1,721,053 research outputs found
Inorganic Nanostructured Materials for Technological Applications
Full text linkFrancesco Antonio Gianturco, Aldo Domenicano, Mariano Venanz
New Hard and Superhard Materials: RhB1.1 and IrB1.35
No full textThe research activity into the designing of superhard materials is in continuous progress. Vickers hardness data obtained in this work reveal IrB1.35 to be superhard, whereas RhB1.1 was found to be hard. The trend in hardness of borides of the transition metals, belonging to the 5th and 6th period of the periodic table, was examd
High-temperature resistivity of aluminum–carbon nanotube composites
No full textThe resistivity and thermal coefficient of resistivity (TCR), of metallic matrix composites, MMCs, aluminum–carbon nanotube, Al-CNT, were studied under
high vacuum in the temperature interval from RT to 800 K. The samples shaped as small cylinders and containing single-walled CNTs or multi-walled CNTs were sintered at 625 C. The resistivity of sintered samples of pure Al was found three orders of magnitude higher with respect to bulk, having the former a density value equal to 98.8 % of bulk Al. The explored range of the CNT concentration was within 5 wt%. At the highest CNT concentrations, the trend of resistivity against temperature was found negative being more pronounced for composites with MWCNTs. For Al-SWCNT composites, at around 3.3 wt%(4.2 vol%), TCR is practically independent from temperature; for Al-MWCNT, the TCR zero-crossing occurs at different compositions depending on temperature. Higher is the temperature, lower is the TCR zero-crossing composition. Resistivity data were discussed in the framework of the Matthiessen’s rule and sound evidences were shown that no Al4C3 formation was detected at working temperatures
Chemical differentiation of carbon nanotubes in a carbonaceous matrix
No full textTo determine the purity of carbon nanotubes (CNTs), the effectiveness of some inorganic substances to catalyze the Boudouard reaction, investigated by thermogravimetric experiments, was studied. The performance, in terms of ability to separate the feature of CNTs from the features of other components, of various catalyst powders (NiO, Rh, Cr2O3), thoroughly mixed with carbonaceous simulated mixtures, was checked under a pure CO2 stream and compared without catalyst in the same operating conditions. The sequential removal of carbon up to 1200 degrees C by pure CO2 from a real sample of MWCNTs was characterized by TEM, XRD, and micro-Raman spectroscopy the carbonaceous fraction residues at each stopping temperature. Some evidence of the poor reliability of Raman spectroscopy in detecting the CNT purity are discussed. The procedure to perform thermogravimetry of simulated mixtures under pure CO2 at different weight compositions constituted by active carbon, MWCNTs, graphite, and Cr2O3 as catalyst was tested. The average relative error between the expected and found weight contents was within +/- 6%. The same procedure was also adopted to solve a simulated mixture of two kinds of MWCNTs having external diameter in a ratio of similar to 1:10 and a very different number of walls
Hydrogen evolution reaction (HER) at thin film and bulk TiC electrodes
No full textThe electrochemical behavior of titanium carbide in the form of sintered rod and thin film was investigated, in relation to the hydrogen evolution reaction (HER), in acidic and alkaline medium. The exchange current. density, io, and the charge transfer coefficient, oc, were measured for the two types of cathode. In the case of sintered TiC, the io values vary from (2.7 +/- 0.4) x 10(-6) (as measured in alkaline medium) to (3.5 +/- 0.2) x 10(-3) A m(-2) (as measured in acidic medium). This latter value (together with the a value) reveals that TiC cathodes are even more efficient than Pd in acid medium. Both types of TiC cathode are very stable in acid medium, also for prolonged electrolysis times. The study of the role played by the electroactive species concentration (i.e., [H3O+] or [OH-]), reveals that the sintered TiC cathode efficiency is higher at low acid concentration or in the presence of a supporting electrolyte. The minimum electric power necessary to drive the HER at a given rate is here evaluated. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved
Experimental Thermodynamics of High Temperature Transformations in Single-Walled Carbon Nanotube Bundles
No full textThe thermodynamic quantities associated to the transformation of carbon in single-walled carbon nanotube (SWCNT) bundles to carbon in graphite were determined from 750 to 1015 K by a CaF2 solid electrolyte galvanic cell: (-) Mo | Cr3C2,CrF2,C '' | CaF(2)s.c. | Cr3C2,CrF2,C' | Mo (+). The trend with temperature of the electromotive force of the cell was found to be greatly dependent on temperature and fully reversible with it. The standard enthalpy Delta H degrees(T) and entropy Delta S degrees(T) changes are 7.1, 6.0, and 60.2 kJ mol(-1) and 8.6, 14.7, and 72.8 J K-1 mol(-1) at 778, 883, and 975 K, respectively. This most likely correlates with the different arrangements and shapes of tubes that deviate from the ideal triangular closed packed structure of the SWCNT bundles. The constraints,to the thermal expansion of bundles in the electrode containing them generated high internal pressures that were responsible for deformations of tube shape and lattice. Stable bundle states were formed that interconvert as a function of temperature. Comparative analyses by low angle XRD, microRaman, and HR-TEM of SWCNT bundles before and after experiments support this scenario. The cohesion energy and associated entropy changes are also reported for such states. The formation enthalpy of unbundled SWCNTs was calculated equal to 9.5 +/- 0.4 KJ mol(-1)
Thermodynamics of Fe-Rich intermetallics along the rare earth series
No full textA set of thermodynamic properties of the Fe-rich part of the Fe/rare earth (RE) systems is presented for almost all the elements in the rare earth series. This set of data comes entirely from electromotive force (emf) vs T experimental measurements obtained by galvanic cells with a CaF2 single crystal as the electrolyte. The standard enthalpy and entropy of formation of RE2Fe17 intermetallics have been obtained and compared with previous results found for the RE2Ni17 intermetallics. The enthalpy of formation of RE2Fe17 intermetallics is decidedly less exothermic than the enthalpy of formation of RE2Ni17 with the exception of the value of Pr2Fe17, which is endothermic. The RE2Fe17 entropy of formation is always positive contrary to the values of RE2Ni17. For comparison purposes, the thermodynamic data of Y2Fe17 and Dy2CO17 have also been determined. The RE solubility in Fe has been evaluated by the shift of the bcc Fe(110) plane spacing with respect to pure Fe as shown by corresponding X-ray diffraction (XRD) data. Along the RE series, the atomic fraction of RE in the Fe solid solution changes from (1.2 +/- 0.1)center dot 10(-4) for Er to (9.2 +/- 0.2)center dot 10(-4) for Ho. The values of the thermodynamic activity of RE coexisting between the RE2Fe17-rich phase and the Fe solid solution are reported along the series, and the related partial excess free energy is given as electronic and dilatation contributions. The electronic partial excess free energy of RE2Fe17 intermetallics is practically independent of the dilatation term contrary to the RE2Ni17 intermetallics
The TiC/TiO2 interface
No full textThe structural and chem. characterization of the TiC/rutile interface that grows in the high temp. and low oxygen partial pressure oxidn. of TiC single crystals is reported. Starting from previous kinetic data, microRaman, and AES-SEM profiles of the TiC/rutile interface cross-section, a Ti oxycarbide phase was expected to form at the TiC/rutile interface. To be consistent with the above mentioned data, Ti oxycarbide should form by partial substitution of C with O without producing any change in the rock-salt structure of TiC except small changes in the lattice parameter. The O-substitution in the C sublattice of TiC was also simulated by synthesizing Ti oxycarbide with electron gun heating of a TiC and TiO2 mixt. The final product was identified by chem. anal. and XRD as Ti1.0±0.1C0.49±0.01O0.4±0.1 having a XRD spectrum almost identical to TiC with a relative change of the lattice parameter equal to -0.8%. This result confirms the reason why no other signals other than the signals of amorphous carbon and rutile appear in the microRaman profiles, and the reliability of the oxidn. mechanism elsewhere proposed
- …
