Showing papers on "Temperature coefficient published in 2002"

Microwave dielectric properties of Ba2−xSm4+2/3xTi9O26 ceramics with zero temperature coefficient

Abstract: Ba 2− x Sm 4+2 x /3 Ti 9 O 26 ( x =0–0.3) was selected as the formula for BaO–Sm 2 O 3 –4.5TiO 2 ceramics based on the structure formula of the tungsten–bronze-type structure. The microstructure and microwave dielectric properties of Ba 2− x Sm 4+2 x/ 3 Ti 9 O 26 and the relationships between them were investigated in this study. The microstructure of Ba 2− x Sm 4+2 x /3 Ti 9 O 26 ceramics consists of a uniform matrix of primary BaO–Sm 2 O 3 –4TiO 2 and BaO–Sm 2 O 3 –5TiO 2 with small amounts of dispersed second phase, TiO 2 and Ba 2 Ti 9 O 20 . Excellent dielectric properties of e r =68–79, Q × f =11 000–12 500 GHz and good temperature stability at the resonant frequency ( τ f =0 ppm per °C) were obtained. The relationships between the microstructure, second phases and microwave dielectric properties were concerned. XRD patterns, energy dispersive X-ray spectrometer (EDS) analysis and scanning electron microscopy are also presented.

(Zn, Ni, Ti) substituted barium ferrite particles with improved temperature coefficient of coercivity

Abstract: The Ba-ferrite particles have a positive temperature coefficient of d H c /d T . From the viewpoint of recording system applications, the d H c /d T should be reduced in order to improve the operating margin for temperature change. It was shown that the coercivity, H c , of Ba-ferrite particles is mainly controlled by the crystalline anisotropy. In the Ba-ferrite there are five distinct crystallographic sites or sublattices, and the Fe 3+ ion located on different site has different contribution to crystalline anisotropy. The replacement of Fe 3+ ions by Zn 2+ , Ni 2+ , and Ti 4+ ion will effect on the saturation magnetization σ s , coercivity H c and their temperature coefficients. The Zn–Ti, Ni–Ti, and Ni–Zn–Ti substituted Ba-ferrite particles were investigated. It was found that the Zn–Ti substituted Ba-ferrite particles have higher σ s , but large temperature coefficient of coercivity d H c /d T . In the case of Ni–Ti and Ni–Zn–Ti substitution the d H c /d T could be reduced to a small value, near to zero or negative value, while the σ s decreases very slowly with increasing the amount of substitution.

High-accuracy measurements of the refractive index and its temperature coefficient of calcium fluoride in a wide wavelength range from 138 to 2326 nm

Abstract: The refractive indices of synthetic calcium fluoride for 69 wavelengths from 138 nm in the deep ultraviolet to 2326 nm in the near infrared were measured by the minimum-deviation method in a nitrogen environment. We made these measurements at 20° and 25 °C, respectively, to determine the thermal coefficients of the refractive index over this wide-wavelength region. These refractive indices were fitted to a four-term Sellmeier dispersion formula. The temperature coefficients of the refractive index were fitted to a Hoffman-type dispersion formula. The standard deviation of the residual between the observed values and the calculated values was 0.6 × 10-6 for the refractive index and was 0.13 × 10-6/°C for the temperature coefficient of the refractive index.

Crystal Structure, Magnetic Properties and Electronic Structure of the MnBi Intermetallic Compound

Abstract: The low-temperature phase of the MnBi alloy has a coercivity μ0Hc of 2.0 T at 400 K and exhibits a positive temperature coefficient from 0 to 400 K. In the higher temperature range it shows a much higher coercivity than that of the NdFeB magnets, which suggests that it has considerable potential as a permanent magnet for use at high temperatures. In the temperature range from 30 to 150 K, the Mn atom is found to change its spin direction from a perpendicular to a parallel orientation with respect to the c axis. The anisotropy field increases with increasing temperature which gives rise to a higher coercivity at the higher temperatures. The maximum energy product (BH)max of the magnet is 7.7 and 4.6 MG Oe at room temperature and 400 K, respectively. The electronic structure of MnBi indicates that the Mn atom possesses a magnetic moment of 3.6μB, and that the Bi atom has a magnetic moment of −0.15μB which is due to the s–d and p–d hybridization between Bi and Mn atoms. We have also investigated the volume dependence of the magnetic moments of Mn and Bi. The results indicate that an increase in the intra-atomic exchange splitting due to the cell volume expansion leads to a large magnetic moment for the Mn atom. The Mn magnetic moment attains a value of 4.6μB at a volume expansion rate of ΔV/V ≈ 100%.

