Pascal Ifeacho

Core and grain boundary sensitivity of tungsten-oxide sensor devices by molecular beam assisted particle deposition

In this study, we investigate the synthesis of WO3 and WOx (2.6≥x≤2.8) by adding different concentrations of tungsten hexafluoride (WF6) into a H2/O2/Ar premixed flame within a low-pressure reactor equipped with a particle-mass spectrometer (PMS). The PMS results show that mean particle diameters dp between 5 and 9 nm of the as-synthesized metal-oxides can be obtained by varying the residence time and precursor concentration in the reactor. This result is further validated by N2 adsorption measurements on the particle surface, which yielded a 91 m2/g surface area, corresponding to a spherical particle diameter of 9 nm (Brunauer-Emmett-Teller technique). H2/O2 ratios of 1.6 and 0.63 are selected to influence the stoichiometry of the powders, resulting in blue-colored WOx and white WO3 respectively. X-ray diffraction (XRD) analysis of the as-synthesized materials indicates that the powders are mostly amorphous, and the observed broad reflexes can be attributed to the orthorhombic structure of β-WO3. Thermal...

Vibrational and defect states in SnOx nanoparticles

We have studied SnOx nanoparticles fabricated by gas-phase condensation and in-flight sintering using Raman and photoluminescence (PL) spectroscopy. We are able to identify various vibrational states of the rutile phase of the SnOx crystal. By thorough analysis of the vibrational modes, we are able to determine the bond lengths of the O–O and Sn–O bonds for the substoichiometric SnO1.5, leading, together with x-ray diffraction data, to a full characterization of the SnO1.5 lattice. In absorption and photoluminescence spectra, we observe a finite density of states inside the band gap due to oxygen vacancies, giving rise to a midgap luminescence peak. Our results suggest that the defect related luminescence efficiency is limited by nonradiative recombination processes and by the oxygen vacancy density. We therefore conclude that the PL intensity has a maximum around a stoichiometry of SnO1.7.

Morphology, structure and electrical properties of iron nanochains

Iron nanoparticles with a diameter of 35 nm are synthesized by the thermal decomposition of iron pentacarbonyl in a hot wall reactor. The particles are ferromagnetic and single magnetic domain. The magnetic forces between the particles cause the formation of iron nanowires of up to 300 µm in length. Morphological characterization using high resolution transmission electron microscopy indicates an oxidic shell with a thickness of 3–4 nm. The electrical properties of single chains are investigated under air using impedance spectroscopy (IS). It is found that the conductivity of the iron is in the same range as that of the bulk material. Temperature dependent measurements on iron nanochain powders performed under H2-atmosphere reveal strong sintering effects and a transition into iron.

Injection of hydrogen peroxide into the combustion chamber of diesel engine : Effects on the exhaust gas behaviour

The improvement of the exhaust gas contents of diesel engines with respect to particles and gaseous emissions is still a very important engineering topic. In this work two different direct injection methods into the combustion chamber of a diesel engine have been investigated and compared. The first system involves the separate injection of H 2 O 2 /water solutions using a second nozzle, while the other is the injection of H 2 O 2 /water/diesel emulsions by a single injection system. It is known that H 2 O 2 forms OH radicals at higher temperatures, whose probability for the oxidation of diesel soot is much higher than that of molecular oxygen [1]. The H 2 O 2 injection into the combustion chamber [2] and the exhaust pipe of diesel engines [3] has proved to be an effective measure towards the reduction of diesel soot. A Differential Mobility Analyzer (DMA) is applied to detect the change in particle diameter as function of H 2 O 2 /water addition. The injected H 2 O 2 concentration in relation to injected diesel fuel was varied from 1 to 10 %, and the injection pressure was also varied between 200 and 500 bars. By constant injection pressure and injection timing, it was found that H 2 O 2 /water/diesel emulsion leads to a higher reduction of unburned hydrocarbons and particulates. Correspondingly, an increase in the NO x emission is observed in comparison to separate injection of H 2 O 2 /water solution. The highest reduction of diesel particles was observed at higher injection pressures, and during delay injection timing of H 2 O 2 /water.