Small but strong: A review of the mechanical properties of carbon nanotube–polymer composites

There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin-mountable, and wearable strain sensors are needed for several potential applications including personalized health-monitoring, human motion detection, human-machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin-mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.

Stretchable, Skin-Mountable, and Wearable Strain Sensors and Their Potential Applications: A Review

The demand for flexible and wearable electronic devices is increasing due to their facile interaction with human body. Flexible, stretchable and wearable sensors can be easily mounted on clothing or directly attached onto the body. Especially, highly stretchable and sensitive strain sensors are needed for the human motion detection. Here, we report highly flexible, stretchable and sensitive strain sensors based on the nanocomposite of silver nanowire (AgNW) network and PDMS elastomer in the form of the sandwich structure (i.e., AgNW thin film embedded between two layers of PDMS). The AgNW network-elastomer nanocomposite based strain sensors show strong piezoresistivity with tunable gauge factors in the ranges of 2 to 14 and a high stretchability up to 70%. We demonstrate the applicability of our high performance strain sensors by fabricating a glove integrated with five strain sensors for the motion detection of fingers and control of an avatar in the virtual environment.

Highly Stretchable and Sensitive Strain Sensor Based on Silver Nanowire–Elastomer Nanocomposite

Owing to their unique mechanical properties, carbon nanotubes are considered to be ideal candidates for polymer reinforcement. However, a large amount of work must be done in order to realize their full potential. Effective processing of nanotubes and polymers to fabricate new ultra-strong composite materials is still a great challenge. This Review explores the progress that has already been made in the area of mechanical reinforcement of polymers using carbon nanotubes. First, the mechanical properties of carbon nanotubes and the system requirements to maximize reinforcement are discussed. Then, main methods described in the literature to produce and process polymer–nanotube composites are considered and analyzed. After that, mechanical properties of various nanotube–polymer composites prepared by different techniques are critically analyzed and compared. Finally, remaining problems, the achievements so far, and the research that needs to be done in the future are discussed.

Mechanical Reinforcement of Polymers Using Carbon Nanotubes

This overview highlights very recent achievements and new trends in one of the most active and developing fields in modern materials science: the production of bulk ultrafine-grained (UFG) materials using severe plastic deformation (SPD). The article also summarizes the chronology of early work in SPD processing and presents clear and definitive descriptions of the terminology currently in use in this research area. Special attention is given to the principles of the various SPD processing techniques as well as the major structural features and unique properties of bulk UFG materials that underlie their prospects for widespread practical utilization.

https://link.springer.com/content/pdf/10.1007%2Fs11837-006-0213-7.pdf

Producing bulk ultrafine-grained materials by severe plastic deformation

The hardness and elastic modulus of a silver nanowire was measured using a nanoindenter. It was found that the silver nanowire has comparable hardness and elastic modulus to bulk silver. An array of nanoscale indents was successfully made on the wire by directly indenting the wire. The shape and size of the indents are controllable. The nanoindentation approach permits the direct machining of nanowires without the complications of conventional lithography.

Nanoindentation of Silver Nanowires

Nanomechanical properties of single-walled carbon nanotube (SWNT) reinforced epoxy composites with varying weight percentage (0, 1, 3, and 5 wt%) of nanotubes were measured by nanoindentation and nanoscratch techniques. Hardness and elastic modulus were measured using a nanoindenter. Scratch resistance and scratch damage were studied using the AFM tip sliding against the SWNT reinforced sample surfaces. Nanoindentation/nanoscratch deformation and fracture behaviour was studied by in situ imaging of the indentation impressions/scratch tracks. Viscoelastic properties of the nanocomposites were measured using nanoindentation dynamic mechanical analysis tests. The reinforcing mechanisms are discussed with reference to the nanotube dispersion, interfacial bonding, and load transfer in the SWNT reinforced polymer composites.

http://iopscience.iop.org/article/10.1088/0957-4484/15/11/005/pdf

Nanomechanical Characterization of Single-Walled Carbon Nanotube Reinforced Epoxy Composites

Deformation-induced grain growth has been reported in nanocrystalline (nc) materials under indentation and severe cyclic loading, but not under any other deformation mode. This raises an issue on critical conditions for grain growth in nc materials. This study investigates deformation-induced grain growth in electrodeposited nc Ni during high-pressure torsion (HPT). Our results indicate that high stress and severe plastic deformation are required for inducing grain growth, and the upper limit of grain size is determined by the deformation mode and parameters. Also, texture evolution suggests that grain-boundary-mediated mechanisms played a significant role in accommodating HPT strain.

/pdf/high-pressure-torsion-induced-grain-growth-in-194sxs9qz9.pdf

High-pressure torsion-induced grain growth in electrodeposited nanocrystalline Ni

Nanoindentation tests were performed directly on solid and hollow cuprous oxide (Cu2O) nanocubes. The hardness and elastic modulus of solid Cu2O nanocubes were measured and compared with the values of bulk Cu2O. It is found that the hollow cube top wall acts as a membrane that bends under an indentation load. The Cu2O nanocubes are more ductile rather than brittle. Deformation behavior and fracture mechanics are discussed in conjunction with the structure of the Cu2O nanocube.

Nanoindentation of Cu2O Nanocubes

Amorphous SiO2 nanowires with diameter ranging from 50 to 100 nm were synthesized using chemical vapor deposition (CVD) under an argon atmosphere at atmospheric pressure. Nanoscale three-point bending tests were performed directly on individual amorphous SiO2 nanowires using an atomic force microscope (AFM). Elastic modulus of the amorphous SiO2 nanowires was measured to be 76.6±7.2GPa, which is close to the reported value of the bulk SiO2 and thermally grown SiO2 thin films, but lower than that of plasma-enhanced CVD SiO2 thin films. The amorphous SiO2 nanowires exhibit brittle fracture failure in bending.

/pdf/elastic-modulus-of-amorphous-sio2-nanowires-1w53x3p5ks.pdf

Hongsheng Gao

Papers

Nanoindentation of Silver Nanowires

Nanomechanical Characterization of Single-Walled Carbon Nanotube Reinforced Epoxy Composites

High-pressure torsion-induced grain growth in electrodeposited nanocrystalline Ni

Nanoindentation of Cu2O Nanocubes

Elastic modulus of amorphous SiO2 nanowires