Showing papers on "Temperature coefficient published in 1996"

Self-ignition of diesel-relevant hydrocarbon-air mixtures under engine conditions

Abstract: The self-ignition behavior of diesel-relevant fuels as homogeneous mixtures with air has been investigated using three high-pressure shock tubes. Because of the low vapor pressure of some diesel-fuel representative hydrocarbons at ambient temperature, a new shock tube equipped with heating facilities was constructed to prevent condensation. The investigated fuels are the aromatic α-methylnaphthalene, the alkane n-decane, and dimethylether. The self-ignition process of α-methylnaphthalene starts deflagrative at 13 bar for the complete investigated temperature range of 840–1300 K. For temperatures above 960 K, a transition to a detonationlike process (DDT, secondary explosion) with strong pressure peaks can be observed. The duration of the transition from deflagration to the initiation of the DDT process decreases with increasing temperature. The self-ignition behavior of n-decane and dimethylether in the temperature range of 650–1300 K is very similar to that of n-heptane [8], with a two-step self-ignition (first step: cool flame process) at lower temperatures. A very short deflagrative phase is followed by a secondary explosion. The time difference between the first pressure rise caused by the cool flame process and the DDT decreases with decreasing temperature, whereas the intensity of the cool flame process increases. The ignition delay times of both n-decane and dimethylether show a negative temperature coefficient (NTC) in the Arrhenius plot. Within the temperature range of this investigation, the shortest ignition-delay times are observed for dimethylether.

Temperature dependence of photovoltaic cells, modules and systems

Abstract: Photovoltaic (PV) cells and modules are often rated in terms of a set of standard reporting conditions defined by a temperature, spectral irradiance and total irradiance Because PV devices operate over a wide range of temperatures and irradiances, the temperature and irradiance-related behavior must be known This paper surveys the temperature dependence of crystalline and thin-film, state-of-the-art, research-size cells, modules and systems measured by a variety of methods The various error sources and measurement methods that contribute to cause differences in the temperature coefficient for a given cell or module measured with various methods are discussed

Capacitance anomaly near the metal—non-metal transition in Cr—hydrogenated amorphous Si—V thin-film devices

Abstract: We present experimental results that show a metal—non-metal (MNM) transition occurring in hydrogenated amorphous Si (a-Si: H) analogue memory devices as a function of temperature. The dc resistance of the devices undergoes a continuous change in the range 65–100 K from semiconductor-like behaviour to metallic behaviour, as the temperature increases. The ac conductivity, measured over the frequency range 1–3·1 × 107 Hz, shows an anomalous change as the temperature is varied over the MNM transition. Ac characteristics were modelled using multicomponent RC and RL equivalent circuits below and above the MNM transition region respectively. It is found that the capacitance increases markedly when the temperature approaches the MNM transition from the semiconductor side. Near the transition temperature this capacitance disappears, and the equivalent circuit now requires an inductive component together with a resistance which has a positive temperature coefficient of resistance equivalent to that of the ...

Polymer composite thermistors for temperature and current sensors

Abstract: The electrical resistivity of polymer composites is studied as a function of temperature. The initial resistivity ρ of thermoplastic or thermoset containing a metallic filler is in the range of 1–10×10−2 Ω cm. Around the curing temperature of epoxies, the resistivity increases by eight to twelve orders of magnitude. For thermoplastic polymers, however, the transition temperature is related to the melting temperature at which a strong volume increase occurs. Hence, the choice of polymer and its processing determine the transition temperature from a conducting state to an insulating state. For a variety of polymers we have observed transitions between 80 and 200 °C. Due to a sharp and strong transition at a predetermined temperature, such materials can be used as temperature sensors. Since the resistivity of the cold state is low, they can also carry rather high currents. The balance between heating and cooling determines then a critical value for the current. Thus, the materials can also serve as current s...

