The Effect of Crosslinking on the Electro-Thermal Properties of Carbon Black and Ethylene-Vinyl Acetate Composites

Ethylene-vinyl acetate (EVA) and highly structured carbon black (CB) composites are promising for self-regulating heating materials, especially, were flexibility is needed. Two types of EVA-CB composites were made. Firstly, non-cross-linked EVA-CB composite was made with different CB concentrations and secondly, cross-linked EVA-CB composite with dicumylperoxide (DCP) as the cross-linking agent. Different concentrations of DCP were used. Samples were tested in two ways – by heating them internally (by applying 5V voltage) and externally (by heating in universal heating oven). The electro-thermal properties (including PTC effect) were determined.

Photothermocapillary Method for the Nondestructive Testing of Solid Materials and Thin Coatings

The photothermocapillary (PTC) effect is a deformation of the free surface of a thin liquid layer on a solid material that is caused by the dependence of the coefficient of surface tension on temperature. The PTC effect is highly sensitive to variations in the thermal conductivity of solids, and this is the basis for PTC techniques in the non-destructive testing of solid non-porous materials. These techniques analyze thermal conductivity and detect subsurface defects, evaluate the thickness of thin varnish-and-paint coatings (VPC), and detect air-filled voids between coatings and metal substrates. In this study, the PTC effect was excited by a “pumped” Helium-Neon laser, which provided the monochromatic light source that is required to produce optical interference patterns. The light of a small-diameter laser beam was reflected from a liquid surface, which was contoured by liquid capillary action and variations in the surface tension. A typical contour produces an interference pattern of concentric rings with a bright and wide outer ring. The minimal or maximal diameter of this pattern was designated as the PTC response. The PTC technique was evaluated to monitor the thickness of VPCs on thermally conductive solid materials. The same PTC technique has been used to measure the thickness of air-filled delaminations between a metal substrate and a coating.

Effect of Ionic Conductors on the Suppression of PTC and Carrier Emission of Semiconductive Composites

The positive temperature coefficient (PTC) effect of the semiconductive layers of high-voltage direct current (HVDC) cables is a key factor limiting its usage when the temperature exceeds 70 °C. The conductivity of the ionic conductor increases with the increase in temperature. Based on the characteristics of the ionic conductor, the PTC effect of the composite can be weakened by doping the ionic conductor into the semiconductive materials. Thus, in this paper, the PCT effects of electrical resistivity in perovskite La0.6Sr0.4CoO3 (LSC) particle-dispersed semiconductive composites are discussed based on experimental results from scanning electron microscopy (SEM), transmission electron microscopy (TEM) and a semiconductive resistance test device. Semiconductive composites with different LSC contents of 0.5 wt%, 1 wt%, 3 wt%, and 5 wt% were prepared by hot pressing crosslinking. The results show that the PTC effect is weakened due to the addition of LSC. At the same time, the injection of space charge in the insulating sample is characterized by the pulsed electroacoustic method (PEA) and the thermally stimulated current method (TSC), and the results show that when the content of LSC is 1 wt%, the injection of space charge in the insulating layer can be significantly reduced.

Synthesis of a Novel Semi-Conductive Composites Doping with La0.8Sr0.2MnO3 for Excellent Electric Performance for HVDC Cable

The semi-conductive layer located between the wire core and the insulation layer in high voltage direct current (HVDC) cable plays a vital role in uniform electric field and affecting space charges behaviors. In this work, the research idea of adding ionic conductive particles to semi-conductive materials to improve the conductive network and reduce the energy of the moving charge inside it and to suppress charge injection was proposed. Semi-conductive composites doped with different La0.8Sr0.2MnO3 (LSM) contents were prepared. Resistivity at different temperatures was measured to investigate the positive temperature coefficient (PTC) effect. Pulse electro-acoustic (PEA) method and thermal-stimulation depolarization currents (TSDC) tests of the insulation layers were carried out. From the results, space charge distribution and TSDC currents in the insulation samples were analyzed to evaluate the inhibitory effect on space charge injection. When LSM content is 6 wt. %, the experimental results show that the PTC effect of the specimen and charge injection are both being suppressed significantly. The maximum resistivity of it is decreased by 53.3% and the insulation sample has the smallest charge amount, 1.85 × 10−7 C under 10 kV/mm—decreased by 40%, 3.6 × 10−7 C under 20 kV/mm—decreased by 45%, and 6.42 × 10−7 C under 30 kV/mm—decreased by 26%. When the LSM content reaches 10 wt. %, the suppression effect on the PTC effect and the charge injection are both weakened, owing to the agglomeration of the conductive particles inside the composites which leads to the interface electric field distortion and results in charge injection enhancement.