Characteristics of chromium doped spinel crystals for a fiber-optic thermometer application

Abstract: The chromium doped spinel crystals, MgAl2O4:Cr3+, have been grown using floating zone technique and its application for the fiber-optic thermometer is discussed on the basis of the temperature dependence of the fluorescence lifetime. Lifetimes of the spinel crystals (τ=10 ms), which decrease with temperature, were about 3 times larger than that of the ruby crystals (τ=4 ms) at room temperature. Temperature coefficients of the photoluminescence (PL) lifetimes for spinel crystals (37 μs/K) are also 3 times longer than that for the ruby crystals (10 μs/K). The spinel crystals are potentially useful sensor probes for the fiber-optic thermometer, which have an advantage of high sensitivity due to long PL lifetime and large temperature coefficient.

Dielectric properties of blood: an investigation of temperature dependence.

Abstract: We have investigated the temperature dependence of the electrical parameters (permittivity and conductivity) of blood. The measuring system, composed of an impedancemeter (HP 4291 A), an open-ended coaxial line and a temperature controlling set, was designed for dielectric measurement in the frequency range of 1 MHz to 1 GHz. Measurements were performed on ex vivo blood of humans and animals (cow and sheep). The results obtained show the weak sensibility and a change of sign of the temperature coefficient of the relative permittivity (about 0.3% degrees C(-1) at 1 MHz and -0.3% degrees C(-1) at 1 GHz). The conductivity presents a more significant variation (of the order of 1% degrees C(-1) over the whole operating frequency range.

Tunability of τf in perovskites and related compounds

Abstract: Tuning the temperature coefficient of resonant frequency (τf) in microwave dielectrics has been attributed to two main mechanisms: (i) dilution of the average ionic polarizability; (ii) the onset of an octahedral tilt transition above room temperature. The contributions of each mechanism have been isolated using ceramics in the Srn+1TinO3n+1, SrxCa1−x)3Ti2O7, and (SrxCa1−x)TiO3 series. In the Srn+1TinO3n+1 series, relative permittivity (er) and τf are linearly proportional over a broad range of values, 100–37 and 800–140 ppm/°C, n = 4 and 1, respectively. No structural phase transitions occur on cooling from the prototype symmetry, and the mechanism of tuning is attributed solely to dilution of the average ionic polarizability as the SrO:SrTiO3 ratio increases. Exchanging Ca for Sr in the (SrxCa1−x)3Ti2O7 series resulted in an 80% reduction in the magnitude of τf from +320 to +50 ppm/°C but only 21% in permittivity (58 to 46). The effect was nonlinear and attributed primarily to the onset of a phase transition involving rotations of the octahedra on cooling. Superlattice reflections associated with the octahedral tilt transition have been identified.

Surface Tension and Its Temperature Coefficient of Molten Tin Determined with the Sessile Drop Method at Different Oxygen Partial Pressures

Abstract: The surface tension of molten tin has been determined by the sessile drop method at The surface tension of molten tin has been determined by the sessile drop method at temperatures ranging from 523 to 1033 K and in the oxygen partial pressure (P-O2) range from 2.85 x 10(-19) to 8.56 x 10(-6) MPa, and its dependence on temperature and oxygen partial pressure has been analyzed. At P-O2 = 2.85 x 10(-19) and 1.06 x 10(-15) MPa, the surface tension decreases linearly with the increase of temperature and its temperature coefficients are -0.151 and -0.094 mNm(-1) K-1, respectively. However, at high P-O2 (3.17 x 10(-10), 8.56 x 10(-6) MPa), the surface tension increases with the temperature near the melting point (505 K) and decreases above 723 K. The surface tension decrease with increasing P-O2 is much larger near the melting point than at temperatures above 823 K. The contact angle between the molten tin and the alumina substrate is 158-173degrees, and the wettability is poor.

