Positive Thermal Coefficient Research Articles

Instantaneous measurement of the internal temperature in lithium-ion rechargeable cells

We demonstrate, in three different rechargeable lithium-ion cells, the existence of an intrinsic relationship between a cell's internal temperature and a readily measurable electrical parameter, namely the phase shift between an applied sinusoidal current and the resulting voltage. The temperature range examined spanned from −20 to 66 °C. The optimum single frequency for the phase measurement is in the 40–100 Hz range, allowing for a measurement time of much less than a second; the phase shift in this range depends predominantly on temperature, and is almost completely independent of the state-of-charge. Literature reports suggest that the observed dependence of the phase shift on temperature arises from the ionic conduction of the so-called solid-electrolyte-interphase layer between the graphite anode and the electrolyte. A meter measuring the phase shift across this interphase is analogous to a thermometer reporting the temperature, thereby providing feedback for rapid corrections of any operating conditions that might lead to the catastrophic destruction of the cell. This level of monitoring and control is distinctly different from the present safety-enabling mechanisms: typically positive thermal coefficient ceramics/plastics, or “shutdown” separators based on polyethylene that act to often permanently shut down current flow through the cell.

Engineering grain size and electrical properties of donor-doped barium titanate ceramics

A semiconducting lanthanum-doped barium titanate ceramic has been fabricated for battery safety applications by simple means from nanoparticles prepared at room temperature by kinetically controlled vapor diffusion catalysis. The material, characterized by electron microscopy, X-ray diffraction and electrical measurements, exhibits a difficult to achieve combination of submicron grain size (∼500 nm) and attractive electrical properties of room temperature resistivity below 100 Ω cm and a 12-fold increase in resistivity through the Curie temperature (positive thermal coefficient of resistivity, PTCR). Systematic investigation of sintering conditions revealed that a short period of heating at 1350 °C under air is necessary to suppress abnormal grain growth, while precise control of the cooling rate is needed to achieve the targeted electrical properties. Cooling must be sufficiently fast to avoid complete back-oxidation, yet slow enough to facilitate oxygen adsorption at the grain boundaries to produce the thin oxide layer apparently responsible for the observed PTCR.

Thermal/electrical modeling for abuse-tolerant design of lithium ion modules

Proper understanding of heat generation and design of heat dissipation paths are critical for ensuring the safety of lithium ion modules during abuse events such as external shorts. Additionally, the behavior of positive thermal coefficient (PTC) current limiting devices—generally effective at the single-cell level—can be difficult to predict for a multi-cell module. To help guide battery pack design, a coupled thermal/electrical model of a commercial 18 650-size cell and a module with 16 cells in parallel (16P) are developed. Cell electrical response is modeled using an equivalent circuit, including the temperature-dependent behavior of the PTC. Cell thermal response is modeled with a high-resolution thermal model from which a simpler 5-node thermal circuit model is extracted. Cell models are integrated into a module-level model considering cell-to-cell electrical and thermal interactions via conduction, convection, and radiation. The module-level model is validated with a 16P external short experiment and applied in a parametric study to assess thermal safety margin. Copyright © 2009 John Wiley & Sons, Ltd.

TRPV1 Gene Required for Thermosensory Transduction and Anticipatory Secretion from Vasopressin Neurons during Hyperthermia

Increases in core body temperature promote thermoregulatory cooling by stimulating sweat production and preemptive renal water reabsorption through the release of vasopressin (VP, antidiuretic hormone). The mechanism by which the hypothalamus orchestrates this anticipatory VP release during hyperthermia is unknown but has been linked to a central thermosensory mechanism. Here, we report that thermal stimuli spanning core body temperatures activate a calcium-permeable, ruthenium red- and SB366791-sensitive nonselective cation conductance in hypothalamic VP neurons. This response is associated with a depolarizing receptor potential and an increase in action potential firing rate, indicating that these neurons are intrinsically thermosensitive. The thermosensitivity of VP neurons isolated from trpv1 knockout (Trpv1(-/-)) mice was significantly lower than that of wild-type counterparts. Moreover, Trpv1(-/-) mice showed an impaired VP response to hyperthermia in vivo. Channels encoded by the trpv1 gene thus confer thermosensitivity in central VP neurons and contribute to the thermal control of VP release in vivo.

