Functional Porous Materials

Modern technology demands new materials with superior properties and novel functionalities. Porous materials have great potential in applications with functionalities, such as energy absorption, sound absorption, thermal insulation, catalysis, molecular separation, and emerging energy and health monitoring technologies. Conventional porous materials have been studied for many decades. In recent years, they have also emerged as novel functional materials with unique properties. This dissertation focused on both three-dimensional (3D) and two-dimensional (2D) porous materials as functional materials. New functional porous materials will focus on the function-led design of the porous structures, including auxetic materials, flexible porous sensors, and flexible porous thermal protection materials. Also, the raw materials of porous structures are another critical parameter to affect the performance of foam materials. The methods of fabricating different kinds of porous materials are described later in detail. The experimental and modeling results focus on the mechanics of the porous structures, tunable mechanical properties, and functional performance, including extraordinary energy absorption capabilities, ultralight thermal insulation capabilities, and flexible sensing properties. Applications in industrial areas and future prospects were also discussed. Followed by a brief introduction (Chapter One) and literature review (Chapter Two), 3D porous materials were explored as novel pseudo-piezoelectric materials in Chapter Three. A low-cost fabrication method was developed to produce hybrid cyclic olefin copolymer (COC) / polypropylene (PP) piezoelectric foams. The hybrid piezoelectric foam was shown to maintain excellent piezoelectric properties. At the same time, the hybrid piezoelectric foams also exhibited markedly improved thermal stability. The mechanics of the hybrid foam was studied and discussed in this chapter. The hybrid piezoelectret combines the advantages of both components, namely, COC's superior charge-storage properties and PP's good flexibility and low cost. In Chapter Four, a new class of 3D hybrid porous materials, auxetic foams, has been designed and fabricated. Here, hybrid auxetic foams with buckling porous architectures were made with polyurethane and phenolic resins. The hybrid foams combine extraordinary mechanical properties, e.g., substantially improved strength, modulus, and energy dissipation. The hybrid foams exhibited a unique near-zero Poisson's ratio behavior. The hybrid foams have superior functions due to the dual-material 3D systems that show high stiffness of the phenolic resin and flexibility of polyurethane-based auxetic porous foams. Finite element analysis (FEA) showed that the near-zero Poisson's ratio behavior resulted from the mismatch of the mechanical properties of the two constituent polymers and the change of the deformation mechanism. Carbon nanotubes (CNT) are advanced materials with extraordinary mechanical, thermal, and electrical performance. In this dissertation study, 2D CNTs porous networks are explored as force sensors (Chapter Five) and thermal protection materials (Chapter Six). Chapter Five describes a flexible piezoresistive strain sensor integrated by carbon fiber and multi-walled carbon nanotubes (MWCNTs) porous thin films. Flexible sensors with tunable Poisson's ratio were designed and validated through different carbon fiber stacking sequences. By varying the fiber layup, flexible carbon fiber composites showed a Negative Poisson's ratio (NPR). The result shows that the negative Poisson's ratio of substrate composites enhances the sensor's sensitivity. The hybrid composite - CNT sensor shows a linear signal response, and the response is highly repeatable and reproducible with excellent sensor performance. This work provides valuable guidelines to design the negative Poisson's ratio of carbon fiber composites. It demonstrates that flexible sensors with negative Poisson's ratio have great potential for use in pressure sensing, health monitoring, and even human-computer interaction. The CNT porous network also exhibits extraordinary thermal conductivity. Chapter Six discusses a systematic design of a porous CNT/phenolic thermal protection layer (TPL) with heat shield functionality while maintaining designed mechanical strength compared to traditional CFRP composites. These TPLs were integrated onto the surface of carbon fiber/bismaleimide composites. Hybrid composites with different TPL volume fractions were produced. The porous CNT network resulted in a 17% decrease in the through-thickness thermal conductivity for hybrid composites. A flame torch test was used to evaluate the thermal shield effectiveness. The hybrid composites showed a noticeable increase in residual flexural strength and modulus compared to the control samples under a heat flux of 20 W/cm2. It also discusses the effects of porous CNT networks' coefficient of thermal expansion and protection mechanisms.