Prof. Dr. Burak Atakan

Institute for Combustion and Gasdynamics
University of Duisburg-Essen


  • Atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) growth of undoped and aluminium-doped ZnO thin film using hot wall reactor
    Nebatti Ech Chergui, A. and Pflitsch, C. and Atakan, B.
    Surfaces and Interfaces 22 (2021)
    In this contribution, a hot wall reactor via economic atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) was adopted for Un-doped and Al-doped Zinc oxide films were deposited on borosilicate glass and silicon substrates. To avoid the use of an expensive vacuum system, all experiments were realized at atmospheric pressure. The chemical reagents used for this experiment are Zinc acetylacetonate (Zn(acac)2) and aluminium acetylacetonates (Al (acac)2) under atmospheric conditions. The obtained films are characterized by X-ray diffraction (XRD),scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and Uv-–vis spectrometer, respectively. As results, it is found that The un-doped ZnO films are polycrystalline. However, a significant enhancement in the intensity of the relevant (100) reflection is observed when Zinc oxide films are doped with Al. It is also observed that the Al-doped Zinc oxide films present higher transparency in the visible region and resistivities of 2.55 ohm cm and 1.44 ohm cm for un-doped and Al doped films respectively © 2020
    view abstract10.1016/j.surfin.2020.100883
  • Investigation of natural gas/hydrogen mixtures for exergy storage in a piston engine
    Rudolph, C. and Atakan, B.
    Energy 218 (2021)
    The conversion of mechanical to chemical energy offers an option for long-term and versatile energy storage. It was already proven that piston engines can be used as flexible reactors for energy conversion. Here, a novel method for energy conversion in piston engines is investigated, the pyrolysis of natural gas/hydrogen mixtures for energy storage. The supplied energy is stored by chemical conversion into hydrogen and higher energy hydrocarbons. The storage efficiency and the product composition are addressed here. To reach sufficiently high temperatures after compression, a dilution with 85–99% argon is used. The main products are hydrogen, acetylene, ethylene and benzene but also soot precursors are formed. The piston engine is simulated as a time-dependent four-stroke single-zone model with detailed chemical kinetics. The intake pressure is kept constant at 2 bar, while intake temperature, intake argon mole fraction and the hydrogen/natural gas ratio is varied. The hydrogen addition allows a reduction of the intake temperature and argon dilution but also reduces the storage power and efficiency. Yields of acetylene or ethylene are increased and the formation of soot precursors is suppressed. A storage power of 1.59 kW is reached with an efficiency of 52%. © 2020 Elsevier Ltd
    view abstract10.1016/
  • Plug-flow reactor and shock-tube study of the oxidation of very fuel-rich natural gas/DME/O2 mixtures
    Kaczmarek, D. and Herzler, J. and Porras, S. and Shaqiri, S. and Fikri, M. and Schulz, C. and Atakan, B. and Maas, U. and Kasper, T.
    Combustion and Flame 225 (2021)
    A polygeneration process with the ability to provide work, heat, and useful chemicals according to the specific demand is a promising alternative to traditional energy conversion systems. By implementing such a process in an internal combustion engine, products like synthesis gas or unsaturated hydrocarbons and very high exergetic efficiencies can be obtained through partial oxidation of natural gas, in addition to the already high flexibility with respect to the required type of energy. To enable compression ignition with natural gas as input, additives such as dimethyl ether are needed to increase the reactivity at low temperatures. In this study, the reaction of fuel-rich natural gas/dimethyl ether (DME) mixtures is investigated to support the further development of reaction mechanisms for these little studied reaction conditions. Temperature-resolved species concentration profiles are obtained by mass spectrometry in a plug-flow reactor at equivalence ratios ϕ = 2, 10, and 20, at temperatures between 473 and 973 K and at a pressure of 6 bar. Ignition delay times and product-gas analyses are obtained from shock-tube experiments, for ϕ = 2 and 10, at 710 – 1639 K and 30 bar. The experimental results are compared to kinetic simulations using two literature reaction mechanisms. Good agreement is found for most species. Reaction pathways are analyzed to investigate the interaction of alkanes and DME. It is found that DME forms radicals at comparatively low temperatures and initiates the conversion of the alkanes. Additionally, according to the reaction pathways, the interaction of the alkanes and DME promotes the formation of useful products such as synthesis gas, unsaturated hydrocarbons and oxygenated species. © 2020
    view abstract10.1016/j.combustflame.2020.10.004
  • An experimental and modeling study on the reactivity of extremely fuel-rich methane/dimethyl ether mixtures
    Porras, S. and Kaczmarek, D. and Herzler, J. and Drost, S. and Werler, M. and Kasper, T. and Fikri, M. and Schießl, R. and Atakan, B. and Schulz, C. and Maas, U.
    Combustion and Flame 212 (2020)
    Chemical reactions in stoichiometric to fuel-rich methane/dimethyl ether/air mixtures (fuel air equivalence ratio ϕ = 1–20) were investigated by experiment and simulation with the focus on the conversion of methane to chemically more valuable species through partial oxidation. Experimental data from different facilities were measured and collected to provide a large database for developing and validating a reaction mechanism for extended equivalence ratio ranges. Rapid Compression Machine ignition delay times and species profiles were collected in the temperature range between 660 and 1052 K at 10 bar and equivalence ratios of ϕ = 1–15. Ignition delay times and product compositions were measured in a shock tube at temperatures of 630–1500 K, pressures of 20–30 bar and equivalence ratios of ϕ = 2 and 10. Additionally, species concentration profiles were measured in a flow reactor at temperatures between 473 and 973 K, a pressure of 6 bar and equivalence ratios of ϕ = 2, 10, and 20. The extended equivalence ratio range towards extremely fuel-rich mixtures as well as the reaction-enhancing effect of dimethyl ether were studied because of their usefulness for the conversion of methane into chemically valuable species through partial oxidation at these conditions. Since existing reaction models focus only on equivalence ratios in the range of ϕ = 0.3–2.5, an extended chemical kinetics mechanism was developed that also covers extremely fuel-rich conditions of methane/dimethyl ether mixtures. The measured ignition delay times and species concentration profiles were compared with the predictions of the new mechanism, which is shown to predict well the ignition delay time and species concentration evolution measurements presented in this work. Sensitivity and reaction pathway analyses were used to identify the key reactions governing the ignition and oxidation kinetics at extremely fuel-rich conditions. © 2019 The Authors
    view abstract10.1016/j.combustflame.2019.09.036
  • An experimental investigation of propane/isobutane mixtures as alternative refrigerants in compression refrigeration machines [Propan/Isobutan-Gemische als alternative Kältemittel in Kompressionskältemaschinen: Eine experimentelle Untersuchung]
    Venzik, V. and Roskosch, D. and Atakan, B.
    Forschung im Ingenieurwesen/Engineering Research 84 (2020)
    Due to political regulations, the currently used refrigerants (e.g., R134a) have to be replaced by fluids with reduced greenhouse warming potential (GWP) in future. Natural refrigerants such as isobutane (R600a) or propane (R290) are such fluids; they are environmentally friendly, inexpensive, available in large quantities and have good thermodynamic properties. If isobutane and propane are mixed, a zeotropic mixture results. Based on basic thermodynamic theory, the non-isothermal phase change of zeotropic mixtures leads to reduced exergy losses in the heat exchangers and thus, to an increased coefficient of performance (COP). Apart from the changes in the heat exchangers, all other process components are also influenced by the mixture composition. This usually remains unconsidered in the large number of theoretical studies. To improve and validate theoretical models, a vapour-compression heat pump test rig was set up. The test rig allows the systematic investigation of different pure fluids and fluid mixtures. In addition, extensive measurement equipment is installed to measure mass flows rates, temperature profiles and pressure levels. Based on these measured values, all important process variables can be derived, such as the COP and the compressor power input as a function of the mixture composition. In this study, the pure fluids isobutane and propane as well as their mixtures are investigated. Discussed parameters are the COP, the isentropic compressor efficiencies, the specific compressor work as well as the temperature glide as a function of the evaporation temperature (0 to 6 °C), for a constant condensation temperature of 30 °C, at different compressor power and rotation speeds (1050, 1500 and 2100 min−1). For steady state conditions the results show that the use of a zeotropic mixture increases the COP compared to the respective pure fluids. The highest COPs are achieved at a composition of xPropane= 0.732; xIsobutane= 0.268 and are between 3.3 and 5.4 depending on the operating conditions. Compared to theory, the experimental results for the mixture xPropane= 0.732; xIsobutane= 0.268 show only slight improvements of the COPs by 5% compared to pure propane. A more detailed analysis shows that mainly two aspects influence the process efficiency. On the one hand the isentropic efficiency of the compressor, which depends strongly on the composition and on the other hand, the pressure losses in the heat exchangers reduce the efficiency. Especially the pressure losses in the evaporator reduces the actually usable temperature glide. These effects are usually not taken into account in simple theoretical approaches. © 2019, Springer-Verlag GmbH Deutschland, ein Teil von Springer Nature.
    view abstract10.1007/s10010-019-00389-8
  • Combined Thermogravimetric Determination of Activity Coefficients and Binary Diffusion Coefficients - A New Approach Applied to Ferrocene/ n-Tetracosane Mixtures
    Lau, S. and Atakan, B.
