Characteristics of a Type of NTC Thermistors for Cryogenic Applications

A new type of miniature negative temperature coefficient (NTC) thermistors has been developed and manufactured with Mn-Ni-Cu-Fe oxides. The prepared NTC thermistors were calibrated in the temperature range from 77 K to 300 K with 1 μA exciting currents. The automatic calibration apparatus as well as thermometric characteristics, stability, calibration equations and interchangeability of the manufactured thermistors were investigated. A mean fit equation was obtained: 1/T = 8.60 × 10−4 + 6.54 × 10−4 ln(R/Rref) + 2.46 × 10−5 ln(R/Rref)2 + 9.48 × 10−7 ln(R/Rref)3 − 2.16 × 10−8 ln(R/Rref)4. All the prepared NTC thermistors agreed with this fit with an error of 1.5 K. If the greater accuracy is required, a calibration is necessary, and the calibration accuracy is estimated to be ±10 mK.

Fabrication of nano-grained negative temperature coefficient thermistors with high electrical stability

Dense nano-grained Ni_(0.7)Mn_(2.3O4) negative temperature coefficient(NTC) thermistors were fabricated by a novel two-step sintering approach that combines rapid sintering and principle of conventional two-step sintering technique.Samples were sintered at 1042℃ for 30 s in the first rapid step and then at 850-950℃ for 20 h in the second soaking step.Crystal phase,microstructure and electrical properties of sintered samples were studied by X-ray diffraction(XRD),scanning electron microscopy(SEM),resistance temperature relationship and aging performance.Sintered samples show a single-phase cubic spinel structure and indicate a high relative density ranging from 84% to 91% of the theoretical density.Moreover,average grain sizes of sintered samples under SEM are distributed between 254 and 570 nm.Meanwhile,the resistivity and the aging coefficient significantly decrease when soaking sintering temperature rises.In addition,the obtained material constant(B) ranges from 3931 to 3981 K.Ni_(0.7)Mn_(2.3)O_(4)-3(soaking at 900℃) and Ni_(0.7)Mn_(2.3O4)-4(soaking at 950℃) present little aging behavior,implying great electrical stability.

Dynamics and coherence resonance in a thermosensitive neuron driven by photocurrent

A feasible neuron model can be effective to estimate the mode transition in neural activities in a complex electromagnetic environment.When neurons are exposed to electromagnetic field,the continuous magnetization and polarization can generate nonlinear effect on the exchange and propagation of ions in the cell,and then the firing patterns can be regulated completely.The conductivity of ion channels can be affected by the temperature and the channel current is adjusted for regulating the excitability of neurons.In this paper,a phototube and a thermistor are used to the functions of neural circuit.The phototube is used to capture external illumination for energy injection,and a continuous signal source is obtained.The thermistor is used to percept the changes of temperature,and the channel current is changed to adjust the excitability of neuron.This functional neural circuit can encode the external heat(temperature)and illumination excitation,and the dynamics of neural activities is investigated in detail.The photocurrent generated in the phototube can be used as a signal source for the neural circuit,and the thermistor is used to estimate the conduction dependence on the temperature for neurons under heat effect.Bifurcation analysis and Hamilton energy are calculated to explore the mode selection.It is found that complete dynamical properties of biological neurons can be reproduced in spiking,bursting,and chaotic firing when the phototube is activated as voltage source.The functional neural circuit mainly presents spiking states when the photocurrent is handled as a stable current source.Gaussian white noise is imposed to detect the occurrence of coherence resonance.This neural circuit can provide possible guidance for investigating dynamics of neural networks and potential application in designing sensitive sensors.

