Positive temperature coefficient of magnetization in AlNiCo magnets with Dy / Gd / Er addition

The temperature effect for the intensity of ,u-mesons at sea-level is investigated by taking 11-,u decays into account. The positive temperature coefficient is calculated by paying special attention to the difference in the attenuation mean free path of the N-component and the absorption mean free path of rr-mesons and to the atmospheric depth which gives main contributions to the coefficient. The magnitude of the coefficient is obtained as 0.05%/C and the depth of main contributions is found to be 200 mb. Influence of heavy mesons is qualitatively discussed § I. Introduction The atmospheric effect of cosmic rays has been interpreted in terms of the absorption of cosmic ray particles and of the spontaneous decay of p.-mesons. The former is correlated with the barometric effect, while the latter with the average height of the production of ,u-mesons. These effects were thoroughly analysed by Olberi> and there remains almost nothing added to his analysis, as far as the role of ,u-mesons is concerned. However, a possible interpretation of the positive temperature effect suggested by him seems to be an open question. Although Olbert tried to attribute the positive temperature effect to the difference in variations of two kinds of average heights introduced in his theory, these two are found to correlate with one another, so that Olbert's suggestion seems to be of little validity, as shown by Wada and Kudo2>. The positive temperature effect was first noticed by Forro3> for cosmic rays underground and was interpreted by Miyazima4> in terms of 1C·,tt decay. A detailed analysis of this effect showed that the upper limit of the positive temperature coefficient is about 0.5% j°C and is attained in the limit of high energies5>. The calculation of the coefficient was extended to lower energies by Barrett et al6> and their result predicted the magnitude of the coefficient appreciably smaller than 0.1% /°C for the bulk of the hard component observed at sea level. This had already been anticipated with a qualitative estimate by Duperier7> who observed a positive temperature coefficient of 0.1% j°C or larger for the hard component at sea level. This was found to be too high to reconcile with the theoretical prediction. On the other hand, several authorss-to> obtained lower values in rough agreement with the theoretical estimate. Such divergent experimental results are thought to be due partly to the varieties of ways of taking correlation with atmospheric temperatures. One can hardly say, at what

The relationship between resistivity and temperature in ordered α2-Ti3Al and γ-TiAl intermetallic compounds single crystals

In this paper, using Positive Temperature Coefficient (PTC) elements, we propose a simple repetitive high-speed PTC Current Limiter (PTC-CL) whose rated voltage is increased by using a series transformer (ST) at levels far beyond their intrinsic voltage rating. The PTC-CL consists of PTC resistors connected in series and in parallel to each winding of the ST. The PTC-CL was numerically studied by using the electro-magnetic transient program (EMTP) and the electrical equivalent model for PTC. The winding resistance effects and whether all of PTCs will trip or not under the fault current condition were studied. The experimental results in case of two PTCs, where the PTC element is rated (60V/40A), show that the sufficient voltage is applied on the non-tripped PTC even after another PTC trippd at first, and all PTCs of PTC-CL tripped within 0.2 ms where the fault current was suppressed from 1200 A to 52 A at rated voltage 120 V. But, the saturation current started to flow after 2 ms and reached above 1500 A because of the small size of the used core. In order to reduce it, RC components were proposed and the measurement results indicate that the saturation current can be eliminated adopting a suitable parameter for RC components.

Multiple enhancement on the magnetic and electric properties of SrM hexaferrites by phase-change perovskite additives

The electrical and microstructural characteristics of 1 kohm/sq. thick-film thermistors with high positive (PTC) and negative (NTC) temperature coefficients of resistivity (TCR) fired on green LTCC (Low-Temperature Cofired Ceramics) substrates were evaluated. Electrical characteristics, i.e., sheet resistivities, TCRs and noise indices were measured. Microstructures of thick films were investigated by scanning electron microscopy (SEM) and analysed by energy dispersive X-ray analysis (EDS). Functional phases in thermistors were determined by X-ray powder diffraction analysis. Obtained results were compared with characteristics of thick film thermistors fired on relatively inert alumina substrates which were used as a reference.

With the rapid development of the power integrated circuit technology, polysilicon resistors have been widely used not only in traditional CMOS circuits, but also in the high voltage applications. However, there have been few detailed reports about the polysilicon resistors’ characteristics, like voltage and temperature coefficients and breakdown behaviors which are critical parameters of high voltage applications. In this study, we experimentally find that the resistance of the polysilicon resistor with a relatively low doping concentration shows negative voltage and temperature coefficients, while that of the polysilicon resistor with a high doping concentration has positive voltage and temperature coefficients. Moreover, from the experimental results of breakdown voltages of the polysilicon resistors, it could be deduced that the breakdown of polysilicon resistors is thermally rather than electrically induced. We also proposed to add an N-type well underneath the oxide to increase the breakdown voltage in the vertical direction when the substrate is P-type doped.

Systematic investigation on neutronics and thermal-hydraulics model for multi-physics analysis of micro molten salt fast reactor with moderating reflector

Three major severe accidents with core meltdown/core disruption occurred at Three Mile Island (USA) in 1979, Chernobyl (Ukraine) in 1986 and Fukushima (Japan) in 2011.

