Ulf Bergmann

An experimental study of the reactions of trimethylgallium with ammonia and water over a wide temperature range

The gas phase reaction of trimethylgallium (TMG) with ammonia was investigated because of its importance in the GaN chemical vapour deposition process. Water is the most important impurity in ammonia and therefore its reaction with TMG was investigated as a source of oxygen impurities in GaN films. Gas phase intermediates were studied in a flow tube reactor in the temperature range 294 to 1000 K by molecular beam sampling and mass spectrometric detection. Two ionisation methods were applied: VUV radiation at 118 nm and electron ionisation at 20 eV. The ionisation pattern of TMG was studied, and the results were used in the interpretation of the TMG–ammonia system. The reaction of deuterated ammonia with TMG was used to evaluate the sum formula of the detected compounds. In the TMG–ammonia system, two signal groups were found mainly at low temperatures, indicating TMG–ammonia complexes (TMG:NH3, TMG(NH3)2). Also, species with two gallium atoms and nitrogen were detected in an extended temperature range and interpreted to be fragments of (CH3)4Ga2NH, and (CH3)4Ga2(NH2)2. Higher mass species were not found. In the reaction between TMG and water, several signals of volatile reaction products were detected up to 670 K. Some of them are consistent with a trimeric dimethylgallium hydroxide, but other compounds might also be present. From the temperature dependence, strategies for the reduction of oxygen in GaN films can be worked out.

Growth of Thin Iron Oxide Films on Si ( 100 ) by MOCVD

The deposition of thin iron oxide films on Si(100) by metallorganic chemical vapor deposition at 55 mbar was systematically studied as a function of temperature between 673 and 1023 K. Ferrocene and oxygen were used as precursors. The growth rate was measured as a function of temperature and the films were characterized by X-ray diffraction (XRD), Auger electron spectroscopy, energy dispersive X-ray analysis, and scanning electron microscopy. The change from the kinetically controlled regime to the transport controlled regime occurs near 750 K. At similar temperatures, a phase change of the deposited material was observed. Films prepared at temperatures higher than 823 K show the structure of α-Fe 2 O 3 , whereas deposition at lower temperature leads to the growth of α-Fe 2 O 3 and other oxide phases. The XRD pattern of these films can be explained by the coexistence of different iron oxide phases, namely α-Fe 2 O 3 , γ-Fe 2 O 3 , and/or β-Fe 2 O 3 .

Partial Oxidation of Methane at Elevated Pressures and Effects of Propene and Ethane as Additive: Experiment and Simulation

Abstract Partial homogeneous oxidation of methane (CH4) within stationary engines may be one concept for conversion of available energy to alternatively mechanical energy, heat, and additional useful chemicals like syngas (CO/H2), formaldehyde (CH2O), methanol (CH3OH) or hydrocarbons (e.g. C2H4). The present study investigates the formation reactions of chemicals experimentally and theoretically. Methane oxidation is studied under fuel-rich conditions (ϕ = 17.50–22.25) at high pressures (6 bar) and high temperatures (T  max  = 1030 K) for long residence times in a tubular reactor. The gas composition is determined experimentally by time-of-flight mass spectrometry for different reactor temperatures. Through variation of reactor temperature an overview of the maximum mole fractions of target chemicals, the temperature of observed reaction onset, and the optimal temperature to increase target yields can be determined. The experimental results are compared to kinetic simulations of the methane conversion using literature mechanisms to assess how well the data are reproduced for these uncommon reaction conditions. The potential of activating the conversion reactions with ethane (C2H6) and propene (C3H6) as additives is investigated. Methanol is chosen as one target compound. Its yield is increased by both additives. In addition, propene as additive reduces the temperature of reaction onset in the experiments and in the simulation. CH2O and C2H4 can be identified as other useful chemicals produced in the experiments and the influence of the additives on the yields is discussed.

Experimental and Numerical Investigations of Ferrocene-Doped Propene Flames

Abstract Ferrocene-doped low-pressure, laminar, premixed propene/oxygen/argon flames of different stoichiometries were investigated experimentally using laser induced fluorescence (LIF) and the results were compared with numerical simulations. The influence of ferrocene on the flame temperature proved to be minimal both in the measurement and in the simulation. The measured flame temperatures were 100–250 K below the adiabatic flame temperature, as expected. However, the modeled flame temperatures were around 450 K below the measurements. It is assumed that the reason for this lies in the incorrect modeling of the flame speed since systematic errors in the LIF measurements should be small, as discussed in the paper. Iron atom concentrations were also measured using LIF. The results show the influence of the stoichiometry on the iron atom concentration, which is also reproduced by the model. The majority of the added iron was observed to exist as atomic iron in the flame. As more oxygen becomes available in leaner flames, iron will tend to form more FeO so that the iron atom concentration decreases with decreasing equivalence ratio.

Heat flux from stagnation-point hydrogen-methane-air flames: experiment and modelling

Hydrogen-methane-air flames were studied in stagnation-point geometry. Light induced phosphorescence from thermographic phosphors was used to study the wall temperatures and heat fluxes from nearly one-dimensional flat premixed flames. The studied flames were stoichiometric methane-air flames with 10%, 25%, 50% and 75% hydrogen as well as a pure hydrogen flame at ambient pressure. The flames were burning in a stagnation-point arrangement against a water cooled plate. The central part of this plate was an alumina ceramic plate coated from both sides with chromium doped alumina (ruby) and excited with a Nd:YAG laser or a green light emitting diode (LED) array to measure the wall temperature from both sides and thus the heat flux rate from the flame. The cold gas velocity was varied from 0.1 m/s to 1.2 m/s. The measured heat flux rates indicate the change of the flame stabilization mechanism from a burner stabilized to a stagnation plate stabilized flame. Flame temperatures were also measured using OH-LIF. The results were compared to the modeling results of a one dimensional stagnation-point flow, with a detailed reaction mechanism. This geometry may be well suited for further studies of the elementary flame wall interaction. The flame temperatures modeled were generally around 200 K lower than those measured.