Continuous-cooling method of specific heat measurement in the temperature range 100-300 K

The authors have developed a simple continuous-cooling method to determine specific heat of liquids and solids in the temperature range 100-300 K. The technique employs very simple instrumentation and continuously records the sample temperature as it cools to the bath temperature through a calibrated heat link. They have obtained specific heat values which agree with the reported data to within 3% for the samples investigated. This method also facilitates easy detection of abrupt changes in specific heat, as demonstrated in the observation of glass transition in some organic glass-forming systems. The method is sensitive to the study of relaxing heat capacity in supercooled liquids.

The oxidation of CH2O in the intermediate temperature range (943-995 K)

New atmospheric pressure flow reactor data on the oxidation of formaldehyde in the temperature range 943-995 K and over equivalence ratios from 0.013 to 36.7 are reported and discussed. A detailed mechanism assembled from previously published results produced acceptable agreement with the experimental data for the fuel-lean conditions, but failed to predict results for oxidative pyrolysis. Analysis or the very fuel-lean conditions, but failed to modelling results are principally sensitive to CH2O+HO2→HCO+H2O2 (6) and H2O2 +M→OH+OH+M (33). Whereas the specific rate of each reaction cannot be independently determined, it is found that the product k33.k6 is a well defined function of temperature: (3.4±3.0).1028 exp(-(26,800±400)/T). Inadequacies in the mechanism which may be responsible for the disagreement under fuel-rich conditions are discussed. © 1991 Combustion Institute.

Investigating the Mechanism and Electrode Kinetics of the Oxygen vertical bar Superoxide (O-2 vertical bar O-2(center dot-)) Couple in Various Room-Temperature Ionic Liquids at Gold and Platinum Electrodes in the Temperature Range 298-318 K

The reduction of oxygen was studied over a range of temperatures (298-318 K) in n-hexyltriethylammonium bis(trifluoromethanesulfonyl)imide, [N-6,N-2,N-2,N-2][NTf2], and 1-butyl-2,3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C(4)dmim][NTf2] on both gold and platinum microdisk electrodes, and the mechanism and electrode kinetics of the reaction investigated. Three different models were used to simulate the CVs, based on a simple electron transfer ('E'), an electron transfer coupled with a reversible homogeneous chemical step ('ECrev') and an electron transfer followed by adsorption of the reduction product ('EC(ads)'), and where appropriate, best fit parameters deduced, including the heterogeneous rate constant, formal electrode potential, transfer coefficient, and homogeneous rate constants for the ECrev mechanism, and adsorption/desorption rate constants for the EC(ads) mechanism. It was concluded from the good simulation fits on gold that a simple E process operates for the reduction of oxygen in [N-6,N-2,N-2,N-2][NTf2], and an ECrev process for [C(4)dmim][NTf2], with the chemical step involving the reversible formation of the O-2(center dot-)center dot center dot center dot [C(4)dmim](+) ion-pair. The E mechanism was found to loosely describe the reduction of oxygen in [N-6,N-2,N-2,N-2][NTf2] on platinum as the simulation fits were reasonable although not perfect, especially for the reverse wave. The electrochemical kinetics are slower on Pt, and observed broadening of the oxidation peak is likely due to the adsorption of superoxide on the electrode surface in a process more complex than simple Langmuirian. In [C(4)dmim][NTf2] the O-2(center dot-) predominantly ion-pairs with the solvent rather than adsorbs on the surface, and an ECrev quantitatively describes the reduction of oxygen on Pt also.

