Showing papers on "Atmospheric temperature range published in 2002"

Polymer-stabilized liquid crystal blue phases

Abstract: Blue phases are types of liquid crystal phases that appear in a temperature range between a chiral nematic phase and an isotropic liquid phase. Because blue phases have a three-dimensional cubic structure with lattice periods of several hundred nanometres, they exhibit selective Bragg reflections in the range of visible light corresponding to the cubic lattice. From the viewpoint of applications, although blue phases are of interest for fast light modulators or tunable photonic crystals, the very narrow temperature range, usually less than a few kelvin, within which blue phases exist has always been a problem. Here we show the stabilization of blue phases over a temperature range of more than 60 K including room temperature (260–326 K). Furthermore, we demonstrate an electro-optical switching with a response time of the order of 10−4 s for the stabilized blue phases at room temperature.

CuO Nanowires Can Be Synthesized by Heating Copper Substrates in Air

Abstract: This paper describes a vapor-phase approach to the facial synthesis of cupric oxide (CuO) nanowires supported on the surfaces of various copper substrates that include grids, foils, and wires. A typical procedure simply involved the thermal oxidation of these substrates in air and within the temperature range from 400 to 700 °C. Electron microscopic studies indicated that these nanowires had a controllable diameter in the range of 30−100 nm with lengths of up to 15 μm by varying the temperature and growth time. Electron diffraction and high-resolution TEM studies implied that each CuO nanowire was a bicrystal divided by a (111) twin plane in its middle along the longitudinal axis. A possible mechanism was also proposed to account for the growth of these CuO nanowires.

Realization of band gap above 5.0 eV in metastable cubic-phase MgxZn1−xO alloy films

Abstract: We report on the realization of wide band gap (5–6 eV), single-phase, metastable, and epitaxial MgxZn1−xO thin-film alloys grown on sapphire by pulsed laser deposition We found that the composition, structure, and band gaps of the MgxZn1−xO thin-film alloys depend critically on the growth temperature The structural transition from hexagonal to cubic phase has been observed for (Mg content greater than 50 at %) (1⩾x⩾05) which can be achieved by growing the film alloys in the temperature range of 750 °C to room temperature Interestingly, the increase of Mg content in the film has been found to be beneficial for the epitaxial growth at relatively low growth temperature in spite of a large lattice mismatch between sapphire and cubic MgZnO alloys

Electrical conductivity for warm, dense aluminum plasmas and liquids.

Abstract: The electrical conductivity of warm, dense aluminum plasmas and liquids is calculated using ab initio molecular dynamics and the Kubo-Greenwood formula. The density range extends from near solid to one-hundredth of solid density, and the temperature range extends from 6000 K to 30 000 K. This density and temperature range allows direct comparison with experimental results obtained with the tamped exploding wire technique.

Thermal conductivity of Si/SiGe and SiGe/SiGe superlattices

Abstract: The cross-plane thermal conductivity of four Si/Si0.7Ge0.3 superlattices and three Si0.84Ge0.16/Si0.76Ge0.24 superlattices, with periods ranging from 45 to 300 and from 100 to 200 A, respectively, were measured over a temperature range of 50 to 320 K. For the Si/Si0.7Ge0.3 superlattices, the thermal conductivity was found to decrease with a decrease in period thickness and, at a period thickness of 45 A, it approached the alloy limit. For the Si0.84Ge0.16/Si0.76Ge0.24 samples, no dependence on period thickness was found and all the data collapsed to the alloy value, indicating the dominance of alloy scattering. This difference in thermal conductivity behavior between the two superlattices was attributed to interfacial acoustic impedance mismatch, which is much larger for Si/Si0.7Ge0.3 than for Si0.84Ge0.16/Si0.76Ge0.24. The thermal conductivity increased slightly up to about 200 K, but was relatively independent of temperature from 200 to 320 K.

Lattice parameters and anisotropic thermal expansion of hexagonal boron nitride in the 10–297.5 K temperature range

Abstract: Lattice parameters for hexagonal boron nitride (hBN) were determined using X-ray powder diffraction at a synchrotron-radiation source (beamline B2, Hasylab/DESY, Hamburg) in the previously uninvestigated temperature range from 10 K up to 297.5 K. The relative change of a and c with rise of temperature in the studied range is anisotropic and amounts to about -0.05% and +0.82%, respectively. The corresponding increase of the unit-cell volume is 0.73%. The evaluated values of thermal expansion coefficients derived from the lattice-parameter dependences on temperature are generally consistent with earlier data determined by interferometryfor T≥82 K.

