Stephan Hamann

Infrared spectroscopic and modeling studies of H2/CH4 microwave plasma gas phase from low to high pressure and power

InfraRed Tunable Diode Laser Absorption Spectroscopy technique has been implemented in a H2/CH4 Micro-Wave (MW frequency f = 2.45 GHz) plasma reactor dedicated to diamond deposition under high pressure and high power conditions. Parametric studies such as a function of MW power, pressure, and admixtures of methane have been carried out on a wide range of experimental conditions: the pressure up to 270 mbar and the MW power up to 4 kW. These conditions allow high purity Chemical Vapor Deposition diamond deposition at high growth rates. Line integrated absorption measurements have been performed in order to monitor hydrocarbon species, i.e., CH3, CH4, C2H2, C2H4, and C2H6. The densities of the stable detected species were found to vary in the range of 1012–1017 molecules cm−3, while the methyl radical CH3 (precursor of diamond growth under these conditions) measured into the plasma bulk was found up to 1014 molecules cm−3. The experimental densities have been compared to those provided by 1D-radial thermoch...

Spectroscopic study of low pressure, low temperature H2–CH4–CO2 microwave plasmas used for large area deposition of nanocrystalline diamond films. Part I: on temperature determination and energetic aspects

In a distributed antenna array (DAA) reactor, microwave H2 plasmas with admixtures of 2.5% CH4 and 1% CO2 used for the deposition of nanocrystalline diamond films have been studied by infrared absorption and optical emission spectroscopy (OES) techniques. The experiments were carried out in order to analyze the dependence of plasma chemical phenomena on power and pressure at relatively low pressures, up to 0.55 mbar, and power values, up to 3 kW. The evolution of the concentration of the methyl radical, CH3, and of five stable molecules, CH4, CO2, CO, C2H2 and C2H6, was monitored in the plasma processes by in situ infrared laser absorption spectroscopy using lead salt diode lasers (TDL) and external-cavity quantum cascade lasers (EC-QCL) as radiation sources. OES was applied simultaneously to obtain complementary information about the degree of dissociation of the H2 precursor gas and of its gas temperature. The experimental results are presented in two separate parts. In Part I, the present paper, the measurement of the gas (T gas), rotational (T rot) and vibrational (T vib) temperatures of the various species in the complex plasma was the main focus of interest. To achieve reliable values for the gas temperature inside and outside the plasma bulk as well as for the rotational and vibrational temperatures in the plasma hot zones, which are of great importance for calculation of species concentrations, five different methods based on the emission and absorption spectroscopy data of H2, CH4, CH3 and CO have been used. In these, line profile analysis has been combined with Boltzmann plot methods. Based on the wide tuning range of the EC-QCL, a variety of CO lines in the ground and three excited states was measured enabling extensive temperature analysis providing new insight into the energetic aspects of this multi-component plasma. Depending on the different plasma zones the gas temperature was found to range between about 360 and 1000 K inside the DAA reactor. In Part II, based on detailed concentration measurements general plasma chemical aspects will be analyzed and discussed.

Spectroscopic study of low pressure, low temperature H2–CH4–CO2 microwave plasmas used for large area deposition of nanocrystalline diamond films. Part II: on plasma chemical processes

In a distributed antenna array (DAA) reactor, microwave H2 plasmas with admixtures of 2.5% CH4 and 1% CO2 used for the deposition of nanocrystalline diamond films have been studied by infrared laser absorption and optical emission spectroscopy (OES) techniques. The experiments were carried out in order to analyze the dependence of plasma chemical phenomena on power and pressure at relatively low pressures, up to 0.55 mbar, and power values, up to 3 kW. The evolution of the concentration of the methyl radical, CH3, of five stable molecules, CH4, CO2, CO, C2H2 and C2H6, and of vibrationally excited CO in the first and second hot band was monitored in the plasma processes by in situ infrared laser absorption spectroscopy using tunable lead salt diode lasers (TDL) and an external-cavity quantum cascade laser (EC-QCL) as radiation sources. OES was applied simultaneously to obtain complementary information about the degree of dissociation of the H2 precursor and of its gas temperature. The experimental results are presented in two separate parts. In Part I, the first paper in a two-part series, the measurement of the gas (T gas), rotational (T rot) and vibrational (T vib) temperatures of the various species in the complex plasma was the main focus of interest. Depending on the different plasma zones the gas temperature was found to range between about 360 and 1000 K inside the DAA reactor (Nave et al 2016 Plasma Sources Sci. Technol. 25 065002). In Part II, the present paper, taking into account the temperatures determined in the first paper, the concentrations of the various species, which were found to be in a range between 1011 and 1015 cm−3, are the focus of interest. The influence of the discharge parameters power and pressure on the molecular concentrations has been studied. To achieve further insight into general plasma chemical aspects the dissociation of the carbon precursor gases including their fragmentation and conversion to the reaction products has been analyzed in detail.

On the influence of carbon contamination of reactor parts in active screen plasma nitrocarburizing processes

Compared to conventional plasma nitrocarburizing the active screen plasma nitrocarburizing technology reduces significantly the risk of soot production and cementite precipitation in the compound layer of nitrocarburized materials. However, in long-time treatments, the uncontrollable contamination even up to full saturation of the metallic active screen and of the walls of the used plasma reactor with carbon compounds is still a remaining factor of uncertainty. It may result in an increased carbon concentration or even in an appearance of the cementite phase in the compound layer of the treated steel surface, as in the case of 42CrMo4 (AISI 4140). The absolute concentration of hydrogen cyanide (HCN), in situ monitored by tunable diode laser absorption spectroscopy, and the emission intensity of the CN (0–0) band of the violet system at a wavelength of λ = 388 nm, recorded by optical emission spectroscopy, were found to be reliable parameters to control the carburizing potential of the nitrocarburizing pla...

Analysis of the Product Gas Composition in Pyrolysis Processes of Single Wood Particles Using FTIR Spectroscopy

Physo-chemical reactions and transport phenomena occurring during pyrolysis of wood chips have not yet been understood to the required extent for its effective utilization for power generation. FTIR spectroscopy has been applied time-resolved to analyse the product gas composition of a single pyrolysing wood particle. A special particle reaction chamber has been built to allow for an in situ measurement of the product gas in close vicinity of the particle surface.