterials or materials at the nanoscale has drastically increased during the last decades. This increase can be explained by (i) the necessity to obtain materials with nanometer-size grains, for instance nanocomposites, and by (ii) the use of nanoparticles in different fields, for instance lubrication applications. A challenge lies in the in situ microstructural characterization of such materials as it can give access to valuable pieces of information regarding the microstructural changes induced by their use.

The availability of dedicated TEM (Transmission Electron Microscopy) holders equipped with force and displacement sensors is of a very high interest to test, in situ, the size-dependent mechanical properties of nanometer-sized objects [1,2]. On crystalline nano-objects, Molecular Dynamics simulations have shown that dislocations nucleate at the surface [3,4]. Therefore, the surface state is of utmost importance in determining the nucleation stresses and types of dislocations. For materials which undergo surface reconstruction or changes in the surface chemistry under vacuum, it is necessary to perform experiments in a controlled environment (i.e. under gas pressure) which reproduces the real one.

Recently a Hysitron PI 95 Picoindenter has been installed on a Cs-corrected FEI TITAN ETEM (Environmental TEM) microscope. It opens the possibility of performing in situ compression under gas pressure, with high resolution imaging. We will present in situ tests of cubic CeO₂, a multifunctional oxide widely used in catalysis. Nanocubes are compressed along either under vacuum or under air pressure. Introducing oxygen inside the chamber limits or avoids the reduction of CeO₂ nanocubes induced more or less rapidly by the electron beam. A comparison of slopes of load-displacement curves obtained under vacuum at different electron doses and under air pressure (see Figure 1) strongly suggests that ceria reduced as Ce₂O₃ under the effect of an intense electron flux has a smaller Young modulus than unreduced or ‘oxidized’ ceria. Atomic resolution observations performed during the compression tests reveal the formation of dislocations and stacking faults (see Figure 2). Simulations are planned to further understand the deformation mechanisms as a function of the oxidation state (native, unreduced or oxidized states), as well as their reversibility [5].

References

[1] Q. Yu, M. Legros, A.M. Minor, MRS Bulletin 40, 62-70 (2015).

[2] E. Calvié, L. Joly-Pottuz, C. Esnouf, P. Clément, V. Garnier, J. Chevalier, Y. Jorand, A. Malchère, T. Epicier, K. Masenelli-Varlot, J. Eur. Ceram. Soc. 32 2067-2071 (2012).

[3] S. Lee, J. Im, Y. Yoo, E. Bitzek, D. Kiener, G. Richter, B. Kim, S.H. Oh, Nature Communications 5:3033 (2014).

[4] I. Issa, J. Amodéo, J. Réthoré, L. Joly-Pottuz, C. Esnouf, J. Morthomas, M. Perez, J. Chevalier, K. Masenelli-Varlot, Acta Materialia 86, 295-304 (2015).

[5] This work is performed within the framework of the LABEX iMUST (ANR-10-LABX-0064) of Université de Lyon, within the program “Investissements d’Avenir” (ANR-11-IDEX-0007) operated by the French National Research Agency (ANR). The authors thank the CLYM (Consortium Lyon-Saint Etienne de Microscopie, www.clym.fr) for the access to the microscope and A.K.P. Mann, Z. Wu and S.H. Overbury (ORNL, USA) for having provided the samples.

Figures:

Figure 1: Comparison of slopes in the pseudo-elastic region of stress-strain curves recorded successively on the same nanocube in high vacuum: a) un-reduced CeO2 (low electron dose), b) reduced Ce2O3 (high dose) and re-oxidized ceria under air at 2.6 mbar.

Figure 2: Dislocation nucleation and residual stacking faults (arrows) after nano-compression along [001] of a ceria nanocube reduced to Ce2O3 in high vacuum under the effect of a condensed electron beam (halo in the micrograph).

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