High Positive Temperature Coefficient Research Articles

Polymer Composites with Self-Regulating Temperature Behavior: Properties and Characterization.

A novel conductive composite material with homogeneous binary polymer matrix of HDPE (HD) and LLDPE (LLD), mixed with conductive filler consisting of carbon black (CB) and graphite (Gr), was tested against a HDPE composite with a similar conductive filler. Even the concentration of the conductive filler was deliberately lower for (CB + Gr)/(LLD + HD), and the properties of this composite are comparable or better to those of (CB + Gr)/HD. The kinetic parameters of the ρ-T curves and from the DSC curves indicate that the resistivity peak is obtained when the polymer matrix is fully melted. When subjected to repeated thermal cycles, the composite (CB + Gr)/(LLD + HD) presented a better electrical behavior than composite CB + Gr)/HD, with an increase in resistivity (ρmax) values with the number of cycles, as well as less intense NTC (Negative Temperature Coefficient) effects, both for the crosslinked and thermoplastic samples. Radiation crosslinking led to increased ρmax values, as well as to inhibition of NTC effects in both cases, thus having a clear beneficial effect. Limitation effects of surface temperature and current intensity through the sample were observed at different voltages, enabling the use of these materials as self-regulating heating elements at various temperatures below the melting temperature. The procedure based on physical mixing of the components appears more efficient in imparting lower resistivity in solid state and high PTC (Positive Temperature Coefficient) effects to the composites. This effect is probably due to the concentration of the conductive particles at the surface of the polymer domains, which would facilitate the formation of the conductive paths. Further work is still necessary to optimize both the procedure of composite preparation and the properties of such materials.

High resistivity‐temperature effect of resistivity for economical and facile conductive polymer composites with low percolation threshold via self‐constructed dual continuous structure

AbstractIn this work, electrically conductive polyethylene/polypropylene‐flake graphite composites with different ratios (HDPE/PP‐FG) were prepared by simple mechanical blending (Mec) and melt blending (Mel) technologies, combining with the grinding technique. According to the effect of preparation technique, dual continuous microstructures for Mec‐HDPE/PP‐FG composites were self constructed by controlling specific molding temperature. As a result, the percolation threshold of 1.33 wt% of Mec‐HDPE1/PP4‐FG was achieved, much lower than that of Mel‐HDPE1/PP4‐FG (7.02 wt%) and the maximum in conductivity was over 10−3 S/m at about FG content of 6 wt%. However, Mel‐HDPE1/PP4‐FG only showed the electrical conductivity of about 8.0 × 10−11 S/m when FG content was 6 wt%. Meanwhile, Mec‐HDPE/PP‐FG composites presented obvious positive temperature coefficient (PTC) effect of electrical resistivity, without negative temperature coefficient (NTC) effect and PTC intensity was up to 6.57 orders of magnitude, far higher than those of Mel‐HDPE/PP‐FG composites. In addition, the excellent repeatability in PTC behaviors for Mec‐HDPE/PP‐FG was achieved after several run cycles. Therefore, the study on economical and facile polymer materials with high performance PTC characteristics had the great significance in practical application.

Polymer positive temperature coefficient composites with room-temperature Curie point and superior flexibility for self-regulating heating devices

Polymer PTC (positive temperature coefficient) materials with low switching temperature point are important for many electronic devices since the electronic devices usually work under room temperature range (0–40 °C). The desired polymer PTC materials for the application in thermal control of flexible electronics should also include room-temperature switching temperature, low room-temperature resistivity, good mechanical flexibility and adaptive thermal control performances. This work designed two new types of PTC materials with room-temperature switching temperature and excellent mechanical properties. They are prepared by mixing the semi-crystalline polymer EVA with low melting temperature (Tm) with the amorphous polymer (PS or PVAc) with low glass transition temperature (Tg), while the low-cost graphite is selected as conductive filler. The PTC materials were obtained with low switching temperature point (<30 °C) and low room temperature resistivity (102 Ω m), high PTC repeatability and flexibility, short heating response time and good adaptive thermal control performances.

