Showing papers on "Atmospheric temperature range published in 2001"

Hydroxylation of camphor by reduced oxy-cytochrome P450cam: mechanistic implications of EPR and ENDOR studies of catalytic intermediates in native and mutant enzymes.

Abstract: We have employed γ-irradiation at cryogenic temperatures (77 K and also ∼6 K) of the ternary complexes of camphor, dioxygen, and ferro-cytochrome P450cam to inject the “second” electron of the catalytic process. We have used EPR and ENDOR spectroscopies to characterize the primary product of reduction as well as subsequent states created by annealing reduced oxyP450, both the WT enzyme and the D251N and T252A mutants, at progressively higher temperatures. (i) The primary product upon reduction of oxyP450 4 is the end-on, “H-bonded peroxo” intermediate 5A. (ii) This converts even at cryogenic temperatures to the hydroperoxo-ferriheme species, 5B, in a step that is sensitive to these mutations.Yields of 5B are as high as 40%. (iii) In WT and D251N P450s, brief annealing in a narrow temperature range around 200 K causes 5B to convert to a product state, 7A, in which the product 5-exo-hydroxycamphor is coordinated to the ferriheme in a nonequilibrium configuration. Chemical and EPR quantitations indicate the ...

Temperature dependence of the thermal conductivity of single-wall carbon nanotubes

Abstract: The thermal conductivity of several single-wall carbon nanotubes has been calculated over a temperature range of 100-500 K using molecular dynamics simulations with the Tersoff-Brenner potential for C-C interactions. In all cases, starting from similar values at 100 K, the thermal conductivities show a peaking behaviour before falling off at higher temperatures. The peak position shifts to higher temperatures for nanotubes with larger diameters and no significant dependence on the tube chirality is observed. It is shown that this phenomenon is due to the onset of Umklapp scattering, which shifts to higher temperatures for nanotubes with larger diameters.

Electrical transport studies of Ag nanoclusters embedded in glass matrix

Abstract: Silver nanoclusters embedded in glass matrices have been obtained by the combined use of ion exchange and subsequent ion implantation. XRD and UV-visible spectro-photometric analysis have confirmed the formation of Ag nano-clusters in the ion-irradiated samples. Temperature-dependent resistivity measurements of the irradiated samples have been found to follow ρ ( T )∝exp√( T 0 / T ) in the temperature range of 80–280 K. The observed behaviour of ρ ( T ) is consistent with charge transport due to hopping between isolated, conducting islands. The separation distance between the conducting islands has been found to be a function of fluence.

Micro-Raman scattering and dielectric investigations of phase transition behavior in the BaTiO3–BaZrO3 system

Abstract: In this study, the phase transition behavior of the BaTiO3–BaZrO3 system was studied using micro-Raman scattering and dielectric measurement techniques. BaZrxTi1−xO3 ceramics were prepared for x=0.00, 0.05, 0.08, 0.15, 0.20, and 1.00 compositions using a solid-state reaction technique. A single-phase perovskite structure of the ceramics was identified by the x-ray diffraction technique. The basic phase transition temperatures in these compositions were studied in the temperature range of 70–575 K. The tetragonal to cubic transition temperature was found to decrease with increasing Zr content. The orthorhombic to tetragonal transition temperature that increases with an initial increase in Zr content merges with the tetragonal–cubic transition for x⩾0.15 compositions. Raman spectra of rhombohedral and orthorhombic phases could not be distinguished. Excellent agreement between the crystallographic transition temperatures obtained by both techniques suggested that Zr substituted octahedra were uniformly distr...

