Positive Temperature Coefficient Effect Research Articles

Cyclic loading effects on the heat-generation performance of carbon nanotube-embedded cement composites

Abstract This study investigates the heat-generation stability of carbon nanotube (CNT)/cement composites after exposing to cyclic loading conditions. The specimens were fabricated with varying CNT contents and levels of fly ash replacement. Results showed that increasing CNT content reduced electrical resistivity, while the impact on the electrical characteristics was found to be insubstantial, even though a considerable portion of fly ash was replaced. In addition, the electrical resistivity of the specimens after exposed to cyclic loading increased. Electrical heating tests revealed both negative and positive temperature coefficient effects depending on the applied voltages. Higher CNT contents improved the heat-generation capability, but the heating capability decreased after exposed to the cyclic loadings which is deduced from the damage of CNT networks during cyclic loadings. In this regard, the authors concluded that the heat-generation stability can be significantly affected by the applied loadings. Thus, the future research will be conducted to improve the heat-generation stability of the cement-based electrical heating systems as exposed to artificial deteriorations.

Temperature-responsive poly(3-decylpyrrole) endowing Li-ion batteries with thermal shutdown function and improved electrochemical performance

Exploring a simple way that can accurately respond to abnormal temperature increase within battery and then promptly shut down the corresponding thermally abused battery is crucial to improving the thermal safety of lithium-ion batteries (LIBs), yet still faces challenges. To counter this, a temperature-sensitive LCO-P3DPY cathode was constructed with a new conductive polymer, i.e., poly(3-decylpyrrole): poly(4-styrenesulfonic acid) (P3DPY: PSS). Safety tests demonstrated that P3DPY exhibited an exceptional positive temperature coefficient (PTC) effect at 100 °C, coinciding with the dangerous occurrence temperature (100 °C) of most LIBs. Consequently, the LCO-P3DPY cathode displayed a significant thermal shutdown in the immediate aftermath of thermal abuse within the battery, showing a capacity output of only 1.7 mAh g−1. Furthermore, LCO-P3DPY cathode not only exhibited excellent electrochemical stability in the battery environment, but also enhanced electrochemical performance, as compared with the traditional batteries at room temperature. After 200 charging-discharging cycles at 0.25C, LCO-P3DPY cathode was able to maintain a reversible capacity of 142.8 mAh g−1, corresponding to a capacity retention rate of 91.83 % that is better than that of blank LCO (62.86 %). This work provides a potential strategy to solve the thermal safety problem and simultaneously improve the electrochemical stability of LIBs.

Investigations on heat-generation stability and PTC effects of self-heating cement composites under diverse temperature conditions

In this study, the novel aspect lies in investigating heat-generation stability expressed as positive temperature coefficient (PTC) effects of self-heating cement under diverse temperature conditions. Self-heating cement composites were fabricated, and subjected to various types of heating tests including monotonic, cyclic, and long-term tests at three distinct temperatures: 25°C, 0°C, and −20°C. The results revealed a novel finding that the heat-generation performance decreased due to the increased electrical resistivity during the heating test, attributed to the PTC effect when high input voltage was applied. Conversely, the absence of PTC effect with low input voltage application resulted in a consistent temperature increase and negligible changes in electrical resistivity, particularly at low temperatures. These experimental findings provide valuable insights into the investigations on stability of heat-generation performance of self-heating cement composites, offering implications for their practical applications.

Simultaneous enhancement of thermal and dielectric properties of Ba0.8Sr0.2TiO3 and h‐BN co‐doped epoxy hybrid composites under high frequencies and temperatures

