Strong Positive Temperature Coefficient Research Articles

Critical Behavior of Electrical Conductivity for Reduced Graphene Oxide/Epoxy Resin Composites

Abstract: In this paper, we analyze the dc and ac electrical conductivities, in the 240 to 400 K temperature range and 102 to 106 Hz frequency range, of a percolating system synthesized by mixing reduced graphene oxide (rGO) particles in insulating epoxy resin matrix, diglycidyl ether of bisphenol A (DGEBA). We found that the dc electrical conductivity of the samples is strongly related to the rGO content, indicating a percolating behavior with percolation threshold ≈ 4 %. The critical behavior of the dc electrical conductivity as a function of the temperature indicates a strong positive temperature coefficient and a negative temperature coefficient of resistivity below and above the transition temperature Tg, respectively. Moreover, the results showed that the dc conductivity obeys the Arrhenius law and the ac electrical conductivity is both frequency and temperature dependent and follows the Jonscher’s power law. Keywords: Composites, Dielectric properties, Fillers, Glass transition, Graphene.

Generalized stacking fault energy of γ-Fe

We investigate the generalized stacking fault energy (-surface) of paramagnetic -Fe as a function of temperature. At static condition, the face-centred cubic (fcc) lattice is thermodynamically unstable with respect to the hexagonal close-packed lattice, resulting in a negative intrinsic stacking fault energy (ISF). However, the unstable stacking fault energy (USF), representing the energy barrier along the -surface connecting the ideal fcc and the intrinsic stacking fault positions, is large and positive. The ISF is calculated to have a strong positive temperature coefficient, while the USF decreases monotonously with temperature. According to the recent plasticity theory, the overall effect of temperature is to move paramagnetic fcc Fe from the stacking fault formation regime ( K) towards maximum twinning ( K) and finally to a dominating full-slip regime ( K). Our predictions are discussed in connection with the available experimental observations.

Novel Positive Temperature Coefficient Effect of Electrical Resistivity in Ni Particles Filled Two Semi-Crystalline Polymer Intermixture Nanocomposite

A novel positive temperature coefficient (PTC) effect nanocomposite by employing both of polyvinylidene fluoride (PP) and polypropylene (PVDF) intermixture in 1-1 volume ratio as host materials and Ni nanoparticles as conductive filler was prepared via a simple hot compaction method. This study focused on the effect of Ni content on percolation threshold and temperature dependence of direct current (DC) resistivity, as well as frequency dependence of alternating current (AC) resistivity of nanocomposites. This kind of material showed a very small percolation threshold value (about 6.0 vol% of Ni) and a novel strong PTC effect of electrical resistivity between temperatures ranging 150-167 °C, which coincided with the both melting temperatures of PP and PVDF. Close to the percolation threshold, abnormal electrical behavior was observed and interpreted based on the tunnel conductivity between Ni clusters. The curves of the AC resistivity versus the frequency, at different loadings of Ni and at different temperatures, offered a further image of the arrangement of conducting clusters within the host.

Tuneable PTC effect in polymer-wax-carbon composite resistors

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

Tuning of the PTC and NTC effects of conductive CB/PA6/HDPE composite utilizing an electrically superfine electrospun network

Temperature-resistivity behaviors of the carbon black (CB)/polyamide 6 (PA6)/high density polyethylene (HDPE) composite with a CB coated electrospun PA6 superfine fibrils network were investigated. A strong positive temperature coefficient (PTC) intensity together with a weak negative temperature coefficient (NTC) intensity has been achieved synchronously by the isothermal treatment (IT). This fascinating phenomenon is attributed to the large size of the conductive component-CB/PA6 fibrous networks and the position evolution of the scattered CB in the conductive polymer composite. A model has been proposed to explain this interesting result. Our work provides an effective approach to prepare a high performance thermal sensitive material.

