Probing Physical Properties of Confined Fluids within Individual Nanobubbles

Abstract

Spatially resolved electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) has been used to investigate as fluidic phase in nanoubbles embedded in a metallic P d90P t10 matrix. Using the 1s → 2p excitation of the He atoms, maps of the He distribution, in particular of its density an pressure in bubbles of different diameter have been realized, thus providing an indication of the involved bubble formation mechanism. However, the short-range Pauli repulsion mechanism between electrons on neighboring atoms seems insufficient to interpret minute variations of the local local measurements performed at the interface between the metal and the He bubble. Simulations relying on the continuum dielectric model have show that these deviations could be interpreted as an interfzce polarization effect on the He atomic transition, which should be accounted for when measuring the densities within the smaller bubbles.Confined fluids in nanosized volumes constitute challenging objects for both basic and technological aspects. The investigation of the structural features and dynamics of nanojets has given rise to spectacular experimental studies and theoretical simulations [1]. Another ideal system is represented by gas confined in nanocavities. It is the case of inert gas atoms coalescing as a fluid or a solid to fill nanocavities in metals, with spherical or faceted morphologies depending of the local pressure. In the case of Xe in Al, an interfacial ordering has been demonstrated by high resolution electron microscopy [2]. These small gas-filled cavities therefore behave as high-pressure cells, providing the boundary conditions for the evaluation of the physical properties of encapsulated gases. A most challenging problem is the evaluation of gas density and pressure in such cavities.Among the possible systems, He nanobubbles in metals have attracted the attention of many researchers, because of their high technological interest in the aging of the mechanical properties of materials involved in nuclear reactors [3]. Measurements averaging the information over large populations of bubbles, the size distribution of which being controlled by TEM, have first been performed by NMR [4] and by a combination of optical absorption and electron energy-loss spectroscopy (EELS) [5]. The first of these studies has revealed a solid-fluid transition at 250K for bubble pressures ranging from 6 to 11 GPa (i.e. He atomic densities from about 100 to 200 nm −3 ). The second study comparing UV absorption spectroscopy on a synchrotron and high energy resolution EELS without spatial resolution on He + implanted Al thin foils, have identified the blue shift of the He 1s → 2p transition (with respect to its value of 21.218 eV for the free atom) as a hint for evaluating the local pressure . Theoretically, Lucas et al. [6] have confirmed that this blue shift of the He K-line should be attributed to the short-range Pauli repulsion between the electrons of neighboring He atoms. Consequently, this effect should increas...

Model for the Formation of Helium Bubbles in Palladium

“…11 The measured He densities span a wide range of values, between 0.005 and 0.25 He atoms/Å 3 , depending on the particular metal host, the conditions of the experiment and the diameter of the bubbles. 1,9,[12][13][14][15] In general, high He densities correspond to small bubble diameters, and low He densities to large bubble diameters. For comparison, the density of bulk liquid He measured by Surko et al 16 at a temperature of 2 K and a pressure of 1 bar is 0.02 atoms/Å 3 .…”

“…In particular, in the case of a Pd-rich alloy (Pd 90 Pt 10 ), 15 , where WS R = 1.52 Å is the Wigner-Seitz cell radius of bulk Pd (the radius of a sphere with the volume of one atom in the metal). Here we have neglected the difference between Pd and Pt atoms, because their atomic radii are similar.…”

Using EELS to Determine He Pressure Inside Nanometer-Scale Bubbles

“…Figure 1 demonstrates an example of He bubbles in 10 keV ion-irradiated Pd (1x10 17 ions/cm 2 ) after being annealed in vacuum for 2 h at 600°C. In this case, a simple windowed mapping can be used to identify the He bubbles, and the shift of the He peak to higher energies is due to the increased He pressure in these bubbles (~1.5 GPa) [5,7]. Figure 2 shows another example in a Pd-5 at.…”

“…Knowledge of the mechanisms of the initial bubble formation and their evolution is critical to predicting long-term performance of materials subjected to these harsh environments. The use of the TEM and EELS provides a unique way to characterize the spatial distribution and properties of these individual bubbles because the TEM's high-spatial resolution can reveal bubbles even at the nanoscale, and EELS' low-energy sensitivity enables He concentration measurements of the 21 eV He K absorption line that can be directly converted to bubble pressure [5,7]. The ability to investigate correlations between bubble size and pressure is critical to testing theories of the mechanisms of the initial nucleation and eventual growth of these bubbles.…”

“…They occur during ion implantation in Si [1,2], nuclear waste disposal [3], in nuclear reactors [4,5], and in tritium storage materials [6]. Knowledge of the mechanisms of the initial bubble formation and their evolution is critical to predicting long-term performance of materials subjected to these harsh environments.…”