AbstractEpoxy resin (EP) is widely employed in power systems and electronic devices due to its low cost and easy processing along with good insulative and adhesive properties. However, EP insulation properties are compromised at elevated frequencies and temperatures because of its negative temperature coefficient effect (NTC) and poor thermal characteristics. In this context, barium strontium titanate (BST‐60), a positive temperature coefficient (PTC) filler, and hexagonal boron nitride (h‐BN), a thermally conductive filler, both surface‐modified with dopamine, are added in EP to simultaneously improve thermal and dielectric insulation properties at high temperatures and high frequencies. This study finds that below the Curie point of BST, the insulating properties of h‐BN fillers significantly contributes to improving insulation properties. Conversely, above the Curie point, the insulating nature of BST fillers leads to an improvement in the insulation performance of composites. Furthermore, at elevated frequency and temperature, the nano h‐BN enhances thermal conductivity of EP by establishing a thermally conductive network, while micro‐BST suppresses the NTC effect of EP by regulating electrical resistivity above the Curie point. The Heywang model elucidates the PTC effect of BST fillers, while a proposed heat conduction‐dissipation mechanism explains the behavior of incorporated fillers at different temperatures.Highlights PDA‐modified Ba0.8Sr0.2TiO3 and h‐BN co‐doped epoxy composites are prepared. Negative temperature coefficient effect of epoxy is mitigated by PTC fillers. The optimal wt.% of filler enhances both dielectric and thermal properties. Dielectric properties are explored at high frequencies and temperatures. Optimized epoxy composites are suitable for high‐frequency applications.

Towards highly homogeneous self-regulating heating of smart nanocomposites

Smart self-regulating heating devices utilising the positive temperature coefficient (PTC) effect have shown great potential in advancing applications across healthcare, soft robotics, and energy-efficient manufacturing. However, achieving homogeneous resistive heating within such temperature self-controllable nanocomposites remains a significant challenge, falling short of meeting the requirements of advanced heating systems. This study explores and evaluates multiple innovative strategies aimed at enhancing the temperature uniformity of PTC nanocomposites. By identifying and analysing the primary physical mechanisms behind the inhomogeneous heating observed in conductive polymer composites, we propose a series of targeted strategies, ranging from customised material formulations to novel electrode configurations. Recycled carbon fibres have also been explored and upcycled as an effective solution for homogenous self-regulating heating. Through a comprehensive analysis of experimental results, the effectiveness of each strategy has been evaluated with a significantly improved temperature uniformity (from 32.6 to 2.7 % variation at 125 °C), providing valuable insights for the design and development of advanced self-regulating heating devices based on conductive polymer nanocomposites, while offering promising prospects for achieving more energy-efficient and uniform heating in various industrial applications.

Electrothermal Instabilities in Barium-Titanate-Based Ceramics

An electrothermal analysis for barium-titanate-based ceramics is presented, combining the Heywang–Jonker model for the electric resistivity with a heat dissipation mechanism based on natural convection and radiation in a one-dimensional model on the device level with voltage as the control parameter. Both positive-temperature-coefficient (PTC) and negative temperature coefficient (NTC) effects are accounted for through the double Schottky barriers at the grain boundaries of the material. The problem formulated in this way admits uniform and non-uniform multiple-steady-state solutions that do not depend on the external circuit. The numerical bifurcation analysis reveals that the PTC effect gives rise to several multiplicites above the Curie point, whereas the NTC effect is responsible for the thermal runaway (temperature blowup). The thermal runaway phenomenon as a potential thermal shock could be among the possible reasons for the observed thermomechanical failures (delamination fracture). The theoretical results for the NTC regime and the thermal runaway are in agreement with the experimental flash sintering results obtained for barium titanate, and 3% and 8% yttria-stabilized zirconia.

Synergistic effect of positive temperature coefficient and thermally conductive micro‐nano fillers on electrical and thermal properties of epoxy resin

AbstractThis study investigates the effect of positive temperature coefficient (PTC) fillers, i.e., barium strontium titanate (BST‐60) and nano hexagonal boron nitride (h‐BN) fillers, on thermal and dielectric insulation properties of epoxy (EP). The electrical resistivity, DC breakdown strength, space charge, thermal conductivity, thermogravimetric analysis, and glass transition temperature (Tg) of EP composites were measured. Results indicate that, below curie temperature, the EP composites containing 0.5 and 1 wt% of BST and 5 wt% of h‐BN have superior electrical resistivity, breakdown strength, and thermal conductivity. However, above curie temperature, the EP composites containing 2 and 2.5 wt% of BST and 5 wt% of h‐BN have superior insulation properties than the pure EP and other composites. While the thermal properties of EP improve with an increase in h‐BN loading to 10 wt%, its dielectric properties are slightly compromised due to the presence of tiny defects in the composite material. It is hypothesized that the BST‐60 suppresses the negative temperature coefficient effect (NTC) of EP by regulating the electrical resistivity above the curie point, whereas nano h‐BN enhances the thermal conductivity of EP by constructing a heat dissipation path. The proposed mechanism for charge dynamics along with heat conduction‐dissipation explains the fillers' behavior at different temperatures.Highlights Analysis of epoxy composites with polydopamine‐modified BST and h‐BN fillers. PTC fillers are used to mitigate the negative temperature effect of epoxy. Optimal fillers wt% improves dielectric and thermal properties. This study shows that optimized epoxy composites can be used for packaging.