Characterization of the BaBiO_3-doped BaTiO_3 positive temperature coefficient of a resistivity ceramic using impedance spectroscopy with T_c=155℃

BaBiO3-doped BaTiO3 （BB-BT） ceramic, as a candidate for lead-free positive temperature coefficient of resistivity （PTCR） materials with a higher Curie temperature, has been synthesized in air by a conventional sintering technique. The temperature dependence of resistivity shows that the phase transition of the PTC thermistor ceramic occurs at the Curie temperature, Tc = 155℃, which is higher than that of BaTiO3 （≤ 130 ℃）. Analysis of ac impedance data using complex impedance spectroscopy gives the alternate current （AC） resistance of the PTCR ceramic. By additional use of the complex electric modulus formalism to analyse the same data, the inhomogeneous nature of the ceramic may be unveiled. The impedance spectra reveal that the grain resistance of the BB-BT sample is slightly influenced by the increase of temperature, indicating that the increase in overall resistivity is entirely due to a grain-boundary effect. Based on the dependence of the extent to which the peaks of the imaginary part of electric modulus and impedance are matched on frequency, the conduction mechanism is also discussed for a BB-BT ceramic system.

AN APPARATUS FOR MEASURING THERMAL CONDUCTIVITY OF LIQUIDS UNDER HIGH PRESSURE

We have reported a method for measuring the thermal conductivity of liguids un-der high pressure previously [1,2].Such data are necessary for improving thetechnologies such like processing as acidity,pressure cracking,thermal exploitation ofpetroleum,etc. An apparatus has thus been constructed,as shown schematically in Fig.1,tomeasure the thermal conductivity of liquids under pressure up to 25 MPa and temperatureranging from 150 to 250℃.The thermal conductivity cell is cylindrical in from,35mmin length and 25mm in inner diameter.The sample is poured into the cell through

The Printing and Firing Process of Al Paste in PTC Ohmic Contact Electrodes

Al electrodes are well known as ohmic contact electrodes for the PTC component , the influence of their thickness on final component properties was investigated by comparing their ohmic characteristics with the ones of InGa electrodes . After observing the Al paste physical and chemical behaviors during rising temperature by thermal analysis (DTA), the firing operation of Al electrodes could be divided into three main subsections: the temperature rising time (t-r), the peak firing temperature (T-p) and the hold time at peak firing temperature (t-h). The effects of these three parameters on final component properties were discussed in detail.

Modified Function Transformation Method for Linearizing Thermistor Characteristics

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A Simulation of the Response of a Sounding Temperature Sensor Based on the Combination of a Genetic Algorithm and Computational Fluid Dynamics

The present study aims at improving the accuracy of weather forecast by providing useful information on the behavior and response of a sounding temperature sensor.A hybrid approach relying on Computational Fluid Dynamics and a genetic algorithm(GA)is used to simulate the system represented by the bead thermistor and the surrounding air.In particular,the influence of different lead angles,sensor lead length,and lead number is considered.The results have shown that when the length of the lead wire of the bead thermistor is increased,the radiation temperature rise is reduced;when the number of lead wire is four and the angle between the lead wires is 180°,the solar radiation angle has a scarce influence on the radiation temperature rise of the sounding temperature sensor.

Ovulation detection mechanism—a microcomputer based approach

This paper presents a real-time microcomputer-based logger for measuring basal body temperature (BBT). BBT is the normal resting body temperature of a healthy person immediately upon waking in the morning. The temperature for women normally rises after ovulation due to hormonal changes. The temperature is logged real-time into the computer over a period and the BBT chart produced is used to predict ovulation. There is high correlation between the approach developed by this study and other standard measuring equipments—correlation of 0.9945with standard thermistor, and correlation of 0.9977 with standard thermocouple. This development enables privacy of use, allowing women to predict their ovulation status at a personal level.

Frequency and temperature dependent electrical characteristics of CaTiO_(3) nano-ceramic prepared by high-energy ball milling

