Reproduzierbare Herstellung und Charakterisierung von plasmagespritzten Waermedaemmschichtsystemen auf ZrO_2-Basis

The present state-of-the-art enables the deposition of plasma-sprayed thermal barrier coating systems, especially in turbine construction, with the aim of improving the efficiency of turbines by higher inlet temperatures and thus increasing profitability and reducing pollutant emissions and (electricity generation) costs. To achieve this improvement, a two-layer system is used, consisting of a metallic, vacuum-plasma-sprayed NiCoCrAlY bond coat for oxidation protection and a ceramic 7-8wt%-Y_2O_3 ZrO_2 thermal barrier coating produced by atmospheric plasma spraying and serving as a heat shield. The studies described in this thesis concern, on the one hand, the production of coating using the F4 plasma torch with an established history in the industry as well as the new triplex torch technology. Parameter variations and parameter studies for the bond coats were carried out exclusively with the F4 torch. The top coat studies were performed with both the F4 and triplex torches. For characterization, on the other hand, extensive studies were carried out using mercury analysis, image analysis, microindenter Young's modulus and hardness measurement, thermal cycling, Raman microscopy, roughness measurements, dilatometry, SEM and optical microscopy. The factor test scheme presented here is a procedure which unambiguously defines the spraying distance as the most important factor influencing porosity and spray efficiency with the triplex torch. For both torches, increasing wear (in particular of the cathods) is detected on the basis of different layer porosities. The triplex torch was subjected to much stronger wear than the F4 torch. Wear reducing measures, such as using lower electrical power, contribute to very good reproducibilities for coating production153 refs.SIGLEAvailable from TIB Hannover: RA 831(3669) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Metall-Glas-Waermedaemmschichten fuer Turbinenschaufeln

Die Oxidation an Bondcoats in Waermedaemmschichtsystemen waehrend des Betriebes ist eines der zentralen Probleme des Einsatzes von Waermedaemmschichten (WDS) auf Gasturbinenschaufeln. Diese Oxidation ruft die Abplatzungen der oxidkeramischen WDS hervor und begrenzt damit die moeglichen Einsaetze der WDS im Turbinenbau. In dieser Arbeit wird ein neuer Werkstoff fuer WDS hergestellt und untersucht, dessen Hauptaufgabe es ist, neben der Waermedaemmfunktion einen verbesserten Oxidationsschutz zu leisten. Als neuer Werkstoff wird Metall-Glas eingesetzt; im Unterschied zu oxidkeramischen Schichten besitzt eine Metall-Glas-WDS keine Rissstruktur und kann deswegen gasdicht hergestellt werden. Eine hohe Thermoschockbestaendigkeit des neues WDS-Systems wird mittels einer genauen Anpassung der Waermeausdehnungskoeffizienten von Schicht und Substrat erreicht. Ausserdem koennen die Metall-Glas-Schichten bei Temperaturen hoeher als 500-600 C die thermischen Spannungen durch plastische Verformung abbauen. Die Metall-Glas-Verbundwerkstoffe besitzen die thermischen, physikalischen und mechanischen Eigenschaften, die weder fuer Metalle noch fuer Glaeser typisch sind: sie bilden also eine neue Werkstoffart, die besonders fuer Schichten neue Perspektiven oeffnet. Im Vergleich zu WDS-Systemen aus YSZ sind die Metall-Glas-Systeme fast gar nicht erforscht. Diese Systeme koennen aus mehreren unterschiedlichen Legierungen und Glaesern bestehen und dabei ganz unterschiedliche Eigenschaften besitzen. Die Metall-Glas-WDS, die in dieser Arbeit untersucht wurden, wurden fuer den Einsatz in Gasturbinen entwickelt und muessen den harten Betriebsbedingungen entsprechen: Temperatur auf der WDS-Oberflaeche ca. 1200 C, starker Thermoschock, Betriebsdauer mehr als 10000h. Es gibt natuerlich andere Einsatzmoeglichkeiten fuer diese neue Schichtart. In Frage kommen die Faelle, in denen es um Gaskorrosionsschutz und eine Waermedaemmung geht. Die Metall-Glas-WDS koennten z.B. fuer Dieselmotoren (Waermedaemmung der Kolben) sehr interessant sein. (orig.)Oxidation on bondcoats in thermal insulation systems during operation is one of the central problems of thermal insulation layers on gas turbine blades. The author presents a new material with improved oxidation protection characteristics. It is based on metal/glass; in contrast to oxide ceramics, metal/glass systems have no crack structure and can be made gastight. High thermoshock resistance can be achieved by matching the thermal extension coefficients of the insulating layer and substrate. At temperatures beyond 500-600 C, thermal stresses are removed by plastic deformation. The systems described here are suited for difficult operating conditions. They are an interesting option also for diesel engines (thermal insulation of pistons).Available from TIB Hannover: RA 831(3818) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Untersuchung mesoskopischer Strukturen an der Grenzflaeche fest-fluessig