    Journal of Chemical and Engineering Data 65 (2020)
    The use of isothermal thermogravimetry is investigated for combined measurements of activity coefficients and diffusion coefficients in binary mixtures, where the vapor pressure is dominated by one compound. The coupling of evaporation with gas phase and condensed phase diffusion leads to an unsteady mass loss, which depends on the activity coefficient and both, gas phase and condensed phase, diffusion coefficients, as is explained. This can be used to measure two of these properties, if the remaining parameters are known. This concept is evaluated for the ferrocene/n-tetracosane system. The former is a flame-retardant, and the latter was regarded as a substitute for a polymer, with simpler behavior. Between 341 and 399 K, the activity coefficient for ferrocene is measured to drop from above 3 to values near 1, while the diffusion coefficients are weakly temperature dependent, with values around 2 × 10-9/m2·s-1. In addition, the vapor pressures of the two pure substances were re-evaluated for this temperature range. This relatively simple method seems to be useful for the determination of such properties of other flame-retardant-related systems. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acs.jced.9b00578
  • Exergoeconomic analysis of an HCCI engine polygeneration process
    Schröder, D. and Hegner, R. and Güngör, A. and Atakan, B.
    Energy Conversion and Management 203 (2020)
    A polygeneration system producing hydrogen, electricity, process steam, and heating water is modeled and studied by conducting an exergoeconomic analysis. This system includes a homogeneous charge compression ignition (HCCI) engine burning a rich methane-air mixture, a water-gas shift reactor (WGSR) and a palladium membrane for hydrogen separation. Different cost-apportioning methods were considered in the present work in order to assess their suitability for the studied system. Furthermore, a global sensitivity analysis was used to identify the relevant system parameters as well as to quantify the influence that the input data uncertainty causes on the costs of the system products. It is shown that these costs are sensitive to the investment costs of only few system components and that the highest exergy destruction rates and costs occur in the engine. With the predicted cost of hydrogen ranging from 3.23 to 3.99 €/kg at EGR ratios of up to 25%, the studied process is promising. © 2019 Elsevier Ltd
    view abstract10.1016/j.enconman.2019.112085
  • Flexible energy conversion and storage via high-temperature gas-phase reactions: The piston engine as a polygeneration reactor
    Atakan, B. and Kaiser, S.A. and Herzler, J. and Porras, S. and Banke, K. and Deutschmann, O. and Kasper, T. and Fikri, M. and Schießl, R. and Schröder, D. and Rudolph, C. and Kaczmarek, D. and Gossler, H. and Drost, S. and Bykov, V. and Maas, U. and Schulz, C.
    Renewable and Sustainable Energy Reviews 133 (2020)
    Piston engines are typically considered devices converting chemical energy into mechanical power via internal combustion. But more generally, their ability to provide high-pressure and high-temperature conditions for a limited time means they can be used as chemical reactors where reactions are initiated by compression heating and subsequently quenched by gas expansion. Thus, piston engines could be “polygeneration” reactors that can flexibly change from power generation to chemical synthesis, and even to chemical-energy storage. This may help mitigating one of the main challenges of future energy systems – accommodating fluctuations in electricity supply and demand. Investments in devices for grid stabilization could be more economical if they have a second use. This paper presents a systematic approach to polygeneration in piston engines, combining thermodynamics, kinetics, numerical optimization, engineering, and thermo-economics. A focus is on the fuel-rich conversion of methane as a fuel that is considered important for the foreseeable future. Starting from thermodynamic theory and kinetic modeling, promising systems are selected. Mathematical optimization and an array of experimental kinetic investigations are used for model improvement and development. To evaluate technical feasibility, experiments are then performed in both a single-stroke rapid compression machine and a reciprocating engine. In both cases, chemical conversion is initiated by homogeneous-charge compression-ignition. A thermodynamic and thermo-economic assessment of the results is positive. Examples that illustrate how the piston engine can be used in polygeneration processes to convert methane to higher-value chemicals or to take up carbon dioxide are presented. Open issues for future research are addressed. © 2020 The Authors
    view abstract10.1016/j.rser.2020.110264
  • Isothermal pyrolysis investigation of aluminum diethylphosphinate mixed as a flame retardant additive into ultra-high molecular weight polyethylene
    Lau, S. and Atakan, B.
    Combustion and Flame 222 (2020)
    Aliphatic polymers such as polyethylene or polypropylene are widely used in spite of their high flammability and forces the introduction of flame retardants (FR), e.g. metal phosphinates, into the polymers. These flame retardants usually act in the condensed phase or may influence the gas phase mechanism of combustion. However, the modes of action are not yet understood in detail and require increasing research. In this study a contribution to the latter is made. The thermal decomposition behavior of aluminum diethylphosphinate (AlPi) as FR, mixed in ultra-high molecular weight polyethylene (UHMWPE) is investigated here by molecular beam mass spectrometry (MBMS) in an oxygen-free atmosphere. The isothermal pyrolysis experiments for this system have been systematically studied by first concentrating on each neat compound, FR and polymer, respectively, followed by the analysis of doped polyethylene blends. The aim is to detect phosphorus-containing species in the gas phase, which is the minimum requirement for a gas phase active FR. It was found that the main product of AlPi is diethylphosphinic acid, which subsequently degrades to lighter species or dimerizes. In the mixture, although the AlPi decomposition is influenced by the polymer in the condensed phase, most of the species responsible for a flame suppressant effect are still present in the gas phase. © 2020
    view abstract10.1016/j.combustflame.2020.08.048
  • Compression–expansion processes for chemical energy storage: Thermodynamic optimization for methane, ethane and hydrogen
    Atakan, B.
    Energies 12 (2019)
    Several methods for chemical energy storage have been discussed recently in the context of fluctuating energy sources, such as wind and solar energy conversion. Here a compression–expansion process, as also used in piston engines or compressors, is investigated to evaluate its potential for the conversion of mechanical energy to chemical energy, or more correctly, exergy. A thermodynamically limiting adiabatic compression–chemical equilibration–expansion cycle is modeled and optimized for the amount of stored energy with realistic parameter bounds of initial temperature, pressure, compression ratio and composition. As an example of the method, initial mixture compositions of methane, ethane, hydrogen and argon are optimized and the results discussed. In addition to the stored exergy, the main products (acetylene, benzene, and hydrogen) and exergetic losses of this thermodynamically limiting cycle are also analyzed, and the volumetric and specific work are discussed as objective functions. It was found that the optimal mixtures are binary methane argon mixtures with high argon content. The predicted exergy losses due to chemical equilibration are generally below 10%, and the chemical exergy of the initial mixture can be increased or chemically up-converted due to the work input by approximately 11% in such a thermodynamically limiting process, which appears promising. © 2019 by the author.
    view abstract10.3390/en12173332
  • Fluid retrofit for existing vapor compression refrigeration systems and heat pumps: Evaluation of different models †
    Roskosch, D. and Venzik, V. and Atakan, B.
    Energies 12 (2019)
    The global warming potential of many working fluids used nowadays for vapor compression refrigeration systems and heat pumps is very high. Many of such fluids, which are used in currently operating refrigerators and heat pumps, will have to be replaced. In order to avoid a redesign of the system, it would be very helpful if efficient and ecological alternative working fluids for a given plant could be found. With modern process simulation tools such a selection procedure seems possible. However, it remains unclear how detailed such a model of a concrete plant design has to be to obtain a reliable working fluid ranking. A vapor compression heat pump test-rig is used as an example and simulated by thermodynamic models with different levels of complexity to investigate this question. Experimental results for numerous working fluids are compared with models of different complexity. Simple cycle calculations, as often used in the literature, lead to incorrect results regarding the efficiency and are not recommended to find replacement fluids for existing plants. Adding a compressor model improves the simulations significantly and leads to reliable fluid rankings but this is not sufficient to judge the adequacy of the heat exchanger sizes and whether a given cooling or heating task can be fulfilled with a certain fluid. With a model of highest complexity, including an extensive model for the heat exchangers, this question can also be answered. © 2019 by the authors.
    view abstract10.3390/en12122417
  • Investigation of the partial oxidation of methane/n-heptane-mixtures and the interaction of methane and n-heptane under ultra-rich conditions
    Kaczmarek, D. and Atakan, B. and Kasper, T.
    Combustion and Flame (2019)
    The homogeneous partial oxidation of methane is an interesting approach to obtain useful chemicals like synthesis gas, higher hydrocarbons, aldehydes or alcohols. Because of the low reactivity of methane, the homogeneous conversion needs high temperatures to proceed at reasonable reaction rates. Additives like n-heptane form reactive intermediates at comparatively low temperatures and initiate the conversion. To study the kinetics of doped conversion reactions, fuel-rich diluted methane/n-heptane/oxygen/argon-mixtures (2 ≤ Φ ≤ 20) were investigated in a plug-flow reactor at a pressure of 6 bar, at intermediate temperatures between 423 and 973 K and at relatively long residence times (7 ≤ τ ≤ 14 s). The product composition at the reactor outlet is analyzed by gas chromatography and mass spectrometry. Species profiles as a function of equivalence ratio and temperature are compared with simulations, and serve as validation data for different reaction mechanisms. Rates of production and reaction paths are analyzed to investigate the interaction of methane and n-heptane during the oxidation process. They show that the chemical interaction of the oxidation products of both fuels has a promoting effect on the formation of different useful products like carbon monoxide, methanol or ethane. To prove this observation, mole fraction profiles as a function of temperature were compared between experiments with an equivalence ratio of Φ = 8 using neat methane, neat n-heptane and methane/n-heptane mixtures as fuels. The results show that the yields of these species are much higher in case of the methane/n-heptane mixture compared to the yields obtained in the neat methane and neat n-heptane conversions or the sum of both. © 2019 The Combustion Institute
    view abstract10.1016/j.combustflame.2019.04.005
  • Modeling study of reactive species formation from C1–C3 alkanes in an HCCI engine
    Saylam, A. and Atakan, B. and Kaiser, S.
