Large Positive Temperature Coefficient Research Articles

PTCR Characteristics in YBa2Cu3O7−x —Linear Low Density Polyethylene (LLDPE) Composite Materials

A series of superconductor/polymer composites is made using high Tc YBa2Cu3O7−x (Y-123) phase and Linear Low Density Polyethylene (LLDPE). The resistivity of the composites is measured as a function of temperature between 293 K and 400 K. A large positive temperature coefficient (PTC) of resistance is observed near 395 K. An attempt has been made to relate the percolation behavior of the YBCO–LLDPE composites and the thermal expansion of the LLDPE matrix. The intensity of the PTC effect for these composites was found to be as large as eight orders of magnitude.

PTCR effect in Li0.12Na0.88Ta y Nb1−y O3 ferroelectric solid solutions

Ceramic samples of Li0.12Na0.88Ta y Nb1−y O3 ferroelectric solid solutions reduced in vacuum in order to raise their electronic conductivity are shown to have a large positive temperature coefficient of resistance in the range 350–400°C, near the superionic transition temperature of analogous unreduced ceramics. This behavior is interpreted in terms of the interplay between electronic and ionic transport processes.

The electrical behavior of thermosetting polymer composites containing metal plated ceramic filler

AbstractThis paper describes the electrical behavior of a thermosetting system, based on epoxy resin, containing metal plated fillers. Ceramic fillers such as chopped glass fibers and mica flakes were coated with copper by electroless plating and incorporated into an epoxy resin based on di‐glycidyl ether of bisphenol A (DGEBA) with tri‐ethylenetetramine (TETA) curing agent. The percolation threshold in these systems is obtained at very low copper contents of 0.11–0.44 vol%. The epoxy/copper coated mica system is characterized by an extremely large positive temperature coefficient (PTC) effect, which is not followed by a negative temperature coefficient (NTC) effect. Increasing the copper coated mica concentration raises the PTC temperature of the first temperature cycle, and exposing the material to continuous heating‐cooling cycles results in a decrease in the PTC temperature and an increase of its room temperature resistivity. Inverse relations were found between the coefficient of thermal expansion and the PTC temperature. Accordingly, the mechanism governing the PTC effect in the epoxy/copper coated mica composite is based on a larger thermal expansion coefficient of the matrix compared with the ceramic filler. POLYM. COMPOS., 26:12–19, 2005. © 2004 Society of Plastics Engineers.

Electrical properties of the binary icosahedral quasicrystal and its approximant in the Cd–Yb system

Abstract Electrical resistivity, magnetoresistance, and specific heat measurements have been performed for the binary icosahedral Cd 5.7 Yb and its cubic approximant Cd 6 Yb. The resistivity of both the icosahedral phase (i-phase) and the approximant is found to show a rather metallic behavior with a large positive temperature coefficient of the resistivity (TCR) below 200–300 K followed by leveling-off at low temperatures below 10 K. For the approximant, we observe a stepwise change of the resistivity at 110 K which is due to a phase transition. Giant magnetoresistance as high as 200% at 9 T below 4.2 K is observed for the i-phase and the electronic specific heat coefficient γ is found to be extraordinary large in both alloys, i.e., 2.87 mJ/(mole K 2 ) for the i-phase and 7.60 mJ/(mole K 2 ) for the approximant. Such large γ values may be attributed to the Yb-derived states at the Fermi level. Furthermore, the Debye temperatures ( Θ D ) are very low and almost the same for both alloys, i.e., 142 and 144 K, respectively, which are indeed the lowest values ever reported in i-phases and approximants. The metallic character of the binary i-phase implies that the negative TCR is not necessarily a consequence of the quasiperiodicity.

Long afterglow phosphorescent sensor materials for fiber-optic thermometer

Long duration phosphorescent SrAl2O4 phosphors doped with various auxiliary activators (Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) have been evaluated for the fiber-optic thermometer application. In this thermometer, temperature was measured based on the temperature dependence of the lifetime and/or intensity of the afterglow phosphorescence which was dominated by the thermal excitation of trapped carriers. Characteristics of the long phosphorescent fiber-optic thermometer are quite different from those of the conventional thermometer using fluorescent lifetime of the phosphors. Lifetime and intensity of the afterglow phosphorescence from the long afterglow phosphorescent sensor heads show large positive temperature coefficients in the narrow temperature region around room temperature. Sensitivity and temperature range of the long phosphorescent fiber-optic thermometer depend strongly on the trap depth and the trap density of phosphors. Since the trap depth and the trap density vary with auxiliary activator elements, a variety of fiber-optic thermometers with high sensitivity in various temperature ranges will be able to be developed using Eu2+ doped SrAl2O4 codoped with various auxiliary activator Ln3+ ions (Ln=Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu).

