Showing papers on "Temperature coefficient published in 1997"

Temperature dependence of 1H chemical shifts in proteins

Abstract: Temperature coefficients have been measured by 2D NMR methods forthe amide and CαH proton chemical shifts in two globularproteins, bovine pancreatic trypsin inhibitor and hen egg-white lysozyme.The temperature-dependent changes in chemical shift are generally linear upto about 15° below the global denaturation temperature, and the derivedcoefficients span a range of roughly −16 to +2 ppb/K for amide protonsand −4 to +3 ppb/K for CαH. The temperaturecoefficients can be rationalized by the assumption that heating causesincreases in thermal motion in the protein. Precise calculations oftemperature coefficients derived from protein coordinates are not possible,since chemical shifts are sensitive to small changes in atomic coordinates.Amide temperature coefficients correlate well with the location of hydrogenbonds as determined by crystallography. It is concluded that a combined useof both temperature coefficients and exchange rates produces a far morereliable indicator of hydrogen bonding than either alone. If an amide protonexchanges slowly and has a temperature coefficient more positive than−4.5 ppb/K, it is hydrogen bonded, while if it exchanges rapidly andhas a temperature coefficient more negative than −4.5 ppb/K, it is nothydrogen bonded. The previously observed unreliability of temperaturecoefficients as measures of hydrogen bonding in peptides may arise fromlosses of peptide secondary structure on heating.

Material characteristics of perovskite manganese oxide thin films for bolometric applications

Abstract: We are optimizing thin films of perovskite manganese oxides for bolometric applications. We have studied the relevant material characteristics of several members of this family namely, La0.7Ba0.3MnO3, La0.7Sr0.3MnO3, La0.7Ca0.3MnO3, and Nd0.7Sr0.3MnO3. Here, we discuss issues related to the choice of material, the influence of deposition parameters, and postdeposition heat treatments on the relevant characteristics such as the resistivity-peak temperature (Tp) and the temperature coefficient of resistance (TCR). For a given material, a higher peak temperature implies a larger temperature coefficient of resistance. In contrast, on comparing different material systems, the TCR tends to decrease as Tp increases.

Dielectric Properties of AB2O6 Compounds at Microwave Frequencies (A=Ca, Mg, Mn, Co, Ni, Zn, and B=Nb, Ta)

Abstract: The microwave dielectric properties of AB2O6 compounds, where A=Ca, Mg, Mn, Co, Ni, or Zn, and B=Nb or Ta, were investigated. All samples were prepared using the mixed oxide method. In particular, AB2O6 compounds with Mg2+ and Zn2+ revealed very good dielectric properties with respect to dielectric resonator applications. However, their temperature coefficient of resonant frequencies should be modified. It was found that the electronegativity of AB2O6 compounds is relevant to their dielectric constant. The temperature coefficient of resonant frequency was discussed in terms of volume contraction.

A respiration based description of plant growth rate responses to temperature

Abstract: The temperature dependence of metabolic rates determines how plant growth rates vary with temperature. This paper shows that equations on physiological relations between respiration rates (i.e. rates of heat loss and CO2 evolution) and growth rates can be used to describe temperature effects on plant growth rate. Incorporating measured values of plant respiratory heat and CO2 rates at a few temperatures into the equations allows description of growth rates as a function of temperature and provides a physiological basis for understanding the effects of temperature on growth rate. The paper presents data on cabbage (Brassica oleracea L. Capitata) and tomato (Lycopersicon esculentum Miller cv. Ace) as model cool-climate and warm-climate cultivars to illustrate application of the methods in determining optimal growth climates for different cultivars, accessions, and ecotypes. The respiration-based calculations of growth rate vs. temperature yield curves for both species that are consistent with known temperature-growth requirements. We conclude that plant responses to temperature can be accurately predicted in detail from respiration rate measurements and the growth-respiration model. These studies demonstrate that the temperature dependence of growth rates is a function of the temperature dependencies of both metabolic rates and metabolic efficiency, which change continuously with temperature. The ultimate cause of high- and low-temperature growth limits is commonly not membrane phase transitions or enzyme denaturation as has been supposed, but is loss of substrate carbon conversion efficiency. The results show that “plant temperature stress” has been misunderstood and must be redefined because there is no “nonstressfull temperature”.