Polymeric positive temperature coefficient (PTC) materials have been prepared by incorporating carbon black (CB) into two different polymer matrices, crystalline high density polyethylene (HDPE) and amorphous polystyrene (PS). The effects of thermal volume expansion on the electrical properties of conductive polymer composites were studied. The volume fraction of conductive particles behaves like a switch from insulator to conductor in the polymeric PTC composite. Our mathematical model and experimental model have proved that the abrupt resistivity increase at PTC transition range and at the percolation curve close to the critical volume fraction for both polymeric PTC composites have the same conductive mechanism. The thermal expansion is one of the key factors responsible for the PTC effect and can be seen by comparing the PTC transition curves from model predictions and experiment. Furthermore, the model predicts PTC curves of CB/PS composite more successfully than it does for the CB/HDPE composite, and the reasons for this are also discussed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3078–3083, 2007

Various thermoplastic and thermo-set based polymers, blended with acetylene carbon blacks and graphite fillers to form electrically conductive polymers, were investigated to determine the switching mechanism under high power short-circuit fault conditions. Presently, a positive temperature coefficient (PTC) effect is the accepted switching mechanism for low power overcurrent devices. However, for high power, distribution level devices, this mechanism has been assumed to be the same under high fault conditions. This work proposes another possible switching mechanism responsible for a steep change in resistance under high power faults. The proposed mechanism of resistance change comes, not from the PTC change in the material, but rather from the vaporization of the conducting polymer contact areas at the polymer/electrode interface. A comparison between low power polypropylene based devices with a positive temperature coefficient (PTC) effect (thermal expansion of the polymer bulk) and high power devices was done to support this theory. A model was developed along with numerous short-circuit tests to support this theory. Modifications with e-beam irradiation were made to alter the PTC characteristics of thermoplastics but had little effect on high fault short-circuit switching. Also, numerous short-circuit testing on thermo-set based materials without any PTC transition still switched under high fault currents. These all supported the proposed theory that switching, under high fault currents, was from vaporization of the contact areas at the polymer surface rather than thermal expansion in the bulk of the conductive polymer as previously accepted.

The effect of conductive network on positive temperature coefficient behaviour in conductive polymer composites

A 3D mathematical model was developed to simulate the weld pool development in a moving A-TIG weld pool with different oxygen and sulfur concentrations. It is shown that the surface active elements—oxygen and sulfur, which change the temperature coefficient of surface tension from a negative value to a positive one, can cause significant changes in fluid flow patterns and the weld penetration. When surface active element content increases, the weld penetration and depth/width ratio increase sharply and then remain nearly a constant. Positive temperature coefficient of surface tension dominates the fluid flow and the weld pool is narrow and deep. The further increasing surface active element content leads to an inappreciable difference in the weld pool size and shape when the oxygen content increases beyond 280 ppm and sulfur content beyond 125 ppm. Positive and negative temperature coefficients of surface tension co-exist in the weld pool when surface active element content is less than the critical value. The fluid flows in the weld pool change apparently with different surface active element. Depending upon the oxygen and sulfur concentrations, three, one or two vortexes that have different positions, strength and directions may be found in the weld pool. The vortexes with opposite direction caused by positive temperature coefficient of surface tension can efficiently transfer the thermal energy from the arc, creating a deep weld pool.

We report on the studies of the temperature dependence of gate–leakage current in AlGaN/GaN high-electron-mobility transistors (HEMTs) for the temperature range 20–400 °C. The results show that the temperature dependence of gate–leakage current for AlGaN/GaN HEMTs at subthreshold regime (VGS=−6.5 V) have both negative and positive trends. It has been observed that the leakage current decreases with the temperature up to 80 °C. Above 80 °C, the leakage current increases with the temperature. The negative temperature dependence of leakage current with the activation energy +0.61 eV is due to the impact ionization. The positive temperature dependence of leakage current with the activation energy −0.20 eV is due to the surface related traps, and the activation energy −0.99 eV is due to the temperature assisted tunneling mechanism. The drain voltage at a fixed drain–leakage current reveals the occurrence of both positive (+0.28 V/K) and negative (−0.53 V/K) temperature coefficients.

Composite ceramics with compositions within the ZnO–NiO–TiO2, ZnO–MgO, and ZnO–Ln2O3(Ln = Nd, Sm) systems were found to exhibit an anomalous positive temperature coefficient of electrical resistivity (PTCR) effect. The investigations revealed, that in all cases when the PTCR effect was identified, the composite ceramics were found to be composed of phases with different electrical resistivities and linear thermal expansion coefficients. Thermal mismatch between the phases in the composites leads to a disconnection of the grains of the low resistivity constituent phase on account of the high thermal expansion of the other high resistivity constituent phase, resulting in a PTCR effect.

Liquid levels must be monitored in almost any process involving liquids. Most level sensors are mounted inside the vessel containing the liquid. Herein, a fully screen-printed level sensor is demonstrated for external use. It consists of a vertical array of 16 pixels, each comprising a voltage divider of a positive temperature coefficient (PTC) element and a shunt resistor. The self-regulating PTC elements are heated with constant voltage. Heat flow out of the PTCs dictate their resistances and enables inference about their thermal surrounding. Water in a polypropylene container changes voltage levels by (33 ± 2) % compared to air. Applications with a glass container and household oil instead of water are also successfully tested. Both liquids yield a distinctive difference in signal and the sensor determines the height of the oil/water interface as well as the surfaces of the liquid. To further demonstrate the capabilities of the sensor, segregation of a water-oil mixture, slowed by a mixing agent, is observed in real time. This work offers an adaptable and simple alternative for external level sensing.