Structure and magnetic properties of the MnBi low temperature phase

Abstract: High purity MnBi low temperature phase has been prepared and analyzed using magnetic measurements and neutron diffraction. The low-temperature phase of the MnBi alloy has a coercivity μ0iHc of 2.0 T at 400 K, and exhibits a positive temperature coefficient from 0 to at least 400 K. The neutron data refinement indicated that the Mn atom changes its spin direction from c axis above room temperature to nearly perpendicular to the c axis at 50 K. A canted magnetic structure has been observed below 200 K. The anisotropy field increases with increasing temperature which gives rise to a high coercivity at the higher temperatures. The anisotropic bonded magnets have maximum energy products (BH)max of 7.7 and 4.6 MGOe at room temperature and 400 K, respectively.

Conductive polymer composites (CPCs): comparison of electrical properties of poly(ethylene-co-ethyl acrylate)-carbon black with poly(butylene terephthalate)/poly(ethylene-co-ethyl acrylate)-carbon black†

Abstract: We have studied the influence of the addition of poly(butylene terephthalate) (PBT) to poly(ethylene-co-ethyl acrylate)-carbon black (EEA-CB) on the electrical properties of the blends, namely the resistivity and power dissipation of the monophasic and diphasic systems. The morphology of the latter blend is characterized by two interpenetrated immiscible phases which provide a good thermal stability to the composite up to at least 170 °C. Power can be regularly delivered without a negative temperature coefficient (NTC) effect at 100 °C where poly(ethylene-co-ethyl acrylate) melts. However, the significant decrease of positive temperature coefficient (PTC) amplitude compared to the monophasic systems reduces the self-regulating heating ability. The PTC effect amplitude decrease was studied by some of its known origins (thermal expansion, melting of crystalline phase, carbon black localization). Correlation of the composites resistivity with linear expansion, the melting process of EEA, and the volume expansion of PBT, show that none of these parameters can alone explain the electrical properties' evolution with temperature. Our interpretation of the PTC effect amplitude decrease is based on the PBT matrix lower thermal expansion and on the increased carbon black content located at the interface. Copyright © 2003 John Wiley & Sons, Ltd.

Compositional dependence of the Néel transition, structural stability, magnetic properties and electrical resistivity in Fe–Mn–Al–Cr–Si alloys

Abstract: Compositional dependence of the Neel transition temperature, TN, structural stability, magnetic properties, and anomaly of resistivity, ρ, have been investigated in γ-Fe–Mn–Al–Cr–Si–C alloys. In the Fe–(20–32.5 at.%)Mn-based alloys, an increase in Mn, Al, Cr or C content will inhibit the γ→e→α′ martensitic transformation and lower the e martensitic transformation temperature Ms or deformation-induced martensitic transformation temperature Md, while an increase in Si content promotes the γ→e martensitic transformation and increases the Ms and Md temperature. Manganese raises TN, decreases the susceptibility, χ, and slightly increases the anomaly of electrical resistivity, ρ. The temperature dependence of χ for Fe–Mn binary alloys shows a break at TN and χ becomes almost independent of temperature above TN. The effects of Cr on χ versus T and ρ versus T are similar to that of Mn but lowering TN. With increasing Al or Si content, TN decreases markedly; above TN, the magnetic state of the alloys changes from Pauli-paramagnetism to the Curie–Weiss behavior, and the anomalous ρ increases rapidly, leading to a negative temperature coefficient below TN. An empirical formula has been developed to express the compositional dependence of TN in γ-Fe–Mn–Al–Cr–Si alloys. The alloying elements that lower TN are in increasing order of severity, Cr, Al, Si and C. Manganese is the only element which both raises TN and decreases χ and Ms. Adding up to 10 at.% Al to Fe–Mn based alloys strongly stabilizes the γ phase and retains antiferromagnetism down to 4 K. Increasing the Ni content of Fe–Cr–Ni steels improves their stability relative to the α′ martensitic transformation but induces a transition from the paramagnetic γ-phase to the ferromagnetic one at very low temperatures. Alloying Fe–Mn based alloys with Al and Cr exhibits an excellent combination of FCC structural stability, corrosion resistance and antiferromagnetism down to liquid helium temperature. The present results are valuable for the design of cryogenic non-magnetic steels and certain functional Fe–Mn based alloys.