Bulk synthesis of silicon nanowires using a low-temperature vapor–liquid–solid method

Abstract: Silicon nanowires will find applications in nanoscale electronics and optoelectronics both as active and passive components. Here, we demonstrate a low-temperature vapor–liquid–solid synthesis method that uses liquid-metal solvents with low solubility for silicon and other elemental semiconductor materials. This method eliminates the usual requirement of quantum-sized droplets in order to obtain quantum-scale one-dimensional structures. Specifically, we synthesized silicon nanowires with uniform diameters distributed around 6 nm using gallium as the molten solvent, at temperatures less than 400 °C in hydrogen plasma. The potential exists for bulk synthesis of silicon nanowires at temperatures significantly lower than 400 °C. Gallium forms a eutectic with silicon near room temperature and offers a wide temperature range for bulk synthesis of nanowires. These properties are important for creating monodispersed one-dimensional structures capable of yielding sharp hetero- or homointerfaces.

Gel Network Structure of Methylcellulose in Water

Abstract: Thermal gelation was studied for aqueous gelling solutions of a methylcellulose. An attempt was made to elucidate the gel network structure and the validity of scaling laws. Thermal gelation was observed on heating, and it reverted to the liquid state on cooling. The thermoreversibility was a heating/cooling rate dependent process. For isothermally stabilized samples, 42.5 °C was found to be the critical temperature differentiating the weak gels from the strong gels. Below 42.5 °C, the gel elasticity evolved by following a scaling law with temperature as Ge ∼ [(T − Tc)/Tc]2.93 where Ge is the equilibrium modulus of the gel and Tc is the critical temperature of 42.5 °C. In contrast, no single scaling laws could be found for Ge when the temperature was above 42.5 °C. In the temperature range from 42.5 to 70 °C, it was observed that the elasticity evolution was a linear function of temperature and the mean bridge length between junctions was independent of temperature. On the basis of the experimental result...

High Performance Thermoelectric Tl 9 BiTe 6 with an Extremely Low Thermal Conductivity

Abstract: $\mathrm{Tl}{}_{9}\mathrm{BiTe}{}_{6}$ exhibits a thermoelectric figure of merit of $\mathrm{ZT}\ensuremath{\sim}1.2$ around 500 K, which significantly exceeds the state-of-the-art materials in this temperature range. The extraordinary thermoelectric performance is mainly due to the extremely low thermal conductivity of $\mathrm{Tl}{}_{9}\mathrm{BiTe}{}_{6}$ [ $0.39\mathrm{W}/(\mathrm{m}\ifmmode \dot{}\else \.{}\fi{}\mathrm{K})$ at 300 K]. In fact, the minimum lifetime of the phonons has to be taken into account to describe the thermal conductivity data.

Temperature dependence for the photovoltaic device parameters of polymer-fullerene solar cells under operating conditions

Abstract: We report on the temperature dependence of various photovoltaic device parameters of solar cells, fabricated from interpenetrating networks of conjugated polymers with fullerenes, in the wide temperature range of their possible operating conditions ~25‐ 60 °C!. The open-circuit voltage was found to decrease linearly with increasing temperature. For the short-circuit current, we observed a monotonic increase with increasing temperature, followed by a saturation region. The rate of this increase ~coupled to a corresponding increase for the fill factor! was found to overtake the corresponding rate of decrease in voltage, resulting in an overall increase of the energy conversion efficiency. The efficiency was observed to reach a maximum value in the approximate range 47‐ 60 °C. The results are discussed with respect to possible mechanisms for photovoltage generation and charge carrier transport in the conjugated polymer-fullerene composite, and in particular, thermally activated charge carrier mobility. © 2001 American Institute of Physics. @DOI: 10.1063/1.1412270#

Dynamic yield and tensile strength of aluminum single crystals at temperatures up to the melting point

Abstract: This article presents experimental results of the dynamic yield strength and dynamic tensile strength (“spall strength”) of aluminum single crystals at shock-wave loading as a function of temperature. The load duration was ∼40 and ∼200 ns. The temperature varied from 20 to 650 °C which is only by 10 °C below the melting temperature. A linear growth of the dynamic yield strength by more than a factor of 4 was observed within this temperature range. This is attributed to the phonon drag effect on the dislocation motion. High dynamic tensile strength was maintained over the whole temperature range, including the conditions at which melting should start in a material under tension. This could be an indication of the existence of superheated states in solid crystals.

