Showing papers on "Atmospheric temperature range published in 2004"

Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 1. Variation of Anionic Species

Abstract: Room-temperature ionic liquids (RTILs) based on 1-butyl-3-methylimidazolium ([bmim]) with a variety of fluorinated anions were prepared, and the thermal behavior, density, viscosity, self-diffusion coefficients of the cations and anions, and ionic conductivity were measured over a wide temperature range. The temperature dependencies of the self-diffusion coefficient, viscosity, ionic conductivity, and molar conductivity have been fitted to the Vogel−Fulcher−Tamman equation, and the best-fit parameters for the self-diffusion coefficient, viscosity, ionic conductivity, and molar conductivity have been estimated, together with the linear fitting parameters for the density. The self-diffusion coefficients determined for the individual ions by pulsed-field-gradient spin−echo NMR method exhibit higher values for the cation compared with the anion over a wide temperature range, even if its radius is larger than that of the anionic radii. The summation of the cationic and anionic diffusion coefficients for the RT...

Effect of Temperature and Illumination on the Electrical Characteristics of Polymer-Fullerene Bulk-Heterojunction Solar Cells

Abstract: The current-voltage characteristics of ITO/PEDOT:PSS/OC1C10-PPV:PCBM/Al solar cells were measured in the temperature range 125-320 K under variable illumination, between 0.03 and 100 mW cm(-2) (white light), with the aim of determining the efficiency-limiting mechanism(s) in these devices, and the temperature and/or illumination range(s) in which these devices demonstrate optimal performance. (ITO: indium tin oxide; PEDOT:PSS: poly(styrene sulfonate)-doped poly(ethylene dioxythiophene); OC1C10-PPV: poly[2-methoxy-5-(3,7-dimethyl octyloxy)-1,4-phenylene vinylene]; PCBM: phenyl-C-61 butyric acid methyl ester.) The short-circuit current density and the fill factor grow monotonically with temperature until 320 K. This is indicative of a thermally activated transport of photogenerated charge carriers, influenced by recombination with shallow traps. A gradual increase of the open-circuit voltage to 0.91 V was observed upon cooling the devices down to 125 K. This fits the picture in which the open-circuit voltage is not limited by the work-function difference of electrode materials used. The overall effect of temperature on solar-cell parameters results in a positive temperature coefficient of the power conversion efficiency, which is 1.9% at T = 320 K and 100 mW cm(-2) (2.5% at 0.7 mW cm(-2)). The almost-linear variation of the short-circuit current density with light intensity confirms that the internal recombination losses are predominantly of monomolecular type under short-circuit conditions. We present evidence that the efficiency of this type of solar cell is limited by a light-dependent shunt resistance. Furthermore, the electronic transport properties of the absorber materials, e.g., low effective charge-carrier mobility with a strong temperature dependence, limit the photogenerated current due to a high series resistance, therefore the active layer thickness must be kept low, which results in low absorption for this particular composite absorber.

Energetics of hydrogen bond network rearrangements in liquid water.

Abstract: A strong temperature dependence of oxygen K-edge x-ray absorption fine structure features was observed for supercooled and normal liquid water droplets prepared from the breakup of a liquid microjet. Analysis of the data over the temperature range 251 to 288 kelvin (–22° to +15°C) yields a value of 1.5 ± 0.5 kilocalories per mole for the average thermal energy required to effect an observable rearrangement between the fully coordinated (“ice-like”) and distorted (“broken-donor”) local hydrogen-bonding configurations responsible for the pre-edge and post-edge features, respectively. This energy equals the latent heat of melting of ice with hexagonal symmetry (ice Ih) and is consistent with the distribution of hydrogen bond strengths obtained for the “overstructured” ST2 model of water.