Low-temperature scaling behavior of BaFe 0.5 Nb 0.5 O 3

Abstract: The ferroelectric ceramic ${\mathrm{BaFe}}_{0.5}{\mathrm{Nb}}_{0.5}{\mathrm{O}}_{3}$ (BFN) is synthesized by a solid-state reaction technique. The x-ray-diffraction of the sample at room temperature shows a monoclinic phase. The field dependences of the dielectric response and the conductivity are measured in a frequency range from 10 Hz to 2 MHz and in a temperature range from 93 to 213 K. The frequency dependence of the loss peak is found to obey an Arrhenius law with an activation energy 0.12 eV. An analysis of the real and imaginary parts of the dielectric permittivity with frequency is performed, assuming a distribution of relaxation times as confirmed by Cole-Cole plots as well as the scaling behavior of the dielectric loss. The scaling behavior of the dielectric loss spectra suggests that the distribution of the relaxation times is temperature independent. The frequency-dependent electrical data are also analyzed in the framework of the conductivity and modulus formalisms. Both these formalisms provided qualitative similarities in the relaxation times. We observe that the hopping frequency can be used for a scaling of the conductivity spectra for BFN. All these observations clearly suggest that BFN is a relaxor ferroelectric.

Nanoparticle luminescence thermometry

Abstract: A new concept of using luminescent nanoparticles for thermometry is described in this paper. To demonstrate this idea, the temperature-dependent emission characteristics of several luminescent nanoparticles have been investigated. The results show that some of them have a linear response above room temperature, which could be applied to temperature sensing. Semiconductor nanoparticles of CdTe and doped nanoparticles of ZnS:Mn2+ show a reversible linear temperature response over the physiological temperature range and have potential for biomedical thermometry. Double-doped nanoparticles of ZnS:Mn2+,Eu3+ show a different temperature response for each dopant; the ratio of their intensities provides a robust temperature measurement approach. Thermal instability of the nanoparticle stabilizer and nanoparticle surface defects is a possible reason for the irreversible thermal response of some nanoparticles.

Hygroscopicity of aerosol particles at low temperatures. 2. Theoretical and experimental hygroscopic properties of laboratory generated aerosols

Abstract: A Hygroscopicity Tandem Differential Mobility Analyzer (H-TDMA) system has been used to measure hygroscopic growth curves and deliquescence relative humidities (DRH) of laboratory generated (NH4)2SO4, NaCl, and NaNO3 particles at temperatures T = 20 °C and −10 °C. Good agreement (better than 3.5%) between measured growth curves and Kohler theory was found using empirical temperature and concentration dependent values for water activity, solution density, and surface tension. The measured growth curves only experience a small temperature dependence in the observed temperature range. Therefore, to a first approximation, it is possible to neglect the temperature dependence of the water activity for theoretical calculations in the temperature range −10 °C < T < 25 °C. The small differences between experiment and theory, which were predominantly observed for NaCl particles, are probably caused by a small amount of water adsorbed on the “dry” crystals. It was also observed that these particles experience a sign...

Very large magnetic entropy change near room temperature in LaFe11.2Co0.7Si1.1

Abstract: A very large magnetic entropy change ΔS has been observed in Fe-based cubic NaZn13-type compound LaFe11.2Co0.7Si1.1 near the Curie temperature TC of 274 K. The value of the entropy change is ∼20.3 J/kg K under a magnetic field of 5 T at TC=274 K. It markedly exceeds that of pure Gd at the corresponding temperature range [V. K. Pecharsky & K. A. Gschneidner, Jr., Phys. Rev. Lett. 78, 4494 (1999)]. The great entropy change produced by the sharp change of magnetization is associated with a large negative lattice expansion at TC. The very large magnetic entropy change and low cost suggest that the compound LaFe11.2Co0.7Si1.1 has great potential for applications as magnetic refrigerants near room temperature.