Electrical properties of polymer composites prepared by sintering a mixture of carbon black and ultra-high molecular weight polyethylene powder

Abstract: Conductive polymer composites were prepared by sintering a mixture of ultrahigh molecular weight polyethylene (UHMWPE) powder and carbon black. Two processing parameters-time and temperature-were shown to have a notable effect on the resistivity of the composites. The relationships between the processing parameters and morphology were studied using optical microscopy and transmission electron microscopy (TEM). The results of the optical microscopy studies indicate that the carbon black is distributed in the interfacial regions between the UHMWPE particles. The dimension of the carbon black channels increases with the carbon black concentration. TEM micrographs show that a high degree of intermixing between the carbon black and the polymer occurs at higher temperatures and longer processing times, resulting in higher resistivities. A positive temperature coefficient (PTC) effect was observed for these materials. A mechanism for the PTC effect in this system is proposed. The magnitude of the PTC effect is found to be inversely proportional to the dimension of the carbon black channels in the composites. The dimension is directly related to the carbon black concentration. The PTC effect is a result of the polymer volume expansion caused by melting of the crystallites. A large PTC effect is observed for the composites with a low carbon black concentration and vice versa. No negative temperature effect (NTC) is observed at temperatures substantially above the melting point of the polymer.

Improvement of the dielectric properties of Ta2O5 through substitution with Al2O3

Abstract: For bulk ceramic materials, small substitutions of Al2O3 in Ta2O5 are shown to cause a moderate enhancement of the dielectric constant. For compositions near 0.95Ta2O5–0.05Al2O3, the temperature coefficient of the dielectric constant at 20 °C dramatically decreases, from more than 200 ppm/°C for pure Ta2O5 to less than 20 ppm/°C. The quality factors (Q) at 1 MHz, are 2000–5000. Measurements to 14 GHz at 20 °C show that the dielectric constant is maintained to microwave frequencies, with a Q of ∼600 at 5 GHz.

Polymeric compositions having a temperature-stable dielectric constant

Abstract: A polymeric composition which has a dielectric constant K' greater than 4 at 20 °C which varies little with temperature is made from a polymer or mixture of polymers and a ceramic or a mixture of ceramics, where the polymer or mixture of polymers has a dielectric constant K' in the range of about 1.5 to about 3.5 and a temperature coefficient of dielectric constant TCK' that is negative and is between 0 and about -300 ppm/°C; and the ceramic includes a first ceramic, which may be one ceramic or a mixture of ceramics, each having a dielectric constant in the range of about 15 to about 200 and a TCK' that is positive and is between 0 and about 3000 ppm/°C; and an optional second ceramic, which may be one ceramic or a mixture of ceramics, each having a dielectric constant in the range from about 3 up to about 15 and a TCK' that is positive and is between 0 and about 300 ppm/°C.

Thermal and electromagnetic behavior of doped poly(3,4-ethylenedioxythiophene) films

Abstract: Structure and electrophysical properties of poly(3,4-ethylenedioxythiophene) (PEDOT) films electrochemically doped with PF6-, BF4-, and CF3SO3- have been studied. According to SEM and XRD studies, doped PEDOT films have amorphous fibrillar structure. Thermal studies show that continuous degradation occurs above 150 °C and complete decomposition above 390 °C. Temperature dependence of the dc conductivity of best doped PEDOT samples was very weak and characteristic resistivity ratio ρr = ρ(1.4 K)/ρ(291 K) was less than 2. For these most conducting PEDOT samples the temperature coefficient of resistivity (TCR = Δρ/ρΔΤ) changes sign below 10 K from negative to positive which is characteristic for normal metals. SQUID data are in good agreement with the conductivity results.

Positive temperature coefficient of breakdown voltage in 4H-SiC pn junction rectifiers

Abstract: It has been suggested that once silicon carbide (SiC) technology overcomes some crystal growth obstacles, superior SiC semiconductor devices would supplant silicon in many high-power applications. However, the property of positive temperature coefficient of breakdown voltage, a behavior crucial to realizing excellent power device reliability, has not been observed in 4H-SiC, which is presently the best-suited SiC polytype for power device implementation. This paper reports the first experimental measurements of stable positive temperature coefficient behavior observed in 4H-SiC pn junction rectifiers. This research indicates that robust 4H-SiC power devices with high breakdown reliability should be achievable after SiC foundries reduce material defects such as micropipes, dislocations, and deep level impurities.