Insight into the interface effect and PTC effect on the temperature-adaptive electrical conductivity of epoxy composite

Doping functional fillers into the polymeric matrix is an effective strategy to improve the electrical, thermal, and other performance of insulating materials. It is imperative to understand the influence of fillers on the charge carrier behavior to achieve better regulation effectiveness. In this work, micrometer-sized and nano-sized ceramic particles with positive temperature coefficient (PTC) electrical resistivity are employed to prepare the epoxy composites, whose electrical conductivity under different temperature and electric field, space charge characteristics, permittivity, and electric field distribution are studied. It is found that the doping of a PTC filler shifts the electrical conduction from bulk-controlled to electrode-limited, determining the quantity of charge carriers within epoxy composites. While the interface effect mainly affects the transport process of charge carriers, it would fail to dominate the electrical conduction since the abundant charge carrier introduced by the semiconductive functional filler. Combined with the reinforced interface effect, the electrical conductivity–temperature characteristic of the epoxy nanocomposite is optimized, leading to the reduction in the maximum electric field within electrical equipment insulation by 55%. These findings emphasize the synergistic regulation of charge carrier amount and transport, which contributes to the precision design of polymeric composites doped with functional fillers.

Electrical and Electro-Thermal Characteristics of (Carbon Black-Graphite)/LLDPE Composites with PTC Effect.

Electrical properties and electro-thermal behavior were studied in composites with carbon black (CB) or hybrid filler (CB and graphite) and a matrix of linear low-density polyethylene (LLDPE). LLDPE, a (co)polymer with low crystallinity but with high structural regularity, was less studied for Positive Temperature Coefficient (PTC) applications, but it would be of interest due to its higher flexibility as compared to HDPE. Structural characterization by scanning electron microscopy (SEM) confirmed a segregated structure resulted from preparation by solid state powder mixing followed by hot molding. Direct current (DC) conductivity measurements resulted in a percolation threshold of around 8% (w) for CB/LLDPE composites. Increased filler concentrations resulted in increased alternating current (AC) conductivity, electrical permittivity and loss factor. Resistivity-temperature curves indicate the dependence of the temperature at which the maximum of resistivity is reached (Tmax(R)) on the filler concentration, as well as a differentiation in the Tmax(R) from the crystalline transition temperatures determined by DSC. These results suggest that crystallinity is not the only determining factor of the PTC mechanism in this case. This behavior is different from similar high-crystallinity composites, and suggests a specific interaction between the conductive filler and the polymeric matrix. A strong dependence of the PTC effect on filler concentration and an optimal concentration range between 14 and 19% were also found. Graphite has a beneficial effect not only on conductivity, but also on PTC behavior. Temperature vs. time experiments, revealed good temperature self-regulation properties and current and voltage limitation, and irrespective of the applied voltage and composite type, the equilibrium superficial temperature did not exceed 80 °C, while the equilibrium current traversing the sample dropped from 22 mA at 35 V to 5 mA at 150 V, proving the limitation capacities of these materials. The concentration effects revealed in this work could open new perspectives for the compositional control of both the self-limiting and interrupting properties for various low-temperature applications.

Effect and mechanism analysis of matrix resin type on thermal aging characteristics of semi‐conductive shielding material for high voltage cable