Achieving a high positive temperature coefficient effect at low temperature through combining spherical HDPE/SBR with MCMB

A novel system by using spherical high-density polyethylene (HDPE) and meso-carbon microbeads (MCMB) to regulate the positive temperature coefficient (PTC) effect is studied, and the elastic styrene-butadiene rubber (SBR) is used as the matrix. This HDPE-MCMB/SBR composite (HMSC) is constructed by a simple solution mixing method. The PTC intensity (PTCI) of HMSC with 1 wt% MCMB (HMSC-1) achieves three orders of magnitude (its value is 3.39) in the low-temperature range (20 °C to 90 °C), and HMSC-1 also shows fantastic PTCI reproducibility during the repeated temperature cycles from −20 °C to 90 °C. The high PTC effect is mainly caused by the collaborative combination of the expansion effect of HDPE spheres and the elasticity of SBR. The excellent stability during the temperature cycle mainly stems from the elastic effect of composite, rather than being destroyed during the temperature cycle. Meanwhile, HMSC-1 has good mechanical properties.

Nd1-xSrxMnO3 (x = 0.3): A perovskite manganite ceramic that exhibits PTC and NTC double properties

Nd1-xSrxMnO3 (NSMO, x = 0.280, 0.300, 0.330, 0.350, and 0.375) polycrystalline ceramics were fabricated using the sol-gel method. The crystal structure, surface morphology, valence state, elements distribution, and electrical properties were examined to understand the effect of Sr doping on NdMnO3 ceramics. The resistivity of the NSMO ceramics was measured using a standard four-probe method. The results obtained revealed that Sr doping significantly decreased the resistivity of the ceramics, which can be explained by the double exchange (DE) interaction and small-polaron hopping (SPH) model. The Nd0.70Sr0.30MnO3 ceramic had a positive temperature coefficient (PTC) of resistivity (16.69% K−1) at 197.5 K, and is expected to be used for preparing electronic switches with high sensitivity. Additionally, the NSMO ceramics maintained a stable negative temperature coefficient (NTC) of resistivity (−1% K−1) for x = 0.300 in the temperature range of 210.6–342.5 K. In conclusion, it is worth exploring materials with a high PTC and NTC over an extended temperature range, possessing the double potential function for high sensitivity or wide-temperature detection for electronic switches or infrared bolometers.

Excellent positive temperature coefficient behavior and electrical reproducibility of HDPE/(TiC-CB) composites

The high-density polyethylene/(titanium carbide--carbon black) (HDPE/(TiC-CB) composites were prepared by meltingblendprocess. Microstructure, room temperature resistivity, positive temperature coefficient (PTC) behavior and electrical reproducibility of composites were investigated. Results show that adding 3.0 vol% CB was in favor of reducing the room temperature resistivity of HDPE/(TiC-CB) composites, and leading to obtain high PTC intensity (the ratio of maximum resistivity to room temperature resistivity is defined as the PTC intensity). When the content of conductive fillers TiC was 32.0 vol% and CB was 3.0 vol%, the room temperature resistivity of composites was 0.8 Ω.m and PTC intensity reached 7.0, which is similar to the composites with 45.0 vol% TiC individually. The reduction of the inorganic material TiC facilitates the more flexibility of the composites, and the effect of PTC was still stability after several heating cycles. When the content of CB was more than 5.0 vol%, the room temperature resistivity was slightly reduced. However, the PTC intensity dropped below 3.7 abruptly. The micrographs of scanning electron microscopy (SEM) indicated that the CB had greatly influence on the formation of the electric conduction channel in the matrix.