Nanocrystalline calcium titanate(CT)ceramic has been synthesized by a combination of solid-state reaction and high-energy ball milling.This nano-ceramic is characterized by X-ray diffraction(XRD),dielectric study and impedance spectroscopy.The XRD pattern shows single phase ceramic of orthorhombic symmetry.The frequency-dependent dielectric study shows that the dielectric constant is maximized at low frequencies and decreases with an increase in frequency.Impedance spectroscopy analyses reveal a non-Debye type relaxation phenomenon.A significant shift in impedance loss peaks toward the higher-frequency side indicates conduction in the material favoring the long-range motion of mobile charge carriers.The grain conduction effect is observed from the complex impedance spectrum by the appearance of one semicircular arc in Nyquist plot.It is also observed that the resistance decreases with an increase in temperature showing a negative temperature coefficient of resistance(NTCR).Various thermistor parameters have been calculated by fitting with Steinhart-Hart equation.The modulus plots represent the presence of temperature-dependent electrical relaxation phenomenon with the material.The frequency-dependent AC conductivity at different temperatures indicates that the conduction process is thermally activated.The activation energy has been calculated from an Arrhenius plot of DC conductivity and relaxation frequency.

Enhanced time response and temperature sensing behavior of thermistor using Zn-doped CaTiO_3 nanoparticles

In the present study, Zn-doped CaTiO_3 nanocrystalline was synthesized to study the thermistor behavior with temperature. The X-ray powder diffraction analysis showed the formation of a single-phase orthorhombic structure at room temperature. The electrical resistance of the Zn-doped CaTiO_3 increased with increasing doping concentration and decreased at higher measuring temperature, showing a negative temperature coefficient of resistance(NTCR) behavior. Different thermistor parameters were calculated using Steinhart–Hart equations, whilst time domain analysis confirmed faster response towards applied voltage.

CaTiO_(3) nano ceramic for NTCR thermistor based sensor application

It is possible to fabricate highly sensitive NTCR(negative temperature coefficient of resistance)thermistor using nano crystalline CaTiO_(3) synthesized by high energy ball milling.Disc shaped green pellets were prepared and effects of sintering on the disc pellets were studied as thermistor by sintering the samples at 1000℃,1100℃and 1200℃.The as-prepared samples were characterized by X-ray diffraction(XRD),impedance analysis and electrical measurement.The resistivity of the prepared samples varies predictably with temperature:this makes them promising material for temperature sensor.The experimental results prove that nano crystalline CaTiO_(3) ceramic is one kind of thermistor with exponential negative temperature coefficient of resistance in the temperature range of 300-500℃.The samples have the advantages of rapid response,high sensitivity and capability to withstand thermal surges over the temperature range of 300-500℃.Resistance-temperature characteristics are described by thermistor equation with thermistor constant around 4003 K to 10795 K and thermal coefficient of resistanceαaround-1%/℃to-13%/℃.The activation energy is in the range of 0.34-0.93 eV.The observed thermistor parameters are found to be comparable with many of the known thermistor materials.This suggests that the electrical properties can be adjusted to desirable values by controlling the temperature parameter.The influence of fabrication process of disc thermistor and electrical properties are discussed.The study shows the potential of nano crystalline CaTiO_(3) to act as an NTCR material for thermistor applications.

Electronic and magnetic stability in correlated transportations of rare-earth nickelate perovskites

Although the thermistor and metal to insulator transition bi-functionalities were discovered for rareearth nickelates(RENiO3),the electronic stability in their correlated transports under impulse voltage or magnetic field remain as open questions.Herein,we demonstrate the thermistor transportations of the electron correlated rare-earth nickelates under impulse direct current voltage and in magnetic environment.The insulating phase of RENiO3 shows zero crossing linear I-V characters,indicating their stable electronic resistance is independent of the imparted voltage up to 10 V and pulse width down to1 us,in spite of their sensitive electronic structures to polarizations.In addition,the high electronic stability associated with the thermistor transportation of RENiO3 is also demonstrated in magnetic fields up to 9 T(i.e.,MR<0.2%).The high electronic stability further paves the way to applying RENiO3 as a broad temperature range thermistor in temperature sensing or circuit protections for correlated electronics.

NUMERICAL ANALYSIS OF NONSTATIONARY THERMISTOR PROBLEM

The thermistor problem is a coupled system of nonlinear PDEs which consists of the heat equation with the Joule heating as a source, and the current conservation equation with temperature dependent electrical conductivity. In this paper we make a numerical analysis of the nonsteady thermistor problem. L(infinity)(OMEGA), W1,infinity(OMEGA) stability and error bounds for a piecewise linear finite element approximation are given.

