We report on highly stretchable piezoelectric structures of electrospun PVDF-TrFE nanofibers. We fabricated nanofibrous PVDF-TrFE yarns via twisting their electrospun ribbons. Our results show that the twisting process not only increases the failure strain but also increases overall strength and toughness. The nanofibrous yarns achieved a remarkable energy to failure of up to 98 J/g. Through overtwisting process, we fabricated polymeric coils out of twisted yarns that stretched up to ∼740% strain. This enhancement in mechanical properties is likely induced by increased interactions between nanofibers, contributed by friction and van der Waals interactions, as well as favorable surface charge (Columbic) interactions as a result of piezoelectric effect, for which we present a theoretical model. The fabricated yarns and coils show great promise for applications in high-performance lightweight structural materials and superstretchable piezoelectric devices and flexible energy harvesting applications.

Smart textiles are envisioned to make a paradigm shift in wearable technologies to directly impart functionality into the fibers rather than integrating sensors and electronics onto conformal substrates or skin in wearable devices. Among smart materials, piezoelectric fabrics have not been widely reported, yet. Piezoelectric smart fabrics can be used for mechanical energy harvesting, for thermal energy harvesting through the pyroelectric effect, for ferroelectric applications, as pressure and force sensors, for motion detection, and for ultrasonic sensing. We report on mechanical and material properties of the plied nanofibrous piezoelectric yarns as a function of postprocessing conditions including thermal annealing and drawing (stretching). In addition, we used a continuous electrospinning setup to directly produce P(VDF-TrFE) nanofibers and convert them into twisted plied yarns, and demonstrated application of these plied yarns in woven piezoelectric fabrics. The results of this work can be an early step toward realization of piezoelectric smart fabrics.

With the ever-growing demand for renewable energy sources, energy harvesting from natural resources has gained much attention. Energy sources such as heat and mechanical motion could be easily harvested based on pyroelectric, thermoelectric, and piezoelectric effects. The energy harvested from otherwise wasted energy in the environment can be utilized in self-powered micro and nano devices, and wearable electronics, which required only µW-mW power. This article reviews pyroelectric energy harvesting with an emphasis on recent developments in pyroelectric energy harvesting and devices at micro/nanoscale. Recent developments are presented and future challenges and opportunities for more efficient materials and devices with higher energy conversion efficiency are also discussed.

Infiltration of a molten metal phase into a ceramic scaffold to manufacture metal−ceramic composites often involves high temperature, high pressure, and expensive processes. Low-cost processes for fabrication of metal−ceramic composites can substantially increase their applications in various industries. In this article, electroplating (electrodeposition) as a lowcost, room-temperature process is demonstrated for infiltration of metal (copper) into a lamellar ceramic (alumina) scaffold. Estimation shows that this is a low energy consumption process. Characterization of mechanical properties showed that metal infiltration enhanced the flexural modulus and strength by more than 50% and 140%, respectively, compared to the pure lamellar ceramic. More importantly, metal infiltration remarkably enhanced the crack initiation and crack growth resistance by more than 230% and 510% compared to the lamellar ceramic. The electrodeposition process for development of metal−ceramic composites can be extended to other metals and alloys that can be electrochemically deposited, as a low-cost and versatile process.

Hybrid composites of layered brittle-ductile constituents assembled in a brick-and-mortar architecture are promising for applications requiring high strength and toughness. Mostly, polymer mortars have been considered as the ductile layer in brick-and-mortar composites. However, low stiffness of polymers does not efficiently transfer the shear between hard ceramic bricks. Theoretical models point to metals as a more efficient mortar layer. However, infiltration of metals into ceramic scaffold is non-trivial, given the low wetting between metals and ceramics. The authors report on an alternative approach to fabricate brick-and-mortar ceramic-metal composites by using electroless plating of nickel (Ni) on alumina micro-platelets, in which Ni-coated microplatelets are subsequently aligned by a magnetic field, taking advantage of ferromagnetic properties of Ni. The assembled Ni-coated ceramic scaffold is then sintered using spark plasma sintering (SPS) to locally create Ni mortar layers between ceramic platelets, as well as to sinter the ceramic microplatelets. The authors report on materials and mechanical properties of the fabricated composite. The results show that this approach is promising toward development of bioinspired ceramic-metal composites.Many structural materials are quickly approaching their performance limits. [1] There is an unmet demand for damage-tolerant, lightweight bulk structural composites for a range of applications including transportation, infrastructure, and for applications requiring operation in extreme environments, such as high temperature and high pressure. Damage tolerance refers to the ability of a material to exhibit simultaneous strength and toughness to resist propagation of cracks.