Mechanism of O3 and NO2 detection and selectivity of In2O3 sensors

Abstract: The gas-sensitive properties of the sol–gel prepared In 2 O 3 , In 2 O 3 –NiO, and In 2 O 3 –MoO 3 thin film sensors at the detection of NO 2 and O 3 were investigated. The differences in the mechanisms of interaction between the oxide surface and O 3 and NO 2 molecules are discussed. The activity and selectivity of oxides substantially depend on the metal–oxygen binding energy in the oxide lattice. The introduction of dopants into In 2 O 3 , which increases the metal–oxygen binding energy as well as creates adsorption centers with the high affinity to oxygen (Mo 5+ ), leads to the shift of NO 2 detection temperature at In 2 O 3 -sensors to the high temperature range. In the case of nonstoichiometric In 2 O 3 films, the optimal ozone detection temperature is low. The observed differences of the interaction between NO 2 and O 3 and the oxide surfaces can be used for measuring the properties of oxide sensor regarding these gases.

Temperature-Controlled Growth of Silicon-Based Nanostructures by Thermal Evaporation of SiO Powders

Abstract: Silicon-based nanostructures with different morphologies, sizes, compositions, and microstructures were grown on Si wafers by thermal evaporation of SiO powders at 1350 °C for 5 h under 300 Torr of a flowing gas mixture of 5% H2-Ar at a flow rate of 50 standard cubic centimeters per minute (sccm). The SiO powders and Si wafers were placed inside an alumina tube, which was heated by a tube furnace. The local temperature inside the tube was carefully calibrated by a thermal couple. After evaporation, Si-containing products with different colors and appearances were formed on the surfaces of the Si wafers over a wide temperature range of 890-1320 °C and a long distance of 85 mm. Basing on the colors and appearances of the products, five distinct zones, which corresponding to different temperature ranges, were clearly identified from the highest temperature of 1320 °C to the lowest temperature of 890 °C. They are zone I (1250-1320 °C), zone II (1230-1250 °C), zone III (1180-1230 °C), zone IV (930-1180 °C), and zone V (890-930 °C). The deposited products were systematically studied by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results show that, besides Si nanowires, many other kinds of Si-based nanostructures such as octopuslike, pinlike, tadpolelike, and chainlike structures were also formed. The temperature distribution inside the alumina tube was found to play a dominant role on the formation of these structures. It is demonstrated that a control over the growth temperature can precisely control the morphologies and intrinsic structures of the silicon-based nanomaterials. This is an important step toward design and control of nanostructures. The growth mechanisms of these products were briefly discussed.

Colloidal Particles of Ca(OH)2: Properties and Applications to Restoration of Frescoes

Abstract: We studied the synthesis and characterization of Ca(OH)2 colloidal particles and their application to the restoration of wall paintings. Submicrometer Ca(OH)2 crystalline particles have been obtained by mixing equal volumes of NaOH and CaCl2 aqueous solutions with different degrees of supersaturation (from 2 to 10) and in the temperature range 60−90 °C. The Ca(OH)2 particles are crystalline and have the appearance of hexagonal platelets with thicknesses of a few nanometers, as determined by scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, and atomic force microscopy techniques. The kinetic stability of dispersions of Ca(OH)2 particles in water and propan-1-ol has also been studied in view of their possible application in wall painting conservation. A successful example of this application to the restoration of Santi di Tito's wall paintings (16th century) in the Santa Maria del Fiore Cathedral in Florence is also reported.

An improved model for determining degree-day values from daily temperature data.