    Combustion Theory and Modelling 23 (2019)
    The production of oxygenated hydrocarbons, hydrogen peroxide, and ethylene by low and intermediate temperature reactions of C1–C3 alkanes in a homogeneous charge compression ignition (HCCI) in an internal combustion engine was explored via single-zone modeling. For lean equivalence ratios, the main operating parameters were successively optimised with respect to intermediate species yield. A combination of 9–13 for compression ratio, 400 rpm for engine speed, and 0.05–0.25 for equivalence ratio was found for fixed intake temperature and pressure of 400 K and 1 bar, respectively. The optimum was sharply delineated in compression ratio, and widest in equivalence ratio. For these optimal parameters, 5–13.3% of the methane fuel was converted to formaldehyde and 1.6–3.4% to hydrogen peroxide, while more than 1% ethylene yield was found for ethane, and somewhat less for propane over that range. At this optimum, adding reactive species to methane as a fuel did not significantly improve yields, nor did varying intake temperature off the chosen 400 K, indicating that in fact the parameters combination is at least near-optimal. Operating conditions of an In-situ production unit of hydrogen peroxide and formaldehyde from methane partial oxidation have been explored to feed highly-efficient combustion and/or easy accessible/stabilised operating conditions of HCCI engine fuelled by low reactive fuel, the methane/natural gas. © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/13647830.2019.1638972
  • Plug-Flow Reactor Study of the Partial Oxidation of Methane and Natural Gas at Ultra-Rich Conditions
    Kaczmarek, D. and Atakan, B. and Kasper, T.
    Combustion Science and Technology 191 (2019)
    The homogeneous partial oxidation of fuel-rich CH4/O2, CH4/C2H6/C3H8/O2 as well as CH4/C2H6/C3H8/H2/O2 mixtures is investigated in a plug-flow reactor at intermediate temperatures (473 ≤ T ≤ 973 K) and a pressure of 6 bar. Experiments are carried out at equivalence ratios (Φ) of 2, 10, and 20. Product species are analyzed using time-of-flight molecular-beam mass spectrometry. The experimental results are further compared with kinetic simulations. It was found that under the investigated conditions, the onset temperature for CH4 oxidation is above 773 K. The highest methane conversion at equivalence ratios of 10 and 20 was between 0–3% for neat methane as fuel and 10–13% for natural gas as fuel. The conversions yield useful chemicals like synthesis gas (H2/CO), C2H4, C2H6, or C3H6. Higher CH4 conversion in the natural gas mixtures results in much higher yields of all products. The natural gas components ethane and propane do not influence the reaction onset temperature. © 2019, © 2019 Taylor & Francis Group, LLC.
    view abstract10.1080/00102202.2019.1577829
  • Power and syngas production from partial oxidation of fuel-rich methane/DME mixtures in an HCCI engine
    Banke, K. and Hegner, R. and Schröder, D. and Schulz, C. and Atakan, B. and Kaiser, S.A.
    Fuel 243 (2019)
    Polygeneration is the coupling of energy conversion and conversion towards useful chemicals, providing a route towards more flexible and efficient energy systems. In this work, we explore a particular concept of polygeneration using an internal combustion engine as a reactor for partial oxidation to generate synthesis gas in variable combinations with mechanical work and heat. Experiments were performed in a single-cylinder engine operated in homogeneous-charge compression-ignition (HCCI) mode on a mixture of methane and air with dimethyl ether (DME) as a reactivity-enhancing additive. For intake temperature from 100 to 190 °C, the range of stable, non-sooting operation with acceptable pressure-rise rates was determined in terms of equivalence ratio and DME mole fraction in the fuel. At 150 °C intake temperature, 8.7–9.5% DME were needed to stabilize operation at equivalence ratios between about 1.3 and 2.7. Experimental results from fuel-rich conditions with equivalence ratios ranging from 1.65 to 2.34 were compared to simulations with a homogeneous, single-zone engine model. The concept of exergy was used to investigate the thermodynamic performance of the polygeneration engine. The effect of the equivalence ratio on work and heat output, thermal and exergetic efficiency, and selectivity towards useful product species was investigated. In the experiments a work output of up to 160 J (ϕ = 1.65) per cycle (IMEP = 4.82 bar) and exergetic efficiencies of up to 81.5% (ϕ = 2.34) were achieved. The simultaneous generation of synthesis gas had a selectivity of up to 72% for hydrogen and 79% for carbon monoxide (both at ϕ = 2.34). © 2019 The Authors
    view abstract10.1016/j.fuel.2019.01.076
  • Thermographic phosphor heat flux measurements of laminar methane/air flame impinging on a cylindrical surface
    Oketch, P.O. and Gonchikzhapov, M. and Bergmann, U. and Atakan, B.
    Measurement Science and Technology 30 (2019)
    The stagnation point heat fluxes of methane/air flames impinging normal on a cylindrical surface were determined experimentally. Light induced phosphorescence from thermographic phosphors was used to investigate surface temperatures at the stagnation point from a nearly 1D laminar premixed flame burning against a water-cooled ceramic tube. The ceramic tube was coated with 1.1% chromium-doped alumina (ruby) at the impingement area and excited with a green light-emitting diode (LED) to measure the surface temperature. The flame temperature profiles were also measured with a thermocouple of type R (Pt/Pt + 13% Rh). Effects on variations in cold gas velocity (0.1 m s-1-0.5 m s-1) relative to the flame speed, equivalence ratio (= 0.85-1.2), burner to impingement surface spacing (H/d = 0.5-2) and surface curvature are reported. The stagnation point heat fluxes are strongly influenced by the flame stabilization mechanism, which changes from burner to wall stabilization, which also is seen from the measured flame temperature profiles. Increasing the cold gas velocity of the reactants leads to higher stagnation point heat fluxes. In addition, decreasing the distance between the burner and impingement surface increases the heat flux, with higher heat fluxes recorded for a tube compared to a flat plate. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-6501/ab217e
  • Combined production of power and syngas in an internal combustion engine – Experiments and simulations in SI and HCCI mode
    Wiemann, S. and Hegner, R. and Atakan, B. and Schulz, C. and Kaiser, S.A.
    Fuel 215 (2018)
    In this work, an internal combustion engine is used as a reactor for partial oxidation to produce syngas together with mechanical work. Experiments were performed in a single-cylinder engine operated on methane/air. Spark-ignition (SI) and homogeneous-charge compression-ignition (HCCI) were investigated. For HCCI, 5 mol% n-heptane were added to the fuel to reduce auto-ignition temperatures. With spark ignition at ϕ = 1.56, the product gas contained up to 8.6 mol% CO and 7.7 mol% H2 at 71.5% exergetic efficiency, while at ϕ = 0.72 roughly the same mechanical work was generated, but with only 42.5% exergetic efficiency. Under the richer conditions achievable in HCCI combustion, syngas content increased to 15.8 mol% CO and 17.9 mol% H2, and the exergetic efficiency to 81.5%. A homogeneous single-zone model coupled with a detailed reaction mechanism was used to simulate the process. The experimental results and the simulation were in good agreement for operating points without frequent misfires. © 2017
    view abstract10.1016/j.fuel.2017.11.002
  • Potential analysis of pumped heat electricity storages regarding thermodynamic efficiency
    Roskosch, D. and Venzik, V. and Atakan, B.
    Renewable Energy (2018)
    The rising share of renewable energy sources in power generation leads to the need of energy storage capacities. In this context, also some interest in thermal energy storages, especially in a concept called pumped heat electricity storage (PHES), arises. One possible design of such a PHES system consists of a compression heat pump (HP), a thermal storage and an organic Rankine cycle (ORC). The present work analyses the general thermodynamic potential and limits of such a system by means of realistic simple Rankine cycles. The potential analysis starts with the optimal case of combining two reversible Rankine cycles with reversible heat transfer. Afterwards Rankine cycles are transferred to more realistic cycles by taking temperature differences between heat sink and source and the cycles, respectively into account (irreversible heat transfer). For this case the relation between power output of the discharging cycle and the efficiency of the entire process is analysed. In order to ensure optimal conditions, the considered working fluids are always optimal hypothetical fluids, which are defined by their critical point and some further parameters. In an inverse engineering approach, these parameters are numerically optimized, with respect to cycle efficiency and meeting several boundary conditions. It is shown that the total or roundtrip efficiencies, which are defined by the quotient of the rejected work and the stored work, are between 56% and 37% for a power output of 80% of the maximal value and decrease with increasing storage temperatures, in contrast to a Carnot cycle analysis. A further expansion of the investigation considers the influence of the isentropic efficiencies of the ORC expander and the HP compressor on the process efficiency. Here, a stronger sensitivity of the isentropic efficiency of the ORC expander on the roundtrip efficiency was found. © 2018 Elsevier Ltd
    view abstract10.1016/j.renene.2018.09.023
  • A polygeneration process concept for HCCI-engines - Modeling product gas purification and exergy losses
    Hegner, R. and Atakan, B.