Electrical and optical properties of TiOx thin films deposited by reactive magnetron sputtering

Abstract: The present study of the electronic properties of titanium monoxide thin films is centered on the electrical and optical properties of nano-grain material. TiOx thin films with x ranging from 0.75 to 1.45 have been deposited by r.f. reactive magnetron sputtering in a mixed Ar/O2 or Ar/H2O atmosphere. All films show a negative temperature coefficient of the resistivity. Spectroscopic ellipsometry measurements were performed in the Vis-UV spectral range. The free carrier and interband contributions to the dielectric function have been sorted out. The most striking feature of the free carrier optical response is the very short scattering time of the order of 10−15 s. Such an intense impurity scattering is beyond the validity range of the semi-classical Boltzmann equation and remains an open problem.

Temperature dependence of commercially available diode detectors.

Abstract: Temperature dependence of commercially available n- and p-type diodes were studied experimentally under both high instantaneous dose rate (pulsed) and low dose rate (continuous) radiation. The sensitivity versus temperature was measured at SSD = 80 or 100 cm, 10 x 10 cm2, and 5 cm depth in a 30 x 30 x 30 cm3 water phantom between 10 degrees C and 35 degrees C. The response was linear for all the diode detectors. The temperature coefficient (or sensitivity variation with temperature, svwt) was dose rate independent for preirradiated diodes. They were (0.30 +/- 0.01)%/degrees C, (0.36 +/- 0.03)%/degrees C, and (0.29 +/- 0.08)%/degrees C for QED p-type, EDP p-type, and Isorad n-type diodes, respectively. The temperature coefficient for unirradiated n-type diodes was different under low dose rate [(0.16 to 0.45)%/degrees C, continuous, cobalt] and high instantaneous dose rate [(0.07 +/- 0.02)%/degrees C, pulsed radiation]. Moreover, the temperature coefficient varies among individual diodes. Similarly, the temperature coefficient for a special unirradiated QED p-type diode was different under low dose rate (0.34%/degrees C, cobalt) and high instantaneous dose rate [(0.26 +/- 0.01)%/degrees C, pulsed radiation]. Sufficient preirradiation can eliminate dose rate dependence of the temperature coefficient. On the contrary, preirradiation cannot eliminate dose rate dependence of the diode sensitivity itself.

Gases delivery conduit

Abstract: A gases delivery conduit for the supply of humidified medical gases as formed from an extruded plastic tube. The extruded plastic tube includes at least one heating element embedded within the wall of the tube. The heating element includes a pair of electrical conductors separated by a positive temperature coefficient material wherein the localised resistance of the material is positively related to the localised temperature.

Thermistor ceramic with negative temperature coefficient based on Zn7Sb2O12: An inverse spinel-type phase

Abstract: The electric properties of the semiconductor ceramic Zn7Sb2O12 were investigated by impedance spectroscopy. The grain resistance exhibits a thermistor behavior with negative temperature coefficient. Two regions on the resistance curve were identified. Each region shows a different thermistor characteristic parameter (β), which is equal to 3170 and 3845 °C, at measurement temperatures up to 350 °C and above 450 °C, respectively. The temperature coefficient of the resistance (α) was derived being equal to −2.59×10−2 °C−1 and −1.89×10−2 °C−1, at 350 and 450 °C, respectively. The anomalous behavior of the resistance is further evidence of phase transition phenomenon.

