Positive Temperature Coefficient Effect Research Articles

The electrical conductive effect of nickel-coated graphite/two-component silicone-rubber sealant

Nickel-coated graphite particles and two-component silicone-rubber were compounded to form a conductive composite system. The electrical volume resistivity of the composites were examined and compared under constant tensile strains, cyclic heating-cooling, electric field and repeated cyclic tensile strains in order to study the mechanism of electrical conductivity behaviors of the conductive composites under stress, temperature and current. The results showed that a peak value of the electrical resistivity appeared previously and then gradually increasing with increasing tensile strain. The electrical resistivity displayed positive temperature coefficient effect during the temperature increasing and decreasing. Applying 5A direct current to the conductive composites resulted in an increase in the electrical resistance immediately, but no changes were detected under lower currents. Under the repeated cyclic strain, the peak value of the electrical resistivity of each cycle increased with the test cycle. All the electrical resistivity changes were attributed to the conductive networks broken-up and rebuilt in the conductive composites.

Influence of morphology on positive temperature coefficient effect for conductive polymer composites with carbon black dispersed at interface

Selective localization of carbon black (CB) at the interface of polymer blends was achieved by the method that EBA‐g‐MAH was first reacted with CB, and then blended with poly(ethylene‐co‐butyl acrylate)/nylon6 (EBA/PA6). In CB‐filled EBA/PA6 blends, EBA and PA6 phases formed cocontinuous morphology and CB was localized in PA6 phase. The percolation threshold was 5 wt%. A single PTC (positive temperature coefficient) effect was observed in this composite. The appearance of PTC effect was originated from the thermal expansion of EBA phase. In the EBA‐g‐MAH filled EBA/PA6 blends, TEM results showed that CB particles were induced by EBA‐g‐MAH to localize at the interface, resulting that the percolation threshold was much lower than that of EBA/PA6/CB. Influence of morphology on PTC effect of EBA/PA6/EBA‐g‐MAH/CB composites was studied. In the composites with sea‐island morphology, the conductive network was fabricated by dispersed phase and CB at the interface. Thermal expansion of matrix interrupted the contact of dispersed phases and conductive network formed by CB particles at the interface, resulting in the double PTC effect. The composites with co‐continuous morphology exhibited single PTC effect due to the fact that conductive network was only fabricated by CB localized at the interface. POLYM. ENG. SCI., 53:2640–2649, 2013. © 2013 Society of Plastics Engineers

Temperature resistivity behaviour in carbon nanotube/ultrahigh molecular weight polyethylene composites with segregated and double percolated structure

A novel conductive polymer composite (CPC) based on carbon nanotube (CNT) and binary polymer blend of low density polyethylene (LDPE) and ultrahigh molecular weight polyethylene (UHMWPE) was successfully fabricated by high speed mechanical mixing and hot pressing. The conductive CNT/LDPE component was only dispersed on the surfaces of UHMWPE particles, manifesting combined segregated and double percolated structure. The temperature resistivity behaviour of the CPC was investigated in this paper. A typical double positive temperature coefficient (PTC) of resistivity was observed near the melting points of LDPE and UHMWPE followed by a negative temperature coefficient (NTC) of resistivity. The resistivity began to rise at the maximum crystallisation temperature during cooling, indicating that the conducting nanoparticles were expelled from the crystalline phase. This CPC also exhibited a relatively low PTC and NTC effect, which was determined by the combined segregated and double percolated conductive network as well as high viscosity of UHMWPE matrix. Moreover, with the assistance of in situ optical micrograph and transmission electron microscopy, we found that the subtle microstructural evolution of the conductive network via Brownian motion and crystallisation induced flow was responsible for the higher room temperature resistivity.