Structure of organoclays—an X-ray diffraction and thermogravimetric analysis study

Abstract: X-ray diffraction has been used to study the changes in the surface properties of a montmorillonitic clay through the changes in the basal spacings of montmorillonite (SWy-2) and surfactant-intercalated organo-clays. Variation in the d-spacing was found to be a step-function of the surfactant concentration. High resolution thermogravimetric analysis (HRTG) was used to study the thermal decomposition surfactant modified SWy-2-MMTs modified with the surfactant octadecyltrimethylammonium bromide. High resolution thermogravimetry shows the decomposition takes place in 4 steps. A mass loss step is observed at room temperature and is attributed to dehydration of adsorption water. A second mass loss step is observed over the 87.9 to 135.5 degrees Celsius temperature range and is also attributed to dehydration of water hydrating metal cations such as Na+ . The third mass loss occurs from 178.9 to 384.5 degrees Celsius and is assigned to a loss of surfactant. The fourth mass loss step is ascribed to the loss of OH units through dehydroxylation over the 556.01 to 636.35 degrees Celsius temperature range.

Er3+-doped BaTiO3 nanocrystals for thermometry: Influence of nanoenvironment on the sensitivity of a fluorescence based temperature sensor

Abstract: Frequency upconverted emissions centered at 526 and 547 nm from two thermodynamically coupled excited states of Er3+ doped in BaTiO3 nanocrystals were recorded in the temperature range from 322 to 466 K using a diode laser emitting at 980 nm as the excitation source. The ensemble measurements of the fluorescence intensity ratio (FIR) of the signals at 526 and 547 nm as a function of the temperature showed that the sensitivity (the rate in which the FIR changes with the temperature) of such sensor depends on the size of the nanocrystal. This is explained taking into consideration modifications of nonraditive relaxation mechanisms with the size of the nanocrystals.

Size dependence of thermal stability of TiO2 nanoparticles

Abstract: Anatase TiO2 nanoparticles with average particle size ranging between 12 and 23 nm were synthesized by metalorganic chemical vapor deposition. The structure and particle size were determined by x-ray diffraction and transmission electron microscopy. The specific surface areas were measured by Brunauer-Emmett-Teller and ranged from 65 to 125m2∕g. The size effects on the stability of TiO2 in the air were studied by x-ray diffraction and transmission electron diffraction for isochronally annealed samples in the temperature range of 700–800 °C. Only anatase to rutile phase transformation occurred. With the decrease of initial particle size the onset transition temperature was decreased. An increased lattice compression of anatase with the raising of temperature was observed by the x-ray peak shifts. Larger distortions existed in samples with smaller particle size. The calculated activation energy for phase transformation decreased from 299 to 180 kJ∕mol with the decrease of initial anatase particle size from ...

The role of Auger recombination in the temperature-dependent output characteristics (T0=∞) of p-doped 1.3 μm quantum dot lasers

Abstract: Temperature invariant output slope efficiency and threshold current (T0=∞) in the temperature range of 5–75 °C have been measured for 1.3 μm p-doped self-organized quantum dot lasers. Similar undoped quantum dot lasers exhibit T0=69K in the same temperature range. A self-consistent model has been employed to calculate the various radiative and nonradiative current components in p-doped and undoped lasers and to analyze the measured data. It is observed that Auger recombination in the dots plays an important role in determining the threshold current of the p-doped lasers.

Electrical conductivity and gas sensing properties of MoO31

Abstract: Electrical conductivity and gas sensing properties of MoO3 are investigated. The electrical conductivity is found to be independent of oxygen partial pressure in the temperature range 510–773 K. Two distinct conduction processes were identified from the conductivity experiments carried out under ambient air, moist oxygen, and moist argon. The conductivity in the low temperature range (510–578 K) are attributed to species arising from the reversibly inserted water molecules into MoO3 lattice. The conduction process in the high temperature region (578–773 K) are attributed to the non-stoichiometry existing in the sample due to the presence of Mo5+ ions which was confirmed by EPR and XPS investigations. Sensing characteristics of MoO3 towards NH3, H2, and LPG were studied. Experiments showed that the ammonia sensing mechanism of MoO3 involved the formation of molybdenum suboxides and nitride.

