Positive Temperature Coefficient Characteristics Research Articles

Effects of the addition of multi-walled carbon nanotubes on the positive temperature coefficient characteristics of carbon-black-filled high-density polyethylene nanocomposites

This study investigates the effects of the addition of multi-walled carbon nanotubes (MWNTs) on the positive temperature coefficient (PTC) characteristics of conventional carbon black (CB)/high-density polyethylene (HDPE) composites. MWNT/CB/HDPE hybrid nanocomposites were prepared by the combined solution and melt-mixing process. The obtained results indicated that the PTC intensity and repeatability of the hybrid nanocomposites were dramatically improved by adding a small amount of MWNT and that the optimal MWNT content corresponded to the CB content. The initial resistivity of the materials decreased with increasing MWNT content, but not always in the PTC intensity.

Effect of processing parameters on electrical resistivity and thermo-sensitive properties of carbon-black/styrene–butadiene–rubber composite membranes

Carbon-black-filled conducting composite membranes were prepared by using styrene–butadiene–rubber as matrices and incorporating with some organic crystals (2,6-di- tert-butyl-4-methyl phenol). A number of composite membranes exhibited a thermo-sensitive property which corresponded to a positive temperature coefficient (PTC) effect. It was found that processing parameters strongly affected the microstructure of composite membranes and in turn, the resistivity of membranes as well as their PTC characteristics. The effect of several main processing parameters, such as mixing parameters (temperature, time and nip gap) and vulcanization parameters (temperature, time and pressure), on the resistivity and PTC characteristic of composite membranes was investigated. The results obtained from resistivity measurements, DSC thermograms, SEM micrographs and X-ray diffraction patterns revealed that, by using optimized mixing and vulcanization parameters, the obtained composite membranes showed a microcrystal-embedded structure and their resistivity against carbon black content exhibited a tangible percolation behavior. It was also found that the PTC characteristics of some composite membranes prepared by optimized processing parameters appeared to be stable for an acceptable long term.

Influence of AST additives on the stability of PTCR characteristics and microstructure in ferroelectric ceramics

(Ba0.69Pb0.31)TiO3 ceramics were prepared using Al2O3, SiO2, additives and excess of TiO2 (AST). The characteristics of positive temperature coefficient of resistivity (PTCR) was studied and the corresponding microstructures were investigated using atomic force microscopy and scanning electron microscopy. The results showed that the PTCR effect was related to the AST additives. The maximum value of resistivity in the ceramics with lower content of or without Al2O3 and SiO2 additives was much lower than in those with AST additives. Ceramics with low AST content, which were heated by electric field to a temperature much higher than their Curie temperature, lost the PTCR effect after the electric field stimulation. The microstructure observations revealed that re-crystallization took place in the ceramics with lower content of or without AST additives resulting in the loss of the PTCR effect.

Microstructure and PTCR effect of La-doped BaPbO3ceramics

La-doped BaPbO3 ceramics were prepared by using BaCO3 and PbO. Electrical properties and microstructure of the ceramics were studied. The results show a thin surface layer with a very low resistivity, and the interior of the ceramics with the characteristics of positive temperature coefficient of resistivity (PTCR). The PTCR behavior was related to the La content. The investigation by scanning electron microscopy and transmission electron microscopy revealed that the low resistivity of the surface layer was due to formation of a nano-size BaPbO3 phase with metallic properties.

Polymeric positive-temperature-coefficient materials: dynamic curing effect

The object of this work was to prepare high-density polyethylene (HDPE)/ethylene–propylene–diene terpolymer (EPDM)/conductive carbon black (CB) composites by dynamic curing and to characterize the positive-temperature-coefficient (PTC) performances of the composites.EPDM and dicumyl peroxide were preblended in a research mill. The roll-milled strands were blended with HDPE and CB in a Haake mixer. The sheet resistivity and morphology of the HDPE/EPDM/CB composites with or without the dynamic curing process were investigated. It was concluded that the dynamically cured blends exhibit better PTC performance than the simple blends without dynamic curing. The effects of shear intensity and dicumyl peroxide content during the dynamic curing process were discussed for the PTC characteristics of the HDPE/EPDM/CB composites.