Achieving High Positive Temperature Coefficient Effect at Low Temperature Through Combining Spherical Hdpe/Sbr with Mcmb

Achieving High Positive Temperature Coefficient Effect at Low Temperature Through Combining Spherical Hdpe/Sbr with Mcmb

Anomalous behavior induced by water insertion in molybdenum disulfide nanoflowers

The coexistence of negative photoconductivity and metallic-like behavior in conventional semiconductors is very uncommon. In this work, we report the existence of such unconventional physical properties in molybdenum disulfide nanoflowers (MoS2-NF). This is achieved by making the surface of MoS2 hygroscopic by alcohol treatment and creating a transport channel that favors protonic over electronic conduction. On cooling the MoS2-NF in a heat sink, the excess water that condenses on the surface forms a proton (H3O+) wire which exhibits pinched hysteresis characteristics. The conductivity of MoS2 increased by two orders of magnitude in the proton-dominated conduction regime with an exceptionally high positive temperature coefficient of 1.3 × 104 Ω K−1. Interestingly, MoS2-NF also exhibits strong negative photoconductivity at room temperature when illuminated with UV and infra-red radiation. This interesting behavior observed in MoS2 NF can be useful for energy harvesting applications and the realization of fast thermal memories and optical switches.

Structure and electrical properties of polymer composites based on tungsten oxide varistor ceramics

Composites based on nonlinear tungsten oxide ceramics and a polyethylene matrix with a volume fraction of ceramic filler from 10 vol % to 43 vol % were studied. It was shown that composites are an isotropic mixture of WO3 grains in a polymer matrix. The current-voltage characteristics of the composites were nonlinear. Composites have a high positive temperature coefficient of resistance up to 15.8 K-1 in the temperature range of 40–75 °C. It is shown that the temperature coefficient of resistance is positive and increases with increasing electric field strength. The decrease in the electrical conductivity of composites with increasing temperature is explained by the expansion of the polymer matrix and the rupture of the current-conducting channels between the conducting grains of the varistor ceramics. The dependence of the electrical conductivity of the composite on the volume fraction of varistor ceramics is well described within the framework of a three-dimensional percolation model for a two-phase system.

Microwave dielectric characterization of Ca0.6(La1-xYx)0.2667TiO3 perovskite ceramics with high positive temperature coefficient

Solid solution Ca0.6(La1-xYx)0.2667TiO3 dielectric ceramic systems with various x values were studied, which were prepared using a solid-state reaction method. X-ray diffraction and X-ray spectroscopy analyses showed that the crystal structure of these samples was orthorhombic perovskite. The microstructures with the substitution amount of Y3+ and the dielectric performances of the Ca0.6(La1-xYx)0.2667TiO3 ceramics were also explored. With x = 0.1, the Ca0.6(La0.9Y0.1)0.2667TiO3 ceramic could be sintered at 1350 °C, and the microwave dielectric performance was found to be strongly correlated with the sintering temperature. A maximum Qf value of 23,100 (GHz), dielectric constant (εr) of 111, and temperature coefficient (τf) of 374.6 ppm/°C were achieved for samples sintered at 1350 °C for 4 h. This dielectric ceramic possessed good potential as a τf compensator to obtain a near-zero τf mixture for high-quality substrates for use in wireless communication systems.

High positive temperature coefficient effect of resistivity in conductive polystyrene/polyurethane composites with ultralow percolation threshold of MWCNTs via interpenetrating structure

In this work, a novel interpenetrating system including amorphous polystyrene/polyurethane and multiwall carbon nanotubes (PS/PU-MWCNTs) was in-situ constructed by the homogeneous bulk polymerization. The ultralow percolation threshold (about 0.01 wt%) and high electrical conductivity were achieved owing to the unique conductive networks of MWCNTs in the interconnected “island chain” of PU. Surprisingly, the high positive temperature coefficient (PTC) effect of resistivity of amorphous PS/PU-MWCNTs composites, which was caused by the polymer segment motion, was achieved, such as PTC intensity of six orders of magnitude, fantastic reproducibility and favorable mechanical properties. Finally, this work provided a new promising in the development of excellent PTC materials.

High-performance PTCR ceramics with extremely low resistivity for multilayer chip thermistor application

It is a great challenge to prepare chip thermistors with high positive temperature coefficient of resistivity (PTCR) jump and simultaneously low room temperature resistivity by reduction-reoxidation method. In this work, the influence of Mn doping on Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3 based chip thermistors were carefully investigated and Mn doping was proved to be an effective way in preparation of high-performance PTCR ceramics with low resistivity. The results showed that Mn can't create a high enough potential barrier in un-oxidized samples but can obviously increase the value of potential barrier height after reoxidation. The trap activation of Mn d states near Curie point helps to reduce room temperature resistivity and increase PTCR jump in reoxidized samples. As a result, samples with extremely low resistivity and high PTCR jump of 103.4 were reported for the first time.