Understanding the role of ductile polymer phase in mechanical behavior of bioinspired hybrid composites is an important step toward development of materials with damage tolerant properties. Herein, the authors report on fabrication and characterization of a bioinspired lamellar composite by incorporation of a semicrystalline polymer into a freeze casted scaffold. The elastic modulus and ductility of the polymer phase can be changed by more than three and 55 times, respectively, in addition to 42 folds decrease in modulus of toughness, by thermal annealing post-processing, after infiltration into the freeze casted ceramic scaffold. The results show that although polymer phase affects the fracture toughness and flexural behavior of the composite, the drastic changes in mechanical properties of the polymer phase has only marginal effect in the resulted properties of the composite. The authors use in situ SEM experiments and finite element simulation to investigate the deformation mechanism and the effect of the polymer phase on the distribution of stress in the fabricated composites.

Rewritable information record materials usually demand not only reversibly stimuli‐responsive ability, but also strong mechanical properties. To achieve one photochromic hydrogel with super‐strong mechanical strength, hydrophobic molecule spiropyran (SP) has been introduced into a copolymer based on ion‐hybrid crosslink. The hydrogels exhibit both photoinduced reversible color changes and excellent mechanical properties, i.e., the tensile stress of 3.22 MPa, work of tension of 12.8 MJ m−3, and modulus of elasticity of 8.6 MPa. Moreover, the SP‐based Ca2+ crosslinked hydrogels can be enhanced further when exposed to UV‐light via ionic interaction coordination between Ca2+, merocyanine (MC) with polar copolymer chain. In particular, hydrogels have excellent reversible conversion behavior, which can be used to realize repeatable writing of optical information. Thus, the novel design is demonstrated to support future applications in writing repeatable optical information, optical displays, information storage, artificial intelligence systems, and flexible wearable devices.

A metal-ceramic composite comprised of %82 vol% alumina (Al 2 O 3 ) and %18 vol% nickel (Ni) is fabricated via co-assembly of alumina micro-platelets with Ni particles using the freeze-casting process followed by the spark plasma sintering (SPS). The SPS processing with a custom-designed temperature-pressure history result in formation of elongated Ni phase between the lamellar-ceramic phase. Results of the mechanical characterization shows that inclusion of Ni improves the flexural strength of the composite by more than 47% compared to the lamellar ceramic. Additionally, the crack initiation (K IC ) and crack growth toughness increase by 20% and 47%, respectively. The inclusion of softer Ni phase does not compromise the indentation modulus and indentation hardness of the composite compared to the pure ceramic.Inclusion of a ductile metal phase into a ceramic matrix can improve its fracture toughness, in addition to other possible desirable properties such as electrical conductivity. [1][2][3] Such metal-ceramic composites have applications and market demand in various industries including automotive, aerospace, oil, and defense, in products such as high performance wearresistance parts, cutting tools, light-weight structural composites, and aero-engine components. [4,5] Metal inclusion can be either in the form of randomly distributed particles, layered (lamellar), or nature-inspired brickand-mortar architecture. Based on the desired structure, there are various processes for preparation of metal-ceramic composites. They include (but not limited to) stir-casting, tape-casting and tape-sintering, squeeze-casting, thermal spraying of multilayered ceramic-metal, hot pressing, freeze-casting, and spark-plasma sintering, among others. [1][2][3][5][6][7][8][9] In certain cases, a combination of these processes is used to fabricate the metal-ceramic composite. Often it is desired that the metal phase to have small volume percentage to achieve a good balance of strength and toughness in the composite. [10] This requirement imposes further processing challenges, in particular for lamellar and brick-and-mortar architecture, since the pore size to be filled with metal mortar is small.The overall toughening mechanism of metal inclusion into ceramic is believed to be a combination of several factors including (but not limited to) energy dissipation associated with the plastic deformation of the metal phase, crack tip blunting, deflection of the crack at the metal-ceramic interface, and reduction of stress concentration by distribution of the crack to a larger area. [1][2][3] In the case of lamellar and brick-and-mortar architecture, the ductile metal phase can also function similar to a lubricant, which relieves stress concentration by allowing for limited sliding. The ductile metal phase may also provide crack bridging as an extrinsic crack-tip shielding mechanism. [7,11] These mechanism overall are manifested in the form of a rising R-curve behavior.In several early works alumina-Ni composites have been reported, in which ...