Morphological evolution and characterization of hydrothermal synthesized Na 2CuP 2O 7 crystals

Na 2CuP 2O 7 crystals were synthesized by hydrothermal technique at moderate pressure–temperature conditions. The maximum degree of crystallinity is observed at critical concentration of initial components, beyond which the crystallinity collapses sharply to minimum. Solubility studies have revealed, positive thermal coefficient of solubility in a H 3PO 4 solvent with relatively high enthalpy. Thermal behaviour and structural coordination of the prepared materials were investigated, respectively, using DTA/TGA and FTIR measurements. Magnetic results have revealed; it is a frequency and temperature dependent prospective paramagnetic material.

The thermodynamic characteristics of point defects and the mechanism of charge transfer in lanthanum cobaltite doped with strontium and nickel

The specific conductivity and thermal electromotive force (EMF) coefficient of lanthanum cobaltite doped with strontium and nickel La0.9Sr0.1Co0.9Ni0.1O3−δ were measured over the temperature and pressure ranges 1023–1223 K and 10−6−1 atm. The oxide was found to exhibit metallic-type conductivity (800 Ω−1 cm−1 ≤ σ ≤ 1150 Ω−1 cm−1) and small positive thermal EMF coefficient values. A joint analysis of the thermodynamic characteristics of formation of point defects and external T and pO2 parameter dependences of conductivity and thermal EMF showed that charge was transferred in the oxide by small-radius polarons according to the hopping mechanism. The role of polarons was played by electrons MeCo′ and holes MeCo• localized on 3d transition metals. The isothermal dependences of the concentrations and mobilities of charge carriers on the degree of oxygen nonstoichiometry of the oxide studied were calculated.

Hydrothermal synthesis and characterization of CsNiP and 2(LiZnHP2O7) crystals

CsNiP and 2(LiZnHP2O7) crystals were synthesized by hydrothermal technique at moderate P-T conditions. Solubility results of both the compound shown positive thermal coefficient and single crystal X-ray studies revealed, CsNiP crystallized in hexagonal system with cell parameters; a = 7.173(2), c = 5.944(9) A, V = 264.87(7) A3 and space group P63/mmc and 2(LiZnHP2O7) crystallized in orthorhombic system with cell parameters; a = 12.3636 A, b = 27.5330 A, c = 6.8647 A and space group, Pca21 exhibiting ring type of cavities with open aperture in the structure. CsNiP is a frequency dependent paramagnetic and 2(LiZnHP2O7) is a diamagnetic.

Hydrothermal synthesis, morphological evolution and characterization of Na 2CoP 2O 7 crystals

Na 2CoP 2O 7 crystals are synthesized by hydrothermal technique at moderate pressure–temperature conditions. The maximum degree of crystallinity is observed at the critical concentration of initial components, beyond which the crystallinity collapses sharply to a minimum. Solubility studies have revealed positive thermal coefficient of solubility in a H 3PO 4 solvent with relatively high enthalpy. DTA results revealed this compound to undergo a phase transition at 380 °C.