PTC Effect and Its Mechanism in a Novel Thermistor Material —— Hot-pressed SiC/SiO_2 Multiphase Ceramic

As functional materials, PTC thermistor ceramics characterized by a drastic increase in resistivity at its transition temperature have been widely used in high-technology areas such as temperature measurement and indication, temperature control, protective switches and so on. At present, the PTC thermistors are fabricated by means of doping BaTiO3 and their PTC effects result from the existence of acceptor state or electronic trap on grain boundary. But the fabrication process

MECHANISM OF THE PTC EFFECT OF V_2O_3-POLYETHYLENE COMPOSITES

The positive temperature coefficient （PTC） effect of ceramics has been widely used in temperature sensors, protective switches and so on, which requests large positive temperature coefficients in narrow region above the curie-point temperature and low resistivities below the curie-point temperature. At present, PTC thermistors are fabricated by means of doping the BaTiO3 to produce a PTC effect by the existence of accep.tor states or

Regularity Results for a Nonlinear Elliptic-Parabolic System with Oscillating Coefficients

In this paper we study the initial boundary value problem for the system div(σ(u)∇φ)=0,u_(t)−∆u=σ(u)|∇φ|2.This problem is known as the thermistor problem which models the electrical heating of conductors.Our assumptions onσ(u)leave open the possibility that lim inf_(u→∞)σ(u)=0,while lim sup_(u→∞)σ(u)is large.This means thatσ(u)can oscillate wildly between 0 and a large positive number as u→∞.Thus our degeneracy is fundamentally different from the one that is present in porous medium type of equations.We obtain a weak solution(u,ϕ)with|∇φ|,|∇u|∈L∞by first establishing a uniform upper bound for eεu for some smallε.This leads to an inequality in∇φ,from which the regularity result follows.This approach enables us to avoid first proving the Holder continuity ofφin the space variables,which would have required that the elliptic coefficientσ(u)be an A2 weight.As it is known,the latter implies that lnσ(u)is“nearly bounded”.

Preparation of Homogeneous Mn-Co-Ni-O Negative Temperature Coefficient Ceramic by Improved Oxalate Coprecipitation

High-quality Mn-Co-Ni oxalate powder was prepared in a mixed solvent of water and ethanol by an improved oxalate coprecipitation approach.The as-prepared oxalate powder appears a controlled homogeneous morphology and narrower particle size distribution by conveniently adjusing the ratio of water to ethanol.The ceramic,moulded with the high quality oxalate precursor,could be densified at a lower temperature of 1150 ℃;and also exhibits a higher density and the more homogeneous microstructurc with smaller grain sizes of 2-5 urn.Accordingly,the NTC thermistors fabricated from the ceramic have a better yield as compared to the traditional method.

Effects of La_2O_3 Doping on Aging Properties of Mn_(2.38)Ni_(0.5)Cu_(0.12)O_4 Type NTC Thermistor Materials

The effects of La_2O_3 doping on the phase structure.electric properties and aging properties of the samples were researched using XRD,SEM and electric property measurements.The results suggest that the aging value of Mn_(2.38)Ni_(0.5)Cu_(0.12)O_4 thermistor ceramic samples decreases from 10%to about 6%by only doping La_2O_3.The electrical stability of the sample has improved as well.Such doped material will broaden the production and application scope of Cu contained negative temperature coefficient thermal ceramics.

Bleeding from thermistor connector indicates entrapment of pulmonary artery catheter

Several complications of insertion and use of pulmonary artery catheter （PAC）, including catheter entrapment, are recognized during or after cardiac surgery. Early detection is very important. We report a case of PAC entrapment because the the catheter was accidentally su- tured to the superior vena cava. Bleeding from the thermistor connector was initially detected after surgery because the needle punctured the lumen of thermistor connector. The PAC was removed through re-exploration. Therefore, bleeding from the thermistor connector could early indicate the PAC entrapment. We suggest that abnormal bleeding from the non-injectate lumen ports of PAC should be checked before sternal closure.