Metal clusters have been deposited on metal substrates and characterized with in-situ electrochemical Scanning Tunneling Microscopy (STM). These samples are model electrodes with defined mesoscopic structure and were used to investigate correlation between structure and electrochemical behavior of nanometer sized metal clusters. Different size distribution (1,5-9 nm), mean particle distance (0-50 nm) and chemical composition (Pt, Ru, PtSn and PtRu alloy) have been used. The model electrodes were electrochemically characterized by cyclic voltammetry and by electrooxidation of absorbed CO. Preliminary experiments have been done to use the tip of the in-situ STM as an analytical tool in order to investigate single metal clusters. Hydrogen has been electrochemically developed at Pt clusters and was detected as a current at the STM tip. A summary is given of the theoretical background concerning in-situ STM, metal clusters and their use as model electrodes. The experimental set-up and methods used are described. First the structural and electrochemical properties of the substrates are presented. Flame annealed Au surfaces, Au and Pt single crystal surfaces were used as substrates. It is demonstrated that clean electrochemical conditions can be achieved in the in-situ STM electrochemical cell by monitoring the characteristic single crystal surface voltamogram of Pt(111). Also electrooxidation of adsorbed CO is performed under in-situ STM conditions. (orig.)SIGLEAvailable from TIB Hannover: RA 831(3827) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Optimierung von Anoden der Hochtemperatur-Brennstoffzelle durch Korrelation von Herstellungsverfahren, Gefuege und Eigenschaften

Solid oxide fuel cells (SOFCs) are electrochemical devices which directly convert the chemical energy of a fuel into electricity. At the Research Centre Juelich, the planar substrate concept has been developed, in which the anode, which is a cermet of nickel and yttria-stabilized zirconia, consists of a supporting, porous anode substrate and a thin finely structured coating. The anode substrate currently used has been studied in detail using various methods of characterization, and its properties and microstructural features were determined for further optimization experiments. Various low-cost starting materials and different processing techniques were used for substrate production. These substrates were evaluated on the basis of their properties and microstructure with a view to application in solid oxide fuel cells. A series of microstructural tests have been carried out on anode substrates and anode layers with respect to their long-term stability. Ageing of the anodes due to Ni agglomeration was described quantitatively and in terms of a model. Low Ni agglomeration was observed whenever both Ni as well as YSZ and pores were as finely distributed as possible. (orig.)23 refs.Available from TIB Hannover: RA 831(3678) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Optische und elektrische Untersuchungen nanokristalliner Halbleiter fuer photoelektrochemische Solarzellen