Microstructure and electrical transport properties of pulse-plated nanocrystalline nickel electrodeposits

Abstract: The microstructure and the electrical transport properties (the electrical resistivity, its temperature coefficient and the thermoelectric power) were investigated for pulse-plated nanocrystalline nickel electrodeposits. Transmission and scanning electron microscopy were used to study the microstructure (grain size and lattice defects) and the surface morphology respectively. The samples were prepared from the same bath as used previously for d.c. plating and the deposition current density was constant, in most cases i dep = 20 A dm −2 . In a given series, the pulse length t on was kept constant at 0.001, 0.01, 0.1, 1 or 10 s and the separation between pulses t off was varied from 0.001 s to 10 s. Systematic variations of the electrical transport parameters with t on and t off were observed, which we attempt to explain in terms of the periodic variation due to pulse-plating of the local Ni 2+ concentration at the cathode-electrolyte interface.

Properties and applications of filled conductive polymer composites

Abstract: The electrical properties of polymers filled with different types of conducting particles are reviewed. Following a theoretical description of a general effective media (GEM) equation, the experimental conductivity–volume fraction data for thermoplastic filled with vanadium oxide particles as well as thermosetting polymer composites, are fitted to the equation. The calculated property-related parameters in the equation are discussed. The electrical conductivity of the composites is combined with an extremely large positive temperature coefficient (PTC) effect, depending on the filler type (V2O3 or carbon black), as well as on its distribution and volume fraction. Both melting and recrystallization behaviour are responsible for the PTC effect. Due to a conductive filamentary network across the medium, a localized thermal effect comes into existence, leading to self-heating of the body. This gives the composites potential application, for example, in plastic welding. Preliminary experimental results are reported. ©1997 SCI

Carbon black/high density polyethylene conducting composite materials: Part II The relationship between the positive temperature coefficient and the volume resistivity

Abstract: Carbon black/polymer composites can be used in a variety of industrial applications, where a high conductivity is an important physical parameter. The relationship between the positive temperature coefficient (PTC) effect and the room temperature volume resistivity has been investigated. These are two of the more important operating parameters. Resistivity-temperature measurements were performed on a radiation crosslinked high structure (H) carbon black mixed at different concentrations in HDPE. The results obtained for composites with a range of carbon black properties showed that there was a linear relationship between log (R2/R1) and log R1, where R2=maximum resistivity and R1=room temperature volume resistivity. The relationship can be taken as a reference line for the assessment of the relative effect that modifying the carbon black structure and changing the mixing variables on the relative magnitude of the PTC effect and the room temperature volume resistivity. The high structure (H) carbon black structural characteristics were modified by both gas phase (carbon dioxide at 900°C) and liquid phase (nitric acid) treatments. The former treatment procedure resulted in a decrease, while the latter an increase in the volume resistivities of the final composites. These results are discussed in terms of the effect of surface functionality and carbon structural characteristics.

Microwave Dielectric Properties of Certain Simple Alkaline Earth Perovskite Compounds as a Function of Tolerance Factor

Abstract: Microwave dielectric properties of some simple alkaline earth perovskite compounds were studied with respect to the variation in the tolerance factor. It was found in the present study that the variation in dielectric constant (e), Qf o product and the temperature coefficient of polarizability with tolerance factor is quite opposite to what is observed in some complex perovskite materials. It was also observed that the temperature coefficient of dielectric constant follows the same trend as in some complex perovskite materials with respect to the tolerance factor.

High-temperature thermistors based on yttria and calcium zirconate

Abstract: Two oxide systems, yttria and calcium zirconate, have been systematically studied for their potential use as thermistor materials above 1000°C Both oxides have a sensitivity (temperature coefficient of resistance) of about 15% °C−1 or higher Doping yttria with zirconia results in a slight decrease in sensitivity, but enables the system to be used in a humid environment without protective coating The electrical response of calcium zirconate is found to be sensitive to methane, but is practically unaffected by humidity and carbon monoxide The use of a calcium zirconate-based thermistor is, therefore, limited to atmospheres without methane and/or possibly other hydrocarbon gases The results of feasibility tests of ac-powered thermistors based on yttria and calcium zirconate suggest that an excitation circuitry can be used for designing a thermistor display device

Positive temperature coefficient effect in carbon black/epoxy polymer composites

Abstract: It is shown that carbon black/epoxy polymer composites can present a positive temperature coefficient effect that may reach three orders of magnitude. This effect is strongly influenced by the thermo-mechanical properties of the material and the structure of the conducting network but the effective contribution of each aspect is difficult to quantify

Fast current limitation by conducting polymer composites

Abstract: The transition of materials from low resistivity to comparatively high resistivity can be utilized for current limitation, enabling permanent fuses that do not have to be replaced after an overload or short-circuit operation. An interesting class of materials for this purpose are particulate filled polymer composites with a strong positive temperature coefficient (PTC) of resistivity. If an applied current becomes too high, the PTC element is heated to its critical temperature and trips from the conducting into the insulating state. The dynamic heating of the composite upon current flow is described by a one-dimensional model. It is predicted that the heating of a composite depends on the size of the filler particles. Smaller filler particles should allow a faster heating and, hence, a better limitation of the current. Experimental verification is performed using composite of TiB2 particles in a polyethylene matrix. Commercial TiB2 powders with different particle-size distributions between 1 and 200 μm we...

