Microstructural transformations and kinetics of high-temperature heterogeneous gasless reactions by high-speed x-ray phase-contrast imaging

Heterogeneous gasless reactive systems, including high-energy density metal-nonmetal compositions, have seen increasing study due to their various applications. However, owing to their high reaction temperature, short reaction time, and small scale of heterogeneity, investigation of their reaction mechanisms and kinetics is very difficult. In this study, microstructural changes and the kinetics of product layer growth in the W-Si system was investigated using a high-speed x-ray phase-contrast imaging technique. Using the Advanced Photon Source of Argonne National Laboratory, this method allowed direct imaging of irreversible reactions in the W-Si reactive system at frame rates up to $36000$ frames per second with $4\mu \text{s}$ exposure and spatial resolution of $10\mu \text{m}$. Details of the Si melt and reactions between W and Si, that are unable to be viewed with visible-light imaging, were revealed. These include processes such as the initiation of nucleated melting and other physical phenomena that provide insight into the mixing of reactants and subsequent reaction. Through the use of this imaging technique and future optimization in the imaging process, a model for accurately identifying kinetics of chemical reactions, both spatially and temporally, is also proposed.

Figure 1

(Color online) SEM images of unreacted and reacted wires. Figure A displays the unreacted $105\mu \text{m}$ diameter W wire diameter with $6.6–6.8\mu \text{m}$ Si layer. Figure B shows the reacted wire sectioned in between droplets. This section consists of a $96.8\mu \text{m}$ W core with a ${\text{WSi}}_2$ product of thickness $\sim 5.4\mu \text{m}$. Figure C shows the reacted wire sectioned at a droplet. The product layer in this section is a thicker layer of ${\text{W}}_5{\text{Si}}_3$ measuring $25.7–31.1\mu \text{m}$. Figure D shows the droplets as they solidified along the wire with the distance between them noted.

Figure 2

Depiction of the x-ray imaging setup.

Figure 3

Plot of heat release rate, $q$, and wire temperature, $T$ versus time. The data shows that the Si melt occurs over a period of $\sim 3\text{ms}$. The reaction is seen to conclude within $\sim 10\text{ms}$ of the onset of heating. The inset images show the evolution of the W-Si reaction with their temporal location indicated by arrows. Melt nucleation sites are distinguished by arrows in figure B. These sites grow, coalesce, and disperse evenly along the periphery of the wire. As the reaction progresses, the bright fringe, seen at the edge of the wire in figure A, decreases in intensity.

Figure 4

Intensity profile in the radial direction for a W-Si composite wire undergoing reaction, where $R$ is the wire’s outer radius. The depletion in fringe intensity with time is related to the progress of reaction in the wire.

Figure 5

Growth of droplet due to formation of ${\text{W}}_5{\text{Si}}_3$ product. Image A is taken at time $t=100.053\text{ms}$ after onset of heating. Images B, C, and D are taken at $t=105.742$, 111.486, and 117.256 ms, respectively.

Figure 6

Intensity plots taken in the radial direction in (A) a large droplet and (B) the narrow wire, as seen in the inset. By adjusting the material properties to account for different chemical compositions, the appropriate material can be selected by selecting the best fitting modeled profile. In figure (B), the results for the modeling of $\text{W}+{\text{WSi}}_2$ and $\text{W}+{\text{W}}_5{\text{Si}}_3$ are almost indistinguishable since the resolution of the optical system is larger than the product layer that is being modeled.