This work focuses on the optical and electrical properties of nanocrystalline TiO_2 and PbS thin films. These materials show a tunable band gap, increasing with decreasing semiconductor particle size (quantum size effect). From electron microscopy pictures we can see, that the particle size and band edge increase with increasing temperature in the tempering process. Non-tempered TiO_2, that was partly doped by noble metal salts, shows no detectable particle size and very low conductivity. It is assumed to be amorphous. The particle size of nanocrystalline PbS-layers varies between 3 and 10 nm and decreases with increasing content of Nafion used as a stabilising agent. The photoelectric properties of these semiconductor layers were determined by electrochemically measuring photocurrent-spectra. Small particles show a hypsochromic shift of the spectrum but also very low photocurrents while noble metal salt doped TiO_2 shows a bathochromic shift. An alternative to the liquid electrolyte in a photoelectrochemical sensibilisation cell is the use of a hole conducting liquid crystalline solid. In this cell a dye, which is adsorbed on the surface of a porous TiO_2-electrode, injects electrons into the TiO_2-electrode upon illumination while the holes are transported by the hole conductor to the back contact. Unfortunately the series resistance is still very high for this type of cell. The high resistance of 3 nm small PbS-particles was examined by temperature-dependent measurements. Accordingly, the conductivity for small particles can be described as an activated process with only one activation energy. This leads to the explanation that the low conductivity is a consequence of a low tunnel-probability between particles in the layer or a high ''inner resistance'' inside the particles. (orig.)98 refs.Available from TIB Hannover: RA 831(3654) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Untersuchungen zur Sauerstoffreduktion an Kathoden fuer Hochtemperatur-Brennstoffzellen

Lanthan-Strontium-Manganite perowskites are the most widespread materials in use for solid oxide fuel cell cathodes. The electrode reaction taking place, i.e. the reduction of oxygen supplied by air, was investigated by electrochemical means to obtain further knowledge about the electrode processes. The high activation energy of this reaction (200 kJ/mol), preventing lower operation temperatures of the SOFC, was the starting point for the investigation. Quasi steady state current voltage measurements and impedance spectroscopy were performed in a three electrode configuration. The electrodes were of circular shape with a diameter of 10 mm. The preparation was made by screen printing as well as wet powder spraying onto plates made of Yttria-stabilized zirconia. Perowskite powders of varying chemical and stoichiometric composition were used. To obtain higher power densities and, more important, lower apparent activation energies, catalytic layers were added at the interface electrode/electrolyte. Additionally, a less complex system, a model electrode/electrolyte setup made from single-crystal YSZ as electrolyte and gold in liquid and solid state as electrode was developed to create a better defined system. This setup was used to investigate the behaviour of the electrode/electrolyte interface. Reliable, reproducible results could be obtained using either setup. The experimental conditions i.e. oxygen partial pressure, temperature and overpotential were varied in order to determine the kinetic properties of the electrodes. Apparent activation energies, pre-exponential factors, apparent charge-transfer coefficients and electrochemical orders of reaction were calculated from the current-voltage data in order to propose possible reaction steps. (orig.)74 refs.Available from TIB Hannover: RA 831(3561) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Ortsaufgeloeste Photostrommessungen (OBIC) am Halbleiter/Elektrolyt-Kontakt im Sub-Mikrometerbereich

When a semiconductor is inserted into an electrolyte, a rectifying electrical contact is produced. This contact is suitable for converting solar into electrical or chemical energy. It also acts as a model system for solid state contacts as are used for example in electronic circuits. In this study, spatial variations of such electrolyte/semiconductor-interfaces were investigated by photocurrent measurements using a newly constructed photoelectrochemical scanning laser microscope (SLM) with a very high spatial resolution in the sub-micrometer range. These measurements provide information about the lateral photocurrent density profile and the lateral quantum efficiency profile of a sample. Physical properties such as lateral changes in the electrical potential in the contact area could be deduced from that. This study focuses on the question: which photoelectrochemical and electronic processes primarily determine the spatial resolution observed. The relevant processes are above all a light-induced flatband potential shift and lateral minority carrier diffusion and drift. These investigations were mainly performed with polycrystalline as well as with partially corroded, defect-rich or gold-structured, single crystalline titanium dioxide (TiO_2). (orig.)183 refs.Available from TIB Hannover: RA 831(3715) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Verfahrensanalyse von Brennstoffzellensystemen zur Stromerzeugung fuer Elektroantriebe in Fahrzeugen bei Nutzung unterschiedlicher Kraftstoffe

