Rab Rens Zijlmans

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...

Molecule synthesis in an Ar-CH4-O2-N2 microwave plasma

The formation of new molecules in a microwave plasma, created from a mixture of Ar, CH4, N2 and O2, is investigated by means of an in-depth study of the molecular abundance in the plasma. The molecules are detected by means of tunable diode laser absorption spectroscopy and by absolute mass spectrometry. Three groups of molecules can be discerned in terms of molecular abundance: CO is predominantly formed, together with H2O, N2 and H2. The molecules CH4 and O2 are significantly depleted, but still abundant in a finite quantity. The third group is formed by several other species like NH3, NO, HCN etc. This tendency is expected to occur in every low temperature plasma containing C, O, H and N atoms. Furthermore, the combination of both techniques also allows us to make a clear distinction between the etching mode and deposition mode of the microwave reactor. Etching mainly occurs when the ratio of admixed gas flows Φ(O2)/Φ(CH4) > 0.5.

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.

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.

Dissociative recombination as primary dissociation channel in plasma chemistry

Molecule formation, surface modification and deposition in plasmas can in first order be described as dissociation in the plasma and association of fragments at the surface. In active plasmas ionization and dissociation by electrons is accompanied by excitation. But besides these direct electron processes also a second dissociation channel is active: that by charge transfer followed by dissociative recombination. This latter route is the dominant one in the colder recombining phase of the plasma. Atomic and molecular radicals diffuse or flow to the surface, where new molecules are formed. As a result the original molecules are, after being dissociated in the plasma, converted at the surface to new simple molecules, as H2, CO, N2, H2O, O2, NO, NH3, HCN, C2H2, CH4, to name a few in C/H/O/N containing plasmas. There is evidence that the molecular fragments resulting from dissociative recombination are ro-vibrationally (and possible electronically) excited. Also the molecules resulting from association at the surface may be ro-vibrationally or electronically excited. This may facilitate follow up processes as negative ion formation by dissociative attachment. These negative ions will be lost by mutual recombination with positive ions, giving again excited fragments. Rotational or other excitation may change considerably plasma chemistry.

Modelling of a pinched cascaded arc light source using argon

In this work, we study a cascaded arc in argon that is used as a broadband light source for spectroscopic applications. In this arc, the arc channel is geometrically constricted. A numerical model is used to investigate the plasma parameters and light output of the arc. It is found that constricting leads to a higher electron density in the constricted area, which strongly enhances the local broadband emission of the plasma. A parameter study, in which the current is varied, is performed. The simulated arc voltages are compared with measured arc voltages, and excellent agreement is found. Furthermore, it is found that the emissivity increases strongly for increasing current, making the current a suitable control parameter to control the light output of the arc.

A new design of a bright cascaded arc light source: measurements and simulations

Profiling the arc channel of a cascaded arc into an hourglass-like shape results in an extremely high light output intensity (3.5 × 1012 Wm−2 m−1 sr−1 in the near-infrared) at an input power of 2.2 kW, a good stability (better than 5 × 10−4) and a small solid angle of the emitted light beam (Ω = 0.015 sr). These optical properties are advantageous for spectroscopic applications that require a high light intensity over a broad wavelength range in a small solid angle. The absolute light output is measured and is compared with a fast and flexible modelling scheme. These simulations are used to verify the arc performance and to guide arc design. The calculated emitted radiance at moderate arc input power agrees with the experiments within 30% for moderate input power.