Heating Performance Enhancement of High Capacity PTC Heater with Modified Louver Fin for Electric Vehicles

Electric vehicles use positive temperature coefficient (PTC) heaters and heat pumps to warm the vehicle cabin. High-capacity PTC heaters are needed because heat pump performance decreases sharply in the winter months due to low outdoor temperatures. The weight of PTC heaters is an important heater design factor for improving the single-charge travel distance of electric vehicles. A fin shape is necessary to improve the heater’s heat transfer performance in comparison to its weight. To develop a 6 kW class high-capacity PTC heater for electric vehicles, this study presents a numerical analysis of heat flow according to a modified louver fin’s geometric shape variables and evaluates heating performance. Based on the geometric shape of an initial plate-shaped fin prototype, a numerical analysis was performed on the width, position, height, and angle to develop a modified louver fin while considering heat transfer performance and ease of manufacturing. An improved prototype was built using the developed modified louver fin, and its heating performance under standard conditions was evaluated. The improved prototype had a heating performance of 6.05 kW, an efficiency of 98.0%, a pressure drop of 18.3 Pa, and a heating density of 3.81 kW/kg. Compared to the initial prototype, its heating performance and heating density were improved by approximately 15.7%.

A New Thermal Controlling Material with Positive Temperature Coefficient for Body Warming: Preparation and Characterization

Positive temperature coefficient (PTC) materials have many applications in self-regulating heating; however, they are generally used in the high temperature range, and their use in the room temperature range have rarely been reported. A new kind of PTC material with the Curie temperature of 37 °C was proposed in this study. The material was prepared by adding 6wt % carbon black (CB) and 5wt % dioctyl phthalate (DOP) into the copolymer of ethylene vinyl acetate (EVA) and lauric acid (LA) (1:3). The results showed that it had an exceptionally high PTC intensity of 5.5 and rationally low electrical resistivity of 600 Ω·cm. at room temperature. And this PTC material also shows a preferable repeatability in the PTC intensity after the thermal cycle of 4th. Moreover, the PTC material shows a great potential application in warming the human body in a cold environment.

Engineering the Charge Transport of Ag Nanocrystals for Highly Accurate, Wearable Temperature Sensors through All-Solution Processes.

All-nanocrystal (NC)-based and all-solution-processed wearable resistance temperature detectors (RTDs) are introduced. The charge transport mechanisms of Ag NC thin films are engineered through various ligand treatments to design high performance RTDs. Highly conductive Ag NC thin films exhibiting metallic transport behavior with high positive temperature coefficients of resistance (TCRs) are achieved through tetrabutylammonium bromide treatment. Ag NC thin films showing hopping transport with high negative TCRs are created through organic ligand treatment. All-solution-based, one-step photolithography techniques that integrate two distinct opposite-sign TCR Ag NC thin films into an ultrathin single device are developed to decouple the mechanical effects such as human motion. The unconventional materials design and strategy enables highly accurate, sensitive, wearable and motion-free RTDs, demonstrated by experiments on moving or curved objects such as human skin, and simulation results based on charge transport analysis. This strategy provides a low cost and simple method to design wearable multifunctional sensors with high sensitivity which could be utilized in various fields such as biointegrated sensors or electronic skin.

Particle size induced tunable positive temperature coefficient characteristics in electrically conductive carbon nanotubes/polypropylene composites

A novel approach, i.e. manipulating the size of incorporated polymer matrix particles, was proposed to tune the positive temperature coefficient (PTC) characteristics of carbon nanotubes (CNTs)/polypropylene (PP) composites with a segregated microstructure. For the conductive properties of CNTs/PP composites, the percolation threshold decreased from 1.32vol% to 0.44vol% when the matrix particle size enlarged from 20 to 1200µm, showing an inverse correlation effect. The controllable PTC characteristics in resistivity are attributed to the microstructure development of conductive pathways and the heat-induced volume expansion of polymer matrix particles. An extremely high PTC material has also been achieved through this method. The present work provides an effective route to acquire a tunable temperature-resistivity sensor.

