Hybrid Metallic Foam with Superior Elasticity, High Electrical Conductivity, and Pressure Sensitivity

Abstract

Growing interest has been received in metallic foams for their combined features of metals and porous structures. Coating metals on polymers have been the most prevalent method to fabricate hybrid metallic foams to inherit both the merits of metals and the mechanical flexibility of polymers. However, direct coating metals on foams is challenging and requires tedious synthesis, such as electrolysis and chemical reduction. This work reported a facile strategy to build hybrid metallic foams via in situ foaming of liquid metals (LM) and polyurethane. The fluidity and incompatibility of LM with porous polyurethane allow the coating of LM on polymers. LM-Foams exhibit high electrical conductivity (3.9 × 104 S/m), low density (ρ < 1 g/cm3), phenomenal elasticity (recover at 95% strain), and excellent mechanical stability (stable with 1000 compressive cycles). Interestingly, the ease of deformation for fluidic fillers in elastic polyurethane generates additional resistive change under pressure, showing unique sensory behaviors which were not observed in conventional conductive foams, such as high response sensitivity (gauge factor >25), short response time (202 ms), and outstanding electrical stability. The nonuniform size distribution of pores leads LM-Foams to show unusual position-dependent sensitivity, enabling advanced applications as password pads and electrical protection foams.

Preparation and application of gallium-based conductive materials in the very recent years

“…Through changing the rheological and any other property of LM, LM-based composites have been prepared to benefit for printing (Figure 2(e-h)). Generally, combined with LM in printing, other rigid metals [44,45] and inorganics [20,46,47] are usually used. Thus the composites can be fixed by direct writing, printing, roller-assisted or spraying-assisted screen printing.…”

“…While during the recovery process, acid or alkali solution was used, which is still not so safe. As known that the rheological properties change with surface tension, and surface tension change with the formation of surface oxide layer, which affecting LM stick to some substrate materials such as PVC, cloth, and paper [30,46,47]. For example, to reduce the surface tension, directly, Ou et al [27] wrote partially oxidized liquid metals (POLMs) on cloth material, filling POLMs into fiber networks to form wearable, stretchable, and flexible electrically conductive elements.…”

“…To make LM circuit more precise and smaller in size, low density LM composites are explored. Among that, Peng et al [47] built a hybrid metallic foam (Figure 3(a ), excellent conformability and electric conductivity. And its stiffness varies under temperature regulation.…”

“…Figure3(Color online) (a) Schematic illustration of the in situ formation of LM-Foam-x. Reproduced with permission from ref [47]…”

Tailorable, Lightweight and Superelastic Liquid Metal Monoliths for Multifunctional Electromagnetic Interference Shielding

“…Construction of LM-based conductive network at low filler contents along with highly self-standing, reliable, and reproducible flexible composite has great significance to extend practical applications of LM. Foaming is an effective method to construct conductive networks and reduce the content of fillers, including freeze-drying [ 22 – 24 ], supercritical carbon dioxide foaming [ 25 , 26 ], chemical foaming [ 27 ], and sacrificial templates [ 3 , 28 ], Unfortunately, the majority of traditional foaming process is not suitable to make LM-based polymer matrix foam structure due to migration and poor compatibility. Therefore, it is still a great challenge to develop an efficient and feasible processing method for the preparation of macroscopically self-standing, reliable LM composites.…”