Magnetic characterization of nanocrystalline Ni-Zn ferrite powder prepared by the glyoxylate precursor method

Abstract: Nanocrystalline Ni0.35Zn0.65Fe2O4 mixed ferrite was obtained from the Fe2(Ni0.35,Zn0.65)(OH)4(C2H2O4)2 · H2O complex combination that corresponds to the atomic ratio Ni(II) : Zn(II) : Fe(III) = 0.35 : 0.65 : 2; the complex combination was decomposed at 325uC and the resulting oxides mixture was annealed in the temperature range of 400–1000uC for 2 h. The thermal analysis of the synthesized complex combination was done by TG–DTA techniques. It has been shown by means of x-ray diffraction that even at 400uC Ni–Zn mixed spinel ferrite is formed with an face-centred cubic structure and a lattice parameter that is in agreement with the reported value. Thus, the formation temperature of ferrite was drastically reduced (by cca. 900uC) compared to that of the conventional ceramic method. The magnetic measurements showed the increase of the saturation magnetization σs and a maximum of the coercivity Hc of the nanocrystalline system with the increase of the annealing temperature. These changes can be attributed to the increase of the average diameter of the nano-sized crystallites from 14.6 to 46.3 nm when the temperature increases from 400uC to 1000uC. The nanocrystallites are single-domain up to ∼28 nm; above this value they have an incipient structure of Weiss domains, a result that is in agreement with the critical diameter of the single-domain deduced from theoretical calculation.

Atomic Layer Deposition of Hafnium Dioxide Films from Hafnium Tetrakis(ethylmethylamide) and Water

Abstract: HfO 2 films were produced from Hf[N(CH 3 )(C 2 H 5 )] 4 and H 2 O, on borosilicate glass, indium-tin-oxide (ITO), and Si(100) substrates, in the temperature range 150-325 °C, using atomic layer deposition (ALD). In the temperature range 200-250 °C, the growth rate of the HfO 2 films was 0.09 nm per cycle, but increased with both increasing and decreasing temperatures. The self-limiting adsorption of Hf[N(CH 3 )(C 2 H 5 )] 4 at 250 °C was verified. The films were stoichiometric dioxides with an O/Hf ratio of 2.0 ± 0.1. The concentrations of residual carbon, nitrogen, and hydrogen, determined using ion beam analysis, were 0.3-0.6 at.-%, 0.1-0.2 at.-%, and 2-3 at.-%, respectively. The films crystallized at growth temperatures exceeding 150-175 °C, and consisted mainly of the monoclinic HfO 2 phase. The refractive index of the films varied between 2.08 and 2.10. The effective permittivities of the HfO 2 films grown in the temperature range 200-300°C varied between 11 and 14.

Temperature-Dependent Growth of Vertically Aligned Carbon Nanotubes in the Range 800−1100 °C

Abstract: We investigated the effect of temperature on the growth rate and structure of carbon nanotubes using scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The carbon nanotubes were grown vertically aligned on iron nanoparticle deposited silicon substrate by thermal chemical vapor deposition of acetylene in the temperature range 800−1100 °C. As the growth temperature increases from 800 to 1100 °C, the average diameter increases from 20 to 150 nm and the growth rate also increases by about 20 times. All carbon nanotubes exhibit a bamboo-like structure over this temperature range. In the carbon nanotubes grown at higher temperature, the thicker compartment layers appear more frequently. The relative amount of crystalline graphitic sheets increases progressively with the growth temperature. The Arrhenius plot provides the activation energy of carbon nanotube growth to be at least 30 kcal/mol. The results indicate that the bulk diffusion of carbons would be an important factor...

Mechanical behavior of Ti–6Al–4V at high and moderate temperatures—Part I: Experimental results

Abstract: This study investigates the plastic deformation of titanium alloy Ti–6%Al–4%V under low and moderate strain rates and various temperature conditions. Mechanical testing is performed in the temperature range 650–1340 K (710–1950 °F) and under constant strain rate loading ranging from 10 −3 to 10 s −1 . The test results are correlated with the evolution of the microstructure and compared to published data. The flow stress of this alloy is strongly dependent on both temperature and deformation rate, with the temperature effect becoming negligible in the upper part of the temperature range investigated. At temperatures above 800 K (980 °F) the flow stress decreases sharply with temperature. The effect of deformation rate on this transition is investigated and the possible mechanisms responsible for the behavior are discussed. Based on these experimental results, a physically-based constitutive law is developed in the sequel of this paper.