Effect of mixed fillers on positive temperature coefficient of conductive polymer composites

This paper investigates the trade-off between low percolation threshold and large positive temperature coefficient (PTC) intensity in conductive polymer composites (CPCs). Conductive particles with low aspect ratios and large dimensions have been demonstrated to induce large PTC intensity in CPCs. Conversely high aspect ratio conductive (nano)particles like carbon nanotubes (CNTs) are desirable because of their extremely low percolation threshold (typically well below 1 wt.%), providing benefits in terms of reduced density, brittleness, costs and improved processability. Herein we report on combinations of different conductive fillers to explore the possibility to obtain both low percolation threshold and high PTC intensity. For the first time we use model systems in which at least one of the two conductive fillers is of relatively homogenous size and shape to facilitate unraveling some of the complicated inter-relationships between (mixed) conductive fillers and the PTC effect.

Self-Heating Effects In Polysilicon Source Gated Transistors.

Source-gated transistors (SGTs) are thin-film devices which rely on a potential barrier at the source to achieve high gain, tolerance to fabrication variability, and low series voltage drop, relevant to a multitude of energy-efficient, large-area, cost effective applications. The current through the reverse-biased source barrier has a potentially high positive temperature coefficient, which may lead to undesirable thermal runaway effects and even device failure through self-heating. Using numerical simulations we show that, even in highly thermally-confined scenarios and at high current levels, self-heating is insufficient to compromise device integrity. Performance is minimally affected through a modest increase in output conductance, which may limit the maximum attainable gain. Measurements on polysilicon devices confirm the simulated results, with even smaller penalties in performance, largely due to improved heat dissipation through metal contacts. We conclude that SGTs can be reliably used for high gain, power efficient analog and digital circuits without significant performance impact due to self-heating. This further demonstrates the robustness of SGTs.

Study of the performance of dielectric resonator antennas based on the matrix composite of Al2O3 – CaTiO3

ABSTRACTIn this article is present the structural and dielectric microwave properties of Alumina – CaTiO3 ceramic composite with the addition of CaTiO3 and your performance as a dielectric resonator antenna (DRA). Alumina has a negative temperature variation of resonant frequency coefficient τf, while CaTiO3 has a high positive temperature coefficient. Also influence of calcium titanate on densification and porosity of the composite is presented. The CaTiO3 was added (2.5% to 10% wt. amount) in order to analyze changes in the dielectric properties. Samples were characterized by X‐ray diffraction. Dielectric properties were investigated by Hakki‐Coleman methods and the experimental characteristics parameters like return loss, bandwidth, and input impedance are presented and compared with numerical simulations. The results obtained shows the formation of secondary phases (Hibonite and TiO2), with the dielectric permittivity and dielectric loss increasing and decreasing with calcium titanate concentration, respectively. The application of DRA was verified and all samples tested presented return loss below −10 dB in the resonant frequency. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:963–969, 2015

Influence of CF on PTC Properties of LDPE/Carbon Fiber Composites

PO/CB (Polyolefin/Carbon Black) PTC (Positive Temperature Coefficient) composite with easy processing, low cost characteristics has been applied widely. But it suffered from a relatively short lifespan because of its NTC (Negative Temperature Coefficient) effect and low PTC intensity. In order to overcome this shortcoming, the CF was calcination-treated to prepare LDPE/CF (Low Density Polyethylene/Carbon Fiber) PTC composite. Influence of length, content and treatment method of CF on PTC properties of composites was investigated. Results showed that 0.5mm length CF in composites had higher PTC intensity than that of 2mm length CF. PTC intensity of the composites was enhanced more effectively by calcination treated CF compared to the untreated CF. The maximum PTC intensity was 8.1 when CF’s content was at 8wt%.