    International Journal of Hydrogen Energy 42 (2017)
    This modeling study addresses the question, whether HCCI piston engines can be used as chemical reactors in a polygeneration process. In this context, a combustion engine is used to produce hydrogen by partial oxidation in the engine and the further auxiliary units assure the purification of hydrogen. Several aspects were addressed during the development of the process regarding fresh-gas- and exhaust gas-treatment and energy integration. The fresh gas preheating for methane ignition was achieved by recirculation of hot exhaust gas, which also contributed to a high flexibility towards power-, heat- and hydrogen-output. Exergetic efficiencies, process-outputs and fuel consumption were calculated and compared for different operation points. Exergetic efficiencies of up to 80% were achievable and this high efficiency leads to reduced fuel consumption, with up to 40% fuel savings compared to the separated production of power, heat and hydrogen. Power and heat flow of the process can be adjusted very flexibly and the ratio can be varied within a factor of two within the investigated operating conditions. © 2016 Hydrogen Energy Publications LLC.
    view abstract10.1016/j.ijhydene.2016.09.050
  • Fuel-Rich HCCI Engines as Chemical Reactors for Polygeneration: A Modeling and Experimental Study on Product Species and Thermodynamics
    Hegner, R. and Werler, M. and Schießl, R. and Maas, U. and Atakan, B.
    Energy and Fuels 31 (2017)
    The usage of a methane-fueled homogeneously charged compression ignition (HCCI) engine process for producing base chemicals like ethylene and synthesis gas together with some work output is investigated. This polygeneration process is studied by numerical modeling, accompanied by rapid compression machine experiments. Studies include the seeding of dimethyl ether (DME) as a reaction enhancer which allows methane conversion already at moderate, technically easily accessible compression ratios and precompression temperatures. The concept is promising for equivalence ratios above 2, predicting product gas mole fractions of up to 25 mol % H2, 20 mol % CO, and 2 mol % C2H4. These simulation results are largely consistent with the outcome of rapid compression machine (RCM) experiments, in which production of up to 20 mol % H2, 16 mol % CO, and 1 mol % C2H4 was detected. In addition to studying the product composition, thermodynamic aspects of the approach were investigated by comparing the exergetic efficiency of fuel-rich and fuel-lean combustion. These calculations also confirmed the advantages of fuel-rich combustion. © 2017 American Chemical Society.
    view abstract10.1021/acs.energyfuels.7b02150
  • Propene/isobutane mixtures in heat pumps: An experimental investigation
    Venzik, V. and Roskosch, D. and Atakan, B.
    International Journal of Refrigeration 76 (2017)
    Zeotropic mixtures in heat pumps, based on thermodynamic analysis, should lead to higher coefficients of performance (COP) due to the temperature glide which decreases exergy losses in the heat exchangers. However, fluid mixtures influence every component of a plant and the total system performance. In addition to the various theoretical studies in this field, a laboratory scale vapour compression heat pump test rig was designed and set up. In the present experimental investigations, the operating performance for the pure fluids isobutane and propene, and their mixtures are systematically investigated. COPs and exergetic efficiencies as a function of evaporation temperature, compressor speed and composition of the mixture are presented and compared with a theoretical approach. Contrary to theoretical expectations, the experimental results show only a slight increase of the COP for the mixture, compared to the better pure fluid, because heat exchanger pressure drops reduce the temperature glide. © 2017 Elsevier Ltd and IIR
    view abstract10.1016/j.ijrefrig.2017.01.027
  • Pumped heat electricity storage: Potential analysis and orc requirements
    Roskosch, D. and Atakan, B.
    Energy Procedia 129 (2017)
    The rising share of renewable energy sources in power generation leads to the need of energy storage capacities. In this context, also some interest in thermal energy storages, especially in a concept called pumped heat electricity storage (PHES), arises. One possible design of such a PHES system consists of a compression heat pump, a thermal storage and an organic Rankine cycle (ORC). The present work analyses the general thermodynamic potential and limits of such a system and deals with the unusual requirements for the ORC. The potential analysis starts with the optimal case of combining two Carnot cycles with irreversible heat transfer. It is found that the efficiency of the entire process increases with increasing storage temperature and in general roundtrip efficiencies up to 70 % are predicted. Afterwards the cycles are transferred to cycles that are more realistic by considering technical aspects and a hypothetical working fluid which is optimized by an inverse engineering approach. This leads to lowered roundtrip efficiencies, which now, decrease with increasing storage temperatures. In a second step the specific ORC requirements as part of a PHES are considered, emphasizing the working fluid parameters. Especially, the use of a latent thermal energy storage leads to an ORC design differing from common (e.g. geothermal) applications. It is shown that the efficiency of the ORC and of the entire process strongly depends on the superheating at the expander inlet; here, the superheating must be held as small as possible, contrary to ORCs using common heat sources. © 2017 The Author(s).
    view abstract10.1016/j.egypro.2017.09.235
  • Thermodynamic model for reciprocating compressors with the focus on fluid dependent efficiencies
    Roskosch, D. and Venzik, V. and Atakan, B.
    International Journal of Refrigeration 84 (2017)
    Fluids with high global warming potential, which are used in existing refrigeration cycles and heat pumps, will have to be replaced soon by less harmful fluids, but the fluid selection is difficult especially due to the unknown compressor performance. In this work a differential compressor model for reciprocating compressors is introduced which predicts volumetric and isentropic efficiencies quickly and can be easily fitted with measured data at only one operation point of an existing compressor. In order to characterise the influence of different fluids two semi-physical correlations for the valve flows are fitted here, and a procedure of transferring them to different compressors is shown. The model is validated on, in total, 63 measured points based on numerous fluids from one semi-hermetic reciprocating compressor which is part of a heat pump cycle. The calculations lead to mean prediction errors of 3.0% for the isentropic and 2.3% for the volumetric efficiency. © 2017 Elsevier Ltd and IIR
    view abstract10.1016/j.ijrefrig.2017.08.011
  • Unusual application of aluminium-doped ZnO thin film developed by metalorganic chemical vapour deposition for surface temperature sensor
    Nebatti, A. and Pflitsch, C. and Atakan, B.
    Thin Solid Films 636 (2017)
    A relatively new promising method for surface temperature measurement is the use of thermographic phosphors. For this purpose, the temperature-dependent photoluminescence (PL) properties of aluminium-doped ZnO thin films were studied. The films have been successfully deposited on substrate of Si(100)-orientation by metalorganic chemical vapour deposition (MOCVD) method. For the use of the films as temperature sensors, the Photoluminescence (PL) properties are most important. Consequently, the emission peaks are observed in the undoped and Al-doped films deposited at 550 °C and annealed at 900 °C for 2 h after ultraviolet laser excitation (355 nm). The results show that with increasing temperature the PL intensity is quenched for the Al-doped ZnO film (n(Al)/n(Zn) = 0.051). As a result, the area under the spectrum changes significantly with temperature, making it useful for temperature evaluation. Al-doped ZnO films can be used as temperature sensors within the range of room temperature to 300 °C. Beyond this range the spectrum is no longer sensitive to temperature change.
    view abstract10.1016/j.tsf.2017.07.002
  • Shock-tube and plug-flow reactor study of the oxidation of fuel-rich CH4/O2 mixtures enhanced with additives
    Sen, F. and Shu, B. and Kasper, T. and Herzler, J. and Welz, O. and Fikri, M. and Atakan, B. and Schulz, C.
    Combustion and Flame 169 (2016)
    Partial oxidation of hydrocarbons under well-controlled conditions opens a path to higher-value chemicals from natural gas with small exergy losses if the chemical conversion proceeds in an internal combustion engine as a polygeneration process (Gossler et al., 2015). For the relevant reaction conditions, kinetics models are not sufficiently validated due to the atypical reaction conditions, e.g., high equivalence ratios and pressures. The purpose of this study is to obtain experimental validation data for chemical reaction mechanisms that can be used to predict polygeneration processes in practical applications. In case of methane these processes proceed under fuel-rich conditions and yield primarily syngas (CO/H2). In this study, the partial oxidation of methane was investigated for an equivalence ratio of φ=2 in a shock-tube and a plug-flow reactor (PFR) in order to cover a wide temperature range. Time-resolved CO mole fractions were measured in shock-heated mixtures between 1600 and 2100K at ~1bar. Good agreement was found between the experiment and the models (Yasunaga et al., 2010; Burke et al., 2015; Zhao et al., 2008). Stable reaction products were monitored by time-of-flight mass spectrometry between 532 and 992K at 6bar in a tubular flow reactor at reaction times >4s. The influence of dimethyl ether (DME) and n-heptane addition on methane reactivity and conversion was investigated. The additives significantly lower the initial reaction temperature by producing significant amounts of OH. The results were compared to simulations and serve as validation data for the development of reaction mechanisms for these atypical reaction conditions. Good agreement was found between the experiment and the models for most of species. © 2016 The Combustion Institute.
    view abstract10.1016/j.combustflame.2016.03.030
  • Chemical vapor deposition and analysis of thermally insulating ZrO2 layers on injection molds
    Atakan, B. and Khlopyanova, V. and Mausberg, S. and Mumme, F. and Kandzia, A. and Pflitsch, C.