Effects of thermal ageing treatment and antioxidants on the positive temperature coefficient characteristics of carbon black/polyethylene conductive composites

AbstractPolyethylene (PE)‐filled with carbon black (CB) is a prototypical composite that displays resistance switching. These materials can exhibit either a positive temperature coefficient (PTC) or negative temperature coefficient (NTC). The CB‐filled semicrystalline polymer composites ideally need antioxidants, which stabilize the composites against thermooxidative degradation, because they should be resistant to the severe conditions of high temperature. The characterization of PTC materials is affected by the crystallinity of the polymer, and the crystallinity of the polymer is changed with thermal ageing treatment. Thermal ageing of PTC samples was conducted in an oven in the range 50–140°C, in air. The composites, containing 0.5–3% (by weight) Irganox 1076 (Ciba‐Geigy), were irradiated under nitrogen at room temperature with different doses of gamma rays from a 60Co source. The resulting composites were analyzed by differential scanning calorimetry, gel fractionation, X‐ray diffraction, and dynamic mechanical analysis. The conductivity of the composites depended on the amounts of antioxidants and the duration of the thermal ageing treatment. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2316–2322, 2003

A method of preparing thin‐film micro‐PTC thermistors based on BaTiO3 using YAG‐laser irradiation

AbstractA technique is proposed for obtaining thin‐film positive temperature coefficient (PTC) microthermistors made of BaTiO3‐based material. In this technique, stacked layers of Ti and BaO thin films are irradiated in air by a YAG laser beam, which creates BaTiO3 semiconductor microregions with PTC characteristics. The experiments indicate that by suitably choosing the optical power of the laser beam and its scanning rate over the stacked layers, it is possible, and in fact easy, to control the resistance of the resulting BaTiO3 semiconductor microregions. We found that the resistance of the BaTiO3 semiconductor material created in this way must be sufficiently low in order for the material to exhibit PTC characteristics. In this case, the material becomes a thermistor, that is, its resistance increases rapidly with increasing temperature. As the temperature increases, the resistance suddenly begins to increase starting at a certain threshold temperature. In an attempt to control this threshold temperature, we modified the technique described in this research by replacing some of the Ti sites with Zr. This modified technique led us to conclude that, just as in bulk BaTiO3, such control was indeed possible. When our technique is used, high‐temperature heating of the entire substrate is not necessary, either during the deposition of the thin film or after its deposition. In this way we confirm that it is possible to create compound device modules by fabricating multiple thin‐film microdevices on a single substrate, including PTC microthermistors. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 85(11): 25–31, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.1113

Low-temperature electrical anisotropy of self-assembled organic films

We have examined the low-temperature characteristics of the charge transport in self-assembled organic films with thickness varying from $5$ to $60$ deposited bilayers of polypyrrole and polythiophene acetic acid. When the temperature is progressively decreased, independent measurements of the longitudinal and transversal components of the resistivity tensor of these thin films reveal that while the conductivity along the surface directions has the expected semiconducting profile, the charge transport in the transversal direction exhibits a positive temperature coefficient (PTC). For the samples with a small number of deposited bilayers we observe that at very low temperatures the PTC characteristics disappear and a tunnelinglike current begins to dominate the transversal charge transport.

Effect of incorporating ethylene‐ethylacrylate copolymer on the positive temperature coefficient characteristics of carbon black filled HDPE systems

AbstractIn order to study the effect of introducing ethylene‐ethylacrylate copolymer (EEA) in carbon black‐HDPE composite systems, two HDPE‐EEA composites prepared by pre‐blending and masterbatch‐blending processes were compared with HDPE and EEA composites in terms of positive temperature coefficient (PTC) characteristics and percolation threshold. The percolation threshold of masterbatch‐blended composites occurred at the lowest carbon black concentration among four kinds of composites. The conduction path in the masterbatch‐blended composite is effectively formed as a result of the localization of carbon black distribution predominantly in the EEA phase, resulting in an increase of conductivity. Ipeak values, the resistivity ratio of the peak to 25°C, of two blend composites were lower than those of HDPE composites. The I85 values, the resistivity ratio of 85°C to 25°C, of masterbatch‐blended composites were higher than those of pre‐blended as well as HDPE composites. It is evident that since most carbon black is dispersed in the EEA phase of the masterbatch‐blended composites, the conduction networks are mainly broken by the crystal melting of EEA before the temperature reaches the crystal melting temperature of HDPE.

Thermal analysis of PTC applied resin heater using 3-D finite element method

This paper introduces a computational method for predicting the temperature characteristics of PTC (positive temperature coefficient) applied resin heater. In this method, a current vector analysis is coupled with a thermal analysis using the 3-D finite element method. Transient thermal analysis is carried out to compute the temperature distribution of the heater after computing the Joule loss by taking the nonlinear temperature characteristics of the resistance of PTC resin into account. The validity of this method is confirmed through the comparison with measured values.

