Self-healing composites: A review

Self-healing composites are composite materials capable of automatic recovery when damaged. They are inspired by biological systems such as the human skin which are naturally able to heal themselves. This paper reviews work on self-healing composites with a focus on capsule-based and vascular healing systems. Complementing previous survey articles, the paper provides an updated overview of the various self-healing concepts proposed over the past 15 years, and a comparative analysis of healing mechanisms and fabrication techniques for building capsules and vascular networks. Based on the analysis, factors that influence healing performance are presented to reveal key barriers and potential research directions

A flexible and highly sensitive pressure sensor based on a PDMS foam coated with graphene nanoplatelets

The demand for high performance multifunctional wearable devices is more and more pushing towards the development of novel low-cost, soft and flexible sensors with high sensitivity. In the present work, we describe the fabrication process and the properties of new polydimethylsiloxane (PDMS) foams loaded with multilayer graphene nanoplatelets (MLGs) for application as high sensitive piezoresistive pressure sensors. The effective DC conductivity of the produced foams is measured as a function of MLG loading. The piezoresistive response of the MLG-PDMS foam-based sensor at different strain rates is assessed through quasi-static pressure tests. The results of the experimental investigations demonstrated that sensor loaded with 0.96 wt.% of MLGs is characterized by a highly repeatable pressure-dependent conductance after a few stabilization cycles and it is suitable for detecting compressive stresses as low as 10 kPa, with a sensitivity of 0.23 kPa−1, corresponding to an applied pressure of 70 kPa. Moreover, it is estimated that the sensor is able to detect pressure variations of ~1 Pa. Therefore, the new graphene-PDMS composite foam is a lightweight cost-effective material, suitable for sensing applications in the subtle or low and medium pressure ranges

Rapid Microwave Polymerization of Porous Nanocomposites with Piezoresistive Sensing Function

In this paper, polydimethylsiloxane (PDMS) and multi-walled carbon nanotube (MWCNT) nanocomposites with piezoresistive sensing function were fabricated using microwave irradiation. The effects of precuring time on the mechanical and electrical properties of nanocomposites were investigated. The increased viscosity and possible nanofiller re-agglomeration during the precuring process caused decreased microwave absorption, resulting in extended curing times, and decreased porosity and electrical conductivity in the cured nanocomposites. The porosity generated during the microwave-curing process was investigated with a scanning electron microscope (SEM) and density measurements. Increased loadings of MWCNTs resulted in shortened curing times and an increased number of small well-dispersed closed-cell pores. The mechanical properties of the synthesized nanocomposites including stress–strain behaviors and Young’s Modulus were examined. Experimental results demonstrated that the synthesized nanocomposites with 2.5 wt. % MWCNTs achieved the highest piezoresistive sensitivity with an average gauge factor of 7.9 at 10% applied strain. The piezoresistive responses of these nanocomposites were characterized under compressive loads at various maximum strains, loading rates, and under viscoelastic stress relaxation conditions. The 2.5 wt. % nanocomposite was successfully used in an application as a skin-attachable compression sensor for human motion detection including squeezing a golf ball.This research received no external funding and The APC was funded by University Libraries Open Access fund. Open Access fees paid for in whole or in part by the University of Oklahoma Libraries.Ye

Electrical conductivity of carbon nanofiber reinforced resins: potentiality of Tunneling Atomic Force Microscopy (TUNA) technique

Epoxy nanocomposites able to meet pressing industrial requirements in the field of structural material have been developed and characterized. Tunneling Atomic Force Microscopy (TUNA), which is able to detect ultra-low currents ranging from 80 fA to 120 pA, was used to correlate the local topography with electrical properties of tetraglycidyl methylene dianiline (TGMDA) epoxy nanocomposites at low concentration of carbon nanofibers (CNFs) ranging from 0.05% up to 2% by wt. The results show the unique capability of TUNA technique in identifying conductive pathways in CNF/resins even without modifying the morphology with usual treatments employed to create electrical contacts to the ground

Multifunctional composite interphase

In this work, carbon nanotubes were deposited onto the insulative glass fibre surface to form a semiconductive network. Utilizing the unique properties of CNTs network, a multifunctional composite interphase could be achieved. The interfacial adhesion strength was improved by CNTs distributed in the interphase. The semiconductive interphase have been used as a chemical/phaysical sensor, strain sensor and microswitch

Fabrication of low electrical percolation threshold multi-walled carbon nanotube sensors using magnetic patterning