Abstract: Although using hourly weather data offers the greatest accuracy for estimating growing degree-day values, daily maximum and minimum temperature data are often used to estimate these values by approximating the diurnal temperature trends. This paper presents a new empirical model for estimating the hourly mean temperature. The model describes the diurnal variation using a sine function from the minimum temperature at sunrise until the maximum temperature is reached, another sine function from the maximum temperature until sunset, and a square-root function from then until sunrise the next morning. The model was developed and calibrated using several years of hourly data obtained from five automated weather stations located in California and representing a wide range of climate conditions. The model was tested against an additional data-set at each location. The temperature model gave good results, the root-mean-square error being less than 2.0 °C for most years and locations. The comparison with published models from the literature showed that the model was superior to the other methods. Hourly temperatures from the model were used to calculate degree-day values. A comparison between degree-day estimates determined from the model and those obtained other selected methods is presented. The results showed that the model had the best accuracy in general regardless of the season.

High-Temperature X-ray Diffraction Studies on the Crystalline Transitions in the α- and γ-Forms of Nylon-6

Abstract: The crystallization of nylon-6 from the melt was monitored in situ by X-ray diffraction. The nylon-6 was found to crystallize into a high-temperature α‘-phase as indicated by the two-peak nature of the diffractogram. On cooling from the crystallization temperature to room temperature, nylon-6 retained the two-peak nature. However, data analysis indicates a change from high-temperature (HT) α‘-phase to low-temperature α-phase at ∼180 °C. On heating, the α-phase transformed into the α‘-phase at about 190 °C and melted in the α‘-phase. The transition took place over a temperature range where both phases coexisted. However, samples crystallized from the melt at temperatures 140 and 180 °C showed the α-phase at room temperature, but on heating the α-phase first transformed into a pseudohexagonal phase and before melting the pseudohexagonal phase further transformed into the α‘-phase. The α-phase was transformed into the γ-phase, by potassium iodide−iodine treatment, and the behavior of the γ-phase with tempera...

Superconducting bulk magnets: Very high trapped fields and cracking

Abstract: Improved trapped fields are reported for bulk melt-textured YBa2Cu3O7−δ (YBCO) material in the temperature range between 20 and 50 K. Trapped fields up to 12.2 T were obtained at 22 K on the surface of single YBCO disks (with Ag and Zn additions). In YBCO minimagnets, maximum trapped fields of 16 T (at 24 K) and of 11,2 T (at 47 K) were achieved using (Zn+Ag) and Zn additions, respectively. In all cases, the YBCO disks were encapsulated in steel tubes in order to reinforce the material against the large tensile stress acting during the magnetizing process and to avoid cracking. We observed cracking not only during the magnetizing process, but also as a consequence of flux jumps due to thermomagnetic instabilities in the temperature range betweeen 20 and 30 K.

Relaxor behaviour of low lead and lead free ferroelectric ceramics of the Na0.5Bi0.5TiO3–PbTiO3 and Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 systems

Abstract: Ceramics with compositions belonging to the Na0.5Bi0.5TiO3–PbTiO3 and Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 systems were fabricated by natural sintering of powders prepared by thermal decomposition of adequate precursor solutions. The ferroelectric to paraelectric phase transitions were studied by variable temperature X-ray diffractometry, differential scanning calorimetry and impedance measurement in a wide range of temperature and frequency. In contrast with pure NBT, both the permittivity and dielectric loss of the NBT-rich solid solutions show a strongly temperature and frequency dependent behaviour. The permittivity decreases and its maximum is shifted towards high temperatures as the frequency increases. In the high temperature range, the thermal variation of the permittivity is well described by a law 1 e − 1 e m =C T−T m γ with γ close to 1.5. Such a relaxor-like behaviour is interpreted in terms of cation disorder due to the statistical repartition of (Na,Bi) and Pb (or Na and K). This would be one very rare case of relaxor phenomena correlated with the A-site occupancy in perovskite-like materials.