The Reduction of Oxygen in Various Room Temperature Ionic Liquids in the Temperature Range 293-318 K: Exploring the Applicability of the Stokes-Einstein Relationship in Room Temperature Ionic Liquids

The voltammetry for the reduction of oxygen at a microdisk electrode is reported in six commonly used RTILs: [C(4)mim][NTf2], [C(4)mpyrr][NTf2], [C(4)dmim][NTf2], [C(4)mim][BF4], [C(4)mim][PF6], and [N-6.2.2.2][NTf2], where [C(4)mim](+) is 1-butyl-3-methylimidazolium, [NTf2](-) is bis(trifluoromethanesulfonyl)imide, [C(4)mpyrr](+) is N-butyl-N-methylpyrrolidinium, [C(4)dmim](+) is 1-butyl-2,3-methylimidazolium, [BF4](-) is tetrafluoroborate, [PF6](-) is hexafluorophosphate, and [N-6.2.2.2](+) is n-hexyltriethylammonium at varying scan rates (50-4000 mV s(-1)) and temperatures (293-318 K). Diffusion coefficients, D, of oxygen are deduced at each temperature from potential-step chronoamperometry, and diffusional activation energies are calculated. Oxygen solubilities are also reported as a function of temperature. In the six ionic liquids, the Stokes-Einstein relationship (D proportional to eta(-1)) was found to apply only very approximately for oxygen. This is considered in relationship to the behavior of other diverse solutes in RTILs.

Thermal conductivities of ionic liquids over the temperature range from 293 K to 353 K

The thermal conductivities of 11 ionic liquids were determined, over the temperature range from 293 K to 353 K, at atmospheric pressure, using an apparatus based on the transient hot-wire method. For each of the ionic liquids studied, the thermal conductivities were found to be between (0.1 and 0.2) W.m(-1).K-1, with a slight decrease observed on increasing temperature. The uncertainty is estimated to be less than +/- 0.002 W.m(-1).K-1. In all cases, a linear equation was found to give a good fit to the data. The effects of water content and chloride content on the thermal conductivities of some of the ionic liquids were investigated. In each case, the thermal conductivities of the water + ionic liquid and chloride + ionic liquid binary mixtures were found to be less than the weighted average of the pure component thermal conductivities. This effect was adequately modeled using the Jamieson correlation. Chloride contamination at typical postsynthesis levels was found to have no significant effect on the thermal conductivities of the ionic liquid studied.

Nafion-based composite electrolytes for proton exchange membrane fuel cells operating above 120C with titania nanoparticles and nanotubes as fillers

FAPESP:5150

High-Temperature (1023 K to 1273 K 750 degrees C to 1000 degrees C]) Plastic Deformation Behavior of B-Modified Ti-6Al-4V Alloys: Temperature and Strain Rate Effects

A minor addition of B to the Ti-6Al-4V alloy, by similar to 0.1 wt pct, reduces its as-cast prior beta grain size by an order of magnitude, whereas higher B content leads to the presence of in situ formed TiB needles in significant amounts. An experimental investigation into the role played by these microstructural modifications on the high-temperature deformation behavior of Ti-6Al-4V-xB alloys, with x varying between 0 wt pct and 0.55 wt pct, was conducted. Uniaxial compression tests were performed in the temperature range of 1023 K to 1273 K (750 degrees C to 1000 degrees C) and in the strain rate range of 10(-3) to 10(+1) s(-1). True stress-true strain responses of all alloys exhibit flow softening at lower strain rates and oscillations at higher strain rates. The flow softening is aided by the occurrence of dynamic recrystallization through lath globularization in high temperature (1173 K to 1273 K 900 degrees C to 1000 degrees C]) and a lower strain rate (10(-2) to 10(-3) s(-1)) regime. The grain size refinement with the B addition to Ti64, despite being marked, had no significant effect on this. Oscillations in the flow curve at a higher strain rate (10(0) to 10(+1) s(-1)), however, are associated with microstructural instabilities such as bending of laths, breaking of lath boundaries, generation of cavities, and breakage of TiB needles. The presence of TiB needles affected the instability regime. Microstructural evidence suggests that the matrix cavitation is aided by the easy fracture of TiB needles.

Determination of the thermodynamic properties of xMgO + (1 x)MnO (s, sln) from a solid-electrolyte galvanic cell in the temperature range 1163 to 1318 K

The thermodynamic activities of MgO in the NaCl-type solid solutions which can exist in xMgO + (1 x)MnO have been determined in the temperature range 1163 to 1318 K from a solid-state galvanic cell incorporating MgF2 as the solid electrolyte. The activities of MnO have been calculated by a graphical Gibbs-Duhem integration method. The activities of both the components exhibit positive deviations from ideality over the entire composition range. The excess molar enthalpies are found to be positive. Further, xMgO + (1 - x)MnO does not conform to regular-solution behaviour. The origin of the excess thermodynamic properties is discussed in relation to the cationic size disparity and the crystal-field effects.