The influence of crystalline and aggregate structure on PTC characteristic of conductive polyethylene/carbon black composite

The relationship between conductivity and morphological structure of a polyethylene/carbon black composite (PE/CB) was investigated by DSC, WAXD and SAXS, and a new interpretation of the PTC (positive temperature coefficient) effect has been proposed. The PTC intensity enhanced with increased crystallinity for the PE/CB system; a substantial PTC effect was not discovered in an amorphous polymer/CB system. The behaviour of melting, crystallization and aggregate “large size” properties of the polymer matrix were modified by annealing, which in turn made the PTC intensity increase. It is pointed out that not only crystallinity, but also long spacing and lamellar size, also affect the PTC performances.

Effect of A-Site Substitution and Firing Temperature on Microstructure and Electrical Properties of BaTiO3 Semiconducting Ceramics Fired by Reduction-Reoxidation Method

The effects of A-site substitution and firing temperature on the microstructure and electrical properties of BaTiO3 semiconducting ceramics fired by the reduction-reoxidation method were experimentally investigated. Ba ions were substituted with Ca or (Sr0.5Pb0.5). Various microstructures were obtained by varying the firing temperature in reducing atmosphere. Using these samples, the recovery of the characteristics of positive temperature coefficient of resistivity (PTCR) was evaluated. In particular, the minimum grain size required for the reoxidation of only grain boundaries during annealing in oxygen atmosphere in the cooling process was investigated.

Thermochemistry and electrical contact properties at the interface between semiconducting BaTiO3 and (Au–Ti) electrodes

The interfacial characteristics of positive temperature coefficient of resistance (PTCR) BaTiO3-electrode interfaces were studied. Sessile drop wetting experiments in combination with measurements of the contact resistance of the interface were used to establish a fundamental perspective of the electrode-ceramic interface. It was shown that the thermodynamic work of adhesion Wad), which is the sum of the strengths of chemical interactions present at the interface, can be manipulated by the addition of chemically active elements to the electrode metal which enhance adhesion. This same procedure is shown to modify the important electrical interfacial properties such as the contact resistance.

Positive temperature coefficient characteristics of poly(3-alkylthiophene)s

Poly(3-alkylthiophene)(P3AT) films were prepared by both electrochemical and chemically oxidative polymerization methods, and their positive temperature coefficient(PTC) characteristics were investigated in the range from 0 to 200°C. The change in the amount of tetrafluoroborate ion(BF 4−) or iron(III) perchlorate(FeCl 3) which were doped into P3AT film was also estimated by infrared(IR) and ultraviolet(UV) spectrophotometry. It was found that the P3AT films chemically polymerized at a low temperature had PTC characteristics, which was not affected by the dedoping involved in thermal treatment

PTCR Characteristics of Semiconducting (Ba, Ca)TiO<sub>3</sub> Ceramics Fired in Reducing-Reoxidizing Atmospheres

Three electrode materials, nickel, palladium and platinum were tested about the ohmic contact to semiconducting BaTiO3 ceramics for the application in multilayer BaTiO3 semiconductors with a positive temperature coefficient (PTC) of resistivity. Semiconducting BaTiO3 ceramics with nickel electrodes showed a good ohmic contact, while palladium and platinum electrodes showed rectifying contacts at the junctions. Semiconducting BaTiO3 ceramics with a PTC characteristic of resistivity were fired at 1350°C in a reducing atmosphere and then annealed at various temperatures in the cooling process and in various oxidizing atmospheres, with a view to fabricate multilayer PTC ceramics having Ni metal or Pd-Ni alloy as internal electrodes. The resistivity anomaly of PTC characteristics increased with increasing oxygen concentration in the annealing atmosphere and with increasing annealing temperature below 1200°C. The resistivities at room temperature were almost independent of the annealing conditions. The reoxidation in annealing occurs probably only at grain boundaries.

The Enhanced Penetration of Oxygen along the Grain Boundary in Semiconducting Barium Titanate

The characteristics of Positive Temperature Coefficient of Resistivity (PTCR) thermistors depend strongly on the atmosphere, especially oxygen, during the fabrication process. The PTCR jump decreases when the sample is heated in a reducing atmosphere, while it increases when the sample is heated in air. The behavior of oxygen which is important to improve the PTCR jump has been examined by a tracer (18O) method. The depth profile of the tracer has been determined by means of Secondary ion mass spectrometry (SIMS). The rate of penetration of oxygen into the grain boundary is very rapid and the PTCR jump depends on the amount of the oxygen penetrated into such a region.