High-temperature thermal stability performance in /spl delta/-doped In/sub 0.425/Al/sub 0.575/As--In/sub 0.65/Ga/sub 0.35/As metamorphic HEMT

We report, to our knowledge, the best high-temperature characteristics and thermal stability of a novel /spl delta/-doped In/sub 0.425/Al/sub 0.575/As--In/sub 0.65/Ga/sub 0.35/As--GaAs metamorphic high-electron mobility transistor. High-temperature device characteristics, including extrinsic transconductance (g/sub m/), drain saturation current density (I/sub DSS/), on/off-state breakdown voltages (BV/sub on//BV/sub GD/), turn-on voltage (V/sub on/), and the gate-voltage swing have been extensively investigated for the gate dimensions of 0.65×200 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The cutoff frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) and maximum oscillation frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> ), at 300 K, are 55.4 and 77.5 GHz at V/sub DS/=2 V, respectively. Moreover, the distinguished positive thermal threshold coefficient (/spl part/V/sub th///spl part/T) is superiorly as low as to 0.45 mV/K.

Thermal dependence of electrical characteristics of micromachined silica microchannel plates

Micromachined silica microchannel plates (MCPs) under development have a number of advantages over standard glass MCPs and open completely new possibilities in detector technologies. In this article we present the results of our studies on the thermal properties of silica microchannel plates (sMCPs). Similar to standard glass microchanel plates the resistance of silica MCPs was measured to change exponentially with temperature with a negative thermal coefficient of −0.036 per °C, somewhat larger than that of standard glass MCPs. The resistance also decreases linearly with the applied voltage, with the voltage coefficient of −3.1×10−4 V−1. With the knowledge of these two coefficients, our thermal model allows the calculation of the maximum voltage, which can be applied to a given MCP without inducing a thermal runaway. A typical 25 mm diam, 240 μm thick sMCP with 6 μm pores has to have the resistance larger than ∼30 MΩ to operate safely at voltages up to 800 V. With this model we can also calculate the time required for a given silica MCP to reach the point of thermal equilibrium after a voltage increase. We hope that the ongoing efforts on a proper modification of the sMCP semiconducting layer will lead to the production of new MCPs with a small negative or even a positive thermal coefficient, reducing the possibility of thermal runaways of low-resistance MCPs required for high count rate applications.

Characterization of Chromatographic Silica Beads by Micro‐Thermal Field‐Flow Fractionation

Micro‐thermal field‐flow fractionation (micro‐TFFF) was used to characterize silica based chromatographic beads within the diameter range from 3 to 10 µm. The main goal of this study was to demonstrate that silica particles that sediment in water suspensions and are usually separated by gravitational field‐flow fractionation (GFFF), can be retained more effectively under the conditions of micro‐TFFF. It has been found, that the retentions of all studied particles substantially increased when the temperature drop, ΔT, across the micro‐TFFF channel was applied, in comparison with the retentions when only the gravitation was acting as an effective field at ΔT = 0 K. This finding confirmed that micrometer size and high‐density silica particles exhibit a positive thermal diffusion coefficient. Various operational conditions were tested up to extreme temperature drop, ΔT = 80 K, at the temperature of the cold wall T c = 300 K. As a result, the time of micro‐TFFF analysis can be as short as 10 min. The samples were also characterized by scanning electron microscopy (SEM). The comparison of the results of micro‐TFFF and SEM showed good agreement of the mean particle sizes and of the particle size distributions (PSD) estimated from the SEM pictures with the peak retentions, widths, and shapes of the micro‐TFFF fractograms. Micro‐TFFF was found as a rapid and easily applicable method for the characterization of the mean particle size and PSD of the chromatographic beads. This is an important conclusion because micro‐TFFF can, thus, become a convenient and inexpensive analytical tool for the quality control of the uniformity of chromatographic beads, which determines the efficiency of the packed columns.

Static and dynamic non-linear behaviors of carbon particle filled polymers with positive thermal coefficient

Polymers filled with conductive particles like carbon black, short carbon fibers or microbeads, may exhibit a very large positive thermal coefficient of the electrical resistance when they are heated above room temperature. Such a behavior is particularly useful in industrial applications for producing electrical safety devices. We present a comprehensive study of the electrical properties of epoxies filled with carbon microbeads when heating is achieved by the Joule effect. In that case, the electrical responses up to a large enough electrical excitation must be non-linear as a result of progressive heating of the material. As expected, we have found that it depends on both the 'intrinsic' properties of the sample (essentially a function of the composition) and the extent of thermal coupling between the investigated sample and the external thermal bath (variable heat leak). We have particularly studied the stationary states under constant applied voltage and the transients following a step like time dependent voltage. Most of our findings agree with the general results of percolation theory combined with theoretical models of rupture on a random fuse network and with a simple model of heat flow through the sample and from the sample to the thermal bath.