Negative thermal expansion materials

Abstract: The ZrW2O8 family of materials has been shown to exhibit the unusual property of negative thermal expansion over a wide temperature range α-ZrW2O8 itself shows negative thermal expansion from 2 K to 1443 K with a coefficient of thermal expansion α1 of -91×10-6 K-1 between 2 and 300 K This behaviour can be related to the unusual structure of these materials, which possess a family of low energy phonon modes with negative Gruneisen parameters α-ZrW2O8 also shows a phase transition at 448 K to β-ZrW2O8 in which oxygen atoms are dynamically disordered These phenomena have been investigated by a number of techniques Results of powder neutron diffraction at 260 different temperatures are presented

The temperature dependence of effective thermal conductivity of open-celled steel alloy foams

Abstract: The effective thermal conductivity of steel alloy FeCrAlY (Fe—20 wt.% Cr—5 wt.% Al—2 wt.% Y—20 wt.%) foams with a range of pore sizes and porosities was measured between 300 and 800 K, under both vacuum and atmospheric conditions. The results show that the effective thermal conductivity increases rapidly as temperature is increased, particularly in the higher temperature range (500–800 K) where the transport of heat is dominated by thermal radiation. The effective conductivity at temperature 800 K can be three times higher than that at room temperature (300 K). Results obtained under vacuum conditions reveal that the effective conductivity increases with increasing pore size or decreasing porosity. The contribution of natural convection to heat conduction was found to be significant, with the effective thermal conductivity at ambient pressure twice the value of vacuum condition. The results also show that natural convection in metal foams is strongly dependent upon porosity.

Temperature dependence of the piezoelectric response in lead zirconate titanate films

Abstract: The temperature dependence of the effective transverse piezoelectric coefficient (e31,f) in lead zirconate titanate (PZT) thin films was measured between −55 and 85 °C. e31,f was calculated by simultaneously monitoring the piezoelectric charge output and strain in the film during wafer flexure. This method was used to characterize the temperature dependence of e31,f in PZT films with 2, 4, and 6 μm thickness and 40/60, 52/48, and 60/40 Zr/Ti ratios. |e31,f| was found to increase with temperature and average increases were 46%, 32%, and 12% for films with PZT 60/40, 52/48, and 40/60 compositions, respectively. Measurement uncertainty ranged from ±3%–12%. The measured temperature dependences of e31,f were consistent with the rapid rise in intrinsic d31 as Tc is approached, suggesting that they were controlled by intrinsic contributions. Additional contributors to the measured variation in the PZT film piezoelectric response over the measured temperature range were identified. Changes in film elastic propert...

Microstructural instability and strength of an AZ31 Mg alloy after severe plastic deformation

Abstract: Equal channel angular pressing (ECAP) technique, which involves a simple large shear deformation during passage through two intersecting channels, was applied to the AZ31 Mg alloy to make an ultrafine-grained microstructure. ECAP temperature was controlled to decrease with pass number to maximize the grain refinement efficiency with preventing cracking. The first and second pressings were conducted at 593 K, while the third and fourth pressings were conducted at 523 and 473 K, respectively. The degree of grain refinement and homogeneity of grain-size distribution increased with pass number. After four passes, the reasonably homogeneous microstructure composed of fine and equiaxed grains was obtained. The stability of the ECAPed structure at elevated temperatures was examined by annealing the four-passed materials over a wide range of temperature between 473 and 773 K. Measurement of activation energies for static grain growth shows the presence of three different values depending on the temperature range investigated: Q = 0.78 Q gb (activation for grain boundary diffusion) in the low temperature range 473–523 K, Q = 0.27 Q gb in the intermediate temperature range 523–673 K and Q = 0.84 Q L (activation for lattice diffusion) or Q = 1.23 Q gb in the high temperature range 673–773 K. The abnormally low Q value in the intermediate temperature range may not represent the true activation energy. Progressive decrease in dislocation density by enhanced recovery with increasing temperature may be the cause of the result. After ECAP, the yield stresses (YS) of the ECAPed AZ31 alloys decreased while their elongations increased. Enlarged strain hardening exponent after ECAP is the key factor considered to bring in the tensile-elongation increase, while modification of texture for easier slip on basal planes during ECAP is believed to be responsible for the yield stress decrease.