A new ytterbium chelate as contrast agent in chemical shift imaging and temperature sensitive probe for MR spectroscopy

Abstract: A paramagnetic Yb(III) complex that is the prototype of a novel class of probes for MRI and MRS has been developed. The complex displays highly shifted 1H resonances that are characterized by short relaxation times and, as such, may prove to be a valuable alternative in applications that currently require fluorine-containing probes. Selective excitation of the paramagnetically shifted resonances allows the spatial distribution of the complex to be mapped. This communication reports the images that were obtained by selectively exciting the most intense methyl group (-14.2 ppm at 27 degrees C) for complex concentrations ranging from 0.003-0.1 M. Spectroscopically, the complex may be used as a temperature probe since the proton chemical shifts exhibit a strong temperature dependence. In human serum the chemical shift difference of a selected pair of proton resonances was observed to follow a gradient of -0.42 +/- 0.01 ppm/degrees C. Furthermore, since the chemical shift of the methyl resonance displays a temperature coefficient of -0.04 +/- 0.01 ppm/degrees C, it should be possible to use the image phase for thermal mapping.

Investigation of semiconducting YBaCuO thin films: A new room temperature bolometer

Abstract: We explore the application of the semiconducting phases of YBaCuO thin films as a bolometer for uncooled infrared detection. For this study, four different structures were built with different types of buffer layers: YBaCuO on a Si substrate with and without a MgO buffer layer, and on an oxidized Si substrate with and without a MgO buffer layer. These films were all amorphous without a detectable long range order. For comparison, crystalline tetragonal YBa 2 Cu 3 O 6.5 and YBa 2 Cu 3 O 6.3 thin films on a LaAlO 3 substrate were included into the study. All six films exhibited semiconducting resistance versus temperature characteristics. The bolometer figures of merit, responsivity, and detectivity were calculated from the measured temperature coefficient of resistance ~TCR! and the inherent noise characteristics of the temperature sensing element. The room temperature TCRs for all four amorphous films were greater than 2.5% K 21 . The highest TCR of 4.02% K 21 was observed on the amorphous YBaCuO thin film deposited on MgO/Si without a SiO2 layer. The TCR of the tetragonal films, on the other hand, remained 2% K 21 or less in the same temperature range. Noise measurements performed in the 1‐100 Hz frequency range revealed a quadratic dependence on the bias current as would be expected from ohmic electrical characteristics. The Johnson and 1/f regions were clearly identified in the noise spectrum. From TCR and noise measurements, we estimated the amorphous semiconducting YBaCuO bolometers would have a responsivity as high as 3.8310 5 V/W and a detectivity as high as 1.6310 9 cm Hz 1/2 /W for 1 mA bias current and frame frequency of 30 Hz if integrated with a typical air-gap thermal isolation structure. © 1996 American Institute of Physics.@S0021-8979~96!02924-6#

Oxygen generating device

Abstract: An oxygen generating device (11, 211) to which a gas mixture containing oxygen is fed, and which is operable to separate the gas mixture into oxygen rich and oxygen depleted gas components, the device (11, 211) including a negative temperature coefficient material (36a 36b 36c; 236) which is active at an elevated temperature above a minimum operating temperature, to separate the gas mixture into oxygen rich and oxygen depleted gas components, there being an electronic control unit coupled to the device to pass an electrical current through the device (11, 211) to produce a heating effect within the device to heat the gas mixture at least towards the minimum operating temperature wherein the device includes a plurality of active sections (A, B, C) through each of which the gas mixture passes in turn, and the electrical current supply is connected to each of the sections (A, B, C) so that the sections electrically are connected in series.