Ordering and microwave dielectric properties of Ba(Ni1/3Nb2/3)O3 ceramics

Abstract: Ordering and microwave dielectric properties of Ba(Ni1/3Nb2/3)O3 have been investigated using x-ray diffraction, transmission electron microscopy, energy-dispersive spectroscopy, and a network analyzer. Samples sintered at 1400 °C for 2 h were disordered and showed the presence of Nb-rich liquid phase at grain boundary junctions. Degree of ordering increased with following annealing at 1300 °C. Growth of the ordered region during the annealing process was discussed in terms of nucleation and growth. A long-range order parameter was calculated using structure factor. Measurements of microwave dielectric properties showed that permittivity and temperature coefficient of resonant frequency decreased with ordering, and quality factor increased with ordering. The correlation between microwave dielectric properties and ordering was discussed in terms of covalency of bonding, inhomogeneous charge distribution, and defects concentration.

Dielectric properties of barium titanium niobates

Abstract: The results of measurements of dielectric constants, in the vicinity of ambient temperature, are presented for eight barium titanium niobium oxides (BaTi1+2nNb4O13+4n for n = 0, 1, 2, 3, 4; Ba3Ti4Nb4O21, Ba3Ti5Nb6O28, and Ba6Ti2Nb8O30) in polycrystalline ceramic form. The dielectric constants are in the range of 30 to 70. The results of dielectric measurements on solid solutions obtained by partial substitution of Ta for Nb are also reported. These substitutions do not dramatically increase the dielectric constants. One material, Ta-substituted Ba3Ti5Nb6O28, has a very low temperature coefficient of dielectric constant at K ≈ 45.

Dielectric frequency - temperature- compensated microwave whispering-gallery-mode resonators

Abstract: In this paper we show that it is possible to annul the frequency - temperature coefficient of a sapphire dielectric resonator using another dielectric with the opposite frequency - temperature dependence. We have successfully annulled the frequency - temperature coefficient of a composite sapphire - strontium titanate ( - ) microwave resonator at 108 K with a resulting Q factor of 20 000 - 50 000 below 150 K.

Microstructure and Temperature Coefficient of Resistance of Platinum Films

Abstract: Platinum thin films are prepared on alumina ceramic substrates by rf magnetron sputtering. Microstructure and morphology are found to be sensitive to the sputtering gases and substrate surface conditions. N2-sputtered films show better microstructure quality than Ar-sputtered ones for use in forming industrial platinum resistance thermometers (IPRTs). The preferred (111) orientation in both as-deposited and recrystallized Pt films are identified using X-ray diffraction analysis. Thin-film Pt resistance thermometers are fabricated by laser beam trimming. The values of temperature coefficient of resistance (TCR) for N2-sputtered films show values close to the desired IPRT specification of 3850 ppm/°C and increase proportionally to the film's thickness. The dependence of microstructure and TCR on various heat treatment conditions is also investigated.

Piezoresistive properties of RuO2-based thick-film resistors: the effect of RuO2 grain size

Abstract: The piezoresistive properties of thick-film resistors are well documented and widely used in sensors of strain-related quantities. However, the origin of the effect is not well established, and correlations between resistor composition and relevant properties (such as strain sensitivity, temperature coefficient of resistance (TCR) and excess noise) have been not worked out yet. This paper reports a systematic study of these correlations in RuO2-based model resistors prepared with the same glass frit and RuO2 powders covering a range of particle sizes from a few nanometres to micrometres. Gauge factors (GFs) from 2 to 30 have been observed in resistors of sheet resistance R□ in the range 1 k Ω /□ to 3 M Ω /□. At a selected R□ value, the GF increases linearly with the logarithm of RuO2 grain size, while the TCR and excess noise do not significantly depend on the RuO2 powder size.

