Showing papers on "Atmospheric temperature range published in 1996"

Negative Thermal Expansion from 0.3 to 1050 Kelvin in ZrW2O8

Abstract: Negative thermal expansion was found for ZrW 2 O 8 from 0.3 kelvin to its decomposition temperature of about 1050 kelvin. Both neutron and x-ray diffraction data were used to solve and refine the structure of this compound at various temperatures. Cubic symmetry persists for ZrW 2 O 8 over its entire stability range. Thus, the negative thermal expansion behavior is isotropic. Essentially the same behavior was found for isostructural HfW 2 O 8 . No other materials are known to exhibit such behavior over such a broad temperature range. These materials are finding applications as components in composites in order to reduce the composites9 overall thermal expansion to near zero.

Activation of acceptors in Mg-doped GaN grown by metalorganic chemical vapor deposition

Abstract: The activation kinetics of acceptors was investigated for heteroepitaxial layers of GaN, doped with Mg. After growth, the samples were exposed to isochronal rapid thermal anneals in the temperature range from 500 to 775 °C. The samples were studied by variable temperature Hall effect measurements and photoluminescence (PL) spectroscopy in the as‐grown condition and after each temperature step. The thermal treatment reduced the resistivity by six orders of magnitude and the p‐type conductivity was found to be dominated by an acceptor with an activation energy of ∼170 meV. This acceptor is attributed to Mg atoms substituting for Ga in the GaN lattice and the activation process is consistent with dissociation of electrically inactive Mg–H complexes. It is shown that the appearance of a blue emission band in the PL spectrum of Mg‐doped GaN does not directly correlate with the increase in p‐type conductivity.

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.

Magnetic Susceptibility of Quasi-One-Dimensional Compound α'- NaV2O5 –Possible Spin-Peierls Compound with High Critical Temperature of 34 K–

Abstract: Stoichiometric powder samples of α'-NaV 2 O 5 were synthesized and the magnetic susceptibility was measured in the temperature range from 2 K to 700 K. The magnetic susceptibility has a good fit to the equation for an S =1/2 antiferromagnetic Heisenberg linear chain with J / k B =280 K and g =2 above 34 K. Below 34 K the magnetic susceptibility rapidly decreases with decreasing temperature to a constant value of 1.49×10 -4 emu/V 4+ -mol which is reasonable for spin-singlet V 4+ –V 4+ pairs. This rapid reduction of the spin susceptibility below 34 K suggests the existence of a spin-Peierls transition. α'-NaV 2 O 5 is a possible spin-Peierls compound with the highest critical temperature yet observed.

In situ X-ray observations of the coesite-stishovite transition: reversed phase boundary and kinetics

Abstract: Using a DIA-type, cubic-anvil, high-pressure apparatus (SAM-85) in conjunction with in situ X-ray diffraction, we have investigated phase relations between coesite and stishovite up to 12 GPa and 1530 °C using synthetic powders of the two phases as the starting materials. The phase transition between coesite and stishovite was identified by observing the first appearance of a phase that did not already exist or by a change in the relative intensity of the two patterns. In most experiments, the diffraction patterns on samples were collected within 10 minutes after reaching a pressure and temperature condition. On this time scale, two phase boundaries associated with the coesite-stishovite transition have been determined: (1) for the stishovite-to-coesite transition, observations were made in the temperature range of 950–1530 °C, and (2) for the coesite-to-stishovite transition from 500 to 1300 °C. These observations reveal that there exists a critical temperature of about 1000 °C to constrain the coesite-stishovite equilibrium phase boundary. Above this temperature, both boundaries are linear, have positive dP/dT slopes, and lie within a pressure interval of 0.4 GPa. Below this temperature, the dP/dT slope for the stishovite-to-coesite phase boundary becomes significantly larger and that for the coesite-tostishovite phase boundary changes from positive to negative. As a result, an equilibrium phase boundary can only be determined from the results above 1000 °C and is described by a linear equation P (GPa)=6.1 (4)+ 0.0026 (2) T (°C). This dP/dT slope is in good agreement with that of Zhang et al. (1993) but more than twice that of Yagi and Akimoto (1976). For the kinetics of the phase transition, preliminary rate data were obtained for the stishovite-to-coesite transition at 1160 and 1430 °C and are in agreement with the simple geometric transformation model of Avrami and Cahn.