Structural, dielectric and electrical properties of ACu3Ti4O12 (A = Ca, Sr and Ba)

Abstract: Polycrystalline compounds of a general formula ACu3Ti4O12 (A = Ca, Sr, Ba) were synthesized by a high-temperature solid-state reaction technique. Crystal structure of the compounds at room temperature studied by an X-ray diffraction (XRD) technique was found to be cubic. Detailed studies of dielectric constant (e) and loss tangent (tan δ) of the compounds as a function of frequency (100 Hz to 1 MHz) and temperature (125 K to 600 K) did not show any ferroelectric phase transition. The ac and dc conductivity studied over a wide range of temperature provided activation energy of the compounds. Variable range phonon-assisted quantum mechanical tunneling technique of correlated barrier hopping (CBH) model has been applied to interpret the temperature and frequency dependent ac conductivity. Using these theoretical models, various model parameters have also been calculated. The temperature variation of resistivity shows that the compounds have negative temperature coefficient of resistance (NTCR).

The polymer–fullerene interpenetrating network: one route to a solar cell approach

Abstract: We analyse recent developments of thin film plastic solar cells, using polymer–fullerene bulk heterojunctions as an absorber, as well as peculiarities of the device behaviour at various temperature and light illumination intensities. To establish the potential of this type of cells in full scale and to identify the loss factors, we studied the electronic transport properties of the conjugated polymer/fullerene based solar cells by means of temperature and illumination intensity-dependent current–voltage characteristics. A positive temperature coefficient of the short-circuit current at all light illumination intensities applied, i.e., from 0.1 to 100mW/cm2 (white light), has been established. An increase of the open-circuit voltage from 850 to 940mV was observed, when cooling down the device from room temperature to 100K. The incerase of the VOC at low temperatures is in contrast to polymer absorbtion red-shift (due to lowering of HOMO-LUMO gap) as well as to the red-shift of external quantum yield spectrum at low temperatures. The fill factor depends strongly on temperature with a positive temperature coefficient in the same temperature range. In contrast, the light intensity dependence of the fill factor shows a maximum of 52% at intermediate illumination intensities (3mW/cm2) and decreases subsequently, with increasing intensity. These observations, clearly formulate the requirements to utilise high-mobility polymers with reduced defect density as the absorber. The spectral matching of the active layer absorption to the solar spectrum needs to be improved, being essential for potential applications of these novel photo-voltaic devices.

Fine-Tolerance Resonator Applications of Bismuth-Layer-Structured Ferroelectric Ceramics

Abstract: The two-layered SrBi2Nb2O9 (SBN) ceramic resonator that generates second harmonics of the thickness extensional vibration is a strong candidate for fine-tolerance-resonator applications, because of its small electromechanical coupling coefficient and high temperature stability. The Nd substitution of Sr in SBN is effective in further decreasing the temperature coefficient of resonance frequency (TCF) while maintaining its high maximum value of electrical quality factor (Qemax) at a resonance frequency. On the other hand, a grain-oriented SBN-type ceramic resonator has an attractive property of thickness shear vibration. The temperature coefficient of oscillation frequency (TCO) could be less than 10 ppm/°C in an ordinary oscillation circuit with grain-oriented SBN-type ceramics. In this paper, we describe how a fine tolerance resonator can be realized using compositional and structural designs of the bismuth-layer-structured ferroelectric (BLSF) ceramic resonator.

High Temperature Stable Acoustic Surface Wave Substrates of SiO2/LiNbO3 Structure with Super High Coupling

Abstract: In this study, in order to improve the temperature coefficient of frequency (TCF), the propagation characteristics of SiO2/Rotated Y-cut, X-propagating LiNbO3 leaky surface acoustic wave (SAW) substrates with a large electromechanical coupling coefficient (k2), zero attenuation and zero TCF are theoretically investigated. The results showed a large k2 of more than 0.2 (zero k2 of Rayleigh mode), zero TCF and zero propagation attenuation for leaky SAW in the case of a short boundary at a thin film thickness of H/λ\fallingdotseq0.15–0.25 (H: SiO2 film thickness, λ: SAW wavelength). The experimental results agreed with the theoretical ones.