AbstractThe type of matrix resin of the semi‐conductive shielding layer directly affects the thermal aging characteristics of the semi‐conductive shielding layer and the high‐voltage cable. In this paper, ethylene vinyl acetate (EVA), ethylene ethyl acrylate (EEA), and ethylene butyl acrylate (EBA) resins were used as matrix to prepare carbon black (CB) + EVA, CB + EEA, and CB + EBA shielding materials. The law of physicochemical, electrical, and mechanical properties of different matrix resin shielding materials with aging time was studied, and the influence mechanism of matrix resin on the aging characteristics of shielding materials was analyzed. The results show: with the increase of aging time, the crystallization area and the number of functional groups of the three shielding materials decreased to varying degrees. The number of functional groups in CB + EBA shielding materials decreased evenly with aging time, but that of CB + EVA and CB + EEA shielding materials changed significantly after 7 days of aging. After 60 days of aging, the crystallization area of CB + EBA shielding material changed slightly, but that of CB + EVA and CB + EEA shielding material decrease significantly. The electrical properties of the three shielding materials showed different decreasing trend with aging time. When the aging time is 7 days, the positive temperature coefficient (PTC) effect of CB + EEA shielding material decreases obviously. When the aging time is 30 days, the resistivity of CB + EVA and CB + EEA shielding material increases slowly (9 Ω cm–12 Ω cm) with the increase of temperature. When the aging time is 60 days, the resistivity of CB + EBA shielding material decreases obviously, and the PTC effect weakens obviously. Taking the mechanical properties of the shielding material as reference, the rapid deterioration stage of the mechanical properties of the three shielding materials is different. The CB + EVA and CB + EEA shielding material rapid deterioration time is 0–7 days, and the tensile strength and elongation of the shielding material are greatly reduced. The rapid deterioration stage of CB + EBA shielding material is 7–30 days, and the tensile strength and elongation decrease from 24.38 MPa and 499.5% to 14 MPa and 155.7%, respectively. This work can provide data support for the selection of matrix resin of shielding material and the fault analysis of shielding layer of high voltage cable.

The effect of Li(Ni0.9Co0.05Mn0.05)O2 on improving electrical properties of semi-conductive LDPE/EVA/CB composites

The conductive mechanism of the semiconductor shielding layer between the conductive core and the insulation layer of high-voltage direct current (HVDC) cables includes tunneling and over-diffusion theories; however, the ethylene vinyl acetate copolymer (EVA) matrix in the semi-conductive composite material expands when heated, leading to a break in the carbon black (CB) conductive networks, which results in an increase in resistivity and the formation of the positive temperature coefficient (PTC) effect. The addition of materials with negative temperature coefficient (NTC) properties can weaken the PTC effect while absorbing some free electrons to enhance the suppression of space charge injection into the insulating layer. Li(Ni0.9Co0.05Mn0.05)O2 (NCM) was prepared by sol–gel method, which was melted and mixed with low-density polyethylene (LDPE), EVA, and CB, and pressed and molded to obtain modified semi-conductive composites. The semiconducting composites were measured by resistivity, thermal stimulation depolarizing current method (TSDC), and pulsed electroacoustic method (PEA). The results showed that the composites had the best performance when the NCM doping was 2 wt%.

Effect of matrix resin components on properties of the semi‐conductive shielding material for high‐voltage cable

AbstractThe semi‐conductive shielding layer is an important part of the high‐voltage cable, playing a role in uniform electric field and reducing the air gap at the interface. The overall performance of the cable is affected by the semi‐conductive shielding layer with different base resin composition. The matrix resins of ethylene‐vinyl acetate (EVA), ethylene‐ethyl acrylate (EEA) and ethylene‐butyl acrylate (EBA) were used to prepare the semi‐conductive shielding materials of CB/EVA, CB/EEA and CB/EBA. The influences of different matrix resins on the chemical, electrical, thermal and mechanical properties of the shielding materials were investigated. The results show that CB has the best dispersion in EBA resin and the worst dispersion in EEA resin. The temperature inflection points of CB/EVA, CB/EEA and CB/EBA shielding materials are 90°C, 100°C and 110°C, respectively, and the CB/EBA shielding material has the lowest resistivity and the weakest positive temperature coefficient effect at room temperature. The thermal conductivity of all three matrix resins is smaller than that of the shielding material, the thermal conductivity of CB/EVA and CB/EBA shielding materials show a slow increase with temperature, and the CB/EEA shielding material is the least affected by the temperature. The mechanical properties of the three matrix resins and the three shielding materials are EBA > EVA > EEA, among them the stress of CB/EBA shielding material is 31.0 MPa and the strain is 570%. This work has important reference significance for the selection of shielding layer matrix resin and material formulation in engineering applications.

Tailored Out-of-Oven Energy Efficient Manufacturing of High-Performance Composites with Two-Stage Self-Regulating Heating via a Double Positive Temperature Coefficient Effect.

