Degang Xie

In situ study of the initiation of hydrogen bubbles at the aluminium metal/oxide interface

The presence of excess hydrogen at the interface between a metal substrate and a protective oxide can cause blistering and spallation of the scale. However, it remains unclear how nanoscale bubbles manage to reach the critical size in the first place. Here, we perform in situ environmental transmission electron microscopy experiments of the aluminium metal/oxide interface under hydrogen exposure. It is found that once the interface is weakened by hydrogen segregation, surface diffusion of Al atoms initiates the formation of faceted cavities on the metal side, driven by Wulff reconstruction. The morphology and growth rate of these cavities are highly sensitive to the crystallographic orientation of the aluminium substrate. Once the cavities grow to a critical size, the internal gas pressure can become great enough to blister the oxide layer. Our findings have implications for understanding hydrogen damage of interfaces.

Wetting characteristics on hierarchical structures patterned by a femtosecond laser

Micro-scale hierarchical structures consisting of parallel grooves decorated by embossed triangle patterns are prepared by femtosecond laser irradiation on silicon (Si) wafers. The effects of surface morphology on wetting properties are investigated, and the results show that increasing the vertex angle of the triangle and groove spacing will lead to the enhancement of wettability and anisotropy, respectively. The structured surfaces also exhibit high adhesive force with droplets remaining attached to the surface even when the sample is turned upside down. Furthermore, the evaporation process of a water droplet on such an anisotropic surface is characterized to study its dynamic wetting behavior.

Hydrogenated vacancies lock dislocations in aluminium

Due to its high diffusivity, hydrogen is often considered a weak inhibitor or even a promoter of dislocation movements in metals and alloys. By quantitative mechanical tests in an environmental transmission electron microscope, here we demonstrate that after exposing aluminium to hydrogen, mobile dislocations can lose mobility, with activating stress more than doubled. On degassing, the locked dislocations can be reactivated under cyclic loading to move in a stick-slip manner. However, relocking the dislocations thereafter requires a surprisingly long waiting time of ∼103 s, much longer than that expected from hydrogen interstitial diffusion. Both the observed slow relocking and strong locking strength can be attributed to superabundant hydrogenated vacancies, verified by our atomistic calculations. Vacancies therefore could be a key plastic flow localization agent as well as damage agent in hydrogen environment.

Thermal treatment-induced ductile-to-brittle transition of submicron-sized Si pillars fabricated by focused ion beam

Si pillars fabricated by focused ion beam (FIB) had been reported to have a critical size of 310–400 nm, below which their deformation behavior would experience a brittle-to-ductile transition at room temperature. Here, we demonstrated that the size-dependent transition was actually stemmed from the amorphous Si (a-Si) shell introduced during the FIB fabrication process. Once the a-Si shell was crystallized, Si pillars would behave brittle again with their modulus comparable to their bulk counterpart. The analytical model we developed has been proved to be valid in deriving the moduli of crystalline Si core and a-Si shell.

Effect of hydrogen on the integrity of aluminium–oxide interface at elevated temperatures

Hydrogen can facilitate the detachment of protective oxide layer off metals and alloys. The degradation is usually exacerbated at elevated temperatures in many industrial applications; however, its origin remains poorly understood. Here by heating hydrogenated aluminium inside an environmental transmission electron microscope, we show that hydrogen exposure of just a few minutes can greatly degrade the high temperature integrity of metal–oxide interface. Moreover, there exists a critical temperature of ∼150 °C, above which the growth of cavities at the metal–oxide interface reverses to shrinkage, followed by the formation of a few giant cavities. Vacancy supersaturation, activation of a long-range diffusion pathway along the detached interface and the dissociation of hydrogen-vacancy complexes are critical factors affecting this behaviour. These results enrich the understanding of hydrogen-induced interfacial failure at elevated temperatures.

