No abstract

Microcapsules are important for the protection, transport, and delivery of cargo in a variety of fields but are often too weak to withstand the high mechanical stresses that arise during the preparation and formulation of products. Although thick-shell strong capsules have been developed to circumvent this issue, the microfluidic or multistep methods utilized thus far limit the ease of fabrication and encapsulation throughput. Here, we exploit the phase separation of ternary liquid mixtures to achieve a high-throughput fabrication of strong bilayer microcapsules using a one-step bulk emulsification process. Phase separation is induced by the diffusion of water from the continuous phase into droplets that initially contain a mixture of monomers, cross-linkers, an initiator, and cosolvent γ-butyrolactone. The double emulsions generated via such a phase separation are converted into microcapsules through a polymerization reaction triggered by UV illumination. Surprisingly, the shells of the consolidated capsules exhibit a hard-soft bilayer structure that can be designed to show a resilient eggshell-like fracture behavior. Our method allows for the production of large volumes of microcapsules with such a strong bilayer shell within a time scale of only a few minutes, thus offering an enticing pathway toward the high-throughput fabrication of mechanically robust encapsulation systems.

in polymers [1,2,[10][11][12] and hydrogels, [17,18] relies on functional groups with strategic room temperature reactivity to create new bonds at the damage site, healing processes in metals usually demand high temperature to promote mass transport [19][20][21] or a phase transformation [22][23][24][25][26] that lead to damage repair. As remarked by Tasan and Grabowski, [27] because of the challenging energy requirements of metals, the design of a healing process where the energy required to promote healing is provided at room temperature is nontrivial. Accordingly, previous work on healing of metals focused on the use of furnace treatments to promote damage recovery: van der Zwaag et al. [19][20][21] successfully demonstrated that controlled heat treatments represent an efficient mean to initiate and promote self-healing behavior in different alloys. Annealingbased healing concepts, however, are incompatible with materials in temperature-sensitive systems. For instance, the prolonged exposure to high temperature (500-800 °C, ≈hours) is detrimental to most thin film-based devices or electronic circuits.Here, we present a concept for on-chip healing of metal films based on the use of reactive multilayers as integrated heat sources. Similar to chemical reactions in liquid media, reactions in solids can be either endothermic (requiring heat) or exothermic (releasing heat). In Ni/Al nanostructured reactive multilayers, the constituents are characterized by a strongly exothermic intermetallic-forming reaction that, when ignited by external stimuli such as a mechanical impact or an electric pulse, generates temperatures as high as 1500 °C. Through the design of reactive multilayers as integrated heat sources, we show that the heat produced by the reaction can be utilized to repair defects in metal thin films with melting point up to 1090 °C. Healing is based on the promotion of crack welding in a strongly confined volume at the crack site. The process can be initiated on-demand at room temperature by a low energy current pulse that is compatible with standard current-voltage operation conditions of microelectronics circuitry.Our concept is tested in a model system that is produced by magnetron sputtering and consisting of two main parts: the heat source and the "to-be-healed" metal layer (Figure 1a). The heat source is a Ni/Al multilayer with 50 nm bilayer thickness (Figure 1b). The metal layer is a 100 nm thick film of either Au or Cu. Upon activation by a current pulse, [28] the heat source Self-healing behavior, the ability to autonomously counteract damage, is observed in some inorganic materials, and it has recently been extended to various artificial systems. In metals, healing usually requires thermal activation by a furnace treatment that stimulates damage repair. High temperature exposure, however, renders these routes incompatible with temperature-sensitive systems such as on-chip microelectronic components. In this work, designing Ni/Al multilayers as on-chip heat sources, a concept for on-demand healing of...

Thermal management is conventionally the design of microelectronics circuitry to maximize heat extraction and minimize local heating. In this work, we investigate a reverse thermal management problem related to understanding and preventing heat dissipation during the propagation of a self-sustained reaction in Ni/Al reactive multilayers, metastable nanostructures that can release heat through a self-sustained propagating exothermic reaction. While it was recently demonstrated that reactive multilayers can serve as on-chip heat sources for on-demand healing of metal films, they still face challenges of device integration due to conductive heat losses to the substrate or adjacent on-chip components, which act as heat sinks and consequently quench the reaction. Here, we study the impact of different heat sink materials, such as gold, copper, and silicon, on the propagation velocity and temperature of the self-sustained heat wave and show that the propagation can be controlled and even stopped by varying the heat sink thickness. Further, we demonstrate that the introduction of a multilayered Al2O3/Zr/Al2O3 thermal barrier enables stable propagation on substrates that would otherwise quench the reaction. The results of this study will facilitate the integration of Ni/Al multilayers as intrinsic heat sources on different substrates for applications in micro/nanodevices.

Metallic reactive multilayers are known as high energy-density storage systems. Conventionally, these multilayers are tailored for high reaction rates with the purpose to achieve high maximum reaction temperatures and explosive-like behavior upon mixing. However, in some instances such as neutralization of biological hazards or chemical energy-storage systems, a low heat flow rate is desired. In the present work, we show that Nb-alloying presents an efficient approach to stabilize the as-deposited state and to form a diffusion barrier in situ, effectively reducing the heat flow rate by more than 50%. The validation of the concept is carried out by a comparative study of thermally induced phase reactions in Ni/Al and (Nb-Ni)/Al reactive multilayers. Kinetics of the phase reactions in these systems were followed by differential scanning calorimetry, analytical scanning transmission electron microscopy, and in situ electron diffraction analysis. The results confirm alloying as a design strategy for tailoring reaction kinetics of reactive multilayers.