Dielectric properties of Ta2O5–ZrO2 polycrystalline ceramics

Abstract: The dielectric properties of (Ta2O5)1−x(ZrO2)x polycrystalline ceramics at 1 MHz and temperatures between 0 and 100 °C are reported for 0.0

Role of Bi2O3 in optimizing the dielectric properties of Ba4.5Nd9Ti18O54 based microwave ceramics

Abstract: Small additions of Bi2O3 or bismuth titanate improve the dielectric properties of Ba4.5Nd9Ti18O54 at microwave frequencies. It was found that Bi3+ can replace Nd3+ up to 15 mol%, the limiting composition of solid solution being Ba4.5(Nd0.85Bi0.15)9Ti18O54. At the solid solution limit the temperature coefficient of resonant frequency attains its minimum value. The modified ceramic is distinguished by high permittivity of 99 and a Q-value of 5500. The temperature coefficient of resonant frequency is low, 15 ppm/K. After exceeding the solid solubility limit, additional Bi2O3 concentrates as a Bi-rich phase at the grain boundaries, causing considerable reduction of the Q-value and an increase of τf.

Electrical resistivity and thermopower of liquid Ge and Si

Abstract: The electrical resistivity and thermopower S of pure liquid silicon and pure liquid germanium have been carefully measured. For silicon, a new containment material was used, namely high-density graphite. This graphite has a low thermopower ( at ) a high resistivity ( at ), and little or no reaction with Si, making it an ideal containment material. The results for each liquid show a metallic value of resistivity, a small but positive temperature coefficient of the resistivity and a small thermopower. In particular, for liquid Si, , and and, for liquid Ge, , and ; all values are for the respective melting temperatures of Si and Ge. We also report a calculation of the resistivity of each liquid, using the Ziman formalism, with a recent pseudopotential and an experimental structure factor. Both our experimental and our calculated results are compared with other work.

Temperature-independent narrowband optical filter at 1.3 [micro sign]m wavelength by an athermal waveguide

Abstract: The authors report the realisation of a temperature independent narrowband optical filter at 1.3 µm wavelength. A ring resonator has been designed and fabricated using an athermal waveguide, in which the optical path length is independent of temperature. The temperature coefficient of the central wavelength was successfully reduced to 7 × 10-4 nm/K (7% of that of a conventional filter).

Undercooling of bulk liquid silicon in an oxide flux

Abstract: Drops of molten silicon surrounded by a SiO2–BaO–CaO flux were undercooled at 350 K below their melting temperature. This undercooling is 75 K greater than the largest one reported so far for bulk silicon. To account for this result as well as the nucleation data from laser‐melted thin films, classical nucleation theory requires a crystal‐melt interfacial tension with a positive temperature coefficient.

Non-stoichiometry and Antiferromagnetic Phase Transition of NaCl-type CrN Thin Films Prepared by Reactive Sputtering

Abstract: Non-stoichiometric CrN 1+x (0.0≤x≤0.2) thin films with a NaCl-type structure have been prepared by reactive sputtering of chromium metal target in Ar/N 2 mixed gas. Nearly stoichiometric CrN thin films are obtained at the N2 mixing ratio of 20% and the total pressure of 0.67 Pa. Magnetic susceptibility, electrical resistivity and X-ray diffraction measurements indicate that these films show an antiferromagnetic first order phase transition at around 260 K. With the increase in N2 partial pressure, metastable over-stoichiometric CrN 1+x thin films are obtained. The compositions of samples prepared in pure N2 gas are almost CrN 1.2 and the lattice constant is 2% greater than that of the nearly-stoichiometric bulk CrN. Over-stoichiometric CrN 1+x thin films do not show the first order antiferromagnetic phase transition. Their X-T curves show a broad maximum at around 90 K. The electrical resistivity at room temperature decreases with increasing the nitrogen composition and the temperature coefficient of resistivity is always negative. The composition dependence of magnetic and electrical properties correlates to the variation in the density of state near the Fermi level detected by XPS.