In situ transmission electron microscopy study of the electrochemical sodiation process for a single CuO nanowire electrode

In this work, an in situ transmission electron microscopy (TEM) study of the electrochemically driven sodiation and desodiation of a CuO nanowire (NW) was performed. Upon sodiation, Na ions first reacted with CuO, yielding a mixture of Cu, Cu2O, and Na2O, and then some Cu2O was subsequently reduced into nanocrystal Cu. The final sodiation product was nanocrystalline Cu mixed with Na2O and Cu2O. Upon extraction of Na+, the nanocrystalline Cu first oxidized into Cu2O and finally transformed back to nanocrystalline CuO. The volume of the CuO NW was found to expand markedly during the first sodiation, but the morphology of the NW remained unchanged during the following cycles. The mechanism by which high-performance CuO NW anodes are used for rechargeable Na ion batteries was analyzed by carrying out an in situ TEM technique based on our results, and this study may be beneficial for designing the optimal structure of the CuO anode, improving its cycle performance, and making CuO more feasible for Na ion batteries.

In situ TEM observing structural transitions of MoS2 upon sodium insertion and extraction

In this study, the sodiation and desodiation processes of MoS2 were characterized by using an in situ TEM technique. The structural evolution of MoS2 and its performance in a coin-type cell are recognized. Our findings provide a fundamental understanding of the reaction mechanism of MoS2 as anode for Na ion batteries.

In Situ TEM Study of the Hydrogen Effect on the Interface between Al and Its Oxide at Room and Elevated Temperature

Fossil fuels, including coal, natural gas and petroleum, are the major energy source for most of the countries. However, the consumption of fossil fuels is expected to generate many kinds of emissions, such as carbon dioxide, carbon monoxide, nitrogen dioxide, sulphur dioxides and industrial dust etc. Once the total amount of these emissions exceeds the upper limit of environmental capacity, heavy air pollution such as haze will occur which has raised serious concerns regarding its potential threat not only on human health, but also on precision machinery and high cleanliness industry [1]. In order to satisfy the global rising energy demands, intense research efforts have been dedicated worldwide to seek other renewable, clean and green energy sources, such as atomic power, solar energy, wind energy and hydrogen energy etc. Among them, hydrogen energy is thought to be promising because it is non-toxic, renewable, bountiful in supply and far more efficient than other sources of energy. However, to make hydrogen energy practically affordable and user friendly, people need to generate, store and transport hydrogen in a safe and low-cost manner.

Investigation of Gas Cooling Effect on the In Situ Heating Stage Inside Environmental TEM

In-situ heating technique has been widely used in studies of temperature related material behaviors, such as phase transformations, solid/gas-solid reactions, microstructural changes, growth of nanostructures, sintering of catalysts, et al. A perfect heating stage for experiments inside TEM requires low spatial drifts and accurate temperature measurement during temperature rise and fall. Current commercial heating stages can be divided into three categories: conventional furnace heating, direct filament heating and MEMS based heating chips. In recent years, although the spatial resolution of current heating stages has been improved to atomic scale, accurate temperature control lags far behind, especially in case of gaseous environments. With the development of the environmental TEM in the past few years, increasing numbers of heating experiments are carried out in gas environments, especially in the field of catalysts. Gas introduction usually cools down the heated parts, and fluctuation of gas pressure and flow state also change the sample temperature dynamically. Therefore, accurate temperature measurement is critical to achieve precise temperature control in environmental TEM. However, most of the current insitu heating stages are designed without real-time temperature sensing, the temperature is only calculated from previous calibrations in vacuum. In this work, a home-made MEMS based in-situ heating stage with real-time temperature sensing and feed-back temperature control will be shown. With exquisite structural optimization, we also achieved so far the lowest thermal drift rate for in situ TEM imaging. Using this new device, the dynamic change of temperature with gas pressure are quantified systematically with several conventional gas species like O2, N2, H2 and Air. Our presentation will introduce this new heating device and would provide a general reference for estimation of the actual sample temperature for device without temperature sensing in gas environments [1].

In situ TEM Investigation of electrical Current Effect on Aluminum Interconnect

Interconnect is a thin wire of copper or aluminum alloy which makes electrical contact between devices, typically on Si substrate. Electro-migration is the phenomenon that metallic atoms are transported by electron wind due to high electrical current density in the metal line. Ever since Blech [1,2] reported a relationship between interconnect length and the rate of electromigrationinduced drift, other factors like crystallographic texture [3-6], grain size and its distribution [7-10], and grain boundary structures [8,9,11-14] have been extensively documented to have a major impact on electron-migration induced plasticity. The continuous scaling down in the dimensions of typical integrated circuits leads to increasing electric current density in interconnects lines. Consequently, understanding the exact mechanism and evolution of electrical current effect on interconnect has become critical than ever for designing reliable devices.