The needs for sustainable development and energy efficient manufacturing are crucial in the development of future composite materials. Out-of-oven (OoO) curing of fiber-reinforced composites based on smart conductive polymers reduces energy consumption and self-regulates the heating temperature with enhanced safety in manufacturing, presenting an excellent example of such energy efficient approaches. However, achieving the desired curing processes, especially for high-performance systems where two-stage curing is often required, remains a great challenge. In this study, a ternary system consisting of graphene nanoplatelets/HDPE/PVDF was developed, with a double positive temperature coefficient (PTC) effect achieved to fulfill stable self-regulating heating at two temperatures (120 and 150 °C). Systematic studies on both single and double PTC effects were performed, with morphological analysis to understand their pyroresistive behaviors. Compared to the oven curing process, up to 97% reduction in the energy consumption was achieved by the ternary system, while comparable thermal and mechanical properties were obtained in the carbon fiber/epoxy laminates. This work presents a new route to achieve OoO curing with two-stage self-regulating heating, which can be utilized in many high-performance composite applications.

Effect of Li1.3Al0.3Ti1.7(PO4)3 on semiconductive composites: Positive temperature coefficient (PTC) performance and inhibition of space charge injection

The semiconductive layer located between the core and insulation layer of a high-voltage DC (HVDC) cable can inhibit the accumulation of space charge in the insulation layer. This helps increasing the electric field inside the cable, thus improving the stress distribution of the electric field on the inner and outer surfaces of the insulation layer and the electrical strength of the cable. An inevitable positive temperature coefficient effect (PTC) in the semiconductor layer leads to aging failure of the insulation layer. To improve this phenomenon, the semiconductive shielding layer was modified for suppressing its PTC effect and reducing the space charge injection into the insulation layer. Li1.3Al0.3Ti1.7(PO4)3(LATP) was prepared via a solid-phase reaction and added to a composite consisting of carbon black (CB), ethylene vinyl acetate copolymer (EVA), and low-density polyethylene (LDPE). Resistivity, thermally stimulated depolarization current (TSDC), and pulsed electroacoustic (PEA) measurements were performed on the modified sample. The results showed that when the LATP content was 4 wt%, the modification effect of the semiconductive shielding layer was most effective, the PTC effect was reduced by 11%, and the space charge injection into the insulating layer was improved significantly.

Enhancement of PTC effect of polymer/CB composite by intermolecular forces between nitrile groups on silica surface and polymer chain

The application of positive temperature coefficient (PTC) materials in sensing and intelligent heating has been widely studied. However, challenges such as poor stability and limited choices of base materials still exist. In this study, functionalized silica containing nitrile groups was synthesized through thiol-ene click reaction and incorporated into HDPE/CB composites to improve the PTC performance by forming non-covalent bonds through molecular interactions between the methyl and nitrile groups. The results showed that these non-covalent bonds were disrupted with increasing temperature, resulting in a 0.2 increase in PTC intensity and improved PTC stability in the composite materials. Furthermore, it was found that introduction of this nitrile-functionalized silica also induced PTC behavior around 140 °C in other polymers such as polystyrene and polycarbonate. This work is believed to provide new insights into understanding PTC behavior, expanding the choices of base materials for PTC materials, and enabling the preparation of high-performance PTC materials.

Использование хвостов Каджаранского медно-молибденового комбината (ХКММК) для синтеза стёкол с магнитными свойствами

The area of glass formation in the systems TKCMC - Li2O - B2O3 and TKCMC - LiFe5O8 - B2O3 has been studied. The most interesting compositions have been determined and the density and temperature coefficient of linear expansion (TCLE) of glasses have been investigated. It was found that the glasses exhibit weak magnetic properties. The study of the magnetic properties of crystallized glass samples showed that crystallization greatly enhances the magnetic properties, reaching values of practical application due to the precipitation of lithium ferrospinel in the samples. Research on the specific volume electrical resistivity of crystallized lithium ferrite system samples showed that they exhibit pronounced positive temperature coefficient effects at different temperatures, which will allow their use in various types of sensors. The work done shows that the tailings of the Kajaran copper-molybdenum production can be successfully used to produce electronic materials.