Dislocation mechanisms for plastic flow of nickel in the temperature range 4.2 – 1200 K

The temperature ranges of thermal and athermal deformation behaviour of nickel are identified by employing the temperature-dependence of flow-stress and strain-rate cycling data. The results are used to present a unified view of dislocation mechanisms of glide encompassing the two thermally activated and the intermediate athermal regimes of plastic flow.In the low-temperature thermally activated region (<250 K) the strain rate is found to be controlled by the repulsive intersection of glide and forest dislocations, in accordance with current ideas. The athermal stress in this region can be attributed mainly to the presence of strong attractive junctions which are overcome by means of Orowan bowing, a small contribution also coming from the elastic interactions between dislocations. The values of activation area and activation energy obtained in the high-temperature region (> 750 K) negate the operation of a diffusion-controlled mechanism. Instead, the data support a thermal activation model involving unzipping of the attractive junctions. The internal (long-range) stress contribution here results solely from the elastic interactions between dislocations. This view concerning the high-temperature plastic flow is further supported by the observation that the Cottrell–Stokes law is obeyed over large strains in the range 750–1200 K.

A ~ 32-70 K formation temperature range for the ice grains agglomerated by Comet 67P/Churyumov-Gerasimenko

Grand Canonical Monte Carlo simulations are used to reproduce the N₂/CO ratio ranging between 1.7 x 10⁻³ and 1.6 x 10⁻² observed in situ in the Jupiter-family comet 67 P/Churyumov-Gerasimenko (67 P) by the ROSINA mass spectrometer on board the Rosetta spacecraft. By assuming that this body has been agglomerated from clathrates in the protosolar nebula (PSN), simulations are developed using elaborated interatomic potentials for investigating the temperature dependence of the trapping within a multiple-guest clathrate formed from a gas mixture of CO and N₂ in proportions corresponding to those expected for the PSN. By assuming that 67 P agglomerated from clathrates, our calculations suggest the cometary grains must have been formed at temperatures ranging between ~ 31.8 and 69.9 K in the PSN to match the N₂/CO ratio measured by the ROSINA mass spectrometer. The presence of clathrates in Jupiter-family comets could then explain the potential N₂ depletion (factor of up to ~ 87 compared to the protosolar value) measured in 67 P/Churyumov-Gerasimenko.

Gold nanoresistors with near-constant resistivity in the cryogenic-to-room temperature range

Using a multiple plasma deposition-annealing (MDA) technique, we have fabricated an Au nanoisland-based thin film nanoresistor with a very low temperature coefficient of electrical resistivity in a cryogenic-to-room temperature range of 10 to 300 K. The nanoislanded gold film was deposited on a SiO2/Si wafer (500 nm SiO2 thickness) between two 300 nm thick Au electrodes which were separated by 100 m. A sophisticated selection of the thickness of the nanoislanded gold film, the annealing temperature, as well as the number of deposition/annealing cycles resulted in the fabrication of a nanoresistor with a temperature coefficient of electrical resistivity of 2.1 × 10-3 K-1 and the resistivity deviation not exceeding 2% in a cryogenic-to-room temperature range. We have found that the constant resistivity regime of the nanoisland-based thin film nanoresistor corresponds to a minimized nanoisland activation energy (approximately 0.3 meV). This energy can be minimized by reducing the nearest neighbor distance and increasing the size of the Au nanoislands in the optimized nanoresistor structure. It is shown that the constant resistivity nanoresistor operates in the regime where the thermally activated electron tunneling is compensated by the negative temperature dependence of the metallic-type conductivity of nanoislands. Our results are relevant to the development of commercially viable methods of nanoresistor production for various nanoelectronics-based devices. The proposed MDA technique also provides the opportunity to fabricate large arrays of metallic nanoparticles with controllable size, shapes and inter-nanoparticle gaps.