Structure and magnetic properties of low-temperature phase Mn-Bi nanosheets with ultra-high coercivity and significant anisotropy

The microstructure, crystal structure, and magnetic properties of low-temperature phase (LTP) Mn-Bi nanosheets, prepared by surfactant assistant high-energy ball milling (SA-HEBM) with oleylamine and oleic acid as the surfactant, were examined with scanning electron microscopy, X-ray diffraction, and vibrating sample magnetometer, respectively. Effect of ball-milling time on the coercivity of LTP Mn-Bi nanosheets was systematically investigated. Results show that the high energy ball milling time from tens of minutes to several hours results in the coercivity increase of Mn-Bi powders and peak values of 14.3 kOe around 10 h. LTP Mn-Bi nanosheets are characterized by an average thickness of tens of nanometers, an average diameter of ∼1.5 μm, and possess a relatively large aspect ratio, an ultra-high room temperature coercivity of 22.3 kOe, a significant geometrical and magnetic anisotropy, and a strong (00l) crystal texture. Magnetization and demagnetization behaviors reveal that wall pinning is the dominant coercivity mechanism in these LTP Mn-Bi nanosheets. The ultrafine grain refinement introduced by the SA-HEBM process contribute to the ultra-high coercivity of LTP Mn-Bi nanosheets and a large number of defects put a powerful pinning effect on the magnetic domain movement, simultaneously. Further magnetic measurement at 437 K shows that a high coercivity of 17.8 kOe and a strong positive temperature coefficient of coercivity existed in the bonded permanent magnet made by LTP Mn-Bi nanosheets.

8인치 Si Power MOSFET Field Ring 영역의 도핑농도 변화에 따른 전기적 특성 비교에 관한 연구

Power Metal Oxide Semiconductor Field Effect Transistor's (MOSFETs) are well known for superior switching speed, and they require very little gate drive power because of the insulated gate. In these respects, power MOSFETs approach the characteristics of an "ideal switch". The main drawback is on-resistance RDS(on) and its strong positive temperature coefficient. While this process has been driven by market place competition with operating parameters determined by products, manufacturing technology innovations that have not necessarily followed such a consistent path have enabled it. This treatise briefly examines metal oxide semiconductor (MOS) device characteristics and elucidates important future issues which semiconductor technologists face as they attempt to continue the rate of progress to the identified terminus of the technology shrink path in about 2020. We could find at the electrical property as variation p base dose. Ultimately, its ON state voltage drop was enhanced also shrink chip size. To obtain an optimized parameter and design, we have simulated over 500 V Field ring using 8 Field rings. Field ring width was <TEX>$3{\mu}m$</TEX> and P base dose was <TEX>$1e15cm^2$</TEX>. Also the numerical multiple <TEX>$2.52cm^2$</TEX> was obtained which indicates the doping limit of the original device. We have simulated diffusion condition was split from <TEX>$1,150^{\circ}C$</TEX> to <TEX>$1,200^{\circ}C$</TEX>. And then <TEX>$1,150^{\circ}C$</TEX> diffusion time was best condition for break down voltage.

Structural and magnetic properties of bulk MnBi permanent magnets

Structural and magnetic properties of bulk nanostructural Mn100−xBix (x = 40, 45, 52) permanent magnets prepared by spark plasma sintering technique were studied. The effect of the Mn/Bi ratio on the MnBi low temperature phase (LTP) formation and its magnetic properties were investigated. An increase of the bismuth amount in the magnets leads to better formation of LTP, resulting in the improvement of both magnetization (Ms) and remanence (Mr), but decreasing the coercivity (Hc) of the magnets. At room temperature, Ms increases from 27.87 emu/g for Mn60Bi40 to 45.31 emu/g for Mn48Bi52, whereas Hc decreases from 12 to 7.9 kOe. The microstructure of Mn48Bi52 magnet is composed of fine and uniform grains with an average size of 140 nm as shown in the TEM image. The Mn48Bi52 magnet shows a high Hc of 19 kOe at 423 K, indicating a strong positive temperature coefficient of coercivity for the MnBi magnet.

