van Jh Jean-Pierre Helden

Detailed study of the plasma-activated catalytic generation of ammonia in N2-H2 plasmas

We investigated the efficiency and formation mechanism of ammonia generation in recombining plasmas generated from mixtures of N2 and H2 under various plasma conditions. In contrast to the Haber-Bosch process, in which the molecules are dissociated on a catalytic surface, under these plasma conditions the precursor molecules, N2 and H2, are already dissociated in the gas phase. Surfaces are thus exposed to large fluxes of atomic N and H radicals. The ammonia production turns out to be strongly dependent on the fluxes of atomic N and H radicals to the surface. By optimizing the atomic N and H fluxes to the surface using an atomic nitrogen and hydrogen source ammonia can be formed efficiently, i.e., more than 10% of the total background pressure is measured to be ammonia. The results obtained show a strong similarity with results reported in literature, which were explained by the production of ammonia at the surface by stepwise addition reactions between adsorbed nitrogen and hydrogen containing radicals a...

Density and production of NH and NH2 in an Ar–NH3 expanding plasma jet

The densities of NH and NH2 radicals in an Ar–NH3 plasma jet created by the expanding thermal plasma source were investigated for various source-operating conditions such as plasma current and NH3 flow. The radicals were measured by cavity ringdown absorption spectroscopy using the (0,0) band of the AΠ3←XΣ−3 transition for NH and the (0,9,0)-(0,0,0) band of the AA12←XB12 transition for NH2. For NH, a kinetic gas temperature and rotational temperature of 1750±100 and 1920±100K were found, respectively. The measurements revealed typical densities of 2.5×1012cm−3 for the NH radical and 3.5×1012cm−3 for the NH2 radical. From the combination of the data with ion density and NH3 consumption measurements in the plasma as well as from a simple one-dimensional plug down model, the key production reactions for NH and NH2 are discussed.The densities of NH and NH2 radicals in an Ar–NH3 plasma jet created by the expanding thermal plasma source were investigated for various source-operating conditions such as plasma current and NH3 flow. The radicals were measured by cavity ringdown absorption spectroscopy using the (0,0) band of the AΠ3←XΣ−3 transition for NH and the (0,9,0)-(0,0,0) band of the AA12←XB12 transition for NH2. For NH, a kinetic gas temperature and rotational temperature of 1750±100 and 1920±100K were found, respectively. The measurements revealed typical densities of 2.5×1012cm−3 for the NH radical and 3.5×1012cm−3 for the NH2 radical. From the combination of the data with ion density and NH3 consumption measurements in the plasma as well as from a simple one-dimensional plug down model, the key production reactions for NH and NH2 are discussed.

Bulk and surface defects in a-Si:H films studied by means of the cavity ring down absorption technique

Abstract The sub gap absorption at 1.17 eV in hydrogenated amorphous silicon (a-Si:H) has been measured by means of the ex situ cavity ring down (CRD) absorption technique. The evolution of defects has been studied as a function of film thickness and deposition temperatures. A comparison with the dual beam photoconductivity (DBP) technique shows that the CRD results are systematically higher. Furthermore, it is shown that the CRD technique is sensitive to surface defects and typical surface dangling bond coverages of 2×10−4 up to 6×10−3 are obtained for air-exposed a-Si:H samples depending on the deposition temperature.

Resemblance in gas composition of Ar–N2–O2 plasmas and Ar–NO plasmas

We measured the steady-state gas composition of plasmas produced from Ar?N2?O2 mixtures and Ar?NO mixtures with quantitative mass spectrometry. In the former, mainly N2 and O2, but also a significant amount of nitric oxide (NO) was formed, i.e. up to 5% of the background gas was NO. In the inverse experiment, in which NO was admixed to an argon plasma, up to 92% of the NO was converted into N2 and O2. The observed molecules are mostly generated in wall association processes but also by gas phase reactions between N atoms and O2 molecules leading to NO. The two types of plasmas show a strong mutual resemblance in the steady-state gas composition if substantial dissociation can be reached in the residence time of the gases in the plasma, i.e. ?5% NO and ?95% N2 and O2, although the starting conditions are completely different. It seems that in first order the system prefers to produce the most thermodynamically stable molecules.

Downstream ion and radical densities in an Ar-NH3 plasma generated by the expanding thermal plasma technique

A full characterization of the Ar–NH3 expanding thermal plasma source applied in industrial processes for high-rate silicon nitride deposition is presented in terms of absolute densities of reactive species such as ions and radicals. The ion composition of the plasma was identified by mass spectrometry, while absolute ion density information was obtained by Langmuir probe measurements. N radicals were detected by threshold ionization mass spectrometry, whereas NH and NH2 radicals were measured by cavity ringdown spectroscopy. It was found that the ion density decreases from 1013 cm−3 in a pure Ar plasma to 1010–1011 cm−3 when NH3 is injected, with ArH+, , , and being the most abundant ions. The densities of N and NH both saturate at a level of 3 × 1012 cm−3 at NH3 flows above 3 sccs while the density of NH2 increases linearly with the NH3 flow and reaches a level of 4 × 1012 cm−3. When the plasma source current is increased, the densities of N and NH increase linearly, while the NH2 density remains approximately constant. Furthermore, it is revealed that most of the consumed NH3 is converted into N2 and H2 in the plasma.

Experimental study of surface contributions to molecule formation in a recombining N2/O2 plasma

A low pressure recombining Ar plasma to which mixtures of N2 and O2 were added has been studied to explore the relevance of surface related processes for the total chemistry. The abundances of the stable molecules N2, O2, NO, N2O and NO2 have been measured by means of a combination of infrared tunable diode laser absorption spectroscopy and mass spectrometry.A gas phase chemical kinetics model was developed in CHEMKIN to investigate the contribution of homogeneous interactions to the conversion of the feedstock gases N2 and O2. At a partial pressure of N2 plus O2 less than 8 Pa, significant discrepancies between measured and calculated concentrations of N2O and NO2 were observed, indicating that heterogeneous processes are dominating the chemistry in this pressure range. At a partial pressure of N2 plus O2 higher than 40 Pa and a relatively high fraction of admixed O2 we observed a fair agreement between measured and calculated concentrations of NO molecules, indicating that homogeneous processes (notably N atom induced) are more dominant than heterogeneous processes.

Molecule formation in N and O containing plasmas

The visual appearance of an expanding nitrogen plasma with or without oxygen is shown. The interaction of the plasma with a substrate leads to the appearance of additional light, which is ascribed to the formation of excited molecules by association of N and/or O atoms at the substrate.