Effect of the carbon black structure on the stability and efficiency of the conductive network in polyethylene composites

AbstractComposites of high‐density polyethylene (HDPE) with different kinds of carbon black (CB) were prepared through melt blending. The influence of the CB structure on the stability and efficiency of the conductive network in HDPE/CB composites were mainly investigated. Scanning electron microscopy was used to observe the morphology of the CB primary aggregates. The relationship between the temperature‐resistivity behaviors of the composites and the crystallization behaviors of the matrix were also investigated. High‐structure CB built an effective conductive network at a low filler content compared to the low‐structure one because of its branched morphology. Therefore, the composite containing high‐structure CB revealed a lower percolation threshold. The composite containing low‐structure CB obtained a stronger positive temperature coefficient (PTC) intensity because the cluster network was fragile and easily damaged during matrix melting. The reproducibility of the results of PTC effect of the composite containing high‐structure CB was better than that of the composite containing a low‐structure one. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Development of Ni particle dispersed poly(methylmethacrylate) composites exhibiting conductor/insulator transition by the positive temperature coefficient effect of electrical resistivity

In this paper, we have provided the first conclusive evidence that the Ni particle dispersed poly(methylmethacrylate) (PMMA) composites, which do not exhibit melting phenomenon, exhibit the positive temperature coefficient (PTC) effect when Ni content were 20 and 25vol.%. In particular, the composite with Ni content of 25vol.% exhibited PTC effect comparable to that of composites composed of crystalline polymer, and it demonstrated conductor/insulator transition. We assumed that these composites have few conductive paths, and so disconnection of conductive path occurred from slight volume expansion of PMMA by temperature rising.

High performance hybrid carbon fillers/binary–polymer nanocomposites with remarkably enhanced positive temperature coefficient effect of resistance

Aiming to enhance the positive temperature coefficient (PTC) effect of resistance, immiscible polymer blends [ultra-high molecular weight polyethylene (UHMWPE)/polyvinylidene fluoride (PVDF) = 4 : 1] based composites containing hybrid fillers [carbon nanotubes (CNTs) and carbon black (CB)] were explored. The conductive fillers were premixed with UHMWPE by melt-mixing and then PVDF was introduced into the blends. The preferential distribution of conductive fillers in the UHMWPE phase was desirably observed. Besides decreasing the electrical resistivity of the single UHMWPE based PTC materials, the addition of PVDF with much higher melting point could improve the temperature range of the PTC materials, which is important for their potential applications. A remarkable synergetic effect arising from the combination of CB and CNTs with different geometric structures and aspect ratios on improving the PTC behavior was demonstrated. By introducing 0.5 vol% CNTs into the 4 vol% CB filled UHMWPE0.8–PVDF0.2 composites, the initial resistivity decreased by about two orders of magnitude and the PTC intensity (PTCI) increased by about 30%. Owing to the 3-dimensional conductive networks provided by tube-shaped CNTs and spherical CB and the high viscosity of the UHMWPE matrix, favorable PTC repeatability was also achieved.

Resistivity–temperature characteristics of filler-dispersed polymer composites

Composites containing carbon nano tube (CNT) or carbon black (CB) conductive particle filler have the special characteristics of positive-temperature-coefficient (PTC) effects of resistivity. We quantitatively studied the relationship between poly(vinylidene fluoride) (PVDF) polymer's thermal volume expansion and the PTC effects of PVDF/CNT and PVDF/CB. The equation to revise filler content at each temperature due to the considerable thermal volume expansion rate of PVDF polymer indicates that filler content decreased with rising temperature. The graphs of filler content at room temperature plotted against apparent filler content with PTC effect were linear and their slopes were constant. From these graphs, we can determine the filler content necessary to occurring PTC effects. For example, the CNT content was 89% at room temperature, and the CB content was 93%. To our knowledge, this study is the first to report such phenomena.

Positive temperature coefficient effect of polymer-carbon filler composites under self-heating evaluated quantitatively in terms of potential barrier height and width associated with tunnel current

A new approach for positive temperature coefficient (PTC) effect of resistivity for polymer-filler composites by self-heating and external forced heating was proposed, based on SEM, ESR and positron annihilation in terms of tunnel effect of electrons through polymer matrix between neighboring short carbon fibers (CFs). The CF content as fillers in polymer matrix was fixed at the concentration slightly higher than the percolation threshold. The fitting between theoretical resistivity calculated by tunnel current density and experimental resistivity indicated that transport hindrance of electrons with elevating temperature was attributed to a drastic increase in potential barrier height resulted from free volume expansion in polymer matrix as well as air volume expansion between neighboring CFs. Different from the well-known concept, this effect was much more predominant than an increase in potential barrier width. The proposed quantitative analysis also satisfied time-dependence of resistivity at a fixed applied electric field and an increase in conductivity by γ-ray irradiation.

Effects of different bismuth-based compounds additives on the positive temperature coefficient effect of BaTiO3 ceramics

As a candidate for lead-free positive temperature coefficient of resistivity (PTCR) ceramics, BaTiO3 (BT) ceramics with different Bi-based compounds (K0.5Bi0.5TiO3, BaBiO3, Bi4Ti3O12 and Bi2O3) were fabricated by a conventional solid state reaction. The effects of Bi-based compounds additives on the phase composition, microstructure and PTCR characteristics of BT ceramics were investigated with X-ray diffraction, scanning electron microscope, and resistivity-temperature measurement instrument. The results showed that the average grain size, the Curie temperature (Tc) and the room temperature resistivity (ρRT) of BT ceramics varied with the additives of Bi-based compounds. The minimum value of ρRT for BT ceramics with different Bi-based compounds could be obtained when the atom concentrations of bismuth were around 0.2%. The ceramic samples showed a moderate PTCR effect, with a resistance jump greater by 1.4–3 orders of magnitude and increased Tc about 120–132°C, along with a low resistance of 4–1500Ωcm.

Electrical properties and EMI shielding characteristics of multiwalled carbon nanotubes filled carbon black-high density polyethylene nanocomposites

Electrically conducting nanocomposites were prepared by melt mixing process using high density polyethylene (HDPE) with 20 wt.% of carbon black (CB) and 0–1 wt.% of multiwalled carbon nanotubes (MWNTs). The effects of MWNTs in HDPE-CB-MWNTs composites were examined using X-ray diffraction and differential scanning calorimeter. DC resistivity (ρ dc), AC conductivity (σ ac), and some dielectric properties and electromagnetic interference (EMI) shielding effectiveness of these composites have been investigated. The results reveal that ρ dc decreases significantly whereas σ ac, , and EMI shielding effectiveness increase with loading of MWNT. The values of , , and σ ac are frequency dependent. Positive temperature coefficient effects are observed to be dominant over temperature range of 60–105 °C and the shielding effectiveness is frequency dependent over the X band (8.2–12.4 GHz).

Triple percolation behavior and positive temperature coefficient effect of conductive polymer composites with especial interface morphology

Selective localization of carbon black (CB) at the interface of polymer blends was achieved by the method that poly(styrene-co-maleic anhydride) (SMA) was first reacted with CB, and then blended with nylon6/polystyrene (PA6/PS). In the PA6/PS blends, CB was localized in PA6 phase and typical double percolation was exhibited. In the PA6/PS/(SMA–CB) blends, TEM results showed that CB particles were induced by SMA to localize at the interface, resulting in the especial interface morphology fabricated by SMA and CB. The especial interface morphology of PA6/PS/(SMA–CB) caused distinct triple percolation behavior and very low percolation threshold. The positive temperature coefficient (PTC) intensity of PA6/PS/(SMA–CB) composites was stronger than that of PA6/PS/CB and the negative temperature coefficient (NTC) effect was eliminated. The elimination of NTC effect was arisen from the especial interface morphology. A stronger PTC intensity was attributed to the low percolation threshold and the morphology.

Anomalous attenuation and structural origin of positive temperature coefficient (PTC) effect in a carbon black (CB)/poly(ethylene terephthalate) (PET)/polyethylene (PE) electrically conductive microfibrillar polymer composite with a preferential CB distribution

AbstractThis article reports the positive temperature coefficient (PTC) and negative temperature coefficient (NTC) effects of a carbon black (CB)‐filled electrically conductive microfibrillar poly(ethylene terephthalate) (PET)/polyethylene (PE) composite (FCMC). The composite contains in situ polymer microfibrils in the matrix of another polymer with CB particles selectively localized at microfibrils' surfaces. Anomalous attenuations of PTC and NTC intensities (IPTC and INTC) of FCMC were observed during heating–cooling runs (HCRs) and long‐term isothermal treatments. Particularly, when the isothermal treatment time was 32 h, the IPTC decreased from 5.5 in the original sample to only 0.5, showing a tremendous attenuation ratio of up to 91%, and the NTC effect was completely eliminated. On the contrary, attenuations of PTC and NTC effects in a common conductive polymer composite (CCPC) were so weak as to be negligible through the same thermal treatments. Microstructural changes of the conductive network by Brownian motion and large size of the conductive component‐CB coated PET microfibrils are both responsible for the great reductions in IPTC and INTC. The present results strongly suggest that thermal field induced microstructural transformation by Brownian motion helps to reveal the origin of PTC and NTC effects. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

The Resistivity Response of an Anisotropically Conductive Polymer Composite with in-situ Conductive Microfibrils During Cooling

The resistivity response of anisotropically conductive polymer composite (ACPC) with carbon black (CB) particles selectively localizing at the surfaces of aligned polyethylene terephthalate (PET) microfibrils was investigated during cooling from 180°C in the parallel and perpendicular directions. The ACPC exhibited a cooling positive temperature coefficient (PTC) around the crystallization temperature of polyethylene (PE) matrix. With the increase of CB loading, the cooling PTC effect decreased gradually. When the ACPC experienced isothermal annealing, the aligned microfibrils disordered to form more conductive pathways. The increase of annealing time caused the gradual attenuation of the cooling PTC effect in both directions.

Temperature and time dependence of electrical resistivity in an anisotropically conductive polymer composite with in situ conductive microfibrils

AbstractAn anisotropically conductive polymer composite (ACPC) based on conductive carbon black (CB) and binary polymer blend of polyethylene (PE) and polyethylene terephthalate (PET) was successfully fabricated under shear and elongational flow fields. The PET phase formed in situ the aligned conductive microfibrils whose surfaces were coated by CB particles. This ACPC material exhibited a strong electrical anisotropy within a broad temperature range. When the ACPC samples were subjected to isothermal treatment (IT), they showed anomalous variations of the positive temperature coefficient (PTC) and negative temperature coefficient (NTC) effects. The PTC intensity was attenuated gradually with the increase of the IT time, and the NTC intensity was nearly eliminated after IT of 8 or 16 h. Beyond 16 h, the resistivity in the NTC region rose anomalously with the temperature after the elimination of NTC effect, which was the result of much transformation from the potential pathways to the intrinsic pathways due to the disordering of oriented conductive microfibrils. When the amount of potential pathways was very small, the effect of the intrinsic pathway separation surmounts that of the potential pathways, leading to the anomalous resistivity increase in the NTC region. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Integrated thermal management strategy and materials for solid oxide fuel cells

The application of positive temperature coefficient (PTC) thermistors to thermal management of solid oxide fuel cells (SOFCs) is considered. A strategy is proposed for eliminating hotspots and reducing temperature gradients in SOFCs via inclusion of a material that exhibits a dramatic change in resistance near the desired maximum cell temperature. Three PTC thermistor materials with transition temperatures near the maximum desirable SOFC operating temperature are identified and screened for compatibility with common SOFC materials. All are found to be unsuitable because of reaction with the SOFC materials or unacceptably small PTC effect.

Positive temperature coefficient effect and interaction based on low‐density polyethylene/graphite powder composites

AbstractIn this article, the positive temperature coefficient (PTC) and interaction based on low‐density polyethylene (LDPE) filled with the loading of graphite (G) powder have been investigated. The dependence of the room temperature resistivity on filler content showed the significant decrease. The PTC behavior enhanced with increasing graphite content but this was not always the case. The maximum PTC effect was observed in LDPE/G composites (G, 45 wt %) with the relatively low room temperature resistivity. The thermal behavior was measured by differential scanning calorimetry (DSC). The structure characteristic for LDPE/G composites was examined by X‐ray diffraction (XRD), field‐emission scanning electron microscopy (SEM), and stress–strain test. The fact was revealed that the slight interaction between LDPE matrix and graphite may lead to change the thermal‐electric properties of the PTC materials. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

(1-x)BaTiO3+(x)La2/3Sr1/3MnO3 复合材料的结构和电子性质

Liquid-phase sintering method was used to prepare BaTiO3 powders and the structure and electrical properties of (1-x)BaTiO3+(x)La2/3Sr1/3MnO3 (LSMO) composites were investigated. The results of X-ray diffraction showed that the prepared BaTiO3 powders were pure and fine, indicating that sintering temperature was effectively lowered when the NaCl sintering aid was added. X-ray diffraction patterns of (1-x)BaTiO3+(x)La2/3Sr1/3MnO3 (LSMO) composites showed that LSMO and BaTiO3 phases were coexistent and no other phases were detected. The impedance spectra showed that the resistance of grain boundaries for LSMO-doped samples was suppressed. The resistivity-temperature measurements showed that roomtemperature resistivity of the composites was lowered from insulator for pure BaTiO3 ceramic to 10 Ω·m for the x=0.3 sample. The sample of x≤0.2 showed the positive temperature coefficient (PTC) effect whereas the x=0.3 sample exhibited PTC effect at temperatures below 68 ◦C and then negative temperature coefficient (NTC) effect at temperatures above 68 ◦C. The related mechanism has been elucidated.

Direct Current Conductivity of Carbon Nanofiber-Based Conductive Polymer Composites: Effects of Temperature and Electric Field

Polymer composites based on high density polyethylene (HDPE) and carbon nanofiber (CNF) were fabricated by melt compounding. The dependences of electrical conductivity of HDPE-CNF composites on filler concentration, temperature, and applied electric field were investigated. The results showed that the conductivity of the HDPE-CNF composites follows the scaling law of percolation theory. Increasing temperature caused a sharp increase in the resistivity of HDPE-CNF composites near the melting temperature of HDPE, yielding a positive temperature coefficient (PTC) effect of resistance. The potential mechanisms involved in the PTC effect of such composites were analyzed. An investigation of the effect of electric field on the conductivity of HDPE-CNF composites revealed the presence of tunneling conduction. The tunneling conductivity increased with increasing filler content because of high tunneling frequency, and decreased with rising temperature as a result of gap widening between conducting CNF fillers.

PVDF/CB composites prepared by novel solvent casting method for low voltage PTC temperature regulation applications

AbstractA novel solvent casting preparation technique utilizing three variants of poly(vinylidene fluoride) (PVDF) to achieve a thermal cut off and a self regulation effect at a low applied voltage is reported in this study. The positive temperature coefficient (PTC) composites were prepared by dissolving PVDF in 1‐methyl‐2‐pyrrolidone (NMP) solvent, blending with Vulcan® XC72 carbon black (CB) filler, crosslinking with vinyl trimethoxysilane (VTMOS) and quenching in water. All composites displayed a highly macrovoidal structure that promoted a PTC effect when subjected to a thermal expansion effect via an electrical current. Subsequently the current was cut off and self regulation behavior was exhibited. Kynar® 761A PVDF resulted in the strongest PTC effect, and displayed temperature regulation at around 100°C which may be attributed to the highly semi crystalline nature and the larger molecular weight of this polymer in comparison with the other PVDF composites studied. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Study on the Temperature-Sensitive Properties of Carbon Fiber-Reinforced Mortar

Carbon fiber (~5mm long)-reinforced mortar is found to be an effective thermal sensor. In this paper, relations between temperature change and resistivity of carbon fiber- reinforced mortar (CFRM for short) with different carbon fiber contents (0.4%~1.2% by mass of cement) are studied. The results show that during the initial period, the resistivity decreases when the temperature increases (Negative Temperature Coefficient effect). After the temperature reaches a certain value, the resistivity increases when the temperature increases (Positive Temperature Coefficient effect). Besides, with the change of carbon fiber content, the transit temperature of NTC/ PTC effect also changes. Based on the experimental results, the CFRM shows a potential use as a thermal sensor. The mechanisms of temperature- sensitive properties and NTC/ PTC transition are also discussed.