In Energieumwandlungssystemen fuer Elektrofahrzeuge mit Brennstoffzellen zur Erzeugung des Stroms fuer den Antrieb koennen unterschiedliche Kraftstoffe als Energietraeger eingesetzt werden. Diese einzelnen Kraftstoffe koennen entweder in einem jeweils darauf zugeschnittenen Verfahren zu einem wasserstoffreichen Brenngas umgesetzt werden oder auch in einem weitgehend einheitlichen Verfahren unter Beruecksichtigung der spezifischen Anforderungen des Kraftstoffs. In der vorliegenden Analyse werden Ergebnisse fuer die Kraftstoffe Methanol, Ethanol, Dimethylether (DME), Benzin, Dieselkraftstoff und Propan fuer ein Verfahren mit autothermer Reformierung mit Hilfe der rechnerischen Simulation erarbeitet. Daneben werden Verfahren mit beheizter Reformierung fuer Ethanol, Methanol und DME untersucht, so dass ein Vergleich der Wasserstofferzeugung dieser Kraftstoffe in den unterschiedlichen Verfahren moeglich wird. Der Wirkungsgrad der Erzeugung elektrischen Stroms im Gesamtsystem wird von den Wirkungsgraden der Gaserzeugung und auch der notwendigen Verdichter und Entspannungsturbinen unter Beruecksichtigung des erreichbaren Zellenwirkungsgrades beeinflusst. (orig.)In energy conversion systems for electric vehicles with fuel cells for generating electricity for the power train, different fuels can be used as energy carriers. These fuels can be converted into hydrogen-rich fuel gas either in a process tailored to the respective fuel or in a largely unified process accounting for the specific requirements of the fuel used. In the present analysis, results for methanol, ethanol, dimethylether (DME), gasoline, diesel fuel and propane obtained with the aid of computer simulation will be described for a process with autothermal reforming. Furthermore, processes with heated reforming are investigated for ethanol, methanol and DME, so that a comparison of the hydrogen production of these fuels in the different processes will be possible. The efficiency of electricity generation in the overall system is influenced by the efficiencies of gas production and of the necessary compressors and expansion turbines taking account of the cell efficiency achievable. (orig.)SIGLEAvailable from TIB Hannover: RA 831(3666) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Photoelektrische Messungen mit der Rastersondenmikroskopie zur Mikroanalyse reiner und modifizierter Halbleiteroberflaechen im Elektrolyten Projektabschlussbericht

A novel method for measuring the local photocurrent together with the topography on semiconductor samples with a resolution less than 1 nm is described. The setup is based on a scanning tunneling microscope and is especially able to measure the short circuit photocurrent in order to distinguish it from pure photoconductivity. With measurements in ambient environment on a tungsten diselenide model semiconductor it is shown that the system consisting of the tunneling tip, the tunnel gap and the semiconductor behaves as a nanosized MIS (metal-insulator-semiconductor) solar cell. The spatially resolved photocurrent is a quite sensitive tool for analyzing the electronic variations on a semiconductor surface. Variations in flat band potential as well as changes in the recombination rate can be detected and linked to structures of the surface, below the surface, or on metal particle modified surfaces. It is possible to visualize the space charge zones along steps as well as around catalyst particles on the surface. Measuring the photocurrent makes it possible to directly observe effective barriers of copper particles and of size effects influencing the width of the space charge layer at the surface. (orig.)16 refs.SIGLEAvailable from TIB Hannover: RA 831(3718) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekStiftung Volkswagenwerk, Hannover (Germany)DEGerman

Auslegung und Bau eines thermischen Wasserstoffkompressors auf der Basis von Metallhydriden