Thermal expansion and crystal structure of cementite, Fe3C, between 4 and 600 K determined by time-of-flight neutron powder diffraction

Abstract: The cementite phase of Fe3C has been studied by high-resolution neutron powder diffraction at 4.2 K and at 20 K intervals between 20 and 600 K. The crystal structure remains orthorhombic (Pnma) throughout, with the fractional coordinates of all atoms varying only slightly (the magnetic structure of the ferromagnetic phase could not be determined). The ferromagnetic phase transition, with Tc ≃ 480 K, greatly affects the thermal expansion coefficient of the material. The average volumetric coefficient of thermal expansion above Tc was found to be 4.1 (1) × 10−5 K−1; below Tc it is considerably lower (< 1.8 × 10−5 K−1) and varies greatly with temperature. The behaviour of the volume over the full temperature range of the experiment may be modelled by a third-order Gruneisen approximation to the zero-pressure equation of state, combined with a magnetostrictive correction based on mean-field theory.

Stabilization of Tetragonal ZrO2 in ZrO2–SiO2 Binary Oxides

Abstract: Gel-glasses of various compositions in the xZrO 2 .(10-x)SiO 2 system were fabricated by the sol-gel process. Precipitation due to the different reactivities between tetraethyl orthosilicate (TEOS) and zirconium(IV) n-propoxide has been eliminated through the use of 2-methoxyethanol as a chelating agent. Thermal treatment of these gels produced crystalline ZrO 2 particles. While monoclinic is the stable crystalline phase of zirconia at low temperatures, the metastable tetragonal phase is usually the first crystalline phase formed on heat treatment. However, stability of the tetragonal phase is low, and it transforms to the monoclinic phase on further heat treatment. In this study, it has been found that the transformation temperature increases as the SiO 2 content in the ZrO 2 -SiO 2 binary oxide increases. The most significant results were from samples containing only 2 mol% SiO 2 , where the metastable tetragonal phase formed at low temperatures and remained stable over a broad temperature range. X-ray diffraction, transmission electron microscopy, and Fourier transform infrared spectroscopy were used to elucidate the structure of these binary oxides as a function of temperature.

Temperature dependence of the force sensitivity of silicon cantilevers

Abstract: The resonance frequency $\ensuremath{\omega}$ and internal friction ${Q}^{\ensuremath{-}1}$ of the first eigenmode of microfabricated silicon cantilevers are measured in the temperature range of 15--300 K. The analysis shows that variation of Young's modulus is responsible for the temperature dependence of the resonance frequency, whereas the dependence of the geometrical dimensions can be neglected. Accordingly, the data can be fitted by the Wachtman equation, yielding a Debye temperature ${\ensuremath{\Theta}}_{D}=634\mathrm{K}.$ The temperature variation of internal friction ${Q}^{\ensuremath{-}1}$ is analyzed in terms of Zener's theory of thermoelastic damping. Due to the temperature dependence of the thermal expansion coefficient $\ensuremath{\alpha},$ thermoelastic damping is expected to vanish at 20 K and 125 K. A minimum of internal friction is observed at 20 K, whereas the minimum at 125 K appears to be hidden by other dissipation effects. A maximum of internal friction at 160 K is observed, which is an activation peak due to phonon scattering by atomic-scale defects. The best force sensitivity is achieved at 20 K, where a factor of 10 is gained compared to room temperature.

Highly processable bulk metallic glass-forming alloys in the Pt–Co–Ni–Cu–P system

Abstract: Highly processable bulk metallic glass alloys in the Pt–Co–Ni–Cu–P system were discovered. The alloys show low liquidus temperature below 900 K, excellent processability with low critical cooling rate reflecting in maximum casting thicknesses in quartz tubes of up to 20 mm, and a large supercooled liquid region. The Pt57.5Cu14.7Ni5.3P22.5 composition has a liquidus temperature of 795 K, a glass transition temperature of 508 K with a supercooled liquid region of 98 K. For medical and jewelry applications a Ni-free alloy, Pt60Cu16Co2P22 was discovered with a liquidus temperature of 881 K, a glass transition temperature of 506 K, and a supercooled liquid region of 63 K. Glass formation was observed in a wider composition range. Vickers hardness of these alloys is in the 400 Hv range. The alloys can be processed in the supercooled liquid region in air without any measurable oxidation. In this region, a large processing window is available in which the material does not embrittle. Embrittlement in these alloys is correlated with crystallization. It can be avoided as long as substantial crystallization does not take place during isothermal processing in the supercooled liquid region. Also, liquid processing can be performed in air when flux with B2O3.