Bulk Nd-Fe-Al Amorphous Alloys with Hard Magnetic Properties

Abstract: An amorphous phase in Nd-Fe-Al system was formed in an extremely wide composition range of 0 to 90 at% Fe and 0 to 93 at% Al by melt spinning. Based on the information on the amorphous formation, ferromagnetic Nd 90-x Fe x Al 10 bulk amorphous alloys with high coercive force at room temperature were obtained by a copper mold casting method. The maximum diameter of the cylindrical amorphous samples with a length of 50 mm is about 7 mm for the 20%Fe alloy and about 4 mm for the 30%Fe alloy. Neither glass transition nor supercooled liquid region is observed in the temperature range before crystallization, being different from previous bulk glassy alloys exhibiting a wide supercooled liquid region before crystallization. The onset temperature of crystallization (T x ) and melting temperature (T m ) are measured to be 778 and 863 K, respectively, for the Nd 70 Fe 20 Al 10 alloy. The resulting reduced ratio of T x /T m is as high as 0.90 and the temperature interval between T x and T m is as small as 85 K. The extremely high T x /T m and small ΔT m (=T m -T x ) values are the reason for the achievement of the large glass-forming ability. The bulk amorphous Nd 70 Fe 20 Al 10 alloy has a ferromagnetism with the Curie temperature (T c ) of about 600 K which is much higher than the highest T c (about 480 K) for the Nd-Fe binary amorphous alloy ribbons. The remanence (B,) and intrinsic coercive force ( i H c ) for the bulk Nd 60 Fe 30 Al 10 alloy are 0.122 T and 277 kA/m, respectively, in the as-cast state and 0.128 T and 277 kA/m, respectively, in the annealed state for 600 s at 600 K. The B r and 1 H c decrease to 0.045 T and 265 kA/m, respectively, for the crystallized Nd 60 Fe 30 Al 10 sample consisting of Nd+Al 2 Nd+δ phases and the maximum hard magnetic properties are achieved in the amorphous state. The hard magnetic properties for the bulk amorphous alloys are presumably due to the homogeneous development of ferromagnetic clusters with large random magnetic anisotropy. The finding of the bulk amorphous alloys exhibiting hard magnetic properties at room temperature is promising for the future development as a new type of metallic amorphous permanent magnet.

Solubility of single gases carbon dioxide and hydrogen sulfide in aqueous solutions of N-methyldiethanolamine in the temperature range 313--413 K at pressures up to 5 MPa

Abstract: Experimental results for the solubility of the single gases carbon dioxide and hydrogen sulfide in aqueous solutions of 2,2‘-methyliminodiethanol (N-methyldiethanolamine (MDEA)) at temperatures between 313 and 413 K and total pressures up to 5 MPa are reported. A model taking into account chemical reactions as well as physical interactions is used to correlate the new data. The correlation is also used to compare the new experimental data with literature data.

Dynamics of ribonuclease H: temperature dependence of motions on multiple time scales.

Abstract: The temperature dependence of the backbone motions in Escherichia coli ribonuclease HI was studied on multiple time scales by 15N nuclear magnetic spin relaxation. Laboratory frame relaxation data at 285, 300, and 310 K were analyzed using the model-free and reduced spectral density approaches. The temperature dependence of the order parameters was used to define a characteristic temperature for the motions of the backbone N-H bond vectors on picosecond to nanosecond time scales. The characteristic temperatures for secondary structure elements, loops, and the C-terminus are approximately 1000, approximately 300, and approximately 170 K, respectively. The observed variation in the characteristic temperature indicates that the energy landscape, and thus the configurational heat capacity, is markedly structure dependent in the folded protein. The effective correlation times for internal motions do not show significant temperature dependence. Conformational exchange was observed for a large number of residues forming a contiguous region of the protein that includes the coiled coil formed by helices alpha A and alpha D. Exchange broadening in the CPMG experiments decreased with increased temperature, directly demonstrating that the microscopic exchange rate is faster than the pulse repetition rate of 1.2 ms. The temperature dependence of the exchange contributions to the transverse relaxation rate constant shows approximately Arrhenius behavior over the studied temperature range with apparent activation enthalpies of approximately 20-50 kJ/mol. Numerical calculations suggest that these values underestimate the activation barriers by at most a factor of 2. The present results obtained at 300 K are compared to those reported previously [Mandel, A. M., Akke, M., & Palmer, A. G., III (1995) J. Mol. Biol. 246, 144-163] to establish the reproducibility of the experimental techniques.

Room-Temperature Pulsed Operation of GaInNAs Laser Diodes with Excellent High-Temperature Performance

Abstract: We have successfully operated GaInNAs laser diodes with a pulsed current at room temperature. The lowest threshold current density was about 0.8 kA/cm2, and the lasing wavelength was about 1.2 µ m. Characteristic parameters such as internal quantum efficiency and the gain constant were measured, and excellent high-temperature performance was observed. The characteristic temperature was 127 K in the temperature range from 25 to 85° C.