High-temperature nanoindentation testing of fused silica and other materials

Abstract: This paper describes a new high-temperature stage for small-scale mechanical property testing. This allows the determination of the load-penetration curve of a diamond tip in a temperature range extending from room temperature to 400°C. Both sample and indenter can be heated separately. Indentation curves show that very low thermal drift can be achieved. Nanoindentation results are presented for gold, soda—lime glass, fused silica and a polyimide and compared with existing microscale and bulk mechanical property data where available. Results from fused silica show that its mechanical properties exhibit a completely different temperature dependence from those of soda-lime glass, as expected since fused silica is an anomalous glass.

Net emission coefficient of air thermal plasmas

Abstract: We have calculated the net emission coefficient of air plasmas at atmospheric pressure in the temperature range between 300 and 40?000?K, in the assumption of local thermodynamic equilibrium and isothermal plasmas. This calculation takes into account the radiation due to the atomic continuum, the molecular continuum, the molecular bands (several systems for O2, N2, NO and N2+) and the atomic lines. Special attention has been devoted in this paper to the description of the molecular bands radiation. The results show that in the high temperature range where molecules are dissociated, the radiation properties of air plasmas are mainly due to those of nitrogen plasmas, the resonance atomic lines playing an important role in spite of their strong self-absorption. At low temperature (T<6000?K) the role of the molecular bands of oxygen (O2) and NO is predominant.

Kinetics of selective CO oxidation in hydrogen-rich mixtures on Pt/alumina catalysts

Abstract: Selective CO oxidation on Pt/alumina catalysts in hydrogen-rich mixtures was studied in a temperature range of 150–350 °C and a concentration range of 0.5–2% for oxygen and carbon monoxide. The intrinsic kinetics of the CO oxidation was well described by a simple power-law expression with an activation energy of 78 kJ/mol and the reaction orders of −0.51 and 0.76 for the CO and the oxygen partial pressure, respectively. The selectivity showed a maximum with increasing temperature and was dependent mainly on the feed composition and the reaction temperature. But the selectivity did not show any appreciable change with respect to flow rate, indicating a constant selectivity along the catalyst bed, and Pt loading of the catalyst in the range of 0.04–1 wt.%, even though the 1 wt.% catalyst exhibited severe internal diffusional limitation. For this, the selectivity once measured for a Pt/alumina catalyst as a function of feed composition and temperature could be used for other Pt/alumina catalysts of different activities and for different conversion levels. A reactor model with an approximation of the internal effectiveness factor was developed to simulate the integral reactor performance with the 1 wt.% catalyst.

Room-temperature magnetism in Cr-doped AlN semiconductor films

Abstract: Synthesis and characterization of magnetic semiconductors Al1−xCrxN, in which the atomic fraction of chromium x is up to 0.357, are reported. The films, grown by reactive co-sputtering on silicon, glass, and kapton substrates, have a crystal structure of aluminum nitride. Magnetic and transport properties were studied in the temperature range of 50 to 340 K. The materials are in the dielectric regime and have variable-range-hopping type of conductance. The films are ferromagnetic at temperatures over 340 K.

Thermal and electrical properties of Nb2AlC, (Ti, Nb)(2)AlC and Ti2AlC

Abstract: The heat capacities, thermal-expansion coefficients, thermal and electrical conductivities of Nb2AlC (actual Nb:Al:C mole fractions: 0.525±0.005, 0.240±0.002, and 0.235±0.005, respectively), Ti2AlC and (Ti, Nb)2AlC (actual Ti:Nb:Al:C mole fractions: 0.244±0.005, 0.273±0.005, 0.240±0.003, and 0.244±0.005, respectively) were measured as a function of temperature. These ternaries are good electrical conductors, with a resistivity that increases linearly with increasing temperatures. The resistivity of (Ti, Nb)2AlC is higher than the other members, indicating a solid-solution scattering effect. The thermal-expansion coefficients, in the 25 °C to 1000 °C temperature range, are comparable and fall in the narrow range of 8.7 to 8.9 × 10−6 K−1, with that of the solid solution being the highest. They are all good conductors of heat, with thermal conductivities in the range between 15 to 45 W/m K at room temperature. The electronic component of the thermal conductivity is the dominant mechanism at all temperatures for Nb2AlC and (Ti, Nb)2AlC. The conductivity of Ti2AlC, on the other hand, is high because the phonon contribution to the conductivity is nonnegligible.

