Failure of elastic‐plastic core–shell microcapsules under compression

Abstract

Characterization of the failure behavior of microcapsules is extremely important to control the release of their core actives by mechanical forces. The strain and stress of elastic-plastic uninflated core-shell microcapsules at failure (rupture or bursting) has been determined using finite element modeling (FEM) and micromanipulation compression experiments. The ductile failure of polymeric microcapsules at high deformations is considered to occur when the maximum strain in the shell exceeds a critical strain, resulting in their rupture. FEM has been used to determine the maximum strains present in the capsule wall at different deformations for three types of shell material: elastic, elastic-perfectly plastic and elasticperfectly plastic with strain hardening at large strains. The results obtained were used to determine the failure strain and stress of melamine-formaldehyde microcapsules, with average population values of $0.48 and $350 MPa, respectively. Thus, the elastic-plastic stress-strain relationship has been determined for the core-shell microcapsules tested. Figure 3. Log-linear plot of maximum von Mises stress (closed symbols) and strain H max (open symbols) in a fully elastic microcapsule (h/r 5 0.62%) vs. fractional deformation.Results from Smith et al. 17 are for inflated core-shell spheres with different initial stretch ratios (SR), shown for comparison.Continuous lines represent H max , the largest of the three plane strains at each fractional deformation. H max for the fully elastic scenario is shown for comparison. Inset shows the position of the most strained nodes as described in Figure 4b.Points are the results for the conditions of Figure 11, with the following best-fitted equation: F/r Y rh ¼ 1.945 þ 1.12 Â 10 À15 exp(48.5) þ 0.0143exp(10.29) for 0.3 \ \ 0.75.

Fe‐Beta@CeO2 core‐shell catalyst with tunable shell thickness for selective catalytic reduction of NOx with NH3

“…55,56 The adsorbed NH 3 species on Brønsted acid site disappear completely at 3008C. However, the bands at 1303 and 1171 cm 21 are still detected even at 5008C. Compared with Fe-Beta, the intensities of the peaks over Fe-Beta@CeO 2 -1 ( Figure 11B) at corresponding temperature are lower, which should be favorable for proceeding of NH 3 -SCR process.…”

“…With the increasing of temperature, almost all bands disappear as temperature reaches to 3008C. However, it worth noting that the intensities of the bands due to bridging monodentate nitrate (1571 cm 21 ) and monodentate nitrate (1540 cm 21 ) are still strong even at 3008C. It is well known that nitrate is an inactive species in NH 3 -SCR process, which can deposit on the surface of catalyst and block active sites, resulting in the decrease of NH 3 -SCR activity.…”

“…NO band assigned to NO x species is detected in the whole region. For Fe-Beta ( Figure 11A), with the elevating of temperature, the intensities of the bands attributed to NH 1 4 species on Brønsted acid sites (1475 cm 21 ) decrease more than those of the bands corresponding to coordinated NH 3 on Lewis acid sites (1303 and 1171 cm 21 ). 55,56 The adsorbed NH 3 species on Brønsted acid site disappear completely at 3008C.…”

“…In the case of Fe-Beta ( Figure 10A), several distinct bands are detected at 1633, 1603, and 1576 cm 21 at room temperature, which are ascribed to adsorbed bridging bidentate nitrate, asymmetric frequency of gaseous NO 2 molecules and bidentate nitrate, respectively. [47][48][49] With the increasing of temperature, the intensities of these bands decrease quickly and almost disappear completely at 2008C.…”

“…[17][18][19][20][21] In general, they could bring unique collective and synergetic functions, endowing the CSCs with superior properties than either core or shell material alone. For example, Cargnello et al 22 developed Pd@CeO 2 /Al 2 O 3 CSCs for catalytic methane combustion, which exhibited better catalytic activity than either Pd/CeO 2 or Pd/CeO 2 /Al 2 O 3 due to the presence of interface effect between Pd and CeO 2 during the catalytic process.…”