Temperature and pressure dependences of the laser-induced fluorescence of gas-phase acetone and 3-pentanone

Abstract: Laser-Induced Fluorescence (LIF) from the S1 state of acetone and 3-pentanone was studied as a function of temperature and pressure using excitation at 248 nm. Additionally, LIF of 3-pentanone was investigated using 277 and 312 nm excitation. Added gases were synthetic air, O2, and N2 respectively, in the range 0–50 bar. At 383 K and for excitation at 248 nm, all the chosen collision partners gave an initial enhancement in fluorescence intensity with added gas pressure. Thereafter, the signal intensity remained constant for N2 but decreased markedly for O2. For synthetic air, only a small decrease occurred beyond 25 bar. At longer excitation wavelengths (277 and 312 nm), the corresponding initial rise in signal with synthetic air pressure was less than that for 248 nm. The temperature dependence of the fluorescence intensity was determined in the range 383–640 K at a constant pressure of 1 bar synthetic air. For 248 nm excitation, a marked fall in the fluorescence signal was observed, whereas for 277 nm excitation the corresponding decrease was only half as strong. By contrast, exciting 3-pentanone at 312 nm, the signal intensity increased markedly in the same temperature range. These results are consistent with the observation of a red shift of the absorption spectra (≈9 nm) over this temperature range. Essentially, the same temperature dependence was obtained at 10 and 20 bar pressure of synthetic air. It is demonstrated that temperatures can be determined from the relative fluorescence intensities following excitation of 3-pentanone at 248 and 312 nm, respectively. This new approach could be of interest as a non-intrusive thermometry method, e.g., for the compression phase in combustion engines.

Oxidation of WC-Co, WC-Ni and WC-Co-Ni hard metals in the temperature range 500–800 °C

Abstract: Oxidation of WC-16%Co, WC-8%Co-8%Ni and WC-16%Ni hard metals in the temperature range between 500 and 800 °C was investigated by TGA, XRD and SEM methods. The thermodynamics of tungsten carbide oxidation reactions was calculated. Oxidation kinetics of the alloys follows a complex behaviour. The WC-16%Co alloy is oxidized to a lesser degree than the WC-16%Ni alloy in the temperature range 650–800 °C. Oxidation resistance of hard metals is governed by phase composition, formation mechanism of the scales and protective properties of the oxidation products (CoO, NiO, WO3 and complex oxides, based on the formula (Ni, Co)WO4).

Dielectric properties and phase transitions of Ba(Ti1-xSnx)O3 solid solution

Abstract: In Ba(Ti1-x Snx )O3 solid solution, for Sn concentration x≤0.1, the permittivity follows the Curie-Weiss law above the Curie point T c. The increase in the peak value of the permittivity at T c is due to the change in ferroelectric transition from first order to second order with increasing x. For x>0.1, some deviations from the Curie-Weiss law are observed in a narrow temperature region above T c. The diffuse phase transition was observed. For x>0.2, the Curie-Weiss law does not hold over a wide temperature range. The dielectric dispersion was observed in the radio-frequency range (10 kHz–10 MHz). With increasing Sn concentration, the change in the phase transition from a normal ferroelectric transition to relaxor through the diffuse phase transition was observed.

Low-cost conversion of a coaxial nozzle arrangement into a stationary low-temperature attachment

Abstract: A stationary nozzle accessory designed for the Siemens/Nicolet LTII low-temperature device with standard ϕ-coaxial nozzle arrangement is described. The attachment is easy to assemble and its compatibility allows interchanging between both systems in less than 1 h, if required. Implementation of the stationary nozzle reduces the consumption of liquid nitrogen by more than 40% and extends the temperature range available for data collection by 60 K in the lower temperature range. Details of construction are presented, and a comparison with the ϕ-coaxial nozzle is discussed.

Diffusivity of ozone in water

Abstract: The diffusivity of ozone in water was determined experimentally by measuring the rate of ozone absorption into an aqueous laminar liquid jet. Penetration theory was used to interpret the results. Experiments were conducted over the temperature range of 10 °C to 45 °C. The results for diffusivity were correlated by an Arrhenius function for temperature dependence with a maximum deviation of ±3.3%: D/m2·s-1 = 1.10 × 10-6 exp[−1896/(T/K)].

Behavior of Benzene in Na-X and Na-Y Zeolites: Comparative Study by 2H NMR and Molecular Mechanics

Abstract: We have performed solid state 2H NMR spin−lattice relaxation measurements on perdeuterated benzene in the zeolites Na-X and Na-Y over the temperature range 155−350 K. The resulting NMR correlation times exhibit Arrhenius temperature dependence, with the following Arrhenius parameters: = (9.6 ± 0.4) × 10-13 s, = 23.5 ± 0.9 kJ mol-1, = (8.8 ± 2.3) × 10-12 s, and = 14.0 ± 0.6 kJ mol-1. This significant enhancement of benzene mobility from Na-Y to Na-X is reproduced with atomistic molecular mechanics simulations, yielding = 35 kJ mol-1 and = 15 kJ mol-1, in quite reasonable agreement with the NMR measurements. We propose a simple length scale model which accounts for both the mobility enhancement and its order of magnitude. We find that the enhancement from Na-Y to Na-X is due to attractive interactions from SIII sites influencing adjacent SII sites, making mobility more energetically favorable in Na-X. The relative magnitudes of our calculated hopping activation energies suggest that, while intracage and in...