Absorption and fluorescence of toluene vapor at elevated temperatures

Abstract: Absorption and fluorescence of the S0 → S1 (π,π*) transition in toluene are studied in the temperature range 300 K to 1130 K and 300 K to 930 K, respectively. Experiments are conducted in a shock-tube and in a heated flow-cell. Fluorescence spectra are investigated after excitation at 248 nm and 266 nm using a nitrogen diluent at a total pressure of 1 bar. Over the temperature range studied the fluorescence quantum yield decreases exponentially by three orders of magnitude for 266 nm excitation and double exponentially by three orders of magnitude for 248 nm excitation. The fluorescence spectrum shifts to the red with increasing temperature. The vibrational structure of the absorption spectrum found at ambient conditions vanishes above 600 K. The absorption feature broadens and the maximum shifts to the red. Taking advantage of the distinctive temperature dependence of the fluorescence, we suggest potential techniques using toluene as a sensitive tracer molecule for temperature imaging in both homogeneously and inhomogeneously mixed flow-fields.

Electrical transport properties of single ZnO nanorods

Abstract: Single ZnO nanorods with diameters of ∼130nm were grown on Au-coated Al2O3 substrates by catalyst-driven molecular beam epitaxy. Individual nanorods were removed from the substrate and placed between Ohmic contact pads and the current–voltage characteristics measured as a function of temperature and gas ambient. In the temperature range from 25to150°C, the resistivity of nanorods treated in H2 at 400°C prior to measurement showed an activation energy of 0.089±0.02eV and was insensitive to the ambient used (C2H4,N2O,O2 or 10% H2 in N2). By sharp contrast, the conductivity of nanorods not treated in H2 was sensitive to trace concentrations of gases in the measurement ambient even at room temperature, demonstrating their potential as gas sensors.

High temperature transport properties of partially filled CaxCo4Sb12 skutterudites

Abstract: Partially filled CoSb3 skutterudite compounds are emerging materials for thermoelectric energy conversion at high temperature. CaxCo4Sb12 with different Ca contents has been prepared by the conventional metallurgical route. The temperature dependences of the electrical resistivity, Seebeck coefficient, and thermal conductivity have been measured on these compounds in the 300–800 K temperature range. These measurements have identified Ca as being a true n-type filler atom and offer, for this family of skutterudite, several valuable insights into the potential of Ca to provide good thermoelectric performance.

Gas-Phase HO•-Initiated Reactions of Elemental Mercury: Kinetics, Product Studies, and Atmospheric Implications

Abstract: Mercury is an environmentally volatile toxic fluid metal that is assumed to have a long atmospheric residence time and hence is subject to long-range transport. The speciation and chemical transformation of mercury in the atmosphere strongly influences its bioaccumulation potential in the human food chain as well as its global cycling. To investigate the oxidation of Hg0 by HO•, the dominant daytime atmospheric oxidant, we performed kinetic and product studies over the temperature range 283−353 K under near atmospheric pressure (100 ± 0.13 kPa) in air and N2 diluents. Experiments were carried out by the relative rate method using five reference molecules and monitored by gas chromatography with mass spectroscopic detection (GC-MS). The HO• were generated using UV photolysis of isopropyl nitrite at 300 ≤ λ ≤ 400 nm in the presence of NO. The room-temperature rate constant was found to be (9.0 ± 1.3) × 10-14 cm3 molecule-1 s-1. The temperature dependence of the reaction can be expressed as a simple Arrheniu...