    Physica Status Solidi (C) Current Topics in Solid State Physics 12 (2015)
    High quality injection molding requires a precise control of cooling rates. Thermal barrier coating (TBC) of zirconia with a thickness of 20-40 μm on polished stainless steel molds could provide the necessary insulating effect. This paper presents results of zirconia deposition on stainless steel substrates using chemical vapor deposition (CVD) aiming to provide the process parameters for the deposition of uniform zirconia films with such a thickness. The deposition was performed with zirconium (IV) acetylacetonate (Zr(C<inf>5</inf>H<inf>7</inf>O<inf>2</inf>)<inf>4</inf>) as precursor and synthetic air as co-reactant, which allows deposition at temperatures below 600 °C. The experiments were carried out in a hot-wall reactor at pressures between 7.5 mbar and 500 mbar and in a temperature range from 450 °C to 600 °C. Important growth parameters were characterized and growth rates between 1 and 2.5 μm/h were achieved. Thick and well adhering zirconia layers of 38 μm could be produced on steel within 40 h. The transient heat transfer rate upon contact with a hot surface was also evaluated experimentally with the thickest coatings. These exhibit a good TBC performance. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssc.201510033
  • Novel silica-based adsorbents with activated carbon structure
    Curdts, B. and Pflitsch, C. and Pasel, C. and Helmich, M. and Bathen, D. and Atakan, B.
    Microporous and Mesoporous Materials 210 (2015)
    The preparation of silica adsorbances from chemical vapor infiltration of activated carbon with tetramethylsilane (TMS) is shown. The method bases on a two step process. In a first step, activated carbons are infiltrated at 943 K in a nitrogen-TMS-vapor atmosphere of 200 mbar. At these conditions, the carbon skeleton is covered with silicon. The resulting material is highly porous and well adapting the surface structure of the carbon templates. It is already partly oxidized at room temperature when coming into contact with air. In a second step, the infiltrated carbons are completely oxidized in air at 853 K. At these conditions, the supporting carbon skeleton is completely burned and the silicon becomes silicon oxide. The resulting materials are highly porous with extremely large surface areas around 835 m2/g. Overall, the novel material seems to well adapt the original macro- and microstructure of the activated carbon used. © 2015 The Authors. Published by Elsevier Inc.
    view abstract10.1016/j.micromeso.2015.02.007
  • NOx conversion properties of a novel material: Iron nanoparticles stabilized in carbon
    Busch, M. and Kompch, A. and Suleiman, S. and Notthoff, C. and Bergmann, U. and Theissmann, R. and Atakan, B. and Winterer, M.
    Applied Catalysis B: Environmental 166-167 (2015)
    Nitrogen oxides (NOx) belong to the most common pollutants from combustion processes and are a major threat to human health. Carbon-based catalysts exhibit strong advantages for NOx removal like low-toxic application and easy handling. However, gasification of the carbon matrix at elevated temperatures is still one of the greatest concerns. Hence, we have directed our focus on especially low temperature NOx-removal using a novel material, iron nanoparticles stabilized in a carbon matrix (nano-Fe/C). The investigations included NO2 uptake properties and catalytic conversion of NO2 in recycle flow at 425K and 328K, scanning transmission electron microscopy and 77K-N2-adsorption. Nano-Fe/C exhibits superior NOx-removal properties compared with untreated or iron-infiltrated activated carbon or magnetite reference catalysts. No severe catalyst deactivation or catalyst aging at 425K is observed. Even at 328K nano-Fe/C still exhibits NO2-conversion, although without converting the product NO. NO2 adsorption at 297K is suggested to occur in three stages with different kinetics: (1) NO2 adsorption and reduction to NO, (2) physisorption on the oxidized catalyst surface and (3) saturation of the catalyst and diffusion into the substrate matrix. At 425K, NO2 is quickly reduced to NO and the resulting NO is further converted to N2O. After complete consumption of NO, the residual NO2 is also converted to N2O. A possible reaction mechanism is suggested based on the conversion kinetics. © 2014 Published by Elsevier B.V.
    view abstract10.1016/j.apcatb.2014.11.013
  • Partial oxidation of methane at elevated pressures and effects of propene and ethane as additive: Experiment and simulation
    Sen, F. and Kasper, T. and Bergmann, U. and Hegner, R. and Atakan, B.
    Zeitschrift fur Physikalische Chemie 229 (2015)
    Partial homogeneous oxidation of methane (CH<inf>4</inf>) within stationary engines may be one concept for conversion of available energy to alternatively mechanical energy, heat, and additional useful chemicals like syngas (CO/H<inf>2</inf>), formaldehyde (CH<inf>2</inf>O), methanol (CH<inf>3</inf>OH) or hydrocarbons (e.g. C<inf>2</inf>H<inf>4</inf>). The present study investigates the formation reactions of chemicals experimentally and theoretically. Methane oxidation is studied under fuel-rich conditions (σ=17.50-22.25) at high pressures (6bar) and high temperatures (T <inf>max</inf>=1030K) 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 (C<inf>2</inf>H<inf>6</inf>) and propene (C<inf>3</inf>H<inf>6</inf>) 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. CH<inf>2</inf>O and C<inf>2</inf>H<inf>4</inf> can be identified as other useful chemicals produced in the experiments and the influence of the additives on the yields is discussed. © 2015 Walter de Gruyter Berlin/Boston.
    view abstract10.1515/zpch-2014-0576
  • Reverse engineering of fluid selection for thermodynamic cycles with cubic equations of state, using a compression heat pump as example
    Roskosch, D. and Atakan, B.
    Energy 81 (2015)
    Fluid selection for thermodynamic cycles like refrigeration cycles, heat pumps or organic Rankine cycles remains an actual topic. Generally the search for a working fluid is based on experimental approaches or on a not very systematic trial and error approach, far from being elegant. An alternative method may be a theory based reverse engineering approach, proposed and investigated here: The design process should start with an optimal process and with (abstract) properties of the fluid needed to fit into this optimal process, best described by some general equation of state and the corresponding fluid-describing parameters. These should be analyzed and optimized with respect to the defined model process, which also has to be optimized simultaneously. From this information real fluids can be selected or even synthesized which have fluid defining properties in the optimum regime like critical temperature or ideal gas capacities of heat, allowing to find new worldng fluids, not considered so far. The number and kind of the fluid-defining parameters is mainly based on the choice of the used EOS (equation of state). The property model used in the present work is based on the cubic Peng-Robinson equation, chosen due to its moderate numerical expense, sufficient accuracy as well as a general availability of the fluid-defining parameters for many compounds. The considered model-process works between the temperature levels of 273.15 and 333.15 K and can be used as heat pump for supplying buildings with heat, typically. The objective functions are the COP (coefficient of performance) and the VHC (volumetric heating capacity) as a function of critical pressure, critical temperature, acentric factor and two coefficients for the temperature-dependent isobaric ideal gas heat capacity. Also, the steam quality at the compressor entrance has to be regarded as a problem variable. The results give clear hints regarding optimal fluid parameters of the analyzed process and deepen the thermodynamic understanding of the process. Finally, for the COP optimization a strategy for screening large databases is explained. Several fluids from different substance groups like hydrogen iodide (COP = 3.68), formaldehyde (3.61) or cyclopropane (3.42) were found to have higher COPs than the often used R134a (3.12). These fluids will also have to fulfill further criteria, prior to their usage, but the method appears to be a good base for fluid selection. (C) 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/
  • Using the acetylacetonates of zinc and aluminium for the Metalorganic Chemical Vapour Deposition of aluminium doped zinc oxide films
    Nebatti, A. and Pflitsch, C. and Curdts, B. and Atakan, B.
    Materials Science in Semiconductor Processing 39 (2015)
    Metalorganic Chemical Vapour Deposition is a promising method for the growth of thin aluminium doped zinc oxide films (ZnO:Al), a material with potential application as transparent conducting oxide (TCO), e.g. for the use as front electrode in solar cells. For the low-cost deposition, the choice of the precursors is extremely important. Here we present the deposition of quite homogeneous films from the acetylacetonates of zinc and aluminium that are rather cheap, commercially available and easy to handle. A user-made CVD-reactor activating the deposition process by the light of halogen lamps was used for film deposition. Well-ordered films with an aluminium content between 0 and 8% were grown on borosilicate glass and Si(100). On both types of substrate, the films are crystalline and show a preferred orientation along the (002)-direction. The 0.3 to 0.5 μm thick films are highly transparent in the visible region. The best films show a low electric resistivity between 2.4 and 8 mΩ cm. © 2015 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.mssp.2015.05.053
  • Chemical vapor infiltration of activated carbon with tetramethylsilane
    Pflitsch, C. and Curdts, B. and Helmich, M. and Pasel, C. and Notthoff, C. and Bathen, D. and Atakan, B.
    Carbon 79 (2014)
    Chemical vapor infiltration of activated carbon with tetramethylsilane (TMS) at 200 hPa total pressure and a gas phase concentration of 15 (mol-)% TMS in nitrogen is studied. The influence of temperature on the infiltration process is discussed in detail. Up to 873 K, the infiltration is performed in the kinetically controlled regime resulting in high loadings up to around 42 (wt.-)%. The modified materials show high values for BET-surface and pore volume indicating a sufficient adoption of the infiltrated silicon layer to the surface morphology of the carbon substrates. Low oxidation resistance of the infiltrated material and EDX measurements give rise to the assumption that the infiltrated material is silicon. At higher infiltration temperatures above 873 K, particles are formed which have the shape of cylindrical nanostructures. EDX measurements reveal that silicon carbide is produced at these temperatures. © 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2014.07.018
  • Effect of preparation of iron-infiltrated activated carbon catalysts on nitrogen oxide conversion at low temperature
    Busch, M. and Schmidt, W. and Migunov, V. and Beckel, A. and Notthoff, C. and Kompch, A. and Bergmann, U. and Winterer, M. and Atakan, B.
    Applied Catalysis B: Environmental 160-161 (2014)
    Nitrogen oxides are toxic and their concentration in human workspace should be reduced to a minimum level. Among the possible catalyst materials activated carbon based catalysts are a cheap and non-toxic alternative of high availability. In this paper we investigate two different methods for the preparation of iron-infiltrated activated carbon catalysts: chemical vapor infiltration (CVI) and the incipient wetness method (IWM). The effects of the preparation method on the structure and catalytical performance are compared with the effects of infiltration load and co-deposition of silicon dioxide. The study elucidates profound differences in the nitrogen dioxide adsorption and catalytic nitrogen oxide decomposition, depending on the catalyst preparation technique. Samples prepared by chemical vapor infiltration exhibit well dispersed iron/iron oxide particles all over the sample cross section. Crystalline iron oxide is only detected in the samples prepared via the gas phase and not in samples prepared by IWM. The nitrogen dioxide adsorption is notably enhanced in samples with a large accessible micropore volume. All samples containing iron catalyze the conversion of nitrogen oxides into nitrous oxide and carbon monoxide, but especially the co-deposition of silica enhances the nitric oxide conversion into less harmful species. The iron/silica-co-deposited activated carbon catalyst prepared via incipient wetness method exhibits the best catalytical performance of all investigated catalysts at 425. K. © 2014 .
    view abstract10.1016/j.apcatb.2014.05.010
  • Heat flux from stagnation-point hydrogen-methane-air flames: Experiment and modelling
    Elmnefi, M.S. and Staude, S. and Bergmann, U. and Atakan, B.
    WIT Transactions on Engineering Sciences 83 (2014)
    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. © 2014 WIT Press.
    view abstract10.2495/HT140351
  • Ruthenium complexes as precursors for chemical vapor-deposition (CVD)
    Gaur, R. and Mishra, L. and Siddiqi, M.A. and Atakan, B.
    RSC Advances 4 (2014)
    Ruthenium and its compounds are often used as thin films and can be deposited by chemical vapor deposition. The quality of the films strongly depends on the inorganic precursors, their evaporation behaviour and thermochemistry. This is an area where different aspects of inorganic chemistry and chemical engineering must fit together to provide good thin films. It was noticed that providing firsthand information in one place especially for a learner of this area of research, and collection of reports on different types of ruthenium complexes as CVD precursors would be timely. Thus, in this review a bird's eye view of ruthenium complexes suitable for CVD technology, together with the presentation of different precursors, their synthesis, evaporation, decomposition and film formation is presented. A brief summary of the CVD technique is also presented with future-design, synthesis and usefulness of CVD precursors. This journal is © the Partner Organisations 2014.
    view abstract10.1039/c4ra04701j
  • Sol-Gel deposited thermographic phosphors as possible thermal history coatings
    Stenders, D. and Karadagli, I. and Pflitsch, C. and Atakan, B.
    IET Conference Publications 2014 (2014)
    In many engineering applications surface temperatures have to be measured accurately. Sometimes the maximum temperature reached by a surface within its lifetime is even more important than its actual temperature. In order to obtain information about such maximum temperatures, so called temperature history sensors can be applied. One idea to obtain such information, which is followed in the actual work, is to deposit doped oxides with phosphorescence properties which are depending on the crystal structure of the host oxide. Thus, if the crystal structure is changing at a certain temperature, this change could also be detected by a simple phosphorescence lifetime measurement afterwards. In the present work, two materials were investigated. Yttria doped with terbium and europium and alumina doped with europium. Both materials were deposited as thin films with the sol gel technique using a newly developed automated spray coating set-up. The samples were heated twice: once for calcination at a given temperature and then a second time by annealing them at different temperatures. After each of the two treatments the phosphorescence lifetimes were investigated. The first procedure is intrinsic for the production of phosphorescing material. The second annealing simulates the high temperature treatment within an application. It is found that the phosphorescence lifetime is changed in most cases, when the second annealing temperature is higher than the first calcination temperature. Thus, a certain design or alignment of the detected maximum temperature is possible. For the doped yttria, the lifetimes increase by ca. 20% after the second treatment at higher temperatures, while for doped alumina a reduction of the lifetimes by more than 50% is observed. In total this may be a promising way for obtaining adjustable temperature history sensors.
    view abstract10.1049/cp.2014.0537
  • Activity coefficients at infinite dilution of organic solutes in the ionic liquid PEG-5 cocomonium methylsulfate at T = (313.15, 323.15, 333.15, and 343.15) K: Experimental results and COSMO-RS predictions
    Reddy, P. and Aslam Siddiqi, M. and Atakan, B. and Diedenhofen, M. and Ramjugernath, D.
    Journal of Chemical Thermodynamics 58 (2013)
    Activity coefficients at infinite dilution γ13∞ have been determined for 25 polar and non-polar organic solutes (alkanes, cycloalkanes, alk-1-enes, alk-1-ynes, aromatic compounds, alcohols, and ketones) in the ionic liquid PEG-5 cocomonium methylsulfate with gas-liquid chromatography at four different temperatures, T = (313.15, 323.15, 333.15, and 343.15) K. Packed columns with phase loadings of 0.28 and 0.34 ionic liquid mass fraction in the stationary phase were employed to obtain γ13∞ values at each temperature investigated. Speed of sound, density, and refractive index values have also been measured for the pure ionic liquid at P = 0.1 MPa and T = (303.15, 313.15, 323.15, 333.15, and 343.15) K. The uncertainties in the sound speeds, densities, and refractive indices are estimated to be values of 0.9 m·s-1, 0.00003 g·cm-3, and 0.00002, respectively. Partial molar excess enthalpies at infinite dilution ΔH1E,∞ were calculated for the solutes from the temperature dependency of the γ13∞ values for the temperature range of this study. The uncertainties in the determinations of γ13∞ and ΔH1E,∞ values are 5% and 10%, respectively. Selectivity values at infinite dilution S12∞ for hexane/benzene, cyclohexane/benzene, hexane/hex-1-ene, and hexane/ethanol separations have been calculated. These have been compared against values obtained from COSMO-RS predictions. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.jct.2012.10.024
  • On surface temperature measurements with thermographic phosphors: A review
    Brübach, J. and Pflitsch, C. and Dreizler, A. and Atakan, B.
    Progress in Energy and Combustion Science 39 (2013)
    Surface temperatures, which are important in many combustion and energy transfer processes, can be measured optically using rare-earth or transition metal doped ceramic materials, so-called thermographic phosphors. For this purpose, the surface is coated with a thin phosphor film, which can be excited by different light sources. The properties of the subsequently emitted luminescence are exploited for temperature determination employing appropriate calibration measurements. The present review introduces the basic principles with regards to combustion and energy science. In this context, a broad overview of phosphor film preparation techniques is presented. For the first time, an entire error analysis is given for this technique, which may sensitise future studies for error sources and encourage an estimation of their total accuracy. Finally, a tabulated survey provides a broad database, which may help future work to identify appropriate phosphor materials. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.pecs.2012.06.001
  • Thermographic phosphor thermometry in transient combustion: A theoretical study of heat transfer and accuracy
    Atakan, B. and Roskosch, D.
    Proceedings of the Combustion Institute 34 (2013)
    Thermographic phosphors (TPs) are used in combustion environments to study wall surface temperatures and heat fluxes. Recently they are also applied in unsteady environments like internal combustion engines to study the heat transfer to walls. The present study investigates theoretically some related effects leading to limitations of the method and thus trying to help experimenters to choose proper conditions for their experiments. The influence of absorptivity and film thickness is studied first. Then the unsteady heat flux of a surrounding gas phase is investigated as a function of film thickness and conductivity, including the effect on the temperatures which would be measured using TPs. The errors in temperature measurements and time resolution are investigated for typical cases. Finally the relation between the surface temperature and film thickness is investigated for combinations of two base materials (quartz and steel) coated with layers of Mg2SiO4 or SiO2, as representatives for TP host materials. It is seen that the maximum surface temperature is influenced in unsteady heat transfer processes even by relatively thin layers. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstract10.1016/j.proci.2012.05.022
  • Upscaling the chemical vapor infiltration process of activated carbon with TMS
    Curdts, B. and Helmich, M. and Pasel, C. and Bathen, D. and Atakan, B. and Pflitsch, C.
    Physics Procedia 46 (2013)
    Activated carbons are important adsorbents covering a broad range of applications from gas to air purification. In order to improve their mechanical stability and their resistance against oxidation they are infiltrated here with an inert material, SiC or SiO2. For this process the chemical vapor infiltration technique with tetramethylsilane as precursor is used. The process is designed for the infiltration of larger quantities (up to 50 g) which allows further analysis of the materials produced: in comparison to the uncoated activated carbon the novel adsorbent have an increased breaking strength and the BET-surface areas and pore volumes are on high levels. The possibility of using the novel material for solvent adsorption is demonstrated for acetone. © 2013 The Authors.
    view abstract10.1016/j.phpro.2013.07.061
  • Alkanes as fluids in Rankine cycles in comparison to water, benzene and toluene
    Siddiqi, M.A. and Atakan, B.