In der vorliegenden Arbeit wird die thermische Wasserstoffkompression auf der Basis von Metallhydriden als Alternative zu konventionellen Wasserstoffverdichtungsverfahren fuer geringe Durchsaetze von einigen Normkubikmetern pro Stunde untersucht. Die Absorptions- und Desorptionsreaktion der Metalle bzw. Legierungen mit Wasserstoff (Hydrierung) erfolgt temperaturabhaengig bei unterschiedlichen Gleichgewichtsdruecken. Dieses Verhalten wird genutzt, um Wasserstoff unter Aufwendung thermischer Energie zu verdichten. Den Schwerpunkt der Arbeit bildet die experimentelle Untersuchung eines zweistufigen thermischen Wasserstoffkompressors, mit dem Wasserstoff auf bis zu 120 bar verdichtet wird. Die verwendeten Metallhydride sind AB_2-Legierungen mit den Hauptlegierungselementen Titan und Mangan. Die Wahl der Metallhydride erfolgte unter der Randbedingung, dass fuer die Desorption Waerme mit einer Temperatur von etwa 100 C zur Verfuegung steht. Die Versuche bilden die Grundlage fuer die Modellierung des Betriebsverhaltens des thermischen Wasserstoffkompressors. Das Modell beschreibt in guter Uebereinstimmung mit den Experimenten die Abhaengigkeit des Durchsatzes und des spezifischen Waermebedarfs von den Desorptionsparametern Druck und Temperatur sowie von der Desorptionszeit. Eine Optimierung des Betriebsverhaltens kann unter Beruecksichtigung sicherheitstechnischer Aspekte durch die Verringerung des Hohlraumanteils im Metallhydridreaktor sowie durch eine veraenderte Konstruktion der Reaktoren erreicht werden. Im Hinblick auf den Einsatz eines thermischen Wasserstoffkompressors spielt die Zyklusstabilitaet der verwendeten Metallhydride eine grosse Rolle. Die Langzeitversuche, in denen das Metallhydrid unter Verwendung unterschiedlicher Wasserstoffqualitaeten zyklisch be- und entladen wurde, zeigen dass eine Reinigung des in einer Elektrolyse produzierten Wasserstoffs im Hinblick auf die Erhaltung der Kapazitaet zwingend notwendig ist. Schliesslich zeigen die Auslegungsrechnungen der thermischen Waermeversorgung mit Vakuum-Roehrenkollektoren, dass die Bedingung einer 100-prozentigen solaren Deckung des Waermebedarfs zu grossen Kollektorflaechen fuehrt, so dass die Kosten der Waermeversorgung in etwa die Haelfte der Gesamtinvestitionen ausmachen. (orig.)In the present study a thermal hydrogen compressor based on metal hydrides is studied as an alternative to conventional processes for the compression of hydrogen with low volumetric flowrates in the range of a few normal cubic meters per hour. The absorption- and desorption-reaction of these alloys with hydrogen takes place at different equilibrium pressures depending on temperature. This behavior is used to compress hydrogen by supplying heat. The main focus of this work is the experimental evaluation of an apparatus that compresses hydrogen in two stages up to 120 bar and that is supplied only with thermal energy. The metal hydrides chosen are of the AB_2-structure consisting mainly of Titanium and Manganese. They have been applied due to the fact that the heat of desorption is provided at a temperature level of 100 C. A model that is derived from the experiments and that is in good agreement with their results describes the volumetric hydrogen flow and the specific heat consumption as a function of the desorption parameters pressure and temperature as well as the desorption time. The performance of the thermal hydrogen compressor can be optimized within the limits of safety consideration by reducing the void space in the reactor vessels that contain the metal hydride and by changing the construction of these vessels. Having in mind the application of a thermal hydrogen compressor the cyclic stability of the metal hydride plays a major role. The long-time studies, in which the metal hydride was loaded and unloaded in a cyclic operation mode using different hydrogen qualities show the need to purify the hydrogen, which is produced in an electrolyser, in order to maintain the capacity of the metal hydride. Finally, the calculations concerning the sizing of the solar collectors for the heat supply lead to the conclusion that almost half of the investment costs are due the heat supply system, when 100% of the heat needed for the compression is covered by solar energy. (orig.)SIGLEAvailable from TIB Hannover: RA 831(3571) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman