Parametric study of the decomposition of NH3 for an induction plasma reactor design

The present paper reports a study of the gas mixing and chemical transformation in an induction plasma reactor under atmospheric pressure, and its dependence on the plasma operating conditions. For this purpose, the thermal dissociation of ammonia into nitrogen and hydrogen was chosen because of the relative simplicity of the reactions involved and its use in a number of studies on plasma synthesis of ultrafine nitride ceramic powders using ammonia as nitriding agent. A hot-wall reactor configuration is investigated in which ammonia is injected radially through multiple orifices into the gases at the exit nozzle of an induction plasma torch. Concentration mapping in the mixing zone was carried out, using a VG-Micromass-PC 300 D quadrupole mass spectrometer, for different plasma power levels, in the range 13–24 kW. A 3-point injection mode is used with the injection ports oriented upstream at 45° to the torch axis. This allows uniform mixing of the injected gas in the plasma jet. A systematic study of the effects of plate power and ammonia and plasma gas flow rates on the mixing and dissociation of NH₃ in the reactor is reported. The results are analyzed and discussed from the viewpoint of their use for optimizing the design of induction plasma reactors, to he applied to the vapor-phase synthesis of ultrafine silicon nitride powders.     

Graph-based machine learning interprets and predicts diagnostic isomer-selective ion–molecule reactions in tandem mass spectrometry

Diagnostic ion–molecule reactions employed in tandem mass spectrometry experiments can frequently be used to differentiate between isomeric compounds unlike the popular collision-activated dissociation methodology. Selected neutral reagents, such as 2-methoxypropene (MOP), are introduced into an ion trap mass spectrometer where they react with protonated analytes to yield product ions that are diagnostic for the functional groups present in the analytes. However, the understanding and interpretation of the mass spectra obtained can be challenging and time-consuming. Here, we introduce the first bootstrapped decision tree model trained on 36 known ion–molecule reactions with MOP. It uses the graph-based connectivity of analytes'' functional groups as input to predict whether the protonated analyte will undergo a diagnostic reaction with MOP. A Cohen kappa statistic of 0.70 was achieved with a blind test set, suggesting substantial inter-model reliability on limited training data. Prospective diagnostic product predictions were experimentally tested for 13 previously unpublished analytes. We introduce chemical reactivity flowcharts to facilitate chemical interpretation of the decisions made by the machine learning method that will be useful to understand and interpret the mass spectra for chemical reactivity.

We combine mass spectrometry with machine learning that is predictive and explainable using chemical reactivity flowcharts for diagnostic ion–molecule reactions.     

Effects of process parameters on ultrafine SiC synthesis using induction plasmas

A study is reported of the formation of ultrafine SiC powder through the reaction of elemental silicon and CH₄ in an induction plasma. The reaction route used involved in the first place the vaporization of a fine elemental silicon powder axially injected into the center of the discharge followed by the carburization reaction through the coinjection of CH₄. The powder obtained was composed of a mixture of α- and β-SiC with varying amounts of free carbon and free silicon. The particle size distribution was typically in the range of 40–60 nm with a corresponding specific surface area of 30–50 m²/g. A parametric study showed that the quality of the powder obtained varied with the plasma plate power and the position of the injection probe. The plasma gas composition employed was found to influence the proportions of α- and β-SiC in the synthesized SiC powder. With an Ar/N₂ mixture as the plasma gas, the ratio of the α to β phases was less than 1.0, whereas the ratio was greater than 1.5 when using a mixture of Ar/H₂ as plasma gas. The Si powder feed rate and the input C/Si molar ratio in the injected reactants significantly affected both the formation of the SiC and the free Si and free C content in the synthesized powder. Lining the cylindrical reactor wall with graphite resulted in improved conversion of Si to SiC. The weight fraction of the powder collected at different sections of the reactor system varied with the reactor operating conditions. The experimental results support the view that the formation mechanism for ultrafine SiC is dominated by the reaction of Si vapor with the thermal decomposition products of CH₄.     

Heating of powders in an r.f. inductively coupled plasma under dense loading conditions

A study was carried out of the heating of powders in an r.f. inductively coupled plasma under dense loading conditions. The results obtained using a mathematical model taking into account plasma-particle interaction effects reveal an important cooling of the plasma caused by the presence of the particles. This, in turn, gave rise to a corresponding drop of the efficiency of the melting of the particles in the plasma. The effect is shown to depend strongly on the thermodynamic properties of the material of the powder.     

Mixing study of the induction plasma reactor: Part II. Radial injection mode

A study was undertaken on the mixing pattern in am induction plasma torch and reactor system. The results presented in this part of the paper relate to the radial injection mode, in which an auxiliary gas is injected into the main plasma stream through a set of 2, 4, or 8 injection ports located in the torch nozzle at the level of the torch exit flange. A much faster mixing of the gases occurred in this mode compared to the axial injection mode investigated in Part I of this paper. As in the case of axial injection, the present study demonstrates that gas mixing, in the presence of the discharge, is considerably more difficult than under ambient temperature conditions. Lower turbulence levels exist in the plasma reactor. due to the considerably higher viscosity of the gases under plasma conditions. Results obtained with a three-point injection flange, in which the injection ports were oriented at 45° to the torch and reactor axes toward the upstream .side, mere most interesting since they achieved essentially the same degree of mixing as was obtained with the radial injection ports without the need to locate the injection ports at the exit nozzle of the plasma torch. This arrangement provides for added flexibility in reactor design.