Hydrodynamic instabilities in laser pulse-produced melts of metal films

The dynamics of melts, as induced by 7 ns laser pulses in Al and Au films, were investigated by in situ time-resolved transmission electron microscopy. Melting, motion of the liquid, and crystallization were observed by tracing the image intensity with a photomultiplier (space/time resolution 100 nm/3 ns) and by streak imaging (streak times 15 ns–4 μs). Films with native oxides/adsorbed atmospheric contaminations and films purified by pulse melting were found to show a completely different behavior of their melts. The melts of purified films either remained almost flat (Al) or revealed a gradual pileup of liquid in cold regions within 500 ns (Au), caused by thermocapillarity with the negative thermal coefficient of the surface tension of pure metals. In contrast, contaminated films showed three distinctly different types of perturbations: (1) a fast expulsion of the melt from the center of the laser spot within 20 ns after the laser pulse; (2) a gradual contraction of liquid at the center within 0.5–1 μs; (3) thickness oscillations with frequencies of 5–10 MHz and time constants of 500 ns. These effects are explained by recoil from evaporating contaminations, by thermocapillary flow with a positive thermal coefficient of the surface tension, caused by surface active impurity atoms, and by thermocapillary waves.

Electron cryomicroscopy of frozen-hydrated biological specimens: analysis of freezing artifacts by x-ray cryocrystallography

Freezing artifacts have been evaluated by X-ray cryocrystallography on pellets of two-dimensional membrane protein crystals: purple membrane and maltoporin. The comparison of the X-ray patterns recorded when the specimens are maintained at room temperature to those obtained when the specimens are maintained at about -160°C shows that (i) membrane proteins have a positive thermal dilatation coefficient: the protein crystal lattice shrinks upon cooling; (ii) the asymmetric unit of crystal containing water is changed upon freezing; the relative intensities of the diffraction rings of such crystals are different after freezing. From these results, it can be postulated that freezing may lead to partial dehydration of biological objects. Electron cryomicroscopy visualizes objects which are structurally influenced by the cooling procedure. However, our microscopy study on maltoporin crystals shows that freezing artifacts are negligible in comparison to artifacts associated with conventional techniques such as negative staining.

Instabilite et flux de chaleur dans le probleme de benard a deux constituants aux coefficients de soret positifs

In this paper, we consider an approximation to the nonlinear analysis of the two component Bénard problem, taking into account thermal diffusion. The present study is restricted to the case of positive thermal diffusion coefficient: no complete calculations were indeed reported till now, in this domain. The results presented in this paper are essentially the variation of the Nusselt number versus the Rayleigh number for different thermal diffusion factors. The experimental results can be explained by the use of the theoretical results.

The effect of spinal and skin temperatures on the firing rate and thermosensitivity of preoptic neurones.