Low-temperature resistance and its temperature dependence in nanostructured silver

Abstract: The dc resistance and the temperature coefficient of resistance (TCR) of bulk nanostructured silver (n-Ag), synthesized by inert gas condensation and in situ vacuum compaction as well as by the sol-gel method, was investigated in the temperature range from 4.2 to 300 K. The results indicated that for all of the n-Ag specimens with larger grain sizes (d>20 nm) and higher densities (relative density D>88%) investigated, their resistivity decreased with decreasing temperature, showing metallic behavior; however, it was found that for the n-Ag with smaller grain sizes and lower density (D=45–50 %), the resistance increased with decreasing temperature (negative TCR) as its mean size d<9 nm, exhibiting nonmetallic behavior. Furthermore, it was found that generally at a certain (fixed) temperature (at 280 K, for instance), there were approximately linear relations (with negative slope) between its TCR and reciprocals of both grain size and density. In addition, the absolute magnitudes of the resistivity of n-Ag were higher than that of polycrystalline silver (poly-Ag), and increased with decreasing both grain size and density. With the model of grain boundary reflection, it was evaluated that the electron mean free path at room temperature was 44 and 33 nm for the n-Ag with grain size 38.5 and 25 nm, respectively, both of which are smaller than that of poly-Ag (51 nm). It was also evaluated that the electron transmission coefficient through boundaries decreased monotonically from 0.83 to 0.42 as n-Ag density decreased from 98.5 to 88%, suggesting greater boundary barriers in the n-Ag’s with lower densities. The fact that transition of TCR sign from positive to negative can be attributed mainly to the dominant scattering caused by interfaces as compared to that caused by intragranular phonons in n-Ag with extremely fine grain sizes and low densities.

The role of dopants in tailoring the microwave properties of Ba6-xR8+2/3x Ti18O54 R = (La–Gd) Ceramics

Abstract: Microwave dielectric ceramics with a high dielectric constant need to satisfy very high technical demands. They should possess extremely low losses to achieve high Q-values (Quality factor) a small temperature coefficient of resonant frequency (τf), and a relative permittivity (er) higher than 80. Industrial applications require very stringent electrical and dimensional tolerances, typically ± 0.5–1.0 ppm K-1 for a specified τf and ± 0.25% for a specified er. To meet such requirements ceramics based on BaO–R2O3 – TiO2 (R = La–Gd) are used. The investigation of this type of ceramic was stimulated by the observation that ceramics based on compositions in the TiO2-rich region of the system exhibit highly temperature stable electrical properties. Especially interesting are compositions within the solid solubility region with the general formula Ba6-xR8+2/3x Ti18O54. As the ionic radius of the rare earth decreases the extent of the solid solubility region becomes narrower, i.e., 0

Electrical properties of nickel-zirconia cermet films for temperature- and flow-sensor applications

Abstract: Low-resistance NiZrO2 cermet thin films with 30–78 vol.% Ni have been produced from metallo-organic deposition (MOD) precursor solutions. The films are spin coated onto Si or fused silica substrates and heat treated at 800–1100°C for 1 h in forming gas (10% H2/N2) to reduce Ni, resulting in Ni and ZrO2 dispersions. Based upon the linearity of the temperature coefficient of resistance (TCR) and the magnetic susceptibility data, the optimum condition for sintering of 1 μm thick NiZrO2 cermet films is determined to be 800–900°C in forming gas. The magnetic response of these cermets, and their increased resistance compared to NiFe-based alloys, make them suitable for use in magnetoresistive sensors. The objective in this work is to develop controlled NiZrO2 thin-film structures for potential temperature- and flow-sensor applications. Predominantly metallic conduction is obtained on both Si and fused SiO2 substrates in 1 μm thick cermet films with Ni content as low as 30 vol.%. TCR values of 3000–6800 ppm °C−1 are obtained for films with room-temperature resistances in the range 4–250 Ω/□. These films readily self heat, are strongly sensitive to air flow, with low power requirements of < 1 W and exhibit little hysteresis in flow response.

A high temperature pressure sensor with /spl beta/-SiC piezoresistors on SOI substrates

Abstract: This paper reports about the first piezoresistive pressure sensor for high operating temperatures using single crystalline, n-type /spl beta/-SiC piezoresistors on Silicon On Insulator (SOI) substrates. The new Silicon Carbide On Insulator (SiCOI) layer structure prevents a leakage current flow through the substrate at high temperatures up to 723 K. The sensor was tested in the temperature range between room temperature and 573 K. The sensitivity of the device at room temperature is approximately 20.2 /spl mu/V/VkPa. This corresponds to a longitudinal gauge factor of -32 in the [100]-direction. The Temperature Coefficient of Sensitivity (TCS) is -0.16 %K/sup -1/ at 573 K.