Junction dynamics in telechelic hydrogen bonded polyisobutylene networks

Abstract: 4-Urazoylbenzoic acid groups are attached to the chain ends of polyisobutylene. The cooperative assembling process of these polar groups is studied by DSC and dielectric and dynamic mechanical spectroscopy. The melting of the ordered clusters occurs in the temperature range 380-390 K. Distortions within the U4A clusters (Σ process) are monitored below the melting temperature T m with dielectric spectroscopy. On a larger length scale, these distortions also lead to stress relaxation which can be probed by dynamic mechanical measurements. Near T m , the relaxation ofU4A multiplets (α* relaxation) is detected with dielectric spectroscopy. In this temperature range, dynamic mechanical measurements show a transition from elastic behavior to viscous flow. The results are rationalized with molecular modeling calculations.

Performances of Mass-Sensitive Devices for Gas Sensing: Thickness Shear Mode and Surface Acoustic Wave Transducers

Abstract: In this work we investigated different thickness shear mode resonators (TSMRs) with fundamental frequencies of 10 and 30 MHz and surface acoustic wave devices with fundamental frequencies of 80 and 433 MHz. Four aspects were of primary interest in this comparison: noise levels and signal-to-noise ratios (S/N), influence of the polymer film thickness, influence of temperature on the transducer signal before and after coating, and minimum threshold values for monitoring different volatile organic compounds in the environment. We limited our investigations to a temperature range between 298 and 308 K, with 303 K the routine measuring temperature. Analyte concentrations (n-octane, tetrachloroethene) were chosen from the minimum detection limit up to 5000 μg/L. The temperature was found to strongly affect the performance of all the devices. The sorption of the analyte vapors into the polymeric films was demonstrated to be transducer-independent (identical partition coefficients for all the devices). The 30 MH...

Spontaneous formation of highly concentrated water-in-oil emulsions (gel-emulsions)

Abstract: Water-in-Oil-type gel-emulsions (or highly concentrated emulsions) are spontaneously formed from oil-swollen micellar solutions or oil-in-water (O/W) microemulsions in a water/tetraoxyethylene dodecyl ether/oil system with an abrupt increase in temperature. The phase change occurs from water-continuous microemulsion to water-in-oil (W/O) gel-emulsion via a lamellar liquid crystal and a bicontinuous surfactant phase (L3 phase). Hence, the spontaneous curvature of the surfactant layer is continuously changed with temperature change because the gel-emulsion consists of a reverse micellar solution and an excess water phase. In a narrow temperature range above the single L3-phase region, there is a two-phase region consisting of L3 and an excess water phase (W) in which emulsions are extremely unstable. The electroconductivity curve as a function of temperature monotonically decreases with increasing temperature when the final temperature is high and the temperature change is fast. If the temperature change is...

Giant magnetoresistance of a two‐dimensional ferromagnet La2−2xCa1+2xMn2O7

Abstract: Bulk samples of La2−2xCa1+2xMn2O7 with the layered Sr3Ti2O7‐type perovskite structure have been successfully synthesized and investigated with respect to their magnetic and electrical properties. It is found that La2−2xCa1+2xMn2O7 (x=0.25) is a metallic ferromagnet with a magnetic transition temperature Tc of 215 K. The large magnetoresistance (MR) effect with Δρ/ρ0 of ∼60% at 1.8 T was observed in a wide temperature range below a cusp temperature in resistivity of 96 K, which is well below the magnetic Tc. This behavior is quite different from that of the well‐known double‐exchange ferromagnets such as La1−xCaxMnO3, where large MR effects are restricted to a narrow temperature range around the ferromagnetic transition. The present result could be interpreted by using the double‐exchange theory incorporating the anisotropy resulting from the two‐dimensional Mn‐O‐Mn networks in La2−2xCa1+2xMn2O7.

Infrared collision-induced absorption by O2 near 6.4 microm for atmospheric applications: measurements and empirical modeling.

Abstract: Accurate measurements of collision-induced absorption by O(2) and O(2)-N(2) mixtures in the fundamental band near 64 microm have been made A Fourier-transform spectrometer was used with a resolution of 05 cm(-1) Absorption has been investigated in the 0-20-atm and 193-293 K pressure and temperature ranges, respectively The current measurements confirm that the broad O(2) continuum carries small features whose attribution is not yet clear Available experimental data in the 190-360 K temperature range have been used to build a simple, low cost computer, empirical model that is well adapted for computation of atmospheric O(2) absorption Tests show that it is accurate, contrary to predictions of widely used atmospheric transmission codes