1. In anaesthetized rabbits, preoptic single unit activity was recorded while preoptic, spinal cord and skin temperatures were independently manipulated.2. The units that were insensitive to preoptic temperature were characterized by low firing rates and also by a very low incidence of extrahypothalamic thermosensitivity.3. Thirty-seven units having positive coefficients to preoptic temperature were tested for their response to spinal or skin temperature. Of these, twenty-two units responded to extrahypothalamic temperature, seventeen with positive thermal coefficients. In addition, the incidence of extrahypothalamic thermosensitivity generally increased among the higher firing units.4. Twenty-two units had negative coefficients for preoptic temperature and were tested for their extrahypothalamic thermosensitivities. Of these, sixteen units had dual thermosensitivities, ten with negative coefficients for the extrahypothalamic temperatures. In addition, there was no correlation between the incidence of extrahypothalamic thermosensitivity and the level of firing rate.5. In the units having positive coefficients for preoptic temperature, an increased firing rate, due to extrahypothalamic temperature, generally resulted in a decreased preoptic thermosensitivity. Conversely, a decreased firing rate usually resulted in an increased preoptic thermosensitivity.6. In the units having negative coefficients for preoptic temperature, an increased firing rate, due to extrahypothalamic temperature, usually increased the preoptic thermosensitivity; while a decreased firing rate tended to decrease the sensitivity to preoptic temperature.

The effect of firing rate on preoptic neuronal thermosensitivity.

1. In anaesthetized rabbits, preoptic single units were recorded having positive or negative thermal coefficients (impulses/sec. degrees C) for changes in preoptic temperature.2. A population of forty-two positive coefficient units was divided into four groups based on their level of firing rate at 38 degrees C. In each group, the average thermoresponse curve was determined by averaging the firing rates of the units at 1 degrees C intervals over the 33-43 degrees C range of preoptic temperatures.3. A population of twenty-six negative coefficient units was divided into three groups based on their firing rates at 38 degrees C. Similar average thermoresponse curves were determined for each group.4. As the level of firing rate increased in the positive coefficient units, the preoptic thermosensitivity progressively decreased at temperatures above 39 degrees C, but generally increased at temperatures below 39 degrees C.5. In the negative coefficient units, preoptic thermosensitivity generally increased (especially above 39 degrees C) as the firing rate at 38 degrees C increased.6. The results indicate that in positive coefficient units the level of firing rate determines the temperature range in which units are most thermosensitive. This range reflects whether a neurone is more likely to function in heat-loss or heat-production responses.7. Since peripheral thermal input affects the level of firing rate of positive coefficient units, a neuronal model is suggested to explain the role of peripheral and central thermal signals in temperature regulation.

Multiple-Curie-Point Capacitor Dielectrics

This paper reports an experimental study of the preparation of screened printed dielectric materials having high dielectric constants and a controlled temperature characteristic. The technique uses mixtures of sintered bodies which have different Curie points and which retain their identity after processing. Barium-strontium and barium-lead titanate mixtures have been prepared having a dielectric constant of about 500. This process has been used to produce negative-positive-zero, negative, and positive thermal coefficient bodies over various temperature ranges.

The instantaneous elasticity of active muscle.

When the tension of a muscle contracting isometrically is rapidly lowered, there is an immediate and proportional rise of temperature. This is not due to physiological shortening, which is a relatively slow process, but is directly connected with the fall of tension. A similar effect occurs in any material possessing a normal (positive) thermal coefficient of linear expansion. It is the opposite of what is observed in bodies with long-range rubber-like elasticity. The experimental relation, in active muscle, between the heat (∆ Q ) immediately produced and the rapid fall of tension (-∆ P ) is ∆ Q = 0∙018 l o (-∆ P ), where l o is the standard length of the muscle. The constant 0∙018 is considerably greater than for metals but about the same as for ebonite and wood. In resting muscle, in the range of moderate tensions, the constant is of the opposite sign, and its absolute size is five to ten times as great. Resting muscle, in this range, has rubber-like elastic properties. During active contraction, therefore, the contractile filaments possess normal and not long-range elasticity. The force exerted by active muscle is not of thermokinetic origin. Unlike resting muscle its entropy and its internal energy both decrease when its tension is rapidly lowered. The power of physiological shortening, at a rate depending on the tension, is not directly derived from elastic properties. In normal relaxation after an isometric contraction there is known to be a substantial production of heat. This is derived partly from elastic energy developed earlier during contraction, in the series elastic component: the balance is fully accounted for by the thermo­elastic heat resulting from the fall of tension.