Optical band gap and Urbach tail in Y‐doped BaCeO3

Abstract: Optical processes at the absorption edge in Y‐doped BaCeO3 films with thicknesses of 160 and 4600 nm have been studied in the temperature region between 20 and 1000 °C. For the first time, optical band‐gap energies of Y‐doped BaCeO3 were determined with a room temperature value of 4.1 eV and a temperature coefficient of −3.88×10−4 eV/K. It was found that the optical absorption coefficient below the band–band transition exhibits an exponential dependence on photon energy following Urbach’s rule, and the Urbach tail is dependent on oxygen partial pressures, i.e., on lattice defects. The corresponding Urbach edge parameters and their temperature dependence were investigated. The data are discussed within the model of structural and thermal disorder and a characteristic phonon energy of 46 meV was obtained according to this disorder model.

Electron paramagnetic resonance of Nb‐doped BaTiO3 ceramics with positive temperature coefficient of resistivity

Abstract: Paramagnetic centers in Nb‐doped BaTiO3 ceramics are measured at 77–500 K by electron paramagnetic resonance (EPR) for investigating the role of the centers on the well‐known positive temperature coefficient of resistivity (PTCR) effect (PTCR at the Curie temperature). EPR detects four signals; an anisotropically broad singlet signal at g=2.005, a sextet signal due to Mn2+, a Cr3+ signal, and a Ti3+ signal. The former two signals arise in the rhombohedral and cubic phases, but disappear in the tetragonal and orthorhombic phases. The Cr3+ signal appears in all of the phases, while the Ti3+ signal is detected only at low temperatures. The singlet signal also arises in undoped, barium‐deficient BaTiO3 ceramics, therefore the signal is attributable to barium‐vacancy‐associated centers rather than Nb4+ ions or Fe3+ ions proposed by several authors. In this article, we propose that the singlet signal is due to vacancy‐pairs of VBa–F+ type, i.e., the vacancy pair of VBa–VO capturing one electron. The electrical resistivity data show a polaronic character of low‐temperature conduction and a high resistivity jump around the Curie temperature. The low‐temperature polaronic conduction is explained in terms of electron‐hopping between Ti4+ and Ti3+ ions. The resistivity jump at the Curie temperature occurs along with the EPR intensity increase of the singlet signal, the Mn2+ signal and the Cr3+ signal. We conclude that the PTCR of Nb‐doped BaTiO3 ceramics is strongly associated with the trap activation of the VBa–VO vacancy‐pairs and manganese centers at the tetragonal‐to‐cubic transition.

Thermally tunable integrated optical ring resonator with poly-Si thermistor

Abstract: Doped polysilicon is applied as a thermo-optic heating element to temperature tune the ring resonator based on silicon nitride rib waveguides on silicon. The ring diameter is 2 mm allowing a large free spectral range of 26 GHz. For the first time, a poly-Si resistor is used as a thermistor to measure waveguide temperature. The temperature coefficient of the poly-Si thermistor is 9.5/spl times/10/sup -4///spl deg/C. The reproducibility of the temperature measurement is 0.3/spl deg/C. The finesse of the ring resonator is 77. The propagation loss of silicon nitride rib waveguide is 0.5 dB/cm at the wavelength of 1.312 /spl mu/m. The temperature tuning of the ring resonator and the in situ supervision of the temperature on chip based on the same poly-Si fabrication process may find use especially in optical sensor applications.

Influences of crystallization histories on PTC/NTC effects of PVDF/CB composites

Abstract: Two switching phenomena of poly (vinylidene fluoride) (PVDF) containing carbon black (CB) with different crystallization histories are studied. Both the percolation threshold (Φpc) and the critical concentration of the positive temperature coefficient (PTC) materials (Φc) increase as the crystallization temperature declines. PTC and the negative temperature coefficient (NTC) effect become obvious when the cooling speed is reduced. The morphologies of PVDF crystals play an important role in modifying the shape of the resistivity-temperature (ρ-T) curves. The PTC and NTC intensities of these systems, whose CB contents lie between Φpc and Φc, increase with increasing crystallinity, which are realized by altering the crystallization histories of the matrices. © 1996 John Wiley & Sons, Inc.