Electrical and mechanical characteristics of composites consisting of fractionated poly(3-hexylthiophene) and conducting particles

The dynamic viscoelasticity of fractionated poly(3- hexylthiophene)titanium carbide (P3HT/TiC) composites was examined with regard to their electrical characteristics. The elastic modulus (E′) at 0°C [i.e., near the glass-transition temperature (Tg) of P3HT] increased with increasing TiC content of the composite. In particular, composites whose TiC content exceeded the threshold concentration showed a high E′. This was caused by the high E′ of TiC and the strong interaction between TiC and P3HT. When the sample was heated above the Tg, E′ decreased rapidly and an increase in the loss tangent appeared near the Tg of P3HT. Mechanical loss was caused by friction between TiC and P3HT. The change in mechanical characteristics affected the electrical conductivity. When the TiC content of the composite approximated to the threshold concentration, a significant change in mechanical characteristics took place, so that a large positive temperature coefficient (PTC) effect was observed near the Tg. To explain the PTC phenomenon, we propose a model of conductive pathway for P3HT/TiC. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1429– 1433, 2002

Structural and electrical characterization of semiconducting barium-lead-titanate ceramics

BaTiO3 is usually doped to achieve the temperature stability required bydevice applications, as well as to obtain a large positive temperature coefficient anomaly of resistivity (PTCR). Uniform distribution of dopants among the submicron dielectric particles is the key for optimal control of grain size and microstructure to maintain a high reliability. The system Ba0.84Pb0.16TiO3 was synthesized from high purity BaCO3, TiO2, PbO oxide powders as raw materials. Sb2O3, MnSO4 and ZnO were used as dopantsand Al2O3, TiO2 and SiO2 as grain growth controllers. Phase composition was analyzed by using XRD and the microstructure was investigated by SEM. EDS attached to SEM was used to analyze phase composition specially related to abnormal grain growth. Electrical resistivities were measured as a function of temperature and the PTCR effect characterized by an abrupt increase on resistivity.

Abnormal temperature dependence of intrinsic coercivity in sintered Sm–Co-based permanent magnets

In search of high-temperature permanent magnet materials, sintered Sm–Co-based magnets with abnormal temperature dependence of intrinsic coercivity have been obtained. These magnets have small negative, or near-zero, or large positive temperature coefficients of intrinsic coercivity. A model of coercivity mechanism has been proposed to explain this abnormal temperature dependence. According to this model, the coercivity in 2:17-type permanent magnets is controlled by two conflicting mechanisms. The first mechanism is thermal activation, which leads to decreased coercivity with increasing temperature. The second mechanism can be explained by a pinning process in which increasing temperature may result in an increased difference of crystalline anisotropy between the 2:17 cell phase and 1:5 cell boundary phase and, in turn, a higher coercivity at high temperature.

Study on effect of carbon black on behavior of conductive polymer composites with positive temperature coefficient

Conductive polymer composites showing large positive temperature coefficient (PTC) are made of semi-crystalline polymer as an insulator and a conducting filler, whose concentration is close to the critical volume fraction. In this study, the resistivity and PTC behavior of high density polyethylene (HDPE) filled with different carbon blacks were studied. Among those composites, N660 carbon black filled PE showed the greatest PTC behavior. Carbon black with large particle size, small surface area and small amount of aggregated structure leads to large amplitude of PTC transition (defined as the ratio of maximum resistivity to the resistivity at 25/spl deg/C). The great PTC behavior is due to some microscopic mechanism under the macroscopic thermal expansion of polymer matrix during melting of polymer crystal.

Low Temperature Coefficient Bulk Dielectrics in the Ca 2Nb 2O 7–Ca 2Ta 2O 7 System

The dielectric properties of bulk polycrystalline ceramics in the binary Ca 2Ta 2O 7–Ca 2Nb 2O 7 system are reported. Measurements of the dielectric constant at 1 MHz were made at temperatures between 0 and 100°C. The large negative (for the niobate) and positive (for the tantalate) temperature coefficients for the dielectric constants in the pure materials in the vicinity of room temperature can be balanced in a two-phase region near x = 0.36 in Ca 2Ta 2−xNb xO 7. At this composition, the dielectric constant is approximately 30, and the temperature coefficient of the dielectric constant is approximately 2 ppm/°C. The Q in our unoptimized polycrystalline ceramic at 1 MHz is approximately 5000.

