2, Department of Mechanical Engineering, Osaka University, Osaka, , Japan
When a sufficient energy is applied at a local point in multilayered nano-films composed of alternating layers with a thickness of 10 nm order, an exothermic reaction occurs and a compound is generated at the local region. The generated heat then spreads into an adjacent unreacted region. If the temperature in the neighboring region becomes high sufficient to cause additional mixing, a self-sustained propagating reaction (SPR) occurs. SPR is induced by not only by electrostatic discharge, electrical heating, and laser irradiation, but also by mechanical impact. SPR by mechanical impact can be divided into two phenomena: i.e., (i) initial reaction by mechanical loading and (ii) ignition and propagation of reaction wave. Many studies have been conducted on (ii) SPR properties such as the ignition threshold and the speed of propagation. On the other hand, for (i), the detailed mechanisms of the initial chemical reaction due to mechanical loading has been unclear. This chemical reaction occurs at well below melting points and eutectic temperature, implying that the chemical reaction can occur by solid-state reaction. The atomic mixing in the solid-state can be brought about by diffusion or plastic deformation. Since the reaction occurs in very short time, we focus the role of plastic deformation.
The purpose of the study is to clarify the mechanisms of initial chemical reaction of Ti/Si multilayered nano-films by mechanical loading. Since the multilayered nano-films have structural anisotropy, the deformation mechanism depends on the loading mode with respect to the stacking direction. In this study, we focus the deformation and chemical reaction under compression loading in the stacking direction as a basic mode. We fabricated truncated cone-shaped specimens by focused ion beam from a polycrystalline-Ti/amorphous-Si multilayered nano-films (bilayer thickness: ~30 nm) deposited by electron beam evaporation, and then conducted compression experiments under in situ observation by field emission scanning electron microscopy. We evaluated the chemical reaction or change in crystal structure by transmission electron microscopy (TEM) observation and selected-area electron diffraction to the deformed specimens.
The specimens yielded at a true stress of ~3 GPa under compression, and showed gradual hardening behavior. TEM observation confirmed that each layer was plastically deformed or thinned, and then the mixing of Ti and Si occurred. The selected-area electron diffraction revealed that a new crystal structure, estimated to be Ti5Si4 or TiSi, was generated on the Ti/Si interface and in the Ti layer. These results indicated that the mixing of each layer was induced by plastic deformation due to compressive stress and the compound of Ti and Si was generated. The exothermic reaction can be controlled or generated at a desired site by the local mechanical loading, and so it can be used for various applications such as local heating in large-scale devices.