Influence of (Bi0.5M0.5)TiO3 (M=Li, Na, K, Rb) Addition on the Curie Temperature and PTC Effect of BaTiO3-Based Ceramics

In order to investigate the influence of (Bi0.5M0.5)TiO3 (M=Li, Na, K, Rb) addition on the Curie temperature (Tc) and positive temperature coefficient (PTC) effect of BaTiO3-based ceramics, BaTiO3-(Bi0.5M0.5)TiO3 (M=Li, Na, K, Rb) solid solutions were prepared by a conventional solid sintering reaction using high-purity reagents. It was found that the Tc of the samples would vary with (Bi0.5M0.5)TiO3 of different alkali ions, in which BT-BKT ceramics had the highest value (about 148 °C). Moreover, the incorporation of alkali ions would influence the PTC effect of the sample, which can be defined by the resistivity jump with the ratio of maximum to minimum resistivity (ρmax/ρmin). Under the present conditions, the ρmax/ρmin of BT-BRT and BT-BLT ceramics were almost equal and higher than those of BT-BNT and BT-BKT ceramics.

Distinct positive temperature coefficient effect of polymer–carbon fiber composites evaluated in terms of polymer absorption on fiber surface

The positive temperature coefficient (PTC) effect for high-density polyethylene (HDPE)/carbon fiber (CF) composites was studied.

Tuneable PTC effect in polymer-wax-carbon composite resistors

Purpose – The purpose of this paper is to study tuneable positive temperature coefficient (PTC) effect in polymer-wax-carbon composite resistors. The resistivity dependence on temperature of composite resistors made of carbon fillers dispersed in an organic matrix is known to be strongly affected by the matrix thermal expansion. High PTC effects, i.e. essentially switching from resistive to quasi-insulating behaviour, can be caused by phase changes in the matrix and the assorted volume expansion, a behaviour that has been previously shown with both simple organic waxes and semi-crystalline polymers. However, waxes become very liquid on melting, possibly resulting in carbon sedimentation, and tuneability of semi-crystalline polymers is limited. Design/methodology/approach – The authors therefore study a ternary polymer-wax-conductor (ethylcellulose-octadecanol-graphite) composite resistor system, where polymer and wax fuse to a viscous liquid on heating, and re-solidify and separate by crystallisation of the wax on cooling. Findings – It is shown that with appropriate formulation, the resulting resistors exhibit strong PTC effects, linked with the melting and crystallisation of the wax component. The behaviour somewhat depends on sample history, and notably cooling speed. Research limitations/implications – The phase equilibria and transformation kinetics of the polymer-wax system (including possible wax polymorphism), as well as the exact mechanism of the conductivity transition, remain to be investigated. Originality/value – As many compatible polymer-wax systems with different melting/solidification behaviours are available, ternary polymer-wax-conductor composite PTC resistors allow a high tuneability of properties. Moreover, the high viscosity in the liquid state should largely avoid the sedimentation issues present with binary wax-conductor systems.

The Influence of Donor-Doped Content on PTC Effect of Ba-Excess BaTiO3 Based Ceramics Fired in Reducing Atmosphere

We investigated the influence of the Sm-doped contentration on the electrical properties and PTC effect of Ba-excess BaTiO3Based Ceramics, which were fired at 1300 °C for 30 min in a reducing atmosphere and then reoxidized at 850 °C for 1 h. The results showed that the donor dopant affected PTC characteristics and the electrical properties of the BSMT ceramics, whose room temperature resistivity first decreased and then increased with an increase in the Sm3+-doped content across the range from 0.1 to 0.5 mol%. The BSMT specimens exhibited a remarkable PTC effect, with a resistance jump greater by 2.7 orders of magnitude, along with a low room temperature resistivity of 128.6 Ω∙cm at the donor-doped content of 0.3 mol%. The influence of the donor dopant on the grain size of the as-fired samples has been also investigated.