Properties and applications of filled conductive polymer composites

The electrical properties of polymers filled with different types of conducting particles are reviewed. Following a theoretical description of a general effective media (GEM) equation, the experimental conductivity–volume fraction data for thermoplastic filled with vanadium oxide particles as well as thermosetting polymer composites, are fitted to the equation. The calculated property-related parameters in the equation are discussed. The electrical conductivity of the composites is combined with an extremely large positive temperature coefficient (PTC) effect, depending on the filler type (V2O3 or carbon black), as well as on its distribution and volume fraction. Both melting and recrystallization behaviour are responsible for the PTC effect. Due to a conductive filamentary network across the medium, a localized thermal effect comes into existence, leading to self-heating of the body. This gives the composites potential application, for example, in plastic welding. Preliminary experimental results are reported. ©1997 SCI

Effect of crosslinking and field strength on the electrical properties of carbon/polyolefin composites with a large positive temperature coefficient of resistivity

AbstractCrosslinking is a promising method for stabilizing the electrical properties of polyolefin composites with a positive temperature coefficient of resistivity (PTCR) for application as self‐controlled heaters. An interesting phenomenon is that the measured room temperature resistivity of such materials is independent of applied voltage at low DC fields, but crosses over to display an apparent decrease with the accretion of voltage above a critical value. Measured current vs. time of voltage application has been recorded that shows an equilibrium varying with temperature and voltage. Theoretical approaches to these results are discussed.

Electrical properties of polymer composites prepared by sintering a mixture of carbon black and ultra‐high molecular weight polyethylene powder

AbstractConductive polymer composites were prepared by sintering a mixture of ultrahigh molecular weight polyethylene (UHMWPE) powder and carbon black. Two processing parameters—time and temperature—were shown to have a notable effect on the resistivity of the composites. The relationships between the processing parameters and morphology were studied using optical microscopy and transmission electron microscopy (TEM). The results of the optical microscopy studies indicate that the carbon black is distributed in the interfacial regions between the UHMWPE particles. The dimension of the carbon black channels increases with the carbon black concentration. TEM micrographs show that a high degree of intermixing between the carbon black and the polymer occurs at higher temperatures and longer processing times, resulting in higher resistivities. A positive temperature coefficient (PTC) effect was observed for these materials. A mechanism for the PTC effect in this system is proposed. The magnitude of the PTC effect is found to be inversely proportional to the dimension of the carbon black channels in the composites. The dimension is directly related to the carbon black concentration. The PTC effect is a result of the polymer volume expansion caused by melting of the crystallites. A large PTC effect is observed for the composites with a low carbon black concentration and vice versa. No negative temperature effect (NTC) is observed at temperatures substantially above the melting point of the polymer.

Preparation and characterization of glass composite using metal particles coated with semiconductive SnO 2 fine particles obtained via sol—gel method

SnO2—glass composites are promising materials for nitrogen-fireable thick film resistor systems. However, the SnO2—glass composite has many undesirable properties which should be improved for industrial application, such as a high electrical resistivity and a large negative temperature coefficient of electrical resistance. This work was undertaken to make a survey of the methods for improvement in the electrical properties of the SnO2—glass composites. The effect of the addition of Cu particles, with a large positive temperature coefficient of electrical resistance, on the electrical properties of SnO2—glass composites has been investigated. Cu particles have been coated with semiconductive SnO2 fine particles by hydrolysing tin and antimony ethoxides then firing. The coated particles have been applied as conductive components in the glass composite. Using SnO2-coated Cu particles, Cu and SnO2 particles are homogeneously dispersed in a glass matrix, and the electrical properties of the glass composites largely depend on the volume fraction of Cu in the glass composite. The volume fraction of Cu in the glass composite is determined by a Cu/SnO2 volume ratio in the coated particles. From the experimental evidence, it is thought feasible to produce the glass composite having well-controlled electrical properties by the suitable selection of the Cu/SnO2 ratio in the coated particles.

Electrical properties of polyethylene highly filled with carbon

Carbon-filled polyethylene composites were fabricated and tested to establish the practical lower limit of their electrical resistivity at room temperature and to investigate the trade-offs between low resistivity and the magnitude of the resistance anomaly (i.e., a large positive temperature coefficient of resistivity) that appears when such composites are heated through the polyethylene crystalline melting transition. Carbon blacks with large particle size and low surface area provided low-resistivity composites having large resistance anomalies. The largest resistance anomalies were found in composites that were well mixed, but the room-temperature resistivity also increased in composites that were cycled repetitively through the crystalline-melting transition. A mixture of carbon blacks of two different sizes provided a lower resistance than was found in a material with the same fill of only the coarser black. By controlling the composition and the processing, composites were made with room-temperature resistivities lower than 0.2 ohm cm and resistance changes of at least 2 orders of magnitude. A resistance change of as much as 5 orders of magnitude was obtained for composites with room-temperature resistivities of only 1 ohm cm.

Excess molar enthalpies and excess molar volumes for (pronane + propan-1-ol) at a temperature of 348.15 K and at pressures of 5, 10, and 15) MPa

Excess molar enthalpies H m Eand excess molar volumes V m Ehave been determined for (propane + propan-l-ol) at a temperature T= 348.15 K and pressures p= (5, 10, and 15) MPa. Large and positive H m Ewith a large positive temperature coefficient (excess molar heat capacity) C p,m Eare explained in terms of hydrogen bonds being broken when the propan-1-ol is diluted with the propane. V m Eis negative with a large positive pressure coefficient, suggesting that the propane fits into cavities in the hydrogen-bonded structure of the propan-1-ol, with the number of cavities decreasing with increasing pressure. The V m Efor (propane + propan-1-ol) is intermediate between the values obtained for (ethane + propan-1-ol) and (butane + propan-1-ol). The relative magnitudes of V m Efor these three systems are explained in terms of the relationship of the temperature of measurement to the critical temperature of the alkanes.

Origin of Appearance of PTCR Properties in Bi–Sr–Ti–O System

(Bi0.85Sr0.15)4(Ti0.95Nb0.05)3O12 shows a large positive temperature coefficient of resistivity (PTCR) properties of almost 4 orders of magnitude at about 543 K. This temperature does not correspond to the Curic point of Bi4Ti3O12, but to the melting point of Bi metal. The microstructure of samples with or without showing PTCR properties observed by optical microscopy, showd the presence of Bi metal distributed in matrix. The various resistivity-temperature characteristics were obtained by controlling the distribution Bi metals in the matrix. Therefore, the new appearance of PTCR properties in Bi–Sr–Ti–O system would be related with the melting and solidification of Bi metal which is finely distributed in samples and isolated for each other. The equivalent electrical circuit for the samples were determind by complex impedance and modulus analysis. A model of this appearance of PTCR properties is proposed from these results as follows; Electrical current would flow in Bi metal at lower temperatures, but would not flow at higher temperatures. The resistivity of the sample at higher temperatures would be determined by matrix. Another RC component which acts only in the vicinity of PTCR temperature was also observed. This component may be due to the interface of Bi metals and the matrix.

Influence of Microstructure and Grain Boundary Potential Barrier Layer on the Electrical Breakdown of Positive Temperature Coefficient BaTiO3 Ceramics

The influence of the microstructure and the grain boundary potential barrier layer on the electrical breakdown of barium titanate positive temperature coefficient (PTC) ceramics was studied. Samples of various grain sizes were prepared by adjusting the amount of Sb substitution into Ba sites in barium titanate PTC ceramics. Samples exhibiting large PTC effect (R max/R min) and fine grain size withstand high applied voltage. It is clarified that grain size effect is related to the number of grain boundaries per unit thickness of the samples because the applied voltage drop is mainly across the potential barriers at grain boundaries. However, the voltage dependence of a single grain boundary potential barrier was more important in determining breakdown voltage in our study. The voltage dependence of the grain boundary potential barrier layer was due to the grain boundary built-in potential.

Influence of polarization spatial distribution on screening in polycrystalline ferroelectrics-semiconductors

Using the Landau-Devonshir thermodynamic potential the spatial distribution of polarization in polycrystalline ferroelectrics-semiconductors is calculated for the case of BaTiO3. It is shown that this distribution plays an important role in the screening of electric fields of charged grain boundaries. When polarization reorientation in the space region takes place the screening is most effective, the potential barrier height and, hence, resistivity values in the ferroelectric phase being the lowest resulting in large positive temperature coefficient of resistivity (PTCR) effect. The value of the PTCR effect as well as the dimensions of the grain boundary space charge region depend on the tnutial orientation of grain boundary electric field and the spontaneous polarization in the bulk of the grain.

Effect of temperature on the electrical resistivity of YBa 2Cu 3O 7−x filled polyethylene

Heterogeneous materials composed of conducting YBa 2Cu 3O 7−x particles randomly dispersed inside an insulating polyethylene matrix were prepared. The resistivity of the materials was measured as a function of temperature between 293 K and 410 K. A large positive temperature coefficient (PTC) of resistance was observed near 404 K, and was a maximum of 9 orders of magnitude for the specimen containing 30 vol.% YBa 2Cu 3O 7−x. The mechanism of the PTC effect in the materials is related to the percolation behaviour of the YBa 2Cu 3O 7−x-polyethylene composites and the thermal expansion of the polyethylene matrix.