Capillary condensation of nitrogen in MCM-41 and SBA-15

Abstract: We performed measurements of a series of adsorption–desorption isotherms of nitrogen into MCM-41 and SBA-15 mesoporous molecular sieves with different pore sizes in a wide temperature range between a bulk triple point (Tt) and a bulk critical point (Tc). The hysteresis loop shrinks with increasing temperature and eventually disappears at the hysteresis critical temperature (Tch), in accord with the appearance of metastable states in a single pore. We estimated the exact pore sizes of our samples from comparing the experimental equilibrium pressure at 77 K with the NLDFT isotherms, where the condensation pressures at 77 K were determined by extrapolating a plot of the equilibrium pressure versus temperature measured above Tch. The pore critical temperature (Tcp) was also determined from the inflection point in a plot of the inverse slope of the adsorption step against temperature. Tch is always lower than Tcp. Both plots of (Tc−Tch)/Tc versus d/rp and (Tc−Tcp)/Tc versus d/rp form straight lines passing thr...

Substitution-induced pinning in MgB2 superconductor doped with SiC nano-particles

Abstract: By doping MgB2 superconductor with SiC nano-particles, we have successfully introduced pinning sites directly into the crystal lattice of MgB2 grains (intra-grain pinning). It became possible due to the combination of counter-balanced Si and C co-substitution for B, leading to a large number of intra-granular dislocations and the dispersed nano-size impurities induced by the substitution. The magnetic field dependence of the critical current density was significantly improved in a wide temperature range, whereas the transition temperature in the sample MgB2(SiC)x having x = 0.34, the highest doping level prepared, dropped only by 2.6 K.

Temperature dependence of the Seebeck coefficient and the potential barrier scattering of n-type PbTe films prepared on heated glass substrates by rf sputtering

Abstract: Thermoelectric properties of polycrystalline degenerate n-type PbTe films have been investigated in order to understand potential barrier scattering. The Seebeck coefficients of the PbTe films obtained in this study were larger than those of bulk samples having the same carrier concentrations in the temperature range from 100 to 450 K. Some of their power factors were larger than those of bulk samples having the same carrier concentrations in the temperature range from 200 to 450 K, while their electrical conductivities were smaller than those of bulk samples. From a comparison of these results with those previously reported, we concluded that potential barrier scattering occurred at grain boundaries in our films, resulting in the above favorable changes in thermoelectric properties. By analyzing their properties using the energy filtering model, we estimated the height of grain-boundary potential barriers, which probably influenced the increases in the Seebeck coefficient. We also examined the origin of the potential barriers accordingly. Consequently, we consider that the origin of the potential barriers was mainly due to point defects, probably Te vacancies, and that the barrier height may be controlled, for example, by the preparation conditions.

Formation of Formaldehyde by the Tunneling Reaction of H with Solid CO at 10 K Revisited

Abstract: The reaction of H atoms with the solid CO thin film was reinvestigated in the temperature region of 10-25 K. H2CO was found to be only the reaction product, and no other products such as CH3OH were detected. This indicates that the tunneling reactions of H with H2CO to form CH3OH are even slower than the slow reaction of H with CO to form H2CO. The CH3OH found in the envelopes of the dark clouds may have other sources for their production in addition to reactions 2H + H2CO → CH3OH, e.g., reaction of O(1D) with CH4 trapped on the dust grains to form CH3OH. The yield of H2CO from the reaction H with solid CO showed a steep increase with a decrease of temperature from 25 to 10 K. This indicates that the dark clouds whose temperature is kept at as low as 10 K are the favorable place for the chemical evolution via tunneling reactions. The erosion of the solid CO film was not observed with the spray of the H atoms over the CO solid film in the temperature range of 10-25 K. This finding suggests that the contribution of the highly exothermic reaction 2H + H2 to desorption of the grain mantle may not be as large as thought before.