A Prosthetic Hand with Integrated Sensing Elements for Selective Detection of Mechanical and Thermal Stimuli

Flexible electronics have gained popularity because of their capability to combine softness and functionality. Soft resistive sensors are susceptible to mechanical stimuli and detecting temperature selectively remains a challenge. In this study, soft flexible thermistors are developed for detecting temperature changes based on the positive temperature coefficient (PTC) effect, selectively. By thermomechanical analysis and differential scanning calorimetry, it is observed that thermoplastic elastomers with higher thermal expansion and semicrystalline morphology result in a sensitive thermistor response. To achieve a high sensitivity in temperature and low sensitivity in the detection of mechanical stimulus, a low‐carbon filler content is required. The opposite trend is seen for the piezoresistive sensors for mechanical strain detection. Both sensory material types are compatible with thermoplastic material extrusion‐based additive manufacturing. The method is used for the fabrication of the sensing elements and an open‐source prosthetic hand. The strain sensor detects the bending of the fingers and the temperature sensor detects the temperature when in contact with a heated surface, successfully. In addition, the temperature sensor is used as a tactile sensor to detect contact with a non‐heated surface. Combining selective multisensory capabilities will significantly affect the future development of sensorized prosthetic devices and wearable electronics.

Fabrication and performance evaluation of the flexible positive temperature coefficient material for self‐regulating thermal control

AbstractIn this study, three kinds of newly flexible positive temperature coefficient (PTC) materials are fabricated by a simple preparation method. Due to introducing the hybrid filler of carbon nanotube and carbon black, these materials exhibit remarkable PTC effect including the PTC intensity of more than four‐orders magnitude and the resistivity‐temperature coefficient of 154%/°C. The Curie points of PTC materials are regulated to room‐temperature range by the low melting phase and influenced by melting onset temperatures of their solid–liquid phase change. With the help of transmission electron microscope, the micro‐capsule structure of the phase change regions is observed, and the hybrid conductive fillers randomly distribute in the blend matrix to construct three‐dimensional conductive network. Using the PTC materials as a heating element, the equilibrium temperature of the controlled device can be maintained around their Curie point without any control method. Moreover, the adaptive thermal control effect becomes more obvious with the increase of ambient temperature and initial heating power. Furthermore, the thermal control accuracy fluctuates within 0.15°C without any external control method, and is improved to round 0.08°C in combination with the switch control method. This study provides a new method for the thermal control of electronic devices in the requirement of lightweight and miniaturization at low temperature environment.

Evolution and Parametric Analysis of Concrete Temperature Field Induced by Electric Heating Curing in Winter

Electric heat treatment is a widely used concrete curing method during the winter. Through direct and indirect heat exchange, the electric heating system tracks and controls the temperature of the heating medium based on a positive temperature coefficient (PTC) effect. In this study, to standardize the application of this treatment in the winter curing of concrete, the thermal energy conversion of an electric heating system and the heat-transfer characteristics of concrete have been studied. Based on the theoretical derivation, a calculation model of the relationship between the thermal energy of the electric heating system and the temperature of the concrete is established. The model is verified using the concrete heating and curing test results. The numerical analysis program COMSOL is used to analyze the effects of various factors on the concrete temperature field, including the electric heating power (e.g., the surface temperature of the electric heating system), concrete casting temperature, thermal conductivity, and heat release coefficient. The results show that decreasing the surface exothermic coefficient and increasing the heating temperature will significantly increase the peak temperature of the concrete. When the heat source temperature increases by 20 °C, the peak temperature could increase by approximately 13 °C. When the heating stops, the concrete volume increases temporarily, particularly in the region where the heating cable is buried. Consequently, an excessive heating power increase may cause cracks on the concrete surface. Compared with the factors of thermal conductivity and surface exothermic coefficient, the ambient temperature has the most significant effect on the concrete cooling rate when the heating stops. When the ambient temperature decreases by −20 °C, the cooling rate of concrete increases by 0.72 °C/h. The role of concrete insulation materials needs to be strengthened to reduce cooling rates during power outages and form removal. The findings from the study provide industry practitioners with a comprehensive guide regarding the specific applications of the electric heating system in early-age concrete curing.

Enhanced reproducibility of positive temperature coefficient effect of TPO/HDPE blends via elastic crosslinking

Enhanced reproducibility of positive temperature coefficient effect of TPO/HDPE blends via elastic crosslinking