    Energy 45 (2012)
    Waste heat recovery from the exhaust gases of gas turbines, geothermal resources, and from industrial plants offers a great opportunity for rational energy usage by productively converting the waste energy. In the present communication we report a systematic study starting with the hydrocarbons for the different temperature ranges at which the waste heat is available from high, medium, or low temperature heat sources (viz., 773.15, 623.15 and 523.15 K). In the present work hydrocarbons from n-pentane to n-dodecane were investigated in comparison to water, benzene and toluene. Model calculations have been performed at various boiler pressures. Higher efficiencies are achieved at higher boiler pressures. In the low temperature range (523.15 K) hydrocarbons such as n-hexane, n-pentane are promising. If the heat is available at higher temperature (773.15 K) n-dodecane and toluene are the suitable fluids. In the case studies for the intermediate temperature range (623.15 K), it turns out that for many conditions, octane, heptanes and water are well suited working fluid for cogeneration systems. The results for the total heat recovery efficiency and the surface area of the heat exchanger have been discussed. T-H Diagrams were used to select the best possible conditions. © 2012.
    view abstract10.1016/
  • Diffusion coefficients for some organic and organometallic compounds using quartz crystal microbalance
    Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B.
    Thermochimica Acta 532 (2012)
    The diffusion coefficient data for organometallic substances are scarce in the literature. A recently developed quartz crystal microbalance (QCM) system has been used to measure the binary diffusion coefficients. The system consists of a QCM with a driving oscillator circuit, a closed Stefan tube, a temperature controlled unit, a data logger, and a frequency counter. The QCM crystal is placed on the top of a Stefan tube and its active surface is coated with a thin layer of the substance. On the other end of the Stefan tube highly adsorptive charcoal powder is placed. Thus a mass concentration gradient is established in the diffusion tube. The rate of mass loss from the QCM was then used to determine the binary diffusion coefficient in air. Diffusion coefficients of polyaromatic hydrocarbons anthracene and phenanthrene and some organometallic compounds (e.g. ferrocene, aluminium acetylacetonate and chromium acetylacetonate) in air at various temperatures are reported. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tca.2011.05.009
  • MOCVD-growth of thin zinc oxide films from zinc acetylacetonate and air
    Pflitsch, C. and Nebatti, A. and Brors, G. and Atakan, B.
    Journal of Crystal Growth 348 (2012)
    The metalorganic chemical vapour deposition (MOCVD) of thin zinc oxide films on borosilicate glass and silicon substrates in a hot-wall CVD-reactor (HWR) was studied. Zinc acetylacetonate (Zn(acac) 2) and air were used as precursors. The aim of this work was to optimize the deposition parameters, such as pressure and deposition temperature, with respect to the film quality, structure, and homogeneity. Most experiments were performed at atmospheric pressure; this approach avoids the usage of an expensive vacuum system. It turned out that polycrystalline zinc oxide is grown at deposition temperatures above 613 K. Above 823 K, they additionally are c-axis orientated. At atmospheric pressure and lower temperature (< 773 K) the film deposition is homogeneously over the whole tube furnace while at higher temperature inhomogeneous film growth and particle formation are observed, indicating a shift of the growth mechanism to the diffusion controlled regime. Although the homogeneity is improved by using higher flow velocities at atmospheric pressure, particle growth cannot be suppressed. Only at reduced pressure, which was 200 mbar in the present case, the deposition at 823 K is kinetically controlled and without particle formation, resulting in the homogeneous growth of well adhering ZnO films with c-axis orientation. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jcrysgro.2012.03.016
  • Atmospheric pressure Chemical Vapor Infiltration (CVI) for the preparation of biomorphic SiC ceramics derived from paper
    Pflitsch, C. and Curdts, B. and Atakan, B.
    Journal of Nanoscience and Nanotechnology 11 (2011)
    Chemical Vapor Infiltration of biological structures such as paper is used here to produce biomorphic SiC ceramics with high temperature resistance. The biological substrate materials are infiltrated with tetramethylsilane (TMS) at atmospheric pressure and elevated temperatures of 790 °C. A simple tube furnace (hot-wall reactor) is used for the infiltration process. As result, porous SiC-ceramics are grown which are around 20% smaller and 70% lighter than the initial substrates. This can be explained by the pyrolytic reaction of the substrates while heating them up to 790 °C, which is necessary for the infiltration process. Nevertheless, besides the shrinking of the substrates the geometrical form remains nearly unchanged. The resulting materials were heated up to 1000 °C in oxygen atmosphere in order to analyze their resistance against oxidation. After this treatment, all of them were still mechanically stable and of unchanged shape while a further mass loss was observed. This confirms the high temperature stability of the prepared materials. Copyright © 2011 American Scientific Publishers All rights reserved.
    view abstract10.1166/jnn.2011.5096
  • Experimental and numerical investigations of ferrocene-doped propene flames
    Staude, S. and Bergmann, U. and Atakan, B.
    Zeitschrift fur Physikalische Chemie 225 (2011)
    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. © by Oldenbourg Wissenschaftsverlag, München.
    view abstract10.1524/zpch.2011.0157
  • Investigation of the criteria for fluid selection in rankine cycles for waste heat recovery
    Siddiqi, M.A. and Atakan, B.
    International Journal of Thermodynamics 14 (2011)
    The organic Rankine cycle is a promising way for the conversion of low temperature heat to electricity. Different fluids can be used in Rankine cycles for the utilization of waste heat. The suitability of a certain fluid will depend on its thermodynamic properties as well as on the conditions at which the heat is available, thus it is often unclear if an organic fluid has any advantage compared to inorganic fluids like water. Various substances starting from the refrigerants to high boiling organic liquids have been investigated as possible working fluids for the different temperature ranges at which the waste heat is available. The present communication reports the results for three different classes of substances: 1) a hydrocarbon (n-heptane); 2) two refrigerants (R245fa) and pentafluoro butane mixture (Solkatherm, SES36); and, 3) water in the intermediate temperature range (473 to 773 K) where the exhaust gases of combustion engines may be used as energy source for cogeneration. In this range it turns out that for many conditions, water and heptane are well suited working fluids for cogeneration systems. In the present investigation, the attention was not laid on the cycle efficiency alone, but also on the total exergy usage from an enthalpy stream (e.g. exhausts gas). This is taken into account while discussing the quality of the process. The results for different thermodynamic parameters and the surface area of the heat exchanger have been discussed. T-H. diagrams were also used for judging the suitability of a fluid. It turns out that water is well suited for many cases in the intermediate temperature regime.
    view abstract10.5541/ijot.323
  • Methylated [(arene)(1,3-cyclohexadiene)Ru(0)] complexes as low-melting MOCVD precursor complexes with a controlled follow-up chemistry of the ligands
    Jipa, I. and Danova, K. and Popovska, N. and Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B. and Cremer, T. and Maier, F. and Marbach, H. and Steinrück, H.-P. and Heinemann, F.W. and Zenneck, U.
    Journal of Materials Chemistry 21 (2011)
    [(Benzene)(2-methyl-1,3-cyclohexadiene)Ru(0)] (1), [(1,3-cyclohexadiene) (toluene)Ru(0)] (2), and [(methyl-cyclohexadiene)(toluene)Ru(0)] (3, mixture of isomers) have been prepared and tested as new metal organic ruthenium precursor complexes for chemical vapor deposition (MOCVD) with favorable properties. 1 is a low-melting precursor complex (mp = 29 °C) and the isomeric mixture 3 forms a liquid at room temperature. X-ray diffraction studies of single crystals of complexes 1 and 2 are characteristic for true Ru(0) π-complexes without molecular structure peculiarities or significant intermolecular interactions in the solid state, which could hinder undecomposed evaporation. Differential thermal analysis (DTA), differential scanning calorimetry (DSC) and vapor pressure data qualify the compounds as almost ideal MOCVD precursors. Thin ruthenium films have been deposited successfully on silicon wafers and substrate temperatures between 200 and 450 °C in inert gas atmospheres. Film growth and properties were evaluated by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and four-point probe conductivity measurements. All films consist of polycrystalline metallic ruthenium with a low surface roughness. © The Royal Society of Chemistry 2011.
    view abstract10.1039/c0jm02652b
  • Methylated [(benzene)(1,3-butadiene)Ru0] derivatives as novel MOCVD precursors with favorable properties
    Jipa, I. and Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B. and Marbach, H. and Cremer, T. and Maier, F. and Steinrück, H.-P. and Danova, K. and Popovska, N. and Heinemann, F.W. and Zenneck, U.
    Chemical Vapor Deposition 17 (2011)
    [(Benzene)(2-methyl-1,3-butadiene)Ru0] (1), [(benzene)(2,3- dimethyl-1,3-butadiene)Ru0] (2), and [(2,3-dimethyl-1,3-butadiene) (toluene)Ru0] (3) are prepared and tested as new metal-organic (MO) ruthenium precursor complexes with favorable deposition properties for the CVD of thin ruthenium films. X-ray diffraction (XRD) studies of single crystals of the complexes are characteristic for true Ru0 π-complexes without molecular structure peculiarities or significant intermolecular interactions in the solid state, which can hinder undecomposed evaporation. Differential thermal analysis (DTA) and vapor pressure data qualify the compounds as almost ideal MOCVD precursors. Thin ruthenium films are deposited successfully on silicon wafers at substrate temperatures between 200 and 400°C in a nitrogen gas atmosphere. X-ray photoelectron spectroscopy (XPS), four-point probe conductivity measurements, and atomic force microscopy (AFM) are used to characterize the films. All films consist of polycrystalline metallic ruthenium with a low surface roughness. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cvde.201006853
  • Synthesis of active carbon-based catalysts by chemical vapor infiltration for nitrogen oxide conversion
    Busch, M. and Bergmann, U. and Sager, U. and Schmidt, W. and Schmidt, F. and Notthoff, C. and Atakan, B. and Winterer, M.