Remarkable selective localization of modified nanoscaled carbon black and positive temperature coefficient effect in binary-polymer matrix composites

Academic and industrial research groups are currently working on materials with high-quality positive temperature coefficient (PTC) effects. They are crucial for designing new switching sensors, self-regulating heaters and shielding structures. Such materials are in fact the basis for a new generation of cheap and large sensors. Here we present a robust and simple procedure to prepare binary-polymer matrix composites with strong PTC effect through melt-mixing and subsequent hot-pressing technology. The intuitive and detailed double-percolation structure is observed on the basis of morphological evidence. Modified nanoscaled carbon black (MNCB) is often selectively localized in one of the binary-polymer phases. When the volume ratio of the two polymers is very close to 1 : 1, we observe remarkable PTC intensity (PTCI), namely PTCI = 3.3 × 107 in MNCB-high density polyethylene/polypropylene (MNCB-HDPE/PP) and PTCI = 8.3 × 107 in MNCB-HDPE/polyvinylidene fluoride (MNCB-HDPE/PVDF). The process for the preparation of these binary-polymer matrix PTC composites is suitable for industrial mass production.

Exploration of unusual electrical properties in carbon black/binary-polymer nanocomposites

Ultralow percolation threshold, strong positive temperature coefficient (PTC) effect, and abnormal alternating current electrical behaviors were obtained in ternary nanocomposites consisting of polypropylene (PP) and polyvinylidene fluoride (PVDF) (volume ratio: 1∕1) loaded with different amounts of nanosized carbon black (CB). These unusual properties could be assigned mainly to the special formation of electrically double-percolation network, the consecutive volume expansion of (PP-PVDF) cocontinuous phase, and the tunnel conduction between CB clusters. Both the low percolation threshold and the strong PTC effect should make the (CB-PP)/PVDF nanocomposites one of the most promising functional dielectric materials.

Sensor properties of SBS block copolymer and carbon composite film structures

Carbon composite thin films were prepared from a styrene butadiene styrene (SBS) block copolymer matrix with dispersed graphitic carbon as the second phase. Precursor solutions in the viscosity range of 150–200 cps were developed from codispersion of SBS–carbon in toluene as solvent base. Film coatings were prepared by spin coat onto glass, alumina or silicon substrates at 2000 rev min−1 followed by drying and vacuum heat treatment at ∼80°C. Characterisation of the films was carried out using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and SEM techniques. Degree of crystallinity in the composite film structures was correlated with the strong positive temperature coefficient (PTC) effect. These composite thin films also showed high resistance sensitivity to temperature in the range from 2 90 to −90 to +90°C. A unique feature of the resistance/temperature response was the observed negative temperature coefficient (NTC) behaviour below ∼0°C combined with a strong PTC response above ∼0°C. When exposed to various organic vapours, the composite films showed high selectivity, indicated by sensitive change in resistivity values.

Ruthenium oxide cryogenic temperature sensors

RuO 2 based cryogenic temperature sensors were developed using a nanopowder approach. Ruthenium oxide nanopowders were consolidated into compacts using the Plasma Pressure Compaction (P 2C ®) process and were tested for resistivity as a function of temperature for determining the reproducibility and stability of the sensors due to thermal stresses. The sensors showed metallic behavior and strong positive temperature coefficient of resistivity (PTCR). Resistance–temperature calibration demonstrated a monotonic response with a positive temperature coefficient of resistivity from 70 to 300 K along with dimensionless sensitivity (approximately 1.0) comparable to that of platinum resistance thermometers.

A Links-Nodes-Blobs Model for Conductive Polymer Composites

A Links-Nodes-Blobs (L-N-B) model, based on the fractal and percolation concepts, is used to study the electrically conductive mechanism of conductive filler loaded polymer composites. The change in the conductivity of polymer composites during the mixing process can be explained as the competition between the breakdown of filler aggregates and the diffusion of ingredients of matrix material and impurities onto the surface of the filler. The value of the fractal dimension μ, which is the exponent in the power-law relationship of the electrical conductivity σ = σ0·(ϕ – ϕc)μ, is calculated as 1.88. This value is close to the values obtained directly from experiments or from other simulations. The positive temperature coefficient (PTC) behavior in the conductivity of composite material is also explained by this model as the breakdown of the conductive filler network. If the thermo-expansion induced strain is greater than the apparent on-set strain εonset = mQ + 2 G/2d G·εb of the L-N-B model, a strong PTC effect would happen.

Spherulites of Tussah Silk Fibroin. Structure, thermal properties and growth rates

The crystal structure, thermal properties and growth rates of spherulites of the Tussah silk fibroin, produced upon drying of the silk taken directly from the lumen which is essentially a poly(L-alanine)polypeptide, are investigated. Depending on casting conditions, spherulites with either αhelical chain conformation or β parallel sheet structure are produced. The growth rates display a strong positive temperature coefficient, with an apparent transition, which however cannot be related with the formation of two different crystal structures at this stage.

Fast current limitation by conducting polymer composites

The transition of materials from low resistivity to comparatively high resistivity can be utilized for current limitation, enabling permanent fuses that do not have to be replaced after an overload or short-circuit operation. An interesting class of materials for this purpose are particulate filled polymer composites with a strong positive temperature coefficient (PTC) of resistivity. If an applied current becomes too high, the PTC element is heated to its critical temperature and trips from the conducting into the insulating state. The dynamic heating of the composite upon current flow is described by a one-dimensional model. It is predicted that the heating of a composite depends on the size of the filler particles. Smaller filler particles should allow a faster heating and, hence, a better limitation of the current. Experimental verification is performed using composite of TiB2 particles in a polyethylene matrix. Commercial TiB2 powders with different particle-size distributions between 1 and 200 μm were used. The specific resistivity of the composites is small, in the range of 0.01–0.02 Ω cm. Around the melting temperature of the polymer, the resistivity increases within only 20 °C by seven orders of magnitude. In order to verify the expected dependence of the switching dynamic on the filler particle size, the tested elements had comparable electrical characteristics. Samples were prepared having, to a certain degree, the same specific resistivity, cross section, and total resistance. Free parameters were the length, and for some samples, the filler content. Short-circuit experiments show that for decreasing particle size the time until the material trips into the high-resistive state becomes shorter. The best current limitation occurs for composites containing particles in the range of 1–45 μm. Current limitation starts already after 150 μs, and a current density of up to 10 kA/cm2 can be switched off within a further 200 μs. The experiments are in excellent agreement with the predictions from theory. Due to the low resistance in the cold state and the very fast limitation of electrical currents, PTC elements based on conducting polymers can be highly attractive for power applications.

Electrical and dynamic mechanical behavior of carbon black filled polymer composites

The effect of processing conditions on the electrical and dynamic behavior of carbon black (CB) filled ethylene/ethylacrylate copolymer (EEA) composites was investigated. The compounds were prepared by two methods, solution blending and mechanical mixing. Compared with the solution counterpart, the mechanical composites have a strong positive temperature coefficient (PTC) effect and a high dynamic elastic modulus, which results from the good dispersion state of carbon black in EEA, i.e. the strong interaction between carbon black and EEA. It can be concluded that the strong interaction between polymer and carbon black is essential for composites to have a high PTC intensity, good electrical reproducibility and high dynamic elastic modulus.

I-V characteristics of carbon black-loaded crystalline polyethylene

The I-V characteristics of carbon black-loaded crystalline polyethylene were investigated at room temperature and at few degrees above the melting temperature, Tm. The material was chemically cross-linked using silane. It had a large carbon black content of 28 parts per hundred by weight and displayed a strong positive temperature coefficient of resistance (PTCR) effect with a resistivity jump of four orders of magnitude. Logarithmic current-voltage plots were found to be linear with unity slopes at temperatures above and below Tm, indicating good ohmic behaviour. This is in contrast with previous theories which explain the PTCR effect on the basis of electron tunnelling as the current conduction mechanism. A new model capable of explaining both the PTCR effect and the steep reduction in resistance above Tm is presented, according to which the PTCR effect is the result of the co-operative effect of changes in crystallinity and volume expansion.