Fabrication, morphology and structural characterization of ordered single-crystal Ag nanowires

Abstract: Highly aligned Ag nanowires have been synthesized by dc electrodeposition within a hexagonal close-packed nanochannel anodic aluminum oxide template. The pore diameter varies from 20 nm to 50 nm depending on the anodization voltage and temperature for the two types of aqueous solutions, sulphuric and oxalic acids, respectively. The size and morphology of the Ag nanowire arrays were measured by scanning electron microscopy and transmission electron microscopy. The images indicate that the highly aligned Ag nanowires grow in the uniform nanochannels of the anodic alumina template and that the size of the nanowires depends on the size of the nanochannels. X-ray diffraction, selected area electron diffraction pattern and high-resolution transmission electron microscopy images show that the Ag nanowires are single-crystal. The temperature coefficient of resistivity (temperature range from 4.2 K to 300 K) of the Ag nanowire arrays decreases with decreasing diameter of the nanowires.

Temperature stable CMOS device

Abstract: A CMOS field effect transistor (FET) is provided with predetermined temperature characteristics. More particularly, the relationship between the channel length, gate width, gate-to-source voltage, and drain current is exploited to create an FET that has relatively constant drain current across a relatively wide range of frequencies. Alternately, the above-mentioned relationship is exploited to create a drain current with a predetermined temperature coefficient across a wide temperature range.

Microwave Dielectric Properties and Microstructures of Ca1-xNd2x/3TiO3–Li1/2Nd1/2TiO3 Ceramics

Abstract: The microwave dielectric properties and microstructures of (1-y)Ca1-xNd2x/3TiO3–yLi1/2Nd1/2TiO3 ceramics, prepared by a mixed oxide route, have been investigated. A maximum quality factor Q×f=8600 GHz (where f is the resonant frequency) was achieved for samples with y=0 and x=0.39, although the dielectric properties varied with sintering temperature. The Q×f value of (1-y)Ca1-xNd2x/3TiO3–yLi1/2Nd1/2TiO3 increased up to 1400°C, after which it decreased. The decrease in dielectric properties was coincident with the onset of rapid grain growth. In the (1-y)Ca1-xNd2x/3TiO3–yLi1/2Nd1/2TiO3 system, as y increased, the temperature coefficient of resonant frequency (τf) decreased from 800 to 8 ppm/°C for y=0.52 (x=0.39). However, the optimum combination of microwave dielectric properties was achieved at 1400°C for samples where y=0.45 (x=0.39) with a dielectric constant er of 101, a Q×f value of 5300 (at 7.2 GHz) and a τf value of +13 ppm/°C.

Temperature sensing apparatus and methods

Abstract: Circuits and methods for generating a temperature dependent signal are described involving: generating a thermal voltage referenced positive temperature coefficient signal using a pair of transistors operating at different current densities; generating a transistor voltage referenced negative temperature coefficient signal using a transistor voltage of one of said pair of transistors; and subtracting one of said positive and negative temperature coefficient signals from the other of said signals to generate said temperature dependent signal, whereby the temperature dependence of said temperature dependent signal is greater than either of said subtracted signals.

Surface Tension and Its Temperature Coefficient of Molten Silicon at Different Oxygen Potentials

Abstract: Previous investigations on the effects of temperature and impurities on the, surface tension of molten silicon, and relevant measurement methods have been reviewed, and the influence of oxygen partial pressure (P-O2) in the atmosphere has been analyzed emphatically, on the basis of the results obtained by the sessile drop method and calculations. In the case of P-O2 less than or equal to P-O2.(sat) (the saturated oxygen partial pressure in the Si(l)-O-2(g)-SiO2(S) system), the surface tension First remains almost constant and then decreases remarkably with the increase of P-O2, the temperature coefficient of surface tension (partial derivativesigma/partial derivativeT) is negative and increases with the oxygen partial pressure, and the molten silicon drop is very sensitive to outside vibrations, However. in the case of P-O2 > P-O2.(sat) the surface tension increases slightly with the oxygen partial pressure, partial derivativesigma/partial derivativeT is higher and also increases with P-O2 the molten silicon drop is not influenced by environmental disturbances and remains stable, and EPMA (electron probe microanalyzer) analysis indicates the formation of a thin SiO2(S) film on the surface of the molten silicon drop which might account fur the surface tension increase.