Showing papers on "Atmospheric temperature range published in 1987"

A structure-activity relationship for the estimation of rate constants for the gas-phase reactions of OH radicals with organic compounds

Abstract: A previous technique for the calculation of rate constants for the gas-phase reactions of the OH radical with organic compounds has been updated and extended to include sulfur- and nitrogen-containing compounds. The overall OH radical reaction rate constants are separated into individual processes involving (a) H-atom abstraction from CH and OH bonds in saturated organics, (b) OH radical addition to >CC NH, >N, SH, and S groups. During its development, this estimation technique has been tested against the available database, and only for 18 out of a total of ca. 300 organic compounds do the calculated and experimental room temperature rate constants disagree by more than a factor of 2. This suggests that this technique has utility in estimating OH radical reaction rate constants at room temperature and atmospheric pressure of air, and hence atmospheric lifetimes due to OH radical reaction, for organic compounds for which experimental data are not available. In addition, OH radical reaction rate constants can be estimated over the temperature range ca. 250–1000 K for those organic compounds which react via H-atom abstraction from CH and OH bonds, and over the temperature range ca. 250–500 K for compounds containing >CC< bond systems.

Tin dioxide gas sensors. Part 1.—Aspects of the surface chemistry revealed by electrical conductance variations

Abstract: Detailed measurements are reported of the dependence of the conductance of porous, sintered pellets of tin dioxide, on temperature, moisture and oxygen partial pressure. Comparison is made with the behaviour of thin layers prepared by radio-frequency sputtering. The effect of introducing a non-equilibrium mixture of a combustible gas in air, on the conductance and conductance activation energy is described. The theoretical basis for current interpretations of the behaviour of this material is reviewed. The results are discussed using a model in which the conductance of the porous pellets is controlled by different sorts of intergrain contact, represented as necks, necks depleted of conduction electrons, and Schottky barriers. The results are best rationalised by postulating that the conductance is in fact controlled by Schottky barriers separating domains or agglomerates, each comprising a rather large number of crystallites. The effects of temperature, moisture, oxygen partial pressure and combustible gases are discussed in terms of their effect on the Schottky barrier height by way of altrations in the area density, charge and occupancy of surface states (chemisorbed oxygen species). The existence of a surface state level located ≳ 1.1 eV below the conduction band edge is deduced. Adsorbed water strongly affects both the conductance and the response to combustible gases. Loss of water from the surface over the temperature range 280–450 °C results in a sigmoidal conductance-temperature relationship in moist air. The effect is to lower the resistance at temperatures below this range to no more than one-tenth of the values observed for dried pellets. A surface transformation O2–↔ OH– is inferred. In dried air, with dried pellets, inflexions in the conductance-temperature behaviour at temperatures below 230 °C are tentatively attributed to the effect of O2–↔ O–↔ O–2 surface transformations. The onset of a conductance response to the presence of a combustible gas coincides with the onset of a surface-catalysed combustion. At higher temperatures an effect of combustible gases is to lower the conductance activation energy for the porous pellets, but not for the sputtered layers. The effect of moisture on the response to CO was to extend the response to lower temperature; on dried pellets in dried air the response disappeared abruptly when the temperature fell below 350 °C.

Determination of Oxygen Nonstoichiometry in a High-Tc Superconductor Ba2YCu3O7-δ

Abstract: In order to elucidate the correlations between composition, structure and electrical properties of a high-Tc superconductor Ba-Y-Cu-O system, thermogravimetric measurement and chemical analysis of oxygen nonstoichiometry were made at a temperature range of 350 to 1000°C and under the oxygen partial pressure of 10-4 to 1 atm. Within a stability range of the composition Ba2YCu3O7-δ, the oxygen deficiency, δ, was found to vary approximately from zero to 0.9, with associated mean valence of copper ions varying from 2.33 to 1.73. At a possible phase boundary region of tetragonal to orthorhombic crystal structures, no sign of discontinuity in δ was observed, suggesting its phase transition was of higher than a first order.

Neutron diffraction study on the structural phase transition in GeTe

Abstract: The structural phase transition in GeTe has been investigated by neutron diffraction in the temperature range 295-716 K. Accurate positional and thermal parameters have been obtained as a function of temperature both in the rhombohedral hR2 phase(binary analogue of grey arsenic structure A7) and in the cubic CF8 phase (NaCl-type, B1). The temperature variation of the rhombohedral distortion and the relative shifts of the Ge and Te substructures show that the structural phase transition in GeTe is a displacive type.

Hydrogen diffusion in amorphous silicon

Abstract: Hydrogen diffusion in doped and compensated a-Si:H has been measured by secondary-ion mass spectrometry profiling in the temperature range 155-300°C. Doping reduces the activation energy and enhances the diffusion coefficient by up to three orders of magnitude at 200°C, and a correlation between the diffusion coefficient and the dangling-bond density is found. An analysis of three different diffusion models indicates that the breaking of weak Si – Si bonds by hydrogen may be an important process. The relation between the diffusion results and the thermal equilibration of the electronic structure is discussed. The hydrogen diffusion coefficient in a-Si:H has been measured over the temperature range 155-300°C, with particular emphasis on the effects of doping and compensation. In all cases D H is thermally activated with an energy 1[sdot]2-1[sdot]5 eV. The diffusion coefficient decreases slowly with time which is attributed to the disorder-induced variation in site energies. We find that D H is gre...

Nitric oxide adsorption, decomposition, and desorption on Rh(100)

Abstract: Nitric oxide adsorption, decomposition, and desorption were studied on Rh(100) in the temperature range from 88 to 1100 K using electron energy loss spectroscopy (EELS) and temperature programmed desorption (TPD) The EEL spectrometer was equipped with a multichannel detector for fast data acquisition There are two adsorption states of NO on Rh(100), designated α1NO and α2NO, characterized by vibrational modes at 114 and 196 meV, respectively, and assigned to a lying down or highly inclined species and a vertically adsorbed species The populations of the two states as functions of the total NO coverage were measured on the clean surface and with coadsorbed oxygen and CO These coadsorbed species, whether adsorbed before or after the NO, increase the α2 population at the expense of α1 A model that includes an adsorbate–adsorbate interaction (range≈7 A) which converts α1NO to α2NO and which permits adsorbing NO to diffuse so as to favor α1 adsorption fits the measured populations of the two species on th

Nature of CO Adsorption during H 2 Oxidation in Relation to Modeling for CO Poisoning of a Fuel Cell Anode

Abstract: To understand the polarization loss due to poisoning by of a porous Pt anode under various conditions, poisoning losses have been measured in a half‐cell in the temperature range 110°–190°C in 100 weight percent for various mixtures of , , and gases. At a fixed current density, the poisoning loss, , was observed to vary linearly with ln . Deviation from linearity was observed at lower temperatures and higher current densities for high ratios. Considering only the linear portions, it has been possible to derive a general relationship for with temperature, concentration, and current density. The surface coverage by was calculated at various temperatures and was found to bear a linear relationship with ln . From the experimental Temkin isotherms, a general adsorption relationship has been obtained. The standard free energies for adsorption were calculated and were found to vary from −14.5 to −12.1 kcal/mol in the temperature range 130°–190°C. The standard entropy for adsorption was calculated to be −39 cal/mol K. Interpretations of the data indicate that adsorption occurs through 1:1 replacement of H by through the process of selective site poisoning. Under conditions where a nonlinearity in the poisoning relationship occurs, molecules undergo some orientation, favored by the increasing positive charges on the Pt surface.

Synthesis and Thermal Expansion of MZr4P6O24 (M=Mg, Ca, Sr, Ba)

Abstract: MZr4P6O24 (M=Mg, Ca, Sr, Ba), which belongs to a new, low-thermal-expansion family of materials known as [NZP] or [CTP], were synthesized by solid-state reaction (oxide mixing) and sol-gel methods, and their sinterability in two cases was investigated and compared. Thermal-expansion measurements were made on sintered samples by dilatometry, high-temperature X-ray diffractometry, and the laser speckle technique. An anisotropy in axial thermal-expansion coefficients was observed in these materials. CaZr4P6O24 and SrZ4P6O24 showed reverse anisotropy as well as opposite bulk thermal expansion; i.e., CaZr4P6O24 had negative bulk thermal expansion and SrZr4P6O24 positive bulk thermal expansion in the temperature range 25° to 500°C. The microstructure of the sintered samples was also examined by scanning electron microscopy.

Shock-tube study of pressure broadening of the A2∑+ - X2Π (0,0) band of OH by Ar and N2

Abstract: A rapid-tuning, frequency-doubled ring dye laser was used to make fully-resolved lineshape measurements of OH generated behind reflected shock waves in H 2 -O 2 -Ar and H 2 -O 2 -N 2 mixtures. Values of the collisional linewidth were deduced by fitting the observed lineshapes to Voigt profiles. Lines corresponding to transitions with lower-state rotational quantum numbers ranging from 1.5 to 17.5 in the A 2 ∑ + - X 2 Π (0,0) band near 307 nm were probed in order to evaluate a possible J ″-dependence in the observed broadening. The temperature range investigated was 1400–4100 K. The collisional broadening parameter, 2γ, was measured at fixed temperature and did not exhibit a strong dependence on the rotational quantum number for either Ar or N 2 . The temperature dependence of 2γ for specific J ″-values was measured and fit to a power law with an average value of the temperature exponent of 0.8 for both species.

Dielectric relaxation measurements of poly(vinyl acetate) in glassy state in the frequency range 10−6–106 Hz

Abstract: Dielectric relaxation measurements covering a wide frequency range extending from 10−6 to 106 Hz were made on poly(vinyl acetate) with conventional glass transition temperature Tg of 31 °C at temperatures between 26.85 and 84.77 °C. It was found that temperature dependence of frequency of the maximum dielectric loss below Tg cannot be described by the Williams–Landel–Ferry equation which gives a complete explanation to the dependence in the temperature range sufficiently higher than Tg, but by a simple expression of Arrhenius type with activation energy 138 kcal/mol. It was also found that distribution of the relaxation times changes abruptly in the vicinity of Tg. Potential barrier height for the chain motion has a distribution to some extent below Tg.

Neutrino Emission by the Pair, Plasma, and Photo Processes in the Weinberg-Salam Model

Abstract: The results of numerical integrations of the rates and emissivities of the photo, pair, and plasma neutrino emission mechanisms in the Weinberg-Salam theory of the weak interaction are presented. The range of densities 10 gm cm/sup -3/ less than or equal to rho < 10/sup 14/ gm cm/sup -3/ and the temperature range 10/sup 8/K less than or equal to T less than or equal to 10/sup 11/K are considered. Fitting formulae, similar to those provided by Beaudet, Petrosian, and Salpeter, which reproduce the numerical result for the total emissivity to within 20% in the temperature range 10/sup 8.2/K less than or equal to T less than or equal to 10/sup 11/K are presented. 24 refs., 21 figs., 1 tab.

Structure and crystallization of n-C21H44, n-C36H74, and low-molecular-weight polyethylene glasses

Abstract: Glassy films of n-CzlH,, n-C36H74a, nd low molecular weight polyethylene were prepared by vacuum sublimation onto a CsI window held at 7 K and were studied by infrared spectroscopy. The conformational disorder achieved for the glass was comparable to that of the liquid near the sublimation temperature. The chain-organizing processes were monitored for Czl and polyethylene as the sample was warmed to 300 K. A number of separate steps are involved. Each step occurs over a more or less broad temperature region, and sometimes the steps overlap. In the case of CZ1, the first step involves a conformational ordering of the chains to their extended all-trans form. The extended chains initially pack in a monoclinic subcell, but they are not in longitudinal register; that is, the end methyl groups do not lie in parallel planes. At a higher temperature, the monoclinic subcell is converted to an orthorhombic subcell, but the chains are still not in register. In the last annealing step, which occurs over a narrow temperature range, the chains come into register so that the Czl finally assumes its stable orthorhombic crystal structure. The annealing of the polyethylene glass proceeds in exactly the same way except that there is no chain-registering step. In general, the transition temperatures are higher and the temperature range over which the transitions occur is broader for the polyethylene sample.

Crystallization of the glassy phase in an Si3N4 material by post-sintering heat treatments

Abstract: It is shown that post-sintering heat treatments in air in the temperature range 1100 to 1400° C result in substantial crystallization of the glassy phase in an Si3N4 material which was produced by the nitridation pressureless sintering (NPS) method using Y2O3 and Al2O3 as sintering aids. X-ray diffraction combined with analytical electron microscopy showed that the secondary crystalline phases which form are strongly dependent upon time and temperature of heat treatment as well @S depth below the oxide scale. This effect is primarily due to the outward diffusion of cations (yttrium, aluminium and impurities) as well as the inward diffusion of oxygen. Small glassy pockets and thin amorphous intergranular films remain in the microstructure after heat treatment.

Raman-Scattering Study of the Soft Phonon Modes in Hexagonal Barium Titanate

Abstract: The Raman spectrum of hexagonal BaTiO 3 has been investigated in the temperature range from 2 K to room temperature. Below the structural-phase-transition temperature of T a (∼222 K), we find the corresponding underdamped soft-phonon modes; the mode frequencies are proportional to ( T a - T ) β with β∼1/3 in a wide temperature range (0.5< T / T a <0.95). Although, in the ferroelectric phase below T c (∼74 K), several new features are observed, no soft mode which correlates with the ferroelectric transition is found.

Size and temperature effects on the Seebeck coefficient of thin bismuth films

Abstract: The electrical resistivity and the Seebeck coefficient of thermally evaporated thin bismuth films of thicknesses from 300 to 1900 A\r{} have been measured in the temperature range 300--470 K. The latter is negative and its magnitude is found to increase initially with increasing temperature, reach a maximum, and then decrease with a further rise in temperature. The temperature at which the Seebeck coefficient is maximum is found to be thickness dependent, decreasing with increasing thickness. The observed dependence is explained by considering that the Fermi energy is temperature dependent. Bismuth films show a negative temperature coefficient of resistivity. The thickness dependence of the electrical resistivity and the Seebeck coefficient of simultaneously prepared films are analyzed using the newer effective mean-free-path model. From the analysis, important material constants like the mean free path, the electron concentration, and the effective mass of electrons have been evaluated.

Electrical transport properties of transition‐metal disilicide films

Abstract: Electrical resistivity in the temperature range of 2–1100 K and Hall‐effect measurements from 10 to 300 K of CoSi2, MoSi2, TaSi2, TiSi2, and WSi2 polycrystalline thin films were studied. Structure, composition, and impurities in these films were investigated by a combination of techniques of Rutherford backscattering spectroscopy, x‐ray diffraction, transmission electron microscopy, and Auger electron spectroscopy. These silicides are metallic, yet there is a remarkable difference in their residual resistivity values and in their temperature dependence of the intrinsic resistivities. For CoSi2, MoSi2, and TiSi2, the phonon contribution to the resistivity was found to be linear in temperature above 300 K. At high temperatures, while a negative deviation from the linearity followed by a quasisaturation was observed for TaSi2, the resistivity data of WSi2 showed a positive deviation from linearity. It is unique that the residual resistivity, ρ(2 K), of the WSi2 films is quite high, yet the temperature depend...

A neutron scattering study of supercooled liquid tellurium

Abstract: Local order in liquid tellurium has been investigated in both liquid and supercooled state. The nearest-neighbours number has been determined, and its variation vs. temperature is discussed in terms of theoretical predictions. The semiconductor-to-metal transition is shown to occur in a narrow temperature range, mainly in the supercooled-state region.

The thermal conductivity of argon, carbon dioxide and nitrous oxide

Abstract: The paper presents new, absolute measurements of the thermal conductivity of three gases: argon (Ar), carbon dioxide (CO 2 ) and nitrous oxide (N 2 O). The measurements have been carried out with a transient hot-wire instrument in the temperature range 308 to 430 K and at pressures up to 11 MPa. For most of the range of thermodynamic states covered by the measurements it is estimated that the accuracy of the thermal conductivity data is one of ±0.3%. However, for carbon dioxide and nitrous oxide near their critical conditions the accuracy is degraded and the uncertainty may be as much as ±2%. The new experimental data for argon confirm the accuracy claimed for the thermal conductivity in the limit of zero-density. The thermal conductivity of the polyatomic gases in the limit of zero-density is used in conjunction with information on other transport cross-sections for the same systems, to extract a consistent set of cross-sections sensitive to the anisotropy of the intermolecular pair potential for use in the testing of proposed potential surfaces. Cross-sections for both of the available formulations of the thermal conductivity of a polyatomic gas due to Wang Chang and Uhlenbeck and Thijsse et al. are derived. For both gases it is shown that the Mason-Monchick approximation breaks down in either formulation. However, the effect of the failure on the formulation of Thijsse et al. is smaller and it is possible to represent the data with the aid of a single cross-section which has a very simple temperature dependence. The analysis also demonstrates that all available high-temperature thermal conductivity data for carbon dioxide are in substantial error. In the moderately dense gas the concept of a temperature-independent excess thermal conductivity is confirmed to a high degree of precision for carbon dioxide and argon when due allowance is made for the critical enhancement. A generalized correlation of the temperature dependence of the first density coefficient of thermal conductivity is broadly confirmed.

Kinetics of Corn Meal Gelatinization at High Temperature and Low Moisture

Abstract: The kinetics of corn meal starch gelatinization and melting was investigated. At 25% moisture, in the temperature range 110–140°C. Arrhenius equation parameters were determined by two calculable procedures which agreed well. Activation energy for gelatinization was calculated to be approximately 86.2 kJ/g-mole. Further experiments in the moisture range 13.4–34.4% and temperature range 10–165°C. yielded results for endotherm enthalpies and melt temperature which agreed with literature reports.

Iron-Excess Manganese Ferrite: Electrical Conductivity and Cation Distributions

Abstract: The electronic conductivity and thermoelectric power of a magnetic ferrite of composition Mn0.863Fe2.137O4 have been measured as functions of oxygen partial pressure and temperature. Within the single-phase region of the spinel ferrite, both electrical properties are essentially independent of the oxygen partial pressure, implying that the electronic charge carrier concentration is governed by the molecularity rather than by oxygen nonstoichiometry. The electrical conduction behavior is successfully interpreted on the basis of a small polaron model, yielding a hopping energy of 0.363 ± 0.008 eV and a drift mobility of 0.05 to 0.10 cm2/(V·s) in the temperature range of 900 to 1300 K.