Microwave Dielectric Properties and Crystal Structure of CaO–Li2O–(1-x)Sm2O3–xLn2O3–TiO2 (Ln: lanthanide) Ceramics System

Abstract: We have investigated the microwave dielectric properties and crystal structure of the CaO–SrO–Li2O–Sm2O3Ln2O3–TiO2 system (Ln: lanthanide). This system is orthorhombic with a GdFeO3–type perovskite structure, which is the same as that of CaTiO3. It was also seen that the lanthanide ionic radius has a strong influence on the dielectric properties. Namely, the dielectric constant is affected by the unit-cell volume and the fQ value is affected by variations in the crystal structure and by the appearance of another phase. Excellent dielectric properties of e r=123 and fQ value=4150 GHz and good temperature stability for the resonant frequency (τ f'=10.8 ppm/° C) at 3 GHz were obtained with a composition of CaO:SrO:Li2O:Sm2O3:Nd2O3:TiO2=15:1:9:6:6:63 (molar ratio).

Thermal dependence of the refractive index of InP measured with integrated optical demultiplexer

Abstract: The temperature dependence of the refractive index of InP in the wavelength range from 1.2 to 1.6 μm has been determined. The method is based on the measurement of the transmission wavelengths of a n−/n+‐InP optical demultiplexer as a function of device temperature. The refractive index of InP is found to depend linearly on temperature with a temperature coefficient between 2.3 and 1.9×10−4/K. The measured absolute value and the dispersion of the refractive index of InP are in agreement with data from the literature.

Temperature Distribution Induced by Pre-lgnition Reactions in a Rapid Compression Machine

Abstract: The temperature after compression of reactive and unreactive gas mixtures are measured at different locations and times in a rapid compression machine by a thin wire thermocouple. Validation is done by laser Rayleigh scattering. Inspection or the temperature fields of unreactive gas and during the delays in three different patterns of spontaneous ignition of isooctane-“air” mixtures shows that the gas temperature is homogeneous for some limes after top dead centre. Then heat losses to the walls and gas recirculation induce temperature inhomogeneities. They are partially levelled as hot ignition is approached. The gas temperature before hot ignition is nearly the same and constant in time in both two-stage or one-stage ignitions. The levelling effect is attributed to the complex chemical kinetics of low temperature alkane oxidation in the range of the negative temperature coefficient of the global reaction rate.

VOX film, wherein X is greater than 1.875 and less than 2.0, and a bolometer-type infrared sensor comprising the VOX film

Abstract: At a heat treatment temperature in a reducing atmosphere of Ar and H 2 , a precursory film of V 2 O 5 is reduced into a VO x film with the heat treatment temperature selected in a predetermined temperature range between 350 ° C. and 450° C., both exclusive, to control a resistivity of the VO x film, where x is greater than 1.875 and less than 2.0. The VO x film is not susceptible to a metal-semiconductor phase transition inevitable in VO 2 at about 70° C. and is excellent for use in a bolometer-type infrared sensor. When reduced at 350° C. and 450° C. the resistivity and its temperature coefficient of the VO x film at room temperature are 0.5 and 0.002 Ω cm and -2.2% and 0.2% per degree Celsius.

Temperature sensing apparatus

Abstract: A temperature sensing apparatus (200), includes a temperature sensor (220), a current source (203), and a temperature independent voltage source (240, 250). The current source (203) provides a biasing current to the temperature sensor (220), and the voltage source (240, 250) is programmable to adjust the output of the temperature sensor (220). Preferably, the voltage source (240, 250) includes a variable current source (240) and a resistive element (250) with counterbalanced temperature coefficients, and the bias current source (203) is selected to have a counterbalancing temperature coefficient to that of the resistive element (250).

