Spinodal Decomposition Underlies Evolution of Nanoporosity in De-Alloying
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sufficient time period (t = 1800 s), two DWs approaching the defects from each side are effectively stationary. The line defects act as “chalk lines” to straighten the DWs in the magnetic recording layers. Only a few defects stabilize large areas of films. Good results were produced in field up to ~2 Hc . The researchers said that situations at higher fields (H ~ 32 Hc ) and speeds of current recording technology (1 Gbit s-1 ) need further studies. LI ZENG
Spinodal Decomposition Underlies Evolution of Nanoporosity in De-Alloying The selective dissolution of an alloy is a well-known de-alloying process with broad applications in the chemical industry. During the de-alloying process, the less noble elements of the alloy are dissolved while the more noble elements remain. After selective dissolution, a nanoporous sponge is formed by the remaining noble elements. Although the chemical phenomenon is well documented, the physical mechanism of selective dissolution has remained unclear. A team of researchers from Johns Hopkins, Harvard, Northeastern, and Arizona State universities have developed a scenario for the de-alloying process, reporting in the March 22 issue of Nature that the effect known as spinodal decomposition is responsible for the characteristic nanoporosity size. Jonah Erlebacher of Johns Hopkins and his research team used critical potential as an index to construct a simulation model. Basing their experiments on the dealloying of Ag-Au alloy, Erlebacher and his research team placed the alloys into an electrolyte and measured the critical potential of the alloys with different alloy compositions. After the dissolution, scanning electron micrographs showed a nanoporous sponge composed of gold on the surface of the alloy with ligament spacings of the order of 10 nm. The researchers proposed the following scenario. Initially, a single silver atom on a de-alloying surface is dissolved. Then, the nearby atoms are more easily dissolved because of the vacancy. But as a more noble element, the gold atoms on the surface do not dissolve. They agglomerate by diffusion into “islands.” At first, the tops of the “islands” are gold-rich while the bases are not de-alloyed material. As more and more silver atoms are dissolved, the bases of the islands become exposed to the electrolyte. “These mounds get undercut, increasing the surface area that gold must cover
to bring about passivation,” according to the researchers. So, by agglomerating together while keeping enough surface area, the gold atoms evolve into a nanoporous sponge. When the nanoporosity size is large, the sponge will grow slowly because of the time required for gold to diffuse. On the other hand, the sponge will also grow slowly if the nanoporosity size is small because, the researchers reported, “shortlength-scale fluctuations create much energetically unfavorable incipient interface between phases, inhibiting their growth.” This effect, known as spinodal decomposition, is the reason why the nanoporosity
can have a characteristic spacing. Based on their
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