    Journal of Nanoscience and Nanotechnology 11 (2011)
    Direct reduction of nitrogen oxides is still a challenge. Strong efforts have been made in developing noble and transition metal catalysts on microporous support materials such as active carbons or zeolites. However, the required activation energy and low conversion rates still limit its break-through. Furthermore, infiltration of such microporous matrix materials is commonly performed by wet chemistry routes. Deep infiltration and homogeneous precursor distribution are often challenging due to precursor viscosity or electrostatic shielding and may be inhibited by pore clogging. Gas phase infiltration, as an alternative, can resolve viscosity issues and may contribute to homogeneous infiltration of precursors. In the present work new catalysts based on active carbon substrates were synthesized via chemical vapor infiltration. Iron oxide nano clusters were deposited in the microporous matrix material. Detailed investigation of produced catalysts included nitrogen oxide adsorption, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Catalytic activity was studied in a recycle flow reactor by time-resolved mass spectrometry at a temperature of 423 K. The infiltrated active carbons showed very homogeneous deposition of iron oxide nano clusters in the range of below 12 to 19 nm, depending on the amount of infiltrated precursor. The specific surface area was not excessively reduced, nor was the pore size distribution changed compared to the original substrate. Catalytic nitrogen oxides conversion was detected at temperatures as low as 423 K. Copyright © 2011 American Scientific Publishers. All rights reserved.
    view abstract10.1166/jnn.2011.5074
  • Heat flux measurements in stagnation point methane/air flames with thermographic phosphors
    Salem, M. and Staude, S. and Bergmann, U. and Atakan, B.
    Experiments in Fluids 49 (2010)
    Light-induced phosphorescence from thermographic phosphors was used to study the wall temperatures and heat fluxes from nearly one-dimensional flat premixed flames. The investigated flames were stoichiometric, lean and rich laminar methane/air flames with equivalence ratios of φ = 1, φ = 0.75 and φ = 1.25 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 outlet velocity of the gases was varied from 0.1 to 1.2 m/s. The burner to plate distance (H) ranged from 0.5 to 2 times the burner exit diameter (d = 30 mm). The measured heat flux rates indicate the change of the flame stabilization mechanism from a burner stabilized to a stagnation plate stabilized flame. The results were compared to modeling results of a one-dimensional stagnation point flow, with a detailed reaction mechanism. In order to prove the model, gas phase temperatures were measured by OH-LIF for a stoichiometric stagnation point flame. It turns out that the flame stabilization mechanism and with it the heat fluxes change from low to high mass fluxes. This geometry may be well suited for further studies of the elementary flame wall interaction. © 2010 Springer-Verlag.
    view abstract10.1007/s00348-010-0910-4
  • Sol-gel deposition of multiply doped thermographic phosphor coatings Al2O3:(Cr3+, M3+) (M = Dy, Tm) for wide range surface temperature measurement application
    Eckert, C. and Pflitsch, C. and Atakan, B.
    Progress in Organic Coatings 68 (2010)
    A promising method of measuring surface temperatures in harsh environments is the use of thermographic phosphor coatings. There, the surface temperature is evaluated from the phosphorescence decay lifetime following a pulsed laser or flash lamp light excitation. Depending on the used dopant, single doped M3+:α-Al2O3 (M = Cr, Dy, Tm) emit at 694 nm (Cr3+), 488 nm (Dy3+), 584 nm (Dy3+), and 459 nm (Tm3+), respectively. However, the accessible temperature range with a single dopant is limited: for the Cr3+-transition from 293 K up to 900 K, and for the Dy3+ and Tm3+-transitions both from 1073 K up to 1473 K. In the present study a new approach is followed to extend these limitations by co-doping two dopants using the sol-gel method and dip coating of α-Al2O3 thin films. For that application (Dy3+ + Cr3+) co-doped thin α-Al2O3 films and (Tm3+ + Cr3+) co-doped α-Al2O3 films with thicknesses of 4-6 μm were prepared, and the temperature-dependent luminescence properties (emission spectra and lifetimes) were analysed after pulsed laser excitation in the UV (355 nm). The phosphorescence lifetime as a function of temperature were measured between 293 K and 1473 K. A considerably extended range for surface temperature evaluation was established following this new approach by combining different dopants on the molecular level. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.porgcoat.2009.08.021
  • Sol-gel-deposition of thin TiO2:Eu3+ thermographic phosphor films
    Nebatti, A. and Pflitsch, C. and Eckert, C. and Atakan, B.
    Progress in Organic Coatings 68 (2010)
    A relatively new promising method for surface temperature measurement is the use of thermographic phosphors. For this application, the temperature-dependent luminescence properties of europium(III)-doped anatase (TiO2:Eu3+) thin films were studied. The films were prepared by the sol-gel method using dip coating. The structures and the morphology of the films were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Electron dispersive X-ray spectroscopy (EDX) was used to verify the europium concentration within the films. For using the films as temperature sensors the optical properties are the main concern. Therefore, the emission spectra of the films were measured after ultraviolet laser excitation (355 nm). They indicate that the red characteristic emission (617 nm) of TiO2:Eu3+ due to the 5D0 →7F2 electric dipole transition is the strongest. The decay time constant of the exponential emission decay under UV excitation with a Nd:YAG laser (355 nm, f = 10 Hz) is strongly temperature dependent in the range from 200 °C up to 400 °C, making it useful for temperature evaluation. The temperature dependence was measured for the emission line at 617 nm; the results demonstrate that anatase doped europium(III) can be used as a thermographic phosphor. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.porgcoat.2009.08.024
  • Thermal stability and sublimation pressures of some ruthenocene compounds
    Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B. and Roth, N. and Lang, H.
    Materials 3 (2010)
    We set out to study the use of a series of ruthenocenes as possible and promising sources for ruthenium and/or ruthenium oxide film formation.The thermal stability of a series of ruthenocenes, including (η5-C5H4R)(η5-C5H4R')Ru (1), R = R' = H (3), R = H, R' = CH2NMe2 (5), R = H, R'= C(O)Me (6), R = R' = C(O)Me (7), R = H, R' = C(O)(CH2)3CO2H (8), R = H, R' = C(O)(CH2)2CO2H (9), R = H, R' = C(O)(CH2)3CO2Me (10), R = H, R'= C(O)(CH2)2CO2Me (11), R = R' = SiMe3), (η5-C4H3O-2,4-Me2)2Ru (2), and (η5-C5H5-2,4-Me2)2Ru (4) was studied by thermogravimetry. From these studies, it could be concluded that 1-4, 6 and 9-11 are the most thermally stable molecules. The sublimation pressure of these sandwich compounds was measured using a Knudsen cell. Among these, the compound 11 shows the highest vapor pressure. © 2010 by the authors.
    view abstract10.3390/ma3021172
  • Thermal stability, vapor pressures, and diffusion coefficients of some metal 2,2,6,6-tetramethyl-3,5-heptandionate [M(tmhd)n] compounds
    Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B.
    Journal of Chemical and Engineering Data 55 (2010)
    Many metal 2,2,6,6-tetramethyl-3,5-heptandionate [M(tmhd)n] compounds are volatile enough to be useful as precursors of the metals in vapor-phase deposition processes, for example, metal organic chemical vapor deposition (MOCVD). The thermal stability, vapor pressures, and gaseous diffusion coefficients of these compounds are, therefore, of fundamental importance for achieving reproducible and effective depositions. The present communication reports the thermal stability, vapor pressures, enthalpies of sublimation, and diffusion coefficients (in nitrogen and/or helium) for some metal 2,2,6,6-tetramethyl-3,5-heptandionate compounds [M(tmhd)n], namely, [Al(tmhd)3], [Cr(tmhd)3], [Cu(tmhd)2], [Fe(tmhd)3], [Mn(tmhd)3], and [Ni(tmhd)2] at temperatures between (341 and 412) K at ambient pressure. All of these are found to be stable under the investigated experimental conditions and thus are suitable precursors for CVD. © 2010 American Chemical Society.
    view abstract10.1021/je9006822
  • [cis-(1,3-diene) 2W(CO) 2] complexes as MOCVD precursors for the deposition of thin tungsten - Tungsten carbide films
    Jipa, I. and Heinemann, F.W. and Schneider, A. and Popovska, N. and Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B. and Marbach, H. and Papp, C. and Steinrück, H.-P. and Zenneck, U.
    Chemical Vapor Deposition 16 (2010)
    Tungsten - tungsten carbide thin films are deposited by metal-organic (MO)CVD on silica-coated silicon wafers using [cis-(1,3-butadiene) 2W(CO) 2] and [cis-(1,3-cyclohexadiene) 2W(CO) 2], respectively, as tunable precursor complexes. The compounds are prepared through photochemical ligand exchange reactions from [W(CO) 6] and fully characterized, including X-ray structure determination and detailed differential thermal analysis (DTA)/thermogravimetry (TG) investigations. Gas-phase diffusion coefficients and the vapor pressure of the compounds are calculated. The MOCVD experiments are performed in a vertical cold-wall reactor and the exhaust gas is analyzed by gas chromatography (GC). X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM) measurements are utilized for film characterization. Consequences of the high oxophilicity of freshly formed tungsten surfaces, consecutive surface reactions of the complex ligands, film growth, and film properties are discussed. Inside the layers, tungsten carbide is identified as the main component. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cvde.201006852
  • chemical vapor deposition

  • combustion

  • scanning electron microscopy

  • surfaces

  • x-ray diffraction

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