Evaluation by tunneling effect for the temperature-dependent electric conductivity of polymer-carbon fiber composites with visco-elastic properties

The influence of DC voltage on the conductivity of chloroprene rubber-carbon black composites for flexible resistive heating elements

In order to acquire a flexible resistive heating element in the temperature range for human body heating, the influence of DC voltage on chloroprene rubber (CR) and carbon black (CB) composites has been investigated. Three conduction mechanisms have been found to occur in the range from 0.5 V to 10 V - charge velocity increase due to the increase of the electric field, matrix thermal expansion that results in decreased tunnelling currents and new electroconductive channel formation at voltages above 7.5 V, where the temperature exceeds the matrix's softening point. As opposed to external heating, during resistive heating, the composite exhibits a negative temperature coefficient of resistivity up to an applied voltage of 5 V. The intrinsic electro-chemical matrix properties play an important role in the overall resistivity of the composite. The material shows cyclical stability when repeatedly applying a voltage of 5 V and can be used as a human body heating element.

Changes on the Surface of the SiO2/C Composite, Leading to the Formation of Conductive Carbon Structures with Complex Nature of DC Conductivity

Sol–gel layers have been the subject of many studies in recent decades. However, very little information exists about layers in which carbon structures are developed in situ. Using the spin-coating method, we obtained thin iron-doped SiO₂/C composite films. The results of Raman spectroscopy showed that our samples consisted of graphitic forms and polymers. The latter’s contribution decreases with rising temperature. FTIR and EDS studies show changes in carbon distribution on top of the layer, depending on the sintering temperature. The samples sintered at 800 °C showed a significant increase in the contribution of carbon forms to the layer’s surface. Therefore, high conductivity can be observed in this sample. The results of XPS spectroscopy showed that the contribution of sp³ hybridized carbon increases after etching. The total electrical conductivity, studied by a DC four-wire technique, increased with the temperature and showed almost linear characteristics with significant changes below 150 K. The reduced activation energy plot has a positive temperature coefficient, which is a characteristic property of the conductive polymers in a metallic regime of conductivity.

Self-Healing Soft Sensors: From Material Design to Implementation

The demand for interfacing electronics in everyday life is rapidly accelerating, with an ever-growing number of applications in wearable electronics and electronic skins for robotics, prosthetics, and other purposes. Soft sensors that efficiently detect environmental or biological/physiological stimuli have been extensively studied due to their essential role in creating the necessary interfaces for these applications. Unfortunately, due to their natural softness, these sensors are highly sensitive to structural and mechanical damage. The integration of natural properties, such as self-healing, into these systems should improve their reliability, stability, and long-term performance. Recent studies on self-healing soft sensors for varying chemical and physical parameters are herein reviewed. In addition, contemporary studies on material design, device structure, and fabrication methods for sensing platforms are also discussed. Finally, the main challenges and future perspectives in this field are introduced, while focusing on the most promising examples and directions already reported.

Considerable different frequency dependence of dynamic tensile modulus between self-heating (Joule heat) and external heating for polymer--nickel-coated carbon fiber composites

Dynamic tensile moduli of polyethylene--nickel-coated carbon fiber (NiCF) composites with 10 and 4 vol % NiCF contents under electrical field were measured by a homemade instrument in the frequency range of 100--0.01 Hz. The drastic descent of the storage modulus of the composite with 10 vol % was verified in lower frequency range with elevating surface temperature (T(s)) by self-heating (Joule heat). The composite was cut when T(s) was beyond 108 °C. On the other hand, the measurement of the composite with 4 vol % beyond 88 °C was impossible, since T(s) did not elevate because of the disruption of current networks. Incidentally, the dynamic tensile moduli by external heating could be measured up to 130 and 115 °C for 10 and 4 vol %, respectively, but the two composites could be elongated beyond the above temperatures. Such different properties were analyzed in terms of crystal dispersions, electrical treeing, and thermal fluctuation-induced tunneling effect.

Appearance of perfect amorphous linear bulk polyethylene under applied electric field and the analysis by radial distribution function and direct tunneling effect

Without melting flow, linear ultrahigh molecular weight polyethylene (UHMWPE) provided X-ray intensity curve from only amorphous halo at 129.0 °C (surface temperature, Ts arisen by Joule heat) lower than the conventionally known melting point 145.5 °C on applying electric field to UHMWPE-nickel-coated carbon fiber (NiCF) composite. Such surprising phenomenon was analyzed by simultaneous measurements of X-ray intensity, electric current, and Ts as a function of time. The calculated radial distribution function revealed the amorphous structure with disordered chain arrangement. The appearance of such amorphous phase was arisen by the phenomenon that the transferring electrons between overlapped adjacent NiCFs by tunneling effect struck together with X-ray photons and some of the transferring electron flown out from the gap to UHMWPE matrix collided against carbon atoms of UHMWPE. The impact by the collision caused disordering chain arrangement in crystal grains.