Effects of temperature on the electrical impedance of piezoelectric sensors

Abstract: Piezoelectric materials have been known to have temperature dependency regarding their basic properties, such as the dielectric constant and the piezoelectric coefficient. In this paper, this temperature dependency is investigated. The motivation of this work is linked to the impedance-based nondestructive evaluation (NDE) technique which employs piezoceramic (PZT) sensors for tracking changes in the structural impedance, by measuring the electrical impedance, to qualitatively identify damage. However, for this NDE technique to be successful in all types of environments, it must be insensitive to temperature variations. As mentioned earlier, piezoelectric materials have strong temperature dependency and a temperature compensation procedure is necessary. The approach used in this paper is empirical due to the complexity of the thermoelectromechanical constitutive modeling. Through experimental investigation, it was found that temperature will have the effect of shifting the electrical impedance magnitude of the piezoelectric sensor, while leaving the impedance phase unaffected. For a PZT PSI-5A, the variation was found to be linear in the 80 degree F to 160 degree F temperature range. To characterize the temperature effects in piezoelectric materials, a temperature coefficient which is independent of frequency has been defined. Finally, based on the defined temperature coefficient, a simple temperature compensation technique has been implemented successfully, eliminating the effects of temperature on PZT sensors while not eliminating the effects of temperature on the structure.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Electrode effects in positive temperature coefficient and negative temperature coefficient devices measured by complex‐plane impedance analysis

Abstract: Impedance spectroscopy in conjunction with capacitance measurements were utilized to study the electrical contact behavior of commercial positive temperature coefficient and negative temperature coefficient ceramics. The impedance spectroscopy data, measured at frequencies ranging from 100 Hz to 40 MHz, exhibited a double semi‐circle shape characteristic of systems which have both barrier and bulk contributions to the impedance of the device. From the impedance data, equivalent capacitive‐resistive circuits were assigned to grain, grain boundary, and barrier elements. These barrier capacitances and resistances were measured for Au, Ag, Pt, and In‐Ga electrodes. The trends in the magnitude of the barrier resistance are discussed with relevance to the thermodynamic work of adhesion and the effect of metal‐oxygen and metal‐metal interactions at the interface.

Low-temperature electrical conductivity of

Abstract: We report the electrical conductivity between 2 and 300 K for across the composition-controlled metal - insulator (m - i) transition. Using a method first suggested by Mobius, we identify the critical concentration to be 0.3 for the m - i transition. The negative temperature coefficient of resistivity observed at low temperatures in the metallic phase follows a temperature dependence characteristic of disorder effects. The semiconducting compositions do not show a simple activation energy but exhibit variable-range hopping at high temperatures confirming that the m - i transition in this system is driven by increasing disorder effects.

X-Ray Investigations and Determination of the Dielectric Properties of the Compound Ba4.5Gd9Ti18O54.

Abstract: The composition of the Gd variant of the Ba6-x R8+2/3x Ti18O54 family of solid solutions ( R=rare earth) was determined by investigating its high temperature phase relations and by wavelength-dispersion spectroscopic analysis. The composition of the compound was determined to be Ba4.5Gd9Ti18O54, which corresponds to x=1.5 without a detectable solid solubility range. The intensities of the diffraction lines are not in accordance with published values in the powder diffraction file (No. 43–233). An investigation of the thermal stability revealed that Ba4.5Gd9Ti18O54 starts to decompose at a temperature around 1350° C. It decomposes into a Gd2Ti2O7, BaTi2O5 solid solution with Gd2O3, and probably Ba4Ti13O30. However, during slowly cooling metastable BaTi2O5(ss) also decomposes into Ba6Ti17O40 and BaTiO3 solid solution with Gd2O3. Single phase Ba4.5Gd9Ti18O54 ceramics displayed a dielectric constant (e r) of 76.1, a Qxf-value of 2050 and a temperature coefficient of the resonant frequency (τ f) of -35 ppm/K.