Experimental proof of major role of magnetic field losses in microwave heating of metal and metallic composites

Abstract: In 1999 we found that powdered metal samples including very complex shaped and large size (100 mm diameter, 1 kilograms) could be fully sintered in 30 min in a 2.45 GHz multi-mode microwave cavity [1]. Moreover, these samples had properties at least as good as, and usually better than, those sintered in conventional furnaces. This finding was outside the experience of a very large number of scientists whose extensive work has been covered in many reviews [2–4]. This achievement was as puzzling to us as to colleagues and efforts to explain this by skin depth absorption etc. did not work. The well known extensive theoretical treatment of microwave-material interaction by many workers (see e.g. Varadan and Varadan [5], Booske et al. [6] and others) have in common that they always treat the energy absorption mechanism as due to the dielectric loss factor. In 1994 Cherradi et al. [7] reported their preliminary work in which they showed that the magnetic field must make substantial contributions to the heating of alumina (at high temperature) and semiconductors, and metallic copper. But in their work, the experimental design of using samples of 120 mm length, where in some cases, the sample was exposed to both magnetic and electric field simultaneously, caused a complicated interplay of the different absorption. In present work, a finely tuned microwave cavity with a cross section dimension of 86 mm by 43 mm which works in TE103 single mode was used to investigate the microwave heating behaviors of various materials in different microwave fields. Fig. 1 shows the scheme of the microwave system, and the distribution of the microwave field within the cavity is sketched in Fig. 2. In the L/2 location along the length of the cavity, the maximum electric (E) field is in the center of the cross section, where the magnetic (H ) field is minimum; and the maximum magnetic field is near the wall, where the electric field is minimum. A quartz tube was introduced in this location to hold the sample and also to enable us to control the atmosphere. A 2.45 GHz, 1.2 kW microwave generator (Toshiba, Japan) with power monitor was used as microwave source. A small cylindrical sample (5 mm diameter and 3 mm thick) was placed inside at two different locations, the maximum electric field area where the magnetic field is minimum, and the maximum magnetic field area where the electric field is minimum, respectively. Sample temperatures were measured using an infrared pyrometer (Mikron Instrument Co., Model M90-BT, temperature range −50 ◦C–1000 ◦C). During the experiments, atmospheric pressure nitrogen gas was passed through the quartz tube to avoid oxidation of metal samples at high temperature. Initially, we tried to use a fixed microwave power for all samples during heating, but for some samples, the temperature increase was too fast and the highest temperature exceeded the measuring range of the pyrometer, and in some cases, discharging and arcing occurred. So we set different microwave powers for different samples to get more stable heating results. Fig. 3a shows the heating observed for a typical commercial powdered metal sample (Keystone Powdered-metal Company, Saint Marys, PA, USA. The

Materials aspects, electrical performance, and scalability of Ni silicide towards sub-0.13 μm technologies

Abstract: Ni-silicide phase formation with and without a Ti capping layer was studied by sheet resistance, x-ray diffraction and transmission electron microscopy. Ni monosilicide is found to be the stable phase in a temperature range from 400 to 600 °C. At lower temperatures the Ni2Si phase is found to be present. For temperatures higher than 700 °C NiSi is converted into NiSi2. Pyramidal NiSi2 precipitates were found to grow epitaxially along the Si〈111〉 planes for annealing temperatures as low as 310 °C. The epitaxial NiSi2 grains were found to disappear when the annealing temperature is increased. Stress buildup during Ni silicidation was measured in situ and could be correlated to the formation of the different Ni-silicide phases. The stress induced by Ni-monosilicide formation compares favorably to the stress induced by Co disilicide and Ti disilicide. The average silicon consumption required to obtain a certain sheet resistance was found to be 35% lower for Ni monosilicide compared for Co disilicide. It was f...

Paramagnetism and antiferromagnetic d–d coupling in GaMnN magnetic semiconductor

Abstract: The magnetization of Ga1−xMnxN (x<0.1) crystals was measured as a function of the magnetic field and temperature. Paramagnetic behavior typical of spin S=5/2 expected for Mn2+ (d5) magnetic centers was observed in the temperature range of 2 K