Soft robotics is an expanding area with multiple applications; however, building low-cost, soft, and flexible robots requires the development of sensors that can be directly integrated into the soft robotics fabrication process. Thus, the motivation for this work was the design of a low-cost fabrication process of flexible sensors that can detect touch and deformation. The fabrication process proposed uses a flexible polymer nanocomposite with permanent magnets strategically placed where the conductive electrodes should be. The nanocomposite is based on poly(dimethylsiloxane) (PDMS) and multi-walled carbon nanotubes (MWCNTs). The MWCNT contains ferromagnetic impurities remaining from the synthesis process, which can be used for magnetic manipulation. Several electrode geometries were successfully simulated and tested. The magnetic patterning was simulated, allowing the fabrication of conductive patterns within the composite. This fabrication process allowed the reduction of the electrical resistivity of the nanocomposites as compared to the composites with homogeneous MWCNT dispersion. It also allowed the fabrication of piezoresistive and triboelectric sensors at MWCNT concentration as low as 0.5 wt.%. The fabrication process proposed is flexible, allows the development of sensors for soft robotics, as well as monitoring large and unconventional areas, and may be adapted to different mould shapes and polymers at low cost.This research is part of the PhD project at the Doctoral Program in Advanced Materials and Processing—FEUP. We would like to thank CeNTI for providing resources (labs, equipment and consumables) to perform the fabrication and characterisation of the samples. The authors thank CEMUP for expert assistance (Rui Rocha) with SEM-EDS. IPC acknowledges the support of FCT through National Funds References UIDB/05256/2020 and UIDP/05256/2020

Flexible Temperature Sensor Array Based on a Graphite-Polydimethylsiloxane Composite

This paper presents a novel method to fabricate temperature sensor arrays by dispensing a graphite-polydimethylsiloxane composite on flexible polyimide films. The fabricated temperature sensor array has 64 sensing cells in a 4 × 4 cm2 area. The sensor array can be used as humanoid artificial skin for sensation system of robots. Interdigitated copper electrodes were patterned on the flexible polyimide substrate for determining the resistivity change of the composites subjected to ambient temperature variations. Polydimethylsiloxane was used as the matrix. Composites of different graphite volume fractions for large dynamic range from 30 °C to 110 °C have been investigated. Our experiments showed that graphite powder provided the composite high temperature sensitivity. The fabricated temperature sensor array has been tested. The detected temperature contours are in good agreement with the shapes and magnitudes of different heat sources

Design, development and characterisation of piezoresistive and capacitive polymeric pressure sensors for use in compression hosiery

The work in this thesis was focused in developing a flexible and cost-effective pressure sensor capable of detecting pressure variations within the low working range (0-6kPa) of compression hosiery. For this cause, both piezoresistive and capacitive pressure sensors were developed and characterised, utilising conductive and non-conductive polymeric elements to sense compressive loads. In the first case, the developed piezoresistive sensor is composed of a conductive filler - polymer composite, with a force-dependent conductivity, encapsulated in between a structured and unstructured configuration of electrodes. Initially, as the sensing element of the sensor a multi-walled carbon nanotubes-polydimethylsiloxane (MWCNT-PDMS) composite was tested. A fabrication process is also proposed for developing the MWCNT-PDMS composite which involves a series of successive direct ultrasonications and shear mixing in order to disperse the two constituents of the composite, with the use of an organic solvent. Developing the composite over a range of different filler concentrations revealed a sharp step-like conductivity behaviour, typical amongst percolating composites. The MWCNT-PDMS sensor exhibited a positive piezoresistive response when subjected to compression, which was substantially enhanced when structured electrode layers were utilised. A Quantum Tunnelling Composite (QTC) material was also tested as the sensing material, which displays a large negative piezoresistive response when deformed. The QTC pressure sensor exhibited an improved performance, which was similarly significantly increased when a structured electrode was employed. In the second case, a parallel-plate capacitive pressure sensor was developed and characterised, which successfully provided a pressure sensitivity within the working range of compression hosiery. The sensor employs an ultra-thin PDMS blend film, with tuneable Young’s modulus, as the dielectric medium of the capacitor, bonded in between two rigid copper-coated glass layers. A casting process is also presented, involving the use of a sacrificial mould, in order to pattern the polymeric film with a micro-pillar structure to assist the deformation of the medium under compressive loads. The performance of the sensor with regards to the polymeric film thickness, structure and mechanical softness was explored. Overall, the combination of an ultra-thin dielectric medium with a very low Young’s modulus and a microstructured surface resulted in a capacitive pressure sensor with a good performance within the desired pressure regime

Tooling design and microwave curing technologies for the manufacturing of fiber-reinforced polymer composites in aerospace applications

The increasing demand for high-performance and quality polymer composite materials has led to international research effort on pursuing advanced tooling design and new processing technologies to satisfy the highly specialized requirements of composite components used in the aerospace industry. This paper reports the problems in the fabrication of advanced composite materials identified through literature survey, and an investigation carried out by the authors about the composite manufacturing status in China’s aerospace industry. Current tooling design technologies use tooling materials which cannot match the thermal expansion coefficient of composite parts, and hardly consider the calibration of tooling surface. Current autoclave curing technologies cannot ensure high accuracy of large composite materials because of the wide range of temperature gradients and long curing cycles. It has been identified that microwave curing has the potential to solve those problems. The proposed technologies for the manufacturing of fiber-reinforced polymer composite materials include the design of tooling using anisotropy composite materials with characteristics for compensating part deformation during forming process, and vacuum-pressure microwave curing technology. Those technologies are mainly for ensuring the high accuracy of anisotropic composite parts in aerospace applications with large size (both in length and thickness) and complex shapes. Experiments have been carried out in this on-going research project and the results have been verified with engineering applications in one of the project collaborating companies